INFLUENCE OF COUNTER IONS OF BINARY DOPED CONDUCTING ...
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INFLUENCE OF COUNTER IONS OF BINARY DOPED CONDUCTING POLYMER ON THE PHOTOVOLTAIC PROPERTIES OF DYE SENSITIZED SOLAR CELLS PhD Thesis By SHEHNA FAROOQ NATIONAL CENTRE OF EXCELLENCE IN PHYSICAL CHEMISTRY, UNIVERSITY OF PESHAWAR, KPK, PAKISTAN 2019
Transcript of INFLUENCE OF COUNTER IONS OF BINARY DOPED CONDUCTING ...
INFLUENCE OF COUNTER IONS OF BINARY DOPED
CONDUCTING POLYMER ON THE PHOTOVOLTAIC
PROPERTIES OF DYE SENSITIZED SOLAR CELLS
PhD Thesis
By
SHEHNA FAROOQ
NATIONAL CENTRE OF EXCELLENCE IN
PHYSICAL CHEMISTRY, UNIVERSITY OF
PESHAWAR, KPK, PAKISTAN
2019
INFLUENCE OF COUNTER IONS OF BINARY DOPED
CONDUCTING POLYMER ON THE PHOTOVOLTAIC
PROPERTIES OF DYE SENSITIZED SOLAR CELLS
PhD Thesis
By
SHEHNA FAROOQ
A dissertation submitted to the University of Peshawar in partial fulfillment of the requirement
for the degree of
DOCTOR OF PHILOSOPHY
IN
PHYSICAL CHEMISTRY
NATIONAL CENTRE OF EXCELLENCE IN
PHYSICAL CHEMISTRY, UNIVERSITY OF
PESHAWAR, KPK, PAKISTAN
2019
NATIONAL CENTRE OF EXCELLENCE IN PHYSICAL CHEMISTRY
UNIVERSITY OF PESHAWAR
2019
APPROVAL CERTIFICATE
It is recommended that the dissertation prepared by Ms. Shehna Farooq entitled “Influence of
Counter Ions of Binary Doped Conducting Polymer on the Photovoltaic Properties of Dye
Sensitized Solar Cells” be accepted as fulfilling this part of the requirement for the degree of Doctor
of Philosophy in Physical Chemistry.
______________________________ _____________________________
Dr. Salma bilal Prof. Dr. Abdul Naeem Khan
Research Supervisor Director
EXAMINATION SATISFACTORY
COMMITTEE ON FINAL EXAMINATION
__________________________________ ______________________________
External Examiner Internal Examiner
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TABLE OF CONTENTS
Table of contents i
List of Abbreviations and symbols vii
Abstract viii
Key words x
Acknowledgment xi
Dedication xiii
Chapter 1
1 Introduction 1
1.1 Current Status of Solar Energy Utilization 1
1.1.1 First Generation 2
1.1.2 Second Generation 2
1.1.3 Third Generation 2
1.2 A Brief History of Dye Sensitized Solar Cells 4
1.3 Operational Principle of the Dye-Sensitized Solar Cell 4
1.4 Electron Transfer in DSSCs 6
1.4.1 Electron Excitation in Dye by Absorption of Photons and Ultrafast….. 6
1.4.2 Reactions at the Surface of Counter Electrode 7
1.4.3. Back Electron Transfer or Recombination Reactions 8
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1.5 Performance Evaluation of DSSCs 9
1.5.1 Short-Circuit Current (Isc) 9
1.5.2 Open Circuit Voltage (OCV) 10
1.5.3 Fill Factor (FF) 10
1.5.4 Power Conversion Efficiency (η) 10
1.6 Components of DSSC 11
1.6.1 Transparent Conducting Glass 12
1.6.2 Photoanode 13
1.6.3 Sensitizer 15
1.6.4 Electrolyte 19
1.6.5 Counter Electrode 19
1.6.5.1 Platinum Counter Electrode 20
1.6.5.2 Carbon Materials 21
1.6.5.3 Inorganic Compounds 21
1.6.5.4 Intrinsically Conducting Polymers 22
1.7 Polyaniline: (an Overview) 23
1.7.1 Chemical Structure of PANI 24
1.7.2 Synthesis of Polyaniline 27
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1.7.2.1 Electrochemical Polymerization 27
1.7.2.2 Chemical Oxidative Polymerization 27
1.7.3 Doping in Polyaniline 28
1.7.4 Binary doping in Polyaniline 29
1.7.5 Polyaniline as Counter Electrode in DSSCs 31
1.7.6 Techniques for the Analysis of Counter Electrode and DSSC 33
1.7.6.1 Cyclic Voltammetry 34
1.7.6.2 Electrochemical Impedance Spectroscopy 35
1.7.6.3 Photovoltaic Measurements 36
1.8 Aim of the Present Work 37
Chapter: 2
2 Experimental 39
2.1 Materials 39
2.2 Synthesis of Pristine PANI 39
2.3 Synthesis of H2SO4 Doped PANI 39
2.4 Synthesis of ALS Doped PANI 40
2.5 Synthesis of Binary Doped PANI 40
2.6 Fabrication of PANI Based Counter Electrodes (CEs) 41
2.7 Fabrication of DSSCs Devices 42
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2.8 Material Characterization 43
2.8.1 Physico-chemical Characterization 43
2.8.1.1 Polymerization Yield 43
2.8.1.2 Focused Ion Beam Scanning Electron Microscope (FIB-SEM) 44
2.8.1.3 Fourier transform infrared spectroscopy 44
2.8.1.4 X-ray diffraction 44
2.8.1.5 DC conductivity 44
2.8.1.6 Elemental Analysis and Mapping 45
2.8.1.7 UV-Vis spectroscopy 45
2.8.1.8 Cyclic Voltammetry 45
2.8.2 Photovoltaic Characterization 45
2.8.2.1 Photocurrent density-voltage (I-V) test 45
2.8.2.2 Start/stop ability test 45
2.8.2.3 Electrochemical impedance spectroscopy 46
Chapter 3:
3 Results and Discussion 47
Part 1: Optimization of Reaction Parameters and Analysis of Different Properties of Resulting
Polymers for Application in Dye Sensitized Solar Cells
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3.1 Effect of Synthesis Parameters in Binary Doped PANI 47
3.1.1 Effect of Monomer Amount on Polymerization Yield 47
3.1.2 Influence of H2SO4 Amount on the Polymerization Yield 48
3.1.3 Effect of ALS Amount on the Yield of Binary Doped PANI 49
3.2 Surface Morphology of Binary Doped PANI Samples and CEs 51
3.3 Elemental Analysis and Mapping 58
3.4 Optical Properties of Binary Doped PANI 61
3.5 DC Conductivity of Binary Doped PANI 63
3.6 Functional Groups Detection of Binary Doped PANI Samples 65
3.7 XRD Analysis 68
3.8 Electrocatalytic Activity of Binary Doped PANI CEs 70
3.9 Photovoltaic Performance of DSSCs Based Binary Doped PANI CEs and Pt CE 76
3.10 Process of Photoexcitation in DSSCs 78
3.11 Durability of Fabricated DSSCs 79
3.12 Charge Transport Properties 80
Part 2: Comparison of Photovoltaic Properties of DSSCs based on Pristine PANI, H2SO4
doped PANI, ALS doped PANI and binary doped PANI CEs (Effect of counter ion)
3.13 Morphology 83
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3.14 XRD Analysis 88
3.15 FTIR Analysis 89
3.16 DC Conductivity 91
3.17 Electronic Spectroscopy 92
3.18 SEM-EDX Analysis and Elemental Mapping 95
3.19 Electrochemical Characterization 98
3.20 Photovoltaic Properties 104
3.21 Electrochemical Impedance spectroscopy 108
3.22 Multiple start/stop proficiency 111
Summary 112
Conclusion 114
Future outlook 114
References 115
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List of Abbreviations and symbols
Abbreviations
OPVCs Organic Photovoltaic Cells
DSSCs Dye-Sensitized Solar Cells
AM 1.5 Air Mass 1.5
TCO Transparent Conducting Oxide
CE Counter Electrode
HOMO Highest Occupied Molecular Orbital
LUMO Lowest Unoccupied Molecular Orbital
CB Conduction Band
ITO Tin-Doped Indium Oxide
FTO Fluorine-Doped Tin Oxide
Jsc Short Circuit Current Density
OCV Open Circuit Voltage
FF Fill Factor
ICPs Intrinsically Conducting Polymers
CV Cyclic Voltammetry
EIS Electrochemical Impedance Spectroscopy
RS Series Resistance
RCT Charge Transfer Reaction
FIB-SEM Focused Ion Beam Scanning Electron Microscope
ECA Electro Catalytic Activity
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Abstract
Dye-sensitized Solar Cells (DSSCs), different in principle from the conventional solar cells
based on p–n junctions, are competitively cost-effective. For development of this kind of emerging
solar cell technology, it is very significant to reduce cost and improve energy conversion efficiency
of these cells to the maximum extent. The work presented in this thesis is focused on the synthesis
of binary doped Polyaniline (PANI) salts and their use as counter electrodes (CEs) to replace the
noble and expensive metal Pt in DSSCs. An effective material should exhibit high conductivity,
high catalytic activity and favorable morphology to be used as a CE in DSSC. Therefore, a novel
dopant couple comprising of an organic acid i.e Ammonium Lauryl Sulphate (ALS) and an
inorganic acid i.e Sulfuric Acid (H2SO4) was employed with the aim to enhance the conductivity,
favorable morphology and electrocatalytic activity of PANI. The synergy of the positive quality
factors of these two acid dopants was realized by concurrent and optimized doping, resulting in a
molecular composite material that renders good photovoltaic performance.
This dissertation comprises of two parts: one is based on the synthesis and optimization of
reaction parameters for obtaining binary doped PANI most suitable for utilization as CE in DSSCs
whereas second part includes the study of the effect of counter ions of the binary dopant ions on
the photovoltaic properties of the DSSC device by comparing its performance with DSSCs based
on Pristine PANI, H2SO4 doped PANI, ALS doped PANI, and Pt CEs.
Different morphological, optical and spectroscopic techniques were employed to
characterize as synthesized PANI salts. The improvement in conductivity and decrease in band
gap illustrated the positive effect of binary dopants on the binary doped PANI salts. These
properties are known to be of great importance to the catalytic electrode material. Electrocatalytic
properties of binary doped PANI CEs were investigated by Cyclic Voltammetry.
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The higher values of reduction current density (Jred) and diffusion coefficient (Dn) and
lower values of peak to peak separation (Epp) also contributed to the desirable properties of the
synthesized material for their better performance as a catalytic material in solar devices. These
synthesized materials were tested for photovoltaic application. It was interesting to realize that
these materials demonstrated good photovoltaic properties as compared to noble metal Pt.
Furthermore, these fabricated devices were also checked through Electrochemical Impedance
Spectroscopy EIS to understand the charge transfer processes and very encouraging results have
been obtained with decrease in charge transfer resistance. In addition to these, the durability of
DSSC based on binary doped PANI was also examined by start/stop switching.
In the second part of dissertation, comparative study of optimized binary doped PANI with
pristine PANI, H2SO4 doped PANI, ALS doped PANI and Pt is discussed. All the above mentioned
techniques were used to study the physical, electrochemical properties and photovoltaic properties
of the synthesized materials. Comparative analysis revealed that the counter ions of the dopant do
have great impact on the properties of PANI based DSSCs. The binary doped PANI showed
superior electrochemical properties, photovoltaic properties and facile charge transfer. At
optimized fabrication conditions, the counter electrode shows significantly high photoelectric
conversion efficiency of 4.54% compared to 4.02% for reference platinum counter electrode.
Charge transfer resistance at the interface between electrolyte and counter-electrode is also
decreased for the binary polyaniline based counter electrode. Furthermore, the devices also present
the characteristics of multiple start/stop ability and fast activity.
The simple preparation procedure, low cost and improved photovoltaic properties permit
fabricated counter electrodes to be a reliable alternative for dye sensitized solar cells for their better
performance as a catalytic material in solar devices.
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Keywords: Dye sensitized solar cells, counter ions, binary doped PANI, counter electrodes,
electrocatalytic activities, start/stop switching.
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Acknowledgements
All praises and virtues to Almighty ALLAH, the most gracious, the most compassionate,
the creator of the universe, who pulled us out of darkness of ignorance and enlightened us with
knowledge. All the favors of ALLAH to Holy Prophet HAZRAT MUHAMMAD (PBUH), who
distinguished between right and wrong and also guides us towards the path of success.
I owe my deepest gratitude to Dr. Salma Bilal, my worthy supervisor, who has been an
inspiration during the course of thesis. I thank her for his patience and encouragement that carried
me on through difficult times, and for her insights and suggestions that helped to shape my research
skills. Her dynamical attitude has empowered me with zeal of energy to conquer the minor details
of my research work. I express my sincere thanks to Dr. Anwar-ul-Haq, Institute of Chemical
Sciences, University of Peshawar, for his valuable guidance in carrying out work under his
meaningful discussions, encouragement and cooperation.
I would like to thank Dr. Asif Ali Tahir, my host supervisor in University of Exeter, UK,
for his assistance and guidance throughout my laboratory work and providing the facilities for
carrying out the research in university of Exeter. I would like to thank the lab mates of
environmental and sustainability laboratory, University of Exeter, especially Prabu, Bander and
Govinder for their pleasant support during my research.
I wish my special thanks to Prof. Dr. Abdul Naeem, Director, National Centre of
Excellence in Physical Chemistry, for providing all other sorts of research facilities. Generally I
wish to give my grateful acknowledgements to all the members of the National Centre of
Excellence in Physical Chemistry, University of Peshawar who provide a pleasant atmosphere.
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The meaning of life and work is incomplete without paying regards to my respected Parents whose
blessings and continuous encouragement have shown me the path to achieve my goals. They have
educated me with aspects from all facets of life offered me unconditional support and
encouragement to pursue my interests.
A special word of thank to my husband Sami-ur-Rahman for being incredibly loving,
understanding, supportive and patient. I am greatly indebted to the parents, brothers and sisters of
my husband for their love, affection and prayers. I am highly obliged to my brothers for their
timely support and help.
I wish to express my sincere feelings to my lab mates and friends Madeeha and Ammara
for their patience, moral support and constant cooperation.
Finally, Engineering and Physical Science Research Council, UK (EPSRC grant No
EP/P510956/1 and EP/R512801/1), Alexender von Humboldt Foundation, Germany, and Higher
Education Commission Pakistan (project No. 20-1647 & 20-3111/NRPU/R&D/HEC) are highly
acknowledged for financial support. NSG Pilkington Glass Ltd. is acknowledged for kindly
providing the FTO substrates for this work.
Shehna Farooq
February, 2019
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DEDICATION
Dissertation dedicated to my husband,
my son, siblings, mother and father
(late)
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Chapter # 1:
1. Introduction
1.1 Current Status of Solar Energy Utilization
The global demand of energy and consequently depletion of fossil fuels associated with
greenhouse effect, make nations to reassess the significance of exploring the renewable energy
sources. In future the world will be facing a limited supply of petroleum and consequently high
prices [1]. Recently, the term “Renewable Energy” has been come out to represent a sustainable
energy future. The International Energy Agency (IEA) explains “Renewable Energy” as energy
derived from the natural processes that are constantly replenished [1]. Renewable energy sources
comprises the solar energy, wind, geothermal, biofuels, hydropower and hydrogen resultant from
renewable assets [2]. These sources are limitless and environmental friendly and have the ability
to subsist the energy demands that are increasing rapidly with industrialization and human
population. As earth surface is constantly facing the ample amount of solar energy therefore, solar
energy is assumed to be the chief and limitless renewable energy resource having the substantial
prospective of fulfilling human necessities [3].
Sunlight is a free and abundant source of energy that can be apprehended by new and modern
technologies and altered into electricity [4]. It has been estimated that sunlight which reaches the earth
surface provide 10000 times more energy than that which the whole world devours [5]. Therefore by
using the solar energy is expected to have the potential to encounter a large portion of future energy
consumption demands.
A solar cell or photovoltaic cell is an energy harvesting device that can directly generate
electricity from sunlight through the photovoltaic effect and plays very important role in various
energy sectors and currently find top position among other technologies. The world’s first
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selenium based solar cell is credited to Charles Fritts in 1883 with power conversion efficiency of
1-2 % [6]. Later in 1954, a group of scientists at Bell labs assembled the first applied solar cell
with 6% efficiency by using silicon tampered with various impurities, which is now referred as 1st
generation solar cell [7].
Solar cells can be divided into three generations on the basis of operational principles and material
used:
1.1.1 First Generation
Silicon based solar cells are the “first generation” solar cells and are reported by Bell
Laboratories in 1954. These cells uses monocrystalline silicon (mc-silicon). The power conversion
efficiency of these cells can be upto 25% but the high cost and complicated manufacturing process
have limited their wide range applications. But still their development continues upto the present-
day. The present price of this generation solar cells is ~US$4/Wp (Fig. 1) [8], still very expensive
to have a significant doubt on the energy production Market [9].
1.1.2 Second Generation
Solar cells based on Polycrystalline silicon (pc-silicon) and thin films, namely, Copper
Indium Gallium Selenide (CIGS), Gallium Arsenid (GaAs), Cadmium Telluride (CdTe) etc. The
second generation solar cells are based on Polycrstalline silicon (pc-silicon) and thin films, namely,
Copper Indium Gallium Selenide (CIGS), Gallium Arsenid (GaAs), Cadmium Telluride (CdTe)
etc. These cells utilize thin-films usually in micrometers, can substantially lower the energy
requirements, and hence the manufacturing cost. These cells utilize thin-films usually in
micrometers, can substantially lower the energy requirements for their fabrication, and hence the
manufacturing cost. However, these cells have low efficiency compared to first generation solar
cells [10, 9].
3
1.1.3 Third Generation
The limitations of 2nd generation solar cells have inspired researchers to investigate a 3rd
generation of solar cell, focusing on cheaper organic and polymer materials with substantially
lower manufacturing costs, greater flexibility and ultimately, higher efficiency. Generally these
cells are semiconductor devices because they do not count on traditional p-n junctions to separate
photo generated charges [11]. The organic photovoltaic cells (OPVCs) are the most auspicious
group of third generation solar cells, which are characteristically divided into two groups based
upon the ingredients used for their assembly. The first group of OPVCs is fully organic because
they are completely made from organic or polymeric materials. This group of solar cells includes
the bulk hetero-junction solar cell (BHJSCs) [12]. The second group of OPVCs consist hybrid
solar cells which are composed from organic and inorganic composites.
Those solar cells which are composed from inter-penetrating mesoscopic semiconductors
show higher efficiency than conventional cells [13]. The prototype cells of this group are called
Dye-sensitized Solar Cells (DSSCs). These cells depend on the process of absorption and
separation of charge with association of sensitizer as light absorbing because of wide band gap
semi-conductors.
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Figure 1.1: Cost efficiency analysis of first, second, and third (IIIa and IIIb) generation
photovoltaic devices. [Adopted from Ref. 18]
1.2 A Brief History of Dye Sensitized Solar Cells
Photo electrochemical cells are very interesting as these are potentially large scale
harvesters of sunlight for many years. As from 1984 the situations about these photo
electrochemical cells still looked depressing because the efficiency and stabilities of these
experimental cells were very low. The work on DSSCs was initiated in 1991 by Michael Gratzel
and Brian O’ Regan, and they reported a cell with efficiency of 9% under AM 1.5 conditions [14].
Furthermore a rapid advanced research has been done on the DSSCs to increase the efficiency
which was utmost necessary for commercial use. In 2010, DSSCs attained 11% efficiency, but still
using the same materials and structure that enabled the 1991 breakthrough [15, 16]. Liyuan and
coworkers reported a cell with 11.4% efficiency in 2012 by using donor acceptor type co-
adsorbents materials, because these overcome the absorption of light with the help of redox
electrolyte and minimize the recombination reactions [17]. Currently the record efficiency for the
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cell is 13% at standard illumination condations using porphyrin dye with combination of cobalt
(II/III) redox electrolyte [18].
1.3 Operational Principle of the Dye-Sensitized Solar Cell
Dye-sensitized solar cell (DSSC) is a photovoltaic device that directly converts light energy
into electrical energy by using photovoltaic phenomenon. DSSC comprises of a mesoporous film
of wide band gap oxide generally called photoanode. Generally, photoanode is constructed by
coating a film of TiO2 on transparent conducting oxide (TCO) for electron transfer. Beside this a
wide band gap oxide materials have also been investigated such as ZnO and Nb2O5 [19, 20].
Photoanode is sensitized by adsorption of a layer of sensitizer (dye) on its surface. Dye undergoes
photo-excitation which results in the injection of electrons to the conduction band of TiO2. This
process is strongly dependent on the energy levels of oxide layer and dye molecule. Dye molecule
should exhibit high energy level relative to oxide layer. The oxidized dye is then regenerated by
accepting electrons from the electrolyte. The mostly used electrolyte in this system an organic
solvent containing redox system such as iodide/triiodide couple as it easily penetrated into the
porous layer of semiconductor [21]. In turn the iodide is regenerated from triiodide by reduction
taking place at the surface of counter electrode (CE), which is usually a conducting glass substrate
coated with a catalytic material and the circuit is completed by electron migration through the
external load. The generation of photovoltage under the process of illumination is attributed to the
potential difference between the electron’s Fermi level in the TiO2 and the electrolyte’s redox
potential. Thus the generation of electric power using light energy by the device happens without
distress any permanent chemical transformation [22].
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Fig. 1.2 depicts the foremost image of DSSC which dates back to 1988 [23]. In this cell a titanium
sheet and platinum wire were used as a working electrode (photo-anode) and counter electrode
respectively, while electrolyte used was an aqueous solution of bromide and bromine.
Figure 1.2 The first fabricated laboratory solar cell. [Adopted from Ref.23]
1.4 Electron Transfer in DSSCs
When DSSCs exposed to visible light, a sequence of reactions cycles take place which are
illustrated bellow.
1.4.1 Electron Excitation in Dye by Absorption of Photons and Ultrafast Electron Injection
Under exposure to sun light, the dye molecule harvests light energy and generated excited
electrons from the HOMO to the LUMO in the excited state (Eq 1). The photo excited electrons
are then injected into the conduction band (CB) of TiO2 results in the generation of oxidized dye
and electron is transferred from CB of TiO2 to the anode (Eq 2). The oxidized dye returns to its
original position by accepting electrons from reduced species (RED) of electrolyte (Eq 3). The
reduced species is then oxidized (OXI) which move towards CE for compensation of its missing
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electron (Eq 4). Moreover, two competitive reactions are involves in this process. The injected
electrons in the TiO2 film is either captured by the excited molecule of the dye (Eq. 5) or captured
by the oxidized species (OXI) of the electrolyte (Eq. 6) [24].
The fast electron insertion in the CB of the semiconducting layer from excited dye
molecule is one of the most amazing discoveries in DSSC. The charge transfer occurs due to the
strong electronic coupling between the dye/TiO2 interfacial bonds. This charge transfer process
occurs in the range of hundreds of picoseconds to tens of femtoseconds (Fig 1.3). Collection of
the electrons from the TiO2 layer to the anode takes micro to millisecond and regeneration of
oxidized dye occurs in the range of nanoseconds. The transportation and recombination of electron
in DSSC are determined by the diffusion length (Ln). The Ln is not influenced by the intensity of
light and its value is in the range of 5-20 micrometer [25, 26]. Scheme 1.1 Illustrate the electron
transfer mechanism of DSSC under the process of illumination.
Figure 1.3 Time constant of the redox processes elaborate in the conversion of light to electricity
by DSSCs. [Adopted from Ref. 25]
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1.4.2 Reactions at the Surface of Counter Electrode
The presence of I-/I3- redox species in electrolyte undergoes oxidation and reduction
reactions. In the process of oxidation, I- produces I3- ions while reduction process involves the I3
-
to I- by taking electrons from the CE. Electrons from the anode to external load are collected at
the surface of CE [27].
Pt is most widely used material for counter electrode in DSSC owing to its excellent
ectrocatalytic activity towards I-/I3- electrolyte. An effective CE should have ability to minimize
the overpotential which is responsible for charge transfer resistance (RCT). In this regard, the
material used for CE must be electro catalytically active and electrochemically stable in the
electrolyte system. The value of RCT should be or less than 1 Ωcm2 to escape significant losses of
electrons in the CE [28].
1.4.3. Back Electron Transfer or Recombination Reactions
Sometimes there is a possibility of back electron transfer to the excited state in the system
in which either the photoexcited electrons from the semiconducting oxide layer recombines with
the oxidized dye or with the oxidized species of electrolyte. These back electron transfer reactions
are also called recombination reactions. [29].
The time scales for recombination with dye molecule and with the electrolyte are in the
range of micro to millisecond and millisecond to second, respectively. Both these recombinations
take place at the TiO2/dye/electrolyte interface. There are different ways to minimize
recombination reactions, by controlling morphology of photoanode i-e by using nanostructured
semiconductors or composites of semiconductor to increase conductivity and/or by treatment of
TiO2 film with TiCl4, etc. [30].
The operational cycles can be summarized by the following equations:
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S + hν S* Photoexcitation (Eq 1)
S* S+ + e- (cb) TiO2 charge injection (Eq 2)
2S+ + 3I- 2S + I3- Regeneration of S (Eq 3)
I3- + 2e- 3I- Regeneration of I- (Eq 4)
S+ + e- (TiO2) S Recombination by dye (Eq 5)
Recombination process
OXI + e- (cb) RED Recombination by OXI (Eq 6)
Scheme 1.1 Schematic diagram of DSSC showing the sequence of electron transfer.
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1.5 Performance Evaluation of DSSCs
The current density-voltage (J-V) relation is a valuable tool for evaluation of solar cell
performance. Evaluation is implemented either in the dark or under standard illumination with 100
mW/cm2 incident power density at 25 ℃. Four basic efficiency parameters can be evaluated from
a typical I-V curve:
1.5.1 Short-Circuit Current (Isc):
The measurement of current produced between the two electrodes (photanode and counter
electrode) when the potential across the solar cell is zero is known to be a short circuit current (Isc).
Isc depends on the illumination intensity, light absorption properties, concentration and oxidized
dye regeneration by the redox electrolyte. Generally, the term short circuit current is replaced by
short circuit current density (Jsc) that is actually the ratio of Isc and illuminated region of the solar
cell [31].
1.5.2 Open Circuit Voltage (OCV)
The open circuit voltage (OCV) is the voltage measured under standard illumination when
both the photoanode and counter electrode are not electrically connected. In other words, at OCV,
no current is produced by the cell. In DSSCs, OCV shows significant dependence on the Fermi
energy level of the semiconducting material i-e TiO2 and redox mediator potential. The difference
of these two factors gives a rough estimation of OCV. The DSSC with lower OCV value provides
an estimation of rate of charge recombination [32].
1.5.3 Fill Factor (FF)
The FF is an important parameter in solar cell that gives an idea about the efficiency of the
cell. The value of FF can be calculated by comparing the power that provided by the cell and the
theoretic power that is the product of Isc and OCV [33].
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The FF is greatly affected by the material resistance and charge recombination. Therefore high
value of FF is an indicator of better internal transport within the fabricated cell [30].
1.5.4 Power Conversion Efficiency (η)
The conversion of part of sunlight energy into electricity through the photovoltaic process
is connected to the power conversion efficiency of the cell. It can be calculated by comparing the
maximum power that is in fact the electrical power output and incident power i-e the solar power
input [34]. The power conversion efficiency is dependent upon Jsc, OCV and FF from device
fabricated [26].
1.6 Components of DSSC
The system consists of the following components which are illustrated in Fig. 1.4 [35, 30].
1. A transparent conducting glass.
2. A wide band gap photoanode generally including semiconducting material such as titanium
dioxide deposited on the conducting glass.
3. A layer of organometallic dye spread on the surface of photoanode for maximum light
absorption.
4. A redox electrolyte usually iodide/triiodide dissolved in an organic solvent for regeneration
of oxidized dye.
5. A counter electrode (CE) coated with a catalytic material, generally platinum, to facilitate
electron collection.
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Figure 1.4: A Sketch of DSSC showing all of its components. [Adopted from Ref. 35]
1.6.1 Transparent Conducting Glass
Transparent conducting glasses act as substrates for DSSCs. These are normally coated with
conducting oxides having high conductivity (104 S cm−1) and transparency with suitable thermal
stability for sintering of deposited material. These properties are beneficial for the maximum collection
of photocurrents generated by DSSCs [35, 36].
Among variety of conducting oxides, Indium doped Tin Oxide (ITO) glass is most widely used.
It is glass is coated with the tin oxide that is doped with indium (SnO2:F). Due to high resistivity (10-4
ohm.cm) and high transparency (more than 80 %), ITO glass is most widely used in optoelectronics.
However, the major disadvantage of using ITO is that its thermal stability is low at high temperature
(typically 200 oC or less), while its low resistivity tends to be lost during sintering process in DSSCs
[37].
Various approaches have been implemented to maintain its low resistivity at high
temperature. Recently, binary compounds of SnO2, In2O3 and ZnO have been investigated [38, 39]
along with Fluorine doped SnO2 (SnO2:F), Tin-doped In2O3 (In2O3:Sn) [40] and aluminium doped
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ZnO (ZnO:Al) [41]. These doped oxides have been sputtered on the ITO surface to form a double
layer. This approach has been proven to be effective in increasing the thermal stability with low
sheet resistance [42]. Recently, Goto et. al [43] prepared multi-layer ITO and Fluorine-doped Tin
Oxide (FTO) substrate and utilized them in DSSC. Good thermal stability was observed by using these
substrates.
Apart from various conducting oxides, FTO has been designed with similar working principle
as ITO, which has fluorine doped tin oxide deposition on the glass. FTO glass have transparency and
resistivity of about 70 – 80 % and 12 ohm cm, respectively which are less than that of ITO [44]. Besides
its comparable properties with ITO, the cost of FTO is low compared to ITO. Literature reveals that this
cost effective FTO is also stable even at high temperature (500 oC) [45]. However, its low transparency
has strongly affected the efficiency of solar devices [8].
As an alternate to ITO and FTO, other metal oxides have been explored. Recently ZnO and
CdO have been used in place of ITO and FTO. Examination on these substrates revealed less
improvement in device efficiency [21] as these undoped oxide films become unstable at high
temperature. A thin layer of graphene coated on glass has also been used in place of FTO in solid
state DSSCs. This film exhibits more than 70 % transparency and high conductivity of 550 S/cm.
But the device efficiency is very low [46]. At present, FTO is commonly used instead of ITO
electrodes because of its low cost, lower sensitivity to surface cleaning methods and good
efficiency [21].
1.6.2 Photoanode
Photoanode (also referred to as working electrode) serves as the support for loading of
sensitizer and transfer excited electrons from sensitizer to external circuit. These two properties
14
are directly related to its high surface area that results in high loading of dye which in turn increases
the rate of electron transfer to ensure high electron collection efficiency [47].
Nanostructured oxide layer deposited on the ITO or FTO acts as photoanode in DSSCs. The most
widely used nanostructured oxide layer in DSSCs is titanium dioxide because of its high surface area,
inertness, biocompatibility, non-toxicity and low cost. In addition to these, its wide bandgap of 3.2 eV
with high Fermi level leads to higher OCV in DSSCs.
Although other semiconducting metal oxides such as Nb2O5, SnO2 and ZnO have been
investigated as photoanodes but still most preferred metal oxide is TiO2 [48]. TiO2 exists in two natural
forms; anatase and rutile. Rutile is the most stable and crystalline form of TiO2 while anatase form is
chemically more active in DSSCs. Literature revealed that the role of rutile form as photoanode in
DSSCs is not so effective. The rutile based DSSCs exhibited low short circuit current which is indication
of low absorption of dye due to small surface area [49]. Whereas anatase form is chemically active
because of its high band gap that results in the reduction of rate of recombination of electrons in DSSCs
[50].
ZnO is another promising semiconducting material used as photoanode. The structure of ZnO
is similar to TiO2 and electron mobility is high (1–5 cm2V-1s -1) relative to TiO2. But the device
fabricated with ZnO showed less efficiency due to dissolution of ZnO in the acidic solution. Such
problems can be overcome by the addition of base in the acidic solution and promising results can be
obtained with improved efficiency [48, 52]. SnO2 is another attractive photoanode material with high
mobility and larger band gap as compared to TiO2. But surprisingly, SnO2 based solar device exhibits
very less efficiency compared to TiO2 [52]. The ideality of photoanode lies in that it should be in nano
range and mesoporous with high surface area, essential for high dye loading [8].
15
Therefore, extensive research has been implemented to attain above mentioned properties of the
semiconducting material with the use of nanotubes, nanowires and nanorods [53, 54]. With the growing
Knowledge of nanomaterials it becomes feasible to develop nanomorphology of various
metal oxides such as ZnO and TiO2. Utilization of these nanostructures as photoanode in DSSCs can
replaced the semiconducting mesoporous layer [53].
1.6.3 Sensitizer
The dye also called sensitizer is an organometallic compound spread over the semiconducting
oxide layer of photoanode. At the time of development of DSSCs, the weakest point of the device was
the dye sensitizer as its function is just like electron pump. It actually forces the electron to jump into
semiconducting layer of photoanode by absorption of incident sunlight, also accepts an electron from
the redox couple in the electrolyte to compensate its electron deficiency and then repeats the cycle [63].
Substantial efforts have been made for the development of best performing dye. An efficient
dye should absorb all the visible light, must have high stability, non-toxicity and anchoring groups for
better adsorption on the surface of semiconductor [39]. Nazzeruddin et al. [55] in 1993 reported DSSC
using N3 dye, namely (4,4′-dicarboxylic acid-2,2′-bipyridine)ruthenium(II) that was actually ruthenium
complex (Fig 1.5), with conversion efficiency of 10.3%. Thus, the N3 dye becomes an archetype of
sensitizers for mesoporous solar cells. In 1997, the results of N3 dye were beaten by the development
of another ruthenium complex [56], generally called the black dye [tri (isothiocyanato)-2,2′,2″-
terpyridyl-4,4′,4″-tricarboxylate) ruthenium(II)] (Fig. 1.5).
Another most promising N719 dye (Fig 1.5) has been investigated in 2005 which is similar to N3 dye
but the efficiency of DSSCs using N719 dye was 11.2 %, higher than N3 dye. Up till now, ruthenium
complexes are the most successful dyes [57, 58].
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16
Despite their high performance, the cost of the ruthenium metal is very high. Therefore,
researchers are attempting to investigate other alternative dyes. The most encouraging and cheaper
alternatives are natural dyes, organic dyes and synthetic dyes, but the device based on these dyes shows
less efficiency and less stability [59, 60]. Chlorophyll (Chl) is a natural pigment used in the process of
photosynthesis having porphyrin ring in its structure. Different forms of chlorophyll and their
derivatives have been utilized as sensitizer in DSSCs [61]. Although these materials are not best
alternative for the ruthenium dyes because they do not exhibit the absorption of red light or near IR.
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17
N
N
HOOC
COOH
Ru
N
N
....C
SC
S
N
N
COOH
COOH
N
N
Bu4+
N-O2C
Ru
N
N
....C
SC
S
N
N
N3 Dye
COOH
N719 Dye
CO2-N
+Bu4
COOH
18
N
N
HOOC
COOH
Ru
N
N
....C
SC
S
N
N
C9H18
N
N
Bu4+
N-O2C
Ru
N
N
N
Z907
COOH
Black Dye
N
C
S
C
S
C
S
CO2-N
+Bu4
C9H18
Figure 1.5: Structures of some of the most efficient Ru-dyes for DSSCs.
19
1.6.4 Electrolyte
The electrolyte being a fundamental component of all DSSCs, acts as imperative mediator
between the two electrodes (counter electrode and photoanode) and is responsible for dye regeneration
at the interface between photoanode and electrolyte. It also regenerates itself by collecting the charges
at the CE. The most widely used electrolyte in DSSCs is triiodide/iodide electrolyte prepared in organic
solvents because of its low viscosity and high conductivity. But the use of organic solvents significantly
decreases the durability of the solar cells on account of solvent evaporation and desorption of dyes [62].
Further, electrolyte can strongly affect the parameters of DSSCs such as Jsc, OCV and FF [14].
For example, The Jsc parameter depends on the transport of charges present in the triiodide/iodide redox
electrolyte. The charge transfer resistance (RCT) between electrode and electrolyte interface and
diffusion of charge carrier can affected the FF whereas redox potential of the electrolyte greatly affected
the OCV.
To overcome these issues, several approaches have been devoted to develop room-
temperature ionic liquids (RTILs) [63], quasi-solid state [64] and solid state electrolytes [65]. Usage of
these electrolytes in solar cells increases the stability but conversion efficiency is constantly low. This
is because of their high viscosity which requires high concentration of redox species in order to maintain
the conductivity [66]. An ideal electrolyte should be less viscous to minimize charge transfer resistance,
should not have significant effect on dissociation of electrolyte, sealing material and adsorbed dye [67].
1.6.5 Counter Electrode
Counter electrode (CE) is an essential part of DSSCs, designed to assemble electrons from
the external circuit and catalyze the reduction of redox couple, which are mediators for dye
regeneration or transporting holes in solid-state electrolyte. Therefore, a very fast process of
reduction of I-/I3- redox couple at the counter electrode is required for highly effective dye
20
regeneration. In the view of these basic functions, an effective CE should exhibit high reflectivity, high
catalytic activity, low-price, high conductivity, optimum thickness, high surface area and good
adhesivity with glass substrates etc. In addition to these, other parameters including 80% transparency,
charge transfer resistance (RCT) of 2-3 Ω cm2 and series resistance Rs of 20 Ω cm2 are also the basic
requirements for ideal CE.
Typically, FTO coated with Pt is utilized as CE in DSSCs as it offers low RCT and superior
catalytic activity. These two properties have significant effect on the parameters of DSSCs [68].
However, Pt metal is undoubtedly expensive and its cost as a CE is over 40% of the whole DSSC.
Moreover, Pt requires high energy requirement for preparation and is susceptible to corrosion by
tri iodide electrolyte. These defects directly make Pt unfavorable for durable stability and practical
manufacture of DSSCs. Subsequently, the substitution of Pt by an alternative material remains a
serious matter in DSSCs development. In this regard, considering these defects, it is essential to
substitute easily fabricated, stable and cost effective Platinized FTO free counter electrodes in
DSSCs.
Several substitute materials such as derivatives of carbon (graphene-based carbon, carbon
nanotubes, carbon nanofibers, etc.), metal alloys [8], transition metal compounds [9, 10] and
intrinsically conductive polymers (ICPs) [1, 3] have been investigated for the replacement of Pt
[8].
1.6.5.1 Platinum Counter Electrode
Pt being precious, ductile and inert white metal, discovered in 1735 by Julius Scaliger,
given the name of platina (a SPANIsh word) means “little silver”. Pt is considered as noble metal
on account of its remarkable physio-chemical properties such as high thermal and electric
conductivity, high electrocatalytic activity and high stability in air or water (even at high
21
temperatures), making Pt promising for various applications such as catalyst in chemical reactions,
in vehicles as catalytic converter, , surgical tools, electrical resistance wires, etc.
Despite of wide applications of bulk Pt, researchers have devoted their attention for the
development of nanostructures of Pt, because Pt nanoparticles have several advantages over bulk
Pt. Pt nanoparticles possess high surface area, high electrical conductivity, low value of RCT, high
transmittance and corrosion resistance than other noble metals. These unique features attracted Pt
to use as a CE/catalytic material in DSSC [69]. However, several defects have been associated
with the utilization of Pt. The high cost of the Pt and its dissolution in electrolyte solution make
Pt unfavorable for durable stability and practical manufacture of DSSCs [62].
1.6.5.2 Carbon Materials
Carbon is most abundant material and is found everywhere. Carbon materials are found to
be an attractive and efficient catalytic material in DSSCs owing to their high thermal stability, low
price and better catalytic activity, high thermal stability, high electrical conductivity and corrosion
resistance. Various carbonaceous materials such as porous carbon, graphene and carbon nanotubes
(CNTs) have been explored and intensively utilized as CEs produced high performance DSSCs
[71]. Researchers are also trying to develop new catalytic materials by combining two carbon
materials such as porous carbon/carbon nanotubes and carbon nanotube/graphene nano-ribbons
for further improvement in electrocatalytic activity of CEs [72, 73]. However various drawbacks
such as carcinogenic effect of carbon black and high production cost of carbon nanotubes insisted
the researchers to develop other low cost and environmentally friendly catalytic materials [72].
1.6.5.3 Inorganic Compounds
Sulfides (CoS2, CuInS2) [74], carbides (TiC) [75], nitrides (TiN, ZrN) [76], phosphides
(Ni5P4) [77], and metal oxides (e.g. WO2 and V2O5) [78] have also been effectively utilized as CE
22
materials on account of their encouraging application in large scale and cost effective DSSCs.
However, still there is a need to further improve the stability and efficiency of inorganic
compounds for DSSCs.
1.6.5.4 Intrinsically Conducting Polymers
Discovery of Conducting polymers (CPs) or, more specifically, Intrinsically Conducting
Polymers (ICPs) in 1977 [79] has led to the development of a new field in electrochemistry and are
categorized into fourth generation of polymers with wide range of applications from products of
research laboratory to mature industrial products [80]. In this regard, in 2000, three brilliant scientists,
Heeger, Shirakawa and MacDiarmid were granted a Nobel Prize for the fundamental discovery of ICPs
[79].
ICPs have metallic conductivity similar to that of semiconductors and also the most
remarkable behavior is that their electronic properties such as bandgap can be tailored by changing
its synthesis techniques [81]. ICPs are promising candidates, to be utilized as Pt-free CEs in DSSCs
owing to their cost effectiveness, simple synthesis, favorable catalytic properties, controllable
conductivity and porous structure [82, 83]. The most favored ICPs employed as CEs in DSSC
includes PEDOT [84], polypyrrole (PPy) [85], polythiophene [86] and polyaniline (PANI) [87] on
account of their transparency, high stability and electrochemical activity [88]. Fig. 1.6 shows
structures of some common ICPs.
23
Polythiophene
S S S
Poly(phenylenesulfide)S
Poly(phenylene vinylene)
Polyaniline
Polyacetylene
H
N
H
N
CH3
NH
CH3
Poly (o-toluidine)
CH3
NH
H
N
NH
CH3
Figure 1.6: Molecular structures of a range of intrinsically conducting polymers.
Versus other conducting polymers, PANI has been assumed as an efficient CE material for
DSSCs as it offers high catalytic activity, better power efficiency, effective cost, high chemical
stability and processability. Diverse varieties of PANI (nanofibers, nanorods, nanotubes etc.) have
been prepared and utilized as CE materials for DSSC [70].
1.7 Polyaniline: (an Overview)
The most preferred ICP is Polyaniline (PANI), discovered over 150 years ago. PANI has gained
strong attention since 1980s from the scientific community and has been intensively studied ICP in the
past 50 years due to its modifiable electrical conductivity, inexpensive monomers, relatively facile
24
processability, structure versatility, favorable environmental stability, unique redox properties,
reversible doping/dedoping [89, 90, 91]. Its electronic and optical properties are comparable with
the metals while retaining processibility and flexibility of conventional polymers. These
outstanding properties justify the use of Polyaniline in various applications like chemical sensors
[92], supercapacitors [93], fuel cells [94], electrically conducted yarns [95] anticorrosion coatings
[96] and solar cells [89].
1.7.1 Chemical Structure of PANI
The chemical structure of PANI can be determined by two most important factors namely,
the redox state and the doping level. Fully reduced (Leucoemeraldine), half oxidized (Emeraldine)
and fully oxidized (Pernigraniline) states are the three main distinguishable oxidation states of
PANI with different colours (Table 1.1). In addition, a number of possible oxidation states exists
in between these three main oxidation states. Therefore, theoretically, PANI can exist in a no. of
oxidation states ranging from a completely reduced to a completely oxidized form [97]. Fig. 1.7
depicts a general structure of PANI with reduced and oxidized repeating units. Where x is a
variable, depicting the fraction of these two repeating units, having values between 0 and 1 and is
used for the description of the degree of oxidation [98].
NH NH N N
x 1-x n
Reduced repeating unit Oxidized repeating unit
Figure 1.7: Structure of polyaniline along with its repeating units.
25
It follows that the value of x for leucoemeraldine is 1, for emeraldine is 0.5 and for
pernigraniline is 0. Looking into the chemical structure of PANI, only the amine nitrogen atoms
are the constituents of reduced state whereas the oxidized state contains only the imine nitrogen
atoms. In the presence of acidic environment imine nitrogen atoms are made protonated to attach
protons with these nitrogen atoms to produce radical cations [97]. Oxidation states of PANI and
pH of the electrolyte solution imparts significant effect on degree of protonation. Originally, imine
nitrogen atoms in PANI chain were considered as main protonation sites. However there are
evidences from the experimental results that some from amine nitrogen atoms can also be
protonated to generate NH2+ groups even if all the imines are not protonated [99, 100]. The three
different oxidation states of unprotonated PANI (base form) and the corresponding protonated
PANI (salt form) [97] are illustrated in Fig. 1.8. Table 1.1 displays different states and colours of
PANI.
Table 1.1 Different states and colours of PANI.
Oxidation state Colour
Leucoemeraldine Pale yellow
Emeraldine base Blue
Emeraldine salt Green
Pernigraniline Purple
26
HN
HN
NH
NH
HN
HN
NH
NHn n
+ +
+ HX
- HX
Red OX
HN
HN
NH
NH n
+.X-
+.X-
Red OX
Emeraldine Salt
HN N
NH
NH n
+ +
+ +
Pernigraniline Salt
Red OX
HN N
NNH n
Red OX
N N
NN n
Emeraldine Base
Pernigraniline Base
Leucoemeraldine Base Leucoemeraldine Salt
+ HX
- HX
+ HX
- HX
Figure 1.8: Different redox states and the corresponding doped forms of PANI.
27
1.7.2 Synthesis of Polyaniline
Polyaniline is generally prepared by oxidative polymerization of monomer units. Oxidation
can be done electrochemically by using different electrode materials or chemically by using a
chemical oxidant. The properties of the polymer depend on the method of synthesis [101].
1.7.2.1 Electrochemical Polymerization
Electrochemical syntheses is done by placing working electrode in the solution containing
doping salts and diluted monomer in an appropriate solvent. The reaction is carried out under
constant potential, potential scanning and/or constant current conditions [102]. Electrodeposition
takes place on the surface of electrode by applying suitable potential to the working electrode,
where oxidation of monomer leads to the formation of radical cations. These cations then combine
with other monomers to produce polymer. Electrochemical synthesis has an important advantage
of preparation. The film deposition of conducting polymer takes place in a single step and resulting
polymer is simple and highly conductive [103]. Electrochemical synthesis of PANI is affected by
various factors like applied potential, solvent, pH, electrode, temperature, concentration of
monomer and cell conditions. Therefore, it is difficult to optimize all the parameters in a single
experiment.
1.7.2.2 in-situ Chemical oxidative Polymerization
In-situ chemical polymerization is a common, simple and useful technique for the synthesis
of PANI. In this process synthesis of PANI takes place in a reactor containing by monomer aniline,
an oxidizing agent and a dopant in an appropriate aqueous or non- aqueous solvent. Various
oxidizing agents used for the polymerization of aniline such as hydrogen peroxide, ammonium
persulfate, potassium dichromate and iodine, initiate the reaction and produce a large amount of
28
PANI [104]. PANI powder obtained by this method has good conductivity, environmental
stability, good solubility, processability and porosity. The main advantage of chemical synthesis
is production of large amount of polymer in good yield and this process requires no special
instrument [102]. In this dissertation, chemical oxidative polymerization technique will be used to
obtain the product with desirable properties such as conductivity and morphology.
1.7.3 Doping in Polyaniline
The doping mechanism of PANI is also unique among other conducting polymers. The
conducting emeraldine state can be obtained either by redox doping or non-redox doping of non-
conducting emeraldine base form of PANI. Redox doping process involves the oxidation of
leucoemeraldine to emeraldine by changes the number of electrons in the polymer chain. This
process is common in almost all ICPs.
However, PANI can also undergoes non-redox doping which involves organic and
inorganic acids for the formation of conducting form of PANI without changes the number of
electrons in the polymer backebone, making the doping process simpler [104]. The significant
enhancement in the electric properties of PANI due to acid doping can be credited to the formation
of radical cations at the imine nitrogen. These radical cations undergo delocalization over the
polymer chains and are thought to be responsible for the electronic conduction in PANI [104, 105].
Consequently, majority of radical cations accumulated on the polymer backbone are neutralized
by anionic species of the dopant used.
Hence, the majority charge carriers in polyaniline are holes which are characteristics of the
P-type semiconductors [106]. Undoped PANI, having large band gap energy of approximately
3.16 eV, is an insulator. When an electron is removed from the PANI backbone during the process
of oxidation, the benzenoid structure of PANI is converted to a quinoid one results in the formation
29
of a polaron [105] (Fig. 1.9). This gives rise to two localized electronic levels within the band gap
while the unpaired electron occupies the bonding state. Removal of second electron from the
polaronic state of PANI leads to the formation of a doubly charged bipolaron [107] (Fig. 1.9). The
benzenoid to quinoid deformation is stronger in the bipolaron than in the polaron. Further oxidation
results in an overlap of bipolarons (Fig. 1.9) which leads to a decrease in the energy gap from 3.16
to 1.4 eV [105].
HN N
NNH n
HN
HN
NH
NH n
+
HN
HN
NH
NH n
+. .+
+
HN
H2N
NH
NH n
+ +
+
2xH+
xH+
Quinoid segments of dedoped Pani
Bipolarons from doping Polarons from doping
Bipolarons from overdoping
Figure 1.9: Scheme of the doping and overdoping processes of polyaniline and formation of
the bipolaron and polaron segments. The anions are not shown.
1.7.4 Binary doping in Polyaniline
The improvement in the electrical conductivity and processibility of PANI can be done
through doping either with organic acids or with inorganic acids. It was reported that inorganic
acids (HCl, H2SO4) doped PANI is more conducting relative to the doping with weak organic acids
30
like acetic acid. But the obtained metallic PANI, in most of the cases, is completely insoluble
[108].
Organic acids when used as dopants provide nanotubes and nanofibrous of various
dimensions, respectively with improved solubility. Possible reasons for providing such properties
are: 1). The organic acids incorporated into the molecular backbone of PANI act as surfactant,
which can provide it a favorable morphology. 2) Functional groups present in organic acids make
contribution in its processibility. Various organic acids such as p-touenesulfonic acid (pTSA),
camphorsulphonic acid (CSA), phytic acid (PA), dodecylbenzenesulfonic acid (DBSA) etc. have
been used to dope PANI [108]. Literature reveal that an interesting research has been carried out
by using PANI as a CE in DSSCs which depends on morphology and electrical conductivity which
in turn depend on synthesis conditions as well as on the type of acid dopant used [109, 110].
The desired morphology of the polymeric system is very difficult to control. If one can
attain the desirable morphology with conductivity then such materials would be capable of
enhancing photovoltaic properties [111]. Therefore, Scientists have adopted a strategy to binary
doped PANI with the simultaneous use of organic and inorganic acids, with the aim to develop
efficient and cost-effective materials with desirable morphology, processibiity and electrical
conductivity for various applications especially in the field of photovoltaics [112].
However, there are very few reports on dual acid doping in PANI. Kuo et al. [113, 112]
co-doped PANI with polyacrylic acid (PAA) and HCl resulted in the effective dispersion of Pt
nanoparticles in polymer matrix as compared to HCl doped PANI alone due to better morphology.
Gawli, Y. et al. obtained the high supercapacitive performance of binary doped PANI with phytic
acid and HCl by attaining both desirable morphology and electrical properties of PANI [108]. In
the present study, PANI will be simultaneously doped with organic acid (ALS) and inorganic acid
31
(H2SO4) to achieve its desirable properties like conductivity and morphology. These desirable
properties are the basic requirements for a highly efficient CE when utilized in DSSCs.
1.7.5 Polyaniline as Counter Electrode in DSSCs
A sensitized nanostructured TiO2 electrode (photoanode), Pt coated counter electrode (CE)
and a redox electrolyte constitutes a DSSC. Among these components, CE is most important part
of DSSC as its performance strongly affect the parameters of DSSCs [45, 47]. An optimal CE
should have high conductivity, large surface area, high catalytic activity, good adhesivity with
transparent conducting oxide (TCO) and must have low cost [114]. Generally, Pt when used as a CE
yielded high performance of DSSCs. However, the high cost of the Pt and its dissolution in
electrolyte solution make Pt unfavorable for durable stability and practical manufacture of DSSCs.
Therefore, researchers have devoted their attention to develop cost effective CE material while
maintaining the other requirements of an ideal CE.
PANI is as the most versatile ICP, has the great potential to be utilized as counter electrode in
DSSCs on account of its low cost, facile synthesis, modified conductivity, interesting redox properties
and high catalytic activity [115]. Different varieties of PANI such as nanofibers, nanorods, nanotubes
etc., have been synthesized and fabricated as CE materials for DSSCs. Generally, PANI CE should
possess a porous morphology with high surface area along with high conductivity. In this regard, Wu
et.al. in 2008 [116], synthesized microporous PANI nanoparticles through chemical oxidative
polymerization using perchloric acid as a dopant and for the first time, utilized these PANI nanoparticles
as a CE in DSSCs. The results revealed high catalytic activity of nanostructured PANI CE relative to Pt
CE. Hou et al. [17] fabricated DSSC using serrated and ultrathin PANI nanoribbon CE. In situ
polymerization method was employed in the presence of electrospun vanadium pentoxide (V2O5)
resulting in the formation of PANI nanoribbons. The PANI NR CE showed high electrocatalytic activity
32
and high efficiency comparable to that of Pt based DSSCs because of its high surface area and good
adhesion with FTO.
By using suitable methods, a thin transparent film of PANI can be synthesized and utilized to
enhance the effective utilization of incident light in bifacial solar cells (Fig. 1.10). A highly uniform and
transparent PANI film was synthesized by Tai et al. [118] through a facile in situ polymerization for its
use as CE in transparent bifacial DSSCs. Front illuminated efficiency was found to be 6.54 comparable
to Pt based DSSCs while rare illuminated efficiency was 4.26 %.
Figure 1.10: Bifacial DSSC assembled with a transparent anode and transparent Pt CEs. [Adopted
from Ref. 118]
Various inorganic dopants such as BF4−, SO4
2−, ClO4−, and Cl− have been used to develop
catalytic materials for CEs [119, 120]. Doping ions imparts significant effect on the doping and
dedoping process, morphologies and electrochemical properties of polymer films. Among the various
dopants, PANI doped with the SO42− exhibits porous morphology along with low value of Rct and high
reduction current density than the Pt CE. High efficiency was obtained by utilizing PANI-SO4 CE based
33
DSSC [119]. High catalytic activity was obtained by doping PANI with sodium dodecyl sulfate (SDS).
Compared to Pt CE, DSSCs fabricated with PANI-SDS CE demonstrated a conversion efficiency of
7.0% [121]. Amin et al. studied the influence of sulfamic acid (SFA) doping on the performance of
DSSCs. PANI nanofibers (NF) were produced by the doping with SFA along with high
conductivity. The cell based on PANI-SFA CE showed a 27% improvement in photoconversion
efficiency [122].
PANI-nanobelts have been prepared by simple chemical polymerization method which
showed high surface area and hence high conductivity. Fabrication of these PANI-nanobelts CEs
in DSSCs exhibited good efficiency [123]. A unique hollow spherical morphology of PANI was
obtained by using environment friendly and facile emulsion polymerization. These hollow
spherical PANI particles manifested high surface area with better catalytic activity. Fabrication of
these particles in DSSCs showed better performance [124].
Excellent electrocatalytic activities, cost effectiveness and simple preparation of PANI mark a
suitable alternative CE material for DSSCs. However, in most cases, it is not easy for one material to
achieve all these properties simultaneously. Deposition of uniform film on transparent conducting oxide
with large surface area and desirable conductivity is one of the main issues in the fabrication of PANI
as CEs. Hence, finding suitable dopants is the key to enhance the efficiency of the material which
despites of increasing the conductivity, also function as a pore former which can increase the
surface area without affecting the uniformity of the film.
1.7.6 Techniques for the Analysis of Counter Electrode and DSSC
Electrochemical and photoelectrochemical techniques [125] are considered to be most powerful
tools to apprehend the mechanism of chemical reaction, such as rate of electron transfer, charge
generation, catalytic activity of the CEs utilized in DSSC and evaluation of photovoltaic performance
34
of the DSSCs. These include Current Density–Voltage (J–V) measurements, Cyclic Voltammetry (CV)
and Electrochemical Impedance Spectroscopy (EIS) [126].
1.7.6.1 Cyclic Voltammetry
Cyclic Voltammetry (CV) is another essential electrochemical technique for exploring the redox
couples and electrocatalytic activities of the counter electrodes [127]. The peak to peak separation is an
important parameter to check the electron transfer rate of the redox electrolyte and is determined from
CV curve (Fig. 1.11) by using the equation [128]
Ep = |Epc − Epa| Eq. 9
Where Epp is peak to peak separation, Epc and Epa is cathodic and anodic peak potentials, respectively.
The reduction peak current densities from the CV curve are direct indications of the electrocatalytic
capability of the CE. In addition CV curves are also useful for estimation of electron transfer kinetics
and for the determination of diffusion co-efficient (Dn) of redox couples by using Randles–Sevcik
equation:
Jred = KAC Eq. 10
Where K indicates the constant of 2.69*105 cm2s-1, n represents the number of electrons
transferred in redox reaction, A stands for electrode area, and ν and C demonstrates the scan rate
and concentration of redox species in bulk, respectively.
35
Figure 1.11: A typical CV curve for counter electrode with reduction current density (Jred), oxidation
current density (Joxi) and peak seaparation (Epp).
1.7.6.2 Electrochemical Impedance Spectroscopy
Electrochemical Impedance Spectroscopy (EIS) provides valuable information regarding the kinetics
of charge transport and evaluation of electrocatalytic activity of CE in DSSCs [129]. The data obtained
from EIS are represented by Nyquist plot (Fig. 1.12) or by Bode plot. For DSSCs, Nyquist plot
commonly exhibits three semicircles. In high frequency region, the x-axis intercept shows series
resistance (RS), connecting wires and glass substrate is mainly responsible for Rs. Resistance generated
between CE and the electrolyte is estimated by the analysis of first semicircle located in high frequency
region and is attributed to the charge transfer resistance (RCT). The second semicircle located at middle
frequency manifests the RCT at the interface of anode/dye/electrolyte and the third semicircle (low
frequency) is attributed to the diffusion resistance of redox species (ZW) [130]. By using suitable
equivalent circuit, fitting of the measured data is done by some EIS software and the EIS parameters
such as RS, RCT, ZW etc. can be obtained, which signifies the electrochemical properties of the cell [131].
36
Figure 1.12: Nquist plot for DSSCs with equivalent circuit showing values of Rs and Rct.
1.7.6.3 Photovoltaic Measurements
The Current Density–Voltage (J–V) curve is the relationship between the output current and
voltage of the solar cell under standard full spectrum irradiation and it is applied to conclude the
photovoltaic device parameters [132].
J–V curves are used to characterize the photoelectric conversion efficiency of solar cells.
From I–V curves (Fig. 1.13), we can calculate the open circuit potential (OCV), short circuit
current density (Jsc), Fill Factor (FF) and the maximum output power point (Imax.Vmax) of a solar
cell. The FF can be calculated from the following equation 1 [33]
𝐹𝐹 =𝑃𝑚𝑎𝑥
𝐼𝑆𝑐𝑉𝑂𝐶=
𝐼𝑚𝑎𝑥𝑉𝑚𝑎𝑥
𝐼𝑆𝐶𝑉𝑂𝐶 Eq. 11
Where, Vmax and Jmax are maximum voltage and current density. Higher value of FF gives
indication of a desirable rectangular shape for J–V curves and excellent performance for the device.
The photoelectron conversion efficiency can be calculated from the following equation 2 [33]:
37
𝜂 =𝑃𝑚𝑎𝑥
𝑃𝑖𝑛=
𝐽𝑆𝐶𝑉𝑂𝐶𝐹𝐹
𝑃𝑖𝑛 Eq. 12
Where Pin is the power density of incident light. Jsc and OCV is short circuit current density and
open circuit potential, respectively. Equation 2 is used to determine the efficiency of the DSSC.
Figure 1.13: J-V characteristic curve for the DSSCs.
1.8 Aim of the Present Work
The aims and objectives of the proposed project are to:
Synthesize binary doped PANI powder.
Optimize the reaction conditions to study the effect of changing reaction parameters (ALS
and H2SO4) on physico-chemical properties such as morphological, structural, optical and
photovoltaic properties of synthesized salts.
Fabricate simple and cost effective counter electrodes modified with thin films of
synthesized PANI.
Study the electrocatalytic activity of fabricated counter electrodes towards I3- reduction.
38
Fabricate dye synthesized solar cells (DSSCs) utilizing the counter electrodes modified
with different PANI samples and examine their performance for energy harvesting.
Analyze the photocurrent response and operational stability of the fabricated solar cells by
alternatively irradiating and darkening the cells.
Investigate the kinetics of electron transfer and photochemical processes of DSSCs.
Utilize the fabricated counter electrodes as cost effective, environment friendly and durable
alternative for platinum counter electrode.
Compare the photovoltaic properties of DSSCs based on Pristine PANI, H2SO4 doped
PANI, ALS doped PANI and binary doped PANI CEs.
39
Chapter: 2
2 Experimental
2.1 Materials
Aniline (ANI) (Merck) was distilled under reduced pressure and stored in a refrigerator.
Other reagents, such as ammonium per sulfate, APS (Merk), H2SO4, ammonium lauryl sulphate,
ALS, DMSO, NMP, Chloroform (Acros), deoxycholic acid, DCA, 4-tert-butylpyridine ,TBP,
Anhydrous lithium iodide (LiI), 3-methoxypropionitrile (MPN), 1,2-dimethyl-3-
propylimidazolium iodide, (DMPIm) (Isolaronix) , and Iodine, I2, (Aldrich), FTO (13 Ω/sq) and
Rhothenium Dye D179 were used as received.
2.2 Synthesis of Pristine PANI
Polymerization of pristine PANI was carried out by dropwise addition of 1.2 mmol aqueous
solution of APS in the round bottom flask containing mixture of 620 mmol of chloroform and 9.8
mmol of aniline. The reaction mixture was kept under constant stirring. After 24 hrs , the mixture
was taken into a separating funnel for separation of organic containing PANI and aqueous layer.
After separation, organic layer is filtered and washed with acetone and water and then dried in
vacuum for further characterizations.
2.3 Synthesis of H2SO4 Doped PANI
Synthesis of H2SO4 doped PANI was done by adding 1.2 mmol aqueous solutions of H2SO4
and APS in the mixture of 9.8 mmol aniline and 620 mmol of chloroform, taken in reaction flask
under stirring. After completion of polymerization, separation and washing was carried out by the
same way as discussed above.
40
2.4 Synthesis of ALS Doped PANI
To a reaction mixture of chloroform (620 mmol) and aniline (9.8 mmol), 8.2 mmol of ALS
was added followed by the dropwise addition of aqueous solution of APS (1.2 mmol) under
constant stirring. After separation and washing, ALS doped PANI was obtained.
2.5 Synthesis of Binary Doped PANI
The typical reaction for the synthesis of binary doped PANI was performed by adding 9.8
mmoles of aniline to 620 mmoles of chloroform into each of four flasks (250 ml, 1-4) which were
kept under constant stirring followed by the addition of 6.8, 7.5, 8.2 and 9.0 mmoles of ALS
respectively. 1.2 mmoles (0.05 M) aqueous solutions of H2SO4 and (0.05 M) APS, were later added
drop wise to the reaction flask. The reaction mixture was kept under constant stirring for 24 h for
complete polymerization. The green precipitates were obtained through filtering and washing with
acetone and deionized water and dried under vacuum. Finally, the binary doped PANI samples
were prepared in powder form.
Following the above procedure, the amount of ALS, aniline and H2SO4 was stepwise changed in
the feed for optimization.
Pristine PANI, H2SO4 doped PANI, ALS doped PANI and various concentrations of binary doped
PANI salts along with their codes are shown in Table 2.1.
41
Table 2.1: Sample composition and their codes
Samples Name Code
Pristine PANI P-N
H2SO4 doped PANI P-H2SO4
ALS doped PANI P-ALS
6.8 mmole ALS- H2SO4 PANI P-Mix 1
7.5 mmole ALS- H2SO4 PANI P-Mix 2
8.2 mmole ALS- H2SO4 PANI P-Mix 3
9.0 mmole ALS- H2SO4 PANI P-Mix 4
As we were interested to study the effect of ALS on various properties of PANI, so we have used
various concentrations of ALS doped PANI salts keeping the conc. of H2SO4 constant. Further the
above mentioned binary doped PANI salts were selected on the basis of percent yield.
2.6 Fabrication of PANI Based Counter Electrodes (CEs)
In a series of experiments, P-N, P-H2SO4, P-ALS and various amounts of P-Mix in the
form of powder were dispersed in NMP at a constant concentration (0.05 g / ml) under stirring for
2 h. The dispersed PANI solutions were coated on the surface of ultrasonically cleaned FTOs by
Doctor Blade technique [133] and finally dried at 50°C for 10 min. The thickness of all the films
was about 8 μm (Fig. 3.7 a).
42
2.7 Fabrication of DSSCs Devices
Different layers of 20 nm opaque TiO2 (Dyesol 18NR-O) paste were deposited on FTO glasses
using screen printing technique (shown in Fig. 2.2a) followed by rapid annealing at 150, 300, 350,
400, 450, and 500 oC for 10, 15, 10, 15, 10 and 15 min, respectively. The thickness and active area
of the obtained TiO2 photoanodes was about 12 μm and 0.28 cm2, respectively as indicated in Fig
1 f. After cooling down to 80 oC, they were immersed in dye solution (0.5 mM N719 dye in
ethanol) for 24 h followed by cool air drying. The as prepared PANI CEs were positioned over the
sensitized TiO2 electrodes with a surlyn between them by using paper clips (scheme 1). A drop of
the redox electrolyte (0.04 M I2, 0.4 M LiI and 0.4 M tetrabutylammonium iodide (TBAI) in 3
methoxypropionitrile (MPN) and acetonitrile (ACN) mixture (1:1) was injected carefully injected
between the electrodes. The overall illustration is presented in scheme 2.1.
43
Scheme. 2.1: Sketch of the representative structure of PANI CE for DSSCs
2.8 Material Characterization
2.8.1 Physico-chemical Characterization
2.8.1.1 Polymerization Yield
The binary doped powdered PANI salts were weighed by subtracting the weight of empty bottle
from the weight of bottle having sample and used to calculate the polymerization yield by using
equation:
% Yield = Weight of binary doped PANI
Weight of monomer× Weight of dopant× 100 [3] Eq. 13
44
2.8.1.2 Focused Ion Beam Scanning Electron Microscope (FIB-SEM)
The FIB-SEM is a dual beam unit that combines an electron beam and a Gillum ion beam.
It is capable of low and high resolution scanning electron microscopy, scanning ion microscopy,
as well as ion beam and electron beam lithography.
The surface morphologies and cross sections of thin films of PANI salts coated on FTO
were observed by xT Nova Nanolab 600 Focussed Ion Beam Scanning Electron Microscopes (FIB-
SEM) shown in Fig. 2.2b.
2.8.1.3 Fourier transform infrared spectroscopy
The structural information about all the synthesized samples were obtained through ATR-
FTIR (Shimadzu) within the range of 400-4000 cm-1 in the Attenuated total reflection (ATR) mode
with resolution of 2 cm−1.
2.8.1.4 X-ray diffraction
The crystal structures of different samples of PANI were measured by using Rigaku X-Ray
diffractometer (Japan) operated at 30 mA and 40 KV with wavelength of 1.5405 A˚ over the range
of 10° to 80°.
2.8.1.5 DC conductivity
Prior to dc conductivity, the dry pellets of powder samples with 13 mm diameter and 5 mm
thickness were prepared under a pressure of 50 MPa. The electrical conductivity of these pellets
was tested by using Jandel RM3000 four-probe resistivity/square resistance tester equipped with
a potentiostate at room temperature.
2.8.1.6 Elemental Analysis and Mapping
45
To study the elemental composition, elemental analysis and mapping of PANI CEs were
performed with the help of Helios G4 CX Dual Beam microscope equipped with Octane Elite.
2.8.1.7 UV-Vis spectroscopy
Absorption spectra of PANI dispersions dissolved in NMP were recorded using a Perkin
Elmer spectrophotometer in a spectral range of 300-900 nm.
2.8.1.8 Cyclic Voltammetry
To study the electro catalytic performance, CV measurements were conducted using
Metrohm Autolab with Nova Software in redox electrolyte containing 0.1 M LiClO4, 1.0 mM I2
and 10 mM LiI in acetonitrile within the potential range of -0.6-1.2 V at various scan rates (30, 50,
75, 100, 125 mV s-1). A three electrode cell, filled with aforementioned electrolyte and equipped
with PANI/FTO or Pt/FTO as working electrode, a Pt wire as a counter electrode and an Ag/AgCl
as a reference electrode, was employed for CV measurements. A constant quantity of synthesized
materials (0.05 g) dissolved in NMP (1ml) were coated on FTO glasses.
2.8.2 Photovoltaic Characterization
2.8.2.1 Photocurrent density-voltage (I-V) test
The photovoltaic tests of Pristine PANI, doped PANI’s and Pt based DSSCs were studied by
measuring the photocurrent density-voltage (J-V) curves using the Metrohm Autolab with a light
source of 100 W Xenon arc lamp in an ambient atmosphere.
2.8.2.2 Start/stop ability test
The ON-OFF plots of the DSSCs based binary doped PANI and Pt was measured under by
alternately irradiating (100 mWcm_2)) and darkening (0 mWcm_2) the DSSC devices at 0 V over
600 s to study the start/stop ability of the cell.
46
2.8.2.3 Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) of assembled DSSCs in the light was performed
using the aforementioned Autolab. The AC signal had an amplitude of 10 mV in the frequency
range from 0.01 Hz to 100 KHz at 0.7 V DC bias. The obtained EIS spectra were simulated by
Autolab impedance analyzer.
Figure 2.2: a) Screen printing (for fabrication of TiO2 film) and b) FIB-SEM (for surface
morphology and thickness) instrumentation
47
Chapter: 3
3. Results and Discussion
Chapter 3 is divided into two parts. Part 1 covers synthesis and optimization of reaction conditions
and characterization of morphological, optical, electrical and photovoltaic properties of binary
doped PANI salts for its utilization in Dye Sensitized Solar Cells (DSSCs). Whereas, part 2
explains the effect of counter ions of various dopants on the morphological, optical and
photovoltaic properties of synthesized material for DSSCs.
Part 1
Optimization of reaction parameters and analysis of different properties of resulting
polymers for application in Dye Sensitized Solar Cells
In this section effect of different reaction parameters on the yield and properties of the
resulting polymers will be discussed in order to fix the optimized parameters for desired properties
of the material.
3.1 Effect of Synthesis Parameters on Binary Doped PANI
The factorial experimental design is a powerful tool to realize the effects of some
independent variables that significantly affect the experimental results. Here we present the results
of the effect of monomer amount and dopants (ALS and H2SO4) amount on the yield of binary
doped PANI. The influence of factors on the yield of binary doped PANI is discussed below.
3.1.1 Effect of Monomer Amount on Polymerization Yield
The effect of amount of aniline on percent yield of binary doped PANI is shown in Fig.
3.1. Aniline with amounts ranging from 7.6 mmol, 9.8 mmol, 12.04 mmol, 14.2 mmol to 16.43
48
mmol was used for polymerization. It was observed that the maximum yield can be obtained at
12.04 mmol of aniline. Above this concentration the percent yield decreases on further increase in
monmer amount. This might be owing to the formation of large concentration of monomer’s active
sites, leads to the generation of oligomers which are soluble in reaction medium. Moreover, the
rapid decrease in efficiency of initiator can also be responsible for low yield [134].
Figure 3.1: Percent yield of PANI at different amounts of monomer.
3.1.2 Influence of H2SO4 Amount on the Polymerization Yield
The effect of amount of H2SO4 on the yield of binary doped PANI is shown in Fig 3.2.
Highest yield was obtained at 4.41 mmol. The yield of PANI increases gradually with increase in
amount of H2SO4. After this, further increase in H2SO4 decreases the yield. The increase in yield
might be due to the strong oxidizing power of H2SO4 which causes the extended conformation of
49
Polymer chain by creating strong electrostatic repulsion between PANI chains [135]. But high
amount of acid beyond a certain level can also results in the hydrolysis of the polymer that affected
the yield of binary doped PANI. [136].
Figure 3.2: Percent yield of PANI at varying H2SO4 amount.
3.1.3 Effect of ALS Amount on the Yield of Binary Doped PANI
The variation of ALS amount shows little effect on the polymerization yield (Fig. 3.3).
Although same trend is observed in the case of effect of monomer and H2SO4 on yield. Increase
50
in amount of ALS in the polymerization mixture increases the doping of PANI resulting in increase
in yield but excessive ALS may aggravate the glutinosity of the system and induced difficulties to
separation and washing [137]. The amount of ALS seems to have no significant effect on the yield
compared to H2SO4. Varying the amount of H2SO4 indeed has increase the yield but the product
has poor electrochemical performance and processibility. On the other hand, the incorporation of
ALS showed little effect on yield but the obtained product showed good processibility and
electrocatalytic activity due to high conductivity.
Figure 3.3: Percent Yield of PANI at different amounts of ALS
51
3.2 Surface Morphology of Binary Doped PANI Samples and CEs
Fig. 3.4 shows SEM images of P-Mix 1, P-Mix 2, P-Mix 3 and P-Mix 4 obtained from
binary acid doping in different amounts of organic acid while keeping inorganic acid constant. An
irregular morphology can be seen in P-Mix 1 (Fig. 3.4 a), since there is no sign of elongation, we
can conclude that fibers are not formed. SEM micrographs reveal that, there are agglomerations of
particles with the formation of a cauliflower like structure. It is expected that the counterions from
the dopant interact with the cation-radicals of aniline. This interaction might act as a driving force
for the self-assembling cauliflower type shape and highly random aggregation of particles in the
cauliflower like structure. Such aggregation results in low electron delocalization and therefore
increases the penetration and charge transfer resistance for exchange of I-/I3-redox couples [128].
With increase in ALS amount, an evolution from dense irregular morphology observed in
P-Mix 1 to micro sized (1 μm) short rod like morphology with large flakes is observed in P-Mix 2
(Fig. 3.4b). Here selective types of aggregation process results in the formation of short needle
formation.
52
Figure 3.4: SEM images of a) P-Mix 1 and b) P-Mix 2.
In case of P-Mix 3 (Fig 3.5a), further increase in amount of ALS induced well organized
long nano sized (diameter range 43 - 50 nm) needle like morphology with some porosity. Similar
morphology of PANI is also reported by Katarzyna Krukiewicz et.al. They synthesized PANI in
m-cresol using camphor sulphonic acid as a surfactant with improved optical and morphological
properties [133].
53
Ting Chen et.al reported granular and tube like morphology of synthesized co-doped PANI
using dodecylbenzene sulfonate (DBSA) and HCl. Co-doped PANI showed good conductivity and
thermal stability [135].
Such structure is known to be of great value to photovoltaic application such as in DSSCs.
The needle like and some porous structure of the material is beneficial for the enhancement of
electrocatalytic activity for I3-/I-redox reaction and is expected to facilitate the migration of redox
couple within the CE by adsorption of the liquid electrolyte by trapping the liquid in the
nanoporomerics [128, 138]. However, with further increase in ALS amount, the needle like
morphology of P-Mix 3 is getting closure with the formation of a large flakes which reduces the
porosity in P-Mix 4 as shown in (Fig. 3.5b). it suggests that needle like morphology is very critical
to the amount of dopants in the binary doped PANI samples.
54
Figure 3.5: SEM images of c) P-Mix 3 d) P-Mix 4
55
The reason for observing different morphology in binary doped PANI system is related to
the micelle formation which is directly related to the concentration of counterions. We can say that
an appropriate amount of counterions is necessary to assist the cation-radical of aniline to
remarkably alter to the other shape. [139].
The reason for observing different morphology in binary doped PANI system is related to
the micelle formation which is directly related to the concentration of counterions. We can say that
an appropriate amount of counterions is necessary to assist the cation-radical of aniline to
remarkably alter to the other shape. [139, 128].
Fig. 3.6 (a and b) displays top view SEM image of P-MIX 3 and TiO2 films. Here it is
observed that needle like morphology with small pores of P-Mix 3 and rough surface of TiO2 are
beneficial for better electrocatalytic activity as porous structure of films can easily adsorbed liquid
electrolyte by trapping it in porous sides [140]. Fig. 3.7 (a and b) FIB cross sectional image of P-
Mix 3 and TiO2 films shows thickness of 8μm and 12μm respectively.
56
Figure 3.6: Top view SEM image of P-Mix 3 CE (a and a*) at 8000 and 50,000 magnification,
Top view SEM image of TiO2 CE (b and b*) 8,000 magnification and 50,000 magnification.
57
‘
Figure 3.7: FIB-SEM cross section of a) P-Mix 3 and b) TiO2 films.
58
3.3 Elemental Analysis and Mapping
Fig. (3.8-3.9) display the SEM- EDX spectra and surface mapping of elements of P-Mix
1, P-Mix 2, P-Mix 3, P-Mix 4. From the SEM-EDX spectra, the elements like C, N, O and S were
examined in a selected area of SEM shown as inset in Fig. (3.8-3.9). The signals from C, N, O and
S showed good agreement with each other and revealed the successful incorporation of binary
dopants in to the polymer backbone. The experimental results showed that compared to other
samples, P-Mix 3 has high content of carbon that is a main element present in the polymer
backbone. Moreover, high percentage of the sulphur (S) and oxygen (O) in P-Mix 3 is assumed to
be responsible for successful incorporation of dopants in the PANI backbone [25]. It can be noticed
that all the spectra exhibited three peaks of S at 2.3 KeV, 2.47 KeV and 0.014 KeV with different
intensities. These peaks can be associated with Kα1, Kedge, and L1edge, respectively [139]. For
a specific absorber, a sharp increase in intensity was observed when the energy of the X-rays
coincides with the energy of an electron shell (K, L, or M) in the absorber. The absorbed energy
(also called ‘critical excitation’ or ‘edge’) creates a vacancy in the specific shell. This vacancy is
then occupied by the electron transferred from another shell. As a result, specific X-Ray lines are
generated from K, L, M, N or O shells, corresponding to the excitation of the K, L, M, N and O
levels. The mapped images also confirmed the distribution of S contents along with other elements.
59
Figure 3.8: SEM-EDX spectrum and Mapping of a) P-Mix 1 and b) P-Mix 2.
60
Figure 3.9: SEM-EDX spectrum and Mapping of c) P-Mix 3 and d) P-Mix 4.
61
3.4 Optical Properties of Binary Doped PANI
The absorption curves of all samples (Fig. 3.10 a) display a similar shape with three
absorption bands at about 338 nm, 418 nm and 790 nm. The band ranging from 338-346 nm can
be ascribed to the electronic π to π* transitions due to excitation of nitrogen in the benzenoid rings.
A shoulder at 418-428 nm ascribed to the polaron to π* transitions indicating that incorporation of
dopants in polymer backbone [141, 142]. The band at infra-red region ranging from 790-822 nm
originates from π - polaron transition (cationic species). This broad band may be caused by
interband charge transfer from benzenoid to quinoid rings of conjugated PANI. Stronger
absorption of this band represents the protonation of synthesized material [143].
The wavelength, intensity and intensity ratios (A2/A1) of A2 (second band) to the A1 (first
band) of different amounts of binary doped PANI are illustrated in Table 2. The values of A2/A1
for P-Mix 3 is larger compared to others indicating the high doping level of the counter anions
than the others. It is illustrious that lambda 2 exhibit a significant red shift with increase in dopant
amount from P-Mix 1 to P-Mix 3, proposing an increase in conjugation length and ordered
structure of PANI backbone as dopant amount increases. This may be due to the fact that some
template effects are produced by dopant ions which improve the ordering of PANI chains. This
confirms an effective incorporation of counter ion in polymer structure [70, 144].
Further increase in dopant amount from P-Mix 3 to P-Mix 4, a blue shift is observed. This
blue shift might be due to the low degree of polymerization or disordered structure of PANI
because of fast polymerization rate [70].
To investigate the effect of dopant amount on band gap energy (Eg) of all PANI’s, Tauc relation
[145] is used to interpret absorption spectra.
αhv = A(hv - Eg)n Eq. 14
62
Where h= Planck's constant, v depicts frequency of photon and α represents the absorption
coefficient. Following equation is used in the calculation of α as:
𝛼 =2.303×𝐴
𝐼 Eq. 15
Where A and I depicted absorbance and path length, respectively. The Eg was obtained by plotting
(αhv)2 versus hv for direct energy transitions through extrapolating the slope to (αhv)2 → 0.The
mentioned plots are shown in Fig. 3.10 b. The calculated values of band gap are illustrated in
Table 2. The calculated band gap values are in good agreement with the reported work [146]. The
band gap values are verified with the conductivity data as shown in Table 3.2
It is known that the low band gap, high degree of polymerization, organization in polymer chains
and high doping degree of counter anions in PANI backbone are responsible for increase in
electrical conductivity [146, 147].
From the analysis of absorption spectra, it can be concluded that P-Mix 3 has highest
doping degree of counter anions, highly ordered structure and the lowest band gap amongst all the
synthesized samples and is expected to have increased conductivity.
63
300 400 500 600 700 800 9000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0
0
1
2
3
4
5
Wavelength (nm)
P-Mix1
P-Mix2
P-Mix3
P-Mix4
aA
bs
orb
an
ce
(h)
P-Mix1
P- Mix2
P-Mix3
P-Mix4
b
h(eV)
Figure 3.10. (a) Absorbance spectra of various binary doped PANI samples. (b) Tauc plot showing
band gap energies of binary doped PANI samples.
3.5 DC Conductivity of Binary Doped PANI
Of particular interest from the standpoint of processing of PANI are the half oxidized
forms-emeraldine base and the emeraldine salt. The emeraldine base is the neutral form with a
conductivity lower than 10−10 S/cm. its conductivity can be enhanced by doping process that yield
Emeraldine salt of PANI which is conducting in nature. The electrical conductivity of conducting
polymers results from mobile charge carriers introduced into the π-electronic system through
doping. The electrical conductivities of binary doped polymers are measured by four probe method
and are illustrated in Table 3.2. Little variation is observed in the conductivity values of various
concentrations of PANI samples. It can be observed that conductivity increases with increase in
64
ALS content from P-Mix 1 (0.09 S/cm) to P-Mix 3 (1.75 S/cm). But its further increase decreases
the conductivity of P-Mix 4. Doping play an important role in the enhancement of conductivity by
protonating the imine nitrogen atoms of PANI resulting in the formation of green Emeraldine salt
state. This state is electrically conducting and is responsible for conductivity. Use of different
dopant ions have significant effect on the process of conductivity. Dopant ions provide the mobility
of charge carriers on which conduction is based [146].
Ting Chen and his coworkers adopted micro emulsion pathway to synthesized co doped
PANI using DBSA and HCl. The co doped PANI demonstrated high conductivity of 1.6 S/cm-1
with heterogeneous morphology and better stability [135].
Binary acid doping of PANI, using phytic acid (PA) and HCl, was performed by Yogesh
Gawli et al. It was found that co-doping rendered a substantial and positive enhancement in the
conductivity of PANI (0.61 S/cm) than separately doped PANI (0.48 S/cm and 0.32 S/cm for HCl
doped PANI and PA doped PANI, respectively) [108]. The dependence of conductivity on the
amount of ALS indicates that the extent of benzenoid structure and needle like morphology due to
incorporation of ALS and H2SO4 are optimized in case of sample P-Mix 3 as shown in Fig. 3.5 a.
This results in maximum value of conductivity (1.72 S/cm) for this sample while in P-Mix 4,
further increase in ALS results in the destruction of favorable morphology, resulting in decreased
value of conductivity. It was reported that orientation of structures and morphology of samples at
the macroscopic level affects the mobility of charge carriers and, thus, influences conductivity
[146].
65
Table 3.2 The parameters derived from the UV-Vis spectra and four probe method.
Sample A1 A2 λ 1 λ 2 A2/A1 Eg Conductivity
Ω-1.cm-1
P-Mix 1 0.485 0.355 347 826 0.73 2.65 0.09
P-Mix 2 0.485 0.402 347 829 0.82 2.643 1.35
P-Mix 3 0.485 0.585 347 830 1.21 2.61 1.72
P-Mix 4 0.485 0.507 347 800 1.04 2.63 1.46
3.6 Functional Groups Detection of Binary Doped PANI Samples
Figure 3.11 shows FTIR transmittance spectra of various amounts of binary doped
PANI’s. A band at 3219-3226 cm-1 represents the stretching vibrations of NH part present in
polymer. Band at 2906-2914 cm-1 is related to NH2+ part in C6H4NH2
+C6H4– groups [148, 149].
The presence of the band at 2836-2850cm-1 is related to symmetrical and asymmetrical stretching
of alkyl substituent of ALS [150]. The building unit of PANI i-e benzene and quinone ring
depicting vibrations in the vicinity of 1545-1563 cm-1and 1468-1470 cm-1 confirming the
successful synthesis of PANI [151]. Their intensity ratio could be used as an indicator of the degree
of oxidation of PANI. The (IQ/ IB) ratio calculated for all spectra are listed in Table 3.3.
The detailed assignment of all the spectra of PANI is depicted in Table 3.3. The intensity
ratio of PANI is in order of P-Mix 1 P-Mix 2 P-Mix 3 P-Mix 4. This indicated that oxidation
level of PANI increases up to P-Mix 3 and after this a decrease is observed. Higher intensity ratio
reveals the greater conductivity and reflecting the emeraldine salt state of PANI [152]. The C-N
66
bending and stretching mode are observed at 1206-1220 cm-1 and 1283-1290 cm-1, respectively.
The bands at 1109-1114 cm-1 and 751-785 cm-1 are assigned to the vibrational band of nitrogen
quinine and 1,4-substituted aromatic rings, respectively [149].
The peak detected at 610-617 cm-1 is allotted to the stretching of S-O bond of H2SO4
dopant. The large descending base line in the spectral region of 4000-2000 cm-1 is attributed to
the free-electron conduction in the doped polymer [153]. The bands at 1631 and 1348 disappeared
in all the doped samples, revealing effective doping of polymers with the polaron formation (C-
N+) [154]. This may be attributed to the fact that both dopants were effectively incorporated in all
samples. Slight shifting of these bands in all spectra depicting a distinctive behaviour of doped
polymers with various concentrations of dopant [155]. This FT-IR analysis qualitatively supports
the presence of dopants on PANI salts. Presence of the bands attributed to the dopant ions and
quinoid-benzenoid rings clearly specify the successful synthesis of conducting salts of binary
doped PANI and support the results of absorption study.
67
Table 3.3: FTIR spectral absorption bands assignments of binary doped PANI.
S. No
FT-IR Frequency of absorption (cm-1)
Assignment
P-Mix 1
P-Mix 2
P-Mix 3
P-Mix 4
1 3226 3233 3229 3226 ν(N−H)
2 2906 2914 2914 2906 ν (C−H) in the dopant
3 2836 2843 2843 2826 Symmetrical and asymmetrical
stretching of alkyl substituent of ALS
4 1545 1552 1563 1545 ν(C=C) Q
5 1468 1470 1468 1468 ν(C=C) B
6 1283 1283 1283 1283 δ (CN)
7 1206 1213 1220 1206 ν (CN)
8 1099 1107 1107 1078 Due to nitrogen quinine
9 603 610 617 610 γ (S─O)
10 780 780 780 780 γ(C─H)
11 544 544 561 532 β(N−H) γ
: out-of-plane deformation mode; B: benzoid; δ: in-plane defomation mode ν: stretching mode;
Q: quinoid type ring;: bending mode; SQ: semiquinone
68
4000 3500 3000 2500 2000 1500 1000 500
20
40
60
80
100
120
140
1283
14681545
3219
d
b
a
c
% T
ran
sm
itta
nc
e
Wavenumber (cm-1)
Figure 3.11: FTIR transmittance spectra of a) P-Mix 1, b) P-Mix 2, c) P-Mix 3 and d) P-Mix 4.
3.7 XRD Analysis
XRD curves of different amounts of binary doped PANI samples are shown in Fig. 3.12.
It can be noted that the P-Mix 1 shows three main peaks at 2θ= 15° , 20 and 25, representing that
sample is partly crystalline and these peaks are characteristics of emeraldine state of binary doped
PANI [150]. The peaks at 2θ= 20° and 2θ= 25° are ascribed to the periodicity parallel and
perpendicular to the polymer chains of PANI, respectively. With increase of ALS in binary doped
PANI from P-Mix 2 to P-Mix 3, few crystalline peaks are appeared. In P-Mix 2, an additional
69
intense peak at 2θ= 29° is observed by suppressing the other peaks. Appearance of the intense peak
counts for slight better structural ordering and hence may exhibit better crystallinity [156]
While P-MIX 3 displays three sharp peaks 2θ= 20°, 2θ= 25°, 2θ= 29° and two small peaks
at 2θ= 15.0° and 2θ= 26.7°. The appearance of sharp peaks owing to high regularity and high
crystallinity compared to P-Mix 1 and P-Mix 2. Further increase in dopant amount results in the
reduction of peaks and hence reduces its degree of crystallinity. It is found that high ordering and
regularity in the arrangement of polymer chains enhances the crystallinity of the material. These
two properties are favorable for intramolecular mobility of charged species along the chain and to
some extent intermolecular hopping because of better and closer packing. Hence, it might be
suggested that high crystallinity leads to high conductivity [157]
From the above discussion, it is evident that intense peaks in P-Mix 3 leads to high
crystallinity and hence leads to high conductivity as evident from results elucidated above.
70
10 20 30 40 50 60 700
200
400
600
800
1000
1200
1400 29025
020
0In
ten
sit
y (
a.u
.)
2
150
200 25
0
26.70
290
a
b
c
d
Figure 3.12: XRD patterns of a) P-Mix 1, b) P-Mix 2 c) P-Mix 3 d) P-Mix 4.
3.8 Electrocatalytic Activity of Binary Doped PANI CEs
The electrocatalytic activity of the different amounts of binary doped PANI CEs and Pt CE
towards the iodide/triiodide (I- /I3-) redox couple was measured from cyclic voltammetry (CV)
based on three electrode system using various P-Mix and Pt as working electrode as shown in
Figure 3.13. These CV curves allow qualitative assessment of the electrode kinetics of the redox
couple. All CV curves have a pair of oxidation and reduction peaks that affect device performance
in DSSCs. The peaks at positive potential (anodic peaks) display an oxidation reaction and is
attributed to the reaction shown in Eq. (16) and cathodic peaks at negative potential exhibit
reduction reaction and is related to the reaction 2 illustrated in Eq. (17) [128].
71
3I2 + 2e− → 2I3− Eq. (16)
I3− + 2e− → 3I− Eq. (17)
Literature reveals that a higher Ired value manifests high electrocatalytic performance and
conductivity of the electrode material and the more striking observation was the separation of the
anodic and the cathodic peak potentials (Epp) which is inversely correlated with the catalytic
activity of the CE. The Epp values can be used to estimate their redox reaction resistances [158].
The peak current density of reduction (Jred) and oxidation (Jox), and peak separation (Epp) of the
five electrodes are mentioned in Table 3.4. It is detected that, the variation of ALS content caused
variance in electro catalytic activity (ECA) of the binary doped PANI CEs. Fig. 29a clearly showed
that the reduction process of P-Mix 1 is not obvious as it displays smaller current density as well
as high Epp compared to others. This clearly depicts relatively weak electrocatalytic ability and
lower conductivity of P-Mix 1 as seen in conductivity analysis [159]. With the increasing content
of ALS, the current densities of the electrodes (P-Mix 2 and P-Mix 3) increases gradually,
indicating that more active sites have been created, which might be due to the role of the ALS
which induces different morphologies in binary doped PANI samples with different amount as
exemplified by Fig 3.4 and 3.5. According to experimental data, P-Mix 3 represents highest value
of Jred, Jox, and the smallest Epp, reflecting its largest surface area for efficient catalytic reduction
of I3- and fastest charge transfer at electrolyte/PANI interfaces, which contributes to its high
conductivity and electrocatalytic behavior to the I−/I3−redox couple [62] as represented in table 4.
72
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-2
-1
0
1
2
3
P-Mix 4
P-Mix 3
P-Mix 2
Cu
rre
nt
De
ns
ity
(m
A c
m-2)
Potential (V)
P-Mix 1
P-Mix 2
P-Mix 4
P-Mix 3
Pt
P-Mix 1
Pt
Figure 3.13: Cyclic voltammograms of iodide species on binary doped PANI films and Pt at scan
rate of 50 mV/s.
73
Table 3.4: The parameters derived from CVs of various concentrations of binary doped PANI CEs
and Pt CE.
CE Joxi
(mV)
Jred
(mV)
Dn×10-7
(cm2.s-1)
Epp (V)
P-Mix 1 1.87 -1.20 0.95 0.91
P-Mix 2 2.11 -1.55 1.23 0.8
P-Mix 3 2.71 -1.92 2.3 0.73
P-Mix 4 2.28 -1.78 1.27 0.78
Pt 2.05 -1.65 1.8 0.76
Fig. 3.14a depicts the influence of the scan rates on the CV of the P-Mix 3 CE in I−/I3−
redox electrolyte. CV curves elaborated that increase in scan rates results in gradual shifting of
cathodic and anodic peaks to the negative and positive directions, respectively. Fig. 3.14b shows
the effect of square root of scan rates [(scan rate)1/2] on Jred and Joxi of P-Mix 3 CE, which
demonstrated that both are linearly proportional to the (scan rate)1/2. The result indicates that the
adsorption of iodide species is little affected by the reaction on the P-Mix 3 CEs' surface, thus
suggesting no specific interaction between the redox couple and the P-Mix 3 CE same as with the
Pt CE [158]. Moreover, the diffusion coefficient (Dn) can be calculated by using Randles–Sevcik
equation [128] as illustrated in Eq. (18) and its values are tabulated in Table 3.4.
Jred = Kn1.5AC(Dn)0.5 0.5
Eq. 18
Where K, n, A,ν and C indicates the constant of 2.69*105, no. of electrons transferred in redox
reaction, electrode area, scan rate and concentration of redox species in bulk, respectively. As seen
from the table, the diffusivity for the P-Mix 3 CE is higher than that of the Pt and other CEs,
74
indicating reversible redox reaction on P-Mix 3 CE with high charge transfer speed and fast rate
of redox reaction, indicating that the reduction reaction of I−/I3−that occurred at the binary doped
PANI and Pt CEs belongs to the diffusion controlled transport process [160].
By examining the results of experimental data, it can be suggested that P-Mix 3 has high
electrocatalytic activity and fast charge transport process, probably originating due to the
simultaneous use of organic and inorganic dopants, inducing rod like morphology with diameter
of 50 nm which is conductive to the charge transmission and I3- diffusion. The CV results together
with absorption analysis and EIS model suggested that P-MIX 3 can be utilized as an efficient CE
in a DSSC than its counter parts.
75
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-4
-2
0
2
4
6
7 8 9 10 11 12-4
-2
0
2
4
6
20 mV/s
50 mV/s
100 mV/s
125 mV/s
aC
urre
nt D
en
sit
y (
mA
cm
-2)
Potential (V)
anodic
cathodic
b
(Scan Rate)1/2 (mV/s)
1/2
Figure 3.14: (a) CV curves of P-Mix 3 at scan rates of 20, 50, 100 and 125 mV/s and (b) relation
of cathodic and anodic peaks of P-Mix 3 vs (scan rate)1/2.
76
3.9 Photovoltaic Performance of DSSCs Based Binary Doped PANI CEs and Pt CE
To evaluate the photovoltaic performance of DSSCs constructed with different amounts of
binary doped PANI CEs, the J-V curves of these devices under standard light irradiation are
illustrated in Fig. 3.15 and corresponding device parameters are summarized in Table 3.5. DSSCs
based on P-Mix 1 CE, exhibited poor conversion efficiency (2%) with low values of Jsc, manifests
low conductivity of the polymer. The low value of FF and OCV depicts low catalytic activity of
P-Mix 1 CE as these two parameters are largely affected by electro catalytic activity of the CE
[116]. When using P-Mix 2 and P-Mix 3, all photovoltaic parameters increases with conversion
efficiency of 2.7% and 4.54 %, which may arise due to increase in ALS content which give an
increase in ALS content results in conversion of needle like morphology to large flakes of P-Mix
4 as represented in Fig. 3.5. Comparing DSSC constructed with P-Mix 3 with Pt based DSSC
(4.02%), all the photovoltaic parameters of P-Mix 3 CE based DSSC are enhanced.
Kezhong Wu and its coworkers [157] reported the effect of transition metal ions (Mn2+,
Ni2+, Co2+, Cu2+) on the photovoltaic properties of PANI used as CE in DSSC. PANI CE doped
with Mn2+ showed maximum power conversion efficiency of 4.41 % amongst others.
Auliya et al. [51] fabricated HCl doped PANI films at various temperatures (from 273 K
to 348 K) which were used as CEs in DSSC. The HCl doped PANI film (fabricated at low
temperature) as CE exhibited high electrocatalytic activity with power conversion efficiency of
1.91%.
The enhancement in the efficiency of DSSC with P-Mix 3 CE can be attributed to three
aspects. Firstly, the increase in content of ALS induces favorable morphology in P-Mix 3 (Fig.
3.5c) creates a high surface area on the CE. This is beneficial for the efficient reduction reaction
in the I−/I3−system [161]. Secondly, the small RCT value of P-Mix 3 CE is responsible for ease in
77
electron transfer and enhancement in Jsc. Thirdly, the higher electrocatalytic activity of P-Mix 3
results in the increasing of FF and hence efficiency of the cell [117, 162]. Therefore, the DSSC
with P-Mix 3 CE has predominant photoelectric performance. The facile synthesis and effective
cost allow binary doped PANI electrode to be a possible alternative CE with improved
photovoltaic and electrocatalytic properties of DSSCs.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
0
2
4
6
8
10
12
14
16
Cu
rren
t D
en
sit
y (
mA
cm
-2)
Potential (V)
P-Mix 1
P-Mix 2
P-Mix 3
P-Mix 4
Pt
Figure 3.15: Photocurrent density–photovoltage (J–V) curves of DSSCs constructed with
different CEs
78
Table 3.5: The detailed parameters obtained from J-V curves of DSSCs based various binary
doped PANI CEs and Pt CE.
CE Jsc (mV) OCV (V) FF η (%)
P-Mix 1 9.1 0.53 0.38 2.0
P-Mix 2 10.3 0.59 0.43 2.7
P-Mix 3 15.13 0.60 0.53 4.54
P-Mix 4 11.90 0.63 0.40 3.0
Pt 12.67 0.64 0.52 4.02
3.10 Process of Photoexcitation in DSSCs
Scheme 3.1 illustrate the process of photo excitation in the DSSCs [26]. A mesoporous
nano sized oxide layer typically TiO2 is coated on FTO glass. High surface area of TiO2 film functions
as a support for the dye, provides path for electron transportation and diffusion of the redox electrolyte.
At the surface of TiO2 film, a layer of dye molecule is bonded covalently. The dye pushes an electron
into the CB of mesoporous TiO2 film by harvesting incident solar light resulting in the formation of
oxidized dye. The oxidized dye undergoes regeneration by triiodide (electron donor) in the electrolyte
(charge mediator) as quickly as possible. The oxidized redox couple migrates to the PANI CE and
reduced by accepting electrons at CE surface. This results in reduction of oxidation state of doped PANI
to emeraldine and/or leucoemeraldine salt. Meanwhile these salts may oxidized by redox couple
because of the redox equivalent. The electrons are collected at CE from the external circuit and return
79
back into the circulation within the cell. The circuit being completed via electron migration through the
external load.
Scheme 3.1 Schematic diagram of DSSC with PANI as CE showing the sequence of electron
transfer.
3.11 Durability of Fabricated DSSCs
To investigate the durability of DSSC/PANI, the start-up behavior and multiple start/stop
(ON/OFF) capability are important parameters. Solar cell performance was monitored for 600 s
with the start/stop switching of the DSSCs using P-Mix 3 by alternating between turning ON and
OFF the illumination, the performance of DSSC/Pt is also examined for comparison (Fig. 3.16).
On turning ON the illumination, the current density increases sharply and there is no time delay in
starting the cell. This confirms a rapid light response and hence high electrocatalytic activity
towards redox electrolyte. About 96 % of the initial photocurrent density was measured after
eleven cycles, implying a superior capability of the P-Mix 3 CE to start multiple times [163].
80
Taş et al. [91] synthesized Aluminum-doped PANI salts in various solvent media. They fabricated
these salts on FTO and utilized as CEs in DSSCs. Aluminum-doped PANI CE in acetone showed
the best electrocatalytic ability and highest efficiency but its stability is low.
0 100 200 300 400 500 600
0
2
4
6
8
10
12
14
16
Cu
rre
nt
De
ns
ity
/ m
A c
m-2
Time (second)
P-Mix 3
PtLight on Light off
Figure 3.16: Start-stop switches of DSSC assembled with P-Mix 3 and Pt CE.
3.12 Charge Transport Properties
Electrochemical Impedance Spectroscopy (EIS) analysis was performed to further
elucidate the electrochemical catalysis of different PANI CEs on the reduction of redox electrolyte,
by using symmetrical cells fabricated with different amounts of binary doped PANI as CEs and
also Pt as a CE to compare the performance of binary doped PANI films. The Nyquist impedance
of DSSCs based on various CEs are shown in Fig 3.17. There is a well-defined semicircle for the
81
plots of binary doped PANI and Pt based cells. The semicircles located in high frequency can be
assigned to impedance (Rct) related with charge transfer processes occurring at the CE/electrolyte
interface. In high frequency region, the x-axis intercept refers to the series resistance (RS) of the
electrode which describes mainly the resistance of the two identical electrodes and the electrolytic
resistance [164].
We mainly emphasized on the RS and RCT of CEs. The RS and RCT parameters of these
electrodes have been evaluated by fitting the measured EIS data using a Randles-type equivalent
circuit in Autolab impedance analyzer shown as an inset in Fig 3.17. The RS and RCT of different
CEs together with other parameters are tabulated in Table. 3.6. The P-Mix 1 possesses a largest
value of Rs and Rct of 5.2 and 18.3Ωcm2, respectively. After increasing the content of ALS, the
Rct of P-Mix 2 and P-Mix 3 is dramatically decreased to17.5Ωcm2 and 16.6Ωcm2, respectively,
which is even below the Rct of commonly used Pt/FTO (17.2 Ωcm2). Lower value of Rct and Rs
of the P-Mix 3 CE corresponds to its needle like structure with small porosity and hence large
surface area, which can endorse the electrocatalytic capability of the PANI CE and enhances the
diffusivity of I3− to the CE while lower Rs value manifests a firm adhesion of electroactive material
on the FTO [62]. This argument agrees with the calculated Dn values from CV analysis (Table
3.4), DC conductivity (Table 3.2).
82
0 5 10 15 20 25 30 35 40
0
10
20
30
40
Z"
/ (o
hm
.cm
2)
Z' / (ohm.cm2)
Pt
P-Mix 1
P-Mix 2
P-Mix 3
P-Mix 4
Figure 3.17: Nyquist plots for the electrochemical cells assembled using various electrode
materials and the equivalent circuit.
Table 3.6: Electrochemical parameters obtained from EIS analysis.
CE Rs (Ωcm2) Rct (Ωcm2)
P-Mix 1 5.2 18.3
P-Mix 2 3.9 15.5
P-Mix 3 4.2 8.26
P-Mix 4 5.0 13.90
Pt 4.6 10.36
83
Part 2
Comparison of Photovoltaic Properties of DSSCs based on Pristine PANI, H2SO4 doped
PANI, ALS doped PANI and binary doped PANI CEs
(Effect of counter ion)
In this section it is discussed that how the type of counter ion of the dopant affect the
properties of synthesized materials.
3.13 Morphology
Fig. 3.18- 3.20, respectively, shows scanning electron micrographs of P-N, P- H2SO4, P-
ALS and P-Mix. It can be observed that the counter ions derived from the dopants has significant
influence on the morphological features of the resulting polymer. The morphological analysis
reveals some fascinating features as a function of organic and inorganic acids used during
synthesis. The microstructure of P-N is examined by SEM, as shown in Fig. 3.18a, proposing
closely packed aggregations probably due to intermolecular and intramolecular hydrogen bonding.
This compact structure might increase interfacial and penetration resistance for charger transfer
and for exchange of I-/I-3 redox couples [165]. P-H2SO4 (Fig. 3.18b) shows interconnected fibrous
like morphology while P-ALS exhibits micro porous structure (Fig. 3.19a).
Interestingly, in the case of P-Mix (Fig. 3.19b) it is represented that some morphological
features are similar to those of P- H2SO4 and P-ALS [128]. Short PANI nano fibers might grow
on porous sites of ALS and long interconnected rod like morphology with porosity is thus observed
in P-Mix [166] The porous structure allows the reactive species continuously nourish into the
depleted regions permitting nanofibers to unceasingly grow and elongate in a one-dimensional
84
direction. This porous and rod like morphology of PANI is beneficial to the diffusion and rapid
exchange of I3- redox couples within the CE material [167].
Fig. 3.20 a and b represents the FIB SEM cross sections of TiO2 and P-Mix CEs. The thickness
of TiO2 film and P-Mix are 12 μm and 8 μm, respectively. Same thickness of other CEs was used
in this study.
85
Figure 3.18: Top view of SEM of a) P-N, b) P-H2SO4,
86
Figure 3.19: Top view of SEM of a) P-ALS and b) P-Mix.
87
Figure 3.20: (a and b) FIB-SEM cross sections of P-Mix and TiO2 films, respectively.
88
3.14 XRD Analysis
The XRD diffraction patterns of P-N, P-H2SO4, P-ALS and P-Mix are shown in Fig. 3.21.
The PANI exhibits two significant peaks located at 2θ= 20.2° and 25.2°, corresponding to the
diffraction of crystallographic planes of PANI which originates from the main chains of the
polymer structure. These peaks are recognized to the periodicity parallel and perpendicular to the
PANI chain, respectively [156].
Similar peaks are also observed in all samples in addition to the shoulder peak at 2θ= 14.4°.
This peak is considered to be specific for an anionic lattice and can be assigned to scattering from
a lattice built up from main PANI chains [168]. With addition of binary dopant into the PANI, the
diffraction peak becomes sharper, indicating a growth of composed crystallites [156]. The
conversion of shoulder peak into intense peak in all doped samples suggests the incorporation of
dopants induces ordered packing of polymer chains. All these peaks are symbolic of semi
crystalline emeraldine salt of PANI [169].
The above discussion clarifies that incorporation of individual dopants i-e H2SO4 and ALS
has little effect on crystallinity while their combination (P-Mix) exhibits intense peaks suggesting
high crystallinity. The higher crystallinity can be attributed to the smaller dopant such as H2SO4
helping in closer chain arrangement while the presence of bulky ALS causes ring distortion in the
material [168].
89
0 10 20 30 40 50 60 70 80
0
400
800
1200
1600
2000
Inte
ns
ity
(a
.u.)
2 Theta
a
b
c
d
Figure 3.21: XRD pattern of a) P-N, b) P-H2SO4, c) P-ALS and d) P-Mix.
3.15 FTIR Analysis
The vibrational bands (Fig. 3.22) observed in Pristine, doped and binary doped PANI are
explained on the basis of normal modes. The contribution from the stretching mode of NH part
present in polymer is observed at 3254 cm-1. Band at 2914 cm-1 is related to NH2+ part in
C6H4NH2+C6H4– groups [148, 149].
The C-N bending mode and stretching mode are appeared at 1248 cm-1 and 1291 cm-1,
respectively. The signals observed at 1348 cm-1, 1144 cm-1 and 806 cm-1 are assigned to the
90
stretching mode of CN part of a secondary aromatic amine, vibrational band of nitrogen quinine
and 1,4-substituted aromatic rings, respectively [149].
The characteristic signals of P-N at 1555 cm-1 and 1469 cm-1 depicting the presence of the
benzene and quinone ring deformations, respectively [152]. In P-H2SO4, P-ALS and P-Mix,
shifting of these bands towards higher wavenumber representing the creation of positive charges
on the polymer chain depicting a distinctive behavior of doped polymers [155]. The bands at 1631,
1348, and 1144 cm-1 disappeared in all the doped samples, revealing effective doping of polymers
with the polaron formation (C-N+) [144]. This may be attributed to the fact that both dopants were
effectively incorporated in all samples. The major characteristic peaks observed at 3206, 1558,
1462, 1295 and 1234 cm-1 are similar to the PANI. In P- H2SO4, the band observed at 1031 cm-1
indicates NH+…..SO3- interaction between polymer and dopant. The peaks detected at 942 and
682 cm-1 are allotted to the stretching of O=S=O and S-O group due to the dopant [170].
In P-ALS, the presence of the bands at 2941 cm-1 and 2841cm-1 are related to symmetrical
and asymmetrical stretching of alkyl substituent of ALS [150]. In P-Mix, similar peaks are
observed as in P-ALS but intensity of the peaks enhanced due to the presence of H2SO4. The FT-
IR spectra clearly indicate the presence of counter ions of dopants on PANI salts and confirms the
formation of emeraldine salt of the polymer and these results of FTIR further supports the UV
data.
91
800 1600 2400 3200
40
80
120
160
200
% T
ran
smit
tan
ce
Wavenumber (cm-1
)
a
b
c
d
Figure 3.22: FTIR spectra of a) P-N, b) P-H2SO4, c) P-ALS and d) P-Mix.
3.16 DC Conductivity
The electrical conductivity of pristine, single doped and binary doped PANI of the pellets
are tested by four probe technique and conductivity measurement results are tabulated in Table
3.7. The values of electrical conductivity fall in the sequence of P-N < P-ALS < P-H2SO4 < P-Mix.
From the order it is clear that there are apparent differences in the electrical conductivity of
polymers and the P-Mix have highest conductivity compared to others. High conductivity is
required for ease of electron transfer from CEs to redox electrolyte. This high conductivity of P-
Mix might be due to the simultaneous addition of both the acidic dopants provide maximum acidic
92
medium for the polymerization due to which maximum transport of electrons take place which
consequently enhances the conductivity [91].
It can be concluded that simultaneous introduction of binary dopants enhances the conductivity of
the material hence resulting in lower Rs value which leads to the high catalytic activity and facile
electron transfer at the CEs [91].
Table 3.7. The wavelength (λ), intensity (A) and band gap calculated from the UV-Vis spectra
and electrical conductivity measured by four probe method, of different PANI samples.
Sample A1 A2 λ 1 λ 2 A2/A1 Eg Conductivity
Ω-1.cm-1
P-N 1.13 0.67 329 610 0.59 2.67 0.03
P-H2SO4 0.3 0.35 346 790 1.16 2.58 0.22
P-ALS 0.36 0.41 344 822 1.13 2.5 0.10
P-Mix 0.48 0.58 344 822 1.21 2.35 1.42
3.17 Electronic Spectroscopy
The UV spectra of all samples (Fig. 3.23 a) reveal a similar shape with three absorption
bands except PANI. PANI exhibits two distinct bands at 329 nm and 610 nm representing a local
charge transfer between a quinoid ring and the adjacent imine-phenyl-amine unit [171].
93
The band ranging from 338-346 nm originates from the π to π* transitions centered on the
benzene rings. The absorption band located at 418-428 nm belongs to the polaron−π* transitions
indicating the protonation of polymer backbone [141]. The band obtained in NIR region ranging
from 790-822 nm corresponds to the protonated PANI which confirms the successful incorporation
of binary dopants in the PANI backbone. Stronger absorption of this band represents the
introduction of dopants into the polymer backbone [142]. On comparison with UV–Vis absorption
spectrum of P-N with that of P- H2SO4, P-ALS and P-Mix, noticeable differences are seen in all
the doped samples with different oxidation states, intensities and positions with respect to the
dopants used. Thus, optical spectroscopic analysis reveals that nature and size of the counter ions
of various dopants are responsible in the creation of different states of doped and binary doped
polymers [143].
It is recognized that notable changes are observed in blue band with various dopants
demonstrating that degree of oxidation is different in all spectra [142]. However, the red region
band of all doped PANI exhibit a significant red shift compared to PANI, proposing an increase in
conjugation length and degree of polymerization of the polymer backbone. This meaningful shift
might reveal the effective integration of sulphate ions in the PANI during polymerization which
results in the ordered structure of PANI backbone [144].
The absorption band’s intensity and wavelength of various PANI samples are presented in Table
3.7.
The intensity ratio of the low energy band (second band) to the high energy band (first
band) (A2/A1) for all the samples is also mentioned in Table 3.7. The A2/A1 ratio for P-Mix is
larger compared to others indicating the high doping level of the counter anions than the others. It
is known that electrical conductivity is directly related to the ordered structure and doping level of
94
PANI backboneTable 3.7 [147]. Thus P-Mix has high doping level and, accordingly, high
conductivity among all the samples (as supported by dc conductivity).
In order to find band gap energy (Eg) of all PANI’s, Tauc relation [172] is used to interpret
absorption spectra.
αhv = A(hv - Eg)n Eq. 19
Where h= Planck's constant, v= Frequency of photon and α refers to the absorption coefficient
and is calculated by the following equation
𝛼 =2.303×𝐴
𝐼 Eq. 20
Where A represents absorbance and I depicted path length.
hv is determined by 1240/wavelength. Eg was calculated by plotting (αhv)2 for direct energy
transitions (n=2) versus hv and by extrapolating the slope to (αhv)2 → 0, Eg was obtained. Fig.
3.23b shows that calculated values of Eg are within the range of 2.35–2.67 eV and are depicted in
Table 3.7. The band gap values for synthesized materials agrees with the reported works [146].
From the analysis of band gap values of PANI films, it is confirmed that P-Mix has the lowest
band gap amongst all the synthesized samples and hence lead to increased conductivity and
effective charge transport, which supports the conductivity and impedance data.
95
300 400 500 600 700 800 900
0.0
0.2
0.4
0.6
0.8
1.0
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4
0
2
4
6
8
10
Ab
sorb
an
ce
Wavelength (nm)
P-Mix
P-ALS
P-H2SO
4
P-N
a
(h)
h(eV)
P-N
P-H2SO4
P-ALS
P-Mix
b
Figure 3.23: a) Absorption spectra of PANI CEs. b) Tauc plots of PANI CEs showing the band
gap energy (Eg).
3.18 SEM-EDX Analysis and Elemental Mapping
To check the presence of different elements and its distribution, SEM-EDX spectra and
elemental mapping of the various doped and binary doped PANI are shown in Fig. 3.24-3.25.
From the EDX spectra of the P-N (Fig. 3.24 a), very less amount of sulfur was detected. This
might be due to the presence of APS [148]. For the other electrodes, the detected weights and
atomic ratios of sulfur were respectively 3.01% and 1.25% for P-H2SO4 (Fig. 3.24 b), 2.58% and
1.07% for P-ALS (Fig. 3.24 c), and 8.31% and 3.53% for P-Mix (Fig. 3.24 d). From the
96
experimental results, high S contents in P-Mix revealed the successful incorporation of binary
dopants (H2SO4 and ALS) in the PANI backbone.
Figure 3.24: SEM-EDX spectrum and mapping of a) P-N and b) P-H2SO4.
97
Figure 3.25: SEM-EDX spectrum and mapping of c) P-ALS and d) P-Mix.
98
3.19 Electrochemical Characterization
Collection of electrons from the external circuit and reduction of iodide species in the
electrolyte are main functions of a CE. Therefore, low charge transfer resistance and high catalytic
activity are ideal to optimize the efficiency of a CE. To explore the electro catalytic performance
of various doped PANI and Pt CEs on I3- reduction, CV analysis were performed (Fig. 3.26 a). All
curves show a pair of anodic and cathodic peak corresponding to the redox reaction i-e I3-+ 2e-→3
I-, which has strong effect on the efficiency of DSSC [160]. The anodic peak at high potential is
attributed for the oxidation reaction i-e 3I- - 2e−….. I3−, and cathodic peak at negative potential is
allotted to the reduction reaction i-e I3- + 2e-……. 3I− [173]. The shape and peak positions of all the
PANI CEs are similar to Pt CE, suggesting high catalytic activity of PANI CEs towards I- to I3-
redaox couple.
Reduction peak current IRed and peak-to peak separation (Epp) are two critical parameters
for comparing the electrocatalytic behavior of various CEs [174]. The values of IRed are in an order
of P-N < P-H2SO4 < P-ALS < Pt < P-Mix and the Epp values (listed in Table 3.8) are in an inverse
order. Compared with the Pt and other counter electrodes, P-Mix electrode shows higher IRed and
lower Epp values for I−/I3− redox reaction. Whereas the low value of IRed is evident in P-N, P-
H2SO4 and P-ALS. Thus, the simultaneous introduction of H2SO4 and ALS into PANI results in
a significant improvement in the electrocatalytic activity. Epp is inversely related to rate of charge
transfer and high rate of charge transfer is favorable for high electrocatalytic activity of a CE [175,
176]. The higher peak current and the lower Epp demonstrate a higher catalytic behavior to I3-
reduction [62, 174]. Therefore, the electrocatalytic activity of various CEs is in an order of P-N<
P- H2SO4 < P-ALS < Pt < P-Mix. The highest current density of P-Mix electrode indicating the
improved catalytic activity of the CE. The differences observed in the catalytic performance of the
99
as prepared electrodes may be affected by different morphologies of the CEs that means different
surface area of the electrode and, thus, different electrocatalytic performance [178].
Moreover, to elucidate the correlation between diffusion of iodide and peak current density
in the binary doped PANI CE, Randles-Sevcik theory is applied to calculate the diffusion
coefficient (Dn) as mentioned in Eq. 18 (Part 1) [128].
As presented in Table 8, the Dn increases in an order of P-N < P- H2SO4 < P-ALS < Pt <
P-Mix, illustrating a similar rank of CE’s catalytic activity for the I3- reduction. It clearly shows
that the diffusion of PANI CE is improved by inclusion of organic and inorganic dopants, the P-
Mix CE exhibits larger Dn, which is even slightly larger than Pt electrode, originating from the
Porous and fibrous morphology which is conductive to the charge transmission and I3- diffusion.
However, the doped and Pristine PANI CEs show relatively lower Dn because of less porosity to
establish less charge pathway [179].
Estimation of fast electron transfer process with diffusion limited is studied by recording
cyclic voltammograms of P-Mix and Pt as the working electrodes under different scan rates (30-
125 mV/s) as illustrated in Fig. 3.26b and Fig. 3.27a. These CV curves illustrated the increase in
cathodic and anodic peaks with scan rates. By analyzing the current density and (scan rates)1/2 of
Pt and P-Mix (Fig. 3.27b), a linear relationship is observed for both the samples indicating that
the electrolyte specie undergoing redox reaction on PANI electrodes is controlled by ionic
diffusion in the electrolyte, that is correlated with the transportation of electrolyte in the bulk
solution and inside the PANI film [159]. Slope of these linear curves are correlated to the diffusion
process, hence estimating the rate of electrochemical reaction. Slope with highest value manifests
a high diffusion rate that is beneficial for ease of charge transfer [163]. It can be observed that P-
Mix depicts the highest slope value, followed by Pt and agrees with the results obtained from Fig.
100
3.27a. Although Pt CE is the best electrocatalyst but in some cases, electrocatalytic activity of Pt
is lower than other CEs. Actually, catalytic activity is intrinsic property of a material but it also
depends on the extrinsic factors and can be varied by changing certain extrinsic factors like
porosity, particle size, crystal structure, morphology and so on [128]. This clarifies that why our
CE shows high electrocatalytic activity relative to Pt.
Dissolution or corrosion of the CE material in electrolytes is major obstacle for its long
term stability. In order to check the stability of P-Mix in iodide electrolyte, the freshly prepared P-
Mix CE was stored for 30 days and then was subjected to successive CV scanning i-e 15 CV cycles
at 50 mVs-1. As illustrated in Fig. 3.28, no noticeable change is observed in the reduction of
current density and peak shifting. This implies that our material is stable and can coexist with
iodide species for long period of time [179].
From the above discussion, we can now conclude that the incorporation of dual dopants
leads to more desirable morphology, band gap and conductivity of the polymer which
consequently can have positive impact on the electrochemical and photocatalytic properties when
used as counter electrodes in dye sensitized solar cells [178, 179]. The CV data along with EIS
model and absorption curves suggested that P-Mix can be used as a more efficient CE in a DSSC
than its counter parts.
101
Table 3.8. CV, J-V and EIS parameters of DSSCs with P-N, P- H2SO4, P-ALS, P-Mix and Pt CEs.
CE Joxi (mA) Jred (mA) Dn (cm2.s-1) Epp (V)
P-N 1.87 -1.10 0.9×10-8 0.82
P-H2SO4 1.67 -1.18 1.45×10-8 0.798
P-ALS 1.67 -1.38 1.70×10-7 0.93
P-Mix 2.39 -1.80 2.3×10-7 0.59
Pt 2.05 -1.41 1.8×10-7 0.69
102
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2-3
-2
-1
0
1
2
3
4
-0.3 0.0 0.3 0.6 0.9 1.2
-2
-1
0
1
2
3C
urr
ent
Den
sity
(m
A c
m-2
)
Potential (V)
30 mV/s
50 mV/s
75 mV/s
100 mV/s
125 mV/s
b
Cu
rren
t D
ensi
ty (
mA
cm
-2)
Potential (V)
P-Mix
P-H2SO4
P-ALS
P-N
Pt
a
Figure 3.26: (a) Cyclic voltammograms of iodide species for Pt and PANI CEs at 50 mV/s scan
rate and (b) CVs of P-Mix at different scan rates (30, 50, 75, 100, 125 mV/s).
103
-0.3 0.0 0.3 0.6 0.9 1.2
-2
-1
0
1
2
3
4
5 6 7 8 9 10 11 12 13 14
-3
-2
-1
0
1
2
3
Potential (V)
30 mV/s
50 mV/s
75 mV/s
100 mV/s
125 mV/s
aC
urr
ent
Den
sity
(m
A c
m-2
)
(Scan Rate)1/2 / (mV/s)1/2
bP-Mix
Pt
P-Mix
Pt
Figure 3.27: a) CVs of Pt at different scan rates (30, 50, 75, 100, 125 mV/s) and b) Relationship
between Jred, Joxi and scan rates.
104
0 2 4 6 8 10 12 14
-2
-1
0
1
2
3
4
-0.3 0.0 0.3 0.6 0.9 1.2
-2
-1
0
1
2
3
4
Cu
rren
t D
ensi
ty (
mA
cm
-2)
Cycle Time
anodic
cathodic
b
Cu
rren
t D
ensi
ty (
mA
cm
-2)
Potential (V)
a
Figure 3.28: a) 15 successive CVs of P-Mix electrode at the scan rate of 50 mVs-2 in iodide
electrolyte, b) anodic and cathodic peak current.
3.20 Photovoltaic Properties
Fig. 3.29 illustrated the J-V graphs of the DSSCs with Pristine PANI, P- H2SO4, P-ALS
and P-Mix CEs and Pt CE for comparing, under light source of 100 W. The photovoltaic
parameters are summarized in Table 3.9. Eqs. 11 and 12 (mentioned in chapter 1) are used to
calculate the FF and η of the DSSCs, respectively [180].
According to the experimental data, the poor performance belongs to P-N CE based DSSC
in terms of the lowest values of η = 1.14 %, OCV= 0.48 mV and FF= 0.45. The lower value of η
105
might be due to the fact that this CE has insufficient pores (as explained in SEM) for diffusion of
redox species resulting in the increased availability of recombination between the photoinjected
electrons and iodide ions at the CE [164]. The P-H2SO4 also demonstrates low value of η
representing that H2SO4 alone is not sufficient to improve its electrochemical properties. A
noticeable difference is observed in Jsc and OCV of P-ALS based DSSC from the PANI and P-
H2SO4 CE based cells.
This might be due to doping of PANI with ALS that can enhance the conductivity and
desirable morphology of counter electrodes. The efficiency of P-ALS increased to 2.79 %. In the
P-Mix, the value of FF increases noticeably with improved Jsc and OCV values; this increment
increased its performance to 4.54 %, a level more efficient than that of DSSCs with Pt CEs (4.03
%). As aforementioned, high conductivity and low RCT due to contribution of binary dopants in
P-Mix is attributed to the higher value of Jsc, leading to high catalytic activity for facile reduction
of electrolyte at the CEs. In addition, the increased electro-catalytic ability for fast reduction of I3-
ions at the P-Mix CEs results in the reduced availability of I3-ions for recombination with
photoinjected electrons, resulting in higher OCV compared to the other CEs [161].
Duan et al. [25] used the PANI CE for DSSCs, and found that it shows power conversion
efficiency (PCE) of 3.1%. Wu et al. [35] studied the effect of different dopants on electrocatalytic
activity of PANI as CE in DSSCs. They fabricated Sodium dodecyl sulfate (SDS),
Ganodermalucidum Polysaccharide (GLP) and Ethylene diaminetetraacetic acid (EDTA)
disodium salt doped PANI CEs for DSSC. The SDS doped PANI exhibited high catalytic activity
towards electrolyte and achieved power conversion efficiency of 4.25 % compared to GLP doped
PANI (2.69 %) and EDTA doped PANI (2.51 %). In another report Wu et al. [10] reported the
electrochemical synthesis of PANI modified with Ni2+, Co2+, Mn2+ and Cu2+ and fabricated as CEs
106
for DSSCs. They found that catalytic activity of fabricated electrodes were largly affected by ion
modification. Maximum power conversion efficiency (4.70 %) was observed by DSSC based on
PANI modified with Ni2+ as CE.
The improvement in the Jsc is might be due to the fact that simultaneous doping of P-Mix
with H2SO4 and ALS increases the conductivity and provide unusual morphology to PANI CE
which may bring more active sites for I3- reduction. These active sites are responsible for serving
a good path for transportation of charges [117, 162]. The findings of EIS and CV analysis also
confirms P-Mix a best electrocatalyst to the I-/I3-redox process. These results clearly demonstrate
that the P-Mix is a potential CE material to replace the expensive Pt CE for low-cost DSSCs.
107
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70
2
4
6
8
10
12
14
P-N
P-H2SO
4
P-ALS
P-Mix
Pt
Cu
rren
t D
ensi
ty (
mA
cm-2
)
Potential (V)
Figure 3.29: Photovoltaic performances of DSSCs based on Pt and PANI CEs.
Table 3.9: Parameters of DSSCs with P-N, P- H2SO4, P-ALS, P-Mix and Pt CEs
CE Jsc (mV) OCV (V) FF η (%)
P-N 4.71 0.48 0.45 1.14
P-H2SO4 7.86 0.53 0.43 1.78
P-ALS 10.84 0.60 0.43 2.79
P-Mix 15.13 0.60 0.53 4.54
Pt 12.67 0.64 0.52 4.02
108
3.21 Electrochemical Impedance spectroscopy
To further elucidate the electron transferability between different counter electrodes and
electrolyte, EIS investigation was performed with DSSCs based on different counter electrodes.
The Nyquists plots (Fig 3.30) illustrate impedance characteristics using a well-supported
equivalent circuit (shown as an inset in Fig 3.30) and are potted in Table 3.10. In Electric circuit,
Rs relates to the ohmic series resistance of the device and is determined directly from the intercept
of the real axis at high frequency region. The fill factor of the device is mainly dependent on this
parameter [178]. Rct observed as a semicircle at high frequency region of Nyquist plot
corresponding the charge transfer process at the electrolyte/CE interface, reflecting catalytic
activity in the I3- reduction at the electrolyte/CE interface and is at the focus of this work. W refers
to Warburg impedance which describes the diffusion resistance of electrolyte [181].
Based on this approach, we find much smaller value of W than Rct and Rs, so it can be
ignored. Since all systems have all the same components, the variation in Rct values can be
ascribed mainly to the change in porous nature of PANI electrode [133]. The Rct value decreases
in the order of P-Mix (8.26 Ω cm2) ˃ Pt (10.36 Ω cm2) ˃ P-ALS (17.16 Ω cm2) ˃ P- H2SO4 (18.17
Ω cm2) ˃ P-N (23.21 Ω cm2) signifying an inverse order of electrocatalytic activity. DSSC based
on P-Mix CE is found to depict lesser Rct value (8.26) compared to that of Pt CE (10.36) and its
counter parts. The lower Rct value implies that reduction of iodide specie is most favorable at P-
Mix CE and performance of this electrode is better than Pt CE. In contrast high RCT values can be
recognized to slow ionic diffusion in the structure of Pristine and doped PANI CEs [164]. The
lower Rct might be due to the addition of ALS and H2SO4 that imparts P-Mix a higher catalytic
activity as well as a more porous structure than its counter parts and thereby, the highest device
efficiency.
109
Rct is an important parameter in order to determine the electrochemical stability of a CE
in DSSC [164]. Fig. 3.31 illustrates the electrochemical stability of P-Mix based DSSC which was
performed by electrochemical impedance at different time intervals i-e from 0 hrs to 30 hrs. Fig.
3.31 shows that there is slight increase in Rct value with time. The Rct value for fresh cell is 8.26Ω
cm2 which increases after 5 hrs to 8.408 Ω cm2 and to 9.20 after 20 hrs and then to 9.33 after 30
hrs while Rs value is not affected with time. These results indicates that there is very small increase
in Rct suggesting an excellent electrochemical stability of DSSC with P-Mix.
0 10 20 30 40
0
10
20
30
40 Pt
P-Mix
P-ALS
P-H2SO
4
P-N
Z"(o
hm
.cm
2)
Z' (ohm.cm2)
Figure 3.30: EIS Nyquist plots of Pt and PANI CEs while inset shows equivalent circuit.
110
Table 3.10: Parameters of pristine, separately doped and binary doped PANI obtained from EIS.
CE Rs (Ωcm2) Rct (Ωcm2)
P-N 8.94 23.21
P-H2SO4 6.4 18.17
P-ALS 4.21 17.16
P-Mix 4.2 8.26
Pt 4.6 10.36
5 10 15 20
0
2
4
6
8
10
Z" (
oh
m.c
m2)
Z' (ohm.cm2)
Fresh
5 hrs
10 hrs
15 hrs
20 hrs
25 hrs
30 hrs
Fig. 3.31: Stability illustration of DSSC with P-Mix through impedance at 0 V from 0.01-105 Hz.
111
3.22 Multiple start/stop proficiency
When applied as windows, roof panels or portable sources, the solar panels should be
expected to have advantages of numerous start/stop proficiency, fast start-stop performance, and
insistent stability. By alternatively darkening (0 mWcm-2) and illuminating (100 mWcm-2) the P-
Mix CE based DSSC, the start/stop switches are recorded to estimate the start-up performances.
The performance of DSSC/Pt is also examined for comparison
As shown in Fig. 3.32, a rapid increase in photocurrent density under irradiation with no
time delay depicts a fast start-up behavior. After twelve start-stop cycles, the photocurrent density
is still unaffected in comparison to its initial state. This confirms a rapid light response and hence
high electrocatalytic activity towards redox electrolyte. About 96 % of the initial photocurrent
density was measured after eleven cycles as compared to Pt based DSSC having 93 %, implying a
superior capability of the P-Mix CE to start multiple times, which is a necessity for a durable solar
panel [180].
112
0 100 200 300 400 500 6000
2
4
6
8
10
12
14
16
18
20 P-N
P-H2SO
4
P-Mix
P-ALS
Pt
Cu
rren
t D
ensi
ty (
mA
cm-2
)
Time (s)
Figure 3.32: start-stop switches of DSSC assembled with P-MIX 3 and Pt CE.
Summary
Binary doped PANI salts (P-Mix salts) were effectively synthesized via the proposed
simple and cost effective chemical oxidative polymerization technique using inorganic (sulfuric
acid) and organic (ammonium lauryl sulphate) binary dopants, simultaneously. These materials
were tested for application in DSSCs as novel counter electrodes. The study indicates that binary
dopants render a positive enhancement in the conductivity and electro catalytic activity with
controlled morphology, which are essential for counter electrodes used in DSSCs. Influence of
113
synthetic parameters such as effect of monomer, H2SO4 and ALS strongly affected the % yield of
the binary doped PANI salts. Different spectroscopic and microscopic techniques (UV, FTIR,
XRD, FIB-SEM, EDX) revealed successful synthesis of different polymeric materials and proper
incorporation of dopants into the polymer chain. The porous morphology of binary doped PANI
salts gave credence to our experimental results because such fibrous morphology with porosity is
known to be of great importance to the catalytic electrode material.
Binary doped PANI CEs were fabricated by coating PANI salts on FTO by doctor blade
technique with constant thickness of 8 micro meter. These fabricated electrodes have high
electrocatalytic activity in iodide redox electrolyte with high current density (Jred), lesser Epp
value and large diffusion constant (Dn).
The fabricated counter electrodes were further utilized as counter electrodes in dye
sensitized solar cells using D719 dye as a sensitizer. As compared to the low Jsc (4.71 mA cm-2)
and η (1.14 %) for Pristine PANI (P-N) and Jsc (12.67 mA cm-2) and η (4.03 %) for Pt based
devices, an increase in Jsc (15.31 mA cm-2) was obtained for binary doped PANI (P-Mix salts)
based devices, yielding an energy conversion efficiency of 4.54 %. The sharp increase in current
density at “light ON” and the lack of a delay in starting the cell suggest a rapid response and a high
catalytic activity toward the reduction of I3-. Furthermore, EIS analysis also showed the decrease
in charge transfer resistance and increase in catalytic performance of DSSC based on binary doped
PANI CE.
Comparison of optimized binary doped PANI with pristine PANI and separately doped
PANI have also been investigated by using all the above mentioned techniques. From the analysis,
it was concluded that binary doped PANI CE manifested lowest charge transfer resistance,
promising eletctrocatalytic activity and highest photovoltaic performance as compared to other
114
CEs including Pt. Thus, it is concluded that the counter ion of the dopant has pronounced effect
on all the desirable properties of PANI and the use of binary dopants can be counted towards a
cost effective and strategy for having enhancement of photocatalytic properties. The study further
suggests binary doped PANI counter electrodes to be a reliable alternative for dye sensitized solar
cells.
Conclusions
Combination of H2SO4 and ALS renders a positive enhancement on the electrical,
morphology and photovoltaic properties of PANI electrodes. Binary doping provides an effective
strategy for accelerating the charge transfer and iodide redox within the CE. A high photovoltaic
performance is realized for the case of P-Mix 3 CE (4.54 %) for DSSC in comparison with Pt
based DSSC (4.03 %). Moreover, the virtues on fast start/up and multiple start/stop capability
motivate the potential applications of such flexible DSSCs in movable power sources and
photovoltaic curtain walls. In view of this facile approach, low-cost, and highly electrochemical
catalysts, the P-Mix 3 CE demonstrates a promising potential in production of DSSCs.
Future outlooks
The results in this thesis are within the experimental limitations. The studies presented in
this thesis further suggest some area of technological importance. The conductivity of PANI varies
with various dopants, but the mechanism of conduction in polymer is not still very clear. Therefore,
the mechanism of conduction in PANI with various dopants needs to be investigated. The
composites of PANI with nanoparticles are of technical importance. Improved efficiency of the
DSSC in the presence of PANI composites counter electrode may provide new insights in
115
designing low cost DSSCs. Further its photovoltaic properties and stability in different dyes need
to be explored.
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