環修飾之紫質的設計與合成以應用於太陽能光敏染

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(1) . Design and Synthesis of Core-modified Porphyrin Sensitizers for Dye-Sensitized Solar Cells. A Thesis Submitted to the National Taiwan Normal University. For the Degree of DOCTOR OF PHILOSOPHY (In Chemistry). By Sandeep Babruwahan Mane (898420121). Adviser Dr. Chen-Hsiung Hung. Department of Chemistry, National Taiwan Normal University . Institute of Chemistry, Academia Sinica, Taipei, Taiwan.

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(3) . CERTIFICATE. . This is to certify that the work incorporated in the thesis entitled “Design and Synthesis of Core-modified Porphyrin Sensitizers for Dyesensitized Solar Cells” submitted by Sandeep Babruwahan Mane was carried out by him under my supervision at the Institute of Chemistry, Academia Sinica, Taipei, Taiwan. . Dr. Chen-Hsiung Hung Institute of Chemistry, Academia Sinica 128, Academia Rd, Sec. 2, Nankang, Taipei 11529, Taiwan (ROC) E-mail: [email protected] TEL: +886-2-27898570 FAX: +886-2-27831237. v .

(4) . CANDIDATE’S DECLARATION . I hereby declare that the work presented in the dissertation entitled “Design and Synthesis of Core-modified Porphyrin Sensitizers for Dyesensitized Solar Cells” submitted for Ph.D. degree to National Taiwan Normal University, Taipei, Taiwan. The work has been carried out by myself at the Institute of Chemistry, Academia Sinica, Taipei, Taiwan (ROC), under the supervision of Dr. Chen-Hsiung Hung. The work is original and any of the part of this work was not submitted by me for another degree or diploma to this or any other university. In keeping with the general practice, due acknowledgements have been made, wherever. the. work. described. based. on. the. findings. of. other. investigators. Any inadvertent omissions that might have occurred, due to oversight or error in judgment are regretted. . Date: June 2014. Sandeep Babruwahan Mane. Department of Chemistry, National Taiwan Normal University, Taipei 11677, TAIWAN (ROC) vi .

(5) . Dedicated to,. My Teachers…… Teachers don't impact for a year or two, but for a lifetime.. And. My Family…… In every conceivable manner, the family is link to our past and bridge to our future.. vii .

(6) . Acknowledgements This thesis has been conducted in the Institute of Chemistry, Academia Sinica under the direction of Prof. Chen-Hsiung Hung. I wish to express my sincere gratitude to Prof. Hung for giving me the opportunity to get into research in his group. During the stay in this lab I enjoyed the freedom and support provided by my supervisor. Much helpful advice and the moral support from Prof. Hung has helped me to complete of the dissertation work within time. My gratitude also goes to Prof. Jiann-T’suen Lin, Prof. Ching-Fa Yao, Prof. Way-Zen Lee and Prof. Eric Diau, who agreed to read my thesis and to participate as a referee in my final oral defense. I also wish to thank our collaborator Dr. Diau for his help for the photovoltaic measurements. Completion of this thesis would not have been possible without the wonderful skills and valuable advices of Dr. Liyang Luo and Gao-Fong Chang. I would also like to thank my past and present colleagues, Dr. Anilkumar Pal, Dr. Amitava Dutta, Dr. Wei-min ching, Dr. Malgorzata Gajewska, Sam Chien, Chuan-Hung Chuang, Chien-Chen Yeh, Li-Ting Fong, Belete Bedemo, Shailaja Pingale, Jay-ar dela Cruz, Gong-Fong Lin, Chen-Yi Yu, Hsin-Han Tsai, Shan-Tung Liu, Yun-Ru Huang, Jia-Shan Li, Yi-Chieh Lo, Chih-Fu Cheng, Min-Hung Tsai, John, Quan-bo Chen, Amy, Denis, Luffy and Kai-tin for maintaining a congenial, fun filled atmosphere in the lab. Dr. Ram, my dear friend and colleague, indeed we had a great collaboration. I also want to express my sincere thanks to Dr. Ashutosh Singh, Dr. Pratap Patil, Surendhar Reddy and Dr. Rajeswara Rao for the untiring discussions about my work, great support in my depressed times and enduring well wishes. My deep gratitude also goes to my teachers Dr. Sunil Joshi, Dr. R. P. Pawar, Dr. W. N. Jadhav, Dr. Ratnalikar, Mr. Santpure, Mr. Khupse, Mrs. Chavan and Mr. Selukar, without your guidance and encouragement I could never achieve what I have today. At each step of my life, I had blessed with good friends who have healthy contribution in developing me as a better person. Life in Taiwan would never been as great without my friends here who helped me to relieve my hassles through small but merry events and concentrate back to my work. Friends from Parbhani and Pune, with whom I enjoyed some of the precious moments of my life, taught me to stay unite and focused in difficult times. I want to take this chance to be grateful to all my friends for always being there for me. Finally, I would like to thank my parents for providing me with the unconditional love and support needed in order to continually push myself to succeed. Without you're love and viii .

(7) . support, I wouldn't be here today! Dear bhauji, tai, pradip and sonali, your continuous encouragement and backing always kept me inspired and in the hunt for greater goals in my life. I preserve my gratitude to my late father-in-law for his blessings, mammy (mother-inlaw), Suraj and Dhiraj for their emotive support and encouragement. Dear deepika, words cannot express how thankful I am for your unwavering care and love. I couldn't have made it through the past year of ups and downs without you! I love you with all my heart. Last but not the least, with a warm heart, I would like to thank my nephews and nieces Nikita, Asmita, Atharva and Rajdeep for keeping me cheerful throughout the PhD.. ix .

(8) . TABLE OF CONTENTS Content. Page. Abbreviations List of publications Abstract. Ch 1. Introduction 1.1 Motivation. 1. 1.2 Natural photosynthesis. 3. 1.3 Basics of photovoltaics. 5. 1.4 Dye-sensitized solar cell. 8. 1.5 Sensitizers. 13. 1.5.1 Ruthenium sensitizers. 13. 1.5.2 Organic Dyes. 15. 1.5.3 Porphyrinoid sensitizers. 16. 1.5.3.1 Chlorins and Bacteriochlorins. 17. 1.5.3.2 Porphyrins. 18. 1.5.3.3 Corroles. 33. 1.5.3.4 Phthalocyanines and Subphthalocyanines. 33. 1.5.3.5 Core-modified Porphyrins. 35. 1.6 Scope of thesis. 36. 1.7 References. 37. Section I: Core-modified porphyrins Ch 2. Effects of core-modification on porphyrin sensitizers to the efficiency of dye-sensitized solar cells 2.1 Introduction. 46. 2.2 Results and Discussion. 48. 2.2.1 Syntheses. 48. 2.2.2 Optical Spectroscopy. 49. 2.2.3 Cyclic Voltammetry. 52. 2.2.4 DFT and TD-DFT calculations. 53. 2.2.5 Dye loading experiments. 55. 2.2.6 Photovoltaic measurements. 56. 2.2.7 TCSPC measurements. 57 x . .

(9) . 2.3 Conclusions. 59. 2.4 Experimental Section. 60. 2.5 References. 67. Ch 3. Synthesis of carboxylate functionalized A3B and A2B2 thiaporphyrins and their application in DSSCs 3.1 Introduction. 69. 3.2 Results and Discussion. 70. 3.2.1 Syntheses. 70. 3.2.2 Optical Spectroscopy. 72. 3.2.3 Electrochemical Spectroscopy. 74. 3.2.4 DFT and TD-DFT calculations. 76. 3.2.5 ATR-FTIR measurements. 77. 3.2.6 Dye loading experiments. 79. 3.2.7 Photovoltaic measurements. 80. 3.2.8 Electrochemical Impedance Spectroscopy. 82. 3.3 Conclusions. 83. 3.4 Experimental Section. 84. 3.5 References. 93. Section II: Core-modified expanded porphyrins Ch 4. Novel expanded porphyrin sensitized solar cell using boryl oxasmaragdyrins as the sensitizers 4.1 Introduction. 94. 4.2 Results and Discussion. 96. 4.2.1 Syntheses. 96. 4.2.2 Optical Spectroscopy. 97. 4.2.3 CV Studies. 100. 4.2.4 DFT and TD-DFT calculations. 102. 4.2.5 Dye loading experiments. 104. 4.2.6 Photovoltaic measurements. 105. 4.2.7 Transient photoelectric and Charge-Extraction Measurements. 106. 4.2.8 Femtosecond transient absorption studies. 108. 4.3 Conclusions. 109. 4.4 Experimental section. 111 xi . .

(10) . 4.5 References. 119. Ch 5. Molecular engineering of boryl oxasmaragdyrin sensitizers. through peripheral substituents: Effect of number and position of donor-acceptor groups 5.1 Introduction. 121. 5.2 Results and Discussion. 122. 5.2.1 Syntheses. 122. 5.2.2 Optical Spectroscopy. 125. 5.2.3 Electrochemical Spectroscopy. 128. 5.2.4 DFT and TD-DFT calculations. 130. 5.2.5 Dye loading measurements. 133. 5.2.6 Photovoltaic measurements. 134. 5.3 Conclusions. 137. 5.4 Experimental Section. 138. Ch 6. Novel carboxylate functionalized N2S3 Sapphyrins:. Synthesis, Photophysical and Photovoltaic properties 6.1 Introduction. 158. 6.2 Results and Discussion. 160. 6.2.1 Syntheses. 160. 6.2.2 Optical properties. 162. 6.2.3 Electrochemical properties. 163. 6.2.4 DFT calculations. 164. 6.2.5 Photovoltaic measurements. 166. 6.2.6 Electrochemical Impedance Spectroscopy. 167. 6.3 Conclusions. 168. 6.4 Experimental Section. 169. 6.5 References. 175. Section III: Miscellaneous dyes Ch 7. Co-sensitization of free-base and zinc porphyrins: An effective strategy to improve the photon to current conversion efficiency of DSSCs 7.1 Introduction. 177. 7.2 Results and Discussion. 179 xii . .

(11) . 7.2.1 Syntheses. 179. 7.2.2 Optical Spectroscopy. 180. 7.2.3 Electrochemical Spectroscopy. 182. 7.2.4 DFT and TD-DFT calculations. 184. 7.2.5 Dye loading measurements. 187. 7.2.6 Photovoltaic measurements. 187. 7.3 Conclusions. 190. 7.4 Experimental Section. 191. 7.5 References. 196. Ch 8. Novel D-π-A type organic dyes with oxadiazole ring as electron transporter for DSSC application 8.1 Introduction. 198. 8.2 Results and Discussion. 199. 8.2.1 Syntheses. 199. 8.2.2 Optical Spectroscopy. 200. 8.2.3 Electrochemical Spectroscopy. 201. 8.2.4 DFT and TD-DFT calculations. 202. 8.2.5 Photovoltaic measurements. 204. 8.2.6 Electrochemical Impedance Spectroscopy. 205. 8.3 Conclusions. 206. 8.4 Experimental Section. 207. 8.5 References. 211. Ch 9. Concluding Remarks. 213. Appendix 1 Publications. 216. xiii .

(12) . Abbreviations DSSCs. Dye-sensitized solar cells. DFT. Density functional theory. TD-DFT. Time-dependent density functional theory. HOMO. Highest occupied molecular orbital. LUMO. Lowest unoccupied molecular orbital. MLCT. Metal to ligand charge transfer. LHE. light harvesting efficiency. IPCE. Incident photon to current efficiency. WE. Working electrode. CE. Counter electrode. CV. Cyclic voltammetry. ATR-FTIR. Attenuated total reflectance-Fourier transform infrared. NMR. Nuclear magnetic resonance. HRMS-FAB. High resolution mass spectra-fast atom bombardment. HRMS-ESI. High resolution mass spectra-electrospray ionization. MALDI. Matrix Assisted Laser Desorption Ionization. ε. Extinction coefficient. η. Solar-to-electric power conversion efficiency. Jsc. Short-circuit current. Voc. Open-circuit voltage. FF. Fill Factor. EIS. Electrochemical impedance spectroscopy. TCSPC. Time-correlated single photon counting. ν (C=O). C=O stretching. νsym(COO¯). C-O symmetric stretching. νasym(COO¯). C-O asymmetric stretching. Hz. Hertz. ml. Milliliter. mg. Milligram. μm. Micrometer. μl. Microliter. ppm. Parts per million xiv . .

(13) . eq.. Equivalents. mp. Melting point. bp. Boiling point. CDCA. Chenodeoxycholic acid. TBP. 4-tert-Butyl pyridine. BMII. 1-butyl-3-methylimidazolium iodide. I2. Diiodide. LiI. Lithium iodide. DMPII. 1,2-Dimethyl-3-propylimidazolium iodide. GuSCN. Guanidinium thiocyanate. TFA. Trifluoroacetic acid. BF3•OEt2. Boron trifluoride etherate. n-BuLi. n-Butyl lithium. TMEDA. Tetramethylethylenediamine. DDQ. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone. TPA. Triphenylamine. NBS. N-Bromosuccinamide. KOH. Potassium hydroxide. THF. Tetrahydrofuran. CH3CN. Acetonitrile. EtOH. Ethanol. MeOH. Methanol. CH2Cl2. Dichloromethane. DMF. N,N-Dimethylformamide. DMSO. Dimethylsulfoxide. Pd2(dba)3. Tris(dibenzylideneacetone)dipalladium(0). AsPh3. Triphenyl Arsine. SI. Supporting information. xv .

(14) . List of publications The work carried out during my doctoral studies has led so far to three first author and one co-author publications, whereas four manuscripts are under preparation (see Appendix 1). First author 1. Synthesis of carboxylate functionalized A3B and A2B2 thiaporphyrins and their application in dye-sensitized solar cells, Sandeep B. Mane and Chen-Hsiung Hung, New Journal of Chemistry (2014), DOI:10.1039/C4NJ00373J. 2. Effects of core-modification on porphyrin sensitizers to the efficiencies of dye-sensitized solar cells, Sandeep B. Mane, Liyang Luo, Gao-Fong Chang, Eric Wei-Guang Diau and Chen- Hsiung Hung, J. Chin. Chem. Soc. (2014), 61, 545-555. 3. Novel expanded porphyrin sensitized solar cells using boryl oxasmaragdyrin as the sensitizer, Sandeep B. Mane, Jyun-Yu Hu, Yu-Cheng Chang, Liyang Luo, Eric WeiGuang Diau and Chen-Hsiung Hung, Chem. Comm. (2013), 49, 6882-6884. 4. Molecular engineering of boryl oxasmaragdyrins through peripheral substituents, Sandeep B. Mane and Chen-Hsiung Hung, manuscript under preparation. 5. Synthesis and photovoltaic properties of carboxylate functionalized A3B and A2B2 type N2S3 sapphyrins, Sandeep B. Mane and Chen-Hsiung Hung, manuscript under preparation. 6. Co-sensitization of free-base and zinc porphyrins: an effective approach to improve the photon-to-current conversion efficiency in DSSCs, Sandeep B. Mane, Liyang Luo and Chen-Hsiung Hung, manuscript under preparation. Co-author 7. Toward carboxylate group functionalized A4, A2B2, A3B oxaporphyrins and zinc complex of oxaporphyrins, Ram Ambre, Chien-Yi Yu, Sandeep B. Mane, Ching-Fa Yao, and Chen-Hsiung Hung, Tetrahedron (2011), 67; 4680-4688. 8. Anchoring Effects of Number and Position of meta-Carboxyphenyl and paraCarboxyphenyl Groups of Zinc Porphyrins in Dye-Sensitized Solar Cells: StructurePerformance Relationship, Ram Ambre, Sandeep B. Mane, Gao-Fong Chang and ChenHsiung Hung, manuscript under preparation.. xvi .

(15) . Abstract in English This thesis reported the design and synthesis of novel core-modified and expanded porphyrin sensitizers for dye-sensitized solar cells. The development of these sensitizers starts off with novel thiaporphyrin and oxaporphyrin, which consists of ethynylphenyl group as a conjugated linker and carboxylic acid as anchoring group. Comparative photovoltaic studies of these heteroporphyrins with regular N4 porphyrins show that core-modification results in decreased performance, with moderate efficiency for thiaporphyrin and minute efficiency for oxaporphyrin. Following parts of this thesis regard the development strategy through coremodification. The periphery of thiaporphyrin was modified to obtain five more derivatives with A3B and A2B2 substitution pattern. The photophysical and electrochemical studies revealed that the π-extension through the ethynylphenyl group not only made the macrocycle planar but also increased their absorption bathochromicaly. Furthermore, substitution of the electron withdrawing cyano group produced higher polarizability required for the effective charge transfer from the porphyrin ring to the anchoring cyanoacrylic group. All these factors contributed to achieve the highest overall power conversion efficiency of 1.69% for N3SECN compared to other thiaporphyrin dyes. We also devote effort to uncover the potential of applying expanded porphyrins to DSSC studies. Boron chelated oxasmaragdyrins, a class of aromatic core-modified expanded porphyrin with a 22 π-electron conjugation were synthesized. These dyes provided desired redox potentials, high absorption coefficients, high stability, and higher power conversion efficiencies. More importantly, broad absorption spreading over the entire visible region and its lower energy Q bands covering part of the NIR region. The factor above made this class of compound an optimistic candidate for being one of the future selections of porphyrinsensitized solar cells. The molecular engineering of these oxasmaragdyrins through peripheral substitution with various donors and acceptors was also discussed. To our surprise, utilizing of triphenylamine unit as a donor and ethynylphenyl group as a linker did not improve the overall performance for these dyes. Finally, novel mono or dual carboxylate functionalized N2S3 sapphyrins were prepared and applied as sensitizers in DSSCs for the first time. From the electrochemical studies it was observed that, the potential difference between the LUMO level of the dyes and conduction band of TiO2 was not sufficient for effective electron injection, which ultimately resulted in minute power conversion efficiencies for this class of dyes.. xvii .

(16) . In an effort to enhance the overall performance of porphyrin dyes, we also evaluated the co-sensitization effect. We combined two discretely moderate dyes, a free-base porphyrin and its zinc derivative, with complementary absorption spectra to construct a mixed porphyrin DSSC. From photo-action spectra, it is evident that the incident photon collection for the mixed dyes is higher than both the individual dyes due to the broader absorption. For the Mix-1, consisting of N4CA (2.71%) and N4ZnCA (3.09%), we observed conversion efficiency of 3.28% which higher than both the individual dyes, while for Mix-2, comprising for N4CN (1.94%) and N4ZnCN (3.52%), overall photon-to-current conversion efficiency of 4.18% was observed. We also evaluated the effectiveness of oxadiazole group as electron transporter. We attached an electron donor, oxadiazole ring and phenyl or thiophenephenyl group as linkers and carboxylic or cyanoacrylic group as anchor in D-π-A fashion to prepare three novel OD dyes. Although, the optical and electrochemical properties and DFT calculations confirmed good absorption features and effective charge separation, only moderate efficiency of 2.72% was observed for OD3 dye. So far, the work from this thesis has led to achieve the highest efficiency for thiaporphyrins and also unleashed a new class of porphyrinoids, the expanded porphyrins as highly efficient sensitizers for DSSCs.. Keywords: Dye-sensitized solar cells, core-modified porphyrins, expanded porphyrins, organic dyes. xviii .

(17) . Abstract in Chinese. 環修飾之紫質的設計與合成以應用於太陽能光敏染此篇論文報導了新的環修飾紫質以及大環紫質的設計與合成，以及其在太陽能光敏染料之應用。此些染料的發展由單硫取代紫質以及單氧取代紫質開始，並且其上有一ethynylphenyl 基團為共振鏈以及以羧基為掛載基團。比較這些異原子取代紫質與傳統的四氮紫質，結果顯示異原子的取代降低了這些染料的光電伏打效率，其中單氧取代的紫質表現更低於單硫取代的紫質。接著我們對單硫取代紫質進行了環修飾得到了擁有A3B以及A2B2之不同取代型式的五種衍生物。經由光物理和電化學的研究顯示以ethynylphenyl基團延伸共振的鏈長不僅增加了環的平面性，同時也增加了光譜的紅位移。而在取代基上加上擁有拉電子能力的cyano基團亦可增加分子的極性而讓分子有較佳的電荷轉移效果。結合上訴修飾，單硫取代紫質的光電轉換效率可由沒有cyano基團的0.2 % 提升到有cyano基團的1.69 %。我們亦致力於研究大環紫質於太陽能光敏染料的應用。我們合成出了有硼配位的. oxasmaragdyrins，其是擁有22 個π電子共振的芳香大環紫質。這些染料擁有合適的還原電位，高消光係數，良好的穩定性，以及高光電轉換效率。更重要的，其在可見光區段的吸光範圍廣泛，且其較低能量的Q band吸收位於在紅外光區段。基於上述條件， oxasmaragdyrins適合用來發展為太陽能光敏染料。我們亦於了oxasmaragdyrins 環上掛載了不同的推拉電子基，並進行研究與討論。令人意外的，掛載上tripheylamine為推電子基以及ethynylphenyl為共振鏈的染料其光電轉換效能相較於未修飾的染料並未增加。最後，我們合成了分別擁有一個和兩個羧基的新的二氮三硫 sapphyrins化合物並且將其應用於太陽能光敏染料。由電化學研究看來，其最低未佔軌域與二氧化鈦之傳導能帶間的差距太相近，使其不夠能讓激發的電子做有效的電子注入至二氧化鈦，故此染料的光電轉換效率不佳。至目前為止，此論文完成了單硫取代紫質最高的光電轉換效率，並且開發出了一系列新的大環紫質為高效率的太陽能光敏染料。. 關鍵字：太陽能光敏染料，環修飾紫質，大環紫質，硼配位的oxasmaragdyrins . xix .

(18) 1. Introduction “Energy is the ‘oxygen’ of the economy and the life-blood of growth”. -Peter Voser, Energy Community Leader, 2011, World Economic Forum. 1.1 Motivation In the old civilizations, energy was called as the source of all ‘life’, however for modern civilizations renewable energy will be the source of a better life. As of today, the world population exceeded 7 billion with a growth rate of 1.1% by 2011. With the increasing population, global energy consumption also grew by 2.5% in 2011. Fossil fuels still dominate energy consumption with 78% share of the market (Figure 1-1). Due to the tumultuous events of the ‘Arab Spring’ the balance between the oil production and sales disrupts resulting in record high oil prices of all-time.[1] The reserves of fossil fuels are limited and one day they will be depleted. A large dependency on fossil fuels is not only causing geopolitical tensions but also damaging our environment severely (Figure 1-2). These environmental and economic concerns have encouraged researchers to find alternative renewable sources of energy that could replace fossil fuels.. Figure 1-1. Renewable Energy Share of Global Energy Consumption, 2011.[2] Solar energy is the most abundant, clean and safe energy source that could also be harnessed in remote areas.Energy is consumed at a rate of about 15 Terawatts (TW) by global civilization, whereas total solar radiation on earth’s surface is around 1.22 × 105 TWs.[3-4] Energy from renewable sources like biomass, geothermal, hydropower, solar and wind currently accounts for about 19% of total energy. In terms of energy generation, the renewable energy comprises more than 25%. In last five years many renewable technologies .

(19) Introduction. . 2. grew at rapid rates. Wind energy clearly outperforms photovoltaics (PV) in terms of installed capacity. However solar PV is the fastest growing power generation technology, with operating capacity increasing at an average of 58% annually.[2] The PV sector shows extraordinary boom owing to a combination of growing demands, decreasing prices ($ 1.26 per watt compared to $ 3 per watt in 2005), huge government subsidies and the development of less expensive thin film technologies.. Figure 1-2. Energy future predictions. (Image courtesy: Y. S. Kang, Hanyang uni. Seoul) Back in my country India, increasing economic development has resulted in growing electricity consumption by the commercial as well as residential sectors. The country is largely dependent on fossil fuel imports to meet its energy requirements, which is estimated to exceed 53% of its total energy consumption by 2030. Out of 100, about 70% of the country’s energy generation capacity is from fossil fuels, with coal accounting for a large share (40%) of its total energy consumption. India being a tropical country receives adequate solar radiation for 300 days, amounting to 3000 hours of sunshine equivalent to over 5,000 trillion kWh. Therefore solar energy can play a pivotal role in the energy sector to fulfill our future energy demands and reduce the fossil fuel imports. Solar cell is the most suitable answer to this energy problem as the solar radiations are free and available all the year. Silicon solar cell is an outstanding performer of solar energy research with lab scale efficiency of 44% and for commercially available module efficiency of approximately 20%. Extensive research is underway all over the world to find efficient, low cost solar cells which can provide electrical energy affordable to all people. Continuous energy supply at economical price is necessary to get a better life. The dye-sensitized solar cell (DSSC), a technology invented by Ecole Polytechnique Federale de Lussane in 1991, fulfills these .

(20) Introduction. . 3. requirements and most probably will be a major contributor to the future commercial photovoltaic scenario. This thesis aims to support the development of the DSSCs by synthesizing efficient, cost effective and stable core modified porphyrin sensitizers. We are bathed in sunshine, and we can profit from it immensely. Thomas Edison told his friends Henry Ford and Harvey Firestone, back in 1931, “I’d put my money on the sun and solar energy. What a source of power! I hope we don’t have to wait until oil and coal run out before we tackle that.”. 1.2 Natural Photosynthesis The average rate of energy captured by photosynthesis globally is immense, approximately 130 TWs per year, which is ten times larger than the power consumption of human civilization (approximately 12 TW).[3-4] Photosynthesis is the process in which light energy is converted in to chemical energy and stored in the form of chemical bonds of sugar. This process occurs in plants, algae and cyanobacteria. Photosynthesis uses carbon dioxide and water to produce sugars releasing oxygen as by product. Although all cells in the green parts of a plant have chloroplasts (Figure 1-3), most of the energy is captured in the leaves.. Figure 1-3. Schematic diagram of Chloroplast. (source:- http://en.wikipedia.org) The cells in the interior tissues of a leaf, called the mesophyll, can contain between 450,000 and 800,000 chloroplasts per square millimeter of leaf. The surface of the leaf is uniformly coated with a water-resistant waxy cuticle that protects the leaf from excessive evaporation of water and decreases the absorption of ultraviolet or blue light to reduce heating. The transparent epidermis layer allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place. There are two parts of photosynthesis:. .

(21) Introduction. . 4. The light reaction occurs in the thylakoid membrane (Figure 1-4) and converts light energy to chemical energy. One molecule of the pigment chlorophyll (PSII) absorbs one photon and loses one electron. This electron is passed to a modified form of chlorophyll called pheophytin, which passes the electron to a quinone molecule, allowing the start of a flow of electrons through an electron transport chain that leads to the ultimate reduction of NADP to NADPH. In addition, this creates a proton gradient across the chloroplast membrane; its dissipation is used by ATP synthase for the concomitant synthesis of ATP. The chlorophyll molecule regains the lost electron from a water molecule through a process called photolysis, which releases a dioxygen molecule. The overall equation for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is as given below.[5] 2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2. (eq. 1). Figure 1-4. Schematic diagram of Thylakoid membrane. (source:- http://en.wikipedia.org) The dark reaction takes place in the storma within the chloroplast (Figure 1-5), and converts carbon dioxide to sugar. This reaction doesn’t need light in order to occur, but it does need the products of the light reaction (ATP and NADPH).The dark reaction involves a cycle called the Calvin cycle in which CO2 and energy from ATP are used to form sugar. The overall equation for the dark reaction is, 3CO2 + 9ATP + 6NADPH + 6H+ → C3H6O3-phosphate + 9ADP + 8Pi + 6NADP+ + 3H2O. (eq. 2). The fixation or reduction of carbon dioxide is a process in which carbon dioxide combines with a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP), to yield two molecules of a three-carbon compound, glycerate-3-phosphate (GP), also known as 3-phosphoglycerate (PGA). GP, in the presence of ATP and NADPH from the light reaction, is reduced to glyceraldehyde-3-phosphate (G3P), sometimes referred to as 3-phosphoglyceraldehyde. Five out of six molecules of the G3P produced are used to regenerate RuBP so the process can continue. .

(22) Introduction. . 5. Figure 1-5. Schematic diagram of Storma. (source:- http://en.wikipedia.org) The remaining one molecule of the G3P condenses to form hexose phosphates, which ultimately yield sucrose, starch and cellulose. Plants usually convert light into chemical energy with a photosynthetic efficiency of 3–6%.[6] Actual plant’s photosynthetic efficiency varies with the frequency of light being converted, light intensity, temperature and proportion of carbon dioxide in the atmosphere, and can vary from 0.1% to 8%.[7] Comparatively, solar panels convert light into electric energy at an efficiency of approximately 6–20% for massproduced panels, and above 40% in laboratory devices. As seen from the above discussion, solar energy is effectively collected by chromophores based on porphyrins (chlorophylls, P680 and P700).. 1.3 Basics of photovoltaic energy conversion PV is a method of generating electrical power by converting solar radiations into current using semiconductors that exhibit the photovoltaic effect. The photovoltaic effect refers to photons of light exciting electrons into a higher state of energy acting as a charge carrier for an electric current. This effect was first observed by Edmond Becquerel in 1839, when he was experimenting with illuminated metal electrodes in an electrolyte.[8] However, the photovoltaic effect was explained in 1905 with the pioneering theoretical work of Albert Einstein on the photoelectric effect for which he received the Noble prize.[9]. .

(23) Introduction. . 6. 1.3.1 The Solar Spectrum Solar irradiation can be well estimated by a black body at a temperature of 5800 K emitting according to Plank’s distribution.[10] The solar radiation is attenuated by absorption, reflection, and scattering. Sunlight is absorbed in the visible and UV region by molecular nitrogen, oxygen, ozone, nitrous oxide, and methane in the mid-infrared region by water vapor and in the infrared region by carbon dioxide. The spectrum is strongly confined between the far infrared and near ultraviolet by the time it reaches the earth’s surface. The air mass coefficient is used to characterize the solar spectrum after the solar radiation travels through the atmosphere and hence it is commonly used to characterize the performance of solar cells under standardized conditions. The Air Mass (AM) is the ratio of the path length (y) of the sun light passing through the atmosphere when the sun is at a given angle θ to the zenith, to the path length (x) when sun is at its zenith (Figure 1-6). This relation is approximated as below,. AM =. y. x. 1 cos θ. (eq. 3). Figure 1-6. Cartoon drawing illustrating AM. The solar irradiance outside the earth’s surface (AM 0), at sea level (AM 1) and the standard reference spectrum are compared in (Figure 1-7). The spectrum outside the atmosphere, the 5,800 K black body, is denoted as AM 0, meaning ‘zero atmospheres’. Solar cells for space power applications are generally characterized using AM 0. The spectrum after travelling through the atmosphere to sea level with the sun directly overhead is referred to as AM 1, meaning ‘one atmosphere’. AM 1 (θ = 0o) to AM1.1 (θ = 25o) range is used to estimate performance of solar cells in equatorial and tropical regions. The standard reference spectrum in PV is denoted by AM 1.5 G, which corresponds to the total global (hemispherical) irradiance under specified atmospheric conditions at an incident angle of 48o. .

(24) Introduction. . 7. 2.5. AM 0 AM 1.5 Direct AM 1.5 Global. -2. Spectral irradiance / Wm nm. -1. 2.0. 1.5. 1.0. 0.5. 0.0 500. 1000. 1500. 2000. 2500. 3000. Wavelength / nm. Figure 1-7. Solar irradiation spectra above atmosphere and at surface. 1.3.2 Photovoltaic Market overview Renewable energy continued to grow strongly as investments increase, prices falls, and policies spread. In the span of last five years, total global installed capacity of many renewable energy technologies grew at very fast rates. Solar PV capacity in operation at the end of 2011 (70 GW) was about ten times the global total (7 GW) just five years earlier (Figure 1-8). It grew the fastest of all with operating capacity increasing at an average of 58% annually. It was followed by concentrating solar thermal power (CSP), which increased almost 37% and wind power increased 26%. In spite of this admirable progress, renewable energy only shares 17% shares of global energy consumption. Biomass, solar and geothermal collectively share a tiny amount of 3.3% among the all renewable resources. By the end of 2011, 30 GW of operating capacity of solar PV was added, increasing the total global capacity by 74% to almost 70 GW, sufficient to generate 85 TW/year.[11]. Figure 1-8. Solar PV Total World Capacity.[2] .

(25) . Introduction. 8 . Solar PV is now the third most important renewable energy source following hydro and wind power, in terms of globally installed capacity. The number of countries having more than 1 GW capacity to their grids increases from three to six. So far the best efficiency solar cell is a multi-junction concentrator solar cell with the overall efficiency 44% (Figure 1-9). The highest efficiency of 35.8% was obtained by sharp corporation using a triple-junction technology in 2009[12] and Boeing Spectrolab have achieved 40.7% using a triple layer design. Crystalline silicon based modules are facing great competition by thin-film solar cells, CdTe, amorphous Si, and microcrystalline Si, which are expected to account for 31% of the global installed power capacity by 2013. San Jose based company Sunpower produces cells with energy conversion ratio of 19.5%, which is well above the market average of 1218%.[13] Solar cell efficiency varies from 6% for amorphous silicon-based solar cells to 44% with multiple-junction concentrated photovoltaics. But for the commercially available photovoltaic modules, the efficiencies are around 14-22%. The cost of PV has already reached well below nuclear power in 2011 and is set to fall further. The average solar cell prices as monitored by Solarbuzz group fell from $3.50/watt to $2.43/watt over the course of 2011 and the prices below $2.00/watt are looking inevitable. For large-scale installations, prices reached below $1.00/watt. The declining prices of PV are directly proportional to the installation capacities. Emerging technologies, such as DSSCs and organic solar cells are expected to grow rapidly in next few years. Although they have lower module efficiencies, their cost per watt is estimated to be three to four times lower than the conventional c-Si based systems. Currently these emerging technologies are being developed industrially in pilot plants and are very close to commercialization.. 1.4 Dye-sensitized solar cell DSSCs made from crystalline TiO2 electrodes is one of the most promising candidates in recent quest for cheap, clean and green alternative to fossil fuels. In recent years a lot of research is focused on the development of highly efficient and stable dyes. It was revealed in the late 1960s that upon illumination, organic dyes can generate electricity at oxide electrodes in electrochemical cells.[14] Current best PV research-cell efficiencies are displayed in Figure 1-9. To understand and simulate the primary processes in photosynthesis, the phenomenon was studied with chlorophyll extracted from spinach (bio-mimetic approach).[15] In 1972, these experiments lead to demonstrate electric power generation from solar cell via the dye sensitization principle.[16] .

(26) Introduction. . 9 . Figure 1-9. Current best photovoltaic research-cell efficiencies.[17] However, The instability of the DSSC was recognized as a main challenge.[18] Nanocrystalline semiconductor films have been used in the direct conversion of solar energy into chemical or electrical energy.[19-20] The conventional PVs having crystalline or amorphous silicon, have exceptional solar energy to electricity conversion efficiency of approximately 20%.[21] However, the fabrication of these PVs is expensive. CuInSe and CdTe thin film PV cells reach efficiencies of around 15%.[22] The scarcity of indium, selenium and tellurium can be a drawback for large scale production of these cells; also the high toxicity of cadmium has to be taken into account. In 1991, Michael Grätzel and Brian O’Regan ignited the solar cell research area with a spark of 7% overall power conversion efficiency using a ruthenium sensitizer and porous TiO2 layer as semiconducting material.[23] Recently DSSCs achieved certified conversion efficiencies of around 11.9% for laboratory-scale devices based on ruthenium sensitizers[24] and 12.7% for porphyrin-based devices[25] and 8.5% with small submodules.[26] The facile assembly, large choice of colors, transparency and mechanical flexibility are some features for extensive attraction towards the DSSCs. 1.4.1 Device structure A schematic diagram of a typical DSSC device is shown in Figure 1-10. Dye-sensitized solar cells separate the two functions provided by silicon in a traditional cell design. Silicon not. .

(27) Introduction. . 10 . only acts as the source of photoelectrons, but also it provides the electric field to separate the charges and create a current. In DSSCs, the semiconductor is used solely for charge transport, the photoelectrons are provided from a separate photosensitive dye. Charge separation occurs at the surfaces between the dye, semiconductor and electrolyte.. Figure 1-10. Schematic representation of Dye-Sensitized Solar Cell. The nanocrystalline semiconductor is normally TiO2 with typical sizes of 20-30 nm, film thickness of ~10 µm with a porosity of ~60%, although other wide band gap oxides like ZnO and SnO can be used.[27] A monolayer of the sensitizer is attached to the surface of the semiconductor. A redox mediator, commonly iodide/tri-iodide redox couple in organic solvent is used as electrolyte. The electrode with the mesoporous film (the photoanode) is sandwiched together with a second conducting glass substrate. The second electrode is coated with catalytically active platinum for efficient reduction of oxidized redox species. 1.4.2 Electron Injection, Transport and Recombination Efficient photon to current conversion occurs in DSSCs because of a judiciously welladjusted interplay of different kinetic processes as illustrated in Figure 1-11. In the dark, the fermi level of electrons in the TiO2 semiconductor is in equilibrium with the redox energy level of the electrolyte. When a photon is absorbed by the sensitizer (S), it is excited to the higher energy level (eq. 4). The excited state molecule (S*) injects an electron into the conduction band (Ec) of the semiconductor in a femto to picosecond timescale (eq. 5) before the dye can relax back to its ground state (eq. 8). The oxidized sensitizer (S+) is regenerated by iodide in the electrolyte within a few microseconds (eq. 6), which generally occurs more rapidly than reduction by photoinjected electrons in the TiO2 (eq. 9). The tri-iodide formed upon the dye regeneration is reduced at the platinized counter electrode (eq. 7). The additional charge in the TiO2 under illumination defines a quasi-Fermi level EFn. Electrons in .

(28) Introduction. . 11. the TiO2 are affected by two competing processes: Recombination with tri-iodide in the electrolyte (eq. 10) and diffusion through the mesoporous TiO2 to the front electrode. The effective time constants for these processes strongly depend on the trapping and detrapping events.. S . → S*. ∗. → . 2I → 2e → 3I . I3 ∗. (eq. 4). Charge injection. (eq. 5). I and 2I●2 → I3 + I. Dye regeneration. (eq. 6). Electrolyte regeneration. (eq. 7). Dye relaxation. (eq. 8). Recombination via dye. (eq. 9). → . 2. Photoexcitation. → I3 → 3I. Recombination via electrolyte (eq. 10). Figure 1-11. Schematic representation of the electron flow in DSSC. Recombination occurs in the millisecond to second range, and diffusion ideally occurs on a timescale one to two orders of magnitude smaller such that a large fraction of electrons is extracted at the front electrode. The differences in the electrochemical potentials (or Fermi energies) of the electrons at the opposite electrodes, i.e. EFn and Eredox, defines the photovoltage generated by the cell, The quasi-fermi level EFn of electrons in the TiO2 depends on the charge generation rate in the TiO2, the transport rate, and the recombination rate. 1.4.3 Incident Photon to Current Efficiency (IPCE) The solar cell performance is measured with several parameters like incident photon to current efficiency (IPCE), short circuit current (Jsc), open circuit voltage (Voc), fill factor (FF) and the overall efficiency of the photovoltaic cell (η). IPCE measures how efficiently .

(29) Introduction. . 12. the incident photons are converted to electrons. The wavelength dependent IPCE can be expressed as the product of the light harvesting efficiency (LHE), quantum yield of charge injection (Φinj), charge collection efficiency (ηcoll) at the back contact and quantum yield of regeneration (Φreg) (eq. 11).. IPCE = LHE· Φinj· ηcoll· Φreg. (eq. 11). Where, Φ and η are dependent on kinetic parameter, LHE depends on the active surface area of the semiconductor and on the light absorption of the sensitizers.[28] In practice the IPCE measurements are carried out with monochromatic light and calculated according to eq. 12,. IPCE %. 1240·Jph λ·Φ. ·100. (eq. 12). Where Jph is the short-circuit photocurrent density for monochromatic irradiation and λ and Φ are the wavelength and the intensity, respectively. 1.4.4 Overall Efficiency of the Photovoltaic Cell (η) The solar energy to electricity conversion efficiency is given by eq. 13,. η =. Jsc·Voc ·FF Φ. (eq. 13). Where Jsc is the short circuit current, Voc the open circuit voltage, FF the cell fill factor, and Φ the intensity of the incident light. The fill factor is defined by the ratio of the current and the voltage at the maximum power point to the short circuit current and the open circuit voltage. The fill factor measures the squareness of the I-V curve (Figure 1-12). DSSCs are advantageous over conventional p-n junction solar cells by features like, less sensitivity to impurities, easy fabrication, operation over a wide range of temperatures, different angles of the incident light, and lower production costs. The applications of these cells are more flexible since they can be made of different substrates such as glass, plastics, ceramics, fabric and metal.[27]. Figure 1-12. I-V curve. .

(30) . Introduction. 13 . 1.5 Sensitizers Design and synthesis of highly efficient sensitizers for DSSC is the most attractive and challenging field. Thousands of ruthenium,[29] porphyrinoids,[30-31] and organic dyes[32-33] have been synthesized and utilized in DSSCs. To be a best candidate, the photosensitizer should fulfill some important characteristics: 1. The absorption spectrum of the sensitizer should cover the whole UV-visible region and even the part of the near-infrared (NIR) region in order to absorb as many photons as possible. 2. Lowest unoccupied molecular orbital (LUMO) of the sensitizer should be more negative than the conduction band (CB) of the semiconductor and highest occupied molecular orbital (HOMO) should be more positive than the redox potential of the electrolyte so as to trigger the efficient electron transfer. 3. The molar extinction coefficient (ε) of the dye must be as high as possible to enable efficient light harvesting with thinner TiO2 film. 4. In order to minimize charge recombination between the injected electrons and the resulting oxidized dye, the positive charge resulting after electron injection should be localized at the donor part, so that it will be away from the TiO2 surface. 5. The dye should not aggregate on TiO2 film. 6. The synthesis should be easy and straightforward with minimum steps so that bulk production will be easy in future. 7. The dye should be enough stable and bind strongly on TiO2. Based on above requirements many sensitizers of ruthenium complexes, porphyrinoids and organic dyes have been designed, synthesized, and applied successfully in DSSC. Some of the best performers from ruthenium and organic sensitizers are discussed below. As this thesis deals with the synthesis of core-modified porphyrin dyes, porphyrinoids are discussed comprehensively. 1.5.1 Ruthenium Sensitizers The Ru complexes have shown the best photovoltaic properties due to their broad absorption spectrum, suitable excited and ground state energy levels, relatively long excited state life time and good electrochemical stability. Many Ru complexes used in DSSCs have achieved solar cell efficiency about 11%. The use of the Ru complexes with carboxylated bipyridine ligands for the TiO2 sensitization was first reported long back in 1979.[34] In 1985, a tris(2,2' .

(31) Introduction. . 14 . bipyridyl-4,4'-di-carboxylate)ruthenium(II) dichloride dye with three carboxylated bipyridine ligands is reported by Grätzel and coworkers to get an IPCE value of 44%.[35] In 1991 O’Regan and Grätzel reported a breakthrough 7.1-7.9% solar cell efficiency using a trinuclear Ru complex, a novel mesoporous TiO2 electrode and an organic solvent (MeCN) based electrolyte.[23] Later in 1993, Grätzel and coworkers reported N3 dye (Scheme 1-1) with solar to electric energy conversion efficiency of 10%.[36-37] N719 dye (Scheme 1-1) having only two free carboxylic acid groups to bind with TiO2 gives high solar cell efficiency of 11%.[3839]. N749 referred as “Black dye” (Scheme 1-1), a Ru complex with a terpyridine ligand. substituted with three carboxyl groups and three thiocyanato ligands having extended spectral response region up to the near-IR region gives improved efficiency 11.1%.[40-42] Until 2012, this was the first certified highest efficiency by a public test center (AIST, Japan). C101 (Scheme 1-1), a high molar extinction coefficient heteroleptic polypyridyl Ru sensitizer having alkyl thiophene unit in ancillary ligand achieved several benchmarks in DSSCs in 2012. It gave 11.3% efficiency with iodide/triiodide electrolyte in acetonitrile, a long term stability using low volatile electrolyte.. Scheme 1-1. Ruthenium Sensitizers. .

(32) Introduction. . 15 . Because of its high extinction coefficients, the TiO2 film thickness can be reduced, which favors the charge collection efficiency.[43-44] Recently, a new record efficiency of 11.4% is achieved by black dye using novel co-adsorbent Y1 (Scheme 1-1). This co-adsorbent effectively reduces the competitive light absorption by I−/I3−, avoids dye aggregation and decreases the charge recombination. This result is certified by AIST, Japan. Though these are the best performers in the DSSCs, their progress would be hindered due to the limited resources, sensitivity towards purity and high cost of ruthenium metal purification, demanding further search for new sensitizers. 1.5.2 Organic Sensitizers Organic dyes are considered as dark horse in the solar cell field due to their easy synthesis, very high molar extinction coefficients compared to the Ru dyes, and lower cost of preparation.[32-33] Their facile optimization can allow extension of the absorption wavelengths and tuning the HOMO-LUMO energy levels. Even though hundreds of organic dyes have been synthesized and applied in the DSSCs till date, only few of them gave efficiencies comparable to Ru dyes (Scheme 1-2).. Scheme 1-2. Organic Sensitizers. .

(33) Introduction. . 16 . Some important classes of the organic dye family are carbazoles,[45-47] coumarins,[48-51] fluorines,[52-55] hemicyanines,[56-58] heteroanthracenes,[59-63] indolines,[64-66] merocyanine,[67] perylenes,[68-69] quinaxolines,[70-71] squarines[72-77] and triphenylamines.[78-81] Some of the best solar cell performers having photon to current conversion efficiencies more than 9% are depicted in Scheme 1-2.[82-84] Dye C219 (Scheme 1-2) is the first and only organic sensitizer to achieve 10% PCE. The performance of organic dyes remain subordinate compared to Ru complexes and porphyrinoid sensitizers due to their low light harvesting capacity, as the absorption spectra is limited up to 600 nm only and lower stability. Owing to the recent higher efficiency, easy structural modification and high molar extinction coefficients organic dyes can still be a popular candidate for commercial solar cell modules. 1.5.3 Porphyrinoid Sensitizers Nature has utilized chlorophylls in plants as antennae to harvest light for the conversion of solar energy in the photosynthetic processes. In the photosynthetic cores of bacteria and plants, solar energy is collected at chromophores based on porphyrin;[85] the captured radiant energy is converted efficiently to chemical energy. Inspired by the natural photosynthesis, scientists utilized artificial chlorophylls, “the porphyrins” as efficient centers to harvest light for solar cells sensitized with a porphyrin. This efficient energy transfer in naturally occurring photosynthetic reaction centers motivated a lot of researchers to design and synthesize numerous porphyrins for DSSC applications.[30-31, 86-90] Porphyrins are heterocyclic macrocycles composed of four modified pyrrole subunits inter connected at their α-carbon atoms via methane bridges as shown in Figure 1-13. Porphyrin macrocycles are highly conjugated systems and as a consequence, they typically have very intense absorption bands in the visible region; the name "porphyrin" comes from a Greek word for purple.. . Figure 1-13. General structure of porphyrin. Due to their aromatic structure porphyrins have a similar chemical behavior as simple aromatics. The inherent advantages of porphyrin-based dyes are their rigid molecular .

(34) . Introduction. 17 . structures with large absorption coefficients in the visible region. Also, their various reaction sites, i.e., four meso and eight β-positions, are available for functionalization through which fine tuning of the optical, physical, electrochemical and photovoltaic properties of porphyrins thus becomes feasible. In principle, the free meso-carbon atoms are more reactive than the βcarbon atoms. Though regular N4 porphyrins are the most efficient and massively studied porphyrin analog, other porphyrinoids such as chlorins, bacteriochlorins, phthalocyanines and subphthalocyanines, corroles and thiaporphyrins have been efficiently used as sensitizers for DSSCs. In this section we reviewed various porphyriniod sensitizers for DSSCs. 1.5.3.1 Chlorins and Bacteriochlorins The use of chlorins in a DSSC was first reported in 1993 by Kay and Grätzel.[91] In that report a variety of metallo and free-base carboxychlorins were prepared from natural chlorophylls via metallation and/or saponification. The best DSSC reported was with a copper chlorophyll derivative, Cu-2-α-Oxymesoisochlorin e4 as sensitizer (Scheme 1-3), which gave an efficiency value of 2.6%.[91-92] Since this first report, chlorins have been studied in DSSCs using both free-base[93] and zinc metalated[94] forms. Ikegami et al. reported in 2008, the best performance of a DSSC using Chlorin-e6 (Scheme 1-3), η = 4.35% was observed after optimizing the co-adsorbent to avoid molecular aggregate formation between dye molecules.[95] The research by Wang et al. has had a profound impact on advances in chlorin and bacteriochlorin DSSCs,[96-104] starting in 2006 with work on chlorin PPB a in which an η = 4.2% was achieved using β-carotene as a co-adsorbent (Scheme 1-3).[97] Later PPB a was tested without the presence of the co-adsorbent and compared to other chlorins, bacteriochlorins, and porphyrins; in these tests an η of 3.8% was achieved and PPB a was the best sensitizer tested.[100] Dyes Chlorin 1-4 (Scheme 1-3) have carboxylic acid groups linked to the chlorin macrocycle via an ethylene moiety and gave η values in the range of 6.5%8.0% in DSSC tests, comparable to N719 which gave η = 9.3% under the same experimental conditions.[104] An improved η value of 7% has been reported for Chlorin 2 compared to Chlorin 1 (η = 6.5%).[102, 104] Chlorin 3 gave an η = 8.0% which is the best η value to date for a chlorin sensitizer.[104] Work on bacteriochlorin sensitizers by Wang et al. has yielded the most efficient bacteriochlorin DSSC to date. The dye BChlorin-1 (Scheme 1-3) uses dialkyl substitution at its second reduced pyrrole ring to increase the stability of the bacteriochlorin skeleton and avoid oxidation to the corresponding chlorin (a general problem with bacteriochlorins). The η value of BChlorin-1 sensitized DSSC was found to be 6.2% and was improved to 6.6% when chenodeoxycholic acid was used as a coadsorbent.[101] .

(35) Introduction. . 18 . . Scheme 1-3. Chlorin and Bacteriochlorin Sensitizers. 1.5.3.2 Porphyrin Sensitizers Regular porphyrins are the most studied sensitizers among porphyrinoids for the application in DSSCs. Through available four meso and eight β-positions, a variety of porphyrin derivatives were designed, synthesized and applied in DSSCs. Most of the design strategies are based on the mode of attachment of anchor either on β-position or on meso-position of porphyrin ring. Several other variations were also tried, like inserting different metals inside the porphyrin core, Zn/free base porphyrin heterodimers, checking the scope with different anchoring groups.[105-109] In this section we have systematically discussed the mode of attachment of the linker strategy. a) Attachment through β-position Modification at β-position with π-extending functional groups can result in red shifts in absorption spectrum and also increase the possibility of electron transfer from the substituent. .

(36) Introduction. . 19 . due to the splitting of the four frontier molecular orbitals. The first example of β-substituted porphyrins was reported by Officer, Grätzel and co-workers in 2004. They reported a series of β-substituted zinc porphyrins, among them Zn-1a (Scheme 1-4) attained promising efficiency of 4.8%.[105] Further they investigated different derivatives, out of which Zn-3 (Scheme 1-4) achieved η = 5.6% in the presence of co-adsorbent chenodeoxycholic acid (CDCA).[110]. N. N. N. N. N. N Zn. Zn N. N. N Zn. N. N. COOH NC. N. COOH HOOC. Zn-1a. Zn-3. GD2. COOH N. N. COOH Zn. N N. N. N. COOH. Zn N. COOH. COOH. N. N. COOH. COOH. 2b-bdta-Zn. tda-2b-bd-Zn. . Scheme 1-4. Molecular structures of β-substituted porphyrins. Later in 2007, the same group reported another series of porphyrin sensitizers with the best performer GD2 (Scheme 1-4) obtaining impressive efficiency of η = 7.1% with liquid electrolytes.[111] Kim and co-workers applied this strategy of attaching the π-conjugated linker at β-position of the porphyrin ring to design doubly anchored porphyrin sensitizers. They revealed that zinc porphyrin 2b-bdta-Zn (Scheme 1-4) with double malonic acid linkers effectively enhance the electron injection and retarded charge recombination.[112] The similar group in 2011, reported β-functionalized push-pull zinc poprhyrins with diarylamino donor, tda-2b-bd-Zn (Scheme 1-4) presenting the best performance of η = 7.5% which is comparable with N3 dye (η = 7.7%) under the similar conditions.[113] In 2013, the same group .

(37) Introduction. . 20 . also reported β-Ethynylbenzoic acid substituted push–pull porphyrins, ZnEP1 and ZnEP2 (Scheme 1-5). Surprisingly ZnEP1 with single anchoring arm (η = 5.9%) performed better than ZnEP2 with η = 4.0%. The overall conversion efficiency of ZnEP1 was comparable with YD1 (η = 6.2%).[114]. . Scheme 1-5. Molecular structures of β-substituted porphyrins. In a report, Pizzotti and co-workers synthesized five β-substituted porphyrins with ethynylphenyl linker and different anchoring acid groups and compared their DSSC performance with meso-substituted porphyrin derivatives.[115] They have shown that, zinc porphyrin 4 (Scheme 1-5) attained η = 4.6%, while the push-pull zinc porphyrin 5 (Scheme 15), achieved overall conversion efficiency of 4.7%. They also stated that the efficiency of 5 is better than the meso derivative (η = 4.2%). Although some of the above mentioned dyes show that β-substituted porphyrins performed better than meso-substituted porphyrins, their progress is limited to the overall conversion efficiency of 7.5%. Also it is noteworthy that the π-conjugation at the β-substituted porphyrins has a narrow effect to extend the absorption spectra to greater wavelengths. These results encourage researchers to change the design of extending the π-conjugation through functionalization the porphyrins at meso-position. .

(38) Introduction. . 21 . b) Attachment through meso-position The concept of the meso ethynyl substituted porphyrins was first reported by Anderson[116] and Therian.[117] The first meso-substituted free base porphyrin for DSSC application was reported in 2000 by Cherain and Wasmer[118] with η = 3.5%. After a long gap, in 2007 Galoppini et al. reported tetrachelated zinc porphyrins with meta-substituted linker on four meso positions to suppress dye aggregation. To solve the porphyrin aggregation, 3,5-di-tertbutylphenyl group were introduced at the meso-positions of the porphyrin ring. Following this concept, Yeh and co-workers designed and synthesized a library of meso- and βsubstituted porphyrins with carboxyl anchoring group. Their study revealed that dyes with meso-substituents are better in terms of efficiency than their β-substituent counterparts. They designed dye YD1 (Scheme 1-6), a push-pull porphyrin with a D-π-A skeleton, having diphenylamine as donor group, porphyrin chromophore as a π spacer and 4-ethynylbenzoic acid as acceptor group, to achieve 6.0% efficiency under AM 1.5 G illumination.[119] It is noteworthy that it is comparable with N3 dye (η = 6.1%) under the similar conditions. The superlative performance of YD1 reflects its remarkable short-circuit photocurrent density (Jsc) which arises from the large IPCEs broadly extending beyond 700 nm. The electron donor in YD1 plays a role not only spectrally to extend the absorption to a greater wavelength but also spatially pushing the excited electrons towards TiO2 for an improved separation of charge. Another promising porphyrin YD2 (Scheme 1-6) based on design of YD1 is reported by the same group with the tert-butyl groups were replaced by hexyl chains in the diphenylamine donor. The device performance of YD2 was improved to η = 6.8%. The electron donating nature of the amino substituents in YD1 and YD2 appears to be accountable for their higher open-circuit voltage (Voc).. H13C6 N. N Zn. N N. N. N COOH. Zn. N. N. N. COOH N. H13C6. YD1. YD2. . Scheme 1-6. Molecular structures of meso-substituted porphyrins YD1 and YD2. .

(39) . Introduction. 22 . Mozer and co-workers, using GD2 as an example noted that the lower Voc of porphyrins compared with the Ru sensitizers is due to the significantly decreased electron lifetime related to the rapid recombination of electrons with electrolyte.[120] In 2010, performance of the device based on YD2 was further improved by Grätzel and coworkers,[121] giving η = 10.9% supported by Jsc/ mA cm-2 = 18.6, Voc/V = 0.77, FF = 0.764. c) Porphyrins coupled with π-extended chromophores The best practical way to enhance Jsc is to harvest a broader region of the solar spectrum. In general, porphyrins show a Soret band at 400–450 nm and Q bands at 500–650 nm. To extend the absorption of porphyrin dyes to the NIR region, the energy gap between the HOMO and the LUMO levels must be reduced. There are two approaches to achieve this: first is to couple a highly conjugated π-extended chromophore with the porphyrin ring, and second is to synthesize fused or dimeric porphyrins. As seen from the previous section the best strategy to extend the π-conjugation is through functionalize the porphyrin at mesopositions. The acene family is most suitable for these purpose, in which the π-conjugation can be effectively increased with increasing number of aromatic rings. Lin and co-workers[122] prepared porphyrins coupled with acenes, from benzene to pentacene as π-extended chromophores, through ethynyl bond. Among these porphyrins, the anthracene substituted porphyrin LAC-3 (Scheme 1-7) displayed the best performance reaching 80% of the N3 dye under the similar conditions. Based on the backbone structure of YD2, Yeh and coworkers[123] designed three acenyl-ethynyl substituted porphyrins YD11-YD13. The bridge between ethyne and carboxyl group is varied from phenylene for YD11 (Scheme 1-7), naphthylene for YD12 (Scheme 1-7) and to anthracenylene for YD13. Among these dyes, YD11 and YD12 both exhibited superior performance relative to N719 dye with Jsc of these two porphyrin-based devices being significantly greater than that of N719 device. Without an added scattering layer the overall power conversion efficiencies of YD11 (η = 6.7%) and YD12 (η = 6.8%) outperforms that of N719 device (η = 6.1%). When the TiO2 films were covered with an additional scattering layer for light penetration, the cell performance of N719 pointedly improved to η = 7.3%, whereas the performances of the porphyrin dyes increased only slightly (η = 6.8% and 7.0% for YD11 and YD12, respectively).[123] These results indicate that a substantial increase in Jsc for the N719 device is a crucial factor for the enhancement of the cell performance with the addition of a scattering layer.[124] Based on those observations, the participation of the partially allowed triplet metal-to-ligand charge transfer (MLCT) states of ruthenium complexes was found to be responsible for the enhanced .

(40) Introduction. . 23 . efficiency in the red shoulder of the IPCE spectrum of N719, whereas the spin–orbit coupling in zinc porphyrins has insufficient effect for the S0-T1 transitions to occur; the additional scattering layer provided no improvement in the IPCE spectra of YD11–YD13 beyond the Q band absorptions.[123]. N. N Zn N. COOH N. LAC-3. H17C8. H17C8 N. N Zn. N N. Zn. N. N. H17C8. N. N COOH. N. COOH N. H17C8. YD12. YD13. . Scheme 1-7. Molecular structures of meso-substituted porphyrins. Functionalized chromophore anthracene plays an important role to extend the π-conjugation in LAC-3 for an improved overall device performance,[122] but the same anthracene group in YD13 with a link shorter than that in LAC-3 exhibited a notable effect to deteriorate significantly the device performance of YD13.[123] Results obtained from femtosecond measurements of fluorescence decay indicate that the presence of the anthracene group in the bridge from YD13 to TiO2 did not hamper the rate of interfacial electron transfer for the observed small injection yield of YD13; rather, it was the anthracene-induced rapid relaxation of intermolecular energy due to dye aggregation that gave the poor device performance of YD13, which was also evident in experiments in the absence and presence of co-adsorbent CDCA.[123] Lin and co-workers further designed cyclic aromatic substituents attached at the porphyrinic meso-position opposite to the anchoring group.[125-126] Among .

(41) Introduction. . 24 . these dyes the fluorene-modified porphyrin LD22 (Scheme 1-8) displayed the device performance η = 8.1%,[125] and the pyrene-substituted porphyrin LD4 (Scheme 1-8) attained an impressive efficiency of η = 10.1%,[126] and was superior to that of a N719 dye (η = 9.3%) under the same conditions. The superior photovoltaic performance of the LD4-based porphyrin-sensitized solar cell (PSSC) was attributed to its enhanced ability to harvest light with the IPCE action spectrum covering the entire visible spectral region and extending beyond 800 nm. Also Voc of LD4 device was much smaller than that of the N719 device, which might because of a much smaller electron lifetime of porphyrin-based solar cells than that of N719 cells.[120]. . Scheme 1-8. Molecular structures of meso-substituted porphyrins. Voc of LD4 was even smaller than that of YD2, which might be rationalized due to efficient electron interception for LD4 than for YD2 because of improved charge separation for the latter.[127] Wang and Wu,[128] reported another porphyrin dyes WW3 and WW5 (Scheme 1-8) coupled with N-annulated perylene (NP) which works as an efficient electron donor and also helps to push the absorption to higher wavelengths. The only difference between WW3 and WW5 was that the incorporation of an ethynylene group between the porphyrin and NP units, which would help to extend the π-conjugation and decrease the HOMO-LUMO gap, thus giving a longer-wavelength absorption spectrum. When tested for the photovoltaic performance, WW3 showed a moderate power conversion efficiency of 5.6%. While, when .

(42) Introduction. . 25 . WW5 was employed, an impressive broad IPCE action spectrum covering the panchromatic visible region and part of the NIR region was achieved. The most distinct character is that the onset of IPCE action spectrum was further red-shifted to 815 nm for WW5 cell, as a result, a high Jsc of 18.4 mA cm-2 was achieved corresponding to η = 10.3%. d) Fused Porphyrins According to above mentioned examples, porphyrins are promising photosensitizers for DSSC due to their intense absorption in the Soret and Q bands to harvest solar energy efficiently over a broad spectral region, but the presence of a wide dip between the Soret and Q bands in porphyrins limits their DSSC performance. To improve the light-harvesting ability of the porphyrins, another useful strategy is fusion of a chromophore with porphyrin for π-elongation. Feasibility of this idea was first proved by Imahori and co-workers, when they reported a meso-β edge naphthalene fused zinc porphyrin fused-Zn-1 (Scheme 1-9) which obtained η = 4.1%, which was improved to 5.0% under co-sensitization relative to the reference cell with an unfused porphyrin.[129-130] The same group[131] reported the unsymmetrically π-elongated quinoxaline-based β–β’-edge fused zinc porphyrins, out of which a fused porphyrin with one anchoring group ZnQMA (Scheme 1-9) exhibited η = 5.2%, attaining 80% performance of a N719 device under the same conditions.. . Scheme 1-9. Molecular structures of fused porphyrins.. .

(43) Introduction. . 26 . Comparison of the IPCE spectra of the two fused porphyrins revealed that, even though the fused-Zn-1 dye involves a broad light-harvesting property extending the IPCE action spectrum to nearly 800 nm, a large gap in the middle of the spectrum limits the growth of photocurrents to an optimal condition. In contrast, the IPCE spectrum of the ZnQMA dye extends to only ~700 nm, but an effective electronic coupling between quinoxaline and porphyrin moieties diminishes the gap between the Soret and Q bands of the spectrum, leading to improved Jsc and consequently device performance to that of fused-Zn-1. In 2013, the same group prepared a push–pull porphyrin ZnPQI (Scheme 1-9) with an electrondonating triarylamino group at the β,β’-edge through a fused imidazole group and an electron-withdrawing carboxyquinoxalino anchoring group at the opposite β,β’-edge.[132] The Soret band of ZnPQI was split into two peaks and becomes broader. The Soret band as well as the Q bands shifted toward longer wavelengths relative to that of ZnQMA. But the overall performance the ZnPQI solar cell (η = 5.5%) is comparable to that of the ZnQMA cell (η = 5.4%) under similar conditions. Wang and Wu[133] designed and synthesized two perylene anhydride fused nickel porphyrin sensitizers WW1 and WW2 (Scheme 1-10) to extend the light absorption towards the near-infrared (~1000 nm) region.. . Scheme 1-10. Molecular structures of perylene fused porphyrins. .

(44) Introduction. . 27 . Since these perylene fused porphyrins suffer from dye aggregation, the IPCE values did not exceed 30%, which was unable to attain a notable performance (η ~1.3%). The same group later in 2014,[128] reported another porphyrin dye fused with N-annulated perylene (NP) which works as an efficient electron donor and also helps to push the absorption to higher wavelengths. The absorption of the NP-fused porphyrin dye WW4 was much more redshifted to 792 nm due to the efficient π-extension after fusion. Two intense absorption bands at 444 nm and 540 nm were also observed. In spite of the red shifted absorption dye WW4 exhibited poorer performance due to its low lying LUMO energy level and non-disjoint HOMO/LUMO profile indicating that its driving force for electron injection was not sufficient. A very weak photocurrent response was observed for WW4, although its absorption is extended to the NIR region up to 920 nm. Both the Jsc and Voc gave very disappointing results (3.00 mA cm-2 and 0.500 V, respectively) corresponding to the overall η = 0.3% revealing that the fusion at opposite side of the anchoring group is less effective. Yeh and Diau[134] proposed another strategy, the fusion of two porphyrins, to extend the πconjugation. They designed and synthesized two fused porphyrins YDD2 and YDD3 (Scheme 1-11), but both of them exhibited poor DSSC performance. For YDD2, the absorption spectrum extends even beyond 1200 nm, but no photocurrent was observed because the energy level of LUMO was substantially lower than the conduction band edge of TiO2 indicating that its driving force for electron injection was not sufficient.. . Scheme 1-11. Molecular structures of fused porphyrins. For YDD3, although a small response was observed in the IPCE action spectrum corresponding to the contribution of broad bands I and II of the fused porphyrin, nearly no response was observed for the broad band III in region 700–900 nm. WW2, WW4 and. .

(45) Introduction. . 28 . YDD3 are, nevertheless, three interesting panchromatic porphyrin sensitizers with the potential to extend the light-harvesting ability toward the near-infrared region for PSSC. From all the example mentioned above, it is clear that the fusion at the acceptor part of the porphyrin is much more effective than the fusion at the donor part. e) Porphyrin Dimers Combination of two porphyrin moieties through a chemical bond is another useful strategy to improve light-harvesting ability of the sensitizer. The first attempt to use dimeric porphyrins as sensitizers for DSSC was made in 2009 by Officer and co-workers.[135] The device based on dimeric porphyrin dyes exhibited light-harvesting efficiencies slightly improved relative to the corresponding monomeric porphyrin. The effect of π-conjugation for the red shift of the IPCE spectra due to porphyrin dimerization was small as the link between the two porphyrins was made at the β-position. The first effort to link the two porphyrins at the meso-position was made in 2009 by Kim and Osuka.[136] Their polyethanediol (PEG)-modified dimer PEG2b-bd-Zn2 (Scheme 1-12) with two β-substituted linkers showed the best device performance of the various dimers, giving an overall efficiency η = 4.2%. Yeh and Diau[134] designed two porphyrin dimers, YDD0 and YDD1 (Scheme 1-12).. . Scheme 1-12. Molecular structures of dimeric porphyrins. .

(46) . Introduction. 29 . Similar to the results of Kim and Osuka,[136] the absorption spectrum of YDD1 exhibited slight red-shift relative to that of the monomeric porphyrin YD0. Also because of effective excitonic coupling between the two nearly perpendicular porphyrin units in YDD1, the gap shown in the IPCE spectrum of YD0 was completely filled in the spectrum of YDD1, making Jsc of the its device become much better resulting in slightly higher η = 5.23%. With the ethynyl linkage between the two porphyrin units, YDD0 showed split Soret bands in the range of 400-500 nm, and red shifts and broadening of the Q bands extending to nearly 800 nm. The broad IPCE spectrum of YDD0 showed much smaller efficiency of η = 4.07% than those of YD0 and YDD1 because of the co-planar structure which results in severe dye aggregation. Similar to the structural design of YDD0, Segawa and co-workers[137] added an electron donating carbazole unit at the meso-position of the porphyrin edge to form a push– pull porphyrin dimer, DTBC (Scheme 1-12). DTBC has an absorption spectrum similar to that of YDD0, but its IPCE spectrum showed much greater efficiency than that of YDD0. In the absence of a TBP additive, the DTBC device exhibited IPCE values up to 80%, yielding a remarkable Jsc/mA cm-2 = 18.2 but also causing a poor Voc/V~0.4. The poor Voc was significantly improved in the presence of TBP to attain an optimized device performance η = 5.2%. f) Enveloping porphyrins with long alkoxyl chains The superior performance of some of the above mentioned porphyrins such as YD2,[121] LD4[126] and WW5[128] was due to their higher light-harvesting ability resulted mainly by the introduction of an electron donating group or a π-extended chromophore at the meso-position of the porphyrin ring. However Voc of these highly efficient porphyrin dyes was significantly less than that of the commonly used ruthenium dyes. The considerably reduced electron lifetime was described to be responsible for the smaller Voc of porphyrins, and the positively charged zinc center of the porphyrin core might attract the I3− ions in the electrolyte causing effectual electron interception from the TiO2 surface.[120] In case of the organic dyes, Tian and co-workers concluded that Voc can be improved by decreasing the charge recombination and increasing the efficiency of electron injection.[138] To solve this problem of aggregation in porphyrins, a new concept was introduced to design a zinc-porphyrin sensitizer with long alkoxyl chains to protect the porphyrin core for retarded charge recombination and also to decrease effectively the dye aggregation for an efficient electron injection. Such a molecular design were first introduced in 2010 by Hupp and co-workers.[139] They reported porphyrin. .