Alkali metal (K, Rb, Cs) doped methylammonium lead iodide ...

i

Alkali metal (K Rb Cs) doped methylammonium lead iodide perovskites for potential

photovoltaic applications

PhD Dissertation Abida Saleem

Department of Physics

Quaid-i-Azam University Islamabad Pakistan

2019

ii



This work is submitted as a dissertation in partial fulfillment of

the requirement for the degree of

DOCTORATE

IN

PHYSICS

by

ABIDA SALEEM

Material Science Laboratory Department of Physics


2019

iii

iv

v

vi

vii

DEDICATED

TO

MY LOVING

FATHER

AND

LATE SISTER

(HAJRA SALEEM)

viii

Acknowledgments This dissertat ion and the work behind were completed with the

selfless support and guidance of many I would like to express my deep

grat itude to them and my sincere wish that even though the

dissertat ion has come to a conclusion our friendships continue to thrive

First I owe sincerest thanks to Prof Nawazish Ali Khan my Ph D

research advisor for the opportunity to part icipate in the dist inguished

research in his group His research guideline of always taking the

challenges has influenced me the greatest During the years I have

never been short of encouragement t rust and inspiration from my

advisor for which I thank him most

I thank Dr Afzal Hussain Kamboh who has also been a constant

support for me these years I am also grateful to my colleagues at NCP

for extending every required help to carry out my research work

I will not forget to thank my dearest senior partners in Dr Nawazish Ali

Khan lab I am glad to have the chance to share the joys and hard

t imes together with my fellow colleagues Ambreen Ayub Muhammad

Imran and Asad Raza I wish them best for their life and career

At last I thank my parents especially my father siblings spouse

my daughter Anaya Javed Khan grandparents (late) and other family

members I would not have reached this step without their belief in me

I would also like to acknowledge Higher education commission

of Pakistan for the financial support under project No 213-58190-2PS2-

071 to carryout PhD project

ABIDA SALEEM

2019

ix

Abstract The hybrid perovskite solar cells have become a key player in third generation

photovoltaics since the first solid state perovskite photovoltaic cell was reported in

2012 Over the course of this work a wide array of subjects has been treated starting

with the synthesis and deposition of different charge transpo rt layers synt hesis of

hybrid perovskite materials optimization of annealing temperature stability of the

material with the addition of inorganic metal ions and photovoltaic device

fabrications The key advantages of methylammonium lead iodide

(CH3NH3PbI3=MAPbI3 CH3NH3=MA) perovskite is the efficient absorption of light

optimum band gap and long carrier life time The organic components ie CH3NH3

in MAPbI3 perovskites bring instabilities even at ambient conditions To address such

instabilities an attempt has been made to replace the organic constituent with

inorganic monovalent cations K+1 Rb+1 and Cs+1 in MAPbI3 (MA)1-xBxPbI3 (B= K

Rb Cs x=0-1) The optical morphological structural chemical optoelectronic

and electrical properties of the materials have been explored by employing different

characterization techniques Initially methylammonium lead iodide (MAPbI3)

compound grows in a tetragonal crystal structure which remains intact with lower

doping concentrations However the crystal structure of the material is found to be

transformed from tetragonal at lower doping to double phase ie simultaneous

existence of tetragonal MAPbI3 and orthorhombic BPbI3 (B=K Rb Cs) structure at

higher doping concentrations These structural phase transformations are also visible

in electron micrographs of the doped samples The resistances of the samples were

seen to be suppressed in lower doping range which can be attributed to the more

electropositive character of inorganic alkali cations A prominent blue shift has seen

in the steady state photoluminescence and optical absorption spectra with higher

alkali cation doping which corresponds to increase in the energy bandgap and this

effect is very small in light doped samples The x-ray photoemission spectroscopy

studies of all the investigated perovskite samples have shown the presence of Pb+2 and

I-1 oxidation states The intercalation of inadvertent carbon and oxygen in perovskite

films was also investigated by x-ray photoemission spectroscopy It is observed that

the respective peaks intensities of carbon and oxygen responsible for

methylammonium lead iodide decomposition has decreased with partial dop ing

which can be attributed to the doping of oxidation stable alkali metal cations (K+1

Rb+1 Cs+1) Following this work some of the properties of the phase pure organic-

inorganic MAPbI3 have been studied The selected devices with pristine as well as

x

doped perovskite ie (MA)1-xBxPbI3 (B= K Rb Cs) based inverted perovskite

photovoltaic cells were fabricated and tested their power conversion efficiencies

Later the power conversion characteristics of the devices were investigated by

developing an electronic circuit allowing versatile power point tracking of solar

devices The device with the best efficiency of 1537 was attained with 30 Cs

doping having device parameters as open circuit voltage value of 108V

photocurrent density (Jsc) of 1970mAcm2 and fill factor of 072 In case of

potassium (K+) based mixed cation perovskite based devices efficiency of about

1332 is obtained with 10 doping Using this approach the stability of the

materials and performances of perovskite based solar devices have been increased

These studies showed that the organic (MA) and inorganic cations (K Rb and Cs) can

be used in specific ratios by wet chemical synthesis procedure for better stability and

efficiency of solar cells We showed that mixed cations lead to a stable perovskite

tetragonal phase in low atomic concentrations with no appreciable variation in energy

bandgap of the photo-absorber allowing the material to intact with the properties of

un-doped perovskite with enhanced efficiency and stability

xi

LIST OF PUBLICATIONS

1 Abida Saleem M Imran M Arshad A H Kamboh NA Khan Muhammad I

Haider Investigation of (MA)1-x Rbx PbI3(x=0 01 03 05 075 1) perovskites as a

Potential Source of P amp N-Type Materials for PN-Junction Solar Cell Applied

Physics A 125 (2019) 229

2 M Imran Abida Saleem NA Khan A H Kamboh Enhanced efficiency and

stability of perovskite solar cells by partial replacement of CH3NH3+ with

inorganic Cs+ in CH3NH3PbI3 perovskite absorber layer Physica B 572 (2019) 1-

11

3 Abida Saleem Naveedullah Kamran Khursheed Saqlain A Shah Muhammad

Asjad Nazim Sarwar Tahir Iqbal Muhammad Arshad Graphene oxide-TiO2

nanocomposites for electron transport application Journal of Electronic

Materials 47 (2018) 3749

4 M Zaman M Imran Abida Saleem AH Kamboh M Arshad N A Khan P

Akhter Potassium doped methylammonium lead iodide (MAPbI3) thin films as

a potential absorber for perovskite solar cells structural morphological

electronic and optoelectric properties Physica B 522 (2017) 57-65

5 Nawazish A Khan Abida Saleem Anees-ur-Rahman Satti M Imran A A

Khurram Post deposition annealing a route to bandgap tailoring of ZnSe thin

films J Mater Sci Mater Electron 27 (2016) 9755ndash9760

6 M Imran Abida Saleem Nawazish A Khan A A Khurram Nasir Mehmood

Amorphous to crystalline phase transformation and band gap refinement in

ZnSe thin films Thin solid films 648 (2018) 31-36

7 N A Khan Abida Saleem S Q Abbas M Irfan Ni Nanoparticle-Added

Nix (Cu05Tl05)Ba2Ca2Cu3O10-δ Superconductor Composites and Their Enhanced

Flux Pinning Characteristics Journal of Superconductivity and Novel

Magnetism 31(4) (2018) 1013

8 M Mumtaz M Kamran K Nadeem Abdul Jabbar Nawazish A Khan Abida

Saleem S Tajammul Hussain M Kamran Dielectric properties of (CuO CaO2

and BaO) y CuTl-1223 composites 39 (2013) 622

9 Abida Saleem and ST Hussain Review the High Temperature

Superconductor (HTSC) Cuprates-Properties and Applications J Surfaces

Interfac Mater 1 (2013) 1-23

corresponding authorship

xii

List of Foreign Referees

This dissertation entitled ldquoAlkali metal (K Rb Cs) doped methylammonium

lead iodide perovskites for potential photovoltaic applicationsrdquo submitted by

Ms Abida Saleem Department of Physics QuaidiAzam University

Islamabad for the degree of Doctor of Philosphy in Physics has been

evaluated by the following panel of foreign referees

1 Dr Asif Khan

Carolina Dintiguished Professor

Professor Department of Electrical Engineering

Director Photonics amp Microelectronics Lab

Swearingen Engg Center

Columbia South Carolina

Emailasifengrscedu

2 Prof Mark J Jackson

School of Integrated Studies

College of Technology and Aviation

Kansas State University Polytechnic Campus

Salina KS67401 United States of America

xiii

Table of Contents 1 Introduction and Literature Review 1

11 Renewable Energy 1

12 Solar Cells 1 121 p-n Junction 2 122 Thin film solar cells 2 123 Photo-absorber and the solar spectrum 3 124 Single-junction solar cells 4 125 Tandem Solar Cells 5 126 Charge transport layers in solar cells 7 127 Perovskite solar cells 8

1271 The origin of bandgap in perovskite 10 1272 The Presence of Lead 11 1273 Compositional optimization of the perovskite12 1274 Stabilization by Bromine13 1275 Layered perovskite formation with large organic cat ions 14 1276 Inorganic cations15 1277 Alternative anions16 1278 Multiple-cat ion perovskite absorber17

13 Motivation and Procedures 17

2 Materials and Methodology 19

21 Preparation Methods 19 211 One-step Methods 19 212 Two-step Methods 20

22 Deposition Techniques 21 221 Spin coating 21 222 Anti-solvent Technique 21 223 Chemical Bath Deposition Method 22 224 Other perovskite deposition techniques 23

23 Experimental 23 231 Chemicals details 23

24 Materials and films preparation 24 241 Substrate Preparation 24 242 Preparation of ZnSe films 24 243 Preparation of GO-TiO2 composite films 24 244 Preparation of NiOx films 25 245 Preparation of CH3NH3PbI3 (MAPb3) films 26 246 Preparation of (MA)1-xKxPbI3 films 26 247 Preparation of (MA)1-xRbxPbI3 films 26 248 Preparation of (MA)1-xCsxPbI3 films 26 249 Preparation of (MA)1-(x+y)RbxCsyPbI3 films 26

25 Solar cells fabrication 27 251 FTONiOxPerovskiteC60Ag perovskite solar cell 27 252 FTOPTAAPerovskiteC60Ag perovskite solar cell 27

26 Characterization Techniques 27 261 X- ray Diffract ion (XRD) 27 262 Scanning Electron Microscopy (SEM) 28 263 Atomic Force Microscopy (AFM) 29 264 Absorption Spectroscopy 29 265 Photoluminescence Spectroscopy (PL) 30 266 Rutherford Backscattering Spectroscopy (RBS) 31 267 Particle Induced X-rays Spectroscopy (PIXE) 31 268 X-ray photoemission spectroscopy (XPS) 32 269 Current voltage measurements 34 2610 Hall effect measurements 34

xiv

27 Solar Cell Characterization 35 271 Current density-voltage (J-V) Measurements 35 272 External Quantum Efficiency (EQE) 36

3 Studies of Charge Transport Layers for potential Photovoltaic Applications 37

31 ZnSe Films for Potential Electron Transport Layer (ETL) 39 311 Results and discussion 39

32 GOndashTiO2 Films for Potential Electron Transport Layer 46 321 Results and discussion 46

33 NiOX thin films for potential hole transport layer (HTL) application 51 331 Results and Discussion 51

34 Summary 67

4 Alkali metal (K Rb Cs RbCs) Based Mixed Cation Perovskites 70

41 Results and Discussion 71 411 Optimization of annealing temperature 71 412 Potassium (K) doped MAPbI3 perovskite 73 413 Rubidium (Rb) doped MAPbI3 perovskite 81 414 Cesium (Cs) doped MAPbI3 perovskite 94 415 Effect of RbCs doping on MAPbI3 perovskite103

42 Summary 105

5 Mixed Cation Perovskite Photovoltaic Devices 107

51 Results and Discussion 108

52 Summary 125

6 References 125

7 Conclusions and further work plan 133

71 Summary and Conclusions 133

72 Further work 138

xv

List of Figures Figure 11 (a) A figure from Shockley and Queisserrsquos work displaying the maximum attainable theoretical efficiency (η)of single-junction semiconductor solar cells as a function of the semiconductor band-gap (Xg) [9] (b) Spectral energy entropy for the diluted and direct AM0 spectrum [10]4 Figure 12 Two-junction tandem solar cell with four contacts 6 Figure 13 (a) Cubic crystal-structure of a ABX3 type perovskite (b) The same perovskite structure but for MAPbI3 MA cation is in the center which is enclosed by 12 iodide ions in corner sharing PbI6 octahedra [77] 10Figure 14 (a)Energy band diagram of MAPbI3 perovskite based solar cell (b) the configuration of the solar cell (c) Photograph of lab-scale MAPbI3 perovskite solar cells with identical layers as in the diagram to the left with a 10 Euro cent coin for scale (d) Side view of the same sample and coin 10Figure 21 One step synthesis method of perovskite The precursor materials are dissolved in the one solution (a single solvent or mixture of two solvents) and the substrate is coated with the solution The perovskite changes its color at annealing 20Figure 22 Schematic illustration of the MAPbI3 perovskite synthesis using a two-step method [142] 20Figure 23 A spin coating instrument the solution to be coated is placed in the micropipette the lid placed down and the spinning cycle started 21Figure 24 Scanning electron microscopy opened sample chamber 29Figure 25 (a) A Schematic diagram of Rutherford Backscattering Spectroscopy Setup (b) working principle of Rutherford Backscattering Spectroscopy 31Figure 26 Particle induced X-rays spectroscopy (PIXE) results of a Specimen containing calcium and titanium 32Figure 27 Basic elements of x-ray photoemission spectroscopy (XPS) experiment 33Figure 28 2-probe Resistivity Measurements 34Figure 29 (a) Current density vs voltage (J-V) characteristics of a typical solar cell under illumination intensity (AM1 AM05 spectrum) (b) Power curve of the solar cell 36Figure 31 (a) Perovskite solar cell configuration (b) Inverted perovskite solar cell configuration 39Figure 32 Rutherford backscattering spectra (RBS) of ZnSe films annealed at different temperatures 40Figure 33 X-ray diffractograms of ZnSe thin films annealed at different temperatures 41Figure 34 Optical transmission spectra of ZnSe thin films at different annealing temperatures 42Figure 35 (αhν)2 versus hν of ZnSe thin films annealed at different temperature 42Figure 36 X-ray diffractograms of ZnSeITO films annealed at various conditions 43Figure 37 Transmittance spectra of ZnSeITO thin films at different annealing temperatures 44Figure 38 (αhν)2 versus hν plots of ZnSeITO thin films 45Figure 39 Current voltage (IndashV) graphs of ZnSeITO thin films 45Figure 310 X-ray diffraction patterns of (a) GO (b) TiO2 nanoparticles (c) xGOndashTiO2 (x = 0 2 4 8 and 12 wt)ITO thin films 47Figure 311 Electron micrographs of xGOndashTiO2 (a) x=0 (b) x = 2 wt (c) x = 4 wt (d) x = 8 wt and (e)x = 12 wt nanocomposite thin films on ITO substrates 48Figure 312 Optical transmission spectra of xGOndashTiO2 (x = 0 2 4 8 and 12 wt) nanocomposite thin films on ITO substrates 49Figure 313 (αhν)2 vs hν plots of xGOndashTiO2 (x = 0 2 4 8 and 12 wt) nanocomposite films on ITO substrates 49Figure 314 Current-voltage characteristics of xGOndashTiO2 (x = 0 2 4 8 and 12 wt)ITO films 50Figure 315 X-ray photoemission spectroscopy (a) survey scan (b) O1s (c) C1s (d) Ti2p core level spectra of as synthesized xGOndashTiO2 (x = 8 wt)ITO films 51Figure 316 X-ray diffraction pattern of 01M NiO films on (a) glass substrates (b) FTO substrates annealed at 150-600 oC 52Figure 317 UV-Vis-NIR (a) Transmission spectra (b) (αhν)2 vs hν plots of 01M NiOx glass films annealed at 150-600oC temperatures 53

xvi

Figure 318 X-ray diffraction scans of 01M yAg NiO (y=0 4 6 8mol)FTO thin films 54Figure 319 XRD scans of NiOx glass films using Ni(NO3)26H2O and Ni(ace)4H2O precursors 54Figure 320 UV-Vis-NIR (a)Transmittance (b) absorption spectra of yAg NiO (y=0 4 6 8mol)FTO thin films 56Figure 321 (a)(αhν)2 vs hν plots (b) Extinction coefficient (n) of yAg NiOx (y=0 4 6 8mol)FTO 56Figure 322 Optical (a)Transmission spectra (b)(αhν)2 vs hν plots of the NiOx thin film Prepared by (01 and 075M) Nickel Nitrate and 075M Nickel acetate precursor on glass substrates 57Figure 323 XRD scans of 075M precursor based yAg NiOx (y=0-8 mol ) films (a )glass substrates (b) FTO substrates (c) Strained picture of yAg NiOx (y=0 4 6 8 mol )FTO thin films 59Figure 324 Optical (a)Transmission (b) (αhν)2 vs hν plots of yAg NiOx (y=0-8 mol)glass thin films 61Figure 325 Scanning electron micrographs of yAg NiOx (y=0 4 6 8 mol)deposited (a) on glass substrates at 50kx (b) on glass substrates at 100kx (c) on FTO substrates at 50kx (d) on FTO substrates at 100kx magnification 63Figure 326 Rutherford backscattering spectra of 075M yAg NiOx (y=0 4 6 8 mol)glass thin film 64Figure 327 Particle induced x-ray emission plot of 075M yAg NiOx (y=0 4 6 8 mol)glass thin films 65Figure 328 (a) Carrier concentration vs doping concentration (b) Hall Mobility vs doping concentration (c) Resistivity vs doping concentration (d) Conductivity vs doping concentration of yAg NiOx (y=0 4 6 8 mol)glass thin films 66Figure 329 Electrochemical impedance spectroscopy Nyquist plots of yAg NiOx (y=0 4 6 8 mol) thin films 67Figure 41 (a) planar perovskite solar cell configuration (b) Inverted planar perovskite solar cell configuration 71Figure 42 X-ray diffraction of MAPbI3 films annealed at 60 80 100 110 and 130ᵒC 72Figure 43 UV-vis absorption spectrum of MAPbI3 annealed at 60 80 100 110 and 130ᵒC 73Figure 44 X-ray diffraction of (MA)1-xKxPbI3(x=0 01 02 03 04 05 075 1) films 74Figure 45 Electron micrographs at magnification (a) 500 x and (b) 50 Kx of (MA)1-xKxPbI3 (x=0 01 03 05) 76Figure 46 Current voltage (I-V) characteristics of (MA)1-xKxPbI3 (x=0 01 02 03 04 1) films 76Figure 47 XPS survey scans of Kx(MA)1-xPbI3(x=0 01 05 1) films 78Figure 48 XPS spectra of (a) C1s (b) K2p (c) Pb4f (d) I3d (e) O1s for (MA)1-xKxPbI3 (x=0-1) films 78Figure 49 XPS valance band spectra of (MA)1-xKxPbI3(x=0 01 05 1) films 79Figure 410 Optical absorption spectra of (MA)1-xKxPbI3 (x=0 01 02 03 04 05 075 1) films 79Figure 411 (αhν)2 vs hν plots with band gap calculations of (MA)1-xKxPbI3 (x=0 01 02 03 04 05 075 1) films 80Figure 412 X-ray diffraction of MA1-xRbxPbI3(x=0 01 03 05 075 1) 82Figure 413 Strained x-ray diffraction of MA1-xRbxPbI3(x=0 01 03 05 075 1) 82Figure 414 X-ray diffraction aging studies of (a) MAPbI3 (b) MA09Rb01PbI3 (c) (MA)025Rb075PbI3 sample for four weeks at ambient conditions 83Figure 415 Scanning electron micrographs of MA1-xRbxPbI3(x=0 01 03 05 075 1) at (a) lower magnification (b) higher magnification 85Figure 416 (a)Absorption spectra (b) (αhν)2 vs hν plots of (MA)1-xRbxPbI3(x=0- 1) samples 85Figure 417 (αhν)2 vs hν plots with bandgap values of (MA)1-xRbxPbI3(x=0 01 03 05 075 1) films 86Figure 418 (a) Steady State Photoluminescence (PL) (b) Time resolved-PL spectra of (MA)1-

xRbxPbI3(x=0 01 03 05 075 1) 88

xvii

Figure 419 (a) Carrier concentration vs Doping concentration (b) Hall mobility vs Doping concentration (c) Sheet conductance vs Doping concentration (d) Magneto Resistance vs Doping concentration of (MA)1-xRbxPbI3(x=0 01 03 075) 89Figure 420 XPS survey scan of (MA)1-xRbxPbI3(x=0 01 03 05 1) films 93Figure 421 XPS Core level spectra of (a) C1s (b) N1s (c) O1s (d) Pb4f (e) I3d (f) Rb3d (g) valence band spectra of (MA)1-xRbxPbI3(x=0 01 03 05 1) films 94Figure 422 X-ray diffraction scans of (MA)1-xCsxPbI3(x=0 01 02 03 04 05 075 1) films 95Figure 423 Electron micrographs of (MA)1-xCsxPbI3 films (x=0 01 02 03 04 05 075 1) 96Figure 424 Atomic force microscopy surface images of (MA)1-xCsxPbI3 films (x=0 01 02 03 04 05 075 1) 98Figure 425 UV-Vis-NIR (a) absorption spectra (b) (αhν)2 vs hν plots of (MA)1-xCsxPbI3 (0-1) films 99Figure 426 Photoluminescence (PL) spectra of (MA)1-xCsxPbI3 (0-1) films 99Figure 427 X-ray photoemission spectroscopy (a) survey scan (b) C1s (c) N1s (d) O1s (e) Pb4f (f) I3d and (g) Cs3d Core level spectra of (MA)1-xCsxPbI3 (x=0 01 1) samples 102Figure 428 X-ray diffraction of (MA)1-(x+y)RbxCsyPbI3 (x=0 005 01 015 y=0 005 01 015) films 104Figure 429 (a) Absorption spectra (b) (αhν)2 vs hν plots of (MA)1-(x+y)RbxCsyPbI3 films 104Figure 51 (a) device configuration (b) energy diagram of MAPbI3 Based Perovskite Solar Cells 109Figure 52 Current density vs Voltage curves for (a) MAPbI3 (b) (MA)09K01PbI3 (c) (MA)09Rb01PbI3 based Perovskite Solar Cells 109Figure 53 I-V Hysteresis factor of MAPbI3 (MA)09K01PbI3 and (MA)09Rb01PbI3 based Perovskite Solar Cells 111Figure 54 EQE of MAPbI3 (MA)09K01PbI3 (MA)09Rb01PbI3 perovskite film based Solar Cells 112Figure 55 (a) Photoluminescence spectra (b) Time resolved PL spectra of MAPbI3 (MA)09K01PbI3 and (MA)09Rb01PbI3 films 113Figure 56 Variation of maximum power point (MPP) short circuit current density (Jsc) and open circuit voltage (Voc) of glassFTONiOx(MA)09K01PbI3C60Ag perovskite solar cells with time under illumination (AM15) in ambient conditions 114Figure 57 (a) Schematic illustration of the device structure (b) energy diagram of (MA)1-

xCsxPbI3 (x=0 01 02 03 04 05 075 1) perovskite solar cells 115Figure 58 The JndashV characteristics of the solar cells obtained using various Cs concentrations (MA)1-xCsxPbI3 (x=0-1) were measured under AM 15G illumination 115Figure 59 (a) Dark current density-voltage (J-V dark) characteristics (b) log(I) vs log(V) plots (c) Ideality factor (n) Vs voltage (V) plots of (MA)1-xCsxPbI3 (0-1) solar cells 119Figure 510 Current density vs Voltage curves for (a)MAPbI3 (b) (MA)08Rb01Cs01PbI3 and (c) (MA)075Rb015Cs01PbI3 based Perovskite Solar Cells 121Figure 511 I-V Hysteresis factor of MAPbI3 MA08Rb01Cs01PbI3 and MA075Rb015Cs01PbI3 based Perovskite Solar Cells 122Figure 512 External quantum efficiency (EQE) of MAPbI3 (MA)08Rb01Cs01PbI3 and (MA)075Rb015Cs01PbI3 based Perovskite Solar Cells 123Figure 513 (a) Photoluminescence spectra (b) Time resolved photoluminescence spectra of MAPbI3 (MA)08Rb01Cs01PbI3 and (MA)075Rb015Cs01PbI3 films 124

xviii

List of Tables Table 11 Effective radii of several ions used and proposed for synthesis of lead based perovskite materials for solar cell purposes [107ndash111] 13Table 31 Thickness of ZnSe films with annealing temperature 40Table 32 Energy bandgap values of ZnSeglass at different annealing temperatures in vacuum 43Table 33 Structural parameters of the ZnSeITO films annealed under various environments 44Table 34 Optical bandgap of the ZnSeITO thin films annealed at 450degC 45Table 35 ZnSe thin films annealed at 450degC 46Table 36 Band gap values of the pristine NiOglass thin film annealed at 150-500ᵒC 53Table 37 Band gap calculation of yAg NiOx (y=0 4 6 8mol )glass thin films 57Table 38 NiOxglass thin film Prepared by nickel nitrate and nickel acetate precursors 57Table 39 Interplanar spacing d(Å) and Lattice constant a(Å) measurement of yAg NiOx (y=0 4 6 8 mol ) thin films on glass substrates 59Table 310 Full width at half maxima values of yAg NiOx (y=0 4 6 8 mol )glass thin films 59Table 311 Crystallite size (D)of yAg NiOx (y=0 4 6 8 mol )glass thin films 60Table 312 Dislocation density parameter (δ) of yAg NiOx (y=0 4 6 8 mol)glass thin films 60Table 313 Micro strain parameter (ε) of yAg NiOx (y=0 4 6 8 mol)glass thin films 60Table 314 Bandgap values of 075M NiO and yAg NiOx (y=0 4 6 8 mol)glass thin films 61Table 315 Elemental composition and thickness of 075M yAg NiOx (y=0 4 6 8 mol) thin films on glass substrates calculated by RBS PIXE spectroscopy 65Table 316 Carrier concentration Hall mobility Resistivity and conductivity of 075M yAg NiOx (y=0 4 6 8 mol) thin films on glass substrates 66Table 317 Resistance of yAg NiOx (y=0 4 6 8 mol) thin films 67Table 41 Resistance values for (MA)1-xKxPbI3 (x=0 01 02 03 04 05 075 1) films 76Table 42 XPS core level binding energies of Pb4f I3d and K with reference to the C1s of (MA)1-xKxPbI3 (x=0 01 05 1) films 79Table 43 Energy bandgap values of (MA)1-xKxPbI3 (x=0 01 02 03 04 05 075 1) films 81Table 44 Binding energy values of Rb3d32 and differences between binding energies of I3d52 and Pb4f72 levels of (MA)1-xRbxPbI3(x=0 01 03 05 075 1) samples 94Table 45 Atomic of C O N Pb I Cs observed in (MA)1-xCsxPbI3(x=0 01 1) films 103Table 46 Band gap values of (MA)1-(x+y)RbxCsyPbI3 (x=005 01 015 y=005 01 015) films 105Table 51 Solar cell parameters of MAPbI3 (MA)09K01PbI3 and (MA)09Rb01PbI3 Solar Cells 110Table 52 Parameters of fitting the time resolved photoluminescence decay curves of the samples 113Table 53 current density-voltage (J-V) parameters of (MA)1-xCsxPbI3 (x=0 01 02 03 04 05 075 1) based devices 115Table 54 J-V parameters of (MA)1-xCsxPbI3 (x=0-1) based devices 116Table 55 Solar cell parameters of MAPbI3 (MA)08Rb01Cs01PbI3 and (MA)075Rb015Cs01PbI3 based Perovskite Solar Cells 122Table 56 Parameters of fitting the time resolved photoluminescence decay curves of MAPbI3 (MA)08Rb01Cs01PbI3 and (MA)075Rb015Cs01PbI3 films 125

xix

LIST OF SYMBOLS AND ABBREVIATIONS

PSCs Perovskite solar cells

MAPbI3 Methylammonium lead iodide

Jsc Short circuit current density

Voc Open circuit voltage

FF Fill factor

PV Photovoltaics

PCE Power conversion efficiency

EQE External quantum efficiency

DRS Diffuse reflectance spectroscopy

XRD X-ray diffraction

XPS X-ray photoemission spectroscopy

AFM Atomic Force Microscopy

SEM Scanning Electron Microscopy

EDX Energy Dispersive X-ray analysis

FWHM Full width at half maximum

TRPL Time resolved photoluminescence

1

CHAPTER 1

1 Introduction and Literature Review

This chapter refers to the short introduction of solar cells in general as well as the

literature review of the development in hybrid perovskite solar cells

11 Renewable Energy The Earthrsquos climate is warming because of increased man made green-house gas

emissions The climate of Earth climate has been warmed by 11degC since 1850 and is

likely to enhance at least 3degC by the end o f this century [12] The main effects caused

by global warming are increase of oceanic acidity more frequent extreme weather

incidences rising sea levels due to melting polar icecaps and inhabitable land areas

due to extreme temperature changes A 3degC increase will result in a sea level increase

of two meters in at the end of this century which is more than enough to sink many

densely populated cities in the world It is disheartening that the climate conditions

will not improve in the next several decades [3] The solution is to simply phase out

fossil fuels and progress towards renewable energy sources Different methods exist

to capture and convert CO2 [4ndash6] but the most permanent solution to climate change

is to evolve to an electric economy based on sustainable energy production

Renewable electricity production is possible through hydro wind geothermal

oceanic-wave and solar power In only one hour amount of energy hits the earth in

the form of sunlight corresponds to the global annual energy demand [7] If 008 of

earthrsquos surface was roofed by solar-panels with 20 power conversion efficiency

(PCE) the energy need could be met globally One of the advantages of solar panels

is that they can be installed anywhere and the wide-range of different solar

technologies makes it possible to employ them in various conditions However there

are regions of the world that do not receive much sunlight Therefore the exploration

for other renewable sources for energy is very significant along with photovoltaic

cells

12 Solar Cells The basic principle of any solarphotovoltaic cell is the photovoltaic effect The photo

absorber absorbs light to excite electrons (e-) and generate electron deficiency (hole

h+) The two contacts separate the photo-generated carriers spatially The physically

2

separated photo-generated charges on opposite electrodes create a photo-voltage The

charge carriers move in oppos ite directions in this electric field and get separated

This separation of charges results in a flow of current with a difference in potential

between electrodes resulting in power generation by connecting to the external circuit

121 p-n Junction

The photovoltaic devices which comprised of p-n junction their charge carries get

separated under the effect of electric field The p-n junction is made by dop ing one

part of Si semiconductor with elements yielding moveable positive (p) charges h+

and another part with elements yielding mobile negative (n) charges e- When those

two parts are combined it results in the development of a p-n junction and the

difference in carrier concentration between the two parts generates an electric field

over the so-called depletion region An electron excites by absorption of photon in

valence band and jump to the conduction band by creating an h+ behind in valence

band resulting in the formation of an exciton (e- h+ pa ir) These exciton further

disassociate to form separated free-charge carriers ie e- and h+ which randomly

diffuse until they either reach their respective contacts or the depletion region When

the excited e- reaches the depletion region it experiences an attraction towards the

electric field and a beneficial potential difference in the conduction band by travelling

from p-doped Si to an n-doped Si Conversely the h+ is repelled by the electric field

and experiences an unfavorable potential difference in the valence band Similarly in

the occurrence of absorption in the n-type layer the h+ that reaches the depletion

region is pulled by the field whereas the e- is repelled This promotes e- to be collected

at the n-contact and h+ at the p-contact resulting in the generation of a current in the

device

122 Thin film solar cells

In thin film solar cells charge transpor t layers are used along with p-i-n or p-n

junctions The charge transpor t layers selectively allow photo-generated h+ and e- to

pass through a specific direction which results in the separation of carriers and

development of potential difference Amorphous silicon copper- indium-gallium-

selenide cadmium telluride and copper zinc-tin-sulfide solar cells are called thin film

solar cells The copper zinc-tin-sulfide and copper- indium-gallium-selenide are p-type

intrinsically and because of the limitations of n-type doping they need a dissimilar

semiconducting material to create a p-n junction These type of p-n junction are called

3

heterojunc tion because n and p-type materials The heterojunction and a

compos itional gradient design of the semiconductor itself shifts and bends the

conduction bandvalence band structure in the material as to make it easy for the

excited e- and hard for h+ to pass through the electron selective layer and vice-versa

for the hole selective layer Organic photovoltaics are a different class of solar cells

in which the most efficient ones are established on bulk heterojunction structures A

bulk-heterojunction organic photovoltaics consists of layers or domains of two

different organic materials which can generate exciton and together separate the e--

h+ pairs In the simplest case the organic photovoltaics consists of a layer of poly (3-

hexylthiophene-25-diyl) (P3HT) as a p-type and phenyl-C61-butyric acid methyl

ester (PCBM) as a n-type material The P3HT absorbs most of the incoming visible

light resulting in exciton formation At the PCBMP3HT interface the energy level

matching of the two materials facilitates charge-separation so that the excited e- is

transferred into the PCBM layer whereas the h+ remains in the P3HT layer The

charge carriers then diffuse to their respective contacts The dye-sensitized solar cells

(DSSCs) devices generally contain organic-molecules as dyes which adhere to a

wide-bandgap semiconductor The excited e- is injected from dyersquos redox level of dye

to the conduction band of the wide-bandgap semiconducting material The

regeneration of h+ in the dye is carried out by means of an electrolyte or by solid state

hole conductor OrsquoRenegan and Graumltzel in 1991 presented a dye sensitized solar cell

prepared by sensitizing a mesoporous network of titanium oxide (TiO2) nanoparticles

with organic dyes as the light absorber with a 71 power conversion efficiency [8]

The use of a porous TiO2 structure is to enhance the surface area and the dye loading

in the solar cell The elegant working mechanism of this system relies on the ultrafast

injection of excited state e- from the redox energy level of the organic dye into the

conduction band of the TiO2

123 Photo-absorber and the solar spectrum

The pe rformance of a photo-absorber is mainly influenced by its photon

absorption capability The energy bandgap is one of the main factors that determine

which photon energies a semiconductor photo absorber absorb The best energy band

gap of a photo absorber for solar cells application depend on light emission spectrum

of the photo-harvesters Shockley and Queisser in 1961 presented a thorough power

conversion efficiency limit of a single-junction solar cell This limit is well-known by

the title of Shockley-Queisser (SQ) limit whose highest attainable limit is around

4

30 for a solar cell having single-junction Figure 11(a) [9] In some more recent

work this has been established to 338 [10] In single junction semiconductor solar

cells all photons of higher energy than the energy bandgap of the semiconductor

produce charge-carriers having energy equal to the bandgap and all the lower energy

photons than band gap remain unabsorbed The sun is our source of light and its

spectral irradiance is showed in Figure 11(b)

The sunlightrsquos spectra at the earthrsquos surface are generally described by using

air mass (AM) coefficient The extra-terrestrial sunlight spectrum which can be

observed in space has not passed through the atmosphere and is called AM0 When

the light pass by the atmosphere at right angles to the surface of earth it is called

AM1 In solar cell research the air mass 15G spectrum which is observed when the

sun is at an azimuthal angle of 4819deg is used as a reference spectrum It is a

standardized value meant to represent the direct and diffuse part of the globa l solar

spectrum (G) The AM 0 and 15 spectra differ somewhat due to light scattering from

molecules and particles in the atmosphere and from absorption from molecules like

H2O CO2 and CO Scattering causes the irradiance to decrease over all wavelengths

but more so for high energy photons and the absorption from molecules cause several

dips in the AM15G spectrum Figure 11(b)

Figure 11 (a) A figure from Shockley and Queisserrsquos work displaying the maximum attainable

theoretical efficiency (η)of single-junction semiconductor solar cells as a function of the

semiconductor band-gap (Xg) [9] (b) Spectral energy entropy for the diluted and direct AM0 spectrum

[10]

124 Single-junction solar cells

Silicon solar cells (SiSCs) are sometimes portrayed as a stagnant technology

despite dominating the photovoltaic market and presenting the one of the cheapest

sources of electricity in the world [11] The structure of the silicon solar cell and the

5

device manufacturing methods have improved tremendous ly since the first versions

The rear cell silicon and passivated-emitter solar cell technologies which yield higher

power conversion efficiencies than the conventional structure are expected to claim

market-domination within 10 years because of their compatibility with the

conventional silicon solar cells manufacturing processes [12] However the present

silicon solar cellsrsquo an efficiency record of over 26 has reached with a combination

of heterojunction and interdigitated back-contact silicon solar cells [13] Market

projections also indicate that these heterojunctions interdigitated back-contact

structures are slowly increasing their market share and will most likely dominate

market sometime after or around 2030 In short silicon solar cells present the

cheapest means to produce electricity and considering the value of the Shockley-

Queisser limit it is unlikely that other solar cell technologies will soon manage to

compete with Si for industrial generation of electricity by single-junction

photovoltaics However this does not mean that all new solar cell technologies are

destined to fail in the shadow of the silicon solar cell For instance the crystalline

silicon solar cells may not be applicable or efficient for all situations Building

integration of solar cells may require low module weight flexibility

semitransparency and aesthetic design Because crystalline-silicone is a brittle

material it requires thick glass substrates to suppor t it which makes the solar cells

relatively heavy and inflexible Semi transparency can be achieved at the cost of

absorber material thickness and hence photocurrent generation and aesthetics are

generally an opinionated subject Organic photovoltaics offer great flexibility and

low weight and can be used on surfaces which are not straight or surfaces which

require dynamic flexibility [14] Dye sensitized solar cells offer a great variety of

vibrant color s due to the use of dyes and they can along with semitranspa rency

provide aesthetic designs Furthermore dye sensitized solar cells have shown to be

superior to other solar cells when it comes to harvesting light of low-intensity such as

for indoor applications [15] The Copper zinc-tin-sulfide and Copper- indium-gallium-

selenide solar cells are generally constructed in a thin film architecture made possible

by the large absorption coefficient of the photo absorbers [1617] The materials used

in this architecture allow for flexible modules of low weight

125 Tandem Solar Cells

A famous saying goes ldquoIf you canrsquot beat them join themrdquo which accurately

describes the mindset one must adopt on new solar cell technologies to adapt to the

6

governing market share of silicon solar cell Tandem solar cells are as the name

describes two or more different solar cells stacked on each other to harvest as much

light as possible with as few energetic losses as possible shown in Figure 12 Light

enters the cell from the top and passes through first cell with large energy bandgap

where the photon with high energy are harvested The photons with low-energy

ideally pass through the top-cell without interaction and into the small energy

bandgap bottom cell where they can be harvested The insulating layer and the

transparent contacts are replaced with a recombination layer in monolithic tandem

solar cells

Figure 12 Two-junction tandem solar cell with four contacts

The top cell through which the light passes first should absorb the photon

having high energy and the bottom cell should absorb the remaining photons with low

energy The benefit of combining the layers in such a way is that with a large energy

band gap top layer the high-energy photons will result in formation of high energy

electrons The tandem solar cells comprising of two solar cells which are connected in

series have higher voltage output generation on a smaller area compared to

disconnected individual single-junction cells and hence a larger pow er output The

30 power conversion efficiency limit does not apply to tandem solar cells but they

are still subjected to detailed balance and the losses thereof The addition of a photo

absorber with an energy band gap of around 17eV on top of the crystalline silicon

solar cell to form a tandem solar cell could result in a device with an efficiency

above 40 The market will likely shift towards tandem modules in the future

because the total cost of the solar energy is considerably lowered with each

incremental percentage of the solar cells efficiency In the tandem solar cell market it

might not be certain that crystalline silicon solar cells will still be able to dominate the

market because the bandgap of crystalline silicon (11 eV) cannot be tuned much

Specifically the low energy bandgap thin film semiconductor technologies are of

specific interest for these purposes as their bandgap can be modified somewhat to

match the ideal bandgap of the top cell photo absorber [1819]

7

126 Charge transport layers in solar cells

Zinc Selenide (ZnSe) is a very promising n-type semiconductor having direct

bandgap of 27 eV ZnSe material has been used for many app lications such as

dielectric mirrors solar cells photodiodes light-emitting diodes and protective

antireflection coatings [20ndash22] The post deposition annealing temperature have

strong influence on the crystal phase crystalline size lattice constant average stress

and strain of the material ZnSe films can be prepared by employing many deposition

techniques [23ndash26] However thermal evaporation technique is comparatively easy

low-cost and particularly suitable for large-area depositions The crystal imperfections

can also be eliminated by post deposition annealing treatment to get the preferred

bandgap of semiconducting material In many literature reports of polycrystalline

ZnSe thin films the widening or narrowing of the energy bandgap has been linked to

the decrease or increase of crystallite size [27ndash31] Metal-oxide semiconductor

transport layers widely used in mesostructured perovskite cells present extra benefits

of resistance to the moisture and oxygen as well as optical transparency The widely

investigated potential electron transport material in recent decades is titanium dioxide

(TiO2) TiO2 having a wide bandgap of 32eV is an easily available cheap nontoxic

and chemically stable material [32] In TiO2 transport layer based perovskite devices

the hole diffusion length is longer than the electron diffusion length [33] The e-h+

recombination rate is pretty high in mesoporous TiO2 electron transport layer which

promotes grain boundary scattering resulting in suppressed charge transport and hence

efficiency of solar cells [3435] To improve the charge transport in such structures

many effor ts have been made eg to modify TiO2 by the subs titution of Y3+ Al3+ or

Nb5+ [36ndash41] Graphene has received close attent ion since its discovery by Geim in

2004 by using micro mechanical cleavage It has astonishing optical electrical

thermal and mechanical properties [42ndash51] Currently graphene oxides are being

formed by employing solution method by exfoliation of graphite The carboxylic and

hydroxyl groups available at the basal planes and edges of layer of graphene oxide

helps in functionalization of graphene permitting tunability of its opto-electronic

characteristics [5253] Graphene oxide has been used in all parts of polymer solar cell

devices [54ndash57] The bandgap can be tuned significantly corresponding to a shift in

absorption occurs from ultraviolet to the visible region due to hybridization of TiO2

with graphene [58]

Another metal oxide material ie nickel oxide (NiO) having a wide

bandgap of about 35-4 eV is a p-type semiconducting oxide material [59] The n-type

8

semiconductors like TiO2 ZnO SnO2 In2O3 are investigated frequently and are used

for different purposes On the other hand studies on the investigation of p-type

semiconductor thin films are rarely found However due to their technological

app lications they need to have much dedication Nickel oxide has high optical

transpa rency nontoxicity and low cost material suitable for hole transport layer

app lication in photovoltaic cells [60] The variation in its band gap can be arises due

to the precursor selection and deposition method [61] Nickel oxide (NiO) thin films

has been synthesized by different methods like sputtering [62] spray pyrolysis [63]

vacuum evaporation [64] sol-gel [65] plasma enhanced chemical vapor deposition

and spin coating [66] techniques Spin coating is simplest and practicable among all

because of its simplicity and low cost [67] Though NiO is an insulator in its

stoichiometry with resistivity of order 1013 Ω-cm at room conditions The resistivity

of Nickel oxide (NiO) can be tailored by the add ition of external incorporation

Literature has shown doping of Potassium Lithium Copper Magnesium Cesium

and Aluminum [68ndash73] Predominantly dop ing with monovalent transition metal

elements can alter some properties of NiO and give means for its use in different

app lications

127 Perovskite solar cells

The perovskite mineral was discovered in the early 19t h century by a Russian

mineralogist Lev Perovski It was not until 1926 that Victor Goldschmidt analyzed

this family of minerals and described their crystal structure [74] The perovskite

crystal structure has the form of ABX3 in which A is an organic cation B is a metal

cation and X is an anion as shown in Figure 13(a) A common example of a crystal

having this structure is calcium titanate (CaTiO3) Many of the oxide perovskites

exhibit useful magnetic and electric properties including double perovskite oxides

such as manganites which show ferromagnetic effects and giant magnetoresistance

effects [75] The methyl ammonium lead triiodide (MAPbI3 MA=CH3NH3) was first

synthesized in 1978 by D Weber along with several other organic- inorganic

perovskites [76] The crystal structure of MAPbI3 is presented in Figure 13(b)

The idea of combining organic moieties with the inorganic perovskites allows

for the use of the full force and flexibility of organic synthesis to modify the electric

and mechanical properties of the perovskites to specific applications [77] In the work

by Weber it was discovered that this specific subclass of perovskite materials could

have photoactive properties and in 2009 first hybrid perovskite photovoltaic cell was

9

fabricated by using MAPbI3 as the photo absorber [78] This first organic- inorganic

perovskite photovoltaic cell was manufactured imitating the conventional dye

sensitized photovoltaic device structure A TiO2 working electrode was coated with

MAPbI3 and MAPbBr3 absorber layer and a liquid electrolyte was used for hole

transport The stability of the device reported by Im et al was very bad because of the

perovskite layer dissociation simply due to the liquid electrolyte causing the

perovskite solar cell to lose 80 of their original power conversion efficiencies after

10 minutes of continuous light exposure [79] Kim and Lee et al published first solid-

state perovskite photovoltaic cell around the same time in 2012 where efficiencies of

97 and 109 were obtained respectively [8081] In both cases the solar cell took

inspiration from the layers of a solid-state dye sensitized photovoltaic devices in

which photo-absorber is loaded in a mesoporous layer of TiO2 nanoparticles and

covered by a hole transpor t material (HTM) Spiro-OMeTAD layer Lee et al obtained

their record power conversion efficiencies (PCEs) by using a mesopo rous Al2O3

nanoparticle scaffold instead of TiO2 which sparked doubt to whether the perovskite

solar cell was an electron- injection based device or not

A plethora of different layer architectures have been published for the

perovskite photovoltaic devices but the world record perovskite photovoltaic device

which have yielded a 227 PCE [82] However recent results also show that the

HTM layer must be modified for improving the solar cell stability In Figure 14(a) a

simplified estimation of the energy band diagram for the TiO2MAPbI3Spiro-

OMeTAD perovskite photovoltaic cell is shown The diagram shows how the

conduction bands of the perovskite and TiO2 match to promote a movement of excited

electrons from MAPbI3 into the TiO2 and all the way to the FTO substrates Similarly

the valence band maxima of the hole transport material matches with the valence band

of the MAPbI3 to allow for efficient regeneration of the holes in the pe rovskite

semiconductor Figure 14 (b) presents a schematic representation of the layered

structure of the perovskite photovoltaic cell It shows how light enters from the

bottom side travels into the photo absorbing MAPbI3 medium in which the excited

charge-carriers are generated and can be reflected from the gold electrode to make a

second pass through the photo absorbing layer In Figure 14 (c) each gold square

represents a single solar cell with dimensions of 03 cm2 The side-view of the same

substrate see Figure 14 (d) shows that the solar cell layer is practically invisible to

the naked eye because its thickness is roughly a million times thinner than the

diameter of an average sand grain

10

Figure 13 (a) Cubic crystal-structure of a ABX3 type perovskite (b) The same perovskite structure but

for MAPbI3 MA cation is in the center which is enclosed by 12 iodide ions in corner sharing PbI6

octahedra [83]

Figure 14 (a)Energy band diagram of MAPbI3 perovskite based solar cell (b) the configuration of the

solar cell (c) Photograph of lab-scale MAPbI3 perovskite solar cells with identical layers as in the

diagram to the left with a 10 Euro cent coin for scale (d) Side view of the same sample and coin

1271 The origin of bandgap in perovskite

The energy bandgap of the hybrid perovskite is characterized through lead-

halide (Pb-X) structure [8485] The valence band edge of methylammonium lead

iodide (MAPbI3) comprise mos t of I5p orbitals covering by the Pb6p and 6s orbitals

whereas the conduction band edge comprises of Pb6p - I5s σ and Pb6p - I5pπ

hybridized orbitals So also bromide and chloride based perovskites are characterized

with 4p orbitals of the bromide atom and 3p orbitals of the chloride atom [86] The

main reason behind the energy bandgap changing when the halide proportion in the

perovskites are changed is because of the distinction in the energy gaps of the halides

p-orbitals Since the cation is not pa rt of the valence electronic structure the energy

gap of perovskite is for most part mechanically instead of electronically influenced by

the selection of the cation [8788] However because of the difference in the cation

size bond lengths and bond angels of the Pb-X will be influenced and ultimately the

a) ABX3 b) MAPbI3

11

valence electronic structure In the case of larger cations for example the

formamidinium (FA+) and cesium (Cs+) cation they push the lead-halide (Pb-X) bond

further apart than resulting in decreased Pb-ha lide (X) binding energy So lower

energy bandgap of the formamidinium lead halide perovskite contrasted with cesium

lead halide perovskites is obtained To summarize although a specific choice of

constituents of perovskite gives t value between 08 and 1 which is not a thumb rule

for the formation of stable perovskite material Moreover if a perovskite can be made

with a specific arrangement of ions we cannot say much regarding its energy bandgap

except we precisely anticipate the structure of its monovalent cation-halide (A-X)

arrangement [89ndash93]

1272 The Presence of Lead

The presence of lead in perovskite material is one of the main disadvantages

of using this material for solar cells application The Restrictions put by European

Unions on Hazardous Subs tances banned lead use in all electronics due to its toxic

nature [94] For this reason replacements to lead in the perovskite photo-absorber

have become a major research avenue A simple approach to replace lead is to take a

step up or down within Group IV elements of the periodic table Flerovium is the next

element in the row below lead a recently discovered element barely radioactively

stable and because of its radioactivity perhaps not a suitable replacement for lead Tin

(Sn) which is in the row above lead would be a good candidate for replacing Pb in

perovskite solar cells because of its less-toxic nature The synthesis of organic tin

halides is as old as that of the lead halides The methylammonium tin iodide

(MASnI3) perovskite which has an energy bandgap value of 13eV was reported for

photovoltaic device application and yielded an efficiency of 64 [9596] However

oxidation state of Sn ie +2 necessary for the perovskite structure formation is

unstable and it gets rapidly oxidized to its another oxida tion state which is +4 under

the interaction with humidity or oxygen It is a problem for the solar cell

manufacturing process as well as for the working conditions of the device However

the Pb-based perovskites have been successfully mixed with Sn and 5050 SnPb

alloyed halide perovskites had produced an efficiency of 136 [97] The most recent

efforts within the Sn perovskite solar cell field have been directed towards

stabilization of the Sn2+ cation with add itives such as SnF2 FASnI3 solar cells have

been prepared with the help of these add itives and yielded an efficiency of 48

[9899] However the focus of perovskite research field has shifted somewhat

12

towards the inorganic cesium tin iodide (CsSnI3) perovskites due to the surprisingly

efficient quantum-dot solar cells prepared with this material resulting in a power

conversion efficiencies of 13 [100] Recently bismuth (Bi) based photo absorbers

have also caught the interest of the scientists in the field as an option for replacing

lead While they do not reach high power conversion efficiencies yet the bismuth

photo-absorbers present a non-toxic alternative to lead perovskites [101ndash103] The

ideal perovskite layer thickness in photovoltaic cell is typically about 500 nm due to

the strong absorption coefficients of the lead halide perovskites From this it can be

supposed that if the lead (Pb) from one Pb acid car battery has to be totally

transformed to use in perovskite solar cells an area of 7000 m2 approximately could

be covered with solar cell [104] The discussion for commercialization of lead

perovskite solar cells is still open Although one should never ignore the toxic nature

of lead the problem of lead interaction with environment could be solved with

appropriate encapsulation and reusing of the lead-based perovskite solar cells

1273 Compositional optimization of the perovskite

Despite the promising efficiency of the methylammonium lead iodide

(MAPbI3) solar cells the material has several flaws which pushed the field towards

exploring different perovskite compositions Concerns regarding the presence of lead

in solar cells were among the first to be voiced Furthermore MAPbI3 perovskite

material did not appear entirely chemically stable and studies showed that it might

also not be thermally stable [105106] Degradation studies by Kim et al in 2017

showed that the methylammonium (MA+) cation evaporated and escaped the

perovskite structure at 80degC a typical working condition temperature for a solar cell

resulting in the development of crystalline PbI2 [106] The instability of MAPbI3

perovskite photovoltaic cell is presumably the greatest concern for a successful

commercialization However this instability of MAPbI3 was not the main reason for

the research field to explore different perovskite compositions In the dawn of the

perovskite photovoltaics field a few organic- inorganic perovskite crystal structures

were known to be photoactive but had not been tested in solar cells yet [77]

Therefore there was a large curiosity driven interest in discovering and testing new

materials possibly capable of similar or greater feats as the MAPbI3 Goldschmidt

introduced a tolerance factor (t) for perovskite crystal structure in his work in 1926

[74] By comparing relative sizes of the ions t can give an ind ication of whether a

certain combination of the ABX3 ions can form a perovskite The equation for t is

13

t= 119955119955119912119912+119955119955119935119935radic120784120784(119955119955119913119913+119955119955119935119935)

11 where rA rB and rX are the radii of the A B and X ions respectively The

values of the ions effective radii for the composition of the lead-based perovskite are

presented in Table 11 Photoactive perovskites have a tolerance factor (t) between 08

and 1 where the crystal structures are tetragonal and cubic and are often referred to

as black or α-phases Hexagonal and various layered perovskite crystal structures are

obtained with t gt 1 and t lt 08 results in orthorhombic or rhombohedral perovskite

structures all of which are not photoactive The photo- inactive phases are generally

referred to as yellow or δ-phases With t lt 071 the ions do not form a perovskite

Ion SymbolFormula Effective Ionic radius (pm) Lead (II) Pb2+ 119

Tin (II) Sn2+ 118 Ammonium NH4+ 146

Methylammonium MA+ 217 Ethylammonium EA+ 274

Formamidinium FA+ 253 Guanidinium GA+ 278

Lithium Li+ 76

Sodium Na+ 102 Potassium K+ 138

Rubidium Rb+ 152 Cesium Cs+ 167

Fluoride F- 133 Chloride Cl- 181

Bromide Br- 196 Iodide I- 220

Thiocyanate SCN- 220 Borohydride BH4- 207

Table 11 Effective radii of several ions used and proposed for synthesis of lead based perovskite materials for solar cell purposes [107ndash111]

1274 Stabilization by Bromine

Noh et al found that by replacing the iodine in the methylammonium lead

iodide (MAPbI3) by a slight addition of Br- served to stabilize and enhance the

photovoltaic properties of the MAPbI3 perovskite and at the same time it increased the

band gap energy of the perovskite [89] In fact they also claimed that the band gap

energy could easily be tuned by replacing I with Br and their energy band gap values

14

are 157 eV for the MAPbI3 to 229 eV for the methylammonium lead bromide

(MAPbBr3) When the material is exposed to light the MAPb(I1-xBrx)3 material tends

to segregate into two separate domains ie I and Br rich domains [112] A domain

with separate energy band gap of this type is of course a problematic phenomenon

for any type of a solar cell However this only seems to affect perovskites with a

composition range of around 02 lt x lt 085 [74]

1275 Layered perovskite formation with large organic cations

Already established in David Mitzis work in 1999 organic cations with

longer carbon chains than MA such as ethylammonium (EA) and propylammonium

(PA) do not yield a three-dimensional (3D) cubic-perovskite with the lead-halides

[77] This is mainly because their ionic radii are too large These cations rather tend to

form two-dimensional (2D) layered perovskite structures Although the 2D-

perovskites are photoactive and due to their chemical tunability can be

functionalized with various organic moieties they have not resulted in as efficient

solar cells compared to the 3D-perovskites unless embedded inside one [113114]

This is possibly due to the layered structure of the 2D-perovskite which presents a

hindrance in terms of charge conduction and extraction However the stability of

these perovskites is generally greater than that of the MAPbX3 due to the higher

boiling point of the longer carbon chain cations

12751 The Formamidinium cation

Cubic lead-halide perovskite structures can also be formed using the

formamidinium cation (FA+) which is somewhat in between the ionic size of

methylammonium (MA+) and ethylammonium (EA+) cation [115] The

formamidinium (FA) cation yields a perovskite with greater thermal stability than the

methylammonium (MA) cation due to a higher boiling point of the formamidinium

(FA) compared with methylammonium (MA) Although the formamidinium lead

halide (FAPbX3) perovskites did not yield record perovskite solar cells at the time

they were first reported researchers in the field were quick to pick up the material

Since then FA-rich perovskite materials have been used in all of the efficiency record

breaking perovskite photovoltaic cells [82116ndash119] Furthermore these materials

most likely present the best opportunity for developing stable organic- inorganic

perovskite photovoltaic cells [120] The formamidinium lead iodide (FAPbI3)

perovskite is ideal for photovoltaic device applications due to its suitable band gap of

15

148eV However the material requires 150degC heating to form the photoactive cubic

perovskite phase and when cooled down below that temperature it quickly

deteriorates to a photo inactive phase known as δ-phase [117] Initially the α-phase of

formamidinium lead iodide (FAPbI3) was stabilized by slight addition of methyl

ammonium forming MAyFA1-yPbI3 [116] Furthermore it was discovered that the

structure could also be stabilized by mixing the perovskite with bromide forming

similar to its methyl ammonium predecessor FAPb(I1-xBrx)3 This lead to the

discovery of the highly efficient and stable compositions of (FAPbI3)1-x(MAPbBr3)x

commonly referred as the mixed- ion perovskite [117] The use of larger organic

cations such as guanidinium (GA) have also been proposed as an additive to the

MAPbI3 crystal structure but the pure GAPbI3 perovskite does not form a 3D-

perovskite owing to the large size of the guanidinium cation (GA+) [121122]

1276 Inorganic cations

The instability of the perovskite materials is found to be mainly due to the

organic ammonium cations Inorganic cations are stable compared with the organic

cations which evaporates easily The organic perovskite should therefore maintain

superior stability at the operational conditions of photovoltaic devices where

temperatures can rise above 80 degC Cesium lead halide perovskites have almost 100

years extended history than organic lead halide perovskites [123] As such it is not

unexpected to use Cs+ cation to be presented into the Pb halide perovskite for making

new photo absorber [123124] Choi et al in 2014 investigated the effect of Cs doping

on the MAPbI3 structure [125] Lee et al introduced cesium into the structure of

formamidinium lead iodide (FAPbI3) and in similar manner as to addition of

bromine this stabilized the perovskite structure at room temperature and increased

power conversion efficiencies of photovoltaic cells [126] Shortly thereafter Li et al

mapped out the entire CsyFA1- yPbI3 range to determine the stability of the alloy [127]

The cubic phase of CsPbI3 at roo m tempe rature is unstable just like formamidinium

lead iodide (FAPbI3) However because formamidinium cation (FA+) is slightly large

for the cubic perovskite phase (t gt 1) and Cs+ is slightly small (t lt 08) for it the

CsFA composites produce a relatively stable cubic perovskite phase Inorganic

perovskite CsPbI3 in its cubic phase has energy band gap value of 173 eV which is

best for tandem solar cell applications with c-Si This also makes its instability at

room-temperature somewhat higher [91] On the other hand the α-phase of CsPbBr3

having energy band gap of around 236 eV is completely stable at room temperature

16

But due to its large bandgap the power conversion efficiencies yield is not more than

15 in single-junction devices but the material may still be useful for tandem solar

cells [92] Sutton et al manufactured a mixed-halide CsPbI2Br perovskite solar cell in

order attempt to combine the stability of CsPbBr3 and the energy band gap of

CsPbI3which resulted in almost a 10 power conversion efficiency [128] However

authors stated that even this material was not completely stable Possibly due to ion-

segregation like in the mixed halide MA-perovskite and subsequent phase transition

of the cesium lead iodide (CsPbI3) domains to a δ-phase Owing to the small cation

size of rubidium cation (Rb+) rubidium lead halide (RbPbX3) retains orthorhombic

crystal structure (t lt 08 ) at room temperature similar to the CsPbX3 [128] At

temperatures above 330degC the cesium lead iodide (CsPbI3) transitions to a cubic

phase but rubidium lead iodide (RbPbI3) does not exhibit a crystal phase transitions

prior to its melting point between 360- 440degC [129] However the use of a smaller

anion such as Cl- can yield a crystal phase transition for the rubidium lead chloride

(RbPbCl3) perovskite at lower temperatures than for the rubidium lead iodide

(RbPbI3) [130]

1277 Alternative anions

A slight addition of bromine to the MAPbI3 or FAPbI3 perovskites serves to

both stabilize the crystal structure and increases bandgap [112] Lead bromide

perovskites generally result in materials with larger energy band gap than the lead

iodide perovskites Synthesis of organic lead chloride perovskites such as the

methylammonium lead chloride (MAPbCl3) has also been reported and studies show

that these materials display an even larger energy band gap than their bromide

counterparts [131] The first high efficiency perovskite devices were reported as a

mixed-halide MAPb(I1-xClx)3 perovskite because PbCl2 was used as the lead precursor

in this procedure [81] However studies have shown that chlorine doping can result in

an Cl- inclusion of roughly 4 in the bulk perovskite crystal structure and that this

inclusion has a substantial beneficial effect on the solar cells performance [132ndash134]

Numerous other non-halide anions ie thiocyanate (SCN-) and borohydride (BH4-)

also known as pseudo-halides had been tried for perovskite materials formation

[135136] But pure organic thiocyanate or borohydrides perovskites are not

effectively produced in spite of the suitable ionic sizes of the anions for the perovskite

structure In fact the first lead borohydride perovskite a tetragonal CsPb(BH4)3 was

only first synthesized in 2014 [135]

17

1278 Multiple-cation perovskite absorber

In 2016 researchers at ldquoThe Eacutecole polytechnique feacutedeacuterale de Lausanne

(EPFL) Switzerlandrdquo reported a triple-cation based perovskite solar cells [137] The

material was synthesized from a precursor solution form in the same manner as for

the best performing (FAPbI3)1-x(MAPbBr3)x perovskite along with cesium iodide

(CsI) addition slightly Deposition of this solution yielded a perovskite material which

can be abbreviated as Cs005MA016FA079Pb(I083Br017)3 As reported by Saliba et al

the inclusion of cesium cation (Cs+) served to further stabilize the perovskite structure

and increase the efficiencies and the manufacturing reproducibility of the solar cells

[137] Further optimization would push the same researchers to add rubidium iodide

(RbI) to the precursor solutions of perovskites [118] This resulted in the formation of

a quadruple-cation perovskite with a material composition of

Rb005Cs005MA015FA075Pb(I083Br017)3

13 Motivation and Procedures Our research work of this thesis started when the field of hybrid perovskite for

photovoltaics had just begun with only a few articles that were published at that time

on this topic The questions and problems raised in 2014 are entirely different than the

ones of 2017 As a consequence the chapters that we present in this work follow

closely the rapid development of the research during this period Over a period of

these years thousands of articles were published on this topic by over a hundred

research groups around the world In the beginning there was a general struggle of

photovoltaic devices fabrication with reasonable efficiencies (PCEs) in a reproducible

fashion

We investigated some of the aspects of the chemical composition of the hybrid

perovskite layer and its influence on its optical electrical and photovoltaic properties

Questions with regards to the stability of the perovskite were raised particularly in

view of the degradation of the organic component methylammonium (MA) in

methylammonium lead iodide (MAPbI3) perovskite material This issue will be

discussed in x-ray photoe lectron spectroscopy studies of mixed cation perovskite

films More specifically the studies we report in this thesis comprise the following

The experimental work and characterizations carried out during our studies

have been reported in chapter 2 The general experimental protocols use for the

synthesis and characterization related to the field is also given in this chapter The

investigation of some of the possible charge transport layers for photovoltaic

18

applications were carried out during these studies and reported in chapter 3 The

preparation of hybrid lead- iodide perovskites hybrid cations based lead halide

perovskites by simple solution based protocols and their investigation for potential

photovoltaic application has been discussed in chapter 4 In this chapter we discussed

inorganic monovalent alkali metal cations incorporation in methylammonium lead

iodide ie (MA)1-xBxPbI3 (B= K Rb Cs x=0-1) perovskite and characterization by

employing different characterization techniques The planar perovskite photovoltaic

devices were fabricated by organic- lead halide perovskite as absorber layer The

organic- inorganic mixed (MA K Rb Cs RbCs) cation perovskites films were also

used and investigated in photovoltaic devices by analyzing their performance

parameters under dark and under illumination conditions and reported in chapter 5

The references and conclusions of the thesis are given after chapter 5

19

CHAPTER 2

2 Materials and Methodology

In this chapter general synthesis and deposition methods of perovskite material as

well as the experimental work carried out during our thesis research work is

discussed A brief background along with the experimental setup used for different

characterizations of our samples is also given in this chapter

21 Preparation Methods The solution based deposition processes of perovskite film in the perovskite

photovoltaic cells can commonly be classified on the basis of number of steps used in

the formation of perovskite material The one-step method involved the deposition of

solution prepared by the perovskite precursors dissolved in a single solution The

solid perovskite film forms after the evaporation of the solvent The so called two-step

methods also known as sequential deposition generally comprise of a step-wise

addition of perovskite precursor to form the perovskite of choice Further

manufacturing steps can be carried out to modify the perovskite material or the

precursors formed sequentially but these are usually not referred to as more-than-

two-step methods

211 One-step Methods

One-step method was the first fabrication process to be published on solid-state

perovskite solar cells [80][81] The high boiling point solvents are used for the

perovskite precursor solution at appreciable concentration [138] Apart from using the

least amount of steps possible involved in one-step method for the manufacturing

compositional control is an additional advantage using this method The general

process of a one-step method is presented in Figure 21 All precursor materials are

dissolve in appropriate ratios in one solution to produce a perovskite solution One

solvent or a mixture of two solvents in appropriate ratios can be used in the solut ion

20

Coating the perovskite solut ion on a subs trate following by post depos ition annealing

to crystallize the material is carried out to get perovskite films [139]

Figure 21 One step synthesis method of perovskite The precursor materials are dissolved in the one

solution (a single solvent or mixture of two solvents) and the substrate is coated with the solution The

perovskite changes its color at annealing

212 Two-step Methods

Synthesis of organic- inorganic perovskite materials by two-step methods was first

described in 1998 and successfully applied to perovskite photovoltaic devices in 2013

[140141] The number of controlled parameters are increased in a two-step method

but so are the number of possible elements that can go wrong While these methods

are generally hailed for greater crystallization control they are far more sensitive to

human error and environmental changes On the other hand a benefit of these

methods is that they allow for a greater choice of deposition techniques The typical

process of a two-step method is presented in Figure 22 and entails

1 Dissolving a precursor in a solution to yield a precursor solution

2 Coating the precursor solution on a substrate

3 Removing the solvent to obtain a precursor substrate

4 Applying the second precursor solution in a solvent to the substrate to start the

perovskite crystallization reaction

Figure 22 Schematic illustration of the MAPbI3 perovskite synthesis using a two-step method [142]

21

The final step of annealing may also not be necessary eg for methods employing

vaporization of the second precursor because the film is already annealed during the

reaction

22 Deposition Techniques

221 Spin coating

Spin coating technique is a common and simplest method for the depositing

perovskite thin films It has earned its favor in the research community due to its

simplicity of application and reliability Therefore it is not surprising that all high-

efficient perovskite photovoltaic devices are fabricated by one or more step spin

coating method for perovskite deposition Spin coating techniques (shown in Figure

23) depend on the solution application method to the substrate and it can be

categorized into two methods (1) static spin coating (2) dynamic spin coating In spite

of the spin coatingrsquos capability to yield thin films of greatly even thickness it is not a

practical option for the deposition of thin film at large scale To give an example

spin-coating is generally used with substrates with around 2x2 cm2 size and it is not

practical to coat substrates larger than 10x10 cm2 During this procedure maximum

solution is thrown away and wasted except specially recycled The greater speed of

the substrate far from the center of motion results in non-uniform thickness for larger

substrates Therefore the chances of forming defects in film with larger substrate is

higher

Figure 23 A spin coating instrument the solution to be coated is placed in the micropipette the lid

placed down and the spinning cycle started

222 Anti-solvent Technique

The anti-solvent method involves the addition of a poor ly soluble solvent to the

precursor solution This inspiration has been taken from single-crystal growth

22

methods The perovskite starts crystalizing by the addition of the anti-solvent into the

precursor solution This implies that all the required perovskite precursor materials

need to be present in the same solution Therefore this method is only used for one-

step method The anti-solvent preparation technique for perovskite photovoltaic

device applications was first reported in 2014 by Jeon et al and at the time it resulted

in a certified world record efficiency of 162 without any hysteresis [118] The

precursor solution of perovskite in a combination of γ-but yrolactone and Dimethyl

sulfoxide solvents was coated onto TiO2 substrates for the perovskite solar cell and

toluene was drop casted during this process The toluene which was used as anti-

solvent cause the γ-but yrolactone Dimethyl sulfoxide solvent to be pushed out from

the film resulted in a very homogeneous film The films deposited by this method

were found to be highly crystalline and uniform upon annealing Later on anti-solvent

technique has been used in all world record efficiency perovskite solar cells There are

many anti-solvents available for perovskite and a few of them have been reported

[118143ndash145] A fairly high reproducibility in the performances of the solar cells has

obtained by using this technique Still it is not an up-scalable method to fabricate the

perovskite photovoltaic devices owing to its combined use with spin-coating

223 Chemical Bath Deposition Method

The most common employed method for the sequential deposition of perovskite is

chemical bath deposition In this method a solid Pb (II) salt precursor film is dip into

a cations and anions solution in chemical bath However this technique sets some

constraints on the chemicals used for the preparation process

1 The lead salt must be very soluble in a solvent to form the precursor film and

it should not be soluble in the chemical bath

2 The resulting perovskite must not be soluble in the chemical bath

Burschka et al presented that a PbI2 based solid film was dipped into a

methylammonium iodide solution in IPA in order to crystallize of methylammonium

lead iodide (MAPbI3) perovskite [141] Methylammonium iodide (MAI) dissolves

simply in isopropanol whereas PbI2 is insoluble in it Moreover the perovskite

MAPbI3 dissolve in isopropanol but the perovskite reaction time is short compared

with the time it takes to dissolve the whole compound

23

224 Other perovskite deposition techniques

Many of the available large-area perovskite deposition techniques use spray coating

blade-coating or evaporation in at least one of the perovskite preparation steps The

major challenge with large-area manufacture of thin film photovoltaic devices is that

the probability of film defects increases with the solar cell size Laboratory scale

perovskite solar cells are generally prepared and measured with small photoactive

areas of less than 02 cm2 but 1 cm2 area solar cells may give a better indication of the

device performance at larger scale than the small cells On that end perovskite

photovoltaic devices have shown efficiency of 20 on a 1 cm2 scale despite the use

of spin-coating and this result shows that the perovskite material can also perform

well on a larger scale [146] Co-evaporation of methylammonium iodide and lead

chloride was the first large-scale technique used for perovskite photovoltaic device

application [147] At the time of publication this technique yielded a world record

perovskite efficiency of 154 for small-area cells [147] The perovskite can also be

formed sequentially by exposing the metal salt film to vaporized ionic salt The metal

salt film be able to be deposited on with any deposition technique and ionic salt is

vaporized and the film is exposed to it which initiates the perovskite crystallization

Blade and spray-coating techniques both require the precursors to be in solution

therefore they can be employed by both one or two-step methods Slot-die coating

technique has also been used to manufacture perovskite photovoltaic devices and it is

a technique that presents a practical way for large-scale solution based preparation of

perovskite photovoltaic devices [148ndash150] On 1 cm2 area this technique has yielded

a power conversion efficiencies (PCEs) of 15 and of 10 on 25 cm2 large area

perovskite solar cell modules (176 cm2 active area)

23 Experimental

231 Chemicals details

Methylammonium iodide (CH3NH3I MAI) having purity of 999 is bought from

Dyesol Potassium Iodide (KI 998 purity) Cesium Iodide (CsI 998 purity)

Rubidium Iodide (RbI 998 purity) Lead iodide (PbI2) bearing purity of 99 Zinc

selenide (ZnSe 99999 pure trace metal basis powder) Titanium isopropoxide

(C12H28O4Ti) Sodium nitrate (NaNO3) Potassium permanganate (KMnO4) Nickel

nitrate (Ni(NO3)26H2O 998 purity) Nickel acetate tetrahydrate

(Ni(CH₃CO₂)₂middot4H₂O) Silver Nitrate (AgNO3) sodium hydroxide pellets

24

(NaOH) graphite powder N N-dimethylformamide Toluene γ-butyrolactone

hydrogen peroxide (H2O2) ethylene glycol monomethyl ether (CH3OCH2CH2OH)

dimethyl sulfoxide (DMSO 99 pure) Hydrochloric acid (purity 35) Sulfuric acid

(purity 9799) Polyvinyl alcohol (PVA) Hydrofluoric acid (HF reagent grade

48) and Ethanol were obtained from Sigma Aldrich Poly(triarylamine) (PTAA

99999 purity) has been purchased from Xirsquoan Polymers All the materials and

solvents except graphite powder were used as received with no additional purification

24 Materials and films preparation

241 Substrate Preparation

Fluorine-doped tin oxide (FTO sheet resistance 7Ω) and Indium doped tin oxide

(ITO sheet resistance 15-25 Ω ) glass substrates were purchased from Sigma

Aldrich FTO substrates were washed by ultra-sonication in detergent water ethanol

and acetone in sequence Indium doped tin oxide substrates were prepared by ultra

sonication in detergent acetone and isopropyl alcohol The washed substrates were

further dried in hot air

242 Preparation of ZnSe films

The zinc selenide (ZnSe) thin films were deposited by physical vapor deposition The

films have been coated by evaporating ZnSe powder in tantalum boat under 10-4 mbar

pressure having substrate- source distance of 12cm The post deposition thermal

annealing experiments were conducted under rough vacuum conditions for 10min in

the temperature range 350ndash500ᵒC The heating and cooling to the sample for post

deposition annealing was carried out slowly

243 Preparation of GO-TiO2 composite films

The graphene oxide-titanium oxide (GO-TiO2) composite solution was prepared by

using different wt of graphene oxide (GO) with TiO2 nanoparticles The

hydrothermal method was employed to prepare nanoparticles of TiO2 The 10ml

Titanium isopropoxide (C12H28O4Ti) precursor was mixed with 100 ml of deionized

water under stirring for 5min followed by addition of 06g and 05g of NaOH and

PVA respectively The prepared solution was kept under sonication for some time and

then put into an autoclave at 100ᵒC for eight hours The purification of graphite flakes

was carried out by hydrogen fluoride (HF) treatment The acid treated flakes were

washed with distilled water to neutralize acid The washed flakes were further washed

25

with acetone once followed by heat treatment at 100ᵒC Graphene oxide powder was

obtained from purified graphite flakes by employing Hummerrsquos method [151] The 1g

of graphite flakes (purified) was mixed with NaNO3(05 g) and concentrated sulfuric

acid (23 ml) under constant stirring at 5ᵒC by using an ice bath for 1 h A 3g of

potassium per manganate (KMnO4) were added slowly to the above solution to avoid

over-heating while keeping temperature less tha n 20ᵒC The follow-on dark-greenish

suspension was further stirred on an oil ba th for 12 h at 35ᵒC The distilled water was

slowly put into the suspension under fast stirring to dilute it for 1h After 1h

suspension was diluted by using 5ml H2O2 solution with constant stirring for more 2h

Afterwards the suspension was washed as well as with 125ml of 10 HCl and dried

at 90ᵒC in an oven Dark black graphene oxide (GO) was finally obtained

The nanocomposite xGOndashTiO2(x = 0 2 4 8 and 12 wt ) solutions were ready by

mixing two separately prepared TiO2 nanopa rticle and graphene oxide (GO)

dispersion solutions and sonicated for 3h TiO2 dispersion was made by adding 50mg

of TiO2 nanoparticles to 1ml ethanol and sonicated for 1h and graphene oxide (GO)

dispersion were formed by sonicating (2 4 8 and 12) wt of graphene oxide (GO)

in 2ml isopropanol The films of xGOndashTiO2 (x = 0 2 4 8 and 12 wt) on the ITO

substrate were prepared by employing spin coating technique The post annealing

treatment of samples was performed at 150ᵒC for 1h Finally post deposition thermal

treatment of xGOndashTiO2 (x = 12 wt) film at 400ᵒC in inert environment is also

performed

244 Preparation of NiOx films

To prepare nickel oxide (NiOx) films two precursor solutions with molarities 01

075 were prepared by dissolving Ni(NO3)2middot6H₂O precursor in 10ml of ethylene

glycol monomethyl and stirred at 50⁰C for 1h For Ag doping silver nitrate (AgNO3)

in 2 4 6 and 8 mol ratios were mixed in above solution 1ml acetyl acetone is

added to the solution while stirring at room temperature for 1h which turned the

solution colorless For film deposition spin coating of the samples was performed at

1000rpm for 10sec and 3000rpm for 30sec The coating was repeated to get the

optimized NiOx sample thickness Lastly the films were annealed in the furnace at

different temperatures (150-600 ⁰C) by slow heating rate of 6⁰C per minute

26

245 Preparation of CH3NH3PbI3 (MAPb3) films

The un-doped perovskite material MAPbI3 was prepared by stirring the mixture of

0395g MAI and 1157g PbI2 in equimolar ratio in 2ml (31) mixed solvent of DMF

GBL at 70o C in glove box filled with nitrogen Thin films were deposited by spin

coating the prepared solutions of prepared precursor solution on FTO substrates The

spin coating was done by two step process at 1000 and 5000 revolutions per seconds

(rpm) for 10 and 30 seconds respectively The films were dried under an inert

environment at room temperature in glove box for few minutes The prepared films

were annealed at various temperature ie 60 80 100 110 130ᵒC

246 Preparation of (MA)1-xKxPbI3 films

The precursor solutions of potassium iodide (KI) added MAPbI3 (MA)1-xKxPbI3(x=

01 02 03 04 05 075 1) were ready by dissolving different weight ratios of KI

in MAI and PbI2 solution in 2ml (31) mixed solvent of DMF GBL and stirred

overnight at 70ᵒC inside glove box Thin films of these precursor solutions were

prepared by spin coating under same conditions The post deposition annealing was

carried out at 100o C for half an hour

247 Preparation of (MA)1-xRbxPbI3 films

The same recipe as discussed above is used to prepare (MA)1-xRbxPbI3(x= 01 03

05 075 1) In this case rubidium iodide (RbI) is used in different weight ratios with

MAI and PbI2 to obtain the (MA)1-xRbxPbI3(x=0 01 03 05 075 1) solutions Thin

films deposition and post deposition annealing was carried out by the same procedure

as discussed above

248 Preparation of (MA)1-xCsxPbI3 films

The same recipe is used as above to prepare (MA)1-xCsxPbI3 In this case cesium

iodide (CsI) is used in different weight ratios with MAI and PbI2 to get (MA)1-

xCsxPbI3 (x=01 02 03 04 05 075 1) solutions The same procedure as

mentioned above was used for film deposition and post deposition annealing

249 Preparation of (MA)1-(x+y)RbxCsyPbI3 films

To prepare (MA)1-(x+y)RbxCsyPbI3(x=005 010 015 y=005 010 015) RbI and

CsI salts are used in different weight percent with respect to MAI and PbI2 under the

same conditions as described in synthesis of previous batch In this case films

27

formation and post deposition annealing was carried out by employing same

procedure as above

25 Solar cells fabrication The planar inverted structured devices were fabrication The fluorine doped tine oxide

(FTO) substrates were washed and clean by a method discussed earlier On FTO

substrates hole transport layer photo-absorber perovskite layer electron transport

layer and finally the contacts were deposited by different techniques discussed in

next sections

251 FTONiOxPerovskiteC60Ag perovskite solar cell

Nicke l oxide (NiOx)FTO films were deposited by the same procedure as discussed in

section 244 above On the top of NiOx (hole transport layer) absorber layers of

200nm film is deposited by already prepared precursor solutions of pure MAPbI3 as

well as alkali metal (K Rb RbCs) mixed MAPbI3 perovskites These precursor

solutions were spin casted in two steps at 2000rpm for 10sec and 6000rpm for 20sec

and drip chlorobenzene 5sec before stopping The prepared films were annealed at

100ᵒC for 15 min A 45nm C60 layer and 100nm Ag contacts were deposited by using

thermal evaporation with a vacuum pressure of 1e-6 mbar

252 FTOPTAAPerovskiteC60Ag perovskite solar cell

In the fabrication of these devices a 20 nm PTAA (HTL) film was deposited by spin

coating a 20mg Poly(triarylamine) (PTAA) solution in 1ml toluene at 6000 rpm

followed by drying at 100o C fo r 10min The rest of the layers were p repared by the

same method as discussed above

26 Characterization Techniques

261 X- ray Diffraction (XRD)

To study the crystallographic prope rties such as the crystal structure and the lattice

parameters of materials xrd technique is employed The wavelength of x-rays is

05Aring-25Aring range which is analogous to the inter-planar spacing in solids Copper

(Cu) and Molybdenum (Mo) are x-ray source materials use in x-ray tubes having

wavelengths of 154 and 08Aring respectively [152] The x-rays which satisfy Braggrsquos

law (equation 21) give the diffraction pattern

2dsinθ = nλ n=1 2 hellip 21

28

For which λ is wavelength of x-rays d represent the distance between two planes and

n is the order of diffraction

The XRD is particularly useful for perovskite materials to determine which crystal

phase it has and to evaluate the crystallinity of the material Another advantage of

this technique is that the progression of the perovskite formation as well as its

degradation can be studied by using this technique Due to crystallinity of the

perovskite precursors specially the lead compounds have a relatively distinct XRD

and it can easily be distinguished from the perovskite XRD pattern The appearance of

crystalline precursor compounds is a typical indication of perovskites degradation or

incomplete reaction In our work we employed D-8 Advanced XRD system and

ldquoXrsquopert HighScoreChekcellrdquo computer softwares to find the structure and the lattice

parameters

262 Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) is one of the best characterization techniques

for thin films The SEM is capable of other extensive material characterizations made

possible by a wide selection of detection systems in the microscope In a normal

scanning electron microscopy instrument tungsten filament is used as cathode from

which electrons are emitted thermoionically and accelerated to an anode shown in

Figure 24 One of the interaction events when a primary electron from the electron

emission source hits the sample electrons from the samples atoms get kicked out

called secondary electrons For imaging the secondary electrons detector is

commonly used because of the generation of the secondary electrons in large

numbers near the surface of the material causing a high resolution topographic image

The detector however is not capable of distinguishing the secondary electrons

energies and the topographic contrast is therefore only based on the number of

secondary electrons detected which results in a monochromatic image Another

interaction event is the backscattering of secondary electrons Depend ing on the

atomic mass of the sample atoms the primary electrons can be sling shot at different

angles to a backscattering detector This results in atomic mass contrast in the electron

image A third interaction event is an atoms emission of an x-ray upon generation of a

secondary electron from the inner-shell of the atom and subsequent relaxation of the

outer shell electron to fill the hole

In our studies SEM experiments were performed using ZEISS system The SEM

scans were recorded at 10 Kx to 200 Kx magnifications with set voltage of 50 kV

29

Figure 24 Scanning electron microscopy opened sample chamber

263 Atomic Force Microscopy (AFM)

A unique sort of microscopy technique in which the resolution of the microscope is

not limited by wave diffraction as in optical and electron microscopes is called

scanning probe microscopy (SPM) In AFM mode of SPM a probe is used to contact

the substrate physically to take topographical data along with material properties The

resolution of an AFM is primarily controlled by the many factors such as the

sensitivity of the detection system the radius of the cantileverrsquos tip and its spring

constant As the cantilever scans across a sample the AFM works by monitoring its

deflection by a laser off the cantileverrsquos back into a photodiode which is sensitive to

the position A piezoelectric material is used for the deflection of probe whose

morphology is affected by electric fields This property is employed to get small and

accurate movements of the probe which allows for a significant feature called

feedback loop of SPM Feedback loops in case of AFM are used to keep constant

current force distance amplitude etc These imaging modes each have advantages

for different purposes In our studies the AFM studies were carried out using Nano-

scope Multimode atomic force microscopy in tapping mode

264 Absorption Spectroscopy

The absorption properties of a semiconductor have a large influence on the materials

functionality and the spectral regions determination where solar cell will harvest light

are of general interest The strongest irradiance and largest photon flux of the solar

spectrum are in and around the visible spectral range Absorption measurements in the

ultra violet visible and near infrared spectral regions (200-2000 nm) therefore yield

information on how the solar cell interacts sunlight A classical UV-Vis-NIR

spectrometer consists of one or more light sources to cover the spectral regions to be

30

measured monochromators to select the wavelength to be measured a sample

chamber and a photodiode detector For solid materials such as parts of or a

complete solar cell it is most appropriate to carry out transmittance and reflectance

measurements to account for the full optical properties of the films It is important to

collect light from all angles for an accurate measurement because the absorbance (Aλ)

of a sample is in principle not measured directly but calculated from the transmittance

(Tλ) and reflectance (Rλ) of the sample The main part of the spectrometer is an

integrating sphere which collect all the light interacting with the sample By

measuring the transmitted and reflected light through the material one can calculate

the Aλ of a material

In our studies the absorption spectroscopy experiments were carried out by with

Specord 200 UV-Vis-NIR spectroscopy system

265 Photoluminescence Spectroscopy (PL)

When a semiconducting material absorbed a photon an e- is excited and jumps to a

higher level Unless the charge is extracted through a circuit the excited photo-

absorber has two possible mechanisms for relaxation to its ground state non-radiative

or radiative Radiative relaxation entails that the material emits a photon having

energy corresponding to its energy bandgap A strong photoluminescence from a

photo-absorber most likely shows less non-radiative recombination pathways present

in the material These non-radiative are due to surface defects and therefore their

reduction is indication of good material quality The photoluminescence of a material

can also be monitored like in absorption spectroscopy Typically material is

illuminated by a monochromatic light having energy greater than the estimated

emission and by means of a second monochromator vertical to the incident

illumination the corresponding emission spectrum can be collected A pulsed light

source is used in time resolved photoluminescence for exciting a sample The

intensity of photoluminescence is rightly associated with population of emitting

states In normal mechanism of time resolved photoluminescence photoluminescence

with respect to time from the excitation is recorded However a few kinetic

mechanisms due to non-radiative recombination in perovskites are not identified in

time resolved photoluminescence but for the study of charge dynamics in pe rovskite

material it is use as the most popular tool In our studies the photoluminescence

spectroscopy was examined by Horiba Scientific using Ar laser system

31

266 Rutherford Backscattering Spectroscopy (RBS)

The Rutherford backscattering spectroscopy (RBS) is used for the

compos itional analysis and to find the approximate thickness of thin films It is based

on the pr inciple of the famous experiment carried out by Hans Giger and Earnest

Marsdon under the supervision of lord Earnest Rutherford in 1914 [153] Doubly

positive Helium nuclei are bombarded on the samples upon interaction with the

nucleus of the target atoms alpha particles (Helium nuc lei) are scattered with different

angles The energy of these backscattered alpha particles can be related to incident

particles by the equation

119844119844 =119812119812119835119835119812119812119842119842

=

⎩⎨

⎧119820119820120783120783119836119836119836119836119836119836119836119836+ 119820119820120784120784120784120784 minus119820119820120783120783

120784120784119826119826119842119842119826119826120784120784119836119836

119820119820120783120783 +119820119820120784120784⎭⎬

⎫120784120784

120784120784120784120784

Where Eb is backscattered alpha particles energy Ei is incident alpha

particles energy k is a kinematic factor M1 is incident alpha particles mass M2 is

target particle mass and θ is the scattering angle [154] The schematic representation

of the whole apparatus of the scattering experiment and the theoretical aspects of the

incident alpha particles on the target sample also penetration of the incident particles

is shown for depth calculation in Figure 25(a b)

Figure 25 (a) A Schematic diagram of Rutherford Backscattering Spectroscopy Setup (b) working

principle of Rutherford Backscattering Spectroscopy

267 Particle Induced X-rays Spectroscopy (PIXE)

The particle induced x-rays spectroscopy is a characterization system in which

target is irradiated to a high energy ion beam (1-2 MeV of He typically) due to which

x-rays are emitted The spectrometry of these x-rays gives the compositional

information of the target It works on the principle that when the specimen is

32

irradiated to an incident ion beam the ion beam enters the target after striking As the

ion moves in the specimen it strikes the atoms of the specimen and ionize them by

giving sufficient energy to turn out the inner shell electrons The vacancy of these

inner shell electrons is filled by the upper shell electrons by emitting a photon of

specific energy falling in the x-rays limit These x-rays contain specific energy for

specific elements

Figure 26 represents the PIXE results of the sample containing calcium (Ca)

and titanium (Ti) Particle induced x-rays spectroscopy is sensitive to 1ppm

depending on characteristics of incoming charged particles and elements above

sodium are detectable by this technique

Figure 26 Particle induced X-rays spectroscopy (PIXE) results of a Specimen containing calcium and

titanium

268 X-ray photoemission spectroscopy (XPS)

The photoelectric effect is the basic principle of XPS In 1907 P D Innes irradiated

x-rays on platinum and recorded photoelectronsrsquo kinetic energy spectrum by

spectrometer [155] Later development of high-resolution spectrometer by Kai M

Siegbahn with colleagues make it possible to measure correctly the binding energy of

photoelectron peaks [156] Afterwards the effect of shift in binding energy is also

observed by the same group [157158] The basic XPS instrument consists of an

electron energy analyzer a light source and an electron detector shown in Figure

27 When x-rays photon having energy hν is irradiated on the surface of sample the

energy absorbed by core level electrons at surface atoms are emitted with kinetic

energy (Ek) This process can be expressed mathematically by Einstein equation (22)

[159]

Ek = hν minus EB minus Ψ 22

Where Ψ is instrumental work function The EB of core level electron can be

determined by measuring Ek of the emitted electron by an analyzer

33

Figure 27 Basic elements of x-ray photoemission spectroscopy (XPS) experiment

In our work a Scienta-Omicron system having a micro-focused monochromatic x-ray

source with a 700-micron spot size is used to perform x-ray photoemission

spectroscopy measurements For survey scan source was operated at 15keV whereas

for high resolution scans it is operated at 20 eV with 100eV analyzer energy To

avoid charging effects a combined low energyion flood source is used for charge

neutralization Matrix and Igor pro softwares were used for the data acquisition and

analysis respectively The fitting was carried out with Gaussian-Lorentzia 70-30

along shrilly background corrections

361 Electrochemical Impedance Spectroscopy (EIS)

Electrochemical impedance spectroscopy (EIS) is a non- destructive technique

used to find the response of a material to periodic alternating current signal It is used

for calculation of complex parameters like impedance The total resistance of the

circuit is known as impedance this includes resistance capacitance inductance etc

During electrochemical impedance spectroscopy measurement a small AC signal is

imposed to the target material and its response on different frequency readings is

analyzed The current and voltage response is observed to calculate the impedance of

the load The real and imaginary values of impedance are plotted against x-axis and y-

axis respectively with variation in the frequency called Nyquist plot Nyquist plot

gives impedance values at a specific frequency [160] The device works on the

fundamental equation given below

119833119833(120538120538) = 119829119829119816119816

= 119833119833120782120782119838119838119842119842empty = 119833119833120782120782(119836119836119836119836119836119836empty+ 119842119842119836119836119842119842119826119826empty) 23 Where V is the AC voltage I is the AC current Z0 is the impedance amplitude

and empty is the phase shift

34

269 Current voltage measurements

The current flowing through any conductor can be found by using Ohms law

By measuring current and voltage resistance (R) can be calculated by using

R = VI 24 If lengt h of a certain conductor is lsquoLrsquo and area of cross section is lsquoArsquo (Figure 28)

then resistivity (ρ) can be calculated by following expression

R α AL

rArr R = ρ AL 25

Resistivity is a geometry independent parameter

Figure 28 2-probe Resistivity Measurements

In our experiments Leybold Heraeus high vacuum heat resistive evaporation system

is the development of metal contact through shadow mask The base pressure was

maintained at ~10-6 mbar One contact was taken from the sample and other with the

conducting substrate on which film was deposited The Kiethley 2420 digital source

meter is used to get current voltage data

2610 Hall effect measurements

In 1879 E H Hall discovered a phenomenon that when a current flow in the

presence of perpendicular magnetic field an electric field is created perpendicular to

the plane containing magnetic field and current called Hall effect The Hall effect is a

reliable steady-state technique which is used to determine whether the semiconductor

under study is n-type or p-type It is also used to determine the intrinsic defect

concentration and carriers mobilities In 2014 Huang et al executed the behavior of

MAPbI3 film as p-type developed by the inter-diffusion method through Hall effect

35

measurements They find a p-type carrier concentration of 4ndash10 ˟ 013 cm3 which may

appeared due to absence of Pb in perovskites or in other words due to extra

methylammonium iodide (MAI) presence in the course of the inter-diffusion [161]

On the other hand materials with high resistivity cannot be measured this technique

normally In our work we performed hall effect measurements by employing ECOPIA

Hall Effect Measurement system with a constant 08T magnetic field

27 Solar Cell Characterization

271 Current density-voltage (J-V) Measurements

The power conversion efficiency (PCE) of solar cell is determined by examining its

current density vs voltage measurements Figure 29 (a) Typically the current is

changed to current density (J) by accounting for the active area of the solar cell thus

resulting in a J-V curve Electric power (P) is simply the current multiplied by the

voltage and the P-V curve is presented in Figure 29 (b) The point at which the solar

cell yields the most power Pmax also known as maximum power point (MPP) is

highly relevant with regards to the operational power output of solar cell The PCE of

solar cell denoted with η can be expressed by following equation

120520120520 = 119823119823119823119823119823119823119823119823119823119823119842119842119826119826

= 119817119817119836119836119836119836119829119829119836119836119836119836 119813119813119813119813119823119823119842119842119826119826

120784120784120789120789 Although η may be the most important value to de termine for a solar cell

A phenomenon known as hys teresis can present itself in current density-voltage

curves which will result in anomalous J-V behavior and affects the efficiency A short

circuit to open circuit J-V scan of a same perovskite cell can yield a different FF and

VOC than a short circuit to open circuit J-V scan The different J-V curves of the same

perovskite solar cell under different scan speeds can also be obtained Although

researchers were aware of it the issue of hysteresis was not thoroughly addressed

until 2014 [162] Quite a few thoughts regarding the cause of the perovskite solar

cells hysteresis have been suggested for example ferroe lectricity in perovskite

material slow filling and emptying of trap states ionic-movement and unbalanced

charge-extraction but consensus seems to have been reached on the last two be ing the

major effects [91162ndash164] If those issues are successfully addressed the hysteresis

appears to be minimal This is the case with most inverted perovskite solar cells

where charge extraction is well balanced and in perovskite solar cells with proper

ionic management [82165166] It has been widely proposed to use either the

stabilized power-output or maximum power point tracking To estimate the actual

36

working performance of a solar cell in a better way it may be necessary to employ

MPP tracking The MPP tracking functions in a similar way except that at specific

time intervals the Pmax of the device is re-evaluated and PCE of the perovskite cell is

calculated from Pmax Pin ratio as opposed to just its current response

In our thesis work the J-V scans are collected by means of a Keithley-2400 meter and

solar simulator (air mass 15 ) with a 100 mWcm2 irradiation intens ity All our

perovskite solar cells have an area between 02- 03cm2 In most measurements of

solar de vices we have used a round shadow mask with 4mm radius corresponding to

an area of 0126 cm2 and J-V scan speeds of either 10 mVs or 50 mVs

Figure 29 (a) Current density vs voltage (J-V) characteristics of a typical solar cell under illumination

intensity (AM1 AM05 spectrum) (b) Power curve of the solar cell

272 External Quantum Efficiency (EQE)

Another general characterization method for solar cells is the external quantum

efficiency (EQE) which is also called incident photon-to-current efficiency (IPCE)

In IPCE measurement cell is irradiated with monochromatic light and its current

output at that specific wavelength is monitored Knowing the incident photon flux

one can estimate the solar cells IPCE at that wavelength using the measured shor t-

circuit current with the following equation

119920119920119920119920119920119920119920119920(120640120640) =119921119921119956119956119956119956(120640120640)119942119942oslash(120640120640) =

119921119921119956119956119956119956(120640120640)119942119942119920119920119946119946119946119946(120640120640)

119945119945119956119956120640120640

= 120783120783120784120784120783120783120782120782119921119921119956119956119956119956(120640120640)120640120640119920119920119946119946119946119946(120640120640) 120784120784120790120790

where e is electron charge λ is the wavelength φ is the flux of photon h is the Planck

constant c is the speed of light The IPCE spectrum for a given solar cell is therefore

highly dependent on absorbing materialrsquos properties and the other layers in the solar

37

cell Longer wavelengths compared to shorter are typically absorbed deeper in the

material due to a lower absorption coefficient of the material

CHAPTER 3

3 Studies of Charge Transport Layers for potential Photovoltaic Applications

In this chapter we studied different types of charge transport layers for potential

photovoltaic application To explore the properties of charge transport layers and

their role in photovoltaic devices was the aim behind this work

Zinc Selenide (ZnSe) is found to be a very interesting material for many

app lications ie light-emitting diodes [22] solar cells [167] photodiodes [21]

protective and antireflection coatings [168] and dielectric mirrors [169] ZnSe having

a direct bandgap of 27eV can be used as electron transpor t layer (ETL) in solar cells

as depicted in Figure 31(a) The preparation protocol and crystalline quality of the

material which play a significant character in many practical applications are mostly

dependent on the post deposition treatments ambient pressure and lattice match etc

[170] The post deposition annealing temperature have a strong influence on the

crystal phase crystalline size lattice constant average stress and strain of the

material ZnSe films can be prepared by employing many deposition techniques [23ndash

26] However thermal evaporation technique is comparatively easy low-cos t and

particularly suitable for large-area depositions The post deposition annealing

environment can affect the energy bandgap which attributes to the oxygen

intercalation in the semiconductor material The crystal imperfections can also be

eliminated by post deposition annealing treatment to get the preferred bandgap of

semiconducting material In many literature reports of polycrystalline ZnSe thin films

the widening or narrowing of the bandgap is attributed to the decrease or increase of

crystallite size [27ndash31171] Metal-oxide semiconductor charge-transport materials

38

widely used in mesos tructured solar PSCs present the extra benefits of resistance to

the moisture and oxygen as well as optical transparency

TiO2 having a wide bandgap of 32eV is a chemically stable easily available

cheap and nontoxic and hence suitable candidate for potential application as the ETL

in PSCs Figure 31(a) [32] In TiO2-based PSCs the hole diffusion length is longer as

compared to the electron diffus ion lengt h [33] The e-h+ recombination rate is pretty

high in mesoporous TiO2 electron transport layer which promotes grain boundary

scattering resulting in suppressed charge transpor t and hence efficiency of solar cells

[3435] To improve the charge transpo rt in such structures many efforts have been

made eg to modify TiO2 by the subs titution of Y3+ Al3+ or Nb5+ [36ndash41] Graphene

has received close attention since its first production by using micro mechanical

cleavage by Geim in year 2004 It has astonishing optical electrical thermal and

mechanical properties [42ndash51] Currently struggles have been directed to prepare

graphene oxide by solution process which can be attained by exfoliation of graphite

The reactive carboxylic-acid groups on layers of graphene oxide helps in

functionalization of graphene permitting tunability to its opto-electronic

characteristics while holding good polar solvents solubility [5253] Graphene oxide

has been used in all part ie charge extraction layer electrode and in active layer of

polymer solar devices [54ndash57] The optical bandgap can be tuned significantly by

hybridization of TiO2 with graphene [58] The hybridization shifts the absorption

threshold from ultraviolet to the visible region Owing to the electrostatic repulsion

among graphene oxide flakes plus folding as well as random wrinkling in the

formation process of films the resulting films are found to be more porous than the

film prepared from reduced graphene oxide All the flakes of graphene oxides are

locked in place after the formation of graphene oxides film and have no more

freedom in the course of the reduction progression [172]

Another metal oxide material ie nickel oxide (NiO) having a wide bandgap

of about 35-4 eV is a p-type material The NiO material can be used as a hole

transport layer (HTL) in perovskite solar cells shown in Figure 31(b) [59] The n-

type semiconducting materials ie ZnO TiO2 SnO2 In2O3 have been investigated

repeatedly and are used for different applications However studies on the

investigation of p-type semiconductor thin films are hardly found and they need to be

studied properly due to their important technological applications One of the p-type

semiconducting oxide ie nicke l oxide has high optical transparency low cos t and

nontoxic can be employed as hole transport material in photovoltaic cell applications

39

[60] The energy bandgap of nickel oxide can vary depending upon the deposition

method and precursor material [61] Thin films of nickel oxide (NiO) can be

deposited by different deposition methods and its resistivity can be changed by the

addition of external incorpo ration

In our present study the post annealing of ZnSe films have been carried under

different environmental conditions The opt ical and structural studies of ZnSe films

on indium tin oxide coated glass (ITO) have been carried out which were not present

in the literature For TiO2 and GO-TiO2 films the cost-effective and facile synthesis

method has been used The nickel oxide films were prepared post deposition

annealing is studied The effect of silver (Ag) doping has also been carried out and

compared with the undoped nickel oxide films The results of these different charge

transport layers are discussed in separate sections of this chapter below

Figure 31 (a) Perovskite solar cell configuration (b) Inverted perovskite solar cell configuration

31 ZnSe Films for Potential Electron Transport Layer (ETL) In this section optical structural morphological electrical compositional studies of

ZnSe thin films has been carried out

311 Results and discussion The Rutherford back scattering spectra were employed to investigate the

thickness and composition and of ZnSe films Figure 32 The intensities of the peaks

at particular energy values are used to measure the concentration of the constituent in

the compound The interface properties of ZnSe film and substrate material are also

studied Rutherford backscattering spectroscopy analysis

40

Figure 32 Rutherford backscattering spectra (RBS) of ZnSe films annealed at different temperatures

The spectra in the range of 250ndash1000 channel is owing to the glass substrate

The high energy edge be longs to Zn and shown in the inset of the figure The next

lower energy edge in spectra is due to Se atoms The simulation softwares ie

SIMNRA and RUMP were used for the compositional and thickness calculations of

the samples The calculated thickness values are given in Table 31 These studies

have also shown that there is no interdiffusion at the interface of film and substrate

material even in the case of post annealed films

Annealing Temp (degC) As-made 350 400 450 500

Thickness (nm) 154 160 148 157 160

Table 31 Thickness of ZnSe films with annealing temperature

The x-ray diffractograms of ZnSeglass films are displayed in Figure 33 The as made

film has shown an amorphous behavior Whereas the well crystalline trend is

observed in the case of post deposition annealed samples The post deposition

annealing has improved the crystallinity of the material which was improved with the

annealing temperature further up to 500˚C The ZnSe films have presented a zinc

blend structure with an orientation along (111) direction at 2Ѳ = 2711ᵒ The peak

shift toward higher 2Ѳ value is observed with post deposition annealing The value of

lattice constant has found to be decreased and crystallite size has increased with

increasing annealing temperature

41

Figure 33 X-ray diffractograms of ZnSe thin films annealed at different temperatures

From xrd analys is crystallite size lattice strain and disloc ation density were

calculated by using following expressions [173]

119915119915 =120782120782 120791120791120783120783120640120640119913119913119920119920119913119913119956119956Ѳ 120785120785120783120783

120634120634 =119913119913119920119920119913119913119956119956Ѳ120783120783 120785120785 120784120784

120633120633 =120783120783120783120783120634120634119938119938119915119915 120785120785 120785120785

The op tical transmission spectra of ZnSeglass films were collected by optical

spectrophotometer in 350ndash1200 nm wavelength range It was observed that ZnSe

films have higher transmission in 400-700 nm wavelength range showing the

suitability of the material for window layer application in thin film solar cells Films

have shown even better optical transmission with the increasing post-annealing

temperature The improvement in optical tramission can be attributed to the reduction

in oxygen due to post-annealing in vacuum Figure 34 This reduction in oxygen

from the material might have remove the trace amounts of oxides ie SeO2 ZnO etc

present in the region of grain boundaries which resulted in enhanced crystallite size

The absorption coefficient (α) is calculated by using expression α= 1119889119889119889119889119889119889 (1

119879119879) where d is

the thickness and T is transmission of the films The optical bandgap is calculated by

(αhυ)2 versus (hυ) plot by drawing an intercept on the linear portion of the graph

Figure 35 The intercept at x-axis (hν) give the energy bandgap value The energy

bandgap value of as made ZnSe is found to be 244eV which has increased with

increasing post-annealing temperature Table 32

The electrical conductivities of ZnSeglass films have been calculated by using

currentndashvoltage (I-V) graphs of the films The films have shown an increasing trend in

42

electrical conductivity with increasing post-annealing temperature The increase in

conductivity with the increase of post-annealing temperature is most probably owing

to the rise in crystallite size as annealing temperature control the growth of the grains

of films The lowest value of the resistance is observed in the samples post deposition

annealed at 450ᵒC Afterwards ZnSeITO were prepared and the samples are post-

annealed at 450ᵒC and compared their structural and optical properties with

ZnSeglass films The structural and optical characteristics of ZnSe films can be

highly affected by the oxygen impurities So post-annealing of ZnSeglass films has

been carried out at 450ᵒC in air as well as in vacuum to see the consequence of

oxygen inclusion on its properties Moreover the contamination of ZnSe films with

oxygen during the preparation process is difficult to prevent due to more reactivity of

ZnSe [174]

Figure 34 Optical transmission spectra of ZnSe thin films at different annealing temperatures

Figure 35 (αhν)2 versus hν of ZnSe thin films annealed at different temperature

43

Table 32 Energy bandgap values of ZnSeglass at different annealing temperatures in vacuum

The x-ray diffraction scans of ZnSeITO are displayed in Figure 36 These

samples have shown a zinc blend structure with 2Ѳ = 3085ᵒ along (222) direction

The ZnSeITOglass samples have shown higher energy bandgap value as compared

with ZnSeglass samples This increase in bandgap is most likely arising because of

the flow of carriers from ZnSe towards ITO due higher ZnSe Fermi- level than that of

ITO Due to difference in the fermi- levels of ZnSe and ITO more vacant states are

created in the conduction band of ZnSe thus encouraging an increase in the energy

gap The crystallinity of ZnSeITO thin films has been improved after post deposition

annealing under vacuum conditions and no peak shift is detected The crystallite size

was found to be increased with post deposition annealing supporting with our

previous argument in earlier discussion The strain and dislocation density have been

decreased with post deposition annealing in vacuum as shown in Table 33

Figure 36 X-ray diffractograms of ZnSeITO films annealed at various conditions

Post deposition annealing Temp (degC) Energy bandgap (eV)

As-made 244

350 248

400 250

450 252

500 253

Sample Annealing 2θ(ᵒ) d

(Å)

a

( Å)

D

(nm)

ε

times10-4

δtimes1015

(lines m-2)

ZnSeITO As-made 3084 288 1001 20 17 124

ZnSeITO Vacuum (450degC) 3064 290 1008 35 10 041

44

Table 33 Structural parameters of the ZnSeITO films annealed under various environments

The transmission spectra of ZnSeITO films were collected in 350-1200 nm

wavelength range Figure 37 The transmission was found to be increased in the case

of sample annealed at 450ᵒC in air The band gap values are shown in Figure 38 and

Table 34 The highest bandgap value is obtained for the sample annealed in air This

increase in bandgap could be most probably due to removal of the vacancies and

dislocations closer to the conduction bandrsquo bottom or valence bandrsquos top due to lower

substrate temperature during deposition which resulted in the lowering of the energy

bandgap The density of states present by inadvertent oxyge n is reduced due to the

removal of oxygen from the material by post-annealing resulting in an increase in the

bandgap of the sample The increase in bandgap of vacuum annealed sample is

smaller as compared with that of air annealed sample which is most likely due to the

recrystallization of ZnSe in the absence of oxygen intake from the atmosphere This

has been resulted in energy bandgap of 293 eV in post deposition annealing ZnSe

sample in vacuum

Figure 37 Transmittance spectra of ZnSeITO thin films at different annealing temperatures

ZnSeITO Air (450degC) 3088 288 1001 23 16 102

45

Figure 38 (αhν)2 versus hν plots of ZnSeITO thin films

Table 34 Optical bandgap of the ZnSeITO thin films annealed at 450degC

The presence of inadvertent oxygen in ZnSe films promotes the oxides

formation which increase films resistance The current voltage (I-V) measurements

supported this argument Figure 39 The films have shown Ohmic behavior and

resistance of the vacuum annealed sample has shown minimum value shown in Table

35

Figure 39 Current voltage (IndashV) graphs of ZnSeITO thin films

Sample Annealing Resistance

(103Ω)

Sheet resistance

(103Ω)

Annealing Energy bandgap (eV)

As prepared 290

Vacuum annealed (450degC) 293

Air annealed (450degC) 320

46

ZnSe-ITO As prepared 015 04

ZnSe-ITO Vacuum annealed (450degC) 010 02

ZnSe-ITO Air annealed (450degC) 017 05

Table 35 ZnSe thin films annealed at 450degC

In sample post deposition annealed in vacuum the decrease in resistance is

most likely due to the decrease in oxygen impurities which promotes the oxide

formation From these post deposition annealing experiments we come to the

conclus ion that by adjusting the post deposition parameters such as annealing

temperature and annealing environment the energy bandgap of ZnSe can easily be

tuned By more precisely adjusting these parameters we can achieve required energy

bandgap for specific applications

32 GOndashTiO2 Films for Potential Electron Transport Layer In this section we have discussed the titanium dioxide (TiO2) films for potential

electron transport applications and studied the effect of graphene oxide (GO)

addition on the properties of TiO2 electron transport layer

321 Results and discussion The x-ray diffraction scans of the synthesized graphene oxide (GO) and

titanium oxide (TiO2) nanoparticles are shown in Figure 310 (a b) The spacing (d)

between GO and reduced graphene oxide (rGO) layers was calculated by employing

Braggrsquos law The peak appeared around 1090ᵒ along (001) direction with interlayer

spacing value of 044 nm was seen to be appeared in the x-ray diffraction scans of

graphene oxides (GO) A few other diffraction peaks of small intensities around

2601ᵒ and 42567ᵒ corresponding to the interlayer spacing of 0176nm and 008nm

respectively were observed The increase in the value of interlayer spacing observed

for the peak around 10ᵒ is because to the presence functional groups between graphite

layers The diffraction peak observed around 4250ᵒ also shows the graphite

oxidation The high intensity peak appeared around 1018ᵒ along (001) direction

having d-spacing value of 080nm and crystallite size of 10nm There was appearance

of small peak around 2690ᵒ along (002) direction owing to the domains of un-

functionalized graphite [175] The x-ray diffractogram of TiO2 nanoparticles is shown

in Figure 310(b) The xrd analysis confirmed the rutile phase of TiO2 nano-particles

The main diffraction peak of rutile phase is observed around 2710ᵒ along (110)

direction [176] The average crystallite size of TiO2 nanoparticles was about 36nm

The x-ray diffraction of the graphene oxide added samples ie xGOndashTiO2 (x = 0 2

47

4 8 and 12 wt)ITO nanocomposite films are displayed in Figure 310(c) The xrd

analysis revealed pure rutile phase of TiO2 nanoparticles film with distinctive peaks

along (110) (200) (220) (002) (221) and (212) directions A few peaks due to

substrate material ie tin oxide (SnO2) indexed to SnO2 (101) and SnO2 (211) are

also appeared In the case of composite films a very weak diffraction peak around

10ᵒ-12ᵒ is present which most likely due to reduced graphene oxide presence in small

amount in the composites films The peak observed around 269ᵒ due to graphene

oxides (GO) is suppressed due to very sharp TiO2 peak present around 27ᵒ

Scanning electron microscopy (SEM) images of xGOndashTiO2(x=0 2 4 8 and

12 wt) nano-composites are presented in Figure 311 It can be seen from

micrographs that with graphene oxide addition the population of voids has reduced

and the quality of films has been improved Figure 311 (bndashd) shows graphene oxide

(GO) sheets on TiO2 nanoparticles

Figure 310 X-ray diffraction patterns of (a) GO (b) TiO2 nanoparticles (c) xGOndashTiO2 (x = 0 2 4 8

and 12 wt)ITO thin films

48

Figure 311 Electron micrographs of xGOndashTiO2 (a) x=0 (b) x = 2 wt (c) x = 4 wt (d) x = 8 wt

and (e)x = 12 wt nanocomposite thin films on ITO substrates

To investigate the optical properties UVndashVis spectroscopy of xGOndashTiO2 (x

=0 2 4 8 12wt) thin films are collected in 200-900nm wavelength range and

shown in Figure 312 The transmission of the films has been decreased with increase

in the graphene oxide (GO) concentration in the composite films The optical

absorption spectra of GOndashTiO2 nanocomposites have shown a red-shift which is

corresponding to a decrease in energy bandgap with addition of graphene oxide The

absorption in the 200-400nm region is increased with graphene oxide (GO) addition

The optical bandgap calculated by us ing beerrsquos law was found to be around 349eV

for TiO2 thin film and 289eV for the GOndashTiO2 nanocompos ites Figure 313 The

add ition of graphene oxide has decreased the bandgap of the composite samples

which is due to TindashOndashC bonding corresponding to the red shift in absorption spectra

[177] Moreover the bandgap has been decreased with increasing GO concentration

which directs the increasing interaction between graphene oxide (GO) and TiO2

The presence of small hump in the tauc plot shows the presence of defect states

which is attributed to the additional states within the energy bandgap of TiO2 [32]

The observed decrease in the transmission spectra owing to GO addition is a

drawback of GO composites electron transport layers This issue could be addressed

by post-annealing of GO-based composite samples at higher temperatures The post

deposition annealing of xGOndashTiO2 (x = 12 wt) film was carried out at 400ᵒC under

nitrogen atmosphere for 20minutes The post deposition thermally annealed films has

49

shown an increase of 10-12 in transmission and shifted the bandgap value from 289

to 338eV depicted in Figure 312 and 313 This increase in transmission of the film

due to thermal post-annealing is associated to the reduction of oxygenated graphene

[178]

Figure 312 Optical transmission spectra of xGOndashTiO2 (x = 0 2 4 8 and 12 wt) nanocomposite

thin films on ITO substrates

Figure 313 (αhν)2 vs hν plots of xGOndashTiO2 (x = 0 2 4 8 and 12 wt) nanocomposite films on ITO

substrates The electrical properties of the xGOndashTiO2 (x = 0 2 4 8 and 12 wt)ITO

thin films were studied by analyzing the currentndashvoltage graphs Figure 314 The

current has increased linearly with increasing voltage in the case of pure TiO2

nanoparticle film which shows the Ohmic contact between TiO2 and ITO layers The

conductivity of the films has decreased with the higher GO addition in the samples

This decrease in conductivity is most likely arises owing to the functional groups

50

presence at the basal planes of the GO layers The existence of such func tional groups

interrupt the sp2 hybridization of the carbon atoms of GO sheets turning the material

to an insulating nature The post deposition thermal annealing of xGOndashTiO2 (x = 12

wt)ITO film at 400ᵒC has also improved the conductivity of the sample which is

found by analyzing current ndashvoltage curves as shown in Figure 314

Figure 314 Current-voltage characteristics of xGOndashTiO2 (x = 0 2 4 8 and 12 wt)ITO films

These changes in the properties of post deposition annealed xGOndashTiO2 (x =

12 wt)ITO film is probably because of the detachment of functional groups from

the graphene oxide basal plane In other words post deposition thermal annealing has

enhanced the sp2 carbon network in the graphene oxide (GO) sheets by removing

oxygenated functional groups thereby increasing the conductivity of the sample [50]

The x-ray photoemission spectroscopy survey scans of as synthesized GOndashTiO2ITO

film are represented in Figure 315(a) The survey scan shows the peaks related to

titanium carbon and oxygen The oxygen 1s core level spectrum can be resolved into

three sub peaks positioned at 5299 5316 and 5328eV shown in Figure 315(b) The

peaks positioned at 5299 5316 and 5328eV correspond to O2- in the TiO2 lattice

oxygen bonded with carbon and OH- on TiO2 surface [179180] The C 1s core level

spectrum has de-convoluted into four sub peaks located at 2846 eV 2856 2873 and

2890eV corresponds to CndashCCndashH CndashOH CndashOndashC andgtC=O respectively shown in

Figure 315(c) This shows that the presence of ndashCOOndashTi bonding which arises by

interacting ndashCOOH groups on graphene sheets with TiO2 to form ndashCOOndashTi bonding

[181ndash183] The core leve l spectrum of Ti2p have shown two peaks at 4587 and

46452eV binding energies which is due to titanium doublet ie 2p32 and 2p12

respectively Figure 315(d) [179184]

51

Figure 315 X-ray photoemission spectroscopy (a) survey scan (b) O1s (c) C1s (d) Ti2p core level

spectra of as synthesized xGOndashTiO2 (x = 8 wt)ITO films

33 NiOX thin films for potential hole transport layer (HTL) application

In this section we have investigated nickel oxide (NiOx) thin films The films

were post annealed at different temperatures for optimization These NiOx films were

further used as HTL in solar cell fabrication

331 Results and Discussion

The x-ray diffraction scans of NiOxglass and NiOₓFTO thin films in 20-70⁰

range with a step size of 002ᵒ sec are displayed in Figure 316(a b) The post-

annealing of the samples was carried out at 150 to 600⁰C A small intensity peak is

observed around 4273⁰ which corresponds to (200) plane The diffraction patterns

were matched with cubic structure having lattice parameter a=421Aring The lower

intensity of peaks designates the amorphous nature of the deposited film With the

increasing post deposition annealing temperature peak position is shifted marginally

towards higher 2θ value In xrd patterns of NiOₓFTO samples two peaks of NiOx

along (111) and (200) planes at 2θ position of 3761ᵒ and 4273ᵒ respectively were

observed The substrate peaks were also found to be appeared along with NiOx peaks

The reflection along (111) plane which is not appeared in NiOxglass sample could

possibly be either due to FTO or NiOx film The increase in the intensity of the peak is

52

witnessed for sample annealed at 500⁰C which shows improvement in the

crystallinity of the films Beyond post annealing temperature of 500⁰C the peak

intensities are decreased So it was concluded that the optimum annealing

temperature is 500oC from these studies

Figure 316 X-ray diffraction pattern of 01M NiO films on (a) glass substrates (b) FTO substrates annealed at 150-600 oC

The transmission spectra of NiOx thin films annealed at 150-500⁰C are

displayed in Figure 317(a) The films have shown 80 transmission in the visible

region with a sudden jump near the ultraviolet region associated to NiOx main

absorption in this region [185] The maximum transmission is observed in sample

with 150⁰C post deposition annealing temperature which is most likely due to cracks

in the film which were further healed with increasing annealing temperatures and

homogeneity of the films have also been improved The band gaps of NiOx films

annealed at different temperatures were calculated and d isplayed in Figure 317(b)

The energy bandgap values were found to be around 385 387 391eV for

150 400 and 500ᵒC respectively shown in Table 36 The bandgap of NiOx films has

increased with post annealing up to 500ᵒC which shows the tunability o f the bandgap

of NiOx with post-annealing temperature The increase in the band gap is supports the

2θ shift towards higher value These band gap values are comparable with literature

values [186]

53

Figure 317 UV-Vis-NIR (a) Transmission spectra (b) (αhν)2 vs hν plots of 01M NiOx glass films

annealed at 150-600oC temperatures

Sample Name Energy bandgap (eV) Refractive Index (n)

NiO (150oC) 385 213

NiO (400oC) 387 213

NiO (500oC) 391 210

Table 36 Band gap values of the pristine NiOglass thin film annealed at 150-500 ᵒC

The x-ray diffraction scans of Ag doped NiOx films yAg NiOx (y=0 4 6 8mol

) on FTO substrates are shown in Figure 318 The molar concentration of pristine

NiOₓ precursor solution was fixed at 01M and doping calculations were carried out

with respect to this molarity The post deposition annealing was carried out at 500ᵒC

The two main reflections due to NiOₓ are observed at 376ᵒ and 4274ᵒ corresponding

to (111) and (200) planes as discussed above The presence of peaks due to FTO

subs trates shows that the NiOx films are ver y thin Another reason may be because of

the low concentration of the NiOx precursor solution has formed a very thin film The

peak intensities of yAg NiOx (y=4 6 8mol) were increased as compared to the

pr istine NiOx thin film which shows that doping of silver has improved the

crystallinity of the film No extra peak due to Ag was observed which shows that the

incorporation of silver has not affected the crystal structure of the NiOx

Figure 319 shows the xrd scans of nickel oxide thin films prepared by using

different precursors ie Ni(NO3)26H2O and Ni(acet)4H2O with different moralities

(01 and 075 M) on glass substrate The films with molar concentration of 01M

shows only a single peak at 4323⁰ correspo nding to (200) direction while other two

reflections along (111) and (220) which were given in the literature were not observed

in this case However all the three reflections are observed when we increase the

54

molarity of the precursor solution to 075M The same reflection planes were also

observed in the films prepared by another precursor salt ie Ni(acet)4H2O The

075M Ni(NO3)26H2O precursor solut ion films have shown better crystallinity than

films based on 075M Ni(acet)4H2O precursor solution

Figure 318 X-ray diffraction scans of 01M yAg NiO (y=0 4 6 8mol)FTO thin films

Figure 319 XRD scans of NiOx glass films using Ni(NO3)26H2O and Ni(ace)4H2O precursors Hence increasing the molarity from 01 to 075M of Ni(NO3)26H2O increases the

crystallinity of the nickel oxide thin film The crystal structure of NiOx was found to

be cubic structure by matching with literature

Optical transmission and absorption spectra of yAg NiOx (y=0 4 6

8mol)FTO thin films in 250-1200 nm wavelength range are displayed in Figure

320(a b) The inset demonstrations are the enlarge picture of 350-750nm region of

transmission and absorption spectra The spectra show transmission above 80 in the

visible region showing that the prepared films are highly transparent The

transmission of the NiOx films have further increased with Ag doping The 4mol

and 6mol Ag doped films exhibited an increased in film transmission up to 85

which is an advantage for using these films for potential window layer application

55

The Figure 321(a) shows the plots of (αhν)2 vs (hν) for pristine NiOx and yAg NiO

(y=4 6 8mol) films on FTO subs trates The graph shows the band gap of pristine

NiOx of 414 eV which displays a good agreement with the bandgap value of pristine

NiOx thin film reported in literature The optical band gap of pristine NiOx is in the

range 34-43eV has been reported in the literature [187] The bandgap values have

been decreased with Ag doping and the minimum bandgap value of 382eV is

obtained for 4 mol Ag doping shown in Table 37 This decrease in bandgap

increases the photocurrent which is good for photovoltaic applications The decrease

in the transmittance with the increased thickness of NiO thin film can be explained by

standard Beers Law [188]

119920119920 = 119920119920120782120782119942119942minus120630120630120630120630 120785120785120783120783

Where α is absorption co-efficient and d is thickness of film The absorption

coefficient (α) can be calculated by equation below [189]

120630120630 =119949119949119946119946(120783120783 119931119931 )

120630120630 120785120785120783120783

Absorption coefficient can also be calculated using the relation [190]

120572120572

=2303 times 119860119860

119889119889 3 Error Bookmark not defined Where T represents transmittance A is the absorbance and d represent the film

thickness of NiO thin film

The refractive index is calculated by using the Herve-Vandamme relation [190]

119946119946120784120784 = 120783120783+ (119912119912

119920119920119944119944 +119913119913)120784120784 120785120785120789120789

Where A and B have constants values of 136 and 347 eV respectively The

calculated values of refractive index and optical dielectric constant are given in the

Table 37 The dielectric values are in the range of 4 showing that the films are of

semiconducting nature These values of refractive index are in line with the reported

literature range [59] The doping of Ag has shown a marginal increase in the dielectric

values showing that the silver doping has increased the conductivity of the NiO thin

films The extinction coefficient of the deposited yAg NiOx (y=0 4 6 8mol) thin

films has also been calculated and plotted in the Figure 321(b) Extinction coefficient

shows low values of in the visible and near infra-red region confirming the

transparent nature of the silver doped NiO thin films

56

The Figure 322(a) shows the transmittance spectra of the nickel oxide thin films

prepared with different molarities and different precursor materials The transmission

spectra show a decrease in the transmission of the NiOx thin film up to 65 in the

visible region as the molarity is increased from 01 to 075M The band gap values

calculated from the transmission spectra are shown in Figure 322(b) The calculated

values of the band gap of NiOx thin films deposited on glass substrates are 39 36

and 37eV for Nickel nitrate (01M) Nickel nitrate (075M) and Nickel acetate

(075M) precursor solutions shown in Table 38 Increase in molarity of the solut ion

has shown a decrease in energy band gap value However 075M Ni(NO3)26H2O

precursor solution based NiOx thin film have shown better crystallinity and suitable

transmission in the visible range so this concentration was opted for further

investigation of NiOx thin films in detail in next section

Figure 320 UV-Vis-NIR (a)Transmittance (b) absorption spectra of yAg NiO (y=0 4 6 8mol)FTO thin films

Figure 321 (a)(αhν)2 vs hν plots (b) Extinction coefficient (n) of yAg NiOx (y=0 4 6 8mol)FTO

57

Figure 322 Optical (a)Transmission spectra (b)(αhν)2 vs hν plots of the NiOx thin film Prepared by (01 and 075M) Nickel Nitrate and 075M Nickel acetate precursor on glass substrates

119962119962119912119912119944119944119925119925119946119946119925119925 Energy bandgap (eV) Refractive Index (n) Optical Dielectric Constant (εꚙ)

119962119962 = 120782120782120782120782119949119949 414 205 437

119962119962 = 120783120783120782120782119913119913119949119949 382 212 462

119962119962 = 120788120788120782120782119913119913119949119949 402 207 449

119962119962 = 120790120790120782120782119913119913119949119949 405 207 449

Table 37 Band gap calculation of yAg NiOx (y=0 4 6 8mol )glass thin films

Sample Energy bandgap (eV) Refractive Index (n)

01M Ni(NO3)26H2O 39 21

075M Ni(NO3)26H2O 36 216

075M Ni(acet)4H2O 37 215

Table 38 NiOxglass thin film Prepared by nickel nitrate and nickel acetate precursors

To investigate the doping effect more clearly the x-ray diffraction patterns of

yAg NiOx (y=0 4 6 8 mol) thin films based on 075M precursor solution on

glass as well as FTO substrates were collected The x-ray diffraction pattern of yAg

NiOx (y=0 4 6 8 mol) thin films on glass substrates are shown in Figure 323(a)

The pristine NiOx films have shown cubic crystal structure by matching with JCPDS

card number 96-432-0509 [191] The main reflections are observed around 3724ᵒ

4329ᵒ and 6282ᵒ positions corresponding to (111) (200) and (220) direction

respectively The preferred orientation of NiOx crystal structure is along (200)

direction The doping of Ag in the NiOx has shifted the peaks slightly towards lower

58

2θ value The peak shifts towards lower value corresponds to the expansion of the

crystal lattice The ionic radius of Ag atom (115Å) is larger than that of N i atom

(069Å) [192] The shifting of the NiOx peaks towards the lower theta positions is also

observed in the NiOx thin film deposited on the FTO substrate and shown in Figure

323(b c) No extra peak was observed in x-ray diffraction pattern of NiOx films with

Ag showing the substitutional type of doping that is the doped silver replaces the

nicke l atom

These results of yAg NiOx (y=0 4 6 8 mol) thin films on glass substrates were

further confirmed by calculating the lattice constant and inter-planar spacing by using

Braggrsquos law (39) [68]

1119889119889ℎ1198961198961198891198892 =

ℎ2 + 1198961198962 + 1198891198892

1198861198862 3 Error Bookmark not defined

2119889119889ℎ119896119896119889119889 119904119904119904119904119889119889119904119904= 119889119889119899119899 3 Error Bookmark not defined

The calculated values of the lattice constant and interplanar spacing is shown

in the Table 39 The lattice constant values are found to be increased with Ag doping

the lattice is expanding with the Ag dop ing in NiOx

59

Figure 323 XRD scans of 075M precursor based yAg NiOx (y=0-8 mol ) films (a )glass substrates (b) FTO substrates (c) Strained picture of yAg NiOx (y=0 4 6 8 mol )FTO thin films

119962119962119912119912119944119944119925119925119946119946119925119925 d(111) (Å) d(200) (Å) d(220) (Å) a (Å)

y=0 mol 24125 20883 14780 41766

y=4mol 24488 21274 14884 4255

y=6mol 24506 21373 14995 4275

y=8mol 24436 21422 1501 4284

Table 39 Interplanar spacing d( Å) and Lattice constant a(Å) measurement of yAg NiOx (y=0 4 6 8 mol ) thin films on glass substrates

Crystallite Size of yAg NiOx (y=0 4 6 8mol)) thin films were calculated using Debye-Scherrer relation [193]

119863119863 =119896119896119899119899


Where k=09 (Scherrer constant) 120573120573 is the full width at half maximum and

λ=15406Å wavelength of the Cu kα radiation

The dislocation density (δ) is calculated by the Williamson-Smallman

relation Similarly microstrain parameter (ε) is calculated by the known relation

[194]

120575120575

=11198631198632 (

1198891198891199041199041198891198891198971198971199041199041198981198982 ) 3 Error Bookmark not defined

120576120576 =120573120573


The calculated values of the 120573120573 119863119863 120575120575 and 120634120634 are given in the following Tables

310 to 313 The FWHM is found to be decreasing as the doping of Ag is increased

which shows the improved crystallinity of the films Table 311 shows that the

crystallite size of the NiOx crystals is increased with the silver incorporation up to 6

mol Beyond 6mol dop ing ratio the crystallite size has shown a decrease This is

possibly due to phase change of the NiOx crystal lattice beyond 6mol Ag doping

119930119930119938119938120782120782119930119930119949119949119942119942 FWHM (2θ )

FWHM (111) (ᵒ) FWHM (200) (ᵒ) FWHM (220) (ᵒ)

y=0 mol 0488 0349 0411

y=4mol 0164 0284 0282

y=6mol 0184 0180 0204

y=8mol 0214 0215 0168

Table 310 Full width at half maxima values of yAg NiOx (y=0 4 6 8 mol )glass thin films

119930119930119938119938120782120782119930119930119949119949119942119942 D (111) nm D (200) nm D (220) nm

60

y=0 mol 17 25 23

y=4mol 51 30 33

y=6mol 45 47 45

y=8mol 39 40 55

Table 311 Crystallite size (D)of yAg NiOx (y=0 4 6 8 mol )glass thin films

119930119930119938119938120782120782119930119930119949119949119942119942 δ (111) (1014

linesm2)

δ (200) (1014

linesm2)

δ (220) (1014

linesm2)

y=0 mol 339 166 1947

y=4mol 387 111 925

y=6mol 485 445 485

y=8mol 655 636 328

Table 312 Dislocation density parameter (δ) of yAg NiOx (y=0 4 6 8 mol)glass thin films

119930119930119938119938120782120782119930119930119949119949119942119942 ε (111) (10-4) ε (200) (10-4) ε (220) (10-4)

y=0 mol 28 3832 293

y=4mol 216 3171 2032

y=6mol 2425 2027 1484

y=8mol 2811 2430 1221

Table 313 Micro strain parameter (ε) of yAg NiOx (y=0 4 6 8 mol)glass thin films

There was a considerable decrease in the values of dislocation density and micro-

strain was observed with the increase of Ag doping up to 6 mol This is most likely

due to subs titutiona l dop ing of Ag which replaces the Ni atoms and no change in the

lattice occurs but as the doping exceeds from six percent defects had showed an

increase giving a sign in phase change of NiOx structure above six percent Results

show that the limit of Ag dop ing in NiO thin films is up to 6mol

The Figure 324(a) represents the transmittance of 075 M yAg NiOx (y=0 4

6 8 mol) thin films deposited on glass substrates in the wavelength range of 200-

1200nm Results showed that the pr istine NiOx thin film is transparent in nature

showing above 75 transparency By the Ag doping into the pristine NiOx the

transmittance of the films is increased that is at 4mol dop ing ratio the transmittance

is above 80 This showed that by the doping of silver the transmittance of the NiO

thin film is increased This increase in the transparency of the NiOx films make them

more useful for window layer in photovoltaic devices

61

Figure 324 Optical (a)Transmission (b) (αhν)2 vs hν plots of yAg NiOx (y=0-8 mol)glass thin films The Figure 324(b) shows the band gap calculation by using the Tauc plot of 075M

yAg NiOx (y=0 4 6 8 mol) thin films on the glass substrates With silver

incorporation the band gap of NiOx thin film is decreased up to 6mol doping level

which may be due to formation of accepter levels near the top edge of valence band o f

NiOx with Ag doping Beyond 6mol doping bandgap value was found to be again

increased as shown in Table 314 These results are consistent with the x-ray

diffraction results that doping of Ag has expanded the NiOx crystal structure

119962119962119912119912119944119944119925119925119946119946119925119925 Energy bandgap (eV) Refractive Index (n)

119962119962 = 120782120782120782120782119913119913119949119949 408 206

119962119962 = 120783120783120782120782119913119913119949119949 397 208

119962119962 = 120788120788120782120782119913119913119949119949 401 208

119962119962 = 120790120790120782120782119913119913119949119949 404 207

Table 314 Bandgap values of 075M NiO and yAg NiOx (y=0 4 6 8 mol)glass thin films

62

The scanning electron microscopy images of yAg NiOx (y=0 4 6 8 mol)

thin films deposited on glass substrates at different magnifications ie 50kx 100kx

are shown in Figure 325(a b) There are visible cracks in pristine NiOx films which

were found to be healed with Ag doping and texture film with low population of voids

were observed with Ag doping of 6 8mol The dop ing of silver has minimized the

cracks of pristine NiOx and the smoothness of the NiOx films has also been improved

with the incorpor ation of Ag The Figure 325(c d) shows the SEM micrographs of

yAg NiOx (y=0 4 6 8 mol) thin films deposited on FTO substrates at different

magnification The thin films on FTO substrates have shown better results as

compared with that on the glass substrates The cracking surface of NiOx thin films

observed on glass substrates was healed in this case With the doping of Ag

smoothness of the film has increased with This is possibly due to the surface

modification due to already deposited FTO thin film as well as with Ag dop ing

The Figure 326 shows the Rutherford backscattering spectroscopy (RBS)

measurements of yAg NiOx (y=0 4 6 8mol ) on glass substrates The spectra

were fitted with SIMNRA software The experimental results are not match well with

the theoretical results which is due to the porous nature of the thin films formed and

the homogeneity of the films The thickness of the deposited thin films was obtained

to be round about 100 to 200 nm shown in Table 315

63

Figure 325 Scanning electron micrographs of yAg NiOx (y=0 4 6 8 mol)deposited (a) on glass

substrates at 50kx (b) on glass substrates at 100kx (c) on FTO substrates at 50kx (d) on FTO substrates at 100kx magnification

64

As the Rutherford backscattering technique is less sensitive technique for the

determination of compositional elements so the detection of Ag doping in the NiOx

thin films is not shown in the Rutherford backscattering spectroscopy (RBS) For this

purpose we have used another technique known as particle induced x-ray emission

(PIXE) The elemental composition and thickness are shown in the Table 315

Figure 326 Rutherford backscattering spectra of 075M yAg NiOx (y=0 4 6 8 mol)glass thin film

The Figure 327 shows the particle induced x-ray emission results of the pristine

NiOx and yAg NiOx (y=0 4 6 8 mol) thin films on glass substrates The results

show the detection of contents of thin film and substrates also in the form of peaks

Results showed a significant peak of Ni (K L) whereas the Ag is also appeared in the

particle induced x-ray emission (PIXE) spectra of doped NiOx thin films Oxygen is

of low atomic number thatrsquos why it is not in the detectable range of particle induced

x-ray emission Particle induced x-ray emission is a sensitive technique as compared

to the RBS thatrsquos why the de tection of Ag doping has observed

65

Figure 327 Particle induced x-ray emission plot of 075M yAg NiOx (y=0 4 6 8 mol)glass thin films

119930119930119938119938120782120782119930119930119949119949119942119942 y=0mol y=4mol y=6mol 119962119962 = 120790120790120782120782119913119913119949119949

Nickel 020 023 0225 021

Silver 00001 00001 0001 001

Silicon 0140 0127 0130 013

Oxygen 0658 0636 0640 065

Thickness (nm) 110 195 183 116

Table 315 Elemental composition and thickness of 075M yAg NiOx (y=0 4 6 8 mol) thin films on glass substrates calculated by RBS PIXE spectroscopy

To examine the effect of silver (Ag) doping on the electrical properties of

NiOx thin films we measured the carrier concentration Hall mobility and the

resistivity using the Hall-effect measurements apparatus at room temperature The

Figure 328 (a b c d) represents the carrier concentration Hall mobility Resistivity

and conductivity values at different dop ing concentrations of Ag in N iOx thin films on

glass substrates

Table 316 shows the values of carrier concentration mobility and resistivity

on Ag doped NiOx thin films With Ag doping in NiOx has enhanced the carrier

concentration of NiOx semiconductor This increase may be due to replacement of Ag

by Ni (substitutional doping) in the NiOx crystal lattice resulting in NiOx lattice

disorder This disorder increases the Oxygen Vacancy with Ag resulting in an

enhanced conductivity of the NiOx Further mob ility measurements showed that by

silver dop ing the charge mobility has decreased up to 4 doping level then an

increase is observed in the thin films up to 6mol Ag dop ing The resistivity has

66

shown a decreasing trend up to the 6mol with Ag doping This is possibly due to

the increased number of hole charge carriers as confirmed by the carrier concentration

values Further from 6 to 8mol a rise in the resistivity of the NiOx thin films This

refers to the phase segregation as confirmed by the x-ray diffraction and optical

results

Figure 328 (a) Carrier concentration vs doping concentration (b) Hall Mobility vs doping concentration (c) Resistivity vs doping concentration (d) Conductivity vs doping concentration of yAg

NiOx (y=0 4 6 8 mol)glass thin films

119930119930119938119938120782120782119930119930119949119949119942119942 Carrier Concentration (cm-3)

Hall Mobility (cm2Vs)

Resistivity (Ω-cm)

Conductivity (1Ω-cm)

119962119962 = 120782120782120782120782119913119913119949119949 2521times1014 6753 3666 2728x10-7

119962119962 = 120783120783120782120782119913119913119949119949 2601times1015 2999 1641 6095x10-7

119962119962 = 120788120788120782120782119913119913119949119949 6335times1014 285 8003 125x10-6

119962119962 = 120790120790120782120782119913119913119949119949 4813times1014 1467 8843 1131x10-6

Table 316 Carrier concentration Hall mobility Resistivity and conductivity of 075M yAg NiOx (y=0 4 6 8 mol) thin films on glass substrates

The electrochemical impedance spectroscopy (EIS) spectroscopy results of yAg

NiOx (y=0 4 6 8 mol) thin films annealed at 500oC with molarity 075M in the

frequency range 10Hz to 10MHz at roo m temperature are presented in Figure 329

The samples show typical semicircles matched well with literature [195] The

67

resistance of the samples can be measured by the difference of the initial and final real

impedance values of the Nyquist plots The calculated values of resistances calculated

by both methods are shown in the The resistance of the NiOx thin films are found to

be decreased by the Ag incorporation Table 317 The minimum resistance value is

obtained for 6mol Ag doping shown in the inset of Figure 329 This is in also

consistent with the hall measurements discussed above The roughness of the

semicircles is also reduced by the silver dop ing showing improvement in the

chemistry of the NiOx thin films

Figure 329 Electrochemical impedance spectroscopy Nyquist plots of yAg NiOx (y=0 4 6 8 mol) thin films

119962119962119912119912119944119944119925119925119946119946119925119925

Resistance (EIS) (Ω)

119962119962 = 120782120782120782120782119913119913119949119949 15 times 106

119962119962 = 120783120783120782120782119913119913119949119949 4 times 105

119962119962 = 120788120788120782120782119913119913119949119949 25 times 103

119962119962 = 120790120790120782120782119913119913119949119949 12 times 106

Table 317 Resistance of yAg NiOx (y=0 4 6 8 mol) thin films

34 Summary The zinc selenide (ZnSe) graphene oxide-titanium dioxide (GO-TiO2)

nanocomposite films are studied for potential electron transport layer in solar cell

application The hole transport nickel oxide (NiO) is also investigated for potential

hole transport layer in solar cell applications The ZnSe thin films were deposited by

thermal evapor ation method The structural optical and electrical properties were

found to be dependent on the post deposition annealing The energy bandgap of

ZnSeITO films was observed to have higher value than that of ZnSeglass films

68

which is most likely due to higher fermi- level of ZnSe than that of ITO This

difference in fermi- level has resulted into a flow of carriers from the ZnSe layer

towards the indium tin oxide which causes the creation of more vacant states in the

conduction band of ZnSe and increased the energy bandgap So it is concluded from

these studies that by controlling the post deposition parameters precisely the energy

bandgap of ZnSe can be easily tuned for desired applications

The GOndashTiO2 nanocomposite thin films were prepared by a low-cost and simplistic

method ie spin coating method The structural studies by employing x-ray

diffraction studies revealed the successful preparation of graphene oxide and the rutile

phase of titanium oxide The x-ray photoemission spectroscopy analysis confirmed

the presence of attached functional groups on graphene oxide sheets The current-

voltage features exhibited Ohmic nature of the contact between the GOndashTiO2 and ITO

substrate in GOndashTiO2ITO thin films The sheet resistance of the films is decreased by

post deposition thermal annealing suggesting to be a suitable candidate for charge

transport applications in photovoltaic cells The UVndashVis absorption spectra have

shown a red-shift with graphene oxide inclusion in the composite film The energy

band gap value is decreased from 349eV for TiO2 to 289eV for xGOndash TiO2 (x = 12

wt) The energy band gap value of xGOndashTiO2 (x = 12 wt) has been increased

again to 338eV by post deposition thermal annealing at 400ᵒC with increased

transmission in the visible range which makes the material more suitable for window

layer applications The reduction of graphene oxides films directly by annealing in a

reducing environment can consequently reduce restacking of the graphene sheets

promoting reduced graphene oxide formation The reduced graphene oxide obtained

by this method have reduced oxygen content high porosity and improved carrier

mobility Furthermore electron-hole recombination is minimized by efficient

transferring of photoelectrons from the conduction band of TiO2 to the graphene

surface corresponding to an increase in electron-hole separation The charge transpo rt

efficiency gets improved by reduction in the interfacial resistance These finding

recommends the use of thermally post deposition annealed GOndashTiO2 nanocomposites

for efficient transport layers in perovskite solar cells

The semiconducting NiO films on FTO substrates were prepared by solution

processed method The x-ray diffraction has shown cubic crystal structure The

morphology of NiO films has been improved when films were deposited on

transparent conducting oxide (FTO) substrates as compared with glass substrates The

doping of Ag in NiOx has expanded the lattice and lattice defects are found to be

69

decreased The surface morphology of the films has been improved by Ag doping

The atomic and weight percent composition was determined by the energy dispersive

x-ray spectroscopy showed the values of the dopant (Ag) and the host (NiO) well

matched with calculated values The film thicknesses calculated by the Rutherford

backscattering spectroscopy technique were found to be in the range 100-200 nm The

UV-Vis spectroscopy results showed that the transmission of the NiO thin films has

improved with Ag dop ing The band gap values have been decreased with lower Ag

doping with lowest values of 37eV with 4mol Ag doping The alternating current

(AC) and direct current (DC) conductivity values were calculated by the

electrochemical impedance spectroscopy (EIS) and Hall measurements respectively

The mobility of the silver (Ag) doped films has shown an increase with Ag doping up

to 6mol doping with values 7cm2Vs for pr istine NiO to 28cm2Vs for 6mol Ag

doping The resistivity of the film is also decreased with Ag doping with minimum

values 1642 (Ω-cm)-1 at 4mol Ag doping At higher doping concentrations ie

beyond 6mol Ag goes to the interstitial sites instead of substitutional type of doping

and phase segregation is occurred leads to the deteriorations of the optical and

electrical properties of the films These results showed that the NiO thin films with 4

to 6mol Ag dop ing concentrations have best results with improved conductivity

mobility and transparency are suggested to be used for efficient window layer

material in solar cells

70

CHAPTER 4

4 Alkali metal (K Rb Cs RbCs) Based Mixed Cation Perovskites

In this chapter mixed cation (MA K Rb Cs RbCs) based perovskites are

discussed The structural morphological optical optoelectronic electrical and

chemical properties of these mixed cation based films were investigated

Perovskites have been the subject of great interest for many years due to the large

number of compounds which crystallize in this structure [80196ndash199] Recently

methylammonium lead halide (MAPbX3) have presented the promise of a

breakthrough for next-generation solar cell devices [78200201] The use of

perovskites have several advantages excellent optical properties that are tunable by

managing ambipolar charge transport [198] chemical compositions [200] and very

long electron-hole diffus ion lengt hs [79201] Perovskite materials have been applied

as light absorbers to thin or thick mesoscopic metal oxides and planar heterojunction

solar devices In spite of the good performance of halide perovskite solar cell the

potential stability is serious issue remains a major challenge for these devices [202]

For synthesis of new perovskite few attempts have been made by changing the halide

anions (X) in the MAPbX3 structure the studies show that these changings have not

led to any significant improvements in device efficiency and stability Also the

optoelectronic properties of perovskite materials have been studied by replacing

methylammonium cation with other organic cations like ethylammonium and

formidinium [115203] The organic part in hybrid MAPbI3 perovskite is highly

volatile and subject to decomposition under the influence of humidityoxygen In our

studies we have investigated the replacement of organic cation with oxidation stable

inorganic monovalent cations and studies the corresponding change in prope rties of

the material in detail

71

In this work we have investigated the monovalent alkali metal cations (K Rb

Cs RbCs) doped methylammonium lead iodide perovskite ie (MA)1-x(D)xPbI3(B=K

Rb Cs RbCs x=0-1) thin films for potential absorber layer application in perovskite

solar cells shown in Figure 41 The cation A in AMX3 perovskite strongly effects the

electronic properties and band gap of perovskite material as it has influence on the

perovskite crystal structure We have tried to replace the cation organic cation ie

methylammonium (MA+) with inorganic alkali metal cations in different

concentrations and studied the corresponding changes arises due to these

replacements The prepared thin film samples were characterized by crystallographic

(x-ray diffraction) morphological (scanning electron microscopyatomic force

microscopy) optoelectronic (UV-Vis-NIR spectroscopy photoluminescence) and

electrical (current voltage and hall measurements) chemical (x-ray photoemission

spectroscopy) measurements The results of each doping are discussed separately in

sections different sections below of this chapter

Figure 41 (a) planar perovskite solar cell configuration (b) Inverted planar perovskite solar cell configuration

41 Results and Discussion This section describes the investigation of alkali metal cations (K Rb Cs)

doping on the structural morphological optical optoelectronic electric and

chemical properties of hybrid organic-inorganic perovskites thin films

411 Optimization of annealing temperature

The x-ray diffraction scans of methylammonium lead iodide (MAPbI3) films

annealed at temperature 60-130ᵒC are shown in Figure 42 The material has shown

72

tetragonal crystalline phase with main reflections at 1428 ᵒ 2866ᵒ 32098ᵒ 2theta

values The crystallinity of the material is found to be increased with increase of

annealing temperature from 60 to 100ᵒC When the annealing temperature is increase

beyond 100oC a PbI 2 peak begin to appear and the substrate peaks become more

prominent showing the onset of degrading of methylammonium lead iodide

(MAPbI3)

Figure 42 X-ray diffraction of MAPbI3 films annealed at 60 80 100 110 and 130 ᵒC

The UV Visible spectra of post-annealed MAPbI3 samples are shown in Figure 43

The MAPbI3 samples post-annealed at 60 80 100oC have shown absorption in 320 -

780 nm visible region The post-annealing beyond 100o C showed two distinct changes

in the absorption of ultraviolet and Visible region edges start appearing The

absorption band in wavelength below 400 nm region indicates the PbI2 rich absorption

also reported in literature [204] Moreover the appearance of PbI2 peak from x-ray

diffraction results discussed above also support these observations A slight shift of

the absorption edge in visible range towards longer wavelength is also apparent form

the absorption spectra of the films annealed at temperatures above 100 ᵒC attributed

to the perovskite band gap mod ification with annealing [205] However sample has

not shown complete degradation at annealing temperatures at 110 and 130oC The

above results show that material start degrading beyond 100ᵒC so the post deposition

annealing temperature of 100ᵒC is opted for further studied

73

Figure 43 UV-vis absorption spectrum of MAPbI3 annealed at 60 80 100 110 and 130 ᵒC

412 Potassium (K) doped MAPbI3 perovskite

X-ray diffraction scans of (MA)1-xKxPbI3 (x=0-1)FTO films are shown in Figure 44

The diffraction lines in the x-ray diffraction of KxMA1-xPbI3 (x= 0 01 02 03)

samples were fitted to the tetragonal crystal structure [206] The main reflections at

1428ᵒ which correspond to the black perovskite phase are oriented along (110)

direction In this diffract gram a PbI2 peak at 127ᵒ is quite weak confirming that the

generation of the PbI2 secondary phase is negligible in the MAPbI3 layer

The refined structure parameters of MAPbI3 showed that conventional

perovskite α-phase is observed with space group I4mcm and lattice parameters

determined by using Braggrsquos law as a=b=87975Aring c=125364Aring Beyond x=02 a

few peaks having small intensities arises which was assigned to be arising from

orthorhombic phase The co-existence of both phases ie tetragonal and

orthorhombic was observed by increasing doping concentration up to x=05 due to

phase segregation For x= 04 05 the intensity of the tetragonal peaks is decreased

and higher intensity diffraction peaks corresponding to orthorhombic phase along

(100) (111) and (121) planes were observed The main reflections along (110) at

1428ᵒ for x=01 sample is slightly shifted towards lower value ie 2Ɵ=14039

Beyond x=01 orthorhombic reflections start growing and tetragonal peak also

increased in intensity up to x=04 The intensity of the main reflection of the

tetragonal phase was maximum in the xrd of x=04 sample be yond that or thorhombic

phase got dominated The pure orthorhombic phase is obtained for (MA)1-xKxPbI3 (x=

075 1) samples forming KPbI3 2H2O final compound which was matched with the

literature reports It is also reported in the literature that this compound has a tendency

to dehydrate slowly above 303K and is stable in nature [34] The attachment of water

74

molecule is most likely arises due to air exposure of the films while taking x-ray

diffraction scan in ambient environment The cell parameters of KPbI32H2O were

calculated with space group PNMA and have va lues of a=879 b= 790 c=1818Aring

and cell volume=1263Aring3 A few small Intensity peaks corresponding to FTO

substrate were also appeared in all samples These studies reveal that at lower doping

concentration ie x=1 K+ has been doped at the regular site of the lattice and

replaced CH3NH3+ whereas for higher doping concentration phase segregation has

occurred The dominant orthorhombic KPbI32H2O compound is obtained for x=075

1 samples [207]

Figure 44 X-ray diffraction of (MA)1-xKxPbI3(x=0 01 02 03 04 05 075 1) films

The electron micrographs of (MA)1-xKxPbI3(x=0 01 03 05) were taken at

magnifications 10Kx and 50Kx are shown in Figure 45 (a b) The fully covered

surfaces of the samples were observed at low resolution micrographs Some strip-like

structure start appearing over the grain like structures of initial material with

potassium added samples and the sizes of these strips have increased with the

increasing potassium concentration in the samples The surface is fully covered with

these strip like structures for x=05 in the final compound The micrograph with

higher resolution have shown similar micro grains at the background of all the

samples and visible changes in the morphology of the film is observed at higher K

doping These changes in the surface morphology can be linked to the phase change

from tetragonal to orthorhombic as discussed in the x-ray diffraction analys is [29]

75

Current-voltage graphs of (MA)1-xKxPbI3 (x=0 01 02 03 04 1)FTO films are

shown Figure 46 The RJ Bennett and Standard characterization techniques were

employed to calculate the values of series resistances (Rs) by using forward bias data

[208] The Ohmic behavior is observed in all samples The resistance of the samples

has decreased with the K doping up to x=04 by an order of magnitude and increased

back to that of un-doped sample for x=10 Table 41 This decrease in the resistance

values is most likely due to higher electro-positive nature of the doped cation ie K

as compared to CH3NH3 The more loosely bound electrons available for participating

in conduction in K than C and N in CH3NH3 makes it more electropos itive which

results in an increase in its conductance

b

a

76

Figure 45 Electron micrographs at magnification (a) 500 x and (b) 50 Kx of (MA)1-xKxPbI3 (x=0 01 03 05)

The better crystallinity and inter-grain connectivity with K doping could be

another reason for decrease in resistance as discussed in xrd and SEM results in last

section The increase in resistance with higher K doping could possibly be due to the

formation of KPbI32H2O compound and other oxides at the surface which may

contribute to the increased resistance of the sample [209]

Figure 46 Current voltage (I-V) characteristics of (MA)1-xKxPbI3 (x=0 01 02 03 04 1) films

(MA)1-xKxPbI3 x=0 x=01 x=02 x=03 x=04 x=1

Rs (Ω) 35x107 15x107 25x106 15x106 53x106 15x107

Table 41 Resistance values for (MA)1-xKxPbI3 (x=0 01 02 03 04 05 075 1) films The x-ray photoemission survey spectra of (x=0 01 05 1) samples are displayed in

Figure 47 All the expected peaks including oxygen (O) carbon (C) nitrogen (N)

iodine (I) lead (Pb) and K were observed The other small intensity peaks due to

substrate material are also seen in the survey spectra Carbon and Oxygen may also

appear due to the adsorbed hydrocarbons and surface oxides through interaction with

humidity as well as due to FTO substrates

The C1s core level spectra for (MA)1-xKxPbI3(x=0 01 05 1) samples have been de-

convoluted into three sub peaks leveled at 2848 eV 2862 eV and 2885 eV

associated to C-CC-H C-O-C and O-C=O bonds respectively as depicted in Figure

48(a) The C-C C-O-C peaks appeared in pure KPbI3 sample are related with

adventitious or contaminated carbon The K2p core level spectra has been fitted very

well to a doublet around 2929 2958 eV corresponding to 2p32 and 2p12

respectively Figure 48(b) The intensity of this doublet increased with the increasing

K doping and the it shifted towards higher binding energy for x=05 In KPbI3 ie

77

x=10 broadening in doublet is observed which is de-convoluted into two sub peaks

around 2929 2937eV and 2955 2964eV respectively which is most likely arises

due to the inadvertent oxygen in the fina l compound The Pb4 f core levels spectra of

(MA)1-xKxPbI3 (x=01 05 1) films are shown in Figure 48(c) The Pb4f (Pb4f72

Pb4f52) doublet is positioned at 1378plusmn01 1426plusmn01eV and is separated by a fixed

energy value ie 49eV The doublet at this energy value corresponds to Pb2+ The

curve fitting is performed with a single peak (full width at half maxima=12 eV) in

case of partially doped sample The Pb4f (4f72 4f52) core level spectrum of KPbI3

film has been resolved into two sub peaks with full width at half maxima of 12 eV for

each peak The first peak positioned at 1378plusmn01 eV linked to Pb2+ while the second

peak at 1386plusmn01eV corresponds to the Pb having higher oxidation state ie Pb4+

which is most likely appeared due to Pb attachment with adsorbed oxygen states This

is also supported by x-ray diffraction studies of KPbI3 sample The core level spectra

of I3d shows two peaks corresponding to the doublet (I3d52 I3d32) around 61910

and 6306eV respectively associated to oxidation state of -1 Figure 48(d) The value

of binding energy for spin orbit splitting for iod ine was found to be around 115eV

which is close to the value reported in the literature [210] The energy values for the

iodine doublet in (MA)1-xKxPbI3 (x=01 05 1) sample are observed at 61910 63060

eV 61859 6301eV and 61818 62969 eV respectively The slight shifts in

energies are observed which is most likely due to the change in phase from tetragonal

to orthorhombic with K doping these samples These changes in structure lead to the

change in the coordination geometry with same oxidation state ie -1 The binding

energy differences between Pb4f72 and I3d52 is 481eV which is close to the value

reported in literature [207] Table 42 The O1s peak has been de-convoluted into

three sub peaks positioned at 5305 5318 and 533eV Figure 48(e) The difference in

the intensities of these peaks is associated to their bonding with the different species

in final compound The small intensity peak at 533e V in O1s core level spectra of

partially K doped films compared with other samples showing improved stability as

all samples were prepared under the same conditions The xps valance band spectra of

(MA)1-xKxPbI3(x=0 01 05 1) samples are shown in Figure 49 The un-doped

sample have shown peak between 0-5 eV which consist of two peaks at 17 eV and

37 eV binding energies The lower energy peak has grown in relative intensity with

the increased doping of K up to x=05 The absorption has been increased while there

is no significant change in bandgap is observed which is attributed to the enhanced

78

crystallinity of the partially doped material which is in agreement with x-ray

diffraction analysis as discussed earlier

Figure 47 XPS survey scans of Kx(MA)1-xPbI3(x=0 01 05 1) films

Figure 48 XPS spectra of (a) C1s (b) K2p (c) Pb4f (d) I3d (e) O1s for (MA)1-xKxPbI3 (x=0-1) films

79

Figure 49 XPS valance band spectra of (MA)1-xKxPbI3(x=0 01 05 1) films

Table 42 XPS core level binding energies of Pb4f I3d and K with reference to the C1s of (MA)1-

xKxPbI3 (x=0 01 05 1) films

The optical absorption spectra of (MA)1-xKxPbI3(x=0-1) samples are shown Figure

410 The absorption of the doped samples has been increased in the visible region up

to x=04

Figure 410 Optical absorption spectra of (MA)1-xKxPbI3 (x=0 01 02 03 04 05 075 1) films

There is no significant change observed in the energy bandgap values for doping up to

x=050 Figure 411 and Table 43 The is due to improvement in crystallinity which

Kx(MA)1-xPbI3 EBPb4f72(eV) EBId52(eV) EB(I3d52-Pb4f72)(eV)

x=0 13733 61832 480099

x=01 13701 61809 48108

x=05 13754 61858 48104

x=1 13684 61817 48133

80

is also clear from x-ray diffraction and x-ray photoemission spectroscopy studies For

K doping beyond x=05 the absorption in ultraviolet (320-450nm) region has been

increased Pure MAPbI3 (x=0) and KPbI3 absorb light in the visible and UV regions

corresponding to bandgap values of 15 and 26eV respectively [38 39] In the case

of partial doped samples no significant change in bandgap is observed but absorption

be likely to increase These studies showing the more suitability of partial K doped

samples for solar absorption applications

Figure 411 (αhν)2 vs hν plots with band gap calculations of (MA)1-xKxPbI3 (x=0 01 02 03 04 05

075 1) films

(MA)1-xKxPbI3 Energy band gap (eV)

81

Table 43 Energy bandgap values of (MA)1-xKxPbI3 (x=0 01 02 03 04 05 075 1) films

413 Rubidium (Rb) doped MAPbI3 perovskite

To investigate the effect of Rb doping on the crystal structure of MAPbI3 the x-ray

diffraction scans of (MA)1-xRbxPbI3(x=0 01 03 05 075 1) perovskite films

deposited on fluorine doped tin oxide (FTO) coated glass substrates were collected

shown in Figure 412 Most of the diffraction lines in the x-ray diffraction of

methylammonium lead iodide (MAPbI3) films are fitted to the tetragonal phase The

main reflections at 1428 ᵒ which correspond to the black perovskite phase are

oriented along (110) direction In this diffract gram a PbI2 peak at 127ᵒ is quite

weak confirming that the generation of the PbI2 secondary phase is negligible in the

methylammonium lead iodide (MAPbI3) layer The refined structure parameters of

MAPbI3 showed that conventional perovskite α-phase is observed with space group

I4mcm and lattice parameters determined by using Braggrsquos law as a=b=87975Aring

c=125364Aring The low angle perovskite peak for methylammonium lead iodide

(MAPbI3) and (MA)1-xRbxPbI3(x=01) occur at 1428ᵒ and 1431ᵒ respectively

showing shift towards larger theta value with Rb incorporation The corresponding

lattice parameters for (MA)1-xRbxPbI3(x=01) are a=b=87352Aring c=124802Aring

showing decrease in lattice parameters The same kind of behavior is also observed

by Park et al [211] in their x-ray diffraction studies In addition to the peak shift in

(MA)1-xRbxPbI3(x=01) sample the intens ity of perovskite peak at 1431ᵒ has also

increased to its maximum revealing that some Rb+ is incorporated at MA+ sites in

MAPbI3 lattice The small peak around 101ᵒ in RbxMA1-xPbI3 (x=01 ) is showing the

presence of yellow non-perovskite orthorhombic δ-RbPbI3 phase [212] This peak is

observed to be further enhanced with higher concentration of Rb showing phase

segregation in the material The further increase of Rb doping for x ge 03 the peak

x=0 155

x=01 153

x=02 153

x=03 152

x=04 154

x=05 159

x=075 160

x=1 26

82

returns to its initial position and the peak intensities of pure MAPbI3 phase become

weaker and a secondary phase with its major peak at 135ᵒ and a doublet peak around

101ᵒ appears which confirms the orthorhombic δ-RbPbI3 phase [211] The intensity

of the doublet gets stronger than α-phase peak for higher doping concentration (ie

beyond xgt05) Figure 413 The refinement of x-ray diffraction data of RbPbI3

yellow phase indicated orthorhombic perovskite structure with space group PNMA

and lattice parameters a=98456 b=46786 and c=174611Aring The above results

showed that the Rb incorporation introduced phase segregation in the material and

this effect is most prominent in higher Rb concentration (xgt01) This phase

segregation is most likely due to the significant difference in the ionic radii of MA+

and Rb+ showing that MAPbI3-RbPbI3 alloy is not a uniform solid system

Figure 412 X-ray diffraction of MA1-xRbxPbI3(x=0 01 03 05 075 1)

Figure 413 Strained x-ray diffraction of MA1-xRbxPbI3(x=0 01 03 05 075 1)

83

The (MA)1-xRbxPbI3(x=0 01 075) films have been further studied to observe the

effect of Rb doping on the stability of perovskite films by taking x-ray diffraction

scans of these samples after one two three and four weeks These experiments were

performed under ambient conditions The samples were not encapsulated and stored

in air environment at the humidity level of about 40 under dark The x-ray

diffraction scans of pure MAPbI3 sample have shown two characteristic peaks of PbI2

start appearing after a week and their intensities have increased significantly with

time shown in Figure 414(a) These PbI2 peak are associated with the degradation of

MAPbI3 material In the case of 10 Rb doped sample (Rb01MA09PbI3) a small

intensity peak of PbI2 is observed as compared with pure MAPbI3 case and the

intensity of this peak has not increased with time even after four weeks as shown in

Figure 414(b) Whereas in 75 Rb doped sample (ie MA025Rb075PbI3) stable

orthorhombic phase is obtained and no additional peaks are appeared with time

Figure 414(c) These observations showed that the Rb doping slow down the

degradation process by passivating the MAPbI3 material

Figure 414 X-ray diffraction aging studies of (a) MAPbI3 (b) MA09Rb01PbI3 (c) (MA)025Rb075PbI3

sample for four weeks at ambient conditions

84

Scanning electron micrographs of MA1-xRbxPbI3(x= 0 01 03 05) recorded at two

different magnifications ie 20 Kx and 1 Kx are shown in Figure 415(a) (b) The

lower magnification micrographs have shown that the surface is fully covered with

MA1-xRbxPbI3 The dendrite structures start forming like convert to thread like

crystallites with the increase dop ing of Rb as shown in high resolution micrographs

Figure 415(b) At higher Rb doping the surface of the samples begins to modify and a

visible change are observed in the morphology of the film due to phase segregation as

observed in x-ray diffraction studies The porosity of the films increases with

enhanced doping of Rb resulting into thread like structures These changes can also be

attributed to the appearance of a new phase that is observed in the x-ray diffraction

scans

To analyze the change in optical properties of perovskite with Rb doping absorption

spectra of MA1-xRbxPbI3(x=0 01 03 05 075 1) films were collected Figure

416(a) The band gaps of the samples were calculated by using Beerrsquos law and the

results are plotted as (αhυ)2 vs hυ The extrapolation of the linear portion of the plot

to x-axis gives the optical band gap [175]

The absorption spectrum of MAPbI3 have shown absorption in 320 -800nm region

For lower Rb concentration ie MA1-xRbxPbI3(x=01 03) samples have shown two

absorption regions ie first at the same 320-800nm region whereas a slight increase

in the absorption is observed in 320-450nm region showing the presence of small

amount of second phase in the material For heavy doping ie in MA1-

xRbxPbI3(x=05 075) the second absorption peak around 320-480 grows to its

maximum which is a clear indication and confirmation of the existence of two phases

in the final compound In x=1 sample ie RbPbI3 single absorption in 320-480nm

region is observed The lower Rb doped phase has shown strong absorption in the

visible region (ie 450-800nm) whereas with the material with higher Rb doping

material has shown strong absorption in the UV reign (ie 320-480nm) This blue

shift in absorption spectra corresponds to the increase in the high optical band gap

materials content in the final compound These changes in band gap of heavy doped

MA1-xRbxPbI3 perovskites are attributed to the change of the material from the black

MAPbI3 to two distinct phases ie black MAPbI3 and yellow δ-RbPbI3 as shown in

Figure 416(b)

85

Figure 415 Scanning electron micrographs of MA1-xRbxPbI3(x=0 01 03 05 075 1) at (a) lower

magnification (b) higher magnification

Figure 416 (a)Absorption spectra (b) (αhν)2 vs hν plots of (MA)1-xRbxPbI3(x=0- 1) samples

86

The band gaps of each sample is calculated separately and shown in Figure 417 The

bandgap of MAPbI3 is found to be around 156eV whereas that of yellow RbPbI3

phase is around 274eV For lower Rb concentration ie RbxMA1-xPbI3(x=01 03)

samples have shown bandgap values of 157 158eV respectively Whereas for

Heavy doping ie MA1-xRbxPbI3(x=05 075) two separate bandgap values

corresponding to two absorption regions are observed These optical results showed

that the optical band gap of MAPbI3 has not increase very significantly in lower Rb

doping whereas for higher doping concentration two bandgap with distinct absorption

in different regions corresponds to the presence of two phases in the material

Figure 417 (αhν)2 vs hν plots with bandgap values of (MA)1-xRbxPbI3(x=0 01 03 05 075 1) films In Figure 418(a) we present the room temperature photoluminescence spectra of MA1-

xRbxPbI3(x=0 01 03 05 075 1) In in un-doped MAPbI3 film a narrow peak

around 774 nm is attributed to the black perovskite phase whereas the Rb doped MA1-

xRbxPbI3(x=01 03 05 075 1) phase have shown photoluminescence (PL) peak

broadening and shifts towards lower wavelength values The photoluminescence

intensity of the Rb doped sample is increases by six orders of magnitude up to 50

doping (ie x=05) However with Rb doping beyond 50 the luminescence

intensity begins to suppress and in 75 Rb doped samples (ie x=075) the weakest

87

intensity of all is observed no peak is observed around 760 nm in 100 Rb doped

samples (ie x=10) Substitution of Rb with MA cation is expected to increase the band

gap of perovskites materials A slight blue shift with the increase Rb doping results in

the widening of the band gap The intensity in the PL spectra is directly associated with

the carrierrsquos concentration in the final compound The PL intensity of up-doped samples is

pretty low whereas its intensity substantially increases for initial doping of Rb The PL

data of Rb doped MA1-xRbxPbI3(x=01 03 05) shows higher density of carriers in the

final compound whereas the heavily doped MA1-xRbxPbI3(x=075 10) samples have

shown relatively lower density of the free carriers These observations lead to a suggestion

that the doping of Rb creates acceptor levels near the top edge of valance band which

increases the hole concentration in the final compound The heavily doped Rb

samples (x=075 10) the acceptor levels near the top edge of valance band starts

touc hing the top edge of valance band thereby supp ressing the density of holes in the

final compound The possible increase in the population of holes increases the

electron holesrsquo recombination processes that suppress the density of free electron in

the final compound in heavily doped samples

Time resolved photoluminescence (TRPL) measurements were carried out to study

the exciton recombination dynamics Figure 418(b) shows a comparison of the time

resolved photoluminescence (TRPL) decay profiles of MA1-xRbxPbI3(x=0 01 03

05 075) films on glass substrates The Time resolved photoluminescence (TRPL)

data were analyzed and fitted by a bi-exponential decay func tion

119860119860(119905119905) = 1198601198601119897119897119890119890119890119890minus11990511990511205911205911+1198601198602119897119897119890119890119890119890

minus11990511990521205911205912 (2)

where A1 and A2 are the amplitudes of the time resolved photoluminescence

(TRPL) decay with lifetimes τ1 and τ2 respectively The average lifetimes (τavg) are

estimated by using the relation [213]

τ119886119886119886119886119886119886 = sum119860119860119904119904 τ119904119904sum 119860119860119904119904

(3)

The average lifetimes of carriers are found to be 098 330 122 101 and 149 ns

for MA1-xRbxPbI3(x=0 01 03 05 075) respectively It is observed that MAPbI3

film have smaller average recombination lifetime as compared with Rb based films

The increase in carrier life time corresponds to the decrease in non-radiative

recombination and ultimately lead to the improvement in device performance From

the average life time (τavg) values of our samples it is clear that in lower Rb

concentration non-radiative recombination are greatly reduced and least non radiative

recombination are observed for (MA)09 Rb01PbI3 having average life time of 330 ns

88

This increase in life time of carriers with Rb addition is also observed in steady state

photoluminescence (PL) in terms of increase in intensity due to radiative

recombination

Figure 418 (a) Steady State Photoluminescence (PL) (b) Time resolved-PL spectra of (MA)1-

xRbxPbI3(x=0 01 03 05 075 1)

To understand the semiconducting properties of samples the hall measurements of

MA1-xRbxPbI3(x=0 01 03 075) were carried out at room temperature The undoped

MAPbI3 exhibited n-type conductivity with carrier concentration 6696times1018 cm-3

which matches well with the literature [214] The doping of Rb at methylammonium

(MA) sites turns material p-type for entire dop ing range and the p-type concentration

of 459times1018 794 times1018 586 times1014 holescm3 for doping of x=01 03 075 as

shown in Figure 419 (a) however the samples with Rb doping of x=10 have not

shown any conductivity type The un-dope MAPbI3 samples have shown Hall

mobility (microh) around 42cm2Vs The magnitude of microh suppresses significantly in all

89

doped samples Figure 419(b) The increase in the mobility of the carriers is most

likely due to the change in the effective mass of the carriers the effective mass

crucially depends on the curvature of E versus k relationship and it has changed

because the carrierrsquos signed has changed altogether majority electrons to majority

holes in all doped samples The sheet conductivity of the material is increases with Rb

doping up to x=03 and decreases with higher doping concentration Figure 419(c) It

follows the trend of carrierrsquos concentration and depends crucially on density of

carriers in various bands and their effective mass in that band The magneto resistance

initially remains constants however its value increases dramatically in the samples

with Rb doping of x=075 MA1-xRbxPbI3(x=0 01 03 075) samples have shown

magneto-resistance values around 8542 5012 1007 454100 ohms Figure 419(d)

The higher dopants offer larger scattering cross section to the carriers in the presence

of external magnetic field

Figure 419 (a) Carrier concentration vs Doping concentration (b) Hall mobility vs Doping

concentration (c) Sheet conductance vs Doping concentration (d) Magneto Resistance vs Doping concentration of (MA)1-xRbxPbI3(x=0 01 03 075)

To investigate the effect of Rb doping on the electronic structure elemental

composition and the stability of the films x-ray photoelectron spectroscopy (XPS) of the

samples has been carried out The x-ray photoelectron spectroscopy survey scans of

MA1-xRbxPbI3(x=0 01 03 05 075 1) films are shown in Figure 420 In the

90

survey scans we observed all expected peaks comprising of carbon (C) oxygen (O)

nitrogen (N) lead (Pb) iodine (I) and Rb The XPS spectra were calibrated to C1s

aliphatic carbon of binding energy 2848 eV [215] The intensity of Sn3d XPS peak in

survey scans indicates that MA1-xRbxPbI3(x=0 01 03 05) thin films have a

uniformly deposited material at the substrate surface as compared with MA1-

xRbxPbI3(x=075 1) samples This could also be seen in the form of increased

intensity of oxygen O xygen may also appear due to formation of surface oxide layers

through interaction with humidity during the transport of the samples from the glove

box to the x-ray photoelectron spectroscopy chamber Additionally the aforementioned

reactions can occur with the carbon from adsorbed hydrocarbons of the atmosphere

The x-ray photoelectron spectroscopy (XPS) core level spectra were collected and fitted

using Gaussian-Lorentz 70-30 line shape after performing the Shirley background

corrections For a relative comparison of various x-ray photoelectron spectroscopy core

levels the normalized intensities are reported here The C1s core level spectra for

MAPbI3 have been de-convoluted into three sub peaks leveled at 2848 28609 and

28885eV are shown in Figure 421(a) The first peak at 2848 eV is attributed to

adventitious carbon or adsorbed surface hydrocarbon species [215216] while the

second peak located at 28609 is attributed to methyl carbons in MAPbI3 The peak at

28609 eV have also been observed to be associated with adsorbed surface carbon

singly bonded to hydroxyl group [217218] The third peak appeared at 28885 eV is

assigned to PbCO3 that confirms the formation of a carbonate as a result of

decomposition of MAPbI3 which form solid PbI2 [216218] The intensity of C1s peak

corresponding to methyl carbon is observed to be decreasing in intensity with the

increase of Rb dop ing that indicates the intrinsic dop ing o f Rb at MA sites The peak

intensity of C1s in the form of PbCO3 decreases with the doping of Rb up to x=05

whereas in the case of the samples with the Rb doping of x=075 the peak at 28885

eV disappear altogether With the disappearance of peak at 28885 eV a new peak

around 28830eV appears in (MA)1-xRbxPbI3(x=075 1) which is associated with

adsorbed carbon species The C1s peaks around 2848 2859 and 28825eV in case of

Rb doping for x=1 are merely due to adventitious carbon adsorbed from the

atmosphere This might also be due to substrates contamination effects arising from

the inhomogeneity of samples

The N1s core level spectra of MA1-xRbxPbI3(x= 0 01 03 05 075 1) are depicted

in Figure 421(b) The peak intensity has systematically decreased with increasing Rb

doping up to x=05 The N1s intensity has vanished for the case when x=075 Rb

91

doping which shows there is no or very less amount of nitrogen on the surface of the

specimen Figure 421(c) shows the x-ray photoelectron spectroscopy analys is of the

O1s region of Rb doped MAPbI3 thin films The O1s core level of x=0 01 samples

are de-convoluted into three peaks positioned at 5304 5318 and 53305 eV which

correspond to weakly adsorbed OHO-2 ions or intercalated lattice oxygen in

PbOPb(OH)2 O=C and O=C=O respectively These peaks comprising of different

hydrogen species with singly as well as doubly bounded oxygen and typical for most

air exposed samples The peak appeared around 5304 eV is generally attributed to

weakly adsorbed O-2 and OH ions This peak has been referred as intercalated lattice

O or PbOPb(OH)2 like states in literature [218] The hybrid perovskite MAPbI3 films

are well-known to undergo the degradation process by surface oxidation where

dissociated or chemisorbed oxygen species can diffuse and distort its structure [219]

In these samples the x-ray photoelectron spectroscopy signals due to the main pair of

Pb 4f72 and 4f52 are observed around 13779 and 1427plusmn002eV respectively

associated to their spin orbit splitting Figure 421(d) This corresponds to Pb atoms

with oxidation states of +2 In film with 10 Rb (x=01) doping an additional pair of

Pb peaks emerges at 1369 and 1418plusmn002eV These peaks are attributed to a Pb atom

with an oxidation state of 0 suggesting that a small amount of Pb+2 reduced to Pb

metal [220] These observations are in agreement with the simulation reported by

miller et al [215] The x-ray photoelectron spectroscopy core level spectra for I3d

exhibits two intense peaks corresponding to doublets 3d52 and 3d32 with the lower

binding component located at 61864plusmn009eV and is assigned to triiodide shown in

Figure 421(e) for higher doping (x=075 1) concentrations the Pb4f and I3d peaks

slightly shifted toward higher binding energy which is attributed to a stronger

interaction between Pb and I due to the decreased cubo-octahedral volume for the A-

site cation caused by incorporating Rb which is smaller than MA The broadening in

the energy is most like ly assoc iated with the change in the crystal structure of material

from tetragonal to orthorhombic reference we may also associate the reason of peak

broadening in perspective of non-uniform thin films therefore x-ray photoelectron

spectroscopy signa l received from the substrate contribution The splitting of iodine

doublet is independent of the respective cation Rb and with 1150 eV close to

standard values for iodine ions [221] These states do not shift in energy appreciably

with the doping of Rb doping The binding energy differences between Pb4f72 and

I3d52 are nearly same (481 eV) for all concentrations (shown in Table 44)

92

indicating that the partial charges of Pb and I are nearly independent of the respective

cation doping

These peaks vary in the intensity with the increased doping of Rb in the material The

change in the crystal structure from tetragonal to or thorhombic unit cell fixes the

attachment abundance of oxygen with the different atoms of the final compound This

implies enhanced crystallinity of the perovskite absorber material which supports the

observations of the x-ray diffraction analysis as discussed earlier The x-ray

photoelectron spectroscopy core level spectra for Rb3d is displayed in Figure 421(f)

fitted very well to a doublet around 110eV and 112eV The intensity of this doublet

progressively grows with the increased doping of Rb and broadening of the peak is

also observed for the case of Rb doping which is most probably due to the increase in

the interaction between the neighboring atoms due to the decrease in the volume of

unit cell by small cation dop ing Thus Rb can stabilize the black phase of MAPbI3

perovskite and can be integrated into perovskite solar cells This stabilization of the

perovskite by can be explained as the fully inorganic RbPbI3 passivates of the

perovskite phase thereby less prone to decomposition These results are aligned with

the x-ray diffraction observations discussed earlier In particular we show that the

addition of Rb+ to MAPbI3 strongly affects the robustness of the perovskite phase

toward humidity

Valence band spectra for MAPbI3 films with doping give understanding into valence

levels and can exhibit more sensitivity to chemical interactions as compared with core

level electrons The valance band spectra of MA1-xRbxPbI3(x=0010305) samples

are shown in Figure 421(g) The valance band spectrum of MAPbI3 sample samples

is observed between 05-6eV and is peaked around 28eV The doping of Rb around

x=01 lowers the intensity of the peak and shifts its peak position to the lower energy

ie to 26eV The intensity of peak around 26eV recovers in MA1-

xRbxPbI3(x=0305) samples to the peak position of undoped samples but the center

of the peak remains shifted to the position of 26eV In undoped samples all the

electrons being raised to the electron-energy analyzed (ie the detector) were most

likely being raised by the x-ray photon were from the valance band whereas in all

doped samples these electrons are being raised to the detector from the trapped states

Since the trap states are above the top edge of the valance band therefore their energy

was lower than that of the electrons in the trap state and that is why they are peaked

around 26eV The lower peak intensity of the MA1-xRbxPbI3(x=01) samples is most

likely due to the lower population of electrons in the trap state the peak intensity

93

recovers for higher Rb doping The observations of valance band spectra also support

our previous thesis that doped Rb atoms introduce their states near the top edge of

valance band thereby turning the material to p-type

Figure 420 XPS survey scan of (MA)1-xRbxPbI3(x=0 01 03 05 1) films

94

Figure 421 XPS Core level spectra of (a) C1s (b) N1s (c) O1s (d) Pb4f (e) I3d (f) Rb3d (g) valence

band spectra of (MA)1-xRbxPbI3(x=0 01 03 05 1) films

Table 44 Binding energy values of Rb3d32 and differences between binding energies of I3d52 and Pb4f72 levels of (MA)1-xRbxPbI3(x=0 01 03 05 075 1) samples

414 Cesium (Cs) doped MAPbI3 perovskite

The Cs ions substitute at the A (cubo-octahedral) sites of the tetragonal unit cell

due to similar size of Cs and MA cations the higher doping concentrations of it

brings about the phase transformation from tetragonal to orthorhombic Figure 422

shows the x-ray diffraction scans of (MA)1-xCsxPbI3(x=0 01 02 03 04 05 075

1) perovskite samples in the form of thin films The x-ray diffractogram of MAPbI3

film sample is fitted to the tetragonal phase with prominent planar reflections

observed at 2θ values of 1428ᵒ 2012ᵒ 2866ᵒ 3198ᵒ and 4056 ᵒ which can be indexed

to the (110) (112) (220) (310) and (224) planes by fitting these planar reflections to

tetragonal crystal structure of MAPbI3 [222] The refined structure parameters of


I4-mcm and lattice parameters a=b=87975 c=125364Aring and cell volume 97026Aring3

In pure CsPbI3 samples the main diffraction peaks were observed at 2θ values of

982ᵒ 1298ᵒ 2641ᵒ 3367ᵒ 3766ᵒ with (002) (101) (202) (213) and (302) crystal

planes of γ-phase indicating an or thorhombic crystal structure [223ndash225] The refined

structure parameters for CsPbI3 are found to be a=738 b=514 c=1797Aring with cell

volume =68166Aring3 and PNMA space group There is small difference in peaks

(MA)1-xRbxPbI3 EBPb4f72

(eV)

EBI3d52

(eV)

EBRb3d32

(eV)

EBI3d52-EBPb4f72

(eV)

x=0 13779 6187 0 48091

x=01 1378 61855 11012 48075

x=03 13781 61857 1101 48076

x=05 13778 61862 11017 48079

x=075 1378 61873 11015 48084

x=1 1378 61872 11014 48092

95

observed between MAPbI3 and CsxMA1-xPbI3 perovskite having 30 Cs replacement

whereas the x-ray diffraction peaks related to the MAPbI3 perovskite are less distinct

in the spectra of (MA)1-xCsxPbI3 (x=04 05 075) The doping of Cs in MAPbI3

tetragonal unit cell effects the peak position and intensity slightly changed This slight

shift in peak pos ition can be attributed to the lattice distortion and strain in the case of

low Cs dop ing concentration in the MAPbI3 perovskite samples Moreover with the

increasing Cs content the diffraction peaks width is also observed to increase This is

attributed to the decrease in the size of crystal domain by Cs dop ing [226] The above

results showed that the Cs dop ing lead to the transformation of the crystal phase from

tetragonal to orthorhombic phase

Figure 422 X-ray diffraction scans of (MA)1-xCsxPbI3(x=0 01 02 03 04 05 075 1) films

To investigate the change in the morphology and surface texture of (MA)1-xCsxPbI3

(0-1) perovskite films scanning electron microscopy and atomic force microscopy

studies were carried out It was found that pure MAPbI3 perovskite crystalline grains

are in sub-micrometer sizes and they do not completely cover the fluorine doped tin

oxide substrate Figure 423 However with Cs doping rod like structures starts

appearing and the connectivity of the grains is enhanced with the increased doing of

Cs up to x=05 The films with Cs doping between 40-50 completely heal and cover

the surface of the substrate In the films with higher doping of Cs ie x=075 1

prominent rod like structure is observed appearance of which is finger print of CsPbI3

structure that appears in the forms of rods

96

Figure 423 Electron micrographs of (MA)1-xCsxPbI3 films (x=0 01 02 03 04 05 075 1)

The AFM studies shows that the surface of the films become smoother with

Cs doping and root mean square roughness (Rrms) decreases dramatically from 35nm

observed in un-doped samples to 11nm in 40 Cs doped film Figure 424 showing

healing of grain boundaries that finally lead to the enhancement of the device

performance It is also well known that lower population of grain boundaries lead to

lower losses which finally result in an efficient charge transport Moreover with

heavy doping ie (MA)1-xCsxPbI3 (075 1) the material tends to change its

morphology to some sharp rod like structures and small voids These microscopic

voids between the rod like structures lead to the deterioration of the device

performance

Figure 425(a) displays the UV-Vis-NIR absorption spectra of (MA)1-xCsxPbI3 (x=0

01 03 05 075 1) perovskite films The band gaps were obtained by using Beerrsquos

law and the results are plotted (αhυ)2 vs hυ The extrapolation of the linear portion on

the x-axis gives the optical band gap [175] The absorption onset of the MAPbI3 is at

790nm corresponding to an optical bandgap of around 156eV Subsequently doping

with Cs hardly effect the bandgap but the absorption is slightly increased with doping

up to x=03 The absorption bandgap is shifted to higher value in the case of higher

97

doping ie x=075 1 The lower Cs doped phase has shown strong absorption in the

visible region (ie 380-780 nm) whereas with the phase with higher Cs doping has

shown absorption in in the lower edge of the visible reign (ie 300-500 nm) This blue

shift in absorption spectra corresponds to the increase in the optical band gap of the

material These changes in band gap of heavy doped (MA)1-xCsxPbI3 perovskites are

attributed to the change of the material from the black MAPbI3 to dominant gamma

phase of CsPbI3 which is yellow at room temperature as shown in Figure 425(b)

The band gap of black MAPbI3 phase is around 155 eV whereas that of yellow

CsPbI3 phase is around 27eV These optical results showed that the optical band gap

of MAPbI3 can be enhanced by doping Cs in (MA)1-xCsxPbI3 films

To study the charge carrier migration trapping transfer and to understand the

electronhole pairs characteristics in the semiconductor particles photoluminescence

is a suitable technique to be employed The photoluminescence (PL) emissions is

observed as a result of the recombination of free carriers In Figure 426 we present the

room temperature PL spectra of (MA)1-xCsxPbI3 (x=0 01 02 03 04 05 075 1) thin

films In un-doped MAPbI3 film a narrow peak around 773 nm is attributed to the

black perovskite phase whereas the Cs doped (MA)1-xCsxPbI3 (x=01 02 03 04 05

075 1) films have shown PL peak broadening and shifts towards lower wavelength

values The photoluminescence intensity of the Cs doped sample is increases by five

orders of magnitude up to 20 doping (ie x=02) However with increase doping of

Cs beyond 20 the luminescence intensity begins to suppress and in 75 Cs doped

samples (ie x=075) the weakest intensity of all doped samples is observed In case

of CsPbI3 (ie x=1) no peak is observed around in visible wavelength region

Substitution of Cs with MA cation is observed to increase the band gap of perovskites

materials

The Cs doping results in the widening of the band gap with the slight blue shift in

photoluminescence spectra These photoluminescence (PL) peak shifts in MAPbI3 with

doping of Cs are identical to the one reported in literature [227] The PL intensity of un-

doped sample is pretty low whereas its intensity substantially increases for initial doping of

Cs The PL spectra of Cs doped (MA)1-xCsxPbI3 (x=01 02 03 04 05) shows higher

radiative recombination or in other words decreased non-radiative recombination which

are trap assisted recombination lead to the deterioration of device efficiency The heavily

doped (MA)1-xCsxPbI3 (x=075) sample have shown relatively lower emission intensity

showing increase in non-radiative recombination

98

Figure 424 Atomic force microscopy surface images o f (MA)1-xCsxPbI3 films (x=0 01 02 03 04

05 075 1)

Rrms=35

Rrms=19

Rrms=21

Rrms=13

Rrms=11

Rrms=14

Rrms=28

99

Figure 425 UV-Vis-NIR (a) absorption spectra (b) (αhν)2 vs hν plots of (MA)1-xCsxPbI3 (0-1) films

Figure 426 Photoluminescence (PL) spectra of (MA)1-xCsxPbI3 (0-1) films

To understand the effect of Cs doping on the electronic structure elemental compo sition

and the stability of the films x-ray photoelectron spectroscopy (XPS) of the (MA)1-

100

xCsxPbI3 (x=0 01 1) samples has been carried out The x-ray photoemission

spectroscopy survey scans of (MA)1-xCsxPbI3 (x=0 01 1)) films Figure 427(a) We

observed all expected elements present in the compound The x-ray photoemission

spectroscopy spectra were calibrated to C1s aliphatic carbon of binding energy 2848

eV [215] The Sn3d peak in survey scans is due to the subs trate material and non-

uniformly of the deposited material at the substrate surface This could also be seen in

the form of increased intensity of oxygen Oxygen may in add ition appeared due to

surface oxides formed through contact with humidity during the transport of the

samples from the glove box to the x-ray photoemission spectroscopy chamber

Additionally the aforementioned reactions can occur with the carbon from adsorbed

hydrocarbons of the atmosphere

The core level photoemission spectra were obtained for detailed understanding of the

surfaces of (MA)1-xCsxPbI3 (x=0 01 1) For a relative comparison of various x-ray

photoemission spectroscopy core levels the normalized intensities are reported here

The C1s core level spectra for MAPbI3 have been de-convoluted into sub peaks at

2848 28609 and 28885eV are shown in Figure 427(b) The first peak located at

2848 eV is ascribed to aliphatic carbonadsorbed surface species [215216] while the

second peak positioned at 28609 is associated to methyl carbons in MAPbI3 This

peak have also been reported to be assigned with adsorbed surface carbon related to

hydroxyl group [217218] The third peak at 28885 eV is associated to PbCO3 which

shows the formation of a carbonate species resulting the degradation of MAPbI3 to

form solid PbI2 [216218] The intensity of C1s peak corresponding to methyl carbon

is observed to be decrease in intensity with the increase doping that indicates the

intrinsic doping of Cs at MA sites The peak intensity of C1s in the form of PbCO3

decreases with the doping of Cs in (MA)09Cs01PbI3 sample and the peak position is

also shifted to the lower binding energy In case of CsPbI3 sample for x=1 C1s peaks

appeared around 2848 2862 and 28832eV are due to the adsorbed carbon from the

atmosphere

The N1s core level spectra of (MA)1-xCsxPbI3 (x=0 01 1) are depicted in Figure

427(c) The peak positioned at level 401 eV is due the methyl ammonium lead

iodide The peak intensity has decreased with Cs doping for x=01 and has been

completely vanished for sample x=1 Figure 427(d) shows the x-ray photoemission

spectroscopy analysis of the O1s core level spectra of Cs doped MAPbI3 thin films

The O1s photoemission core level are de-convoluted into three sub peaks For sample

x=0 the peaks are leveled at 5304 5318 and 53305 eV which agrees to weakly

101

adsorbed OHO-2 ions or intercalated lattice oxygen in PbOPb(OH)2 O=C and

O=C=O respectively These peaks comprising of different hydrogen species with

singly as well as doubly bounded oxygen and typical for most air exposed samples

The peak appeared around 5304 eV is generally recognized as weakly adsorbed O-2

and OH ions This peak has been referred as PbOPb(OH)2 or intercalated lattice O

like states in literature [218] The hybrid perovskite MAPbI3 films are well-known to

undergo the degradation process by surface oxidation where dissociated or

chemisorbed oxygen species can diffuse and distort its structure [219] For sample

x=01 1 the peak positions associated with O=C and O=C=O are shifted about 02

eV towards lower binding energy indicating the more stable of thin film Moreover

the quantitative analysis also shows the reduction in the intens ity of oxygen species

attached to the surfaces Table 45

The x-ray photoemission spectroscopy signals associated with doublet due to spin orbit

splitting of Pb 4f72 and 4f52 are observed around 13779 and 14265plusmn002eV

respectively Figure 427(e) These peak positions correspond to the +2 oxidation state

of Pb atom In Cs doped ie x=01 1 perovskite films Pb doublet has broadened

which is most probably due to change of the electronic cloud around Pb atom showing

the inclusion of Cs atom in the lattice

The detailed values of energy corresponding to each peak has shown in Table 45 The

x-ray photoemission spectroscopy core level spectra for I3d exhibits two intense peaks

corresponding to doublets 3d52 and 3d32 with the lower binding component located at

6187plusmn009 eV and is assigned to triiodide shown in Figure 427(f) for higher doping

(x=075 1) concentrations the Pb4f and I3d peaks slightly shifted toward higher

binding energy This shift is attributed to the decreased in cubo-octahedral volume

due to the A-site cation caused by incorporating Cs resulting in a stronger interaction

between Pb and I The broadening in the energy is most likely associated with the

change in the crystal structure of material from tetragonal to or thorhombic The

splitting of iodine doublet is independent of the respective cation Cs and with 1150

eV close to standard values for iodine ions [221]

102

Figure 427 X-ray photoemission spectroscopy (a) survey scan (b) C1s (c) N1s (d) O1s (e) Pb4f (f) I3d and (g) Cs3d Core level spectra of (MA)1-xCsxPbI3 (x=0 01 1) samples

The x-ray photoemission spectroscopy core level spectra for Cs3d is presented in

Figure 427(g) which is very well fitted to a doublet around 72419 and 7383 eV

103

The intensity of the peaks has grown high with higher Cs doping which is most

probably due to the increase in the interaction between the neighboring atoms due to

the decrease in the volume of unit cell by cation dop ing This stabilization as well as

the performance of the perovskite can be improved with Cs doping and can be

explained as the fully inorganic CsPbI3 passivates of the perovskite phase thereby less

prone to decomposition

Atomic C O N Pb I Cs MAPI3 1973 5153 468 38 2019 0

(MA)01Cs09PbI3 3385 4434 232 171 1720 058

CsPbI3 3628 4261 0 143 1372 594 Table 45 Atomic of C O N Pb I Cs observed in (MA)1-xCsxPbI3(x=0 01 1) films

415 Effect of RbCs doping on MAPbI3 perovskite In this section we have investigated the structural and optical properties of

mixed cation (MA Rb Cs) perovskites films

To investigate the effect of mixed cation doping of RbCs on the crystal

structure of MAPbI3 the x-ray diffraction scans of (MA)1-(x+y)RbxCsyPbI3 (x=0 005

01 015 y=0 005 01 015) films have been collected in 2θ range of 5o to 45o

shown in Figure 428 The undoped MAPbI3 has shown tetragonal crystal structure as

described in x-ray diffraction discussions in previous sections The peak at ~ 98o -10ᵒ

is due to the orthorhombic phase introduced due to doping with Cs and Rb in

MAPbI3 The x-ray diffraction studies of both Rb and Cs doped MAPbI3 perovskite

showed that orthorhombic reflections appeared with the main reflections at 101ᵒ and

982ᵒ respectively By investigating mixed cation (Rb Cs) x-ray diffraction patterns

we observed a wide peak around 98-10ᵒ showing the reflections due to both phases

in the material

Figure 429(a) shows the UV-Vis-NIR absorption spectra of (MA)1-

(x+y)RbxCsyPbI3 (x= 005 01 015 y=005 01 015) films It was observed that

absorption is enhanced with increased doping concentration The band gaps were

obtained by using Beerrsquos law and the results are plotted as (αhυ)2 vs hυ The

absorption onset of the MAPbI3 was at 790nm corresponding to an optical bandgap

of 1566 eV as discussed in last section The calculated bandgaps values for doped

samples calculated from (αhυ)2 vs hυ graphs shown in Figure 429(b) are tabulated in

Table 46 The bandgap values are found to be increased slightly with doping due to

low doping concentration The slight increase in bandgap values are attributed to

doping with Rb and Cs cations The ionic radii of Rb and Cs is smaller than MA the

bandgap values are slightly towards higher value

104

Figure 428 X-ray diffraction of (MA)1-(x+y)RbxCsyPbI3 (x=0 005 01 015 y=0 005 01 015) films

Figure 429 (a) Absorption spectra (b) (αhν)2 vs hν plots of (MA)1-(x+y)RbxCsyPbI3 films

105

Table 46 Band gap values of (MA)1-(x+y)RbxCsyPbI3 (x=005 01 015 y=005 01 015) films

42 Summary We prepared mixed cation (MA)1-xBxPbI3 (B=K Rb Cs x=0-1) perovskites by

replacing organic monovalent cation (MA) with inorganic oxidation stable alkali

metal (K Rb Cs) cations in MAPbI3 by solution processing synthesis route Thin

films of the synthesized precursor materials were prepared on fluorine doped tin oxide

substrates by spin coating technique in glove box The post deposition annealing at

100ᵒC was carried out for every sample in inert environment The structural

morphological optical optoelectronic electrical and chemical properties of the

prepared films were investigated by employing x-ray diffraction scanning electron

microscopyatomic force microscopy UV-Vis-NIR spectroscopyphotoluminescence

spectroscopy current voltage (IV)Hall measurements and x-ray photoemission

spectroscopy techniques respectively The undoped ie MAPbI3 showed the

tetragonal crystal structure which begins to transform into a prominent or thorhombic

structure with higher doping The or thorhombic distor tion arises from the for m of

BPbI3(B=K Rb Cs) The materials have shown phase segregation in with increased

doping beyond 10 The aging studies of (MA)1-xRbxPbI3 were carried out by

collecting x-ray diffraction patterns of the samples for 30 days with one-week

interval The studies showed that low degradation of the material in case of Rb doped

sample as compared with pristine MAPbI3 The morphology of the films was also

observed to change from cubic like grains of MAPbI3 to strip like structures for

potassium (K) doped dendritic structure for rubidium (Rb) doped and rod like

structures for cesium (Cs) doped samples The atomic force microscopy studies show

that the surface of the films become smoother with Cs doping and root mean square

roughness (Rrms) decreases dramatically from 35nm observed in un-doped samples to

11nm in 40 Cs doped film showing healing of grain boundaries that finally lead to

the improvement of the device performance The band gap of pristine material

observed in UV visible spectroscopy is around 155eV which increases marginally for

the higher doped samples The associated absorbance in doped samples increases for

lower doping and then supp resses for heavy doping attributed to the increase in

(MA)1-

(x+y)RbxCsyPbI3

X=0

y=0

X=005

y=005

X=005

y=01

X=01

y=005

X=015

y=01

X=01

y=015

Band gap (eV) 1566 1570 1580 1580 1581 1584

106

bandgap and corresponding blue shift in absorption spectra The band gap values and

blue shift was also confirmed by photoluminescence spectra of our samples The IV

characteristics of (MA)1-xKxPbI3 (x=0 01 02 03 04 1) thin films have shown

Ohmic behavior associated with a decrease in the resistance with the doping K up to

x=04 This decrease in the resistance is suggested to be arising from the higher

electro-positive nature of K+1 and via improved film morphology The resistance of

KPbI3 sample is increased which might be due to formation of KPbI3 dihydrate and its

oxide derivatives at the surface of the sample which was also obvious from x-ray

diffraction and x-ray photoemission spectroscopy studies Time resolved spectroscopy

studies showed that the lower Rb doped samples have higher average carrier life time

than pristine samples suggesting efficient charge extraction These Rb doped samples

in hall measurements have shown that conductivity type of the material is tuned from

n-type to p-type by dop ing with Rb Carrier concentration and mobility of the material

could be controlled by varying doping concentration This study helps them to control

the semiconducting properties of perovskite lighter absorber and it tuning of the

carrier type with Rb doping This study also opens a way to the future study of hybrid

perovskite solar cells by using perovskite film as a PN- junction

X-ray photoemission spectroscopy ana lysis has shown that no app reciable

change in the binding energies and hence the oxidation states of lead and iodine core

levels with doping up to 50 K doping showed their charge state remain same In the

valance band spectrum peaks intensity shifts to lower energy for partially K doping up

to 50 but no change is observed in the band gap Also from optical analysis it is

observed that there is no significant change in energy band gap (around 15) up to

50 doping whereas it increased significantly for KPbI3 ie 260eV These results

show that with partial K doping ie Kx(MA)1-xPbI3(x=01 02 03 04 05) materials

having better crystallinity low resistance increased absorption better stability and

optimum bandgaps are suitable for solar cell application The intercalation of

inadvertent carbon and oxyge n in (MA)1-xRbxPbI3(x= 0 01 03 05 075 1)

materials are investigated by x-ray photoemission spectroscopy It is observed that

oxygen related peak which primary source of decomposition of pristine MAPbI3

material decreases in intensity in all doped samples showing the enhanced stability

with doping These partially doped perovskites with enhanced stability and improved

optoelectronic properties could be attractive candidate as light harvesters for

traditional perovskite photovoltaic device applications Similarly Cs doped samples

107

have shown better stability than pr istine sample by x-ray photoemission spectroscopy

studies

CHAPTER 5

5 Mixed Cation Perovskite Photovoltaic Devices

In this chapter we discussed the inverted perovskite solar devices based on MA K

Rb Cs and RbCs mixed cation perovskite as light absorber material Summary of the

results is presented at the end of the chapter

Organo- lead halide based solar cells have shown a remarkable progress in some

recent years While developing the solar cells researchers always focus on its

efficiency cost effectiveness and stability These perovskite materials have created

great attention in photovoltaics research community owing to their amazingly fast

improvements in power conversion efficiency [117228] their flexibility to low cost

simple solution processed fabrication [81229] The perovskites have general

formula of ABX3 where in organic- inorganic hybrid perovskite case A is

monovalent cation methylammonium (MA)formamidinium (FA) B divalent cation

lead and X is halide Cl Br I In some recent years the organic- inorganic

methylammonium lead iodide (MAPbI3 MA=CH3NH3) is most studies perovskite

material for solar cell application The organic methylammonium cation (MA+) is

very hygroscopic and volatile and get decomposed chemically [230ndash233] Currently

the mos t of the research in perovskite solar cell is dedicated to investigate new

methods to synthesize perovskite light absorbers with enhanced physical and optical

properties along with improved stability So as to modify the optical properties the

organic cations like ethylammonium and formamidinium are exchanged instead of

organic component methylammonium (MA) [234235] The all inorganic perovskite

can be synthesized by replacement of organic component with inorganic cation

species ie potassium rubidium cesium at the A-site avoiding the volatility and

chemical degradation issues [118236237] But it has been observed that the poor

photovoltaic efficiency (009) is achieved in case of cesium lead iodide (CsPbI3)

and the mix cation methylammonium (MA) and Cs has still required to show

improved performances [5]

CsPbI3 has a band gap near the red edge of the visible spectrum and is known as a

semiconductor since 1950s [124] The black phase of CsPbI3 perovskite is unstable

108

under ambient conditions and is recently acknowledged for photovoltaic applications

[91] The equilibrium phase at high temperature conditions is cubic perovskite

Moreover under ambient conditions its black phase undergoes a rapid phase

transition to a non-perovskite and non-functional yellow phase [124129225238] In

cubic perovskite geometry the lead halide octahedra share corners whereas the

orthorhombic yellow phase is contained of one dimensional chains of edge sharing

octahedral like NH4CdCl3 structure The first reports of CsPbI3 devices however

showed a potential and CsPbBr3 based photovoltaic cell have displayed a efficiency

comparable to MAPbBr3 [91237239] Here we have used selective compositions of

K Rb Cs doped (MA)1-xBx PbI3(B=K Rb Cs RbCs x=0 01 015) perovskites in

the fabrication of inverted perovskite photovoltaic devices and investigated their

effect on perovskite photovoltaic performance The fabricated devices are

characterized by current-voltage (J-V) characteristics under darkillumination

conditions incident photon to current efficiency and steady statetime resolved

photoluminescence spectroscopy experiments

51 Results and Discussion The inverted methylammonium lead iodide (MAPbI3) perovskite solar cell structure

and corresponding energy band diagram are displayed in Figure 51(a b) The device

is comprised of FTO transparent conducting oxide substrate NiOx hole transpo rt

(electron blocking) layer methylammonium lead iodide (MAPbI3) perovskite light

absorber layer C60 electron transport (hole blocking) layer and silver (Ag) as counter

electrode The composition of absorber layer has been varied by doping MAPbI3 with

K and Rb ie (MA)1-xBxPbI3 (B=K Rb x=0 01) while all other layers were fixed in

each device The photovoltaic performance parameters of the (MA)1-xBxPbI3 (B=K

Rb x=0 01) devices were calculated by current density-voltage (J-V) curves taken

under the illumination of 100 mWcm2 in simulated irradiation of AM 15G Figure

52(a) displayed the current density-voltage (J-V) curves of MAPbI3 based perovskite

solar cell device The current density-voltage (J-V) curves with K+ and Rb+

compositions are displayed in Figure 52(b c)

109

Figure 51 (a) device configuration (b) energy diagram of MAPbI3 Based Perovskite Solar Cells

Figure 52 Current density vs Voltage curves for (a) MAPbI3 (b) (MA)09K01PbI3 (c) (MA)09Rb01PbI3

based Perovskite Solar Cells The corresponding solar cell parameters such as short circuit current density (Jsc)

open circuit voltage (Voc) series resistance (Rs) shunt resistance (Rsh) fill factor (FF)

and power conversion efficiencies (PCE) are summarized in Table 51 The devices

with MAPbI3 perovskite films have shown short circuit current density (Jsc) of

1870mAcm2 open circuit voltage (Voc) of 086V fill factor (FF) of 5401 and

power conversion efficiency of 852 An increase in open circuit voltage (Voc) to

106V was obtained for the photovoltaic device fabricated with K+ composition of

10 with short circuit current density (Jsc) of 1815mAcm2 FF of 6904 and power

conversion efficiency of 1323 The increase in open circuit voltage (Voc) is

attributed to the increase shunt resistance showing reduced leakage current and

improved interfaces In the case of (MA)09Rb01PbI3 perovskite films based solar cell

110

power conversion efficiency of 757 was achieved with open circuit voltage (Voc) of

083V short circuit current density (Jsc) of 1591mAcm2 and FF of 5815

Apparently the short circuit current density (Jsc) showed a decrease in the case of Rb+

compared with MAPbI3 and K+ based devices The incorporation of Rb+ results in the

reduction of absorption in the visible region of light which eventually results in the

reduction of Jsc The decrease in the absorption might be due to the presence of yellow

non-perovskite phase ie δ-RbPbI3 The appearance of this phase was also obvious in

the x-ray diffraction of MA1-xRbxPbI3 perovskites films discussed in chapter 4 Also

bandgap of Rb based samples was found to be marginally increased as compared with

pristine MAPbI3 samples attributed to the shift in absorption edge towards lower

wavelengt h value

Perovskite Scan

direction

Jsc

(mAcm2)

Voc

(V)

FF

PCE Rsh

(kΩ)

Rs

(kΩ)

MAPbI3 Reverse 1870 086 5401 851 093 13

Forward 1650 078 5761 726 454 14

(MA)09K01PbI3 Reverse 1815 106 6904 1332 3979 9

Forward 1763 106 6601 1237 3471 11

(MA)09Rb01PbI3 Reverse 1591 0834 58151 7567 454 22

Forward 13314 0822 59873 6426 079 25

Table 51 Solar cell parameters of MAPbI3 (MA)09K01PbI3 and (MA)09Rb01PbI3 Solar Cells From J-V curves of both reverse and forward scans shown in Figure 52 we

can quantify the current voltage hysteresis factor of our MA K Rb based cells The I-

V hysteresis factor (HF) can be represented by following equation [240]

119919119919119919119919 =119920119920119920119920119920119920119929119929119942119942119929119929119942119942119955119955119956119956119942119942 minus 119920119920119920119920119920119920119919119919119913119913119955119955119917119917119938119938119955119955120630120630

119920119920119920119920119920119920119929119929119942119942119929119929119942119942119955119955119956119956119942119942 120783120783120783120783

Where hysteresis factor value of 0 corresponds to the solar cell without hysteresis

The calculated values of HF from equation 51 resulted into minimum value for K+

based cell shown in Figure 53 The reduction of hysteresis factor (HF) in case of K+

can be associated with mixing of the K+ and MA cations in perovskite material [241]

The maximum value of HF is found in Rb+ based cell which can be attributed to the

phase segregation which is also supported and confirmed by x-ray diffraction studied

already discussed in chapter 4

111

Figure 53 I-V Hysteresis factor of MAPbI3 (MA)09K01PbI3 and (MA)09Rb01PbI3 based Perovskite

Solar Cells The external quantum efficiency which is incident photon-current conversion

efficiency (EQE) or the ratio of extracted electrons to incident photons at a given

wavelength is directly related to the Jsc of device The external quantum efficiency

spectra of FTONiOxMAPbI3C60Ag FTONiOx(MA)09K01PbI3C60Ag and

FTONiOx(MA)09Rb01PbI3C60Ag perovskite photovoltaic devices are presented in

Figure 54 The device with (MA)09K01PbI3 perovskite layer has shown maximum

external quantum efficiency value reaching upto 80 in region 450-770nm compared

with MAPbI3 based device Whereas (MA)09Rb01PbI3 based device exhibited

minimum external quantum efficiency which is in agreement with the minimum value

of Jsc measured in this case The K+ based cell has shown maximum external quantum

efficiency value which is consistent with the Jsc values obtained in forward and

reverse scans of J-V curves

The steady state photoluminescence (PL) measurements were executed to understand

the behavior of photo excited charge carriersrsquo recombination mechanisms shown in

Figure 55 (a) The highest PL intensity was observed in MA09K01PbI3 perovskite

films The increase in PL intensity reveals the decrease in non-radiative

recombination which shows the supp ression in the population of defects in the

MA09K01PbI3 perovskite film as compared with MAPbI3 and MA09Rb01PbI3 films

The time resolved photoluminescence (TRPL) spectra of the films have also been

carried out to study the photo-excited charge carrier recombination dynamics Figure

55(b)

The time resolved photoluminescence (TRPL) spectra has been analyzed and

fitted by bi-exponential function represented by equation 52

119912119912(119957119957) = 119912119912120783120783119942119942119942119942119930119930minus119957119957120783120783120649120649120783120783+119912119912120784120784119942119942119942119942119930119930

minus119957119957120784120784120649120649120784120784 51

112

where A1 and A2 are the amplitudes of the time resolved photoluminescence decay

with lifetimes τ1 and τ2 respectively Usually τ1 and τ2 gives the information for

interface recombination and bulk recombination respectively [242ndash244] The average

lifetime (τavg) which gives the information about whole recombination process is


120533120533119938119938119929119929119944119944 =sum119912119912119946119946120533120533119946119946sum119912119912119946119946

120783120783120785120785

The average lifetimes of carriers obtained are 052 978 175ns for MAPbI3

MA09K01PbI3 and MA09Rb01PbI3 perovskite films respectively shown in Table 52

As revealed MAPbI3 film has an average life time of 05ns whereas a substantial

increase is observed for MA09K01PbI3 film sample This increase in carrier life time

of K doped sample is suggested to have longer diffusion length of the excitons with

suppressed recombination and defect density in MA09K01PbI3 sample as compared

with pristine and Rb based sample The increase in carrier life time of K+ mixed

perovskites corresponds to the decrease in non-radiative recombination which is also

visible in steady state photoluminescence This decrease in non-radiative

recombination leads to the improvement in device performance

Figure 54 EQE o f MAPbI3 (MA)09K01PbI3 (MA)09Rb01PbI3 perovskite film based Solar Cells

113

Figure 55 (a) Photoluminescence spectra (b) Time resolved PL spectra of MAPbI3 (MA)09K01PbI3 and (MA)09Rb01PbI3 films

These photoluminescence studies showed that the MA09K01PbI3 perovskite film

exhibits less irradiative defects which might be due to high crystallinity of the films

due to K+ incorporation which was confirmed by the x-ray diffraction data

Sample A1 τ1(ns) A2 τ2(ns) τavg(ns)

MAPbI3 5678 050 091 185 05

(MA)09K01PbI3 105 370 030 3117 978

(MA)09Rb01PbI3 136 230 026 1394 175 Table 52 Parameters of fitting the time resolved photoluminescence decay curves of the samples

The stability of the glassFTONiOx(MA)09K01PbI3C60Ag perovskite solar cell

for short time intrinsic stability like trap filling effect and light saturation under

illumination was investigated by finding and setting the maximum power voltage and

measured the efficiency under continuous illumination for encapsulated devices The

maximum power point data was recorded for 300s and represented in Figure 56(a)

The Jsc and Voc values were also track for the stipulated time and plotted in Figure 56

(b) and 56 (c) It is evident from these results that the devices are almost stable under

continuous illumination for 300 sec

114

Figure 56 Variation of maximum power point (MPP) short circuit current density (Jsc) and open

circuit voltage (Voc) of glassFTONiOx(MA)09K01PbI3C60Ag perovskite solar cells with time under illumination (AM15) in ambient conditions

To study the Cs effect of doping on the performance of solar cell we

fabricated eight devices with different Cs dop ing in perovskite layer in the

configuration of FTOPTAA(MA)1-xCsxPbI3C60Ag (x=0 01 02 03 04 05

0751) Figure 57(a) Figure 57(b) presents the energy band diagram of different

materials used in the device structure We have employed whole range of Cs+ doping

at MA+ sites of MAPbI3 to optimize its concentration for solar cell performance The

current voltage (J-V) curves of the fabricated devices utilizing Cs doped MAPbI3 as

light absorber are shown in Figure 58 The device with un-doped MAPbI3 perovskite

showed the short-circuit current density (JSC) of 1968 mAcm2 an open circuit

voltage (Voc) of 104V and a fill factor (FF) of 65 corresponding to the power

conversion efficiency of 1324 An increase in short-circuit current density to 2091

mAcm2 was observed for the photovoltaic device prepared with Cs doping of 20

whereas short circuit current density (JSC) of 970 1889 2038 1081 020 mAcm2

are obtained with Cs doping of 30 40 50 75 100 respectively The corresponding

open circuit voltage (Voc) values for (MA)1-xCsxPbI3 (x=02 03 04 05 06 075 1)

based devices are recorded as 104 105 108 104 104 102 and 007V

respectively

115

Figure 57 (a) Schematic illustration of the device structure (b) energy diagram of (MA)1-xCsxPbI3

(x=0 01 02 03 04 05 075 1) perovskite solar cells

Figure 58 The JndashV characteristics of the solar cells obtained using various Cs

concentrations (MA)1-xCsxPbI3 (x=0-1) were measured under AM 15G illumination

Amongst all the Cs dop ing concentrations (MA)07Cs03PbI3 have shown maximum Voc

of 108 V and its fill factor (FF) has shown the similar trend as Voc of the device and

also shown highest value of shunt resistance (Rsh) Table 53-54 The device made

out of (MA)07Cs03PbI3 have shown maximum efficiency around 1537 the

efficiencies of (MA)1-xCsxPbI3 (x=0 01 02 04 05 075) are 1324 1334 1403

1313 1198 494 respectively This shows that 30 Cs doping is optimum for

such devices made out of these materials

MA1-xCsxPbI3 Jsc (mAcm2) Voc (V) FF Efficiency () x= 0 1968 104 065 1324 x= 01 2034 104 063 1334 x= 02 2091 105 064 1403 x= 03 1970 108 072 1537 x= 04 1889 104 067 1313 x= 05 2038 104 056 1198 x= 075 1081 102 045 494 x= 1 020 007 022 000

Table 53 current density-voltage (J-V) parameters of (MA)1-xCsxPbI3 (x=0 01 02 03 04 05 075 1) based devices

MA1-xCsxPbI3 Rs(kΩ) Rsh(kΩ)

x= 0 79 15392

x= 01 3 1076

x= 02 4 1242

x= 03 276 79607

x= 04 17 2391

x= 05 6 744

116

x= 075 25 6338

x= 1 10 78

Table 54 J-V parameters of (MA)1-xCsxPbI3 (x=0-1) based devices

It is most likely Cs introduces additional acceptor levels near the top edge of valance

band which get immediately ionized at room temperature thereby increasing the hole

concentration in the final compound Whereas in heavily doped Cs samples (x=075

10) the acceptor levels near the top edge of valance band begin to touch the top edge

of valance band and as result the density of holes suppresses due to recombination of

hole with the extra electron of ionized Cs atoms

To understanding the charge transpor t mechanism within the device in detail we have

collected IV data under dark as shown in Figure 59(a) The logarithmic plot of

current vs voltage plots (logI vs logV) in the forward bias are investigated

Figure 59(b) Three distinct regions with different slopes were observed in log(I)-

log(V) graph of the IV data for the devices with Cs doping up to 50 corresponding

to three different charge transport mechanisms The power law (I sim Vm where m is

the slope) can be employed to analyzed the forward bias log(I) ndash log(V) plot The m

value close to 1 shows Ohmic conduction mechanism [245] If the value of m lies

between 1 and 2 it indicates Schottky conduction mechanism The value of m gt 2

implies space charge limited current (SCLC) mechanism [246][247] Considering

these facts we can determine that in (MA)1-xCsxPbI3 (0 01 02 03 04 05) based

devices there are three distinct regions Region-I 0 lt V lt 03 V Region-II

03 lt V lt 06 V and Region-III 06 V lt V lt 12 V) corresponding to Ohmic

conduction mechanism (msim1) Schottky mechanism (msim18) and SCLC (m gt 2)

mechanism respectively The density of thermally generated charge carriers is much

smaller than the charge carriers injected from the metal contacts and the transpo rt

mechanism is dominated by these injected carriers in the SCLC region At low

voltage the Ohmic IV response is observed With further increase in voltage IV

curve rises fast due to trap-filled limit (TFL) In TFL the injected charge carriers

occupy all the trap states Whereas in (MA)1-xCsxPbI3 (075 1) based devices only

Ohmic conduction mechanism is prevalent

It is also observed from IV dark characteristics that at low voltages the current is due

to low parasitical leakage current whereas a sharp increase of the current is observed

above at approximately 05V The quantitative analyses of IV characteristics

employed by us ing the standard equations of thermo ionic emission of a Schottky

117

diode The steep increment of the current observed from the slope of the logarithmic

plots are due diffusion dominated currents [248] usually defined by the Schottky

diode equation [249]

119921119921 = 119921119921s119942119942119954119954119954119954119946119946119951119951119931119931 minus 120783120783 52

where Js n k and T represents the saturation current density ideality factor

Boltzmann constant and temperature respectively Equation 55 represents

differential ideality factor (n) which is described as

119946119946 = 119951119951119931119931119954119954

120655120655119845119845119836119836119845119845 119921119921120655120655119954119954

minus120783120783

53 n is the measure of slope of the J-V curves In the absence of recombination

processes the ideal diode equation applies with n value equal to unity The same

applies to the direct recombination processes where the trap assisted recombination

change the ideality factor and converts its value to 2 [250][251] The dark current

density-voltage (J-V) characteristic above 08 V are shown in Figure 59(b)

manifesting a transition to a less steep JV behavior that most likely arises due to drift

current or igina ting either by the formation of space charges or the charge injection

The voltages be low the built- in potential are very significant due to solar cell

ope ration in that voltage regions therefore the IV characteristics in such regimes are

of our main interest In Figure 59(c) the differential ideality factor which can be

derived from the diffusion current below the built- in voltage using Equation 55 is

plotted for Cs doped devices It has values larger than unity for different Cs doped

devices which is considerably higher than ideal diode indicating the prevalence of

trap assisted recombination process [249252] and its source is a too high leakage

current [251252] Stranks et al [253] found from photoluminescence measurements

that the trap states are the main source of recombination under standard illumination

conditions that is in line with the ideality factor measurements for our system

Therefore we can increase the power-conversion efficiency of perovskite solar cells

by eliminating the recombination pathways This could be achieved by maintaining

the doped atomic concentration to a tolerable limit (ie x=03 in current case) that can

simultaneously increase the hole concentration and limit the leakage current

118

119

-02 00 02 04 06 08 10 12

10-4

10-3

10-2

10-1

100

101

102

J(m

Ac

m2 )

V (V)

x=0 x=01 x=02 x=03 x=04 x=05 x=075 x=1

J-V Dark

a

001 01 11E-8

1E-7

1E-6

1E-5

1E-4

1E-3

001

x=0 x=01 x=02 x=03 x=04 x=05 x=075 x=1

log(

J)

Log (V)

Region IRegion II

Region III

b

08 10 12

2

4

x=0 x=01 x=02 x=03 x=04 x=05D

iffer

entia

l Ide

ality

Fac

tor

V (V)

c

Figure 59 (a) Dark current density-voltage (J-V dark) characteristics (b) log(I) vs log(V) plots (c)

Ideality factor (n) Vs voltage (V) plots of (MA)1-xCsxPbI3 (0-1) solar cells

120

The RbCs mixed cation based perovskite solar cells are fabricated in the same

configuration as shown in Figure 51(a b) The device comprises of FTO NiOx

electron blocking layer perovskite light absorber layer C60 hole blocking layer and

silver (Ag) as counter electrode For device fabrication solution process method was

followed by mixing MAI and PbI2 directly for the preparation of MAPbI3 solut ion

Likewise RbI CsI and MAI were taken in stoichiometric ratios with PbI2 for mixed

cation perovskite ie (MA)1-xRbxCsyPbI3 (x=0 005 01 y=0 005 01) The

photovoltaic performance parameters of the devices were calculated by current

density-voltage (J-V) curves taken under the illumination of 100 mWcm2 in

simulated irradiation of AM 15G Figure 510(a) displayed the J-V curves of MAPbI3

based perovskite solar cell device The J-V curves with Rb+Cs+ perovskite

compositions are displayed in Figure 510 (b c) The corresponding solar cell

parameters such as short circuit current density (Jsc) open circuit voltage (Voc) series

resistance (Rs) shunt resistance (Rsh) fill factor (FF) and power conversion

efficiencies (PCE) are summarized in Table 56 The devices with MAPbI3 perovskite

films have shown JSC of 1870mAcm2 Voc of 086 V fill factor (FF) of 5401 and

power conversion efficiency of 852 The shirt circuit current density values of

1569mAcm2 open circuit voltage of 089V with fill factor of 6298 and Power

conversion efficiency of 769 are obtained for (MA)08Rb01Cs01PbI3 perovskite

composition In the case of (MA)075Rb015Cs01PbI3 perovskite film based solar cell

power conversion efficiency of 338 was achieved with Voc of 089V Jsc of

689mAcm2 and FF of 5493 The Jsc showed a decrease in the case of

MA075Rb015Cs01PbI3 compared with MA08Rb01Cs01PbI3 and MAPbI3 Perovskite

based devices The incorporation of Rb+ results in the decrease in the absorption in

the visible region due to presence of non-perovskite yellow phase of RbPbI3 which

ultimately results into decrease in short circuit current density (Jsc) of the device The

appearance non perovskite orthorhombic phase was also confirmed from the x-ray

diffraction patterns of these perovskite films discussed in the last section of chapter 4

Also bandgap of RbCs based samples was found to be marginally increased as

compared with pristine MAPbI3 samples attributed to the shift in absorption edge

towards lower wavelength value which results in the decrease in the solar absorption

in the visible region due to blue shift (towards lower wavelength) The hysteresis in

the current density-voltages graphs is decreased in the solar device with

MA075Rb015Cs01PbI3 perovskite layer From J-V curves of bo th reverse and forward

121

scans shown in Figure 510 we can quantify the current voltage hysteresis factor of

our devices

Figure 510 Current density vs Voltage curves for (a)MAPbI3 (b) (MA)08Rb01Cs01PbI3 and (c)

(MA)075Rb015Cs01PbI3 based Perovskite Solar Cells The I-V hysteresis factor (HF) can be calculated by using equation 51 The calculated

values of hysteresis factor are 0147 0165 and 0044 which shows the minimum

value of hysteresis in the case of (MA)075Rb015Cs01PbI3 based device shown in

Figure 511

Sample Scan

direction Jsc

(mAcm2) Voc (V)

FF

PCE

Rsh(kΩ) Rs(Ω)

MAPbI3 Reverse 1870

08

6 5401 851 454 17

Forward 1650

07

8 5761 726 193 20

(MA)080Rb0

1Cs01PbI3 Reverse 1569

07

9 6298 769 241 23

Forward 1125

08

0 7304 642 143 6

(MA)075Rb0 Reverse 689 08 5493 338 068 31

122

Table 55 Solar cell parameters of MAPbI3 (MA)08Rb01Cs01PbI3 and (MA)075Rb015Cs01PbI3 based Perovskite Solar Cells

Figure 511 I-V Hysteresis factor of MAPbI3 MA08Rb01Cs01PbI3 and MA075Rb015Cs01PbI3 based Perovskite Solar Cells

The external quantum efficiency (EQE) which is incident photon-current conversion

efficiency (IPCE) or the ratio of extracted electrons to incident photons at a given


spectra of FTONiOxMAPbI3C60Ag FTONiOxMA08Rb01Cs01PbI3C60Ag and

FTONiOxMA075Rb015Cs01PbI3C60Ag perovskite photovoltaic devices are

presented in Figure 512 The device with MA075Rb015Cs01PbI3 perovskite layer has

shown minimum external quantum efficiency and blue shift in wavelength is observed

as compared with MAPbI3 based device corresponding to minimum short circuit

current density value Whereas MA08Rb01Cs01PbI3 based device exhibited EQE in

between MAPbI3 and MA075Rb015Cs01PbI3 based devices which is in agreement with

the value of Jsc measured in this case

15Cs01PbI3 9

Forward 688

08

9 5309 323 042 33

123

400 500 600 700 8000

20

40

60

80

Wave length (nm)

EQE

()

MAPbI3 MA080Rb01Cs01PbI3 MA075Rb015Cs01PbI3

Figure 512 External quantum efficiency (EQE) of MAPbI3 (MA)08Rb01Cs01PbI3 and

(MA)075Rb015Cs01PbI3 based Perovskite Solar Cells

The steady state photoluminescence (PL) spectra are shown in Figure 513(a) The

highest PL intensity was observed in MA08Rb01Cs01PbI3 perovskite films The

increase in PL intensity reveals the decrease in non-radiative recombination which

shows the suppression in the pop ulation of de fects in the MA08Rb01Cs01PbI3

perovskite film as compared with MAPbI3 and MA075Rb015Cs01PbI3 films



513(b) presents the time resolved photoluminescence (TRPL) spectra of MAPbI3

MA08Rb01Cs01PbI3 and MA075Rb015Cs01PbI3 films analyzed and fitted by bi-

exponential function represented by equation 52

124

Figure 513 (a) Photoluminescence spectra (b) Time resolved photoluminescence spectra of MAPbI3 (MA)08Rb01Cs01PbI3 and (MA)075Rb015Cs01PbI3 films

The average lifetime (τavg) which gives the information about whole recombination

process is estimated by using the equation 53 and its values are 052 742 065 ns for

MAPbI3 MA08Rb01Cs01PbI3 and MA075Rb015Cs01PbI3 perovskite films respectively

shown in Table 57 The increased average life time is observed for

MA08Rb01Cs01PbI3 film sample This increase in carrier life time of doped samples is

suggested to have longer diffusion length of the excitons with suppressed

recombination and defect density in doped sample as compared with pristine MAPbI3

sample The increase in carrier life time of mixed cation perovskites corresponds to

the decrease in non-radiative recombination which is also visible in steady state

photoluminescence This decrease in non-radiative recombinations lead to the

improvement in device performance

125

Table 56 Parameters of fitting the time resolved photoluminescence decay curves of MAPbI3

(MA)08Rb01Cs01PbI3 and (MA)075Rb015Cs01PbI3 films

52 Summary The hybrid organic- inorganic MAPbI3 perovskite as well as mixed cation (MA+ K+1

Rb+1 Cs+1) based perovskite photovoltaic devices has been fabricated in inverted

device configuration The current density-voltage (J-V) characteristics obtained under

illumination is analyzed to get photovoltaic device parameters such as short circuit

current density (Jsc) open circuit voltage (Voc) fill factor (FF) and power conversion

efficiency (PCE) The best efficiencies were recorded in the case of mixed cation

(MA)07Cs03PbI3 and (MA)08K01PbI3 cation perovskites photovoltaic devices with

power conversion efficiency of 15 and 1332 respectively The current density-

voltage (J-V) hysteresis has found to be minimum in mixed cation (MA)08K01PbI3

and (MA)075Rb015Cs01PbI3 perovskite solar cells which shows the doping of the

alkali metals in the lattice of MAPbI3 perovskite material The highest external

quantum efficiency is obtained for (MA)08K01PbI3 based perovskite solar cells which

is in consistence with the short circuit current density (Jsc) value The increased

photoluminescence intensity and average decay life times of the carriers in the case of

(MA)08K01PbI3 (978ns) and (MA)08Rb01Cs01PbI3(745ns) films shows decease in

non-radiative recombinations and suggested to have longer diffusion length of the

excitons with supp ressed recombination and de fect dens ity in dope d samples as

compared with pristine MAPbI3 samples

6 References

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MAPbI3 5678 05 091 185 052

MA08Rb01Cs01PbI3 094 41 051 1442 745

MA075Rb015Cs01PbI3 1501 065 1502 065 065

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Katz Journal of Physical Chemistry Letters 6 326 (2015) 233 G Niu W Li F Meng L Wang H Dong and Y Qiu Journal of Materials Chemistry A 2 705 (2014) 234 J H Im J Chung S J Kim and N G Park Nanoscale Research Letters 7 1 (2012) 235 J W Lee D J Seol A N Cho and N G Park Advanced Materials 26 4991 (2014) 236 M Muzammal uz Zaman M Imran A Saleem A H Kamboh M Arshad N A Khan and P Akhter Physica B Condensed Matter 522 57 (2017) 237 M Kulbak D Cahen and G Hodes Journal of Physical Chemistry Letters 6 2452 (2015) 238 I Chung J H Song J Im J Androulakis C D Malliakas H Li A J Freeman J T Kenney and M G Kanatzidis Journal of the American Chemical Society 134 8579 (2012) 239 T S Ripolles K Nishinaka Y Ogomi Y Miyata and S Hayase Solar Energy Materials and Solar Cells 144 532 (2016) 240 Z Li J Tinkham P Schulz M Yang D H Kim J Berry A Sellinger and K Zhu Advanced Energy Materials 7 1601451 (2016) 241 Z Tang T Bessho F Awai T Kinoshita and M M Maitani Scientific Reports 1 (2017) 242 S-H Turren-Cruz M Saliba M T Mayer H Juaacuterez-Santiesteban X Mathew L Nienhaus W Tress M P Erodici M-J Sher M G Bawendi M Graumltzel A Abate A Hagfeldt and J-P Correa-Baena Energy amp Environmental Science 11 78 (2018) 243 Z Fang H He L Gan J Li and Z Ye Advanced Science 1800736 1 (2018) 244 Y Guo J Jiang S Zuo F Shi J Tao and Z Hu Solar Energy Materials amp Solar Cells 178 186 (2018) 245 S M Sze and K K Ng Physics of Semiconductor Devices (John wiley amp sons 2006) 246 M S Sheikh A P Sakhya A Dutta and T P Sinha Journal of Alloys and Compounds 727 238 (2017) 247 B Tatar A E Bulgurcuoǧlu P Goumlkdemir P Aydoǧan D Yilmazer O Oumlzdemir and K Kutlu International Journal of Hydrogen Energy 34 5208 (2009) 248 P De Bruyn A H P Van Rest G A H Wetzelaer D M De Leeuw and P W M Blom Physical Review Letters 111 1 (2013) 249 G J A H Wetzelaer M Scheepers A M Sempere C Momblona J Aacutevila and H J Bolink Advanced Materials 27 1837 (2015) 250 C t Sah R N Noyce and W Shockley Proceedings of the IRE 45 1228 (1957) 251 G A H Wetzelaer M Kuik H T Nicolai and P W M Blom Physical Review B - Condensed Matter and Materials Physics 83 1 (2011) 252 G A H Wetzelaer L J A Koster and P W M Blom Physical Review Letters 107 1 (2011) 253 S D Stranks V M Burlakov T Leijtens J M Ball A Goriely and H J Snaith Physical Review Applied 2 1 (2014)

7 Conclusions and further work plan

71 Summary and Conclusions The light absorber organo- lead iodide in perovskite solar cells has stability

issues ie organic cation (Methylammonium MA) in methylammonium lead iodide

(MAPbI3) perovskite is highly volatile and get decomposed easily To address the

134

problem we have introduced the oxidation stable inorganic monovalent cations (K+1

Rb+1 Cs+1) in different compositions in methylammonium lead iodide (MAPbI3)

perovskite and studied its properties The investigation of Zinc Selenide (ZnSe)

Graphene Oxide-Titanium Oxide (GO-TiO2) for potential electron transport layer and

Nicke l Oxide (NiO) for hole transport layer applications has been carried out The

photovoltaic devices based on organic-inorganic mixed cation perovskite are

fabricated The following conclusions can be drawn from our studies

The structural optical and electrical properties of ZnSe thin films are sensitive

to the post deposition annealing treatment and the substrate material The ZnSe thin

films deposition on substrates like indium tin oxide (ITO) has resulted in higher

energy band gap as compared with ZnSe films deposited on glass substrate The

difference in energy band gap of ZnSe thin films on two different substrates is due to

the fact that fermi- level of ZnSe is higher than that of transparent conducting indium

tin oxide (ITO) which is resulted in a flow of carriers from the ZnSe towards the

indium tin oxide (ITO) This flow of carriers towards indium tin oxide (ITO)causes

the creation of more vacant states in the conduction band of ZnSe and increased the

energy band gap Whereas the post deposition annealing treatment in vacuum and air

is resulted in the shift of optical band gap to higher value It is therefore concluded

that precisely controlling the post deposition parameters ie annealing temperature

and the annealing environment the energy band gap of ZnSe can be easily tuned for

desired properties

The titanium dioxide (TiO2) studies for electron transport applications showed

that spin coating is facile and an inexpensive way to synthesize TiO2 and its

nanocomposite with graphene oxide ie GOndashTiO2 thin films The x-ray diffraction

studies confirmed the formation of graphene oxide (GO) and rutile TiO2 phase The

electrical properties exhibited Ohmic type of conduction between the GOndashTiO2 thin

films and indium tin oxide (ITO) substrate The sheet resistance of film has decreased

after post deposition thermal annealing suggesting improved charge transpor t for use

in solar cells The x-ray photoemission spectroscopy analysis revealed the presence of

functional groups attached to the basal plane of graphene sheets UVndashVis

spectroscopy revealed a red-shift in wavelength for GOndashTiO2 associated with

bandgap tailoring of TiO2 The energy band gap has decreased from 349eV for TiO2

to 289 eV for xGOndash TiO2 (x = 12 wt) The energy band gap is further increased to

338eV after post deposition thermal annealing at 400ᵒC for xGOndashTiO2 (x = 12 wt)

with increased transmission in the visible range being suitable for use as window

135

layer material These results suggest that post deposition thermally annealed GOndash

TiO2 nanocomposites could be used to form efficient transport layers for application

in perovskite solar cells

To investigate hole transport layer for potential solar cell application NiO is selected

and to tune its properties silver in different mol ie 0-8mol is incorporated The

x-ray diffraction analys is revealed the cubic structure of NiOx The lattice is found to

be expanded and the lattice defects have been minimized with silver dop ing The

smoot h NiO2 surface with no significant pin holes were obtained on fluorine doped tin

oxide glass substrates as compared with bare glass substrates The NiO2 film on glass

substrates have shown enhanced uniformity with silver doping The film thickness

calculated by the Rutherford backscattering (RBS) technique was found to be in the

range 100-200 nm The UV-Vis spectroscopy results showed that the transmission of

the NiO thin films has improved with Ag dop ing The energy band gap values are

decreased with lower silver doping The mobility of films has been enhanced up to

6mol Ag doping The resistivity of the NiO films is also decreased with lower

silver doping with minimum value 16times103 Ω-cm at 4mol silver doping

concentration as compared with 2times106 Ω-cm These results showed that the NiO thin

films with 4-6mol Ag doping have enha nced conductivity mobility and

transparency are suggested to be used for efficient window layer material in solar

cells

The undoped ie methylammonium lead iodide (MAPbI3) showed the

tetragonal crystal structure With alkali metal cations (K Rb Cs) doping material

have shown phase segregation beyond 10 dop ing concentration The or thorhombic

distor tion arise from the BPbI3(B=K Rb Cs) crystalline phase The current voltage

(I-V) characteristics of (MA)1-xKxPbI3 (x=0 01 02 03 04 1) thin films have

shown Ohmic behavior associated with a decrease in the resistance with the doping K

up to x=04 This decrease in the resistance is suggested to be arising from the higher




diffraction and x-ray photoemission spectroscopy studies

The aging studies of alkali metal doped ie (MA)1-xRbxPbI3 were carried out by

collecting x-ray diffraction patterns of the samples with a week interval for four

weeks The reduced degradation of the material in case of Rb doped sample as

136

compared with pristine methylammonium lead iodide (MAPbI3) perovskite is

observed

The morphology of the films was also observed to change from cubic like

grains of methylammonium lead iodide (MAPbI3) to strip like structures for


structures for cesium (Cs) doped samples

The energy band gap of pr istine material observed in UV-visible spectroscopy

is around 155eV which increases marginally for the heavily alkali metal based

perovskite samples The associated absorbance in doped samples increases for lower

doping and then suppresses for heavy doping attributed to the increase in energy band

gap and corresponding blue shift in absorption spectra The band gap values and blue

shift was also confirmed by photoluminescence spectra of our samples

Time resolved spectroscopy studies showed that the lower Rb doped samples

have higher average carrier life time than pristine samples suggesting efficient charge

extraction These rubidium (Rb+1) doped samples in Hall measurements have shown

that conductivity type of the material is tuned from n-type to p-type by doping with

Rb It is concluded that carrier concentration and mobility of the material could be

controlled by varying doping concentration This study helps to control the

semiconducting properties of perovskite light absorber and tuning of the carrier type

with Rb doping This study also opens a way to the future study of hybrid perovskite

solar cells by using perovskite film as a PN-junction










optimum bandgaps are suitable for solar cell application

The intercalation of inadvertent carbon and oxygen in (MA)1-xRbxPbI3(x= 0

01 03 05 075 1) materials are investigated by x-ray photoemission spectroscopy It

is observed that oxygen related peak which primary source of decomposition of

pristine MAPbI3 material decreases in intensity in all doped samples showing the

137

enhanced stability with Rb doping These partially doped perovskites with enhanced

stability and improved optoelectronic properties could be attractive candidate as light

harvesters for traditional perovskite photovoltaic device applications Similarly Cs

doped samples have shown better stability than pristine sample by x-ray photoemission

spectroscopy studies

Monovalent cation Cs doped MAPbI3 ie (MA)1-xCsxPbI3 (x=0 01 02 03

04 05 075 1) films have been deposited on FTO substrates and their potential for

solar cells applications is investigated The x-ray diffraction scans of these samples

have shown tetragonal structure in pr istine methylammonium lead iodide (MAPbI3)

perovskite material which is transformed to an orthorhombic phase with the doping of

Cs The orthorhombic distortions in (MA)1-xCsxPbI3 arise due to the formation of

CsPbI3 phase along with MAPbI3 which has orthorhombic crystal structure It was

found that pure MAPbI3 perovskite crystalline grains are in sub-micrometer sizes and

they do not completely cover the FTO substrate However with Cs doping rod like

structures starts appearing and the connectivity of the grains is enhanced with the

increased doing of Cs up to x=05 The inter-grain voids are completely healed in the

films with Cs doping between 40-50 The inter-grain voids again begin to appear for

the higher doping of Cs (ie x=075 1) and disconnected rod like structures are

observed The atomic force microscopy (AFM) studies showed that the surface of the

films become smoother with Cs doping and root mean square roughness (Rrms)

decreases dramatically from 35nm observed in un-doped samples to 11nm in 40 Cs

doped film showing healing of grain boundaries that finally lead to the improvement

of the device performance The band gap of pristine material observed in UV visible

spectroscopy is around 155 eV which increases marginally (ie155+003 eV) for the

higher doping of Cs in (MA)1-xCsxPbI3 samples The band gap of (MA)1-xCsxPbI3 (x=

0 01 03 05 075) samples is also measured by photoluminescence measurements

which confirmed the UV-visible spectroscopy measurements

The hybrid organic- inorganic MAPbI3 perovskite as well as mixed cation (MA+ K+1



illuminationdark condition is analyzed to get photovoltaic device parameters such as

short circuit current density (Jsc) open circuit voltage (Voc) fill factor (FF) and power

conversion efficiency (PCE) The best efficiencies were recorded in the case of mixed

cation (MA)07Cs03PbI3 and (MA)08K01PbI3 cation perovskites photovoltaic devices

with power conversion efficiency of 15 and 1332 respectively The current

138

density-voltage (J-V) hysteresis has found to be minimum in mixed cation

(MA)08K01PbI3 and (MA)075Rb015Cs01PbI3 perovskite solar cells which shows the

doping of the potassium in the MAPbI3 perovskite material The highest external


is in consistence with the short circuit current density (Jsc) value The Cs doped

devices have shown improvement in the power conversion efficiency of the device

and best efficiency is achieved with 30 doping (ie PCE ~15) The dark-current

ideality factor values are in the range of asymp22 -24 for all devices clearly indicating

that trap-assisted recombination is the dominant recombination mechanism It is

concluded that the power-conversion efficiency of perovskite solar cells can be

enhanced by eliminating the recombination pathways which could be achieved either

by maintaining the doped atomic concentration to a tolerable limit (ie x=03 in Cs

case) or by limiting the leakage current

The increased photoluminescence intensity and average decay life times of the

carriers in the case of (MA)08K01PbI3 (978ns) and (MA)08Rb01Cs01PbI3 (745ns)

films shows decease in non-radiative recombinations and suggested to have longer

diffus ion length of the excitons with supp ressed recombination and de fect density in

doped samples as compared with pristine methylammonium lead iodide (MAPbI3)

sample

The organic inorganic mixed cation based perovskites are better in terms of enhanced

stability and efficiency compared with organic cation lead iodide (MAPbI3)

perovskite These studies open a way to explore mixed cation based perovskites with

inorganic electron and hole transpor t layers for stable and efficient PN-junction as

well as tandem perovskite solar cells

72 Further work A clear extension to this work would be the investigation of the role of various charge

transport layers with these mixed cation perovskites in the performance parameters of

the solar cells The fabrication of these devices on flexible substrates by spin coating

would be carried out

The P and N-type perovskite absorber materials obtained by different dop ing

concentrations would be used to fabricate P-N junction solar cells Beyond single

junction solar cells these material would be used for the fabrication of multijunction

solar cells in which perovskite films would be combined with silicon or other

139

materials to increase the absorption range and open circuit voltage by converting the

solar photons into electrical potential energy at a higher voltage

A study on lead free perovskites could be undertaken due to toxicology concerns

by employing single as well as double cation replacement The layer structured

perovskite (2D) and films with mixed compositions of 2D and 3D perovskite phases

would be investigated The approach of creating stable heterojunctions between 2D

and 3D phases may also enable better control of perovskite surfaces and electronic

interfaces and study of new physics

Introduction and Literature Review

Renewable Energy

Solar Cells

p-n Junction

Thin film solar cells

Photo-absorber and the solar spectrum

Single-junction solar cells

Tandem Solar Cells

Charge transport layers in solar cells

Perovskite solar cells

The origin of bandgap in perovskite

The Presence of Lead

Compositional optimization of the perovskite

Stabilization by Bromine

Layered perovskite formation with large organic cations

The Formamidinium cation

Inorganic cations

Alternative anions

Multiple-cation perovskite absorber

Motivation and Procedures

Materials and Methodology

Preparation Methods

One-step Methods

Two-step Methods

Deposition Techniques

Spin coating

Anti-solvent Technique

Chemical Bath Deposition Method

Other perovskite deposition techniques

Experimental

Chemicals details

Materials and films preparation

Substrate Preparation

Preparation of ZnSe films

Preparation of GO-TiO2 composite films

Preparation of NiOx films

Preparation of CH3NH3PbI3 (MAPb3) films

Preparation of (MA)1-xKxPbI3 films

Preparation of (MA)1-xRbxPbI3 films

Preparation of (MA)1-xCsxPbI3 films

Preparation of (MA)1-(x+y)RbxCsyPbI3 films

Solar cells fabrication

FTONiOxPerovskiteC60Ag perovskite solar cell

FTOPTAAPerovskiteC60Ag perovskite solar cell

Characterization Techniques

X- ray Diffraction (XRD)

Scanning Electron Microscopy (SEM)

Atomic Force Microscopy (AFM)

Absorption Spectroscopy

Photoluminescence Spectroscopy (PL)

Rutherford Backscattering Spectroscopy (RBS)

Particle Induced X-rays Spectroscopy (PIXE)

X-ray photoemission spectroscopy (XPS)

Current voltage measurements

Hall effect measurements

Solar Cell Characterization

Current density-voltage (J-V) Measurements

External Quantum Efficiency (EQE)

Studies of Charge Transport Layers for potential Photovoltaic Applications

ZnSe Films for Potential Electron Transport Layer (ETL)

Results and discussion

GOndashTiO2 Films for Potential Electron Transport Layer


NiOX thin films for potential hole transport layer (HTL) application

Results and Discussion

Summary

Alkali metal (K Rb Cs RbCs) Based Mixed Cation Perovskites


Optimization of annealing temperature

Potassium (K) doped MAPbI3 perovskite

Rubidium (Rb) doped MAPbI3 perovskite

Cesium (Cs) doped MAPbI3 perovskite

Effect of RbCs doping on MAPbI3 perovskite

Summary

Mixed Cation Perovskite Photovoltaic Devices


Summary

References

Conclusions and further work plan

Summary and Conclusions

Further work

ii



This work is submitted as a dissertation in partial fulfillment of

the requirement for the degree of

DOCTORATE

IN

PHYSICS

by

ABIDA SALEEM

Material Science Laboratory Department of Physics


2019

iii

iv

v

vi

vii

DEDICATED

TO

MY LOVING

FATHER

AND

LATE SISTER

(HAJRA SALEEM)

viii
























ABIDA SALEEM

2019

ix



































x

















xi





Physics A 125 (2019) 229




11




Materials 47 (2018) 3749














Magnetism 31(4) (2018) 1013








xii







1 Dr Asif Khan






Emailasifengrscedu






xiii













xiv






34 Summary 67



42 Summary 105



52 Summary 125

6 References 125



72 Further work 138

xv


xvi


xRbxPbI3(x=0 01 03 05 075 1) 88

xvii



xviii


xix






FF Fill factor

PV Photovoltaics











1

CHAPTER 1

























cells




2





121 p-n Junction



















device










3



































4






















[10]





5



































6









solar cells





















7

































with graphene [58]



8

















app lications


















9




































10



octahedra [83]

















a) ABX3 b) MAPbI3

11



































12



































13

t= 119955119955119912119912+119955119955119935119935radic120784120784(119955119955119913119913+119955119955119935119935)













Lithium Li+ 76













14


































15



































16
















(RbPbI3) [130]



















17













Rb005Cs005MA015FA075Pb(I083Br017)3









fashion












18













19

CHAPTER 2















two-step methods










20





















21



reaction


221 Spin coating















higher






22
































23

























23 Experimental









24

































25





















performed











26






















as discussed above










27


procedure as above





next sections
























28












parameters























29


























30


































31










119844119844 =119812119812119835119835119812119812119842119842

=

⎩⎨

⎧119820119820120783120783119836119836119836119836119836119836119836119836+ 119820119820120784120784120784120784 minus119820119820120783120783

120784120784119826119826119842119842119826119826120784120784119836119836

119820119820120783120783 +119820119820120784120784⎭⎬

⎫120784120784

120784120784120784120784














32






specific elements






titanium












[159]




33






















119833119833(120538120538) = 119829119829119816119816



34






R α AL

rArr R = ρ AL 25
















35

















120520120520 = 119823119823119823119823119823119823119823119823119823119823119842119842119826119826

= 119817119817119836119836119836119836119829119829119836119836119836119836 119813119813119813119813119823119823119842119842119826119826

















36



















119920119920119920119920119920119920119920119920(120640120640) =119921119921119956119956119956119956(120640120640)119942119942oslash(120640120640) =

119921119921119956119956119956119956(120640120640)119942119942119920119920119946119946119946119946(120640120640)

119945119945119956119956120640120640

= 120783120783120784120784120783120783120782120782119921119921119956119956119956119956(120640120640)120640120640119920119920119946119946119946119946(120640120640) 120784120784120790120790




37



CHAPTER 3























38




































39





















40










Thickness (nm) 154 160 148 157 160











41




119915119915 =120782120782 120791120791120783120783120640120640119913119913119920119920119913119913119956119956Ѳ 120785120785120783120783

120634120634 =119913119913119920119920119913119913119956119956Ѳ120783120783 120785120785 120784120784

120633120633 =120783120783120783120783120634120634119938119938119915119915 120785120785 120785120785











119879119879) where d is








42












ZnSe [174]



43
















As-made 244

350 248

400 250

450 252

500 253


(Å)

a

( Å)

D

(nm)

ε

times10-4

δtimes1015

(lines m-2)

ZnSeITO As-made 3084 288 1001 20 17 124


44















sample in vacuum


ZnSeITO Air (450degC) 3088 288 1001 23 16 102

45







35



(103Ω)

Sheet resistance

(103Ω)


As prepared 290



46

































47
















48























49




[178]










50

























51




















52



















values [186]

53




NiO (150oC) 385 213

NiO (400oC) 387 213

NiO (500oC) 391 210




















54

















55











119920119920 = 119920119920120782120782119942119942minus120630120630120630120630 120785120785120783120783



120630120630 =119949119949119946119946(120783120783 119931119931 )

120630120630 120785120785120783120783


120572120572

=2303 times 119860119860




119946119946120784120784 = 120783120783+ (119912119912

119920119920119944119944 +119913119913)120784120784 120785120785120789120789











56















57



119962119962 = 120782120782120782120782119949119949 414 205 437

119962119962 = 120783120783120782120782119913119913119949119949 382 212 462

119962119962 = 120788120788120782120782119913119913119949119949 402 207 449

119962119962 = 120790120790120782120782119913119913119949119949 405 207 449



01M Ni(NO3)26H2O 39 21

075M Ni(NO3)26H2O 36 216

075M Ni(acet)4H2O 37 215











58







nicke l atom




1119889119889ℎ1198961198961198891198892 =

ℎ2 + 1198961198962 + 1198891198892






59


119962119962119912119912119944119944119925119925119946119946119925119925 d(111) (Å) d(200) (Å) d(220) (Å) a (Å)

y=0 mol 24125 20883 14780 41766

y=4mol 24488 21274 14884 4255

y=6mol 24506 21373 14995 4275

y=8mol 24436 21422 1501 4284



119863119863 =119896119896119899119899






[194]

120575120575

=11198631198632 (


120576120576 =120573120573








119930119930119938119938120782120782119930119930119949119949119942119942 FWHM (2θ )


y=0 mol 0488 0349 0411

y=4mol 0164 0284 0282

y=6mol 0184 0180 0204

y=8mol 0214 0215 0168


119930119930119938119938120782120782119930119930119949119949119942119942 D (111) nm D (200) nm D (220) nm

60

y=0 mol 17 25 23

y=4mol 51 30 33

y=6mol 45 47 45

y=8mol 39 40 55


119930119930119938119938120782120782119930119930119949119949119942119942 δ (111) (1014

linesm2)

δ (200) (1014

linesm2)

δ (220) (1014

linesm2)

y=0 mol 339 166 1947

y=4mol 387 111 925

y=6mol 485 445 485

y=8mol 655 636 328


119930119930119938119938120782120782119930119930119949119949119942119942 ε (111) (10-4) ε (200) (10-4) ε (220) (10-4)

y=0 mol 28 3832 293

y=4mol 216 3171 2032

y=6mol 2425 2027 1484

y=8mol 2811 2430 1221
















61









119962119962 = 120782120782120782120782119913119913119949119949 408 206

119962119962 = 120783120783120782120782119913119913119949119949 397 208

119962119962 = 120788120788120782120782119913119913119949119949 401 208

119962119962 = 120790120790120782120782119913119913119949119949 404 207


62




















63



64















65


119930119930119938119938120782120782119930119930119949119949119942119942 y=0mol y=4mol y=6mol 119962119962 = 120790120790120782120782119913119913119949119949

Nickel 020 023 0225 021

Silver 00001 00001 0001 001

Silicon 0140 0127 0130 013

Oxygen 0658 0636 0640 065

Thickness (nm) 110 195 183 116







glass substrates









66





results





Resistivity (Ω-cm)


119962119962 = 120782120782120782120782119913119913119949119949 2521times1014 6753 3666 2728x10-7

119962119962 = 120783120783120782120782119913119913119949119949 2601times1015 2999 1641 6095x10-7

119962119962 = 120788120788120782120782119913119913119949119949 6335times1014 285 8003 125x10-6

119962119962 = 120790120790120782120782119913119913119949119949 4813times1014 1467 8843 1131x10-6






67










119962119962119912119912119944119944119925119925119946119946119925119925


119962119962 = 120782120782120782120782119913119913119949119949 15 times 106

119962119962 = 120783120783120782120782119913119913119949119949 4 times 105

119962119962 = 120788120788120782120782119913119913119949119949 25 times 103

119962119962 = 120790120790120782120782119913119913119949119949 12 times 106









68




































69





















70

CHAPTER 4

























71






















72






(MAPbI3)















73


























74









1 samples [207]













75











b

a

76








(MA)1-xKxPbI3 x=0 x=01 x=02 x=03 x=04 x=1

Rs (Ω) 35x107 15x107 25x106 15x106 53x106 15x107















77



































78





79






to x=04





x=0 13733 61832 480099

x=01 13701 61809 48108

x=05 13754 61858 48104

x=1 13684 61817 48133

80









075 1) films


81


























x=0 155

x=01 153

x=02 153

x=03 152

x=04 154

x=05 159

x=075 160

x=1 26

82














83


















84











scans





















Figure 416(b)

85




86


















87






















119860119860(119905119905) = 1198601198601119897119897119890119890119890119890minus11990511990511205911205911+1198601198602119897119897119890119890119890119890

minus11990511990521205911205912 (2)




τ119886119886119886119886119886119886 = sum119860119860119904119904 τ119904119904sum 119860119860119904119904

(3)









88



recombination


xRbxPbI3(x=0 01 03 05 075 1)










89




















90




































91



































92


cation doping

















toward humidity

















93





94






















(eV)

EBI3d52

(eV)

EBRb3d32

(eV)

EBI3d52-EBPb4f72

(eV)

x=0 13779 6187 0 48091

x=01 1378 61855 11012 48075

x=03 13781 61857 1101 48076

x=05 13778 61862 11017 48079

x=075 1378 61873 11015 48084

x=1 1378 61872 11014 48092

95






















96











performance








97

























materials










98


05 075 1)

Rrms=35

Rrms=19

Rrms=21

Rrms=13

Rrms=11

Rrms=14

Rrms=28

99





100




























atmosphere








101






























102




103








(MA)01Cs09PbI3 3385 4434 232 171 1720 058














in the material












104



105






























(MA)1-

(x+y)RbxCsyPbI3

X=0

y=0

X=005

y=005

X=005

y=01

X=01

y=005

X=015

y=01

X=01

y=015

Band gap (eV) 1566 1570 1580 1580 1581 1584

106



































107


studies

CHAPTER 5






























108





























109














110











wavelengt h value

Perovskite Scan

direction

Jsc

(mAcm2)

Voc

(V)

FF

PCE Rsh

(kΩ)

Rs

(kΩ)

MAPbI3 Reverse 1870 086 5401 851 093 13

Forward 1650 078 5761 726 454 14

(MA)09K01PbI3 Reverse 1815 106 6904 1332 3979 9

Forward 1763 106 6601 1237 3471 11

(MA)09Rb01PbI3 Reverse 1591 0834 58151 7567 454 22

Forward 13314 0822 59873 6426 079 25




119919119919119919119919 =119920119920119920119920119920119920119929119929119942119942119929119929119942119942119955119955119956119956119942119942 minus 119920119920119920119920119920119920119919119919119913119913119955119955119917119917119938119938119955119955120630120630

119920119920119920119920119920119920119929119929119942119942119929119929119942119942119955119955119956119956119942119942 120783120783120783120783








111






















55(b)



119912119912(119957119957) = 119912119912120783120783119942119942119942119942119930119930minus119957119957120783120783120649120649120783120783+119912119912120784120784119942119942119942119942119930119930

minus119957119957120784120784120649120649120784120784 51

112






120533120533119938119938119929119929119944119944 =sum119912119912119946119946120533120533119946119946sum119912119912119946119946

120783120783120785120785












113






MAPbI3 5678 050 091 185 05

(MA)09K01PbI3 105 370 030 3117 978










114



















respectively

115















x= 0 79 15392

x= 01 3 1076

x= 02 4 1242

x= 03 276 79607

x= 04 17 2391

x= 05 6 744

116

x= 075 25 6338

x= 1 10 78

































117




119921119921 = 119921119921s119942119942119954119954119954119954119946119946119951119951119931119931 minus 120783120783 52




119946119946 = 119951119951119931119931119954119954

120655120655119845119845119836119836119845119845 119921119921120655120655119954119954

minus120783120783






















118

119

-02 00 02 04 06 08 10 12

10-4

10-3

10-2

10-1

100

101

102

J(m

Ac

m2 )

V (V)

x=0 x=01 x=02 x=03 x=04 x=05 x=075 x=1

J-V Dark

a

001 01 11E-8

1E-7

1E-6

1E-5

1E-4

1E-3

001

x=0 x=01 x=02 x=03 x=04 x=05 x=075 x=1

log(

J)

Log (V)

Region IRegion II

Region III

b

08 10 12

2

4

x=0 x=01 x=02 x=03 x=04 x=05D

iffer

entia

l Ide

ality

Fac

tor

V (V)

c



120



































121


our devices





Figure 511

Sample Scan

direction Jsc

(mAcm2) Voc (V)

FF

PCE

Rsh(kΩ) Rs(Ω)

MAPbI3 Reverse 1870

08

6 5401 851 454 17

Forward 1650

07

8 5761 726 193 20

(MA)080Rb0


07

9 6298 769 241 23

Forward 1125

08

0 7304 642 143 6

(MA)075Rb0 Reverse 689 08 5493 338 068 31

122














15Cs01PbI3 9

Forward 688

08

9 5309 323 042 33

123

400 500 600 700 8000

20

40

60

80

Wave length (nm)

EQE

()














124













125





















6 References



MAPbI3 5678 05 091 185 052

MA08Rb01Cs01PbI3 094 41 051 1442 745

MA075Rb015Cs01PbI3 1501 065 1502 065 065

126


127


128


129


130


131


132


133






134





















desired properties















135




















cells















136


observed


































137




































138



















sample














139










Renewable Energy

Solar Cells

p-n Junction




Tandem Solar Cells









Inorganic cations

Alternative anions




Preparation Methods

One-step Methods

Two-step Methods


Spin coating




Experimental

Chemicals details



































Summary








Summary



Summary

References



Further work

iii

iv

v

vi

vii

DEDICATED

TO

MY LOVING

FATHER

AND

LATE SISTER

(HAJRA SALEEM)

viii
























ABIDA SALEEM

2019

ix



































x

















xi





Physics A 125 (2019) 229




11




Materials 47 (2018) 3749














Magnetism 31(4) (2018) 1013








xii







1 Dr Asif Khan






Emailasifengrscedu






xiii













xiv






34 Summary 67



42 Summary 105



52 Summary 125

6 References 125



72 Further work 138

xv


xvi


xRbxPbI3(x=0 01 03 05 075 1) 88

xvii



xviii


xix






FF Fill factor

PV Photovoltaics











1

CHAPTER 1

























cells




2





121 p-n Junction



















device










3



































4






















[10]





5



































6









solar cells





















7

































with graphene [58]



8

















app lications


















9




































10



octahedra [83]

















a) ABX3 b) MAPbI3

11



































12



































13

t= 119955119955119912119912+119955119955119935119935radic120784120784(119955119955119913119913+119955119955119935119935)













Lithium Li+ 76













14


































15



































16
















(RbPbI3) [130]



















17













Rb005Cs005MA015FA075Pb(I083Br017)3









fashion












18













19

CHAPTER 2















two-step methods










20





















21



reaction


221 Spin coating















higher






22
































23

























23 Experimental









24

































25





















performed











26






















as discussed above










27


procedure as above





next sections
























28












parameters























29


























30


































31










119844119844 =119812119812119835119835119812119812119842119842

=

⎩⎨

⎧119820119820120783120783119836119836119836119836119836119836119836119836+ 119820119820120784120784120784120784 minus119820119820120783120783

120784120784119826119826119842119842119826119826120784120784119836119836

119820119820120783120783 +119820119820120784120784⎭⎬

⎫120784120784

120784120784120784120784














32






specific elements






titanium












[159]




33






















119833119833(120538120538) = 119829119829119816119816



34






R α AL

rArr R = ρ AL 25
















35

















120520120520 = 119823119823119823119823119823119823119823119823119823119823119842119842119826119826

= 119817119817119836119836119836119836119829119829119836119836119836119836 119813119813119813119813119823119823119842119842119826119826

















36



















119920119920119920119920119920119920119920119920(120640120640) =119921119921119956119956119956119956(120640120640)119942119942oslash(120640120640) =

119921119921119956119956119956119956(120640120640)119942119942119920119920119946119946119946119946(120640120640)

119945119945119956119956120640120640

= 120783120783120784120784120783120783120782120782119921119921119956119956119956119956(120640120640)120640120640119920119920119946119946119946119946(120640120640) 120784120784120790120790




37



CHAPTER 3























38




































39





















40










Thickness (nm) 154 160 148 157 160











41




119915119915 =120782120782 120791120791120783120783120640120640119913119913119920119920119913119913119956119956Ѳ 120785120785120783120783

120634120634 =119913119913119920119920119913119913119956119956Ѳ120783120783 120785120785 120784120784

120633120633 =120783120783120783120783120634120634119938119938119915119915 120785120785 120785120785











119879119879) where d is








42












ZnSe [174]



43
















As-made 244

350 248

400 250

450 252

500 253


(Å)

a

( Å)

D

(nm)

ε

times10-4

δtimes1015

(lines m-2)

ZnSeITO As-made 3084 288 1001 20 17 124


44















sample in vacuum


ZnSeITO Air (450degC) 3088 288 1001 23 16 102

45







35



(103Ω)

Sheet resistance

(103Ω)


As prepared 290



46

































47
















48























49




[178]










50

























51




















52



















values [186]

53




NiO (150oC) 385 213

NiO (400oC) 387 213

NiO (500oC) 391 210




















54

















55











119920119920 = 119920119920120782120782119942119942minus120630120630120630120630 120785120785120783120783



120630120630 =119949119949119946119946(120783120783 119931119931 )

120630120630 120785120785120783120783


120572120572

=2303 times 119860119860




119946119946120784120784 = 120783120783+ (119912119912

119920119920119944119944 +119913119913)120784120784 120785120785120789120789











56















57



119962119962 = 120782120782120782120782119949119949 414 205 437

119962119962 = 120783120783120782120782119913119913119949119949 382 212 462

119962119962 = 120788120788120782120782119913119913119949119949 402 207 449

119962119962 = 120790120790120782120782119913119913119949119949 405 207 449



01M Ni(NO3)26H2O 39 21

075M Ni(NO3)26H2O 36 216

075M Ni(acet)4H2O 37 215











58







nicke l atom




1119889119889ℎ1198961198961198891198892 =

ℎ2 + 1198961198962 + 1198891198892






59


119962119962119912119912119944119944119925119925119946119946119925119925 d(111) (Å) d(200) (Å) d(220) (Å) a (Å)

y=0 mol 24125 20883 14780 41766

y=4mol 24488 21274 14884 4255

y=6mol 24506 21373 14995 4275

y=8mol 24436 21422 1501 4284



119863119863 =119896119896119899119899






[194]

120575120575

=11198631198632 (


120576120576 =120573120573








119930119930119938119938120782120782119930119930119949119949119942119942 FWHM (2θ )


y=0 mol 0488 0349 0411

y=4mol 0164 0284 0282

y=6mol 0184 0180 0204

y=8mol 0214 0215 0168


119930119930119938119938120782120782119930119930119949119949119942119942 D (111) nm D (200) nm D (220) nm

60

y=0 mol 17 25 23

y=4mol 51 30 33

y=6mol 45 47 45

y=8mol 39 40 55


119930119930119938119938120782120782119930119930119949119949119942119942 δ (111) (1014

linesm2)

δ (200) (1014

linesm2)

δ (220) (1014

linesm2)

y=0 mol 339 166 1947

y=4mol 387 111 925

y=6mol 485 445 485

y=8mol 655 636 328


119930119930119938119938120782120782119930119930119949119949119942119942 ε (111) (10-4) ε (200) (10-4) ε (220) (10-4)

y=0 mol 28 3832 293

y=4mol 216 3171 2032

y=6mol 2425 2027 1484

y=8mol 2811 2430 1221
















61









119962119962 = 120782120782120782120782119913119913119949119949 408 206

119962119962 = 120783120783120782120782119913119913119949119949 397 208

119962119962 = 120788120788120782120782119913119913119949119949 401 208

119962119962 = 120790120790120782120782119913119913119949119949 404 207


62




















63



64















65


119930119930119938119938120782120782119930119930119949119949119942119942 y=0mol y=4mol y=6mol 119962119962 = 120790120790120782120782119913119913119949119949

Nickel 020 023 0225 021

Silver 00001 00001 0001 001

Silicon 0140 0127 0130 013

Oxygen 0658 0636 0640 065

Thickness (nm) 110 195 183 116







glass substrates









66





results





Resistivity (Ω-cm)


119962119962 = 120782120782120782120782119913119913119949119949 2521times1014 6753 3666 2728x10-7

119962119962 = 120783120783120782120782119913119913119949119949 2601times1015 2999 1641 6095x10-7

119962119962 = 120788120788120782120782119913119913119949119949 6335times1014 285 8003 125x10-6

119962119962 = 120790120790120782120782119913119913119949119949 4813times1014 1467 8843 1131x10-6






67










119962119962119912119912119944119944119925119925119946119946119925119925


119962119962 = 120782120782120782120782119913119913119949119949 15 times 106

119962119962 = 120783120783120782120782119913119913119949119949 4 times 105

119962119962 = 120788120788120782120782119913119913119949119949 25 times 103

119962119962 = 120790120790120782120782119913119913119949119949 12 times 106









68




































69





















70

CHAPTER 4

























71






















72






(MAPbI3)















73


























74









1 samples [207]













75











b

a

76








(MA)1-xKxPbI3 x=0 x=01 x=02 x=03 x=04 x=1

Rs (Ω) 35x107 15x107 25x106 15x106 53x106 15x107















77



































78





79






to x=04





x=0 13733 61832 480099

x=01 13701 61809 48108

x=05 13754 61858 48104

x=1 13684 61817 48133

80









075 1) films


81


























x=0 155

x=01 153

x=02 153

x=03 152

x=04 154

x=05 159

x=075 160

x=1 26

82














83


















84











scans





















Figure 416(b)

85




86


















87






















119860119860(119905119905) = 1198601198601119897119897119890119890119890119890minus11990511990511205911205911+1198601198602119897119897119890119890119890119890

minus11990511990521205911205912 (2)




τ119886119886119886119886119886119886 = sum119860119860119904119904 τ119904119904sum 119860119860119904119904

(3)









88



recombination


xRbxPbI3(x=0 01 03 05 075 1)










89




















90




































91



































92


cation doping

















toward humidity

















93





94






















(eV)

EBI3d52

(eV)

EBRb3d32

(eV)

EBI3d52-EBPb4f72

(eV)

x=0 13779 6187 0 48091

x=01 1378 61855 11012 48075

x=03 13781 61857 1101 48076

x=05 13778 61862 11017 48079

x=075 1378 61873 11015 48084

x=1 1378 61872 11014 48092

95






















96











performance








97

























materials










98


05 075 1)

Rrms=35

Rrms=19

Rrms=21

Rrms=13

Rrms=11

Rrms=14

Rrms=28

99





100




























atmosphere








101






























102




103








(MA)01Cs09PbI3 3385 4434 232 171 1720 058














in the material












104



105






























(MA)1-

(x+y)RbxCsyPbI3

X=0

y=0

X=005

y=005

X=005

y=01

X=01

y=005

X=015

y=01

X=01

y=015

Band gap (eV) 1566 1570 1580 1580 1581 1584

106



































107


studies

CHAPTER 5






























108





























109














110











wavelengt h value

Perovskite Scan

direction

Jsc

(mAcm2)

Voc

(V)

FF

PCE Rsh

(kΩ)

Rs

(kΩ)

MAPbI3 Reverse 1870 086 5401 851 093 13

Forward 1650 078 5761 726 454 14

(MA)09K01PbI3 Reverse 1815 106 6904 1332 3979 9

Forward 1763 106 6601 1237 3471 11

(MA)09Rb01PbI3 Reverse 1591 0834 58151 7567 454 22

Forward 13314 0822 59873 6426 079 25




119919119919119919119919 =119920119920119920119920119920119920119929119929119942119942119929119929119942119942119955119955119956119956119942119942 minus 119920119920119920119920119920119920119919119919119913119913119955119955119917119917119938119938119955119955120630120630

119920119920119920119920119920119920119929119929119942119942119929119929119942119942119955119955119956119956119942119942 120783120783120783120783








111






















55(b)



119912119912(119957119957) = 119912119912120783120783119942119942119942119942119930119930minus119957119957120783120783120649120649120783120783+119912119912120784120784119942119942119942119942119930119930

minus119957119957120784120784120649120649120784120784 51

112






120533120533119938119938119929119929119944119944 =sum119912119912119946119946120533120533119946119946sum119912119912119946119946

120783120783120785120785












113






MAPbI3 5678 050 091 185 05

(MA)09K01PbI3 105 370 030 3117 978










114



















respectively

115















x= 0 79 15392

x= 01 3 1076

x= 02 4 1242

x= 03 276 79607

x= 04 17 2391

x= 05 6 744

116

x= 075 25 6338

x= 1 10 78

































117




119921119921 = 119921119921s119942119942119954119954119954119954119946119946119951119951119931119931 minus 120783120783 52




119946119946 = 119951119951119931119931119954119954

120655120655119845119845119836119836119845119845 119921119921120655120655119954119954

minus120783120783






















118

119

-02 00 02 04 06 08 10 12

10-4

10-3

10-2

10-1

100

101

102

J(m

Ac

m2 )

V (V)

x=0 x=01 x=02 x=03 x=04 x=05 x=075 x=1

J-V Dark

a

001 01 11E-8

1E-7

1E-6

1E-5

1E-4

1E-3

001

x=0 x=01 x=02 x=03 x=04 x=05 x=075 x=1

log(

J)

Log (V)

Region IRegion II

Region III

b

08 10 12

2

4

x=0 x=01 x=02 x=03 x=04 x=05D

iffer

entia

l Ide

ality

Fac

tor

V (V)

c



120



































121


our devices





Figure 511

Sample Scan

direction Jsc

(mAcm2) Voc (V)

FF

PCE

Rsh(kΩ) Rs(Ω)

MAPbI3 Reverse 1870

08

6 5401 851 454 17

Forward 1650

07

8 5761 726 193 20

(MA)080Rb0


07

9 6298 769 241 23

Forward 1125

08

0 7304 642 143 6

(MA)075Rb0 Reverse 689 08 5493 338 068 31

122














15Cs01PbI3 9

Forward 688

08

9 5309 323 042 33

123

400 500 600 700 8000

20

40

60

80

Wave length (nm)

EQE

()














124













125





















6 References



MAPbI3 5678 05 091 185 052

MA08Rb01Cs01PbI3 094 41 051 1442 745

MA075Rb015Cs01PbI3 1501 065 1502 065 065

126


127


128


129


130


131


132


133






134





















desired properties















135




















cells















136


observed


































137




































138



















sample














139










Renewable Energy

Solar Cells

p-n Junction




Tandem Solar Cells









Inorganic cations

Alternative anions




Preparation Methods

One-step Methods

Two-step Methods


Spin coating




Experimental

Chemicals details



































Summary








Summary



Summary

References



Further work

iv

v

vi

vii

DEDICATED

TO

MY LOVING

FATHER

AND

LATE SISTER

(HAJRA SALEEM)

viii
























ABIDA SALEEM

2019

ix



































x

















xi





Physics A 125 (2019) 229




11




Materials 47 (2018) 3749














Magnetism 31(4) (2018) 1013








xii







1 Dr Asif Khan






Emailasifengrscedu






xiii













xiv






34 Summary 67



42 Summary 105



52 Summary 125

6 References 125



72 Further work 138

xv


xvi


xRbxPbI3(x=0 01 03 05 075 1) 88

xvii



xviii


xix






FF Fill factor

PV Photovoltaics











1

CHAPTER 1

























cells




2





121 p-n Junction



















device










3



































4






















[10]





5



































6









solar cells





















7

































with graphene [58]



8

















app lications


















9




































10



octahedra [83]

















a) ABX3 b) MAPbI3

11



































12



































13

t= 119955119955119912119912+119955119955119935119935radic120784120784(119955119955119913119913+119955119955119935119935)













Lithium Li+ 76













14


































15



































16
















(RbPbI3) [130]



















17













Rb005Cs005MA015FA075Pb(I083Br017)3









fashion












18













19

CHAPTER 2















two-step methods










20





















21



reaction


221 Spin coating















higher






22
































23

























23 Experimental









24

































25





















performed











26






















as discussed above










27


procedure as above





next sections
























28












parameters























29


























30


































31










119844119844 =119812119812119835119835119812119812119842119842

=

⎩⎨

⎧119820119820120783120783119836119836119836119836119836119836119836119836+ 119820119820120784120784120784120784 minus119820119820120783120783

120784120784119826119826119842119842119826119826120784120784119836119836

119820119820120783120783 +119820119820120784120784⎭⎬

⎫120784120784

120784120784120784120784














32






specific elements






titanium












[159]




33






















119833119833(120538120538) = 119829119829119816119816



34






R α AL

rArr R = ρ AL 25
















35

















120520120520 = 119823119823119823119823119823119823119823119823119823119823119842119842119826119826

= 119817119817119836119836119836119836119829119829119836119836119836119836 119813119813119813119813119823119823119842119842119826119826

















36



















119920119920119920119920119920119920119920119920(120640120640) =119921119921119956119956119956119956(120640120640)119942119942oslash(120640120640) =

119921119921119956119956119956119956(120640120640)119942119942119920119920119946119946119946119946(120640120640)

119945119945119956119956120640120640

= 120783120783120784120784120783120783120782120782119921119921119956119956119956119956(120640120640)120640120640119920119920119946119946119946119946(120640120640) 120784120784120790120790




37



CHAPTER 3























38




































39





















40










Thickness (nm) 154 160 148 157 160











41




119915119915 =120782120782 120791120791120783120783120640120640119913119913119920119920119913119913119956119956Ѳ 120785120785120783120783

120634120634 =119913119913119920119920119913119913119956119956Ѳ120783120783 120785120785 120784120784

120633120633 =120783120783120783120783120634120634119938119938119915119915 120785120785 120785120785











119879119879) where d is








42












ZnSe [174]



43
















As-made 244

350 248

400 250

450 252

500 253


(Å)

a

( Å)

D

(nm)

ε

times10-4

δtimes1015

(lines m-2)

ZnSeITO As-made 3084 288 1001 20 17 124


44















sample in vacuum


ZnSeITO Air (450degC) 3088 288 1001 23 16 102

45







35



(103Ω)

Sheet resistance

(103Ω)


As prepared 290



46

































47
















48























49




[178]










50

























51




















52



















values [186]

53




NiO (150oC) 385 213

NiO (400oC) 387 213

NiO (500oC) 391 210




















54

















55











119920119920 = 119920119920120782120782119942119942minus120630120630120630120630 120785120785120783120783



120630120630 =119949119949119946119946(120783120783 119931119931 )

120630120630 120785120785120783120783


120572120572

=2303 times 119860119860




119946119946120784120784 = 120783120783+ (119912119912

119920119920119944119944 +119913119913)120784120784 120785120785120789120789











56















57



119962119962 = 120782120782120782120782119949119949 414 205 437

119962119962 = 120783120783120782120782119913119913119949119949 382 212 462

119962119962 = 120788120788120782120782119913119913119949119949 402 207 449

119962119962 = 120790120790120782120782119913119913119949119949 405 207 449



01M Ni(NO3)26H2O 39 21

075M Ni(NO3)26H2O 36 216

075M Ni(acet)4H2O 37 215











58







nicke l atom




1119889119889ℎ1198961198961198891198892 =

ℎ2 + 1198961198962 + 1198891198892






59


119962119962119912119912119944119944119925119925119946119946119925119925 d(111) (Å) d(200) (Å) d(220) (Å) a (Å)

y=0 mol 24125 20883 14780 41766

y=4mol 24488 21274 14884 4255

y=6mol 24506 21373 14995 4275

y=8mol 24436 21422 1501 4284



119863119863 =119896119896119899119899






[194]

120575120575

=11198631198632 (


120576120576 =120573120573








119930119930119938119938120782120782119930119930119949119949119942119942 FWHM (2θ )


y=0 mol 0488 0349 0411

y=4mol 0164 0284 0282

y=6mol 0184 0180 0204

y=8mol 0214 0215 0168


119930119930119938119938120782120782119930119930119949119949119942119942 D (111) nm D (200) nm D (220) nm

60

y=0 mol 17 25 23

y=4mol 51 30 33

y=6mol 45 47 45

y=8mol 39 40 55


119930119930119938119938120782120782119930119930119949119949119942119942 δ (111) (1014

linesm2)

δ (200) (1014

linesm2)

δ (220) (1014

linesm2)

y=0 mol 339 166 1947

y=4mol 387 111 925

y=6mol 485 445 485

y=8mol 655 636 328


119930119930119938119938120782120782119930119930119949119949119942119942 ε (111) (10-4) ε (200) (10-4) ε (220) (10-4)

y=0 mol 28 3832 293

y=4mol 216 3171 2032

y=6mol 2425 2027 1484

y=8mol 2811 2430 1221
















61









119962119962 = 120782120782120782120782119913119913119949119949 408 206

119962119962 = 120783120783120782120782119913119913119949119949 397 208

119962119962 = 120788120788120782120782119913119913119949119949 401 208

119962119962 = 120790120790120782120782119913119913119949119949 404 207


62




















63



64















65


119930119930119938119938120782120782119930119930119949119949119942119942 y=0mol y=4mol y=6mol 119962119962 = 120790120790120782120782119913119913119949119949

Nickel 020 023 0225 021

Silver 00001 00001 0001 001

Silicon 0140 0127 0130 013

Oxygen 0658 0636 0640 065

Thickness (nm) 110 195 183 116







glass substrates









66





results





Resistivity (Ω-cm)


119962119962 = 120782120782120782120782119913119913119949119949 2521times1014 6753 3666 2728x10-7

119962119962 = 120783120783120782120782119913119913119949119949 2601times1015 2999 1641 6095x10-7

119962119962 = 120788120788120782120782119913119913119949119949 6335times1014 285 8003 125x10-6

119962119962 = 120790120790120782120782119913119913119949119949 4813times1014 1467 8843 1131x10-6






67










119962119962119912119912119944119944119925119925119946119946119925119925


119962119962 = 120782120782120782120782119913119913119949119949 15 times 106

119962119962 = 120783120783120782120782119913119913119949119949 4 times 105

119962119962 = 120788120788120782120782119913119913119949119949 25 times 103

119962119962 = 120790120790120782120782119913119913119949119949 12 times 106









68




































69





















70

CHAPTER 4

























71






















72






(MAPbI3)















73


























74









1 samples [207]













75











b

a

76








(MA)1-xKxPbI3 x=0 x=01 x=02 x=03 x=04 x=1

Rs (Ω) 35x107 15x107 25x106 15x106 53x106 15x107















77



































78





79






to x=04





x=0 13733 61832 480099

x=01 13701 61809 48108

x=05 13754 61858 48104

x=1 13684 61817 48133

80









075 1) films


81


























x=0 155

x=01 153

x=02 153

x=03 152

x=04 154

x=05 159

x=075 160

x=1 26

82














83


















84











scans





















Figure 416(b)

85




86


















87






















119860119860(119905119905) = 1198601198601119897119897119890119890119890119890minus11990511990511205911205911+1198601198602119897119897119890119890119890119890

minus11990511990521205911205912 (2)




τ119886119886119886119886119886119886 = sum119860119860119904119904 τ119904119904sum 119860119860119904119904

(3)









88



recombination


xRbxPbI3(x=0 01 03 05 075 1)










89




















90




































91



































92


cation doping

















toward humidity

















93





94






















(eV)

EBI3d52

(eV)

EBRb3d32

(eV)

EBI3d52-EBPb4f72

(eV)

x=0 13779 6187 0 48091

x=01 1378 61855 11012 48075

x=03 13781 61857 1101 48076

x=05 13778 61862 11017 48079

x=075 1378 61873 11015 48084

x=1 1378 61872 11014 48092

95






















96











performance








97

























materials










98


05 075 1)

Rrms=35

Rrms=19

Rrms=21

Rrms=13

Rrms=11

Rrms=14

Rrms=28

99





100




























atmosphere








101






























102




103








(MA)01Cs09PbI3 3385 4434 232 171 1720 058














in the material












104



105






























(MA)1-

(x+y)RbxCsyPbI3

X=0

y=0

X=005

y=005

X=005

y=01

X=01

y=005

X=015

y=01

X=01

y=015

Band gap (eV) 1566 1570 1580 1580 1581 1584

106



































107


studies

CHAPTER 5






























108





























109














110











wavelengt h value

Perovskite Scan

direction

Jsc

(mAcm2)

Voc

(V)

FF

PCE Rsh

(kΩ)

Rs

(kΩ)

MAPbI3 Reverse 1870 086 5401 851 093 13

Forward 1650 078 5761 726 454 14

(MA)09K01PbI3 Reverse 1815 106 6904 1332 3979 9

Forward 1763 106 6601 1237 3471 11

(MA)09Rb01PbI3 Reverse 1591 0834 58151 7567 454 22

Forward 13314 0822 59873 6426 079 25




119919119919119919119919 =119920119920119920119920119920119920119929119929119942119942119929119929119942119942119955119955119956119956119942119942 minus 119920119920119920119920119920119920119919119919119913119913119955119955119917119917119938119938119955119955120630120630

119920119920119920119920119920119920119929119929119942119942119929119929119942119942119955119955119956119956119942119942 120783120783120783120783








111






















55(b)



119912119912(119957119957) = 119912119912120783120783119942119942119942119942119930119930minus119957119957120783120783120649120649120783120783+119912119912120784120784119942119942119942119942119930119930

minus119957119957120784120784120649120649120784120784 51

112






120533120533119938119938119929119929119944119944 =sum119912119912119946119946120533120533119946119946sum119912119912119946119946

120783120783120785120785












113






MAPbI3 5678 050 091 185 05

(MA)09K01PbI3 105 370 030 3117 978










114



















respectively

115















x= 0 79 15392

x= 01 3 1076

x= 02 4 1242

x= 03 276 79607

x= 04 17 2391

x= 05 6 744

116

x= 075 25 6338

x= 1 10 78

































117




119921119921 = 119921119921s119942119942119954119954119954119954119946119946119951119951119931119931 minus 120783120783 52




119946119946 = 119951119951119931119931119954119954

120655120655119845119845119836119836119845119845 119921119921120655120655119954119954

minus120783120783






















118

119

-02 00 02 04 06 08 10 12

10-4

10-3

10-2

10-1

100

101

102

J(m

Ac

m2 )

V (V)

x=0 x=01 x=02 x=03 x=04 x=05 x=075 x=1

J-V Dark

a

001 01 11E-8

1E-7

1E-6

1E-5

1E-4

1E-3

001

x=0 x=01 x=02 x=03 x=04 x=05 x=075 x=1

log(

J)

Log (V)

Region IRegion II

Region III

b

08 10 12

2

4

x=0 x=01 x=02 x=03 x=04 x=05D

iffer

entia

l Ide

ality

Fac

tor

V (V)

c



120



































121


our devices





Figure 511

Sample Scan

direction Jsc

(mAcm2) Voc (V)

FF

PCE

Rsh(kΩ) Rs(Ω)

MAPbI3 Reverse 1870

08

6 5401 851 454 17

Forward 1650

07

8 5761 726 193 20

(MA)080Rb0


07

9 6298 769 241 23

Forward 1125

08

0 7304 642 143 6

(MA)075Rb0 Reverse 689 08 5493 338 068 31

122














15Cs01PbI3 9

Forward 688

08

9 5309 323 042 33

123

400 500 600 700 8000

20

40

60

80

Wave length (nm)

EQE

()














124













125





















6 References



MAPbI3 5678 05 091 185 052

MA08Rb01Cs01PbI3 094 41 051 1442 745

MA075Rb015Cs01PbI3 1501 065 1502 065 065

126


127


128


129


130


131


132


133






134





















desired properties















135




















cells















136


observed


































137




































138



















sample














139










Renewable Energy

Solar Cells

p-n Junction




Tandem Solar Cells









Inorganic cations

Alternative anions




Preparation Methods

One-step Methods

Two-step Methods


Spin coating




Experimental

Chemicals details



































Summary








Summary



Summary

References



Further work

v

vi

vii

DEDICATED

TO

MY LOVING

FATHER

AND

LATE SISTER

(HAJRA SALEEM)

viii
























ABIDA SALEEM

2019

ix



































x

















xi





Physics A 125 (2019) 229




11




Materials 47 (2018) 3749














Magnetism 31(4) (2018) 1013








xii







1 Dr Asif Khan






Emailasifengrscedu






xiii













xiv






34 Summary 67



42 Summary 105



52 Summary 125

6 References 125



72 Further work 138

xv


xvi


xRbxPbI3(x=0 01 03 05 075 1) 88

xvii



xviii


xix






FF Fill factor

PV Photovoltaics











1

CHAPTER 1

























cells




2





121 p-n Junction



















device










3



































4






















[10]





5



































6









solar cells





















7

































with graphene [58]



8

















app lications


















9




































10



octahedra [83]

















a) ABX3 b) MAPbI3

11



































12



































13

t= 119955119955119912119912+119955119955119935119935radic120784120784(119955119955119913119913+119955119955119935119935)













Lithium Li+ 76













14


































15



































16
















(RbPbI3) [130]



















17













Rb005Cs005MA015FA075Pb(I083Br017)3









fashion












18













19

CHAPTER 2















two-step methods










20





















21



reaction


221 Spin coating















higher






22
































23

























23 Experimental









24

































25





















performed











26






















as discussed above










27


procedure as above





next sections
























28












parameters























29


























30


































31










119844119844 =119812119812119835119835119812119812119842119842

=

⎩⎨

⎧119820119820120783120783119836119836119836119836119836119836119836119836+ 119820119820120784120784120784120784 minus119820119820120783120783

120784120784119826119826119842119842119826119826120784120784119836119836

119820119820120783120783 +119820119820120784120784⎭⎬

⎫120784120784

120784120784120784120784














32






specific elements






titanium












[159]




33






















119833119833(120538120538) = 119829119829119816119816



34






R α AL

rArr R = ρ AL 25
















35

















120520120520 = 119823119823119823119823119823119823119823119823119823119823119842119842119826119826

= 119817119817119836119836119836119836119829119829119836119836119836119836 119813119813119813119813119823119823119842119842119826119826

















36



















119920119920119920119920119920119920119920119920(120640120640) =119921119921119956119956119956119956(120640120640)119942119942oslash(120640120640) =

119921119921119956119956119956119956(120640120640)119942119942119920119920119946119946119946119946(120640120640)

119945119945119956119956120640120640

= 120783120783120784120784120783120783120782120782119921119921119956119956119956119956(120640120640)120640120640119920119920119946119946119946119946(120640120640) 120784120784120790120790




37



CHAPTER 3























38




































39





















40










Thickness (nm) 154 160 148 157 160











41




119915119915 =120782120782 120791120791120783120783120640120640119913119913119920119920119913119913119956119956Ѳ 120785120785120783120783

120634120634 =119913119913119920119920119913119913119956119956Ѳ120783120783 120785120785 120784120784

120633120633 =120783120783120783120783120634120634119938119938119915119915 120785120785 120785120785











119879119879) where d is








42












ZnSe [174]



43
















As-made 244

350 248

400 250

450 252

500 253


(Å)

a

( Å)

D

(nm)

ε

times10-4

δtimes1015

(lines m-2)

ZnSeITO As-made 3084 288 1001 20 17 124


44















sample in vacuum


ZnSeITO Air (450degC) 3088 288 1001 23 16 102

45







35



(103Ω)

Sheet resistance

(103Ω)


As prepared 290



46

































47
















48























49




[178]










50

























51




















52



















values [186]

53




NiO (150oC) 385 213

NiO (400oC) 387 213

NiO (500oC) 391 210




















54

















55











119920119920 = 119920119920120782120782119942119942minus120630120630120630120630 120785120785120783120783



120630120630 =119949119949119946119946(120783120783 119931119931 )

120630120630 120785120785120783120783


120572120572

=2303 times 119860119860




119946119946120784120784 = 120783120783+ (119912119912

119920119920119944119944 +119913119913)120784120784 120785120785120789120789











56















57



119962119962 = 120782120782120782120782119949119949 414 205 437

119962119962 = 120783120783120782120782119913119913119949119949 382 212 462

119962119962 = 120788120788120782120782119913119913119949119949 402 207 449

119962119962 = 120790120790120782120782119913119913119949119949 405 207 449



01M Ni(NO3)26H2O 39 21

075M Ni(NO3)26H2O 36 216

075M Ni(acet)4H2O 37 215











58







nicke l atom




1119889119889ℎ1198961198961198891198892 =

ℎ2 + 1198961198962 + 1198891198892






59


119962119962119912119912119944119944119925119925119946119946119925119925 d(111) (Å) d(200) (Å) d(220) (Å) a (Å)

y=0 mol 24125 20883 14780 41766

y=4mol 24488 21274 14884 4255

y=6mol 24506 21373 14995 4275

y=8mol 24436 21422 1501 4284



119863119863 =119896119896119899119899






[194]

120575120575

=11198631198632 (


120576120576 =120573120573








119930119930119938119938120782120782119930119930119949119949119942119942 FWHM (2θ )


y=0 mol 0488 0349 0411

y=4mol 0164 0284 0282

y=6mol 0184 0180 0204

y=8mol 0214 0215 0168


119930119930119938119938120782120782119930119930119949119949119942119942 D (111) nm D (200) nm D (220) nm

60

y=0 mol 17 25 23

y=4mol 51 30 33

y=6mol 45 47 45

y=8mol 39 40 55


119930119930119938119938120782120782119930119930119949119949119942119942 δ (111) (1014

linesm2)

δ (200) (1014

linesm2)

δ (220) (1014

linesm2)

y=0 mol 339 166 1947

y=4mol 387 111 925

y=6mol 485 445 485

y=8mol 655 636 328


119930119930119938119938120782120782119930119930119949119949119942119942 ε (111) (10-4) ε (200) (10-4) ε (220) (10-4)

y=0 mol 28 3832 293

y=4mol 216 3171 2032

y=6mol 2425 2027 1484

y=8mol 2811 2430 1221
















61









119962119962 = 120782120782120782120782119913119913119949119949 408 206

119962119962 = 120783120783120782120782119913119913119949119949 397 208

119962119962 = 120788120788120782120782119913119913119949119949 401 208

119962119962 = 120790120790120782120782119913119913119949119949 404 207


62




















63



64















65


119930119930119938119938120782120782119930119930119949119949119942119942 y=0mol y=4mol y=6mol 119962119962 = 120790120790120782120782119913119913119949119949

Nickel 020 023 0225 021

Silver 00001 00001 0001 001

Silicon 0140 0127 0130 013

Oxygen 0658 0636 0640 065

Thickness (nm) 110 195 183 116







glass substrates









66





results





Resistivity (Ω-cm)


119962119962 = 120782120782120782120782119913119913119949119949 2521times1014 6753 3666 2728x10-7

119962119962 = 120783120783120782120782119913119913119949119949 2601times1015 2999 1641 6095x10-7

119962119962 = 120788120788120782120782119913119913119949119949 6335times1014 285 8003 125x10-6

119962119962 = 120790120790120782120782119913119913119949119949 4813times1014 1467 8843 1131x10-6






67










119962119962119912119912119944119944119925119925119946119946119925119925


119962119962 = 120782120782120782120782119913119913119949119949 15 times 106

119962119962 = 120783120783120782120782119913119913119949119949 4 times 105

119962119962 = 120788120788120782120782119913119913119949119949 25 times 103

119962119962 = 120790120790120782120782119913119913119949119949 12 times 106









68




































69





















70

CHAPTER 4

























71






















72






(MAPbI3)















73


























74









1 samples [207]













75











b

a

76








(MA)1-xKxPbI3 x=0 x=01 x=02 x=03 x=04 x=1

Rs (Ω) 35x107 15x107 25x106 15x106 53x106 15x107















77



































78





79






to x=04





x=0 13733 61832 480099

x=01 13701 61809 48108

x=05 13754 61858 48104

x=1 13684 61817 48133

80









075 1) films


81


























x=0 155

x=01 153

x=02 153

x=03 152

x=04 154

x=05 159

x=075 160

x=1 26

82














83


















84











scans





















Figure 416(b)

85




86


















87






















119860119860(119905119905) = 1198601198601119897119897119890119890119890119890minus11990511990511205911205911+1198601198602119897119897119890119890119890119890

minus11990511990521205911205912 (2)




τ119886119886119886119886119886119886 = sum119860119860119904119904 τ119904119904sum 119860119860119904119904

(3)









88



recombination


xRbxPbI3(x=0 01 03 05 075 1)










89




















90




































91



































92


cation doping

















toward humidity

















93





94






















(eV)

EBI3d52

(eV)

EBRb3d32

(eV)

EBI3d52-EBPb4f72

(eV)

x=0 13779 6187 0 48091

x=01 1378 61855 11012 48075

x=03 13781 61857 1101 48076

x=05 13778 61862 11017 48079

x=075 1378 61873 11015 48084

x=1 1378 61872 11014 48092

95






















96











performance








97

























materials










98


05 075 1)

Rrms=35

Rrms=19

Rrms=21

Rrms=13

Rrms=11

Rrms=14

Rrms=28

99





100




























atmosphere








101






























102




103








(MA)01Cs09PbI3 3385 4434 232 171 1720 058














in the material












104



105






























(MA)1-

(x+y)RbxCsyPbI3

X=0

y=0

X=005

y=005

X=005

y=01

X=01

y=005

X=015

y=01

X=01

y=015

Band gap (eV) 1566 1570 1580 1580 1581 1584

106



































107


studies

CHAPTER 5






























108





























109














110











wavelengt h value

Perovskite Scan

direction

Jsc

(mAcm2)

Voc

(V)

FF

PCE Rsh

(kΩ)

Rs

(kΩ)

MAPbI3 Reverse 1870 086 5401 851 093 13

Forward 1650 078 5761 726 454 14

(MA)09K01PbI3 Reverse 1815 106 6904 1332 3979 9

Forward 1763 106 6601 1237 3471 11

(MA)09Rb01PbI3 Reverse 1591 0834 58151 7567 454 22

Forward 13314 0822 59873 6426 079 25




119919119919119919119919 =119920119920119920119920119920119920119929119929119942119942119929119929119942119942119955119955119956119956119942119942 minus 119920119920119920119920119920119920119919119919119913119913119955119955119917119917119938119938119955119955120630120630

119920119920119920119920119920119920119929119929119942119942119929119929119942119942119955119955119956119956119942119942 120783120783120783120783








111






















55(b)



119912119912(119957119957) = 119912119912120783120783119942119942119942119942119930119930minus119957119957120783120783120649120649120783120783+119912119912120784120784119942119942119942119942119930119930

minus119957119957120784120784120649120649120784120784 51

112






120533120533119938119938119929119929119944119944 =sum119912119912119946119946120533120533119946119946sum119912119912119946119946

120783120783120785120785












113






MAPbI3 5678 050 091 185 05

(MA)09K01PbI3 105 370 030 3117 978










114



















respectively

115















x= 0 79 15392

x= 01 3 1076

x= 02 4 1242

x= 03 276 79607

x= 04 17 2391

x= 05 6 744

116

x= 075 25 6338

x= 1 10 78

































117




119921119921 = 119921119921s119942119942119954119954119954119954119946119946119951119951119931119931 minus 120783120783 52




119946119946 = 119951119951119931119931119954119954

120655120655119845119845119836119836119845119845 119921119921120655120655119954119954

minus120783120783






















118

119

-02 00 02 04 06 08 10 12

10-4

10-3

10-2

10-1

100

101

102

J(m

Ac

m2 )

V (V)

x=0 x=01 x=02 x=03 x=04 x=05 x=075 x=1

J-V Dark

a

001 01 11E-8

1E-7

1E-6

1E-5

1E-4

1E-3

001

x=0 x=01 x=02 x=03 x=04 x=05 x=075 x=1

log(

J)

Log (V)

Region IRegion II

Region III

b

08 10 12

2

4

x=0 x=01 x=02 x=03 x=04 x=05D

iffer

entia

l Ide

ality

Fac

tor

V (V)

c



120



































121


our devices





Figure 511

Sample Scan

direction Jsc

(mAcm2) Voc (V)

FF

PCE

Rsh(kΩ) Rs(Ω)

MAPbI3 Reverse 1870

08

6 5401 851 454 17

Forward 1650

07

8 5761 726 193 20

(MA)080Rb0


07

9 6298 769 241 23

Forward 1125

08

0 7304 642 143 6

(MA)075Rb0 Reverse 689 08 5493 338 068 31

122














15Cs01PbI3 9

Forward 688

08

9 5309 323 042 33

123

400 500 600 700 8000

20

40

60

80

Wave length (nm)

EQE

()














124













125





















6 References



MAPbI3 5678 05 091 185 052

MA08Rb01Cs01PbI3 094 41 051 1442 745

MA075Rb015Cs01PbI3 1501 065 1502 065 065

126


127


128


129


130


131


132


133






134





















desired properties















135




















cells















136


observed


































137




































138



















sample














139










Renewable Energy

Solar Cells

p-n Junction




Tandem Solar Cells









Inorganic cations

Alternative anions




Preparation Methods

One-step Methods

Two-step Methods


Spin coating




Experimental

Chemicals details



































Summary








Summary



Summary

References



Further work

vi

vii

DEDICATED

TO

MY LOVING

FATHER

AND

LATE SISTER

(HAJRA SALEEM)

viii
























ABIDA SALEEM

2019

ix



































x

















xi





Physics A 125 (2019) 229




11




Materials 47 (2018) 3749














Magnetism 31(4) (2018) 1013








xii







1 Dr Asif Khan






Emailasifengrscedu






xiii













xiv






34 Summary 67



42 Summary 105



52 Summary 125

6 References 125



72 Further work 138

xv


xvi


xRbxPbI3(x=0 01 03 05 075 1) 88

xvii



xviii


xix






FF Fill factor

PV Photovoltaics











1

CHAPTER 1

























cells




2





121 p-n Junction



















device










3



































4






















[10]





5



































6









solar cells





















7

































with graphene [58]



8

















app lications


















9




































10



octahedra [83]

















a) ABX3 b) MAPbI3

11



































12



































13

t= 119955119955119912119912+119955119955119935119935radic120784120784(119955119955119913119913+119955119955119935119935)













Lithium Li+ 76













14


































15



































16
















(RbPbI3) [130]



















17













Rb005Cs005MA015FA075Pb(I083Br017)3









fashion












18













19

CHAPTER 2















two-step methods










20





















21



reaction


221 Spin coating















higher






22
































23

























23 Experimental









24

































25





















performed











26






















as discussed above










27


procedure as above





next sections
























28












parameters























29


























30


































31










119844119844 =119812119812119835119835119812119812119842119842

=

⎩⎨

⎧119820119820120783120783119836119836119836119836119836119836119836119836+ 119820119820120784120784120784120784 minus119820119820120783120783

120784120784119826119826119842119842119826119826120784120784119836119836

119820119820120783120783 +119820119820120784120784⎭⎬

⎫120784120784

120784120784120784120784














32






specific elements






titanium












[159]




33






















119833119833(120538120538) = 119829119829119816119816



34






R α AL

rArr R = ρ AL 25
















35

















120520120520 = 119823119823119823119823119823119823119823119823119823119823119842119842119826119826

= 119817119817119836119836119836119836119829119829119836119836119836119836 119813119813119813119813119823119823119842119842119826119826

















36



















119920119920119920119920119920119920119920119920(120640120640) =119921119921119956119956119956119956(120640120640)119942119942oslash(120640120640) =

119921119921119956119956119956119956(120640120640)119942119942119920119920119946119946119946119946(120640120640)

119945119945119956119956120640120640

= 120783120783120784120784120783120783120782120782119921119921119956119956119956119956(120640120640)120640120640119920119920119946119946119946119946(120640120640) 120784120784120790120790




37



CHAPTER 3























38




































39





















40










Thickness (nm) 154 160 148 157 160











41




119915119915 =120782120782 120791120791120783120783120640120640119913119913119920119920119913119913119956119956Ѳ 120785120785120783120783

120634120634 =119913119913119920119920119913119913119956119956Ѳ120783120783 120785120785 120784120784

120633120633 =120783120783120783120783120634120634119938119938119915119915 120785120785 120785120785











119879119879) where d is








42












ZnSe [174]



43
















As-made 244

350 248

400 250

450 252

500 253


(Å)

a

( Å)

D

(nm)

ε

times10-4

δtimes1015

(lines m-2)

ZnSeITO As-made 3084 288 1001 20 17 124


44















sample in vacuum


ZnSeITO Air (450degC) 3088 288 1001 23 16 102

45







35



(103Ω)

Sheet resistance

(103Ω)


As prepared 290



46

































47
















48























49




[178]










50

























51




















52



















values [186]

53




NiO (150oC) 385 213

NiO (400oC) 387 213

NiO (500oC) 391 210




















54

















55











119920119920 = 119920119920120782120782119942119942minus120630120630120630120630 120785120785120783120783



120630120630 =119949119949119946119946(120783120783 119931119931 )

120630120630 120785120785120783120783


120572120572

=2303 times 119860119860




119946119946120784120784 = 120783120783+ (119912119912

119920119920119944119944 +119913119913)120784120784 120785120785120789120789











56















57



119962119962 = 120782120782120782120782119949119949 414 205 437

119962119962 = 120783120783120782120782119913119913119949119949 382 212 462

119962119962 = 120788120788120782120782119913119913119949119949 402 207 449

119962119962 = 120790120790120782120782119913119913119949119949 405 207 449



01M Ni(NO3)26H2O 39 21

075M Ni(NO3)26H2O 36 216

075M Ni(acet)4H2O 37 215











58







nicke l atom




1119889119889ℎ1198961198961198891198892 =

ℎ2 + 1198961198962 + 1198891198892






59


119962119962119912119912119944119944119925119925119946119946119925119925 d(111) (Å) d(200) (Å) d(220) (Å) a (Å)

y=0 mol 24125 20883 14780 41766

y=4mol 24488 21274 14884 4255

y=6mol 24506 21373 14995 4275

y=8mol 24436 21422 1501 4284



119863119863 =119896119896119899119899






[194]

120575120575

=11198631198632 (


120576120576 =120573120573








119930119930119938119938120782120782119930119930119949119949119942119942 FWHM (2θ )


y=0 mol 0488 0349 0411

y=4mol 0164 0284 0282

y=6mol 0184 0180 0204

y=8mol 0214 0215 0168


119930119930119938119938120782120782119930119930119949119949119942119942 D (111) nm D (200) nm D (220) nm

60

y=0 mol 17 25 23

y=4mol 51 30 33

y=6mol 45 47 45

y=8mol 39 40 55


119930119930119938119938120782120782119930119930119949119949119942119942 δ (111) (1014

linesm2)

δ (200) (1014

linesm2)

δ (220) (1014

linesm2)

y=0 mol 339 166 1947

y=4mol 387 111 925

y=6mol 485 445 485

y=8mol 655 636 328


119930119930119938119938120782120782119930119930119949119949119942119942 ε (111) (10-4) ε (200) (10-4) ε (220) (10-4)

y=0 mol 28 3832 293

y=4mol 216 3171 2032

y=6mol 2425 2027 1484

y=8mol 2811 2430 1221
















61









119962119962 = 120782120782120782120782119913119913119949119949 408 206

119962119962 = 120783120783120782120782119913119913119949119949 397 208

119962119962 = 120788120788120782120782119913119913119949119949 401 208

119962119962 = 120790120790120782120782119913119913119949119949 404 207


62




















63



64















65


119930119930119938119938120782120782119930119930119949119949119942119942 y=0mol y=4mol y=6mol 119962119962 = 120790120790120782120782119913119913119949119949

Nickel 020 023 0225 021

Silver 00001 00001 0001 001

Silicon 0140 0127 0130 013

Oxygen 0658 0636 0640 065

Thickness (nm) 110 195 183 116







glass substrates









66





results





Resistivity (Ω-cm)


119962119962 = 120782120782120782120782119913119913119949119949 2521times1014 6753 3666 2728x10-7

119962119962 = 120783120783120782120782119913119913119949119949 2601times1015 2999 1641 6095x10-7

119962119962 = 120788120788120782120782119913119913119949119949 6335times1014 285 8003 125x10-6

119962119962 = 120790120790120782120782119913119913119949119949 4813times1014 1467 8843 1131x10-6






67










119962119962119912119912119944119944119925119925119946119946119925119925


119962119962 = 120782120782120782120782119913119913119949119949 15 times 106

119962119962 = 120783120783120782120782119913119913119949119949 4 times 105

119962119962 = 120788120788120782120782119913119913119949119949 25 times 103

119962119962 = 120790120790120782120782119913119913119949119949 12 times 106









68




































69





















70

CHAPTER 4

























71






















72






(MAPbI3)















73


























74









1 samples [207]













75











b

a

76








(MA)1-xKxPbI3 x=0 x=01 x=02 x=03 x=04 x=1

Rs (Ω) 35x107 15x107 25x106 15x106 53x106 15x107















77



































78





79






to x=04





x=0 13733 61832 480099

x=01 13701 61809 48108

x=05 13754 61858 48104

x=1 13684 61817 48133

80









075 1) films


81


























x=0 155

x=01 153

x=02 153

x=03 152

x=04 154

x=05 159

x=075 160

x=1 26

82














83


















84











scans





















Figure 416(b)

85




86


















87






















119860119860(119905119905) = 1198601198601119897119897119890119890119890119890minus11990511990511205911205911+1198601198602119897119897119890119890119890119890

minus11990511990521205911205912 (2)




τ119886119886119886119886119886119886 = sum119860119860119904119904 τ119904119904sum 119860119860119904119904

(3)









88



recombination


xRbxPbI3(x=0 01 03 05 075 1)










89




















90




































91



































92


cation doping

















toward humidity

















93





94






















(eV)

EBI3d52

(eV)

EBRb3d32

(eV)

EBI3d52-EBPb4f72

(eV)

x=0 13779 6187 0 48091

x=01 1378 61855 11012 48075

x=03 13781 61857 1101 48076

x=05 13778 61862 11017 48079

x=075 1378 61873 11015 48084

x=1 1378 61872 11014 48092

95






















96











performance








97

























materials










98


05 075 1)

Rrms=35

Rrms=19

Rrms=21

Rrms=13

Rrms=11

Rrms=14

Rrms=28

99





100




























atmosphere








101






























102




103








(MA)01Cs09PbI3 3385 4434 232 171 1720 058














in the material












104



105






























(MA)1-

(x+y)RbxCsyPbI3

X=0

y=0

X=005

y=005

X=005

y=01

X=01

y=005

X=015

y=01

X=01

y=015

Band gap (eV) 1566 1570 1580 1580 1581 1584

106



































107


studies

CHAPTER 5






























108





























109














110











wavelengt h value

Perovskite Scan

direction

Jsc

(mAcm2)

Voc

(V)

FF

PCE Rsh

(kΩ)

Rs

(kΩ)

MAPbI3 Reverse 1870 086 5401 851 093 13

Forward 1650 078 5761 726 454 14

(MA)09K01PbI3 Reverse 1815 106 6904 1332 3979 9

Forward 1763 106 6601 1237 3471 11

(MA)09Rb01PbI3 Reverse 1591 0834 58151 7567 454 22

Forward 13314 0822 59873 6426 079 25




119919119919119919119919 =119920119920119920119920119920119920119929119929119942119942119929119929119942119942119955119955119956119956119942119942 minus 119920119920119920119920119920119920119919119919119913119913119955119955119917119917119938119938119955119955120630120630

119920119920119920119920119920119920119929119929119942119942119929119929119942119942119955119955119956119956119942119942 120783120783120783120783








111






















55(b)



119912119912(119957119957) = 119912119912120783120783119942119942119942119942119930119930minus119957119957120783120783120649120649120783120783+119912119912120784120784119942119942119942119942119930119930

minus119957119957120784120784120649120649120784120784 51

112






120533120533119938119938119929119929119944119944 =sum119912119912119946119946120533120533119946119946sum119912119912119946119946

120783120783120785120785












113






MAPbI3 5678 050 091 185 05

(MA)09K01PbI3 105 370 030 3117 978










114



















respectively

115















x= 0 79 15392

x= 01 3 1076

x= 02 4 1242

x= 03 276 79607

x= 04 17 2391

x= 05 6 744

116

x= 075 25 6338

x= 1 10 78

































117




119921119921 = 119921119921s119942119942119954119954119954119954119946119946119951119951119931119931 minus 120783120783 52




119946119946 = 119951119951119931119931119954119954

120655120655119845119845119836119836119845119845 119921119921120655120655119954119954

minus120783120783






















118

119

-02 00 02 04 06 08 10 12

10-4

10-3

10-2

10-1

100

101

102

J(m

Ac

m2 )

V (V)

x=0 x=01 x=02 x=03 x=04 x=05 x=075 x=1

J-V Dark

a

001 01 11E-8

1E-7

1E-6

1E-5

1E-4

1E-3

001

x=0 x=01 x=02 x=03 x=04 x=05 x=075 x=1

log(

J)

Log (V)

Region IRegion II

Region III

b

08 10 12

2

4

x=0 x=01 x=02 x=03 x=04 x=05D

iffer

entia

l Ide

ality

Fac

tor

V (V)

c



120



































121


our devices





Figure 511

Sample Scan

direction Jsc

(mAcm2) Voc (V)

FF

PCE

Rsh(kΩ) Rs(Ω)

MAPbI3 Reverse 1870

08

6 5401 851 454 17

Forward 1650

07

8 5761 726 193 20

(MA)080Rb0


07

9 6298 769 241 23

Forward 1125

08

0 7304 642 143 6

(MA)075Rb0 Reverse 689 08 5493 338 068 31

122














15Cs01PbI3 9

Forward 688

08

9 5309 323 042 33

123

400 500 600 700 8000

20

40

60

80

Wave length (nm)

EQE

()














124













125





















6 References



MAPbI3 5678 05 091 185 052

MA08Rb01Cs01PbI3 094 41 051 1442 745

MA075Rb015Cs01PbI3 1501 065 1502 065 065

126


127


128


129


130


131


132


133






134





















desired properties















135




















cells















136


observed


































137




































138



















sample














139










Renewable Energy

Solar Cells

p-n Junction




Tandem Solar Cells









Inorganic cations

Alternative anions




Preparation Methods

One-step Methods

Two-step Methods


Spin coating




Experimental

Chemicals details



































Summary








Summary



Summary

References



Further work

vii

DEDICATED

TO

MY LOVING

FATHER

AND

LATE SISTER

(HAJRA SALEEM)

viii
























ABIDA SALEEM

2019

ix



































x

















xi





Physics A 125 (2019) 229




11




Materials 47 (2018) 3749














Magnetism 31(4) (2018) 1013








xii







1 Dr Asif Khan






Emailasifengrscedu






xiii













xiv






34 Summary 67



42 Summary 105



52 Summary 125

6 References 125



72 Further work 138

xv


xvi


xRbxPbI3(x=0 01 03 05 075 1) 88

xvii



xviii


xix






FF Fill factor

PV Photovoltaics











1

CHAPTER 1

























cells




2





121 p-n Junction



















device










3



































4






















[10]





5



































6









solar cells





















7

































with graphene [58]



8

















app lications


















9




































10



octahedra [83]

















a) ABX3 b) MAPbI3

11



































12



































13

t= 119955119955119912119912+119955119955119935119935radic120784120784(119955119955119913119913+119955119955119935119935)













Lithium Li+ 76













14


































15



































16
















(RbPbI3) [130]



















17













Rb005Cs005MA015FA075Pb(I083Br017)3









fashion












18













19

CHAPTER 2















two-step methods










20





















21



reaction


221 Spin coating















higher






22
































23

























23 Experimental









24

































25





















performed











26






















as discussed above










27


procedure as above





next sections
























28












parameters























29


























30


































31










119844119844 =119812119812119835119835119812119812119842119842

=

⎩⎨

⎧119820119820120783120783119836119836119836119836119836119836119836119836+ 119820119820120784120784120784120784 minus119820119820120783120783

120784120784119826119826119842119842119826119826120784120784119836119836

119820119820120783120783 +119820119820120784120784⎭⎬

⎫120784120784

120784120784120784120784














32






specific elements






titanium












[159]




33






















119833119833(120538120538) = 119829119829119816119816



34






R α AL

rArr R = ρ AL 25
















35

















120520120520 = 119823119823119823119823119823119823119823119823119823119823119842119842119826119826

= 119817119817119836119836119836119836119829119829119836119836119836119836 119813119813119813119813119823119823119842119842119826119826

















36



















119920119920119920119920119920119920119920119920(120640120640) =119921119921119956119956119956119956(120640120640)119942119942oslash(120640120640) =

119921119921119956119956119956119956(120640120640)119942119942119920119920119946119946119946119946(120640120640)

119945119945119956119956120640120640

= 120783120783120784120784120783120783120782120782119921119921119956119956119956119956(120640120640)120640120640119920119920119946119946119946119946(120640120640) 120784120784120790120790




37



CHAPTER 3























38




































39





















40










Thickness (nm) 154 160 148 157 160











41




119915119915 =120782120782 120791120791120783120783120640120640119913119913119920119920119913119913119956119956Ѳ 120785120785120783120783

120634120634 =119913119913119920119920119913119913119956119956Ѳ120783120783 120785120785 120784120784

120633120633 =120783120783120783120783120634120634119938119938119915119915 120785120785 120785120785











119879119879) where d is








42












ZnSe [174]



43
















As-made 244

350 248

400 250

450 252

500 253


(Å)

a

( Å)

D

(nm)

ε

times10-4

δtimes1015

(lines m-2)

ZnSeITO As-made 3084 288 1001 20 17 124


44















sample in vacuum


ZnSeITO Air (450degC) 3088 288 1001 23 16 102

45







35



(103Ω)

Sheet resistance

(103Ω)


As prepared 290



46

































47
















48























49




[178]










50

























51




















52



















values [186]

53




NiO (150oC) 385 213

NiO (400oC) 387 213

NiO (500oC) 391 210




















54

















55











119920119920 = 119920119920120782120782119942119942minus120630120630120630120630 120785120785120783120783



120630120630 =119949119949119946119946(120783120783 119931119931 )

120630120630 120785120785120783120783


120572120572

=2303 times 119860119860




119946119946120784120784 = 120783120783+ (119912119912

119920119920119944119944 +119913119913)120784120784 120785120785120789120789











56















57



119962119962 = 120782120782120782120782119949119949 414 205 437

119962119962 = 120783120783120782120782119913119913119949119949 382 212 462

119962119962 = 120788120788120782120782119913119913119949119949 402 207 449

119962119962 = 120790120790120782120782119913119913119949119949 405 207 449



01M Ni(NO3)26H2O 39 21

075M Ni(NO3)26H2O 36 216

075M Ni(acet)4H2O 37 215











58







nicke l atom




1119889119889ℎ1198961198961198891198892 =

ℎ2 + 1198961198962 + 1198891198892






59


119962119962119912119912119944119944119925119925119946119946119925119925 d(111) (Å) d(200) (Å) d(220) (Å) a (Å)

y=0 mol 24125 20883 14780 41766

y=4mol 24488 21274 14884 4255

y=6mol 24506 21373 14995 4275

y=8mol 24436 21422 1501 4284



119863119863 =119896119896119899119899






[194]

120575120575

=11198631198632 (


120576120576 =120573120573








119930119930119938119938120782120782119930119930119949119949119942119942 FWHM (2θ )


y=0 mol 0488 0349 0411

y=4mol 0164 0284 0282

y=6mol 0184 0180 0204

y=8mol 0214 0215 0168


119930119930119938119938120782120782119930119930119949119949119942119942 D (111) nm D (200) nm D (220) nm

60

y=0 mol 17 25 23

y=4mol 51 30 33

y=6mol 45 47 45

y=8mol 39 40 55


119930119930119938119938120782120782119930119930119949119949119942119942 δ (111) (1014

linesm2)

δ (200) (1014

linesm2)

δ (220) (1014

linesm2)

y=0 mol 339 166 1947

y=4mol 387 111 925

y=6mol 485 445 485

y=8mol 655 636 328


119930119930119938119938120782120782119930119930119949119949119942119942 ε (111) (10-4) ε (200) (10-4) ε (220) (10-4)

y=0 mol 28 3832 293

y=4mol 216 3171 2032

y=6mol 2425 2027 1484

y=8mol 2811 2430 1221
















61









119962119962 = 120782120782120782120782119913119913119949119949 408 206

119962119962 = 120783120783120782120782119913119913119949119949 397 208

119962119962 = 120788120788120782120782119913119913119949119949 401 208

119962119962 = 120790120790120782120782119913119913119949119949 404 207


62




















63



64















65


119930119930119938119938120782120782119930119930119949119949119942119942 y=0mol y=4mol y=6mol 119962119962 = 120790120790120782120782119913119913119949119949

Nickel 020 023 0225 021

Silver 00001 00001 0001 001

Silicon 0140 0127 0130 013

Oxygen 0658 0636 0640 065

Thickness (nm) 110 195 183 116







glass substrates









66





results





Resistivity (Ω-cm)


119962119962 = 120782120782120782120782119913119913119949119949 2521times1014 6753 3666 2728x10-7

119962119962 = 120783120783120782120782119913119913119949119949 2601times1015 2999 1641 6095x10-7

119962119962 = 120788120788120782120782119913119913119949119949 6335times1014 285 8003 125x10-6

119962119962 = 120790120790120782120782119913119913119949119949 4813times1014 1467 8843 1131x10-6






67










119962119962119912119912119944119944119925119925119946119946119925119925


119962119962 = 120782120782120782120782119913119913119949119949 15 times 106

119962119962 = 120783120783120782120782119913119913119949119949 4 times 105

119962119962 = 120788120788120782120782119913119913119949119949 25 times 103

119962119962 = 120790120790120782120782119913119913119949119949 12 times 106









68




































69





















70

CHAPTER 4

























71






















72






(MAPbI3)















73


























74









1 samples [207]













75











b

a

76








(MA)1-xKxPbI3 x=0 x=01 x=02 x=03 x=04 x=1

Rs (Ω) 35x107 15x107 25x106 15x106 53x106 15x107















77



































78





79






to x=04





x=0 13733 61832 480099

x=01 13701 61809 48108

x=05 13754 61858 48104

x=1 13684 61817 48133

80









075 1) films


81


























x=0 155

x=01 153

x=02 153

x=03 152

x=04 154

x=05 159

x=075 160

x=1 26

82














83


















84











scans





















Figure 416(b)

85




86


















87






















119860119860(119905119905) = 1198601198601119897119897119890119890119890119890minus11990511990511205911205911+1198601198602119897119897119890119890119890119890

minus11990511990521205911205912 (2)




τ119886119886119886119886119886119886 = sum119860119860119904119904 τ119904119904sum 119860119860119904119904

(3)









88



recombination


xRbxPbI3(x=0 01 03 05 075 1)










89




















90




































91



































92


cation doping

















toward humidity

















93





94






















(eV)

EBI3d52

(eV)

EBRb3d32

(eV)

EBI3d52-EBPb4f72

(eV)

x=0 13779 6187 0 48091

x=01 1378 61855 11012 48075

x=03 13781 61857 1101 48076

x=05 13778 61862 11017 48079

x=075 1378 61873 11015 48084

x=1 1378 61872 11014 48092

95






















96











performance








97

























materials










98


05 075 1)

Rrms=35

Rrms=19

Rrms=21

Rrms=13

Rrms=11

Rrms=14

Rrms=28

99





100




























atmosphere








101






























102




103








(MA)01Cs09PbI3 3385 4434 232 171 1720 058














in the material












104



105






























(MA)1-

(x+y)RbxCsyPbI3

X=0

y=0

X=005

y=005

X=005

y=01

X=01

y=005

X=015

y=01

X=01

y=015

Band gap (eV) 1566 1570 1580 1580 1581 1584

106



































107


studies

CHAPTER 5






























108





























109














110











wavelengt h value

Perovskite Scan

direction

Jsc

(mAcm2)

Voc

(V)

FF

PCE Rsh

(kΩ)

Rs

(kΩ)

MAPbI3 Reverse 1870 086 5401 851 093 13

Forward 1650 078 5761 726 454 14

(MA)09K01PbI3 Reverse 1815 106 6904 1332 3979 9

Forward 1763 106 6601 1237 3471 11

(MA)09Rb01PbI3 Reverse 1591 0834 58151 7567 454 22

Forward 13314 0822 59873 6426 079 25




119919119919119919119919 =119920119920119920119920119920119920119929119929119942119942119929119929119942119942119955119955119956119956119942119942 minus 119920119920119920119920119920119920119919119919119913119913119955119955119917119917119938119938119955119955120630120630

119920119920119920119920119920119920119929119929119942119942119929119929119942119942119955119955119956119956119942119942 120783120783120783120783








111






















55(b)



119912119912(119957119957) = 119912119912120783120783119942119942119942119942119930119930minus119957119957120783120783120649120649120783120783+119912119912120784120784119942119942119942119942119930119930

minus119957119957120784120784120649120649120784120784 51

112






120533120533119938119938119929119929119944119944 =sum119912119912119946119946120533120533119946119946sum119912119912119946119946

120783120783120785120785












113






MAPbI3 5678 050 091 185 05

(MA)09K01PbI3 105 370 030 3117 978










114



















respectively

115















x= 0 79 15392

x= 01 3 1076

x= 02 4 1242

x= 03 276 79607

x= 04 17 2391

x= 05 6 744

116

x= 075 25 6338

x= 1 10 78

































117




119921119921 = 119921119921s119942119942119954119954119954119954119946119946119951119951119931119931 minus 120783120783 52




119946119946 = 119951119951119931119931119954119954

120655120655119845119845119836119836119845119845 119921119921120655120655119954119954

minus120783120783






















118

119

-02 00 02 04 06 08 10 12

10-4

10-3

10-2

10-1

100

101

102

J(m

Ac

m2 )

V (V)

x=0 x=01 x=02 x=03 x=04 x=05 x=075 x=1

J-V Dark

a

001 01 11E-8

1E-7

1E-6

1E-5

1E-4

1E-3

001

x=0 x=01 x=02 x=03 x=04 x=05 x=075 x=1

log(

J)

Log (V)

Region IRegion II

Region III

b

08 10 12

2

4

x=0 x=01 x=02 x=03 x=04 x=05D

iffer

entia

l Ide

ality

Fac

tor

V (V)

c



120



































121


our devices





Figure 511

Sample Scan

direction Jsc

(mAcm2) Voc (V)

FF

PCE

Rsh(kΩ) Rs(Ω)

MAPbI3 Reverse 1870

08

6 5401 851 454 17

Forward 1650

07

8 5761 726 193 20

(MA)080Rb0


07

9 6298 769 241 23

Forward 1125

08

0 7304 642 143 6

(MA)075Rb0 Reverse 689 08 5493 338 068 31

122














15Cs01PbI3 9

Forward 688

08

9 5309 323 042 33

123

400 500 600 700 8000

20

40

60

80

Wave length (nm)

EQE

()














124













125





















6 References



MAPbI3 5678 05 091 185 052

MA08Rb01Cs01PbI3 094 41 051 1442 745

MA075Rb015Cs01PbI3 1501 065 1502 065 065

126


127


128


129


130


131


132


133






134





















desired properties















135




















cells















136


observed


































137




































138



















sample














139










Renewable Energy

Solar Cells

p-n Junction




Tandem Solar Cells









Inorganic cations

Alternative anions




Preparation Methods

One-step Methods

Two-step Methods


Spin coating




Experimental

Chemicals details



































Summary








Summary



Summary

References



Further work

Alkali metal (K, Rb, Cs) doped methylammonium lead iodide ...

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Transcript of Alkali metal (K, Rb, Cs) doped methylammonium lead iodide ...