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
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
Dedicated To
My parents
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
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)
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
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
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
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
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
i
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,
ii
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 10-3
cm2/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.
iii
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
iv
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.
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
2
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
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
E
Chapter 1
3
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
4
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
Chapter 1
5
% 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.
6
n-Si
p-Si
Metal (Front)
Metal (Back)
Metal (Back)
TCO
TCO (front)
n-a-Si
i-µc-Si
p-µc-Si
glass
TCO
Glass, metal foil
CdS
CIGS
Mo (Back)
Metal (Back)
TCO (front)
CdTe
CdS
glass
(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.
Chapter 1
7
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
η %
Obtained 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
8
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
Chapter 1
9
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.
10
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
Chapter 1
11
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
12
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/cm
2 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
Chapter 1
13
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
tetrapods
0.75
9.1
0.52 2.8
B. Sun et al., J Appl Phys
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
14
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
Chapter 1
15
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 (10-16
s) 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.
16
ABSORPTION
INTERNAL CONVERSION (10 ps)
FLUORESCENCE (1-10 ns)
PHOSPHORESCENCE (> 100 ns)
INTERSYSTEM CROSSING
S0
S1
T1
Figure 1.7 Jablonsky diagram of organic molecules depicting typical energy levels and energy
transfer.
1A* + B A + 1B*
Forster dipole-dipole interaction
Long range (30 – 100 Å)
Coulomb
Interaction
Dexter Electron exchange
Short range (6 – 20 Å)
3A* + D A + 3D*
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.
Chapter 1
17
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
18
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
Chapter 1
19
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.
1
2
3
445
5
Donor Acceptor(b)
LUMO
HOMO
AcceptorDonor
Anode Cathode
16
2 3
4
55
(a)
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
20
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
Chapter 1
21
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.
S
S
S
C6H13
S
S
S
S
S
C6H13 C6H13C6H13 C6H13
C6H13 C6H13 C6H13C6H13
h+
e-
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
22
-1.0 -0.5 0.0 0.5 1.0 1.5
-20
-15
-10
-5
0
5
10
15
20
25
Cu
rren
t D
en
sity
Applied bias
Illumination
Dark
VOC
JSC Pmax=(VI)max
FF
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.
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-
Chapter 1
23
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.
24
OCSCtheor VJ
VJ
P
PFF
max
max
max )(
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).
in
SCOC
inin
out
P
FFJV
P
IV
P
P
max)(
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
Chapter 1
25
spectrum of the sun, measured on a clear sunny day with a radiant intensity of 100 W/cm2 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
26
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].
(b)(a)
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
Chapter 1
27
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.
28
Chapter 3 includes the PV performance of P3HT, P3OT and their copolymer P3HT-OT.
The chapter contains the investigations of FTIR, 1H NMR, XRD, TGA, DSC, UV-vis. absorption,
photoluminescence, properties of these polymers The energy level positions have been evaluated
by the cyclic voltammetry. Finally, the photovoltaic performance of P3HT-OT has been studied
and results were compared with the homopolymer P3HT and P3OT.
Chapter 4 deals with the in-situ growth of CdTe NCs in P3HT matrix without use of any
surfactant. The CdTe NCs have been incorporated in-situ in P3HT matrix with the aim of
improving the photovoltaic properties of P3HT by broadening of solar absorption spectrum,
enhancing the charge carrier mobility and improving the interaction between polymer-
nanocrystals. Growth of CdTe nanocrystals has been confirmed by the structural (HRTEM, SEM,
AFM) and spectral properties (FTIR, UV-Vis absorption). Photoluminescence quenching and
decrease in the quantum yield, confirm the charge transfer between P3HT/CdTe. Finally, PV
parameters of P3HT/CdTe hybrid system have been investigated and results were compared with
those of pristine P3HT.
In chapter 5 electrical and optical properties of P3HT/CdS hybrid system have been
studied. The particle shape, size and distribution of CdS QDs 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 nanocrystals. At the end of the chapter J-V
characteristics of P3HT and P3HT/CdS system with PCBM have been investigated under AM 1.5
light as well as in the dark.
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 theoretical
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 section the hole transport
mechanisms in all the polymers (P3HT, P3OT and P3HT-OT) and polymer/nanocrystals
(P3HT/CdS and P3HT/CdTe) hybrid systems in the device configuration ITO/
PEDOT:PSS/Active layer/Au have been studied in girth. Current-voltage characteristics of these
devices have been studied in the temperature range of 300-110 K.
Finally, chapter 7 presents the major conclusions derived from the present work and the
scope of the future study in this field.
Chapter 1
29
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CHAPTER 2
EXPERIMENTAL DETAILS: MATERIALS, METHODS AND CHARACTERIZATION
TECHNIQUES
2.1. INTRODUCTION
2.2. SYNTHESIS OF POLY(3-ALKYTHIOPHENE)S
2.3. SYNTHESIS OF SEMICONDUCTOR NANOCRYSTALS
2.3.1. In-situ Growth of Cadmium Telluride Nanocrystals in P3HT Matrix
2.3.2. Synthesis of Cadmium Sulphide Quantum Dots
2.4. DEVICE FABRICATION
2.4.1. Patterning and Cleaning of ITO Substrates
2.4.2. Glove Box System for Device Fabrication
2.4.3. Active Layer Deposition on ITO Substrate
2.5. CHARACTERIZATION TECHNIQUES
2.5.1 UV-Vis Absorption
2.5.2 Photoluminescence
2.5.3 Fourier Transforms Infrared Spectroscopy
2.5.4 Thermal Analysis
2.5.5 Electrochemical Studies: Cyclic Voltammetry
2.5.6 X-Ray Diffractometer
2.5.7 Scanning Electron Microscopy
2.5.8 Transmission Electron Microscopy
2.5.9 I-V Characterization Technique
2.5.10 Temperature Dependent I-V Measurements Setup
References
2.1. INTRODUCTION
resent chapter describes the synthesis of various conjugated polymers such as poly(3-
hexylthiophene) (P3HT), poly(3-octylthiophene) (P3OT) and the copolymer poly[(3-
hexylthiophene)-co-(3-octylthiophene)] (P3HT-OT). Besides this the synthesis methods
of semiconducting nanocrystals (NCs) of Cadmium Telluride (CdTe) and Cadmium Sulphide
(CdS) have also been discussed. It also describes the fabrication process of bulkheterojunction
solar cells as well as hole only devices for charge transport study. Attempts have also been made
P
40
to describe the experimental setups and working principles for the various characterization
techniques utilized to characterize the organic and organic/inorganic hybrid system.
2.2. SYNTHESIS OF POLY(3-ALKYTHIOPHENE)S
The experimental setup used for the polymerization of poly(3-alkylthiophenes) (P3ATs) are
described here. The low temperature synthesis has been performed using the assembly as shown
in Figure 2.1. The setup has a Julabo low temperature bath, (Model No: Julabo FP-50) a PCi
Nitrogen gas generator (Model: NG-02), a specially designed double walled glass container and a
stirrer.
Figure 2.1 Experimental setup for the polymerization of Poly(3-alkylthiophenes).
The P3ATs were synthesized via chemical oxidative polymerization technique by drop-
wise addition of monomer 3-alkylthiophenes (3ATs) in suspension of ferric chloride (FeCl3,
0.4M) and chloroform (CHCl3) [1-3]. The syntheses were carried out at 228 K under inert
atmosphere (N2 atmosphere) in a double walled glass container, by constant stirring with a glass
stirrer. To maintain the desired temperature, methanol was continuously circulated through the
double wall container with the help of temperature bath running in a temperature range from 323
K down to 223 K with an accuracy of ± 0.1 K.
The homopolymers P3HT, P3OT, and the copolymer P3HT-OT have been synthesized using
the oxidative coupling method shown in scheme 2.1.
Chapter 2
41
Scheme 2.1 Synthesis route for polymers P3HT, P3OT, and P3HT-OT. For P3HT, R= R’ =
C6H13, for P3OT, R= R’ = C8H17, and for P3HT-OT, R = C6H13, R’= C8H17.
For the polymerization, the monomer to the oxidant ratio were taken as 1:4. In a typical
synthesis of P3HT-OT, equal molar ratio of 3HT (0.05M) (0.1M for P3HT) and 3OT (0.05 M)
(0.1M for P3OT) was added drop wise in FeCl3-CHCl3 suspension. The 3HT and 3OT monomer
having desired concentration was slowly added to the continuously stirred FeCl3-CHCl3
suspension for about 6 hours and the whole process was carried out for 24 hrs in order to give
sufficient time for complete polymerization. After mixing of the reactants, the solution turned
green, which after 24 hrs was precipitated by adding plenty of methanol in a polymer-oxidant
mixture. Repeated purification was performed by methanol and distilled water to remove
oligomers and excess oxidant till the filtrate became colorless. The resultant polymer is green
after drying at 333 K for two hrs. P3HT-OT thus obtained contains FeCl3 as an impurity. In order
to get P3HT-OT in pristine form, a rigorous purification process has been described below which
removes FeCl3.
After chemical synthesis, the resultant polymer contains unreacted monomer or oligomers
and oxidant used for polymerization. Unreacted monomers, oligomers and oxidants are removed
from the as grown polymer by successive washing by chemicals which show specific affinity for
the molecules to be removed. In the present case, the polymerization has been carried out using
3HT, 3OT and FeCl3 in CHCl3. To get pristine P3HT-OT, the purification of polymer requires
removal of any leftover 3HT, 3OT monomers, oligomers and FeCl3. In order to remove these
impurities, the as grown polymer was treated with aqueous ammonia (aqueous NH3) and
ethylene-diamine-tetraacetic acid (EDTA) (liquid–liquid extraction) in separate steps. These steps
are as follows.
1. As grown P3HT-OT polymer in solid form is suspended in CHCl3.
2. Copious amounts of NH3 is being poured into the P3HT-OT–CHCl3 suspension.
3. The solution having two phases of aqueous NH3 and P3HT-OT–CHCl3 are slowly heated to the
60 ˚C. Due to continuous heating, the more volatile CHCl3 evaporates first, leaving P3HT-OT
solid with lower chloride content (as NH3 removes the chloride part of FeCl3 intercalated to
P3HT-OT) floating over aqueous NH3.
42
4. P3HT-OT obtained in step 3 is dissolved in CHCl3 and aqueous EDTA of the desired
concentration was poured into the P3HT-OT–CHCl3 solution. The two phase solution is again
heated to the boiling point of CHCl3. As heating continuous, the more volatile CHCl3 evaporates
first, leaving P3HT-OT solid with lower iron content (as EDTA removes the iron part of FeCl3
intercalated to P3HT-OT) floating over aqueous EDTA.
5. Steps (3) and (4) were repeated several times to minimize the FeCl3 impurity present in the
polymer matrix.
Continuous repetition of the aqueous NH3 and EDTA treatment steps reduces the amount
of residual FeCl3 in the polymer matrix and was confirmed by energy dispersive x-ray analysis
(EDAX). This copolymer is termed as „pristine P3HT-OT‟. The pristine P3HT-OT is completely
soluble in CHCl3, chlorobenzene and toluene. The resultant P3HT-OT copolymer solution is cast
in a flat glass substrate. The solution is covered by another glass plate keeping a narrow opening
to allow the evaporated solvent to escape. On complete evaporation of the solvent, the P3HT-OT
film is peeled off from the glass substrate by pouring methanol into the film growing chamber, so
that the polymer film leaves the glass substrate on its own, without any mechanical stretching and
tearing of the film during the separation from the glass substrate. The film is then dried at 353K
for 1 h to remove any solvent trapped inside the film. A good quality film of pristine P3HT-OT
having excellent surface smoothness, free from pinholes and good mechanical strength (Figure
2.2) was obtained and cut into pieces, which were then subsequently used for all electronic and
electrical studies.
Figure 2.2 Solution cast film of copolymer P3HT-OT
2.3. SYNTHESIS OF SEMICONDUCTOR NANOCRYSTALS
The experimental setup used for the growth of semiconductor QDs has been shown in Figure 2.3.
The synthesis process requires a 3-neck and a 2-neck round bottom (RB) flask (100 ml), two
condensers, two magnetic stirrers with hot plates which can achieve 500 ˚C temperature and a
syringe. The synthesis has been carried out under inert (Nitrogen/Argon) atmosphere.
Chapter 2
43
2.3.1. In-situ Growth of Cadmium Telluride Nanocrystals in P3HT Matrix
In-situ growth of CdTe nanocrystals in P3HT matrix was carried out as schematically illustrated
in scheme 2.2 [4, 5]. In a typical synthesis of PHTCdTe1, 0.5 wt.% of P3HT has been dissolved in
tri-chlorobenzene to which 0.1 mmol of cadmium acetate dihydrade in chlorobenzene was added.
The reaction mixture was heated for 2 hrs at 160 0C. The tellurium precursor has been prepared by
treating 0.2 mmol of tellurium powder (Acros Organics) in trioctylphosphine (TOP) (Sigma
Aldrich, USA), at 160°C for 2 hrs under argon or nitrogen flow.
The Te precursor was then injected in to the P3HT-Cd solution and the resultant bright
orange reaction mixture was allowed to react for 2 hrs at 160°C under argon atmosphere. Growth
of CdTe NCs got completed when color of the solution turned black. After the completion of the
reaction, the unreacted cadmium acetate and precursor of tellurium were removed by treating
nanocomposites with hexane. The reaction mixture was separated by centrifugation and dried in
vacuum at 80 °C.
Ar
Oil Bath
Cdacetate+
P3HT+TCB solution
TOPTe solution
Thermocouple
Hot plate with
magnetic stirrer
Figure 2.3 Experimental setup used for the synthesis of CdTe NCs.
Similarly, other compositions of P3HT containing different molar ratios of Cd-acetate
were synthesized and are designated as PHTCdTe2, PHTCdTe3, PHTCdTe4, and PHTCdTe20
for 0.2 mmol, 0.4 mmol, 0.6 mmol and 3.6mmol, of Cd-acetate, respectively. These composites
have the Te precursor in the ratios of 0.4 mmol for PHTCdTe2, 0.8 mmol for PHTCdTe3, 1.2
mmol for PHTCdTe4 and 7.2 mmol for PHTCdTe20. The syntheses of different P3HT-CdTe
compositions were also carried out at 220 °C using the same procedure discussed above.
44
Scheme 2.2 Proposed mechanism for in-situ growth of the CdTe QDs in the P3HT matrix. (a)
P3HT has been synthesized by chemical oxidative polymerization route. (b) Schematic of Cd2+
ions has been assumed to be coupled with the unpaired S atom along the P3HT planar chain
network. (c) Schematic diagram of P3HT capped CdTe nanocrystals after reaction of TOPTe with
Cd2+
ions coupled P3HT.
2.3.2. Synthesis of Cadmium Sulphide Quantum Dots
The synthesis of CdS quantum dots (QDs) was carried out by wet chemical method [6-8]. Two
hexane solutions of Aerosol OT (AOT) (0.2 M, 50 ml) were prepared. An aqueous solution of
cadmium nitrate tetra-hydrate (Cd(NO3)2.4H2O) (0.4 M) was added to one hexane solution, while
an aqueous solution of Na2S (0.4 M) was added to the other solution in order to achieve a
[H2O]/[AOT] ratio of 6 for both solutions. The solutions were stirred for 3 h. The micellar
solution containing cadmium nitrate was then added slowly to the micelle solution containing
Na2S at room temperature under nitrogen atmosphere. CdS QDs were obtained after the solution
was stirred for 3 h. 1-Decanethiol (DT) molecules (4.3 mmol) were added to a hexane solution of
CdS QDs (1.5 M). This solution was stirred for 5 h, and methanol was subsequently added in
order to remove the AOT molecules. After the methanol phase was removed, the hexane phase
was evaporated. The residual solution was then dropped into a large volume of methanol, and the
resultant yellow precipitate was filtered off using a 0.2-µm membrane filter, yielding purified DT-
caped CdS QDs.
Chapter 2
45
2.4. DEVICE FABRICATION
The devices studied in the present investigations for the photovoltaic characterization as well as
for charge transport study have same fabrication steps. The processing steps of these devices have
been discussed below:
2.4.1. Patterning and Cleaning of ITO Substrates
Indium-tin-oxide (ITO) coated glass sheets (with a sheet resistance < 20 Ω/cm2) were cut into the
small pieces of the area 1.5×1.5 cm2. These substrates were patterned by etching method using Zn
dust and hydrochloric acid (HCl). The etched substrates were cleaned twice with soap solution,
and then washed by distilled water. After washing with distilled water, the substrates were
ultasonicated for 30 min in acetone at 50 ˚C, followed by boiling in trichloroethylene and iso-
propanol for 20 min, separately. Finally these substrates were dried in vacuum oven at 120 ˚C for
2 hrs. Prior to use, the cleaned substrate were treated with oxygen plasma. Glass substrates for
UV-Vis absorption, photoluminescence, SEM, and AFM measurements were also cleaned in the
similar manner.
2.4.2. Glove Box System for Device Fabrication
Since the device properties of diodes based on organic compounds are extremely sensitive to the
environmental conditions, in particular to the presence of oxygen and moisture. This sensitivity of
organic semiconductors towards exposition to oxygen and moisture is a strong limiting factor in
the operation of semiconductor elements. Special measures need to be taken during preparation
and further treatment of the manufactured devices. To ensure oxygen and moisture free
environment the device fabrications have been carried out under inert atmosphere by using Hind
Hi-Vac glove box system. The system consists of two interconnected glove-boxes filled with dry
nitrogen gas as shown in Figure 2.4.
One glove-box (Box A) is fitted with a spin coater and a hot plate, used for deposition of
active layer and baking of active layer, respectively. The other glove box (Box B) is equipped
with a thermal evaporator for the deposition of small organic molecules and metals. The two
boxes are connected via a T-anti chamber with translation rails and a loading gate. The glove-box
system includes a gas purifier based on a copper catalyst and molecular sieves with closed gas
circulation. The wet processing box A contains a purification system that is separated from the
other box, protecting the latter from solvent contamination. Box B also shares the purification
system that either allowed independent gas circulation in the two boxes or parallel flow. The
water and oxygen content is measured by a H2O/O2 analyzer and is typically below 1 ppm for
both the boxes.
46
Figure 2.4 Hind Hi-Vac glove box system with box A and box B.
The box A is operated in a purification operation mode, and the gas circulation in the box
was connected via a charcoal-trap to the glove-box. By permanently removing the polluted
nitrogen gas with the protection pump, the vapors of used solvent were captured in the trap.
Simultaneously, the box is refilled with dry nitrogen. In the purifier mode, the nitrogen gas is
cleaned from solvent vapors by an activated charcoal solvent trap that preceded the purification
system. In the metal evaporator (Figure 2.5), a vacuum of 10-6
mbar may be achieved by using a
turbo pump. Venting was initiated by an automatic venting mode with time delay, in order to
protect the turbo pump. Subsequently, the evaporation chamber is filled with dry nitrogen gas out
of the glove-box. This mode of operation allowed us to deposit metals such as Al and Au.
The metal evaporation system included four evaporation boats as sources located at the
bottom of the chamber supported by two automatic/manual shutter for loading the source
material. Tungsten and molybdenum boats were employed, depending on the metal to be
deposited. Above the four sources, the sample holder was positioned, which could support four
samples with the typical size of 1.5×1.5 cm2. Just below the sample holder, two quartz balance
sensors allow on-line measurement of the evaporation rate and thickness by an externally situated
deposition monitor controller.
Chapter 2
47
Figure 2.5 Thermal evaporator system with four boats and two shutters and a sample holder on
the top.
2.4.3. Active Layer Deposition on ITO Substrate
A poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Sigma Aldrich,
USA) layers were spin-coated at onto the pre-cleaned ITO substrate and cured in vacuum. The
ITO-coated glass substrate with a layer of PEDOT:PSS serves as the transparent anode through
which light is incident on the device. For the preparation of solar cells donor materials such as
P3HT, P3OT, P3HT-OT and acceptor materials such as PCBM or QDs both have been taken in
the ratio of x:y with a concentration of z wt.% in chlorobenzene or tri-chlorobenzene or toluene
were dissolved by ultrasonication. The active layer was spin casted from these solutions on the
top of PEDOT:PSS layer in glove box, followed by annealing. Finally, Aluminum (Al, 150 nm,
for solar cell) or Au (200 nm, for hole transport study) contacts were deposited via thermal
evaporation through a shadow mask at 2×10-6
Torr. The device active area is ~0.1 cm2 for all the
devices discussed in this work.
2.5. CHARACTERIZATION TECHNIQUES
This section describes the characterization techniques used for deciphering the structure (XRD,
TEM, and HRTEM), spectroscopic (UV-Visible Photoluminescence, and FTIR), electrical
properties of the polymer and polymer/nanocrystal hybrid system.
48
2.5.1. UV-Visible Absorption Spectra
The absorption of ultraviolet (200-400 nm)/visible (400-800 nm) radiation [9, 10] by a material is
caused by the transitions between the electronic energy levels of the molecules of that material.
When electrons are excited from one energy band to other by making optical transitions that are
dictated by selection rules it is called inter-band absorption [10]. Figure 2.6 shows the Inter-band
optical absorption from initial state to the final state of a molecule.
Figure 2.6 Inter-band optical absorption between an initial state Ei to the final state Ef.
Experimental setup of absorption
Absorption spectroscopy is a technique where the intensity of a beam of light measured before
and after interaction with a sample is compared as a function of wavelength. There are four main
components of a spectrophotometer: (1) a light source which is usually a tungsten filament or gas-
discharge lamp. (2) A monochromator; the input to the monochromator is the broadband light
from the light source; the output is tunable and highly monochromatic light. (3) A sample
chamber which holds the sample under investigation and (4) a detector which measures the
amount of light that passes through the sample. Typically, detectors are either solid state
photodiodes (silicon, germanium, etc.) or photomultiplier tubes. The basic setup for measuring the
absorption or transmission of light through a sample is shown in Figure 2.7. When light of some
wavelength λ with intensity Io passes through the sample the intensity of the light is reduced to a
value I, due to absorption within the sample and reflection at the surfaces of the sample.
Comparison of Io and I, can be used to determine the transmission of the sample at wavelength λ.
In addition to transmission, another useful way to report the optical absorption is in optical
absorbance or optical density. Absorbance (A) is a dimensionless quantity defined as the negative
of the base-ten logarithm of the transmission (T) [11].
TA 10log
Chapter 2
49
Figure 2.7 Light of intensity Io incident upon a sample undergoes a loss in intensity upon passing
through the sample. The intensity measured after passing through the sample is I.
For the experimental absorption spectra measurements of polymer and
polymer/nanocrystals, thin films have been prepared by spin coating from chlorobenzene solution
on to a glass substrate. The UV-Visible absorption spectra have been recorded by Shimadzu UV-
1601 spectrophotometer. The schematic is shown in Figure 2.8.
Figure 2.8 Schematic of double beam UV-Visible spectrometer.
50
2.5.2. Photoluminescence
When the light of sufficient energy is incident on a material, photons are absorbed and electronic
excitations are created. Photo-excitation causes electrons within the material to move into
permissible excited states. When these electrons return to their equilibrium states, the excess
energy is released by emission of light (a radiative process) or via a nonradiative process. If
radiative relaxation occurs, the emitted light is called photoluminescence (PL). The energy of the
emitted light is related to the difference in energy levels between the two electron states involved
in the transition between the emitted states and excited states.
Experimental Setup: PL is simple, versatile, and nondestructive. The instrumentation that is
required for ordinary PL work is modest: an optical source (laser), mirror, collection lenses,
optical power meter or spectrophotometer, and a photodetector. A typical PL set-up is shown in
Figure 2.9. For the PL spectra measurements of polymer and polymer/nanocrystals, thin films
have been prepared by spin coating from chlorobenzene solution on to a glass substrate. PL
measurement was carried out at room temperature. The samples were excited with the wavelength
of 510 nm optical beam and the PL signal was detected with the Perkin Elmer LF 55 having
Xenon source spectrophotometer (in the wavelength region of 530–850 nm).
Figure 2.9 (a) Luminescence process and (b) schematic diagram of the vibrational electronic
transitions in a molecule between the ground state and an excited state (1) absorption (2) non-
radiative relaxation (3) emission (4) non-radiative relaxation [12, 13].
Chapter 2
51
Figure 2.10 Typical schematic diagram and experimental setup for PL measurements.
2.5.3 Fourier Transforms Infrared (FTIR) Spectroscopy
Infrared spectroscopy is powerful tool for the confirmation of functional groups present in the
compound. Infrared radiation spans a section of the electromagnetic spectrum having frequency
range 3x1012
- 3x1014
Hz. The infrared spectroscopy involves the absorption of infrared radiation,
which results in changes in the vibration energy levels of a molecule. Since, usually all molecules
will be having vibrations in the form of stretching, bending, etc., the absorbed energy will be
utilized in changing the energy levels associated with them. It is a valuable and formidable tool in
identifying organic compounds, which have polar chemical bonds (such as OH, NH, CH etc.)
with good charge separation (strong dipoles) [14].
Theory of Infrared Absorption: At temperatures above absolute zero, all the atoms in molecules
are in continuous vibration with respect to each other. The major types of molecular vibrations are
illustrated in Table 2.1. The frequency of vibration ʋ is given by
k
c2
1
Where c is the velocity of light, k is the force constant and µ is the reduce mass.
52
Table 2.1 The major types of molecular vibrations.
The two conditions that must be fulfilled for infrared absorption to occur are (1) the
frequency of a specific vibration of a molecule is equal to the frequency of the incident infrared
radiation and (2) the vibration must entail a net change in the dipole moment of the molecule.
Absorbed infrared radiation leads to the change in the amplitude of molecular vibration.
Molecules composed of several atoms, vibrate not only according to the frequency of the bonds
but also with overtones of these frequencies. When one bond vibrates, the rest of the molecule is
also involved. The harmonic vibrations have frequency which is approximately integral multiple
of a fundamental frequency. A combination band is the sum or difference between the frequencies
of two or more fundamental or harmonic vibrations. The uniqueness arises from those bands
which are characteristics of whole molecule. The intensity of infrared absorption is proportional
to square of the rate of the change of dipole moment with respect to displacement of atoms.
The basic components of an FTIR are shown schematically in Figure 2.11. The infrared
source emits a broad band of different wavelength of infrared radiation. The infrared radiation
goes through an interferometer that modulates the infrared radiation. The interferometer performs
an optical inverse fourier transform on the entering infrared radiation. The modulated infrared
beam passes through the sample where it is absorbed to various extents at different wavelengths
by the various molecules present. Finally the intensity of the infrared beam is detected by a
detector, the detected signal is digitized and Fourier transformed by the computer to get the I
infrared spectrum of the sample gas.
Figure 2.11 Basic components of FTIR.
In the present investigation the FTIR spectra of P3HT, P3OT, P3HT-OT, P3HT-CdTe and P3HT-
CdS films having equal thickness, were recorded on Nicolet 5700 in transmission mode in the
wavenumber range 400-4000 cm-1
.
Chapter 2
53
2.5.4 Thermal Analysis
Thermal analysis involves the study of rate and temperature at which materials undergo physical
and chemical transitions as they are heated and cooled. This is accompanied by the change in
energy and weight involved during the process. Thermogravimetric analysis is the branch of
thermal analysis which examines the mass change of a sample as a function of temperature in the
scanning mode or as a function of time in the isothermal mode. Thermogravimetric is used to
characterize the decomposition and thermal stability of materials under a variety of conditions and
to examine the kinetics of the physicochemical processes occurring in the sample. The mass
change characteristics of a material are strongly dependent on the experimental condition
employed. Factors such as samples mass, volume and physical form, the shape and nature of
sample holder, the nature and pressure of atmosphere in the sample chamber, and the scanning
rate, all have important influences on the characteristics of the recorded thermogravimetric curve.
Thermogravimetric curves are recorded using a thermo balance. The principal elements of a
thermo balance are – an electronic microbalance, a furnace, a temperature programmer and an
instrument for simultaneously recording the outputs from these devices. In the present
investigation the thermogravimetric analysis of P3HT, P3OT and P3HT-OT have been carried out
using Mettler Toledo TGA 851e in nitrogen atmosphere with a flow rate of 60 mL/min. To study
the complete thermal behavior, samples have been heated from 25-700°C with heating rate
10°C/min so that every volatile material could get detached from the samples.
Differential scanning calorimetry (DSC) is another thermal analysis technique in which
the difference in the amount of heat required to increase the temperature of a sample and
reference are measured as a function of temperature. Both the sample and reference are
maintained at very nearly the same temperature throughout the experiment. The basic
experimental set up used for measurement of DSC has been shown in Figure 2.12.
The basic principle underlying this technique is that, when the sample undergoes a physical
transformation such as phase transitions, more (or less) heat, will need to flow to it from the
reference to maintain both at the same temperature.
54
Counter
electrode
Working
electrode
Reference
electrode
Analyte &
electrolyte
Nitrogen tank
Figure 2.12 Basic set-ups for DSC measurement [15].
2.5.5. Electrochemical Studies: Cyclic Voltammetry
All cyclic voltammetry (CV) data were obtained using a three electrode cell assembly as shown in
Figure 2.13. Experiments have been performed using an Autolab 30, Potentiostat/Galvanostat in
acetonitrile solution containing, 0.1 M tetra-n-butylammonium-tetrafluoroborate (TBATFB) at
scan rate 20 mV/s. The Ag/AgCl has been used as the reference electrode while Pt as a counter
electrode. Pt has been used as the working electrode on which the polymer films have been
deposited by drop coating and dried in vacuum at 120 ˚C.
Figure 2.13 Experimental setup for the electrochemical studies.
Chapter 2
55
2.5.6. X-Ray Diffraction Spectroscopy
X-ray diffraction (XRD) is a material characterization technique that can be useful to characterize
the crystallographic structure, crystalline size (grain size) and preferred orientation in
polycrystalline or powder solid sample. It may also be used to characterize heterogeneous solid
mixture to determine the relative abundance of crystalline compound and when coupled with the
lattice refinement technique such as relative refinement, can provide the structure information in
unknown sample [16].
Basic Theory: Diffraction and Bragg’s Law
Diffraction can occur when any electromagnetic radiation interacts with a periodic structure. The
repeat distance of the periodic structure must be about the same wavelength of the radiation. In
crystals, the ions or molecules are arranged in well-defined positions in planes in 3-dimensions.
X-rays have wavelengths of the order of inter-atomic distance in crystalline solids; which make
them appropriate for diffraction from atoms of crystalline materials.
Figure 2.14 Bragg’s diffraction law.
Figure 2.14 schematically shows Bragg‟s law of diffraction. Two beams with identical
wavelength and phase approach a crystalline solid and are scattered by two different atoms within
it. The lower beam traverses an extra length of 2dsinθ. When X-rays are scattered, they can
constructively interfere, producing a diffracted pattern. The relationship describing the angle at
which a beam of X-rays of a particular wavelength diffracts from a crystalline surface was
discovered by Sir William H. Bragg and Sir W. Lawrence Bragg and is known as Bragg‟s Law of
diffraction, and given by [16-19]
nd sin2
56
Where λ is the wavelength of the X-ray, θ is the angle between incident ray and surface of the
crystal, d is the inter-plane spacing and constructive interference occurs when n is the integer.
The mean size of the NCs is determined from the peak broadening in the XRD pattern by
using the Debye-Scherrer equation. In Figure 2.15 the rays A, D and M make precisely this angle
with the reflecting planes. Ray D′, scattered by the first plane below the surface, is one
wavelength out of phase with A′, ray M′ is m wavelengths out of phase with it. At the diffraction
angle 2θB all these rays are in phase and unite to form a beam of maximum amplitude. Ray B
makes a slightly larger angle θ1 with the reflecting plane, such that ray L′ from the mth
plane is (m
+ 1) wavelengths out of phase with B′. This means that in the middle of the crystal there is a plane
scattering, a ray that is exactly an integer plus one-half wavelength out of phase with B′. So the
rays scattered by the upper half of the crystal cancel exactly with those scattered by the lower half
of the crystal and θ1 is the smallest angle where complete destructive interference occurs. This is
also the case for an angle θ2 which is a bit smaller than θB so that the path difference between the
ray scattered by the first and the last plane is (m − 1) wavelengths. These are the two limiting
angles where the intensity of the diffracted beam drops to zero.
Figure 2.15 Scattering from a finite number of equidistant planes.
Chapter 2
57
The width of diffraction curves increases as the thickness of the crystal decreases, because the
angular range (2θ1 − 2θ2) increases as m decreases. As a measure of the peak width, the full width
at half maximum FWHM, denoted by β, is used. As an approximation β = 1/2 (2θ1 − 2θ2) = θ1 −
θ2 is chosen, since this yields the exact FWHM for a Gaussian. The path difference equations for
these two angles related to the entire thickness of the crystal are given by:
2t sin θ1 = (m + 1)λ
2t sin θ2 = (m − 1)λ
Subtracting the above equations yields:
t(sin θ1 − sin θ2) = λ
Since θ1 and θ2 are very close to θB it is reasonable to make the following approximations:
Using the definition of the FWHM introduced above gives a crystal depth t = m·d [20]:
d = 0.9λ / β cos θ
where, d is the average crystallite size (Å), λ is the wavelength of X-rays
(Cu Kα:), θ is the Bragg diffraction angle. By using the above equation one can calculate the size.
The one drawback of the above simple method is that it works only if stress-related and
instrument-related broadening are negligible in comparison to particle size effects. This condition
is often met with particle sizes that are in the 10 - 100 nm range.
Incident X-ray θTransmitted X-ray
2θ
Figure 2.16 Schematic configuration of an X-ray diffraction machine.
58
In Figure 2.16 a schematic configuration of an XRD machine can be seen. The X-ray hits
the sample under an adjustable angle θ. The intensity of the reflected beam is measured with the
detector. The detector moves with a varying glancing angle θ on the measuring circuit in the way
that the angle between the beam direction and the detector is always 2θ. In the present
investigations the XRD patterns were recorded on D8 Advance X-Ray diffractometer (Bruker)
using Cu Kα: radiation λ = 1.5418 Å) in scattering range (2 θ) of 10-800 with a scan rate of
0.0250/sec and slit width of 0.1mm.
2.5.7. Scanning Electron Microscopy
Scanning Electron Microscopy (SEM) is a very useful technique and widely used to study the
surface morphology, surface topography, composition and other surface properties of the samples
and it offers a better resolution than that of optical microscope. It provides high-magnification and
can have resolution of a few nanometers [21].
In a typical SEM instrument, Tungsten or LaB6 is used to emit monochromatic electrons
with typical energy of 20-30 keV. These electrons are focused by condenser lenses to form a
beam with a very fine spot size ~ 1 to 5 nm. This beam passes through a pair of scanning coils in
the objective lenses, which deflects the beam in a raster fashion over the sample surface. This
beam of primary electrons interacts with sample volume ranging from less than 100 nm to 5 m
and generates secondary electrons (Figure 2.17). These secondry electron signals are detected by
appropriate detectors. The final image is produced on the screen through cathode ray tube. In the
present investigation, samples for SEM study were prepared by spin casting of material on a glass
substrate. A thin layer of precious metal was sputtered prior to loading the samples in the
microscope probe.
Another possible way in which a beam of incident electron can interact with an atom is by
the ionisation of an inner shell electron. The resultant vacancy is filled by an outer electron, which
can release its energy either via an Auger electron or by emitting an X-ray (Figure 2.17). This
produces characteristic lines in the X-ray spectrum corresponding to the electronic transitions
involved. Since these lines are specific to a given element, the composition of the material can be
deduced. This can be used to provide quantitative information about the composition near the
surface and is known as Energy Dispersive Auger X-ray (EDAX) Spectroscopy.
Chapter 2
59
Figure 2.17 Schematics of Scanning Electron Microscope.
2.5.8 Transmission Electron Microscopy
Transmission electron microscopy (TEM) is a powerful tool for doing structural and
morphological characterization of materials in the micron, nanometer and subnanometer regimes.
TEMs offer information about morphology (the size, shape and arrangement of the particles),
crystallographic information (the arrangement of atoms in the specimen and their degree of order,
detection of atomic-scale defects in areas a few nanometers in diameter), and compositional
information [22]. Figure 2.18 shows the schematic diagram of a typical transmission electron
microscope [23].
Working principle: TEM works like a slide projector. A projector shines a beam of light which
transmits through the slide. The patterns painted on the slide only allow certain parts of the light
beam to pass through. Thus the transmitted beam replicates the patterns on the slide, forming an
enlarged image of the slide when falling on the screen. TEMs work the same way except that they
shine a beam of electrons (like the light in a slide projector) through the specimen (like the slide).
However, in TEM, the transmission of electron beam is highly dependent on the properties of the
material being examined. Such properties include density, composition, etc. For example, porous
material will allow more electrons to pass through while dense material will allow less. As a
60
result, a specimen with a non-uniform density can be examined by this technique. Whatever part
is transmitted is projected onto a phosphor screen for the user to see.
Figure 2.18 Schematics of Transmission Electron Microscope.
Figure 2.19 Electron source of a TEM
Chapter 2
61
A key requirement for TEM samples is the electron transparency, as a thick sample would
cause too many interactions leaving no intensity in the transmitted beam. A thick sample also
increases the risk that an electron is scattered on multiple occasions and the resulting image would
be difficult to interpret. In the present work, samples have been prepared by dispersing sample in
ethanol or chloroform using sonification and a small drop of that solution was casted onto the
carbon coat copper grid. The images were taken using a Tecnai G2 F30 S-Twin instrument
operated at an accelerating voltage of 300 kV, having a point resolution of 0.2 nm and a lattice
resolution of 0.14 nm.
2.5.9 I-V Characterization Technique
In order to calculate the different parameters of a solar cell, it is desirable to measure the I-V
characteristics under dark and light, which can give information about the VOC, JSC, FF efficiency
as well as defect states and transport properties of the material. For electrical property
measurements, using I-V technique it is necessary to make provisions for electrical contacts which
requires: (1) probe station with needles and sometimes with a microscope attached, to probe very
small devices, (2) a source meter to apply voltage and measure current or vice versa, (3) a
computer with appropriate program to collect data and analyze them. Figure 2.20 shows a
schematic of our J-V setup.
2.5.10 Temperature Dependent I-V Measurements Set Up
For temperature dependent I-V measurements, a Janis cryogenic system model Wilmington, MA
01887 has been used which can go from 20K to 325K with pressurized Helium gas. For I-V
measurements the device has been loaded into the cryostat with proper contact as shown in Figure
2.21. The cryogenic system is connected with a rotary pump. The sample in the cryostat was
connected to the Keithley‟s source measure unit for biasing the device. Data has been collected
with a computer connected to the source meter with GPIB connector.
62
Figure 2.20 Schematic representation of experimental arrangement of current-voltage
measurements of solar cell.
Figure 2.21 Schematic of temperature dependent current-voltage measurements setup.
Chapter 2
63
References
[1] R. Singh, J. Kumar, R. K. Singh, R. C. Rastogi and V. Kumar, New Journal of Physics 9
(2007) 40.
[2] R. K. Singh, J. Kumara, R. Singh,R. Kant, S. Chand, V. Kumar Materials Chemistry and
Physics 104 (2007) 390.
[3] M. T. Khan, M. Bajpai, A. Kaur, S. K. Dhawan, and S. Chand Synthetic Metals 160 (2010)
1530.
[4] M. T. Khan, A. Kaur, S. K. Dhawan, S. Chand J. Appl. Phys. 109 (2011) 114509.
[5] M. T. Khan, A. Kaur, S. K. Dhawan, S. Chand J. Appl. Phys. 110 (2011) 044509.
[6] T. Nakanishi, B. Ohtani, K. Uosaki, J. Phys. Chem. B 102 (1998) 1571.
[7] T. Tsuruoka, K. Akamatsu, H. Nawafune, Langmuir 20 (2004) 25.
[8] M. T. Khan, R. Bhargav, A. Kaur, S. K. Dhawan, S. Chand, Thin Solid Films 519 (2010)
1007.
[9] P. Atkins, J. de Paula, Physical Chemistry, (Oxford University Press), 7th
Edition, (2002) 291.
[10] C. N. Banwell, E. M. McCash, “Fundamentals of Molecular Spectroscopy”, (Tata McGraw-
Hill Publishing Company Limited, New Delhi), 4th Edition, (1994).
[11] www.physicscourses.okstate.edu
[12] Mark Fox, “Optical absorption of solids”, Oxford University Press Inc., (2001).
[13] Ph.D. dissertation of M. A. I. Arif the Faculty of the Graduate School University of Missouri-
Columbia August 2007.
[14] H. F. Shurvell in Handbook of vibrational spectroscopy, Ed., J. M. Chalmer and P. R. Griffith,
John Willey and Sons, Ltd. Vol. 3, 2002, 1783.
[15] http://www.mmsconferencing.com/pdf/eyp/c.rawlinson.pdf
[16] L. V. Azarof, X-ray diffraction, McGraw Company, 1974.
[17] Charles Kittel, Introduction to Solid State Physics, 7th Edition, John Wiley and Sons, Inc.
[18] A J Dekkar, Solid State Physics, Macmillan India Limited, 2000.
[19] M. Ali Omar, Elementary solid state physics: principles and applications, (Pearson Education,
1999)
[20] A. L. Patterson, Phys. Rev. 56 (1939) 978.
[21] G. Lawes, Scanning electron microscopy and X-ray microanalysis: Analysis chemistry by
open learning, John Willey and Sons, 1987.
64
[22] A. P. Rambu, L. P. Curecheriu, G. Mihalache based on the lecture of Prof. Andrew Watt,
High Resolution Electron Microscopy of Soft Condensed Matter Systems, Physics of Advanced
Materials Winter School 2008.
[23] http://www.hk-phy.org/atomic_world/tem/tem02_e.html
[24] D. B. Williams, Transmission electron microscopy, A textbook for material science, Plenum
Press. New York and London, 1996.
65
CHAPTER 3
STUDY OF THE PHOTOVOLTAIC PERFORMANCE OF COPOLYMER POLY[(3-
HEXYLTHIOPHENE)-CO-(3-OCTYLTHIOPHENE)]
3.1 INTRODUCTION
3.2 RESULT AND DISCUSSION
3.2.1 FTIR Spectra
3.2.2 1H NMR Spectrum
3.2.3 Thermal Studies
3.2.4 XRD Studies
3.2.5 Evaluation of Energy Levels
3.2.6 UV–Vis Absorption
3.2.7 Photoluminescence Quenching With Respect to Different P3AT:PCBM
Ratios
3.2.8 J-V characteristics of Solar Cells
3.3. CONCLUSIONS
Reference
3.1 INTRODUCTION
oly(3-hexylthiophene) (P3HT) and poly(3-octylthiophene) (P3OT) are the conjugated
polymers, well known [1-6] to be used in polymer solar cells as electron donor
materials. Owing to its high regio-regularity and high mobility, P3HT is so far
extremely attractive donor material in combination with [6, 6]-phenyl C61 butyric acid methyl
ester (PCBM) as the electron acceptor. Power conversion efficiency ɳ ~ 6% has already been
realized [7] in polymer solar cells based on P3HT:PCBM donor:acceptor interpenetrating bulk
heterojunction (BHJ) with suitable charge transport and collection interface layers. However,
most of the P3OT is used in combination with carbon nanotubes (CNTs) rather than the PCBM.
This may be due to its energetic compatibility with CNTs. There are hardly any significant reports
in literature about P3OT:PCBM combination based solar cells. It may be primarily due to lower
[8] hole mobility of P3OT as compared to P3HT [9].
P
66
The chemical nature and the length of the alkyl side chains have a great effect on the
charge carrier mobility in poly(3-alkylthiophenes) (P3ATs) [10, 11]. In general, the attachment of
branched, bulky side chains led to a low crystallinity of the solid layers. Also, the π-π overlap
distance between the conjugated backbones within the main chain layers is larger in these
polymers, resulting in low carrier mobility [10, 12]. For linear alkyl chains, it is observed that the
mobility decreases with increasing alkyl chain length [11]. This has been attributed to the
isolating nature of the alkyl substituent [13]. In fact, the largest carrier mobility reported for P3OT
in field effect transistor (FET) configuration is 10-3
cm2/Vs [14], approximately 1-2 orders of
magnitude lower than the typical mobilities of P3HT. However, a critical length of the alkyl side
chain is needed for a sufficient solubility and processability of the polymer from solution. For
example, higher-molecular-weight batches of regioregular P3HT are well soluble in chlorinated
solvents such as chloroform, toluene but only weakly soluble in non-chlorinated solvents such as
toluene or xylene. On the other hand, P3OT dissolves quickly in toluene at room temperature. At
the moment, P3HT is considered to present the best compromise with respect to solubility, layer
formation, and overall photovoltaics performance.
Babel and Jenekhe presented binary blends of semiconducting polymers as a novel
approach to tune the properties of polymer FETs [15, 16]. In the first set of experiments, a series
of 10 binary blends of regioregular poly(3-hexylthiophene)s and poly(3-decylthiophene)s have
been prepared and the dependence of the charge carrier mobility on the blend composition has
been studied [15]. They found that the field-effect mobility of these blends relatively higher
(2×10-3
cm2/Vs) and constant over a broad composition range (5-80 wt % of poly(3-
decylthiophene)).
An alternative approach to combine desirable properties of two polymers is by
copolymerization of the respective monomer units. In the present investigations, in order to
incorporate both the features of better solubility plus mobility within a single component, the
regioregular copolymer poly[(3-hexylthiophene)-co-(3-octylthiophene)] (P3HT-OT) has been
used in combination with PCBM in organic solar cells. The molar ratio of 3-hexylthiophene
(3HT):3-octylthiophene (3OT) is 50:50 in copolymer P3HT-OT. The device performance based
on P3HT-OT is compared with the performances of devices based on homopolymers P3HT and
P3OT.
Chapter 3
67
Figure 3.1: Structural formula of homopolymers (a) P3HT (b) P3OT and (c) copolymer P3HT-
OT.
3.2. RESULT AND DISCUSSION
3.2.1. FTIR Spectra
Fourier transform infrared spectroscopy (FTIR) spectra have been recorded on Nicolet 5700 in
transmission mode, wavenumber range 400-4000 cm-1
with a resolution of 4 cm-1
performing 32
scans. The FT-IR spectra of P3HT, P3OT and P3HT-OT are shown in Figure 3.2. A comparative
study of the FT-IR spectra of P3ATs polymer synthesized for the present investigation with those
reported earlier for P3AT synthesized by various routes [19] shows the quality of P3ATs. The
reported band for aromatic CH out of plain vibration is at 820 to 823 cm−1
, which is the
characteristics of 2,5-disubstituted-3-alkylthiophene for rr-P3AT whereas the corresponding band
for rdm-P3AT occurs at 827 to 830 cm−1
[20, 21].
The aromatic CH out of plain vibration in the present study has been observed in
between the 820 to 822 cm-1
(Table 3.1), which confirms the regioregularity of homo polymers
P3HT, P3OT as well as copolymer P3HT-OT. Strong absorption bands of P3HT-OT at 2952,
2921 and 2852 cm-1
have been assigned, respectively, to the asymmetric C–H stretching
vibrations in –CH3 and –CH2–, and the symmetric C–H stretching vibration in –CH2–. They have
been ascribed to the alkyl-side chain. The bands at 1457, 1374 cm_1
are due to the thiophene ring
stretching and methyl deformation respectively. The C-C vibrations appear at 1165 and 1088
cm_1
. The absorption at 720 cm_1
is assigned to the methyl rocking. A measure of the conjugation
length can be determined by FTIR spectra. The intensity ratio of the symmetric FTIR band at
~1460 cm-1
to the asymmetric band at ~1510 cm-1
C=C ring stretches decreases with increasing
conjugation length. For regioregular PATs this ratio is 6-9, less than half of the 15-20 value
68
measured for regiorandom samples [17-20]. In the present investigations we have observed this
ratio in the range of 6-9, for P3HT, P3OT and P3HT-OT, confirm their regioregularity.
Table 3.1 FTIR bands for P3HT, P3OT and P3HT-OT.
1000 1500 2000 2500 3000
20
40
60
80
100
2955
2952
2953
2916
2921
2921
2852.4
2852
2852
1509
1510
1508
1454
1454.6
1463
1377
1375
1374
722
720
723
822
822
820
% T
ran
smit
tan
ce
Wavenumber (cm-1)
P3HT
P3OT
P3HTOT
Figure 3.2 FT-IR spectrum of pristine P3HT, P3OT and copolymer P3HT-OT films.
3.2.2. 1H NMR Spectrum
NMR is a powerful tool for providing information concerning configuration and conformation of
polymer. It has been extensively used for studying regio-chemistry of P3AT. The main elements
of regio-chemistry of P3AT are thiophene dyad and triad configuration, which are shown in
Sample Aromatic
C-H
stretching
Aliphatic C-
H stretching Ring
stretchin
g
Methyl
deformation Aromatic
C-H out of
plane
Methyl
rocking
P3HT-
OT 3054.8 2952.8,
2921.0, 2852 1510.1,
1457.0 1374.7 822.8 720.2
P3OT 3053 2955, 2916.1,
2852.4 1509.6,
1463.5 1377.5 822.5 722.1
P3HT 3055.9 2953, 2921.7,
2852.8 1508.8,
1454.6 1375.6 820.4 723.9
Chapter 3
69
Figure 3.3. Thiophene triads are used to determine the configuration of polymer based on NMR
chemistry of β-proton (4-position) of thiophene ring. Dyad configurations are discussed in terms
of chemical shift of α-methylene-H of the alkyl side chain. 1H NMR spectra of all the polymers
used in the present investigation in CDCl3 solution at 300 MHz are shown in Figure 3.4. it has
been reported in the literature [18, 19] that in a regioregular, HT-PAT, there is only one aromatic
proton signal in the 1H NMR spectrum, due to the β-proton on the aromatic thiophene ring, at δ =
6.98, corresponding to only the HT-HT triad sequence. Proton NMR investigations of
regiorandom PAT reveal that four singlets exist in the aromatic region that can clearly be
attributed to the protons on the β-position of the central thiophene ring in each configurational
triad: HT-HT(δ = 6.98), TT-HT(δ = 7.00), HT-HH(δ = 7.03),and TT-HH(δ = 7.05) [18, 19]. In
this analysis the HT-HT, TT-HT, HT-HH, TT-HH couplings are readily distinguished by a 0.02-
0.03 ppm shift [Table 3.2(a)]. In the present investigations, β-proton aromatic thiophene ring
signal for P3HT, P3OT and P3HT-OT has been observed at 6.978, 6.977, 6.977 ppm,
respectively, which suggest the HT-HT coupling in these polymers.
The relative ratio of HT–HT coupling can also be determined by an analysis of the α-
methylene-H of the 3-substituent on thiophene. As per literature survey [19, 20], resonances in the
spectral region 2.5-3.0 ppm are attributed to of α-methylene-H of the alkyl side and are observed
to HH (2.58ppm) and HT (2.80ppm) [Table 3.2 (b)]. In case of all our polymers, resonances of α-
methylene-H are observed at 2.805ppm which further confirms the HT-HT coupling in these
polymers.
The same information can also be obtained from the β-methylene-H of the 3-substituent.
As shown in Table 3.2(b) the 1H NMR resonance for the HT coupled β-methylene-H appears at δ
=1.72 ppm [19], and that of the HH coupled β-methylene-H appears at δ =1.63 ppm. In the
present investigation, the β-methylene-H signal for P3HT, P3OT, and P3HT-OT has been
observed at 1.704 ppm, 1.708ppm and 1.707ppm respectively. These results again indicate that
polymers having HT–HT couplings. Resonances due to methyl protons are reported in literature
in the spectral region 0.885-0.912 ppm [19, 20]. In the present study these resonance have been
observed at δ= 0.912, 0.884 and 0.887 ppm for P3HT, P3OT, and copolymer P3HT-OT,
respectively. Resonance at 0.887ppm, 1.293ppm, 1.707ppm and 2.806ppm of copolymer are
broader and seem doublet like structure, because methyl proton of both hexyl and octyl side chain
overlap. The doublets in copolymer further indicate the copolymerization of 3HT and 3OT.
70
HT TT HH
HT-HT HT-HH
HH-TT TT-HT
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
Figure 3.3 Dyad and triad configuration of P3HT.
Table 3.2 Chemical shift of (a) β-H (4-position) of thiophene ring and (b) α and β -methylene-H
of the alkyl side chain [19, 20].
Head-to-tail Head-to-head
α-methylene-H 2.80 2.58
β-methylene-H 1.72 1.63
Linkage β H4
HT-HT 6.98
TT-HT 7.00
HT-HH 7.02
TT-HH 7.05
(a) (b)
Chapter 3
71
Figure 3.4 (a) 1H NMR spectra of P3HT.
Figure 3.4 (b) 1H NMR spectra of P3HT-OT.
(a)
(b)
P3HT
P3HT-OT
72
Figure 3.4 (c) 1H NMR spectra of P3OT.
3.2.3. Thermal Studies
Prior to thermogravimetric analysis (TGA) measurements, materials have been dried in vacuum at
elevated temperatures to remove residual solvent/moisture. Dynamic TGA has been carried out on
a METTLER TOLEDO, TGA/SDTA 851e with heating rate of 10
0C/min under nitrogen
atmosphere to assess the thermal stability of the polymers. Differential scanning calorimetry
(DSC) measurement has been performed on a METTLER TOLEDO, DSC822e with heating rate
of 100C/min under nitrogen atmosphere. Thermal stability of the polymers is generally reported as
the temperature at which 5% weight loss has been observed. Figure 3.5(a) shows the TGA graph
of P3HT, P3OT and copolymer P3HT-OT. As shown in TGA graph, the onset point of weight
loss for P3HT, P3HT-OT and P3OT are observed at 440 ºC, 434ºC and 427 ºC, respectively,
indicating that all the polymers have good thermal stability. From above results it has been
concluded that long alkyl side group P3OT decompose at lower temperature than short alkyl side
group P3HT, also thermal stability of copolymer P3HT-OT is in-between of the two
homopolymers. The weight losses in polymers have been observed due to decomposition of the
alky side groups. As long alkyl side group decompose at lower temperature as compared to short
alkyl group, this is why P3HT is more stable than other two polymers.
(c)
P3OT
Chapter 3
73
75 150 225 300 375 450 525
30
45
60
75
90%
Weig
ht
loss
Temperature (0
C)
P3OT
P3HT
P3HTOT
(a)
50 100 150 200 250 300
-1.6
-1.2
-0.8
-0.4
Hea
t F
low
(m
W)
Temperature (0
C)
P3HT
P3HTOT
P3OT (b)
Figure 3.5 (a) TGA and (b) DSC graph of P3HT, P3OT and copolymer P3HT-OT
DSC scan of the polymers are shown in Figure 3.5(b). In the DSC of the copolymer
P3HT-OT, two melting transitions with endothermic peaks at 164 ºC and 228 ºC were observed.
The observed two melting transitions are characteristic of its copolymer architecture composed of
P3HT and P3OT (as suggested in Figure 3.1(c)), which have melting transitions at 215ºC and
186ºC, respectively.
3.2.4. XRD Studies
Figure 3.6 shows X-ray diffraction (XRD) pattern of solution cast films of all the polymers,
precured at 120°C. The strong first order reflections, (100), of P3HT, P3HT-OT, and P3OT, are at
2Ɵ angle 5.08°, 4.7
°, and 4.24
°, correspond to interlayer spacing 17.38 Å, 18.786 Å, and 20.83 Å,
respectively [20]. Observed intensity of copolymer has decreased compared to P3HT, and P3OT,
may be due to random structure (random structure of copolymer is attributed to the random
repeating of hexyl, and octyl group attached to the polymer matrix) of copolymer P3HT-OT. The
second order reflection (200) of P3HT, P3HT-OT, and P3OT are observed at 2Ɵ angle 10.52°,
9.48°, and 8.62
° corresponding to interlayer spacing 8.40 Å, 9.34 Å and 10.25 Å, respectively.
Observed dP3HT-OT values (18.786 Å, and 9.34 Å) in the copolymer P3HT-OTare smaller than the
homopolymer P3OT and larger than the homopolymer P3HT, suggesting partial inter-digitation
between the side chains and/or the occurrence of tilting of the octyl chains in P3HT-OT. XRD
study shows that the interlayer spacing increases with elongation of alkyl side chain. This shows
that the stacks of planer thiophene main chain were uniformly spaced by alkyl side chain.
Copolymer P3HT-OTshows two strong peaks at 2Ɵ angle 16.860°, 14.04° which corresponds to
different two d020 values of 5.254 Å and 6.303 Å respectively. The 6.303 Å spacing is due to the
74
interlayer stacking distance between P3OT in a layered packing structure (dP3OT), whereas the
5.254 Å spacing is corresponds to the interlayer stacking distance between P3HT (dP3HT). These
peaks confirm the formation of copolymer P3HT-OT.
Table 3.3 d-values corresponds to different 2θ angles of P3HT, P3OT and P3HT-OT
P3HT P3OT P3HT-OT
2Ɵ d (A0
) 2Ɵ d (A0
) 2Ɵ d (A0
)
5.080 17.381 4.24 20.823 4.700 18.786
10.520 8.402 8.620 10.250 9.480 9.341
16.00 5.535 9.480 6.725 14.040 16.860
6.303 5.254
5 10 15 20 25
(020)
(100)
P3HT
2 (degree)
(020)
dP3OT
(100) P3HTOT
Lin
(C
ou
nts
)
(020)
d
P3HT
(100)
P3OT
Figure 3.6 XRD spectra of solution cast polymer films, annealed at 120 0C.
3.2.5. Evaluation of Energy Levels
The electronic energy levels, highest occupied molecular orbital (HOMO) level, and lowest
unoccupied molecular orbital (LUMO) level, of polymers are one of the most significant
properties for polymer solar cells. However, their values differ significantly in different literature
reports. Cyclic voltammetry has been performed to estimate the HOMO and LUMO levels of all
Chapter 3
75
the synthesized polymers. Cyclic voltammetry of synthesized polymers and their copolymer have
been carried out on the surface of Pt by applying the potential in the range -1.5 to 1.5 V.
Experiment has been performed using an Autolab 30, Potentiostat/Galvanostat in acetonitrile
solution containing, 0.1 M tetra-n-butylammonium-tetrafluoroborate (TBATFB) at scan rate 20
mV/s. The Ag/AgCl has been used as the reference electrode while Pt as a counter electrode. The
cyclic voltammgram of chemically synthesized polymers film on the surface of Pt have been
shown in Figure 3.7.
-1.0 -0.5 0.0 0.5 1.0 1.5
-0.0008
-0.0006
-0.0004
-0.0002
0.0000
0.0002
0.0004
0.0006
Cu
rren
t (A
)
E/V vs Ag/AgCl
P3OT
Figure 3.7. Cyclic voltammograms of P3HT, P3OT and P3HT-OT thin films in 0.1 mol of
TBATFB-acetonitrile solution, with a scan rate of 20 mV/s. Pt and Ag/AgCl have been used as
working and reference electrode, respectively.
The HOMO level has been calculated from the oxidation onset according to the equation
[21-23]:
eVEeHOMO ox
AgClAgvsonset )71.4( )/.(
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-0.00045
-0.00030
-0.00015
0.00000
0.00015
0.00030
Cu
rren
t (A
)
E/V vs. Ag/AgCl
P3HT
-1.0 -0.5 0.0 0.5 1.0 1.5
-0.0012
-0.0009
-0.0006
-0.0003
0.0000
0.0003
0.0006
P3HTOT
Cu
rren
t (A
)
E/V vs. Ag/AgCl
76
The oxidation onsets of P3HT, P3OT and P3HT-OT and the corresponding HOMO levels
calculated in this way are listed in Table 3.4. The LUMO level (electron affinity) can in principle
be calculated using the reduction onset; however, these measurements were difficult to perform
reliably for our materials. We have therefore estimated the electron affinity simply by subtraction
of the band gap energy Eg from the ionization potential following gHOMOLUMO EEE . There are
significant uncertainties inherent in this method, for example due to the neglect of excitonic
binding energy and other screening effects. Nevertheless the trends between different materials
are of interest, as shown in Table 3.4.
Table 3.4 Optical and Electrochemical Properties of P3HT, P3HT-OT and P3OT.
3.3.6. UV–Vis Absorption Spectra
UV-Vis absorption spectra of all the polymers have been recorded by Shimadzu UV-1601
spectrophotometer. Absorption spectra of all the polymer thin films are shown in Figure 3.8(a). In
conjugated polymers, the extent of conjugation directly affects the observed energy of the π-π*
transition, which appears as the maximum absorption [24]. The wavelengths of maximum
absorption (λmax) in the solid films of the P3HT, P3HT-OT, and P3OT have been observed at 518
nm, 512 nm, and 511 nm, respectively. The blue-shift in the absorption of the P3HT-OT and
P3OT with respect to P3HT has been attributed to steric hindrance of octyl side chain attached to
these polymer matrixes. This octyl side chain may be difficult to rotate compared to hexyl side
chain to form the more advantageous arrangement. Polymer film also shows an absorption
shoulder at 600nm, 595nm, and 598nm for P3OT, P3HT-OT, and P3HT, respectively, which are
assigned to the interchain excitation and 1Bu vibronic sidebands [24, 25] and confirm the
interchain absorption in these polymers [26, 27]. Most remarkably the intensity of the shoulder at
600 nm drops substantially when going from P3HT to P3HT-OT to P3OT, which indicates the
decrease of the interchain interaction between these polymers.
Material Eox onset vs. Ag/AgCl (V) HOMO (eV) Eg optical (eV) LUMO (eV)
P3HT +0.56 -5.27 1.9 -3.37
P3HT-OT +0.60 -5.31 1.99 -3.32
P3OT +0.64 -5.35 1.95 -3.4
Chapter 3
77
Figure 3.8 UV-visible absorption spectra of all polymers (a) thin solid films on glass substrate
and (b) solution in toluene.
Figure 3.8(b) shows the absorption spectra of all the polymers in toluene solution. The
maximum absorption of P3OT, P3HT-OT, and P3HT in toluene appeared at 445 nm, 450 nm, and
457 nm, respectively, which have been attributed to HOMO (π)- LUMO (π*) transition [24]. The
absorption spectra of polymer solutions showed blue-shift with respect to the solid films. The blue
shift in the solution is attributed to coil like structure in solution whereas solid films have rod like
structure. Coil like structure have short effective conjugation length as compared to rod like
structure. This results in decrease of π-π stacking and blue shift in solution phase.
The effect of thermal annealing on the absorption spectrum of P3HT-OT film has been
also studied. The as-prepared film was annealed at 90 °C and 120 °C at an inert atmosphere for 10
min, respectively. Figure 3.9 shows the changes in absorption spectra before and after annealing.
After annealing at 90 °C, the absorption of the films is broadened and red-shifted, and their
absorption intensity also increases. Annealing process also leads to bathochromic shift of the
absorption band edges, resulting in narrowed bandgaps, which are useful for better light
absorption.
When annealing temperature rose to 120 °C, the absorption spectrum became more
featured and exhibited a faint vibration structure at 600 nm, indicating its more regular
arrangements. The thermochromism effect indicates that some steric rearrangement of the
polymer chains was further removed and conjugation degree has extended in P3HT-OT film after
thermal annealing. The similar behaviour has been also reported for homopolymers P3HT and
P3OT [27-31].
300 350 400 450 500 550 600
0.0
0.5
1.0
1.5
2.0
2.5
Ab
sorp
tio
n
Wavelength (nm)
P3HT
P3HTOT
P3OT
(b)
300 400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Ab
sorp
tio
n
Wavelength (nm)
P3HT
P3HTOT
P3OT
(a)
78
300 375 450 525 600 675
0.0
0.2
0.4
0.6
0.8
Ab
sorp
tio
n
Wavelength (nm)
As prepared
annealed at 900C
annealed at 1200C
Figure 3.9 Absorption spectra of annealed P3HT-OT thin films.
For studying the inter donor-acceptor charge transfer process, the blends of PCBM with
P3HT-OT has been prepared. In Figure 3.10 the absorption spectra of P3HT-OT, PCBM, and
blend of P3HT-OT/PCBM all in toluene solution are reported. The absorption spectrum of P3HT-
OT has main peaks at ∼ 449 nm, PCBM spectra shows a peak at ∼ 330 nm and then decays
smoothly in the visible region with a pronounced shoulder. The P3HT-OT/PCBM blends show
more complex shapes where the main peaks of the component materials can be identified,
however, the resultant intensity is not in agreement with a linear combination of the intensity of
polymer and PCBM.
300 375 450 525 600
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Ab
sorp
tio
n
Wavelength (nm)
P3HTOT:PCBM1:0
P3HTOT:PCBM1:1
P3HTOT:PCBM1:2
P3HTOT:PCBM0:1
Figure 3.10 Absorption spectra of P3HT-OT, PCBM and P3HT-OT/PCBM blends in proportion
of 1:1 and 1:2 in toluene solution.
The absorption spectra of the blends of P3HT/PCBM and P3OT/PCBM in same
composition ratios as discussed above, are shown in Figure 3.11(a) and Figure 3.11(b),
Chapter 3
79
respectively. The absorption of P3HT and P3OT shows main peaks at ∼ 457 nm and at ∼ 445 nm,
respectively. The 1:1 composites of P3HT/PCBM and P3OT/PCBM show the main peaks at ∼
330 nm and at ∼ 332 nm, respectively. For the 1:2 composites of P3HT/PCBM and P3OT/PCBM
the main peaks are slightly shifted towards the shorter wavelength and have been observed at ∼
329 nm for both the composites, whereas the position of the fullerene bands as well as the
polymer band edges remain nearly uninfluenced.
Figure 3.11 Absorption spectra of (a) P3HT/PCBM and (b) P3OT/PCBM blends in proportion of
1:1 and 1:2 in toluene solution.
3.3.7. Photoluminescence Quenching With Respect to Different P3AT:PCBM
Ratios
Induced donor-acceptor (D–A) charge transfer processes in P3AT/PCBM composites have been
detected by photoluminescence (PL) quenching. The PL spectra of P3ATs systems at different
PCBM ratios are shown in Figure 3.12. All pure polymers show a broad emission band peaked at
580 nm under the excitation wavelength of 450 nm.
In P3AT/PCBM blends of three polymers (P3HT, P3HT-OT, P3OT), PL relative to the
pristine polymer has been quenched upon addition of PCBM to the blends. The PL quenching in
polymers increases gradually by addition of PCBM, as shown in Figure 3.12. The magnitude of
the quenching of the dominant polymer emission is similar for all the three polymer/PCBM
blends. The PL is due to photoinduced singlet exciton states which undergo a radiative
recombination. The measurement of this PL quenching gives a strong indication of the potential
of this material combination for a charge transfer, which is an important prerequisite for organic
photovoltaic devices.
300 350 400 450 500 550 600
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Ab
sorp
tio
n
Wavelength (nm)
P3OT:PCBM1:0
P3OT:PCBM1:1
P3OT:PCBM1:2
(b)
300 375 450 525 600
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Ab
sorp
tio
n
Wavelength (nm)
P3HT:PCBM1:0
P3HT:PCBM1:1
P3HT:PCBM1:2
(a)
80
The graph presented in Figure 3.12 shows PL quenching for different P3AT:PCBM ratios.
This indicates a very efficient charge transfer from donor to acceptor. The HOMO and LUMO
levels of the two components [Figure 3.12(d)] in these blends are such that in the ground state the
extent of charge transfer is relatively small, and on photoexcitation, a fast electron transfer occurs.
This is the initial step of charge separation and charge carrier collection.
Figure 3.12 PL spectra of (a) P3HT/PCBM, (b) P3HT-OT/PCBM and (c) P3OT/PCBM. (d) Right
bottom shows the energy levels of different materials used in solar cells.
3.3.8. J-V Characteristics of Solar Cells
Regioregular P3HT, P3HT-OT, and P3OT have been used as the donors in combination with
PCBM as the accepter. Current–voltage (J-V) characteristic of P3ATs (P3HT, P3HT-OT, and
copolymer P3OT) have been studied in the device configuration viz.
ITO/PEDOT:PSS/P3AT:PCBM (1:1)/Al. The photovoltaic devices consist of four layers as
shown in Figure 3.13. A glass substrate coated with indium tin oxide (ITO) is used as substrate
(the device area amounts to 1 mm2, 4 to 6 cells have been fabricated at one 1.5×1.5 cm
2
substrate).
500 550 600 650 700 750
0.0
5.0x104
1.0x105
1.5x105
2.0x105
2.5x105
3.0x105
PL
In
ten
sity
Wavelength (nm)
P3HT:PCBM1:0
P3HT:PCBM1:1
P3HT:PCBM1:2
(a)
500 550 600 650 700 750
0.0
4.0x104
8.0x104
1.2x105
1.6x105
2.0x105
PL
In
ten
sity
Wavelength (nm)
P3HTOT:PCBM1:0
P3HTOT:PCBM1:1
P3HTOT:PCBM1:2
(b)
500 550 600 650 700 750
0.0
5.0x104
1.0x105
1.5x105
2.0x105
2.5x105
PL
In
ten
sity
Wavelength (nm)
P3OT:PCBM1:0
P3OT:PCBM1:1
P3OT:PCBM1:2
(c)
ITO PEDOT:PSS P3HT P3HTOT P3OT
-4.8 -5.2
-5.27 -5.31
-6.0
-4.3
Al
-4.2
-3.32-3.37
-6.0
-5.0
-4.0
-3.0
En
erg
y (eV
))
PCBM
-5.35
-3.4
d
Chapter 3
81
-1.0 -0.5 0.0 0.5 1.0
-4
-2
0
2
4
J (
mA
/cm
2)
Voltage (Volts)
Dark
Illuminated(a)
-1.0 -0.5 0.0 0.5 1.0
-4
-2
0
2
4
6
8
10
J (
mA
/cm
2)
Voltage (Volts)
Dark
Illuminated
(b)
Figure 3.13 Device architecture of solar cell in the configuration of ITO/PEDOT:PSS/Active
layer/Al.
Figure 3.14 (a) shows the J-V characteristics of the solar cell based on P3HT/PCBM (1:1
wt.%) both in the dark as well as under light intensity of 100 mW/cm2 with AM1.5 conditions at
room temperature [32, 33]. The cell has an open-circuit voltage (VOC) of 0.396 V, a short-circuit
current (JSC) of 2.00 mA/cm2 and a calculated fill factor (FF) of 0.30. The overall efficiency (ɳ)
for this solar cell has been calculated to be 0.2399%.
Figure 3.14 J–V characteristics of a ITO/PEDOT:PSS/P3HT:PCBM (1:1)/Al cell in the dark and
under illumination of AM1.5 conditions with light intensity of 100 mW/cm2. (a) Without annealed
(b) annealed at 120˚C for 10 min.
As reported earlier by Heeger et al. [34], the performance of device made from P3HT
could be further improved by post-production thermal annealing of device at a sufficiently high
temperature. The above same device has been thermally annealed at 120 ˚C for 10 min. The
performance of thermally annealed device is shown in Figure 3.14 (b). After thermal treatment,
Al
82
device delivers VOC, JSC, FF all increases such that it delivers a power conversion efficiency of
0.4977%. Post-production thermally annealed device exhibited VOC of 0.495 V, JSC of 2.64
mA/cm2, and FF of 0.38.
Figure 3.15 and Figure 3.16 shows the J-V characteristics of the solar cell based on P3HT-
OT/PCBM (1:1 wt.%) and P3OT/PCBM (1:1 wt.%) in the dark and under AM1.5 conditions
applying a light intensity of 100 mW/cm2 at room temperature.
Figure 3.15 J–V characteristics of a ITO/PEDOT:PSS/P3HT-OT:PCBM (1:1)/Al cell in the dark
and under illumination. (a) Without annealed (b) annealed at 120˚C for 10 min.
Figure 3.16 J–V characteristics of a ITO/PEDOT:PSS/P3OT:PCBM (1:1)/Al cell in the dark and
under illumination. (a) Without annealed (b) annealed at 120˚C for 10 min.
J-V characteristics of unannealed devices based on P3HT-OT/PCBM and P3OT/PCBM
are shown in Figures 15(a) and Figure 16(a), respectively. Same J-V characteristic for the devices
which represent the characteristics for devices annealed at 120 C for 10 min are shown in Figures
-1.0 -0.5 0.0 0.5 1.0
-3
-2
-1
0
1
2
3
J (
A/m
2)
Voltage (Volts)
Dark
Illuminated
(a)
Without annealed
-1.0 -0.5 0.0 0.5 1.0
-4
-3
-2
-1
0
1
2
J (
mA
/cm
2)
Voltage (Volts)
Dark
Illuminated
(b)
Annealed
-1.0 -0.5 0.0 0.5 1.0
-2
-1
0
1
2
J (
mA
/cm
2)
Voltage (Volts)
Dark
Illuminated
Annealed
(b)
-1.0 -0.5 0.0 0.5 1.0
-3
-2
-1
0
1
2
3
J (
mA
/cm
2)
Voltage (Volts)
Dark
Illuminated
Without annealed
(a)
Chapter 3
83
15(b) and Figure 16(b), respectively. Table 3.5 summaries the photovoltaic performance
parameters of the cells depicted in Figures 14–16 on AM1.5 conditions.
The open-circuit voltage of the three cells annealed [P3HT (495 mV) < P3HT-OT (503
mV) < P3OT (516 mV)] as well as without annealed [P3HT (396 mV) < P3HT-OT (409 mV) <
P3OT (423 mV)] devices increases gradually with the increase of alkyl side chain length. The
maximum VOC has been observed for P3OT, whereas P3HT shows the minimum VOC. The
copolymer P3HT-OT has value in between the two homopolymers as listed in Table 3.5.
Table 3.5 Photovoltaic performance parameters of the cells depicted in Figures 14–16.
It has been observed by various reporters [35-40], that the open-circuit voltage depends on
the acceptor strength of the fullerenes applied. This result fully does support the assumption, that
the open-circuit voltage of a donor–acceptor bulk-heterojunction cell is directly related to the
energy difference between the HOMO level of the donor and the LUMO level of the acceptor
component [35-39]. In agreement with this result and from the realizable trend comparing Eox onset
of P3HT, P3HT-OT, P3OT (Table 3.4) a possible explanation could be that the relatively smaller
differences in the HOMO levels of the three polythiophenes slightly affect their donor strength.
This corresponds with the energy difference between HOMO level of the donor polymers and
LUMO level of PCBM.
Device Remark VOC
(Volts)
JSC
(mA/cm2)
FF ɳ (%)
P3HT:PCBM Without annealed 0.396 2.00 0.30 0.2399%
P3HT-OT:PCBM Without annealed 0.409 1.61 0.32 0.2093%
P3OT:PCBM Without annealed 0.423 1.32 0.28 0.1564%
P3HT:PCBM Annealed at 120˚C 0.495 2.64 0.38 0.4977%
P3HT-OT:PCBM Annealed at 120˚C 0.503 2.36 0.33 0.3959%
P3OT:PCBM Annealed at 120˚C 0.516 1.46 0.40 0.3002%
84
The cell based on P3HT possesses a higher short-circuit current (2.64 mA/cm2) than the
cells based on P3HT-OT (2.36 mA/cm2) and P3OT (1.46 mA/cm
2). Regioregular head-to-tail
P3HT is well known for a high degree of intermolecular order leading to high charge carrier
mobilities (1.4×10-2 cm
2V
-1s
-1) [41].
The hole mobilities for P3HT-OT (7.2×10-3 cm
2V
-1s
-1) and for P3OT (1.3×10-
3 cm
2V
-1s
-1)
measured form FET geometries have been reported by A. Zen et al [41] which are lower than that
of P3HT. Assuming the same degree of regioregularity as well as of polymerization degree for all
three P3ATs, the hole mobility should increase as the length of the side chains decreases. This is
expected due to the contribution of side chain to the degree of intermolecular order and chain
packaging density. The smaller mobility of charges in P3HT-OT, and P3OT compared to those in
P3HT is due to the isolating nature of the side chain layers. Most remarkably the intensity of the
shoulder at 600 nm in UV-Vis absorption drops substantially when going from P3HT to P3HT-
OT to P3OT. The shoulder at 600 nm has been assigned to an interchain excitation [42, 43].
Therefore, it has been proposed that besides the thickness of the isolating side chain layers, the
packing of the polymer chains in the main chain layers significantly controls the mobility of the
homo- and copolymers studied here.
Furthermore, the potential barrier of P3HT-OT/ITO is slightly higher than that of
P3HT/ITO and somewhat lower than that of P3OT/ITO (see Figure 3.10(d) and Table 3.4). Thus
the hole injection from the HOMO of the polymers into ITO becomes less restricted in the case of
P3HT compared to the other two polythiophenes. P3HT shows a higher absorption coefficient
than P3HT-OT and P3OT (see Figure 3.7). Thus P3HT absorb more photon and has small hole
injection barrier, hence have higher current than other two polymers.
3.3. CONCLUSION
1. The homopolymers P3HT, P3OT, and their copolymer P3HT-OT have been synthesized
by chemical oxidative polymerization techniques. The regioregularity of these synthesized
polymers has been confirmed by FTIR, 1H NMR, and XRD analysis.
2. These polymers have been studied regarding their structural, optical, and electrical
properties as well as used as electron donor material in polymer solar cells.
3. 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.
4. Photovoltaic performance of P3HT-OT exhibit an open-circuit voltage VOC of 0.50V,
short-circuit current of 2.36 mA/cm2 and the overall power conversion efficiency of 0.4%,
Chapter 3
85
which is in between the performance of solar cell fabricated from P3HT ( = 0.5%) and
P3OT ( = 0.3%).
5. Open-circuit voltage systematically increases in the order P3HT:PCBM<P3HT-
OT:PCBM<P3OT:PCBM cells, which is probably due to the slightly lower HOMO levels
of P3OT and P3HT-OT compared with P3HT.
6. JSC of the P3HT:PCBM cell (2.64 mA/cm2) is higher than that of P3HT-OT:PCBM (2.36
mA/cm2) and P3OT:PCBM device (1.46 mA/cm
2). These values are determined by an
increased hole mobility and by a lower energy transition barrier for holes undergoing
transfer from the HOMO level into ITO anode regarding P3HT against P3HT-OT and
P3OT.
7. The performances of devices have been improved by post-production thermal annealing of
device at a sufficiently high temperature. Postproduction thermal annealing decreases the
series resistance and improves the contact between active layer and Al, which results into
enhanced device efficiency.
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88
CHAPTER 4
STUDY OF PHOTOVOLTAIC PERFORMANCE OF ORGANIC/INORGANIC HYBRID
SYSTEM BASED ON IN-SITU GROWN CdTe NANOCRYSTALS IN P3HT MATRIX
4.1 INTRODUCTION
4.2 FABRICATION AND MEASUREMENT OF DEVICE
4.3 RESULT AND DISCUSSION
4.3.1. High Resolution Transmission Electron Microscope images
4.3.2. Study of Surface Morphology
4.3.3. UV-Vis. Absorption Spectra
4.3.4. Photoinduced Charge Transfer at the Donor Acceptor Interface
4.3.5. J-V Characteristics of Solar Cells
4.4. CONCLUSIONS
References
4.1. INTRODUCTION
norganic II-VI semiconductor nanocrystals are of great interest for both fundamental
research and technical applications, due to their strong size dependent properties and
excellent chemical processability. Cadmium chalcogenide (CdX, X= S, Se, Te) are the most
attractive nanocrystals due to their good chemical processability, mono-dispersed size
distribution, and having strong quantum confinement effects. Their optical property can be tuned
as a function of size. Murray et al. [1] reported the synthesis of high quality cadmium
chalcogenides nanocrystals using dimethyl cadmium [(Cd(CH3)2] as the cadmium precursor in the
presence of tri-n-octylphosphine oxide (TOPO) as a coordinating solvent. Talapin et al. [2]
synthesized CdTe quantum dots (QDs) using primary amines and tri-n-octylphosphine (TOP) as
coordinating solvents at 200 ◦C. However, Cd(CH3)2 used in these synthesis is extremely toxic,
pyrophoric, expensive and unstable at room temperature. Moreover, it is explosive at elevated
temperatures due to releasing of large amount of gas [3-6]. Therefore, the Cd(CH3)2-related
schemes require very restricted equipments and conditions and are not suited for large-scale
synthesis. As a result, many researchers who are studying the cadmium-based QDs, welcome the
new synthetic methods that replace dimethyl cadmium with cadmium salts. Peng et al. [3-5]
successfully synthesized CdX QDs using less hazardous cadmium sources such as cadmium oxide
(CdO), cadmium acetate [(CH3COO)2Cd], and cadmium carbonate (CdCO3) at relatively higher
I
90
temperatures (240–360˚C). Among all the tested compounds (CH3COO)2Cd is proven to be the
best cadmium precursor, since it is free from pyrophoric and explosive properties [6]. In all the
above procedures, CdTe was synthesized at fairly high temperatures (>200 ◦C) and utilize
expensive raw materials such as organic phosphines, octadecene (ODE), and aliphatic amines [7-
9].
Environmentally, organic phosphine ligands should be avoided because of their high toxicity,
which would increase the control cost of chemical pollution [10]. Alternatively, a preparation
pathway employing cheap paraffin [11, 12] or plant oil [13] or commercial diesel [14] as a solvent
without any organic phosphines, aliphatic amines, and ODE was introduced.
In all of the above synthesis procedure, nanocrystals have been capped with organic
aliphatic ligands, such as TOPO or oleic acid. It has been shown that when the nanocrystals are
capped with organic ligands, they hinder the efficient electron transfer from the photoexcited
polymer to the nanocrystals [15], as shown in Figure 4.1. To remove the organic ligands,
polymer-nanocrystals 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 device performance [16].
Figure 4.1 Charge transfer between polymer (P3HT) and nanocrystals (CdTe).
To overcome the effects of the capping ligands on charge transport, the nanocrystals of
CdTe have been in-situ synthesized in the polymer matrix as discussed in chapter 2. The in-situ
growth of the nanocrystals in polymer templates controls the dispersion of the inorganic phase in
Chapter 4
91
the organic one, thus ensuring a large, distributed surface area for charge separation. Moreover,
nanocrystals are uniformly distributed to the entire device thickness and thus contains a built in
percolation pathway for transport of charge carriers to the respective electrodes.
In surfactant-assisted synthesis, nanocrystals growth is controlled by electrostatic
interactions induced by the surfactant functional group and steric hindrance from the surfactant
side alkyl chains. P3HT provides a combination of both effects, as it contains an electron donating
sulfur functionality, a potential anchorage for the nucleation, and growth of nanoparticles along
with steric hindrance due to long hexyl side chains [17, 18]. The in-situ growth of CdS [17]
nanorods in P3HT matrix, CdSe [18] nanocrystals in P3HT matrix, and PbS [19] nanorods in
poly(2-methoxy-5-(2-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV), have been reported
previously [17-19]. As CdTe has optimal band gap for solar cells and absorb higher amount of
solar radiation as compared to the CdSe, and PbS nanocrystals. Therefore, replacement of these
nanocrystals with CdTe would enable these hybrid devices for further enhancement in power
conversion efficiency. This new photovoltaic element could provide a new nanoscale criterion for
the investigation of photoinduced energy/charge transport at the organic-inorganic interfaces.
The present chapter deals with the photovoltaic performance of P3HT-CdTe hybrid
system. The various P3HT-CdTe compositions used in the present investigations are PHTCdTe1,
PHTCdTe2, PHTCdTe3, PHTCdTe4, and PHTCdTe20. The respective molar ratios of Cd-acetate
in PHTCdTe1, PHTCdTe2, PHTCdTe3, PHTCdTe4, and PHTCdTe20 are 0.1 mmol, 0.2 mmol,
0.4 mmol, 0.6 mmol and 3.6mmol, respectively. The Te were taken in the molarities of 0.2 mmol
for PHTCdTe1 , 0.4 mmol for PHTCdTe2, 0.8 mmol for PHTCdTe3, 1.2 mmol for PHTCdTe4,
and 7.2 mmol for PHTCdTe20. These nanocomposites are synthesized as discussed in chapter 2.
The aim of in-situ incorporation of CdTe nanocrystals in P3HT matrix is to improve the
photovoltaic properties of P3HT by broadening the absorption in the UV-Visible spectrum,
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 solar cells. The
photovoltaic performances of P3HT:PCBM as well as PHTCdTe2:PCBM have been investigated
in the device configuration viz. indium tin oxide (ITO)/ poly(3,4-ethylendioxythiophene)-
poly(styrene sulfonate) (PEDOT:PSS)/P3HT:PCBM/Al and
ITO/PEDOT:PSS/PHTCdTe2:PCBM/Al, respectively. These devices are designated as device A
and device B, respectively. Based on these investigations it has been observed that the current-
92
density (JSC) and open-circuit voltage (VOC) of device B have increased as compared to device A.
Improvement in JSC is attributed to enhancement of solar absorption and the formation of charge
transfer complex (CTC), which reduces the defect states and barrier height at the polymer-
nanocrystals interfacial boundaries. The enhancement in VOC is explained in the light of the
increase in the energy level offset between the LUMO of the acceptor and the HOMO of the
donor.
4.2. FABRICATION AND MEASUREMENT OF DEVICES
For optical, and morphological studies (scanning electron microscopy and atomic force
microscope), P3HT and P3HT-CdTe nanocomposites were dissolved in tri-chlorobenzene and
thin films of these solutions were deposited on glass substrates by spin casting at 1500 rpm for
120 s, and annealed at 120 °C for 30 min.
For the fabrication of device A and device B, ITO substrates have been carefully cleaned
as discussed in chapter 2. Prior to use, substrate have been treated with oxygen plasma.
PEDOT:PSS (Sigma Aldrich, USA) layers were spin-coated at 2000 rpm for 2 min, onto the ITO
substrate and cured at 120°C for 60 min in vacuum. P3HT:PCBM and P3HT2:PCBM both have
been taken in the ratio of 1:0.8 with a concentration of 1 wt. % in tri-chlorobenzene. The solution
containing P3HT plus PCBM was designated as solution A and other containing P3HT-CdTe
nanocomposite plus PCBM was designated as solution B. The tri-chlorobenzene solution A and B
have been spin casted at 1500 rpm for 2 min on the top of PEDOT:PSS layer in an inert
atmosphere, followed by annealing at 130°C for 30 min. Finally, Aluminum (Al) contacts 150 nm
has been applied via evaporation through a shadow mask at 2×10-6
Torr. The device active area is
~0.1 cm2 for all the devices discussed in this work. The J-V characteristics of device A and device
B have been recorded in the dark and under halogen lamp illumination with irradiance of 80
mWcm−2
, using a Keithley 2400 Source-Measure unit, interfaced with a computer.
4.3. RESULTS AND DISCUSSION
4.3.1. High Resolution Transmission Electron Micrograph Images
HRTEM images and electron diffraction (insets) patterns of the synthesized P3HT-CdTe
nanocomposites PHTCdTe1, PHTCdTe2, and PHTCdTe3 at 160 ˚C are shown in Figure 4.1(a-
b), 4.1(c-d) and 4.1(e-f), respectively. HRTEM images reveal that ratio of P3HT and cadmium
acetate plays a significant role in controlling the size and shape of the nanocomposites. The
difference in contrast at different areas in HRTEM images, indicates that the CdTe nanocrystals
are capped by P3HT. It is evident from the Figures 4.1 (a) and 4.1 (b) that at low CdTe
concentration the P3HT matrix shows more binding with CdTe nanocrystals and formation of
even nanorods structure of P3HT-CdTe as seen by enlarged image.
Chapter 4
93
Figure 4.1. HRTEM images and electron diffraction (ED; insets) patterns of (a)-(b)
PHTCdTe1, (c)-(d) PHTCdTe2 and (e)-(f) PHTCdTe3 nanocomposites synthesized at 160˚C.
Bar scale: 20 nm for (a), (c), (e) and 5 nm for (b) (d) and (f).
However, as the CdTe concentration increases [Figure 4.1 (c) and 4.1 (e)], the binding
between CdTe and P3HT reduces and the precipitation of CdTe nanocrystals appear rather than
percolated network. The optimum percolation and interaction between P3HT and CdTe take
place in PHTCdTe2 as shown in Figure 4.1 (c), where the nanorods formation as well as
individual CdTe precipitation has been suppressed. Hence further device investigation has been
carried out in PHTCdTe2. This interaction between polymer and nanocrystals indicates that
94
nanocomposites have potential for the charge transfer at polymer-nanocrystals interfaces, which
results in the PL quenching and the improvement of short circuit current density.
The mechanism of this interaction has revealed that the sulfur atom of P3HT can interact
with the CdTe nanoparticles by dipole-dipole interaction and CdTe nanocrystals have been
deposited uniformly and compactly on or in-between the P3HT chains to form nanoparticles as
suggested in scheme 2.2 (c) (chapter 2). The selected area electron diffraction patterns of
PHTCdTe1, PHTCdTe2 and PHTCdTe3 are shown in the inset of Figures 4.1 (b), 4.1 (d), and
4.1 (f), respectively, which confirmed the high crystallinity of the CdTe in P3HT.
HRTEM images of the synthesized P3HT-CdTe nanocomposites at 220˚C are shown in
Figure 4.2. In this case, nanorod formation of P3HT-CdTe is absent, may be due to decrease
in the bonding between P3HT and CdTe, hence nanocrystals show better crystallinity.
Moreover, the particle size in the present case is larger, as compared with that of CdTe
nanocrystals synthesized at 160˚C, which is attributed to aggregation of the particles at higher
temperature.
Figure 4.2 HRTEM images and electron diffraction (ED; insets) patterns of (a)-(b)
PHTCdTe1, (c)-(d) PHTCdTe2 and (e)-(f) PHTCdTe3 nanocomposites synthesized at 220˚C.
4.3.2. Surface Morphology
Chapter 4
95
The nanoscale morphology is a crucial parameter to understand the effectiveness of the interface
for exciton splitting into free charge carriers, and the formation of a percolation network for
efficient transport of charge carriers to the electrodes. The surface morphology of the pristine
P3HT and P3HT-CdTe nanocomposite films have been examined by atomic force microscopy
(AFM) and scanning electron microscopy (SEM) images. Figure 4.3 shows the AFM images for
the films of pristine P3HT [Figure 4.3 (a)] as well as of PHTCdTe2 [Figure 4.3 (b)]. Figure 4.3 (a)
shows the fibrillar structures of P3HT which represents the crystalline domains of P3HT. The
nanocomposite PHTCdTe2 film provides a very different phase wherein, an island-like structures
appear instead of fibrillar features. In this image light-colored particles can be seen which are of
the CdTe. These CdTe particles construct percolation network for the transport of charge. These
images show that the change in the surface morphology is a result of incorporation of CdTe
nanocrystals in P3HT matrix.
Figure 4.3. AFM images of spin casted thin films of (a) P3HT, (b) PHTCdTe2 annealed at 120 °C
for 30 minutes.
SEM micrograph of P3HT and P3HT-CdTe are presented in Figure 4.4. Figure 4.4 (a)
shows nearly flat surface morphology of pristine P3HT film. The SEM images, with different
P3HT and CdTe compositions (PHTCdTe1, PHTCdTe2, PHTCdTe3) are shown in Figures 4.4
(b)-(d). At low concentration of CdTe (PHTCdTe1) the nanocrystals aggregate to form mud like
structure due to binding between P3HT and CdTe as shown in Figure 4.4 (b). However, with
increase of the CdTe concentration [Figure 4.4 (c)], the binding between CdTe and P3HT reduces,
leading to the formation of multifoliated leaf like structures. The further increase in CdTe
a b
96
concentration, multifoliated leaf like structure, reduces, leading to the precipitation of CdTe
nanocrystals (as evident from the difference in contrast) as shown in Figure 4.4 (d).
Figure 4.4. SEM micrograph of spin casted thin films of (a) P3HT, (b) PHTCdTe1, (c)
PHTCdTe2 and (d) PHTCdTe3 annealed at 120 °C for 30 minutes.
4.3.3. Fourier Transform Infrared Spectroscopy Analysis
The success of formation of P3HT-CdTe nanocomposites have been confirmed by the FT-IR
spectra as shown in Figure 4.5. Strong absorption bands of P3HT at 2953, 2920 and 2854 cm-1
have been assigned to the asymmetric C–H stretching vibrations in –CH3, –CH2, and the
symmetric C–H stretching vibration in –CH2, respectively [20, 21]. They are ascribed to the alkyl-
side chains. The bands at 1456, 1377 cm-1
, are due to the thiophene ring stretching, and methyl
deformation respectively. The C-C vibration appears at 1260 cm-1
. The characteristic C-S band
stretching has been observed at 1111 cm-1
while absorption band at 822 cm-1
and 725 cm-1
have
been assigned to the aromatic C-H out-of plane stretching and methyl rocking, respectively. In
Chapter 4
97
nanocomposites of P3HT-CdTe, the intensity of peaks corresponding to C-S bond and aromatic
C-H out-of plane stretching decreases. Also a shift by 25 cm-1
(from 1110 to 1135 cm-1
), to the
higher energy region of C-S characteristic band has been observed in P3HT-CdTe, indicating the
enhancement of the C-S bond energy. Moreover, the characteristic band of thiophene ring shows a
red shift from 822 to 816 cm-1
, with the increase of concentration of CdTe in polymer matrix.
These findings suggest additional intermolecular interaction between polymer and nanocrystals,
which arises due to strong dipole-dipole interaction between the Cd2+
ions and S atoms as shown
in scheme 2.2 (b) [17, 21].
800 1200 1600 2000 2400 2800 3200
2953
2920
2854
1126
1135816
819
720
722
723 821 1120
1111725 1260
822
1456
1377 1510
PHTCdTe3
PHTCdTe2
PHTCdTe1
P3HT
Tra
nsm
itta
nce (
a.u
.)
Wavenumber (cm-1
)
Figure 4.5 FT-IR spectra of P3HT and P3HT-CdTe nanocomposites.
4.3.4. UV-Vis Absorption Spectra
The normalized UV-Vis absorption spectra of the pristine P3HT, P3HT-CdTe nanocomposites
films as well as in tri-chlorobenzene solutions are shown in Figure 4.6 and 4.7, respectively. The
maximum absorption of pristine P3HT films has been observed at 510 nm which corresponds to
the π-π* transition of the conjugated chain in the P3HT [22-24]. For the P3HT-CdTe composite
films, the absorption spectrum has been broader as compared to pristine P3HT. The broadness in
absorption spectra indicates the presence of CdTe nanocrystals in polymer matrix [18]. Maximum
98
absorption for PHTCdTe1 and PHTCdTe2 were red shifts to 515 nm and 518 nm, respectively.
this red shift in P3HT-CdTe nanocomposites suggest the formation of charge transfer states in
P3HT-CdTe nanocomposites resulting in partial electron transfer from P3HT to CdTe [25]. On
further increase of the concentration of CdTe in P3HT (PHTCdTe3) there has been a blue shift in
absorption spectra observed as compared to PHTCdTe1 and PHTCdTe2, which is observed at 514
nm. This means at higher concentration of CdTe in P3HT, there is smaller shift in absorption
spectra. The smaller shift in absorption at higher concentration of CdTe in P3HT is due to weak
interaction between polymer-nanocrystals, as evident from HRTEM images.
300 400 500 600 700 800 900
0.0
0.2
0.4
0.6
0.8
1.0
PHTCdTe3
PHTCdTe2
PHTCdTe1
P3HT
No
rm
ali
sed
Ab
sorp
tio
n
Wavelength (nm)
Figure 4.6 Normalized absorption spectra of P3HT and P3HT-CdTe nanocomposites films.
Figure 4.7 shows the absorption spectra of P3HT and P3HT-CdTe hybrid systems in tri-
chlorobenzene solution. The maximum absorption has been observed around at 467 nm for all
solutions. Moreover, on the incorporation of CdTe nanocrystals in P3HT matrix, the absorption
spectra start to broaden, and the broadness increases further with the increase of CdTe
concentration. The second maxima have been observed at 305 nm which is the characteristics of
CdTe nanocrystals. At higher concentration of CdTe (PHTCdTe20, Cd-acetate 3.6mmol, Te 7.2
mmol) the absorption of CdTe is dominating and characteristic maxima of P3HT diminish.
Chapter 4
99
300 375 450 525 600 675
0.0
0.3
0.6
0.9
1.2
1.5
1.8
No
rm
ali
zed
Ab
sorp
tio
n
Wavelength (nm)
P3HT
PHTCdTe1
PHTCdTe2
PHTCdTe3
PHTCdTe20
Figure 4.7 Normalized absorption spectra of P3HT and P3HT-CdTe solution in tri-
chlorobenzene.
4.3.5. Photoinduced Charge Transfer at the Donor Acceptor Interface
The PL quenching can be used as a powerful tool for the evaluation of charge transfer from the
excited polymer to the nanocrystals [26, 27]. Once the photogenerated excitons are dissociated,
the probability for recombination should be significantly reduced. In Figure 4.8 the PL spectra of
pristine P3HT film have been compared with that of different P3HT-CdTe nanocomposites films.
These P3HT and P3HT-CdTe nanocomposites films exhibited emission maximum around 660
nm. PL intensity of the nanocomposite films significantly reduces as compared to that of the
P3HT film. With increase of CdTe concentration in polymer, the PL intensity decreases further.
Reduced PL intensity of the composites relative to the pristine P3HT, indicates that charge
transfer, thereby exciton dissociation at interface of CdTe and P3HT (Figure 4.9) [28]. This PL
quenching experiment provides us with good evidence that the nanocrystals will be able to
transfer their excited state hole to the polymer.
100
600 650 700 750 800
0.0
5.0x105
1.0x106
1.5x106
2.0x106
2.5x106
3.0x106
PL
In
ten
sity
Wavelength (nm)
(a) P3HT
(b) PHTCdTe1
(c) P3HT-PCBM
(d) PHTCdTe2
(e) PHTCdTe2-PCBM
(f) PHTCdTe3
a
b
c
d
e
f
Figure 4.8 Photoluminescence spectra of P3HT, P3HT-CdTe nanocomposites, P3HT-PCBM and
P3HT-CdTe-PCBM films after excitation by radiation of 510 nm wavelengths.
Charge transfer takes place in the conjugated polymer-semiconductor nanocrystals
composites at the interface, where the P3HT with a higher electron affinity (-3.37 eV) transferred
electron onto CdTe with relatively lower electron affinity (-3.71) (Figure 4.9). In this transfer, the
polymer absorb the solar photons (exciton generation), the electron is transferred to the CdTe
nanocrystals and the hole potentially can transfer to the polymer (charge separation). This is a
well known effect of the ultrafast electron transfer from the donor to acceptor, and it is expected
to increase the exciton dissociation efficiency in photovoltaic devices [29, 30]. The PL spectra of
P3HT-PCBM and PHTCdTe2-PCBM are also shown in Figure 4.8. On incorporation of PCBM in
P3HT and PHTCdTe2, the PL spectrum further quenched relative to the P3HT and PHTCdTe2.
The PL quenching upon addition of PCBM in P3HT and PHTCdTe2 further confirm the electron
transfer from P3HT to CdTe or PCBM and CdTe to PCBM.
Figure 4.10 shows the PL spectra of P3HT and different P3HT-CdTe composites solution
in tri-chlorobenzene. The P3HT and P3HT-CdTe nanocomposites exhibited emission maximum
around 580 nm. Like P3HT-CdTe composites films, PL intensity of the nanocomposite solution
significantly reduces as compared with the value of the P3HT solution. Also PL intensity further
decreases with the CdTe concentration in the polymer. Reduced PL intensity of the composites
relative to the pristine P3HT indicates that exciton dissociation, thereby charge transfer at P3HT-
CdTe interface.
Chapter 4
101
Figure 4.9 Schematic illustration of the energy diagram of configuration of device B. The P3HT,
CdTe and PCBM have HOMO levels at 4.27, 4.48 and 6.0 eV while LUMO levels at 3.37, 3.71
and 4.2 eV, respectively for facilitating the charge transfer at the P3HT-CdTe nanocomposites
and PCBM interface. The arrows indicate the expected charge transfer process in solar cell.
500 550 600 650 700 750
0.0
5.0x105
1.0x106
1.5x106
2.0x106
2.5x106
3.0x106
PL
In
ten
sity
Wavelength (nm)
P3HT
PHTCdTe1
PHTCdTe2
PHTCdTe3
PHTCdTe4
Figure 4.10 Photoluminescence spectra of P3HT, P3HT-CdTe nanocomposites in tri-
chlorobenzene solution after excitation by radiation of 450 nm wavelengths.
102
The quantum yield (QY) is defined as the ratio of photons absorbed to photons emitted.
For the measurement of QY the solutions of the standard and test samples have been prepared.
Rhodamine B has been taken as the standard sample, as it has approximately absorption and
emission in the same range as of P3HT. For the measurement of QY, the UV-Vis absorbance and
photoluminescence spectrum have been recorded for Rhodamine B (Figure 4.11) and P3HT
(Figure 4.12) in three different concentration. Then graphs of integrated PL intensity vs.
absorbance have been plotted as shown in Figure 4.13. The QY of the samples have been
estimated according to the equation:
2
)(
)(
)(
)()()(
R
S
RGrad
SGradRQYSQY
Where ‘S’ and ‘R’ represents for test and reference samples, respectively, Grad is the gradient
from the plot of integrated PL intensity vs. absorbance, and ɳ the refractive index of solvent.
Figure 4.11 Absorption and emission data of Rhodamine B dye for three concentrations in
ethanol solution.
The QY(R) of Rhodamine B is 0.7 [31], the calculated QY of P3HT is 26%. Similarly QY
of other samples have been estimated (results are not shown). The QY of P3HT decreases from
initially 26% to 11% on incorporation of CdTe nanocrystals into the P3HT matrix. The
PHTCdTe1, PHTCdTe2, PHTCdTe3 shows the QY of 26%, 20%, 17%, 14%, respectively.
Reduction in QY of polymer/nanocrystal composites compared to that of pristine P3HT, is that a
large amount of singlet excitons are not able to radiate onto ground state and they dissociate at the
polymer/nanocrystals interface as suggested in Figure 4.9.
450 475 500 525 550 575 600
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Ab
sorp
tio
n
Wavelength (nm)
Rhodamine B
(a)
500 550 600 650 700
0
1x105
2x105
3x105
4x105
PL
In
ten
sity
Wavelength (nm)
Rhodamine B(b)
Chapter 4
103
Figure 4.12 Absorption and emission data of P3HT for three concentrations in tri-chlorobenzene.
Figure 4.13 linear plots for (a) Rhodamine B and (b) P3HT.
4.3.6. J-V Characteristics of Solar Cells
Figure 4.14 (a) shows the J-V characteristics of device A and B under AM 1.5 illuminations with
intensity of 80mWcm-2
. The performance of device A showed a short-circuit photocurrent (JSC) of
2.25 mAcm-2
, an open-circuit voltage (VOC) of 0.58 V, a fill factor (FF) of 0.44, and a power
conversion efficiency (PCE) of 0.72%. However, in case of in-situ growth of CdTe nanocrystals
in P3HT matrix (device B), the PCE value increased up to 0.79%, thereby improving the JSC to
3.88 mAcm-2
, VOC of 0.80 V, while FF diminishing to 0.32. Table 4.1 summaries the photovoltaic
performance of these solar cells.
500 550 600 650 700 750
0.0
5.0x105
1.0x106
1.5x106
2.0x106
2.5x106
3.0x106
PL
In
ten
sity
Wavelength (nm)
P3HT(b)
300 350 400 450 500 550 600 650
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Ab
sorp
tio
n
P3HT
Wavelength (nm)
(a)
0.04 0.06 0.08 0.10 0.12 0.14
6.0x106
9.0x106
1.2x107
1.5x107
1.8x107
Inte
gra
ted
PL
In
ten
sity
Absorbance
Rhodamine B(a)
0.07 0.08 0.09 0.10 0.11 0.12 0.13
1.4x108
1.6x108
1.8x108
2.0x108
2.2x108
2.4x108
Inte
gra
ted
PL
In
ten
sity
Absorbance
P3HT(b)
104
-0.50 -0.25 0.00 0.25 0.50 0.75 1.00
-6
-4
-2
0
2
4(a)
J (
mA
/cm
2)
V (Volts)
device A
device B
4 8 12 16 20
0.2
0.4
0.6
0.8
(b)
V (Volts)
J (
A/c
m2)
P3HT
P3HT-CdTe
Table 4.1 Photovoltaic performance of device A and device B
The increase in the value of JSC of device B can be understood in terms of host (P3HT)
and guest (CdTe) charge transfer type interaction. In fact there are various possibilities by which
CdTe can interact with host P3HT. It can either go into P3HT structure main chain or forms donor
acceptor charge transfer complexs or form molecular aggregates. However, the enhancement in
JSC in device B indicates that CTCs formation between the host and guest may be the dominant
mechanism of interaction. This suggested mechanism is indeed supported by the PL quenching in
P3HT-CdTe nanocomposites, decrease in QY and energy levels of different materials used shown
in Figure 4.9. On incident of light, both P3HT and CdTe absorb light and generate excitons. Here,
electron affinities of P3HT, CdTe and PCBM are 3.37 eV, 3.71 eV and 4.2, respectively, hence it
is energetically favorable for electron transfer from P3HT to CdTe or PCBM and CdTe to PCBM
or hole injection from CdTe to P3HT as indicated by arrows in Figure 4.9 [32].
Figure 4.14 (a) J-V curves obtained from device A and device B under AM 1.5 illuminations at
irradiation intensity of 80 mW/cm2 (b) J-V characteristics of pristine and P3HT-CdTe
nanocomposites films in hole only device configuration viz. ITO/PEDOT:PSS/P3HT or P3HT-
CdTe/Au at room temperature in dark.
Device VOC (Volts) JSC (mA/cm2) FF ɳ (%)
Device A 0.58 2.25 0.44 0.72
Device B 0.80 3.88 0.32 0.79
Chapter 4
105
Moreover, enhancement in JSC may result in improvement in the light absorption in P3HT-
CdTe composites, as compared to pristine P3HT. In the device based on P3HT and CdTe both
component absorb light unlike in P3HT:PCBM device where PCBM contribution is very small.
Hence, light harvesting is more in hybrid system so that number of excitons generated upon
incidence of light increases and as a result current density increases.
The enhancement in current density on in-situ incorporation of CdTe nanocrystals is
supported by J-V measurement in dark as shown in Figure 4.14 (b). Figure 4.14 (b) shows the J-V
characteristics in dark of P3HT and PHTCdTe2 nanocomposites thin films in hole only device
configuration viz. ITO/PEDOT:PSS/P3HT/Au and ITO/PEDOT:PSS/PHTCdTe2/Au. The nature
of J-V characteristics of composites thin film is different from that of pristine P3HT. In case of
composites film, it has been observed that the hole current is more than that in pristine P3HT. The
enhancement in the hole current in PHTCdTe2 composites compared to that of pristine P3HT can
be understood in terms of host (P3HT) and guest (CdTe) charge transfer type interaction. In the
composite film the CdTe nanocrystals are bound with P3HT via dipole-dipole interaction and
form a CTC. The charge carriers which had to jump from one chain to another to transport
through P3HT are now assisted by the CdTe nanocrystals. The calculated value of activation
energy of localized states is 52 meV for P3HT and 11 meV for P3HT-CdTe nanocomposites [33].
As activation energy in P3HT-CdTe is lower, compared to the pristine P3HT, the CdTe
nanocrystals support transportation of holes which improves their mobility and results into
enhancement in the hole current.
The enhancement in VOC in device B can be understood in terms of lower HOMO level of
CdTe as compared to P3HT (Figure 4.9). VOC is correlated with the energy difference between the
HOMO of the donor polymer and the LUMO of the acceptor [34, 35]. Clearly, a lower HOMO
energy level provides a higher Voc. The measured difference (0.21 eV) of the HOMO energy
levels between P3HT and CdTe almost completely translated into the observed difference in Voc
(∼0.22 V).
The cells suffered from low fill factors (Table 4.1), which may be caused by shunting and
a high series resistance [36-38]. The presence of polymer or nanocrystal pathways that connect
the anode to the cathode is a source of current leakage or electrical shorts, depending on the
conductivity of the pathway [39]. The incorporation of CdTe nanocrystals into a P3HT–PCBM
matrix results in enhancement in photoconductivity of the active layer [40]. Thus increased
photoconductivity of the active layer is responsible for the decrease in fill factor and change of J-
V shape of device B from device A. The addition of one hole-blocking layer at cathode and
another electron-blocking layer at anode can prevent the polymer and nanocrystal from shorting
the two electrodes under illumination.
106
4.4 CONCLUSIONS
1. In order to improve the photovoltaic properties of P3HT by broadening the absorption in
the UV-Visible spectrum, enhancing the charge carrier mobility, and improving the polymer-
nanocrystals interaction, the CdTe nanocrystals have been in-situ grown in the P3HT matrix
without use of any surfactant.
2. Structural (HRTEM, SEM, AFM) and spectroscopic (FTIR, UV-Vis absorption, PL)
studies confirmed the successfully incorporation of CdTe nanocrystals in P3HT matrix.
3. Structural and morphological studies reveal that CdTe works as transport media
along/between the polymer chains, which facilitate percolation pathways for charge transport.
4. Optical measurements show that photoinduced charge generation on the incident of light
which are dissociated at the P3HT-CdTe interfaces.
5. The solar cell performance of device based on P3HT-CdTe:PCBM show a better device
performance as compared to P3HT:PCBM, by increasing JSC from 2.25 mAcm-2
to 3.88 mAcm-2
,
and VOC from 0.58 V to 0.80 V.
6. The enhancement in VOC in P3HT-CdTe:PCBM based device can be understood in terms
of lower HOMO level of CdTe as compared to P3HT. The measured difference (0.21 eV) of the
HOMO energy levels between P3HT and CdTe almost completely translated into the observed
difference in Voc (∼0.22 V).
7. Enhancement in JSC may result in improvement in the solar absorption spectra and
decrease in the activation energy of localizes states.
8. The cells suffered from low fill factors, which may be caused by shunting and a high
series resistance of P3HT-CdTe as compared to pristine P3HT.
9. The present investigation given in this chapter indicates that the in-situ incorporation of
nanocrystals in polymer matrix is a promising approach for the fabrication of efficient organic-
inorganic hybrid photovoltaic devices.
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CHAPTER 5
STUDY OF THE EFFECT OF CADMIUM SULPHIDE QUANTUM DOTS ON THE
PHOTOVOLTAIC PERFORMANCE OF POLY(3-HEXYLTHIOPHENE)
5.1 INTRODUCTION
5.2. FABRICATION AND MEASUREMENT OF DEVICE
5.3 RESULT AND DISCUSSION
5.3.1 Structural Characterization
5.3.1.1 XRD analysis
5.3.1.2. High resolution transmission electron microscope images
5.3.1.3. Scanning electron micrograph
5.3.2. Optical Study
5.3.2.1. UV-Vis. absorption spectra
5.3.2.2. Photoinduced charge transfer at the donor/acceptor interface
5.3.3. J-V Characteristics of Solar Cells
5.4. CONCLUSIONS
References
5.1 INTRODUCTION
ince the discovery of photoinduced charge transfer between conjugated polymer and
inorganic nanocrystals (NCs) [1], hybrid solar cells have been intensively studied for
large-area, flexible, low-cost solar cells [2-5]. By combining p-type conjugated polymers
with n-type inorganic colloidal NCs, the hybrid system can show the increase in the device
performance relative to either of the non-hybrid counterparts. This is possible due to inherent
advantages of organic conjugated polymers and inorganic nanostructures [6-10].
Various NCs including CdSe [11-15], CdTe [16], PbS [17], PbSe [18], CuInSe2 [19], ZnO
[20] and TiO2 [21] have been widely studied for hybrid solar cell fabrication. However, very few
studies have been reported for utilization of CdS as an important II-VI semiconductor in
nanocrystal-conjugated polymer composite photovoltaic devices. Probably, this may be due to the
relatively large band gap of CdS and mismatches with the solar terrestrial radiation. So far the
highest power conversion efficiency for a CdS/P3HT hybrid solar cell has been reported by Liao
S
110
et al., who fabricated a hybrid solar cell with in-situ grown CdS NCs in P3HT matrix and
obtained a power conversion efficiency of 2.9% [22]. However, since CdS has higher electron
mobility, we believe there is a much room for further improvement in device efficiency for hybrid
CdS/conjugated polymer photovoltaic devices.
Furthermore, the preparation methods of CdSe quantum dots (QDs) utilize expensive raw
materials such as organic phosphines, octadecenes, and aliphatic amines [23]. Environmentally,
organic phosphine ligands should be avoided because of their high toxicity, which would increase
the control cost of chemical pollution [24]. If the production cost of QDs could be decreased
greatly through deploying cheap raw materials with lower toxicity and decreasing reaction
temperatures, large-scale preparation and practical application of QDs would be accessible.
The present chapter deals with the fundamental issue, whether dispersion of CdS QDs into
P3HT matrix causes any noticeable improvement or deterioration of device efficiency. The
particle shape, size and distribution of CdS QDs in P3HT matrix have been investigated by
HRTEM, SEM and XRD. Optical studies [UV-Vis absorption and photoluminescence (PL)]
suggest the electronic interaction between P3HT and CdS QDs. Photovoltaic performances of
device based on pure P3HT as well as dispersed with CdS QDs in the device configuration viz.
ITO/PEDOT:PSS/P3HT:PCBM/Al and ITO/PEDOT:PSS/P3HT:CdS:PCBM/Al have been
investigated. These devices are designated as device X and device Y, respectively. On
incorporation of CdS QDs in P3HT matrix, the power conversion efficiency increased from
0.45% to 0.87% due to enhancement in short-circuits photocurrent, open-circuit voltage, and fill
factor. These effects have been explained on the basis of the formation of charge transfer complex
(CTC) between the host (P3HT) and guest (CdS QDs), duly supported by UV-Vis absorption and
PL quenching studies. The effect of post thermal annealing on device performance has also been
investigated and improved efficiency of devices was observed after thermal treatment at 1500C for
10 min due to their improved nanoscale morphology, crystallinity and contact to the electron-
collecting electrode.
5.2. FABRICATION AND MEASUREMENT OF DEVICE
For the fabrication of solar cells devices, the %wt. ratio of P3HT:PCBM (Sigma-Aldrich) in
device X is 1:0.8 and for the deviceY the %wt. ratio of P3HT:CdS:PCBM, is 1:1:0.8. Two
solutions of P3HT were prepared in chlorobenzene and in one of them, CdS was added and
sonicated for 4 hrs in order to well disperse CdS in P3HT. PCBM solution in chlorobenzene was
added in the above solutions and mixed solution was ultrasonicated for 2 hrs. The solution
Chapter 5
111
containing P3HT plus PCBM is designated as solution X and other containing P3HT plus CdS
and PCBM is designated as solution Y. For preparation of device X and device Y, the ITO-coated
glass substrate was first cleaned with detergent, ultrasonicated in acetone, trichloroethylene and
isopropyl alcohol, and subsequently dried in an vacuum oven as described in chapter 2. Highly
conducting PEDOT:PSS (Aldrich, USA) was spin casted on the ITO surface. The substrate was
dried for 10 min at 1500C in vacuum and then moved into a glove box for spin casting the
photoactive layer. The chlorobenzene solutions X and Y have been then spin-casted at 1500 rpm
for 2 min on the top of PEDOT:PSS layer. Subsequently 120 nm Al film was deposited on top of
the active layer. Thermal annealing has been carried out by directly placing the complete device at
150˚C in a vacuum oven. The performance of these devices was studied by their J-V
characteristics in the dark and under halogen lamp illumination with irradiance of 80 mWcm−2
,
using a Keithley 2400 Source-Measure unit, interfaced with a computer.
5.3. RESULT AND DISCUSSION
5.3.1. Structural Characterization
5.3.1.1. XRD analysis
Figure 5.1 shows X-ray diffraction patterns for pure P3HT, P3HT/CdS nanocomposite, and CdS
powder. In XRD spectrum of CdS, three broad peaks at 2θ ~ 27◦, 44
◦ and 52
◦ have been observed,
which are corresponds to the (111), (220) and (311) planes, respectively, of cubic CdS [25]. The
XRD peaks are broad due to the small size of QDs. The average crystallite size determined from
the peak at 27◦ using Debye–Scherrer formula:
cos/9.0d
where λ is the wavelength of the X- rays used, β is the full width at half maximum and θ is the
angle of reflection. The crystalline size of CdS QDs has been estimated to be about 2.33 nm.
The strong first order reflection, (100), of P3HT has been observed at 2θ angle 5.45◦ [26]
and corresponds to interlayer spacing 16.4 Å, as calculated from XRD spectrum of P3HT. The
second order reflection corresponds to the plane (200) of P3HT [26], has been observed at 2θ
angle 10.86◦, and corresponds to interlayer spacing 8.402 Å. In comparison, XRD data of
CdS/P3HT shows that the 2θ values matching the (100), (100), (111), (220) and (311) planes. The
appearances of few additional peaks in the composites are attributed to the presence of QDs in
P3HT matrix.
112
Figure 5.1 XRD spectra of CdS QDs, P3HT and P3HT/CdS nanocomposites films.
5.3.1.2. High resolution transmission electron microscope images
High resolution transmission electron microscopy (HRTEM) images of CdS QDs and P3HT-CdS
nanocomposite are shown in Figures 5.2 (a-c) and 5.2 (d-f), respectively. It has been observed
from Figure 5.2 (a) that the size of the QDs ranges from 5 to 6 nm and their shape is spherical. In
addition, it is seen from Figure 5.2 (b) that at higher resolution there exists (1 1 1), (2 2 0) and (3 1
1) planes of cubic CdS having interplaner spacing 3.36, 2.06, and 1.76 Å, respectively. This
formation of different planes is explicitly confirmed by diffraction pattern shown in Figure 5.2
(c). Further, Figure 5.2 (d-f) shows the HRTEM images of P3HT-CdS composites prepared by
physically mixing of CdS QDs in P3HT matrix. The P3HT-CdS composites exhibited a
significant phase separation as evidenced in Figure 5.2 (d) and (e). Difference in the contrast in
the HRTEM images of the composites indicates that the CdS QDs are well dispersed in P3HT
matrix. Dark and light phase represents the presence of CdS QDs and P3HT, respectively. Both
phases are eventually well dispersed within hybrid nanocomposites films. Different planes of CdS
QDs in P3HT matrix are shown by diffraction pattern in Figure 5.2 (e).
Chapter 5
113
Figure 5.2 High resolution TEM images of (a) CdS nanoparticles in the range of 5–6 nm (b)
lattice resolution of cubic CdS QDs (c) Diffraction image of CdS QDs (d-e) CdS nanoparticles
dispersed in P3HT matrix and (f) Diffraction image of CdS QDs in P3HT matrix.
5.3.1.3. Scanning electron micrograph
For the recording the images of scanning electron microscopy (SEM), P3HT and P3HT-CdS
nanocomposites were dissolved in 1wt.% of chloform. Thin films of these solutions were
deposited on glass substrates by drop casting, and annealed at 120 °C for 120 min. Figure 5.3
shows the SEM images for the P3HT and P3HT-CdS nanocomposites films. It is apparent from
the Figure 5.3 (a) that the P3HT has 3-D shapeless porous network but when CdS nanoparticles
are incorporated in P3HT [Figure 5.3(b)], nanocrystals masked these cavities and porous network
diminishes. When excess of QDs are incorporated in polymer matrix, the nanocrystals appear to
be buried into the porous surface of polymer and rest of QDs are lying over the surface film.
114
Figure 5.3 SEM images of (a) P3HT and (b) P3HT/CdS nanocomposites thin films, casted from
chloroform solution by drop coating.
5.3.2. Optical Study
5.3.2.1. UV-Vis. absorption spectra
UV–Vis absorption of P3HT and P3HT/CdS composite solution in chloroform is shown in Figure
5.4(a). Regio-regular P3HT has solid-state absorptions ranging from λmax = 520-530 nm and
solution absorption ranging 442-448 nm [26-28]. In the present study, the maximum absorption of
P3HT has been observed at 448 nm for solution and at 526 nm for thin film which confirms its
regio-regularity. Strong absorption band at 448 nm for P3HT is attributed to the excitation of
electrons in the π-conjugated system. P3HT/CdS nanocomposite shows maximum absorption at
438 nm, which is 10 nm blue shifted relative to the pristine P3HT. The blue shift in absorption
spectrum of P3HT/CdS nanocomposite can be attributed to the quantum confinement effect from
the CdS nanoparticles [29-31]. Maximum absorption intensity in the nanocomposite is slightly
lower due to scattering caused by the QDs in the P3HT matrix. As shown in the inset of Figure
5.4 (a), CdS quantum dots show a broad absorption from 290 to 700 nm, with a maximum
absorption peak at 292 nm and an edge at 440 nm. The absorption spectra of P3HT and
P3HT/CdS thin films are shown in Figure 5.4(b). The maximum absorption of P3HT/CdS
composites is observed at 511 nm, which exhibits a 15 nm blue shift relative to pristine P3HT.
This indicates that the CdS nanocrystals in the film also have a quantum confinement effect [29].
The absorption spectra of polymers showed blue-shift in solution compared with that of the solid
films. The blue shift in solution is attributed to coil like structure in solution whereas solid films
have rod like structure. Coil like structures have short effective conjugation length compared to
Chapter 5
115
rod like structure with higher conjugation length, this results in the increase of π-π stacking in
film form of P3HT. Absorption spectra of films also show absorption shoulder at 605nm for
P3HT and at 595nm for P3HT/CdS. These shoulders are assigned to the 1Bu vibronic sidebands
[32] which confirm the interchain absorption in polymer [33, 34].
Figure 5.4 UV-Visible absorption spectra of P3HT and P3HT/CdS QDs nanocomposites (a) in
solution and (b) in solid state.
5.3.2.2. Photoinduced charge transfer at the donor/acceptor interface
Semiconducting nanocrystals are known to accept electrons from an excited polymer and then
transfer the electrons to another acceptor molecule (PCBM). The demonstration of semiconductor
nanocrystals mediated electron transfer between donor and acceptor molecules bound to its
surface is shown in Figure 5.5. Photoluminescence quenching in a bulk heterojunction is a useful
indication of the degree of success of exciton dissociation and efficiency of charge transfer
between the donor-acceptor composite materials [35, 36]. P3HT has a photoluminescence
property, [37, 38] and the photoluminescence spectra of P3HT and P3HT/CdS solution in
chloroform at excitation wavelength 448 nm, are presented in Figure 5.6 (a). Significant PL
quenching has been observed for the nanocomposite solution. The PL intensity of the composite
solution is significantly reduced as compared to pristine P3HT in Figure 5.6 (a). This indicates
that charge transfer, thereby exciton dissociation at interface between CdS and P3HT, is taken
place. Higher exciton dissociation efficiency accounts for higher device performance. For an
excitation wavelength of 448 nm, the maximum emission at 587 nm for P3HT and 583 nm for
P3HT/CdS composites solution have been observed. The reason for the photoluminescence
116
quenching of P3HT/CdS may be due to the π-π interaction of P3HT with CdS [39], forming
additional decaying paths of the excited electrons through the CdS. The small blue shift (4 nm) in
the nanocomposite PL emission spectra indicates that the ground state energy level is more stable
in the nanocomposite than that of pristine P3HT. This may be possible through the resonance
stability of π clouds of P3HT and CdS through π-π interaction.
Figure 5.5 Modulation of photoinduced charge transfer between the P3HT-CdS-PCBM.
Figure 5.6 Photoluminescence spectra of P3HT and P3HT-CdS composites at different weight
ratio of P3HT and CdS in (a) solution of chloroform and (b) thin films casted from chloroform
solution and annealed at 120 ˚C for 30 min. Here P3HT0, P3HT10, P3HT20 and P3HT50
represents the 0 wt.%, 10 wt.%, 20 wt.% and 50 wt.% of CdS in P3HT.
Chapter 5
117
Figure 5.6 (b) shows the PL spectra of the same samples in solid states (film form). The
P3HT and P3HT-CdS nanocomposites exhibited PL emission maximum around 640 nm. PL
intensity of the nanocomposite thin films significantly reduces with increase of CdS concentration
in the pristine P3HT. For the 50 wt.% of CdS, the PL intensity almost diminishes. Reduced PL
intensity of the composites relative to the reference P3HT indicates the charge transfer, thereby
exciton dissociation at P3HT-CdS interface, as shown in Figure 5.5. This PL quenching
experiment provides us with good evidence that the CdS QDs will be able to transfer their excited
state hole to the polymer. In this conversion, the polymer absorbs the solar photons (exciton
generation), the electron is transferred to the CdS QDs and the hole potentially can transfer to the
polymer (exciton dissociation).
5.3.3. J-V characteristics of Solar Cells
Figure 5.7 (a) and 5.7 (b) shows the J-V characteristics of device X and Y under AM 1.5
illuminations with intensity of 80 mWcm-2
. The performance of device X showed a short-circuit
photocurrent (Jsc) of 2.57 mAcm-2
, an open-circuit voltage (VOC) of 0.45 V, a fill factor (FF) of
0.30, and overall power conversion efficiency (PCE) of 0.45%. When CdS QDs have been
incorporated in P3HT matrix (device Y), the PCE value increased up to 0.87% by improving the
JSC of 4.65 mAcm-2
, VOC of 0.45 V, FF of 0.32. The performance of devices X and Y after
thermal annealing at 150 0C for 10 min are shown in Figure 5.7 (c) and 5.7 (d), respectively. After
thermal treatment, device X delivers VOC of 0.58 V, JSC of 2.26 mA/cm2, FF of 0.45 and device
efficiency of 0.74%, whereas device Y gives VOC of 0.58 V, JSC of 2.98 mA/cm2 and a FF of 0.44,
resulting in an estimated device efficiency of 0.95 %. These data are summarized in Table 5.1.
Table 5.1 Performance of P3HT/PCBM solar cells with and without CdS QDs
Devices Voc (Volts) Jsc (mA/cm2) FF (%) Efficiency (%)
Device X 0.45 2.57 30.0 0.45
Device Y 0.45 4.65 32.0 0.87
Device X annealed 0.58 2.26 45.0 0.74
Device Y annealed 0.58 2.98 43.99 0.95
118
Figure 5.7 J-V curve of P3HT:PCBM and P3HT:CdS:PCBM solar cells under AM 1.5
illumination at an irradiation intensity of 80 mW/cm2
. Figure (a) and (b) represents devices
without thermal annealing and Figure (c) and (d) represents devices with post production heat
treatment at 150 0C.
The modulation of device parameters i.e. increase in the value of VOC, JSC, and FF, in
device Y can be understood in terms of host P3HT and guest CdS QDs charge transfer type
interaction. In fact there are various possibilities by which doped CdS can interact with host
P3HT. It can either go structurally into P3HT main chain or forms donor-acceptor charge transfer
complex (CTCs) or form molecular aggregates. However, the enhancement in JSC in P3HT on
CdS dispersion indicates that CTCs formation between the host and the guest may be the
dominant mechanism of interaction between the two. This suggested mechanism is indeed
supported by the UV-Vis absorption and PL emission studies in pure P3HT and CdS dispersed
P3HT as shown in Figures 5.4 and 5.6, respectively.
Chapter 5
119
Blue shift in UV-Vis absorption (Figure 5.4) on incorporation of CdS QDs in P3HT
matrix may be attributed to the CTCs/quantum confinement effect from the CdS nanoparticles
[29]. Also small blue shift (4 nm) in the nanocomposite PL spectra indicates that during the CTCs
formation the ground state energy level is more stable in the nanocomposite than that of pristine
P3HT. This may be possible through the resonance stability of π clouds of P3HT and CdS through
π-π interaction [30] as a result of CTCs formation.
Similarly PL quenching seen in Figure 5.6 on CdS dispersion in P3HT is a direct evidence
of CTCs formation between the host and guest, since PL quenching is an indication of the degree
of success of exciton dissociation and efficiency of charge transfer between the donor-acceptor
composite materials. The PL quenching in P3HT/CdS has been attributed to the π-π* interaction
of P3HT with CdS, forming additional decaying paths of the excited electrons through the CdS.
To be more precise, during CTCs formation CdS QDs may diffuse into the amorphous-crystalline
boundaries of the P3HT polymer and the QDs introduce the conducting path thus reducing the
defect states and barrier height at these interfacial boundaries.
The thermally induced morphology modification has led to increase in PCE and the
improved FF which implies a significant decrease in the series resistance [36], thermally induced
crystallization and improved transport across the interface between the bulk heterojunction
material and aluminum (Al) electrode [5]. This improved nanoscale morphology results in more
efficient charge generation. The higher crystallinity and improved transport across the interface,
result in better charge collection at the electrodes with reduced series resistance and hence the
higher fill factor.
5.4. CONCLUSIONS
1. In order to reduce charge recombination and increase the carrier mobilities in
P3HT:PCBM based devices, the CdS QDs have been incorporated in the P3HT matrix.
2. HRTEM images reveal that the size of CdS QDs ranges from 5 to 6 nm and their shape is
spherical. The average crystallite size determined from the Debye–Scherrer formula is estimated
to be about 2.33nm.
3. The P3HT/CdS nanocomposite shows blue shift in the absorption spectra relative to the
pristine P3HT which is attributed to the quantum confinement effect from the CdS nanocrystals.
120
4. The PL quenching in the P3HT/CdS nanocomposite indicates that charge transfer, thereby
exciton dissociation at P3HT/CdS interface.
5. On incorporation of CdS QDs in P3HT matrix, the power conversion efficiency increases
from 0.45% to 0.87% due to enhancement in short-circuit current, and fill factor.
6. The enhancement in JSC have been explained on the basis of the formation of charge
transfer complex between the host (P3HT) and guest (CdS QDs), duly supported by blue shift in
UV-Vis absorption and PL quenching studies.
7. The effect of post thermal annealing on device performance has also been investigated and
found improved efficiency of devices after thermal treatment. This increase in efficiency may be
due to improved nanoscale morphology, increased crystallinity and improved contact to the
electron-collecting electrode.
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CHAPTER 6
STUDY ON THE CHARGE TRANSPORT MECHANISM IN ORGANIC AND
ORGANIC/INORGANIC HYBRID SYSTEM
6.1. INTRODUCTION
6.2. BASIC CONCEPTS OF THE CHARGE TRANSPORT PROCESSES
6.2.1. Intra-molecular and Inter-molecular perspective
6.2.2. Role of Disorder
6.2.3. Hopping Transport
6.2.4. Charge Carriers in Conjugated Polymers: Concept of Polaron
6.3. CHARGE CARRIER MOBILITY
6.3.1 Factors Influencing the Charge Mobility
6.3.1.1. Disorder
6.3.1.2. Impurities/Traps
6.3.1.3. Temperature
6.3.1.4. Electric field
6.3.1.5. Charge carrier density
6.4 SPACE CHARGE LIMITED CONDUCTION
6.4.1 Trap Free SCLC
6.4.2. SCLC with Exponential Distribution of Traps
6.5. UNIFIED MOBILITY MODEL
6.6. RESULTS AND DISCUSSION
6.6.1. Hole Transport Mechanism in P3HT
6.6.2. Hole Transport Mechanism in P3OT
6.6.3. Hole Transport Mechanism in P3HT-OT
6.6.4. Hole Transport Mechanism in P3HT/CdTe hybrid System
6.6.5. Hole Transport Mechanism in P3HT/CdS hybrid System
6.7. CONCLUSIONS
References
124
6.1. INTRODUCTION
nderstanding of the charge transport mechanism in organic semiconductors is of vital
importance for the development of devices and the realizations of the promises they
hold. In the present chapter, the charge transport mechanisms that occur in organic and
organic-inorganic hybrid systems have been studied. The transport properties in thin-film device
structures made up of conjugated polymers have been well characterized using space-charge
limited current models with field dependent mobilities [1-4]. Nanocrystals are discrete particles,
which can be physically separated from one another, either by the surrounding medium or by a
ligand shell. In fact, the temperature dependence of the conductivity in the films of nanocrystals
has been observed to be thermally activated, which suggests that an activated hopping transport
model can be used to describe the charge transport [5]. This is similar to the hopping model
described for organic semiconductors, but in this case, energetic disorder arises from the size
distribution of the particles and geometric disorder from the separation of particles, spatially or by
ligands [6]. Unlike most conjugated polymer, nanocrystals can transport both electrons and holes
with comparable mobilities [7]. The individual transport properties of both nanocrystals and
polymers have been studied separately in various electronic devices [6-10]. The carrier transport
behavior of these materials in composite devices, in particular, photovoltaic cells, has not been
sufficiently characterized. It is of particular interest to study charge transport in the films of
nanocrystals-polymer hybrid systems, since these systems represent a combination of the
disordered transport in organic materials and the band like transport in inorganic semiconductors.
In this work, the hole transport in organic and organic/inorganic (P3HT/CdS, P3HT-CdTe) hybrid
systems has been investigated and a quantitative explanation is provided for the observed
electrical characteristics in these hybrid systems.
6.2. BASIC CONCEPTS OF THE CHARGE TRANSPORT PROCESSES
6.2.1. Intra-molecular and Inter-molecular Perspective
Organic semiconductors are made up of molecules which consist mainly of carbon and hydrogen
atoms. The carbon atoms in these compounds are sp2 hybridized. The s, px and the py orbitals
hybridize and reorient themselves along a plane separated from each other by 120 degrees. The
remaining pz orbital extends perpendicularly above and below the plane. Two neighboring carbon
atoms covalently bond with each other using an in-plane overlap of the hybridized orbital, called
the σ bond, and another out of plane overlap of the pz orbitals termed as π-bond [Figure 6.1(a)].
When this structure is repeated over a large number of carbon atoms, the π-electrons are
delocalized above and below the plane which is termed as conjugation. Charge transport along a
conjugated polymer chain is called intra-molecular transport while charge transport between
U
Chapter 6
125
adjacent polymer chains called inter-molecular transport [Figure 6.1(b)]. The former which is
specific to conjugated polymers is the most efficient.
In the organic semiconductors, instead of two levels there are two bands of highest
occupied molecular orbital (HOMO) (π band) and lowest unoccupied molecular orbital (LUMO)
(π* band). The HOMO and the LUMO are associated in the polymeric semiconductor to the
“valence band” and the “conductance band”, respectively. The conjugated polymers, as long
chains, tend to create an amorphous solid without any long range order - a "spaghetti pile" like
structure. As a result, there are interferences in the conjugation of the π-orbitals, and the electronic
wave-function continuity is limited in length. This average length is defined as the conjugation
length. A short conjugation length characterizes conjugated polymers and conjugated amorphous
organic materials, similar to the potential barriers in poly-crystalline in-organic semiconductors or
amorphous semiconductors.
Intra-
molecular
Inte
r-m
ole
cu
lar(a) (b)
Figure 6.1 Pictorial representation of (a) formation of σ and π bonds in organic molecule and (b)
intra-molecular and inter-molecular charge transport in organic semiconductors.
6.2.2. The Role of Disorder
The electronic properties of a fully periodic system can be described in terms of Bloch-functions,
energy bands, E-k dispersion relation, and electrons and holes as "free particles like" charge
carriers [11-15]. Inserting a local disorder to such a system will result in the appearance of
scattering centers and energy states in the forbidden gap (deep or shallow levels). A strong
interaction with the scattering centers and many scattering centers results in a decrease of the
mean free path (λ) [16]. When the mean free path is in the order of the typical distance in the
material (kλ~1), the description of "free particle like" charge carriers that can be described in
terms of the Bloch wave functions, is not valid anymore [16]. Such a situation is expected in
126
amorphous materials. In these materials, the short range order is kept but the long range order
breaks down. Explicitly, there is a typical distance between electronic sites nearest neighbors, but
the long range symmetry is weaker or absent. The first concept, equally valid to crystalline and
non-crystalline materials, is the density of states (DOS) g(E). The quantity g(E) denotes the
energy and spatial density of electronic states (per unit energy and per unit volume). There is a
variety of possible shapes and the character of DOS. For instance, the electronic states may be
localized at a certain energy range while beyond this range the states are free. Figure 6.2 shows
the three possible types of DOS that are used to describe non-crystalline materials.
Figure 6.2 Three possible types of density of states in an amorphous material: (a) Free states
band with a localized band at the forbidden energy gap (trap band), (b) free states band with a
localized tail, (c) fully localized band. The shaded shapes denote localized states, where the
energy separating between localized and free states is the mobility edge (EM). A possible position
of the Fermi level EF is marked [16].
The first model [Figure 6.2(a)] is the closest to the crystalline material: two bands of free
states (for holes and electrons) and a distribution of a localized, deep traps, band in the forbidden
gap. The second model [Figure 6.2(b)] is of electronic band that contains localized states at the
lower energy range, and free states at the upper energy range. The energy that separates between
localized and free states is referred as the mobility edge (EM). The third model [Figure 6.2(c)] is of
fully localized band.
6.2.3. Hopping Transport
Most of the organic materials display low-conductance behaviour. The hole mobility in these
materials are typically ranging from 10-7
to 10-3
cm2/(Vs) [Si hole mobility is 1400 cm
2/(Vs)], and
the values for electron mobility are commonly reported lower by a factor of 10-100 [Si electron
mobility is 450 cm2/(Vs)]. The lower mobility in organic semiconductor, in comparison with their
inorganic counterpart, is due to the disorder presented in these materials. Motion of a charge
carrier in the organic semiconductors can be described using hopping transport. Hopping is
defined as a phonon assisted tunneling between two localized electronic states centered at
Chapter 6
127
different locations [17, 18]. It is usually observed in disordered semiconductors due to localization
of charges. This hopping transport takes place around the Fermi level. Many of the hopping
models are based on the single phonon jump rate description as proposed by Miller and Abrams
[19]. In the Miller Abrams hopping model the hopping rate between an occupied site i and an
adjacent unoccupied site j , which are separated in energy by Ei − Ej and in distance by Rij, is
described by
ij
ji
B
ji
ijji EE
EE
Tk
EE
R ,
,1
exp2exp0 (6.1)
where Rij is the intersite distance, ν0 is a prefactor and kB is the Boltzmanns constant. When a field
E is applied, the site energies also include the electrostatic energy. In addition to the energetic
disorder of the transporting sites, positional disorder can be taken into account by regarding the
overlapping parameter γ. As a matter of fact, the transition rate νij from one site to another
depends on their energy difference and on the distance between them. The carriers may hop to a
site with a higher energy only by absorbing a phonon of appropriate energy.
6.2.4. Charge Carriers in Conjugated Polymers: Concept of Polaron
The charge delocalization in the inorganic semiconductors is supported by the large transfer
integrals (around 1eV) calculated between neighboring atoms. It implies a description in terms of
Bloch wavefunctions. However, this picture holds true for organic molecular crystals only at very
low temperature, since the transfer integrals between neighboring molecules are quite low (20 to
80 meV) due to weak Van der Waals interactions, which results in narrow bandwidths. As a
consequence, perturbation effects with the same order of magnitude as the bandwidth can induce
the localization of the charge carriers [20].
The validity of the band model can be verified by calculating the mean free path λ, which
has to be much larger than the crystalline cell parameter a. In general, this condition fails for
organic crystals and a different transport mechanism such as hopping must be invoked. Since the
importance of the phonons is not-negligible in organic conjugated materials, strong charge carrier-
phonon interactions lead to the formation of quasi-particles called polarons.
Thus, polaron [21, 22] is a quasi-particle composed of an electron or a hole and its
associated lattice distortion. It can be defined as a slow moving electron or a hole traveling in a
dielectric medium, that interacts with the lattice ion through the long range forces producing a
polarization field around itself, that travels with the electron or hole. In other words, it can be
described as a cloud of phonon accompanying an electron/hole as it carries its lattice distortion
while moving through the medium.
128
6.3 CHARGE CARRIER MOBILITY
Mobility is measured in (cm/sec) per (volt/cm); i.e. the average velocity of a charge carrier per
unit applied field. In absolute terms mobility varies enormously from one semiconductor to
another. The concept of mobility is very important because it provides us with information on
how fast a charge carrier will move per unit applied field. Achievable fields for a given solar cell
maybe limited by the energetic of the materials employed and dopant concentration, but the
current that can be collected will depend strongly on how fast the charge carriers move under the
influence of the generated external voltage. Electric current measures the number of charge
carriers that cross a unit cross sectional area per unit time. Area of a solid state device may be
considered constant, so mobility becomes the important comparison parameter.
6.3.1 Factors Influencing the Charge Mobility
6.3.1.1. Disorder
In a disordered solid, disorder can be modeled by assigning random site energies from a
probability distribution function. These disorders can be of two kinds: diagonal and non-diagonal
disorders. Diagonal disorder related to the distribution of the energy transporting levels, HOMO
and LUMO of the different molecular sites and is often related to the presence of chemical
impurities [28] or trap states [29]. In the case of flexible molecules, a major contributor to
diagonal disorder is the large conformational degree of freedom (leading for instance to a
distribution of torsion angles between adjacent units). In polymer chains, such a distribution of
torsion angles results in a diagonal disorder via the formation of finite-size conjugated segments
with different lengths and therefore different HOMO and LUMO energies. In addition, diagonal
disorder might be induced by electrostatic/polarization effects from surrounding molecules,
induced by fluctuations in the local packing; this effect is amplified when the molecules repeat
units contain local dipole moments [30-33]. This also holds true when the molecule or the
polymer repeat unit does not carry a permanent dipole moment [34].
In theoretical simulations of transport in disordered materials such as amorphous films,
energetic disorder is generally modeled by a Gaussian distribution of localized states with
standard deviations on the order of 50-100 meV. The non-diagonal disorder reflects fluctuations
in the strength of the intermolecular interactions (i.e. transfer integrals) which depend on the
orientation of the interacting units. If the energetic distribution can be accessed experimentally,
the positional disorder cannot be measured and is accessible only from theoretical calculations
[35]. The off-diagonal disorder promotes either highly conductive pathways or dead-ends for
charge depending on the values of the transfer integrals.
Chapter 6
129
EF
HOMO
LUMO
Et
Shallow trap
EF
HOMO
LUMO
EtDeep trap
EF
HOMO
LUMO
Et
Distribution
of trap
6.3.1.2. Impurities/Traps
The definition of a trap depends on the nature of the charge carrier. For holes (electrons), the
presence of a molecular site characterized by a higher (lower) HOMO (LUMO) with respect to
the levels of the valence (conduction) band is called a trap. Indeed, the chance for these levels to
be filled by a charge carrier is high because it represents a thermodynamically more stable
situation. The lifetime of a hole or electron in a trap state is function of the trap depth. We can
distinguish, shallow traps with a depth of the order of a few kBT and deep traps with depth much
higher than kBT.
The most common defect in an organic crystal is a schottky defect, which is a point defect
formed by a vacancy; an empty site in the crystal structure. Any molecule which has its ionization
energy lower or its electron affinity higher than that of the molecule of interest, behaves as a hole
trap or an electron trap, respectively. These unwanted molecules when present in small amount
among the host molecules, are termed as impurities and create favorable energy states inside the
band gap of the material. These favorable energy states are called as traps and can be shallow or
deep. Shallow or deep trap is defined depending upon the position of the Fermi level with respect
to the trap energy level. For hole traps if the Fermi energy level lies above the trap energy level it
is called shallow trap, on the other hand if the Fermi energy level lies below the trap level, it is
called deep trap with respect to valance band (shown in Figure 6.3). The reverse is true for the
electron with respect to conduction band edge [3].
Impurities are often generated as side products of synthetic reactions. The presence of
impurities can influence the packing of the molecules and create regions with different
polarization energies [36], resulting in a local perturbation of the energy transport levels. The
intrinsic properties of the impurity namely their ionization potential and electron affinity can also
make them acting as a trap.
Figure 6.3 Schematic of typical hole traps.
130
The traps are distributed spatially and energetically in a semiconducting layer. There are two
important distribution functions that are used to characterize the dispersion in trap energies in the
forbidden energy gap. One is the exponential distribution function proposed by Rose [23] and
modified by Mark and Helfrich [24]. It is given by equation:
Tlk
EE
Tlk
HEH
B
tC
B
tt exp)( (6.2)
Where H(Et) is the density of trapping states at energy Et and Ht is the total trap density, l is an
empirical parameter [25], greater than unity, defines how the trap density changes with trap
energy. EC is assumed to be above Et.
The other is a Gaussian distribution proposed by Silinsh [26] is of the form
2
2
2 2exp
2)(
mttt
EEHEH (6.3)
Where Em is the center of the distribution and σ is the dispersion of trap energies around Em.
The exponential distribution is simpler to use and in many cases, the results are close to
that obtained with the Gaussian distribution. Hence in most cases it is experimentally difficult to
differentiate between the two trap distributions given by the above Equations. In the organic
semiconductors the width of the bands can be very narrow and extended states are rarely
observed. Especially in amorphous layers of organic thin films the density of states (DOS) is quite
well represented by a Gaussian-like distribution of localized states of individual molecules as
presented in Figure 6.4.
DOS
HOMO
LUMO
Energy
Figure 6.4 Distribution of HOMO and LUMO levels in organic semiconductors [27].
Chapter 6
131
Experimentally, the distribution of trap depth is measured by Thermally Stimulated
Current measurements. The principle of these measurements is that, after cooling the sample,
charges are created upon exposure to light at a determined wavelength. The sample is then heated
slowly and the current coming from the de-trapped charges is measured as a function of
temperature to estimate the trap depths as well as their distribution [37].
6.3.1.3. Temperature
The temperature dependence charge carrier mobility has been extensively studied in the literature
and has often been turned to a discussion whether a band model or a hopping picture prevails. In
ultra pure organic crystals, the charge carrier mobility often decreases with temperature according
to the power law T-n
[38], with n a positive number. A thermally activated mobility is
characteristic of the presence of shallow traps; when a critical temperature is reached all charges
are de-trapped and the mobility reaches a maximum before decreasing with a power law [39].
In disordered materials, charge carriers are localized due to the presence of energetic and
positional disorder. Charge transport occurs by hopping and is thermally activated. A higher
temperature leads to a larger mobility, the thermal energy helping in crossing of the energetic
barrier between adjacent molecular sites [40]. The mobility is often fitted by an Arrhenius-like
relationship
)exp(0TkB
(6.4)
where µ0, is the mobilities at zero electric field, µ∞ is the high temperature limit of mobility, and ∆
is the activation barrier.
However, Bässler and coworkers [17] showed that the temperature dependence of the mobility in
presence of a Gaussian energetic disorder fits the following expression:
2
0 exp
TkB
(6.5)
where σ is the width of the energetic distribution.
6.3.1.4. Electric Field
In disordered materials, an increase in the mobility is observed at high fields. The field
dependence in the range between 104
-106 V/cm generally obeys a Poole-Frenkel behavior [41-
43]:
FTTF )(exp),0()( (6.6)
where µ(0,T) is the zero-field mobility, γ(T) the field activation factor, which reflects the lowering
of the hopping barriers in the direction of the applied electric field F. The increase of F gives rise
132
to increase of the charge carrier density. The following expression for γ(T) usually allows for a
good fit of the experimental data [42,44]:
oBTkT
1
k
1(T)
B
(6.7)
where T0 a parameter with unit of temperature. Generally, T0 is much higher than room
temperature.
6.3.1.5. Charge-Carrier Density
Experimentally, two main effects demonstrate that the charge transport properties in amorphous
organic semiconductors depend on the charge carrier density. The doping of organic matrices
represents a first clear demonstration of such an effect. It is seen in such experiments that the
mobility first decreases, when the doping ratio is between 0.01 and 1% as explained by the
increase in the concentration of deep traps [45]. However, the mobility increases at higher doping
ratio (up to 10%), due to increased spatial overlap between the trap levels, which lower the
activation barriers [46]. Phillips et al. [47] has shown experimentally that the mobility measured
in PPV and polythiophene derivatives is much lower in diode than in FETs by two or three orders
of magnitude. The explanation lies in the fact that the density of injected charges is much larger in
transistors than in diodes. The observed behavior can be interpreted in terms of a Gaussian DOS.
At lower densities, all the carriers occupy the lower energy states of the DOS and are thus
affected by trapping. At higher carrier densities, only a portion of the carriers are necessary to fill
all the traps, the remaining carriers can access easily to higher energy states. Since these states are
more numerous, trap-free transport is achieved and an increase of the mobility is noticed.
6.4 SPACE CHARGE LIMITED CONDUCTION
Space charge is generally referred to as the space filled with net positive or negative charge. The
space charge limited conduction (SCLC) occurs when the contacting electrodes are capable of
injecting either electrons into the conduction band or holes into the valance band of a
semiconductor or insulator, where the initial rate of such charge carrier injection is higher than the
rate of recombination [2, 3, 48].
An approximate theory of SCLC in a trap-free insulator was proposed by Mott and Gurney
[49] and later extended by Rose, Lampert and Mark, and others [3, 50] to describe currents
limited by the space-charge confined in a single discrete energy level and in localized states with
a distribution of energy. The simplified SCLC theory, which is usually applied to model the I-V
characteristics in organic devices, is based on two main approximations: firstly diffusion currents
are neglected to describe the current flow and secondly the ohmic contact is taken to be an infinite
Chapter 6
133
reservoir of charges available for injection. The first approximation simplifies the theory to
mathematically manageable elementary analysis. The second approximation makes the theory
independent of any detailed properties of the contact and thereby makes a universal theory
possible.
The distribution function for the hole trap density as a function of energy level E above the
valence band, and a distance x from the injecting contact for holes can be written as: [2]
)()(),( xSEnxEh (6.8)
where n(E) and S(x) represent the energy, and spatial distribution functions of traps, respectively.
An assumption of uniform spatial trap distribution within the specimen from injecting
electrode to collecting electrode implies that the effective thickness of the device, under space
charge conditions, remains the thickness itself, and S(x) = 1. The specific functional form of the
SCLC, J-V curve depends on the distribution of charge traps in the band gap. If the traps capture
only holes, the electric field F(x) inside the specimen follows the Poisson’s equation:
)]()([)( xpxpq
dx
xdF t (6.9)
The current density may be expressed as:
)()( xFxpqJ (6.10)
Where p(x) and pt(x) are, respectively, the densities of injected free and trapped holes, and they
are given by
F
l
E
Ept dEEfxEhxp )(),()( (6.11)
and
kT
ENxp
Fp
v exp)( (6.12)
and fp is the Fermi-Dirac distribution function.
6.4.1 Trap Free SCLC
The perfect trap free insulator is the solid state analog of the thermionic vacuum diode. There are
neither thermal free carriers nor trapping states in the solid, that is pt(x)=0. The Poisson’s
equation now can be expressed by
J
dx
xFd
dx
xdFxF
2)]([)()(2
2
(6.13)
Integrating the above equation using the boundary condition
d
dxxFV0
)( (6.14)
134
yields 3
2
8
9
d
VJ
(6.15)
This equation is referred to as the trap-free square law, the Mott-Gurney square law and or
Child’s law for solids.
6.4.2. SCLC with Exponential Distribution of Traps
When the traps are distributed exponentially in the energy space within the forbidden gap,
distribution function [Equation (6.8)] can be written as:
)(exp),( xSE
E
E
HxEh
tt
b
(6.16)
where, Hb is the density of traps at the edge of valence band, and Et is characteristic trap energy.
Et is also often expressed in terms of the characteristics temperature TC of trap
distribution CBt TkE .
If TC˃T we can assume that fp(E)=1 for EFp˂E˂∞. and fp(E)=0 for E ˂ EFp as if we take T=0.
With this assumption
FpECBCB
bt dExS
Tk
E
Tk
Hxp )(exp)(
)(exp)( xSTk
EHxp
CB
Fp
bt
)()( xSN
pHxp
CTT
v
bt
(6.17)
Using continuity Equation (6.10), and boundary condition (Equation 6.14) in Equation 6.9, the
expression for J is given by:
12
1
0
1
1
11
12
l
ll
b
r
l
v
l
d
V
Hl
l
l
lNqJ
(6.18)
where F(x) is the electric field inside the film, Nv is the effective density of states and
TTTkEl CBt // . The parameter l determines the distribution of traps in the forbidden gap.
From the Equation (6.18), the slope of the current-voltage characteristics on a log-log plot is l+1.
Therefore, from the slopes on the log-log plots of current density versus voltage, one can extract
the trap energy width Et.
6.5. UNIFIED MOBILITY MODEL
This model is based on percolation in a variable range hopping (VRH) system with an exponential
distribution of localized states [51-53]. Percolation is the term used for movement of charge
Chapter 6
135
carriers through a random network of obstacles. Consider a square lattice, where each site is
randomly occupied or empty. Occupied sites are assumed to be electrical conductors while the
empty sites represent insulators, and that electrical current can flow between nearest neighbor
conductor sites. Percolation paths are the most optimal paths for current and transport of charge
carriers which are governed by the hopping of charge carriers between these conducting sites. The
system can be described as a random resistor network [54], a system made up of individual
disconnected clusters of conducting sites, whose average size is dependent on a reference
conductance G. The conductance between sites is given by:
ijsGG exp0 (6.19)
with Tk
EEEEEErs
B
FjFiij
ijij2
2
(6.20)
All conductive pathways between sites with GGij are electrical insulators while conductive
pathways between sites with GGij are electrical conductors. At some critical conductance in
between, therefore, a threshold conductance GC exist where the first time electrical current can
percolate from one edge to the other.
A bond is defined as a link between two sites which have a conductance GGij . The
average number of bonds B is equal to the density of bonds (Nb), divided by the density of sites
that form bonds, (Ns), in the material. Critical bond number BC is the average number of bonds
per site for which threshold percolation occurs. The onset of percolation is determined by
calculating the critical average number of bonds per site [53].
S
bCC
N
NBGGB )( (6.21)
Vissenberg and Matters [55], set the critical bond number to Bc = 2.8, The total density of
bonds is given by
ijjiijcjiijb drdEdEssEgEgrN )()()(4 2 (6.22)
The density of sites Ns
dEEETksEgN FBcs )()(
(6.23)
At low carrier concentration exponential density of states in amorphous organic semiconductors is
given by [53, 55]:
0
0,0
,exp)(
00
0
E
E
Tk
E
Tk
N
EgBB
(6.24)
136
where No is the total density of states (molecular density) per unit volume and To is a
characteristic temperature that determines the width of the exponential distribution.
Combining Equation (6.20)-(6.24), the expression for Bc
0
max
3
0
0 exp2 Tk
E
T
TNB
B
C
(6.25)
where TksEE BCF max is the maximum energy that participates in bond formation. According
to the percolation theory, the conductivity of the system can be expressed as
]exp[0 Cs (6.26)
where σ0 is the prefactor and sc is the critical exponent of the critical conductance when
percolation first occurs (when B = Bc).Using Equation (6.25) and (6.26) we get
TT
C
pB
T
T
T
T
0
3
0
4
0
02
sin
(6.27)
The conductivity can be converted into mobility by dividing by e.p, where e is the electronic
charge and p the carrier density [56]:
1
3
0
4
00
0
0
2
sin
),,(
T
T
TT
C
pB
T
T
T
T
qFpT
(6.28)
The average charge carrier density as a function of the applied bias voltage V is given by [3]
2
075.0)(qd
VVp r
(6.29)
6.6. RESULTS AND DISCUSSION
The J-V characteristics of organic and organic/inorganic hybrid systems have been investigated in
the device configuration viz. indium tin oxide (ITO)/poly(3,4-ethylendioxythiophene)-
poly(styrene sulfonate) (PEDOT:PSS)/Active layer/Au. Work function of Au and ITO are close to
the HOMO energy level of active layer (P3HT, P3OT, P3HT-OT, P3HT-CdTe and P3HT-CdS) as
well as far below the LUMO energy level as shown in Figure 6.5. It is clear from Figure 6.5 that
the electron injection barrier is quite higher as compared to the holes injection barrier, from both
Chapter 6
137
Due to low carrier
mobility, injected carrier
form a space charge. Au
ITO
HOMO
LUMO
low p, high E
Two high workfunction
electrodes to prevent
electron injection
0 tx
Collecting contact
Injecting contact
the electrodes. As a result, the transport is dominated by holes in the Au:ITO based device, and
so-called hole only device.
For the fabrication of hole only devices, ITO coated glass substrates have been carefully
cleaned as discussed in section 2.4.1 and dried at 120°C for 2 hrs in vacuum. Prior to use, the
cleaned substrates were treated with oxygen plasma. A PEDOT: PSS layers were spin-coated at
onto the ITO substrate and cured at 120°C for 60 min in vacuum. Active materials were spin
casted in an inert atmosphere, followed by annealing at 120°C for 30 min. Finally, gold (Au)
contacts (200 nm) was applied via evaporation through a shadow mask at 2×10-6
Torr. The device
active areas were ~0.1 cm2 for all the devices discussed in this work. J-V characteristics of the
devices were measured with Keithley 2400 Source-Measure unit, interfaced with a computer.
Figure 6.5 Schematic illustration of the hole only device.
6.6.1. Hole Transport Mechanism in P3HT
Figure 6.6 shows the J-V characteristics in temperature range 290-150 K of a device based on
P3HT. On lowering down the temperature, the decrease in current was observed. In the organic
semiconductors charge transport is governed by hopping of a carrier from site-to-site of an empty
density of states. The thermal energy helps to cross the energetic barrier between two adjacent
sites. This implies that the charge transport in organic semiconductor is thermally activated.
Therefore, the decrease in current is obvious on lowering down the temperature.
At low applied bias, the J-V characteristics follow the ohm’s law: d
VqnJ , as injected
carriers are negligible compared to that of the applied bias [57]. At moderate field, the injected
carrier density becomes so high that the field due to the carriers dominates the applied bias. At
this point the J-V characteristics may switch to pure SCLC and follow the Child-law (Equation
6.15). On further enhancement of field, the quasi-Fermi level intersects the exponential trap
138
distribution, and characteristics will begin to follow Equation 6.18. The hole mobility up to this
field is constant and also independent of the hole density. The fit of the J-V characteristics of the
P3HT device using the Equation 6.18 is poor at high applied bias where current density deviates
strongly as expected from Equation 6.18. This discrepancy has been analyzed by unified mobility
model given by Equation 6.28. This model accounts the influence of temperature, carrier density
and applied field on the carrier mobility [53]. The solid curves in Figure 6.6 have been obtained
by combining Equation 6.18 and Equation 6.28 using a computer program. The value of different
parameters for solid curves are; d=110 nm, r = 3, 0 = 8.8510-14
F/cm, Hb = 2.81018
cm-3
, Nv =
11019
cm-3
, TC=400K, KT 3250 , σ0=4×104 S/m, α-
1=1.12 Å, and Bc = 2.8.
0.01 0.1 1 10
1E-5
1E-4
1E-3
0.01
0.1
1
J (
A/c
m2)
Voltage (V)
150 K
170 K
195 K
225 K
260 K
290 K
Figure 6.6 Experimental (symbols) and calculated (solid lines) J-V characteristic of P3HT thin
film at different temperatures in hole only device configuration viz. ITO/PEDOT:PSS/P3HT/Au.
6.6.2. Hole Transport Mechanism in P3OT
Figure 6.7 shows the experimental J-V characteristics of hole only device of P3OT thin film in the
temperature range 150-290K. At low applied voltages (i.e. below 1V), the J-V relationship
initially exhibits typical Ohmic behavior, with a slope of about one, and then follows the trap-
filling SCLC law, where the slope is larger than two (i.e., l is larger than one). These results have
been analysed in terms of SCLC model. Generally, in most cases, the charge transport
mechanisms in amorphous organic semiconductors has been well explained by SCLC and trapped
Chapter 6
139
charge limited current model (TCLC) and J-V behavior beyond Ohm’s law follows the Equation
6.18.
The theoretically generated curves from Equation (6.18) in the Figure 6.7 gives a perfect
fit to the experimental curves for all analyzed temperatures with fitting parameters Nv =
1.0×1019
cm-3
, Hb = 2.5×1018
cm-3
, and TC = 720K. We obtained the l values in this trap-filling
SCLC regime and plotted them as a function of the inverse of the temperature in the inset of
Figure 6.8. From this plot we have evaluated the value of the width of the exponential trap
distribution i.e. lkT. The values of Et obtained from the SCL diodes is 63 meV.
0.01 0.1 1 10
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
J (
A/c
m2)
Voltage (Volts)
150 K
190 K
220 K
250 K
290 K
Figure 6.7 Experimental (symbols) and calculated (solid lines) J-V characteristic of P3OT thin
film at different temperatures in hole only device configuration viz. ITO/PEDOT:PSS/P3OT/Au.
In this analysis, mobility is found to be field and temperature dependent according to the Equation
[17]:
F
TkC
TkTF
BB
2
2
0
2
3
2exp),(
(6.30)
where is the high-temperature limit of the charge mobility and C is an empirical constant and
and Σ are energetic disorder and positional disorder respectively. The energetic disorder
140
parameter σ arises from distribution of the conjugation length, while the positional disorder
parameter Σ arises from fluctuation of the intermolecular distances or morphological variations.
3.5 4.0 4.5 5.0 5.5 6.0 6.5
3
4
5
6
7
8
9
Pa
ra
mete
r l
1000/T (K-1)
Figure 6.8 Temperature dependence of l obtained from theoretical fit according to the SCLC law
to the experimental data (shown in Figure 6.7).
Figure 6.9 gives the field dependent mobility at different temperatures, which shows that
the hole mobility increases exponentially with the square root of electric field F, consistent with
Equation 6.30. There is a gradual variation of the slope as the temperature increases from 150K to
290K. This indicates that the positional and geometrical disorders are present in the P3OT.
200 400 600 800 1000 120010
-9
10-8
10-7
10-6
10-5
[
cm2
/(V
-s)]
F1/2
(V/cm)1/2
290 K
250 K
220 K
190 K
150 K
Figure 6.9 Field dependence of mobility µ(0,T) at different temperatures, obtained from the
theoretical fit to the experimental data.
Chapter 6
141
Figure 6.10 shows the temperature dependence of zero field mobility and field activation
factor γ. The zero-field mobility μ(0, T) increases with the increase of temperature while the slope
γ decreases with increasing temperature, which is characteristic for hopping transport in
disordered organic solids. When charges transport in disordered organic materials by hopping, we
can describe it with disorder formalism, assuming that charge transport takes place by hopping
through localized states subject to fluctuation of both the hopping site energy and intermolecular
distance following the Gaussian distributions.
Figure 6.10 Temperature dependence of (a) field activation γ(T) and (b) zero field mobility µ(0,
T) and the obtained from the theoretical fit to the experimental data shown in Figure 6.7, are
plotted according to the Equation 6.30 with the fitting parameters = 9.3 × 10-6
cm2/V-s, σ
=69, Σ = 2.1 and C = 1.01 × 10-3
(cm/V) ½
.
By plotting lnµ(0, T) against 105/T
2 [Figure 6.10(b)] and conducting a linear fit according
to Equation 6.30, we can obtain the Gaussian distribution model parameters as µ∞= 9.3 × 10-6
cm2/V -s and σ =69 meV. The positional disorder parameter Σ and the empirical constant C are
obtained from the linear fit of the curves by plotting γ against 105/T
2 [Figure 6.10(a)]. The
parameters Σ and C are found to be Σ = 2.1 and C = 1.01 × 10-3
(cm/V) ½
.
6.6.3. Hole Transport Mechanism in P3HT-OT
Figure 6.11 shows the J-V characteristics of copolymer P3HT-OT thin film in hole only
configuration as mentioned above at different temperatures. These experimental results were
analyzed based on the theory of SCLC with traps distributed exponentially in energy and space.
4 5 6 7
1E-8
1E-7
1E-6
1E-5
[0
,T]
(cm
2/V
-s)
105
/T2(K-2)
(b)
3.5 4.0 4.5 5.0 5.5 6.0 6.5
0.005
0.010
0.015
0.020
(
cm
/V1
/2)
105
/T2(K-2)
(a)
142
0.1 1 101E-5
1E-4
1E-3
0.01
0.1
1
J(A
/cm
2)
Voltage (Volts)
290 K
240 K
190 K
150 K
110 K
Figure 6.11 Experimental (Symbols) and calculated [solid line using Equation (6.6) and Equation
(6.18)] J-V characteristics of hole only device of copolymer P3OT-HT for the temperature range
290 -110K.
When the experimental data in Figure 6.11 has been analyzed in terms of Equation 6.18, it
has been found that the theory fits up to intermediate fields and at high fields (corresponding to
V6 ), the current gradually deviates from the above proposed theory and becomes larger than as
expected from Equation 6.18. This discrepancy has been analyzed in terms of field dependent
mobility model given by Equation 6.6. In order to describe the hole conduction in P3HT-OT at
high fields, we combine the SCLC (Equation 6.18) with the field dependent mobility (Equation
6.6).
Temperature dependence of zero field mobility is shown in Figure 6.12(a) in an Arrhenius
plot, which decreases with lowering down the temperature. This thermally activated behaviour of
zero field mobility follows the Equation 6.4. Temperature dependent high field J-V characteristics
can be understood in terms of the coefficient γ(T). From the J-V curve (Figure 6.11), Equation 6.6
and Equation 6.18 the values of γ(T) has been calculated at each temperature. Figure 6.12(b)
shows the variation of γ(T) as a function of temperature. The experimental results show that there
is a linear dependence according to Equation 6.7.
Chapter 6
143
Figure 6.12 (a) Experimental (Symbols) and calculated [solid line using Equation (6.4) with
activation energy Δ= 0.21eV and Vscm /106.3 25
0
] Arrhenius plot of the zero-field mobility
versus temperature T. (b) The coefficient γ (which described the field dependence of the mobility)
as a function of temperature T. The solid line is according to Equation (6.7), using KT 5000
and 2/12/15 /109.6 cmVeV .
Expressions (6.4) to (6.7) describe the Arrhenius dependence of the mobility, which arises if
moving charges must hop over a coulomb barrier of height in energy. In such a case, electric-
field dependence arises because the barrier height is lowered on applying the electric field by an
amount F .
The set of J-V characteristics of copolymer P3HT-OT, as a function of temperature can be
fully described by combining Equation (6.4), (6.6), (6.7) and (6.18), using the parameters Hb =
3.81018
cm-3
,Nv=31019
cm-3
, Tc=560K, Et=46meV, d=150nm, µ0= 3.6×10-5
cm2/Vs, T0=500K,
∆=21meV and β=6.9×10-5
eV/V1/2
cm1/
.2
A microscopic interpretation of this ubiquitous mobility is that the charge transport in
disordered organic conductors is thought to proceed by means of hopping in a Gaussian site-
energy distribution. This DOS reflects the energetic disorder of hopping site due to fluctuation in
conjugation lengths, structural disorder [58, 59]. Copolymerizing of P3OT and P3HT could create
random structural disorder due to random repetition of hexyl and octyl side group and energetic
disorders due to different energy levels P3HT and P3OT. Due to these structural and energetic
3 4 5 6 7 8 9
0.0
2.0x10-9
4.0x10-9
6.0x10-9
8.0x10-9
1000/T (K-1)
(0
,T) [
cm
2/V
-s]
(a)
3 4 5 6 7 8 9
0.00
0.01
0.02
0.03
0.04
0.05
0.06
(c
m-V
-1/2
)
1000/T (K-1)
(b)
144
disorder in copolymer, the hole mobility is strongly dependent on temperature and electric field.
The introduction of the hexyl group into the P3OT matrix can also lead to structural defects and
hence increase of trap density, so that a fraction of the charges moving inside the P3OT-HT films
are trapped thereby reducing the mobility. Thus copolymerization is expected to diminish the
mobility and increase its electric field dependence for hole.
It is thus explicitly established from above that hole transport in P3HT-OT copolymer
thin films shows field and temperature dependent mobility at higher fields with hole transport
fitting parameters as 2/12/15 /109.6 cmVeV , Vscm /106.3 25
0
, KT 5000 and
meV21 , respectively.
6.6.4. Hole Transport Mechanism in P3HT-CdTe Hybrid System
Figure 6.13 shows the J-V characteristics of hole only device based P3HT-CdTe in the
configuration viz. ITO/PEDOT:PSS/P3HT-CdTe/Au, measured at different temperatures.
Interestingly, the nature of P3HT-CdTe composite thin film is different from that of pristine
P3HT, shown in Figure 6.6. In case of composite film the hole current has been observed to be
more than that in device based on pristine P3HT at all temperatures. Inset of Figure 6.13 shows
the comparison of J-V characteristics of P3HT and P3HT-CdTe at 150 K. The composites
exhibited S shape characteristic and the rate of reduction of current with temperature is low
compared to that in pristine P3HT. We tried to fit the experimental data with unified mobility
model [Equation (6.28)]. The data did not show agreement with the mobility model for single set
of parameter values. On the other hand, the comparison of experimental data with Equation (6.18)
showed a good agreement with same value of parameters at different temperatures. Solid curves
in Figure 6.13 represent the plot of Equation (6.18) at respective temperatures. The values of
parameters used in the calculations are; Hb=5.0×1018
cm−3
, Nv=6.0×1018
cm−3
, µ=6.0×10-5
cm2
V−1
s−1
, d=110 nm, and Tc=400 K. For the characteristics measured at 250 K, 220 K, 195 K, 175
K, 150 K, the agreement was obtained for µ=7.8×10-5
, 1.16×10-4
, 2.4×10-4
, 3.55×10
-4, 7.5×10
-4
cm2 V
−1 s
−1, respectively.
Chapter 6
145
1E-3 0.01 0.1 1 10
1E-5
1E-4
1E-3
0.01
0.1
0.01 0.1 1 10
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
150 K
Cu
rren
t d
en
sity
(A
/cm
2)
Voltage (V)
P3HT
P3HT-CdTe
0.01 0.1 1 10
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
150 K
Cu
rren
t d
en
sity
(A
/cm
2)
Voltage (V)
P3HT
P3HT-CdTe
J (
A/c
m2
)
Voltage (Volts)
280 K
250 K
220 K
195 K
175 K
150 K
P3HT-CdTe
Figure 6.13 Experimental (symbols) and calculated (solid lines) J-V characteristics of device B at
different temperature in hole only device configuration viz. ITO/PEDOT:PSS/P3HT-CdTe/Au.
The inset shows the comparison of J-V characteristics of P3HT and P3HT-CdTe at 150 K.
The enhancement in current density in P3HT-CdTe thin film can be understood in terms of
reduction of activation energy. The calculated values of activation energy of localized states have
been found to be 52 meV for P3HT and 11 meV for P3HT-CdTe (Figure 6.14). As activation
energy in P3HT-CdTe is lower compared to the pristine P3HT, the CdTe nanocrystals support
transportation of holes which improves their mobility and results into enhancement in the current.
The change of mobility from field dependent in P3HT to field independent in the P3HT-CdTe thin
film can be explained on the basis of increase of trap density (Hb) and reduction in activation
energy (Figure 6.15).
Usually, an electric field raises the mobility because it lowers the activation barriers. In
organic semiconductors most of the charge carriers are trapped in localized states. An applied
field gives rise to the accumulation of charge in the region of the semiconducting layer. As these
charges are accumulated (i) spatial overlap between the trap potential increases, that lowers the
activation barriers [60] and (ii) only a fraction of total charge carriers are required to fill all the
traps, the remaining carriers will on average require less activation energy to hop away to a
neighboring site (Figure 6.15). This results in a higher mobility with increasing field.
146
-eFx
∆=
52
meV
-eFx
P3HTP3HT-CdTe
3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
1E-5
1E-4
1E-3
0.01
0.1
1
J (
A/c
m2)
1000/T (K-1)
2V
5V
10V
P3HT
3 4 5 6 7 8
1E-4
1E-3
0.01
0.1
J (
A/c
m2)
1000/T (K-1)
2V
5V
10V
P3HT-CdTe
Figure 6.14 Temperature dependent current density of P3HT and P3HT-CdTe at different applied
bias.
Incorporation of CdTe nanocrystals in P3HT matrix simultaneously enhance the value of
trap density from 2.8×1018
to 5.0×1018
cm-3
and produces extrinsic charge carriers. At high trap
density, the trap potential wells overlap which results in decreasing activation energies (from 52
meV to 11 meV) as shown in Figure 6.15. Furthermore, increase in the charge carrier density on
incorporation of CdTe nanocrystals in P3HT matrix, results, only in partial filling of carriers even
in deeper intrinsic states, this leads to an upward shift of the Fermi level to the effective transport
level and concomitant increase of the jump rate. This implies that even at low field larger numbers
of free charge carriers are available for transport and hence the mobility in P3HT-CdTe films is
independent of applied field.
Figure 6.15 Distribution of trap density in P3HT and P3HT-CdTe. The value of activation
energy decreases from 52 meV to 11 meV on incorporation of CdTe in P3HT matrix.
Chapter 6
147
6.6.5. Hole Transport Mechanism in P3HT-CdS Hybrid System
Figure 6.16 shows the J-V characteristics of device based on P3HT-CdS, measured at different
temperatures. Experimental data in Figure 6.16 are represented by symbols, whereas the solid
curves represent the theoretically generated curves from Equation (6.18). The nature of P3HT-
CdS composite thin film has been different from that of pristine P3HT (Figure 6.6). In case of
composite film the hole current has been observed to be more than that in pristine P3HT at all
temperatures. Inset of Figure 6.17(a) shows the comparison of J-V characteristics of P3HT and
P3HT-CdS at 190 K. We tried to fit the experimental data with unified mobility model. The data
did not show agreement with the mobility model, however, shows a good agreement with
Equation (6.15) and (6.18). As a result, the hole mobility is constant, and thus also independent of
the hole density.
0.1 1 10
1E-5
1E-4
1E-3
0.01
0.1
1
J (
A/c
m2
)
Voltage (Volts)
290K
260K
225K
195K
170K
150K
Figure 6.16 Experimental (symbols) and calculated (solid lines) J-V characteristics of P3HT-CdS
at different temperature in hole only device configuration viz. ITO/PEDOT:PSS/P3HT-CdS/Au.
It is seen from these J-V curves that the characteristics showed ohmic behavior at low
applied bias, which can be attributed to the background doping and thermally generated charge
carriers. These J-V characteristics switched to non-ohmic behavior at higher applied bias, which is
attributed to the formation of space charge near the injecting electrode. It is further seen from
these curves that the slope of high-field conduction region decreases slightly with the increase in
the temperature.
148
Figure 6.17 (a) Experimental (symbols) and calculated (solid lines from Equation 6.15) J-V
characteristics of P3HT-CdS. The inset shows the comparison of J-V characteristics of P3HT
and P3HT-CdS at 190 K. (b) The Arrhenius plot of the current density vs. temperature with the
associated activation energies.
It is observed from Figure 6.16 that for the higher temperatures (290K and 260K) the
experimental curves did not show agreement with the theoretical curves generated from Equation
6.18. For the temperatures 290K and 260K [Figure 6.17(a)] the current density of the P3HT-CdS
diode depends quadratically on applied voltages and follows the Equation 6.15. The Charge
carrier mobility at the temperature 290K and 260K was calculated to µ=6.0×10-5
cm2 V
−1 s
−1and
µ=7.5×10-5
cm2 V
−1 s
−1, respectively, from Equation 6.15. For the temperatures below 260K the
experimental data fitting is according to the Equation 6.18. In this case hole transport fitting
parameters get modulated: Hb=3.0×1018
cm−3
, Nv=1.0×1019
cm−3
, µ=9.0×10-5
cm2 V
−1 s
−1,
d=110 nm, and Tc=500 K.
The two different activation energies of the charge carriers responsible for above
conduction, which have been evaluated by usual Arrhenius type log J vs. 1/T plots (using the data
from Figure 6.16) and shown in Figure 6.17(b). The corresponding Arrhenius plot of J vs. 1/T is
thermally activated with two activation energies and a transition at around 225K (35meV and
18meV). The larger one of the two corresponds to the hopping in P3HT, whereas lower one may
be explained by the hopping between the P3HT and CdS nanocrystals.
The switching of conduction mechanism from mobility model in pristine P3HT to band
conduction in P3HT-CdS can be understood in terms of host (P3HT) and guest (CdS) charge
transfer type interaction. In fact there are various possibilities by which CdS can interact with host
P3HT. It can either go into the P3HT main chain structure or forms donor-acceptor charge
transfer complex (CTCs) or form molecular aggregates. However, the enhancement in J in device
0.1 1 10
1E-4
1E-3
0.01
0.1
1
0.1 1 10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
J (
A/c
m2
)
Voltage (Volts)
P3HT
P3HT-CdS
190 K
J (
A/c
m2
)
Voltage (Volts)
290K
260K
(a)
4 5 6 7 8
1E-3
0.01
0.1
18 meV
J (
A/c
m2
)
1000/T(K-1)
35 meV
P3HT-CdS
(b)
Chapter 6
149
based on P3HT-CdS indicates that the formation of CTCs between the host and guest and may be
the dominant mechanism of interaction between the two.
The PL quenching observed in Figure 5.6 (chapter 5) on CdS dispersion in P3HT is a
direct evidence of CTCs formation between the host and guest since PL quenching is an
indication of the degree of success of exciton dissociation and efficiency of charge transfer
between the donor-acceptor composite materials. The PL quenching in P3HT-CdS has been
attributed to the π-π interaction of P3HT with CdS [61], forming additional decaying paths of the
excited electrons through the CdS. To be more precise during CTCs formation, CdS QDs may
diffuse into the amorphous-crystalline boundaries of the P3HT polymer and introduce the
conducting path, thus reducing the defect states and barrier height (activation energy from 52meV
in P3HT to 18meV in P3HT-CdS) at these interfacial boundaries.
The holes which had to jump from one polymer chain to other to transport through P3HT,
are now assisted by the CdS nanocrystals. Also CdS improves the interchain-interchain interaction
of P3HT. The switching of mobility model in P3HT to band conduction mechanism in the
composites is probably due to improvement in the electron wave function overlap between two
polymer chains. It suggests that CdS works as transport bridge between two polymer chains. Due
to enhancement in the electron wave function overlap the charge carriers do not move from one
molecule to other via hoping but via drift in the extended states of P3HT and valance band of
CdS.
6.6. CONCLUSION
1. In order to understand the charge transport mechanism in the organic and organic-
inorganic hybrid systems, the J-V characteristics have been studied in the hole only device
configuration at different temperatures.
2. The hole transport mechanism in P3HT thin film is governed by space charge limited
conduction wherein the charge carrier mobility is dependent on temperature, carrier density, and
applied field, given by unified mobility model.
3. Thin films of copolymer P3OT-HT exhibited agreement with the space charge limited
conduction with traps distributed exponentially in energy and space. Hole mobility is both
temperature and electric field dependent, arising due to octyl groups attached to these polymer
backbone. The estimated value of zero field mobility is of the order of 3.6×10-5
cm2/V-s.
4. The hole transport mechanism in P3OT thin film is governed by space charge limited
conduction model. The hole mobility follow the Gaussian distribution model with the zero field
mobility of 9.3 × 10-6
cm2/V –s.
150
5. Incorporation of CdTe nanocrystals in P3HT matrix results enhancement current density,
attributed to increase in the value of 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.
6. 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 decrease of temperature.
7. 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 trap potential wells, which results in
decrease in activation energies from 52 meV to 18meV.
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CHAPTER 7
CONCLUSIONS AND FUTURE SCOPE
7.1. SUMMARY
7.2. SUGGESTIONS FOR FUTURE INVESTIGATIONS
7.1. SUMMARY
In this thesis the photovoltaic performances as well as the charge transport mechanism in organic
and organic/inorganic hybrid system have been investigated by a variety of optical, electrical and
numerical techniques. The aim of the present work is to develop and improve the performance of
organic and hybrid solar cells.
The homopolymers P3HT, P3OT, copolymer P3HT-OT have been studied regarding their
optical and structural properties and used as electron donor materials in polymer solar cells. 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. Photovoltaic performance of P3HT-OT exhibit an open-circuit voltage VOC of 0.50V,
short-circuit current of 2.36 mA/cm2 and the overall power conversion efficiency of 0.4%, which
is in between the performance of solar cell fabricated from P3HT ( = 0.5%) and P3OT ( =
0.3%). The open-circuit voltage systematically increases in the order P3HT:PCBM < P3HT-
OT:PCBM < P3OT:PCBM cells, which is probably due to the slightly lower HOMO levels of
P3OT and P3HT-OT compared with P3HT. The short-circuit current JSC of the P3HT:PCBM cell
(2.64 mA/cm2) is higher than that of P3HT-OT:PCBM (2.36 mA/cm
2) and P3OT:PCBM device
(1.46 mA/cm2). These values are governed by an increased hole mobility and by a lower energy
transition barrier for holes undergoing transfer from the HOMO level into ITO anode regarding
P3HT against P3HT-OT and P3OT. The performances of these devices have been improved by
post-production thermal annealing of device at a sufficiently high temperature.
In order to reduce charge recombination and increase the carrier mobilities in
P3HT:PCBM based device, the CdS QDs have been incorporated in the P3HT matrix. HRTEM
images reveal that the size of CdS QDs ranges from 5 to 6 nm and their shape is spherical. The
average crystallite size determined from the Debye–Scherrer formula is estimated to be about
2.33nm. The P3HT/CdS nanocomposite shows blue shift in the absorption spectra relative to the
pristine P3HT, which is attributed to the quantum confinement effect from the CdS nanocrystals.
154
The photoluminescence quenching in the P3HT/CdS nanocomposite indicates the charge transfer,
thereby exciton dissociation at P3HT/CdS interface. On incorporation of CdS QDs in P3HT
matrix, the power conversion efficiency increased from 0.45% to 0.87% due to enhancement in
short-circuit current, and fill factor. The enhancement in JSC have been explained on the basis of
the formation of charge transfer complex between the host (P3HT) and guest (CdS QDs), duly
supported by blue shift in UV-Vis absorption and PL quenching studies. The investigation on the
effect of post thermal annealing on device performance had shown that improved efficiency of
devices after thermal treatment at 1500C for 10 min due to improved nanoscale morphology,
crystallinity and contact to the electron-collecting electrode.
To further improve the photovoltaic properties of P3HT by broadening the solar
absorption, enhancing the charge carrier mobility, and improving the polymer-nanocrystals
interaction, the CdTe nanocrystals have been in-situ grown in the P3HT matrix without use of any
surfactant. Structural and spectroscopic studies confirmed the successfully incorporation of CdTe
nanocrystals in P3HT matrix. Structural and morphological studies reveal that CdTe works as
transport media along/between the polymer chains, which facilitate percolation pathways for
charge transport. Optical measurements show that photoinduced charge generation on the
absorption of light and these are dissociated at the P3HT-CdTe interfaces. The solar cell
performance of device based on P3HT-CdTe:PCBM showed a better performance compared to
P3HT:PCBM, due to increased JSC from 2.25 mAcm-2
to 3.88 mAcm-2
, and VOC from 0.58 V to
0.80 V. The enhancement in VOC in P3HT-CdTe:PCBM based device attributed lower HOMO
level of CdTe compared to P3HT. The measured difference (0.21 eV) of the HOMO energy levels
between P3HT and CdTe almost completely translated into the observed difference in Voc (∼0.22
V). Moreover, enhancement in JSC may result in improvement in the solar absorption spectra and
decrease in the activation energy. This cell suffered from low fill factors, which may be caused by
shunting and a high series resistance of P3HT-CdTe as compared to pristine P3HT.
In order to understand the charge transport mechanism in the photovoltaic devices based
on organic and organic-inorganic hybrid systems, the J-V characteristics have been studied in the
hole only device configuration, at different temperatures. 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.
Hole mobility is both temperature and electric field dependent, arising due to octyl groups
attached to these polymer backbones. The estimated value of zero field mobility of P3HT-OT is
of 3.6×10-5
cm2/V-s. The hole transport mechanism in P3OT thin film is govern by space charge
Chapter 7
155
limited conduction model. The hole mobility follow the Gaussian distribution model with the zero
field mobility of 9.3 × 10-6
cm2/V –s.
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
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.
7.2. SUGGESTIONS FOR FUTURE INVESTIGATIONS
1. A number of mechanisms in organic photovoltaics are still poorly understood, such as the
mechanism by which an exciton dissociates into a free electron and free hole at a heterojunction.
Further study and a better understanding of this mechanism would allow researchers and
engineers to carefully design an efficient heterojunction between the organic and inorganic phases
that reduces the series resistance of the junction and optimizes the band offset between materials.
This study can be done by time resolved spectroscopy. So in future, time-resolved fluorescence
spectroscopy (TRFS) and time-resolved microwave conductivity (TRMC) investigation can be
carried out in donor-acceptor composites to better understand the exciton dissociation process at
donor-acceptor interface in the organic and hybrid solar cells.
2. Exciton and hole mobility in organic solar cells is yet another huge limitation on the
efficiency of organic photovoltaics, restricting excitons to traveling only nanometer distances
prior to recombination and placing strict requirements on the morphology and geometry of the
organic-inorganic photovoltaic cell. An increase in carrier mobilities would relax the
requirements placed on the spacing and geometry of the nanocrystalline phase, and at the same
time allow for the devices to be built thicker and more light-absorbent. In the present
investigation, the carrier mobility has been improved by incorporation of inorganic nanocrystals
(CdTe, CdS) in polymer matrix. In future, the incorporation of rod-shape nanocrystals in
polymer matrix will further improve the carrier mobility, because, charge carrier will have large
transport path to travel in nanorod as compared to spherical nanocrystals.
3. The CdSe/CdTe core/shell structures are electrical insulators in the dark but when exposed
to sunlight, they undergo a dramatic increase in electrical conductivity—as much as three orders
156
of magnitude. Therefore, use of CdSe/CdTe core/shell structure in polymer matrix will further
improve the device performance.
4. The hybrid solar cells suffered from low fill factors which may be caused by low shunting
and a high series resistance. The presence of polymer or nanocrystal pathways that connect the
anode to the cathode, is a source of current leakage or electrical shorts, depending on the
conductivity of the pathway. The incorporation of inorganic nanocrystals into a polymer matrix
results enhancement in photoconductivity of the active layer. This increased photoconductivity
of the active layer is responsible for the decreasing fill factor. The addition of one hole-blocking
layer at cathode and another electron-blocking layer at anode can prevent the polymer and
nanocrystal from shorting the two electrodes under illumination.
5. Further improvement can be achieved by controlling over the morphology of the
photoactive layer, improving the contacts between photoactive layer and cathode and reducing
the current leakage by introducing the electron and hole blocking layers before respective
electrodes.
6. The mechanisms of device degradation require better understanding as degradation plague
organic photovoltaics and are a major factor in their slow entry into the photovoltaic market. To
prevent premature device degradation, both the active materials must have better resistance to
environmental attack, as well as the encapsulation systems should effectively keep air and
moisture away from the active material.
157
Peer Reviewed Publications in International Journals
(1) In-Situ growth of CdTe nanocrystals in P3HT matrix for photovoltaic application,
Mohd Taukeer Khan, Amarjeet Kaur, S K Dhawan, and Suresh Chand, J. Appl. Phys. 110,
044509 (2011).
(2) Hole transport mechanism in organic/inorganic hybrid system based on in-situ grown CdTe
nanocrystals in poly(3-hexylthiophene),
Mohd Taukeer Khan, Amarjeet Kaur, S K Dhawan, and Suresh Chand, J. Appl. Phys. 109,
114509 (2011).
(3) Effect of cadmium sulphide quantum dot processing and post thermal annealing on
P3HT/PCBM PV device,
Mohd Taukeer Khan, Ranoo Bhargav, Amarjeet Kaur, S K Dhawan, and Suresh Chand, Thin
Solid Films 519 1007 (2010).
(4) Electrical, optical and hole transport mechanism in thin films of poly(3-octylthiophene-co-3-
hexylthiophene): Synthesis and characterization,
Mohd Taukeer Khan, Manisha Bajpai, Amarjeet Kaur, S. K. Dhawan, and Suresh Chand,
Synth. Met. 160 1530 (2010).
Papers Presented in National/International Conferences/Symposia
(1) Study on the Solar Cells Performance of P3HT-CdTe Hybrid System
Mohd Taukeer Khan, AmarjeetKaur, S.K. Dhawan, and Suresh Chand,
National Symposium on “Recent Advances in Materials and Devices for Solar Energy
Applications” (1st- 2
nd , Sept. 2011), National Physical Laboratory, New Delhi.
(2) In-situ growth of quantum dots in polymer template: photophysics of organic/inorganic hybrid
solar cells,
Mohd Taukeer Khan, AmarjeetKaur, S.K. Dhawan, and Suresh Chand,
International Conference on Quantum Effect in Solids of Today (I-ConQUEST), Dec. 20-23,
2010, National Physical Laboratory, New Delhi(India).
(3) In-situ growth of ZnTe nanocrystals in polymer template: structural,
optical, and electrical study,
Mohd Taukeer Khan, Amarjeet Kaur, S.K. Dhawan, and Suresh Chand,
M A C R O 2 0 1 0 , Dec. 15 - 17, 2010 India Habitat Centre, New Delhi, India.
(4) Enhancement of open circuit voltage in polymeric solar cell on doping QDs of CdS
Mohd Taukeer Khan, Amarjeet Kaur, S.K. Dhawan, and Suresh Chand,
IJBWME- 2009, Dec, 17-20, 2009, NPL, New Delhi.
158
(5) Optical and electrical properties of poly(3-hexylthiophene)/ZnO nanocomposites,
MohdTaukeer Khan, Amarjeet Kaur, S.K. Dhawan, and Suresh Chand,
Second International Conference on Frontiers in Nanoscience and Technology, Cochin Nano–
2009, January 3-6, 2009 Cochin, India.
(6) Dielectric and electrical behaviour of conjugated polythiophenes for photovoltaic applications,
Mohd Taukeer Khan, Amarjeet Kaur, S.K. Dhawan, and Suresh Chand,
APAM- 18-20 November 2008, NPL, New Delhi.
(7) Soluble poly-p-phenylene for organic photovoltaic application,
Mohd Taukeer Khan, Amarjeet Kaur, S.K. Dhawan, and Suresh Chand,
International conference on electroactive polymer (ICEP), 12th
-17th
Oct-2008, Jaipur, India.
Participation in Workshop/Short Course
1. Short Course on Polymer Characterization, 14th
Feb-2008 at IIT Delhi Delhi.
2. Organic and Molecular Electronics-2008, 07-18 July, 2008 at IIT Kanpur, Kanpur.
3. Short Course on Organic Electronics and PV Systems-2009, 06-14 July, 2009, at IIT Kanpur,
Kanpur.