Mohd Taukeer Khan Ph.D Thesis

1. STUDY OF CHARGE TRANSPORT MECHANISM
IN ORGANIC AND ORGANIC/INORGANIC
HYBRID SYSTEMS WITH APPLICATION TO
ORGANIC SOLAR CELLS
A THESIS
SUBMITTED TO THE
DEPARTMENT OF PHYSICS AND ASTROPHYSICS,
UNIVERSITY OF DELHI
DELHI-110007 INDIA
FOR THE AWARD OF DEGREE OF
DOCTOR OF PHILOSOPHY
IN PHYSICS
BY
MOHD TAUKEER KHAN
SEPTEMBER 2011

2. CERTIFICATE
This is to certify that subject matter presented in this thesis titled “Study of Charge Transport
Mechanism in Organic and Organic/Inorganic Hybrid Systems with Application to Organic
Solar Cells” is the original contribution of the candidate. This work has not been submitted
anywhere for the award of any degree, diploma, fellowship or similar title of any university or
institution.
The extent of information derived from existing literature has been indicated in the body
of the thesis at appropriate places giving the source of information.
Mohd Taukeer Khan
(Candidate)
Dr. Amarjeet Kaur Dr. S. K. Dhawan
Department of Physics & Astrophysics Polymeric & Soft Material Section
University of Delhi National Physical Laboratory
Delhi-110007 New Delhi-110012
Dr. Suresh Chand
Organic & Hybrid Solar Cell Group
National Physical Laboratory
New Delhi-110012
Prof. R. P. Tandon
(Head)
Department of Physics and Astrophysics
University of Delhi
Delhi-110007

3. Dedicated
To
My parents

4. ACKNOWLEDGMENTS
At the outset, I offer my prayers and thanks to the Almighty Allah, for He is good; His love
endures forever. The Almighty Allah is my strength and shield. My heart trusts in Him, and i am
helped. My heart leaps for joy, and i am grateful and give thanks to Him forever...
I shall always remain grateful to my supervisors, Dr. S. K. Dhawan, Dr. Amarjeet Kaur,
and, Dr. Suresh Chand for their never ending support. Without their valuable suggestions,
inspiring guidance, constant supervision and encouragement throughout the whole period of my
thesis work, it would not have been possible for me to complete the job with my little endeavor.
Their friendly behaviour in teaching and advising, always encourage me to work hard. This thesis
is the product of many hours of our critical discussions.
Support from Prof. R. P. Tandon, Head, Department of Physics & Astrophysics,
University of Delhi, Prof. R. C. Budhani, Director, National Physical Laboratory (NPL) and, Prof.
Vikram Kumar, Ex-director, NPL, New Delhi, is highly acknowledge.
I am grateful to Dr. S. S. Bawa, Dr. A. M. Biradar, Dr. M. N. Kamlasanan, Dr. Ritu
Srivastav, Dr. Renu Pasricha, Dr. Vinay Gupta, and Dr. Shailesh Sharma, at National Physical
Laboratory, New Delhi, for supporting me in my research work.
I would also like to thank my thesis advisory committee: Dr. S.A. Hashmi, Dr. Poonam
Silotia, Department of Physics and Astrophysics, University of Delhi, for their continuous
suggestions throughout this work.
I sincerely thank Mr. Parveen Saini, Dr. Pankaj Kumar, and Dr. Rajeev K. Singh for
giving the time to teach me the essentials of organic photovoltaics and how to use the necessary
equipment.
I would like to thank all the past and present group members, Dr. Anil Ohlan, Dr. Kuldeep
Singh, Dr. Hema Bhandari, Mr. Anoop Kumar S, Mr. Avinash Pratap Singh, Ms. Ranoo Bhargav,
Ms. Monika Misjra, Ms. Renchu Scaria, Mrs. Rajni and Mr. Firoz Alam for their support,
encouragement and helpful discussions.
My sincere thanks to, Dr. Anju Dhillon, Dr. Ravikant Prasad, Mr. Ishpal Rawal, Mr.
Manoj Srivastava, Ms. Ritu Saharan and Mr. Beerandra, my colleagues from University of Delhi
for supporting me throughout.
I heartily acknowledge the support of my friends Dr. J. P. Rana, Dr. Ajeet Kaushik, Dr.
Kusum Kumari, Mrs. Manisha Bajpai, and Mr. Ajay Kumar.
I am thankful to Mr. Brijesh Sharma, Mr. Devraj Joshi and Mrs. Barkha for their technical
help during my work. Special mention goes to Dr. G. D. Sharma, Mr. Ramil Bharadwaj, Mr.
Neeraj Chaudhary and Mr. K. N. Sood for technical assistance and recording the SEM and AFM

5. images. I wish to express my sincere thanks to all the staff members, Department of Physics and
Astrophysics, University of Delhi, Delhi for providing necessary help and research facilities.
Last but not the least, financial assistance in form of Junior Research Fellowship and
Senior Research Fellowship by Council of Scientific and Industrial Research (CSIR), New Delhi
is gratefully acknowledged.
Finally, my deepest gratitude goes to my parents, and wife. I really appreciate their
continuous support and endless love throughout all my life. I would like to dedicate this thesis to
them. Their lifelong support and selfless caring has been instrumental in my life.
To all those, not mentioned by name, who in one way or the other helped in the successful
realization of this work, I thank you all.
(Mohd Taukeer Khan)

6. Table of Contents
Chapter 1: Introduction: A Selective History and Working Principle of
Organic and Hybrid Solar Cells…………………………………………………..1
1.1. Introduction..............................................................................................................................2
1.2. Photovoltaic Solar Energy Development and Current Research.........................................3
1.2.1. First Generation................................................................................................................3
1.2.2. Second Generation...........................................................................................................4
1.2.3. Third Generation..............................................................................................................5
1.2.4. Fourth Generation............................................................................................................6
1.3. Polymer Solar Cells..................................................................................................................8
1.3.1. Economical expectations of OPV....................................................................................8
1.3.2. Device Architectures........................................................................................................8
1.3.2.1. Single layer devices............................................................................................8
1.3.2.2. Bilayer devices....................................................................................................9
1.3.2.3. Bulk-heterojunction devices.............................................................................10
1.4. Organic-Inorganic Hybrid Solar Cells.................................................................................11
1.5. Device Physics of Organic and Hybrid Solar Cells.............................................................15
1.5.1. Basics of Molecular Photophysics...................................................................................15
1.5.2. The need for two semiconductors....................................................................................17
1.5.3. Fundamental Physical Process in Bulk Heterojunction Solar Cells................................18
1.5.3.1. Light absorption and exciton generation...........................................................19
1.5.3.2. Diffusion of excitons in conjugated polymers....................................................19
1.5.3.3. Dissociation of charge carriers at the donor/acceptor interface......................20
1.5.3.4. Charge transport in donor: acceptor blends.....................................................20
1.5.3.5. Extraction of the charge carriers at the electrodes...........................................21
1.6. Electrical Characteristics Parameters..................................................................................22
1.6.1. Short‐ circuit Current....................................................................................................22
1.6.2. Open‐ Circuit Voltage..................................................................................................23
1.6.3. Fill Factor.....................................................................................................................23
1.6.4. Power Conversion Efficiency.......................................................................................24
1.6.5. Dark Current.................................................................................................................24

7. 1.6.6. Standard Test Conditions.............................................................................................24
1.6.7. Equivalent Circuit Diagram..........................................................................................25
1.7. Objective of the Present Thesis.............................................................................................26
1.8. Thesis Plan..............................................................................................................................27
References......................................................................................................................................29
Chapter 2: Experimental Details: Materials, Methods and Characterization
Techniques...............................................................................................................39
2.1. Introduction............................................................................................................................39
2.2. Synthesis of Poly(3-Alkythiophene)s.....................................................................................40
2.3. Synthesis of Semiconductor Nanocrystals............................................................................42
2.3.1. In-situ Growth of Cadmium Telluride Nanocrystals in P3HT Matrix...........................43
2.3.2. Synthesis of Cadmium Sulphide Quantum Dots............................................................44
2.4. Device Fabrication..................................................................................................................45
2.4.1. Patterning and Cleaning of ITO Substrates....................................................................45
2.4.2. Glove Box System for Device Fabrication....................................................................45
2.4.3. Active Layer Deposition on ITO Substrate…................................................................47
2.5. Characterization Techniques................................................................................................47
2.5.1 UV-Vis Absorption.......................................................................................................48
2.5.2 Photoluminescence........................................................................................................50
2.5.3 Fourier Transforms Infrared Spectroscopy....................................................................51
2.5.4 Thermal Analysis...........................................................................................................53
2.5.5 Electrochemical Studies: Cyclic Voltammetry..............................................................54
2.5.6 X-Ray Diffractometer....................................................................................................55
2.5.7 Scanning Electron Microscopy......................................................................................58
2.5.8 Transmission Electron Microscopy...............................................................................59
2.5.9 I-V Characterization Technique.....................................................................................61
2.5.10 Temperature Dependent I-V Measurements Setup......................................................61
References......................................................................................................................................63
Chapter 3: Study of the Photovoltaic Performance of Copolymer
Poly[(3-Hexylthiophene)-Co-(3-Octylthiophene)]............................................65

8. 3.1 Introduction.............................................................................................................................65
3.2 Result and Discussion..............................................................................................................67
3.2.1 FTIR Spectra....................................................................................................................67
3.2.2 1H NMR Spectrum...........................................................................................................68
3.2.3 Thermal Studies................................................................................................................72
3.2.4 XRD Studies.....................................................................................................................73
3.2.5 Evaluation of Energy Levels............................................................................................74
3.2.6 UV–Vis Absorption..........................................................................................................76
3.2.7 Photoluminescence Quenching With Respect to Different P3AT:PCBM
Ratio..............................................................................................................................................79
3.2.8 J-V characteristics of Solar Cells......................................................................................80
3.3. Conclusions………………………………………………………………………………….84
Reference………………………………………………………………………………………...85
Chapter 4: Study of Photovoltaic Performance of Organic/Inorganic Hybrid
System Based on In-Situ Grown CdTe Nanocrystals in P3HT
Matrix.......................................................................................................................89
4.1 Introduction………………………………………………………………………………….89
4.2 Fabrication and Measurement of Device…………………………………………………..92
4.3 Result and Discussion……………………………………………………………………….92
4.3.1. High Resolution Transmission Electron Microscope images……………………..…...92
4.3.2. Surface Morphology……………………………………………………………………95
4.3.3. Fourier Transform Infrared Spectroscopy Analysis……………………………………96
4.3.4. UV-Vis. Absorption Spectra…………………………………………………………...97
4.3.5. Photoinduced Charge Transfer at the Donor/Acceptor Interface………………………99
4.3.6. J-V Characteristics of Solar Cells…………………………………………..…………103
4.4. Conclusions………………………………………………………………………………...106
References………………………………………………………………………………………106
Chapter 5: Study of the Effect of Cadmium Sulphide Quantum Dots on the
Photovoltaic Performance of Poly(3-Hexylthiophene)…..................................109

9. 5.1. Introduction………………………………...……………………………………………...109
5.2. Fabrication and Measurement of Device………………………………………………...110
5.3. Result and Discussion…………………...…………………………………………………111
5.3.1 Structural Characterization………………..…………………………………………...111
5.3.1.1 XRD analysis……………………..……..…………………………………….111
5.3.1.2. High resolution transmission electron microscope images…………….……112
5.3.1.3. Scanning electron micrograph………………………..……………………...113
5.3.2. Optical Study………………………...………………………………………….……114
5.3.2.1. UV-Vis. absorption spectra…………………………………………………..114
5.3.2.2. Photoinduced charge transfer at the donor/acceptor interface……………...115
5.3.3. J-V characteristics of Solar Cells……………………………………………………117
5.4. Conclusions……………………………………………………………………………… 119
References…………………………………………………………………………………… 120
Chapter 6: Study on the Charge Transport Mechanism in Organic and
Organic/Inorganic Hybrid System......................................................................123
6.1. Introduction………………………………………………………………………………..124
6.2. Basic Concepts of the Charge Transport Processes..........................................................124
6.2.1. Intra-molecular and Inter-molecular perspective………………………..……………124
6.2.2. Role of Disorder………………………………………………………………………125
6.2.3. Hopping Transport……………………………………………………………………126
6.2.4. Charge Carriers in Conjugated Polymers: Concept of Polaron………………………127
6.3. Charge Carrier Mobility…………………………………………………………………..128
6.3.1 Factors Influencing the Charge Mobility………………………….………………….128
6.3.1.1. Disorder……………………………………………………………………...128
6.3.1.2. Impurities/Traps……………………………………………………………...129
6.3.1.3. Temperature………………………………………………………………….131
6.3.1.4. Electric Field…………………………………………………………………131
6.3.1.5. Charge-Carrier Density……………………………………………………...132
6.4 Space Charge Limited Conduction………………………………………………………..132
6.4.1 Trap Free SCLC ……………………………………………………………………...133
6.4.2. SCLC with Exponential Distribution of Traps………………………………………134

10. 6.5. Unified Mobility Model……………………………………………………………………134
6.6. Results and Discussion …………………………………………………………………....136
6.6.1. Hole Transport Mechanism in P3HT……………………………………………….137
6.6.2. Hole Transport Mechanism in P3OT……………………………………………….138
6.6.3. Hole Transport Mechanism in P3HT-OT…………………………………………...141
6.6.4. Hole Transport Mechanism in P3HT/CdTe hybrid System………………………...144
6.6.5. Hole Transport Mechanism in P3HT/CdS hybrid System………………………….147
6.7 Conclusions…………………………………………………………………………………149
References………………………………………………………………………………………150
Chapter 7: Conclusions and Future Scope.........................................................153
7.1. Summary…………………………………………………………………………………...153
7.2. Suggestions for Future Investigations……………………………………………………155
List of Publications......................................................................................................................157

11. ABSTRACT
In recent years organic photovoltaics has shown a great promise of delivering cost effective,
flexible, light weight, large area and easy processable solar cells. Power conversion efficiency
(PCE) ~ 8.5% have already been realized in polymer solar cells based on donor-acceptor
interpenetrating bulk heterojunction. More recently international R & D efforts are focused
towards the development of hybrid organic-inorganic nanostructured solar cells as it holds a
further promise due to added optical absorption (due to presence of inorganic component), better
charge transport, better physical and chemical stability, easy tailoring of bandgap, cost
effectiveness etc. These solar cells make use of hybrid combinations of various materials such as
poly(3-hexylthiophene), poly(3-octylthiophene), poly[2-methoxy,5-(2-ethylhexoxy)-1,4-
phenylenevinylene], poly[2-methoxy-5-(3’,7’-dimethyloctyloxyl)]-1,4-phenylene vinylene etc.,
and inorganic semiconducting nanoparticles of cadmium telluride, cadmium selenide, cadmium
sulphide, lead sulphide, lead selenide, zinc oxide, titanium oxide, etc.
The hybrid polymer-nanocrystals solar cells that have recently shown the highest PCEs
utilize CdSe nanostructures. The highest PCE achieved ~ 3.2% has been achieved for poly[2,6-
(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3benzothiadiazole)]
(PCPDTBT):CdSe tetrapod blend solar cells, and ~ 2.0 % for P3HT:CdSe quantum dot composite
based solar cells. However, in order to enhance further the PCE of hybrid organic-inorganic
nanostructured solar cells, one needs to understand the fundamental and applied facets of the
materials and devices. The present thesis addresses these issues by way of systematic and detailed
studies of structural, optical and charge transport properties of some of the conjugated polymers,
and their respective polymer-nanocrystals composites for solar cell applications.
The first chapter of the thesis deals with the history and working principle of solar cells
which comprises of the literature survey and overview of various generations of solar cells. It also
includes discussion on various basic and applied concepts of solar cells, such as device
architectures, polymer fullerene bulk-heterojunction, donor-acceptor concept, etc. The main
processes which contribute towards the working of solar cells are given in details. At the end of
the chapter, a thorough discussion of different electrical characteristics parameters of solar cells
for example JSC, VOC, FF, PCE, Rs, Rsh are given.
Chapter 2 describe the synthesis methods and experimental techniques used in the present
work. It also includes the fabrication process of bulk-heterojunction solar cells and hole only
device for charge transport study. The description of techniques used for confirming the synthesis
of polymer, inorganic nanocrystals and incorporation of nanocrystals in polymer matrix, is given.
These techniques include Fourier transform infrared spectroscopy (FTIR), UV-Vis absorption,
i

12. photoluminescence (PL), X-ray diffraction (XRD), and transmission electron microscopy (TEM).
The measurement techniques of J-V characteristics under light, in dark, as well as at different
temperatures are discussed in details.
Chapter 3 includes the photovoltaics performance of devices based on P3HT, P3OT and
their copolymer poly[(3-hexylthiophene)-co-(3-octylthiophene)] (P3HT-OT)]. The largest carrier
mobility reported for P3OT in field effect transistor configuration is 10-3cm2/Vs, which is
approximately 1-2 orders of magnitude lower than the typical mobilities of P3HT. P3HT is very
well soluble in chlorinated solvents such as chloroform, chlorobenzene, however, weakly soluble
in non-chlorinated solvents such as toluene or xylene. On the other hand, P3OT dissolves quickly
in toluene, xylene at room temperature. In order to incorporate both the properties (mobility and
solubility) within a single polymer, in the present investigation, the regioregular copolymer
P3HT-OT has been used as a donor material in combination with PCBM as acceptor. The chapter
also contains the investigations of FTIR, 1H NMR, XRD, thermal analysis, UV-vis. absorption,
photoluminescence properties of these polymers. The composites of the three polymers with
PCBM show a distinctive photoluminescence quenching effect, which confirm the photoinduced
charge generation and charge transfer at P3AT/PCBM interface. Moreover, the energy level
positions have been evaluated by the cyclic voltammetry. Finally, the photovoltaics performance
of P3HT-OT has been studied and results were compared with the homopolymer P3HT and
P3OT. Photovoltaics performance of P3HT-OT exhibit an open-circuit voltage VOC of 0.50V,
short-circuit current of 1.57 mA/cm2 and the overall power conversion efficiency is in between
the performance of solar cell fabricated from P3HT and P3OT.
Chapter 4 discusses the photovoltaics performance of P3HT-CdTe hybrid system. The
aim of in-situ incorporation of CdTe nanocrystals in P3HT matrix is to improve the photovoltaics
properties of P3HT by broadening the solar absorption, enhancing the charge carrier mobility, and
improving the polymer-nanocrystals interaction. Incorporation of CdTe nanocrystals has been
confirmed by the structural (HRTEM, SEM) and spectroscopic (FTIR, UV-Vis absorption, PL)
studies. Optical measurements (UV-Vis and PL) of nanocomposites films show that photoinduced
charge separation occurs at the P3HT-CdTe interfaces. This indicates that the in-situ incorporation
of nanocrystals in polymer matrix is a promising approach for the fabrication of efficient organic-
inorganic hybrid photovoltaics devices. Photovoltaics performance of P3HT:PCBM as well as
P3HT-CdTe:PCBM have been investigated in device configuration viz. indium tin oxide (ITO)/
poly(3,4-ethylendioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)/P3HT:PCBM/Al and
ITO/PEDOT:PSS/P3HT-CdTe:PCBM/Al, respectively. Based on these investigations it has been
found wherein the current-density and open-circuit voltage of device based on P3HT-CdTe have
increased as compared to the device based on pristine P3HT.
ii

13. Chapter 5 deals with the fundamental issue, whether incorporation of CdS nanocrystals
into P3HT matrix causes any noticeable improvement or deterioration of device efficiency. The
particle shape, size and distribution of CdS nanocrystals in P3HT matrix have been investigated
by HRTEM, SEM and XRD. Optical studies (UV-Vis absorption and PL) suggest the electronic
interaction between P3HT and CdS quantum dots. Photovoltaic performances of device based on
pure P3HT as well as dispersed with CdS nanocrystals in the device configuration viz.
ITO/PEDOT:PSS/P3HT:PCBM/Al and ITO/PEDOT:PSS/P3HT:CdS:PCBM/Al have been
investigated. On incorporation of CdS nanocrystals in P3HT matrix, the PCE efficiency increased
due to enhancement in short-circuit current, open-circuit voltage and fill factor. These effects have
been explained on the basis of the formation of charge transfer complex between the host (P3HT)
and guest (CdS), duly supported by UV-Vis absorption and PL quenching studies. The effect of
post thermal annealing on device performance has also been investigated and found improved
efficiency of devices after thermal treatment due to improved nanoscale morphology, increased
crystallinity and improved contact to the electron-collecting electrode.
Chapter 6 gives the theoretical and experimental details of the charge transport processes
in organic semiconductors as well as in organic-inorganic hybrid systems. In the theory section of
the chapter space charge limited conduction which is dominant mechanism for charge transport in
disordered materials has been discussed in details. This chapter also discusses the factors
influencing the charge carrier mobility. In the experimental part we have studied the hole
transport mechanism in all the polymer (P3HT, P3OT, P3HT-OT) and polymer/nanocrystals
hybrid systems (P3HT/CdS and P3HT/CdTe) in the device configuration ITO/
PEDOT:PSS/Active layer/Au.. Current-voltage characteristics of these devices have been studied
in the temperatures range of 110K-300K. The hole transport mechanism in P3HT thin film is
governed by space charge limited conduction with temperature, carrier density, and applied field
dependent mobility. Thin films of copolymer P3HT-OT exhibited agreement with the space
charge limited conduction with traps distributed exponentially in energy and space. The hole
mobility is both temperature and electric field dependent. The hole transport mechanism in P3OT
thin film is governed by space charge limited conduction model and hole mobility is given by
Gaussian distribution model.
Incorporation of CdTe nanocrystals in P3HT matrix results into enhancement in current
density which attributed to increase in the trap density (from 2.8×1018 to 5.0×1018 cm-3) and
decrease of activation energies (from 52 meV to 11 meV). At high trap density, trap potential
wells start overlapping which results in decrease of activation energies. In contrary to P3HT, the
hole mobility in P3HT-CdTe has been found to be independent to charge carrier density and
applied field. The charge carrier mobility depends only on temperature and it increases with the
iii

14. decrease of temperature. On incorporation of CdS nanocrystals in P3HT matrix the mobility is
again independent to applied field and carrier density and exhibited agreement with the band
conduction mechanism. This is attributed to the enhancement in the overlapping of traps potential
wells, which results in the decrease in activation energies from 52 meV to 18meV.
iv

15. CHAPTER 1
INTRODUCTION: A SELECTIVE HISTORY AND WORKING PRINCIPLE OF
ORGANIC & HYBRID SOLAR CELLS
1.1 INTRODUCTION
1.2. PHOTOVOLTAIC SOLAR ENERGY DEVELOPMENT AND CURRENT
RESEARCH
1.2.1. First Generation
1.2.2. Second Generation
1.2.3. Third Generation
1.2.4. Fourth Generation
1.3. POLYMER SOLAR CELLS
1.3.1. Economical Expectations of OPV
1.3.2. Device Architectures
1.3.2.1. Single layer devices
1.3.2.2. Bilayer devices
1.3.2.3. Bulk-heterojunction devices
1.4. ORGANIC-INORGANIC HYBRID SOLAR CELLS
1.5. DEVICE PHYSICS OF ORGANIC AND HYBRID SOLAR CELLS
1.5.1. Basics of Molecular Photophysics
1.5.2. The Need for Two Semiconductors
1.5.3. Fundamental Physical Process in Bulk Heterojunction Solar Cells
1.5.3.1. Light absorption and exciton generation
1.5.3.2. Diffusion of excitons in conjugated polymers
1.5.3.3. Dissociation of charge carriers at the donor:acceptor interface
1.5.3.4. Charge transport in donor:acceptor blends
1.5.3.5. Extraction of the charge carriers at the electrodes
1.6. ELECTRICAL CHARACTERISTICS PARAMETERS
1.6.1. Short‐ Circuit Current
1.6.2. Open‐ Circuit Voltage
1.6.3. Fill Factor
1.6.4. Power Conversion Efficiency
1.6.5. Dark Current

16. 1.6.6. Standard Test Conditions
1.6.7. Equivalent Circuit Diagram
1.7. OBJECTIVE OF THE PRESENT THESIS
1.8. THESIS PLAN
References
1.1. INTRODUCTION
E
nergy forms a very vital componant for sustaining the diverse processes of nature. The
progress of humans from prehistoric to modern times has seen manifold increase in
energy consumption. At one level, various energies help us to sustain our daily
existance. At the other level, our quest for invention and explorations require more energy to
achieve the respective aim. The international energy outlook 2010 (IEO2010) reports that the
world energy consumption would grow by 49% during the period 2007 to 2035 [1]. The world
wide energy demands would rise from 495 quadrillion British thermal units (Btu) in 2007 to 590
quadrillion Btu in 2020 and 739 quadrillion Btu in 2035 [Figure 1.1 (a)] [2].
Figure 1.1 (a) World marketed energy consumption, 2007-2035 (quadrillion Btu) (b) World
marketed energy use by fuel type, 1990-2035 (quadrillion Btu). (Source: IEO2010).
The energy can be non-renewable and renewable. Right now the energy requirement are
fulfilled mostly by non-renewable sources like coal, oil, and natural gas [Figure 1.1 (b)]. As a
result, due to their high demand, these sources are depleting at very fast rate. Moreover, burning
of these fossil fuels lead to the emission of carbon dioxide (CO2) [3-5]. Global warming is a direct
result of the CO2 emission, and this will cause a change in the weather as well as increase the
mean sea level [6, 7]. This emphasizes the need for carbon free power production. The most
2

17. Chapter 1
commercially-viable alternative, available today is nuclear energy [8-10]. Uranium does not cause
CO2 emissions but has always been under intensive public discussions because of the imminent
danger of nuclear power stations and the disposal of hazardous nuclear waste.
Figure 1.2 World energy-related carbon dioxide emissions, 2007-2035 (billion metric tons).
(Source: IEO2010).
On the other hand renewable energy is harvested from a source that will never run out e.g.
photovoltaic, solar thermal, wind, geothermal, and hydroelectric. Also they do not emit CO2,
which means that such systems are environmental friendly. The main advantage of solar cells over
other renewable energy systems involve their elegent operation, i.e. just converting daylight into
electricity. No other fuels, water are required for their operation. Moreover, the solar cells or
photovoltaics systems are noise free and without any technical heavy machinery, so therefore
their maintenance requirement is minima as compared to other renewable system [11].
1.2. PHOTOVOLTAIC SOLAR ENERGY DEVELOPMENT AND CURRENT
RESEARCH
Conventional solar cells based on silicon technology, have low operation and maintenance costs,
but their main drawback is the high initial costs of fabrication [12-18]. In order to generate cost-
effective solar energy, either the efficiency of the solar cells must be improved or alternatively the
fabrication cost must be lowered. Hence continuous research has been carried out in this direction
and has led to four generations of PV technologies.
1.2.1 First Generation
The first generation photovoltaic cells are the dominant technology in the commercial production
of solar cells and account for nearly 80% of the solar cell market [19]. These cells are typically
3

18. made using a crystalline silicon (c-Si) wafer, in which a semiconductor junction is formed by
diffusing phosphorus into the top surface of the silicon wafer. Screen-printed contacts are applied
to the front and rear of the cell. The typical efficiency of such silicon-based commercial
photovoltaic energy systems is in the order of 15% [20]. In these cells a substantial increase of
their efficiency up to 33% is theoretically possible, but the best laboratory cells have power
conversion efficiency (PCE) only about 25% [21-23]. The starting material used to prepare c-Si
must be refined to a purity of 99.9999 % [24]. This process is very laborious, energy intensive; as
a result manufacturing plant capital cost is as high as 60% of manufacturing cost [25]. The cost of
generating electricity using silicon solar modules is typically 10 times higher than that from fossil
fuel which inhibits their widespread application. The main advantages of first generation solar
cells are broad spectral absorption range, high carrier mobility, high efficiency [26, 27]. However,
the main disadvantages are: they require expensive manufacturing technologies [28], most of the
energy of higher energy photons, at the blue and violet end of the spectrum is wasted as heat, and
poor absorber of light.
1.2.2. Second Generation
Second generation solar cells are usually called thin-film solar cells. This generation basically has
three types of solar cells, amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium
gallium diselenide (CIGS). Thin film production market share in the global solar PV market grew
from a mere 2.8% in 2001 to 25% in 2009; this indicates a growing share of these solar cells in
coming future (see Figure 1.3). These technologies are typically made by depositing a thin layer
of photo-active material onto the glass or a flexible substrate. The driving force for the
development of thin film solar cells has been their potential for the reduction of manufacturing
costs. Moreover, as these semiconductors have direct band which leads to higher absorption
coefficient, as a result less than 1 µm thick semiconductor layer is required to absorb complete
solar radiation, which is 100-1000 times less than as compared to Si.
Amorphous silicon solar cell structure has a single sequence of p-i-n layers [see Figure
1.4(b)]. The best commercial a-Si cells utilize a stacked three-layer structure with stabilized
efficiencies of 10.1% [29, 30]. Such cells suffer from significant degradation in their power
output when exposed to the light. Thinner layers can be used to increase the electric field strength
across the material and hence can provide better stability. However, the use of thinner layers
reduces light absorption, and hence cell efficiency. CdTe has a nearly optimal band gap and can
be easily deposited with thin film techniques. Over 16.7% efficiencies have been achieved in the
laboratory for the CdTe solar cells [30]. CdTe usually deposited on cadmium sulfide (CdS) to
form a p-n junction photovoltaic solar cell as shown in Figure 1.4(c). When copper indium
diselenide (CIS) is modified by adding gallium, it exhibits the record laboratory efficiency of 20.3
4

19. Chapter 1
% among thin film materials [30] and shows excellent stability. At the moment CIGS is the most
promising candidate for the solar cells based on this technologies.
Figure 1.3 Market shares of different solar PV technologies (Source: GBI Research).
Although thin films solar cells absorbs incident radiation more efficiently compared to
monocrystalline silicon. The photovoltaic devices based on these materials have shown
efficiencies of 15-20% [31-34], somewhat less than that of solar cells based on mono-crystalline
silicon [8]. This is due to the relatively poor charge transport in these materials compared to
monocrystalline silicon. So the promise of the low cost power has not been realized yet by these
technologies. Research is being conducted into several alternative types of solar cells.
1.2.3. Third Generation
Third generation technologies aim to enhance poor electrical performance of second generation
thin films technologies while maintaining very low production costs. Currently, most of the work
on third generation solar cells is being done in the laboratory and being developed by new
companies and most part of it is still not commercially available. Today, the third generation
approaches being investigated include nanocrystal solar cells, photo electrochemical cells ( PEC),
Dye-sensitized hybrid solar cells (DSSC), Tandem cells, organic photovoltaic (OPV), and the
cells based on the materials that generate multiple electron-hole pairs.
5

20. Metal (Front) Metal (Back) Metal (Back) TCO
TCO
n-Si n-a-Si CdS
i-µc-Si CdTe CIGS
p-Si
p-µc-Si CdS
Mo (Back)
TCO (front) TCO (front)
Metal (Back) glass glass Glass, metal foil
(a) (b) (c) (d)
Figure 1.4 Device configurations for (a) c-Si, (b) a-Si, (c) CdTe and, (d) CIGS. i is intrinsic,
TCO is transparent conductive oxide, and, Mo is molybdenum.
These cells are based on low energy, high-throughput processing technologies e.g. OPV are:
chemically synthesized, solution processable, low material cost, large area, light weight and
flexible. Graetzel cells are attractive replacement for existing technologies in “low weight”
applications like rooftop solar collectors; work even in low-light conditions. However,
efficiencies of all of their cells are lower as compared to first and second generation of PV
technologies. And secondly their efficiency decay with time due to degradation effects under the
environmental conditions.
1.2.4. Fourth Generation
Today a lot of research has been focused on organic-inorganic hybrid materials. The researchers
are finding them a promising candidate to enhance the efficiency of solar cells through a better
use of the solar spectrum, a higher aspect ratio of the interface, and the good processability of
polymers. This has led to the development of fourth generation solar cells. Hybrid polymer-
nanocrystal solar cells, [35-38] consists of conjugated polymers such as P3HT, MEH-PPV,
PCPDTBT, etc. and semiconducting nanocrystals such as CdTe [39-43], titanium dioxide (TiO2)
[44-50], lead selenide (PbSe) [51-53], lead sulphide (PbS) [54], zinc oxide (ZnO) [55-57],
cadmium selenide telluride (CdSeTe) [58], CdS [59, 60], carbon nanotubes (CNT) [61, 62],
cadmium selenide (CdSe) [63-77], etc. Hybrid PV systems have attracted considerable research
attention because of their potential for large area, flexible, easily processable, and low-cost
photovoltaic devices. Moreover, hybrid materials have the ability to tune each component in order
to achieve composite films optimized for solar energy conversion [78, 79]. Year-wise progresses
on the PCE of different PV devices are shown in Figure 1.5.
6

21. Chapter 1
Figure 1.5 Year-wise progress on the efficiencies of different photovoltaic device, under AM 1.5
simulated solar illumination. (Source: http://howisearth.files.wordpress.com/2010/02/best-
research-cell-efficiencies-nationalrenewable-energy-laboratory-usa1.jpg).
Table 1.1 Theoretical and experimental PCE of different types of solar cells [28, 75, 81, 82].
Photovoltaic device Abbreviation Theoretical Obtained η
η% %
Mono-crystalline Si c-Si 28.9 25.0
µ-crystalline Si µc-Si 28.9 20.4
Amorphous Si a-Si 22 10.1
Copper indium gallium diselenide CIGS 28 19.6
Cadmium telluride CdTe 28 16.7
Gallium arsenide GaAs 28 27.6
GaInP/GaAs/Ge GaInP/GaAs/Ge 32
Dye sensitized DSSC 22 10.4
Small molecule 22 8.3
Polymer:fullerene OPV 8.5
Hybrid Systems HOIPV 4.08
7

22. 1.3. POLYMER SOLAR CELLS
Polymer-based PV systems hold the promise for environmentally safe, flexible, lightweight, and
cost-effective, solar energy conversion platform. π-conjugated polymers offer the advantage of
facile chemical tailoring and can be easily processed by wet-processing techniques. Molecular
engineering enables highly efficient active plastics with a wide range of colors. This opens up a
whole new area of solar cell applications not achievable by the traditional solar cells [80, 81].
1.3.1. Economical expectations of OPV
The cost reduction in OPV devices mainly results from the addressing of the 3 major issues:
(1) Lower cost of raw material: The conjugated polymers used as the active layer in OPV are
synthesized by cost effective techniques.
(2) Low material usage: Due to the high absorption coefficient of organic materials, organic
solar cells (OSCs) have a typical active layer thickness of only ~100 nm (1/1000 of Si solar cells),
which means that with only one tenth of a gram of a material an active area of 1 m2 can be
covered. Thus material cost is significantly lowered.
(3) Low manufacturing cost: The organic materials are solution processable and can be easily
processed by wet‐processing techniques, such as ink-jet printing, micro-contact printing, and
other soft lithography techniques. These techniques are very cost effective and fabrication of
devices can be done even at room temperature which reduces the amount of energy consumption
in the manufacturing process. The production of large area OPV (1m2) can be done at a cost 100
times lower than that of mono-crystalline silicon solar cells.
1.3.2. Device Architectures
The polymer solar cells reported in the literature can be categorized by their device architecture as
having single layer, bilayer, blend, or bulk-heterojunction structure. The reason behind the
development of these structures is to achieve higher cell efficiencies by enhancing charge
separation and collection processes in the active layer.
1.3.2.1. Single layer devices
The first investigation of an OPV cell came as early as 1959, when an anthracene single crystal
was studied. The cell exhibited a photovoltage of 200 mV with an extremely low efficiency [83].
Since then, many years of research has shown that the typical PCE of PV devices based on single
layer organic materials will remain below 0.1 %, making them unsuitable for any possible
application.
In the first generation of the OPV devices, a single layer of pure conjugated polymer were
sandwiched between two electrodes with different work functions, such as ITO and Al as shown
in Figure 1.6 (a). The efficiency of such a device remains below 1%. The low efficiency of these
8

23. Chapter 1
devices is primarily due to the fact that absorption of light in the organic materials almost always
results in the production of a mobile excited state (referred to as exciton), rather than free
electron–hole (e-h) pairs as produced in the inorganic solar cells. This occurs because of their low
dielectric constant typically in the range of 2–4 [84], combined with weak intermolecular
coupling. The Coulombic binding energy of an e–h pair separated by 0.6 nm in a system with
εr=3 is 0.6 eV [85-88]. Therefore, the electric field provided by asymmetrical work functions of
the electrodes is not sufficient to break up these photogenerated excitons. Hence, they diffuse
within the organic layer before reach the electrode, where they may dissociate to supply separate
charges, or recombine. Since the exciton diffusion lengths are typically 1–10 nm [89–93], much
shorter than the device thicknesses, exciton diffusion limits charge-carrier generation in the single
layer devices because most of them are lost through recombination.
(a) (b) (c)
Figure1.6 Device architecture for (a) Single layer (b) Bilayer and (c) Bulk-heterojunction OPV.
1.3.2.2. Bilayer devices
A major breakthrough in the OPV performance came in 1986 when Tang discovered that much
higher efficiencies (about 1%) can be attained when an electron donor (D) and an electron
acceptor (A) are brought together in one cell [94], as shown in Figure 1.6 (b). The idea behind a
heterojunction is to use two materials with different electron affinities and ionization potentials.
At the interface, the resulting potentials are strong and may favor exciton dissociation: the
electron will be accepted by the material with the larger electron affinity and the hole will be
accepted by the material with the lower ionization potential. In this device the excitons should be
formed within the diffusion length of the interface. Otherwise, the excitons will decay, yielding,
luminescence instead of a contribution to the photocurrent. Since the exciton diffusion lengths in
the organic materials are much shorter than the absorption depth of the film, this limits the width
of effective light-harvesting layer.
9

24. 1.3.2.3. Bulk-heterojunction devices
To date, the most successful method to construct the active layer of an OPV devices is to blend a
photoactive donor polymer in combination with an electron acceptor in a bulk-heterojunction
(BHJ) configuration as shown in Figure 1.6 (c). BHJ configuration maximizes interfacial surface
area for exciton dissociation [95]. If the length scale of the blend is similar to the exciton diffusion
length, the exciton decay process is dramatically reduced as in the proximity of every generated
exciton there is an interface with an acceptor where fast dissociation takes place. Hence, charge
generation takes place everywhere in the active layer, provided that there exist a percolation
pathways in each material from the interface to the respective electrodes. In BHJ device
configuration a dramatic increase of photon to electron conversion efficiency has been observed
[95].
The brief history of BHJ solar cells can be roughly divided into three phases [96]. Phase
one centered on poly-(phenylene vinylene)s, whose structures and related BHJ morphology were
optimized to achieve an efficiency as high as 3.3% in the case of poly[2-methoxy-5-(3′,7′-
dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) [97, 98]. As a result of its relatively
lower highest-occupied molecular orbital (HOMO) energy level of -5.4 eV, BHJ devices made
from MDMO-PPV offered open circuit voltages (Voc) as high as 0.82 V; however, the relatively
larger band gap of MDMO-PPV limited the short circuit current density (JSC) to 5-6 mA/cm2. As
a result, a smaller band gap polymer, regioregular poly(3-hexylthiophene) (rr-P3HT), took center
stage in phase two.
P3HT based BHJ devices delivered a much higher current density (> 10 mA/cm2), which
was attributed to both its relatively low band gap (1.9 eV) as well as to its increased crystallinity,
which yields a higher hole mobility [99-101]. In addition to P3HT’s favorable intrinsic
characteristics, together with important advances in material processing such as the control of the
morphology of the BHJ blend via thermal [101] or solvent annealing [102], which lead to an
impressive total energy conversion efficiency of 6% [103]. Unfortunately, the high HOMO (- 5.1
eV) energy level of P3HT has restricted the VOC to 0.6 V, which consequently limits the overall
efficiency. Presently, in phase three, the BHJ PV community has adopted two separate approaches
to improve the efficiency of low cost BHJ PV cells.
The first approach places emphasis on the VOC by designing polymers with a low HOMO
energy level. This approach has resulted in VOC greater than 1 V in a few cases [104-106], though
the overall efficiency has been less than 4% because of the mediocre JSC. The second approach,
which is disproportionally favored, is to develop lower band gap polymers for harvesting more
influx photons and enhancing the JSC [107, 108]. By this method, JSC as high as 17.5 mA/cm2 has
been achieved by using poly[(4,4-didodecyldithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-
benzothiadiazole)-4,7-diyl] as the donor in combination with [6, 6]-phenyl C61 butyric acid
10

25. Chapter 1
methyl ester (PCBM) as acceptor [109]. This demonstrates the effectiveness of low-band-gap
polymers in generating more current. However, a low VOC (0.57 V) was observed because of the
relatively high HOMO energy level of donor material [109]. Only a few fine-tuned polymers
developed recently achieved a combination of a low HOMO energy level and a small band gap,
hence over 6% PCE were obtained [110-114]. Recently Samuel et al [113] fabricated a BHJ solar
cell based on using PBnDT-FTAZ/PC61BM, which show a VOC of 0.79 V, a JSC of 12.45 mA/cm2,
FF of 72.2%, and PCE of 7.1%. Yongye et al. [114] reported highest overall efficiency of 7.4%,
with JSC of 14.50 mAcm-2, VOC = 0.74 V and FF of 0.69 in PTB7/PC71BM BHJ solar cell. Year-
wise development in efficiency of polymer BHJ solar cells has been given below:
2003 – P3HT:PCBM (1:4), ɳ=0.2%, not annealed
 J.C. Hummelen et al., Synthetic Metal, 2003, 138, 299
2003 – P3HT:PCBM (1:1), ɳ=3.5%, annealed at 75˚C for 4min
F. Padingger et al., Adv. Funct. Mater., 2003, 13, 85
2004 – P3HT:PCBM (1:1), ɳ=5%, Christoph J. Brabec (SIEMENS)
2005 – P3HT:PCBM (1:0.6), ɳ=5.2%, annealed at 155˚C for 3min
 M.Reyes-Reyes et al., Org. Lett. 2005, 7, 5749
2005 – P3HT:PCBM (1:0.8), ɳ=4.9%, annealed at 155˚C for 5min
 K. Kim et al., Appl. Phys. Lett., 2005, 87, 083506
2006 – P3HT:PCBM (1:1), ɳ=5%, Ca/Ag electrode/Xylene solution casting
 P. Schilinsky et al Adv. Funct. Mater., 2006, 16, 1669
2006 – P3HT:PCBM (1:0.8), ɳ=5%, TiOx Optical spacer
 K. Lee et al, Adv. Funct. Mater., 2006, 18, 572
2007 – PCPDTBT:PCBM (1:0.8), ɳ=5.5%, dithiol treatment
 G. C.Bazan et al Nature Mater., 2007, 6, 1
2007 – P3HT:PCBM (1:0.8)/PCPDTBT:PC71BM (1:0.8), ɳ=6%, TiOx Optical
spacer, Tandem, K. Lee et al Science, 2007, 317, 222
2008 – P3HT:New Acceptor, ɳ>5.98%, Plextronicis
2008 - New Low bandgap donor, ɳ>6.23% Konarke
2009 - New Low bandgap donor, ɳ>6% K. Lee, Y. Yang, Y.Lian
2009 - New Low bandgap donor, ɳ>7.9 Solarmer
2010 - PTB7:PC71BM, ɳ=7.4%, Y. Liang, et al, Adv. Mater. 2010, 22, 1.
2010 -New Low bandgap donor, ɳ=8.13%, Solarmer
2010 - New Low bandgap donor, ɳ>8.5% Konarke
2011 - PBnDT-FTAZ:PC61BM, ɳ=7.1%,
S. C. Price et al, J. Am. Chem. Soc., 2011, 133, 4625
1.4. ORGANIC-INORGANIC HYBRID SOLAR CELLS
Polymer-based solar cells suffer from lower efficiencies and the limited lifetime as compared to
silicon-based solar cell. The limited efficiency of the BHJ polymer solar cell is due to the poor
carrier mobility [115], the short exciton diffusion length [116], the charge trapping [117], and the
mismatch of the absorption spectrum of the active layer and the solar emission [118, 119]. To
11

26. address these fundamental limitations of polymer solar cells, new strategies have been developed
by blending of inorganic nanocrystals (NCs) with organic materials which integrate the benefits of
both classes of materials [120-125]. These hybrid materials are potential systems for OPV devices
because it includes the desirable characteristics of organic and inorganic components within a
single composite. They have advantage of tunability of photophysical properties of the inorganic
NCs and also retain the polymer properties like solution processing, fabrication of devices on
large and flexible substrates [126-130]. Blends of conjugated polymers and NCs are similar to that
of used in organic BHJ solar cells. Excitons created upon photoexcitation are separated into free
charge carriers at organic-inorganic interfaces. Electrons will then be accepted by the material
with the higher electron affinity (acceptor/NCs), and the hole by the material with the lower
ionization potential (donor/polymer) [67]. The usage of inorganic semiconductor NCs embedded
into semiconducting polymer is promising for several reasons such as [131]:
1) Inorganic NCs have high absorption coefficients.
2) They are superb electron acceptors having high electron affinity and high electron mobility.
3) Band gap of NCs is a function of the size of the NCs, so they have size tunable optical and
electrical properties [132-136].
4) A substantial interfacial area for charge separation is provided by NCs, which have high
surface area to volume ratios [120].
5) In hybrid devices light is absorbed by both components, unlike polymer-fullerene BHJ where
the PCBM contributes very little to the spectral response.
6) NCs are prepared by inexpensive wet chemical synthesis route, hence NCs are cost effective.
7) The NCs are easily dispersed in the polymers which can be spin casted for large area and
flexible devices.
8) They show good physical and chemical stability.
Huynh et al. reported the hybrid devices from a blend of 8×13 nm, CdSe NCs, and rr-P3HT
[120]. Under 4.8 W/m2 monochromatic illumination at 514 nm, a JSC of 0.031 mA/cm2 and a VOC
of 0.57 V have been observed. For a similar device, Huynh et al. [64] achieved a PCE of 1.7%
under AM 1.5 illumination with CdSe NCs of 7× 60 nm size.
Hybrid solar cells based on NCs of CuInS2 in the organic matrices were reported by Elif
Arici et al. [137-139]. Nanocrystalline CuInS2 was used with fullerene derivatives to form
interpenetrating interfacial donor–acceptor heterojunction solar cells. Also BHJ cell of CuInS2
and p-type polymer PEDOT:PSS showed better photovoltaic response with external quantum
efficiencies up to 20% [138, 139]. Zhang et al. [140] demonstrated hybrid solar cells from blends
of MEH-PPV and PbS NCs. They investigated the effect of different surfactants on the
photovoltaic performance of the hybrid devices. The device exhibit 250 nA short-circuit current
and an open circuit voltage of 0.47 V. Beek et al. [141] reported hybrid device based on blending
12

27. Chapter 1
of rr-P3HT and ZnO. A PCE of 0.9% with JSC of 2.4 mA/cm2 and a VOC of 685 mV have been
achieved. The best performance of the device based on ZnO nanofiber/P3HT composite [141], a
PCE of 0.53% have been achieved. Incorporation of a blend of P3HT and (6,6)-phenyl C61 butyric
acid methyl ester (PCBM) into the ZnO nanofibers produced an efficiency of 2.03% [142].
Zhou et al. [143] reported a PCE of 2% with JSC of 5.8 mA/cm2 and a VOC of 0.67 V in a
hybrid device fabricated using rr-P3HT and CdSe QDs. In 2005, Sun et al. [144] used CdSe
tetrapods in combination with P3HT and the films prepared from 1,2,4-trichlorobenzene (TCB)
solutions resulted in devices with efficiencies of 2.8%. In 2010 Jilian et al. [145] have studied the
effect of incorporation of CdSe QDs in poly(9,9-n-dihexyl-2,7-fluorenilenevinylene-alt-2,5-
thienylenevinylene) (PFT)/PCBM system. In this work, they found that incorporation of CdSe
QDs in the mixture PFT/PCBM changes the film morphology, which is responsible for the
improvement in device photocurrent and efficiency. In a similar on work P3HT/CdTe/C60 system
a PCE 0.47 % , with JSC of 2.775 mAcm-2, VOC = 0.442 V and FF of 0.38 were obtained [146]. To
date the highest PCE reported for hybrid PV system is ~ 3.2% using poly[2,6-(4,4-bis-(2-
ethylhexyl)-4Hcyclopenta[2,1-b;3,4-b]dithiophene)-alt-4,7-(2,1,3benzothiadiazole)]
(PCPDTBT):CdSe tetrapod blend [76]. Therefore, hybrid polymer-nanocrystal solar cells have
recently gained a lot of attention in scientific community and have also shown considerable PCEs.
Table 1.2 gives the PV performance of a range of selected hybrid solar cells.
Table 1.2 Device configuration and parameters for a range of selected hybrid solar cells.
Device Configuration Voc ( V) Jsc (mA/cm2) EQE PCE (%) References
PCPDTBT: CdSe tetrapods 0.67 10.1 0.55 3.2% S. Dayal et al., Nano Lett.
10 (2010) 239
P3HT: CdSe QDs 0.62 5.8 2% Y. Zhou et al., APL, 96
(2010) 013304
P3HT: CdSe hbranch 0.60 7.10 2.2 I. Gur et al., NanoLett.,7
(2007) 409–14
P3HT: CdSe nanorods 0.62 8.79 0.70 2.6 B. Sun et al., Phys. Chem
Chem. Phys 8 (2006) 3557
OC1C10-PPV: CdSe 0.75 9.1 0.52 2.8 B. Sun et al., J Appl Phys
tetrapods 97 (2005) 014914
APFO-3: CdSe nanorods 0.95 7.23 0.44 2.4 P. Wang et al., Nano Lett
6 (2006) 1789
P3HT: CdSe hbranch 0.60 7.10 2.2 I. Gur et al., NanoLett
7 (2007) 409–14
P3HT: CdSe nanorods 0.71 6.07 0.56 1.7 W. U. Huynh et al.,
Science 295 (2002) 2425–7
MDMO-PPV:ZnO 0.81 2.40 0.39 1.6 WJE Beek et al., Adv
Mater 16 (2004) 1009–13
P3HT:PbS 0.35 1.08 0.21 0.14 D. Cui et. al., Appl. Phys.
Lett. 88, (2006)183111
MEH-PPV: CdTe NCs 0.77 0.19 0.42 T. Shiga et al., Sol.
Energy Mater. Sol. Cells
90 (2006) 1849
P3HT:PCBM:Pt QDs 0.64 10 4.08 M. Y. Chang et al J.
Electrochem. Soc. 156
(2009) B234
13

28. PCBM:PbS 0.24 14.0 1.68 N. Zhao et al. ACS Nano
4 (2010) 3743.
P3HT:GaAs-TiOx 0.59 7.16 2.36 S. Ren et al. Nano Lett.
11 ( 2011) 408
MDMO-PPV:TiO2 0.52 0.6 0.11 V. Hal et al. Adv. Mater.
15 (2003) 118
P3HT:CdS(in-situ) 0.64 2.9 H-C. Liao et al.
Macromol. 42 (2009) 6558
P3HT:ZnO (in-situ) 0.75 5:2 0.44 2.0 S. D. Oosterhout et al.
Nat. Mater. 8 (2009) 818
P3HT:CdS(in-situ) 0.611 3.54 0.72 H. C. Leventis et al. Nano
Lett. 10 (2010) 1253.
The PCEs (ɳ) of hybrid devices based on organic/inorganic NCs are smaller compare to
organic/organic system where ɳ ~8.5% have already been achieved by Mitsubishi Chemical Corp.
[147]. The lower ɳ in hybrid system is because of the inadequate charge transfer between
polymer-NCs and poor nanoscale morphology of the composites film. In conventional synthesis
of QDs (CdTe, CdS), they were capped with organic aliphatic ligands, such as TOPO or oleic
acid. It has been shown that when the QDs are capped with organic ligands, they hinder the
efficient electron transfer from the photoexcited polymer to the NCs [67]. To remove the organic
ligands, polymer-NCs were treated with pyridine. However, pyridine is an immiscible solvent for
the polymer and flocculation of the P3HT chains in an excess of pyridine may lead to the large-
scale phase separation resulting in poor photovoltaic performance [148].
To overcome the effects of the capping ligands many researchers in-situ synthesized the
nanocrystals in polymer matrices. The in-situ growth of the nanocrystals in polymer templates
controls the dispersion of the inorganic phase in organic phase, as a result ensuring a large surface
area for charge separation. Moreover, nanocrystals are uniformly distributed into the entire device
thickness and thus their exist a percolation path for transport of charge carriers to the respective
electrodes.
At an early stage, Van Hal et al. [149] reported hybrid devices based on in-situ grown
TiO2 nanocrystals in to the MDMO-PPV matrix. To prepare bulk heterojunctions they have
blended MDMO-PPV with titanium(iv)-isopropoxide, a precursor for preparation of TiO2
nanocrystals. Subsequent conversion of titanium(iv)isopropoxide precursor via hydrolysis in the
air in the dark resulted in the formation of a TiO2 phase in the polymer film. Such a device
exhibited a JSC of 0.6mA/cm2 and a VOC of 0.52V with a FF of 0.42. External quantum efficiency
up to 11% has been achieved for this device. A similar approach has been recently studied by S.
D. Oosterhout et al. [150] and W. Van Beek et al. [151], with the use of soluble zinc complexes,
which, during and after the deposition process, decompose by reaction with water from the
surrounding atmosphere to yield bi-continuous, interpenetrating ZnO and polymer networks
within the resulting film. An impressive PCE of over 2% has been reported for ZnO/P3HT solar
cells using this fabrication approach. Liao et al. [152] have successfully in-situ synthesized NCs
14

29. Chapter 1
of CdS in P3HT templates using cadmium acetate precursor for Cd and sulphur powder for S. The
device made from P3HT-CdS nanocomposites exhibited a PCE up to 2.9%. Recently H. C.
Leventis et al. [153] thermally decompose the metal xanthate precursor inside P3HT film. Such
device exhibited a PCE of 0.72 %, VOC of 611 mV and JSC of the 3.54 mAcm-2.
1.5. DEVICE PHYSICS OF ORGANIC AND HYBRID SOLAR CELL
1.5.1. Basics of Molecular Photophysics
The main process which occurs in OSCs is based on the photoexcitation of electrons due to
absorption of the light energy. The basic principles of photophysics of a molecule are necessary
for the understanding of organic solar cell operation mechanism.
Π-conjugated polymers generally possess a singlet ground state (S0), (a state in which all
electron spins are paired). Absorption of light usually involves a π‐π* transition to a singlet
excited state of the polymer (S0 + hν → Sn). During absorption, the geometry of the molecule
does not change, although the electrons may undergo rapid motions. This transition to the upper
excited singlet states is referred as Franck-Condon transition [154]. As the mass of the electron
is smaller than the mass of the nucleus, the electronic transition proceeds much faster (10-16s) than
the typical nuclear vibration (10-12-10-14 s). After its formation, the Franck-Condon state
undergoes some vibrational relaxation to attain equilibrium geometry. Usually this process
happens in a time interval of 10-12-10-14 s. The singlet excited state is a very reactive species and it
may release energy or undergo charge transfer. The dominant energy transitions are described
usually by the Jablonsky diagram shown in Figure 1.7 [155]. Decay processes from the singlet
excited state include fluorescence (S1 → S0 + hν), internal conversion (S1 → S0 + thermal energy),
and inter system crossing (ISC) forming triplet excited states (S1 → T1 + thermal energy) [155,
156].
In addition, besides above discussed radiative and nonradiative transitions, one excited
state can participate in a number of inter- and intra-molecular processes. Examples of intra-
molecular processes include ejection of an electron (photo-ionization), decomposition into smaller
fragments (photo-decomposition) or spontaneous isomerization (photo-isomerization). Inter-
molecular pathways, involve reactions with ground state molecules. Among all these reactions,
the most relevant for the understanding of the operation of OSCs are the energy transfer and the
charge transfer. Energy and charge transfer are classified as quenching pathways. In the
photophysics, quenching is defined as the deactivation of an excited sensitizer by an external
component. The external component is called quencher and is usually a molecule in the ground
state.
15

30. S1
ABSORPTION
INTERNAL CONVERSION (10 ps)
T1
FLUORESCENCE (1-10 ns)
PHOSPHORESCENCE (> 100 ns)
INTERSYSTEM CROSSING
S0
Figure 1.7 Jablonsky diagram of organic molecules depicting typical energy levels and energy
transfer.
Coulomb
Interaction
3A* 3D* 1A* + B A + 1B*
+ D A +
Dexter Electron exchange Forster dipole-dipole interaction
Short range (6 – 20 Å) Long range (30 – 100 Å)
Figure 1.8 Illustration of the two mechanisms of energy transfer of an excited molecule: (a)
Dexter electron exchange, (b) Forster dipole-dipole interaction between donor and acceptor.
In case of energy transfer, the quencher (acceptor A) receives the energy from the excited
sensitizer (donor D) and becomes excited (as shown in Figure 1.8).
In the case of charge transfer, the donor is excited first, the excitation is delocalized on the
D–A complex before charge transfer is initiated, leading to an ion radical pair and finally charge
separation can be stabilized possibly by carrier delocalization on the D+. or A-. species by
structural relaxation as shown in Figure 1.9.
16

31. Chapter 1
Figure 1.9 Illustration of the electron transfer between donor and acceptor.
1.5.2. The Need of Two Semiconductors
Photovoltaic cell configurations based on hybrid organic-inorganic materials differ from those
based on inorganic semiconductors, because of the physical properties of inorganic and organic
semiconductors are significantly different. The main differences between organic and inorganic
semiconductors are listed in the Table 1.3.
Table 1.3 A comparison between Organic & Inorganic semiconductors
Semiconductor Inorganic Organic
Interaction energy Covalent (1-4 eV) Van der Waals (10-3 - 10-2 eV)
Dielectric constant 10 2-4
Transport Mechanism Band transport Hopping transport
Mobility (cm2/V.s) RT 100-1000 10-7-1
Mean Free Path (100-1000)ao l=ao lattice constant
Effective Mass (m*/ m) 0.1 Bloch Electrons 100-1000 Polarons
Exciton Type Mott-Wannier Frenkel
Excitonic radius 10-100 nm 1 nm
Exciton binding energy 10 meV 0.1-1 eV
Absorption coefficient --------- >105 cm-1
17

32. Inorganic semiconductors generally have a high dielectric constant of the order of 10, as
compared to 3 in organic semiconductors and a low exciton binding energy. Hence, the thermal
energy at room temperature (kBT = 0.025 eV) is sufficient to dissociate the Wannier-type excitons
(see Figure 1.10) in the inorganic semiconductors. These dissociated electrons and holes are easily
transported within the active layer under the influence of internal field caused by p-n junction.
The organic solids are held by weak Van der Waals interactions, unlike strong covalent
bonds in the inorganic semiconductors. Concomitantly, the relative dielectric constant is low (of
the order of 2-4), which leads to the formation of strongly bound Frenkel-like localized excitons
(Figure 1.10). Hence, dissociation into free charge carriers does not occur at room temperature.
To overcome this problem, OSCs commonly utilize two different materials that differ in electron
donating and accepting properties. Charges are then created by photoinduced electron transfer
between the two components. This photoinduced electron transfer between donor and acceptor
boosts the photo-generation of free charge carriers compared to the individual, pure materials, in
which the formation of bound e-h pairs, or excitons is generally favored.
Figure 1.10 Representation of Frenkel- and Wanier-type exciton.
1.5.3. Fundamental Physical Process in Bulk Heterojunction Solar Cells
The fundamental physical processes in the BHJ PV devices are schematically represented in
Figure 1.11. Sunlight photons which are absorbed by the active layer, excite the donor (1), leading
to the creation of excitons in the conjugated polymer. The created excitons start to diffuse (2)
within the donor phase and if they come across the interface with the acceptor then a fast
dissociation takes place (3) leading to charge separation [157, 158]. Subsequently, the separated
free charge carriers are transported (4) with the aid of the internal electric field (caused by the use
of electrodes with different work functions). These dissociated charge carriers moves towards the
electrodes where they are collected (5) and driven into the external circuit. However, the excitons
18

33. Chapter 1
can decay (6), yielding, e.g., luminescence, if they are generated too far from the interface. Thus,
the excitons should be formed within the diffusion length of the interface, being an upper limit for
the size of the conjugated polymer phase in the BHJ. The comprehensive physics behind
light‐to‐electric energy conversion process in polymer solar cells and some related issues are
discussed below.
LUMO
2 3 1
6
1
5 5
4
5 4
4 5
3
HOMO
Anode Cathode
2
Donor (a) Acceptor Donor (b) Acceptor
Figure 1.11 Fundamental operation process in BHJs solar cells, the numbers (1 to 6) refer to the
operation processes explained in the text (a) Schematic band diagram and (b) Blend of OPV.
1.5.3.1. Light absorption and exciton generation
For an efficient collection of photons, the absorption spectrum of the photoactive organic layer
should match the solar emission spectrum and the layer should be sufficiently thick to absorb all
the incidents light. When the incident photon has an energy hν ≥ Eg, an electron in the HOMO of
the donor would be excited to the LUMO, leaving a hole in the HOMO level. This e-h pair is
called singlet exciton having opposite spin. In an OSC, only a small region of the solar spectrum
is covered. For example, a bandgap of 1.1 eV is required to cover 77% of the AM1.5 solar photon
flux, whereas most solution processable semiconducting polymers (PPVs, P3HT) have bandgaps
larger than 1.9 eV, which covers only 30% of the AM1.5 solar photon flux. In addition, because
of the low charge-carrier mobilities of most polymers, the thickness of the active layer is limited
to ~ 100 nm, which, in turn, results in absorption of only ≈ 60% of the incident light at the
absorption maximum [84]. Thus, an efficient solar cell should have a wide absorption spectrum,
so as to create as many e-h pairs as possible.
1.5.3.2. Diffusion of excitons in conjugated polymers
Because of the high exciton binding energy in the conjugated polymers, the thermal energy at
room temperature is not sufficient to dissociate a photogenerated exciton into free charge carriers.
Consequently, the configuration and operation principle of PV devices based on organic
19

34. semiconductors differ significantly from those based on inorganic materials. Typically, in OSCs
an efficient electron acceptor is used in order to dissociate the strongly bound exciton into free
charge carriers [87] as discussed in section 1.6.2.
1.5.3.3. Dissociation of charge carriers at the donor/acceptor interface
Organic semiconductors are characterized by high excitonic binding energy of the order of 0.2-0.5
eV [159, 160]. As a result, photogenerated excitons dissociation occurs only when the potential
drop at donor and acceptor interface is larger than the exciton binding energy [161-167]. After
photo-excitation of an electron from the HOMO to the LUMO, the electron can jump from the
LUMO of the donor to the LUMO of the acceptor. However, this process, which is called
photoinduced charge transfer, can lead to free charges only if the hole remains on the donor due to
its higher HOMO level. In contrast, if the HOMO of the acceptor is higher, the exciton transfers
itself completely to the material of lower-band gap accompanied by energy loss (Figure 1.12).
Figure 1.12 The interface between donor and acceptor can facilitate either charge transfer by
splitting the exciton or energy transfer, where the whole exciton is transferred from the donor to
the acceptor.
1.5.3.4. Charge transport in donor/acceptor blends
After photoinduced electron transfer at the donor/acceptor interface and subsequent dissociation,
the electrons are localized in the acceptor phase whereas the holes remain in the polymer chains
as shown in Figure 1.13. Subsequently, the free electrons and holes must be transported via
percolated donor and acceptor pathways towards the electrodes to produce the photocurrent.
In order to collect the photogenerated charges, the carriers have to migrate through the
active materials to the electrodes. The active layer in polymer solar cells is usually deposited by
spin-coating. In such a spin-coated film, the polymer chains are arranged in a disordered fashion.
Conformational and chemical defects in the polymer chains and molecules will restrict the charge
20

35. Chapter 1
carriers to small segments. As a result, the delocalization length of the charge carriers is limited to
almost molecular dimensions. The distribution of the π-conjugation lengths of the polymer
segments, results in a distribution of the energies of the localized states available to the charge
carriers.
e-
C6H13 C6H13 C6H13 C6H13
S h+ S S S
S S S S
C6H13 C6H13 C6H13 C6H13
Figure 1.13 Pictorial representation of electron transfer from P3HT to PCBM.
Charge transport in the energetically disordered materials has been successfully described
within the Gaussian disorder model [168]. In this model, energetic disorder is modeled by a
Gaussian distribution of energy levels of the sites. After photo-generation of the charge carriers
in the disordered system, the charge carriers relax towards tail states of the Gaussian distribution
while performing a random walk throughout the disordered potential energy landscape. During
this random walk, the carriers may get trapped on a low energy site. The charge can either be
freed by thermal activation [168, 169] or it may tunnel to a nearby site, without thermal
activation [170].
1.5.3.5. Extraction of the charge carriers at the electrodes
In addition to the attempts for optimizing the components and composition of the active layer,
modification of the electrodes has also lead to an improvement in the device performance [171-
173]. It is evident that the work function of the negatively charged electrode is relevant for the
open-circuit voltage (VOC) of the cells. In the classical metal–insulator–metal (MIM) concept, in
the first order approximation VOC is governed by the work function difference of the anode and
the cathode, respectively. It should be noted that this only holds for the case where the Fermi
levels of the contacts are within the bandgap of the insulator and are sufficiently far away from
the HOMO and LUMO levels, respectively. However, in OSCs, where the ohmic contacts
(negative and positive electrodes match the LUMO level of the acceptor and the HOMO level of
the donor, respectively) are used, the situation is different. Charge transfer of electrons or holes
from the metal into the semiconductor occurs in order to align the Fermi level at the negative and
21

36. positive electrode, respectively. As a result, the electrode work functions become pinned close to
the LUMO/HOMO level of the semiconducting materials [171]. Because of this pinning, the VOC
will be governed by the energies of the LUMO of the acceptor and the HOMO of the donor.
Indeed, in BHJ solar cells, a linear correlation of the VOC with the reduction potential of the
acceptor has been reported [172]. The fact that a slope of unity was obtained indicates a strong
coupling of the VOC to the reduction strength of the acceptors [172]. Remarkably, the presence of
the coupling between the VOC and the reduction potential of the PCBM has been interpreted as a
proof against the MIM concept, although it is in full agreement with a MIM device with two
ohmic contacts. In contrast, only a very weak variation of the VOC (160 meV) has been observed
when varying the work function of the negative electrode from 5.1 eV (Au) to 2.9 eV (Ca) [172].
This has been explained by pinning of the electrode Fermi level to the reduction potential value of
the fullerene. However, it has been pointed out that when the metal work function is reduced to
such an extent that it is below the LUMO, the electrode work function will remain pinned close to
the LUMO level of the semiconductor [173]. This explains why the VOC only increases slightly
when going from Al (4.2 eV) to Ca (2.9 eV), because the Ca work function will be pinned to the
LUMO of the PCBM (3.7 eV).
1.6. ELECTRICAL CHARACTERISTICS PARAMETERS
A solar cell under illumination is characterized by the following parameters: the short circuit
current (JSC), the open‐ circuit voltage (VOC), the fill factor (FF) and the PCE (ɳ). These
parameters are indicated on the J-V characteristic of a solar cell shown in Figure 1.14.
25
20 Illumination
15 Dark
10
Current Density
5
0
-5 V
FF OC
-10
-15
JSC Pmax=(VI)
max
-20
-1.0 -0.5 0.0 0.5 1.0 1.5
Applied bias
Figure 1.14 Definitions of JSC, VOC, FF, Jmax, and Vmax
1.6.1. Short‐ circuit current (JSC)
The short circuit current is the photogenerated current of a solar cell, which is extracted at zero
applied bias. In this case, exciton dissociation and charge transport is driven by the so-called built-
22

37. Chapter 1
in potential. The JSC is heavily dependent on the number of absorbed photons which originates
from two different facts. Firstly, JSC shows a linear dependence on the incident light intensity as
long as no saturation effects occur within the active layer. Secondly, JSC can be maximized by
enlarging the absorption spectrum of the photoactive layer to harvest more photons within the
terrestrial sun spectrum. The JSC also depends on the charge carrier mobilities of the active layer
[174,175].
1.6.2. Open‐Circuit Voltage (VOC)
The open‐circuit voltage is the bias voltage to be applied in order to annihilate the current
generated by the illumination. So, at the VOC there is no external current which flows through the
device under illumination (J=0). For a solar cell with a single conjugated polymer active layer,
the Voc scales with the work function difference of the electrodes and thus follow the MIM model
under consideration of clean polymer/electrode interfaces [176, 177]. Here, clean
polymer/electrode interface refers to absence of dipoles or other entities that changes interface
conditions, usually resulting into shift of charge injection barriers. In a single-layer device, the
VOC cannot exceed the difference in the work functions of the two electrodes [176]. The
experimentally determined VOC is generally somewhat lower, owing to the recombination of free
charge carriers. At open-circuit conditions, all charge carriers recombine within the photoactive
layer. Thus, if recombination can be minimized, the VOC can more closely approach the theoretical
limit. However, based on thermodynamic considerations of the balance between photo-generation
and recombination of charge carriers, it has been found that charge recombination cannot be
completely avoided, resulting in a lower open-circuit voltage [178].
In bilayer, the Voc scales linearly with the work function difference of the electrodes plus
an additional contribution from the dipoles created by photoinduced charge transfer at the
interface of the two polymers [179]. On the other hand, this does not explain the VOC observed for
BHJ solar cells. The Voc of BHJ solar cells mainly originates from the difference between the
LUMO of the acceptor [180] and the HOMO of the donor [181], indicating the importance of the
electronic levels of donor and acceptor in determining the efficiency of such solar cells. In the
case of polymer-polymer BHJ solar cells, it has been demonstrated that the VOC significantly
exceeded the difference in electrode work function with values as large as 0.7 V [182, 183].
1.6.3. Fill factor (FF)
The purpose of a solar cell is to deliver power (V×I). The fourth quadrant of the J‐V curve shows
where the cell can deliver power. In this quadrant, a point can be found where the power reaches
its maximum value, is called the maximum deliverable power (Pmax). The fill factor is defined by
the Equation.
23

38. Pmax ( J  V ) max
FF  
Ptheor max J SC  VOC
The FF is a measure for the diode characteristics of the solar cell. The higher the number, the
more ideal the diode is. Ideally, the fill factor should be unity, but due to losses caused by
transport and recombination its value generally found in between 0.2–0.7 for OPV devices. The
direct relation of FF with current density indicates that it is greatly affected by the mobility of the
charge carriers. Moreover, series and shunt resistance are also observed as limiting factors in BHJ
solar cells [184]. In order to obtain a high fill factor FF the shunt resistance of a photovoltaic
device has to be very large in order to prevent leakage currents and series resistance has to be very
low.
1.6.4. Power Conversion Efficiency (ɳ)
In order to determine the PCE of a PV device, the maximum power Pmax that can be extracted
from the solar cell has to be compared to the incident radiation intensity. It is the ratio of delivered
power (Pin), to the irradiated light power (Plight).
Pout (V  I ) max VOC  J SC  FF
  
Pin Pin Pin
The η reflects how good the solar cell can convert light in to the electrical current.
1.6.5. Dark Current (Idark)
The dark current is the current through the diode in the absence of light. This current is due to the
ideal diode current, the generation/recombination of carriers in the depletion region and any
surface leakage, which occurs in the diode.
When a load is applied in forward bias, a potential difference develops between the
terminals of the cell. This potential difference generates a current which acts in the opposite
direction to the photocurrent, and the net current is reduced from its short circuit value. This
reverse current is usually called dark current in analogy with the current Idark(V) which flows
across the device under an applied voltage in the dark. Most solar cells behave like a diode in the
dark, admitting a much larger current under forward bias (V>0) than under reverse bias (V<0).
This rectifying behavior is a feature of photovoltaic devices, since an asymmetry is needed to
achieve charge separation.
1.6.6. Standard Test Conditions
The efficiency of a solar cell depends upon temperature, excitation, spectrum and illumination
intensity. Therefore, test conditions have been designed to obtain meaningful and comparable
values. These test conditions are based on a spectral distribution, reflection of the emission
24

39. Chapter 1
spectrum of the sun, measured on a clear sunny day with a radiant intensity of 100 W/cm 2 that is
received on a tilted plane surface with an angle of incidence of 48.2°. This spectrum that also
counts for a model atmosphere containing specified concentrations of, e.g., water vapour, carbon
dioxide, and aerosol is referred to as an “Air Mass 1.5 Global” (AM1.5G, IEC 904-3) spectrum
(Figure 1.15). These standard test conditions also include a measuring temperature of 25 °C [185].
Figure1.15 Definition of AM0, AM1.0 , AM1.5 and AM2.0 solar spectra (left) and the
corresponding AM 1.5 spectrum (right).(Source: http://www.eyesolarlux.com/Solar-simulation-
energy.htm).
1.6.7. Equivalent Circuit Diagram
The equivalent circuit diagram (ECD) of an organic solar cell can be represented by a diode in
parallel of a photocurrent source (IPh), a capacitor (C), a resistor called shunt resistor (RSh) and in
series another resistor called series resistor (RS) [186]. The ECD of a solar cell is shown in Figure
1.16.
Figure 1.16 Equivalent circuit diagram of an organic solar cell.
In Figure 1.16, diode represents the diode character of the solar cell which is a result of the
built in field from the donor/acceptor interface. This diode is responsible for the nonlinear shape
25

40. of the I-V curves. The photocurrent source generates current (Iph) upon illumination and equals to
the number of dissociated excitons per second without any recombination effects [187].
The shunt resistor (RSh) represents the current lost due to recombination of e–h pairs at the
site of exciton dissociation, before any charge transport can occur. RSh is correlated with the
amount and character of the impurities and defects in the active organic semiconductor layer
because impurities and defects cause charge recombination and leakage current [188]. Moreover,
during the deposition of the electrodes on thin organic films, the top electrode might short through
to the bottom electrode causing pinhole shorts. These are ohmic contacts that reduce the diode
nature of the device and are represented by the shunt resistor. RSh determines from the inverse
slope of the J-V curve in the fourth quadrant, as shown in Figure 1.17(a) [189].
(a) (b)
Figure 1.17 (a) Impact of the variation of the shunt resistance (RSh) on the FF. (b) Impact of
the variation of the series resistance (RS) on the FF.
The series resistance (RS), is related with the intrinsic resistance, morphology, and
thickness of the semiconductor layer. RS is analogous to conductivity i.e. mobility of the specific
charge carriers in the respective transport medium. RS also increases with a longer traveling
distance of the charges for example in thicker transport layers. The series resistance, Rs, can be
calculated from the inverse slope of the J-V curve in the first quadrant as shown in Figure 1.17(b)
[189]. Organic semiconductors are characterized by low charge carrier mobility. Due to low
carrier mobility in these materials, injected carriers will form a space charge. This space charge
creates a field that opposes the transport of other free charges, acting like a capacitor. This is
represented by the capacitor C in ECD shown in Figure 1.16.
1.7. OBJECTIVE OF THE PRESENT THESIS
The objective of the present work is to develop and improve the performance of organic and
hybrid solar cells, consequently it is necessary to (i) understand the fundamental physical
26

41. Chapter 1
properties of the organic and hybrid systems, (ii) understand the charge transport mechanism in
these devices, (iii) improve the charge transfer at donor/acceptor interface. To attain these
objectives following studies have been carried out.
1. Synthesis of various conjugated polymers such as P3HT, poly(3-octylthiophene) (P3OT)
and copolymer poly[(3-hexylthiophene)-co-(3-octylthiophene)] (P3HT-OT). Besides this
semiconducting NCs of CdS has also been synthesised. To improve the poor charge transfer at
organic/inorganic interface, the NCs of CdTe are in-situ grown in P3HT matrix without use of any
surface ligands.
2. The study has also been carried out to understand the basic physics underlying the
morphological [scanning electron microscopy (SEM), atomic force microscopy (AFM)],
structural [X-ray diffraction (XRD), transmission electron microscopy (TEM)], and spectral
[Fourier transform infrared spectroscopy (FTIR) UV-Vis absorption, Photoluminescence]
behaviors of these materials which are essential for the optimization of PV devices.
3. The PV performance of various organic and hybrid devices has been investigated. The
effect of CdS and CdTe NCs on the solar cells parameters has been studied. The effect of post-
production thermal annealing on the device performance has also been studied.
4. Charge transport study has been carried out to understand the working principle of these
devices. Also the modulation of the charge transport parameters of P3HT on incorporation of
inorganic NCs (CdS and CdTe) has been studied.
1.8. THESIS PLAN
The present thesis explores the structural, optical, charge transport properties of P3HT, P3OT, and
copolymer of 3-hexylthiophene and 3-octylthiophene namely P3HT-OT as well as P3HT/CdTe
and P3HT/CdS hybrid systems for their application in the solar cells. The thesis comprises of 7
chapters.
The present chapter (chapter 1) deals with the introduction which comprises of the
literature survey and overview of various generations of solar cells. Besides this, it also describes
the working principle of photovoltaic devices. It also includes discussion on various basic and
applied concepts, such as solar cell device architectures, polymer fullerene bulk-heterojunction,
donor-acceptor concept.
Chapter 2 discusses the details of the synthesis of conjugated polymers (P3HT, P3OT and
P3HT-OT), semiconducting NCs (CdTe, CdS) and polymer-nanocrystals hybrid systems. It
includes the fabrication process of bulk heterojunction solar cells and hole only device for charge
transport study. Besides this, the basic working principles of various characterization techniques
utilized to characterize organic-inorganic hybrid systems have also been discussed.
27