Study of charge transport mechanism in organic and organicinorganic hybrid systems with application to organic solar cells

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.