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

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Mohd Taukeer Khan Ph.D Thesis

Mohd Taukeer Khan Ph.D Thesis

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    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 organicinorganic hybrid systems with application to organic solar cells Document Transcript

    • 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
    • CERTIFICATEThis is to certify that subject matter presented in this thesis titled “Study of Charge TransportMechanism in Organic and Organic/Inorganic Hybrid Systems with Application to OrganicSolar Cells” is the original contribution of the candidate. This work has not been submittedanywhere for the award of any degree, diploma, fellowship or similar title of any university orinstitution. The extent of information derived from existing literature has been indicated in the bodyof the thesis at appropriate places giving the source of information. Mohd Taukeer Khan (Candidate)Dr. Amarjeet Kaur Dr. S. K. DhawanDepartment of Physics & Astrophysics Polymeric & Soft Material SectionUniversity of Delhi National Physical LaboratoryDelhi-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 ToMy parents
    • ACKNOWLEDGMENTSAt the outset, I offer my prayers and thanks to the Almighty Allah, for He is good; His loveendures forever. The Almighty Allah is my strength and shield. My heart trusts in Him, and i amhelped. 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 mythesis 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 thesisis 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. RituSrivastav, Dr. Renu Pasricha, Dr. Vinay Gupta, and Dr. Shailesh Sharma, at National PhysicalLaboratory, New Delhi, for supporting me in my research work. I would also like to thank my thesis advisory committee: Dr. S.A. Hashmi, Dr. PoonamSilotia, Department of Physics and Astrophysics, University of Delhi, for their continuoussuggestions throughout this work. I sincerely thank Mr. Parveen Saini, Dr. Pankaj Kumar, and Dr. Rajeev K. Singh forgiving the time to teach me the essentials of organic photovoltaics and how to use the necessaryequipment. I would like to thank all the past and present group members, Dr. Anil Ohlan, Dr. KuldeepSingh, 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 Delhifor 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 technicalhelp 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 andAstrophysics, University of Delhi, Delhi for providing necessary help and research facilities. Last but not the least, financial assistance in form of Junior Research Fellowship andSenior Research Fellowship by Council of Scientific and Industrial Research (CSIR), New Delhiis gratefully acknowledged. Finally, my deepest gratitude goes to my parents, and wife. I really appreciate theircontinuous support and endless love throughout all my life. I would like to dedicate this thesis tothem. 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 successfulrealization of this work, I thank you all. (Mohd Taukeer Khan)
    • Table of ContentsChapter 1: Introduction: A Selective History and Working Principle ofOrganic and Hybrid Solar Cells…………………………………………………..11.1. Introduction..............................................................................................................................21.2. 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............................................................................................................61.3. 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.............................................................................101.4. Organic-Inorganic Hybrid Solar Cells.................................................................................111.5. 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...........................................211.6. 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..........................................................................................251.7. Objective of the Present Thesis.............................................................................................261.8. Thesis Plan..............................................................................................................................27References......................................................................................................................................29Chapter 2: Experimental Details: Materials, Methods and CharacterizationTechniques...............................................................................................................392.1. Introduction............................................................................................................................392.2. Synthesis of Poly(3-Alkythiophene)s.....................................................................................402.3. 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............................................................442.4. 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…................................................................472.5. 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......................................................61References......................................................................................................................................63Chapter 3: Study of the Photovoltaic Performance of CopolymerPoly[(3-Hexylthiophene)-Co-(3-Octylthiophene)]............................................65
    • 3.1 Introduction.............................................................................................................................653.2 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:PCBMRatio..............................................................................................................................................79 3.2.8 J-V characteristics of Solar Cells......................................................................................803.3. Conclusions………………………………………………………………………………….84Reference………………………………………………………………………………………...85Chapter 4: Study of Photovoltaic Performance of Organic/Inorganic HybridSystem Based on In-Situ Grown CdTe Nanocrystals in P3HTMatrix.......................................................................................................................894.1 Introduction………………………………………………………………………………….894.2 Fabrication and Measurement of Device…………………………………………………..924.3 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…………………………………………..…………1034.4. Conclusions………………………………………………………………………………...106References………………………………………………………………………………………106Chapter 5: Study of the Effect of Cadmium Sulphide Quantum Dots on thePhotovoltaic Performance of Poly(3-Hexylthiophene)…..................................109
    • 5.1. Introduction………………………………...……………………………………………...1095.2. Fabrication and Measurement of Device………………………………………………...1105.3. 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……………………………………………………1175.4. Conclusions……………………………………………………………………………… 119References…………………………………………………………………………………… 120Chapter 6: Study on the Charge Transport Mechanism in Organic andOrganic/Inorganic Hybrid System......................................................................1236.1. Introduction………………………………………………………………………………..1246.2. 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………………………1276.3. 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……………………………………………………...1326.4 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……………………………………………………………………1346.6. 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………………………….1476.7 Conclusions…………………………………………………………………………………149References………………………………………………………………………………………150Chapter 7: Conclusions and Future Scope.........................................................1537.1. Summary…………………………………………………………………………………...1537.2. Suggestions for Future Investigations……………………………………………………155List of Publications......................................................................................................................157
    • ABSTRACTIn 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-acceptorinterpenetrating bulk heterojunction. More recently international R & D efforts are focusedtowards the development of hybrid organic-inorganic nanostructured solar cells as it holds afurther promise due to added optical absorption (due to presence of inorganic component), bettercharge transport, better physical and chemical stability, easy tailoring of bandgap, costeffectiveness etc. These solar cells make use of hybrid combinations of various materials such aspoly(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, cadmiumsulphide, lead sulphide, lead selenide, zinc oxide, titanium oxide, etc. The hybrid polymer-nanocrystals solar cells that have recently shown the highest PCEsutilize 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 compositebased solar cells. However, in order to enhance further the PCE of hybrid organic-inorganicnanostructured solar cells, one needs to understand the fundamental and applied facets of thematerials and devices. The present thesis addresses these issues by way of systematic and detailedstudies 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 cellswhich comprises of the literature survey and overview of various generations of solar cells. It alsoincludes discussion on various basic and applied concepts of solar cells, such as devicearchitectures, polymer fullerene bulk-heterojunction, donor-acceptor concept, etc. The mainprocesses which contribute towards the working of solar cells are given in details. At the end ofthe chapter, a thorough discussion of different electrical characteristics parameters of solar cellsfor example JSC, VOC, FF, PCE, Rs, Rsh are given. Chapter 2 describe the synthesis methods and experimental techniques used in the presentwork. It also includes the fabrication process of bulk-heterojunction solar cells and hole onlydevice for charge transport study. The description of techniques used for confirming the synthesisof polymer, inorganic nanocrystals and incorporation of nanocrystals in polymer matrix, is given.These techniques include Fourier transform infrared spectroscopy (FTIR), UV-Vis absorption, i
    • 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 differenttemperatures are discussed in details. Chapter 3 includes the photovoltaics performance of devices based on P3HT, P3OT andtheir copolymer poly[(3-hexylthiophene)-co-(3-octylthiophene)] (P3HT-OT)]. The largest carriermobility reported for P3OT in field effect transistor configuration is 10-3cm2/Vs, which isapproximately 1-2 orders of magnitude lower than the typical mobilities of P3HT. P3HT is verywell soluble in chlorinated solvents such as chloroform, chlorobenzene, however, weakly solublein non-chlorinated solvents such as toluene or xylene. On the other hand, P3OT dissolves quicklyin toluene, xylene at room temperature. In order to incorporate both the properties (mobility andsolubility) within a single polymer, in the present investigation, the regioregular copolymerP3HT-OT has been used as a donor material in combination with PCBM as acceptor. The chapteralso contains the investigations of FTIR, 1H NMR, XRD, thermal analysis, UV-vis. absorption,photoluminescence properties of these polymers. The composites of the three polymers withPCBM show a distinctive photoluminescence quenching effect, which confirm the photoinducedcharge generation and charge transfer at P3AT/PCBM interface. Moreover, the energy levelpositions have been evaluated by the cyclic voltammetry. Finally, the photovoltaics performanceof P3HT-OT has been studied and results were compared with the homopolymer P3HT andP3OT. 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 betweenthe performance of solar cell fabricated from P3HT and P3OT. Chapter 4 discusses the photovoltaics performance of P3HT-CdTe hybrid system. Theaim of in-situ incorporation of CdTe nanocrystals in P3HT matrix is to improve the photovoltaicsproperties of P3HT by broadening the solar absorption, enhancing the charge carrier mobility, andimproving the polymer-nanocrystals interaction. Incorporation of CdTe nanocrystals has beenconfirmed by the structural (HRTEM, SEM) and spectroscopic (FTIR, UV-Vis absorption, PL)studies. Optical measurements (UV-Vis and PL) of nanocomposites films show that photoinducedcharge separation occurs at the P3HT-CdTe interfaces. This indicates that the in-situ incorporationof 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 asP3HT-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 andITO/PEDOT:PSS/P3HT-CdTe:PCBM/Al, respectively. Based on these investigations it has beenfound wherein the current-density and open-circuit voltage of device based on P3HT-CdTe haveincreased as compared to the device based on pristine P3HT. ii
    • Chapter 5 deals with the fundamental issue, whether incorporation of CdS nanocrystalsinto P3HT matrix causes any noticeable improvement or deterioration of device efficiency. Theparticle shape, size and distribution of CdS nanocrystals in P3HT matrix have been investigatedby HRTEM, SEM and XRD. Optical studies (UV-Vis absorption and PL) suggest the electronicinteraction between P3HT and CdS quantum dots. Photovoltaic performances of device based onpure 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 beeninvestigated. On incorporation of CdS nanocrystals in P3HT matrix, the PCE efficiency increaseddue to enhancement in short-circuit current, open-circuit voltage and fill factor. These effects havebeen 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 ofpost thermal annealing on device performance has also been investigated and found improvedefficiency of devices after thermal treatment due to improved nanoscale morphology, increasedcrystallinity and improved contact to the electron-collecting electrode. Chapter 6 gives the theoretical and experimental details of the charge transport processesin organic semiconductors as well as in organic-inorganic hybrid systems. In the theory section ofthe chapter space charge limited conduction which is dominant mechanism for charge transport indisordered materials has been discussed in details. This chapter also discusses the factorsinfluencing the charge carrier mobility. In the experimental part we have studied the holetransport mechanism in all the polymer (P3HT, P3OT, P3HT-OT) and polymer/nanocrystalshybrid systems (P3HT/CdS and P3HT/CdTe) in the device configuration ITO/PEDOT:PSS/Active layer/Au.. Current-voltage characteristics of these devices have been studiedin the temperatures range of 110K-300K. The hole transport mechanism in P3HT thin film isgoverned by space charge limited conduction with temperature, carrier density, and applied fielddependent mobility. Thin films of copolymer P3HT-OT exhibited agreement with the spacecharge limited conduction with traps distributed exponentially in energy and space. The holemobility is both temperature and electric field dependent. The hole transport mechanism in P3OTthin film is governed by space charge limited conduction model and hole mobility is given byGaussian distribution model. Incorporation of CdTe nanocrystals in P3HT matrix results into enhancement in currentdensity which attributed to increase in the trap density (from 2.8×1018 to 5.0×1018 cm-3) anddecrease of activation energies (from 52 meV to 11 meV). At high trap density, trap potentialwells start overlapping which results in decrease of activation energies. In contrary to P3HT, thehole mobility in P3HT-CdTe has been found to be independent to charge carrier density andapplied field. The charge carrier mobility depends only on temperature and it increases with the iii
    • decrease of temperature. On incorporation of CdS nanocrystals in P3HT matrix the mobility isagain independent to applied field and carrier density and exhibited agreement with the bandconduction mechanism. This is attributed to the enhancement in the overlapping of traps potentialwells, which results in the decrease in activation energies from 52 meV to 18meV. iv
    • CHAPTER 1 INTRODUCTION: A SELECTIVE HISTORY AND WORKING PRINCIPLE OF ORGANIC & HYBRID SOLAR CELLS1.1 INTRODUCTION1.2. PHOTOVOLTAIC SOLAR ENERGY DEVELOPMENT AND CURRENTRESEARCH 1.2.1. First Generation 1.2.2. Second Generation 1.2.3. Third Generation 1.2.4. Fourth Generation1.3. 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 devices1.4. ORGANIC-INORGANIC HYBRID SOLAR CELLS1.5. 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 electrodes1.6. 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
    • 1.6.6. Standard Test Conditions 1.6.7. Equivalent Circuit Diagram1.7. OBJECTIVE OF THE PRESENT THESIS1.8. THESIS PLANReferences1.1. INTRODUCTIONE 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 dailyexistance. At the other level, our quest for invention and explorations require more energy toachieve the respective aim. The international energy outlook 2010 (IEO2010) reports that theworld energy consumption would grow by 49% during the period 2007 to 2035 [1]. The worldwide energy demands would rise from 495 quadrillion British thermal units (Btu) in 2007 to 590quadrillion 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) Worldmarketed energy use by fuel type, 1990-2035 (quadrillion Btu). (Source: IEO2010). The energy can be non-renewable and renewable. Right now the energy requirement arefulfilled mostly by non-renewable sources like coal, oil, and natural gas [Figure 1.1 (b)]. As aresult, due to their high demand, these sources are depleting at very fast rate. Moreover, burningof these fossil fuels lead to the emission of carbon dioxide (CO2) [3-5]. Global warming is a directresult of the CO2 emission, and this will cause a change in the weather as well as increase themean sea level [6, 7]. This emphasizes the need for carbon free power production. The most 2
    • Chapter 1commercially-viable alternative, available today is nuclear energy [8-10]. Uranium does not causeCO2 emissions but has always been under intensive public discussions because of the imminentdanger 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 overother renewable energy systems involve their elegent operation, i.e. just converting daylight intoelectricity. No other fuels, water are required for their operation. Moreover, the solar cells orphotovoltaics systems are noise free and without any technical heavy machinery, so thereforetheir maintenance requirement is minima as compared to other renewable system [11].1.2. PHOTOVOLTAIC SOLAR ENERGY DEVELOPMENT AND CURRENTRESEARCHConventional 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 thefabrication cost must be lowered. Hence continuous research has been carried out in this directionand has led to four generations of PV technologies.1.2.1 First GenerationThe first generation photovoltaic cells are the dominant technology in the commercial productionof solar cells and account for nearly 80% of the solar cell market [19]. These cells are typically 3
    • made using a crystalline silicon (c-Si) wafer, in which a semiconductor junction is formed bydiffusing phosphorus into the top surface of the silicon wafer. Screen-printed contacts are appliedto the front and rear of the cell. The typical efficiency of such silicon-based commercialphotovoltaic energy systems is in the order of 15% [20]. In these cells a substantial increase oftheir efficiency up to 33% is theoretically possible, but the best laboratory cells have powerconversion efficiency (PCE) only about 25% [21-23]. The starting material used to prepare c-Simust be refined to a purity of 99.9999 % [24]. This process is very laborious, energy intensive; asa result manufacturing plant capital cost is as high as 60% of manufacturing cost [25]. The cost ofgenerating electricity using silicon solar modules is typically 10 times higher than that from fossilfuel which inhibits their widespread application. The main advantages of first generation solarcells 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 theenergy of higher energy photons, at the blue and violet end of the spectrum is wasted as heat, andpoor absorber of light.1.2.2. Second GenerationSecond generation solar cells are usually called thin-film solar cells. This generation basically hasthree types of solar cells, amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indiumgallium diselenide (CIGS). Thin film production market share in the global solar PV market grewfrom a mere 2.8% in 2001 to 25% in 2009; this indicates a growing share of these solar cells incoming future (see Figure 1.3). These technologies are typically made by depositing a thin layerof photo-active material onto the glass or a flexible substrate. The driving force for thedevelopment of thin film solar cells has been their potential for the reduction of manufacturingcosts. Moreover, as these semiconductors have direct band which leads to higher absorptioncoefficient, as a result less than 1 µm thick semiconductor layer is required to absorb completesolar 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 Figure1.4(b)]. The best commercial a-Si cells utilize a stacked three-layer structure with stabilizedefficiencies of 10.1% [29, 30]. Such cells suffer from significant degradation in their poweroutput when exposed to the light. Thinner layers can be used to increase the electric field strengthacross the material and hence can provide better stability. However, the use of thinner layersreduces light absorption, and hence cell efficiency. CdTe has a nearly optimal band gap and canbe easily deposited with thin film techniques. Over 16.7% efficiencies have been achieved in thelaboratory for the CdTe solar cells [30]. CdTe usually deposited on cadmium sulfide (CdS) toform a p-n junction photovoltaic solar cell as shown in Figure 1.4(c). When copper indiumdiselenide (CIS) is modified by adding gallium, it exhibits the record laboratory efficiency of 20.3 4
    • Chapter 1% among thin film materials [30] and shows excellent stability. At the moment CIGS is the mostpromising 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 tomonocrystalline silicon. The photovoltaic devices based on these materials have shownefficiencies of 15-20% [31-34], somewhat less than that of solar cells based on mono-crystallinesilicon [8]. This is due to the relatively poor charge transport in these materials compared tomonocrystalline silicon. So the promise of the low cost power has not been realized yet by thesetechnologies. Research is being conducted into several alternative types of solar cells.1.2.3. Third GenerationThird generation technologies aim to enhance poor electrical performance of second generationthin films technologies while maintaining very low production costs. Currently, most of the workon third generation solar cells is being done in the laboratory and being developed by newcompanies and most part of it is still not commercially available. Today, the third generationapproaches being investigated include nanocrystal solar cells, photo electrochemical cells ( PEC),Dye-sensitized hybrid solar cells (DSSC), Tandem cells, organic photovoltaic (OPV), and thecells based on the materials that generate multiple electron-hole pairs. 5
    • Metal (Front) Metal (Back) Metal (Back) TCO TCO n-Si n-a-Si CdS i-µc-Si CdTe CIGS p-Si p-µc-Si CdS Mo (Back) TCO (front) TCO (front) Metal (Back) glass glass Glass, metal foil (a) (b) (c) (d)Figure 1.4 Device configurations for (a) c-Si, (b) a-Si, (c) CdTe and, (d) CIGS. i is intrinsic,TCO is transparent conductive oxide, and, Mo is molybdenum.These cells are based on low energy, high-throughput processing technologies e.g. OPV are:chemically synthesized, solution processable, low material cost, large area, light weight andflexible. 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 PVtechnologies. And secondly their efficiency decay with time due to degradation effects under theenvironmental conditions.1.2.4. Fourth GenerationToday a lot of research has been focused on organic-inorganic hybrid materials. The researchersare finding them a promising candidate to enhance the efficiency of solar cells through a betteruse of the solar spectrum, a higher aspect ratio of the interface, and the good processability ofpolymers. 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 researchattention because of their potential for large area, flexible, easily processable, and low-costphotovoltaic devices. Moreover, hybrid materials have the ability to tune each component in orderto achieve composite films optimized for solar energy conversion [78, 79]. Year-wise progresseson the PCE of different PV devices are shown in Figure 1.5. 6
    • Chapter 1Figure 1.5 Year-wise progress on the efficiencies of different photovoltaic device, under AM 1.5simulated solar illumination. (Source: http://howisearth.files.wordpress.com/2010/02/best-research-cell-efficiencies-nationalrenewable-energy-laboratory-usa1.jpg).Table 1.1 Theoretical and experimental PCE of different types of solar cells [28, 75, 81, 82]. Photovoltaic device Abbreviation Theoretical Obtained η η% %Mono-crystalline Si c-Si 28.9 25.0µ-crystalline Si µc-Si 28.9 20.4Amorphous Si a-Si 22 10.1Copper indium gallium diselenide CIGS 28 19.6Cadmium telluride CdTe 28 16.7Gallium arsenide GaAs 28 27.6GaInP/GaAs/Ge GaInP/GaAs/Ge 32Dye sensitized DSSC 22 10.4Small molecule 22 8.3Polymer:fullerene OPV 8.5Hybrid Systems HOIPV 4.08 7
    • 1.3. POLYMER SOLAR CELLSPolymer-based PV systems hold the promise for environmentally safe, flexible, lightweight, andcost-effective, solar energy conversion platform. π-conjugated polymers offer the advantage offacile chemical tailoring and can be easily processed by wet-processing techniques. Molecularengineering enables highly efficient active plastics with a wide range of colors. This opens up awhole new area of solar cell applications not achievable by the traditional solar cells [80, 81].1.3.1. Economical expectations of OPVThe 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 aresynthesized by cost effective techniques.(2) Low material usage: Due to the high absorption coefficient of organic materials, organicsolar 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 becovered. Thus material cost is significantly lowered.(3) Low manufacturing cost: The organic materials are solution processable and can be easilyprocessed by wet‐processing techniques, such as ink-jet printing, micro-contact printing, andother soft lithography techniques. These techniques are very cost effective and fabrication ofdevices can be done even at room temperature which reduces the amount of energy consumptionin the manufacturing process. The production of large area OPV (1m2) can be done at a cost 100times lower than that of mono-crystalline silicon solar cells.1.3.2. Device ArchitecturesThe polymer solar cells reported in the literature can be categorized by their device architecture ashaving single layer, bilayer, blend, or bulk-heterojunction structure. The reason behind thedevelopment of these structures is to achieve higher cell efficiencies by enhancing chargeseparation and collection processes in the active layer.1.3.2.1. Single layer devicesThe first investigation of an OPV cell came as early as 1959, when an anthracene single crystalwas 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 singlelayer organic materials will remain below 0.1 %, making them unsuitable for any possibleapplication. In the first generation of the OPV devices, a single layer of pure conjugated polymer weresandwiched between two electrodes with different work functions, such as ITO and Al as shownin Figure 1.6 (a). The efficiency of such a device remains below 1%. The low efficiency of these 8
    • Chapter 1devices is primarily due to the fact that absorption of light in the organic materials almost alwaysresults in the production of a mobile excited state (referred to as exciton), rather than freeelectron–hole (e-h) pairs as produced in the inorganic solar cells. This occurs because of their lowdielectric constant typically in the range of 2–4 [84], combined with weak intermolecularcoupling. The Coulombic binding energy of an e–h pair separated by 0.6 nm in a system withεr=3 is 0.6 eV [85-88]. Therefore, the electric field provided by asymmetrical work functions ofthe electrodes is not sufficient to break up these photogenerated excitons. Hence, they diffusewithin the organic layer before reach the electrode, where they may dissociate to supply separatecharges, or recombine. Since the exciton diffusion lengths are typically 1–10 nm [89–93], muchshorter than the device thicknesses, exciton diffusion limits charge-carrier generation in the singlelayer 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 devicesA major breakthrough in the OPV performance came in 1986 when Tang discovered that muchhigher efficiencies (about 1%) can be attained when an electron donor (D) and an electronacceptor (A) are brought together in one cell [94], as shown in Figure 1.6 (b). The idea behind aheterojunction 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: theelectron will be accepted by the material with the larger electron affinity and the hole will beaccepted by the material with the lower ionization potential. In this device the excitons should beformed 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 inthe organic materials are much shorter than the absorption depth of the film, this limits the widthof effective light-harvesting layer. 9
    • 1.3.2.3. Bulk-heterojunction devicesTo date, the most successful method to construct the active layer of an OPV devices is to blend aphotoactive 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 surfacearea for exciton dissociation [95]. If the length scale of the blend is similar to the exciton diffusionlength, the exciton decay process is dramatically reduced as in the proximity of every generatedexciton there is an interface with an acceptor where fast dissociation takes place. Hence, chargegeneration takes place everywhere in the active layer, provided that there exist a percolationpathways in each material from the interface to the respective electrodes. In BHJ deviceconfiguration 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]. Phaseone centered on poly-(phenylene vinylene)s, whose structures and related BHJ morphology wereoptimized 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 relativelylower highest-occupied molecular orbital (HOMO) energy level of -5.4 eV, BHJ devices madefrom MDMO-PPV offered open circuit voltages (Voc) as high as 0.82 V; however, the relativelylarger band gap of MDMO-PPV limited the short circuit current density (JSC) to 5-6 mA/cm2. Asa result, a smaller band gap polymer, regioregular poly(3-hexylthiophene) (rr-P3HT), took centerstage in phase two. P3HT based BHJ devices delivered a much higher current density (> 10 mA/cm2), whichwas 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 intrinsiccharacteristics, together with important advances in material processing such as the control of themorphology of the BHJ blend via thermal [101] or solvent annealing [102], which lead to animpressive total energy conversion efficiency of 6% [103]. Unfortunately, the high HOMO (- 5.1eV) energy level of P3HT has restricted the VOC to 0.6 V, which consequently limits the overallefficiency. Presently, in phase three, the BHJ PV community has adopted two separate approachesto improve the efficiency of low cost BHJ PV cells. The first approach places emphasis on the VOC by designing polymers with a low HOMOenergy level. This approach has resulted in VOC greater than 1 V in a few cases [104-106], thoughthe 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 moreinflux photons and enhancing the JSC [107, 108]. By this method, JSC as high as 17.5 mA/cm2 hasbeen achieved by using poly[(4,4-didodecyldithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] as the donor in combination with [6, 6]-phenyl C61 butyric acid 10
    • Chapter 1methyl ester (PCBM) as acceptor [109]. This demonstrates the effectiveness of low-band-gappolymers in generating more current. However, a low VOC (0.57 V) was observed because of therelatively high HOMO energy level of donor material [109]. Only a few fine-tuned polymersdeveloped 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 solarcell 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 Opticalspacer, 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, 46251.4. ORGANIC-INORGANIC HYBRID SOLAR CELLSPolymer-based solar cells suffer from lower efficiencies and the limited lifetime as compared tosilicon-based solar cell. The limited efficiency of the BHJ polymer solar cell is due to the poorcarrier mobility [115], the short exciton diffusion length [116], the charge trapping [117], and themismatch of the absorption spectrum of the active layer and the solar emission [118, 119]. To 11
    • address these fundamental limitations of polymer solar cells, new strategies have been developedby blending of inorganic nanocrystals (NCs) with organic materials which integrate the benefits ofboth classes of materials [120-125]. These hybrid materials are potential systems for OPV devicesbecause it includes the desirable characteristics of organic and inorganic components within asingle composite. They have advantage of tunability of photophysical properties of the inorganicNCs and also retain the polymer properties like solution processing, fabrication of devices onlarge and flexible substrates [126-130]. Blends of conjugated polymers and NCs are similar to thatof used in organic BHJ solar cells. Excitons created upon photoexcitation are separated into freecharge carriers at organic-inorganic interfaces. Electrons will then be accepted by the materialwith the higher electron affinity (acceptor/NCs), and the hole by the material with the lowerionization potential (donor/polymer) [67]. The usage of inorganic semiconductor NCs embeddedinto semiconducting polymer is promising for several reasons such as [131]:1) Inorganic NCs have high absorption coefficients.2) They are superb electron acceptors having high electron affinity and high electron mobility.3) Band gap of NCs is a function of the size of the NCs, so they have size tunable optical and electrical properties [132-136].4) A substantial interfacial area for charge separation is provided by NCs, which have high surface area to volume ratios [120].5) In hybrid devices light is absorbed by both components, unlike polymer-fullerene BHJ where the PCBM contributes very little to the spectral response.6) NCs are prepared by inexpensive wet chemical synthesis route, hence NCs are cost effective.7) The NCs are easily dispersed in the polymers which can be spin casted for large area and flexible devices.8) They show good physical and chemical stability. Huynh et al. reported the hybrid devices from a blend of 8×13 nm, CdSe NCs, and rr-P3HT[120]. Under 4.8 W/m2 monochromatic illumination at 514 nm, a JSC of 0.031 mA/cm2 and a VOCof 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 ElifArici et al. [137-139]. Nanocrystalline CuInS2 was used with fullerene derivatives to forminterpenetrating interfacial donor–acceptor heterojunction solar cells. Also BHJ cell of CuInS2and p-type polymer PEDOT:PSS showed better photovoltaic response with external quantumefficiencies up to 20% [138, 139]. Zhang et al. [140] demonstrated hybrid solar cells from blendsof MEH-PPV and PbS NCs. They investigated the effect of different surfactants on thephotovoltaic performance of the hybrid devices. The device exhibit 250 nA short-circuit currentand an open circuit voltage of 0.47 V. Beek et al. [141] reported hybrid device based on blending 12
    • Chapter 1of rr-P3HT and ZnO. A PCE of 0.9% with JSC of 2.4 mA/cm2 and a VOC of 685 mV have beenachieved. The best performance of the device based on ZnO nanofiber/P3HT composite [141], aPCE of 0.53% have been achieved. Incorporation of a blend of P3HT and (6,6)-phenyl C61 butyricacid 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 ahybrid device fabricated using rr-P3HT and CdSe QDs. In 2005, Sun et al. [144] used CdSetetrapods 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 theeffect 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 CdSeQDs in the mixture PFT/PCBM changes the film morphology, which is responsible for theimprovement in device photocurrent and efficiency. In a similar on work P3HT/CdTe/C60 systema PCE 0.47 % , with JSC of 2.775 mAcm-2, VOC = 0.442 V and FF of 0.38 were obtained [146]. Todate 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 haverecently 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 (%) ReferencesPCPDTBT: CdSe tetrapods 0.67 10.1 0.55 3.2% S. Dayal et al., Nano Lett. 10 (2010) 239P3HT: CdSe QDs 0.62 5.8 2% Y. Zhou et al., APL, 96 (2010) 013304P3HT: CdSe hbranch 0.60 7.10 2.2 I. Gur et al., NanoLett.,7 (2007) 409–14P3HT: CdSe nanorods 0.62 8.79 0.70 2.6 B. Sun et al., Phys. Chem Chem. Phys 8 (2006) 3557OC1C10-PPV: CdSe 0.75 9.1 0.52 2.8 B. Sun et al., J Appl Phystetrapods 97 (2005) 014914APFO-3: CdSe nanorods 0.95 7.23 0.44 2.4 P. Wang et al., Nano Lett 6 (2006) 1789P3HT: CdSe hbranch 0.60 7.10 2.2 I. Gur et al., NanoLett 7 (2007) 409–14P3HT: CdSe nanorods 0.71 6.07 0.56 1.7 W. U. Huynh et al., Science 295 (2002) 2425–7MDMO-PPV:ZnO 0.81 2.40 0.39 1.6 WJE Beek et al., Adv Mater 16 (2004) 1009–13P3HT:PbS 0.35 1.08 0.21 0.14 D. Cui et. al., Appl. Phys. Lett. 88, (2006)183111MEH-PPV: CdTe NCs 0.77 0.19 0.42 T. Shiga et al., Sol. Energy Mater. Sol. Cells 90 (2006) 1849P3HT:PCBM:Pt QDs 0.64 10 4.08 M. Y. Chang et al J. Electrochem. Soc. 156 (2009) B234 13
    • 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) 408MDMO-PPV:TiO2 0.52 0.6 0.11 V. Hal et al. Adv. Mater. 15 (2003) 118P3HT:CdS(in-situ) 0.64 2.9 H-C. Liao et al. Macromol. 42 (2009) 6558P3HT:ZnO (in-situ) 0.75 5:2 0.44 2.0 S. D. Oosterhout et al. Nat. Mater. 8 (2009) 818P3HT: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 toorganic/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 betweenpolymer-NCs and poor nanoscale morphology of the composites film. In conventional synthesisof QDs (CdTe, CdS), they were capped with organic aliphatic ligands, such as TOPO or oleicacid. It has been shown that when the QDs are capped with organic ligands, they hinder theefficient electron transfer from the photoexcited polymer to the NCs [67]. To remove the organicligands, polymer-NCs were treated with pyridine. However, pyridine is an immiscible solvent forthe 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 thenanocrystals in polymer matrices. The in-situ growth of the nanocrystals in polymer templatescontrols the dispersion of the inorganic phase in organic phase, as a result ensuring a large surfacearea for charge separation. Moreover, nanocrystals are uniformly distributed into the entire devicethickness and thus their exist a percolation path for transport of charge carriers to the respectiveelectrodes. At an early stage, Van Hal et al. [149] reported hybrid devices based on in-situ grownTiO2 nanocrystals in to the MDMO-PPV matrix. To prepare bulk heterojunctions they haveblended MDMO-PPV with titanium(iv)-isopropoxide, a precursor for preparation of TiO2nanocrystals. Subsequent conversion of titanium(iv)isopropoxide precursor via hydrolysis in theair in the dark resulted in the formation of a TiO2 phase in the polymer film. Such a deviceexhibited a JSC of 0.6mA/cm2 and a VOC of 0.52V with a FF of 0.42. External quantum efficiencyup 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 thesurrounding atmosphere to yield bi-continuous, interpenetrating ZnO and polymer networkswithin the resulting film. An impressive PCE of over 2% has been reported for ZnO/P3HT solarcells using this fabrication approach. Liao et al. [152] have successfully in-situ synthesized NCs 14
    • Chapter 1of CdS in P3HT templates using cadmium acetate precursor for Cd and sulphur powder for S. Thedevice 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. Suchdevice 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 CELL1.5.1. Basics of Molecular PhotophysicsThe main process which occurs in OSCs is based on the photoexcitation of electrons due toabsorption of the light energy. The basic principles of photophysics of a molecule are necessaryfor the understanding of organic solar cell operation mechanism. Π-conjugated polymers generally possess a singlet ground state (S0), (a state in which allelectron spins are paired). Absorption of light usually involves a π‐π* transition to a singletexcited state of the polymer (S0 + hν → Sn). During absorption, the geometry of the moleculedoes not change, although the electrons may undergo rapid motions. This transition to the upperexcited singlet states is referred as Franck-Condon transition [154]. As the mass of the electronis smaller than the mass of the nucleus, the electronic transition proceeds much faster (10-16s) thanthe typical nuclear vibration (10-12-10-14 s). After its formation, the Franck-Condon stateundergoes some vibrational relaxation to attain equilibrium geometry. Usually this processhappens in a time interval of 10-12-10-14 s. The singlet excited state is a very reactive species and itmay release energy or undergo charge transfer. The dominant energy transitions are describedusually by the Jablonsky diagram shown in Figure 1.7 [155]. Decay processes from the singletexcited 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 excitedstate 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 smallerfragments (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 thecharge transfer. Energy and charge transfer are classified as quenching pathways. In thephotophysics, quenching is defined as the deactivation of an excited sensitizer by an externalcomponent. The external component is called quencher and is usually a molecule in the groundstate. 15
    • S1 ABSORPTION INTERNAL CONVERSION (10 ps) T1 FLUORESCENCE (1-10 ns) PHOSPHORESCENCE (> 100 ns) INTERSYSTEM CROSSING S0Figure 1.7 Jablonsky diagram of organic molecules depicting typical energy levels and energytransfer. Coulomb Interaction 3A* 3D* 1A* + B A + 1B* + D A + Dexter Electron exchange Forster dipole-dipole interaction Short range (6 – 20 Å) Long range (30 – 100 Å)Figure 1.8 Illustration of the two mechanisms of energy transfer of an excited molecule: (a)Dexter electron exchange, (b) Forster dipole-dipole interaction between donor and acceptor.In case of energy transfer, the quencher (acceptor A) receives the energy from the excitedsensitizer (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 theD–A complex before charge transfer is initiated, leading to an ion radical pair and finally chargeseparation can be stabilized possibly by carrier delocalization on the D+. or A-. species bystructural relaxation as shown in Figure 1.9. 16
    • Chapter 1 Figure 1.9 Illustration of the electron transfer between donor and acceptor.1.5.2. The Need of Two SemiconductorsPhotovoltaic cell configurations based on hybrid organic-inorganic materials differ from thosebased on inorganic semiconductors, because of the physical properties of inorganic and organicsemiconductors are significantly different. The main differences between organic and inorganicsemiconductors are listed in the Table 1.3. Table 1.3 A comparison between Organic & Inorganic semiconductorsSemiconductor Inorganic OrganicInteraction energy Covalent (1-4 eV) Van der Waals (10-3 - 10-2 eV)Dielectric constant 10 2-4Transport Mechanism Band transport Hopping transportMobility (cm2/V.s) RT 100-1000 10-7-1Mean Free Path (100-1000)ao l=ao lattice constantEffective Mass (m*/ m) 0.1 Bloch Electrons 100-1000 PolaronsExciton Type Mott-Wannier FrenkelExcitonic radius 10-100 nm 1 nmExciton binding energy 10 meV 0.1-1 eVAbsorption coefficient --------- >105 cm-1 17
    • Inorganic semiconductors generally have a high dielectric constant of the order of 10, ascompared to 3 in organic semiconductors and a low exciton binding energy. Hence, the thermalenergy 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 easilytransported 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 covalentbonds in the inorganic semiconductors. Concomitantly, the relative dielectric constant is low (ofthe 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 electrondonating and accepting properties. Charges are then created by photoinduced electron transferbetween the two components. This photoinduced electron transfer between donor and acceptorboosts the photo-generation of free charge carriers compared to the individual, pure materials, inwhich 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 CellsThe fundamental physical processes in the BHJ PV devices are schematically represented inFigure 1.11. Sunlight photons which are absorbed by the active layer, excite the donor (1), leadingto 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 fastdissociation takes place (3) leading to charge separation [157, 158]. Subsequently, the separatedfree charge carriers are transported (4) with the aid of the internal electric field (caused by the useof electrodes with different work functions). These dissociated charge carriers moves towards theelectrodes where they are collected (5) and driven into the external circuit. However, the excitons 18
    • Chapter 1can 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 forthe size of the conjugated polymer phase in the BHJ. The comprehensive physics behindlight‐to‐electric energy conversion process in polymer solar cells and some related issues arediscussed below. LUMO 2 3 1 6 1 5 5 4 5 4 4 5 3 HOMOAnode Cathode 2 Donor (a) Acceptor Donor (b) AcceptorFigure 1.11 Fundamental operation process in BHJs solar cells, the numbers (1 to 6) refer to theoperation processes explained in the text (a) Schematic band diagram and (b) Blend of OPV.1.5.3.1. Light absorption and exciton generationFor an efficient collection of photons, the absorption spectrum of the photoactive organic layershould match the solar emission spectrum and the layer should be sufficiently thick to absorb allthe incidents light. When the incident photon has an energy hν ≥ Eg, an electron in the HOMO ofthe donor would be excited to the LUMO, leaving a hole in the HOMO level. This e-h pair iscalled singlet exciton having opposite spin. In an OSC, only a small region of the solar spectrumis covered. For example, a bandgap of 1.1 eV is required to cover 77% of the AM1.5 solar photonflux, whereas most solution processable semiconducting polymers (PPVs, P3HT) have bandgapslarger than 1.9 eV, which covers only 30% of the AM1.5 solar photon flux. In addition, becauseof the low charge-carrier mobilities of most polymers, the thickness of the active layer is limitedto ~ 100 nm, which, in turn, results in absorption of only ≈ 60% of the incident light at theabsorption 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 polymersBecause of the high exciton binding energy in the conjugated polymers, the thermal energy atroom temperature is not sufficient to dissociate a photogenerated exciton into free charge carriers.Consequently, the configuration and operation principle of PV devices based on organic 19
    • semiconductors differ significantly from those based on inorganic materials. Typically, in OSCsan efficient electron acceptor is used in order to dissociate the strongly bound exciton into freecharge carriers [87] as discussed in section 1.6.2.1.5.3.3. Dissociation of charge carriers at the donor/acceptor interfaceOrganic semiconductors are characterized by high excitonic binding energy of the order of 0.2-0.5eV [159, 160]. As a result, photogenerated excitons dissociation occurs only when the potentialdrop at donor and acceptor interface is larger than the exciton binding energy [161-167]. Afterphoto-excitation of an electron from the HOMO to the LUMO, the electron can jump from theLUMO of the donor to the LUMO of the acceptor. However, this process, which is calledphotoinduced charge transfer, can lead to free charges only if the hole remains on the donor due toits higher HOMO level. In contrast, if the HOMO of the acceptor is higher, the exciton transfersitself 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 bysplitting the exciton or energy transfer, where the whole exciton is transferred from the donor tothe acceptor.1.5.3.4. Charge transport in donor/acceptor blendsAfter 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 chainsas shown in Figure 1.13. Subsequently, the free electrons and holes must be transported viapercolated donor and acceptor pathways towards the electrodes to produce the photocurrent. In order to collect the photogenerated charges, the carriers have to migrate through theactive materials to the electrodes. The active layer in polymer solar cells is usually deposited byspin-coating. In such a spin-coated film, the polymer chains are arranged in a disordered fashion.Conformational and chemical defects in the polymer chains and molecules will restrict the charge 20
    • Chapter 1carriers to small segments. As a result, the delocalization length of the charge carriers is limited toalmost molecular dimensions. The distribution of the π-conjugation lengths of the polymersegments, results in a distribution of the energies of the localized states available to the chargecarriers. e- C6H13 C6H13 C6H13 C6H13 S h+ S S S S S S S C6H13 C6H13 C6H13 C6H13 Figure 1.13 Pictorial representation of electron transfer from P3HT to PCBM. Charge transport in the energetically disordered materials has been successfully described within the Gaussian disorder model [168]. In this model, energetic disorder is modeled by a Gaussian distribution of energy levels of the sites. After photo-generation of the charge carriers in the disordered system, the charge carriers relax towards tail states of the Gaussian distribution while performing a random walk throughout the disordered potential energy landscape. During this random walk, the carriers may get trapped on a low energy site. The charge can either be freed by thermal activation [168, 169] or it may tunnel to a nearby site, without thermal activation [170].1.5.3.5. Extraction of the charge carriers at the electrodesIn 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 theopen-circuit voltage (VOC) of the cells. In the classical metal–insulator–metal (MIM) concept, inthe first order approximation VOC is governed by the work function difference of the anode andthe cathode, respectively. It should be noted that this only holds for the case where the Fermilevels of the contacts are within the bandgap of the insulator and are sufficiently far away fromthe 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 ofthe donor, respectively) are used, the situation is different. Charge transfer of electrons or holesfrom the metal into the semiconductor occurs in order to align the Fermi level at the negative and 21
    • positive electrode, respectively. As a result, the electrode work functions become pinned close tothe LUMO/HOMO level of the semiconducting materials [171]. Because of this pinning, the VOCwill 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 theacceptor has been reported [172]. The fact that a slope of unity was obtained indicates a strongcoupling of the VOC to the reduction strength of the acceptors [172]. Remarkably, the presence ofthe coupling between the VOC and the reduction potential of the PCBM has been interpreted as aproof against the MIM concept, although it is in full agreement with a MIM device with twoohmic contacts. In contrast, only a very weak variation of the VOC (160 meV) has been observedwhen 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 ofthe fullerene. However, it has been pointed out that when the metal work function is reduced tosuch an extent that it is below the LUMO, the electrode work function will remain pinned close tothe LUMO level of the semiconductor [173]. This explains why the VOC only increases slightlywhen going from Al (4.2 eV) to Ca (2.9 eV), because the Ca work function will be pinned to theLUMO of the PCBM (3.7 eV).1.6. ELECTRICAL CHARACTERISTICS PARAMETERSA solar cell under illumination is characterized by the following parameters: the short circuitcurrent (JSC), the open‐ circuit voltage (VOC), the fill factor (FF) and the PCE (ɳ). Theseparameters are indicated on the J-V characteristic of a solar cell shown in Figure 1.14. 25 20 Illumination 15 Dark 10 Current Density 5 0 -5 V FF OC -10 -15 JSC Pmax=(VI) max -20 -1.0 -0.5 0.0 0.5 1.0 1.5 Applied bias Figure 1.14 Definitions of JSC, VOC, FF, Jmax, and Vmax1.6.1. Short‐ circuit current (JSC)The short circuit current is the photogenerated current of a solar cell, which is extracted at zeroapplied bias. In this case, exciton dissociation and charge transport is driven by the so-called built- 22
    • Chapter 1in potential. The JSC is heavily dependent on the number of absorbed photons which originatesfrom two different facts. Firstly, JSC shows a linear dependence on the incident light intensity aslong as no saturation effects occur within the active layer. Secondly, JSC can be maximized byenlarging the absorption spectrum of the photoactive layer to harvest more photons within theterrestrial 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 currentgenerated by the illumination. So, at the VOC there is no external current which flows through thedevice 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 modelunder consideration of clean polymer/electrode interfaces [176, 177]. Here, cleanpolymer/electrode interface refers to absence of dipoles or other entities that changes interfaceconditions, usually resulting into shift of charge injection barriers. In a single-layer device, theVOC cannot exceed the difference in the work functions of the two electrodes [176]. Theexperimentally determined VOC is generally somewhat lower, owing to the recombination of freecharge carriers. At open-circuit conditions, all charge carriers recombine within the photoactivelayer. Thus, if recombination can be minimized, the VOC can more closely approach the theoreticallimit. However, based on thermodynamic considerations of the balance between photo-generationand recombination of charge carriers, it has been found that charge recombination cannot becompletely avoided, resulting in a lower open-circuit voltage [178]. In bilayer, the Voc scales linearly with the work function difference of the electrodes plusan additional contribution from the dipoles created by photoinduced charge transfer at theinterface of the two polymers [179]. On the other hand, this does not explain the VOC observed forBHJ solar cells. The Voc of BHJ solar cells mainly originates from the difference between theLUMO of the acceptor [180] and the HOMO of the donor [181], indicating the importance of theelectronic levels of donor and acceptor in determining the efficiency of such solar cells. In thecase of polymer-polymer BHJ solar cells, it has been demonstrated that the VOC significantlyexceeded 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 showswhere the cell can deliver power. In this quadrant, a point can be found where the power reachesits maximum value, is called the maximum deliverable power (Pmax). The fill factor is defined bythe Equation. 23
    • Pmax ( J  V ) max FF   Ptheor max J SC  VOCThe FF is a measure for the diode characteristics of the solar cell. The higher the number, themore ideal the diode is. Ideally, the fill factor should be unity, but due to losses caused bytransport and recombination its value generally found in between 0.2–0.7 for OPV devices. Thedirect relation of FF with current density indicates that it is greatly affected by the mobility of thecharge carriers. Moreover, series and shunt resistance are also observed as limiting factors in BHJsolar cells [184]. In order to obtain a high fill factor FF the shunt resistance of a photovoltaicdevice has to be very large in order to prevent leakage currents and series resistance has to be verylow.1.6.4. Power Conversion Efficiency (ɳ)In order to determine the PCE of a PV device, the maximum power Pmax that can be extractedfrom the solar cell has to be compared to the incident radiation intensity. It is the ratio of deliveredpower (Pin), to the irradiated light power (Plight). Pout (V  I ) max VOC  J SC  FF    Pin Pin PinThe 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 theideal diode current, the generation/recombination of carriers in the depletion region and anysurface leakage, which occurs in the diode. When a load is applied in forward bias, a potential difference develops between theterminals of the cell. This potential difference generates a current which acts in the oppositedirection to the photocurrent, and the net current is reduced from its short circuit value. Thisreverse current is usually called dark current in analogy with the current Idark(V) which flowsacross the device under an applied voltage in the dark. Most solar cells behave like a diode in thedark, 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 toachieve charge separation.1.6.6. Standard Test ConditionsThe efficiency of a solar cell depends upon temperature, excitation, spectrum and illuminationintensity. Therefore, test conditions have been designed to obtain meaningful and comparablevalues. These test conditions are based on a spectral distribution, reflection of the emission 24
    • Chapter 1spectrum of the sun, measured on a clear sunny day with a radiant intensity of 100 W/cm 2 that isreceived on a tilted plane surface with an angle of incidence of 48.2°. This spectrum that alsocounts for a model atmosphere containing specified concentrations of, e.g., water vapour, carbondioxide, 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 thecorresponding AM 1.5 spectrum (right).(Source: http://www.eyesolarlux.com/Solar-simulation-energy.htm).1.6.7. Equivalent Circuit DiagramThe equivalent circuit diagram (ECD) of an organic solar cell can be represented by a diode inparallel of a photocurrent source (IPh), a capacitor (C), a resistor called shunt resistor (RSh) and inseries another resistor called series resistor (RS) [186]. The ECD of a solar cell is shown in Figure1.16. 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 thebuilt in field from the donor/acceptor interface. This diode is responsible for the nonlinear shape 25
    • of the I-V curves. The photocurrent source generates current (Iph) upon illumination and equals tothe 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 thesite of exciton dissociation, before any charge transport can occur. RSh is correlated with theamount and character of the impurities and defects in the active organic semiconductor layerbecause 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 throughto the bottom electrode causing pinhole shorts. These are ohmic contacts that reduce the diodenature of the device and are represented by the shunt resistor. RSh determines from the inverseslope of the J-V curve in the fourth quadrant, as shown in Figure 1.17(a) [189]. (a) (b) Figure 1.17 (a) Impact of the variation of the shunt resistance (RSh) on the FF. (b) Impact of the variation of the series resistance (RS) on the FF. The series resistance (RS), is related with the intrinsic resistance, morphology, andthickness of the semiconductor layer. RS is analogous to conductivity i.e. mobility of the specificcharge carriers in the respective transport medium. RS also increases with a longer travelingdistance of the charges for example in thicker transport layers. The series resistance, Rs, can becalculated 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 lowcarrier mobility in these materials, injected carriers will form a space charge. This space chargecreates a field that opposes the transport of other free charges, acting like a capacitor. This isrepresented by the capacitor C in ECD shown in Figure 1.16.1.7. OBJECTIVE OF THE PRESENT THESISThe objective of the present work is to develop and improve the performance of organic andhybrid solar cells, consequently it is necessary to (i) understand the fundamental physical 26
    • Chapter 1properties of the organic and hybrid systems, (ii) understand the charge transport mechanism inthese devices, (iii) improve the charge transfer at donor/acceptor interface. To attain theseobjectives 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 thissemiconducting NCs of CdS has also been synthesised. To improve the poor charge transfer atorganic/inorganic interface, the NCs of CdTe are in-situ grown in P3HT matrix without use of anysurface ligands.2. The study has also been carried out to understand the basic physics underlying themorphological [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. Theeffect 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 thesedevices. Also the modulation of the charge transport parameters of P3HT on incorporation ofinorganic NCs (CdS and CdTe) has been studied.1.8. THESIS PLANThe present thesis explores the structural, optical, charge transport properties of P3HT, P3OT, andcopolymer of 3-hexylthiophene and 3-octylthiophene namely P3HT-OT as well as P3HT/CdTeand P3HT/CdS hybrid systems for their application in the solar cells. The thesis comprises of 7chapters. The present chapter (chapter 1) deals with the introduction which comprises of theliterature survey and overview of various generations of solar cells. Besides this, it also describesthe working principle of photovoltaic devices. It also includes discussion on various basic andapplied 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 andP3HT-OT), semiconducting NCs (CdTe, CdS) and polymer-nanocrystals hybrid systems. Itincludes the fabrication process of bulk heterojunction solar cells and hole only device for chargetransport study. Besides this, the basic working principles of various characterization techniquesutilized to characterize organic-inorganic hybrid systems have also been discussed. 27
    • 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 evaluatedby the cyclic voltammetry. Finally, the photovoltaic performance of P3HT-OT has been studiedand 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 anysurfactant. The CdTe NCs have been incorporated in-situ in P3HT matrix with the aim ofimproving 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 anddecrease in the quantum yield, confirm the charge transfer between P3HT/CdTe. Finally, PVparameters of P3HT/CdTe hybrid system have been investigated and results were compared withthose of pristine P3HT. In chapter 5 electrical and optical properties of P3HT/CdS hybrid system have beenstudied. The particle shape, size and distribution of CdS QDs in P3HT matrix have beeninvestigated by HRTEM, SEM and XRD. Optical studies (UV-Vis absorption and PL) suggest theelectronic interaction between P3HT and CdS nanocrystals. At the end of the chapter J-Vcharacteristics of P3HT and P3HT/CdS system with PCBM have been investigated under AM 1.5light as well as in the dark. Chapter 6 gives the theoretical and experimental details of the charge transport processesin organic semiconductors as well as in organic-inorganic hybrid systems. In the theoreticalsection of the chapter, space charge limited conduction which is dominant mechanism for chargetransport in disordered materials has been discussed in details. This chapter also discusses thefactors influencing the charge carrier mobility. In the experimental section the hole transportmechanisms 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 thesedevices have been studied in the temperature range of 300-110 K. Finally, chapter 7 presents the major conclusions derived from the present work and thescope of the future study in this field. 28
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    • CHAPTER 2EXPERIMENTAL DETAILS: MATERIALS, METHODS AND CHARACTERIZATIONTECHNIQUES2.1. INTRODUCTION2.2. SYNTHESIS OF POLY(3-ALKYTHIOPHENE)S2.3. 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 Dots2.4. 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 Substrate2.5. 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 SetupReferences2.1. INTRODUCTIONP 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 methodsof semiconducting nanocrystals (NCs) of Cadmium Telluride (CdTe) and Cadmium Sulphide(CdS) have also been discussed. It also describes the fabrication process of bulkheterojunctionsolar cells as well as hole only devices for charge transport study. Attempts have also been made
    • to describe the experimental setups and working principles for the various characterizationtechniques utilized to characterize the organic and organic/inorganic hybrid system.2.2. SYNTHESIS OF POLY(3-ALKYTHIOPHENE)SThe experimental setup used for the polymerization of poly(3-alkylthiophenes) (P3ATs) aredescribed here. The low temperature synthesis has been performed using the assembly as shownin Figure 2.1. The setup has a Julabo low temperature bath, (Model No: Julabo FP-50) a PCiNitrogen gas generator (Model: NG-02), a specially designed double walled glass container and astirrer.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 inertatmosphere (N2 atmosphere) in a double walled glass container, by constant stirring with a glassstirrer. To maintain the desired temperature, methanol was continuously circulated through thedouble wall container with the help of temperature bath running in a temperature range from 323K down to 223 K with an accuracy of ± 0.1 K. The homopolymers P3HT, P3OT, and the copolymer P3HT-OT have been synthesized usingthe oxidative coupling method shown in scheme 2.1. 40
    • Chapter 2Scheme 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 typicalsynthesis 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 monomerhaving desired concentration was slowly added to the continuously stirred FeCl3-CHCl3suspension for about 6 hours and the whole process was carried out for 24 hrs in order to givesufficient time for complete polymerization. After mixing of the reactants, the solution turnedgreen, which after 24 hrs was precipitated by adding plenty of methanol in a polymer-oxidantmixture. Repeated purification was performed by methanol and distilled water to removeoligomers and excess oxidant till the filtrate became colorless. The resultant polymer is greenafter drying at 333 K for two hrs. P3HT-OT thus obtained contains FeCl3 as an impurity. In orderto get P3HT-OT in pristine form, a rigorous purification process has been described below whichremoves FeCl3. After chemical synthesis, the resultant polymer contains unreacted monomer or oligomersand oxidant used for polymerization. Unreacted monomers, oligomers and oxidants are removedfrom the as grown polymer by successive washing by chemicals which show specific affinity forthe molecules to be removed. In the present case, the polymerization has been carried out using3HT, 3OT and FeCl3 in CHCl3. To get pristine P3HT-OT, the purification of polymer requiresremoval of any leftover 3HT, 3OT monomers, oligomers and FeCl3. In order to remove theseimpurities, the as grown polymer was treated with aqueous ammonia (aqueous NH3) andethylene-diamine-tetraacetic acid (EDTA) (liquid–liquid extraction) in separate steps. These stepsare 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 the60 ˚C. Due to continuous heating, the more volatile CHCl3 evaporates first, leaving P3HT-OTsolid with lower chloride content (as NH3 removes the chloride part of FeCl3 intercalated toP3HT-OT) floating over aqueous NH3. 41
    • 4. P3HT-OT obtained in step 3 is dissolved in CHCl3 and aqueous EDTA of the desiredconcentration was poured into the P3HT-OT–CHCl3 solution. The two phase solution is againheated to the boiling point of CHCl3. As heating continuous, the more volatile CHCl3 evaporatesfirst, leaving P3HT-OT solid with lower iron content (as EDTA removes the iron part of FeCl3intercalated to P3HT-OT) floating over aqueous EDTA.5. Steps (3) and (4) were repeated several times to minimize the FeCl3 impurity present in thepolymer matrix. Continuous repetition of the aqueous NH3 and EDTA treatment steps reduces the amountof 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 completelysoluble in CHCl3, chlorobenzene and toluene. The resultant P3HT-OT copolymer solution is castin a flat glass substrate. The solution is covered by another glass plate keeping a narrow openingto allow the evaporated solvent to escape. On complete evaporation of the solvent, the P3HT-OTfilm is peeled off from the glass substrate by pouring methanol into the film growing chamber, sothat the polymer film leaves the glass substrate on its own, without any mechanical stretching andtearing of the film during the separation from the glass substrate. The film is then dried at 353Kfor 1 h to remove any solvent trapped inside the film. A good quality film of pristine P3HT-OThaving excellent surface smoothness, free from pinholes and good mechanical strength (Figure2.2) was obtained and cut into pieces, which were then subsequently used for all electronic andelectrical studies. Figure 2.2 Solution cast film of copolymer P3HT-OT2.3. SYNTHESIS OF SEMICONDUCTOR NANOCRYSTALSThe 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), twocondensers, two magnetic stirrers with hot plates which can achieve 500 ˚C temperature and asyringe. The synthesis has been carried out under inert (Nitrogen/Argon) atmosphere. 42
    • Chapter 22.3.1. In-situ Growth of Cadmium Telluride Nanocrystals in P3HT MatrixIn-situ growth of CdTe nanocrystals in P3HT matrix was carried out as schematically illustratedin scheme 2.2 [4, 5]. In a typical synthesis of PHTCdTe1, 0.5 wt.% of P3HT has been dissolved intri-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 bytreating 0.2 mmol of tellurium powder (Acros Organics) in trioctylphosphine (TOP) (SigmaAldrich, 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 brightorange reaction mixture was allowed to react for 2 hrs at 160°C under argon atmosphere. Growthof CdTe NCs got completed when color of the solution turned black. After the completion of thereaction, the unreacted cadmium acetate and precursor of tellurium were removed by treatingnanocomposites with hexane. The reaction mixture was separated by centrifugation and dried invacuum at 80 °C. Ar ThermocoupleTOPTe solution Cdacetate+ Oil Bath P3HT+TCB solution 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-acetatewere synthesized and are designated as PHTCdTe2, PHTCdTe3, PHTCdTe4, and PHTCdTe20for 0.2 mmol, 0.4 mmol, 0.6 mmol and 3.6mmol, of Cd-acetate, respectively. These compositeshave the Te precursor in the ratios of 0.4 mmol for PHTCdTe2, 0.8 mmol for PHTCdTe3, 1.2mmol for PHTCdTe4 and 7.2 mmol for PHTCdTe20. The syntheses of different P3HT-CdTecompositions were also carried out at 220 °C using the same procedure discussed above. 43
    • 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 chainnetwork. (c) Schematic diagram of P3HT capped CdTe nanocrystals after reaction of TOPTe withCd2+ ions coupled P3HT.2.3.2. Synthesis of Cadmium Sulphide Quantum DotsThe synthesis of CdS quantum dots (QDs) was carried out by wet chemical method [6-8]. Twohexane solutions of Aerosol OT (AOT) (0.2 M, 50 ml) were prepared. An aqueous solution ofcadmium nitrate tetra-hydrate (Cd(NO3)2.4H2O) (0.4 M) was added to one hexane solution, whilean 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 micellarsolution containing cadmium nitrate was then added slowly to the micelle solution containingNa2S at room temperature under nitrogen atmosphere. CdS QDs were obtained after the solutionwas stirred for 3 h. 1-Decanethiol (DT) molecules (4.3 mmol) were added to a hexane solution ofCdS QDs (1.5 M). This solution was stirred for 5 h, and methanol was subsequently added inorder to remove the AOT molecules. After the methanol phase was removed, the hexane phasewas evaporated. The residual solution was then dropped into a large volume of methanol, and theresultant yellow precipitate was filtered off using a 0.2-µm membrane filter, yielding purified DT-caped CdS QDs. 44
    • Chapter 22.4. DEVICE FABRICATIONThe devices studied in the present investigations for the photovoltaic characterization as well asfor charge transport study have same fabrication steps. The processing steps of these devices havebeen discussed below:2.4.1. Patterning and Cleaning of ITO SubstratesIndium-tin-oxide (ITO) coated glass sheets (with a sheet resistance < 20 Ω/cm2) were cut into thesmall pieces of the area 1.5×1.5 cm2. These substrates were patterned by etching method using Zndust 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 wereultasonicated 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 for2 hrs. Prior to use, the cleaned substrate were treated with oxygen plasma. Glass substrates forUV-Vis absorption, photoluminescence, SEM, and AFM measurements were also cleaned in thesimilar manner.2.4.2. Glove Box System for Device FabricationSince the device properties of diodes based on organic compounds are extremely sensitive to theenvironmental conditions, in particular to the presence of oxygen and moisture. This sensitivity oforganic semiconductors towards exposition to oxygen and moisture is a strong limiting factor inthe operation of semiconductor elements. Special measures need to be taken during preparationand further treatment of the manufactured devices. To ensure oxygen and moisture freeenvironment the device fabrications have been carried out under inert atmosphere by using HindHi-Vac glove box system. The system consists of two interconnected glove-boxes filled with drynitrogen 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 ofactive layer and baking of active layer, respectively. The other glove box (Box B) is equippedwith a thermal evaporator for the deposition of small organic molecules and metals. The twoboxes are connected via a T-anti chamber with translation rails and a loading gate. The glove-boxsystem includes a gas purifier based on a copper catalyst and molecular sieves with closed gascirculation. The wet processing box A contains a purification system that is separated from theother box, protecting the latter from solvent contamination. Box B also shares the purificationsystem that either allowed independent gas circulation in the two boxes or parallel flow. Thewater and oxygen content is measured by a H2O/O2 analyzer and is typically below 1 ppm forboth the boxes. 45
    • 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 boxwas connected via a charcoal-trap to the glove-box. By permanently removing the pollutednitrogen 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 iscleaned from solvent vapors by an activated charcoal solvent trap that preceded the purificationsystem. In the metal evaporator (Figure 2.5), a vacuum of 10-6 mbar may be achieved by using aturbo pump. Venting was initiated by an automatic venting mode with time delay, in order toprotect the turbo pump. Subsequently, the evaporation chamber is filled with dry nitrogen gas outof 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 thebottom of the chamber supported by two automatic/manual shutter for loading the sourcematerial. Tungsten and molybdenum boats were employed, depending on the metal to bedeposited. Above the four sources, the sample holder was positioned, which could support foursamples with the typical size of 1.5×1.5 cm2. Just below the sample holder, two quartz balancesensors allow on-line measurement of the evaporation rate and thickness by an externally situateddeposition monitor controller. 46
    • Chapter 2Figure 2.5 Thermal evaporator system with four boats and two shutters and a sample holder onthe top.2.4.3. Active Layer Deposition on ITO SubstrateA 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. TheITO-coated glass substrate with a layer of PEDOT:PSS serves as the transparent anode throughwhich light is incident on the device. For the preparation of solar cells donor materials such asP3HT, P3OT, P3HT-OT and acceptor materials such as PCBM or QDs both have been taken inthe ratio of x:y with a concentration of z wt.% in chlorobenzene or tri-chlorobenzene or toluenewere dissolved by ultrasonication. The active layer was spin casted from these solutions on thetop 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 thermalevaporation through a shadow mask at 2×10-6 Torr. The device active area is ~0.1 cm2 for all thedevices discussed in this work.2.5. CHARACTERIZATION TECHNIQUESThis section describes the characterization techniques used for deciphering the structure (XRD,TEM, and HRTEM), spectroscopic (UV-Visible Photoluminescence, and FTIR), electricalproperties of the polymer and polymer/nanocrystal hybrid system. 47
    • 2.5.1. UV-Visible Absorption SpectraThe absorption of ultraviolet (200-400 nm)/visible (400-800 nm) radiation [9, 10] by a material iscaused 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 aredictated by selection rules it is called inter-band absorption [10]. Figure 2.6 shows the Inter-bandoptical 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 absorptionAbsorption spectroscopy is a technique where the intensity of a beam of light measured beforeand after interaction with a sample is compared as a function of wavelength. There are four maincomponents 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 lightfrom the light source; the output is tunable and highly monochromatic light. (3) A samplechamber which holds the sample under investigation and (4) a detector which measures theamount of light that passes through the sample. Typically, detectors are either solid statephotodiodes (silicon, germanium, etc.) or photomultiplier tubes. The basic setup for measuring theabsorption or transmission of light through a sample is shown in Figure 2.7. When light of somewavelength λ with intensity Io passes through the sample the intensity of the light is reduced to avalue 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 opticalabsorbance or optical density. Absorbance (A) is a dimensionless quantity defined as the negativeof the base-ten logarithm of the transmission (T) [11]. A   log 10 T 48
    • Chapter 2Figure 2.7 Light of intensity Io incident upon a sample undergoes a loss in intensity upon passingthrough the sample. The intensity measured after passing through the sample is I. For the experimental absorption spectra measurements of polymer andpolymer/nanocrystals, thin films have been prepared by spin coating from chlorobenzene solutionon 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. 49
    • 2.5.2. PhotoluminescenceWhen the light of sufficient energy is incident on a material, photons are absorbed and electronicexcitations are created. Photo-excitation causes electrons within the material to move intopermissible excited states. When these electrons return to their equilibrium states, the excessenergy is released by emission of light (a radiative process) or via a nonradiative process. Ifradiative relaxation occurs, the emitted light is called photoluminescence (PL). The energy of theemitted light is related to the difference in energy levels between the two electron states involvedin the transition between the emitted states and excited states.Experimental Setup: PL is simple, versatile, and nondestructive. The instrumentation that isrequired 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 inFigure 2.9. For the PL spectra measurements of polymer and polymer/nanocrystals, thin filmshave been prepared by spin coating from chlorobenzene solution on to a glass substrate. PLmeasurement was carried out at room temperature. The samples were excited with the wavelengthof 510 nm optical beam and the PL signal was detected with the Perkin Elmer LF 55 havingXenon source spectrophotometer (in the wavelength region of 530–850 nm).Figure 2.9 (a) Luminescence process and (b) schematic diagram of the vibrational electronictransitions 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]. 50
    • Chapter 2Figure 2.10 Typical schematic diagram and experimental setup for PL measurements.2.5.3 Fourier Transforms Infrared (FTIR) SpectroscopyInfrared spectroscopy is powerful tool for the confirmation of functional groups present in thecompound. Infrared radiation spans a section of the electromagnetic spectrum having frequencyrange 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 moleculeswill be having vibrations in the form of stretching, bending, etc., the absorbed energy will beutilized in changing the energy levels associated with them. It is a valuable and formidable tool inidentifying 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 moleculesare in continuous vibration with respect to each other. The major types of molecular vibrations areillustrated in Table 2.1. The frequency of vibration ʋ is given by 1 k  2c Where c is the velocity of light, k is the force constant and µ is the reduce mass. 51
    • Table 2.1 The major types of molecular vibrations. The two conditions that must be fulfilled for infrared absorption to occur are (1) thefrequency of a specific vibration of a molecule is equal to the frequency of the incident infraredradiation 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 bondsbut also with overtones of these frequencies. When one bond vibrates, the rest of the molecule isalso involved. The harmonic vibrations have frequency which is approximately integral multipleof a fundamental frequency. A combination band is the sum or difference between the frequenciesof two or more fundamental or harmonic vibrations. The uniqueness arises from those bandswhich are characteristics of whole molecule. The intensity of infrared absorption is proportionalto 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 infraredsource emits a broad band of different wavelength of infrared radiation. The infrared radiationgoes through an interferometer that modulates the infrared radiation. The interferometer performsan optical inverse fourier transform on the entering infrared radiation. The modulated infraredbeam passes through the sample where it is absorbed to various extents at different wavelengthsby the various molecules present. Finally the intensity of the infrared beam is detected by adetector, the detected signal is digitized and Fourier transformed by the computer to get the Iinfrared 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 thewavenumber range 400-4000 cm-1. 52
    • Chapter 22.5.4 Thermal AnalysisThermal analysis involves the study of rate and temperature at which materials undergo physicaland chemical transitions as they are heated and cooled. This is accompanied by the change inenergy and weight involved during the process. Thermogravimetric analysis is the branch ofthermal analysis which examines the mass change of a sample as a function of temperature in thescanning mode or as a function of time in the isothermal mode. Thermogravimetric is used tocharacterize the decomposition and thermal stability of materials under a variety of conditions andto examine the kinetics of the physicochemical processes occurring in the sample. The masschange characteristics of a material are strongly dependent on the experimental conditionemployed. Factors such as samples mass, volume and physical form, the shape and nature ofsample holder, the nature and pressure of atmosphere in the sample chamber, and the scanningrate, all have important influences on the characteristics of the recorded thermogravimetric curve.Thermogravimetric curves are recorded using a thermo balance. The principal elements of athermo balance are – an electronic microbalance, a furnace, a temperature programmer and aninstrument for simultaneously recording the outputs from these devices. In the presentinvestigation the thermogravimetric analysis of P3HT, P3OT and P3HT-OT have been carried outusing Mettler Toledo TGA 851e in nitrogen atmosphere with a flow rate of 60 mL/min. To studythe complete thermal behavior, samples have been heated from 25-700°C with heating rate10°C/min so that every volatile material could get detached from the samples. Differential scanning calorimetry (DSC) is another thermal analysis technique in whichthe difference in the amount of heat required to increase the temperature of a sample andreference are measured as a function of temperature. Both the sample and reference aremaintained at very nearly the same temperature throughout the experiment. The basicexperimental 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 physicaltransformation such as phase transitions, more (or less) heat, will need to flow to it from thereference to maintain both at the same temperature. 53
    • Figure 2.12 Basic set-ups for DSC measurement [15].2.5.5. Electrochemical Studies: Cyclic VoltammetryAll cyclic voltammetry (CV) data were obtained using a three electrode cell assembly as shown inFigure 2.13. Experiments have been performed using an Autolab 30, Potentiostat/Galvanostat inacetonitrile solution containing, 0.1 M tetra-n-butylammonium-tetrafluoroborate (TBATFB) atscan rate 20 mV/s. The Ag/AgCl has been used as the reference electrode while Pt as a counterelectrode. Pt has been used as the working electrode on which the polymer films have beendeposited by drop coating and dried in vacuum at 120 ˚C. Nitrogen tank Working electrode Reference electrode Counter electrode Analyte & electrolyte Figure 2.13 Experimental setup for the electrochemical studies. 54
    • Chapter 22.5.6. X-Ray Diffraction SpectroscopyX-ray diffraction (XRD) is a material characterization technique that can be useful to characterizethe crystallographic structure, crystalline size (grain size) and preferred orientation inpolycrystalline or powder solid sample. It may also be used to characterize heterogeneous solidmixture to determine the relative abundance of crystalline compound and when coupled with thelattice refinement technique such as relative refinement, can provide the structure information inunknown sample [16].Basic Theory: Diffraction and Bragg’s LawDiffraction can occur when any electromagnetic radiation interacts with a periodic structure. Therepeat distance of the periodic structure must be about the same wavelength of the radiation. Incrystals, 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 makethem 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 identicalwavelength and phase approach a crystalline solid and are scattered by two different atoms withinit. The lower beam traverses an extra length of 2dsinθ. When X-rays are scattered, they canconstructively interfere, producing a diffracted pattern. The relationship describing the angle atwhich a beam of X-rays of a particular wavelength diffracts from a crystalline surface wasdiscovered by Sir William H. Bragg and Sir W. Lawrence Bragg and is known as Bragg‟s Law ofdiffraction, and given by [16-19] 2d sin   n 55
    • Where λ is the wavelength of the X-ray, θ is the angle between incident ray and surface of thecrystal, 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 byusing the Debye-Scherrer equation. In Figure 2.15 the rays A, D and M make precisely this anglewith the reflecting planes. Ray D′, scattered by the first plane below the surface, is onewavelength out of phase with A′, ray M′ is m wavelengths out of phase with it. At the diffractionangle 2θB all these rays are in phase and unite to form a beam of maximum amplitude. Ray Bmakes 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 planescattering, a ray that is exactly an integer plus one-half wavelength out of phase with B′. So therays scattered by the upper half of the crystal cancel exactly with those scattered by the lower halfof the crystal and θ1 is the smallest angle where complete destructive interference occurs. This isalso the case for an angle θ2 which is a bit smaller than θB so that the path difference between theray scattered by the first and the last plane is (m − 1) wavelengths. These are the two limitingangles where the intensity of the diffracted beam drops to zero. Figure 2.15 Scattering from a finite number of equidistant planes. 56
    • Chapter 2The width of diffraction curves increases as the thickness of the crystal decreases, because theangular range (2θ1 − 2θ2) increases as m decreases. As a measure of the peak width, the full widthat 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 forthese two angles related to the entire thickness of the crystal are given by: 2t sin θ1 = (m + 1)λ 2t sin θ2 = (m − 1)λSubtracting the above equations yields: t(sin θ1 − sin θ2) = λSince 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 θwhere, 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 andinstrument-related broadening are negligible in comparison to particle size effects. This conditionis often met with particle sizes that are in the 10 - 100 nm range. 2θ Incident X-ray θ Transmitted X-ray Figure 2.16 Schematic configuration of an X-ray diffraction machine. 57
    • In Figure 2.16 a schematic configuration of an XRD machine can be seen. The X-ray hitsthe sample under an adjustable angle θ. The intensity of the reflected beam is measured with thedetector. The detector moves with a varying glancing angle θ on the measuring circuit in the waythat the angle between the beam direction and the detector is always 2θ. In the presentinvestigations 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 of0.0250/sec and slit width of 0.1mm.2.5.7. Scanning Electron MicroscopyScanning Electron Microscopy (SEM) is a very useful technique and widely used to study thesurface morphology, surface topography, composition and other surface properties of the samplesand it offers a better resolution than that of optical microscope. It provides high-magnification andcan have resolution of a few nanometers [21]. In a typical SEM instrument, Tungsten or LaB6 is used to emit monochromatic electronswith typical energy of 20-30 keV. These electrons are focused by condenser lenses to form abeam with a very fine spot size ~ 1 to 5 nm. This beam passes through a pair of scanning coils inthe objective lenses, which deflects the beam in a raster fashion over the sample surface. Thisbeam of primary electrons interacts with sample volume ranging from less than 100 nm to 5 mand generates secondary electrons (Figure 2.17). These secondry electron signals are detected byappropriate detectors. The final image is produced on the screen through cathode ray tube. In thepresent investigation, samples for SEM study were prepared by spin casting of material on a glasssubstrate. A thin layer of precious metal was sputtered prior to loading the samples in themicroscope probe. Another possible way in which a beam of incident electron can interact with an atom is bythe ionisation of an inner shell electron. The resultant vacancy is filled by an outer electron, whichcan release its energy either via an Auger electron or by emitting an X-ray (Figure 2.17). Thisproduces characteristic lines in the X-ray spectrum corresponding to the electronic transitionsinvolved. Since these lines are specific to a given element, the composition of the material can bededuced. This can be used to provide quantitative information about the composition near thesurface and is known as Energy Dispersive Auger X-ray (EDAX) Spectroscopy. 58
    • Chapter 2 Figure 2.17 Schematics of Scanning Electron Microscope.2.5.8 Transmission Electron MicroscopyTransmission electron microscopy (TEM) is a powerful tool for doing structural andmorphological 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 compositionalinformation [22]. Figure 2.18 shows the schematic diagram of a typical transmission electronmicroscope [23].Working principle: TEM works like a slide projector. A projector shines a beam of light whichtransmits through the slide. The patterns painted on the slide only allow certain parts of the lightbeam to pass through. Thus the transmitted beam replicates the patterns on the slide, forming anenlarged image of the slide when falling on the screen. TEMs work the same way except that theyshine 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 thematerial being examined. Such properties include density, composition, etc. For example, porousmaterial will allow more electrons to pass through while dense material will allow less. As a 59
    • result, a specimen with a non-uniform density can be examined by this technique. Whatever partis 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 60
    • Chapter 2 A key requirement for TEM samples is the electron transparency, as a thick sample wouldcause too many interactions leaving no intensity in the transmitted beam. A thick sample alsoincreases the risk that an electron is scattered on multiple occasions and the resulting image wouldbe difficult to interpret. In the present work, samples have been prepared by dispersing sample inethanol or chloroform using sonification and a small drop of that solution was casted onto thecarbon coat copper grid. The images were taken using a Tecnai G2 F30 S-Twin instrumentoperated at an accelerating voltage of 300 kV, having a point resolution of 0.2 nm and a latticeresolution of 0.14 nm.2.5.9 I-V Characterization TechniqueIn order to calculate the different parameters of a solar cell, it is desirable to measure the I-Vcharacteristics under dark and light, which can give information about the VOC, JSC, FF efficiencyas well as defect states and transport properties of the material. For electrical propertymeasurements, using I-V technique it is necessary to make provisions for electrical contacts whichrequires: (1) probe station with needles and sometimes with a microscope attached, to probe verysmall devices, (2) a source meter to apply voltage and measure current or vice versa, (3) acomputer with appropriate program to collect data and analyze them. Figure 2.20 shows aschematic of our J-V setup.2.5.10 Temperature Dependent I-V Measurements Set UpFor temperature dependent I-V measurements, a Janis cryogenic system model Wilmington, MA01887 has been used which can go from 20K to 325K with pressurized Helium gas. For I-Vmeasurements the device has been loaded into the cryostat with proper contact as shown in Figure2.21. The cryogenic system is connected with a rotary pump. The sample in the cryostat wasconnected to the Keithley‟s source measure unit for biasing the device. Data has been collectedwith a computer connected to the source meter with GPIB connector. 61
    • Figure 2.20 Schematic representation of experimental arrangement of current-voltagemeasurements of solar cell. Figure 2.21 Schematic of temperature dependent current-voltage measurements setup. 62
    • Chapter 2References[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 andPhysics 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 byopen learning, John Willey and Sons, 1987. 63
    • [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 AdvancedMaterials 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, PlenumPress. New York and London, 1996. 64
    • CHAPTER 3STUDY OF THE PHOTOVOLTAIC PERFORMANCE OF COPOLYMER POLY[(3-HEXYLTHIOPHENE)-CO-(3-OCTYLTHIOPHENE)]3.1 INTRODUCTION3.2 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 Cells3.3. CONCLUSIONSReference3.1 INTRODUCTIONP 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 farextremely attractive donor material in combination with [6, 6]-phenyl C61 butyric acid methylester (PCBM) as the electron acceptor. Power conversion efficiency ɳ ~ 6% has already beenrealized [7] in polymer solar cells based on P3HT:PCBM donor:acceptor interpenetrating bulkheterojunction (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 reportsin 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]. 65
    • The chemical nature and the length of the alkyl side chains have a great effect on thecharge carrier mobility in poly(3-alkylthiophenes) (P3ATs) [10, 11]. In general, the attachment ofbranched, bulky side chains led to a low crystallinity of the solid layers. Also, the π-π overlapdistance between the conjugated backbones within the main chain layers is larger in thesepolymers, resulting in low carrier mobility [10, 12]. For linear alkyl chains, it is observed that themobility decreases with increasing alkyl chain length [11]. This has been attributed to theisolating nature of the alkyl substituent [13]. In fact, the largest carrier mobility reported for P3OTin field effect transistor (FET) configuration is 10-3cm2/Vs [14], approximately 1-2 orders ofmagnitude lower than the typical mobilities of P3HT. However, a critical length of the alkyl sidechain is needed for a sufficient solubility and processability of the polymer from solution. Forexample, higher-molecular-weight batches of regioregular P3HT are well soluble in chlorinatedsolvents such as chloroform, toluene but only weakly soluble in non-chlorinated solvents such astoluene or xylene. On the other hand, P3OT dissolves quickly in toluene at room temperature. Atthe moment, P3HT is considered to present the best compromise with respect to solubility, layerformation, and overall photovoltaics performance. Babel and Jenekhe presented binary blends of semiconducting polymers as a novelapproach to tune the properties of polymer FETs [15, 16]. In the first set of experiments, a seriesof 10 binary blends of regioregular poly(3-hexylthiophene)s and poly(3-decylthiophene)s havebeen prepared and the dependence of the charge carrier mobility on the blend composition hasbeen studied [15]. They found that the field-effect mobility of these blends relatively higher(2×10-3cm2/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 bycopolymerization of the respective monomer units. In the present investigations, in order toincorporate both the features of better solubility plus mobility within a single component, theregioregular copolymer poly[(3-hexylthiophene)-co-(3-octylthiophene)] (P3HT-OT) has beenused 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 basedon P3HT-OT is compared with the performances of devices based on homopolymers P3HT andP3OT. 66
    • Chapter 3Figure 3.1: Structural formula of homopolymers (a) P3HT (b) P3OT and (c) copolymer P3HT-OT.3.2. RESULT AND DISCUSSION3.2.1. FTIR SpectraFourier transform infrared spectroscopy (FTIR) spectra have been recorded on Nicolet 5700 intransmission mode, wavenumber range 400-4000 cm-1 with a resolution of 4 cm-1 performing 32scans. The FT-IR spectra of P3HT, P3OT and P3HT-OT are shown in Figure 3.2. A comparativestudy of the FT-IR spectra of P3ATs polymer synthesized for the present investigation with thosereported earlier for P3AT synthesized by various routes [19] shows the quality of P3ATs. Thereported band for aromatic CH out of plain vibration is at 820 to 823 cm−1, which is thecharacteristics of 2,5-disubstituted-3-alkylthiophene for rr-P3AT whereas the corresponding bandfor rdm-P3AT occurs at 827 to 830 cm−1 [20, 21]. The aromatic CH out of plain vibration in the present study has been observed inbetween the 820 to 822 cm-1 (Table 3.1), which confirms the regioregularity of homo polymersP3HT, 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 stretchingvibrations in –CH3 and –CH2–, and the symmetric C–H stretching vibration in –CH2–. They havebeen ascribed to the alkyl-side chain. The bands at 1457, 1374 cm_1 are due to the thiophene ringstretching and methyl deformation respectively. The C-C vibrations appear at 1165 and 1088cm_1. The absorption at 720 cm_1 is assigned to the methyl rocking. A measure of the conjugationlength 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 increasingconjugation length. For regioregular PATs this ratio is 6-9, less than half of the 15-20 value 67
    • measured for regiorandom samples [17-20]. In the present investigations we have observed thisratio 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.Sample Aromatic Aliphatic C- Ring Methyl Aromatic Methyl C-H H stretching stretchin deformation C-H out of rocking stretching g planeP3HT- 3054.8 2952.8, 1510.1, 1374.7 822.8 720.2OT 2921.0, 2852 1457.0P3OT 3053 2955, 2916.1, 1509.6, 1377.5 822.5 722.1 2852.4 1463.5P3HT 3055.9 2953, 2921.7, 1508.8, 1375.6 820.4 723.9 2852.8 1454.6 100 723 1375 1508 80 820 % Transmittance 1454 2852 2953 2921 60 720 1510 822 1374 2952 1454.6 2852 2921 40 P3HT 722 822 1377 P3OT 1509 1463 P3HTOT 2852.4 2955 2916 20 1000 1500 2000 2500 3000 -1 Wavenumber (cm ) Figure 3.2 FT-IR spectrum of pristine P3HT, P3OT and copolymer P3HT-OT films.3.2.2. 1H NMR SpectrumNMR is a powerful tool for providing information concerning configuration and conformation ofpolymer. It has been extensively used for studying regio-chemistry of P3AT. The main elementsof regio-chemistry of P3AT are thiophene dyad and triad configuration, which are shown in 68
    • Chapter 3Figure 3.3. Thiophene triads are used to determine the configuration of polymer based on NMRchemistry of β-proton (4-position) of thiophene ring. Dyad configurations are discussed in termsof chemical shift of α-methylene-H of the alkyl side chain. 1H NMR spectra of all the polymersused in the present investigation in CDCl3 solution at 300 MHz are shown in Figure 3.4. it hasbeen reported in the literature [18, 19] that in a regioregular, HT-PAT, there is only one aromaticproton 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 ofregiorandom PAT reveal that four singlets exist in the aromatic region that can clearly beattributed to the protons on the β-position of the central thiophene ring in each configurationaltriad: HT-HT(δ = 6.98), TT-HT(δ = 7.00), HT-HH(δ = 7.03),and TT-HH(δ = 7.05) [18, 19]. Inthis 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 ringsignal 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 thespectral region 2.5-3.0 ppm are attributed to of α-methylene-H of the alkyl side and are observedto 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 thesepolymers. 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 thepresent investigation, the β-methylene-H signal for P3HT, P3OT, and P3HT-OT has beenobserved at 1.704 ppm, 1.708ppm and 1.707ppm respectively. These results again indicate thatpolymers having HT–HT couplings. Resonances due to methyl protons are reported in literaturein the spectral region 0.885-0.912 ppm [19, 20]. In the present study these resonance have beenobserved 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 arebroader and seem doublet like structure, because methyl proton of both hexyl and octyl side chainoverlap. The doublets in copolymer further indicate the copolymerization of 3HT and 3OT. 69
    • C6H13 C6H13 C6H13 S S S S S S C6H13 C6H13 C6H13 HT TT HH C6H13 C6H13 C6H13 C6H13 S S S S S S C6H13 C6H13 HT-HT HT-HH C6H13 C6H13 C6H13 C6H13 S S S S S S C6H13 C6H13 HH-TT TT-HT 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-Hof the alkyl side chain [19, 20]. (a) (b) Linkage β H4 Head-to-tail Head-to-head HT-HT 6.98 α-methylene-H 2.80 2.58 TT-HT 7.00 β-methylene-H 1.72 1.63 HT-HH 7.02 TT-HH 7.05 70
    • Chapter 3 P3HT (a) Figure 3.4 (a) 1H NMR spectra of P3HT. P3HT-OT (b) Figure 3.4 (b) 1H NMR spectra of P3HT-OT. 71
    • P3OT (c) Figure 3.4 (c) 1H NMR spectra of P3OT.3.2.3. Thermal StudiesPrior to thermogravimetric analysis (TGA) measurements, materials have been dried in vacuum atelevated temperatures to remove residual solvent/moisture. Dynamic TGA has been carried out ona METTLER TOLEDO, TGA/SDTA 851e with heating rate of 100C/min under nitrogenatmosphere to assess the thermal stability of the polymers. Differential scanning calorimetry(DSC) measurement has been performed on a METTLER TOLEDO, DSC822e with heating rateof 100C/min under nitrogen atmosphere. Thermal stability of the polymers is generally reported asthe temperature at which 5% weight loss has been observed. Figure 3.5(a) shows the TGA graphof P3HT, P3OT and copolymer P3HT-OT. As shown in TGA graph, the onset point of weightloss 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 beenconcluded that long alkyl side group P3OT decompose at lower temperature than short alkyl sidegroup P3HT, also thermal stability of copolymer P3HT-OT is in-between of the twohomopolymers. The weight losses in polymers have been observed due to decomposition of thealky side groups. As long alkyl side group decompose at lower temperature as compared to shortalkyl group, this is why P3HT is more stable than other two polymers. 72
    • Chapter 3 -0.4 90 P3OT Heat Flow (mW) -0.8 75 P3HT % Weight loss P3HTOT 60 -1.2 45 P3HT P3HTOT -1.6 30 (a) P3OT (b) 50 100 150 200 250 300 75 150 225 300 375 450 525 0 0 Temperature ( C) Temperature ( C) 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 copolymerP3HT-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 ofP3HT and P3OT (as suggested in Figure 3.1(c)), which have melting transitions at 215ºC and186ºC, respectively.3.2.4. XRD StudiesFigure 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 at2Ɵ 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 randomrepeating of hexyl, and octyl group attached to the polymer matrix) of copolymer P3HT-OT. Thesecond 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 thehomopolymer P3OT and larger than the homopolymer P3HT, suggesting partial inter-digitationbetween the side chains and/or the occurrence of tilting of the octyl chains in P3HT-OT. XRDstudy shows that the interlayer spacing increases with elongation of alkyl side chain. This showsthat 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 todifferent two d020 values of 5.254 Å and 6.303 Å respectively. The 6.303 Å spacing is due to the 73
    • interlayer stacking distance between P3OT in a layered packing structure (dP3OT), whereas the5.254 Å spacing is corresponds to the interlayer stacking distance between P3HT (dP3HT). Thesepeaks 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 0 0 0 2Ɵ d (A ) 2Ɵ d (A ) 2Ɵ d (A ) 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 6.303 16.860 5.254 (100) P3OT (020) Lin (Counts) dP3HT dP3OT (020) (100) P3HTOT (100) P3HT (020) 5 10 15 20 25 2 (degree) Figure 3.6 XRD spectra of solution cast polymer films, annealed at 120 0C.3.2.5. Evaluation of Energy LevelsThe electronic energy levels, highest occupied molecular orbital (HOMO) level, and lowestunoccupied molecular orbital (LUMO) level, of polymers are one of the most significantproperties for polymer solar cells. However, their values differ significantly in different literaturereports. Cyclic voltammetry has been performed to estimate the HOMO and LUMO levels of all 74
    • Chapter 3 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. 0.00030 P3HTOT P3HT 0.0006 0.00015 0.0003 Current (A) 0.00000 0.0000Current (A) -0.00015 -0.0003 -0.00030 -0.0006 -0.0009 -0.00045 -0.0012 -1.0 -0.5 0.0 0.5 1.0 1.5 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 E/V vs. Ag/AgCl E/V vs. Ag/AgCl 0.0006 P3OT 0.0004 0.0002 0.0000 Current (A) -0.0002 -0.0004 -0.0006 -0.0008 -1.0 -0.5 0.0 0.5 1.0 1.5 E/V vs Ag/AgCl 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]: HOMO  e( Eonset( vs. Ag / AgCl)  4.71)eV ox 75
    • The oxidation onsets of P3HT, P3OT and P3HT-OT and the corresponding HOMO levelscalculated in this way are listed in Table 3.4. The LUMO level (electron affinity) can in principlebe calculated using the reduction onset; however, these measurements were difficult to performreliably for our materials. We have therefore estimated the electron affinity simply by subtractionof the band gap energy Eg from the ionization potential following E LUMO  E HOMO  E g . There aresignificant uncertainties inherent in this method, for example due to the neglect of excitonicbinding energy and other screening effects. Nevertheless the trends between different materialsare of interest, as shown in Table 3.4.Table 3.4 Optical and Electrochemical Properties of P3HT, P3HT-OT and P3OT. 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 -3.43.3.6. UV–Vis Absorption SpectraUV-Vis absorption spectra of all the polymers have been recorded by Shimadzu UV-1601spectrophotometer. Absorption spectra of all the polymer thin films are shown in Figure 3.8(a). Inconjugated 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 maximumabsorption (λmax) in the solid films of the P3HT, P3HT-OT, and P3OT have been observed at 518nm, 512 nm, and 511 nm, respectively. The blue-shift in the absorption of the P3HT-OT andP3OT with respect to P3HT has been attributed to steric hindrance of octyl side chain attached tothese polymer matrixes. This octyl side chain may be difficult to rotate compared to hexyl sidechain to form the more advantageous arrangement. Polymer film also shows an absorptionshoulder at 600nm, 595nm, and 598nm for P3OT, P3HT-OT, and P3HT, respectively, which areassigned to the interchain excitation and 1Bu vibronic sidebands [24, 25] and confirm theinterchain absorption in these polymers [26, 27]. Most remarkably the intensity of the shoulder at600 nm drops substantially when going from P3HT to P3HT-OT to P3OT, which indicates thedecrease of the interchain interaction between these polymers. 76
    • Chapter 3 1.2 2.5 P3HT P3HT (b) (a) 1.0 P3HTOT P3HTOT 2.0 P3OT P3OT 0.8 1.5 AbsorptionAbsorption 0.6 1.0 0.4 0.5 0.2 0.0 0.0 300 400 500 600 700 300 350 400 450 500 550 600 Wavelength (nm) Wavelength (nm) 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]. 77
    • 0.8 0.6 Absorption 0.4 As prepared 0.2 annealed at 900C annealed at 1200C 0.0 300 375 450 525 600 675 Wavelength (nm) Figure 3.9 Absorption spectra of annealed P3HT-OT thin films. For studying the inter donor-acceptor charge transfer process, the blends of PCBM withP3HT-OT has been prepared. In Figure 3.10 the absorption spectra of P3HT-OT, PCBM, andblend 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 decayssmoothly in the visible region with a pronounced shoulder. The P3HT-OT/PCBM blends showmore 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 ofpolymer and PCBM. 3.5 P3HTOT:PCBM1:0 3.0 P3HTOT:PCBM1:1 P3HTOT:PCBM1:2 2.5 P3HTOT:PCBM0:1 Absorption 2.0 1.5 1.0 0.5 0.0 300 375 450 525 600 Wavelength (nm)Figure 3.10 Absorption spectra of P3HT-OT, PCBM and P3HT-OT/PCBM blends in proportionof 1:1 and 1:2 in toluene solution. The absorption spectra of the blends of P3HT/PCBM and P3OT/PCBM in samecomposition ratios as discussed above, are shown in Figure 3.11(a) and Figure 3.11(b), 78
    • Chapter 3 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. 3.0 3.0 (a) P3HT:PCBM1:0 (b) P3OT:PCBM1:0 P3HT:PCBM1:1 P3OT:PCBM1:1 2.5 2.5 P3HT:PCBM1:2 P3OT:PCBM1:2 2.0 2.0 AbsorptionAbsorption 1.5 1.5 1.0 1.0 0.5 0.5 0.0 0.0 300 375 450 525 600 300 350 400 450 500 550 600 Wavelength (nm) Wavelength (nm) 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. 79
    • 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. 5 5 3.0x10 2.0x10 (a) P3HT:PCBM1:0 (b) P3HTOT:PCBM1:0 5 2.5x10 P3HT:PCBM1:1 5 P3HTOT:PCBM1:1 1.6x10 P3HT:PCBM1:2 P3HTOT:PCBM1:2PL Intensity 5 2.0x10 PL Intensity 5 1.2x10 5 1.5x10 4 8.0x10 5 1.0x10 4 4 4.0x10 5.0x10 0.0 0.0 500 550 600 650 700 750 500 550 600 650 700 750 Wavelength (nm) Wavelength (nm) 5 2.5x10 (c) P3OT:PCBM1:0 -3.0 5 P3OT:PCBM1:1 2.0x10 -3.32 P3OT:PCBM1:2 -3.37 -3.4 PL Intensity -4.0 Energy (eV)) 5 1.5x10 -4.2 -4.3 -5.0 5 1.0x10 -4.8 -5.2 -5.27 -5.31 5.0x10 4 -5.35 d -6.0 -6.0 0.0 ITO PEDOT:PSS P3HT P3HTOT P3OT PCBM Al 500 550 600 650 700 750 Wavelength (nm) 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 cm2 substrate). 80
    • Chapter 3 Al 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%. 10 4 Dark (a) Dark (b) Illuminated 8 Illuminated 2 6 J (mA/cm2)J (mA/cm2) 4 0 2 -2 0 -2 -4 -4 -1.0 -0.5 0.0 0.5 1.0 -1.0 -0.5 0.0 0.5 1.0 Voltage (Volts) Voltage (Volts) 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, 81
    • 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. 3 2 Dark (a) Dark (b) 2 Illuminated 1 Illuminated Without annealed Annealed 1 0 J (mA/cm2)J (A/m2) 0 -1 -1 -2 -2 -3 -3 -4 -1.0 -0.5 0.0 0.5 1.0 -1.0 -0.5 0.0 0.5 1.0 Voltage (Volts) Voltage (Volts) 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. 3 2 Dark (a) Dark (b) 2 Illuminated Illuminated 1 Without annealed Annealed 1 J (mA/cm2) J (mA/cm2) 0 0 -1 -1 -2 -2 -3 -1.0 -0.5 0.0 0.5 1.0 -1.0 -0.5 0.0 0.5 1.0 Voltage (Volts) Voltage (Volts) 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 82
    • Chapter 315(b) and Figure 16(b), respectively. Table 3.5 summaries the photovoltaic performanceparameters 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 (503mV) < 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. Themaximum VOC has been observed for P3OT, whereas P3HT shows the minimum VOC. Thecopolymer 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. Device Remark VOC JSC FF ɳ (%) (Volts) (mA/cm2)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% It has been observed by various reporters [35-40], that the open-circuit voltage depends onthe acceptor strength of the fullerenes applied. This result fully does support the assumption, thatthe open-circuit voltage of a donor–acceptor bulk-heterojunction cell is directly related to theenergy difference between the HOMO level of the donor and the LUMO level of the acceptorcomponent [35-39]. In agreement with this result and from the realizable trend comparing Eox onsetof P3HT, P3HT-OT, P3OT (Table 3.4) a possible explanation could be that the relatively smallerdifferences 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 andLUMO level of PCBM. 83
    • The cell based on P3HT possesses a higher short-circuit current (2.64 mA/cm2) than thecells based on P3HT-OT (2.36 mA/cm2) and P3OT (1.46 mA/cm2). Regioregular head-to-tailP3HT is well known for a high degree of intermolecular order leading to high charge carriermobilities (1.4×10-2 cm2V-1s-1) [41]. The hole mobilities for P3HT-OT (7.2×10-3 cm2V-1s-1) and for P3OT (1.3×10-3 cm2V-1s-1)measured form FET geometries have been reported by A. Zen et al [41] which are lower than thatof P3HT. Assuming the same degree of regioregularity as well as of polymerization degree for allthree P3ATs, the hole mobility should increase as the length of the side chains decreases. This isexpected due to the contribution of side chain to the degree of intermolecular order and chainpackaging density. The smaller mobility of charges in P3HT-OT, and P3OT compared to those inP3HT is due to the isolating nature of the side chain layers. Most remarkably the intensity of theshoulder 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, thepacking of the polymer chains in the main chain layers significantly controls the mobility of thehomo- and copolymers studied here. Furthermore, the potential barrier of P3HT-OT/ITO is slightly higher than that ofP3HT/ITO and somewhat lower than that of P3OT/ITO (see Figure 3.10(d) and Table 3.4). Thusthe hole injection from the HOMO of the polymers into ITO becomes less restricted in the case ofP3HT compared to the other two polythiophenes. P3HT shows a higher absorption coefficientthan P3HT-OT and P3OT (see Figure 3.7). Thus P3HT absorb more photon and has small holeinjection 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%, 84
    • Chapter 3 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/cm2). 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.REFERENCES[1] Y. Kim, S .Cook, S. M. Tuladhar, S. A. Choulis, J. Nelson, J. R. Durrant, D. D. C. Bradley, M.Giles, I. McCulloch, C. Ha, M. Ree, Nat. Mater. 5 (2006) 197.[2] M. Reyes-Reyes, K. Kim, D. L. Carroll, Appl. Phys. Lett. 87 (2005) 083506.[3] W. U. Huynh, J. J. Dittmer, A. P. Alivisatos, Science 295 (2002) 2425.[4] Y Kang, N-G. Park, D. Kimb, Appl. Phys. Lett. 86 (2005) 113101.[5] I. Khatri, S. Adhikari, H. R. Aryal, T. Soga, T. Jimbo, and M. Umeno, Appl. Phys. Lett. 94(2009) 093509.[6] Z. Liu, Q. Liu, Y. Huang, Y. Ma, S. Yin, X. Zhang, W. Sun , Y. Chen Adv. Mater. 20 (2008)3924.[7] K. Kim, J. Liu, M. A. G. Namboothiry, D. L. Carroll, Appl. Phys. Lett. 90 (2007) 163511.[8] E. Kymakis, P. Servati, P. Tzanetakis, E. Koudoumas,N. Kornilios, I. Rompogiannakis, Y.Franghiadakis, G. A. J. Amaratunga, Nanotechnology 18 (2007) 435702.[9] V. D. Mihailetchi, H. Xie, B. D. Boer, L. J. A. Koster, P. W. M. Blom, Adv. Funct. Mater.16 (2006) 699.[10] Z. Bao, J. A. Rogers, H. E. Katz, J. Mater. Chem. 9 (1999) 1895.[11] H. E. Katz, Z. J. Bao, Phys. Chem. B 104 (2000) 671.[12] Z. Bao, A. J. Lovinger, Chem. Mater.11 (1999) 2607. 85
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    • 88
    • CHAPTER 4STUDY OF PHOTOVOLTAIC PERFORMANCE OF ORGANIC/INORGANIC HYBRID SYSTEM BASED ON IN-SITU GROWN CdTe NANOCRYSTALS IN P3HT MATRIX4.1 INTRODUCTION4.2 FABRICATION AND MEASUREMENT OF DEVICE4.3 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 Cells4.4. CONCLUSIONSReferences4.1. INTRODUCTIONI 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 mostattractive nanocrystals due to their good chemical processability, mono-dispersed sizedistribution, and having strong quantum confinement effects. Their optical property can be tunedas a function of size. Murray et al. [1] reported the synthesis of high quality cadmiumchalcogenides nanocrystals using dimethyl cadmium [(Cd(CH3)2] as the cadmium precursor in thepresence 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) ascoordinating 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 elevatedtemperatures due to releasing of large amount of gas [3-6]. Therefore, the Cd(CH3)2-relatedschemes require very restricted equipments and conditions and are not suited for large-scalesynthesis. As a result, many researchers who are studying the cadmium-based QDs, welcome thenew 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
    • temperatures (240–360˚C). Among all the tested compounds (CH3COO)2Cd is proven to be thebest cadmium precursor, since it is free from pyrophoric and explosive properties [6]. In all theabove procedures, CdTe was synthesized at fairly high temperatures (>200 ◦C) and utilizeexpensive 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 preparationpathway employing cheap paraffin [11, 12] or plant oil [13] or commercial diesel [14] as a solventwithout any organic phosphines, aliphatic amines, and ODE was introduced. In all of the above synthesis procedure, nanocrystals have been capped with organicaliphatic ligands, such as TOPO or oleic acid. It has been shown that when the nanocrystals arecapped with organic ligands, they hinder the efficient electron transfer from the photoexcitedpolymer 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 forthe 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 ofCdTe have been in-situ synthesized in the polymer matrix as discussed in chapter 2. The in-situgrowth of the nanocrystals in polymer templates controls the dispersion of the inorganic phase in 90
    • Chapter 4the 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 inpercolation pathway for transport of charge carriers to the respective electrodes. In surfactant-assisted synthesis, nanocrystals growth is controlled by electrostaticinteractions induced by the surfactant functional group and steric hindrance from the surfactantside alkyl chains. P3HT provides a combination of both effects, as it contains an electron donatingsulfur functionality, a potential anchorage for the nucleation, and growth of nanoparticles alongwith 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 inpoly(2-methoxy-5-(2-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV), have been reportedpreviously [17-19]. As CdTe has optimal band gap for solar cells and absorb higher amount ofsolar radiation as compared to the CdSe, and PbS nanocrystals. Therefore, replacement of thesenanocrystals with CdTe would enable these hybrid devices for further enhancement in powerconversion efficiency. This new photovoltaic element could provide a new nanoscale criterion forthe investigation of photoinduced energy/charge transport at the organic-inorganic interfaces. The present chapter deals with the photovoltaic performance of P3HT-CdTe hybridsystem. The various P3HT-CdTe compositions used in the present investigations are PHTCdTe1,PHTCdTe2, PHTCdTe3, PHTCdTe4, and PHTCdTe20. The respective molar ratios of Cd-acetatein 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 mmolfor 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 thephotovoltaic 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) andspectroscopic (FTIR, UV-Vis absorption, PL) studies. Optical measurements (UV-Vis and PL) ofnanocomposites films show that photoinduced charge separation occurs at the P3HT-CdTeinterfaces. This indicates that the in-situ incorporation of nanocrystals in polymer matrix is apromising approach for the fabrication of efficient organic-inorganic hybrid solar cells. Thephotovoltaic performances of P3HT:PCBM as well as PHTCdTe2:PCBM have been investigatedin the device configuration viz. indium tin oxide (ITO)/ poly(3,4-ethylendioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)/P3HT:PCBM/Al andITO/PEDOT:PSS/PHTCdTe2:PCBM/Al, respectively. These devices are designated as device Aand device B, respectively. Based on these investigations it has been observed that the current- 91
    • 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 chargetransfer 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 theincrease in the energy level offset between the LUMO of the acceptor and the HOMO of thedonor.4.2. FABRICATION AND MEASUREMENT OF DEVICESFor optical, and morphological studies (scanning electron microscopy and atomic forcemicroscope), P3HT and P3HT-CdTe nanocomposites were dissolved in tri-chlorobenzene andthin films of these solutions were deposited on glass substrates by spin casting at 1500 rpm for120 s, and annealed at 120 °C for 30 min. For the fabrication of device A and device B, ITO substrates have been carefully cleanedas 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 ITOsubstrate and cured at 120°C for 60 min in vacuum. P3HT:PCBM and P3HT2:PCBM both havebeen taken in the ratio of 1:0.8 with a concentration of 1 wt. % in tri-chlorobenzene. The solutioncontaining P3HT plus PCBM was designated as solution A and other containing P3HT-CdTenanocomposite plus PCBM was designated as solution B. The tri-chlorobenzene solution A and Bhave been spin casted at 1500 rpm for 2 min on the top of PEDOT:PSS layer in an inertatmosphere, followed by annealing at 130°C for 30 min. Finally, Aluminum (Al) contacts 150 nmhas 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 deviceB have been recorded in the dark and under halogen lamp illumination with irradiance of 80mWcm−2, using a Keithley 2400 Source-Measure unit, interfaced with a computer.4.3. RESULTS AND DISCUSSION4.3.1. High Resolution Transmission Electron Micrograph ImagesHRTEM images and electron diffraction (insets) patterns of the synthesized P3HT-CdTenanocomposites 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 cadmiumacetate plays a significant role in controlling the size and shape of the nanocomposites. Thedifference in contrast at different areas in HRTEM images, indicates that the CdTe nanocrystalsare capped by P3HT. It is evident from the Figures 4.1 (a) and 4.1 (b) that at low CdTeconcentration the P3HT matrix shows more binding with CdTe nanocrystals and formation ofeven nanorods structure of P3HT-CdTe as seen by enlarged image. 92
    • Chapter 4 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 bindingbetween CdTe and P3HT reduces and the precipitation of CdTe nanocrystals appear rather thanpercolated network. The optimum percolation and interaction between P3HT and CdTe takeplace in PHTCdTe2 as shown in Figure 4.1 (c), where the nanorods formation as well asindividual CdTe precipitation has been suppressed. Hence further device investigation has beencarried out in PHTCdTe2. This interaction between polymer and nanocrystals indicates that 93
    • nanocomposites have potential for the charge transfer at polymer-nanocrystals interfaces, whichresults 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 interactwith the CdTe nanoparticles by dipole-dipole interaction and CdTe nanocrystals have beendeposited uniformly and compactly on or in-between the P3HT chains to form nanoparticles assuggested in scheme 2.2 (c) (chapter 2). The selected area electron diffraction patterns ofPHTCdTe1, PHTCdTe2 and PHTCdTe3 are shown in the inset of Figures 4.1 (b), 4.1 (d), and4.1 (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 94
    • Chapter 4The nanoscale morphology is a crucial parameter to understand the effectiveness of the interfacefor exciton splitting into free charge carriers, and the formation of a percolation network forefficient transport of charge carriers to the electrodes. The surface morphology of the pristineP3HT 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 forthe 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. Thenanocomposite PHTCdTe2 film provides a very different phase wherein, an island-like structuresappear instead of fibrillar features. In this image light-colored particles can be seen which are ofthe CdTe. These CdTe particles construct percolation network for the transport of charge. Theseimages show that the change in the surface morphology is a result of incorporation of CdTenanocrystals in P3HT matrix. a bFigure 4.3. AFM images of spin casted thin films of (a) P3HT, (b) PHTCdTe2 annealed at 120 °Cfor 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 differentP3HT 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 likestructure due to binding between P3HT and CdTe as shown in Figure 4.4 (b). However, withincrease 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 95
    • concentration, multifoliated leaf like structure, reduces, leading to the precipitation of CdTenanocrystals (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 AnalysisThe success of formation of P3HT-CdTe nanocomposites have been confirmed by the FT-IRspectra as shown in Figure 4.5. Strong absorption bands of P3HT at 2953, 2920 and 2854 cm-1have been assigned to the asymmetric C–H stretching vibrations in –CH3, –CH2, and thesymmetric 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 methyldeformation respectively. The C-C vibration appears at 1260 cm-1. The characteristic C-S bandstretching has been observed at 1111 cm-1 while absorption band at 822 cm-1 and 725 cm-1 havebeen assigned to the aromatic C-H out-of plane stretching and methyl rocking, respectively. In 96
    • Chapter 4nanocomposites of P3HT-CdTe, the intensity of peaks corresponding to C-S bond and aromaticC-H out-of plane stretching decreases. Also a shift by 25 cm-1 (from 1110 to 1135 cm-1), to thehigher energy region of C-S characteristic band has been observed in P3HT-CdTe, indicating theenhancement of the C-S bond energy. Moreover, the characteristic band of thiophene ring shows ared 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 shownin scheme 2.2 (b) [17, 21]. PHTCdTe3 816 1135 Transmittance (a.u.) 720 PHTCdTe2 1126 722 819 PHTCdTe1 723 821 1120 1111 1377 725 1260 1510 P3HT 1456 822 2854 2953 2920 800 1200 1600 2000 2400 2800 3200 -1 Wavenumber (cm ) Figure 4.5 FT-IR spectra of P3HT and P3HT-CdTe nanocomposites.4.3.4. UV-Vis Absorption SpectraThe normalized UV-Vis absorption spectra of the pristine P3HT, P3HT-CdTe nanocompositesfilms as well as in tri-chlorobenzene solutions are shown in Figure 4.6 and 4.7, respectively. Themaximum absorption of pristine P3HT films has been observed at 510 nm which corresponds tothe π-π* transition of the conjugated chain in the P3HT [22-24]. For the P3HT-CdTe compositefilms, the absorption spectrum has been broader as compared to pristine P3HT. The broadness inabsorption spectra indicates the presence of CdTe nanocrystals in polymer matrix [18]. Maximum 97
    • 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 inP3HT-CdTe nanocomposites resulting in partial electron transfer from P3HT to CdTe [25]. Onfurther increase of the concentration of CdTe in P3HT (PHTCdTe3) there has been a blue shift inabsorption spectra observed as compared to PHTCdTe1 and PHTCdTe2, which is observed at 514nm. This means at higher concentration of CdTe in P3HT, there is smaller shift in absorptionspectra. The smaller shift in absorption at higher concentration of CdTe in P3HT is due to weakinteraction between polymer-nanocrystals, as evident from HRTEM images. 1.0 Normalised Absorption 0.8 0.6 0.4 PHTCdTe3 PHTCdTe2 PHTCdTe1 0.2 P3HT 0.0 300 400 500 600 700 800 900 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 allsolutions. Moreover, on the incorporation of CdTe nanocrystals in P3HT matrix, the absorptionspectra start to broaden, and the broadness increases further with the increase of CdTeconcentration. The second maxima have been observed at 305 nm which is the characteristics ofCdTe nanocrystals. At higher concentration of CdTe (PHTCdTe20, Cd-acetate 3.6mmol, Te 7.2mmol) the absorption of CdTe is dominating and characteristic maxima of P3HT diminish. 98
    • Chapter 4 1.8 P3HT Normalized Absorption PHTCdTe1 1.5 PHTCdTe2 PHTCdTe3 1.2 PHTCdTe20 0.9 0.6 0.3 0.0 300 375 450 525 600 675 Wavelength (nm)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 InterfaceThe PL quenching can be used as a powerful tool for the evaluation of charge transfer from theexcited 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 ofpristine P3HT film have been compared with that of different P3HT-CdTe nanocomposites films.These P3HT and P3HT-CdTe nanocomposites films exhibited emission maximum around 660nm. PL intensity of the nanocomposite films significantly reduces as compared to that of theP3HT 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 chargetransfer, thereby exciton dissociation at interface of CdTe and P3HT (Figure 4.9) [28]. This PLquenching experiment provides us with good evidence that the nanocrystals will be able totransfer their excited state hole to the polymer. 99
    • 6 3.0x10 (a) P3HT a (b) PHTCdTe1 6 (c) P3HT-PCBM 2.5x10 b (d) PHTCdTe2 6 c (e) PHTCdTe2-PCBM 2.0x10 (f) PHTCdTe3 PL Intensity 6 d 1.5x10 e 6 1.0x10 f 5 5.0x10 0.0 600 650 700 750 800 Wavelength (nm)Figure 4.8 Photoluminescence spectra of P3HT, P3HT-CdTe nanocomposites, P3HT-PCBM andP3HT-CdTe-PCBM films after excitation by radiation of 510 nm wavelengths. Charge transfer takes place in the conjugated polymer-semiconductor nanocrystalscomposites at the interface, where the P3HT with a higher electron affinity (-3.37 eV) transferredelectron onto CdTe with relatively lower electron affinity (-3.71) (Figure 4.9). In this transfer, thepolymer absorb the solar photons (exciton generation), the electron is transferred to the CdTenanocrystals and the hole potentially can transfer to the polymer (charge separation). This is awell known effect of the ultrafast electron transfer from the donor to acceptor, and it is expectedto increase the exciton dissociation efficiency in photovoltaic devices [29, 30]. The PL spectra ofP3HT-PCBM and PHTCdTe2-PCBM are also shown in Figure 4.8. On incorporation of PCBM inP3HT 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 electrontransfer from P3HT to CdTe or PCBM and CdTe to PCBM. Figure 4.10 shows the PL spectra of P3HT and different P3HT-CdTe composites solutionin tri-chlorobenzene. The P3HT and P3HT-CdTe nanocomposites exhibited emission maximumaround 580 nm. Like P3HT-CdTe composites films, PL intensity of the nanocomposite solutionsignificantly reduces as compared with the value of the P3HT solution. Also PL intensity furtherdecreases with the CdTe concentration in the polymer. Reduced PL intensity of the compositesrelative to the pristine P3HT indicates that exciton dissociation, thereby charge transfer at P3HT-CdTe interface. 100
    • Chapter 4Figure 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, 3.71and 4.2 eV, respectively for facilitating the charge transfer at the P3HT-CdTe nanocompositesand PCBM interface. The arrows indicate the expected charge transfer process in solar cell. 6 3.0x10 P3HT 6 2.5x10 PHTCdTe1 PHTCdTe2 6 2.0x10 PHTCdTe3 PL Intensity PHTCdTe4 6 1.5x10 6 1.0x10 5 5.0x10 0.0 500 550 600 650 700 750 Wavelength (nm)Figure 4.10 Photoluminescence spectra of P3HT, P3HT-CdTe nanocomposites in tri-chlorobenzene solution after excitation by radiation of 450 nm wavelengths. 101
    • 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 andemission in the same range as of P3HT. For the measurement of QY, the UV-Vis absorbance andphotoluminescence 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 beenestimated according to the equation: 2  Grad ( S )   ( S )  QY ( S )  QY ( R)  Grad ( R)   ( R)      Where ‘S’ and ‘R’ represents for test and reference samples, respectively, Grad is the gradientfrom the plot of integrated PL intensity vs. absorbance, and ɳ the refractive index of solvent. 0.14 (a) 5 (b) 4x10 Rhodamine B 0.12 Rhodamine B 5 0.10 3x10 PL IntensityAbsorption 0.08 5 2x10 0.06 0.04 5 1x10 0.02 0.00 0 450 475 500 525 550 575 600 500 550 600 650 700 Wavelength (nm) Wavelength (nm)Figure 4.11 Absorption and emission data of Rhodamine B dye for three concentrations inethanol solution. The QY(R) of Rhodamine B is 0.7 [31], the calculated QY of P3HT is 26%. Similarly QYof other samples have been estimated (results are not shown). The QY of P3HT decreases frominitially 26% to 11% on incorporation of CdTe nanocrystals into the P3HT matrix. ThePHTCdTe1, 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 alarge amount of singlet excitons are not able to radiate onto ground state and they dissociate at thepolymer/nanocrystals interface as suggested in Figure 4.9. 102
    • Chapter 4 6 3.0x10 0.14 P3HT (a) (b) P3HT 6 0.12 2.5x10 6 0.10 2.0x10 PL Intensity Absorption 0.08 6 1.5x10 0.06 6 1.0x10 0.04 5 5.0x10 0.02 0.0 0.00 500 550 600 650 700 750 300 350 400 450 500 550 600 650 Wavelength (nm) Wavelength (nm) Figure 4.12 Absorption and emission data of P3HT for three concentrations in tri-chlorobenzene. 8 2.4x10 (a) (b) P3HT 7 1.8x10 Rhodamine B Integrated PL Intensity 8Integrated PL Intensity 2.2x10 7 1.5x10 8 2.0x10 7 1.2x10 8 1.8x10 6 9.0x10 8 1.6x10 6 6.0x10 8 1.4x10 0.04 0.06 0.08 0.10 0.12 0.14 Absorbance 0.07 0.08 0.09 0.10 0.11 0.12 0.13 Absorbance 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. 103
    • Table 4.1 Photovoltaic performance of device A and device B 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 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]. (a) (b) 4 device A P3HT device B 0.8 P3HT-CdTe 2 0.6J (mA/cm )2 0 J (A/cm ) 2 0.4 -2 0.2 -4 -6 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00 4 8 12 16 20 V (Volts) V (Volts) 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. 104
    • Chapter 4 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 bothcomponent 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 uponincidence of light increases and as a result current density increases. The enhancement in current density on in-situ incorporation of CdTe nanocrystals issupported by J-V measurement in dark as shown in Figure 4.14 (b). Figure 4.14 (b) shows the J-Vcharacteristics in dark of P3HT and PHTCdTe2 nanocomposites thin films in hole only deviceconfiguration viz. ITO/PEDOT:PSS/P3HT/Au and ITO/PEDOT:PSS/PHTCdTe2/Au. The natureof J-V characteristics of composites thin film is different from that of pristine P3HT. In case ofcomposites film, it has been observed that the hole current is more than that in pristine P3HT. Theenhancement in the hole current in PHTCdTe2 composites compared to that of pristine P3HT canbe understood in terms of host (P3HT) and guest (CdTe) charge transfer type interaction. In thecomposite film the CdTe nanocrystals are bound with P3HT via dipole-dipole interaction andform a CTC. The charge carriers which had to jump from one chain to another to transportthrough P3HT are now assisted by the CdTe nanocrystals. The calculated value of activationenergy 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 CdTenanocrystals support transportation of holes which improves their mobility and results intoenhancement in the hole current. The enhancement in VOC in device B can be understood in terms of lower HOMO level ofCdTe as compared to P3HT (Figure 4.9). VOC is correlated with the energy difference between theHOMO of the donor polymer and the LUMO of the acceptor [34, 35]. Clearly, a lower HOMOenergy level provides a higher Voc. The measured difference (0.21 eV) of the HOMO energylevels 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 anda high series resistance [36-38]. The presence of polymer or nanocrystal pathways that connectthe anode to the cathode is a source of current leakage or electrical shorts, depending on theconductivity of the pathway [39]. The incorporation of CdTe nanocrystals into a P3HT–PCBMmatrix results in enhancement in photoconductivity of the active layer [40]. Thus increasedphotoconductivity 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 andanother electron-blocking layer at anode can prevent the polymer and nanocrystal from shortingthe two electrodes under illumination. 105
    • 4.4 CONCLUSIONS1. In order to improve the photovoltaic properties of P3HT by broadening the absorption inthe 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 matrixwithout 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 mediaalong/between the polymer chains, which facilitate percolation pathways for charge transport.4. Optical measurements show that photoinduced charge generation on the incident of lightwhich are dissociated at the P3HT-CdTe interfaces.5. The solar cell performance of device based on P3HT-CdTe:PCBM show a better deviceperformance 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 termsof lower HOMO level of CdTe as compared to P3HT. The measured difference (0.21 eV) of theHOMO energy levels between P3HT and CdTe almost completely translated into the observeddifference in Voc (∼0.22 V).7. Enhancement in JSC may result in improvement in the solar absorption spectra anddecrease in the activation energy of localizes states.8. The cells suffered from low fill factors, which may be caused by shunting and a highseries resistance of P3HT-CdTe as compared to pristine P3HT.9. The present investigation given in this chapter indicates that the in-situ incorporation ofnanocrystals in polymer matrix is a promising approach for the fabrication of efficient organic-inorganic hybrid photovoltaic devices.References[1] C. B. Murray, D. J. Norris, M. G. Bawendi, J. Am. Chem. Soc., 114 (1993) 8706.[2] D.V. Talapin, S. Haubold, A.L. Rogach, A. Kornowski, M. Haase, H. Weller: J. Phys. Chem.104, (2001) 2260.[3] L. Qu, X. Peng: J. Am. Chem. Soc. 124 (2002) 2049.[4] Z.A. Peng, X. Peng: J. Am. Chem. Soc. 123 (2001) 183.[5] W.W. Yu, Y.A. Wang, X. Peng: Chem. Mater. 14 (2003) 430 014.[6] L. Qu, Z.A. Peng, X. Peng: Nano Lett. 1 (2001) 333. 106
    • Chapter 4[7] X. G. Peng, J. Wickham, A. P. Alivisatos, J. Am. Chem. Soc. 120 (1998) 4343.[8] Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 102 (1998) 3644.[9] T. Vossmeyer, L. Katsikas, M. Giersig, I. G. Popovic, K. Diesner, A. Chemseddine, A.Eychmuller, H. Weller, J. Phys. Chem. 98 (1994) 7664.[10] W. W. Yu, X. G. Peng, Angew. Chem., Int. Ed. 41 (2002) 2368.[11] Z. Deng, L. Cao, F. Tang, B. Zou, J. Phys. Chem. B 109 (2004) 16671.[12] Bin Xing, Wan-wan Li, Kang Sun, Materials Letters 62 (2008) 3178.[13] S. Sapra, A. L. Rogach, J. Feldmann, J. Mater. Chem. 16 (2006) 3391.[14] Jin-Hua Liu, Jun-Bing Fan, Zheng Gu, Jing Cui, Xiao-Bo Xu, Zhi-Wu Liang, Sheng-LianLuo, and Ming-Qiang Zhu, Langmuir, 24 (2008) 4241.[15] W. U. Huynh, J. J. Dittmer, W. C. Libby, G. L. Whiting, A. P. Alivisatos, Adv. Funct.Mater. 13 (2003) 73.[16] D. Cui, J. Xu, T. Zhu, G. Paradee, S. Ashok, M. Gerhold, Appl. Phys. Lett. 88 (2006)183111.[17] H.C. Liao, S.Y. Chen, D.M. Liu, Macromolecules, 42 (2009) 6448.[18] S. Dayal, N. Kopidakis, D. C. Olson, D. S. Ginley, and G. Rumbles, J. Am. Chem. Soc. 131,(2009) 17726.[19] A. Stavrinadis, R. Beal, J. M. Smith, H. E. Assender, A. A. R. Watt, Adv. Mater. 20 (2008)3104.[20] M. T. Khan, R. Bhargav, A. Kaur, S.K. Dhawan, S. Chand, Thin Solid Film, 519 (2010)1007.[21] M. T. Khan, A. Kaur, S.K. Dhawan, S. Chand J. Appl. Phys 110 (2011) 044509.[22] M. T. Khan, M. Bajpai, A. Kaur, S. K. Dhawan, S. Chand, Synthetic Met. 160 (2010) 1430.[23] B. K. Kuila, A. K. Nandi, J. Phys. Chem. B 110 (2006) 1621.[24] R. D. McCullough, Adv. Mater. 2 (1998) 93.[25] J. Xu, J. Hu, X. Liu, X. Qiu, Z. Wei, Macromol. Rapid Comm. 30 (2009) 1419.[26] J. Xu, J. Wang, M. Mitchell, P. Mukherjee, M. Jeffries-EL, J. W. Petrich, Z. Lin, J. Am.Chem. Soc. 129 (2007) 12828.[27] J. Yu D. H. Hu, P. F. Barbara, Science 289 (2000) 1327.[28] D. S. Ginger, N. C. Greenham, Phys. Rev. B 49 (1999) 10622.[29] G. D. Scholes, D. S. Larsen, G. R. Fleming, G. Rumbles, P. L. Burn, Phys. Rev. B 61 (2000)13670.[30] N. C. Greenham, X. Peng, A. P. Alivisatos, Phys. Rev. B 44 (1996) 17628.[31] M. Grabolle, M. Spieless, V. Lesnyak, N. Gaponik, A. Eychmuller, U. Resch-Genger, Anal.Chem., 81, (2009) 6285. 107
    • [32] J. N. Freitas, I. R. Grova, L.C. Akcelrud, E. Arici, N. S. Sariciftci, A.F. Nogueira, J. Mater.Chem. 20 (2010) 4845.[33] M. T. Khan, A. Kaur, S.K. Dhawan, S.Chand, J. Appl. Phys., 109 (2011) 114509.[34] H. Zhou, L. Yang, S. Stoneking, W. You, ACS applied Material and Interface 2 (2010) 1377.[35] Z. T. Liu, M. F. Lo, H. B. Wang, T. W. Ng, A. L. V. Roy, C. S. Lee, S. T. Lee, Appl. Phys.Lett. 95 (2009) 093307.[36] C. Ulzhöfer, S. Hermann N. P. Harder P. P. Altermatt, R. Brendel, Physica status solidi(RRL) Rapid Research Letters. 6 (2008) 251.[37] M. S. Kim, B. G. Kim, J. Kim, ACS applied Material and Interface 6 (2009) 1264.[38] D. Gupta, M. Bag, K. S. Narayan, Appl. Phys. Lett. 92 (2008) 093301.[39] W. U. Huynh, J. J. Dittmer, N. Teclemariam, D. J. Milliron, and A. P. Alivisatos, Phys. Rev.B 67 (2003) 115326.[40] H.Y. Chen, M. K. F. Lo, G. Yang, H. G. Monbouquette, and Y. Yang, Nat. Nanotech. 3(2008) 543. 108
    • CHAPTER 5STUDY OF THE EFFECT OF CADMIUM SULPHIDE QUANTUM DOTS ON THEPHOTOVOLTAIC PERFORMANCE OF POLY(3-HEXYLTHIOPHENE)5.1 INTRODUCTION5.2. FABRICATION AND MEASUREMENT OF DEVICE5.3 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 Cells5.4. CONCLUSIONSReferences5.1 INTRODUCTIONS 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 polymerswith n-type inorganic colloidal NCs, the hybrid system can show the increase in the deviceperformance relative to either of the non-hybrid counterparts. This is possible due to inherentadvantages 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 fewstudies have been reported for utilization of CdS as an important II-VI semiconductor innanocrystal-conjugated polymer composite photovoltaic devices. Probably, this may be due to therelatively large band gap of CdS and mismatches with the solar terrestrial radiation. So far thehighest power conversion efficiency for a CdS/P3HT hybrid solar cell has been reported by Liao
    • et al., who fabricated a hybrid solar cell with in-situ grown CdS NCs in P3HT matrix andobtained a power conversion efficiency of 2.9% [22]. However, since CdS has higher electronmobility, we believe there is a much room for further improvement in device efficiency for hybridCdS/conjugated polymer photovoltaic devices. Furthermore, the preparation methods of CdSe quantum dots (QDs) utilize expensive rawmaterials such as organic phosphines, octadecenes, and aliphatic amines [23]. Environmentally,organic phosphine ligands should be avoided because of their high toxicity, which would increasethe control cost of chemical pollution [24]. If the production cost of QDs could be decreasedgreatly through deploying cheap raw materials with lower toxicity and decreasing reactiontemperatures, large-scale preparation and practical application of QDs would be accessible. The present chapter deals with the fundamental issue, whether dispersion of CdS QDs intoP3HT matrix causes any noticeable improvement or deterioration of device efficiency. Theparticle shape, size and distribution of CdS QDs in P3HT matrix have been investigated byHRTEM, SEM and XRD. Optical studies [UV-Vis absorption and photoluminescence (PL)]suggest the electronic interaction between P3HT and CdS QDs. Photovoltaic performances ofdevice 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 beeninvestigated. These devices are designated as device X and device Y, respectively. Onincorporation of CdS QDs in P3HT matrix, the power conversion efficiency increased from0.45% to 0.87% due to enhancement in short-circuits photocurrent, open-circuit voltage, and fillfactor. 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 andPL quenching studies. The effect of post thermal annealing on device performance has also beeninvestigated and improved efficiency of devices was observed after thermal treatment at 1500C for10 min due to their improved nanoscale morphology, crystallinity and contact to the electron-collecting electrode.5.2. FABRICATION AND MEASUREMENT OF DEVICEFor the fabrication of solar cells devices, the %wt. ratio of P3HT:PCBM (Sigma-Aldrich) indevice X is 1:0.8 and for the deviceY the %wt. ratio of P3HT:CdS:PCBM, is 1:1:0.8. Twosolutions of P3HT were prepared in chlorobenzene and in one of them, CdS was added andsonicated for 4 hrs in order to well disperse CdS in P3HT. PCBM solution in chlorobenzene wasadded in the above solutions and mixed solution was ultrasonicated for 2 hrs. The solution 110
    • Chapter 5containing P3HT plus PCBM is designated as solution X and other containing P3HT plus CdSand PCBM is designated as solution Y. For preparation of device X and device Y, the ITO-coatedglass substrate was first cleaned with detergent, ultrasonicated in acetone, trichloroethylene andisopropyl alcohol, and subsequently dried in an vacuum oven as described in chapter 2. Highlyconducting PEDOT:PSS (Aldrich, USA) was spin casted on the ITO surface. The substrate wasdried for 10 min at 1500C in vacuum and then moved into a glove box for spin casting thephotoactive layer. The chlorobenzene solutions X and Y have been then spin-casted at 1500 rpmfor 2 min on the top of PEDOT:PSS layer. Subsequently 120 nm Al film was deposited on top ofthe active layer. Thermal annealing has been carried out by directly placing the complete device at150˚C in a vacuum oven. The performance of these devices was studied by their J-Vcharacteristics 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 DISCUSSION5.3.1. Structural Characterization5.3.1.1. XRD analysisFigure 5.1 shows X-ray diffraction patterns for pure P3HT, P3HT/CdS nanocomposite, and CdSpowder. 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]. TheXRD peaks are broad due to the small size of QDs. The average crystallite size determined fromthe peak at 27◦ using Debye–Scherrer formula: d  0.9 /  cos where λ is the wavelength of the X- rays used, β is the full width at half maximum and θ is theangle 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. Thesecond order reflection corresponds to the plane (200) of P3HT [26], has been observed at 2θangle 10.86◦, and corresponds to interlayer spacing 8.402 Å. In comparison, XRD data ofCdS/P3HT shows that the 2θ values matching the (100), (100), (111), (220) and (311) planes. Theappearances of few additional peaks in the composites are attributed to the presence of QDs inP3HT matrix. 111
    • Figure 5.1 XRD spectra of CdS QDs, P3HT and P3HT/CdS nanocomposites films.5.3.1.2. High resolution transmission electron microscope imagesHigh resolution transmission electron microscopy (HRTEM) images of CdS QDs and P3HT-CdSnanocomposite are shown in Figures 5.2 (a-c) and 5.2 (d-f), respectively. It has been observedfrom Figure 5.2 (a) that the size of the QDs ranges from 5 to 6 nm and their shape is spherical. Inaddition, it is seen from Figure 5.2 (b) that at higher resolution there exists (1 1 1), (2 2 0) and (3 11) planes of cubic CdS having interplaner spacing 3.36, 2.06, and 1.76 Å, respectively. Thisformation 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 byphysically mixing of CdS QDs in P3HT matrix. The P3HT-CdS composites exhibited asignificant phase separation as evidenced in Figure 5.2 (d) and (e). Difference in the contrast inthe HRTEM images of the composites indicates that the CdS QDs are well dispersed in P3HTmatrix. Dark and light phase represents the presence of CdS QDs and P3HT, respectively. Bothphases are eventually well dispersed within hybrid nanocomposites films. Different planes of CdSQDs in P3HT matrix are shown by diffraction pattern in Figure 5.2 (e). 112
    • Chapter 5Figure 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 nanoparticlesdispersed in P3HT matrix and (f) Diffraction image of CdS QDs in P3HT matrix.5.3.1.3. Scanning electron micrographFor the recording the images of scanning electron microscopy (SEM), P3HT and P3HT-CdSnanocomposites were dissolved in 1wt.% of chloform. Thin films of these solutions weredeposited on glass substrates by drop casting, and annealed at 120 °C for 120 min. Figure 5.3shows the SEM images for the P3HT and P3HT-CdS nanocomposites films. It is apparent fromthe Figure 5.3 (a) that the P3HT has 3-D shapeless porous network but when CdS nanoparticlesare incorporated in P3HT [Figure 5.3(b)], nanocrystals masked these cavities and porous networkdiminishes. When excess of QDs are incorporated in polymer matrix, the nanocrystals appear tobe buried into the porous surface of polymer and rest of QDs are lying over the surface film. 113
    • Figure 5.3 SEM images of (a) P3HT and (b) P3HT/CdS nanocomposites thin films, casted fromchloroform solution by drop coating.5.3.2. Optical Study5.3.2.1. UV-Vis. absorption spectraUV–Vis absorption of P3HT and P3HT/CdS composite solution in chloroform is shown in Figure5.4(a). Regio-regular P3HT has solid-state absorptions ranging from λmax = 520-530 nm andsolution absorption ranging 442-448 nm [26-28]. In the present study, the maximum absorption ofP3HT has been observed at 448 nm for solution and at 526 nm for thin film which confirms itsregio-regularity. Strong absorption band at 448 nm for P3HT is attributed to the excitation ofelectrons in the π-conjugated system. P3HT/CdS nanocomposite shows maximum absorption at438 nm, which is 10 nm blue shifted relative to the pristine P3HT. The blue shift in absorptionspectrum of P3HT/CdS nanocomposite can be attributed to the quantum confinement effect fromthe CdS nanoparticles [29-31]. Maximum absorption intensity in the nanocomposite is slightlylower due to scattering caused by the QDs in the P3HT matrix. As shown in the inset of Figure5.4 (a), CdS quantum dots show a broad absorption from 290 to 700 nm, with a maximumabsorption peak at 292 nm and an edge at 440 nm. The absorption spectra of P3HT andP3HT/CdS thin films are shown in Figure 5.4(b). The maximum absorption of P3HT/CdScomposites 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 solidfilms. The blue shift in solution is attributed to coil like structure in solution whereas solid filmshave rod like structure. Coil like structures have short effective conjugation length compared to 114
    • Chapter 5rod like structure with higher conjugation length, this results in the increase of π-π stacking infilm form of P3HT. Absorption spectra of films also show absorption shoulder at 605nm forP3HT 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) insolution and (b) in solid state.5.3.2.2. Photoinduced charge transfer at the donor/acceptor interfaceSemiconducting nanocrystals are known to accept electrons from an excited polymer and thentransfer the electrons to another acceptor molecule (PCBM). The demonstration of semiconductornanocrystals mediated electron transfer between donor and acceptor molecules bound to itssurface is shown in Figure 5.5. Photoluminescence quenching in a bulk heterojunction is a usefulindication of the degree of success of exciton dissociation and efficiency of charge transferbetween the donor-acceptor composite materials [35, 36]. P3HT has a photoluminescenceproperty, [37, 38] and the photoluminescence spectra of P3HT and P3HT/CdS solution inchloroform at excitation wavelength 448 nm, are presented in Figure 5.6 (a). Significant PLquenching has been observed for the nanocomposite solution. The PL intensity of the compositesolution is significantly reduced as compared to pristine P3HT in Figure 5.6 (a). This indicatesthat charge transfer, thereby exciton dissociation at interface between CdS and P3HT, is takenplace. Higher exciton dissociation efficiency accounts for higher device performance. For anexcitation wavelength of 448 nm, the maximum emission at 587 nm for P3HT and 583 nm forP3HT/CdS composites solution have been observed. The reason for the photoluminescence 115
    • quenching of P3HT/CdS may be due to the π-π interaction of P3HT with CdS [39], formingadditional decaying paths of the excited electrons through the CdS. The small blue shift (4 nm) inthe nanocomposite PL emission spectra indicates that the ground state energy level is more stablein the nanocomposite than that of pristine P3HT. This may be possible through the resonancestability 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 weightratio of P3HT and CdS in (a) solution of chloroform and (b) thin films casted from chloroformsolution and annealed at 120 ˚C for 30 min. Here P3HT0, P3HT10, P3HT20 and P3HT50represents the 0 wt.%, 10 wt.%, 20 wt.% and 50 wt.% of CdS in P3HT. 116
    • Chapter 5 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 0.45Device Y 0.45 4.65 32.0 0.87Device X annealed 0.58 2.26 45.0 0.74Device Y annealed 0.58 2.98 43.99 0.95 117
    • Figure 5.7 J-V curve of P3HT:PCBM and P3HT:CdS:PCBM solar cells under AM 1.5illumination at an irradiation intensity of 80 mW/cm2 . Figure (a) and (b) represents deviceswithout thermal annealing and Figure (c) and (d) represents devices with post production heattreatment at 150 0C. The modulation of device parameters i.e. increase in the value of VOC, JSC, and FF, indevice Y can be understood in terms of host P3HT and guest CdS QDs charge transfer typeinteraction. In fact there are various possibilities by which doped CdS can interact with hostP3HT. It can either go structurally into P3HT main chain or forms donor-acceptor charge transfercomplex (CTCs) or form molecular aggregates. However, the enhancement in JSC in P3HT onCdS dispersion indicates that CTCs formation between the host and the guest may be thedominant mechanism of interaction between the two. This suggested mechanism is indeedsupported by the UV-Vis absorption and PL emission studies in pure P3HT and CdS dispersedP3HT as shown in Figures 5.4 and 5.6, respectively. 118
    • Chapter 5 Blue shift in UV-Vis absorption (Figure 5.4) on incorporation of CdS QDs in P3HTmatrix 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 CTCsformation the ground state energy level is more stable in the nanocomposite than that of pristineP3HT. This may be possible through the resonance stability of π clouds of P3HT and CdS throughπ-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 evidenceof CTCs formation between the host and guest, since PL quenching is an indication of the degreeof success of exciton dissociation and efficiency of charge transfer between the donor-acceptorcomposite materials. The PL quenching in P3HT/CdS has been attributed to the π-π* interactionof 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-crystallineboundaries of the P3HT polymer and the QDs introduce the conducting path thus reducing thedefect states and barrier height at these interfacial boundaries. The thermally induced morphology modification has led to increase in PCE and theimproved FF which implies a significant decrease in the series resistance [36], thermally inducedcrystallization and improved transport across the interface between the bulk heterojunctionmaterial and aluminum (Al) electrode [5]. This improved nanoscale morphology results in moreefficient 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 thehigher fill factor.5.4. CONCLUSIONS1. In order to reduce charge recombination and increase the carrier mobilities inP3HT: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 isspherical. The average crystallite size determined from the Debye–Scherrer formula is estimatedto be about 2.33nm.3. The P3HT/CdS nanocomposite shows blue shift in the absorption spectra relative to thepristine P3HT which is attributed to the quantum confinement effect from the CdS nanocrystals. 119
    • 4. The PL quenching in the P3HT/CdS nanocomposite indicates that charge transfer, therebyexciton dissociation at P3HT/CdS interface.5. On incorporation of CdS QDs in P3HT matrix, the power conversion efficiency increasesfrom 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 chargetransfer complex between the host (P3HT) and guest (CdS QDs), duly supported by blue shift inUV-Vis absorption and PL quenching studies.7. The effect of post thermal annealing on device performance has also been investigated andfound improved efficiency of devices after thermal treatment. This increase in efficiency may bedue to improved nanoscale morphology, increased crystallinity and improved contact to theelectron-collecting electrode.REFERENCES[1] B. C. Thompson and J. M. J. Fr_chet, Angew. Chem. Int. Ed. 47 (2008) 58.[2] W. Ma, C. Yang, X. Gong, K. Lee and A. J. Heeger, Adv. Funct. Mater. 15 (2005) 1617.[3] M. Reyes-Reyes, K. Kim, D. L. Carroll, Appl. Phys. Lett. 87 (2005) 083506.[4] P. Schilinsky, U. Asawapirom, U. Scherf, M. Biele, and C. J. Brabec, Chem. Mater. 17 (2005)2175.[5] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery and Y. Yang, Nature Mater. 4(2005) 864.[6] A. L. Briseno, T. W. Holcombe, A. I. Boukai, E. C. Garnett, S. W. Shelton, J. J. M. Frechetand P. Yang, Nano Lett. 10 (2010) 334.[7] A. A. Lutich, G. Jiang, A. S. Susha, A. L. Rogach, F. D. Stefani and J. Feldmann, Nano Lett. 9(2009) 2636.[8] A. P. Alivisatos, Science 271 (1996) 933.[9] W. U. Huynh, J. J. Dittmer and A. P. Alivisatos Science 295 (2002) 2425.[10] S. Coe, W. K. Woo, M. Bawendi and V. Bulovis, Nature 420 (2002) 800.[11] S. Dayal, N. Kopidakis, D. C. Olson, D. S. Ginley, G. Rumbles, Nano Lett. 10 (2010) 239.[12] K. Kumari, S. Chand, P. Kumar, S. N. Sharma, V. D. Vankar,and V. Kumar Appl. Phys. Lett.92 (2008) 263504. 120
    • Chapter 5[13] S. Bhattacharya, S. Malik, A. K. Nandi and A. Ghosh, J. Chem. Phys. 125 (2006) 174717.[14] J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T. Q. Nguyen, M. Dante, A. J. Heeger, Science317 (2007) 222.[15] L. Wang, Y. S. Liu, X. Jiang, D. H. Qin, and Y. Cao, J. Phys. Chem. C 111 (2007) 9538.[16] T. Shiga, K. Takechi, T. Motohiro, Sol. Energy Mater. Sol. Cells 90 (2006) 1849.[17] S. Gunes, K. P. Fritz, H. Neugebauer, N. S. Sariciftci, S. Kumar, G. D. Scholes, Sol. EnergyMater. Sol. Cells 91 (2005) 420.[18] Z. Tan, T. Zhu, M. Thein, S. Gao, A. Cheng, F. Zhang, C. Zhang, H. Su, J. Wang, R.Henderson, J. Hahm, Y. Yang, J. Xu, Appl. Phys. Lett. 95 (2009) 063510.[19] E. Arici, H. Hoppe, F. Schaffler, D. Meissner, M. A. Malik, N. S. Sariciftci, Thin SolidFilms 451 (2004) 612.[20] S. D. Oosterhout, M. M. Wienk, S. V. Bavel, R. Thiedmann, L. J. A. Koster, J. Gilot, J. Loos,V. Schmidt, R. A. J. Janssen, Nat. Mater. 8 (2009) 818.[21] T. Zeng, H. Lo, C. Chang, Y. Lin, C. Chen, W. Su, Sol. Energy Mater. Sol. Cells 93 (2009)952.[22] C.R. Bullen and P. Mulvaney, Nano Lett. 4 (2004) 2303.[23] W. W. Yu and X. G. Peng, Angew. Chem., Int. Ed., 41 (2002) 2368.[24] T. Nakanishi, B. Ohtani, K. Uosaki, J. Phys. Chem. B 102 (1998) 1571.[25] M. N. Kalasad, M. K. Rabinal, B. G. Mulimani, G. S. Avadhani, Semicond. Sci. Technol. 23(2008) 045009.[26] T. A. Chen, X. Wu, R.D. Rieke, J. Am. Chem. Soc. 117 (1995) 233.[27] R. D. McCullough, Adv. Mater. 10 (1998) 93.[28] R.R. Prabhu, and M.A. Khadar, J. Phys. 65 (2005) 801.[29] P. Sonar, K.P. Sreenivasan, T. Madddanimath, and K. Vijayamohanan, Mater. Res. Bull. 41(2006) 198.[30] R. Maity, U.N. Maiti, M. K. Mitra, and K. K. Chattopadhyay Physica. E 33 (2006) 104.[31] Y. Kim, S. A. Choulis, J. Nelson, D. D. C. Bradley, S. Cook, J. R. Durrant, J. Mater. Sci. 40(2005) 1371.[32] M. Al-Ibrahim H.K. Roth, M. Schroedner, A. Konkin, U. Zhokhavets, G. Gobsch, P. Scharff,S. Sensfuss, Org. Electron. 6 (2005) 65–77.[33] E. Kucur, J. Riegler, G. A. Urban, T. Nann, J. Chem. Phys. 120 (2004) 1500–1505. 121
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    • CHAPTER 6 STUDY ON THE CHARGE TRANSPORT MECHANISM IN ORGANIC AND ORGANIC/INORGANIC HYBRID SYSTEM6.1. INTRODUCTION6.2. 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 Polaron6.3. 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 density6.4 SPACE CHARGE LIMITED CONDUCTION 6.4.1 Trap Free SCLC 6.4.2. SCLC with Exponential Distribution of Traps6.5. UNIFIED MOBILITY MODEL6.6. 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 System6.7. CONCLUSIONSReferences
    • 6.1. INTRODUCTIONU 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 andorganic-inorganic hybrid systems have been studied. The transport properties in thin-film devicestructures made up of conjugated polymers have been well characterized using space-chargelimited 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 aligand shell. In fact, the temperature dependence of the conductivity in the films of nanocrystalshas been observed to be thermally activated, which suggests that an activated hopping transportmodel can be used to describe the charge transport [5]. This is similar to the hopping modeldescribed for organic semiconductors, but in this case, energetic disorder arises from the sizedistribution of the particles and geometric disorder from the separation of particles, spatially or byligands [6]. Unlike most conjugated polymer, nanocrystals can transport both electrons and holeswith comparable mobilities [7]. The individual transport properties of both nanocrystals andpolymers have been studied separately in various electronic devices [6-10]. The carrier transportbehavior of these materials in composite devices, in particular, photovoltaic cells, has not beensufficiently characterized. It is of particular interest to study charge transport in the films ofnanocrystals-polymer hybrid systems, since these systems represent a combination of thedisordered 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) hybridsystems has been investigated and a quantitative explanation is provided for the observedelectrical characteristics in these hybrid systems.6.2. BASIC CONCEPTS OF THE CHARGE TRANSPORT PROCESSES6.2.1. Intra-molecular and Inter-molecular PerspectiveOrganic semiconductors are made up of molecules which consist mainly of carbon and hydrogenatoms. The carbon atoms in these compounds are sp2 hybridized. The s, px and the py orbitalshybridize and reorient themselves along a plane separated from each other by 120 degrees. Theremaining pz orbital extends perpendicularly above and below the plane. Two neighboring carbonatoms covalently bond with each other using an in-plane overlap of the hybridized orbital, calledthe 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 aredelocalized above and below the plane which is termed as conjugation. Charge transport along aconjugated polymer chain is called intra-molecular transport while charge transport between 124
    • Chapter 6adjacent polymer chains called inter-molecular transport [Figure 6.1(b)]. The former which isspecific to conjugated polymers is the most efficient. In the organic semiconductors, instead of two levels there are two bands of highestoccupied 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 longchains, tend to create an amorphous solid without any long range order - a "spaghetti pile" likestructure. As a result, there are interferences in the conjugation of the π-orbitals, and the electronicwave-function continuity is limited in length. This average length is defined as the conjugationlength. A short conjugation length characterizes conjugated polymers and conjugated amorphousorganic materials, similar to the potential barriers in poly-crystalline in-organic semiconductors oramorphous semiconductors. (a) (b) Inter-molecular Intra- molecularFigure 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 DisorderThe 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" chargecarriers [11-15]. Inserting a local disorder to such a system will result in the appearance ofscattering centers and energy states in the forbidden gap (deep or shallow levels). A stronginteraction with the scattering centers and many scattering centers results in a decrease of themean free path (λ) [16]. When the mean free path is in the order of the typical distance in thematerial (kλ~1), the description of "free particle like" charge carriers that can be described interms of the Bloch wave functions, is not valid anymore [16]. Such a situation is expected in 125
    • amorphous materials. In these materials, the short range order is kept but the long range orderbreaks down. Explicitly, there is a typical distance between electronic sites nearest neighbors, butthe long range symmetry is weaker or absent. The first concept, equally valid to crystalline andnon-crystalline materials, is the density of states (DOS) g(E). The quantity g(E) denotes theenergy and spatial density of electronic states (per unit energy and per unit volume). There is avariety of possible shapes and the character of DOS. For instance, the electronic states may belocalized at a certain energy range while beyond this range the states are free. Figure 6.2 showsthe 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 statesband with a localized band at the forbidden energy gap (trap band), (b) free states band with alocalized tail, (c) fully localized band. The shaded shapes denote localized states, where theenergy separating between localized and free states is the mobility edge (EM). A possible positionof the Fermi level EF is marked [16]. The first model [Figure 6.2(a)] is the closest to the crystalline material: two bands of freestates (for holes and electrons) and a distribution of a localized, deep traps, band in the forbiddengap. The second model [Figure 6.2(b)] is of electronic band that contains localized states at thelower energy range, and free states at the upper energy range. The energy that separates betweenlocalized and free states is referred as the mobility edge (EM). The third model [Figure 6.2(c)] is offully localized band.6.2.3. Hopping TransportMost of the organic materials display low-conductance behaviour. The hole mobility in thesematerials are typically ranging from 10-7 to 10-3 cm2/(Vs) [Si hole mobility is 1400 cm2/(Vs)], andthe values for electron mobility are commonly reported lower by a factor of 10-100 [Si electronmobility is 450 cm2/(Vs)]. The lower mobility in organic semiconductor, in comparison with theirinorganic counterpart, is due to the disorder presented in these materials. Motion of a chargecarrier in the organic semiconductors can be described using hopping transport. Hopping isdefined as a phonon assisted tunneling between two localized electronic states centered at 126
    • Chapter 6different locations [17, 18]. It is usually observed in disordered semiconductors due to localizationof charges. This hopping transport takes place around the Fermi level. Many of the hoppingmodels 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 anadjacent unoccupied site j , which are separated in energy by Ei − Ej and in distance by Rij, isdescribed by   Ei  E j  exp     i j   0 exp  2Rij    k BT  , E j  Ei  (6.1)  1, E  E  i jwhere Rij is the intersite distance, ν0 is a prefactor and kB is the Boltzmanns constant. When a fieldE is applied, the site energies also include the electrostatic energy. In addition to the energeticdisorder of the transporting sites, positional disorder can be taken into account by regarding theoverlapping parameter γ. As a matter of fact, the transition rate νij from one site to anotherdepends on their energy difference and on the distance between them. The carriers may hop to asite with a higher energy only by absorbing a phonon of appropriate energy.6.2.4. Charge Carriers in Conjugated Polymers: Concept of PolaronThe charge delocalization in the inorganic semiconductors is supported by the large transferintegrals (around 1eV) calculated between neighboring atoms. It implies a description in terms ofBloch wavefunctions. However, this picture holds true for organic molecular crystals only at verylow temperature, since the transfer integrals between neighboring molecules are quite low (20 to80 meV) due to weak Van der Waals interactions, which results in narrow bandwidths. As aconsequence, perturbation effects with the same order of magnitude as the bandwidth can inducethe localization of the charge carriers [20]. The validity of the band model can be verified by calculating the mean free path λ, whichhas to be much larger than the crystalline cell parameter a. In general, this condition fails fororganic crystals and a different transport mechanism such as hopping must be invoked. Since theimportance 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 itsassociated lattice distortion. It can be defined as a slow moving electron or a hole traveling in adielectric medium, that interacts with the lattice ion through the long range forces producing apolarization field around itself, that travels with the electron or hole. In other words, it can bedescribed as a cloud of phonon accompanying an electron/hole as it carries its lattice distortionwhile moving through the medium. 127
    • 6.3 CHARGE CARRIER MOBILITYMobility is measured in (cm/sec) per (volt/cm); i.e. the average velocity of a charge carrier perunit applied field. In absolute terms mobility varies enormously from one semiconductor toanother. The concept of mobility is very important because it provides us with information onhow fast a charge carrier will move per unit applied field. Achievable fields for a given solar cellmaybe limited by the energetic of the materials employed and dopant concentration, but thecurrent that can be collected will depend strongly on how fast the charge carriers move under theinfluence of the generated external voltage. Electric current measures the number of chargecarriers that cross a unit cross sectional area per unit time. Area of a solid state device may beconsidered constant, so mobility becomes the important comparison parameter.6.3.1 Factors Influencing the Charge Mobility6.3.1.1. DisorderIn a disordered solid, disorder can be modeled by assigning random site energies from aprobability distribution function. These disorders can be of two kinds: diagonal and non-diagonaldisorders. Diagonal disorder related to the distribution of the energy transporting levels, HOMOand LUMO of the different molecular sites and is often related to the presence of chemicalimpurities [28] or trap states [29]. In the case of flexible molecules, a major contributor todiagonal disorder is the large conformational degree of freedom (leading for instance to adistribution of torsion angles between adjacent units). In polymer chains, such a distribution oftorsion angles results in a diagonal disorder via the formation of finite-size conjugated segmentswith different lengths and therefore different HOMO and LUMO energies. In addition, diagonaldisorder might be induced by electrostatic/polarization effects from surrounding molecules,induced by fluctuations in the local packing; this effect is amplified when the molecules repeatunits contain local dipole moments [30-33]. This also holds true when the molecule or thepolymer 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 withstandard deviations on the order of 50-100 meV. The non-diagonal disorder reflects fluctuationsin the strength of the intermolecular interactions (i.e. transfer integrals) which depend on theorientation 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 forcharge depending on the values of the transfer integrals. 128
    • Chapter 66.3.1.2. Impurities/TrapsThe definition of a trap depends on the nature of the charge carrier. For holes (electrons), thepresence of a molecular site characterized by a higher (lower) HOMO (LUMO) with respect tothe levels of the valence (conduction) band is called a trap. Indeed, the chance for these levels tobe filled by a charge carrier is high because it represents a thermodynamically more stablesituation. The lifetime of a hole or electron in a trap state is function of the trap depth. We candistinguish, shallow traps with a depth of the order of a few kBT and deep traps with depth muchhigher than kBT. The most common defect in an organic crystal is a schottky defect, which is a point defectformed by a vacancy; an empty site in the crystal structure. Any molecule which has its ionizationenergy lower or its electron affinity higher than that of the molecule of interest, behaves as a holetrap or an electron trap, respectively. These unwanted molecules when present in small amountamong the host molecules, are termed as impurities and create favorable energy states inside theband gap of the material. These favorable energy states are called as traps and can be shallow ordeep. Shallow or deep trap is defined depending upon the position of the Fermi level with respectto the trap energy level. For hole traps if the Fermi energy level lies above the trap energy level itis called shallow trap, on the other hand if the Fermi energy level lies below the trap level, it iscalled deep trap with respect to valance band (shown in Figure 6.3). The reverse is true for theelectron with respect to conduction band edge [3]. Impurities are often generated as side products of synthetic reactions. The presence ofimpurities can influence the packing of the molecules and create regions with differentpolarization energies [36], resulting in a local perturbation of the energy transport levels. Theintrinsic properties of the impurity namely their ionization potential and electron affinity can alsomake them acting as a trap. LUMO LUMO LUMO Deep trap Et EF EF EF Distribution Shallow trap of trap Et Et HOMO HOMO HOMO Figure 6.3 Schematic of typical hole traps. 129
    • The traps are distributed spatially and energetically in a semiconducting layer. There are twoimportant distribution functions that are used to characterize the dispersion in trap energies in theforbidden energy gap. One is the exponential distribution function proposed by Rose [23] andmodified by Mark and Helfrich [24]. It is given by equation: Ht  E  Et  H ( Et )  exp   C    (6.2) k BTl  k BTl Where H(Et) is the density of trapping states at energy Et and Ht is the total trap density, l is anempirical parameter [25], greater than unity, defines how the trap density changes with trapenergy. EC is assumed to be above Et.The other is a Gaussian distribution proposed by Silinsh [26] is of the form Ht  Et  E m 2  H ( Et )  exp   (6.3) 2 2   2 2  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 tothat obtained with the Gaussian distribution. Hence in most cases it is experimentally difficult todifferentiate between the two trap distributions given by the above Equations. In the organicsemiconductors the width of the bands can be very narrow and extended states are rarelyobserved. Especially in amorphous layers of organic thin films the density of states (DOS) is quitewell represented by a Gaussian-like distribution of localized states of individual molecules aspresented in Figure 6.4. Energy LUMO HOMO DOSFigure 6.4 Distribution of HOMO and LUMO levels in organic semiconductors [27]. 130
    • Chapter 6 Experimentally, the distribution of trap depth is measured by Thermally StimulatedCurrent 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 heatedslowly and the current coming from the de-trapped charges is measured as a function oftemperature to estimate the trap depths as well as their distribution [37].6.3.1.3. TemperatureThe temperature dependence charge carrier mobility has been extensively studied in the literatureand has often been turned to a discussion whether a band model or a hopping picture prevails. Inultra pure organic crystals, the charge carrier mobility often decreases with temperature accordingto the power law T-n [38], with n a positive number. A thermally activated mobility ischaracteristic of the presence of shallow traps; when a critical temperature is reached all chargesare 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 andpositional disorder. Charge transport occurs by hopping and is thermally activated. A highertemperature leads to a larger mobility, the thermal energy helping in crossing of the energeticbarrier between adjacent molecular sites [40]. The mobility is often fitted by an Arrhenius-likerelationship   0    exp(  ) k BT (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 inpresence of a Gaussian energetic disorder fits the following expression: 2     0    exp    k T  B  (6.5)where σ is the width of the energetic distribution.6.3.1.4. Electric FieldIn disordered materials, an increase in the mobility is observed at high fields. The fielddependence in the range between 104 -106 V/cm generally obeys a Poole-Frenkel behavior [41-43]:   ( F )   (0, T ) exp  (T ) F  (6.6)where µ(0,T) is the zero-field mobility, γ(T) the field activation factor, which reflects the loweringof the hopping barriers in the direction of the applied electric field F. The increase of F gives rise 131
    • to increase of the charge carrier density. The following expression for γ(T) usually allows for agood fit of the experimental data [42,44]:  1 1   (T)         k BT k B To  (6.7)where T0 a parameter with unit of temperature. Generally, T0 is much higher than roomtemperature.6.3.1.5. Charge-Carrier DensityExperimentally, two main effects demonstrate that the charge transport properties in amorphousorganic semiconductors depend on the charge carrier density. The doping of organic matricesrepresents a first clear demonstration of such an effect. It is seen in such experiments that themobility first decreases, when the doping ratio is between 0.01 and 1% as explained by theincrease in the concentration of deep traps [45]. However, the mobility increases at higher dopingratio (up to 10%), due to increased spatial overlap between the trap levels, which lower theactivation barriers [46]. Phillips et al. [47] has shown experimentally that the mobility measuredin PPV and polythiophene derivatives is much lower in diode than in FETs by two or three ordersof magnitude. The explanation lies in the fact that the density of injected charges is much larger intransistors 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 thusaffected by trapping. At higher carrier densities, only a portion of the carriers are necessary to fillall the traps, the remaining carriers can access easily to higher energy states. Since these states aremore numerous, trap-free transport is achieved and an increase of the mobility is noticed.6.4 SPACE CHARGE LIMITED CONDUCTIONSpace charge is generally referred to as the space filled with net positive or negative charge. Thespace charge limited conduction (SCLC) occurs when the contacting electrodes are capable ofinjecting either electrons into the conduction band or holes into the valance band of asemiconductor or insulator, where the initial rate of such charge carrier injection is higher than therate 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 currentslimited by the space-charge confined in a single discrete energy level and in localized states witha distribution of energy. The simplified SCLC theory, which is usually applied to model the I-Vcharacteristics in organic devices, is based on two main approximations: firstly diffusion currentsare neglected to describe the current flow and secondly the ohmic contact is taken to be an infinite 132
    • Chapter 6reservoir of charges available for injection. The first approximation simplifies the theory tomathematically manageable elementary analysis. The second approximation makes the theoryindependent of any detailed properties of the contact and thereby makes a universal theorypossible. The distribution function for the hole trap density as a function of energy level E above thevalence band, and a distance x from the injecting contact for holes can be written as: [2] h( E, x)  n( E)S ( x) (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 injectingelectrode to collecting electrode implies that the effective thickness of the device, under spacecharge conditions, remains the thickness itself, and S(x) = 1. The specific functional form of theSCLC, J-V curve depends on the distribution of charge traps in the band gap. If the traps captureonly holes, the electric field F(x) inside the specimen follows the Poisson’s equation: dF ( x) q[ p( x)  pt ( x)]    (6.9) dx  The current density may be expressed as: J  qp( x) F ( x) (6.10)Where p(x) and pt(x) are, respectively, the densities of injected free and trapped holes, and theyare given by EF pt ( x)   h( E, x) f p ( E )dE (6.11) El  E Fp and p( x)  N v exp    kT   (6.12)  and fp is the Fermi-Dirac distribution function.6.4.1 Trap Free SCLCThe perfect trap free insulator is the solid state analog of the thermionic vacuum diode. There areneither thermal free carriers nor trapping states in the solid, that is pt(x)=0. The Poisson’sequation now can be expressed by dF ( x) d [ F ( x)]2 2 J 2 F ( x)   (6.13) dx dx Integrating the above equation using the boundary condition d V   F ( x)dx (6.14) 0 133
    • 9 V2yields J   3 (6.15) 8 dThis equation is referred to as the trap-free square law, the Mott-Gurney square law and orChild’s law for solids.6.4.2. SCLC with Exponential Distribution of TrapsWhen the traps are distributed exponentially in the energy space within the forbidden gap,distribution function [Equation (6.8)] can be written as: H   E h( E , x )   b E  exp    S ( x)   E  (6.16)  t   t 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 trapdistribution Et  k BTC .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  Hb  E  pt ( x )   exp    k T  S ( x)dE  EFp k T B C  B C   E Fp  pt ( x)  H b exp    k T  S ( x)   B C  T  p  TC pt ( x)  H b  N   S ( x) (6.17)  v Using continuity Equation (6.10), and boundary condition (Equation 6.14) in Equation 6.9, theexpression for J is given by: l 1 l  2l  1   l  r 0  V l 1 J q 1l N v     l 1 H  2l 1  d (6.18)  l 1   b where F(x) is the electric field inside the film, Nv is the effective density of states andl  Et / k BT  TC / T . 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 extractthe trap energy width Et.6.5. UNIFIED MOBILITY MODELThis model is based on percolation in a variable range hopping (VRH) system with an exponentialdistribution of localized states [51-53]. Percolation is the term used for movement of charge 134
    • Chapter 6carriers through a random network of obstacles. Consider a square lattice, where each site israndomly occupied or empty. Occupied sites are assumed to be electrical conductors while theempty sites represent insulators, and that electrical current can flow between nearest neighborconductor sites. Percolation paths are the most optimal paths for current and transport of chargecarriers which are governed by the hopping of charge carriers between these conducting sites. Thesystem can be described as a random resistor network [54], a system made up of individualdisconnected clusters of conducting sites, whose average size is dependent on a referenceconductance G. The conductance between sites is given by:   G  G0 exp  sij (6.19) E j  Ei  Ei  E F  E j  E Fwith sij  2rij  (6.20) 2k B TAll conductive pathways between sites with Gij  G are electrical insulators while conductivepathways between sites with Gij  G are electrical conductors. At some critical conductance inbetween, therefore, a threshold conductance GC exist where the first time electrical current canpercolate from one edge to the other. A bond is defined as a link between two sites which have a conductance Gij  G . Theaverage number of bonds B is equal to the density of bonds (Nb), divided by the density of sitesthat form bonds, (Ns), in the material. Critical bond number BC is the average number of bondsper site for which threshold percolation occurs. The onset of percolation is determined bycalculating the critical average number of bonds per site [53]. Nb B(G  GC )  BC  (6.21) NS Vissenberg and Matters [55], set the critical bond number to Bc = 2.8, The total density ofbonds is given by N b  4  rij2 g ( Ei ) g ( E j ) (sc  sij )dEi dE j drij (6.22)The density of sites Ns N s   g ( E ) ( sc k BT  E  E F )dE (6.23)At low carrier concentration exponential density of states in amorphous organic semiconductors isgiven by [53, 55]:  N0  E   exp  , g ( E )   k B T0  k B T0     E  0 (6.24)  0, E  0 135
    • where No is the total density of states (molecular density) per unit volume and To is acharacteristic temperature that determines the width of the exponential distribution.Combining Equation (6.20)-(6.24), the expression for Bc E  3  T  BC  N 0  0  exp  max  (6.25)  2T   k B T0 where Emax  E F  sC k BT is the maximum energy that participates in bond formation. Accordingto the percolation theory, the conductivity of the system can be expressed as    0 exp[sC ] (6.26)where σ0 is the prefactor and sc is the critical exponent of the critical conductance whenpercolation first occurs (when B = Bc).Using Equation (6.25) and (6.26) we get T0   T   T   4 sin       T0     T0  p   0 (6.27)  T  B 2 3   C     The conductivity can be converted into mobility by dividing by e.p, where e is the electroniccharge and p the carrier density [56]: T0   T  T   4 sin      T0   0  T0   T0     1  (T , p, F )    p T  (6.28) q  T  BC 2 3       The average charge carrier density as a function of the applied bias voltage V is given by [3]   V  p(V )  0.75 0 r2  (6.29)  qd 6.6. RESULTS AND DISCUSSIONThe J-V characteristics of organic and organic/inorganic hybrid systems have been investigated inthe 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 tothe HOMO energy level of active layer (P3HT, P3OT, P3HT-OT, P3HT-CdTe and P3HT-CdS) aswell as far below the LUMO energy level as shown in Figure 6.5. It is clear from Figure 6.5 thatthe electron injection barrier is quite higher as compared to the holes injection barrier, from both 136
    • Chapter 6the electrodes. As a result, the transport is dominated by holes in the Au:ITO based device, andso-called hole only device. For the fabrication of hole only devices, ITO coated glass substrates have been carefullycleaned as discussed in section 2.4.1 and dried at 120°C for 2 hrs in vacuum. Prior to use, thecleaned substrates were treated with oxygen plasma. A PEDOT: PSS layers were spin-coated atonto the ITO substrate and cured at 120°C for 60 min in vacuum. Active materials were spincasted 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 deviceactive areas were ~0.1 cm2 for all the devices discussed in this work. J-V characteristics of thedevices were measured with Keithley 2400 Source-Measure unit, interfaced with a computer. Two high workfunction LUMO electrodes to prevent electron injection Due to low carrier low p, high E mobility, injected carrier form a space charge. HOMO Au Collecting contact Injecting contact ITO x 0 t Figure 6.5 Schematic illustration of the hole only device.6.6.1. Hole Transport Mechanism in P3HTFigure 6.6 shows the J-V characteristics in temperature range 290-150 K of a device based onP3HT. On lowering down the temperature, the decrease in current was observed. In the organicsemiconductors charge transport is governed by hopping of a carrier from site-to-site of an emptydensity of states. The thermal energy helps to cross the energetic barrier between two adjacentsites. This implies that the charge transport in organic semiconductor is thermally activated.Therefore, the decrease in current is obvious on lowering down the temperature. V At low applied bias, the J-V characteristics follow the ohm’s law: J  qn , as injected dcarriers are negligible compared to that of the applied bias [57]. At moderate field, the injectedcarrier density becomes so high that the field due to the carriers dominates the applied bias. Atthis point the J-V characteristics may switch to pure SCLC and follow the Child-law (Equation6.15). On further enhancement of field, the quasi-Fermi level intersects the exponential trap 137
    • distribution, and characteristics will begin to follow Equation 6.18. The hole mobility up to thisfield is constant and also independent of the hole density. The fit of the J-V characteristics of theP3HT device using the Equation 6.18 is poor at high applied bias where current density deviatesstrongly as expected from Equation 6.18. This discrepancy has been analyzed by unified mobilitymodel given by Equation 6.28. This model accounts the influence of temperature, carrier densityand applied field on the carrier mobility [53]. The solid curves in Figure 6.6 have been obtainedby combining Equation 6.18 and Equation 6.28 using a computer program. The value of differentparameters for solid curves are; d=110 nm, r = 3, 0 = 8.8510-14 F/cm, Hb = 2.81018 cm-3, Nv =11019 cm-3, TC=400K, T0  325K , σ0=4×104 S/m, α-1=1.12 Å, and Bc = 2.8. 1 150 K 0.1 170 K 195 K 225 K J (A/cm ) 2 0.01 260 K 290 K 1E-3 1E-4 1E-5 0.01 0.1 1 10 Voltage (V)Figure 6.6 Experimental (symbols) and calculated (solid lines) J-V characteristic of P3HT thinfilm at different temperatures in hole only device configuration viz. ITO/PEDOT:PSS/P3HT/Au.6.6.2. Hole Transport Mechanism in P3OTFigure 6.7 shows the experimental J-V characteristics of hole only device of P3OT thin film in thetemperature range 150-290K. At low applied voltages (i.e. below 1V), the J-V relationshipinitially 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 havebeen analysed in terms of SCLC model. Generally, in most cases, the charge transportmechanisms in amorphous organic semiconductors has been well explained by SCLC and trapped 138
    • Chapter 6charge limited current model (TCLC) and J-V behavior beyond Ohm’s law follows the Equation6.18. The theoretically generated curves from Equation (6.18) in the Figure 6.7 gives a perfectfit to the experimental curves for all analyzed temperatures with fitting parameters Nv =1.0×1019cm-3, Hb = 2.5×1018cm-3, and TC = 720K. We obtained the l values in this trap-fillingSCLC regime and plotted them as a function of the inverse of the temperature in the inset ofFigure 6.8. From this plot we have evaluated the value of the width of the exponential trapdistribution i.e. lkT. The values of Et obtained from the SCL diodes is 63 meV. 0.01 150 K 1E-3 190 K 220 K J (A/cm ) 250 K 2 1E-4 290 K 1E-5 1E-6 1E-7 0.01 0.1 1 10 Voltage (Volts)Figure 6.7 Experimental (symbols) and calculated (solid lines) J-V characteristic of P3OT thinfilm 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]:   2  2     2     ( F , T )    exp     C0    2  F  (6.30)   3k BT    k BT          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 139
    • parameter σ arises from distribution of the conjugation length, while the positional disorderparameter Σ arises from fluctuation of the intermolecular distances or morphological variations. 9 8 7 Parameter l 6 5 4 3 3.5 4.0 4.5 5.0 5.5 6.0 6.5 1000/T (K-1)Figure 6.8 Temperature dependence of l obtained from theoretical fit according to the SCLC lawto the experimental data (shown in Figure 6.7). Figure 6.9 gives the field dependent mobility at different temperatures, which shows thatthe hole mobility increases exponentially with the square root of electric field F, consistent withEquation 6.30. There is a gradual variation of the slope as the temperature increases from 150K to290K. This indicates that the positional and geometrical disorders are present in the P3OT. -5 10 -6 10  [cm /(V-s)] 2 -7 10 290 K 250 K -8 10 220 K 190 K 150 K -9 10 200 400 600 800 1000 1200 1/2 1/2 F (V/cm)Figure 6.9 Field dependence of mobility µ(0,T) at different temperatures, obtained from thetheoretical fit to the experimental data. 140
    • Chapter 6 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. 0.020 (a) 1E-5 (b) 0.015 [0,T] (cm2/V-s) 1E-6 (cm/V1/2) 0.010 1E-7 0.005 1E-8 3.5 4.0 4.5 5.0 5.5 6.0 6.5 4 5 6 7 5 5 10 /T2(K-2) 10 /T2(K-2) 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 ½ =69, Σ = 2.1 and C = 1.01 × 10-3 (cm/V) . By plotting lnµ(0, T) against 105/T2 [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/T2 [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. 141
    • 1 0.1 0.01 J(A/cm2) 1E-3 290 K 240 K 190 K 1E-4 150 K 110 K 1E-5 0.1 1 10 Voltage (Volts)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 range290 -110K. When the experimental data in Figure 6.11 has been analyzed in terms of Equation 6.18, ithas been found that the theory fits up to intermediate fields and at high fields (corresponding to 6V ), the current gradually deviates from the above proposed theory and becomes larger than asexpected from Equation 6.18. This discrepancy has been analyzed in terms of field dependentmobility model given by Equation 6.6. In order to describe the hole conduction in P3HT-OT athigh fields, we combine the SCLC (Equation 6.18) with the field dependent mobility (Equation6.6). Temperature dependence of zero field mobility is shown in Figure 6.12(a) in an Arrheniusplot, which decreases with lowering down the temperature. This thermally activated behaviour ofzero field mobility follows the Equation 6.4. Temperature dependent high field J-V characteristicscan be understood in terms of the coefficient γ(T). From the J-V curve (Figure 6.11), Equation 6.6and 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 thereis a linear dependence according to Equation 6.7. 142
    • Chapter 6 -9 0.06 8.0x10 (a) (b) 0.05[cm2/V-s] -9 6.0x10 0.04 (cm-V-1/2) -9 4.0x10 0.03 (0,T) -9 2.0x10 0.02 0.01 0.0 0.00 3 4 5 6 7 8 9 3 4 5 6 7 8 9 1000/T (K-1) 1000/T (K-1) Figure 6.12 (a) Experimental (Symbols) and calculated [solid line using Equation (6.4) with activation energy Δ= 0.21eV and  0  3.6  10 5 cm 2 / Vs ] 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 T0  500K and   6.9 10 5 eV / V 1/ 2 cm1/ 2 . 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 = 3.81018cm-3,Nv=31019cm-3, Tc=560K, Et=46meV, d=150nm, µ0= 3.6×10-5cm2/Vs, T0=500K, ∆=21meV and β=6.9×10-5eV/V1/2cm1/.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 143
    • 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 andhence increase of trap density, so that a fraction of the charges moving inside the P3OT-HT filmsare trapped thereby reducing the mobility. Thus copolymerization is expected to diminish themobility and increase its electric field dependence for hole. It is thus explicitly established from above that hole transport in P3HT-OT copolymerthin films shows field and temperature dependent mobility at higher fields with hole transportfitting parameters as   6.9 10 5 eV / V 1/ 2 cm1/ 2 ,  0  3.6  10 5 cm 2 / Vs , T0  500K and  21meV , respectively.6.6.4. Hole Transport Mechanism in P3HT-CdTe Hybrid SystemFigure 6.13 shows the J-V characteristics of hole only device based P3HT-CdTe in theconfiguration 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 pristineP3HT, shown in Figure 6.6. In case of composite film the hole current has been observed to bemore than that in device based on pristine P3HT at all temperatures. Inset of Figure 6.13 showsthe comparison of J-V characteristics of P3HT and P3HT-CdTe at 150 K. The compositesexhibited S shape characteristic and the rate of reduction of current with temperature is lowcompared to that in pristine P3HT. We tried to fit the experimental data with unified mobilitymodel [Equation (6.28)]. The data did not show agreement with the mobility model for single setof 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 curvesin Figure 6.13 represent the plot of Equation (6.18) at respective temperatures. The values ofparameters used in the calculations are; Hb=5.0×1018 cm−3, Nv=6.0×1018 cm−3, µ=6.0×10-5 cm2V−1 s−1, d=110 nm, and Tc=400 K. For the characteristics measured at 250 K, 220 K, 195 K, 175K, 150 K, the agreement was obtained for µ=7.8×10-5 , 1.16×10-4 , 2.4×10-4, 3.55×10-4, 7.5×10-4cm2 V−1 s−1, respectively. 144
    • Chapter 6 0.1 0.1 P3HT Current density (A/cm ) P3HT-CdTe 2 P3HT-CdTe 0.01 1E-3 1E-4 0.01 1E-5 1E-6 150 K 1E-7 J (A/cm2) 0.01 0.1 1 Voltage (V) 10 1E-3 280 K 250 K 220 K 1E-4 195 K 175 K 150 K 1E-5 1E-3 0.01 0.1 1 10 Voltage (Volts) 0.1 P3HT Current density (A/cm ) 2 P3HT-CdTe 0.01Figure 6.13 Experimental (symbols) and calculated (solid lines) J-V characteristics of device B at 1E-3 1E-4different temperature in hole only device configuration viz. ITO/PEDOT:PSS/P3HT-CdTe/Au. 1E-5 1E-6 150 KThe inset shows the comparison of J-V characteristics of1E-7 P3HT and P3HT-CdTe at 150 K. 0.01 0.1 1 10 Voltage (V) The enhancement in current density in P3HT-CdTe thin film can be understood in terms ofreduction of activation energy. The calculated values of activation energy of localized states havebeen found to be 52 meV for P3HT and 11 meV for P3HT-CdTe (Figure 6.14). As activationenergy in P3HT-CdTe is lower compared to the pristine P3HT, the CdTe nanocrystals supporttransportation 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 thinfilm can be explained on the basis of increase of trap density (Hb) and reduction in activationenergy (Figure 6.15). Usually, an electric field raises the mobility because it lowers the activation barriers. Inorganic semiconductors most of the charge carriers are trapped in localized states. An appliedfield gives rise to the accumulation of charge in the region of the semiconducting layer. As thesecharges are accumulated (i) spatial overlap between the trap potential increases, that lowers theactivation barriers [60] and (ii) only a fraction of total charge carriers are required to fill all thetraps, the remaining carriers will on average require less activation energy to hop away to aneighboring site (Figure 6.15). This results in a higher mobility with increasing field. 145
    • 1 P3HT 2V 0.1 P3HT-CdTe 0.1 5V 10V 2V 0.01 0.01 5V J (A/cm )J (A/cm ) 22 10V 1E-3 1E-3 1E-4 1E-5 1E-4 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 3 4 5 6 7 8 -1 1000/T (K ) -1 1000/T (K ) 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-3and 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. -eFx ∆=52 meV -eFx P3HT P3HT-CdTe 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. 146
    • Chapter 66.6.5. Hole Transport Mechanism in P3HT-CdS Hybrid SystemFigure 6.16 shows the J-V characteristics of device based on P3HT-CdS, measured at differenttemperatures. Experimental data in Figure 6.16 are represented by symbols, whereas the solidcurves 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 ofcomposite film the hole current has been observed to be more than that in pristine P3HT at alltemperatures. Inset of Figure 6.17(a) shows the comparison of J-V characteristics of P3HT andP3HT-CdS at 190 K. We tried to fit the experimental data with unified mobility model. The datadid not show agreement with the mobility model, however, shows a good agreement withEquation (6.15) and (6.18). As a result, the hole mobility is constant, and thus also independent ofthe hole density. 1 290K 260K 0.1 225K 195K J (A/cm2) 0.01 170K 150K 1E-3 1E-4 1E-5 0.1 1 10 Voltage (Volts)Figure 6.16 Experimental (symbols) and calculated (solid lines) J-V characteristics of P3HT-CdSat 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 lowapplied bias, which can be attributed to the background doping and thermally generated chargecarriers. These J-V characteristics switched to non-ohmic behavior at higher applied bias, which isattributed to the formation of space charge near the injecting electrode. It is further seen fromthese curves that the slope of high-field conduction region decreases slightly with the increase inthe temperature. 147
    • 1 0.1 P3HT (a) (b) 0.01 P3HT-CdS 1E-3 0.1 J (A/cm 2) 1E-4 0.1 1E-5 35 meV 1E-6 1E-7J (A/cm2) 1E-8 190 K J (A/cm2) 1E-9 0.01 0.1 1 Voltage (Volts) 10 0.01 18 meV 1E-3 290K 260K P3HT-CdS 1E-4 1E-3 0.1 1 10 4 5 6 7 8 Voltage (Volts) 1000/T(K-1) 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 148
    • Chapter 6based on P3HT-CdS indicates that the formation of CTCs between the host and guest and may bethe dominant mechanism of interaction between the two. The PL quenching observed in Figure 5.6 (chapter 5) on CdS dispersion in P3HT is adirect evidence of CTCs formation between the host and guest since PL quenching is anindication of the degree of success of exciton dissociation and efficiency of charge transferbetween the donor-acceptor composite materials. The PL quenching in P3HT-CdS has beenattributed to the π-π interaction of P3HT with CdS [61], forming additional decaying paths of theexcited electrons through the CdS. To be more precise during CTCs formation, CdS QDs maydiffuse into the amorphous-crystalline boundaries of the P3HT polymer and introduce theconducting path, thus reducing the defect states and barrier height (activation energy from 52meVin 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 interactionof P3HT. The switching of mobility model in P3HT to band conduction mechanism in thecomposites is probably due to improvement in the electron wave function overlap between twopolymer chains. It suggests that CdS works as transport bridge between two polymer chains. Dueto enhancement in the electron wave function overlap the charge carriers do not move from onemolecule to other via hoping but via drift in the extended states of P3HT and valance band ofCdS.6.6. CONCLUSION1. 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 deviceconfiguration at different temperatures.2. The hole transport mechanism in P3HT thin film is governed by space charge limitedconduction wherein the charge carrier mobility is dependent on temperature, carrier density, andapplied field, given by unified mobility model.3. Thin films of copolymer P3OT-HT exhibited agreement with the space charge limitedconduction with traps distributed exponentially in energy and space. Hole mobility is bothtemperature and electric field dependent, arising due to octyl groups attached to these polymerbackbone. The estimated value of zero field mobility is of the order of 3.6×10-5cm2/V-s.4. The hole transport mechanism in P3OT thin film is governed by space charge limitedconduction model. The hole mobility follow the Gaussian distribution model with the zero fieldmobility of 9.3 × 10-6 cm2/V –s. 149
    • 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 ofactivation energies from 52 meV to 11 meV. At high trap density, trap potential wells startoverlapping which results in decrease of activation energies.6. In contrary to P3HT, the hole mobility in P3HT-CdTe has been found to be independentto charge carrier density and applied field. The charge carrier mobility depends only ontemperature and it increases with the decrease of temperature.7. On incorporation of CdS nanocrystals in P3HT matrix the mobility is again independent toapplied 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 indecrease in activation energies from 52 meV to 18meV.References[1] W. Brütting, Physics of Organic Semiconductors, (WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim: 2005).[2] K. C. Kao and W. Hwang, Electrical Transport in Solids, (Pergamon, Oxford, 1981).[3] M. A. Lampert and P. Mark, Current Injection in Solids (Academic, New York, 1970).[4] H. S. Nalwa, Handbook of Advanced Electronic and Photonic Materials and Devices (Vol. 8,Academic Press, San Diego: 2001).[5] D. S. Ginger and N. C. Greenham, J. Appl. Phys. 87 (2000) 1361.[6] W. U. Huynh, J. J. Dittmer, N. Teclemariam, D. J. Milliron, A. Paul Alivisatos, Phys. Rev. B67 (2003) 115326.[7] M. C. Schlamp, X. G. Peng, A. P. Alivisatos, J. Appl. Phys. 82 (1997) 5837.[8] D. L. Klein, R. Roth, A. K. L. Lim, A. P. Alivisatos, P. L. McEuen, Nature 389 (1997) 699.[9] H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meijer, P. Herwig, D. M. de Leeuw, Nature 401(1999) 685.[10] P. W. M. Blom, M. C. J. M. Vissenberg, Phys. Rev. Lett. 80 (1998) 3819.[11] C. K. Chiang, C. R. Fincher, Y. W. Park, A. J. Heeger, H. Shirakawa, E. J. Louis, S. C. Gau,A. G. MacDiarmid, Phys. Rev. Lett. 39 (1977) 1098.[12] J. H. Burroughes, C. A. Jones, and R. H. Friend, Nature 335 (1988) 137.[13] H. Scher and E. W. Montroll, Physical Review B (Solid State) 12 (1975) 2455.[14] H. Scher, M. F. Shlesinger, and J. T. Bendler, Physics Today 44 (1991) 26-34.[15] G. Horowitz and P. Delannoy, Journal of Applied Physics 70 (1991) 469. 150
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    • CHAPTER 7 CONCLUSIONS AND FUTURE SCOPE7.1. SUMMARY7.2. SUGGESTIONS FOR FUTURE INVESTIGATIONS7.1. SUMMARYIn this thesis the photovoltaic performances as well as the charge transport mechanism in organicand organic/inorganic hybrid system have been investigated by a variety of optical, electrical andnumerical techniques. The aim of the present work is to develop and improve the performance oforganic and hybrid solar cells. The homopolymers P3HT, P3OT, copolymer P3HT-OT have been studied regarding theiroptical and structural properties and used as electron donor materials in polymer solar cells. Thecomposites of the three polymers with PCBM show a distinctive photoluminescence quenchingeffect, which confirm the photoinduced charge generation and charge transfer at P3AT/PCBMinterface. 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%, whichis 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 ofP3OT 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/cm2) and P3OT:PCBM device(1.46 mA/cm2). These values are governed by an increased hole mobility and by a lower energytransition barrier for holes undergoing transfer from the HOMO level into ITO anode regardingP3HT against P3HT-OT and P3OT. The performances of these devices have been improved bypost-production thermal annealing of device at a sufficiently high temperature. In order to reduce charge recombination and increase the carrier mobilities inP3HT:PCBM based device, the CdS QDs have been incorporated in the P3HT matrix. HRTEMimages reveal that the size of CdS QDs ranges from 5 to 6 nm and their shape is spherical. Theaverage crystallite size determined from the Debye–Scherrer formula is estimated to be about2.33nm. The P3HT/CdS nanocomposite shows blue shift in the absorption spectra relative to thepristine P3HT, which is attributed to the quantum confinement effect from the CdS nanocrystals.
    • 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 P3HTmatrix, the power conversion efficiency increased from 0.45% to 0.87% due to enhancement inshort-circuit current, and fill factor. The enhancement in JSC have been explained on the basis ofthe formation of charge transfer complex between the host (P3HT) and guest (CdS QDs), dulysupported by blue shift in UV-Vis absorption and PL quenching studies. The investigation on theeffect of post thermal annealing on device performance had shown that improved efficiency ofdevices 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 solarabsorption, enhancing the charge carrier mobility, and improving the polymer-nanocrystalsinteraction, the CdTe nanocrystals have been in-situ grown in the P3HT matrix without use of anysurfactant. Structural and spectroscopic studies confirmed the successfully incorporation of CdTenanocrystals in P3HT matrix. Structural and morphological studies reveal that CdTe works astransport media along/between the polymer chains, which facilitate percolation pathways forcharge transport. Optical measurements show that photoinduced charge generation on theabsorption of light and these are dissociated at the P3HT-CdTe interfaces. The solar cellperformance of device based on P3HT-CdTe:PCBM showed a better performance compared toP3HT:PCBM, due to increased JSC from 2.25 mAcm-2 to 3.88 mAcm-2, and VOC from 0.58 V to0.80 V. The enhancement in VOC in P3HT-CdTe:PCBM based device attributed lower HOMOlevel of CdTe compared to P3HT. The measured difference (0.21 eV) of the HOMO energy levelsbetween P3HT and CdTe almost completely translated into the observed difference in Voc (∼0.22V). Moreover, enhancement in JSC may result in improvement in the solar absorption spectra anddecrease in the activation energy. This cell suffered from low fill factors, which may be caused byshunting 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 basedon organic and organic-inorganic hybrid systems, the J-V characteristics have been studied in thehole only device configuration, at different temperatures. The hole transport mechanism in P3HTthin film is governed by space charge limited conduction with temperature, carrier density, andapplied field dependent mobility. Thin films of copolymer P3HT-OT exhibited agreement withthe 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 groupsattached to these polymer backbones. The estimated value of zero field mobility of P3HT-OT isof 3.6×10-5cm2/V-s. The hole transport mechanism in P3OT thin film is govern by space charge 154
    • Chapter 7limited conduction model. The hole mobility follow the Gaussian distribution model with the zerofield mobility of 9.3 × 10-6 cm2/V –s. Incorporation of CdTe nanocrystals in P3HT matrix results into enhancement in currentdensity which attributed to increase in the trap density (from 2.8×1018 to 5.0×1018 cm-3) anddecrease of activation energies (from 52 meV to 11 meV). At high trap density, trap potentialwells start overlapping which results in decrease of activation energies. In contrary to P3HT, thehole mobility in P3HT-CdTe has been found to be independent to charge carrier density andapplied field. The charge carrier mobility depends only on temperature and it increases with thedecrease of temperature. On incorporation of CdS nanocrystals in P3HT matrix the mobility isagain independent to applied field and carrier density and exhibited agreement with the bandconduction mechanism. This is attributed to the enhancement in the overlapping of traps potentialwells, which results in the decrease in activation energies from 52 meV to 18meV.7.2. SUGGESTIONS FOR FUTURE INVESTIGATIONS1. A number of mechanisms in organic photovoltaics are still poorly understood, such as themechanism 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 andengineers to carefully design an efficient heterojunction between the organic and inorganic phasesthat 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 fluorescencespectroscopy (TRFS) and time-resolved microwave conductivity (TRMC) investigation can becarried out in donor-acceptor composites to better understand the exciton dissociation process atdonor-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 155
    • 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 plagueorganic photovoltaics and are a major factor in their slow entry into the photovoltaic market. Toprevent premature device degradation, both the active materials must have better resistance toenvironmental attack, as well as the encapsulation systems should effectively keep air andmoisture away from the active material. 156
    • 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 CdTenanocrystals 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 onP3HT/PCBM PV device,Mohd Taukeer Khan, Ranoo Bhargav, Amarjeet Kaur, S K Dhawan, and Suresh Chand, ThinSolid 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 SystemMohd Taukeer Khan, AmarjeetKaur, S.K. Dhawan, and Suresh Chand,National Symposium on “Recent Advances in Materials and Devices for Solar EnergyApplications” (1st- 2nd , Sept. 2011), National Physical Laboratory, New Delhi.(2) In-situ growth of quantum dots in polymer template: photophysics of organic/inorganic hybridsolar 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 CdSMohd Taukeer Khan, Amarjeet Kaur, S.K. Dhawan, and Suresh Chand,IJBWME- 2009, Dec, 17-20, 2009, NPL, New Delhi. 157
    • (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 Course1. 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. 158