STUDY OF CHARGE TRANSPORT MECHANISM   IN ORGANIC AND ORGANIC/INORGANIC  HYBRID SYSTEMS WITH APPLICATION TO         ORGANIC...
CERTIFICATEThis is to certify that subject matter presented in this thesis titled “Study of Charge TransportMechanism in O...
Dedicated   ToMy parents
ACKNOWLEDGMENTSAt the outset, I offer my prayers and thanks to the Almighty Allah, for He is good; His loveendures forever...
images. I wish to express my sincere thanks to all the staff members, Department of Physics andAstrophysics, University of...
Table of ContentsChapter 1: Introduction: A Selective History and Working Principle ofOrganic and Hybrid Solar Cells………………...
1.6.6. Standard Test Conditions..............................................................................................
3.1 Introduction.............................................................................................................
5.1. Introduction………………………………...……………………………………………...1095.2. Fabrication and Measurement of Device………………………………………………...1105...
6.5. Unified Mobility Model……………………………………………………………………1346.6. Results and Discussion …………………………………………………………………....136      ...
ABSTRACTIn recent years organic photovoltaics has shown a great promise of delivering cost effective,flexible, light weigh...
photoluminescence (PL), X-ray diffraction (XRD), and transmission electron microscopy (TEM).The measurement techniques of ...
Chapter 5 deals with the fundamental issue, whether incorporation of CdS nanocrystalsinto P3HT matrix causes any noticeabl...
decrease of temperature. On incorporation of CdS nanocrystals in P3HT matrix the mobility isagain independent to applied f...
CHAPTER 1       INTRODUCTION: A SELECTIVE HISTORY AND WORKING PRINCIPLE OF                      ORGANIC & HYBRID SOLAR CEL...
1.6.6. Standard Test Conditions    1.6.7. Equivalent Circuit Diagram1.7. OBJECTIVE OF THE PRESENT THESIS1.8. THESIS PLANRe...
Chapter 1commercially-viable alternative, available today is nuclear energy [8-10]. Uranium does not causeCO2 emissions bu...
made using a crystalline silicon (c-Si) wafer, in which a semiconductor junction is formed bydiffusing phosphorus into the...
Chapter 1% among thin film materials [30] and shows excellent stability. At the moment CIGS is the mostpromising candidate...
Metal (Front)           Metal (Back)             Metal (Back)              TCO                               TCO          ...
Chapter 1Figure 1.5 Year-wise progress on the efficiencies of different photovoltaic device, under AM 1.5simulated   solar...
1.3. POLYMER SOLAR CELLSPolymer-based PV systems hold the promise for environmentally safe, flexible, lightweight, andcost...
Chapter 1devices is primarily due to the fact that absorption of light in the organic materials almost alwaysresults in th...
1.3.2.3. Bulk-heterojunction devicesTo date, the most successful method to construct the active layer of an OPV devices is...
Chapter 1methyl ester (PCBM) as acceptor [109]. This demonstrates the effectiveness of low-band-gappolymers in generating ...
address these fundamental limitations of polymer solar cells, new strategies have been developedby blending of inorganic n...
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 performa...
PCBM:PbS                     0.24      14.0                   1.68        N. Zhao et al. ACS Nano                         ...
Chapter 1of CdS in P3HT templates using cadmium acetate precursor for Cd and sulphur powder for S. Thedevice made from P3H...
S1      ABSORPTION      INTERNAL CONVERSION (10 ps)                                                                       ...
Chapter 1            Figure 1.9 Illustration of the electron transfer between donor and acceptor.1.5.2. The Need of Two Se...
Inorganic semiconductors generally have a high dielectric constant of the order of 10, ascompared to 3 in organic semicond...
Chapter 1can decay (6), yielding, e.g., luminescence, if they are generated too far from the interface. Thus,the excitons ...
semiconductors differ significantly from those based on inorganic materials. Typically, in OSCsan efficient electron accep...
Chapter 1carriers to small segments. As a result, the delocalization length of the charge carriers is limited toalmost mol...
positive electrode, respectively. As a result, the electrode work functions become pinned close tothe LUMO/HOMO level of t...
Chapter 1in potential. The JSC is heavily dependent on the number of absorbed photons which originatesfrom two different f...
Pmax        ( J  V ) max                                           FF                                                  ...
Chapter 1spectrum of the sun, measured on a clear sunny day with a radiant intensity of 100 W/cm 2 that isreceived on a ti...
of the I-V curves. The photocurrent source generates current (Iph) upon illumination and equals tothe number of dissociate...
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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Study of charge transport mechanism in organic and organicinorganic hybrid systems with application to organic solar cells

  1. 1. STUDY OF CHARGE TRANSPORT MECHANISM IN ORGANIC AND ORGANIC/INORGANIC HYBRID SYSTEMS WITH APPLICATION TO ORGANIC SOLAR CELLS A THESIS SUBMITTED TO THE DEPARTMENT OF PHYSICS AND ASTROPHYSICS, UNIVERSITY OF DELHI DELHI-110007 INDIA FOR THE AWARD OF DEGREE OF DOCTOR OF PHILOSOPHY IN PHYSICS BY MOHD TAUKEER KHAN SEPTEMBER 2011
  2. 2. 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
  3. 3. Dedicated ToMy parents
  4. 4. 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
  5. 5. 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)
  6. 6. 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
  7. 7. 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
  8. 8. 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
  9. 9. 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
  10. 10. 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
  11. 11. 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
  12. 12. photoluminescence (PL), X-ray diffraction (XRD), and transmission electron microscopy (TEM).The measurement techniques of J-V characteristics under light, in dark, as well as at 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
  13. 13. 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
  14. 14. 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
  15. 15. 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
  16. 16. 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
  17. 17. 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
  18. 18. 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
  19. 19. 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
  20. 20. Metal (Front) Metal (Back) Metal (Back) TCO TCO n-Si n-a-Si CdS i-µc-Si CdTe CIGS p-Si p-µc-Si CdS Mo (Back) TCO (front) TCO (front) Metal (Back) glass glass Glass, metal foil (a) (b) (c) (d)Figure 1.4 Device configurations for (a) c-Si, (b) a-Si, (c) CdTe and, (d) CIGS. i is intrinsic,TCO is transparent conductive oxide, and, Mo is molybdenum.These cells are based on low energy, high-throughput processing technologies e.g. OPV are:chemically synthesized, solution processable, low material cost, large area, light weight 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
  21. 21. 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
  22. 22. 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
  23. 23. 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
  24. 24. 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
  25. 25. 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
  26. 26. 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
  27. 27. 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
  28. 28. PCBM:PbS 0.24 14.0 1.68 N. Zhao et al. ACS Nano 4 (2010) 3743.P3HT:GaAs-TiOx 0.59 7.16 2.36 S. Ren et al. Nano Lett. 11 ( 2011) 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
  29. 29. 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
  30. 30. 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
  31. 31. 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
  32. 32. 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
  33. 33. 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
  34. 34. 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
  35. 35. 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
  36. 36. 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
  37. 37. 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
  38. 38. 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
  39. 39. 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
  40. 40. 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

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