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From perfectly alternating to random multichromophoric conjugated polymers for ternary blend bulk heterojunction solar cells

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From perfectly alternating to random multichromophoric conjugated polymers for ternary blend bulk heterojunction solar cells

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Content FROM PERFECTLY ALTERNATING TO RANDOM MULTICHROMOPHORIC CONJUGATED POLYMERS FOR TERNARY BLEND BULK HETEROJUNCTION SOLAR CELLS by Seyma Ekiz A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (CHEMISTRY) December 2017 Copyright 2017 Seyma Ekiz ii DEDICATION To myself… iii ACKNOWLEDGEMENTS I am very happy that my Ph.D. is finally ending. I would like to thank my advisor Prof. Barry C. Thompson for his supervision throughout all these years. He was a very focused, patient, and extremely hardworking mentor trying to teach me how to survive in academia. I am grateful for his support and I hope he enjoyed the past five years as much as I did. I would like to thank Prof. Surya G. Prakash, Prof. Smaranda Marinescu, Prof. Chao Zhang, and Prof. Jongseung Yoon for serving on my screening and qualifying examination. Thank you Prof. Surya G. Prakash and Prof. Jongseung Yoon for also serving on my defense committee. I was very lucky working with great mentors when I first joined the group. I would like to thank Dr. Jenna Howard and Dr. Andrey Rudenko for their guidance in the lab. Jenna, we worked so hard during all these years and we shared many memories together (and a pretty cool office). Thank you for your endless friendship and support. Andrey, thank you for your friendship and support both in the east coast and west coast. Our former group members Dr. Petr Khlyabich, Alia Latif, Dr. Bing Xu, and Dr. Sangtaik Noh, thank you for all your support, guidance, and friendship throughout my Ph.D. Petr, you made the entire lab super fun. Thank you for being a great friend. Alia, you were the coolest BCT member. I am very lucky being your friend. Bing, thank you for being very approachable, kind and helpful all the time. Sangtaik, you were and will always be my best friend. I would like to thank current BCT group members, Dr. Nemal Gobalasingham, Betsy Melenbrink, Robert Pankow, Sanket Samal, Qingpei Wan, and John Luke McConn iv for making BCT group a great team. Dr. Nemal Gobalasingham, thank you for your friendship, support, and help throughout our Ph.D. We started our journey in BCT group together. We had a lot of fun. You were an amazing colleague and friend. Many thanks to Betsy Melenbrink for her friendship, support, and advice. You are one of the most dedicated scientists I have ever met. I will always appreciate your effort to make the world a better place, and I will miss our long conversations. I would also like to thank Robert Pankow for his friendship, support and help. You made the group more fun and our LHI trips merrier. And John Luke McConn, thank you for your hard work, support, and friendship. I was lucky being your mentor, and I will miss our deep conversations. Many thanks to my Hillview friends Rasha Hamze, Kavita Belligund, Dan Fang, Dr. Eric Driscoll, Subhasish Sutradhar, Dr. Haipeng Lu, and Chayan Dutta for making Los Angeles transition easier. I would like to thank Dr. Jennifer Moore for a great teaching experience at USC. I would also like to thank Michele Dea, Magnolia Benitez, David Hunter, Jessy May, and Carole Phillips for their help during my time at USC. I am grateful to many other amazing friends at USC who made Los Angeles my home; Beril Kiragasi, Gozde Barim, Serra Ongun, Sebnem Baran, Guher Camliyurt, Ugur Akyol, Kaan Onuk, Samet Keserci, Alperen Ozdemir, Nil Simsek, and Gunce Ezgi Cinay. Beril, I am so lucky that I met you on the first day of USC. You are an amazing friend and you will be. I will remember all our memories in LA and I will truly miss the past five years. Serra, I am grateful for your friendship and support throughout my Ph.D. and I know it will be a lifelong friendship. My labmates from my Masters program at Bilkent University; Dr. Vusala Ibrahimova, Dr. Muge Artar, Ozlem Unal, Meltem Ayguler, and Dr. Eda Kocak, thank v you for your friendship and support throughout my studies at Bilkent University. I also would like to thank all of my friends at Bilkent for making my two years there awesome. My dear college friends; Dr. Ozden Celikbilek, Esra Eroglu, Dr. Murat Kadir Deliomeroglu, and Dr. Cagatay Dengiz, thank you for your endless support and friendship since the first year of college. We shared many amazing moments together, and we will share many more. Special thanks to my sister, Esra Ekiz, for her friendship, support, and advice any time I needed. I know you will always be there for me. vi TABLE OF CONTENTS DEDICATION…………………………………………………………………………….ii ACKNOWLEDGEMENTS………………………………………………………..……..iii LIST OF TABLES………………………………………………………………………..ix LIST OF FIGURES……………………………………………………………………...xii LIST OF SCHEMES……………………………………………………………………xix ABSTRACT……………………………………………………………………………...xx CHAPTER 1: Ternary Blend Bulk Heterojunction Solar Cells…………………………1 1.1 Introduction……………………………………………………………………………1 1.2 State-of-the-art Ternary Blend Systems……………………………………………….6 1.3 Proposed Mechanisms for Ternary Blend Solar Cell Operations……………………12 1.3.1 Organic alloy model……………………………………………………………13 1.3.2 Parallel-like model……………………………………………………………..29 1.3.3 Sensitization/cascade model…………………………………………………...33 1.4 Functions of Third Component………………………………………………………34 1.4.1 Increase in light absorption…………………………………………………….34 1.4.2 V oc tuning………………………………………………………………………40 1.5 Challenges and Outlook……………………………………………………………...46 1.6 References for Chapter 1…………………………………………………………….48 CHAPTER 2: Random Multi-Acceptor Poly(2,7-carbazole) Derivatives Containing the Pentacyclic Lactam Acceptor Unit TPTI for Bulk Heterojunction Solar Cells………….57 2.1 Introduction…………………………………………………………………………..57 2.2 Experimental…………………………………………………………………………61 2.3 Results and Discussion………………………………………………………………63 2.4 Conclusion…………………………………………………………………………...70 2.5 References for Chapter 2…………………………………………………………….72 vii CHAPTER 3: Exploring the Influence of Acceptor Content on Semi-Random Conjugated Polymers…………………………………………………………………….75 3.1 Introduction…………………………………………………………………………..75 3.2 Experimental………………………………………………………………………....77 3.3 Results and Discussion………………………………………………………………79 3.4 Conclusion…………………………………………………………………………...96 3.5 References for Chapter 3…………………………………………………………….98 CHAPTER 4: Wide Band Gap Polymers Incorporating TPTI Acceptor for Bulk Heterojunction Solar Cells……………………………………………………………...102 4.1 Introduction…………………………………………………………………………102 4.2 Experimental………………………………………………………………………..104 4.3 Results and Discussion……………………………………………………………..106 4.4 Conclusion………………………………………………………………………….113 4.5 References for Chapter 4…………………………………………………………...115 BIBLIOGRAPHY………………………………………………………………………118 APPENDIX 1: Random Multi-Acceptor Poly(2,7-carbazole) Derivatives Containing the Pentacyclic Lactam Acceptor Unit TPTI for Bulk Heterojunction Solar Cells …………………………………………………………………………………………..125 A1.1 Materials and Methods……………………………………………………………125 A1.2 Synthesis………………………………………………………………………….127 A1.3 Polymer Characterization…………………………………………………………132 A1.4 Device Fabrication and Characterization…………………………………………138 A1.5 References for Appendix 1……………………………………………………….142 APPENDIX 2: Exploring the Influence of Acceptor Content on Semi-Random Conjugated Polymers…………………………………………………………………...144 A2.1 Synthesis………………………………………………………………………….144 A2.2 Polymer Characterization…………………………………………………………146 viii A2.3 Device Fabrication and Characterization…………………………………………153 A2.4 References for Appendix 2……………………………………………………….158 APPENDIX 3: Wide Band Gap Polymers Incorporating TPTI Acceptor for Bulk Heterojunction Solar Cells……………………………………………………………...160 A3.1 Synthesis………………………………………………………………………….160 A3.2 Polymer Characterization…………………………………………………………162 A3.3 Device Fabrication and Characterization…………………………………………165 A3.4 References for Appendix 3……………………………………………………….167 ix LIST OF TABLES Table 1.1. Photovoltaic properties of ternary blend bulk heterojunction solar cells based on P3HTT-DPP-10%:P3HT 75 -co-EHT 25 :PC 61 BM………………………………………..7 Table 1.2. Photovoltaic properties of the ternary blend bulk heterojunction solar cells of PTB7-Th:PID2:PC 71 BM…………………………………………………………………..9 Table 1.3. Photovoltaic properties of the ternary blend bulk heterojunction solar cells of PTB7-Th:P-DTS(FBTTH 2 ):PC 71 BM…………………………………………………….10 Table 1.4. Photovoltaic properties of the ternary blend bulk heterojunction solar cells of PBTZT-STAT-BDTT-8:PTB7-Th:PC 70 BM. a Current density calculated over the total area……………………………………………………………………………………….12 Table 1.5. Photovoltaic properties of P3HTT-DPP- 10%:PCDTBT:PC 61 BM ternary blend BHJ solar cells at different polymer ratios……………………………….………..23 Table 1.6. Photovoltaic properties of ternary blend solar cells from TAZ:DTBT and DTffBT:DTPyT………………………………………………………………………….32 Table 2.1. Molecular weights, polydispersity indices, electrochemical HOMO values, optical band gaps and surface energies of the polymers…………………………………63 Table 2.2. Photovoltaic properties of D/A alternating copolymers and random multi- acceptor copolymers……………………………………..………………………………68 Table 3.1. Molecular weights, polydispersity indices, electrochemical HOMO values, optical band gaps and melting/crystallization temperatures of the polymers…….……...82 x Table 3.2. Photovoltaic properties of P3HT, P3HTT-DPP 10%, P3HTT-DPP 20%, P3HTT-DPP 30%, and P3HTT-DPP 40%.........................................................................92 Table 4.1. Molecular weights, polydispersity indices, electrochemical HOMO values, optical band gaps and surface energies of the polymers………………………………..108 Table 4.2. Photovoltaic properties of PCTPTI (DArP), PCTPTI (Suzuki), PFTPTI (DArP), PFTPTI (Suzuki), PPTPTI (DArP)……………………………………………111 Table A1.1. Surface energies of neat polymer films…………………………………...137 Table A1.2. Hole and electron mobilities of polymer:PC 61 BM blends in thin films spin- coated from o-DCB……………………………………………………………………..140 Table A1.3. Raw short-circuit current densities (J sc ), spectral mismatch factor (M), spectral mismatch-corrected short-circuit current densities (J sc,corr ) and integrated short- circuit current densities (J sc,EQE ). All devices were spin-coated from o-DCB and placed to the N 2 cabinet before aluminum deposition for 30 mins………………………………..141 Table A2.1. Molecular weights and polydispersity indices of the polymers……..……146 Table A2.2. Relevant information derived from GIXRD patterns of the polymers…....151 Table A2.3. Hole mobility of neat polymers in thin films spin-coated from o-DCB a Average of two sets of devices…………………………………………………………155 Table A2.4. Raw device data of only DTD-DPP incorporated polymers (P3HTT- DTDDPP 10%, P3HTT-DTDDPP 20%, P3HTT-DTDDPP 30%, and P3HTT-DTDDPP 40%). All devices were spin-coated from o-DCB and placed to the N 2 cabinet before aluminum deposition for 30 mins. a, b, c, d Average of four pixels………………………156 xi Table A2.5. Raw device data of the semi-random copolymers that were fabricated from fully-dried polymers. All devices were spin-coated from o-DCB and placed to the N 2 cabinet before aluminum deposition for 30 mins. a, b, c, d Average of four pixels……….156 Table A2.6. Averages and standard deviations for Table 2. for the photovoltaic properties of P3HT, P3HTT-DPP 10%, P3HTT-DPP 20%, P3HTT-DPP 30%, and P3HTT-DPP 40% (MeOH treated wet polymers). All devices were spin-coated from o-DCB and placed to the N 2 cabinet before aluminum deposition for 30 mins……………………..157 Table A2.7. Raw short-circuit current densities (J sc ), spectral mismatch factor (M), spectral mismatch-corrected short-circuit current densities (J sc,corr ) and integrated short- circuit current densities (J sc,EQE ) of the MeOH treated wet polymers. All devices were spin-coated from o-DCB and placed to the N 2 cabinet before aluminum deposition for 30 mins……………………………………………………………………………………..157 Table A3.1. Raw short-circuit current densities (J sc ), spectral mismatch factor (M), spectral mismatch-corrected short-circuit current densities (J sc,corr ) and integrated short- circuit current densities (J sc,EQE ). All devices were spin-coated from o-DCB and placed to the N 2 cabinet before aluminum deposition for 30 mins……………………………………………………………………………………..167 xii LIST OF FIGURES Figure 1.1. Bulk heterojunction solar cell device architecture…………………..……….4 Figure 1.2. Representation of organic donor:acceptor solar cell energy diagram………..5 Figure 1.3. Device architecture of a ternary blend BHJ solar cell………………………..6 Figure 1.4. Structures, HOMO energies and absorption profiles of (P3HT 75 -co-EHT 25 ), (P3HTT-DPP-10%) and PC 61 BM…………………………………………………………8 Figure 1.5. Structures of PID2, PTB7-Th and PC 71 BM…………………………………..9 Figure 1.6. Structures of p-DTS(FBTTH 2 ) 2 , PTB7-Th, and PC 71 BM…………………..10 Figure 1.7. Structures of PBTZT-STAT-BDTT-8, PTB7-Th, and PC 70 BM……………11 Figure 1.8. Scheme of the organic alloy model in ternary blend solar cells (a) D1:D2:A, (b) D:A1:A2……………………………………………………………………………...14 Figure 1.9. Representation of the composition averaged HOMO level of the donor alloy of different donor:donor ratios…………………………………………………………...15 Figure 1.10. Representation of the composition averaged LUMO level of the acceptor alloy of different acceptor:acceptor ratios……………………………………………….15 Figure 1.11. Structures of P3HT, PC 61 BM, and ICBA………………………………….17 Figure 1.12. Photocurrent spectral response (PSR) and energy of CT state compared to the values of the V oc data for the P3HT:PC 61 BM:ICBA (D:A1:A2)…………………….18 xiii Figure 1.13. Structures of P3HT 75 -co-EHT 25 , P3HTT-DPP-10%, and PC 61 BM………..18 Figure 1.14. PSR and energy of CT state compared to the values of the V oc data for the P3HTT-DPP-10%:P3HT 75 -co-EHT 25 :PC 61 BM (D1:D2:A)……………………………...19 Figure 1.15. Expanded plot of the peaks for the P3HT:PC 61 BM:ICBA ternary system near 1.7 eV with the background subtracted. The peak centered above 1.7 eV corresponds to PCBM absorption, and the peak centered below 1.7 eV corresponds to ICBA……………………………………………………………………………………..20 Figure 1.16. High-energy PSR data of P3HTT-DPP-10%:P3HT 75 -co-EHT 25 :PC 61 BM showing the exciton peaks from the donor mixture……………………………………...20 Figure 1.17. Structures of PCDTBT, P3HTT-DPP-10%, and PC 61 BM………………...23 Figure 1.18. V oc of the P3HTT-DPP-10%:PCDTBT:PC 61 BM ternary blend BHJ solar cells as a function of the amount of PCDTBT in the blends…………………………….24 Figure 1.19. Structures of P3HTT-DPP-10% (A), P3HT 75 -co-EHT 25 (1), P3HT 50 -co- 3HET 50 (2), and P3HTTDPP-MEO40% (B)……………………………………………..25 Figure 1.20. Open-circuit voltage (V oc ) of (a) ternary systems of P3HTTDPP- 10%:P3HT 75 -co-EHT 25 :PC 61 BM (A1) and P3HTT-DPP-MEO40%:P3HT 75 -co- EHT 25 :PC 61 BM (B1), (b) corresponding A2 and B2 systems (with P3HT 50 -co-3HET 50 ), and (c) ΔV between tunable and pinned V oc values as a function of random copolymer content……………………………………………………………………………………27 Figure 1.21. Ionization potentials of the blends of P3HTT-DPP-10%:P3HT 75 -co-EHT 25 as a function of the fraction of P3HT 75 -co-EHT 25 ……………………………………….28 Figure 1.22. Scheme of parallel-like mechanism in (a) D1:D2:A and (b) D:A1:A2 ternary xiv blend solar cells…………………………………………………………………………..30 Figure 1.23. Structures of TAZ, DTBT, DTffBT, and DTPyT………………………….31 Figure 1.24. Schematic representation of the charge transfer in a cascade model……...34 Figure 1.25. The EQE spectra of P3HT:PCBM (1:1) and ternary cells with concentrations of 10 wt%, 20 wt%, 30 wt% and 40 wt% Si-PCPDTBT in donors……...36 Figure 1.26. EQE values of P3HTT-DPP-10%:P3HT 75 -co-EHT 25 :PC 61 BM ternary blend BHJ solar cells with various ternary blend ratios: (i) 1:0:1.3 (red), (ii) 0.9:0.1:1.1 (green), (iii) 0.8:0.2:1.0 (blue), (iv) 0.7:0.3:1.0 (cyan), (v) 0.6:0.4:1.0 (magenta), (vi) 0.5:0.5:0.9 (wine-red), (vii) 0.4:0.6:0.9 (olive), (viii) 0.3:0.7:0.8 (dark-yellow), (ix) 0.2:0.8:0.8 (purple), (x) 0.1:0.9:0.9 (yellow), and (xi) 0:1:0.8 (black)………………………………37 Figure 1.27. UV-Vis absorption spectra of P3HT:PCBM:DPSQ films as functions of DPSQ concentration……………………………………………………………………...37 Figure 1.28. Structures of polymer donors and fullerene acceptor that were used for ternary blend solar cells………………………………………………………………….38 Figure 1.29. EQE values of (a) PBDTT-C:PBDTT-DPP:PC 71 BM and (b) PTB7:PBDTT- SeDPP:PC 71 BM ternary blend solar cells………………………………………………..39 Figure 1.30. Structures of BDT-3T-CNCOO, PBDTTPD-HT, and PC 71 BM…………...40 Figure 1.31. Absorption spectra of the active layer with BDT-3T-CNCOO ratio of 40%, small molecule-based binary OSCs (labeled as 100%), polymer-based binary OSCs (labeled as 0%), and corresponding EQE curves of the OSCs…………………………..40 Figure 1.32. (a) Chemical structures and corresponding energy levels of P3HT, ICBA xv and PC 61 BM and (b) V oc of the ternary blend solar cells………………………………...41 Figure 1.33. Chemical structures of P3HT 75 -co-EHT 25 , P3HTT-DPP-10%, and PC 61 BM………………………………………………………………………………..…42 Figure 1.34. V oc of the individually optimized ternary blend BHJ solar cells (squares), with overall polymer:PCBM ratio fixed at 1:1.0 (green triangles), and with overall polymer:PCBM ratio fixed at 1:1.1 (blue stars)………………………………………….43 Figure 1.35. Chemical structures of OXCMA, OXCBA, OXCTA, and P3HT………....44 Figure 1.36. V oc tuning in P3HT:OXCMA:OXCBA ternary blend solar cells………….44 Figure 1.37. Chemical structures of the AnE-PVab, the semi-crystalline version, on the left side and AnE-PVba, the amorphous version, on the right side………….…………..45 Figure 1.38. V oc tuning in AnE-PVba:AnE-PVab:PC 61 BM ternary blend BHJ solar cells………………………………………………………………………………………45 Figure 2.1. Absorption profiles of thin films of D/A alternating copolymers (a), random two-acceptor copolymers (b), and random three-acceptor copolymer (c)……………….66 Figure 2.2. External quantum efficiencies of the BHJ solar cells……………………….70 Figure 3.1. UV-Vis absorption spectra of polymers in thin films spin-coated from o- DCB: (i) P3HT (black, squares), (ii) P3HTT-DPP 10% (red, circles), (iii) P3HTT-DPP 20% (blue, upward triangles), (iv) P3HTT-DPP 30% (green, downward triangles), (v) P3HTT-DPP 40% (pink, diamonds)……………………………………………………..84 Figure 3.2. HOMO levels (blue line, triangles) and V oc (red line, squares) of the P3HT, P3HTT-DPP 10%, P3HTT-DPP 20%, P3HTT-DPP 30%, and P3HTT-DPP 40%...........85 xvi Figure 3.3. GIXRD patterns of (i) P3HT (black), (ii) P3HTT-DPP 10% (red), (iii) P3HTT-DPP 20% (blue), (iv) P3HTT-DPP 30% (green), (v) P3HTT-DPP 40% (pink)..86 Figure 3.4. External quantum efficiencies of the BHJ solar cells: (i) P3HTT-DPP 10% (black, squares), (ii) P3HTT-DPP 20% (red, circles), (iii) P3HTT-DPP 30% (blue, upward triangles), (iv) P3HTT-DPP 40% (green, downward triangles)………………...96 Figure 4.1. UV-Vis absorption spectra of the polymers in thin films spin-coated from o- DCB: (i) PCTPTI (DArP) (black, squares), (ii) PCTPTI (Suzuki) (red, circles), (iii) PFTPTI (DArP) (blue, upward triangles), (iv) PFTPTI (Suzuki) (green, downward triangles), (v) PPTPTI (DArP) (pink, diamonds)………………………………………110 Figure 4.2. External quantum efficiencies of the BHJ solar cells: (i) PFTPTI (Suzuki) (black, squares), (ii) PFTPTI (DArP) (red, circles), (iii) PPTPTI (DArP) (blue, upward triangles)………………………………………………………………………………..113 Figure A1.1. 1 H NMR spectrum of PCDTBT in C 2 D 2 Cl 4 ……………………………..132 Figure A1.2. 1 H NMR spectrum of PCBTDPP in CDCl 3 ……………………………...132 Figure A1.3. 1 H NMR spectrum of PCTPTI in C 2 D 2 Cl 4 ………………………………133 Figure A1.4. 1 H NMR spectrum of PCDTBT-DPP in CDCl 3 …………………………133 Figure A1.5. 1 H NMR spectrum of PCDTBT-TPTI in C 2 D 2 Cl 4 ………………………134 Figure A1.6. 1 H NMR spectrum of PCBTDPP-TPTI in CDCl 3 ………………………134 Figure A1.7. 1 H NMR spectrum of PCDTBT-TPTI-DPP in C 2 D 2 Cl 4 …………………134 xvii Figure A1.8. CV traces for the oxidation of PCDTBT vs. Fc/Fc + ……………………..135 Figure A1.9. CV traces for the oxidation of PCBTDPP vs. Fc/Fc + ……………………135 Figure A1.10. CV traces for the oxidation of PCTPTI vs. Fc/Fc + ……………………..135 Figure A1.11. CV traces for the oxidation of PCDTBT-DPP vs. Fc/Fc + ……………...136 Figure A1.12. CV traces for the oxidation of PCDTBT-TPTI vs. Fc/Fc + ……………..136 Figure A1.13. CV traces for the oxidation of PCTPTI-DPP vs. Fc/Fc + ……………….136 Figure A1.14. CV traces for the oxidation of PCDTBT-TPTI-DPP vs. Fc/Fc + …….....137 Figure A1.15. J-V curves of the BHJ solar cells spin-coated from o-dichlorobenzene and placed to N 2 cabinet for 30 min………………………………………………………...142 Figure A2.1. 1 H NMR spectrum of P3HTT-DPP 10% in C 2 D 2 Cl 4 …………………….147 Figure A2.2. 1 H NMR spectrum of P3HTT-DPP 20% in C 2 D 2 Cl 4 .................................147 Figure A2.3. 1 H NMR spectrum of P3HTT-DPP 30% in C 2 D 2 Cl 4 …………………….148 Figure A2.4. 1 H NMR spectrum of P3HTT-DPP 40% in C 2 D 2 Cl 4 ……………………148 Figure A2.5. CV traces for the oxidation of P3HTT-DPP 10% vs. Fc/Fc + ……………149 Figure A2.6. CV traces for the oxidation of P3HTT-DPP 20% vs. Fc/Fc + ……………149 Figure A2.7. CV traces for the oxidation of P3HTT-DPP 30% vs. Fc/Fc + ……………150 xviii Figure A2.8. CV traces for the oxidation of P3HTT-DPP 40% vs. Fc/Fc + ……………150 Figure A2.9. CV traces for the oxidation of P3HT vs. Fc/Fc + …………………………151 Figure A2.10. DSC traces of a) P3HT, b) P3HTT-DPP 10%, c) P3HTT-DPP 20%, d) P3HTT-DPP 30%, e) P3HTT-DPP 40%.........................................................................152 Figure A2.11. J-V curves of the BHJ solar cells spin-coated from o-dichlorobenzene and placed to N 2 cabinet for 30 min………………………………………………………...155 Figure A3.1. 1 H NMR spectrum of PFTPTI (Suzuki) in C 2 D 2 Cl 4 ……………………..162 Figure A3.2. 1 H NMR spectrum of PFTPTI (DArP) in C 2 D 2 Cl 4 ………………………163 Figure A3.3. 1 H NMR spectrum of PPTPTI (DArP) in C 2 D 2 Cl 4 ………………………163 Figure A3.4. CV traces for the oxidation of PFTPTI (DArP) vs. Fc/Fc + ……………...164 Figure A3.5. CV traces for the oxidation of PFTPTI (Suzuki) vs. Fc/Fc + ……………..164 Figure A3.6. CV traces for the oxidation of PPTPTI (DArP) vs. Fc/Fc + ……………...165 xix LIST OF SCHEMES Scheme 2.1. Chemical structures of PCDTBT, PCBTDPP, and TPTI………………….59 Scheme 2.2. Structures of the D/A alternating and random multi-acceptor copolymers………………………………………………………………………………61 Scheme 3.1. Synthesis and structures of P3HTT-DPP 10%, P3HTT-DPP 20%, P3HTT- DPP 30%, and P3HTT-DPP 40%......................................................................................81 Scheme 4.1. Chemical structures of TPTI and PCTPTI……………………………….103 Scheme 4.2. Syntheses of the WBG conjugated polymers……………….……………107 Scheme 4.3. Chemical structure of PBPTPTI………………………………………….108 Scheme A1.1. Suzuki polymerizations of the D/A alternating and random multi-acceptor copolymers……………………………………………………………………………...129 Scheme A2.1. Synthesis of the Semi-Random copolymers…………….……………...144 Scheme A3.1. Synthesis of the wide band gap polymers…………...………………….160 xx ABSTRACT World energy use is predicted to grow by 48% between 2010 and 2040 and current trends in energy supply are economically and environmentally unsustainable. Consequently, there is a growing realization of the necessity for clean and renewable energy sources. Currently, renewable energy is the world’s fastest growing energy source, increasing by 2.6% per year. Among other renewable energy sources, solar energy has attracted increased attention. It can provide enough energy to the surface of the earth in a single hour to meet the energy demand of the world population for an entire year. Organic solar cells have attracted significant attention due to their low weight, flexibility, low cost and semitransparency. They are based on either conjugated polymers, small molecules, or both. This dissertation is mainly focused on different polymer structures to be used in a class of organic solar cells; ternary blend bulk heterojunction solar cells. In ternary blend bulk heterojunction solar cells, active layer is composed of three components, providing a promising route to high solar cell efficiencies. A comprehensive overview of this class of solar cells is presented in Chapter 1. In Chapter 2, a family of donor/acceptor (D/A) alternating copolymers and random two-acceptor and three-acceptor copolymers are synthesized via Suzuki polymerization based on heptadecan-9-yl substituted carbazole as a donor and 4,7-Bis(5- bromothiophene-2-yl)benzo[c][1,2,5]thiadiazole (DTBT), 2,5-Diethylhexyl-3,6-bis(5- bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione (DPP) and 2,8-Dibromo-4,10- bis(2-ethylhexyl)thieno[2’,3’:5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)- dione (TPTI) as acceptors. For the first time, a relatively new electron-deficient TPTI unit xxi is used as an acceptor in carbazole-based conjugated polymers. Introduction of the electron-deficient TPTI unit into the polymer backbone increases the open-circuit voltage (V oc ) of the resulting polymer solar cells up to 0.96 V. PCTPTI and PCDTBT-TPTI exhibit external quantum efficiencies (EQE) up to 75%. All random two-acceptor copolymers show broadened absorption profiles compared to the D/A alternating analogues. In order to further improve the light absorption, a random three-acceptor copolymer is synthesized for the first time, resulting in the broadest absorption in the range of 350-750 nm. Highest occupied molecular orbital (HOMO) energies and V oc values of the resulting polymers are successfully tuned by introducing different monomer units into the polymer backbone in different ratios. These results indicate that TPTI is a promising acceptor unit for efficient conjugated polymers and that the random copolymer approach is a successful tool for fine tuning of polymer properties. In Chapter 3, a family of diketopyrrolopyrrole (DPP)-incorporated P3HT based semi-random copolymers is synthesized and their optical, electronic and photovoltaic properties are investigated. For the first time, the influence of acceptor content on semi- random copolymers is explored in the broad range of 10% to 40% acceptor. A mixture of DPP acceptor units with different side chains [ethylhexyl (EH) and decyltetradecyl (DTD)] is incorporated into each copolymer in order to improve solubility and film quality. Increased DPP content in the polymer backbone results in broadened absorption between 350 - 900 nm, resulting in a monotonic decrease in optical band gap (E g ) of the polymers from 1.49 eV to 1.37 eV. Highest occupied molecular orbital (HOMO) energy levels show an increase from 10% DPP to 20%-30% DPP, while decreasing for 40% DPP. V oc values follow a consistent trend with HOMO energy levels. Semi-random xxii copolymers show significantly improved photovoltaic properties compared to P3HT. Bulk heterojunction (BHJ) solar cells fabricated from the semi-random copolymers blended with PC 61 BM exhibit high short-circuit current densities (J sc ) up to 10.29 mA/cm 2 and efficiencies up to 4.43%. A new method of methanol treatment is developed and applied to the semi-random copolymers resulting in high fill factors (FF) approaching 0.70 under ambient conditions. In Chapter 4, a set of wide band gap (WBG) perfectly alternating copolymers is synthesized and their optical, electronic and photovoltaic properties are investigated. A relatively new electron-deficient 2,8-Dibromo-4,10-bis(2- ethylhexyl)thieno[2’,3’:5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione (TPTI) unit is used as an acceptor in all copolymers while 9,9-Dioctyl-9H-fluorene-2,7- diboronic acid bis(pinacol) ester (fluorene) and 1,4-Dibromo-2,5-bis(2- ethylhexyloxy)benzene (phenylene) are used as donors. Suzuki Polymerization and Direct Arylation Polymerization (DArP) are both performed for the synthesis of the polymers, and their properties are compared to the recently reported PCTPTI polymers. Resulting copolymers show significantly wide optical band gaps between 2.09-2.30 eV, absorbing light in the range of 375-650 nm. Highest occupied molecular orbital (HOMO) energy levels of the polymers vary between -5.57 eV to -5.63 eV, while open-circuit voltage (V oc ) values change between 0.68 V and 0.90 V. These results indicate that TPTI is a promising acceptor unit for WBG conjugated polymers and that the DArP is an effective alternative tool to Suzuki polymerization. 1 CHAPTER 1: TERNARY BLEND BULK HETEROJUNCTION SOLAR CELLS 1.1 Introduction According to the U.S. Energy Information Administration, world energy use will grow by 48% between 2010 and 2040. 1 With the world energy demand increasing, current trends in energy supply are economically and environmentally unsustainable. Consequently, there is a growing realization of the necessity for clean and renewable energy sources such as hydropower, geothermal, wind and solar. 1, 2 As of 2016, renewable energy is the world’s fastest growing energy source, increasing by 2.6% per year. 1 Among other renewable energy sources, solar energy has attracted increased attention because the sun is the planet’s most abundant potential source of energy. It can provide enough energy to the surface of the earth in a single hour to meet the energy demand of the world population for an entire year. 3, 4 It is a clean energy source and has no impact on the global climate. Solar cells are used to convert sunlight into electricity. They have been categorized into different classes according to the materials including inorganic solar cells, dye sensitized solar cells, hybrid solar cells and organic solar cells. 5, 6, 7 Silicon based inorganic solar cells are the oldest and most dominant branch of solar cells in the energy market. 8 Compared to the other photovoltaic technologies, silicon solar cells are very well studied and understood. Ultimately, 60 years of silicon solar cell research has resulted a 25% power conversion efficiency. Recent developments in inorganic solar cells showed CdTe, CIGS and GaAs are strong competitors for silicon solar cells with their efficiency around 20%. 9 Nonetheless, silicon solar cells are dominant in the solar cell 2 market. One main limitation of silicon solar cells is the production cost. For next generation silicon solar cells, cost effective manufacturing technologies should be the primary interest. 10 Dye sensitized solar cells (DSSC) has been studied in the last two decades. 11 Since the first proposed concept of DSSC in 1991, they have shown high efficiencies up to 13%. 12,13,14,15,16 However, there are several disadvantages preventing them from being widely used. First of all, the active layer of the DSSC is mostly high cost materials making the overall cell significantly expensive. 17 Moreover, temperature-sensitive liquid electrolytes limit the applications of the solar cell. 18 Additionally, 13.4% of estimated maximum efficiency of DSSCs with a high cost of manufacturing make them economically less attractive. 19 Hybrid organic-inorganic solar cells combine organic nanoparticles (mostly conjugated polymers) and inorganic nanoparticles in order to possess the attractive features of both organic and inorganic materials. 20,21,22 Organic materials in the solar cell provide tunable absorption and inorganic materials increase the stability and charge transfer process in the device. 23,24,25,26 Drawbacks of these materials are increased density of trap states associated with the chemistry of the nanoparticle surface and limited control over the donor-acceptor morphology. 25 As a result, efficiency of these solar cells are limited to 3- 5.5%. 27,28 Another promising emerging technology is based on perovskites, which have reached efficiencies above 20%. 29 Much is still not known about the stability of these solar cells, despite their extensive study. 30 3 Solution processable thin film organic cells have attracted significant attention as a promising competitor to silicon solar cells. 31, 32, 33 Several properties such as low weight, flexibility, low cost and semitransparency make them highly attractive in the solar cell market. 34 Their low cost fabrication such as spin coating, spray deposition and printing make them desirable photovoltaics compared to inorganic analogues. 35 Organic solar cells are based on either conjugated polymers, small molecules, or both. 34 A band gap (E g ) of 1 to 3 eV of a typical organic molecule or a polymer results in a semiconducting behavior and visible and near-IR light absorption. One of the major advantages of organic photovoltaics is that their optical and electronic properties can be tuned via modification of the chemical structures accordingly. 34 Many studies related to structure-property relationship contributed to remarkable development of organic solar cells in the last 15 years. 36 Today, efficiencies are reaching 12% making the organic photovoltaics a potentially important part of the energy market. 37 After the first report of an organic photovoltaic device by Tang in 1986, with the efficiency of 1%, the Heeger and Wudl groups observed ultrafast electron transfer from the conjugated polymer poly[2-methoxy- 5-(2-ethylhexyloxy)]-1,4-phenylenevinylene (MEH-PPV) to fullerene (C60), suggesting the use of conjugated polymers as electron donors and fullerene derivatives as electron acceptors in polymer-based organic solar cells. 32,38,39 This study opened a new door for bulk heterojunction (BHJ) solar cells to address the limited exciton diffusion length in organic solar cells that has been a major problem for the previous organic solar cells. 40 Exciton diffusion length of most organic compounds is on the order of 10 nm, which is significantly lower than the optical absorption length of 100 nm. 41, 42 In order to overcome this trade-off between the optical 4 absorption and exciton diffusion, the BHJ architecture has been utilized with a blend of donor and acceptor components with a bicontinuous phase separation in the active layer. 23, 43 As such, it is possible to maximize the all-important interfacial area between the donor and acceptor. This device architecture is described as an interpenetrating network of donor and acceptor compounds that is sandwiched between the two different electrodes (Figure 1.1.). 23 Figure 1.1. Bulk heterojunction solar cell device architecture. Increased interfacial area improves the exciton diffusion process resulting in efficient solar cells. 43 The active layers of BHJ solar cells are processed in solution in a single step that requires minimum energy and can be done at ambient temperatures and pressures. This feature makes BHJ solar cells eligible for low cost mass production. 44 Critical topics including photocurrent generation, stability, device physics and BHJ morphology have been the focus of extensive work. 45,46,47 Even though organic solar cells have many advantages compared to inorganic solar cells, their low power conversion efficiency (PCE) compared to inorganic cells makes them less attractive in the photovoltaic market. An estimated efficiency limit for 5 organic solar cells is calculated as 12% which is significantly lower than the current silicon solar cells. 48 Efficiency of the solar cell (η = (Jsc × Voc × FF)/P in ) is directly proportional to the J sc and V oc of the cell, which are opposing quantities. J sc is proportional to the absorption breadth and intensity of the donor polymer as well as active layer thickness. A high J sc value is obtained with broader wavelength of light absorption, which is possible with smaller band gap donors that have a shallow HOMO level. Conversely, a high V oc value is obtained with a deep HOMO level donor because V oc is the energetic difference between the HOMO of the donor polymer and LUMO of the acceptor fullerene (Figure 1.2.). 49 In order to overcome this efficiency limit, alternative strategies to the simple binary blend (donor/acceptor blend) BHJ solar cells have been studied. Figure 1.2. Representation of organic donor:acceptor solar cell energy diagram. Ternary blend BHJ solar cells have attracted significant attention recently due to their potential to increase the efficiency of binary BHJ solar cells. 50 Their active layer is composed of two donors and one acceptor (D1:D2:A) or one donor and two acceptors (D:A1:A2). 51,52 One of the most important advantage of ternary blend BHJ solar cells is 6 that their active layer is processed in solution in a single step keeping the simplicity of the binary blend BHJ solar cells. 51 The device architecture is the same as binary blend BHJ solar cells (Figure 1.3.). Addition of third component provides tunable properties of the active layer and helps to improve the overall efficiency. Increase in short-circuit current density (J sc ) via broadening the absorption and tunable composition-dependent open circuit voltage (V oc ) are the two main factors that result in the improved efficiencies in ternary blend BHJ solar cells. 53,54 There are several concepts that need to be studied further including the operating mechanism and structure-function relationship in the active layer. 55 Nonetheless, ternary blend BHJ solar cells are promising candidates for high efficiency solar cells. Figure 1.3. Device architecture of a ternary blend BHJ solar cell. 1.2. State-of-the-art Ternary Blend Systems There are many examples of ternary blend bulk heterojunction solar cells that exhibited improved device properties compared to their corresponding binary solar cells. One of the first examples of improved solar cell efficiency via the ternary approach has been demonstrated by Khlyabich et al. in 2012. 52 Two P3HT analogues, high-band-gap poly(3-hexylthiophene-co-3-(2- ethylhexyl)thiophene) (P3HT 75 -co-EHT 25 ) and low- band- gap poly(3-hexylthiophene−thiophene−diketopyrrolopyrrole) (P3HTT-DPP-10%) 7 were used as donor polymers and phenyl-C 61 -butyric acid methyl ester (PC 61 BM) was used as an acceptor (Figure 1.4.). Increased composition of the high-band-gap polymer (P3HT 75 -co-EHT 25 ) in ternary blend resulted in an increased V oc , showing a composition tunable V oc (Table 1.1.). In addition to tunable V oc values, complementary absorption profiles of the polymers increased the J sc values of the device at certain compositions resulting higher J sc than the corresponding binary blends. As a result, overall device efficiency increased by using the ternary approach. Table 1.1. Photovoltaic properties of ternary blend bulk heterojunction solar cells based on P3HTT-DPP-10%:P3HT 75 -co-EHT 25 :PC 61 BM. 52 8 Figure 1.4. Structures, HOMO energies and absorption profiles of (P3HT 75 -co-EHT 25 ), (P3HTT-DPP-10%) and PC 61 BM. 52 (Reprinted with permission from Ref. 52. Copyright 2012, American Chemical Society.) Another high efficiency two donor one acceptor ternary blend solar cell was reported by Lu et al. 56 Poly-3-oxothieno[3,4-d]isothiazole-1,1- dioxide/benzodithiophene (PID2) and poly[4,8-bis(5-(2- ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene- co-3-fluorothieno[3,4-b]thiophene-2 carboxylate] (PTB7-Th) were used as polymer donors while phenyl C 71 butyric acid methyl ester (PC 71 BM) was used as an acceptor (Figure 1.5.). A PCE of 9.20% was achieved for the ternary blend solar cell being higher than the corresponding binary blend solar cells (Table 1.2.). Improved efficiency was attributed to the enhanced light absorption, improved hole mobility and reduced trap- 9 assisted recombination resulting from the addition of PID2 to the PTB7Th:PC 71 BM binary blend. Figure 1.5. Structures of PID2, PTB7-Th and PC 71 BM. 56 Table 1.2. Photovoltaic properties of the ternary blend bulk heterojunction solar cells of PTB7-Th:PID2:PC 71 BM. 56 Addition of a small molecule to a polymer:fullerene binary solar cell is another method to make ternary blend bulk heterojunction solar cells. Zhang et al. has reported a high-efficiency ternary blend solar cell including PTB7-Th as a donor polymer, PC 71 BM as an acceptor, and 7,7-(4,4-bis(2-Ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene- 2,6- diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo-[c] [1,2,5]thiadiazole) (p- DTS(FBTTH 2 ) 2 ) as a small molecule (Figure 1.6.). 57 The small molecule was chosen due 10 to its high crystallinity. The donor polymer and small molecule were miscible within each other, resulting in alloy formation in the active layer with enhanced crystallinity due to the highly crystalline small molecule in the blend. The effect of the small molecule resulted in increased PCE as high as 10.5% in optimized conditions (Table 1.3.). Figure 1.6. Structures of p-DTS(FBTTH 2 ) 2 , PTB7-Th, and PC 71 BM. 57 Table 1.3. Photovoltaic properties of the ternary blend bulk heterojunction solar cells of PTB7-Th:P-DTS(FBTTH 2 ):PC 71 BM. 57 11 One of the most recent studies from Gasparini et al. has demonstrated a champion ternary blend device efficiency of 11.03% (Table 1.4.). 58 The novelty of their study was the fabrication of ternary blend solar cells from uncommonly thick active layers of around 300nm. For the active layer of the device, PBTZT-stat-BDTT-8 was used as a donor and PC 70 BM as an acceptor, with the use of NIR-absorber polymer PTB7-Th as a sensitizer (Figure 1.7). Devices showed internal quantum efficiencies higher than 90% even though the active layer thicknesses exceeded 300nm in some devices. Efficient ternary blend solar cells were supported by solar modules with three solar cells connected in series, which gave 8.2% and 6.8% efficiencies on glass and flexible substrate, respectively. Figure 1.7. Structures of PBTZT-STAT-BDTT-8, PTB7-Th, and PC 70 BM. 58 12 Table 1.4. Photovoltaic properties of the ternary blend bulk heterojunction solar cells of PBTZT-STAT-BDTT-8:PTB7-Th:PC 70 BM. a Current density calculated over the total area. 58 There are more examples of improved photovoltaic properties via ternary approach. In recent years, P3HT-based and PTB7-based ternary blend solar cells have been studied more commonly, as can be seen from the previous examples. In P3HT- based ternary blend solar cells; An et al. 59 , Bi et al. 60 , Keawsongsaeng et al. 61 , Wang et al. 62 , and Sun et al. 63 reported improved solar cell efficiencies between 10% to 25% compared to their corresponding binary solar cells. When PTB7-based ternary blends are considered; Nian et al. 64 , Bharti et al. 65 , Goh et al. 66 and Gupta et al. 67 have reported ternary blend solar cells with improved efficiencies of around 20% relative to their binary blends. 1.3 Proposed Mechanisms for Ternary Blend Solar Cell Operations There are many research groups today studying ternary blend BHJ solar cells focusing on donor-acceptor interactions in the active layer and the morphology of the 13 bulk. While numerous systems have shown enhancement of efficiency with ternary blends, there is still some debate as to how the third component influences device performance. Three models have emerged as proposed explanations: the organic alloy model, the parallel-like model, and the cascade model. While a third component generally provides broadened absorption and thus higher J sc, a composition tunable V oc also provides a route to higher V oc at intermediate compositions. Any complete model must explain these two phenomena. A summary of these models is presented in the following section. 1.3.1 Organic alloy model The organic alloy model was proposed after the composition-dependent tunable V oc was obtained in the ternary blend BHJ solar cells. 68 D1:D2:A or D: A1:A2 ternary blends exhibit a tunable V oc depending on the composition of each component in the active layer. 51,52 While this is an expected behavior in inorganic alloys, it is extraordinary for an organic molecular mixture where the different donor/acceptor compounds have distinct energy levels. 68 In fact, conjugated polymers that are used in solar cells generally bear an absorption profile that is the sum of two individual polymers’ absorption profiles instead of an average. This feature is significantly different than an inorganic alloy. Nonetheless, tunable V oc in ternary blends was attributed to an alloy formation in an organic blend, as is described in detail below. 53,69 The most significant feature of the organic alloy model is that electronically similar components of the active layer (either donors or acceptors) form an electronic alloy with unique and composition averaged frontier orbital energies depending on the composition of these two components (Figure 1.8). 55 In a BHJ solar cell the V oc is 14 proportional to the difference between the donor HOMO and LUMO acceptor. More specifically the V oc is proportional to the energy of the charge transfer state (CT state) which is envisioned as a bound state initially formed in the charge transfer process in which a hole in donor HOMO is bound to an electron in the acceptor LUMO. This is a delocalized intermolecular state that exists at the donor-acceptor interface. The organic alloy model postulates that in a highly miscible blend of two donors in D1:D2:A system or two acceptor in a D:A1:A2 system, the HOMO level of the donor alloy or LUMO level of the acceptor alloy exhibits a composition averaged value (Figure 1.9. and Figure 1.10., respectively). As such the energy of the CT state (and thus the V oc ) will also be composition averaged. Figure 1.8. Scheme of the organic alloy model in ternary blend solar cells (a) D1:D2:A, (b) D:A1:A2. 15 Figure 1.9. Representation of the composition averaged HOMO level of the donor alloy of different donor:donor ratios. Figure 1.10. Representation of the composition averaged LUMO level of the acceptor alloy of different acceptor:acceptor ratios. It follows that a requirement for alloy formation and tunable V oc is an intimate mixing of synergistic components. This model also offers an explanation for the ability to simultaneously tune J sc and V oc . While the V oc is based on a composition averaged CT 16 state energy, the J sc is based on the additive nature of the absorption spectra. This dichotomy is due to the differing spacial extents of the CT state and dissociated charge carriers as compared to the molecular exciton. The CT state is by nature a delocalized intermolecular species as are the free charge carriers in these systems. A recent study has shown that in D:A1:A:2 systems the electron wavefunction can extend over 30 fullerene molecules. 70 Thus the composition averaged HOMO and LUMO levels in D1:D2:A and D:A1:A2 systems, respectively are expected to result in composition averaged V oc in an alloy (highly miscible systems). In contrast, the exciton is a molecular species with a significantly reduced spatial extent that is not expected to take on a composition averaged character even in totally mixed systems. As such the absorption is expected to retain the characteristic of both synergistic components and be additive, resulting in the possibility of an increased J sc relative to the corresponding binary blends. The organic alloy model is the only model that can explain both J sc increase and V oc tuning in ternary blends by using the difference between the nature of the exciton and CT state. Low band gap donor components used in ternary blends enhance the absorption by more effectively blanketing the solar spectrum. Higher bandgap components used in the ternary blend push the LUMO states of the donor mixture to a higher energy level as a result of the organic alloy formation, resulting in large V oc and increasing the band offset at the interface resulting in a more efficient exciton dissociation at the interface. Composition-dependent V oc change is explained by the continuous change of the HOMO and LUMO levels of the two donor or two acceptor components in the ternary, consistent with an alloy formation. 17 In order to understand the origin of tunable V oc , Street et al. studied two ternary blend systems including P3HT:PC 61 BM:ICBA (D:A1:A2) (Figure 1.11) and P3HTT- DPP-10%:P3HT 75 -co-EHT 25 :PC 61 BM (D1:D2:A) (Figure 1.13.). Photocurrent spectral response (PSR) measurements were performed by measuring the optical absorption of the heterojunction interface (CT state energy). Figure 1.12. shows that CT state energy increases by increasing the high-LUMO ICBA acceptor while Figure 1.14 shows an increase in the CT state energy by increasing the deep-HOMO P3HT 75 -co-EHT 25 fraction. These results indicate that donor-acceptor interface and corresponding interface band gap (CT state) exhibit a material-averaged electronic structure as a result of the more delocalized one-electron states. This results in a V oc showing the average composition of the interface that is consistent with the V oc measurements in ternary blend BHJ solar cells. In addition to composition dependent CT state and V oc , it was reported that the V oc from quasi-Fermi level (qV oc ) in BHJ solar cells are 0.55 eV smaller than the CT state energy which is consistent with the same energy difference in binary blend BHJ solar cells reflecting less energy loss under the same voltage. 68 All these results show that ternary blend active layer is forming an organic alloy from an electronic standpoint. Figure 1.11. Structures of P3HT, PC 61 BM, and ICBA. 18 Figure 1.12. Photocurrent spectral response (PSR) and energy of CT state compared to the values of the V oc data for the P3HT:PC 61 BM:ICBA (D:A1:A2). 68 (Reprinted with permission from Ref. 68. Copyright 2013, American Chemical Society.) Figure 1.13. Structures of P3HT 75 -co-EHT 25 , P3HTT-DPP-10%, and PC 61 BM. 19 Figure 1.14. PSR and energy of CT state compared to the values of the V oc data for the P3HTT-DPP-10%:P3HT 75 -co-EHT 25 :PC 61 BM (D1:D2:A). 68 (Reprinted with permission from Ref. 68. Copyright 2013, American Chemical Society.) In ternary blend BHJ solar cells, donor-acceptor components of the active layer are chosen according to their absorption profiles that are complementary with each other. An interesting feature of the ternary blend is that even though V oc of the active layer is composition-dependent and tunable, individual components of the ternary blend keep their individual absorption profiles as described above. Street et al. studied P3HT:PC 61 BM:ICBA (Figure 1.11.) ternary blend and P3HTTDPP-10%:P3HT 75 -co- EHT 25 :PC 61 BM (Figure 1.13.) ternary blend in order to further investigate the alloy formation in the active layer, as mentioned before. The P3HT:PC 61 BM:ICBA ternary system showed that the absorption of the acceptor blend is a weighted sum of the two absorptions of each individual acceptor (Figure 1.15.). P3HTTDPP-10%:P3HT 75 -co- EHT 25 :PC 61 BM ternary blend also showed the same behavior supporting that the optical transitions in ternary blends keep the properties of individual components while CT state 20 energy of the ternary blend is the average of the each components in the active layer (Figure 1.16.). Figure 1.15. Expanded plot of the peaks for the P3HT:PC 61 BM:ICBA ternary system near 1.7 eV with the background subtracted. The peak centered above 1.7 eV corresponds to PCBM absorption, and the peak centered below 1.7 eV corresponds to ICBA. 68 (Reprinted with permission from Ref. 68. Copyright 2013, American Chemical Society.) Figure 1.16. High-energy PSR data of P3HTT-DPP-10%:P3HT 75 -co-EHT 25 :PC 61 BM showing the exciton peaks from the donor mixture. 68 (Reprinted with permission from Ref. 68. Copyright 2013, American Chemical Society.) 21 The origin of the organic alloy model has been studied in several ternary blend systems in order to understand what makes the synergistic components mix and form an alloy. Khlyabich et al. has carefully investigated two different ternary blend systems including D:A1:A2 system of P3HT, PC 61 BM, and ICBA, and the D1:D2:A system of P3HTT-DPP-10%, P3HT 75 -co-EHT 25 and PC 61 BM. 52 As discussed previously, both systems nominally formed organic alloys resulting composition-dependent tunable V oc . Both ternary systems had structurally very similar synergistic components such as PC 61 BM and ICBA in one system, and P3HT derivatives in the other system. These chemical similarities support the miscibility of the synergistic components and alloy formation. In another study, Angmo et al. investigated the composition dependent properties for the P3HT:PC 61 BM:ICBA ternary blends by using cross-polarization magic- angle-spinning solid-state NMR techniques. 71 It was reported that when the blended fullerenes are mixed with a third polymer component, the system exhibits pseudo-binary phase behavior instead of the expected ternary phase behavior. This finding offered more strong evidence of alloy formation between the synergistic components in ternary blends. Conceptually, several other aspects support the miscibility of synergistic polymer components in the described system. Surface energy is a phenomenon affecting the miscibility in blends. 72 In the study mentioned above, surface energies of the synergistic components were reported as 27.6 mN/m for PC 61 BM and 24.9 mN/m for ICBA, while 19.9 mN/m for P3HTT-DPP-10% and 22.1 mN/m for P3HT 75 -co-EHT 25 . Close surface energies of the synergistic components make the miscibility more favorable and support the alloy formation. In addition to surface energy, the random copolymer effect is also thought to be critical for alloy formation as the random incorporation of common 22 monomers in the synergistic components of the ternary blend. 73,74 Overall, miscibility between the synergistic components plays a crucial role for the alloy formation and several parameters including similarities in structures, surface energies and the random copolymer effect affect the miscibility significantly. Organic alloy formation was further investigated by comparison with a different ternary blend system in order to test the hypothesis that miscibility between the synergistic components plays a crucial role in alloy formation. Khlyabich et al. has studied a D1:D2:A system of poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2- thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), P3HTT-DPP-10% and PC 61 BM (Figure 1.17.). 53 The idea in this study was to pick two significantly different polymers from each other in the case of chemical structure, surface energy and crystallinity and to see if organic alloy formation would be possible. Structurally, PCDTBT and P3HTT-DPP-10% are significantly different from each other and they do not have a common monomer in their backbone to engender the random copolymer effect. Also PCDTBT is an amorphous polymer while P3HTT-DPP-10% is semicrystalline. In addition to these differences, PCDTBT has a significantly higher surface energy compared to P3HTT-DPP-10% being 29.5 mN/m and 19.9 mN/m respectively. These differences make the miscibility between the polymers less favorable. Photovoltaic properties of the ternary blends supported this hypothesis by showing pinned V oc in the solar cells limited to the higher lying HOMO of P3HTT-DPP-10% up to 95/5 ratio of PCDTBT:P3HTT-DPP-10% as seen in Table 1.5. and Figure 1.18.. This study showed that surface energy is a strong predictor of the miscibility in ternary blends and the resulting organic alloys. 23 Figure 1.17. Structures of PCDTBT, P3HTT-DPP-10%, and PC 61 BM. Table 1.5. Photovoltaic properties of P3HTT-DPP- 10%:PCDTBT:PC 61 BM ternary blend BHJ solar cells at different polymer ratios. 53 24 Figure 1.18. V oc of the P3HTT-DPP-10%:PCDTBT:PC 61 BM ternary blend BHJ solar cells as a function of the amount of PCDTBT in the blends. 53 (Reprinted with permission from Ref. 53. Copyright 2014, American Chemical Society.) The influence of surface energy on organic alloy formation in ternary blends was further studied by Gobalasingham et al. based on D1:D2:A system. 69 Four different ternary blends were studied including P3HT 75 -co-EHT 25 , poly(3-hexylthiophene-co-3- hexylesterthiophene) (P3HT 50 - co-3HET 50 ), P3HT-DPP-10%, an analog of P3HTTDPP- 10% with 40% of 3-hexylthiophene exchanged for 2-(2- methoxyethoxy)ethylthiophen-2- yl (3MEO-T) (featuring an electronically decoupled oligoether side-chain), referred to as P3HTT-DPP-MEO40% as donor polymers and PC 61 BM as acceptor (Figure 1.19.). All the polymers are rich in rr-P3HT content and perform well with PC 61 BM in binary solar cells. The polymer surface energies are almost identical (21-22nm) excluding P3HTT- DPP-MEO40% which exhibits a higher surface energy of 26 mM/m due to oxygen-rich oligo-ether side chains. It was reported that the two analogous polymers (P3HTT-DPP- 10% and P3HTT-DPP-MEO40%) that individually have been shown perform and mix well with fullerene, exhibit distinct behaviors in ternary blend solar cells due to their surface energy differences. 75 The relatively different surface energy of P3HTT-DPP- 25 MEO40% resulted in a pinned V oc in two different sets of ternary blend devices with P3HT 75 -co-EHT 25 and P3HT 50 - co-3HET 50 while P3HTT-DPP-10% exhibited a tunable V oc in the same sets of solar cells (Figure 1.20.). Figure 1.19. Structures of P3HTT-DPP-10% (A), P3HT 75 -co-EHT 25 (1), P3HT 50 -co- 3HET 50 (2), and P3HTTDPP-MEO40% (B). 69 (Reprinted with permission from Ref. 69. Copyright 2016, American Chemical Society.) Four sets of ternary blends were analyzed in this study. In the first set of ternary blends, P3HTT-DPP-10% (A) was mixed with P3HT 75 -co-EHT 25 (1) and PC 61 BM. Due to the similar surface energies of A and 1, ternary blend system demonstrated composition-dependent V oc values consistent with organic alloy formation (Figure 1.20. (a)). In the second set of ternary blends, A was mixed with P3HT 50 -co-HET 50 (2) and PC 61 BM. As seen in the previous system, the A2 blend also exhibited a tunable V oc due to the similar surface energies of the polymers (Figure 1.20. (b)). However, in the third set of ternary blends with P3HTTDPP-MEO40% (B), 1, and PC 61 BM, a pinned V oc was 26 observed for all B:1 compositions (Figure 1.20. (a)). Consistent with the previous studies, different surface energies of B and 1 resulted in poor miscibility between the polymers and did not exhibit a tunable V oc . 53 B was also mixed with 2 showing a pinned V oc similar to the previous set and attributed to differences in surface energy (Figure 1.20. (b)). Another key point of this study was showing the use of ternary blends for improved V oc values. Figure 1.20. showed that two miscible polymers in ternary blends significantly increase the overall V oc of the solar cell compared to immiscible polymers with different surface energies. ΔV oc demonstrated the increase in the V oc values clearly. This study suggested that even though the chemical structures, crystallinity, and device performances of the polymers are similar, the surface energy is a critical parameter predicting organic alloy formation and increasing the V oc in ternary blends. 27 Figure 1.20. Open-circuit voltage (V oc ) of (a) ternary systems of P3HTTDPP- 10%:P3HT 75 -co-EHT 25 :PC 61 BM (A1) and P3HTT-DPP-MEO40%:P3HT 75 -co- EHT 25 :PC 61 BM (B1), (b) corresponding A2 and B2 systems (with P3HT 50 -co-3HET 50 ), and (c) ΔV between tunable and pinned V oc values as a function of random copolymer content. 69 (Reprinted with permission from Ref. 69. Copyright 2016, American Chemical Society.) Another study performed on three different D1:D2:A ternary blends of P3HTT- DPP-10%:P3HT 75 -co-EHT 25 :PC 61 BM, P3HTT-DPP-10%:P3HTT-TPD-10% and PCDTBT:P3HTT-DPP-10%:PC 61 BM provided more fundamental evidence of alloy formation and its electronic consequences. Specifically, Grazing-incidence X-ray diffraction (GIXD) measurements were used to show the intimate interaction of miscible polymers. 76 The morphology of the different ternary blends, which exhibit tunable V oc values, were studied by GIXD followed by a comparison with a ternary blend which showed pinned V oc values, in order to see the morphological differences. The P3HTT- DPP-10%:P3HT 75 -co-EHT 25 :PC 61 BM ternary blend that was reported as a miscible blend with composition-dependent V oc was analyzed first by GIXD. Both P3HTT-DPP-10% and P3HT 75 -co-EHT 25 polymers were crystalline, showing multiple orders of out-of-plane reflections in their x-ray patterns. When these polymers were mixed, blends retained crystallinity and order, and the x-ray patterns of the blend exhibited monotonically shifting reflections to smaller q z ’s with the addition of P3HT 75 -co-EHT 25. The retention of distinct d-spacings and higher order reflections suggested cocrystallization of P3HTT- DPP-10% and P3HT 75 -co-EHT 25 polymers. 28 Morphology studies were supported by the ionization potentials to show the mixing of ternary components at a fundamental electronic level. Ionization potentials of the P3HTT-DPP-10%:P3HT 75 -co-EHT 25 :PC 61 BM ternary blends were analyzed by photoelectron spectroscopy (PES) (Figure 1.21.). It was demonstrated that the IP of the P3HTT-DPP-10%:P3HT 75 -co-EHT 25 blend decreased progressively with increasing P3HT 75 -co-EHT 25 content. Results were consistent with the increased V oc values of P3HTT-DPP-10%:P3HT 75 -co-EHT 25 :PC 61 BM ternary blends with the increased P3HT 75 - co-EHT 25 content. Being the first direct structural study of ternary blend BHJ active layers, it suggests that some amount of molecular mixing between the synergistic components is crucial for the tunable V oc in the solar cells. This conclusion was supported by ternary blends in which the two donor polymers do not interact physically, that exhibit a pinned V oc value to the smallest V oc value possible. Their ionization potentials did not show any averaging between the ternary components. Figure 1.21. Ionization potentials of the blends of P3HTT-DPP-10%:P3HT 75 -co-EHT 25 as a function of the fraction of P3HT 75 -co-EHT 25 . 76 29 The organic alloy model explains the composition-dependent behavior of V oc and the increase in J sc of the ternary blends using the difference between the nature of exciton and the CT state. In the case of composition-dependent V oc , HOMO and LUMO energy levels change continuously in the two donor or two acceptor blend by changing the composition of the one of the components, showing an organic alloy formation. In the case of increase in J sc , the low band gap polymer helps to fully cover the solar energy spectrum, while the high band gap material pushes the LUMO state of the donor mixture higher in energy with the alloying effect. In order to obtain both increased J sc and V oc from a ternary system, there are several parameters to be considered carefully while choosing ternary blend components such as structural similarity of the components, similar crystallinities, and similar surface energies. There are other models including the parallel-like model and sensitization/cascade model to explain the ternary blend mechanism. However, they fail to explain the composition-dependent V oc behavior even though they explain the J sc increase in ternary blend solar cells. 1.3.2 Parallel-like model The parallel-like model was first proposed by You et al. 77 The proposed mechanism of parallel-like ternary blends is totally different from the alloy model as well as the charge transfer and energy transfer mechanisms. In the parallel-like model, it is postulated that for a D1:D2:A system, the two donors form two separate interpenetrating networks that interfere with a common acceptor component. In contrast to the alloy model, a complete segregation of the synergistic components is thus envisioned. For a D1:D2:A system, the excitons generated in each donor domain diffuse to the respective donor/acceptor interface followed by the dissociation into free electrons and holes. Figure 1.22. shows the exciton dissociation in parallel-like model. 55 As can be seen, generated 30 holes are transported to the anode through their corresponding donors in a parallel-like fashion, while electrons are transported to the cathode through the acceptor. According to the proposed mechanism in this system, holes generated in one donor cannot travel through the other donor. Even though the energy between the HOMO levels may be suitable for such transfer, electron-hole migration works as in parallel connection of the two individual binary blend systems. For D:A1:A2 system, generated excitons can be dissociated into electrons and holes at D/A1 and D/A2 interfaces. In this system, the holes travel to the anode only through the donors, while the electrons are transported to the cathode through the two acceptors. Figure 1.22. Scheme of parallel-like mechanism in (a) D1:D2:A and (b) D:A1:A2 ternary blend solar cells. 55 In the first report of the parallel-like model, You and co-workers studied two sets of polymers to show the parallel-like model in ternary blends. 77 The first set was poly(benzodithiophene–dithienylbenzotriazole) (TAZ) and poly(benzodithiophene– 31 dithienylbenzothiadiazole) (DTBT), and the second set was poly(benzodithiophene– dithienylthiadiazolopyridine) (DTPyT) and poly-(benzodithiophene– dithienyldifluorobenzothiadiazole) (DTffBT) (Figure 1.23.). Polymers in each set had different band gaps and HOMO energy levels. The V oc values of the ternary blends fabricated from the given polymers were measured to be intermediate to the V oc values of each individual sub-cells (Table 1.6.). It was reported that generated holes from each individual donors were transported to the anode through their corresponding channels, as in the parallel connection of two single junction binary blends, giving a V oc value in between the V oc values of the sub-cells, although no direct evidence for this was given. 55 Additionally, no evidence for the proposed morphology was provided. Figure 1.23. Structures of TAZ, DTBT, DTffBT, and DTPyT. 61 32 Table 1.6. Photovoltaic properties of ternary blend solar cells from TAZ:DTBT and DTffBT:DTPyT. 61 The explanation of the tunable V oc in the parallel-like model remains unclear. If there are parallel connected sub-cells in the active layer, the V oc of the ternary system should follow the trend as in tandem solar cells with parallel connection of the sub-cells, where the V oc is pinned to the minimum V oc of each sub-cells. 78, 79 Belcher and co- workers reported a ternary blend system of P3HT:PC 6 PT:PC 61 BM, where PC 6 PT is poly(2,3-dihexyl thieno(3,4-b)pyrazine). 80 They showed that V oc is pinned to the smallest V oc value of the corresponding binary blend system when parallel-connected binary sub- cells were used. Recently, Felekidis et al. pointed the major problems with the parallel- like model while quantitatively explaining the tunable V oc via organic alloy formation in ternary blend bulk heterojunctions. 81 It was discussed that parallel-like model relies on a specific morphology in the active layer where different regions in the active layer act as independent binary cells that are electrically decoupled. It is a very low possibility to get such morphology in the active layer that favors the two (hole-) permeable networks which are electrically isolated from each other. Even though such networks were obtained, metal contacts would definitely enforce equal quasi-Fermi levels in the 33 subcells. It is not possible to treat the subcells independently, considering the equivalent circuit of two parallel photodiodes. 1.3.3 Sensitization/cascade model Ternary blend solar cells include a third organic semiconductor in addition to the electron donor and electron acceptor in order to broaden the absorption across the solar spectrum. In the sensitization/cascade model, it is envisioned that the third material should have compatible energy levels with the electron donor and electron acceptor in the ternary system. It should be selected carefully to provide ionization energies (IEs) and electron affinities (EA) of the three components forming an energy cascade for the charge transportation. 82 The sensitization/cascade model was first proposed by Brabec et al. 4, 83 In this system, the HOMO and LUMO energy levels of the third component are assumed to be intermediate, which allows for a cascade exciton dissociation and charge transfer. For instance, in a D1:D2:A ternary blend solar cell, energy levels of the three components are aligned as HOMO D1 <HOMO D2 <HOMO A and LUMO D1 >LUMO D2 >LUMO A . In this alignment, free charges can travel through the individual domains, and they can also hop from one donor to another donor as long as the energy difference is sufficient. This system allows the use of the three diode model with two parallel diodes for the charge transportation. 84 The energy cascade improves the charge separation and also helps to reduce the geminate charge recombination by spatially separating charges. 85 Moreover, it can also be used to improve exciton collection. If three components of the ternary blend have a cascade of band gaps, they can facilitate the exciton movement to the interfaces. The main idea of this model is that excitons generated in a larger optical band gap component 34 of the ternary blend can be transferred to another component with a smaller optical band gap via Forster energy transfer. 58,86 The one point the cascade model fails to explain is the V oc of the resulting solar cells. For the sensitization/cascade model, all holes should be transported to the highest-lying HOMO level of the donors, then, they can be collected at the electrode (Figure 1.24.). However, this mechanism conceptually results in a pinned V oc value to the smallest V oc of the corresponding binary blend solar cells. There is however significant strong evidence to support this model across a number of systems. 86 Similar to the alloy model suggesting that these two models may both operate under different circumstances. Figure 1.24. Schematic representation of the charge transfer in a cascade model. 4 1.4 Functions of the Third Component 1.4.1. Increase in light absorption Improved light harvesting via broader and stronger light absorption is one of the key factors for improved solar cell efficiencies in ternary blends. Single-junction organic solar cells can go up to 12% efficiency. 48, 87 However, organic materials mostly have a narrow absorption width limiting the photon harvesting and overall energy. The initial 35 interest in ternary blend solar cells started with this point. In order to obtain a wide absorption spectrum of the active layer, three components can be used with complementary absorption profiles instead of just two components. The J sc of the solar cell is proportional to the absorption breadth and the absorption intensity of the active layer, and a rational selection of a third component could enhance the photon harvesting. Although the first report of using multiple donor polymers in fullerene based BHJ solar cell appeared in 2005, 88 dedicated studies focused on this concept began in 2010 with the work of Brabec et al. 89 In this approach, two polymers with different band gaps or a polymer and an organic dye molecule have been used for improved light collection in the active layer. They detected a spectral response from two donor materials and PC 61 BM acceptor showing that an efficient charge transfer is possible from a three components system. Ameri et al. has also performed several studies on enhancing J sc via ternary blends. 4, 90, 91 Si-PCPDTBT was used as a sensitizer for the P3HT/PCBM binary system varying the composition of sensitizer from 10% to 40%. Expanded absorption up to 850 nm was detected after the increased addition of Si-PCPDTBT to the binary system resulting an improved J sc from 8.6 mA/cm 2 to 11 mA/cm 2 . Improved absorption was also confirmed by increased EQE in the region of Si-PCPDTBT absorption (Figure 1.25.). 55 36 Figure 1.25. The EQE spectra of P3HT:PCBM (1:1) and ternary cells with concentrations of 10 wt%, 20 wt%, 30 wt% and 40 wt% Si-PCPDTBT in donors. 55 (Reprinted with permission from Ref. 55. Copyright 2015, Royal Society of Chemistry.) Early examples of improved J sc encouraged the further studies on ternary blends targeting increased absorption of the solar spectrum with increased J sc values. 52, 77, 88, 90, 92, 93, 94, 95 After the addition of low band gap polymer or small molecule, more than 30% increase in J sc with respect to the corresponding binary blend was achieved in several studies. 96, 94, 95 EQE measurements have also demonstrated improved photocurrent in ternary blends. The relative ratio between the two donors significantly affects the EQE values. Figure 1.26. shows the EQE values of some binary and ternary blend solar cells. Khlyabich et al. has reported ternary blends using two polymers (P3HTT-DPP- 10%:P3HT 75 -co-EHT 25 ) with complementary absorptions that resulted significant increase in NIR absorption. 52 Cho et al. studied the effect of a squaraine additive on the photocurrent and efficiency a P3HT:PC 61 BM solar cell. 97 It is possible to see a decrease in the absorption intensity of the visible region while increasing the near-IR absorption (Figure 1.27.). In order to maximize absorption, it is important to optimize the film 37 thicknesses in order to get strong photoresponse from all three components of the ternary blend. Figure 1.26. EQE values of P3HTT-DPP-10%:P3HT 75 -co-EHT 25 :PC 61 BM ternary blend BHJ solar cells with various ternary blend ratios: (i) 1:0:1.3 (red), (ii) 0.9:0.1:1.1 (green), (iii) 0.8:0.2:1.0 (blue), (iv) 0.7:0.3:1.0 (cyan), (v) 0.6:0.4:1.0 (magenta), (vi) 0.5:0.5:0.9 (wine-red), (vii) 0.4:0.6:0.9 (olive), (viii) 0.3:0.7:0.8 (dark-yellow), (ix) 0.2:0.8:0.8 (purple), (x) 0.1:0.9:0.9 (yellow), and (xi) 0:1:0.8 (black). 52 (Reprinted with permission from Ref. 52. Copyright 2012, American Chemical Society.) Figure 1.27. UV-Vis absorption spectra of P3HT:PCBM:DPSQ films as functions of DPSQ concentration. 97 (Reprinted with permission from Ref. 97. Copyright 2013, Elsevier.) 38 Until 2014, most of the ternary blend studies were based on a P3HT donor. Recently, several low band gap polymers have been used in ternary blends. In 2015, Yang et al. reported several dual-donor and multi-donor ternary blend BHJ solar cells with different absorptions. 98 PBDTTT-C, PBDTT-DPP, PTB7 and PBDTT-SeDPP contained the planar BDT unit in their backbone resulting in face-on orientation of the active layer relative to the substrate (Figure 1.28.). Figure 1.29. shows the EQE spectrum of PBDTT-C:PBDTT-DPP:PC 71 BM and PTB7:PBDTT-SeDPP:PC 71 BM ternary blends compared with their corresponding binary blends. PBDTT-C:PBDTT-DPP:PC 71 BM showed significantly higher photoresponse than PBDTT-DPP:PC 71 BM binary blend increasing the J sc value up to 15.7 mA/cm 2 . PTB7:PBDTT-SeDPP:PC 71 BM also showed an increased J sc of 18.7 mA/cm 2 being a combined photoresponse of PTB7 and PBDTT- SeDPP polymers. In addition to J sc values, all the ternary blends exhibited improved efficiency compared to their binary blends showing that compatible polymer donors can coexist harmoniously and result in high-efficiency solar cells. Figure 1.28. Structures of polymer donors and fullerene acceptor that were used for 39 ternary blend solar cells. 98 Figure 1.29. EQE values of (a) PBDTT-C:PBDTT-DPP:PC 71 BM and (b) PTB7:PBDTT- SeDPP:PC 71 BM ternary blend solar cells. 98 (Reprinted with permission from Ref. 98. Copyright 2015, Nature Publishing Group.) Small molecule donors have also been used for improved absorption in ternary blend solar cells. Early studies include several small molecule-incorporated P3HT:PCBM solar cells resulting in relatively low PCE. 97,99,100 In 2015, Wei et al. reported a ternary blend solar cell containing PBDTTPD-HT as a donor polymer, BDT-3T-CNCOO as a new small molecule and PC 71 BM as an acceptor (Figure 1.30.). 101 PBDTTPD-HT-based solar cells show high V oc up to around 1V and BDT-3T-CNCOO exhibits high crystallinity in thin films. Moreover, all three components of the ternary blend have complementary absorption profiles in the visible region. Careful selection of the ternary components resulted in increased absorption intensity (Figure 1.31.) and J sc values compared to the corresponding binary solar cells followed by an increased PCE of 8.40% relative to 6.85%. 40 Figure 1.30. Structures of BDT-3T-CNCOO, PBDTTPD-HT, and PC 71 BM. 101 Figure 1.31. Absorption spectra of the active layer with BDT-3T-CNCOO ratio of 40%, small molecule-based binary OSCs (labeled as 100%), polymer-based binary OSCs (labeled as 0%), and corresponding EQE curves of the OSCs. 55,101 (Reprinted with the permission from Ref. 101. Copyright 2015, Royal Society of Chemistry.) 1.4.2. V oc tuning As discussed previously, addition of the third component to the active layer can result in tunable V oc values making ternary blends highly attractive tools for improved efficiencies. In 2011, Thompson et al. demonstrated for the first time that V oc of the ternary blend solar cells can be tuned by changing the composition of the active layer components with retention of high FF at all compositions. In this system, P3HT was used 41 as a donor, ICBA and PCBM were used as acceptors. 51 Donor to acceptor ratio was kept as 1:1 in all cells while the composition of ICBA and PCBM acceptors were changed from 0% to 100%. Figure 1.32. shows the composition-dependent V oc tuning in this system with the corresponding energy levels and chemical structures. The motivation for using ICBA and PCBM acceptors is based on the fact that both exhibit high efficiencies in binary blend solar cells of P3HT, they show different V oc values with P3HT donor and they have similar chemical structures. Their ternary blends showed tunable V oc starting from 0.61 V going up to 0.84 V by increasing the composition of ICBA in the active layer. All ternary blends exhibited high FFs indicating favorable morphology and good charge transport. This study was a turning point for ternary blend studies proving that V oc of the ternary solar cell is not limited to the smallest V oc of the limiting binary blends. Figure 1.32. (a) Chemical structures and corresponding energy levels of P3HT, ICBA and PC 61 BM and (b) V oc of the ternary blend solar cells. 51 (Reprinted with the permission from Ref. 51. Copyright 2011, American Chemical Society.) 42 Thompson et al. have demonstrated another ternary system with tunable V oc by using two donor polymers P3HTT-DPP-10% and P3HT 75 -co-EHT 25 , and PC 61 BM acceptor as described previously (Figure 1.33.). 52 The V oc values of the ternary blends showed an essentially linear tuning when the overall polymer:fullerene ratio was individually optimized at each polymer (Figure 1.34.). When the overall polymer fullerene ratio was held constant, the V oc values either evolved non-linearly (1:1) or were generally decreased but still linear (1:1.1) as seen in Figure 1.34.. In addition to the V oc decrease, J sc , FF and PCE of the solar cells dropped with the individually unoptimized conditions, showing the importance of optimization. Figure 1.33. Chemical structures of P3HT 75 -co-EHT 25 , P3HTT-DPP-10%, and PC 61 BM. 43 Figure 1.34. V oc of the individually optimized ternary blend BHJ solar cells (squares), with overall polymer:PCBM ratio fixed at 1:1.0 (green triangles), and with overall polymer:PCBM ratio fixed at 1:1.1 (blue stars). 52 (Reprinted with permission from Ref. 52. Copyright 2012, American Chemical Society.) Kim et al. has studied ternary solar cells based on P3HT as a donor, with either o- xylenyl C 60 mono-adduct (OXCMA), o-xylenyl C 60 bisadduct (OXCBA), or o-xylenyl C 60 trisadduct (OXCTA) as acceptors Figure 1.35.). 102 Similar to P3HT:ICBA:PC 61 BM ternary blends, the V oc of the P3HT:OXCMA:OXCBA ternary blends could be tuned from 0.634 V to 0.843 V by increasing the composition of OXCBA in the system (Figure 1.36.). J sc values of the ternary blends were around 10 mA/cm 2 for all OXCMA:OXCBA compositions because of the similar absorptions of both acceptors. FF values were also high being higher than 0.51 in all compositions, showing efficient ternary blends as in the case of P3HT:ICBA:PC 61 BM. However, in the case of P3HT:OXCMA:OXCTA, the V oc followed a non-linear trend with the composition of OXCTA. This difference was attributed to the larger offsets of the LUMO levels of OXCMA and OXCTA (0.33 eV) than between OXCMA and OXCBA (0.17 eV). 44 Figure 1.35. Chemical structures of OXCMA, OXCBA, OXCTA, and P3HT. 81 (Reprinted with the permission from Ref. 81. Copyright 2015, American Chemical Society.) Figure 1.36. V oc tuning in P3HT:OXCMA:OXCBA ternary blend solar cells. 102 (Reprinted with the permission from Ref. 101. Copyright 2013, American Chemical Society.) Hoppe et al. used a different approach for improved V oc and efficiency in ternary blends. 103 Use of amorphous and semi-crystalline polymer mix as a donor with PCBM acceptor resulted in increased efficiencies higher than the corresponding binary blends. Anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) 45 (PPE-PPV) copolymers (AnE-PVs) were used in an amorphous form (AnE-PVba) and semi-crytalline form (AnE-PVab) (Figure 1.37.). Studies showed that small additions of the amorphous AnE-PVba improved the overall cell performance. When the composition of AnE-PVba changed between 10%-70% in the active layer, V oc of the ternary solar cells increased being higher than the both of corresponding binary blends (Figure 1.38.). In addition to increase in V oc , small addition of amorphous AnE-PVba also increased the FF, resulting improved PCEs. Figure 1.37. Chemical structures of the AnE-PVab, the semi-crystalline version, on the left side and AnE-PVba, the amorphous version, on the right side. 103 (Reprinted with the permission from Ref. 103. Copyright 2013, Royal Society of Chemistry.) Figure 1.38. V oc tuning in AnE-PVba:AnE-PVab:PC 61 BM ternary blend BHJ solar cells. 103 (Reprinted with the permission from Ref. 103. Copyright 2013, Royal Society of Chemistry.) 46 Another system with tunable V oc was reported by Zhang and co-workers that showed simultaneous improvement in J sc , V oc and FF. 59 P3HT:PC 71 BM blend was doped with a conjugated small molecule (SMPV1) as a second donor. The V oc of the ternary blend solar cells increased from 0.59 V to 0.73 V by increasing the composition of SMPV1 up to 50% in the active layer. The increased V oc value was attributed to the decreased HOMO energy levels of P3HT:SMPV1 alloy with the increase in SMPV1 composition. This study is a good example of an organic alloy formation between the two donors with high miscibility. 1.5 Challenges and Outlook Recent studies have shown that the ternary blend approach is a simple and effective strategy to overcome the limitations of the binary blends; specifically limited absorption. Addition of the third component can enhance the light harvesting properties and overall efficiency of the solar cells without any complication in the device fabrication keeping the attractive features of the BHJ solar cells. Detailed studies of organic alloy formation have also shown that the ternary blend approach is a promising way to tune the V oc value of the solar cell between the V oc values of the corresponding binary blends allowing an intermediate and non-limiting V oc . At the end, it is possible to get solar cells with improved J sc , V oc and PCE. However, there are still some challenges to be overcome for the better understanding and further improvement of ternary blend BHJ solar cells. The fundamental working mechanism of the ternary blends is more complicated than binary blends due to the addition of the third component in the active layer and it should be understood better. The origin of the different V oc behaviors in ternary blends should 47 also be further investigated and understood better. Another challenge is understanding the morphology of the ternary blend which is also related to compatibility and miscibility between the active layer components. Ternary blend BHJ solar cells are one of the most attractive approaches to organic solar cells with improved photon harvesting via three-component active layer while they keep their simplicity in processing conditions. Careful selection of the donors and acceptors can offer many advantages such as optimized film morphology, improved charge transportation and device stability, enhanced J sc , V oc , FF and overall efficiency. PCE of the ternary blend solar cells is up to 12% showing that ternary blend approach is a promising tool for improved efficiencies. 48 1.6 References for Chapter 1 (1) U.S. Energy Information Administration, 2016. (2) U.S. Energy Information Administration, 2016. (3) Tao, M. Electrochem. Soc. Interface 2008, 30–35. (4) Ameri, T.; Khoram, P.; Min, J.; Brabec, C. J. Adv. Mater. 2013, 25 (31), 4245– 4266. (5) Sharma, S.; Jain, K. K.; Sharma, A. Materials Sciences and Applications 2015, 6, 1145–1155. (6) Nazeeruddin, M. K.; Baranoff, E.; Grätzel, M. 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A 2013, 1 (12), 3961–3969. 57 CHAPTER 2: RANDOM MULTI-ACCEPTOR POLY(2,7- CARBAZOLE) DERIVATIVES CONTAINING THE PENTACYCLIC LACTAM ACCEPTOR UNIT TPTI FOR BULK HETEROJUNCTIONS SOLAR CELLS 2.1 Introduction Polymer solar cells (PSCs) have been studied over the past twenty years offering flexible, low-cost and large area electronic devices with an active layer generally composed of a conjugated polymer as an electron donor and a fullerene derivative as an electron acceptor. 1,2,3 Recent single-junction bulk-heterojunction (BHJ) solar cells have reached 11.7% efficiency showing a significant progress towards the practical efficiency limit of around 12%. 4,5,6 Developing new conjugated polymer structures for further improvement of solar cells has relied heavily on the perfectly alternating donor-acceptor (D/A) approach resulting in low band gap polymers with improved efficiencies. 7,8,9 Poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’- benzothiadiazole)] (PCDTBT) (Scheme 2.1.) is one of the most studied D/A copolymers in BHJ solar cells with high open-circuit voltage (V OC ≈ 0.90 V) , high power conversion efficiency (PCE ≈ 7.5%) and excellent photochemical stability. 10, 11, 12 One of the most attractive features of PCDTBT is its internal quantum efficiency (IQE) reaching nearly 100%. Moreover, the estimated lifetime of 5-10 years makes PCDTBT an attractive copolymer for many applications. 12 Desirable properties of PCDTBT led to the synthesis of PCBTDPP (Scheme 1), an analogue where highly electron-deficient 3,6-bis(5-bromo- thiophene-2-yl)-2,5-di-n-octylpyrrolo[3,4-c]pyrrole-1,4-dione (DPP) unit was used as an acceptor with 9-(Heptadecan-9-yl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)- 9H-carbazole (Cbz) donor. 13 PCBTDPP is a low band gap polymer with high hole- 58 mobility, good solubility, relatively high molecular weight and air stability. 4,7-Bis(5- bromothiophene-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) and DPP are very well known and commonly used acceptor moieties in conjugated polymers. However, many studies are ongoing for the design and synthesis of new acceptor units for further improvement of the PSCs. Polycylic conjugated building blocks have attracted increased attention due to their promising performances in PSCs. 14 Their planar structures facilitate improved charge carrier mobility while extended conjugation results in improved absorption of the polymer. 15, 16 In 2013, Ding’s group reported a pentacyclic aromatic lactam unit thieno[2 ’ ,3 ’ :5,6]pyrido[3,4-g]thieno[3,2-c]- isoquinoline-5,11(4H,10H)-dione (TPTI). 14 The TPTI unit (Scheme 2.1.) bears five fused aromatic rings with benzene in the middle and two thiophene units at the ends connected via two pyridines (Figure 2.1.). Electron- withdrawing lactam moieties provide good electron affinity and solubility. 17, 18 Copolymerization of the TPTI unit with 2,5-bis(trimethylstannyl)thiophene resulted a high V OC of 0.93 V in conventional devices with a PC 71 BM acceptor while short-circuit current density (J SC ) needed further improvement. 14 Since then, several TPTI- incorporated conjugated polymers have been studied for BHJ solar cells and resulted in promising efficiencies. 19, 20 59 SCHEME 2.1. Chemical structures of PCDTBT, PCBTDPP, and TPTI. Design and synthesis of new monomer units is important for further improvement of PSCs, which is heavily based on D/A alternating approach. 21,22,23 As an alternative to the D/A approach, our group has focused on random and semi-random conjugated polymers where multiple chromophores are incorporated into the polymer backbone in a random/semi-random fashion. 23, 24, 25, 26 Multiple components in the polymer backbone can provide broadened absorption, tunable electronic properties, improved solubility and film morphology. 27 A random copolymer approach is a promising method for fine-tuning the absorption profile, molecular packing and HOMO-LUMO energy levels of the polymer while keeping the synthetic simplicity of the donor-acceptor approach. 28 However, there are relatively a few examples of random copolymers developed for PSCs. 28,29,30,31,32 A wider scope of polymers should be studied with new monomer units for further development of random copolymers. Herein, we developed a family of D/A and random poly(2,7-carbazole) derivatives by incorporating multiple acceptor units into the polymer backbone in order to fine-tune the optical and electronic properties of the conjugated polymers. Toward this 60 end, we synthesized a family of copolymers (Scheme 2.2.) based on Cbz as a donor and DTBT, DPP and TPTI as acceptors. We compared the properties of D/A alternating copolymers with the random multi-acceptor copolymers. DTBT, DPP and TPTI are chosen as acceptor moieties because of their different electron affinities and good solubility in common solvents. For the first time, we investigated the effects of TPTI on carbazole-based conjugated polymers. PCTPTI, a new carbazole-TPTI polymer, exhibited a deep HOMO energy level of -5.65 eV and a significantly high V OC of 0.96 V with PC 61 BM. Four new random copolymers were synthesized including two acceptors or three acceptors in the polymer backbone in order to broaden the absorption and tune the HOMO energy levels. To the best of our knowledge, a three-acceptor incorporated random copolymer was synthesized for the first time in this study, showing a broad absorption covering the entire visible region between 350nm-750nm. Random two- acceptor copolymer PCDTBT-TPTI showed the best solar cell efficiency among the other conjugated polymers being 3.45%. Introduction of the electron-deficient TPTI unit into the polymer backbones decreased the HOMO energy levels from -5.45 eV to -5.65 eV. 61 SCHEME 2.2. Structures of the D/A Alternating and Random Multi-Acceptor Copolymers. 2.2 Experimental 2.2.1 Materials All reagents from commercial sources were used without further purification, unless otherwise noted. All reactions were performed under dry N 2 , unless otherwise noted. All dry reactions were performed with glassware that was flamed under high vacuum and backfilled with N 2 . Flash chromatography was performed using a Teledyne CombiFlash Rf instrument in combination with RediSep Rf normal phase disposable columns. Solvents were purchased from VWR and used without further purification except for THF, which was dried over sodium/benzophenone before being distilled. 2.2.2 Measurements All compounds were characterized by 1 H NMR (400 MHz) and 13 C NMR (400 MHz) on a Varian 400. Polymer 1 H NMRs (600 MHz) were obtained on a Varian 600 NMR spectrometer either at 80 °C using C 2 D 2 Cl 4 as a solvent or at 50 °C using CDCl 3 as a solvent. For polymer molecular weight determination, polymer samples were dissolved in HPLC grade o-dichlorobenzene at a concentration of 0.5 mg/ml, briefly heated and then allowed to turn to room temperature prior to filtering through a 0.2 µm PTFE filter. SEC was performed using HPLC grade o-dichlorobenzene at a flow rate of 1 ml/min on one 300 x 7.8 mm TSK-Gel GMHH R-H column (Tosoh Corporation) at 70 °C using a Viscotek GPC Max VE 2001 separation module and a Viscotek TDA 305 RI detector. The instrument was calibrated vs. polystyrene standards (1,050 – 3,800 000 g/mol) and data was analyzed using OmniSec 4.6.0 software. 62 2.2.3 Device Fabrication and Characterization All steps of device fabrication and testing were performed in air. ITO-coated glass substrates (10 Ω/sq, Thin Film Devices Inc.) were sequentially cleaned by sonication in detergent, deionised water, tetrachloroethylene, acetone, and isopropyl alcohol, and dried in a nitrogen stream. A thin layer of PEDOT:PSS (Baytron® P VP AI 4083, filtered with a 0.45 µm PVDF syringe filter – Pall Life Sciences) was first spin-coated on the pre- cleaned ITO-coated glass substrate and annealed at 120 °C for 60 minutes under vacuum. Polymer:fullerene solutions were prepared in o-dichlorobenzene and stirred for 24 hours at 60 °C. The polymer:PC 61 BM active layer was spin-coated (with a 0.45 µm PTFE syringe filter – Whatman) on top of the PEDOT:PSS layer. Concentrations of the polymers were 10 mg/mL in polymer. Films were placed in a nitrogen cabinet for 30 minutes before being transferred to a vacuum chamber. The substrates were pumped down to a high vacuum and aluminum (100 nm) was thermally evaporated at 4 – 5 Å/s using a Denton Benchtop Turbo IV Coating System onto the active layer through shadow masks to define the active area of the devices are 5.2 mm 2 for all devices. The current-voltage (I-V) characteristics of the photovoltaic devices were measured under ambient conditions using a Keithley 2400 source-measurement unit. An Oriel® Sol3A class AAA solar simulator with xenon lamp (450 Watt) and an AM 1.5G filter was used as the solar simulator. An Oriel PV reference cell system 91150V was used as the reference cell. To calibrate the light intensity of the solar simulator (to 100 mW/cm 2 ), the power of the xenon lamp was adjusted to make the short-circuit current (J SC ) of the reference cell under simulated sun light as high as it was under the calibration condition. 63 2.3 Results and Discussion 2.3.1 Synthesis The synthesis of all D/A and random copolymers was carried out from AA (Bpin) and BB (Br) functionalized donor and acceptor monomers via Suzuki polymerization using Pd(PPh 3 ) 4 and K 2 CO 3 in THF/water mixture at 80-85 o C (see Appendix 1, Scheme A1.1.). 13 THF/water mixture was chosen as a solvent system over toluene/water mixture due to the higher molecular weight polymers obtained at the end. The composition of donor and acceptor units in the two-acceptor copolymers and three-acceptor copolymers were 2:1:1 D:A1:A2 and 3:1:1:1 D:A1:A2:A3, respectively. The resulting polymers were precipitated in methanol and purified by Soxhlet extraction using methanol, hexane and chloroform. Molecular weights were determined by dissolving the polymers in o- dichlorobenzene (o-DCB) and running the samples through gel permeation chromatography (GPC) against polystyrene standards (Table 2.1.). Introduction of DPP unit into the polymer backbone resulted in increased molecular weights likely due to the DPP unit’s higher solubility compared to DTBT and TPTI analogues, while the D/A alternating copolymer PCBTDPP showed the highest molecular weight of 27.5 kDa. Polymer structures and acceptor compositions were analyzed using 1 H-NMR spectroscopy by integrating distinct acceptor peaks. Actual monomer ratios in the polymer backbone correspond closely with the monomer feed ratios result in high molecular weight polymers. 64 Polymer M n a (kDa) Đ a HOMO b (eV) Optical E g c γ d (mN/m) PCDTBT 11.8 2.43 -5.45 1.88 29.32 PCBTDPP 27.5 3.68 -5.51 1.69 23.96 PCTPTI 16.1 2.20 -5.65 2.24 25.95 PCDTBT-TPTI 17.3 3.50 -5.57 1.80 26.62 PCDTBT-DPP 23.8 3.41 -5.57 1.72 26.83 PCBTDPP-TPTI 18.7 4.28 -5.51 1.67 22.48 PCDTBT-TPTI-DPP 15.1 3.30 -5.56 1.68 25.06 TABLE 2.1. Molecular weights, polydispersity indices, electrochemical HOMO values, optical band gaps and surface energies of the polymers a Determined by GPC with polystyrene as standard and o-DCB as eluent. b Cyclic voltammetry (vs. Fc/Fc + ) in 0.1M TBAPF 6 . c Calculated from the absorption band edge in thin films, Eg = 1240/λ edge . d Measured for as-cast polymer films. 2.3.2. Optical and Electronic Properties The optical properties of the polymers in thin films are demonstrated in Figure 2.1., using UV-vis absorption spectroscopy. The optical band gaps were determined based on the absorption onsets (Table 2.1.). Absorption coefficients were determined from the film thicknesses estimated by GIXRD in the reflectivity mode. D/A alternating copolymers showed complementary absorption profiles in the range of 350 nm-800 nm encouraging the incorporation of related acceptor units together in a polymer backbone for a full absorption of the solar spectrum. PCTPTI and PCBTDPP showed the highest 65 absorption coefficients reaching 0.9-1.0 x 10 -5 cm -1 . As can be seen, random copolymers exhibited broadened absorption profiles compared to the D/A alternating copolymers. DPP-incorporated polymers showed a red-shift in the absorption profiles due to the high electron-deficiency of the DPP unit. PCDTBT-DPP and PCBTDPP-TPTI showed dual band absorptions indicating π- π * transitions in the short wavelength region. A significant shoulder was observed in the PCDTBT-TPTI absorption profile. The broadest absorption profile was observed in three-acceptor copolymer PCDTBT-TPTI-DPP between 350 nm 750 nm, showing the combination of individual absorption profiles of the D/A alternating copolymers. DPP-incorporated D/A alternating and random copolymers showed lower optical E g compared to the rest of the copolymers due to the strong electron-withdrawing feature of the DPP unit. Importantly, the new polymer PCTPTI exhibited a significantly high optical band gap of 2.24 eV being a promising wide band gap D/A copolymer for tandem and ternary blend solar cells. 66 Figure 2.1. Absorption profiles of thin films of D/A alternating copolymers (a), random two-acceptor copolymers (b), and random three-acceptor copolymer (c). The highest occupied molecular orbital (HOMO) energy levels of the polymers were measured as a film via cyclic voltammetry (CV), and reported in Table 2.1. Introduction of TPTI acceptor into the polymer backbone significantly decreased the 67 HOMO energy level of the polymers compared to the HOMO energy of PCDTBT, -5.45 eV. PCTPTI showed the deepest HOMO energy level of -5.65 eV that would result in a high V OC in solar cells. HOMO values of all the copolymers have varied between -5.45 eV to -5.65 eV. Interestingly, random copolymers exhibited almost the average HOMO values of the corresponding D/A copolymer pairs, which is consistent with the absorption profiles. One important conclusion from the HOMO energy levels of the copolymers is that it is possible to tune the HOMO energies by incorporating different type of acceptor units into polymer backbone even though the overall acceptor composition is same, which is consistent with the previous studies. 24 Surface energies of the polymers were determined using a contact angle goniometer by measuring the contact angles of water and glycerol on pristine as-cast polymer films. Consistent with the previous studies, PCDTBT had the highest surface energy of 29.3 mN/m and DPP incorporated copolymers exhibited lower surface energies compared to rest of the polymers. 33, 34 Surface energy of the conjugated polymers is an important factor to determine the polymer-polymer miscibility and polymer-PCBM miscibility in BHJ solar cells. 34 Overall, surface energies of the synthesized polymers varied between 22.48 mN/m to 29.32 mN/m. Considering the surface energy of PC 61 BM being 27.6 mN/m, there are several copolymers that could favor a good mixing with PC 61 BM in solar cells. 2.3.3. Device Performance In order to investigate the effect of different HOMO energy levels and broadened absorptions on device performances, PSCs were fabricated and tested. All devices were fabricated in air with the structure of ITO/PEDOT:PSS/BHJ active layer/Al and tested 68 under AM 1.5G simulated solar illumination. Different polymer:PCBM ratios between 1:1 to 1:4, different concentrations, solvents, and spin-coating conditions were studied for the optimization of the solar cells (See Appendix 1 for detailed solar cell fabrication procedures). The optimized polymer:PC 61 BM ratios were 1:1.3 for all the polymers. Optimal processing conditions were based on slow solvent evaporation from the polymer:PCBM blends of the thin films under N 2 for 30 min after spin coating the active layer, followed by aluminum deposition. Resulting J SC, V OC , FF and PCE values of the polymers are reported in Table 2.2. Polymer a :PC 61 BM J sc b (mA/cm 2 ) V oc c (V) FF d PCE e (%) PCDTBT 7.20 0.90 0.36 2.32 PCBTDPP 7.99 0.83 0.40 2.64 PCTPTI 6.19 0.96 0.52 3.06 PCDTBT-TPTI 8.31 0.83 0.50 3.43 PCDTBT-DPP 5.66 0.80 0.32 1.45 PCBTDPP-TPTI 3.28 0.80 0.36 1.12 PCDTBT-TPTI-DPP 8.12 0.84 0.39 2.62 TABLE 2.2. Photovoltaic Properties of D/A Alternating Copolymers and Random Multi- Acceptor Copolymers a Spin-coated from o-dichlorobenzene, dried under N 2 before aluminum deposition. b Mismatch corrected. b, c, d, e Average of four pixels. PCTPTI has a significantly high V OC value of 0.96 V, consistent with its deep HOMO energy level of -5.65 eV. Being a new copolymer in literature, PCTPTI exhibited the highest FF among the rest of the polymers indicating a balanced charge-carrier 69 mobility and optimized morphology. Branched solubilizing chains on TPTI provided good solubility in o-dichlorobenzene improving the polymer:PCBM processing in solution. The two-acceptor random copolymer PCDTBT-TPTI showed the second highest FF, 0.50, consistent with the improved processability of the polymer after TPTI incorporation. The J SC value of PCDTBT-TPTI was 8.31 mA/cm 2 , followed by the highest efficiency of 3.43%, showing that the random copolymer approach is a promising way to design efficient polymers for BHJ solar cells. Several low FF values of the synthesized polymers can possibly be attributed to unoptimized film morphology of the active layers, which is the current focus of ongoing studies. V OC values of the PSCs varied between 0.80 V to 0.96 V, providing a set of polymers with different V OC values. PCTPTI showed a significantly high V OC value of 0.96 V. It is known organic photovoltaics rarely exceed the V OC value of 1.0 V, making high V OC PCTPTI a promising polymer in order to achieve improved efficiencies in solar cells. 35 . This family also provides a set of PCDTBT derivatives with similar structures for ternary blend BHJ studies offering different V OC values, HOMO energy levels and complementary absorptions. 33 The external quantum efficiency of the solar cells is shown in Figure 2.2. PCTPTI and PCDTBT-TPTI exhibited the strongest photocurrent between 350 nm to 700 nm with EQE values reaching up to 75%. Photocurrent peaks around 350 nm are attributed to PC 61 BM light absorption. Consistent with the absorption profiles, random copolymers showed broader photocurrent response in the visible region. The random copolymers generally showed smaller peaks in EQE than the perfectly alternating polymers, similar to previous results in our group. 36 The integrated photocurrents from EQE match within 70 5% to that of the mismatch corrected photocurrents measured under simulated AM 1.5G illumination (see Appendix 1 for J SC,corr and J SC,EQE ) Figure 2.2. External quantum efficiencies of the BHJ solar cells. 2.4. Conclusion In conclusion, a series of new D/A alternating and random copolymers were synthesized from electron-rich carbazole donor and electron-deficient DTBT, DPP, and TPTI acceptors via Suzuki polymerization. For the first time, the electron-deficient TPTI unit was used as an acceptor in carbazole-based conjugated polymers. D/A alternating copolymer PCTPTI showed a significantly deep HOMO energy level of -5.65 eV, a very high V OC of 0.96 V and an efficiency of 3.06% in the BHJ solar cells. Introduction of multiple acceptors into the polymer backbone in a random fashion significantly affected their optical and electronic properties, resulting in broadened absorption profiles in the visible and near-IR regions, different HOMO energy levels and a range of V OC values. Three-acceptor random copolymer PCDTBT-TPTI-DPP showed a significantly broad absorption profile in the range of 350nm-750nm, while PCDTBT-TPTI showed the 71 highest J SC of 8.31 mA/cm 2 and PCE of 3.43%. Even though the resulting polymers exhibited different optical and electronic properties, their chemical structures and surface energies were kept very similar to each other which can facilitate polymer miscibility. Ongoing studies are focused on optimization of film morphologies and fabrication of ternary blend BHJ solar cells from the resulting polymers. 72 2.5 References for Chapter 2 (1) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270 (5243), 1789–1791. (2) Thompson, B. 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Macromolecules 2012, 45 (2), 607–632. (23) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules 2011, 44 (13), 5079–5084. (24) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. ACS Macro Lett. 2012, 1 (6), 660–666. (25) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C. Macromolecules 2011, 44 (6), 1242–1246. (26) Howard, J. B.; Ekiz, S.; Cuellar De Lucio, A. J.; Thompson, B. C. Macromolecules 2016, 49 (17), 6360–6367. (27) Kim, K.-H.; Park, S.; Yu, H.; Kang, H.; Song, I.; Oh, J. H.; Kim, B. J. Chem. Mater. 2014, 26 (24), 6963–6970. 74 (28) Hendriks, K. H.; Heintges, G. H. L.; Wienk, M. M.; Janssen, R. A. J. J. Mater. Chem. A 2014, 2 (42), 17899–17905. (29) Kang, T. E.; Choi, J.; Cho, H.-H.; Yoon, S. C.; Kim, B. J. Macromolecules 2016, 49 (6), 2096–2105. (30) Duan, C.; Gao, K.; van Franeker, J. J.; Liu, F.; Wienk, M. M.; Janssen, R. A. J. J. Am. Chem. Soc. 2016, 138 (34), 10782–10785. (31) Jung, J. W.; Liu, F.; Russell, T. P.; Jo, W. H. 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C 2014, 118 (13), 6650–6660. 75 CHAPTER 3: EXPLORING THE INFLUENCE OF ACCEPTOR CONTENT ON SEMI-RANDOM CONJUGATED POLYMERS 3.1 Introduction Polymer bulk heterojunction (BHJ) solar cells are a promising photovoltaic technology offering flexible, low-cost and large area devices with efficiencies up to 12%. 1,2,3,4,5,6,7 Optimization of the active layer components plays a crucial role for further improvement of the efficiency. One of the parameters that can be optimized is the light absorption of the donor polymer to extend across the solar spectrum with a high absorption coefficient (around 10 5 cm -1 ), which is usually targeted via the perfectly alternating donor/acceptor (D/A) approach. 8,9,10 In this approach, electron-rich donor and electron-deficient acceptor units are polymerized in an alternating fashion along the polymer backbone, resulting in a narrow polymer band gap (E g ). However, the primary weakness of the perfectly alternating D/A strategy is that the polymer absorption profile is frequently observed to simply red-shift as opposed to truly broadening across the visible and near-infrared (near-IR) regions. 11 A resulting decrease in the polymer absorption in the visible region of the solar spectrum can hinder the desired increase in the J SC , and ultimately the efficiency. In order to improve the photovoltaic properties, several polymer backbones have been studied as alternatives to the perfectly alternating D/A approach such as random and block copolymers. 12,13, 14,15,16,17,18 As a promising alternative approach, semi-random conjugated polymers have attracted significant attention in the past five years. 11,19,20,21 Our group has developed regioregular poly(3-hexylthiophene) (rr-P3HT or simply P3HT) based semi-random copolymers where a small amount of acceptor units (5-15%) were incorporated into the 76 polymer backbone in a randomized fashion. 11 The small amount of acceptor units improve the absorption in the visible and near-IR region, while retaining the advantages of P3HT such as semi-crystallinity, good solubility in common solvents, and favorable polymer:fullerene ratios in devices. 22,23 Since the first example of a semi-random copolymer, several acceptor units have been incorporated into the P3HT backbone such as thieno[3,4-b]pyrazine (TP), thienopyrroledione (TPD), benzothiadiazole (BTD), and most commonly diketopyrrolopyrrole (DPP). 12,19,21, 24 DPP-incorporated conjugated polymers have been widely used for solar cell applications due to their low band gap, increased photocurrent response up to 1100 nm, excellent efficiencies and high hole-mobilities. 25, 26, 27 Moreover, the DPP unit decreases the HOMO and LUMO energy levels of the polymers due to its electron withdrawing property, rendering it an attractive acceptor unit to tune the electronic properties of the polymers. 28,29 Our group developed DPP-incorporated semi-random P3HT derivatives with various contents of the ethylhexyl (EH)-functionalized DPP unit varying between 5% to 15%, resulting in solar cell efficiencies up to 5.7%. 21 DPP incorporation into the polymer backbone improved the absorption and efficiency of the polymer relative to P3HT. However, the content of the EH-DPP unit was limited to at most 15% due to the decreased solubility and processability of the resulting polymer. Herein, we developed a family of DPP-incorporated P3HT based semi-random copolymers in order to investigate the influence of a broad range of acceptor content on polymer properties and solar cell performance. Toward this end, we synthesized semi- random copolymers via Stille polymerization where the content of DPP acceptor was varied from 10% to 40%, approaching the perfectly alternating analogue. P3HT was used 77 as a reference in order to study the effect of the DPP content. For the first time, we were able to investigate the influence of the acceptor content in a broad range of 10% to 40%. A mixture of EH-functionalized and decyltetradecyl (DTD)-functionalized DPP units were incorporated into each semi-random polymer backbone in order to strike a balance between good solubility and effective processability leading to the formation of high quality films. Increased DPP content in the polymer backbone from 10% to 40% resulted in steadily broadened absorption spectra between 350 - 900 nm, resulting in a monotonic decrease in optical band gap (E g ) of the polymers from 1.49 eV to 1.37 eV. The highest occupied molecular orbital (HOMO) energy levels showed an increase from 10% DPP content (-5.33 eV) to 20% DPP content (-5.27 eV) and 30% DPP content (-5.28), followed by a decrease for 40% DPP content (-5.37 eV). This interesting trend was attributed to various backbone conformations as supported by the grazing-incidence X- ray diffraction (GIXRD) patterns and differential scanning calorimetry (DSC) data. Semi- random copolymers showed significantly improved photovoltaic properties compared to P3HT. BHJ solar cells fabricated from the semi-random copolymers exhibited high J SC values up to 10.29 mA/cm 2 and efficiencies up to 4.43%. A new methanol treatment method was also developed and applied to the semi-random copolymers leading to improved device performance and significantly high FF values approaching 0.70 for devices fabricated in air. 3.2 Experimental 3.2.1. Materials All reagents from commercial sources were used without further purification, unless otherwise noted. All reactions were performed under dry N 2 , unless otherwise 78 noted. All dry reactions were performed with glassware that was flamed under high vacuum and backfilled with N 2 . Flash chromatography was performed using a Teledyne CombiFlash Rf instrument in combination with RediSep Rf normal phase disposable columns. Solvents were purchased from VWR and used without further purification except for THF, which was dried over sodium/benzophenone before being distilled. 3.2.2. Measurements All compounds were characterized by 1 H NMR (400 MHz) and 13 C NMR (400 MHz) on a Varian 400. Polymer 1 H NMRs (600 MHz) were obtained on a Varian 600 NMR spectrometer at 80 °C using C 2 D 2 Cl 4 as a solvent. For polymer molecular weight determination, polymer samples were dissolved in HPLC grade o-dichlorobenzene at a concentration of 0.5 mg/ml, briefly heated and then allowed to turn to room temperature prior to filtering through a 0.2 µm PTFE filter. SEC was performed using HPLC grade o- dichlorobenzene at a flow rate of 0.6 ml/min on one 300 x 7.8 mm TSK-Gel GMHH R-H column (Tosoh Corporation) at 60 °C using a Viscotek GPC Max VE 2001 separation module and a Viscotek TDA 305 RI detector. The instrument was calibrated vs. polystyrene standards (1,050 – 3,800 000 g/mol) and data was analyzed using OmniSec 4.6.0 software. 3.2.3. Device Fabrication and Characterization All steps of device fabrication and testing were performed in air. ITO-coated glass substrates (10 Ω/sq, Thin Film Devices Inc.) were sequentially cleaned by sonication in detergent, deionised water, tetrachloroethylene, acetone, and isopropyl alcohol, and dried in a nitrogen stream. A thin layer of PEDOT:PSS (Clevios PH 500, filtered with a 0.45 µm PVDF syringe filter – Pall Life Sciences) was first spin-coated on the pre-cleaned 79 ITO-coated glass substrate and annealed at 120 °C for 60 minutes under vacuum. Polymer:fullerene solutions were prepared in o-dichlorobenzene and stirred for 24 hours at 60 °C. The polymer:PC 61 BM active layer was spin-coated (with a 0.45 µm PTFE syringe filter – Whatman) on top of the PEDOT:PSS layer. Concentrations of the polymers were 10 mg/mL in polymer. Films were placed in a nitrogen cabinet for 30 minutes before being transferred to a vacuum chamber. The substrates were pumped down to a high vacuum and aluminum (100 nm) was thermally evaporated at 4 – 5 Å/s using a Denton Benchtop Turbo IV Coating System onto the active layer through shadow masks to define the active area of the devices are 5.2 mm 2 for all devices. The current- voltage (I-V) characteristics of the photovoltaic devices were measured under ambient conditions using a Keithley 2400 source-measurement unit. An Oriel® Sol3A class AAA solar simulator with xenon lamp (450 Watt) and an AM 1.5G filter was used as the solar simulator. An Oriel PV reference cell system 91150V was used as the reference cell. To calibrate the light intensity of the solar simulator (to 100 mW/cm 2 ), the power of the xenon lamp was adjusted to make the short-circuit current (J SC ) of the reference cell under simulated sun light as high as it was under the calibration condition. 3.3 Results and Discussion 3.3.1 Synthesis The synthesis of the semi-random polymers has been performed via Stille polycondensation according to our previously reported studies. 11 2-bromo-5-trimethyltin- 3-hexylthiophene, 2,5-bis(trimethyltin)thiophene, and varying amounts of (10% to 40%) dibromo-bisthiophene-diketopyrrolopyrrole were reacted in DMF at 95 °C with Pd(PPh 3 ) 4 as the catalyst (Scheme 3.1.). Previously, our group has reported a family of 80 semi-random copolymers incorporating the EH-functionalized DPP unit. 11 Due to decreasing solubility of the polymer with increasing EH-DPP, the influence of the acceptor composition on the polymer properties was not studied beyond 15% DPP. Here, our first approach to improve the polymer solubility was to incorporate the DTD- functionalized DPP (DTD-DPP) unit into the polymer backbone as a replacement for EH- DPP, which resulted in highly soluble polymers in common solvents with good molecular weights (See Appendix 2 Table A2.1.). However, the significantly improved solubility of the polymers lowered the solution viscosity decreasing the thin film quality, even though different solvents such as DCB, CB, and CHCl 3 were tried. Devices fabricated from DTD-DPP incorporated semi-random copolymers exhibited poor J SC and PCE due to poor film quality (See Appendix 2 Table A2.4.). Beyond the poor filming qualities, in 2014, Li et al. studied the effect of the solubilizing chains on the efficiency of DPP-based polymers in solar cells. 30 It was reported that long solubilizing chains such as DTD form extended semicrystalline network in the thin film as wide fibrils. When the fibril width becomes larger than the exciton diffusion length, the percentage of the excitons that reach the polymer:fullerene interface decreases, limiting the charge generation. They reported moderate device properties with DTD-functionalized DPP polymers while polymers with shorter side chains such as hexyldecyl (HD) showed the best performance in devices. Taking these observations into account, we synthesized semi-random copolymers incorporating both EH-functionalized and DTD-functionalized DPP units in the polymer backbone in order to improve the solubility via the DTD chain and improve the viscosity, film quality, and device performance via the EH chain. 30,31 The ratio of EH-DPP to DTD- DPP was 1:1 in all polymers with the overall DPP ratio varying from 10% to 40% 81 matched by the thiophene content. As a result, the ratio of P3HT varied from 80% to 20%. After Stille polymerization (Scheme 3.1.), the resulting polymers were precipitated in methanol and purified by Soxhlet extraction using methanol, hexane and chloroform followed by re-precipitation into methanol. SCHEME 3.1. Synthesis and structures of P3HTT-DPP 10%, P3HTT-DPP 20%, P3HTT-DPP 30%, and P3HTT-DPP 40%. Molecular weights were determined by dissolving the polymers in o- dichlorobenzene (o-DCB) and using gel permeation chromatography (GPC) against polystyrene standards (Table 3.1.). Molecular weights varied between 10.6 kDa to 14.7 kDa for semi-random copolymers, while the reference polymer P3HT was 11.2 kDa which was synthesized with the same Stille polymerization conditions. This relatively low molecular weight P3HT sample was used to ensure accurate comparison with the semi-random polymers which were all in a similar molecular weight range. The resulting molecular weights are lower than the molecular weights of the semi-random P3HTT-DPP copolymers reported in our previous study. 11 However, we incorporated four different 82 monomers into the polymer backbone in this study while only three monomers were used for the polymerization in the previous case. Stoichiometric balance becomes more challenging for the copolymers incorporating an increasing number of monomers which could result in lower molecular weights. Polymer structures and acceptor contents were analyzed using 1 H-NMR spectroscopy by integrating distinct aromatic DPP peaks along with distinct 3-hexylthiophene alkyl peaks. Actual monomer ratios in the polymer backbone correspond closely with the monomer feed ratios. Polymer M n a (kDa) Đ a HOMO b (eV) Optical E g c (eV) T m ;T c (°C) P3HT 11.2 2.0 -5.22 1.89 212; 178 P3HTT-DPP 10% 13.5 2.6 -5.33 1.49 194; 183 P3HTT-DPP 20% 13.8 3.3 -5.27 1.45 231; 223 P3HTT-DPP 30% 14.7 4.4 -5.28 1.40 255; 232 P3HTT-DPP 40% 10.6 6.3 -5.37 1.37 241; 214 TABLE 3.1. Molecular weights, polydispersity indices, electrochemical HOMO values, optical band gaps and melting/crystallization temperatures of the polymers. a Determined by GPC with polystyrene as standard and o-DCB as eluent. b Cyclic voltammetry (vs. Fc/Fc + ) in 0.1M TBAPF 6 . c Calculated from the absorption band edge in thin films, Eg = 1240/λ edge . 3.3.2 Optical and Electronic Properties The optical absorption of the polymers in thin films is illustrated in Figure 3.1., using UV-Vis absorption spectroscopy. The optical band gaps were determined based on 83 the absorption onsets (Table 3.1.). DPP-incorporated semi-random copolymers showed gradually extended absorption profiles further into the near-IR region with increasing DPP content compared to P3HT absorption, due to the donor-acceptor effect induced by the electron deficient DPP. Our previous report on semi-random P3HTT-DPP copolymers showed absorption profiles red shifted up to 850 nm with 15% DPP. Here, we demonstrated that DPP incorporation beyond 15% results in improved absorption in the range of 350 nm to 900 nm. DPP incorporation resulted in significant dual band absorptions indicating π- π * transitions in the short wavelength region and intramolecular charge transfer (ICT) transitions in the long wavelength region. More specifically, π- π * transitions in the short wavelength range were attributed to thiophene-rich segments of the semi-random copolymers, while ICT transitions were assigned to donor-acceptor interactions between the thiophene and DPP units. Absorption coefficients of the ICT band of the semi-random copolymers significantly increased from P3HTT-DPP 10% to P3HTT-DPP 20%, and showed minor increase beyond the P3HTT-DPP 20%, approaching 10 5 cm -1 . Increased DPP content in the polymer backbone leads to a gradual decrease in the optical band gap (E g ) due to the strong electron-withdrawing character of the DPP unit. Table 1 shows that 10% DPP incorporation decreases the optical band gap from 1.89 eV to 1.49 eV, followed by a gradual decrease to 1.37 eV with 40% DPP content in the polymer backbone. In the literature, it was reported that the perfectly alternating D/A thiophene-DPP polymer with DTD side chains had an optical band gap of 1.36 eV which follows the decreased optical band gap trend with increased DPP content but also indicates that at 40% acceptor content the absorption property is essentially 84 saturated. 31 These results further indicate that semi-random copolymers with different acceptor contents can be used for precise tuning of optical band gaps. Figure 3.1. UV-Vis absorption spectra of polymers in thin films spin-coated from o- DCB: (i) P3HT (black, squares), (ii) P3HTT-DPP 10% (red, circles), (iii) P3HTT-DPP 20% (blue, upward triangles), (iv) P3HTT-DPP 30% (green, downward triangles), (v) P3HTT-DPP 40% (pink, diamonds). HOMO energy levels of the polymers were measured in thin films via cyclic voltammetry (CV), and reported in Table 3.1. Ferrocene was used as a reference taking the ferrocene redox couple as -5.1 eV relative to vacuum (See Appendix 2 for the CV traces). 32,33 Measured HOMO levels did not follow a monotonic compositional trend according to DPP content (Figure 3.2.). From P3HTT-DPP 10% to P3HTT-DPP 20%, the HOMO level of the polymers showed a small increase from -5.33 eV to -5.27 eV. The HOMO level of P3HTT-DPP 30% was the essentially same as P3HTT-DPP 20% (-5.28 eV). However, the HOMO level of P3HTT-DPP 40% showed a significant decrease to - 5.37 eV. In our previous studies, semi-random polymers with 5% to 15% DPP content did not show any difference in the HOMO energy levels, being -5.2 eV for all the 85 polymers, even though they did show a decreasing V oc trend in solar cells. 11 However, one of our previous studies on the influence of the branched side chains on polymer properties showed that increased branched chain density on the polymer led to monotonically decreasing HOMO levels. 34 Here, we see an interesting trend with the acceptor content between 10% to 40%. In order to gain insight into this interesting trend, we performed GIXRD analysis and DSC measurements, which are discussed in the following section. Figure 3.2. HOMO levels (blue line, triangles) and V oc (red line, squares) of the P3HT, P3HTT-DPP 10%, P3HTT-DPP 20%, P3HTT-DPP 30%, and P3HTT-DPP 40%. 3.3.3 Morphological and Thermal Properties GIXRD was used to study the morphology of the polymer films. P3HTT-DPP polymers were spin-coated from o-DCB solutions and thermally annealed at 200 ° C for 30 minutes to induce crystallinity. All polymers showed a distinct (100) diffraction peak (reflection in the 2θ range of 3.5°-6°), showing semi-crystallinity associated with lamellar spacing in rr-P3HT (Figure 3.3.). This peak gradually shifts in a monotonic fashion from a 2θ of 5.0° (for P3HTT-DPP 10%) to 4.4° (for P3HTT-DPP 40%), corresponding to an 86 increased lamellar distance from 17.8 Å to 20.3 Å. In the case of the intensity, P3HTT- DPP 20% showed the highest intensity after P3HT, followed by P3HTT-DPP 10%, P3HTT-DPP 30%, and finally P3HTT-40%. Figure 3.3. GIXRD patterns of (i) P3HT (black), (ii) P3HTT-DPP 10% (red), (iii) P3HTT-DPP 20% (blue), (iv) P3HTT-DPP 30% (green), (v) P3HTT-DPP 40% (pink). The increased lamellar distance from P3HTT-DPP 10% to P3HTT-DPP 40% is attributed to the increased content of DTD-DPP in the polymer backbone. Specifically, from P3HT to P3HTT-DPP 10%, lamellar distance increases due to the DPP incorporation with long branched side chains. When the DPP content increases to 20%, 30%, and 40%, lamellar distance continues a monotonic increase due to the higher DTD- DPP content in the polymer backbone. In our previous studies, semi-random DPP copolymers with EH side chains exhibited decreased lamellar spacing between the polymer chains with increasing DPP content. 11 In that case, planar DPP units in the polymer backbone provided improved backbone packing and the EH side chains did not prevent that interaction. Here, larger lamellar packing distances with increased DPP content were expected due to the significantly longer branched side chains of DPP (DTD) decreasing the polymer packing. 87 In order to further investigate the bulk polymer crystallization and melting behavior, differential scanning calorimetry (DSC) traces were evaluated (See Appendix 2 Figure A2.10.). For all the semi-random copolymers, melt and crystallization thermal transitions were observed, suggesting semi-crystallinity and supporting the GIXRD data. Melting and crystallization temperatures gradually increased with increasing DPP composition in the polymer backbone, being T m = 194 °C, T c = 183 °C for P3HTT-DPP 10%, T m = 231 °C, T c = 223 °C for P3HTT-DPP 20%, and T m = 255 °C, T c = 232 °C for P3HTT-DPP 30%. However, P3HTT-DPP 40% showed a slight decrease in melting and crystallization temperatures (T m = 241 °C, T c = 214 °C). P3HTT-DPP 10% showed a decrease in the T m value (194 °C) compared to P3HT with the T m = 212 °C. This behavior was explained by the disruption of the P3HT packing due to the DPP units with the branched side chains, forming a more disordered structure supported by GIXRD. 34 Increased DPP content in the polymer backbone after 10% DPP content seemingly improved the polymer packing again due to the improved local planarity with higher amounts of DPP and thiophene incorporation, resulting in an increase in the T m values. A slight decrease in T m of P3HTT-DPP 40% can be attributed to the high content of branched side chains decreasing the molecular packing. The results indicate that semi- random copolymers still exhibited semi-crystallinity; one of the key features of rr-P3HT. Analysis of polymer crystallinity allows us to propose an explanation for the unusual trend in HOMO energies with DPP contents (Figure 3.2.). When comparing P3HTT-DPP 10% with P3HT, a decrease in HOMO energy from -5.22 eV to -5.33 eV is observed. At the simplest level, this can be attributed to the incorporation of the more electronegative DPP unit. However, in our previous work with EH-DPP, 10% 88 incorporation resulted in no change in the HOMO level relative to P3HT. The difference in behavior here is attributed to DTD-DPP with its significantly larger branched chains. Considering GIXRD, in the present system incorporation of 10% DPP (5% EH-DPP and 5% DTD-DPP) results in an increase in lamellar spacing from 17.0 Å (P3HT) to 17.8 Å. With 10% EH-DPP and no DTD-DPP, the lamellar spacing actually decreased to 15.3 Å indicating improved, tighter packing of chains and a likely enhancement of backbone planarity. As such, with the previously reported EH-DPP, 10% incorporation had apparently offsetting effects of electronegativity and improved packing, which resulted in no change in HOMO energy level relative to P3HT. With DTD-DPP however, the bulky DTD side chain resulted in poorer packing and likely more backbone twisting which when combined with the electronegativity of the DPP unit resulted in a deeper HOMO level. Upon increasing the DPP content from 10% to 20%, the HOMO level in the present case increased from -5.33 eV to -5.27 eV. Despite the presence of more electronegative DPP units, the increased intensity of the diffraction peak relative to the 10% sample (Figure 3.3.) suggests a more crystalline polymer and the DSC data (Table 3.1.) suggests significantly stronger intermolecular forces as evidenced by an increase in T m from 194 °C with 10% to 231 °C with 20%. Considering these observations, enhanced π-stacking due to higher DPP and unsubstituted thiophene content is inferred and is suspected to result in an increase in polymer backbone planarity and an increased HOMO. Little change is observed in HOMO energy upon increasing from 20% DPP to 30% DPP. Here offsetting effects of increasing electronegative monomer content and suspected reduction in backbone planarity (inferred from decreased crystallinity and 89 increased lamellar spacing due to increased DTD-DPP content) are suspected of resulting in no net change in the HOMO energy. Finally, the large decrease in HOMO energy when going from 30% DPP (-5.28 eV) to 40% DPP (-5.37 eV) is expected to result from the now very high content of DTD-DPP (20%) which reduces crystallinity, increases lamellar spacing and likely results in an overall more twisted backbone that also has a high content (40%) of electronegative monomer. 3.3.4. Photovoltaic Properties In order to investigate the effect of different acceptor contents on device performances, PSCs were fabricated and tested. All devices were fabricated in air with the conventional architecture of ITO/PEDOT:PSS/BHJ active layer/Al and tested under AM 1.5G simulated solar illumination. We have tried several different processing conditions in order to optimize the device properties. At first, we fabricated solar cells with the conventional architecture from fully-dried polymer samples analogous to our previous reports. 11 Devices performed moderately well with the J sc values between 4.8 mA/cm 2 to 8.2 mA/cm 2 , V oc values between 0.52 V to 0.62 V, FF values between 0.50 to 0.58, and efficiencies between 1.4% to 2.8% (See Appendix 2 Table A2.5.). The device performances did not compare favorably with our previously reported semi-random EH- DPP copolymers which showed efficiencies of 3.60% for 5% DPP, 5.73% for 10% DPP, and 4.10% for 15% DPP under the same processing conditions. 11,21 In order to improve device performances, we considered a number of modifications to the processing procedure. In the literature, methanol treatment is reported as a favorable strategy to enhance device performance. The common procedure for methanol treatment is that after spin- 90 coating the polymer:PCBM blend on to ITO, methanol is dripped on the active layer until the film surface is fully covered. Then it is spun off from the active layer immediately by spin-coating at high rates (around 2000 rpm) for less than a minute. 35 Another procedure for methanol treatment is solvent-soaking treatment where polymer:PCBM thin films are immersed in the mixed solvents of methanol and a second solvent for about a minute and then dried under nitrogen. 36 There are two major proposed explanations for the improvement in device properties, being i) an increase in built-in voltage (V bi ) across the device due to the passivation of surface traps, and ii) improvement of the film morphology leading to a desired interpenetrating network of polymer and fullerene. 35,37 This method has been shown to form phase separated structures with smaller domains, and thus likely improves the charge separation, transport and extraction efficiency. 35 It was also reported that methanol affects the internal electric field in the solar cell increasing the surface charge density. With decreased series resistance and increased mobilities, solar cells exhibited reduced charge recombination. 37 Although there is still debate about exactly how methanol treatment works, it is reported in many studies that methanol treatment enhances the device properties including J sc , V oc , FF, and PCE. 35,35,37,38 In order to enhance our device properties, we performed methanol treatment for all the polymer solar cells. However, we did not get good polymer films leading to non-functioning devices. It is possible that the significant content of long alkyl chains on the polymer backbone causes the polymer:PCBM film surface to become very hydrophobic preventing the methanol penetration through the active layer, however this does not explain the poor film quality that resulted for which there is not a clear explanation. 91 In order to improve the methanol penetration through the active layer, we developed a new methanol treatment in this study in which the polymers were treated with methanol before device fabrication. Contrary to the reported methods in the literature, we treated the polymers with methanol after soxhlet extraction but before film formation. Specifically, chloroform fractions of the polymers from soxhlet were precipitated into methanol in which the chloroform/methanol ratio was approximately 1:7. The mixtures were then left to stand for two days at 10 °C. After filtration (and without drying under vacuum), polymer:PCBM solutions were immediately prepared and then spin-coated on to pre-cleaned ITO glasses according to the reported procedure (See Appendix 2). Different polymer:PC 61 BM ratios varying between 1:1 to 1:2, different concentrations, solvents such as DCB, CB, and CHCl 3 , and spin-coating conditions were studied for the optimization of the solar cells (See Appendix 2 for detailed solar cell fabrication procedures). The optimized polymer:PC 61 BM ratios were 1:1.3 for all the polymers. Processing conditions were based on slow solvent evaporation from the polymer:PC 61 BM blends under N 2 after spin coating the active layer from DCB, followed by aluminum deposition. Resulting J SC, V OC , FF and PCE values of the polymers are reported in Table 3.2. For all the polymer solar cells, thin film quality and device performance were significantly improved relative to those prepared with polymer samples that had been fully dried. However, by comparing the data in Table 3.2. with those in Table A2.5., it is clear that the effect of methanol treatment is less significant for P3HT and more significant for the higher DPP contents (20%, 30%, 40%). Analogous to the literature, it is still not well understood how methanol is affecting the solar cells and causing the significant improvement in the device parameters. 92 Polymer a :PC 61 BM J sc b,c (mA/cm 2 ) V oc d (V) FF e PCE f (%) P3HT 7.54 0.55 0.53 2.16 P3HTT-DPP 10% 8.31 0.62 0.61 3.12 P3HTT-DPP 20% 7.98 0.55 0.67 2.91 P3HTT-DPP 30% 10.23 0.57 0.67 3.88 P3HTT-DPP 40% 10.29 0.62 0.69 4.43 TABLE 3.2. Photovoltaic properties of P3HT, P3HTT-DPP 10%, P3HTT-DPP 20%, P3HTT-DPP 30%, and P3HTT-DPP 40%. a Spin-coated from o-dichlorobenzene, dried under N 2 before aluminum deposition. b Mismatch corrected. c, d, e, f Average of four pixels. For the optimized devices, J SC values of the semi-random copolymers significantly increased with higher DPP content in the polymer backbone. P3HTT-DPP 30% and P3HTT-DPP 40% showed J SC values of 10.23 mA/cm 2 and 10.29 mA/cm 2 , respectively. High J SC values are consistent with their high absorption coefficients, broad absorption profile up to 900 nm, and lower band gaps compared to P3HT. For P3HTT- DPP 10% and P3HTT-DPP 20%, J SC values were measured as 8.31 mA/cm 2 and 7.98 mA/cm 2 , respectively. Our previous semi-random P3HTT-DPP copolymers exhibited higher J SC values than the reported J SC values here. The difference is attributed at least in part to the higher molecular weight of the previous polymers. V OC values followed a consistent trend with the HOMO energy levels of the polymers (Figure 3.2.). P3HTT- DPP 10% showed a V OC value of 0.62 V, being higher than the V OC of P3HT (0.55 V). This increase is attributed to the deeper HOMO level of P3HTT-DPP 10% compared to P3HT. The V OC value of P3HTT-DPP 20% (0.55 V) showed a decrease compared to 93 P3HTT-DPP 10%, being consistent with its higher HOMO level compared to P3HTT- DPP 10%. Additionally, further increase in DPP content in the polymer backbone resulted in increased V OC values of 0.57 V and 0.62 V for P3HTT-DPP 30% and P3HTT- DPP 40%, respectively, being consistent with the HOMO energy levels. In our previous study with semi-random P3HTT-DPP copolymers considering only EH-DPP, we observed a monotonic decrease in V OC with increased DPP composition in the polymer backbone from 5% to 15%, even though the HOMO levels remained the same for all the semi-random polymers. We postulate that the contrasting V oc and HOMO trends are due to differences in polymer packing and chain planarity in PCBM blends and pristine films. The highest observed V oc with 5% EH-DPP (0.66 V) is suspected to be due to a more disrupted and less crystalline polymer that is more highly mixed with PCBM, where in effect the small DPP content is acting similarly to regiodefects in P3HT which results in a more amorphous polymer and higher V oc . 39 The 10% EH-DPP polymer is observed to be more crystalline and has a smaller lamellar spacing (15.2 Å vs. 16.0 Å for 5% EH-DPP) supporting the picture of a more densely packed polymer with likely a more planar backbone that is less disrupted by PCBM, resulting in a lower V oc . Upon increasing the DPP content to 15%, the V oc was observed to decrease further possibly due to enhanced π-stacking in the polymer engendered by the planar DPP unit and higher thiophene content. Here, we observed a similar decrease in the V oc values from 10% to 20% DPP composition, followed by an increase for 30% and 40% DPP content. This unusual trend in V oc values is consistent with the HOMO energy levels and was attributed to suspected differences in the backbone planarity. For 10% DPP, the polymer backbone was more 94 twisted compared to the 20% DPP, 30% DPP and 40% DPP, resulting in a low HOMO value and a high V oc value. 40 When we go the 20% DPP, backbone planarity increased resulting in an increase in the HOMO level and a decrease in the V oc level. For 30% DPP, the HOMO level is slightly decreasing with a slightly increased V oc value. When we go up to 40% DPP, due to the higher amount of twisting in the polymer backbone, the HOMO level decreased followed by an increase in the V oc values consistent with our explanation of the HOMO trend. The most significant feature of the solar cells was their higher FF values compared to our previous semi-random copolymers which were in the range of 0.58 to 0.65. 11,21 Considering that the devices were fabricated and tested in air with a PC 61 BM acceptor, reported FF values approaching 0.70 are highly promising. Improved FF values are clearly related to the new methanol treatment method. FF values of all the semi- random copolymers were between 0.60 and 0.69, showing significant improvement compared to P3HT. P3HTT-DPP 40% showed the highest FF of 0.69, followed by P3HTT-DPP 30% and P3HTT-DPP 20% with the FF values of 0.67. P3HTT-DPP 10% with a FF of 0.61 exhibited a slight increase compared to P3HT. The maximum solar cell efficiency in this set of semi-random copolymers was obtained from P3HTT-DPP 40% at 4.43%. Overall device efficiencies varied between 2.91% to 4.43%, showing promising photovoltaic properties. Photovoltaic properties of the semi-random P3HTT-DPP copolymers were compared with the perfectly alternating D/A copolymer of DT-PDPP3T from the literature. 31 Li et al. reported the polymer DT-PDPP3T where 2,5- bis(trimethyltin)thiophene and DTD-functionalized dibromo-bisthiophene- 95 diketopyrrolopyrrole were reacted forming a perfectly alternating thiophene-DPP polymer. They have reported a molecular weight of 136.8 kDa for the resulting polymer and fabricated solar cell with PC 71 BM acceptor. In order to improve the photovoltaic properties, CHCl 3 :DIO mixture was used as a solvent additive. They reported an efficient solar cell with J SC of 10.6 mA/cm 2 , V OC of 0.65 V, FF of 0.71, and PCE of 4.8% in inert atmosphere. Our semi-random P3HTT-DPP 40% copolymer showed very similar device parameters to the reported D/A alternating copolymer with J SC of 10.29 mA/cm 2 , V OC of 0.62 V, FF of 0.69, and PCE of 4.43% in air. Considering that semi-random copolymers reported in this study have significantly smaller molecular weights than DT-PDPP3T, their device performances are very encouraging and open to further improvement. Moreover, device parameters could be improved by the use of PC 71 BM rather than PC 61 BM. Comparison of the polymers reported here with the photovoltaic properties of DT-PDPP3T also demonstrates that the semi-random copolymer approach is an effective tool to tune device properties. J SC and V OC values of the D/A thiophene-DPP copolymer fit our observed trend when extrapolating to increased DPP composition, demonstrating continuously tunable device parameters. The external quantum efficiency data is shown in Figure 3.4. All devices showed a photocurrent response in the range of 350 nm to 900 nm. Photocurrent peaks around 400 nm are assigned to PC 61 BM, while the peaks around 500 nm and 700 nm are attributed to P3HT rich chromophores and D/A rich chromophores absorption, respectively. Higher J SC values obtained from P3HTT-DPP 30% and P3HTT-DPP 40% can be explained by the more intense and more balanced photocurrent response in the visible and near-IR regions. The integrated photocurrents from EQE match within less 96 than 5% to that of the mismatch corrected photocurrents measured under simulated AM 1.5G illumination (see Appendix 2 Table A2.7.). Figure 3.4. External quantum efficiencies of the BHJ solar cells: (i) P3HTT-DPP 10% (black, squares), (ii) P3HTT-DPP 20% (red, circles), (iii) P3HTT-DPP 30% (blue, upward triangles), (iv) P3HTT-DPP 40% (green, downward triangles). 3.4 Conclusion A series of semi-random P3HTT-DPP copolymers containing the DPP acceptor in the range of 10% to 40% were synthesized via Stille polymerization. For the first time, the influence of the acceptor content in semi-random polymers was explored in a broad range of acceptor composition. Increased DPP content in the polymer backbone resulted in broadened absorption spectra between 350 - 900 nm. The optical E g of the polymers showed a gradual decrease from 1.49 eV to 1.37 eV. HOMO energy levels and V oc values showed an interesting trend for all the polymers likely due to the variations in the backbone conformations as inferred from GIXRD and DSC data. Semi-random polymers 97 showed significantly improved polymer and photovoltaic properties compared to P3HT, while P3HTT-DPP 40% showed very similar polymer properties and device performances to a perfectly alternating thiophene-DPP polymer reported in the literature. 31 P3HTT-DPP 40% showed the best device performance with the J SC of 10.29 mA/cm 2 , V OC of 0.62 V, FF of 0.69, and efficiency of 4.43%. A new methanol treatment method was developed and applied to polymers leading to significantly high FF values approaching 0.70 under ambient conditions in air with a conventional architecture without any other special treatments/additives or interlayers. This study indicated that the semi-random copolymer approach is a simple and efficient tool for precise tuning of optical, electronic, and photovoltaic properties. It was demonstrated in this study that very specific energy levels and optical band gaps can be easily obtained by incorporating the right amount of acceptor unit into the polymer backbone, leading to targeted photovoltaic properties useful for ternary blend and tandem solar cells. Ongoing studies are focused on the fabrication of ternary blend BHJ solar cells based on the reported polymers. 98 3.5 References for Chapter 3 (1) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270 (5243), 1789–1791. (2) Thompson, B. C.; Fréchet, J. M. J. 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Soc. 2012, 134 (11), 5222–5232. 102 CHAPTER 4: WIDE BAND GAP POLYMERS INCORPORATING TPTI ACCEPTOR FOR BULK HETEROJUNCTION SOLAR CELLS 4.1 Introduction Polymer solar cells (PSCs) have been studied over the past twenty years offering flexible, low-cost and large area electronic devices with an active layer generally composed of a conjugated polymer as an electron donor and a fullerene derivative as an electron acceptor. 1,2,3 Recent single-junction bulk-heterojunction (BHJ) solar cells have reached up to 12% efficiency which can be further improved by the optimization of the active layer components. 4,5 Developing new conjugated polymer structures for further improvement of solar cells has relied heavily on the perfectly alternating donor-acceptor (D/A) approach resulting in low band gap (optical E g ≤1.5 eV) polymers with improved efficiencies. 6,7,8 While low band gap polymers have been promising to reach high device performances especially in single-cell BHJ devices with PCBM, there is an urgent need for the development of new conjugated polymers with wide band gaps (WBG) for several applications including ternary blend BHJ solar cells, tandem solar cells, and fullerene- free polymer solar cells. 9,10,11,12,13,14,15,16,17 Today, P3HT is still the most commonly used WBG (optical E g ≈ 1.9 eV) conjugated polymer for BHJ solar cells. 1,18,19 However, the high lying HOMO energy level of P3HT (HOMO ≈ -5.2 eV) results in low V oc values (V oc ≈ 0.6 V), limiting the overall device efficiency. Recently, we have reported a series of poly(2,7-carbazole) derivatives incorporating a pentacyclic aromatic lactam unit 2,8-Dibromo-4,10-bis(2- ethylhexyl)thieno[2’,3’:5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione (TPTI) acceptor. 20 TPTI was first reported in 2013 by Ding, bearing five fused aromatic 103 rings with benzene in the middle and two thiophene units at the ends connected via two pyridones (Scheme 4.1.). 21 Copolymerization of the TPTI unit with 9-(Heptadecan-9-yl)- 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole (carbazole) via Suzuki polymerization resulted in PCTPTI polymer (Scheme 4.1.) with a deep HOMO energy level of -5.65 eV and a high V OC of 0.96 V in conventional devices with a PC 61 BM acceptor while power conversion efficiencies (PCE) were exceeding 3%. 20 Optical band gap (E g ) of PCTPTI was 2.24 eV, making it a highly promising WBG polymer. SCHEME 4.1. Chemical structures of TPTI and PCTPTI. 20 For carbazole-based polymers, Suzuki polymerization is a traditional method where carbazole donor is boronated while the acceptor is halogenated. 22,23 However, we recently developed successful Direct Arylation polymerization (DArP) conditions toward carbazole-based polymers that are traditionally made via Suzuki polymerization. 24 DArP has attracted significant interest as a simplifying synthetic alternative to the traditional cross-coupling polymerizations toward conjugated polymers, being promising materials for polymer BHJ solar cells. 25,26,27,28 It has been widely used as an alternative to Stille polymerization, but the investigation of DArP as an alternative to Suzuki polymerization is very uncommon. 29,30,31 We recently have demonstrated that DArP is a promising 104 alternative to Suzuki toward perfectly alternating copolymers. 24 PCTPTI synthesized via DArP exhibited e deep HOMO energy level of -5.57 eV followed by a high V oc of 0.96 V. It was an efficient polymer with an E g of 2.25 eV while the solar cell efficiency was reaching up to 3%, being very consistent with its Suzuki analogue. Herein, we developed a set of perfectly alternating WBG copolymers via Suzuki polymerization and DArP (Scheme 4.2.), comparing their optical, electronic, and photovoltaic properties. We used TPTI as a common acceptor in all polymers, while 9,9- Dioctyl-9H-fluorene-2,7-diboronic acid bis(pinacol) ester (fluorene) and 1,4-Dibromo- 2,5-bis(2-ethylhexyloxy)benzene (phenylene) were used as donors. PFTPTI, a new fluorene-TPTI polymer, exhibited a deep HOMO energy level of -5.63 eV for both Suzuki and DArP analogues, followed by a high V OC of around 0.90 V with PC 61 BM. PPTPTI, a new phenylene-TPTI polymer, exhibited a HOMO energy level of -5.57 eV, with a V oc of 0.68 V. We synthesized PPTPTI only via DArP due to the synthetic intolerance of alkoxy benzene to boronation on the 1,4 position. Optical E g of the polymers were significantly high varying between 2.30 eV and 2.09 eV. Solar cell efficiencies of the polymers changed between 0.90-1.59%. 4.2 Experimental 4.2.1. Materials All reagents from commercial sources were used without further purification, unless otherwise noted. All reactions were performed under dry N 2 , unless otherwise noted. All dry reactions were performed with glassware that was flamed under high vacuum and backfilled with N 2 . Flash chromatography was performed using a Teledyne CombiFlash Rf instrument in combination with RediSep Rf normal phase disposable 105 columns. Solvents were purchased from VWR and used without further purification except for THF, which was dried over sodium/benzophenone before being distilled. 4.2.2. Measurements All compounds were characterized by 1 H NMR (400 MHz) and 13 C NMR (400 MHz) on a Varian 400. Polymer 1 H NMRs (600 MHz) were obtained on a Varian 600 NMR spectrometer at 80 °C using C 2 D 2 Cl 4 as a solvent. For polymer molecular weight determination, polymer samples were dissolved in HPLC grade o-dichlorobenzene (o- DCB) at a concentration of 0.5 mg/ml, briefly heated and then allowed to turn to room temperature prior to filtering through a 0.2 µm PTFE filter. SEC was performed using HPLC grade o-dichlorobenzene at a flow rate of 0.6 ml/min on one 300 x 7.8 mm TSK- Gel GMHH R-H column (Tosoh Corporation) at 60 °C using a Viscotek GPC Max VE 2001 separation module and a Viscotek TDA 305 RI detector. The instrument was calibrated vs. polystyrene standards (1,050 – 3,800 000 g/mol) and data was analyzed using OmniSec 4.6.0 software. 4.2.3. Device Fabrication and Characterization All steps of device fabrication and testing were performed in air. ITO-coated glass substrates (10 Ω/sq, Thin Film Devices Inc.) were sequentially cleaned by sonication in detergent, deionised water, tetrachloroethylene, acetone, and isopropyl alcohol, and dried in a nitrogen stream. A thin layer of PEDOT:PSS (Clevios PH 500, filtered with a 0.45 µm PVDF syringe filter – Pall Life Sciences) was first spin-coated on the pre-cleaned ITO-coated glass substrate and annealed at 120 °C for 60 minutes under vacuum. Polymer:fullerene solutions were prepared in o-dichlorobenzene and stirred for 24 hours at 60 °C. The polymer:PC 61 BM active layer was spin-coated (with a 0.45 µm PTFE 106 syringe filter – Whatman) on top of the PEDOT:PSS layer. Concentrations of the polymers were 10 mg/mL in polymer. Films were placed in a nitrogen cabinet for 30 minutes before being transferred to a vacuum chamber. The substrates were pumped down to a high vacuum and aluminum (100 nm) was thermally evaporated at 4 – 5 Å/s using a Denton Benchtop Turbo IV Coating System onto the active layer through shadow masks to define the active area of the devices are 5.2 mm 2 for all devices. The current-voltage (I-V) characteristics of the photovoltaic devices were measured under ambient conditions using a Keithley 2400 source-measurement unit. An Oriel® Sol3A class AAA solar simulator with xenon lamp (450 Watt) and an AM 1.5G filter was used as the solar simulator. An Oriel PV reference cell system 91150V was used as the reference cell. To calibrate the light intensity of the solar simulator (to 100 mW/cm 2 ), the power of the xenon lamp was adjusted to make the short-circuit current (J SC ) of the reference cell under simulated sun light as high as it was under the calibration condition. 4.3 Results and Discussion 4.3.1. Synthesis PFTPTI polymer was synthesized via both Suzuki polymerization and DArP, while PPTPTI was only synthesized via DArP (Scheme 4.2.). Suzuki polymerization was performed with AA (Bpin) and BB (Br) functionalized donor and acceptor monomers using Pd(PPh 3 ) 4 and K 2 CO 3 in THF/water mixture at 80-85 o C (see Appendix 3). 20 THF/water mixture was chosen as a solvent system over toluene/water mixture due to the higher molecular weight polymers obtained at the end. DArP was performed with AA (Br) and BB (H) functionalized donor and acceptor monomers using neodecanoic acid, 107 Cs 2 CO 3 , P(o-OMePH) 3 and Pd 2 dba 3 in dry toluene or MeTHF at 120 o C (see Appendix 3). DArP completely eliminated the boronation of the donor molecule and halogenation of the acceptor molecule, excluding two synthetic steps towards the polymer formation. Resulting polymers were precipitated in methanol and purified by Soxhlet extraction using methanol, hexane and chloroform. SCHEME 4.2. Syntheses of the WBG conjugated polymers. We performed Suzuki polymerization for the synthesis of PFTPTI copolymer only. In order to synthesize PPTPTI via Suzuki polymerization, dibromo-functionalized alkoxy benzene unit should be boronated through lithiation-boronation step. However, likely due to the bulky alkoxy side chains on the 2,5 position of the 1,4-dibromo benzene ring, boronic ester formation does not occur on the 1,4 position, supported by the lack of literature evidence of boronated alkoxy benzene. We also wanted to look at the polymer properties of biphenyl and TPTI perfectly alternating copolymer (PBPTPTI) that would be another WBG polymer (Scheme 4.3.). However, resulting polymer was insoluble in common solvents due to the lack of solubilizing chains on the biphenyl ring, decreasing polymer solubility drastically. 108 SCHEME 4.3. Chemical structure of PBPTPTI. Molecular weights were determined by dissolving the polymers in o-DCB and running the samples through gel permeation chromatography (GPC) against polystyrene standards (Table 4.1.). PPTPTI had the highest molecular weight of 20.0 kDa likely due to the highly soluble alkoxy benzene unit with long branched alky chains. Replacement of the alkoxy benzene unit with fluorene decreased the molecular weights for both PFTPTI (DArP) (8.3 kDa) and PFTPTI (Suzuki) (10.7 kDa). This was attributed to the limited solubility of the fluorene unit resulting in low molecular weight polymers. Structures of the polymers were analyzed using 1 H-NMR spectroscopy. Polymer Mn a (kDa) Đ a HOMO b (eV) Optical Eg c PCTPTI (DArP) 12.1 2.3 -5.57 2.25 PCTPTI (Suzuki) 16.1 2.2 -5.65 2.24 PFTPTI (DArP) 8.3 1.6 -5.63 2.30 PFTPTI (Suzuki) 10.7 2.3 -5.63 2.30 PPTPTI (DArP) 20.0 3.1 -5.57 2.09 109 TABLE 4.1. Molecular weights, polydispersity indices, electrochemical HOMO values, optical band gaps and surface energies of the polymers. a Determined by GPC with polystyrene as standard and o-DCB as eluent. b Cyclic voltammetry (vs. Fc/Fc + ) in 0.1M TBAPF 6 . c Calculated from the absorption band edge in thin films, E g = 1240/λ edge . 4.3.2. Optical and Electronic Properties The optical properties of the polymers in thin films are demonstrated in Figure 4.1., using UV-Vis absorption spectroscopy. The optical band gaps were determined based on the absorption onsets (Table 4.1.). Absorption coefficients were determined from the film thicknesses estimated by GIXRD in the reflectivity mode. PFTPTI polymers exhibited almost the same absorption profiles in the range of 375 nm-550 nm for both Suzuki and DArP analogues, being very similar to PCTPTI, while absorption coefficients were reaching 0.9-1.0 x 10 -5 cm -1 . As can be seen, PPTPTI polymer exhibited a red-shifted absorption profile in the range of 400 nm-650 nm. This was attributed to the better electron donating feature of the alkoxy benzene unit compared to the fluorene and carbazole, decreasing the band gap of the PPTPTI and resulting in a red-shift in the absorption profile. PPTPTI also showed a dual band absorption indicating π- π * transitions in the short wavelength region. Optical E g of the polymers were significantly high changing between 2.09-2.30 eV, making the resulting polymers a promising candidate for ternary blend solar cell, tandem solar cell, and fullerene-free solar cell applications. PFTPTI polymers showed the highest optical E g of 2.30 eV, consistent with its absorption profile and HOMO energy level. In our recent study, we reported the optical E g of PCTPTI as 2.25 eV for the DArP analogue and 2.24 eV for the Suzuki analogue, being very close to PFTPTI due to the structural and electronic similarities. 20,24 110 Figure 4.1. UV-Vis absorption spectra of the polymers in thin films spin-coated from o- DCB: (i) PCTPTI (DArP) (black, squares), (ii) PCTPTI (Suzuki) (red, circles), (iii) PFTPTI (DArP) (blue, upward triangles), (iv) PFTPTI (Suzuki) (green, downward triangles), (v) PPTPTI (DArP) (pink, diamonds). The HOMO energy levels of the polymers were measured as a film via cyclic voltammetry (CV), and reported in Table 4.1. Introduction of the TPTI acceptor into the polymer backbone resulted in significantly low HOMO energy levels. HOMO values of PFTPTI for both DArP and Suzuki analogues were same being -5.63 eV. PPTPTI showed the HOMO energy level of -5.57 eV. Measured HOMO energies contributed to the wide optical band gap of the polymers and high V oc values. 4.3.3. Device Performance In order to investigate the effect of deep HOMO energy levels and wide band gaps on device performances, PSCs were fabricated and tested. All devices were fabricated in air with the structure of ITO/PEDOT:PSS/BHJ active layer/Al and tested 111 under AM 1.5G simulated solar illumination. Different polymer:PC 61 BM ratios between 1:1 to 1:2, different concentrations between 10 mg/mL to 12 mg/mL, solvents including o-DCB and chloroform, and spin-coating conditions were studied for the optimization of the solar cells (See Appendix 3 for detailed solar cell fabrication procedures). The optimized polymer:PC 61 BM ratios were 1:1.3 for all the polymers. Optimal processing conditions were based on slow solvent evaporation from the polymer:PCBM blends of the thin films under N 2 for 30 min after spin coating the active layer, followed by aluminum deposition. Resulting J SC, V OC , FF and PCE values of the polymers are reported in Table 4.2. Polymer a :PC 61 BM J sc b (mA/cm 2 ) V oc c (V) FF d PCE e (%) PCTPTI (DArP) 6.19 0.96 0.47 2.98 PCTPTI (Suzuki) 6.62 0.96 0.52 3.06 PFTPTI (DArP) 3.03 0.90 0.33 0.90 PFTPTI (Suzuki) 4.92 0.88 0.37 1.59 PPTPTI (DArP) 4.17 0.68 0.47 1.31 TABLE 4.2. Photovoltaic properties of PCTPTI (DArP), PCTPTI (Suzuki), PFTPTI (DArP), PFTPTI (Suzuki), PPTPTI (DArP). a Spin-coated from o-dichlorobenzene, dried under N 2 before aluminum deposition. b Mismatch corrected. b, c, d, e Average of four pixels. In our previous studies, PCTPTI exhibited a significantly high V OC value of 0.96 V for both DArP and Suzuki analogues, consistent with its deep HOMO energy level of - 5.57 eV and -5.65 eV, respectively. 20,24 It is known that organic photovoltaics rarely exceed the V OC value of 1.0 V, making high V OC PCTPTI a promising polymer in order 112 to achieve improved efficiencies in solar cells. Here, we showed that PFTPTI is also a high V oc polymer with 0.90 V for the DArP analogue and 0.88 V for the Suzuki analogue, making it a promising candidate for several applications. Interestingly, PABTPTI showed the lowest V oc value even though it had a low HOMO energy level of -5.57 eV. This could be attributed to the disturbed planarity of the PPTPTI polymer in the bulk with PC 61 BM, increasing the HOMO of the blend and decreasing the V oc . The J SC values of the polymers were lower compared to the J sc value of PCTPTI. PFTPTI (DArP) exhibited a J sc value of 3.03 mA/cm 2 , likely due to the low molecular weight of the polymer. PFTPTI (Suzuki) showed higher J sc value of 4.92 mA/cm 2 , which was attributed to the higher molecular weight of the polymer than the DArP analogue, resulting in a good thin film formation and better charge extraction in the active layer. PPTPTI exhibited a J sc value of 4.17 mA/cm 2 , being comparable with the PFTPTI derivatives. However, PPTPTI exhibited a higher FF than the PFTPTI analogues, being 0.47, which is the FF value for the PCTPTI (DArP) polymer. Branched solubilizing chains on alkoxy benzene and TPTI provided good solubility in o- dichlorobenzene improving the polymer:PC 61 BM processing in solution. They both indicated a balanced- charge-carrier mobility and optimized morphology. PFTPTI polymers exhibited poor FF values of 0.33 and 0.37 for DArP analogue and Suzuki analogue, respectively. Results could be attributed to the poor processing of polymer:PC 61 BM in solution due to the low solubility of the fluorene, as well as the low molecular weight of the polymers. The external quantum efficiency data is shown in Figure 4.2. Devices showed a photocurrent response in the range of 350 nm to 625 nm. Photocurrent peaks around 400 nm are assigned to PC 61 BM, while the peaks around 500 nm are attributed to D/A 113 chromophores absorption. Higher J SC values obtained from PFTPTI (Suzuki) can be explained by the more intense and more balanced photocurrent response in the visible region. The integrated photocurrents from EQE match within less than 5% to that of the mismatch corrected photocurrents measured under simulated AM 1.5G illumination (see Appendix 3 Table A3.1.). Figure 4.2. External quantum efficiencies of the BHJ solar cells: (i) PFTPTI (Suzuki) (black, squares), (ii) PFTPTI (DArP) (red, circles), (iii) PPTPTI (DArP) (blue, upward triangles). 4.4 Conclusion In conclusion, a set of new perfectly alternating WBG copolymers was synthesized from electron-rich fluorene and phenylene donors and electron-deficient TPTI acceptor via Suzuki polymerization and DArP. Their optical, electronic, and photovoltaic properties were compared. For the first time, the electron-deficient TPTI unit was used as an acceptor in fluorene-based and phenylene-based conjugated polymers. In addition, DArP was shown to be a promising alternative to Suzuki Polymerization. PFTPTI polymer exhibited a deep HOMO energy level of -5.63 eV for 114 both DArP and Suzuki analogues, followed by a high V OC of around 0.90 V with PC 61 BM. PPTPTI showed a HOMO energy level of -5.57 eV, with a V oc of 0.68 V. Optical E g of the polymers were significantly high changing between 2.09 eV and 2.30 eV. Solar cell efficiencies of the polymers changed between 0.90-1.59%. Resulting polymer properties were also compared with the recently reported copolymer PCTPTI. Ongoing studies are focused on the applications of reported WBG polymers on different solar cell structures. 115 4.5 References for Chapter 4 (1) Thompson, B. C.; Fréchet, J. M. J. Angewandte Chemie International Edition 2008, 47 (1), 58–77. (2) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270 (5243), 1789–1791. (3) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Chem. 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Macromolecules 2009, 42 (8), 2891–2894. 125 APPENDIX 1: Random Multi-Acceptor Poly(2,7-carbazole) Derivatives Containing the Pentacyclic Lactam Acceptor Unit TPTI for Bulk Heterojunction Solar Cells A1.1 Materials and Methods All reagents from commercial sources were used without further purification, unless otherwise noted. All reactions were performed under dry N 2 , unless otherwise noted. All dry reactions were performed with glassware that was flamed under high vacuum and backfilled with N 2 . Flash chromatography was performed using a Teledyne CombiFlash Rf instrument in combination with RediSep Rf normal phase disposable columns. Solvents were purchased from VWR and used without further purification except for THF, which was dried over sodium/benzophenone before being distilled. All compounds were characterized by 1 H NMR (400 MHz) and 13 C NMR (400 MHz) on a Varian 400. Polymer 1 H NMRs (600 MHz) were obtained on a Varian 600 NMR spectrometer either at 80 °C using C 2 D 2 Cl 4 as a solvent or at 50 °C using CDCl 3 as a solvent. For polymer molecular weight determination, polymer samples were dissolved in HPLC grade o-dichlorobenzene at a concentration of 0.5 mg/ml, briefly heated and then allowed to turn to room temperature prior to filtering through a 0.2 µm PTFE filter. SEC was performed using HPLC grade o-dichlorobenzene at a flow rate of 1 ml/min on one 300 x 7.8 mm TSK-Gel GMHH R-H column (Tosoh Corporation) at 70 °C using a Viscotek GPC Max VE 2001 separation module and a Viscotek TDA 305 RI detector. The instrument was calibrated vs. polystyrene standards (1,050 – 3,800 000 g/mol) and data was analyzed using OmniSec 4.6.0 software. 126 Cyclic voltammetry was collected using an EG&G instruments Model 263A potentiostat under the control of PowerSuite Software. A standard three electrode cell based on a Pt wire working electrode, a silver wire pseudo reference electrode (calibrated vs. Fc/Fc+ which is taken as -5.1 eV vs. vacuum) 1,2 and a Pt wire counter electrode was purged with nitrogen and maintained under nitrogen atmosphere during all measurements. Acetonitrile was distilled over CaH 2 prior to use. Tetrabutyl ammonium hexafluorophosphate (0.1 M) was used as the supporting electrolyte for polymer films. Polymer films were made by repeatedly dipping the Pt wire in a 1% (w/w) polymer solution in chloroform or o-dichlorobenzene and dried under nitrogen prior to measurement. Polymer solutions were prepared in chloroform at a concentration of 0.1 mg/mL and tetrabutyl ammonium hexafluorophosphate (0.1 M) was used as the supporting electrolyte. For thin film measurements polymers were spin coated onto pre-cleaned glass slides from o-dichlorobenzene solutions (10 mg/mL). UV-vis absorption spectra were obtained on a PerkinElmer Lambda 950 spectrophotometer. The thickness and crystallinity of the thin films and GIXRD measurements were obtained using Rigaku Diffractometer Ultima IV using Cu Kα radiation source (λ= 1.54 Å) in the reflectivity and grazing incidence X-Ray diffraction mode, respectively. Crystallinity data is not included here because all seven polymers are amorphous, showing no crystallinity peak. Surface energy studies of the neat polymers film, using the static sessile drop method, were performed on Ramé-Hart Instrument Co. contact angle goniometer model 290-F1 and analyzed using Surface Energy (one liquid) tool implemented in DROPimage 2.4.05 software. Polymer films were prepared from o-dichlorobenzene solutions (10 127 mg/mL), spin-coated on the pre-cleaned glass slides. Water and glycerol were used as two solvents in the two-liquid model to measure the static contact angle and harmonic mean Wu model was used to calculate the average surface energy values for each film according to following set of equations: where Z w and Z g are the contacts angles with water and glycerol, respectively; γ tot is the total surface energy, γ p and γ d are the polar and dispersive surface energy components. A1.2 Synthesis Synthetic procedures for the synthesis of 9-(Heptadecan-9-yl)-2,7-bis(4,4,5,5- tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole, 4,7-Bis(5-bromothiophen-2- yl)benzo[c][1,2,5]thiadiazole, 2,5-Diethylhexyl-3,6-bis(5-bromothiophene-2- yl)pyrrolo[3,4-c]-pyrrole-1,4-dione, 2,8-Dibromo-4,10-bis(2- hexyldecyl)thieno[2',3':5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione were used without modifications as reported in the literatures. 3, 4, 5 128 N S N S S Br Br N B B O O O O C 8 H 17 C 8 H 17 Pd(PPh 3 ) 4 K 2 CO 3 THF/H 2 O, 80-85 o C 24h N C 8 H 17 C 8 H 17 S N S N S n N B B O O O O C 8 H 17 C 8 H 17 N N O O S S Br Br S N O N O S N C 8 H 17 C 8 H 17 n Pd(PPh 3 ) 4 K 2 CO 3 THF/H 2 O, 80-85 o C 24h N B B C 8 H 17 C 8 H 17 O O O O N N S S O O Br Br N C 8 H 17 C 8 H 17 N N S S O O n Pd(PPh 3 ) 4 K 2 CO 3 THF/H 2 O, 80-85 o C 24h N S N S S Br Br N B B O O O O C 8 H 17 C 8 H 17 Pd(PPh 3 ) 4 K 2 CO 3 THF/H 2 O, 80-85 o C 24h N C 8 H 17 C 8 H 17 S N S N S n N N O O S S Br Br m S N O N O S p 129 Scheme A1.1. Suzuki polymerizations of the D/A alternating and random multi-acceptor copolymers. Suzuki Polymerizations for the Polymers. Monomers 9-(Heptadecan-9-yl)-2,7- N B B C 8 H 17 C 8 H 17 O O O O N N S S O O Br Br Pd(PPh 3 ) 4 K 2 CO 3 THF/H 2 O, 80-85 o C 24h N N O O S S Br Br N C 8 H 17 C 8 H 17 N N S S O O N N O O S S m n o N S N S S Br Br N N O O S S Br Br N B B C 8 H 17 C 8 H 17 O O O O N N S S O O Br Br Pd(PPh 3 ) 4 K 2 CO 3 THF/H 2 O, 80-85 o C 24h N C 8 H 17 C 8 H 17 N N S S O O N S N S S N N O O S S m n o p 130 bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole, 4,7-Bis(5- bromothiophene-2-yl)benzo[c][1,2,5]thiadiazole, 2,5-Diethylhexyl-3,6-bis(5- bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione and 2,8-Dibromo-4,10-bis(2- hexyldecyl)thieno[2’,3’:5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione were added to 3-necked rbfs at varied molar ratios [(m=0.5, n=0.25, and n=0.25 for PCDTBT-DPP, PCDTBT-TPTI, PCBTDPP-TPTI), and (m=0.5, n=0.17, o=0.17 and p=0.16 for PCDTBT-TPTI-DPP)]. Monomers were dissolved in THF/H 2 O mixture under N 2 . K 2 CO 3 was added in one portion and stirred for 10 mins. Pd(PPh 3 ) 4 was added quickly in one portion and the reaction was refluxed at 80-85 o C for 24h. 6 Reaction mixtures were cooled to room temperature and precipitated into cold MeOH followed by ammonium hydroxide addition. Polymers were decanted into a thimble and purified via soxhlet extraction with methanol, hexanes and then collected in chloroform. Polymer chloroform solutions were concentrated in vacuo and precipitated into cold MeOH and collected via filtration. PCDTBT: Yield 76%. M n =11.8 kDa, Đ = 2.43. 1 H NMR (600 MHz, C 2 D 2 Cl 4 ) δ 8.26 (m, 2H), 8.16 (m, 2H), 8.01 (m, 2H), 7.86 (m, 2H), 7.66 (m, 2H), 7.58 (m, 2H), 4.73 (m, 1H), 2.45 (m, 2H), 2.15 (m, 2H), 1.39 (m, 8H), 1.26 (m, 16H), 0.87 (t, 6H). PCBTDPP: Yield 83%. M n =27.5 kDa, Đ = 3.68. 1 H NMR (600 MHz, CDCl 3 ) δ 9.05 (s, 2H), 8.13 (m, 2H), 7.88 (m, 1H), 7.69 (m, 1H), 7.59 (m, 4H), 4.66 (m, 1H), 4.17 (d, 4H), 2.37 (m, 2H), 2.05 (m, 4H), 1.44 (m, 6H), 1.33 (m, 14H), 1.16 (m, 20H), 1.00 (t, 6H), 0.91 (t, 6H), 0.81 (t, 6H). 131 PCTPTI: Yield 80%. M n =16.1 kDa, Đ = 2.20. 1 H NMR (600 MHz, C 2 D 2 Cl 4 ) δ 8.92 (m, 2H), 8.20 (m, 2H), 7.95 (m, 1H), 7.75 (m, 1H), 7.63 (m, 2H), 7.43 (m, 2H), 4.72 (m, 1H), 4.36 (d, 4H), 2.44 (m, 2H), 2.16 (m, 4H), 1.42 (m, 16H), 1.22 (m, 24H), 1.08 (t, 6H), 0.99-0.82 (m, 12H). PCDTBT-DPP: m = 0.5, n = 0.25, p = 0.25. Yield 74%. M n =23.8 kDa, Đ = 3.41. 1 H NMR (600 MHz, CDCl 3 ) δ 9.05 (m, 1H), 8.20 (m, 1H), 8.10 (m, 2H), 7.65-7.98 (m, 2H), 7.61 (m, 1H), 7.57 (m, 4H), 4.66 (m, 1H), 4.16 (m, 2H), 2.38 (m, 2H), 2.04 (m, 3H), 1.43 (m, 8H), 1.38-1.24 (m, 18H), 1.17 (m, 6H), 0.98 (t, 3H), 0.90 (t, 3H), 0.80 (t, 6H). PCDTBT-TPTI: m = 0.5, n = 0.24, p = 0.26. Yield 75%. M n =17.3 kDa, Đ = 3.50. 1 H NMR (600 MHz, C 2 D 2 Cl 4 ) δ 8.93 (m, 1H), 7.32-8.30 (m, 10H), 4.74 (m, 1H), 4.36 (d, 2H), 2.45 (m, 2H), 2.19 (m, 3H), 1.59 (m, 6H), 1.44 (m, 8H), 1.27 (m, 18H), 1.10 (t, 6H), 0.86 (m, 6H). PCBTDPP-TPTI: m = 0.5, n = 0.24, o = 0.26. Yield 80%. M n =18.7 kDa, Đ = 4.28. 1 H NMR (600 MHz, CDCl 3 ) δ 9.05 (s, 1H), 8.93 (s, 1H), 8.13 (m, 2H), 7.88 (m, 1H), 7.70 (m, 2H), 7.59 (m, 2H), 7.40 (m, 1H), 4.67 (m, 2H), 4.36 (d, 2H), 4.17 (m, 2H), 2.39 (m, 2H), 2.13-2.04 (m, 4H), 1.33 (m, 14H), 1.18 (m, 18H), 1.10-0.87 (m, 14H), 0.80 (m, 12H). PCDTBT-TPTI-DPP: m = 0.5, n = 0.14, o = 0.18, p = 0.18. Yield 79%. M n =15.1 kDa, Đ = 3.30. 1 H NMR (600 MHz, C 2 D 2 Cl 4 ) δ 9.03 (s, 0.72H), 8.94 (s, 0.56H), 8.26 (m, 0.72H), 8.17 (m, 2H), 8.01 (m, 1H), 7.86 (m, 2H), 7.64 (m, 2H), 7.58 (m, 1H), 7.45 (m, 132 0.56H), 4.72 (m, 1H), 4.40 (m, 1.12H), 4.19 (m, 1.44H), 2.45 (m, 2H), 2.15 (m, 3.28H), 1.65-1.24 (m, 34H), 1.10 (t, 3H), 1.05 (m, 3H), 1.00-0.94 (m, 4H), 0.86 (m, 6H). A1.3 Polymer Characterization Figure A1.1. 1 H NMR spectrum of PCDTBT in C 2 D 2 Cl 4 Figure A1.2. 1 H NMR spectrum of PCBTDPP in CDCl 3 133 Figure A1.3. 1 H NMR spectrum of PCTPTI in C 2 D 2 Cl 4 Figure A1.4. 1 H NMR spectrum of PCDTBT-DPP in CDCl 3 134 Figure A1.5. 1 H NMR spectrum of PCDTBT-TPTI in C 2 D 2 Cl 4 Figure A1.6. 1 H NMR spectrum of PCBTDPP-TPTI in CDCl 3 Figure A1.7. 1 H NMR spectrum of PCDTBT-TPTI-DPP in C 2 D 2 Cl 4 135 Figure A1.8. CV traces for the oxidation of PCDTBT vs. Fc/Fc + Figure A1.9. CV traces for the oxidation of PCBTDPP vs. Fc/Fc + Figure A1.10. CV traces for the oxidation of PCTPTI vs. Fc/Fc + 136 Figure A1.11. CV traces for the oxidation of PCDTBT-DPP vs. Fc/Fc + Figure A1.12. CV traces for the oxidation of PCDTBT-TPTI vs. Fc/Fc + Figure A1.13. CV traces for the oxidation of PCTPTI-DPP vs. Fc/Fc + 137 Figure A1.14. CV traces for the oxidation of PCDTBT-TPTI-DPP vs. Fc/Fc + Table A1.1. Surface energies of neat polymer films. Polymer One-Liquid Surface Energy (mN/m) Wu-Model Surface Energy (mN/m) PCDTBT 26.43 29.32 PCBTDPP 23.82 23.96 PCTPTI 25.07 25.95 PCDTBT-TPTI 26.03 26.62 PCDTBT-DPP 27.37 26.83 PCBTDPP-TPTI 24.22 22.48 PCDTBT-TPTI-DPP 25.78 25.06 138 A1.4 Device Fabrication and Characterization All steps of device fabrication and testing were performed in air. ITO-coated glass substrates (10 Ω/sq, Thin Film Deivces Inc.) were sequentially cleaned by sonication in detergent, deionised water, tetrachloroethylene, acetone, and isopropyl alcohol, and dried in a nitrogen stream. A thin layer of PEDOT:PSS (Baytron® P VP AI 4083, filtered with a 0.45 µm PVDF syringe filter – Pall Life Sciences) was first spin-coated on the pre- cleaned ITO-coated glass substrate and annealed at 120 °C for 60 minutes under vacuum. Polymer:fullerene solutions were prepared in o-dichlorobenzene and stirred for 24 hours at 60 °C. The polymer:PC 61 BM active layer was spin-coated (with a 0.45 µm PTFE syringe filter – Whatman) on top of the PEDOT:PSS layer. Concentrations of the polymers were 10 mg/mL in polymer. Films were placed in a nitrogen cabinet for 30 minutes before being transferred to a vacuum chamber. The substrates were pumped down to a high vacuum and aluminum (100 nm) was thermally evaporated at 4 – 5 Å/s using a Denton Benchtop Turbo IV Coating System onto the active layer through shadow masks to define the active area of the devices are 5.2 mm 2 for all devices. The current-voltage (I-V) characteristics of the photovoltaic devices were measured under ambient conditions using a Keithley 2400 source-measurement unit. An Oriel® Sol3A class AAA solar simulator with xenon lamp (450 Watt) and an AM 1.5G filter was used as the solar simulator. An Oriel PV reference cell system 91150V was used as the reference cell. To calibrate the light intensity of the solar simulator (to 100 mW/cm2 ), the power of the xenon lamp was adjusted to make the short-circuit current (JSC) of the reference cell under simulated sun light as high as it was under the 139 calibration condition. Spectral mismatch corrections were performed for each device according to previously described conditions. 1 External quantum efficiency measurements were performed using a 300 W Xenon arc lamp (Newport Oriel), chopped and filtered monochromatic light (250 Hz, 10 nm FWHM) from a Conerstone 260 1/4 M double grating monochromator (Newport 74125) together with a lightbias lock-in amplifier. A silicon photodiode calibrated at Newport was utilized as the reference cell Mobility was measured using a hole-only device configuration of blend polymer films (ITO/PEDOT:PSS/Polymer:PC 61 BM/Al) and an electron-only device configuration of Al/Polymer:PC 61 BM/Al in the space charge limited current regime. The device preparations for a hole-only device were the same as described above for solar cells. In the case of electron-only device, Al was deposited on the pre-cleaned glass followed with the same steps as in the case of a hole-only device. The dark current was measured under ambient conditions. At sufficient potential the mobilities of charges in the device can be determined by fitting the dark current to the model of SCL current and described by equation 4: where J SCLC is the current density, ε 0 is the permittivity of space, ε R is the dielectric constant of the polymer (assumed to be 3), µ is the zero-field mobility of the majority charge carriers, V is the effective voltage across the device (V = V applied – V bi – V r ), and L is the polymer layer thickness. The series and contact resistance of the hole-only device (35 – 45 Ω) was measured using a blank (ITO/PEDOT/Al) configuration and the voltage drop due to this resistance (V r ) was subtracted from the applied voltage. The built-in 140 voltage (V bi ), which is based on the relative work function difference of the two electrodes, was also subtracted from the applied voltage. The built-in voltage can be determined from the transition between the ohmic region and the SCL region and is found to be about 0.6 V. The series and contact resistance of the electron-only device (1.5-2 Ω) was measured using a blank (Al/Al) configuration. The built-in voltage (V bi ) determined from the transition between the ohmic region and the SCL region and is found to be about 2.3 V. Polymer film thicknesses were measured using GIXRD in the reflectivity mode. Polymer:PCBM (1:1.3) Hole Mobility (µ h ) (cm 2 V -1 s -1 ) Electron Mobility (µ e ) (cm 2 V -1 s -1 ) PCDTBT 1.82x10 -3 3.52x10 -4 PCBTDPP 3.36x10 -3 3.78x10 -6 PCTPTI 5.70x10 -2 6.39x10 -6 PCDTBT-TPTI 1.15x10 -2 5.13x10 -6 PCDTBT-DPP 1.69x10 -2 1.27x10 -4 PCBTDPP-TPTI 1.03x10 -2 5.83x10 -3 PCDTBT-TPTI-DPP 6.08x10 -3 2.23x10 -4 Table A1.2. Hole and electron mobilities of polymer:PC 61 BM blends in thin films spin- coated from o-DCB. 141 Table A1.3. Raw short-circuit current densities (J sc ), spectral mismatch factor (M), spectral mismatch-corrected short-circuit current densities (J sc,corr ) and integrated short- circuit current densities (J sc,EQE ). All devices were spin-coated from o-DCB and placed to the N 2 cabinet before aluminum deposition for 30 mins. Polymer:PCBM (1:1.3) J sc (mA/cm 2 ) M J sc,corr (mA/cm 2 ) J sc,EQE (mA/cm 2 ) J sc error (%) PCDTBT 7.27 1.01 7.20 7.57 4.9 PCBTDPP 6.55 0.82 7.99 7.81 2.3 PCTPTI 5.76 0.93 6.19 6.17 0.3 PCDTBT-TPTI 8.06 0.97 8.31 8.41 1.2 PCDTBT-DPP 5.14 0.91 5.66 5.82 2.7 PCBTDPP-TPTI 3.28 0.84 3.90 3.85 1.3 PCDTBT-TPTI-DPP 7.39 0.91 8.12 8.24 1.5 142 Figure A1.15. J-V curves of the BHJ solar cells spin-coated from o-dichlorobenzene and placed to N 2 cabinet for 30 min. A1.5 References for Appendix 1 (1) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater. Weinheim 2011, 23 (20), 2367–2371. (2) Thompson, B. C.; Kim, Y.-G.; McCarley, T. D.; Reynolds, J. R. J. Am. Chem. Soc. 2006, 128 (39), 12714–12725. (3) Khlyabich, P. P.; Rudenko, A. E.; Street, R. A.; Thompson, B. C. ACS Appl. Mater. Interfaces 2014, 6 (13), 9913–9919. (4) Howard, J. B.; Ekiz, S.; Noh, S.; Thompson, B. C. ACS Macro Lett. 2016, 5 (8), 977–981. -1.0 -0.5 0.0 0.5 1.0 -12 -9 -6 -3 0 3 6 9 Current density (mA/cm 2 ) Voltage (V) PCDTBT PCBTDPP PCTPTI PCDTBT-DPP PCDTBT-TPTI PCTPTI-DPP PCDTBT-TPTI-DPP 143 (5) Cao, J.; Liao, Q.; Du, X.; Chen, J.; Xiao, Z.; Zuo, Q.; Ding, L. Energy & Environmental Science 2013, 6 (11), 3224–3228. (6) Zou, Y.; Gendron, D.; Badrou-Aïch, R.; Najari, A.; Tao, Y.; Leclerc, M. Macromolecules 2009, 42 (8), 2891–2894. (7) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules 2011, 44 (13), 5079–5084. (8) Lee, J.; Cho, S.; Yang, C. Journal of Materials Chemistry 2011, 21 (24), 8528– 8531. (9) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C. Macromolecules 2011, 44 (6), 1242–1246. (10) Ji, Z.; Li, S.; Li, Y.; Sun, W. Inorg. Chem. 2010, 49 (4), 1337–1346. (11) Livi, F.; Gobalasingham, N. S.; Thompson, B. C.; Bundgaard, E. J. Polym. Sci. Part A: Polym. Chem. 2016, 54 (18), 2907–2918. (12) Ekiz, S.; Thompson, B. C. J. Polym. Sci. Part A: Polym. Chem. 2017, 55 (17), 2781–2786. 144 APPENDIX 2: Exploring the Influence of Acceptor Content on Semi-random Conjugated Polymers A2.1 Synthesis Synthetic procedures for the synthesis of 2-bromo-5-trimethyltin-3- hexylthiopehene, poly(3-hexyl thiophene), 2,5- bis(trimethyltin)thiophene, 2,5- Diethylhexyl-3,6-bis(5-bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione, and 3,6- di(2-bromothien-5-yl)-2,5-di(2-decyltetradecyl)-pyrrolo [3,4-c]pyrrole-1,4-dione were used without modifications as reported in the literature. 7, 8, 9 Scheme A2.1. Synthesis of the semi-random copolymers. Stille Polymerizations for the Polymers: Monomers 2-bromo-5-trimethyltin-3- hexylthiopehene (m), 2,5- bis(trimethyltin)thiophene (n), 3,6-di(2-bromothien-5-yl)-2,5- di(2-decyltetradecyl)-pyrrolo [3,4-c]pyrrole-1,4-dione (o), and 2,5-Diethylhexyl-3,6- bis(5-bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione (p) were added to 3-necked rbfs at varied molar ratios as shown in Scheme S1. Monomers were dissolved in dry DMF (0.04 M) under N 2 . The solution was then degassed for 20 min followed by 145 Pd(PPh 3 ) 4 addition in one portion. The reaction was heated to 95 o C and stirred for 48h. 7 Reaction mixtures were cooled to room temperature and precipitated into cold MeOH followed by ammonium hydroxide addition. Polymers were decanted into a thimble and purified via soxhlet extraction with methanol, hexanes and then collected in chloroform. Polymer chloroform solutions were concentrated in vacuo and precipitated into cold MeOH and collected via filtration. P3HTT-DPP 10%: m = 0.78, n = 0.11, o = 0.055, p = 0.055. M n =13.5 kDa, Đ = 2.6. 1 H NMR (600 MHz, C 2 D 2 Cl 4 ) δ 8.88 (d, 1H), 7.33 (s, 1.12H), 7.18 (m, 1.92H), 7.04 (m, 2.70 H), 4.07 (br, 1.81H), 2.84 (s, 6.56H), 2.65 (s, 0.42H), 2.44 (q, 1.58H), 1.75 (m, 7.03H), 1.57 (m, 1.98H), 1.48-1.25 (m, 53.40H), 1.07 (t, 2.79H), 0.95-0.89 (m, 19.05H). P3HTT-DPP 20%: m = 0.62, n = 0.19, o = 0.095, p = 0.095. M n =13.8 kDa, Đ = 3.3. 1 H NMR (600 MHz, C 2 D 2 Cl 4 ) δ 8.87 (s, 1H), 7.35 (m, 1.33H), 7.17 (m, 1.81H), 7.04 (m, 0.88), 4.09 (br, 1.89H), 2.85 (s, 3.20H), 2.45 (q, 10.37H), 2.04 (br, 1.87), 1.73 (m, 3.51H), 1.50-1.23 (m, 56.24H), 1.08 (t, 15.92H), 0.99-0.87 (m, 15.01H). P3HTT-DPP 30%: m = 0.42, n = 0.29, o = 0.145, p = 0.145. M n =14.7 kDa, Đ = 4.4. 1 H NMR (600 MHz, C 2 D 2 Cl 4 ) δ 8.87 (s, 1H), 7.36 (m, 1.31H), 7.21 (m, 1.49H), 4.08 (br, 1.65H), 2.87 (br, 1.40H), 2.45 (q, 6.33H), 2.00 (br, 1.63H), 1.78 (m, 1.69), 1.58-1.25 (m, 53.16H), 1.10 (t, 9.93H), 1.01-0.90 (br, 11.42H). P3HTT-DPP 40%: m = 0.28, n = 0.36, o = 0.18, p = 0.18. M n =10.6 kDa, Đ = 6.3. 1 H NMR (600 MHz, C 2 D 2 Cl 4 ) δ 8.87 (s, 1H), 7.34 (m, 1.01H), 7.19 (m, 1.93H), 4.09 (br, 146 1.56H), 2.87 (m, 0.76), 2.45 (q, 2.01), 1.99 (br, 1.17H), 1.78 (m, 0.85H), 1.58-1.23 (m, 38.55H), 1.10 (t, 3.31H), 1.04-0.90 (m, 9.68H). A2.2 Polymer Characterization Table A2.1. Molecular Weights and Polydispersity Indices of the Polymers. a Determined by GPC with polystyrene as standard and o-DCB as eluent. b Semi-random Polymer M n a (kDa) Đ a P3HT 11.2 2.0 P3HTT-DPP 10% b 13.5 2.6 P3HTT-DPP 20% b 13.8 3.3 P3HTT-DPP 30% b 14.7 4.4 P3HTT-DPP 40% b 10.6 6.3 P3HTT-DPP 10% c 16.7 3.2 P3HTT-DPP 20% c 14.5 3.6 P3HTT-DPP 30% c 15.6 3.8 P3HTT-DPP 40% c 19.4 3.7 147 polymers with EH and DTD functionalized DPP in the polymer backbone. c Semi-random polymers with only DTD functionalized DPP in the polymer backbone. Figure A2.1. 1 H NMR spectrum of P3HTT-DPP 10% in C 2 D 2 Cl 4 Figure A2.2. 1 H NMR spectrum of P3HTT-DPP 20% in C 2 D 2 Cl 4 148 Figure A2.3. 1 H NMR spectrum of P3HTT-DPP 30% in C 2 D 2 Cl 4 Figure A2.4. 1 H NMR spectrum of P3HTT-DPP 40% in C 2 D 2 Cl 4 149 Figure A2.5. CV traces for the oxidation of P3HTT-DPP 10% vs. Fc/Fc + Figure A2.6. CV traces for the oxidation of P3HTT-DPP 20% vs. Fc/Fc + -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -6.0x10 -4 -4.0x10 -4 -2.0x10 -4 0.0 2.0x10 -4 4.0x10 -4 6.0x10 -4 Current (A) Potential (V) P3HTT-DPP 10% -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -8.0x10 -4 -6.0x10 -4 -4.0x10 -4 -2.0x10 -4 0.0 2.0x10 -4 4.0x10 -4 6.0x10 -4 8.0x10 -4 Current (A) Potential (V) P3HTT-DPP 20% 150 Figure A2.7. CV traces for the oxidation of P3HTT-DPP 30% vs. Fc/Fc + Figure A2.8. CV traces for the oxidation of P3HTT-DPP 40% vs. Fc/Fc + -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -4.0x10 -4 -2.0x10 -4 0.0 2.0x10 -4 4.0x10 -4 6.0x10 -4 Current (A) Potential (V) P3HTT-DPP 30% -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -1.0x10 -3 -8.0x10 -4 -6.0x10 -4 -4.0x10 -4 -2.0x10 -4 0.0 2.0x10 -4 4.0x10 -4 6.0x10 -4 8.0x10 -4 1.0x10 -3 Current (A) Potential (V) P3HTT-DPP 40% 151 Figure A2.9. CV traces for the oxidation of P3HT vs. Fc/Fc + Table A2.2. Relevant information derived from GIXRD patterns of the polymers. -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -4.0x10 -4 -2.0x10 -4 0.0 2.0x10 -4 4.0x10 -4 6.0x10 -4 Current (A) Potential (V) P3HT Polymer 2θ (deg) d 100 (Å) P3HT 5.20 16.97 P3HTT-DPP 10% 4.95 17.83 P3HTT-DPP 20% 4.85 18.22 P3HTT-DPP 30% 4.55 19.40 P3HTT-DPP 40% 4.35 20.28 152 Figure A2.10. DSC traces of a) P3HT, b) P3HTT-DPP 10%, c) P3HTT-DPP 20%, d) P3HTT-DPP 30%, e) P3HTT-DPP 40%. 153 A2.3 Device Fabrication and Characterization All steps of device fabrication and testing were performed in air. ITO-coated glass substrates (10 Ω/sq, Thin Film Deivces Inc.) were sequentially cleaned by sonication in detergent, de-ionised water, tetrachloroethylene, acetone, and isopropyl alcohol, and dried in a nitrogen stream. A thin layer of PEDOT:PSS (Clevios PH 500, filtered with a 0.45 µm PVDF syringe filter – Pall Life Sciences) was first spin-coated on the pre- cleaned ITO-coated glass substrate and annealed at 120 °C for 60 minutes under vacuum. Polymer:fullerene solutions were prepared in o-dichlorobenzene and stirred for 24 hours at 60 °C. The polymer:PC 61 BM active layer was spin-coated (with a 0.45 µm PTFE syringe filter – Whatman) on top of the PEDOT:PSS layer. Concentrations of the polymers were 10 mg/mL in polymer. Films were placed in a nitrogen cabinet for 30 minutes before being transferred to a vacuum chamber. The substrates were pumped down to a high vacuum and aluminum (100 nm) was thermally evaporated at 4 – 5 Å/s using a Denton Benchtop Turbo IV Coating System onto the active layer through shadow masks to define the active area of the devices are 5.2 mm 2 for all devices. The current-voltage (I-V) characteristics of the photovoltaic devices were measured under ambient conditions using a Keithley 2400 source-measurement unit. An Oriel® Sol3A class AAA solar simulator with xenon lamp (450 Watt) and an AM 1.5G filter was used as the solar simulator. An Oriel PV reference cell system 91150V was used as the reference cell. To calibrate the light intensity of the solar simulator (to 100 mW/cm2 ), the power of the xenon lamp was adjusted to make the short-circuit current (JSC) of the reference cell under simulated sun light as high as it was under the 154 calibration condition. Spectral mismatch corrections were performed for each device according to previously described conditions. 1 External quantum efficiency measurements were performed using a 300 W Xenon arc lamp (Newport Oriel), chopped and filtered monochromatic light (250 Hz, 10 nm FWHM) from a Conerstone 260 1/4 M double grating monochromator (Newport 74125) together with a lightbias lock-in amplifier. A silicon photodiode calibrated at Newport was utilized as the reference cell Mobility was measured using a hole-only device configuration of polymer films (ITO/PEDOT:PSS/Polymer/Al) in the space charge limited current regime. The device preparations for a hole-only device were the same as described above for solar cells. The dark current was measured under ambient conditions. At sufficient potential the mobilities of charges in the device can be determined by fitting the dark current to the model of SCL current and described by equation 1: where J SCLC is the current density, ε 0 is the permittivity of space, ε R is the dielectric constant of the polymer (assumed to be 3), µ is the zero-field mobility of the majority charge carriers, V is the effective voltage across the device (V = V applied – V bi – V r ), and L is the polymer layer thickness. The series and contact resistance of the hole-only device (30 – 35 Ω) was measured using a blank (ITO/PEDOT/Al) configuration and the voltage drop due to this resistance (V r ) was subtracted from the applied voltage. The built-in voltage (V bi ), which is based on the relative work function difference of the two electrodes, was also subtracted from the applied voltage. The built-in voltage can be determined from the transition between the ohmic region and the SCL region and is 155 found to be about 0.6 V. Polymer film thicknesses were measured using GIXRD in the reflectivity mode. Table A2.3. Hole mobility of neat polymers in thin films spin-coated from o-DCB. a Average of two sets of devices. Polymer Hole Mobility a (cm 2 V -1 s -1 ) P3HTT-DPP 10% 9.00x10 -5 P3HTT-DPP 20% 6.81x10 -5 P3HTT-DPP 30% 3.16x10 -4 P3HTT-DPP 40% 1.47x10 -4 P3HT 4.40x10 -4 156 Figure A2.11. J-V curves of the BHJ solar cells spin-coated from o-dichlorobenzene and placed to N 2 cabinet for 30 min. Polymer:PC 61 BM J SC a (mA/cm 2 ) V OC b (V) FF c PCE d (%) P3HTT-DTDDPP 10% 4.71 ± 0.69 0.67 ± 0.02 0.43 ± 0.01 1.31 ± 0.25 P3HTT-DTDDPP 20% 1.83 ± 0.55 0.63 ± 0.01 0.48 ± 0.02 0.55 ± 0.20 P3HTT-DTDDPP 30% 1.29 ± 0.33 0.67 ± 0.00 0.40 ± 0.02 0.35 ± 0.11 P3HTT-DTDDPP 40% 1.20 ± 0.45 0.69 ± 0.00 0.36 ± 0.01 0.30 ± 0.12 Table A2.4. Raw device data of only DTD-DPP incorporated polymers (P3HTT- DTDDPP 10%, P3HTT-DTDDPP 20%, P3HTT-DTDDPP 30%, and P3HTT-DTDDPP 40%). All devices were spin-coated from o-DCB and placed to the N 2 cabinet before aluminum deposition for 30 mins. a, b, c, d Average of four pixels. Polymer a :PC 61 BM J sc b (mA/cm 2 ) V oc c (V) FF d PCE e (%) P3HT 7.09 ± 0.66 0.54 ± 0.06 0.50 ± 0.01 1.93 ± 0.18 P3HTT-DPP 10% 6.72 ± 0.34 0.62 ± 0.00 0.54 ± 0.05 2.26 ± 0.34 P3HTT-DPP 20% 4.84 ± 0.10 0.52 ± 0.00 0.58 ± 0.01 1.44 ± 0.03 P3HTT-DPP 30% 5.46 ± 0.50 0.56 ± 0.06 0.50 ± 0.03 1.52 ± 0.23 P3HTT-DPP 40% 8.17 ± 0.27 0.62 ± 0.00 0.55 ± 0.01 2.79 ± 0.14 Table A2.5. Raw device data of the semi-random copolymers that were fabricated from 157 fully-dried polymers. All devices were spin-coated from o-DCB and placed to the N 2 cabinet before aluminum deposition for 30 mins. a, b, c, d Average of four pixels. Polymer a :PC 61 BM J sc b (mA/cm 2 ) V oc c (V) FF d PCE e (%) P3HT 7.54 ± 0.39 0.55 ± 0.01 0.53 ± 0.02 2.16 ± 0.06 P3HTT-DPP 10% 8.31 ± 0.09 0.62 ± 0.00 0.61 ± 0.01 3.12 ± 0.07 P3HTT-DPP 20% 7.98 ± 0.43 0.55 ± 0.01 0.67 ± 0.01 2.91 ± 0.13 P3HTT-DPP 30% 10.23 ± 0.26 0.57 ± 0.00 0.67 ± 0.06 3.88 ± 0.08 P3HTT-DPP 40% 10.29 ± 0.41 0.62 ± 0.01 0.69 ± 0.00 4.43 ± 0.20 Table A2.6. Averages and standard deviations for Table 2. for the photovoltaic properties of P3HT, P3HTT-DPP 10%, P3HTT-DPP 20%, P3HTT-DPP 30%, and P3HTT-DPP 40% (MeOH treated wet polymers). All devices were spin-coated from o-DCB and placed to the N 2 cabinet before aluminum deposition for 30 mins. Polymer:PCBM (1:1.3) J sc (mA/cm 2 ) M J sc,corr (mA/cm 2 ) J sc,EQE (mA/cm 2 ) J sc error (%) P3HT 7.46 0.99 7.54 7.92 4.8 P3HTT-DPP 10% 6.73 0.81 8.31 8.53 2.6 P3HTT-DPP 20% 6.30 0.79 7.98 7.74 3.1 P3HTT-DPP 30% 7.57 0.74 10.23 10.15 0.8 P3HTT-DPP 40% 7.41 0.72 10.29 9.92 3.7 158 Table A2.7. Raw short-circuit current densities (J sc ), spectral mismatch factor (M), spectral mismatch-corrected short-circuit current densities (J sc,corr ) and integrated short- circuit current densities (J sc,EQE ) of the MeOH treated wet polymers. All devices were spin-coated from o-DCB and placed to the N 2 cabinet before aluminum deposition for 30 mins. A2.4 References for Appendix 2 (1) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater. Weinheim 2011, 23 (20), 2367–2371. (2) Thompson, B. C.; Kim, Y.-G.; McCarley, T. D.; Reynolds, J. R. J. Am. Chem. Soc. 2006, 128 (39), 12714–12725. (3) Khlyabich, P. P.; Rudenko, A. E.; Street, R. A.; Thompson, B. C. ACS Appl. Mater. Interfaces 2014, 6 (13), 9913–9919. (4) Howard, J. B.; Ekiz, S.; Noh, S.; Thompson, B. C. ACS Macro Lett. 2016, 5 (8), 977–981. (5) Cao, J.; Liao, Q.; Du, X.; Chen, J.; Xiao, Z.; Zuo, Q.; Ding, L. Energy & Environmental Science 2013, 6 (11), 3224–3228. (6) Zou, Y.; Gendron, D.; Badrou-Aïch, R.; Najari, A.; Tao, Y.; Leclerc, M. Macromolecules 2009, 42 (8), 2891–2894. (7) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules 2011, 44 (13), 5079–5084. (8) Lee, J.; Cho, S.; Yang, C. Journal of Materials Chemistry 2011, 21 (24), 8528– 8531. 159 (9) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C. Macromolecules 2011, 44 (6), 1242–1246. (10) Ji, Z.; Li, S.; Li, Y.; Sun, W. Inorg. Chem. 2010, 49 (4), 1337–1346. (11) Livi, F.; Gobalasingham, N. S.; Thompson, B. C.; Bundgaard, E. J. Polym. Sci. Part A: Polym. Chem. 2016, 54 (18), 2907–2918. (12) Ekiz, S.; Thompson, B. C. J. Polym. Sci. Part A: Polym. Chem. 2017, 55 (17), 2781–2786. 160 APPENDIX 3: Wide Band Gap Polymers Incorporating TPTI Acceptor For Bulk Heterojunction Solar Cells A3.1 Synthesis Synthetic procedures for the synthesis of 9,9-Dioctyl-9H-fluorene-2,7-diboronic acid bis(pinacol) ester (fluorene), 1,4-Dibromo-2,5-bis(2-ethylhexyloxy)benzene (phenylene), and 2,8-Dibromo-4,10-bis(2-ethylhexyl)thieno[2’,3’:5,6]pyrido[3,4- g]thieno[3,2-c]isoquinoline 5,11(4H,10H)-dione (TPTI) were used without modifications as reported in the literatures. 10,11,5 SCHEME A3.1. Synthesis of the wide band gap polymers. Suzuki Polymerization for PFTPTI. Monomers 9,9-Dioctyl-9H-fluorene-2,7-diboronic acid bis(pinacol) ester and 2,8-Dibromo-4,10-bis(2- ethylhexyl)thieno[2’,3’:5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline 5,11(4H,10H)-dione were added to a 3-necked rbf at 1:1 molar ratios. Monomers were dissolved in THF/H 2 O mixture under N 2 . K 2 CO 3 was added in one portion and stirred for 10 mins. Pd(PPh 3 ) 4 161 was added quickly in one portion and the reaction was refluxed at 80-85 o C for 24h. 12 Reaction mixture was cooled to room temperature and precipitated into cold MeOH followed by ammonium hydroxide addition. Polymers were decanted into a thimble and purified via soxhlet extraction with methanol, hexanes and then collected in chloroform. Polymer chloroform solutions were concentrated in vacuo and precipitated into cold MeOH and collected via filtration. Direct Arylation Polymerization for PFTPTI and PPTPTI. Monomers (0.25 mmol each), neodecanoic acid, and Cs 2 CO 3 were dissolved in dry toluene (PPTPTI) or MeTHF (PFTPTI) to yield a monomer concentration of 0.4M in a high pressure vessel. Reaction mixtures were degassed with nitrogen for 5 minutes before Pd 2 dba 3 was added and the vessels were further degassed for 5 minutes before the reactions were heated to 120°C for 12h. Then the reactions were cooled, a small about of chlorobenzene was added to the reaction mixtures and they were precipitated into methanol. Polymers were filtered and purified by Soxhlet extraction with methanol, hexanes, and finally chloroform. The final chloroform fractions were concentrated in vacuo and precipitated into methanol. The polymers were filtered and dried overnight under high vacuum. PFTPTI (Suzuki): 1 H NMR (600 MHz, C 2 D 2 Cl 4 ) δ 8.93 (m, 1H), 7.87 (m, 0.79H), 7.75 (m, 1.87H), 7.42 (m, 1.01H), 4.42 (m, 0.72H), 4.31 (m, 0.93H), 2.08 (m, 0.59H), 1.57 (m, 4.74H), 1.45 (m, 3.11H), 1.21 (m, 10.73H), 1.10 (t, 5.08H), 1.00 (m, 5.47H), 0.85 (t, 3.58H). PFTPTI (DArP): 1 H NMR (600 MHz, C 2 D 2 Cl 4 ) δ 8.93 (m, 1H), 7.87 (m, 0.75H), 7.75 (m, 1.85H), 7.42 (m, 0.93H), 4.42 (m, 0.77H), 4.32 (m, 1.08H), 2.17 (m, 2.54H), 2.07 (m, 162 0.74H), 1.33 (m, 3.52H), 1.21 (m, 10.49H), 1.09 (m, 3.71H), 1.00 (m, 3.86H), 0.94 (m, 2.65H), 0.85 (t, 3.59H). PPTPTI (DArP): 1 H NMR (600 MHz, C 2 D 2 Cl 4 ) δ 8.95 (m, 1H), 7.73 (s, 0.88H), 7.43 (s, 0.94H), 4.35 (m, 1.33H), 4.18 (m, 2.09H), 2.20 (m, 1.19H), 2.08 (m, 1.29H), 1.65 (m, 2.35H), 1.33 (m, 26.60H), 1.06 (t, 3.61H), 0.96 (t, 4.49H), 0.87 (t, 6.96H). A3.2 Polymer Characterization Figure A3.1. 1 H NMR spectrum of PFTPTI (Suzuki) in C 2 D 2 Cl 4 163 Figure A3.2. 1 H NMR spectrum of PFTPTI (DArP) in C 2 D 2 Cl 4 Figure A3.3. 1 H NMR spectrum of PPTPTI (DArP) in C 2 D 2 Cl 4 164 Figure A3.4. CV traces for the oxidation of PFTPTI (DArP) vs. Fc/Fc + Figure A3.5. CV traces for the oxidation of PFTPTI (Suzuki) vs. Fc/Fc + -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 -3.0x10 -4 -2.0x10 -4 -1.0x10 -4 0.0 1.0x10 -4 2.0x10 -4 3.0x10 -4 Current (A) Potential (V) PFTPTI (DArP) -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 -1.0x10 -4 -8.0x10 -5 -6.0x10 -5 -4.0x10 -5 -2.0x10 -5 0.0 2.0x10 -5 4.0x10 -5 6.0x10 -5 8.0x10 -5 1.0x10 -4 Current (A) Potential (V) PFTPTI (Suzuki) 165 Figure A3.6. CV traces for the oxidation of PPTPTI (DArP) vs. Fc/Fc + A3.3 Device Fabrication and Characterization All steps of device fabrication and testing were performed in air. ITO-coated glass substrates (10 Ω/sq, Thin Film Deivces Inc.) were sequentially cleaned by sonication in detergent, de-ionised water, tetrachloroethylene, acetone, and isopropyl alcohol, and dried in a nitrogen stream. A thin layer of PEDOT:PSS (Clevios PH 500, filtered with a 0.45 µm PVDF syringe filter – Pall Life Sciences) was first spin-coated on the pre- cleaned ITO-coated glass substrate and annealed at 120 °C for 60 minutes under vacuum. Polymer:fullerene solutions were prepared in o-dichlorobenzene and stirred for 24 hours at 60 °C. The polymer:PC 61 BM active layer was spin-coated (with a 0.45 µm PTFE syringe filter – Whatman) on top of the PEDOT:PSS layer. Concentrations of the polymers were 10 mg/mL in polymer. Films were placed in a nitrogen cabinet for 30 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 -8.0x10 -5 -6.0x10 -5 -4.0x10 -5 -2.0x10 -5 0.0 2.0x10 -5 4.0x10 -5 6.0x10 -5 8.0x10 -5 1.0x10 -4 Current (A) Potential (V) PPTPTI (DArP) 166 minutes before being transferred to a vacuum chamber. The substrates were pumped down to a high vacuum and aluminum (100 nm) was thermally evaporated at 4 – 5 Å/s using a Denton Benchtop Turbo IV Coating System onto the active layer through shadow masks to define the active area of the devices are 5.2 mm 2 for all devices. The current-voltage (I-V) characteristics of the photovoltaic devices were measured under ambient conditions using a Keithley 2400 source-measurement unit. An Oriel® Sol3A class AAA solar simulator with xenon lamp (450 Watt) and an AM 1.5G filter was used as the solar simulator. An Oriel PV reference cell system 91150V was used as the reference cell. To calibrate the light intensity of the solar simulator (to 100 mW/cm2 ), the power of the xenon lamp was adjusted to make the short-circuit current (JSC) of the reference cell under simulated sun light as high as it was under the calibration condition. Spectral mismatch corrections were performed for each device according to previously described conditions. 1 External quantum efficiency measurements were performed using a 300 W Xenon arc lamp (Newport Oriel), chopped and filtered monochromatic light (250 Hz, 10 nm FWHM) from a Conerstone 260 1/4 M double grating monochromator (Newport 74125) together with a lightbias lock-in amplifier. A silicon photodiode calibrated at Newport was utilized as the reference cell. 167 Polymer:PCBM (1:1.3) J sc (mA/cm 2 ) M J sc,corr (mA/cm 2 ) J sc,EQE (mA/cm 2 ) J sc error (%) PCTPTI (Suzuki) 5.76 0.93 6.19 6.17 0.3 PCTPTI (DArP) 6.02 0.91 6.62 6.66 0.1 PFTPTI (Suzuki) 4.23 0.86 4.92 4.99 1.4 PFTPTI (DArP) 2.61 0.86 3.03 3.14 3.5 PPTPTI (DArP) 3.79 0.91 4.17 4.30 3.0 Table A3.1. Raw short-circuit current densities (J sc ), spectral mismatch factor (M), spectral mismatch-corrected short-circuit current densities (J sc,corr ) and integrated short- circuit current densities (J sc,EQE ). All devices were spin-coated from o-DCB and placed to the N 2 cabinet before aluminum deposition for 30 mins. A3.4 References for Appendix 3 (1) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater. Weinheim 2011, 23 (20), 2367–2371. (2) Thompson, B. C.; Kim, Y.-G.; McCarley, T. D.; Reynolds, J. R. J. Am. Chem. Soc. 2006, 128 (39), 12714–12725. (3) Khlyabich, P. P.; Rudenko, A. E.; Street, R. A.; Thompson, B. C. ACS Appl. Mater. Interfaces 2014, 6 (13), 9913–9919. 168 (4) Howard, J. B.; Ekiz, S.; Noh, S.; Thompson, B. C. ACS Macro Lett. 2016, 5 (8), 977–981. (5) Cao, J.; Liao, Q.; Du, X.; Chen, J.; Xiao, Z.; Zuo, Q.; Ding, L. Energy & Environmental Science 2013, 6 (11), 3224–3228. (6) Zou, Y.; Gendron, D.; Badrou-Aïch, R.; Najari, A.; Tao, Y.; Leclerc, M. Macromolecules 2009, 42 (8), 2891–2894. (7) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules 2011, 44 (13), 5079–5084. (8) Lee, J.; Cho, S.; Yang, C. Journal of Materials Chemistry 2011, 21 (24), 8528– 8531. (9) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C. Macromolecules 2011, 44 (6), 1242–1246. (10) Ji, Z.; Li, S.; Li, Y.; Sun, W. Inorg. Chem. 2010, 49 (4), 1337–1346. (11) Livi, F.; Gobalasingham, N. S.; Thompson, B. C.; Bundgaard, E. J. Polym. Sci. Part A: Polym. Chem. 2016, 54 (18), 2907–2918. (12) Ekiz, S.; Thompson, B. C. J. Polym. Sci. Part A: Polym. Chem. 2017, 55 (17), 2781–2786.

Abstract (if available)

Abstract World energy use is predicted to grow by 48% between 2010 and 2040 and current trends in energy supply are economically and environmentally unsustainable. Consequently, there is a growing realization of the necessity for clean and renewable energy sources. Currently, renewable energy is the world’s fastest growing energy source, increasing by 2.6% per year. Among other renewable energy sources, solar energy has attracted increased attention. It can provide enough energy to the surface of the earth in a single hour to meet the energy demand of the world population for an entire year. Organic solar cells have attracted significant attention due to their low weight, flexibility, low cost and semitransparency. They are based on either conjugated polymers, small molecules, or both. ❧ This dissertation is mainly focused on different polymer structures to be used in a class of organic solar cells

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Creator Ekiz, Seyma (author)

Core Title From perfectly alternating to random multichromophoric conjugated polymers for ternary blend bulk heterojunction solar cells

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 11/14/2017

Defense Date 10/20/2017

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag conjugated polymers,OAI-PMH Harvest,solar cells,ternary blend

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Barry C. (committee chair), Prakash, Surya G. (committee member), Yoon, Jongseung (committee member)

Creator Email sekiz@usc.edu,seymaekiz@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c40-457165

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