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Design, Development and Fabrication of Thiophene and Benzothiadiazole Based Conjugated Polymers for Photovoltaics” Is Divided Into Five Chapters

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Design, Development and Fabrication of Thiophene and Benzothiadiazole Based Conjugated Polymers for Photovoltaics” Is Divided Into Five Chapters Design, Development and Fabrication of Thiophene and Benzothiadiazole Based Conjugated Polymers for Photovoltaics A thesis submitted by Radhakrishna Ratha Roll No. 11615303 to Indian Institute of Technology Guwahati For the award of the degree of Doctor of Philosophy Center for Nanotechnology Indian Institute of Technology Guwahati Guwahati - 781039 India February, 2018 Statement INDIAN INSTITUTE OF TECHNOLOGY GUWAHATI Guwahati, Assam-781039, India Centre for Nanotechnology STATEMENT I do hereby declare that the work contained in the thesis entitled ‘Design Development and Fabrication of Thiophene and Benzothiadiazole Based Conjugated Polymers for Photovoltaics’ is the result of investigations carried out by me in the Center for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati, Assam India under the supervision of Dr. Parameswar Krishnan Iyer, Professor, Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati, Assam, India. This work has not been submitted elsewhere for the award of any degree. Radhakrishna Ratha Center for Nanotechnology Indian Institute of Technology Guwahati Guwahati-781039, Assam, India February 2018 I TH-1906_11615303 Certificate INDIAN INSTITUTE OF TECHNOLOGY GUWAHATI Guwahati, Assam-781039, India Centre for Nanotechnology CERTIFICATE This is to certify that the work contained in the thesis entitled ‘Design Development and Fabrication of Thiophene and Benzothiadiazole Based Conjugated Polymers for Photovoltaics by Radhakrishna Ratha, a Ph.D. student of Center for Nanotechnology, Indian Institute of Technology Guwahati, for the award of degree of Doctor of Philosophy has been carried out under my supervision and this work has not been submitted elsewhere for any degree. Prof. Parameswar Krishnan Iyer Thesis Supervisor Department of Chemistry, Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati -781039, Assam, India February, 2018 II TH-1906_11615303 ACKNOWLEDGEMENTS At this stage of ending truly memorable and overwhelming journey towards my intellectual destination, it is really hard to list all the people who sincerely helped me and I would like to thank all of them who have made this thesis possible. First of all, I would like to express my sincere gratitude to my thesis supervisor professor Parameswar Krishnan Iyer for his kind support, advice, encouragement and introducing me to an interdisciplinary area of research. I earnestly thank him for his astute guidance, freedom to work, encouragement, inspiration and creative and scientific ideas, which helped me to enhance my knowledge. I sincerely thank and would like to acknowledge my sincere gratitude to my doctoral committee members, Professor Aditya Narayan Panda, Professor Mohd. Qureshi and Dr. Tapas Kumar Mandal for their insightful advices and suggestions and crucial comments which certainly helped me a lot in betterment of my thesis. I am thankful to all faculty members in the Centre for Nanotechnology and Department of Chemistry IIT Guwahati for their help and encouragement and also the non-teaching staff of the Centre for Nanotechnology and Department for their technical support. I am thankful to the Central Instruments Facility (CIF), IIT Guwahati for different characterization facilities. I like to thank few of my friends and colleagues, Anargha, Biswa, Manu, Bikash, Banaja, Mona, Smita, Nandita, Moumita, Mili, Saumya bhai, Rosalin didi, Himanshu bhai, Santosh bhai, Deepankar bhaiya, who were there with me at those demanding times of highs and lows. With my friends, seniors and juniors my Ph.D. period became cherishably wonderful and will be echoing forever in corridors my mind. Definitely I am going to treasure the fantastic moments shared. I am very lucky to have extremely cooperative and friendly seniors–Gunin bhaiya, Prasanta bhaiya, Atul bhaiya, Muthuraj, Subarao and Meenakshi. My lab juniors Bhim, Suresh, Sameer, Anamika Dey, Anamika Kalita, Debojit, Ashish, Dipjyoti, Arvin, Akhtar, Sayan, Gopi, Raman, Adil, Subrata, Niranjan, Maimur, Debashish, Nehal, Rabindra, Ritesh, Indrani, Rahul, Nystha, Ramesh, Priyanka, Himani, Ekta, Retwick, Nasima, Viki, Anmesha who made my experiences with them unforgettable with their help and care. Thanks go to all other departmental seniors, batch mates and juniors for making my days memorable with them. III TH-1906_11615303 ACKNOWLEDGEMENTS I gratefully acknowledge all my teachers at School and specially Rabi sir, Giri sir and Sabat sir at K.S.U.B College, Bhanjanagara, Orissa, India and Dr. Sandhya A. Mishra, Dr. Naveen Ratha, Chakrapani sir, Basudev sir, Bibhu sir, at Khallikote Autonomous College, Berhampur, Orissa, India for their best teaching, constant motivations and precious advices which are of great impact on me. Finally, my Ph.D. endeavour could not be completed without the endless love, unending support, tolerance and blessings from my family. I would like to express my sincere gratitude to my parents, family members and relatives. Radhakrishna Ratha IV TH-1906_11615303 Contents Statement I Certificate II Acknowledgements III Synopsis VIII Chapter 1: Introduction 1.1 Introduction 1 1.2 Objective of the present work and summary 28 1.3 References 30 Chapter 2: Photo stability enhancement of poly(3-hexylthiophene)- PCBM nano-composites by addition of multi walled carbon nanotubes under ambient conditions Abstract 38 1.1 Introduction 38 1.2 Results and discussion 39 1.3 Conclusion 47 1.4 Experimental 48 1.5 References 49 Chapter 3: Insight into the synthesis and fabrication of 5,6-alt- benzothiadiazole based D-π-A CPs for bulk-heterojunction solar cell Abstract 51 1.1 Introduction 51 1.2 Results and discussion 52 1.3 Conclusion 59 1.4 Experimental 60 V TH-1906_11615303 1.5 References 64 Chapter 4: Substituting non-conjugating ester group into side chain of benzothiadiazole improves optical, electrochemical properties, morphology of active layer and solar cell performance of D-A polymer for photovoltaics Abstract 67 1.1 Introduction 67 1.2 Results and discussion 69 1.3 Conclusion 78 1.4 Experimental 79 1.5 References 85 Chapter 5: 6,7-Di(thiophen-2-yl)naphtho[2,3-c][1,2,5]thiadiazole and 4,6,7,9-tetra(thiophen-2-yl)naphtho[2,3-c][1,2,5]thiadiazole as new acceptor units for D-A type co-polymer towards fabrication of polymer solar cell Abstract 88 1.1 Introduction 88 1.2 Results and discussion 90 1.3 Conclusion 98 1.4 Experimental 98 1.5 References 102 Appendix A1 105 Appendix A2 108 Appendix A3 109 VI TH-1906_11615303 Appendix A4 120 Appendix A5 138 Thesis overview 146 Publication and conferences 147 Shockley–Queisser limit 148 Future Prospective 149 VII TH-1906_11615303 VIII TH-1906_11615303 SYNOPSIS The content of this synopsis report entitled “Design, development and fabrication of thiophene and benzothiadiazole based conjugated polymers for photovoltaics” is divided into five chapters. In chapter 1 the respective research area, where design synthesis and fabrication of conjugated polymers, along with the scope and significance of the subsequent chapters are discussed. In chapter 2 photostability enhancement of P3HT-PCBM by using appropriate ratio of MWCNT in ambient conditions have been discussed. Chapter 3 discusses about synthesis of poly(ortho-arylene-vinylene) type of polymer alternating at 5,6-position of BT namely P1 and P2 in the polymer main chain. Chapter 4 demonstrates that on functionalization of methyl acetate group at 5,6-positions of BT red-shift absorbance, lowered the LUMOoptical, improves phase separation in active layer of PSC and hence improves solar cell performance compared to its methyl counterpart. Chapter 5 results in synthesis of newer napthothiadiazole (NT) based D-A polymer and their BHJ solar cell performance. Chapter 1: Introduction to polymer solar cell Polymer solar cells (PSCs) have gained much attention owing to its several advantages such as large area solution processability, fine tuning of band gap on polymer backbone and amorphous nature of polymer that allow fabrication on light weight flexible substrates. These properties make CPs attractive for solar cell researchers and a PCE of ~13% has been achieved recently. These polymer solar cells will be cheaper and flexible in nature compared to present commercialized expensive silicon solar cells. Generally a blend of p-type CP (Figure 2) and an n-type acceptor (Figure 2) are used to sandwich between an anode and a cathode (Device geometry and mechanism of a polymer solar cell has been shown in Figure 1). VIII TH-1906_11615303 SYNOPSIS Step 2 Step 3 (a) (b) Work e- LUMO Work function function hʋ 3.37 Step 3 Step 1 3.7 Step 2 4.2 ITO PEDOT:PSS POLYMER Al Solar PCBM photons Cathode 4.8 e- h+ Anode 5.0 Substrate 5.5 HOMO 6.1 Bulk-heterojunction geometry + - Figure 1 Bulk-heterojunction polymer solar cell (a) device structure (b) mechanism. R O R S R C C7H15 R C6H13 R R S S F O N N S S O S S S S S n S S S S S n n S n R = 2-Ethylhexyl S R = 2-Ethylhexyl S R = 2-Ethylhexyl R R P3HT CPDT-BT PTB7-Th PBDB-T Solar energy harvesting polymers (donor/ p-type polymer/ hole transporting) I I 1,8-Diiodooctane (DIO) Additive to improve morphology MWCNT PC BM ITC 61 PC71BM Electron transporting acceptor (n-type material) used as blend with CPs Used as ternary component Figure 2 Structure of frequently studied polymers and acceptor units for PSC, along with additive and ternary component. Generally CPs have been synthesized by the combination of π-electron acceptor, such as [2,1,3-benzothiadiazole (BT), thieno[3,4-b]pyrazine (Tp), quinoxaline (Q)] and π-electron donor, such as [thiophene (Th), cyclopentadithiophene (CPDT), benzo[1,2-b:4,5- bˊ]dithiophene (BDT), fluorene (FL), carbazole (Cz)] either as D-A, D-A-D or D-π-A (π refers to extended conjugation). D-A combination creates intra molecular charge transfer between donor and acceptor leading to red shift of absorbance, a spectral region where earth receives the most solar photon flux.
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