Morphological Studies of Organometal Halide Thin Films for Perovskite Solar Cells

by

Donghan Chen

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Department of Materials Science and Engineering University of Toronto

© Copyright by Donghan Chen 2014

ii

Morphological Studies of Organometal Halide Thin Films for

Perovskite Solar Cells

Donghan Chen

Master of Applied Science

Department of Materials Science and Engineering University of Toronto

2014

Abstract

Thin film solar cells are important for making photovoltaic technologies affordable and

easily fabricated. The main objective of research into thin film solar cells is to achieve high-

energy convert efficiency with easy manufacturing methods and accessible elements. Thin film

solar cells based on organometal halide perovskites have demonstrated outstanding efficiency

among photovoltaics. The morphologies of solution-processed perovskite thin films, such as

uniformity of thickness and surface coverage, have been shown to be important factors for device

performance. Perovskite thin films were prepared with uniform thickness and full coverage by

using vapour deposition methods. The morphology features and crystal quality of the perovskite

thin films were examined to optimize the preparation conditions. Influence of the annealing

temperatures was investigated to shed light on the stability of perovskite films in post-

preparation treatment. Atomic force microscopy (AFM) was used to capture high-resolution

morphology changes during annealing treatment and film formation.

iii

Acknowledgements

First and foremost, I would like to express my gratitude to my supervisor, Dr. M. Cynthia

Goh, for her continuous guidance and patience through this journey. This thesis would not be

possible without her encouragement and consideration. Also, I would like to thank Dr. Zheng-

Hong Lu for his generous advices and help on conducting the experiments in this study. I have

been very much honored to work with Professor Goh and Professor Lu.

I am very grateful to my colleagues in Goh group who I have worked with for last two

years. Special thanks go to Dr. Richard Loo, Dr. Jane Goh, Dr. Cheng Lu and Dr. Stanley Wong

for their advices and discussions in research. And it is my honor to have Dr. Alon Eisenstein,

Nari Kim, Calvin Cheng and Dr. Zhe She as my lab mates. I also appreciate all the help provided

by Emmanuel Thibau and Robin White from Lu group. It was great pleasure to work with them.

Last but not least, I would like to thank my friends and family for their supports and love.

My parents, my brother and my dear wife have always been there for me. There are no words to

express my truly gratitude for their love and support.

iv

Table of Contents

Acknowledgements ........................................................................................................................ iii 

Table of Contents ........................................................................................................................... iv 

List of Figures ................................................................................................................................ vi 

List of Abbreviations ..................................................................................................................... ix 

Chapter 1 – Introduction ................................................................................................................. 1 

1.1  Introduction ......................................................................................................................... 1 

1.2  Perovskites for High Performance Solar Cells ................................................................... 3 

1.2.1  Crystal Structure of Perovskites .............................................................................. 5 

1.2.2  Perovskite Distortions ............................................................................................. 6 

1.3  Dye Sensitized Solar Cells (DSSCs) and Perovskite Solar Cells ....................................... 8 

1.3.1  Device Structure of DSSC ...................................................................................... 8 

1.3.2  Working Principles ................................................................................................. 9 

1.3.3  Evolution from DSSCs to Perovskite Solar Cells ................................................. 11 

1.4  Achievements in Morphological Studies of Perovskite Thin Film ................................... 14 

1.5  Motivations and Thesis Overview .................................................................................... 16 

Chapter 2 – Materials and Experimental Methods ....................................................................... 17 

2.1  Materials ........................................................................................................................... 17 

2.2  Methods for Perovskite Thin Film Preparation ................................................................ 17 

2.2.1  Solution Processed Technique .............................................................................. 17 

2.2.2  Vapour Deposition Technique .............................................................................. 18 

2.3  Characterization ................................................................................................................ 22 

2.3.1  Powder X-Ray Diffraction (XRD) ........................................................................ 22 

2.3.2  Scanning Electron Microscopy (SEM) ................................................................. 24 

2.3.3  Atomic Force Microscopy (AFM) ........................................................................ 25 

v

Chapter 3 – Results: Morphological Studies of Perovskite Thin Films ........................................ 28 

3.1  Introduction ....................................................................................................................... 28 

3.2  Experimental Details ......................................................................................................... 30 

3.3  Results and Discussion ..................................................................................................... 33 

3.3.1  Characterization of Perovskite and Related Materials .......................................... 34 

3.3.2  Annealing Effects for Perovskite Thin Films ....................................................... 36 

3.3.3  The Effects of Annealing Temperature on Perovskite Thin Films ....................... 39 

3.3.4  Degradation Study of the Perovskite Thin Films (in Air) ..................................... 49 

3.3.5  High Resolution Study of Perovskite Thin Film with AFM ................................. 53 

3.4  Summary ........................................................................................................................... 55 

Chapter 4 – Conclusions and Future Work ................................................................................... 56 

References: .................................................................................................................................... 58 

vi

List of Figures

Figure 1.1 Comparison of the efficiency of several third-generation photovoltaic technologies

[3]. ................................................................................................................................................... 2 

Figure 1.2 Illustration of ideal ABX3 cubic perovskite crystal structure. ....................................... 5 

Figure 1.3 Illustrative sketches of possible distortions in perovskite structure: (a) ideal cubic

structure of perovskite SrTiO3; (b) rotated octahedra structure of perovskite GdFeO3 (Side view);

(c) rotated octahedra structure of perovskite BaNiO3; (d) Jahn-Teller distorted octahedra

structure of perovskite LaMnO3. ..................................................................................................... 7 

Figure 1.4 Illustrative sketch of a typical DSSC structure. ............................................................. 9 

Figure 1.5 Sketch of a perovskite solar cell with a simplified planar structure. ........................... 13 

Figure 1.6 Thin-film topology characterization: (a-d) SEM images of top-view and cross

sectional view of perovskite thin film and perovskite solar cells made by solution processed

technique and vapour deposition technique. (e-f) SEM images of large cross sectional view

images of perovskite make by solution processed technique and vapour deposition technique

[11]. ............................................................................................................................................... 15 

Figure 2.1 Illustrative sketches of organic chamber used for vapor deposition. (a) Side view of

the organic chamber (b) top view of the organic chamber. The label of each part a – h is

described in page 18. ..................................................................................................................... 20 

Figure 2.2 Illustrative sketch of vapour deposition process. ........................................................ 21 

Figure 2.3 (a) Illustrative diagram of Bragg’s Law; (b) Principle of the XRD measurement [46].

....................................................................................................................................................... 23 

Figure 2.4 Scheme of a typical AFM system. ............................................................................... 25 

Figure 2.5 The dependence of force on the probe distance. ......................................................... 26 

vii

Figure 3.1 Photos of 100-nm thin film perovskite samples made by the vapour deposition

technique: right sample, a typical failure deposited sample film on glass; left sample, a typical

film with deposited with organometal halide perovskite. ............................................................. 33 

Figure 3.2 XRD patterns: (a) Pure perovskite thin film of CH3NH3PbI3; (b) PbI2 powder used for

film preparation; (c) PbCl2 powder used for film preparation. ..................................................... 35 

Figure 3.3 XRD patterns of (a) perovskite sample before annealing treatment and (b) perovskite

ample after annealing treatment in 140 ºC for 20 minutes. .......................................................... 38 

Figure 3.4 XRD patterns of perovskite thin films annealed at temperature ranging from 100 ºC to

400 ºC. Peaks labeled with (*) are characteristic peaks of perovskite; peaks labeled with (↓) are

PbI2 peaks; peaks label with (▼) are PbCl2 peaks. ....................................................................... 40 

Figure 3.5 SEM micrographs of perovskite samples: (a) Sample without annealing treatment; (b)

Sample annealed at 100 ºC. ........................................................................................................... 44 

Figure 3.6 SEM micrographs of perovskite samples: (c) Sample annealed at 140 ºC; (d) Sample

annealed at 180 ºC. ........................................................................................................................ 45 

Figure 3.7 SEM micrographs of perovskite samples: (e) Sample annealed at 260 ºC; (f) Sample

annealed at 300 ºC. ........................................................................................................................ 46 

Figure 3.8 Perovskite surface coverage as a function of annealing temperatures. ....................... 49 

Figure 3.9 XRD patterns of perovskite thin films: (a) Perovskite film made by vapour deposition

technique and annealed at 140 ºC; (b) The same sample from (a) exposure in air for 14 days. ... 50 

Figure 3.10 SEM images of perovskite films: (a) Perovskite film made by vapour deposition

technique and annealed at 140 ºC; (b) The same sample from (a) exposure in air for 14 days. ... 52 

Figure 3.11 AFM images of perovskite films annealed at different temperature from 85 ºC to 140

ºC. .................................................................................................................................................. 53 

viii

Figure 3.12 AFM images of perovskite films with10 nm thickness (left) and 100 nm thickness

(right). ........................................................................................................................................... 54 

ix

List of Abbreviations

AFM Atomic force microscopy

a-Si Amorphous Silicon

CIGS Copper indium gallium selenide

CIS Copper indium selenide

DSSC Dye sensitized solar cells

OPV Organic photovoltaics

PV Photovoltaic

PVD Physical Vapour Deposition

QCM Quantz crystal microbalance

SEM Scanning electron microscopy

1

Chapter 1 – Introduction

1.1 Introduction

For the last two decades, renewable energy has drawn great attention from research to

investments. As the most easily accessible energy, solar energy can be used at the most place of

the world. Therefore the use of solar energy would be an ideal solution for the increasing energy

demands. Specifically, photovoltaics (PVs), or solar cells, are ideal solutions since they have

many advantages. First of all, solar energy is an abundant energy form that does not rely on the

geographic conditions like other renewable energy. Second, solar cell devices are usually easy to

install and they require relatively less maintaining operation compared to others. Third, solar

cells convert solar energy into electricity so they can directly supply energy for most of our

needs today. Photovoltaic industry has been one of the fastest growing industries in renewable

energy development.

For effective photovoltaic production, it is very important to develop efficient and

affordable techniques. Traditional silicon based photovoltaic techniques cannot decrease

production costs due to the high price of materials and complex manufacturing requirements.

Second-generation photovoltaics, based on thin film technologies, reduce the cost significantly

by simplifying the device manufacturing procedures. Even though this generation’s efficiency is

not as high as silicon based solar cells, the ultra-low manufacturing costs make it a much more

cost efficient technology. Therefore, thin film solar cells have become a rapidly growing, and

increasingly important photovoltaic production type in industry [1, 2].

In terms of categorization, thin film solar cells can be classified into different types

according to their photovoltaic materials, such as cadmium telluride solar cells (CdTe), copper

indium gallium selenide solar cells (CIS or CIGS), and amorphous silicon (a-Si) solar cells.

Several relatively new types of solar cells have emerged in recent years, including quantum dots

solar cells, dye sensitized solar cells (DSSC), organic photovoltaics (OPV, organic molecules or

conjugated polymers), and perovskite solar cells, all of which can also be classified as thin film

solar cells – these are referred to as the third generation of solar cells.

2

Figure 1.1 Comparison of the efficiency of several third-generation photovoltaic technologies [3].

Despite being relatively recent, the popularity of perovskite solar cells has soared since

its efficiency has increased dramatically in a very short time. Figure 1.1 compares the efficiency

development of several new solar cell types (third generation solar cells). The power conversion

efficiency (PCE) of perovskite solar cells reached 15% by the end of 2013, and 19% by 2014 [3],

becoming the highest among third generation solar cells. Perovskites are considered to be great

candidates for solar cell production because of they use inexpensive materials and the same

manufacturing techniques used for other thin film solar cells [4].

3

1.2 Perovskites for High Performance Solar Cells

Today, the term “perovskite” refers to crystal species with structures of ABX3. However,

the term initially only referred to CaTiO3, and was discovered by German mineralogist Gustav

Rose in the Ural Mountains in 1839, and named after Russian mineralogist Count Lev Perovskite

(1792 – 1856). Along with many other compounds found with the same ABX3 crystal structure,

Perovskite became one of most common class of minerals on earth. In 1926, Victor Goldschmidt

described the crystal structure of perovskite for the first time, and illustrated its tolerance factors

[5]. An accurate crystal structure was later published in 1945 using a X-ray diffraction study by

Irish crystallographers [6].

Materials in the perovskites family already presented a wide range of applications, such

as conductors, semiconductors, insulators, and even superconductors [7]. Other physical

properties of perovskite also drew substantial interest, especially in magneto-resistance, ionic

conductivity, and a multitude of dielectric properties [7-9]. One can achieve these properties,

which are of great importance in microelectronics and telecommunications, by either keeping or

modifying the ideal perovskite structure. Due to the flexibility of bond angles inherent in the

perovskite structure, many different types of distortions can occur in the ideal structure,

including tilting of the octahedra, displacements of the cations out of the centers of their

coordination polyhedra, and distortions of the octahedra driven by electronic factors [9, 10].

A group of organic-metal halide perovskites has been found with ideal photovoltaic

properties. Particularly in recent years, methylammonium lead halides (CH3NH3PbI3 and

CH3NH3PbI3-XClX) have shown extremely high power conversion efficiency. Within just two

years, the device efficiency tested in laboratory increased from 8% to 19.3%, which is the

highest among the third generation thin film solar cells [3] . The organometal halide perovskites

have high charge carrier mobility and charge carrier lifetime, which allows light-generated

electrons and holes to move far enough to be extracted as current, instead of creating heat and

losing energy. The effective diffusion lengths of CH3NH3PbI3 are several hundred for both

electrons and holes [3]. For photovoltaic performance of CH3NH3PbI3, open-circuit voltage (VOC)

can approach 1 V, while for CH3NH3PbI3-xClx, VOC > 1.1 V has been reported [11]. Since the

band gaps (Eg) for both materials are 1.55 eV, the ratios of VOC to Eg are higher than what is

4

usually obtained from similar thin film cells. By tuning the band-gap of perovskite, VOC can

reach up to 1.3V, which is the highest performing among thin film photovoltaic devices [3, 12].

Another attention-catching advantage of perovskite solar cells is that the device can be

fabricated in simple planar structure, which drastically simplifies the production process. The

most common spin-coating technique has been shown to be a sufficient method of efficiently

preparing perovskite solar cells. On the other hand, the vapor deposition technique is also

considered as a potential fabrication method that simplifies the production and enhances the

degree of thin film quality control. For example, simple planar heterojunction perovskite solar

cells have already been fabricated without complex nanostructures using the vapor

deposition technique [14]. In addition, compared with other thin film solar cells, perovskite is

free from rare elements requirement, which result in relatively low manufacturing costs.

There remain two areas of concerns for perovskite use in solar cells. The first is the use of

heavy metal (lead) in cell fabrication. Since other substitutions with low toxicity (such as tin

[13]) can potentially be used, this may not be a major concern. Also, compared with the large

amount of lead used in lead-related industries every year, the amount used in perovskite solar

cells is relatively small. A second concern for perovskite solar cell application is the stability of

the organic-inorganic hybrid materials. Since the hybrid materials are very sensitive, the

perovskite films degrade quickly in exposure of ambient environment, and the cell durability is

currently insufficient for commercial use [14]. Since the length of device’s lifetime depends on

the stability of the perovskite thin film, studies of the stability of perovskite are of great value for

understanding the device’s durability, and controlling the production conditions. This research

focuses attention mainly on the investigations of stability in the perovskite materials excluded

from the solar devices and photovoltaic performance. The preparation conditions of pure

perovskites have been optimized and the material tolerances for annealing temperature and air

exposure time have been studied. The discoveries in materials properties will benefit by

controlling the production and post-treatment conditions.

To better understand the fundamentals of perovskite, the following section introduces the

perovskite crystal structure and its distortion features.

5

1.2.1 Crystal Structure of Perovskites

A general perovskite structure in ideal cubic model is shown in Fig. 1.2. The crystalline

architecture consists of three elements A, B and X. Each “A” atom sits at the central position of

the unit cell coordinating to 12 “X” atoms, where “X” atoms are usually O2–, F–, Cl– or other

large ions. “B” cations and “X” ions are coordinated and form [BX6] octahedra. There are 8 [BX6]

octahedra in each unit cell. Therefore, “A” and “X” atoms are close-packed with B is occupying

the centre of octahedra. In the organometal halide perovskites studied in this paper, “A”

represents the methylammonium cation (Ch3NH3+), while “B” stands for the metal cation (Pb2+),

and “X” is the halide ions (I– or Cl–).

In ideal perovskite structure, the unit cell axis, a, can be described by the ionic radii of A,

B and X (rA, rB and rx) by the following equation:

)2()(2a XBXA rrrr . (1)

Figure 1.2 Illustration of ideal ABX3 cubic perovskite crystal structure.

6

1.2.2 Perovskite Distortions

Because of the flexibility of bond angles inherent in the perovskite structure, many

different types of distortions can occur from the ideal model [15, 16]. Only a few compounds,

such as CaRbF3 and SrTiO3, have ideal perovskite structures, and even mineral perovskite

CaTiO3 itself is also distorted [16]. There are three major causes for the distortions in perovskite:

1) the size effect of ions and cations; 2) non-ideal stoichiometric deviation; 3) geometrical

distortion caused by electron configuration, also known as Jahn-Teller effect [15].

First, the perovskite structure easily gets distorted by varying the sizes of A, B or X. To

estimate the degree of distortion in a particular ionic perovskite, the Goldschmidt Tolerance

Factor [5] has been defined as t :

rrrr

XB

XA

2

1t (2)

The ideal cubic perovskite structure has t = 1, due to the high symmetry in cubic system. The

factor t gets smaller with the decrease in the cation A size. When the size of cation A dropped

below a certain value, t will be smaller than 1. In this case the octahedra will tilt to make space

for the large ions, such as the cases in CaTiO3 and GdFeO3 (t = 0.81). But t cannot be smaller

than 0.81, since the structure will then be assigned to an ilmenite structure in that range. For a

system with large A or small B ions than in ideal cubic system, the tolerance factor is larger than

1. The close-packing structure will be stable as a varied hexagonal perovskite, such as BaNiO3.

However, the tolerance factor assumes only ionic bonds existing in the structure so it is limited

to distortion from the ideal perovskite structure.

Second, the non-ideal stoichiometric deviation can cause distortion in perovskite

structures. Taking SrFeOx as an example, the valency of the Fe can be different depending on

either a sample heated in an oxidizing or a reducing environment, whereby the oxygen content

varies from 2.5 to 3 in the perovskite structure. Both +3 and +4 oxidation states can be assigned

to Fe in SrFeO2.875: thus, the FeO5 pyramid structure can be formed instead of octahedra. The

original structure distortion takes place because of the deviations from ideal structure [16].

7

The third distortion type is Jahn-Teller effect, which is a geometrical distortion caused by

a certain electron configuration in order to lower the structure’s overall energy. Particularly,

some perovskite systems hava Jahn-Teller active ions at B position [17]. For example, in the

LnMnO3 in which Ln is La, Pr or Nd structure, Jahn-Teller active Mn3+ ions result in the

elongation of the [MnO6] octahedra [16]. All three types of distortions are illustrated as sketches

in Fig 1.3.

The distortions in perovskite structure have influences on the stability of properties. For

organometal halide perovskites, their hybrid composition enlarges the flexibility of the crystal

structure, which could be one of the causes of their low stability.

Figure 1.3 Illustrative sketches of possible distortions in perovskite structure: (a) ideal cubic structure of perovskite SrTiO3; (b) rotated octahedra structure of perovskite GdFeO3

(Side view); (c) rotated octahedra structure of perovskite BaNiO3; (d) Jahn-Teller distorted octahedra structure of perovskite LaMnO3.

8

1.3 Dye Sensitized Solar Cells (DSSCs) and Perovskite Solar Cells

As a new type of thin film photovoltaics, perovskite solar cells emerged from the

previous generation, DSSCs [18]. It is important to understand the basics of thin film DSSCs and

their connections with perovskite solar cells. Here, we simply demonstrated the typical device

structure of DSSC and its work principle. Also, a historic evolution from the DSSCs to

perovskite solar cells is reviewed in order to illustrate the development of perovskite application

in solar cells, the present device designs, and possible future directions.

1.3.1 Device Structure of DSSC

A typical DSSC is commonly built with a series of layers of materials serving for

specific functionalities for transforming light into electrical energy. Figure 1.4 shows a typical

DSSC structure [19]. The substrate that supports the whole cell structure is usually transparent

glass. A layer of transparent conductive material is coated on glass as anode, which is usually

indium tin oxide (ITO) or fluorine doped tin oxide (FTO). The transparency of the anode allows

the light injecting into the device to provide the energy.

On top of the anode material, n-type semiconductor, TiO2 is typically built for electron

transporting. Commonly, two layers of TiO2 are coated in different morphologies [20, 21]. The

compact TiO2 layer works as electron transport layer in solar cells, and the mesoporous TiO2

layer is usually made into nano-scale structure to provide necessary morphology for dye

sensitizer exhibiting the photovoltaic into its pores. Mixing with the organic dye molecules [22-

26], quantum dots [27-29], or other light sensitizers [30, 31], the porous layer is the essential

layer for absorbing light and generating electron-hole pairs in the device. On top of the TiO2

there is a layer of p-type semiconductor, typically Spiro-OMeTAD, which transports holes from

the photovoltaic material structure. A highly conductive cathode layer finalizes the cell

preparation.

9

Figure 1.4 Illustrative sketch of a typical DSSC structure.

1.3.2 Working Principles

Thin film solar cells are connected to conventional solar cells in their working principle.

For traditional solar cells, both n-type and p-type materials are silicon based layers. The new

generation of thin films solar cells, however, use thin film light absorbing materials.

For traditional solar cells, the performance of photovoltaic cells depends on its core

material, semiconductor, which performs as insulator in their pure form but is able to conduct

electricity in high temperature or combine with other materials. A host semiconductor combined

with electron donor materials develops an excess of free electrons, known as an n-type

semiconductor. A host semiconductor combined with acceptor materials develops excess of

“holes” (or equivalently the removal of electrons), known as a p-type semiconductor. A

photovoltaic cell contains adjacent n-type and p-type materials, and the interface between is

known as a P-N junction. Resulting from the nature of two types of semiconductor, a small

number of electrons always move across the junction from the n-type to the p-type

semiconductor, producing a small voltage output, even in dark. With the presence of light, a

great number of electrons can be activated and flow across the junction thus creating an electric

potential difference at each side. Photovoltaic devices are designed to apply this principle, and

convert solar energy into electrical energy.

10

For the new generation of solar cells, which is the focus of this research, the process of

electricity generation is different from P-N junction-based photovoltaics. After passing through

the transparent glass substrate and anode, the sunlight striking onto the absorbing layer excites

electrons from the materials valence band to the conduction band. Accordingly, the excitation of

the electrons creates a free electron in conduction band and a “hole” in the valence band, which

is referred to as “excitons”. After generating excitons, electrons attract into the TiO2 layer and

conducted into metallic anode. The hole will be replaced by an electron provided by the p-type

material layer, and thus be conducted to the cathode. A potential difference, VOC, for an open

circuit formed from anode to cathode and a current density JSC can be generated by continuous

power supplied form light.

11

1.3.3 Evolution from DSSCs to Perovskite Solar Cells

In thin film solar cells, a variety of organic dyes and inorganic quantum dots were used as

photon sensitizers. Perovskite was also considered as a sensitizer in DSSCs but then discovered

with electron transporting property. This section briefly introduces how perovskites become

photovoltaic material that enables simple planar structured solar cells.

Mitzi and co-workers discovered organic-inorganic halide perovskite as a possible

candidate for thin film transistor and light-emitting diode (LEDs) early 1990s [4, 32]. Since then,

properties of photovoltaic are anticipated but have not sufficiently studied due to concerns of

lead toxicity and low stability [4]. Miyasaka [33] was the first to report the photovoltaic

performance of perovskite in the nanoporous TiO2 layer of dye-sensitized cells. The method used

for their study was to spin coat the sensitized layer of perovskite with solution of CH3NH3I and

PbI2 on top of the TiO2 film. The initial efficiency of their CH3NH3PbI3 cell was 2.2%, and they

were then able to increase efficiency to 3.8% by replacing bromine with iodine. This result was

not a groundbreaking achievement in terms of efficiency; however, it demonstrated the potential

for a new light absorbing material in photovoltaic cells. With the further investigation, perovskite

was proven to be an ideal candidate of light sensitizer, which could achieve appreciable

efficiency.

Subsequently, Park [34] implemented perovskite in similar structure by depositing

sparsely spaced hemispherical nanoparticles that were approximately 2.5 nm in diameter. Along

with surface treatment on TiO2, they achieved an efficiency of 6.5% in 2011 [34]. The

performance of perovskite became comparable to that of organic dyes at this time, but degraded

rapidly since perovskite material can easily dissolve in its electrolyte cell. This lead Park, Gratzal

and his co-works to consider replacing the electrolyte with a solid-state hole transport material,

spiro-MeOTAD (2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene) [19,

35]. The perovskite material penetrates the nanoporous structure of the TiO2 layers in all their

structures, and is thus considered a sensitizing layer rather than a fully functioned photovoltaic

layer. It not only improved the stability, as expected, but also it improved efficiency to 9.7% at

2012 [35].

12

Around the same time, Snaith and his workers [19] also reported success in using spiro-

MeOTAD in perovskite structure, and developed photovoltaic cells in several ways. First, they

employed a mixed-halide material CH3NH3PbI3-xClx and improved both the stability and

performance-efficiency of the cell, compared to the pure iodine equivalent. Furthermore, they

capped the TiO2 porous structure with a thin perovskite layer and the device was still functional.

By replacing the TiO2 layer with a similar but non-conducting Al2O3 network, VOC of the device

increased and efficiency boosted to 10.9%. This research demonstrated that perovskite has the

potential to transport both electrons and holes between cell electrodes. The single-step perovskite

deposition process created large morphological inconsistency, and low stability in device

performance. With the method using both perovskite in TiO2 scaffold structure and a capping

layer of pure perovskite overlaying the scaffold, the efficiency jumped to 12.0% [36]. Same

efficiency was observed in a similar structure with Br content in perovskite, and this structure

was proven with high humidity stability [37]. There exists a structural transition from tetragonal

to pseudo-cubic mainly due to a higher t factor, which is caused by the smaller ionic radius of Br.

In 2013, Gratzel’s group used TiO2 scaffold along with two-step iodide deposition, and

improved the efficiency up to 14%. Snaith’s group creatively used a two-source deposition

method, but avoided the previous scaffold structure, and achieved an increased efficiency of

15.4%. This method greatly enhanced the morphology by physical vapor deposition (PVD) and

encouraged our work in this research. A similar structured solar cell with an efficiency of 19.3%

was reported in May 2014 [38].

In summary of the evolution path of perovskite solar cells, perovskite was initially used

as an organic-inorganic hybrid dye in DSSC and also as a solid state DSSC (ssDSSC). It was

substituted for the organic dye molecules, and acted as active material extremely thin absorbers

(ETAs) in DSSC [39]. The discovery of perovskite’s ability to transport charge carriers enabled

it to act as the photovoltaic materials, without depending on the TiO2 electron transporting

materials. Perovskite meso-superstructured solar cells (MSSC) exhibited a high photovoltaic

efficiency on insulation AlO3[40]. Therefore, the possible future directions of perovskite solar

cells could be MSSC, simple planar “p-i-n” thin film solar cell, or p-n heterojunction cells [4, 41,

42]. Figure 1.5 shows the perovskite solar cell with a simplified planar structure, which is a “p-i-

n” thin film solar cell.

13

Figure 1.5 Sketch of a perovskite solar cell with simplified planar structure.

14

1.4 Achievements in Morphological Studies of Perovskite Thin Film

The most progressive morphological study in surface uniformity and consistency is the

co-vapour deposited perovskite thin film, conducted by Snaith’s group in 2013 [43]. This study

discovered significant differences regarding film morphology between solution-processed and

vapour deposited perovskite films. In this work, Snaith’s research team achieved a 5% increase

in efficiency by removing scaffold structure, and simply building the solar cell structure with a

planar heterojunction thin film. The vapour deposition method produced extremely

homogeneous perovskite film with highly uniform thickness[44], which provided contrasts to the

inconsistent thickness and uneven morphology of the solution-processed films (shown in Fig. 1.6

[43]).

Figure 1.6 shows SEM images comparing perovskite solar cells prepared by vapour

deposition and solution processed techniques. The top row photos show that vapour deposition

technique achieved full surface coverage of the film, and the solution processed material

unevenly distributed. The vapour deposited sample is fully covered with perovskite materials and

no vacancies or voids could be observed, whereas the solution processed sample showed a

significant portion of area with substrate exposure. The cross-sectional figure also indicates that

some part of the solution processed film may suffer from short circuits by the varying thickness

from 0 nm to 410 nm, which hinders the performance of the device because of the pinhole

formation. But the vapour deposited film with evenly covered layer of perovskite material is of

no concern in this problem.

This morphological study stressed the advantage of the vapour deposition, in that this

method allows precise control of the thickness of the perovskite film. It is important to optimize

the film thickness since a thicker film is needed to absorb enough light for generating excitons.

However, one needs to maintain the thickness at a thin enough level to allow for the transport of

electrons and holes. Last, optimized perovskite with film thickness of 330 nm indicates that

electron-hole diffusion length exceeds such a length in perovskite material.

The investigation of stability of the perovskite films is also discussed in this report

regarding solution processed perovskite thin film [44]. Using a scanning electron microscope

15

(SEM) is the proper technique for examining the morphology change of the perovskites surface.

Particularly, the surface coverage rate is an important parameter that can be used to describe the

film’s formation, or the surface loss in post-treatment. SEM has thus been used in this research,

and changes of surface morphology and coverage are used to evaluate the degree of degradation.

Figure 1.6 Thin-film topology characterization: (a-d) SEM images of top-view and cross sectional view of perovskite thin film and perovskite solar cells made by solution processed technique and vapour deposition technique. (e-f) SEM images of large cross sectional view

images of perovskite make by solution processed technique and vapour deposition technique [11].

16

1.5 Motivations and Thesis Overview

Thin film solar cell technologies are important in making photovoltaics more affordable.

One goal of studies in thin film solar cells is to achieve of high efficiency devices with easy

processing methods and accessible element resources. Organometal halide perovskites based

solar cells have been demonstrated to have significantly high efficiency. The morphology

conditions of solution-processed perovskite, such as uniformity of thickness and surface

coverage, are key factors to control film quality and device performance. Using the vapour

deposition method in high vacuum system, we prepared perovskite thin films with uniform

thickness and full surface coverage.

In this study, we aimed to optimize the vapor deposition conditions for preparing

perovskite thin films that have been used in a number of solar cells. Perovskite samples were

prepared and annealed under a series of temperatures. By checking the crystal structures with X-

ray diffraction (XRD), and surface morphologies with scanning electron microscope (SEM), the

effect of different annealing temperatures was investigated. The stability of the perovskite thin

film was studied by examining the changes of the material exposing in air. AFM was used to

check high-resolution surface information that cannot be captured by SEM.

In sum, this chapter introduced the fundamentals of perovskite structures, and provided a

brief recount of the evolution from DSSC to perovskite solar cells. This chapter also reviewed a

study of morphological control of the thin film perovskite. Next, chapter 2 illustrates the

materials used in preparing the perovskite thin film, as well as the experimental details of our

study. Chapter 3 shows the experimental results and the conclusions of our investigation. Last,

Chapter 4 is a brief summary of the entire thesis and several suggestions on future research.

17

Chapter 2 – Materials and Experimental Methods

2.1 Materials

Our organometal halide perovskite is made from organic and inorganic sources through

the vapour deposition technique. The organic source is methylammonium iodide (CH3NH3I), and

the inorganic source is lead chloride (PbCl2). CH3NH3I was synthesized by reacting methylamine

with hydroiodic acid at 0 °C for two hours, while stirring. PbCl2 was purchased from sigma-

Aldrich. This chapter will describe materials and experimental methods, including both solution

processing and vapour deposition. Principles of characterization method used in this study will

also be introduced.

2.2 Methods for Perovskite Thin Film Preparation

There are two main techniques for preparing the perovskite thin films, solution processed

technique and vapour deposition. The solution processed method is more commonly used

because of the simplicity of equipment set-up. Vapour-deposition requires complex experimental

conditions, but it pays off by providing the thin film with highly uniformed thickness. In this

study, we mainly focused on perovskite thin films that were made by vapour deposition in high

vacuum system.

2.2.1 Solution Processed Technique

For perovskite thin film preparation, the most commonly used technique is spin coating

the precursor solution on substrate, followed with an annealing treatment. In early studies of thin

film perovskite solar cells, perovskite precursor solution was prepared and spin coated directly

onto substrate to form thin films [19]. The precursor solution is usually the methylammonium

lead halide (CH3NH3PbI2Cl or CH3NH3PbI3),) in N, N-dimethylformaide. Spin coating is usually

conducted at ambient conditions, and the annealing procedure is carried out on sample

immediately after spin coating. Before annealing, the thin film has lower crystallinity (smaller

crystal size) and contains an excess of PbI2 component mixed in the thin film. With the annealing

process, the perovskite film forms larger crystal grains, and pure perovskite crystal structures [3].

Some other methods, based on the solution processed technique, were developed by

varying part of the original procedures in order to obtain perovskite thin films more easily, or

18

with higher performance. A sequential solution deposition method was carried out with high

efficiency in device performance, based on the previous study [14]. Instead of spin coating

perovskite solution directly onto the substrate, the inorganic component PbI2 solution was first

spin coated, and then the substrate was dipped into a CH3NH3I solution. Further modification

was made to introduce the organic component by vapour treatment on the substrate, spin coated

with an inorganic source [11]. CH3NH3I powder was then spread out around the PbI2 coated

substrates in a covered petri dish. By heating the petri dish for desired time, perovskite formed

from the CH3NH3I vapor-treated PbI2 substrate. Although preparation methods seem to differ

from each other in details, all have been proven to be effective ways for perovskite film

preparation. High performance devices with efficiency higher than 12% can be obtained by all

the methods discussed above.

2.2.2 Vapour Deposition Technique

A novel method is to use the vapour deposition technique to produce highly pure

perovskite thin films with high device efficiency. For the vapour deposition technique, thin films

are prepared onto substrates through condensation, or the reaction of vaporized materials. The

preparation procedure is conducted in a multi-technique vacuum system, which has a central-

distribution chamber (CDC) with several sub-chambers located around it. All chambers are

connected and kept in a vacuum at a pressure between 1×10-10 to 5×10-9 Torr. Perovskite thin

films were deposited in an organic deposition chamber, which allows for relatively low

temperature deposition compared to the oxides deposition chamber and the metal deposition

chamber. In the organic deposition chamber, chemical sources are evaporated from the

permanently mounted Knudsen cells (K-cells) whose heating temperature can be precisely

controlled. The two source materials for perovskite preparation are vaporized in the vacuum

system as the gas phase. They interact on the surface of substrate located on the top of K-cells,

and where the chemical reaction takes place forming the thin film.

The illustrative diagrams of organic deposition chamber are shown in Fig. 2.1. Each part

of the chamber is labeled with a letter. Part (a) is the chamber body where the evaporation

deposition takes place. Part (b) is a transfer-arm evaporator cell (TAE-cell) that allows substrate

moving into and out of the chamber body. A sample holder, part (c), is connected at the end of

the transferring arm and it can carry four pieces of substrate for each deposition. Substrates

19

prepared for perovskite deposition are placed at the sample holder so that the substrate can be

delivered into the chamber. Part (d) is a quartz crystal microbalance (QCM) used for monitoring

the films’ thickness. Part (e) is the shutter that controls the exposure of substrate to the chemical

vapour. Part (f) is the connector to the central chamber, while part (g) is the K-cell. These K-

cells are isolated from one another and the temperature of K-calls can be individually controlled.

Each K-cell has its own shutter that controls the evaporation for coming out from the K-cell. Part

(h) is the ion pump used to maintain the required vacuum. The lower panel of Fig. 2.1b illustrates

the top view of the organic chamber.

Figure 2.2 describes the vapour deposition process for perovskite thin film preparation.

During the deposition process, organic source CH3NH3I and inorganic source PbCl2 are placed in

different K-cells. Once the K-cells are heated to the preset temperature, the chemicals start

evaporating and moving upward to the substrate. When both PbCl2 and CH3NH3I vapor reaches

onto the substrate, a chemical reaction is initiated, and begins to form perovskite films on the

substrate’s surface. At the same height level of the substrate holder, there is a QCM device serving

as the monitor for film thickness. QCM measures the mass per unit area by measuring the change

in frequency of a quartz crystal resonator. The resonance changes with increases of a small mass

due to the film deposition at the surface of the acoustic resonator. Film thickness can be

monitored based on the mass addition on unit area. Usually, there is some difference from the

real deposited thickness to the QCM readings. The ratio of the real deposition rate over the QCM

reading is defined as the tooling factor of the QCM. Each QCM has a specific tooling factor in a

certain deposition system. During the process, the system is maintained at pressure of 10-6 – 10-7

Torr. As such, only chemical sources evaporated upon the film participate in the chemical

reaction, which ensures the purity of the reagents and uniformity of the film thickness.

For solution processed perovskite, it is known that the annealing procedure assists film

formation and material crystallization[14, 34]. Most of the films after spin coating need an

annealing process for better crystallization and film formation. To prepare an ideal perovskite

thin film with solution processed method, it is very important to control the annealing

temperature and treatment time. For vapour deposited perovskite, however, there is little research

that addresses the annealing effect from previous studies. This dearth of information provided

motivation for us to conduct an investigation on the annealing effects for perovskite over vapour

deposited thin films.

20

Figure 2.1 Illustrative sketches of organic chamber used for vapor deposition: (a) Side view of the organic chamber (b) top view of the organic chamber. The label of each part a – h is

described in page 18.

21

Figure 2.2 Illustrative sketch of vapour deposition process.

22

2.3 Characterization

2.3.1 Powder X-Ray Diffraction (XRD)

XRD, a traditional technique for crystal characterization, examines long-range ordering

and phase purity. The periodicity of the electron density in a crystal structure generates X-ray

scattering from electrons, and causes coherent “diffraction pattern”. The intensity of the

diffracted X-ray can be plotted as the function of angle 2θ (θ is the angle of electron and the

crystal plane) and thus depict the powder diffraction pattern. Peaks appear in the powder

diffraction pattern at angles with maximum interference that satisfy Bragg’s Law [45]:

sin2dn (5)

Each crystal structure has a unique diffraction pattern with characteristic peaks and

relative intensity. The XRD patterns of known structures has been tested and archived in

databases. Therefore, one can compare the experimental XRD patterns with the standard

patterns in order to identify the sample structure. In this paper, all X-ray diffraction patterns of

the perovskite thin films were obtained on an X-ray diffractometer (Panalytical X’Pert Pro), with

Cu-Kα radiation (λ=1.54056Å). Figure 2.3 shows an illustrative diagram of Bragg’s Law, as

well as sketches of the working principle of a XRD measurement.

To verify the crystal structure of our perovskite thin film samples, we referred to the

database and labeled the theoretical peak positions of perovskite structure. We can thus compare

our XRD patterns with standard XRD patterns, and check if the thin film prepared by vapour

deposition also has perovskite crystal structure. To investigate the effect of other impurity crystal

structure, XRD patterns of possible lead halide (PbI2 and PbCl2) are also obtained. As noted,

XRD is an ideal tool for checking the crystal structure evolution in annealing process; results of

XRD tests provide strong support for the investigation of our thin film and thin film changes.

23

Figure 2.3 (a) Illustrative diagram of Bragg’s Law; (b) Principle of the XRD measurement [46].

24

2.3.2 Scanning Electron Microscopy (SEM)

SEM is an imaging tool that uses a beam of focused electrons, scanning on sample

surfaces, to produce images. The scanning electrons interact with atoms on sample surface

producing signals of surface topography. There are several types of signals produced by SEM,

such as secondary electrons (SE), back-scattered electrons (BSE), characteristic X-rays, cathode-

luminescence (CL), specimen current and transmitted electrons. Secondary electrons are the

most commonly used signal type. During the measurement, the electron beam scans over the

surface of the sample dot-by-dot and line-by-line, producing many types of signals. Secondary

electrons that are dislodged from the surface atoms have unique patterns at each dot. A detector

that counts the secondary electrons scattered from the sample surface receives information of

secondary electrons of the sample. Other sensors also detect BSE, X-ray, and other signals. By

detecting the information along with the scanning beam by different sensors, a great deal of

information is processed by the computer, and displayed by different levels of brightness on a

monitor.

SEM images were collected at the Centre for Nanostructure Imaging at Chemistry

Department of University of Toronto using the Quanta FEG 250 environment SEM with both a

bright field and dark field detector. The crossbeam combines a high resolution SEM for imaging,

with a focused ion beam (FIB) for micromachining by sputter milling with a sub-100 nm lateral

resolution. The spatial resolution of the images can focus down to 1 nm, depending on the

material’s conductivity.

SEM is a powerful imaging tool that can be used in conductive and semi-conductive

materials. For our perovskite sample, it is the proper choice with which to image surface

topography. By detecting the surface of perovskite sample, we are able to understand the surface

uniformity, thickness, and even chemical components in a rough scale. In addition, the change of

the surface morphology observed from SEM benefits to explain the possible material affect.

25

2.3.3 Atomic Force Microscopy (AFM)

Distinguished from its predecessor – Scanning Tunneling Microscopy (STM), which relies

on tunneling current between scanning tip and sample surface atoms – AFM mechanically

contacts with materials directly. Therefore, AFM affords the possibility of observing on almost

any type of materials.

Working principle of AFM is also relatively simple among advanced characterization

techniques. Typically, AFM contains three main parts, as shown in Figure 2.4: the scanning

system, controller, and computer. Samples are located on a piezoelectric scanner that provides

precise movement on x, y and z direction. A very sharp tip attached to a soft cantilever scans on

the sample surface line-by-line with the motion of the piezoelectric scanner. During scanning,

the controller adjusts the z canner to maintain a constant tip-sample force value to make sure the

tip-sample distance has a constant value. At the backside of the tip, a beam comes out of the

laser source and reflects to a photodiode detector. With the monitoring of the laser beam signal,

height change and deflection information of tips can be precisely captured by the controller.

Thus, morphology images can be constructed in three dimensions by recording height change

during lateral dimensional scanning.

Figure 2.4 Illustrative sketch of a typical AFM system.

26

AFM achieves extremely high resolution on sample surface morphology, depending on

sharpness of the probe tip, precision of the scanner, and the optical detection system. The probe

tip usually has a radius of a few nanometers (~10nm), which only has a small amount of atoms at

the end. High sensitivity piezoelectric ceramic provides accurate three-dimensional

displacement. The long path of the laser beam amplifies slight changes of the tip, and the laser

signal is precisely captured by photodiode. Tip-sample interaction also helps AFM achieve

atomic level resolution. Tip-sample interaction may be described by the Lennard-Jones potential

[ω(r)] [47], which considers the ideal interaction between two atoms:

rr

126

BAω(r) (6)

Where r is the distance between two bodies and A, B are constants for fixed atoms. The

force interaction is then:

rr

127

B12A6

dr

dωF (7)

Figure 2.5 The dependence of force on the probe distance.

27

As shown in the above equation, as well as in Figure 2.5, the Lennard-Jones force depends

directly on the probe distance. The curve increases steeply when separation distance is less than a

certain benchmark, which provides the sensitivity to detect slight height changes from repulsive

force.

Different scanning modes are available, depending on several sample-tip relative motion

types. In contact mode, the tip scans under an applied force, and a feedback system keeps the

tip-sample interaction force in a constant value. In dynamic mode, oscillating cantilever beats on

the sample surface to minimize the tip-sample contact effect, and improves the ability to detect

soft materials. The oscillation amplitude and phase change are taken as parameters to report

surface information. Therefore, AFM not only depicts morphology information, but also

presents material surface property by phase shift information.

Although AFM is a powerful tool for characterizing a variety of materials in different

environments, it still has some limitations. As a result of the sharpness of the tip, the AFM used

in this research can only scan within 100μm2 in image size, and the scanning speed is much

slower than other characterization methods, such as electron microscopy. Unsuitable tip shape

introduces image artifacts in the morphology image. Artifacts are mainly due to changes in tip

shape, such as sticking impurities from sample surface, or a small piece knocked off from the tip

end. As a detector originally designed for flat materials, AFM works poorly on morphology with

steep walls or overhangs.

28

Chapter 3 – Results: Morphological Studies of Perovskite Thin Films

3.1 Introduction

Organo-metal halide perovskites have garnered great attention over the last three

years[48, 49] because they provide the highest efficiency among third-generation photovoltaic

materials. The vapour deposition technique allows for the making of perovskite thin film, which

can potentially be assembled into photovoltaic devices through easy fabrication. Compared with

solution-processed methods that were previously common, vapour deposition possesses several

advantages over controlling the film thickness and morphology. First, by controlling the

deposition rate of two chemical sources, vapour deposition precisely controls thickness at the

nanometer level [43]. Second, vapour deposited perovskite thin films have uniform morphology

suitable for further device assembly [36, 44]. For an ideal perovskite thin film, uniform

morphology effectively avoids the possible short circuit in the device, and provides

homogeneous performance across the entire device. As such, the uniformity of morphology

contributes to enhanced efficiency of such devices. Lastly, vapour deposition techniques are

easier for large-scale production, which is a key factor when one considers the potential for

future manufacturing.

The primary goal of this research is to make perovskite structures with organic and

inorganic source chemicals in a PVD system, which has been described in the experimental

section. There are several parameters that need to be optimized in order to obtain the perovskite

thin film. Two major factors for making highly crystalized perovskite are: deposition rate, and

ratio of two source chemicals. By varying the temperature of the K-cells, we controlled the

deposition rate of the film as well as the organic-inorganic ratio. Optimum temperature of each

K-cell was decided by checking the crystal structure of the thin film with XRD.

Besides preparing perovskite thin films, another important issue is perovskite stability.

The stability of perovskite is the main factor determining device performance and lifetime. It is

also decisive in choosing fabrication conditions (e.g. vacuum requirement). Therefore,

investigations on stability of perovskite thin film will benefit from understanding the thin film

properties, and predicting device performance and lifetime. Instead of checking the perovskite

29

performance and stability in solar devices, two studies were conducted directly on the perovskite

thin film. The first study focused on the effects of annealing temperature on film formation and

degradation. Annealing is one of the key procedures aiming to improve perovskite performance.

By heating the film above a certain temperature, materials become more homogeneous and

exhibit better properties. In this study, we observed the effect of annealing temperatures, and

examined the temperature tolerance of perovskite thin film. To further investigate the stability of

the perovskite film, we studied the degradation behavior of perovskite thin film exposed to air.

Morphological information such as film thickness, surface coverage, and crystal grain

sizes can assist in evaluating the rate of film formation, consistency of the preparation

conditions, and changes upon treatments. Solution processed thin film with rough surface

morphology is not suitable for monitoring the change of film surface [43]. However, vapour

deposited film can achieve very uniform surfaces as a baseline for morphology studies, and

enables capturing morphology changes. With a uniform surface, we clearly observed thin film

morphology changes with varying annealing temperature, air exposure time, and other conditions

that may cause the perovskite degradation.

It is important to note that the vapour deposition method confronts several obstacles

during experimental procedures. First, methylammonium iodide (CH3NH3I) does not stick on the

substrate alone, so it is very hard to monitor its deposition rate, and precisely control the organic

component independently. Second, the CH3NH3I powder easily gets clustered upon heating. The

ideal solid material should disperse freely as powder; thus, the clustering makes it difficult to

maintain a stable evaporation rate. Last, the deposition system needs to be calibrated often so as

to keep the same experimental condition from batch to batch, especially the tooling factor of the

QCM. In all, preparation of perovskite sample is time consuming, and a great number of samples

are needed to obtain significant data points.

In this chapter, we described conditions for preparing perovskite thin film with uniform

thickness by the vapour deposition method. By controlling the heating temperature of K-cells,

we successfully optimized the deposition rate of the CH3NH3I and PbCl2 in order to obtain

perovskite thin film. The relationship of the deposition rate and the temperature was also studied.

The crystal structure was examined using XRD, while the surface morphology was studied using

SEM and AFM. Surface coverage changes were also investigated.

30

3.2 Experimental Details

Materials for vapour deposition of perovskite thin films are prepared separately.

CH3NH3I was usually synthesized with following method: 24 mL of CH3NH2 reacted with 10

mL of HI in a 250 mL round-bottom flask at 0 °C for two hours, with constant stirring. The

precipitate of the reaction was then collected using a rotary evaporator, by carefully removing

the solvents at 50 °C. The obtained product was re-dissolved in 80 mL absolute ethanol, and

precipitated with the addition of 300 mL diethyl ether; this procedure was repeated twice for

purification. The final product of CH3NH3I was collected and dried at 60 °C in a vacuum oven

for 24 hours. The Ted Sargent group from the Department of Electrical and Computer

Engineering provided the CH3NH3I, and PbCl2 was purchased from Sigma-Aldrich.

Substrate cleaning is a key procedure in guaranteeing final thin film uniformity in

thickness. A substrate with insufficient cleaning will cause uneven film patterns, which lack

uniformity. As we aimed to image the morphology of the perovskite film with SEM and AFM,

very flat substrates were desired. As such, we chose a silicon wafer as the substrate that would

provide flat surface, and would also allow the film to form on the surface.

The cleaning procedure of the silicon wafer consists of three steps. The first step is to

thoroughly wash the substrates with water, acetone, and methanol, in that order; extra wiping or

sonication may be required depending on the surface condition. The use of a different solvent is

to ensure that the substrate was cleaned with solvents of different polarity. The next step is to dry

the substrate with a flow of nitrogen gas right after the solvent wash. Finally, the substrate is

treated with UV-Ozone radiation. The UV-Ozone treatment not only cleans the surface with

high-energy radiation, but also it oxidizes the surface in order to gain higher work function and

surface potential, which helps the deposition process and enhances the adhesive property. The

whole cleaning procedure was conducted immediately before the deposition, ensuring the least

air exposure after cleaning. In addition, the color of the film is also a key indication of film

quality. So glass slides were used as substrate to prepare sample for colour check.

31

Vapour deposition process is carried out on substrates following sufficient clean. Varying

the heating temperature of the K-cells that contain source materials – CH3NH3I and PbCl2 –

controlled the evaporation rate. The common temperature used for heating PbCl2 is around 330

ºC, and for CH3NH3I it is 125 ºC in the chamber, with the pressure of 10-6 – 10-7 Torr. These

optimal temperatures were decided by varying the temperatures of K-cells and recording the

deposition rate of each source. The device performance and the thin film quality depend greatly

on the component of the CH3NH3I and PbCl2. To optimize the component ratio of CH3NH3I to

PbCl2, the ratio of 1:1 to 10:1 were conducted, and the optimum ratio was found to be around 8:1

for making perovskite thin film in this system.

Since it is difficult to deposit CH3NH3I on a silicon substrate when deposited alone, we

set the QCM as the set of PbCl2, and the deposition rate of CH3NH3I was calculated accordingly.

The optimum perovskite film thickness is around 100 nm, and each deposition takes around two

hours. This thickness is proper for subsequent characterizations [43].

After the deposition process, the sample was taken out from the chamber for annealing.

As discussed, the perovskite is very sensitive especially to moisture; as such, we kept the sample

in a glove box except when annealing or characterizing.

The samples are annealed in a vacuum oven to separate the sample from air and humidity

during heating. Some reported studies did not keep the sample in vacuum during annealing [44,

50]. Instead, the device was annealed in air with the relative humidity at around 30%. Our study

kept the same from exposure to air, since moisture in air was considered to be a major factor in

degradation of perovskite film [50]. All annealed samples were heated in the determined

temperature for 20 minutes. To anneal samples at different temperatures, certain number of

choice sample were taken out of the glove box. During the annealing process of the first sample,

other samples were exposed to air. As such, the second sample actually stayed in air for 20

minutes more than the first sample.

X-ray diffraction (XRD) is a powerful technique with which to study the long range

ordering of perovskite crystal. It is used to examine the crystallinity of perovskite material,

which indicates the purity of the thin film and the grain size of individual crystal. The tolerance

of annealing temperature was detected by the XRD, and showed how the structure changes along

32

with increases to the annealing temperature. Taking advantage of the morphology uniformity

provided by vapour deposition, it is convenient to study perovskite thin film morphology with a

series of characterization methods. Using SEM, we can observe morphology of the perovskite

thin film. AFM is a great tool with which to present morphological changes in very high

resolution.

Characterization: X-ray diffraction pattern (2θ scans) of the perovskite thin film were

obtained on an X-ray diffractometer (Panalytical X’Pert Pro), with Cu-Kα radiation

(λ=1.54056Å). SEM analysis was performed on a Quant FEG 250 environmental SEM. The

AFM used is a JPK NanoWizard II AFM. A cantilever with nominal spring constants between 40

and 50 N/m (NCH probes, Nanoworld Innovative Technologies) was operated under the

dynamic force mode. In this mode, the cantilever is vibrated at around the resonant frequency,

and its amplitude reduces when the tip is in proximity to the sample surface, which is caused by

tip – sample interaction. Reduced amplitude is set as the feedback parameter (set point) so that

the AFM system scans the surface contour of the sample with minimized error signals (the

difference between the set point and the amplitude measured) by adjusting the distance between

the tip and the sample surface. Mapping of this distance constructs a topographic image of the

surface morphology. Mapping the error signal resulted in an image removing the height

contribution, and stressing only the shape of surface features. When the height range is large,

surface features with small height differences are obscured. In this case, it is advantageous to use

the error signal image in order to show the shapes of surface features, while using the

topographic image to estimate the height distribution. The scan rate for obtaining images is 1 Hz.

The experiment was conducted in air with a relative humidity of ~40%.

33

3.3 Results and Discussion

For each deposition, four pieces of substrates were aligned in the sample holder so that

four identical thin films could be obtained. Usually, one of the four substrates is a glass slide that

allows for a quick check of the film colour. The samples of 100 nm-thick perovskite film made

by vapour deposition technique are shown in Fig. 3.1. Only samples deposited on glass are

shown here because of the significant contrast between film and substrate. Samples deposited on

silicon wafers are not well depicted on a photo with clear contrast. The left sample in Fig. 3.1

with light yellow colour is a typical failure deposition due to insufficient perovskite formation on

the substrate surface. The yellow colour is mainly due to PbI2 mixed in the film, indicating that

PbI2 is the intermediate in the reaction between CH3NH3I and PbCl2.

A successfully-deposited perovskite film deposition should have dark color, indicating it

absorbed most of the visible light (Fig. 3.1 sample on the right). The color of the film also gets

darker with the increase of the thickness. From the colour of the sample, we can quickly check if

the deposition constitutes a desired perovskite film.

Figure 3.1 Photos of 100-nm thin film perovskite samples made by the vapour deposition technique: left photo, a typical failure-deposition on glass; right photo, a typical successful

deposition of the organometal halide perovskite.

34

The ideal temperatures for making a good perovskite CH3NH3PbI3 are around 330 ºC for

PbCl2 and 125 º C for CH3NH3I. To set the deposition condition, we fixed the deposition rate of

PbCl2, and adjust the deposition rate of CH3NH3I. Controlling the temperature at these conditions

provides the ratio of deposition rate of CH3NH3I:PbCl2 is 8:1. Under this ratio the deposited thin

film easily shows good perovskite structure as examined with XRD. For another previous study

using vapour deposition, the temperature of PbCl2 and CH3NH3I are 320 ºC and 116 ºC [43]. The

possible reasons for the different deposition temperatures could be the result of many factors.

Different chamber geometry, especially the distance from the K-cells to the substrate, has an

influence on the deposition rate. Also, if the vacuum conditions are not exactly the same the

deposition temperature may change form one system to another.

3.3.1 Characterization of Perovskite and Related Materials

According to previous studies of perovskites, the main XRD peaks assigned to the (110),

(220) and (330) crystal planes are at 14.1º, 28.4º, and 43.2º, respectively [19, 36]. Based on

theoretical calculations (100), (200), (300) peaks are also closely aligned at these three respective

positions with lower intensity. At 2θ = 14.1º, (100) the peak is usually covered by the (110) peak,

so a small peak should would be expected. And similar situations happen at 28.4º for (200) and

(220) peaks, and 43.2º for (300) and (330) peaks. Fig. 3.2a shows an XRD pattern of a typical

pure CH3NH3PbI3 perovskite thin film. In this pattern, three main diffraction peaks at 14.1º,

28.4º, and 43.2º presents with strong intensity. But each peak’s width is broader than a typical

single peak, indicating that the signal is a doublet of two peaks. Solution processed perovskite

have shown identical peaks in previous studies, which demonstrate that both techniques are able

to produce perovskite with same structure [19].

In order to interpret the XRD results of perovskite made by vapour deposition, several

standard patterns of related materials are also examined, including the inorganic source PbCl2

and the possible side product PbI2. PbI2 has a very high intensity of (110) at 12.65º, as shown in

Fig. 3.2b; with two other notable peaks at 39º and 52.5º. PbCl2 has a more complex pattern (Fig.

3.2c), two broad peaks at around 14º and 27.8º, five sharp peaks from 22.5º to 30º, and some

other minor peaks around 40º - 50º. With the XRD patterns of PbI2 and PbCl2, the deposited

perovskite structure can be explained if there are mixed components of PbI2 and PbCl2 in

resultant thin films.

35

.

Figure 3.2 XRD patterns: (a) Pure perovskite thin film of CH3NH3PbI3; (b) PbI2 powder purchased from Sigma-Aldrich; (c) PbCl2 powder used for film preparation purchased

from Sigma-Aldrich.

36

3.3.2 Annealing Effects for Perovskite Thin Films

The annealing process is one of the key procedures during solar cell fabrication. In

materials science, annealing is a heat treatment that alters physical and sometimes chemical

properties in order to improve the working properties. This heat treatment usually consists of

heating a material to above a certain temperature, maintaining that for certain time, and then

cooling to room temperature. Commonly, annealing is applied to soften material, relieve internal

stresses, and refine material structure for making it homogeneous, which eventually makes the

material workable with certain ideal properties. This study investigated the annealing effects on

perovskite thin films. As described in the experimental section (chapter 2), the annealing

procedure is conducted in vacuum oven under 260 ºC, and on a hot plate over the same

temperature – this is done because of the heating limitations of the vacuum oven.

Perovskite preparation requires precise control of the evaporation system, including its K-

cells temperature and QCM readings. Only several batches of samples are presenting signature

peaks of perovskite, so the study is limited to discuss only a few samples with significant

perovskite structures.

Powder X-ray diffraction was used to study the effect of annealing, and selected patterns

are shown in Fig. 3.3. Pattern (a) is the as-made sample (having been just removed from the

deposition chamber), and (b) is a sample annealed at 140 ºC in a vacuum oven. The pattern of

as-made sample shows (110), (220), and (330) peaks of perovskite at 14.1º, 28.5º, and 43.2º,

respectively, demonstrating that perovskite material has been synthesized by the vapour

deposition method. These sharp peaks are dominant signals in the pattern, which means that

CH3NH3PbI3 perovskite has been prepared as the main composition of the thin film. And as

discussed in last section, the widths of all three characteristic peaks are broadening by some

shoulder peaks. These peaks are (100), (200), and (300) peaks, covered under (110), (220), and

(330) peaks. However, this pattern also presents a PbI2 (110) peak at 12.6º, 27º, and 39º, which

reveals that the thin films are actually a mixed composite of CH3NH3PbI3 perovskite and PbI2.

Previous XRD studies have observed peaks of CH3NH3PbCl3, indicating the existence of a

mixed halide perovskite of CH3NH3Pb3-xClx. In this study, however, a peak of CH3NH3PbCl3

was not noticeable. This is mainly because the “x” is usually much smaller than 0.1, so XRD is

not necessarily able to detect its existence.

37

According to the observation of PbI2 peaks in perovskite thin films, the chemical process

of the film formation reaction of organic and inorganic source chemicals can be explained.

During the vapour deposition, the reactions taking place on the substrate can be described

through the equation below:

2CH3NH3I + PbCl2 = 2CH3NH3Cl + PbI2 (1)

PbI2 + CH3NH3I =CH3NH3PbI3 (2)

Pattern (b) is the sample annealed at 140 ºC in a vacuum oven. The pattern has the same

peak positions with a slight enhancement in the signal intensity. This enhancement attributes to

two changes in perovskite thin film. First, the annealing process helps the crystallinity. Second,

the perovskite crystal grains tend to gain size during the annealing process, because it provides a

condition for recrystallization of small grains into bigger crystals. In all, the study supports the

finding that annealing assists the crystallinity of perovskite thin film. This conclusion was

confirmed by further study on the annealing process, and will be explained in detail in the next

section.

38

Figure 3.3 XRD patterns of: (a) perovskite sample before annealing treatment and (b) perovskite ample after annealing treatment in 140 ºC for 20 minutes.

39

3.3.3 The Effects of Annealing Temperature on Perovskite Thin Films

3.3.3.1 XRD Results

To further examine the influence of annealing temperatures, we used an XRD to examine

the crystal structure of the perovskite film after annealing at various temperatures. In order to

guarantee that the thin films are identical before annealing, four pieces of film samples were

deposited in one batch so that each sample could be regarded as one part of the same film. After

each deposition, one of the film samples was used to check the purity of the perovskite with

XRD. To make deposition conditions the same, all parameters were maintained between each

deposition, and depositions in this section were conducted continuously without pauses or

sources evaporating.

The selected patterns of perovskite thin films after annealing at different temperatures are

shown in Fig. 3.4. The 100 ºC annealed sample has identical peak alignments with the 140 ºC

annealed sample, as well as the non-annealed sample shown in Fig. 3.3. Moreover, its peak

intensities are also comparable with the 140 ºC annealed sample, indicating improvement of

crystallinity after annealing at 100 ºC. This study found that for temperature ranges from 90 ºC to

160 ºC, 140 ºC is the optimal temperature for annealing treatment because the crystal structure

detected by the XRD has the greatest-intensity peaks, whereas the surface coverage still stays at

ideal level for devices.

The sample annealed at 180 ºC exhibits the disappearance of the main (110), (220), and

(330) peaks of perovskite at 14.1º, 28.5º, and 43.2°. Comparably, the peaks of PbI2 become

dominant at 12.8º, indicating the PbI2 is the major remaining crystal structure at this stage. This

is indicative that perovskite has completely disappeared after annealing at 180 ºC

After annealing at 220 ºC, the XRD pattern only shows the peak of PbI2 at 12.5º.

However, samples heated at temperature beyond 260 ºC led to the emergence of a very broad

peak at 27.0º, and a noticeable sharp peak at around 33.5º. These two peaks are identical to those

of PbCl2. The sample annealed at 400 ºC only shows weak signal of PbCl2.

40

Figure 3.4 XRD patterns of perovskite thin films annealed at temperature ranging from 100 ºC to 400 ºC. Peaks labeled with (*) are characteristic peaks of perovskite; peaks

labeled with (↓) are PbI2 peaks; peaks label with (▼) are PbCl2 peaks.

The XRD results support the perovskite thermal dynamic property observed in our daily

work. As mentioned, prepared thin films with darker color show identical perovskite peaks in

their XRD patterns. Along with the increase of annealing temperatures, the sample thin films

tend to become a lighter yellowish colour, and almost transparent after high temperature

annealing (300-400 ºC). This suggests that the dark color of the sample film is mainly

attributable to the perovskite material, rather than PbI2.

In addition, the study clearly indicates that perovskite material CH3NH3PbI3 is eliminated

from the film sample as the temperature increases. The threshold temperature is between 140 –

180 ºC, and the disappearing happens in a small temperature range without involving any new

structural evolution in the 180 ºC sample. The main concern of CH3NH3PbI3 dissolving concerns

their reaction with moisture in air, as noted in previous studies [3, 51]. However, the samples are

41

annealed in a vacuum oven, which is a condition lacking of moisture. The possible dissolving

process can be described as:

CH3NH3PbI3 = CH3NH3I + PbI2 (3)

Since the dissolving process happens between 140 -180 ºC, the mechanism is that

CH3NH3I is actively reacting to other form, and thus accelerating the dissolving of perovskite.

Limited by a lack of information on how CH3NH3I further reacts, or behaves in high

temperatures, here we cannot exhibit the whole dissolving process. However, it can be concluded

that the CH3NH3PbI3 perovskite is dissolving at a specific temperature range.

As shown in the XRD results, there are characteristic peaks of PbCl2 appearing in the

XRD of samples annealed 260 ºC, 300 ºC and 400 ºC. However, there is no Cl source contacting

the samples after the deposition, and no Cl related structure has been observed in previous XRD

studies. As mentioned, these samples are actually annealed on hot plates in ambient conditions,

instead of in a vacuum oven. As such, one possible sources of the Cl is the dirt chemicals on the

surface of the hot plate.

The as-made samples were stored in a glove box that maintained a low humidity and

oxygen environment, to ensure that samples remained free from degradation. For measuring the

XRD patterns, a certain number of samples were taken out of the glove box and transferred into

the XRD test room. The samples were exposed to air during the transferring process, and all

pieces not annealed at once. During the annealing of the first sample, the rest of samples are also

exposed in air. Therefore, the annealed sample in the second order stayed in air for around 20

minutes. The influence of this time gap may affect the crystal structure since the perovskite is

very sensitive to moisture in the air. But this possible influence is not avoidable due to the

limitations of the experimental procedure.

This hypothesis would not be valid, however, if all of the PbI2 comes from the original

deposition, and not the dissolving process. In order to confirm whether the dissolving of the

perovskite produces more PbI2, quantitative examination needs to be carried out. A very easy and

straightforward method to understand the material change is to monitor the morphology of the

sample surface. Morphological quantitative information, such as coverage and material volume,

can thus be used as tools with which to evaluate quantitative information. We therefore used

42

SEM to further examine the morphological changes of the perovskite thin films, along with the

temperature increases.

43

3.3.3.2 SEM Results

XRD results show the influence of annealing temperatures on perovskite thin films. It can

be determined that the highest tolerant temperature of perovskite is about 140-180 ºC. A series of

crystal structural changes became clear from the XRD results. This would be helpful to predict

tolerant conditions of perovskite during the fabrication of electronic devices. Based on the crystal

structural changes observed in the XRD study, thin film morphology is very likely to shed some

light on the annealing temperature change. Combining knowledge of thermal dynamic behavior

with the morphological information, a solid prediction of the perovskite thin film thermal

dynamic property could be made. In the SEM study on the influence of annealing temperatures,

we were still using the same samples examined under XRD for section 3.3.2. We observed

perovskite thin films closely under SEM with different resolutions. In Fig. 3.5 we only show

selected images with present significant features on sample surfaces.

Figure 3.5a is the SEM image of the perovskite thin film without annealing treatment.

The thin film shows a full coverage on top of the substrate surface. Unlike other thin films made

by vapour deposition, our perovskite thin film does not show extremely uniform and flat film

surface; this is possibly because this thin film is a mixture of aimed perovskite and its

intermediate PbI2. For samples without annealing, we noticed that the film degrades rapidly in a

moist atmosphere, possibly due to the hygroscopicity of CH3NH3+ cation. We discovered 140 ºC

to be the ideal temperature for annealing, but the sample still suffered from reduction of surface

coverage at this temperature. Therefore, the as-made sample without annealing is the only thin

film that has a full coverage. For solution processed perovskite films [44], the film morphology

has different behavior from sample made in air and inert gas environment. Low temperature

annealing in an inert gas environment also prevents surface coverage reduction [3, 14].

The micrograph of 100 ºC annealed thin film is shown in Fig.3.5b. The thin film started

losing surface coverage, and a portion area of silicon substrate exposure can be observed. During

the heating process, many small pores formed rapidly, then either disappearing or joining

together to form a void area that can be observed from the image. In addition, the thin film

annealed at 100 ºC was homogenous, since the film was likely pieced together by crystal grains.

As XRD verified two mixed crystal structure, perovskite and PbI2, the film displayed in the SEM

44

image seems not clear enough to distinguish one crystal from the other. Nevertheless, it is

notable that the surface coverage is slightly dropped to 94% from the fully covered thin film.

Figure 3.5 SEM micrographs of perovskite samples: (a) Sample without annealing treatment; (b) Sample annealed at 100 ºC.

Upon heating the thin film to 140 ºC, the pore kept enlarging in size, and surface

coverage reduced to around 87% (Fig. 3.6c). Besides this main coverage change, the image

shows two types of crystal morphology: one kind is big crystal grains similar to those observed

45

in 100 ºC annealed sample; the other is the smaller-grain groups gathering at the gap area

between the big grains. According to the XRD results, this film still consists of two crystal

structures, perovskite CH3NH3PbI3 and PbI2, mixing together. The emergence of the smaller-

grain groups suggests that one kind of the crystal may not mix as well as in previous samples.

But further research is needed to determine and confirm which crystal is separating from the

mixture.

Figure 3.6 SEM micrographs of perovskite samples: (c) Sample annealed at 140 ºC; (d) Sample annealed at 180 ºC.

46

The thin film sample that annealed at 180 ºC is shown in Fig 3.6d. The materials lost half

of their surface coverage over the whole substrate surface, and 51 % of the area is still covered

with the remaining materials. The crystal morphology of this sample is small crystal grain groups

that look similar to those small crystal grain groups between big crystal grains (Fig 3.6c). If the

small grain groups observed in Fig. 3.6c are one pure crystal, then this morphology verifies it as

PbI2. However, it is hard to identify the chemical composition from the morphological

observation.

Figure 3.7 SEM micrographs of perovskite samples: (e) Sample annealed at 260 ºC; (f) Sample annealed at 300 ºC.

47

Surface coverage kept decreasing as annealing temperatures increases; the thin film

sample annealed at 260 ºC has a surface coverage of 44%. The surface morphology is mainly

small crystal particles with sizes around 1 μm (Fig 3.7e). XRD results support the idea of only

PbI2 crystal left in this sample.

At very high temperature, the material only remains spherical crystals with sizes smaller

than 500 nm. Shown in Fig 3.7f, the particles only cover 7 % of the surface. According to the

XRD result, the remaining crystal is mainly PbI2 at high annealing temperatures. Again, we were

not able to detect exact chemical composition, so more study is needed to address and explain

the particle composites. As shown in the images of samples annealed over 260 ºC, no significant

morphology appears as a new crystal form. So the PbCl2 cannot be assigned as any morphology

features.

Surface coverage is one of the parameters we can use to describe the thin film formation

over a surface area. To calculate the coverage, we enlarged the film, and divided part of the

image in to 20×20 small squares. By counting the squares covered with thin film, we can

calculate the thin film morphology coverage. Table 1 shows the detailed numbers of squares

filled with materials, and also the coverage percentage of thin films. The data were then plotted

in a diagram that shows the decreasing trend of the surface coverage (Fig. 3.8).

Combining both the XRD and SEM results, we determined that the annealing

temperature greatly affects the crystallinity and morphology of perovskite. The ideal annealing

temperature assists a thin film sample for better crystallinity. In XRD results, perovskite

completely disappeared beyond 180 ºC and the major component of the remaining film is PbI2.

Since the PbI2 has been presented in the film before the perovskite is gone, it is hard to conclude

if the PbI2 is from the film preparation or is from the dissolution of perovskite. From the SEM

results, even chemical components cannot be detected; here, a study of surface coverage sheds

light on the quantitative information. The surface still had more than 50% coverage at 180 ºC, so

that at least half of the thin film’s volume was PbI2. Along with temperature increases, most of

the perovskite disappears from the sample. Our study can be referenced for the future fabrication

of mixed perovskite annealing process, although interesting questions remain open for study.

.

48

Table 1: The surface coverage analysis of samples annealed at different temperatures.

Annealing

temperature

(ºC)

Surface area

(Counts of squares)

Occupied area

(Counts of squares)

Surface

coverage

(%)

Non-annealed 400 399 99.75

100 400 377 94.25

140 400 349 87.25

180 400 206 51.50

220 400 177 44.25

300 400 31 7.75

49

Figure 3.8 Perovskite surface coverage as a function of annealing temperatures.

3.3.4 Degradation Study of the Perovskite Thin Films (in Air)

Perovskite is known as an unstable material that can degrade in air [44]. During the

fabrication of a solar cell, the exposure of perovskite to air can hardly be avoided, so

understanding the change of the material is very important. Here we studied the degradation

behavior of perovskite sample by exposing it in air, and monitoring the changes in crystal

structure and morphology.

Figure 3.9 shows the XRD patterns of the same sample in two stages. The first stage is

right after annealing, and the second stage is after exposure in air for 14 days. The relative

humidity is around 30% on average. Fig. 3.9a shows the XRD pattern of the sample after

annealing at 140 ºC. The characteristic peaks whose position at 14.1º, 28.5º, and 43.2º

correspond to (110), (220) and (330) reflections belong to perovskite structure. However, peaks

of PbI2 are also shown at 14.1º, 27.0º and 39.0º, specifying that samples made from our

technique were mixture of CH3NH3PbI3 perovskite, and PbI2. After the XRD measurement, the

samples were kept in a container that was not airtight for 14 days, which allowed the sample to

50

gain some exposure to air, but would not be contaminated by dust. Degradation took place

during the 14-day process, and we measured the XRD pattern after 14 days, which is shown in

Fig 3.9b. Comparing the XRD pattern of 14-day degraded sample with the one just after

annealing, we found that three characteristic peaks of perovskite all completely disappeared. The

remaining peaks were all due to PbI2, indicating that perovskite degraded in air, and the

remaining material is PbI2 that exists in the film from beginning. Therefore, the result

demonstrates that perovskite CH3NH3PbI3 degrades in air spontaneously.

Figure 3.9 XRD patterns of perovskite thin films: (a) Perovskite film made by vapour deposition technique and annealed at 140 ºC; (b) The same sample of (a) exposed to air for

14 days.

51

According to the XRD results, the only degraded component of the annealed film was the

perovskite. Through observation of morphology changes during the process, it is possible to

identify the morphology composition of perovskite and other components of the thin film. In this

study, SEM was used to examine the morphology in the nano scale level. The information

provided by SEM is more straightforward than a structural analysis of XRD in determining

surface morphology. The SEM images of these two stages are as shown in Fig. 3.10. Both

images were taken immediately after the XRD measurement. At the first stage (Fig. 3.10a), the

majority of the image is the material-covered film, while only a small portion of area is the

substrate exposure. By analyzing the area of the covered surface and uncovered surface, we

determined that the thin film covered area is 87.3%. In this image, we observed that the thin film

is not totally uniform with a certain degree of coverage. Moreover, there are two kinds of

morphology features on the film. One morphological feature, circled with red on the left, has

solid thin film fully covered in a certain area. This feature is identical with those of samples that

had not been annealed. On the other hand, morphology as circled in green, on the right, has a

structure of connected small grains sitting at the edge of a big solid film. Correlated to the XRD

results, these two morphological features may strongly relate to crystal of perovskite and PbI2.

Figure 3.10b shows the second stage of the degradation process. It highlights that after exposure

in air for a 14-day period, the thin film coverage drops to 33.8%, which is less than half of its

original coverage. Also, the thin film morphology consists of small grain sized crystals with

disappearance of the bigger area solid thin film. This is indicative of the fact that the solid thin

film pieces degraded in air. We may be able to confirm that the perovskite structure that changed

in XRD and the bigger solid thin film in morphology are closely related. It is possible that the

disappeared solid crystal pieces are perovskite, and the remaining smaller sized grains are PbI2.

Based on the XRD and SEM results, we are able to calculate the degradation rate of

perovskite in air over time. Over 14 days, the surface coverage percentage dropped from 87.5%

to 33.8%, equivalent to a daily average drop of 5.37% on the surface area. We believe that the

degradation rate is related to the specific humidity condition and thus may vary each day.

Nevertheless, we conducted an efficient method to quantitatively evaluate the degradation rate of

perovskite thin film, which may contribute to the manufacturing control in large scale

production.

52

Figure 3.10 SEM images of perovskite films: (a) Perovskite film made by vapour deposition technique and annealed at 140 ºC; (b) The same sample from (a) exposure in air for 14

days.

53

3.3.5 High Resolution Study of Perovskite Thin Film with AFM

To investigate the surface features in high resolution, we used AFM to gain surface

information that explained the annealing process and growth mechanisms. AFM is a powerful

characterization method used to investigate surface morphology, and is designed for scanning on

small-scale surfaces with height distribution within its capable z-direction adjustment. The image

quality heavily relies on the sample surface conditions and the lateral limitation is very small

(100 μm × 100 μm). For our study, only suitable samples were selected to analyze surface

morphology with AFM.

As discussed in the XRD and SEM results sections (chapter 2), we only observed a surface

coverage loss of 12 % during the heating process, but the crystal morphology remained almost the

same. In addition, the film that showed grain size change in XRD cannot be characterized using SEM.

The detail morphological information on the crystal is studied by one of high resolution scanning

probe microscopy, AFM.

Figure 3.11 AFM images of perovskite films annealed at different temperature from 85 ºC to 140 ºC.

Here, AFM provided additional high-resolution information of surface features. Figure

3.11 is a collection of AFM images of perovskite, showing a significant small region (1 μm×1

μm) of perovskite surface, with different annealing temperatures. The samples annealed at 85 ºC,

100 ºC and 140ºC, as well as the sample that was not annealed, present morphological

differences. The non-annealed and 85 º C annealed samples are both fully covered by materials.

The thin film surface consists of uniform spherical grains with an average grain size of around

100 nm. The 100 ºC annealed sample shows a more flat shape in each grain, tending to have a

sheet-like shape. The 140 ºC annealed sample tends to increase this effect. The grains are even

flatter and exhibit a trend of amalgamating all grains into bigger crystal sheets. This phenomenon

54

indicates a recrystallization of the thin film during the annealing treatment. Grain sizes of the 140

ºC annealed sample become larger due to the recrystallization. This observation is consistent

with the XRD results. The recrystallization effects of annealing can only be observed in high

resolution AFM images, whereas SEM cannot differentiate morphological change at this scale.

We can also use AFM morphological images to study the growth process by comparing

images for films with different thickness, as shown in Fi. 3.12. The left panel shows the top-

down-view of the surface of a 10-nm thick sample, while the right panel depicts a 100 nm thin

film. The enlarged images (1 μm × 1 μm) are inserted for a detailed observation in high image

resolution. The image shows the grain sizes of the 10-nm thick sample range from 10 nm to 200

nm. At the same magnification, the surface of the 100-nm thick sample shows much more

uniform thin film structures, and all the grains have a sized of 50 nm (± 5 nm). The difference

between the two images indicates that the film formation is not always uniform along the

deposition process. The grain sizes change along the deposition, but eventually become uniform

when the film is deposited with enough thickness..

Figure 3.12 AFM images of perovskite films with10 nm thickness (left) and 100 nm thickness (right).

55

3.4 Summary

Organometal halide perovskite thin films were prepared using the vapour deposition

technique. First, this study optimized the conditions for perovskite preparation in a vapour

deposition system. The ideal deposition temperatures of PbCl2 and CH3NH3I are 125 °C and

330 °C, respectively, and the ratio of the organic source to the inorganic source is 8:1. In addition,

a standard for quickly checking the crystal film by its colour assisted this study in deciding if a

batch of deposition was successful. The XRD shows perovskite contained in most of our samples,

even though PbI2 was mixed in with the film. The XRD results compare the effects of different

annealing temperatures, and show that perovskite is a thermally unstable material. The films

demonstrated lack of tolerance for high temperature, and the characteristic peaks of CH3NH3PbI3

perovskite in the XRD disappeared after 180 °C annealing. The XRD results also support the

finding that the remaining part of the thin film is PbI2. Further observation of SEM images

compared the morphological change along with annealing temperature increases. Here, we found

that the surface coverage of the thin film decreased with the increase in annealing temperature.

From the results of both XRD and SEM. we concluded that perovskite was decomposed while

only PbI2 remained. We also identified that PbCl2 evaporates from the film in high temperatures.

Air exposure investigation illustrated that the perovskite was unstable when exposed to

air for long periods of time, and also only PbI2 remained after degradation. This observation

confirmed that perovskite is very sensitive to the ambient environment, and the mixed thin film

of perovskite and PbCl2 usually resulted only in PbCl2. Naturally, SEM has limitations on

observing the surface morphology in a high resolution, so AFM was used to watch closely of the

thin film surface. AFM showed detailed morphological change in high resolution.

56

Chapter 4 Conclusions and Future Work

In this study, organometal halide CH3NH3PbI3 thin films for perovskite solar cells were

prepared using the vapour deposition technique. The deposition temperature and the ratio of two

source chemicals were optimized. The resultant perovskite films still contained a certain portion

of PbI2, which indicates that PbI2 is the intermediate of the chemical reaction. The presence of

the PbI2 intermediate also verified the proposed chemical reactions for perovskite formation. The

study also discovered that controlling the deposition rate and ratio of chemical sources are key

factors in preparing uniform perovskite thin film.

The tolerance temperature of annealing treatment was examined, and an ideal

temperature for the annealing process was decided to be around 140 ºC. A series of perovskite

samples were prepared and annealed at a wide range of temperatures (100 - 400 °C). Results

showed that the perovskite has a low tolerance, and the thin film was not very stable upon

heating to high temperatures. Annealing at 180 °C eliminated all perovskite in the thin film, and

only PbI2 remained. At very high annealing temperatures, PbCl2 was observed, but it was very

hard to explain the source of the Cl. One possible reason is that the hot plate was not clean from

a previous use.

In terms of morphological control, SEM micrographs of non-annealed perovskite showed

the full coverage of perovskite thin film. It indicated that the vapour deposition technique is ideal

for making perovskite uniform thickness, and fully covered thin film. The surface kept losing the

material coverage with the increase of the annealing temperature. Prior 180 °C, it was observed

that perovskite dissolved in the temperature range of 140 – 180 °C. The remaining PbCl2 covered

more than half of the surface, even over 180 °C.

The stability and durability of perovskite thin films was studied by exposing the sample

to air for a relatively long period of time. The observations demonstrated the transformation of

perovskite to PbI2 intermediate. The use of AFM technique allowed for the detailed observation

on surface features, so the annealing effect that was unobtainable in SEM became apparent.

AFM results also showed high-resolution details, which indicated that the film gets more

uniform in thicker thin film samples.

57

There are still a number of topics worth investigation, beyond the results and findings of

this study. Regarding the control of perovskite thin film preparation, further optimization of

preparation conditions may offer perovskites with purer and higher crystallinity. Thus, the film

can get larger crystal grains, and a higher purity of perovskite. Since the crystal quality is a key

factor for photovoltaic performance, it would be helpful to obtain a device with higher

performance capabilities. Moreover, further studies of morphological controls are crucial for

optimizing the manufacturing procedure. In terms of the morphological effect in a real solar cell

device, a study of the interfaces of different functional layers in photovoltaic device may be of

great interest and importance.

Advanced microscopic technology will assist in understanding a series of questions about

surface and interface of thin film devices. By displaying the high-resolution morphology in ultra-

thin films solar cells, a number of unrevealed surface features can be captured. These features

would greatly benefit on explaining thin film properties and the device performance.

58

References:

1. Wen, W.-T.D., Hybrid Organic/Inorganic Solar Cells based on Electrodeposited ZnO Nanowire Arrays on ITO and AZO Cathodes. 2011, University of Toronto.

2. Dennler, G., N.S. Sariciftci, and C.J. Brabec, Conjugated polymer-based organic solar cells. Technology, 2006. 11: p. 2.

3. Hodes, G., Perovskite-Based Solar Cells. Science, 2013. 342(6156): p. 317-318.

4. Snaith, H.J., Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells. The Journal of Physical Chemistry Letters, 2013. 4(21): p. 3623-3630.

5. Goldschmidt, V.M., Die gesetze der krystallochemie. Naturwissenschaften, 1926. 14(21): p. 477-485.

6. Wenk, H.-R. and A. Bulakh, Minerals: their constitution and origin. 2004: Cambridge University Press.

7. Tejuca, L.G. and J.L. Fierro, Properties and applications of perovskite-type oxides. 2000: CRC Press.

8. Lemanov, V., et al., Perovskite CaTiO3 as an incipient ferroelectric. Solid state communications, 1999. 110(11): p. 611-614.

9. Luxová, J., P. Šulcová, and M. Trojan, Study of perovskite compounds. Journal of thermal analysis and calorimetry, 2008. 93(3): p. 823-827.

10. Lufaso, M.W. and P.M. Woodward, Jahn-Teller distortions, cation ordering and octahedral tilting in perovskites. Acta Crystallographica Section B, 2004. 60(1): p. 10-20.

11. Chen, Q., et al., Planar heterojunction perovskite solar cells via vapor-assisted solution process. Journal of the American Chemical Society, 2013. 136(2): p. 622-625.

12. Park, N.-G., Organometal perovskite light absorbers toward a 20% efficiency low-cost solid-state mesoscopic solar cell. The Journal of Physical Chemistry Letters, 2013. 4(15): p. 2423-2429.

13. Hao, F., et al., Lead-free solid-state organic-inorganic halide perovskite solar cells. Nature Photonics, 2014.

14. Burschka, J., et al., Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature, 2013. 499(7458): p. 316-319.

15. Lufaso, M.W. and P.M. Woodward, Jahn-Teller distortions, cation ordering and octahedral tilting in perovskites. Acta Crystallographica Section B: Structural Science, 2004. 60(1): p. 10-20.

59

16. Johnsson, M. and P. Lemmens, Perovskites and thin films—crystallography and chemistry. Journal of Physics: Condensed Matter, 2008. 20(26): p. 264001.

17. Jahn, H.A. and E. Teller, Stability of polyatomic molecules in degenerate electronic states. I. Orbital degeneracy. Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, 1937: p. 220-235.

18. O’regan, B. and M. Grfitzeli, A low-cost, high-efficiency solar cell based on dye-sensitized. nature, 1991. 353: p. 737-740.

19. Lee, M.M., et al., Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science, 2012. 338(6107): p. 643-647.

20. Chen, G., X. Liu, and C. Su, Distinct effects of humic acid on transport and retention of TiO2 rutile nanoparticles in saturated sand columns. Environmental science & technology, 2012. 46(13): p. 7142-7150.

21. Bian, Z., et al., Superior electron transport and photocatalytic abilities of metal-nanoparticle-loaded TiO2 superstructures. The Journal of Physical Chemistry C, 2012. 116(48): p. 25444-25453.

22. Kalyanasundaram, K., Dye-sensitized solar cells. 2010: EPFL press.

23. Zeng, W., et al., Efficient dye-sensitized solar cells with an organic photosensitizer featuring orderly conjugated ethylenedioxythiophene and dithienosilole blocks. Chemistry of Materials, 2010. 22(5): p. 1915-1925.

24. Chung, I., et al., All-solid-state dye-sensitized solar cells with high efficiency. Nature, 2012. 485(7399): p. 486-489.

25. Feldt, S.M., et al., Design of organic dyes and cobalt polypyridine redox mediators for high-efficiency dye-sensitized solar cells. Journal of the American Chemical Society, 2010. 132(46): p. 16714-16724.

26. Ning, Z., Y. Fu, and H. Tian, Improvement of dye-sensitized solar cells: what we know and what we need to know. Energy & Environmental Science, 2010. 3(9): p. 1170-1181.

27. Rühle, S., M. Shalom, and A. Zaban, Quantum ‐ dot ‐ sensitized solar cells. ChemPhysChem, 2010. 11(11): p. 2290-2304.

28. Zhang, Q., et al., Highly efficient CdS/CdSe-sensitized solar cells controlled by the structural properties of compact porous TiO2 photoelectrodes. Physical Chemistry Chemical Physics, 2011. 13(10): p. 4659-4667.

29. Barea, E.M., et al., Design of injection and recombination in quantum dot sensitized solar cells. Journal of the American Chemical Society, 2010. 132(19): p. 6834-6839.

30. Tang, J., et al., New starburst sensitizer with carbazole antennas for efficient and stable dye-sensitized solar cells. Energy & Environmental Science, 2010. 3(11): p. 1736-1745.

60

31. Burschka, J., et al., Tris (2-(1 H-pyrazol-1-yl) pyridine) cobalt (III) as p-type dopant for organic semiconductors and its application in highly efficient solid-state dye-sensitized solar cells. Journal of the American Chemical Society, 2011. 133(45): p. 18042-18045.

32. Mitzi, D., et al., Conducting layered organic-inorganic halides containing< 110>-oriented perovskite sheets. Science, 1995. 267(5203): p. 1473-1476.

33. Kojima, A., et al., Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. Journal of the American Chemical Society, 2009. 131(17): p. 6050-6051.

34. Im, J.-H., et al., 6.5% efficient perovskite quantum-dot-sensitized solar cell. Nanoscale, 2011. 3(10): p. 4088-4093.

35. Kim, H.-S., et al., Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Scientific reports, 2012. 2.

36. Heo, J.H., et al., Efficient inorganic-organic hybrid heterojunction solar cells containing perovskite compound and polymeric hole conductors. Nature photonics, 2013. 7(6): p. 486-491.

37. Noh, J.H., et al., Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells. Nano letters, 2013. 13(4): p. 1764-1769.

38. Service, R., Perovskite Solar Cells Keep On Surging. Science (New York, NY), 2014. 344(6183): p. 458.

39. Tsujimoto, K., et al., TiO2 surface treatment effects by Mg2+, Ba2+, and Al3+ on Sb2S3 extremely thin absorber solar cells. The Journal of Physical Chemistry C, 2012. 116(25): p. 13465-13471.

40. Qiu, Y., W. Chen, and S. Yang, Facile hydrothermal preparation of hierarchically assembled, porous single-crystalline ZnO nanoplates and their application in dye-sensitized solar cells. Journal of Materials Chemistry, 2010. 20(5): p. 1001-1006.

41. Mei, A., et al., A hole-conductor–free, fully printable mesoscopic perovskite solar cell with high stability. science, 2014. 345(6194): p. 295-298.

42. Hardin, B.E., H.J. Snaith, and M.D. McGehee, The renaissance of dye-sensitized solar cells. Nature Photonics, 2012. 6(3): p. 162-169.

43. Liu, M., M.B. Johnston, and H.J. Snaith, Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature, 2013. 501(7467): p. 395-398.

44. Eperon, G.E., et al., Morphological Control for High Performance, Solution‐Processed Planar Heterojunction Perovskite Solar Cells. Advanced Functional Materials, 2014. 24(1): p. 151-157.

61

45. Bragg, W.L., The structure of some crystals as indicated by their diffraction of X-rays. Proceedings of the Royal Society of London. Series A, 1913. 89(610): p. 248-277.

46. Dann, S.E., Reactions and Characterization of Solids. 2002: Royal Society of Chemistry, USA.

47. Lennard-Jones, J.E., Proc. R. Soc. Lond. A, 1924. 106: p. 463.

48. Qin, P., et al., Inorganic hole conductor-based lead halide perovskite solar cells with 12.4% conversion efficiency. Nature communications, 2014. 5.

49. Grätzel, M., The light and shade of perovskite solar cells. Nature materials, 2014. 13(9): p. 838-842.

50. Zhou, H., et al., Interface engineering of highly efficient perovskite solar cells. Science, 2014. 345(6196): p. 542-546.

51. Green, M.A., A. Ho-Baillie, and H.J. Snaith, The emergence of perovskite solar cells. Nature Photonics, 2014. 8(7): p. 506-514.