以簡易且低成本的多掛載基之紫質於作為高效率之太陽能電池光敏染料

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(1)Facile and Low-cost Multi-anchoring Porphyrin Dyes as Efficient Sensitizers for Dye-Sensitized Solar Cells Thesis Submitted to the National Taiwan Normal University For the Degree of DOCTOR OF PHILOSOPHY In Chemistry By Ram Babruvan Ambre. Adviser Dr. Chen-Hsiung Hung Institute of Chemistry, Academia Sinica, Taipei, Taiwan.. Prof. Ching-Fa Yao Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan..

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(3) Acknowledgments. It gives me immense pleasure to express my deep sense of gratitude to my Ph.D. advisor Dr. Chen-Hsiung Hung for his valuable guidance, scientific suggestions, moral support, constant encouragement and financial support during my Ph. D. period. Your endless enthusiasm and receptive attitude will always remain a source of inspiration for me. I would like to express my sincere and humble gratitude to Prof. Ching-Fa Yao for believing me in my hard time and helping me in my tough situation. My sincere thanks to Chang Gao-Fong, for helping me at each step in every aspect, without your help it would have been very difficult to end up. Sandeep my colleague and friend, we had an excellent collaboration. Dear Manoj, your help is equally important. I am thankful to my all the past and present lab members, Dr. Liyang Luo, Dr. Amitawa Dutta, Dr. Anil kumar Pal, Dr. Maggie, Dr. Seesaw, Jaylene, Sam, Fred, Chen-yi, Chaun-Hung, Dev, Quan-Bo, Tami, Luffy, Belete, Jay-Ar, Anjali, Cora, and Kaiting, Ja-San for your cooperation and keeping our lab environment very friendly and healthy. You guys are the best company in and outside the lab. I am thankful to all my friends Dr. Nagendra Kondekar, Dr. Rakesh Kestwal, Dr. Veerababurao Kavala, Dr. Arun Soma, Dr. Shivaji More, Dr. Pratap Patil, Dr. Deepak Patil, Dr. Ramesh CHintakunta, Dr. Ravi Deore, Dr. Suhas Shinde, Dr. Vijay Paike, Dr. Umesh Jadhav, Dr. Santosh Salunke, Dr. Rajeswara Rao, Dr. Naren Singh, Dr. Deepak Barange, Dr. Damodar Janmanchi, Dr. Shobhit Charan, Dr. Sheshu Babu, Dr. Jamil Pateliya, Dr. Mustafa, Dr. Raju, Dr. Shastri, Dr. Dharam Singh, Deepak Huple, Sachin Shivtare, Sachin Gawande, Balraj, Jan Reddy, Lucky, Ajit, Milind, Suri, Himansu, Raja, Jonty, Ramesh Dateer, and Sagar Gawade for helping and making life more easy to live up here with the bless of your friendship. My Indian friends, Dr. Laxman, Dr. Satish biradar, Dr. Abasaheb, Dr, Suleman, Dr. Manmath, Dr. Sharad, Vikas Kadam, Harshal Santan, Akshay, Vinay, Kedar, Pramod, Avinash, Hari, Dahanji, Ramaknat, Vittal, Balaji, Prashant, Pradeep, Anoop, Maruti, Rameshwar, and Sachin, for your wonderful friendship. I also like to extend my thanks to all the professors of the Department of Chemistry, National Taiwan Normal University. Prof. Way-Zen Lee, Prof. Kwunmin Chen, Prof. Tun-Cheng Chien, II.

(4) and Prof. Jia-Jen Ho for their excellent guidance. I would like to thanks NMR operators, X-ray crystallographer, and mass facility operator and in charge Prof. Mei-Chun Tseng for providing quality analytical data. I am thankful to our librarian for her generous help. I wish to thank all the office staff members of Department of Chemistry, NTNU and Institute of Chemistry, Academia Sinica for all the help. Office of International Affairs, NTNU is like second home for every international student, many thanks to all of you for standing for us in our difficult time. You are the most enthusiastic and cooperative staff I have ever seen. I thank to Ministry of Education (MOE), government of Taiwan for providing me Taiwan Scholarship for doctoral study. Furthermore I am deeply indebted to my former Professors, Dr. U. R. Kalkote, Dr. S. P Joshi, Dr. S. P Chavan, Dr. B. P. Bandgar, Dr. P. K. Zubaidha, Dr. R. H. Tale, Dr. A. D. Sagar, Dr. S. S. Makone, Dr. S. M. Muley, Dr. A. V. Nawle, and teachers, Banshelkikar sir, Tamboli sir and Madane Madam, for their excellent teaching valuable guidance and motivation. I express my deep sense of gratitude to my loving parents (aai-dada), whatever I am and whatever I will is just because of your enormous blessings and selfless sacrifices. I preserve my everlasting gratitude for my brothers Deepak and Munna for their unconditional love and support throughout my life journey. Dear Munna, you have proved my belief, I am proud of you. I am thankful to my aaji (grandmother), attya (aunty), aai (mother in law) papa (father in law), bhavji (Brother in law), and Mama (Mr. kalidas Jadhav) for their emotional support and encouragement. My beautiful wife Reshu for your love, affection, and great deal of support, you bring enormous happiness to my life. Last but not least I express my hearty thank to almighty Lord Shiva for his immense blessings on me throughout my life.. Ram Babruvan Ambre. III.

(5) Abbreviations. DSSC. Dye-sensitized solar cell. η. Solar to electric power conversion efficiency. IPCE. Incident photon to current efficiency. LHE. Light harvesting efficiency. JSC. Short circuit current. VOC. Open circuit voltage. FF. Fill Factor. CB. Conduction band. ε. Molar extinction coefficient. DFT. Density functional theory. HOMO. Highest occupied molecular orbital. LUMO. Lowest unoccupied molecular orbital. CDCA. Chenodeoxycholic acid. ATR-FTIR. Attenuated total reflectance-Fourier transform infrared. ν (C=O). C=O stretching. νsym(COO¯). C-O symmetric stretching. νasym(COO¯). C-O asymmetric stretching. UV‒visible. Ultra violet-visible. NMR. Nuclear magnetic resonance. HRMS-FAB. High resolution mass spectra-fast atom bombardment. HRMS-ESI. High resolution mass spectra-electrospray ionization. Hz. Hertz. mL. Milliliter. nmol. Nanomol. ppm. Parts per million. Equiv.. Equivalents. µm. Micrometer. mg. Milligram. IV.

(6) bp. Boiling point. Bu4NPF6. Tetrabutylammonium hexafluorophosphate. n-BuLi. n-Butyl lithium. TBP. 4-tert-Butyl pyridine. DDQ. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone. TMEDA. Tetramethylethylenediamine. KOH. Potassium hydroxide. TPA. Triphenylamine. NBS. N-Bromosuccinimide. BMII. 1-Butyl-3-methylimidazolium iodide. SCN¯. Thiocyanate. GuSCN. Guanidinium thiocyanate. I2. Diiodide. LiI. Lithium iodide. DMPII. 1,2-Dimethyl-3-propylimidazolium iodide. KOH. Pottasium hydroxide. TEA. Triethylamine. THF. Tetrahydrofuran. CH3CN. Acetonitrile. EtOH. Ethanol. MeOH. Methanol. CH2Cl2. Dichloromethane. DMF. N, N-Dimethylformamide. SI. Supporting information. V.

(7) Abstract. Dye-sensitized solar cell (DSSC) technology is a potential game changing player in today’s solar energy related discipline. Nowadays tremendous effort has been applied for the development of highly efficient DSSCs. This thesis deals with the design and synthesis of facile, straightforward, scalable, low-cost and stable porphyrin sensitizers for DSSCs. The results have been divided into six chapters in this thesis. The first chapter includes introduction of DSSC, chapter 2‒5 describes author’s original works and the last chapter comprised of conclusions for this thesis. In the first chapter various components of DSSC such as working electrode, sensitizers, electrolyte, and counter electrode are described in order to explain the working principles of DSSCs. General trends and current developments in the DSSC research field are highlighted with the reviewing of recent literature. The common parameters used to define the efficiency of DSCC, incident Photon to current efficiency (IPCE), open circuit voltage (VOC), short circuit current (JSC), fill factor (FF) and solar to electric power conversion efficiency (η) are described. Highly efficient ruthenium, porphyrin, and organic dyes are sorted out with the help of recent literature survey. In the second chapter a series of porphyrin dyes Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A were synthesized and studied systematically. The effect of number and the position of thienyl and p-carboxyphenyl substituents of the zinc porphyrin dyes on the photovoltaic properties have been explained systematically. The solar energy conversion efficiency of Zn1S3A (3.01%) and cis-Zn2S2A (2.50%) is superior than trans-Zn2S2A (1.80%) and Zn3S1A (0.20%). These results elucidate significant effects of position and number of thienyl and pcarboxyphenyl group on electronic structure, electrochemical properties, and photovoltaic properties. The porphyrin sensitizers consisting of cis-Zn2T2A, cis-Zn2U2A, cis-Zn2S2A, cis-Zn2TH2A, cis-Zn2TC2A, cis-Zn2BC2A, and cis-Zn2TPA2A all with a 2D-π-2A donor-acceptor framework have synthesized in the third chapter. The device using 2D-π-2A porphyrin containing two cisVI.

(8) oriented electron-donating and two cis-oriented anchoring groups on their meso positions as the sensitizing dyes displayed functional group dependent electron injection properties. The highest efficiency of 4.07% has been achieved using cis-Zn2BC2A as the sensitizer. The photo-stability studies on these 2D-π-2A dual anchoring dyes showed higher dye stability in comparison with their mono-anchored porphyrin analogs. The fourth chapter described synthesis, characterization, and DSSC performance of NOVEL porphyrin sensitizers, namely Zn1T3A, Zn1U3A, Zn1S3A, Zn1TH3A, Zn1TC3A, Zn1BC3A, and Zn1TPA3A possessing three p-carboxyphenyl groups and one electron donating group on their four meso positions. Electrochemical properties and energy level diagram demonstrated the feasibility of studied dyes for DSSC application. ATR-FTIR spectra of porphyrins on TiO2 shows only two p-carboxyphenyl groups behaving as the anchoring group to attached to TiO2 surface. Although containing only one single electron donation substituents. the higher efficiencies 5.26% for Zn1TPA3A and 5.36% for Zn1TH3A than DSSV using 2D-π-2A dyes have been achieved. In the fifth chapter newly designed oxaporphyrins and their Zn2+ complexes were synthesized and characterized by optical spectroscopy, high resolution mass spectrometry, NMR spectroscopy, temperature variable NMR, and X-ray crystallography. Although the effieiency is still extremely low, these new oxaporphyrins and Zn2+ oxaporphyrins with terminal carboxylic acid as the functional groups is a step forward toward using a core-modified porphyrins as the sensitizers in DSSC related research areas. The last chapter outlines the results from each chapter to summarize as the concluding remarks. Overall a broab range of power conversion efficiency from lowest 0.20% (Zn3S1A) to highest 5.36% (Zn1TH3A) has been achieved. The competitiveness of our dyes among other is the cost effective synthesis and better photo-stability. The thesis formulates the guidelines for the development of cost-effective, highly efficient and stable sensitizers for DSSC. Keywords: Dye-sensitized solar cells, binding effect, double donor acceptor, low-cost, multianchored, oxaporphyrins, Zinc(II)oxaporphyrins, porphyrin sensitizers.. VII.

(9) Contents Acknowledgments. II. Abbreviations. IV. Abstract. VI. 1 Introduction 1.1 Introduction. 1. 1.2 The Configuration of Dye-Sensitized Solar Cell. 2. 1.3 Working Electrode. 3. 1.4 Sensitizers. 3. 1.5 Electrolyte. 7. 1.6 Counter Electrode. 8. 1.7 Principles of Operation. 8. 1.8 Incident Photon to Current Conversion Efficiency (IPCE). 8. 1.9 Open Circuit Photovoltage (VOC). 8. 1.10 Short Circuit Photocurrent (JSC). 9. 1.11 Fill Factor (FF). 9. 1.12 Solar to Electric Conversion Efficiency (η). 9. 1.13 Aim of the Thesis. 10. 1.14 References. 11. 2 Binding and Electron-Mediation Effects 2.1 Introduction. 14. 2.2 Experimental Section. 16. 2.3 Synthesis of Sensitizers. 18. 2.4 Optical Properties. 25. 2.5 Electrochemical Properties and Energy Levels. 28. 2.6 Fluorescence Lifetime Measurements. 29. 2.7 Density Functional Theory Calculations. 30. 2.8 Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy. 31. VIII.

(10) 2.9 Dye Loading Measurements. 33. 2.10 Photovoltaic Measurements. 33. 2.11 Conclusion. 37. 2.12 References. 38. 2.13 Supporting Information. 40. 3 Double Donor Acceptor Sensitizers 3.1 Introduction. 65. 3.2 Experimental Section. 66. 3.3 Synthesis of Sensitizers. 68. 3.4 Optical Properties. 78. 3.5 Electrochemical Properties and Energy Levels. 80. 3.6 Density Functional Theory Calculations. 82. 3.7 Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy. 84. 3.8 Photovoltaic Measurements. 85. 3.9 Stability Study. 89. 3.10 Conclusion. 90. 3.11 References. 90. 3.12 Supporting Information. 91. 4 Three Anchoring Group Possessing Porphyrin Sensitizers 4.1 Introduction. 136. 4.2 Experimental Section. 137. 4.3 Synthesis of Sensitizers. 137. 4.4 Optical Properties. 144. 4.5 Electrochemical Properties and Energy Levels. 147. 4.6 Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy. 148. 4.7 Density Functional Theory Calculations. 150. 4.8 Photovoltaic Measurements. 152. 4.9 Stability Study. 156. 4.10 Comparison between 2D-π-2A and 1D-π-3A porphyrins. 158. 4.11 Conclusion. 158. IX.

(11) 4.12 References. 159. 4.13 Supporting Information. 160. 5 Oxaporphyrins 5.1 Introduction. 197. 5.2 Synthesis. 198. 5.3 Photovoltaic Measurements. 207. 5.4 Experimental Section. 208. 5.5 Conclusion. 219. 5.6 References. 219. 5.7 Supporting Information. 221. 6 Concluding Remarks. X.

(12) 1 Introduction 1.1 Introduction The world energy consumption is increasing rapidly. Currently fossil fuel consumption increases at an average annual rate of 0.9% whereas total energy demand is increasing by 1.4 percent per year. Oil, coal, and natural gases are the dominant energy sources for decades, however their resources are limited and they are causing serious impact on global ecosystem. In the long run we have to rely on other renewable and nonrenewable energy sources. Among all types of power, nuclear fission, hydrogen gas, biofuel, biomass, geothermal, wind energy, and solar energy have potential of being the next generation green energy resources. Nuclear fission is one of the highly efficient energy production processes. Currently nuclear power provides about 20% of the world's energy. Making of nuclear power plant is quite expensive, prolonged and tedious. A recent accident happened at Japan’s Fukushima nuclear power plant has raised serious questions over the safety and future of nuclear energy. Producing energy from hydrogen burning is highly efficient which does not emits any hazardous waste but hydrogen needs to be separated from water via electrolysis which takes lot of energy. Alcohol extracted from sugar, starch crops, and Jatropa oil can be used as biofuel, but the production is not environmental friendly and bulk production is not possible. Earth contains tremendous amount of heat at some specific places which is highly efficient energy source but making of geothermal pumping station is expensive. Wind energy is highly environmental friendly, it doesn’t produce any greenhouse gases such as carbon dioxide or methane. However the source is highly variable, nature dependent, and relatively low efficient. Hydro energy is also environmental friendly and highly efficient but making of big dams on rivers and lakes affects ecological cycle. Each system has its own merits and demerits but none of them appear to be the ultimate option. The earth receives incredible amount of solar energy from sun. Capturing less than 0.02% energy from sun would be enough to meet our current energy demand. This abundant solar energy attracts lots of attention of global community for electricity production. Silicon, indium, and cadmium photovoltaic solar cells have been firstly developed in 1950. Currently crystalline silicon solar panels occupy the largest market share of solar cell. The commercial crystalline or 1.

(13) amorphous silicon solar cells reach approximate 20% solar to electric power conversion efficiency. Even if the efficiency is good enough, making of silicon solar cells is too expensive. The high demand of silicon for making of silicon solar cell will increase the price of silicon metalloid dramatically in near future. Silicon solar cells are thick, rigid, and fragile thus are inconvenient to use. The availability of indium and cadmium is limited which is major hurdle for bulk production. High toxicity of cadmium also cannot be neglected. The factors discussed above leads to the great interest of searching new options for cheaper and non-conventional solar cells. DSSC has great potential because of low production cost, environment friendly, flexibility, short energy payback, and relatively good solar to electric power conversion efficiency. In 1991 Michael Grätzel and Brian O’Regan triggered the DSSC research area with overall power conversion efficiency of 7% using a ruthenium sensitizer and TiO2 layer as semiconducting material.1 This important development had overcome several drawbacks of silicon solar cells such as high sensitivity to impurities, high production cost, rigid and fragile texture of solar cell. Operation over a wide range of temperature, easy fabrication, low production cost, and the applications of the cells on flexible substrates such as glass, plastics, ceramics, fabric, and metal make DSSC highly attractive for users. 1.2 The Configuration of Dye-Sensitized Solar Cell The major component of DSSC contains (i) working electrode, (ii) sensitizer, (iii) electrolyte, and (iv) counter electrode. The optimization of each and every component has great importance in order to improve the overall efficiency. Figure 1 provides a schematic presentation of the DSSC with its operating principle.. 2.

(14) Figure 1. Schematic presentation of dye-sensitized solar cell with operating principle.2 1.3 Working Electrode (WE) The working electrode consists of TiO2 nanoparticle layer deposited on a transparent ITO conducting glass or a plastic substrate. The use of opened-end TiO2 nanotube array enhances the overall efficiency.3 ZnO nanowires and nanorods are also useful because of their characteristic band gaps and electron affinity and their electron injection efficiency are nearly identical to TiO2.4,5 The typical TiO2 nanoparticle size varies from 10‒30 nm and thickness from 10‒20 µm. The most common technique used for deposition of TiO2 nanoparticle on substrate is repetitive screen printing. The material is sintered to improve the electronic conduction. Titania paste of TiO2 nanoparticle is available commercially from Solaronix. Readily printed TiO2 photoanode are commercially available from Yingkou Opvtech. 1.4 Sensitizer The layer of sensitizer is adsorbed on the surface of semiconductor by chemical bonding. The commonly used organic solvents for dissolving dye are THF, MeOH, EtOH, DMF, and CH2Cl2. Immersion time, solvent, and temperature play crucial roles in determining the overall efficiency. The function of sensitizer is to absorb the incident light, inject the excited electron into the semiconductor and get regenerated by the redox couple in the electrolyte. The most commonly used sensitizers for DSSC are ruthenium complexes, porphyrins and organic dyes.2,6-10 Design and synthesis of highly efficient sensitizers for DSSC is the most attractive and challenging field. Thousands of ruthenium, porphyrins and phthalocyanines metal complexes, and organic dyes have been synthesized and utilized for dye-sensitized solar cell.2,6-10 To obtain the best efficiency, sensitizers must meet a few important criteria. (I) Energy level of sensitizers should match with the conduction band of the semiconductor and the redox potential of the electrolyte. (II) The sensitizer should have broad absorption range in the UV‒visible range in order to absorb as many photons as possible. (III) The molar extinction coefficient (ε) of the dye must be as high as possible to enable efficient light harvesting with TiO2 film.. 3.

(15) (IV) 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, which is further away from the TiO2 surface. (V) The dye should not aggregate on TiO2 film. (VI) The synthesis should be easy and straightforward with minimum steps so that mass production will be easy. (VII) The dye should be stable enough and bind strongly on TiO2. Based on above requirements many sensitizers of ruthenium complexes, porphyrins, phthalocyanines and organic dyes have been designed, synthesized, and applied successfully in DSSC. Ruthenium sensitizers Ruthenium sensitizers are proved as highly efficient photosensitizers because of their broad absorption spectra, suitable excited and ground state energy levels, relatively long excited-state lifetimes, and good electrochemical stabilities. As represented in Chart 1, the most famous ruthenium sensitizers N3 and N719 achieved highest efficiency of 11.2%11 whereas the record efficiency of 11.7% has been reported for C106.12. Chart 1. Most efficient ruthenium sensitizers. Many efforts have been taken to change peripheral ligands in order to tune the physicochemical properties of complexes.13-15 Alkyl, alkoxyl and phenylene groups have been tried in order to increase the molar extinction coefficient and suppress dye aggregation on the semiconductor 4.

(16) which in turn optimizes the redox potential of the photosensitizer.16-18 Although ruthenium sensitizers have been studied extensively with successes, their practical application will be hindered substantially because of limited resources and high cost of ruthenium metal. Porphyrins and Phthalocyanines The unique properties of strong Soret and moderate Q bands, fast electron injection, photophysical, and thermal stability of porphyrins and phthalocyanines make them potential candidates for photovoltaic applications.2,19 Porphyrins are highly versatile molecules whose physicochemical properties can be tuned by selective functionalization and chemical modification on their meso or β-pyrrolic positions. The impressive efficiency of 7.8% was achieved by β-pyrrolic malonic acid substituted zinc porphyrins.20 Recently, porphyrins with a push-pull system (D-π-A) are becoming popular because of their easy modulation on electrondonating and anchoring sites, through which optical, electrochemical, and photochemical properties can be tuned to get high efficiency.21-26 By utilizing the similar concept Bessho et al. reported porphyrin sensitizer YD2 (Chart 2) gives solar to electric power conversion efficiency 11%.25 The record breaking 12.3% efficiency has been achieved by mixing YD2-o-C8 porphyrin sensitizer with an organic dye under a cobalt based electrolyte.27. Chart 2. Record performing D-π-A framework containing porphyrin sensitizers. The synthetic route given for YD2-o-C8 in Scheme 1 shows that the porphyrin molecules like YD2 and YD2-o-C8 require several steps including key reactions of Sonogashira and BuchwaldHartwig cross coupling reactions which might make a mass production of these porphyrin dyes infeasible.. 5.

(17) a. Reaction conditions: (I) 1-bromooctane, K2CO3, acetone; (II) TMEDA, n-BuLi, DMF; (III) dipyrromethane, TFA, DDQ, CH2Cl2; (IV) NBS, CH2Cl2, Zn(OAc)2•2H2O, CH2Cl2/MeOH; (V) triisopropylacetylene, Pd(PPh3)2Cl2, CuI, TEA/THF; (VI) ) NBS, CH2Cl2; (VII) bis(4hexylphenyl)amine, DPEphos, Pd(OAc)2, toluene, (VIII) TBAF, CH2Cl2, 4-iodobenzoic acid, Pd2(dba)3, AsPh3, TEA/THF.. Organic Dyes Organic dyes have attracted much attention in recent years because of their ease of synthesis, higher molar extinction coefficients and lower synthetic cost.8,9 Their facile modification allows tuning of not only photophysical and electrochemical properties but also stereochemical structures. Many derivatives of coumarin dyes,28-32 indoline dyes,33-38 carbazole dyes,39-44 triarylamine dyes,45-50 hemicyanine dyes,51-53 and squarains dyes,54-56 have been synthesized and applied in DSSC. As represented in Chart 3, the indoline containing D20533 and. the. triphenylamine (TPA) containing C21957 give record efficiencies of 9.5% and 10.3%, respectively.. 6.

(18) Chart 3. Highly efficient organic dyes. The performance of organic dyes remains inferior compared to ruthenium and porphyrin complexes because of poor light-harvesting capacity and high tendency of π-stacked aggregation on the TiO2 surface. Furthermore, the stability of organic dyes is generally lower than that of ruthenium and porphyrin sensitizers. 1.5 Electrolyte Electron injections to TiO2 and dye regeneration are the most important processes in DSSC. Typical redox electrolyte for DSSC is I-/I3- redox pair in acetonitrile in combination with other compounds such as tert-butylpyridine (TBP) or 1-butyl-3-methylimidazolium iodide (BMII). Cobalt bipyridine based redox pair58-60 and ferrocene/ferrocenium (Fc/Fc+)61 pair are effective electrolytes too. In several cases, the using of cobalt electrolyte offers a higher open-circuit voltage because of a more positive redox potential of cobalt electrolyte in comparison with the potential of traditional I-/I3-. The use of liquid electrolyte in DSSC brings certain limitations because of corrosive and volatile nature. Changing of the volatile electrolyte to a nonvolatile ionic liquid will be challenging.62 1.6 Counter Electrode The counter electrode consists of a conducting layer of platinum on a conducting glass. Other counter electrodes such as carbon black and polymers have also been tested. Spin coating of. 7.

(19) alcoholic solution of H2PtCl6 followed by thermal decomposition is the most common technique for making of Pt counter electrode. 1.7 Principles of Operation The schematic representation of DSSC with operating principle is represented in Figure 1. The solar energy is absorbed by the sensitizer and excited electron in the LUMO level rapidly injects in conduction band of TiO2 leaving the dye in its oxidized state. The electron then transfers to platinum counter electrode through external circuit. The electrolyte in oxidation state is reduced on counter electrode and circuit is closed by the regeneration of the dye by electron transfer from the electrolyte. Electric power is generated in the process without permanent chemical transformation. The performance of a solar cell can be quantified with parameters such as incident photon to current efficiency (IPCE), open circuit photovoltage (VOC), short circuit photocurrent (JSC), fill factor (FF), and the solar energy to electric power conversion efficiency (η). In order to compare the results of different research groups, a standard solar spectrum of AM 1.5G, 100 mW cm-2 is being used for the illumination of the cell. 1.8 Incident photon to current efficiency (IPCE) IPCE is one of the fundamental measurements of the performance of the solar. It is also known as the “external quantum efficiency” and describes how efficiently the light of a specific wavelength is converted to current. The IPCE can be calculated according to equation. IPCE=LHE × Φinj × ηcol Here, LHE (λ) is the light-harvesting efficiency for photons of wavelength λ, Φinj is the quantum yield for electron injection from the excited sensitizer to the conduction band of the semiconductor oxide, and ηcol is the electron collection efficiency. 1.9 Open Circuit Photovoltage (VOC) The VOC is the difference in potential between the two terminals in the cell under light illumination when the circuit is open. It is dependent on both the Fermi level of the semiconductor and the level of dark current. The theoretical maximum of the cell is determined by the difference between the Fermi level of the semiconductor and the electrolyte. 8.

(20) 1.10 Short Circuit Photocurrent (JSC) JSCis the photocurrent per unit area (mA/cm2) when an illuminated cell is short circuited. It is dependent on several factors such as the light intensity, light absorption, injection efficiency, dye loading, and regeneration of the oxidized dye. 1.11 Fill Factor (FF) The fill factor measures the ideality of the device and is defined as the ratio of the maximum power output per unit area to the product of Voc and Jsc. Several factors can influence the FF, such as a high inner resistance (e.g. a bad counter electrode), which will give a low fill factor and a decreased overall efficiency. 1.12 Solar to Electric Conversion Efficiency (η) The overall solar energy to electricity conversion efficiency of a solar cell is defined by the ratio of the maximum output of the cell divided by the power of the incident light. It can be calculated by the equation. . J SC  VOC  FF Pin. Where JSC is photocurrent density measured at short circuit, VOC is open circuit photovoltage, FF is the fill factor of the cell, and Pin is intensity of the incident light. Overall efficiency is directly depending on all the first three factors. Thus optimizing each one of them is of most important.. 9.

(21) 1.13 Aim of the Thesis “After tremendous efforts by global researchers in laboratories it is still equivocal that why class of homologous sensitizers have drastically variable efficiencies. To solve the problem we aim to design and synthesize series of sensitizers, compares them comprehensively, come out with some necessary conclusions and develop new sensitizers. The prime effort has been applied to formulate the guideline for the development of cost-effective, highly efficient and stable sensitizers for DSSC.”. 10.

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(25) 2 Binding and Electron-Mediation Effects 2.1 Introduction The introduction of porphyrins in DSSC is inspired from the primary role of chlorophyll in photosynthesis. Porphyrins are serving as potential candidate for the DSSC1,2 because their LUMO level lie above the conduction band edge of the semiconductor electrode and HOMO level lie below the energy level of electrolyte. They exhibit strong absorption for the Soret band in the 400‒450 nm region as well as moderate absorption for the Q band in the 500‒700 nm region. Optical and electrochemical properties of porphyrins can be tuned easily by adding the substituents at meso or β-pyrrol position of porphyrin ring. Higher value of extinction coefficient and thermal stability can also be accounted. In the extensive search of highly efficient porphyrin dyes researchers realize that electronic structure of ligand,3-5 anchoring group,6-8 the bridging distance between dye and TiO2,9 and type of central metal,8,10,11 play important roles in determining the efficiency. Carboxyl is the most commonly employed anchoring group in DSSC. Phosphonate and sulphonate bind more strongly on TiO2 but efficiency is lower compare to carboxyl group.8,12 Derivate of the carboxylic acids such as cyanoacrylic acids, esters, and carboxylate salts also been employed. Chart 1 represents several commonly used anchoring groups and their derivatives on meso-position of porphyrins. As represented in Figure 1 unidentate, chelating, and bridging bidentate are the possible modes proposed for the attachment of carboxyl on TiO2. Precise ATR-FTIR study ruled out possibility of unidentate attachment9 and suggested that bridging bidentate attachment is more likely possible.8,9,13 The most famous N3 ruthenium dye is known to anchor on TiO2 through two of its carboxyl groups from the same or different ligands which ensures effective electron injection.14 The effects on the number of carboxyl groups in ruthenium bipyridyl complexes of the photovoltaic properties have been systematically investigated. After intensive literature search it is interesting to learn that there are very few porphyrin dyes with more than one carboxyl bipyridyl group at meso or β-pyrolic position in DSSC.15-18 In this article we have synthesized a series of meso-thienyl and meso-p-carboxyphenyl substituted zinc porphyrins, namely Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A, bearing up to three carboxyl groups on a porphyrinic. 14.

(26) macrocycle, as represented in Chart 2. Density functional theory (DFT) calculations have proved that the electronic structure of. Chart 1. Commonly used anchoring groups and derivatives on meso position of porphyrins porphyrin will be strongly influenced by peripheral electron donating or electron withdrawing substituents. The electron rich thienyl, which can increase donor property and can provide better coplanarity to the porphyrin core than a phenyl group, has been used as the complementary meso substituent. This systematic study, supported by optical spectra, cyclic voltammetry, DFT calculations, ATR-FTIR, and photovoltaic measurements, elucidates the significant influence of position and number of p-carboxyphenyl and thienyl groups on electronic structure, electrochemical, and photovoltaic properties.. Figure 1. Possible modes of attachment of carboxyl on TiO2.. 15.

(27) Chart 2. Molecular structures of porphyrins used in this study. 2.2 Experimental section Synthesis. The zinc porphyrins used in this study Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A were synthesized in three steps by literature reported methods such mixed condensation,19 Zn2+ metalation,20 and hydrolysis.21 The mixed condensation of 4-methylformyl benzoate, 2-thiophenecarboxyaldehyde, and pyrrole in the presence of boron trifluoride-diethyl etherate as catalyst, followed by subsequent oxidation by 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) gave a mixture of six porphyrins, which were purified by column chromatography. Out of these six porphyrins, the four studied porphyrins were dissolved separately in CH2Cl2 and refluxed with zinc acetate in MeOH to obtain corresponding zinc complexes. The purification by column chromatography and subsequent hydrolysis by KOH isolated analytical pure products. All of the porphyrins were fully characterized by optical spectroscopy, ATR-FTIR, NMR, and high-resolution mass spectrometry. Optical Spectroscopy. Transmittance and reflection UV−visible absorption spectra of the porphyrins in THF and porphyrins adsorbed on TiO2 electrodes were recorded using a JASCO V-670 UV−visible/NIR spectrophotometer. For the thin film TiO2 absorption spectra, 1 × 1 cm2 area and ~1 µm thickness films were prepared to obtain accurate shape and position of peaks.3 The films were immersed in THF solution of 2 × 10-4 M porphyrin for 12 h, the films were rinsed with THF, dried and measure the absorbance. Steady-state fluorescence spectra were acquired by using a Varian Cary Eclipse fluorescence spectrophotometer. Cyclic Voltammetry. The cyclic voltammetry measurements of all porphyrins were carried out on CHI 600D electrochemical analyzer (CH Instruments, Austin, TX, USA) in degassed THF 16.

(28) containing 0.2 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) as the supporting electrolyte. The cell assembly consists of a platinum working electrode, a Ag/AgCl reference electrode, and a platinum wire as the auxiliary electrode. The scan rate for all measurements was fixed at 100 mV/s. A ferrocene+1/0 couple (0.56 V vs SCE) is used as the internal reference for correcting the applied potential. Fluorescence Lifetime Measurements. The picosecond fluorescence transients were measured with time-correlated single photon counting (PicoQuant, FluoTime 200). The samples were excited at 445 nm with a picosecond laser diode, and fluorescence decays were monitored at the maximum of fluorescence Q(0,0) band for all zinc porphyrins (1 × 10−5 M) dissolved in atmospheric THF. The fluorescence lifetimes in THF solution were fit by single exponential function. DFT Calculations. Geometric optimization and electronic structure of the porphyrins were performed with DFT in B3LYP level and the 6-31G* basis set in the Gaussian 03 program package. ATR-FTIR Measurements. ATR-FTIR spectra for the zinc porphyrins were recorded on a VERTEX 70 spectrometer by using Golden Gate diamond ATR accessory on solid powder of porphyrin samples. For the preparation of samples with zinc porphyrins adsorbed on TiO2, THF solution containing 5 ×10-4 M porphyrin was mixed with 5 mg TiO2 powder and kept for 12 h. Excess solvent were dripped out by pipet. TiO2 powder was washed twice by THF and dried in vacuo, and the obtained powder sample was used for measurement. ATR-FTIR spectra for zinc porphyrin adsorbed on TiO2 were recorded at a resolution of 4 cm−1 and 320 scans. Photovoltaic Measurements. To characterize the photovoltaic performance of the DSSC devices, a fluorine-doped tin oxide (FTO; 30 Ω/sq, Sinonar, Taiwan) glass (typical size 1.0 × 2.0 cm2), used as a cathode, was coated with Pt particles by using the thermal platinum nanocluster catalyst method. The Pt catalyst was deposited from a precursor solution composed of 5 × 10‒4 M solution of hexachloroplatinic acid in anhydrous isopropanol. The precursor solution was spin-coated on FTO glass (10 L/cm2) and dried in air for 3 min. The coated Pt electrode was placed in an oven, and the temperature was gradually increased to 360 °C in 15 min. The porphyrin/TiO2 layer was served as a working electrode (anode). We immersed the TiO2 coated FTO (TiO2 thickness 10 μm, active size 0.4 × 0.4 cm2) in a THF solution containing Zn3S1A, 17.

(29) trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A (1 × 10-4 M) with 3 × 10-4 M of chenodeoxycholic acid (CDCA) at 40 °C for 3 h. To fabricate the DSSC device, the two electrodes were assembled into a sandwich-type cell, spaced, and sealed with a hot-melt film (SX1170, Solaronix, thickness 30 μm). The thin layer of electrolyte was introduced into the space between the two electrodes. A typical redox electrolyte contained lithium iodide (LiI, 0.1 M), diiodine (I2, 0.01 M), 4-tertbutylpyridine (TBP, 0.5 M), 1-butyl-3-methylimidazolium iodide (BMII, 0.6 M), and guanidinium thiocyanate (GuNCS, 0.1 M) in a mixture of acetonitrile and valeronitrile (15/1, v/v). The photocurrent and voltage curves were recorded with a digital source meter (Keithley 2400) under AM1.5 one-sun irradiation from a solar simulator (Sanei Electric XES-502S) calibrated with a Si-based reference cell (Hamamatsu S1133). IPCE measurements were carried out with a homebuilt system, which includes a Xe lamp (PTi A-1010, 150 W), a monochromator (Dongwoo DM150i, 1200 gr/mm blazed at 500 nm), and a source meter (Keithley 2400). A standard Si photodiode (ThorLabs FDS1010) was used as a reference to calibrate the power density of the light source at each wavelength so that the IPCE of the DSSC device could be obtained.. 18.

(30) 2.3 Synthesis of sensitizers All chemicals were purchased from Acros Organics and Sigma Aldrich and used without further purification. 1H NMR spectra were recorded on a Bruker 400 MHz spectrometer and performed in CDCl3 (δ = 7.26 ppm) or DMSO-D6 (δ = 2.50 ppm) solution. Chemical shifts are reported in ppm. Coupling constants J are reported in Hz. The signals are described as s: singlet; d: doublet; dd: doublet of doublet. HRMS-FAB or HRMS-ESI was conducted on a JMS-700 double focusing mass spectrometer (JEOL, Tokyo, Japan). Flash chromatography was carried out by using silica gel (40‒63 μm, Merck). Analytical TLC was performed on Merck silica gel plates. Melting points were recorded using an Electrothermal capillary melting point apparatus. The zinc porphyrins used in this study Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A were synthesized by the simplest possible route in three steps, mixed condensation,19 zinc metalation,20 and hydrolysis,21 by literature reported methods: Condensation of pyrrole, methyl 4-formylbenzoate, and thiophene-2-carboxyaldehyde under Lindsey’s conditions catalyzed by boron trifluoride-diethyl etherate followed by subsequent oxidation by DDQ afforded the mixture of six porphyrins. All six porphyrins were purified and characterized by various spectroscopic techniques. Each porphyrin can be distinguished by its unique splitting pattern in NMR spectroscopy. Notably, tran-2S2E and cis-2S2E porphyrins in Table 1, display characteristic splitting patterns for the resonances of β-pyrrolic protons. In the case of the tran-2S2E, eight protons on pyrrole rings split as two resonances, 9.87 (d, J = 4.8 Hz, 4H) and 8.87 (d, J = 4.8 Hz, 4H), whereas in the case of the cis-2S2E, four resonances, 9.08 (s, 2H), 9.07 (d, J = 4.8 Hz, 2H), 8.78 (d, J = 2.8 Hz, 2H), and 8.80 (s, 2H), were observed. The IR spectra of porphyrins shows stretching frequency at around 1720 cm−1 supporting the presence of ester carbonyl group. Zinc metalation has been readily achieved in high yields by reacting free base porphyrins with zinc acetate. The success of zinc metalation was confirmed through the complete disappearance of the NMR resonance of inner NH with slight upfield shifts for all remaining protons. Hydrolysis of metal complexes has been achieved by reacting metal complexes in a mixture of THF and methanol with excess aqueous KOH. The success of hydrolysis was confirmed through complete disappearance of methyl protons of ester group and appearance of acidic protons of pcarboxyphenyl in highly downfield rejoin at around 13 ppm. ATR-FTIR spectra of final acid products show shifting of carbonyl peaks in the range of 1675−1700 cm−1 because of intermolecular hydrogen bonding. 19.

(31) (I) Mixed condensation: In a 1000 ml round bottom flask equipped with a magnetic stirrer, nitrogen inlet and outlet, dichloromethane (750 ml) was added and purged for 10 min. Pyrrole (0.500 g, 7.5 mmol), methyl 4-formylbenzoate (0.615 g, 3.75 mmol), and thiophene-2carboxyaldehyde (2.62 g, 3.75 mmol) were added and degassed. Then, 48% boron trifluoridediethyl ether (10 mol%, 0.142 mL) was added. The reaction mixture was protected from light. After stirring at room temperature for 1h, DDQ (1.70 g, 7.5 mmol) was added and the solution was continuously stirred at room temperature for another 1 h in open atmosphere. Excess dichloromethane was removed completely on rotary evaporator. The dark powder was eluted through short silica gel column with 2% methanol/dichloromethane as the eluent. The porphyrin fraction was concentrated and then separated on a second silica gel column eluted with a solvent gradient from 7:3 dichloromethane/hexane to pure dichloromethane. If necessary, mixed fraction of tran-2S2E and cis-2S2E was further separated on a third silica gel column eluted with 1:1 dichloromethane/hexane. The yields of the obtained porphyrins 3S1E, tran-2S2E, cis-2S2E and 1S3E are listed in Table1.. Reaction conditions: (i) methyl 4-formylbenzoate (3.75 mmol), thiophene-2-carboxyaldehyde (3.75 mmol), pyrrole, BF3.OEt2 (10 mol%), CH2Cl2 (750 ml), 1 h (ii) DDQ (7.5 mmol), 1 h. b Yield of analytically pure product. 20.

(32) (II) Zn metalation: The respective free-base porphyrin 3S1E, tran-2S2E, cis-2S2E, or 1S3E (60 mg) was dissolved in 30 ml dichloromethane. To this solution, Zn(OAc)2·2H2O (1.5 equivalent) dissolved in 10 mL methanol was added. Reaction mixture was refluxed for 1 h. Reaction progress was monitored by TLC and UV‒visible spectroscopy. The solvent was removed under reduced pressure and purified directly by silica gel column chromatography eluted with dichloromethane to give zinc(II) porphyrins Zn3S1E, tran-Zn2S2E, cis-Zn2S2E, or Zn1S3E. The yields of the reactions are reported in Table 2.. a. Reaction conditions: free base porphyrin (60 mg), Zn(OAc)2·2H2O (1.5 equivalent), CH2Cl2/MeOH (3/1, v/v), reflux 1 h, b Yields of analytically pure product.. 21.

(33) (III) Hydrolysis: The respective zinc(II) porphyrin Zn3S1E, tran-Zn2S2E, cis-Zn2S2E or Zn1S3E (50 mg) was dissolved in 40 ml solvent (THF/MeOH,. V/V ,. 3/1), to this. solution 20 equiv. of 1 M aqueous KOH was added and refluxed for 10 h. After cooling, the reaction mixture was treated slowly with 0.1 M HCl. The precipitation formed was filtered off and washed with distilled water. The residue remained was dried on vacuum to yield analytically pure Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, or Zn1S3A in good yields as reported in Table 3.. a. Reaction conditions: zinc(II) porphyrin (50 mg), 1 M KOH (20 equivalent), THF/MeOH (3/1, v/v), reflux, 10 h. b Yield of analytically pure product. The R1, R2, R3, R4 are same as in Table 2.. Characterization data 5-(4-methoxycarbonylphenyl)-10,15,20-tris(2-thienyl)porphyrin (3S1E). mp ˃ 300 oC; 1H NMR (400 MHz, CDCl3) δ = 9.05 (m, 6H), 8.77 (d, J = 4.5 Hz, 2H), 8.45 (d, J = 8.8 Hz, 2H), 8.29 (d, J = 8.8 Hz, 2H ), 7.92 (d, J = 2.4 Hz, 3H), 7.86 (d, J = 5.2 Hz, 3H), 7.50 (dd, J = 4.4, 6.0 22.

(34) Hz, 3H), 4.12 (s, 3H), -2.66 (s, 2H); IR (Neat, cm-1): 3318, 1710, 1604, 1403, 1276, 1177, 1100, 973, 857, 721; λabs/nm (CH2Cl2): 424, 520, 559, 596, 655; HRMS-ESI calcd for C40H26N4O2S3([M+H]+): 691.1296, found 691.1294. 5,15-bis(4-methoxycarbonylphenyl)-10,20-bis(2-thienyl)porphyrin (trans-2S2E). mp ˃ 300 o. C; 1H NMR (400 MHz, CDCl3) δ = 9.87 (d, J = 4.8 Hz, 4H), 8.87 (d, J = 4.8 Hz, 4H), 8.45 (d, J. = 7.6 Hz, 4H), 8.29 (d, J = 7.6 Hz, 4H ), 7.92 (d, J = 3.6 Hz, 2H), 7.85 (d, J = 4.8 Hz, 2H), 7.50 (dd, J = 3.2, 5.4 Hz, 2H), 4.12 (s, 6H), -2.70 (s, 2H); IR (Neat, cm-1): 3319, 1713, 1604, 1473, 1279, 1111, 1020, 949, 796, 706; λabs/nm (CH2Cl2): 422, 519, 556, 595, 652; HRMS-ESI calcd for C44H30N4O4S2([M+H]+):743.1787, found 743.1793. 5,10-bis(4-methoxycarbonylphenyl)-15,20-bis(2-thienyl)porphyrin (cis-2S2E). mp ˃ 300 oC; 1. H NMR (400 MHz, CDCl3) δ = 9.08 (s, 2H), 9.07 (d, J = 4.8 Hz, 2H), 8.78 (d, J = 2.8 Hz, 2H),. 8.80 (s, 2H), 8.44 (d, J = 7.6 Hz, 4H), 8.29 (d, J = 8.4 Hz, 4H ), 7.92 (d, J = 3.2 Hz, 2H), 7.86 (d, J = 5.6 Hz, 2H), 7.50 (dd, J = 2.8, 5.2 Hz, 2H), 4.12 (s, 6H), -2.63 (s, 2H); IR (Neat, cm-1): 3313, 1716, 1606, 1473, 1271, 1176, 1099, 978, 818, 679; λabs/nm (CH2Cl2): 421, 518, 556, 594, 650; HRMS-ESI calcd for C44H30N4O4S2([M+H]+): 743.1787, found 743.1780. 5,10,15-tris(4-methoxycarbonylphenyl)-20-(2-thienyl)porphyrin (1S3E). mp ˃ 300 oC; 1H NMR (400 MHz, CDCl3) δ = 9.09 (m, 2H), 8.82 (s, 2H), 8.80 (d, J = 5.6 Hz, 4H), 8.45 (d, J = 8.0 Hz, 6H ), 8.30 (d, J = 8.0 Hz, 6H), 7.93 (d, J = 3.2 Hz, 1H), 7.86 (d, J = 5.2 Hz, 1H), 7.50 (dd, J = 2.4, 5.2 Hz, 1H), 4.12 (s, 9H), -2.72 (s, 2H); IR (Neat, cm-1): 3316, 1720, 1606, 1475, 1272, 1110, 962, 818, 761; λabs/nm (CH2Cl2): 420, 517, 554, 593, 649; HRMS-ESI calcd for C48H35N4O6S([M+H]+): 795.2277, found 795.2269. 5-bis(4-methoxycarbonylphenyl)-10,15,20-bis(2-thienyl)porphyrinato. zinc(II). (Zn3S1E).. mp ˃ 300 oC; 1H NMR (400 MHz, DMSO-D6) δ = 9.01 (m, 6H), 8.76 (d, J = 4.8 Hz, 2H), 8.39 (d, J = 8.0 Hz, 2H), 8.33 (d, J = 8.0 Hz, 2H ), 8.12 (d, J = 5.6 Hz, 3H), 7.96 (d, J = 2.8 Hz, 3H), 7.57 (dd, J = 3.2, 5.2 Hz, 3H), 4.04 (s, 3H); IR (Neat, cm-1): 1719, 1605, 1490, 1231, 1203, 1000, 973, 809, 715; λabs/nm (CH2Cl2): 425, 554, 596; HRMS-FAB calcd for C40H24N4O2S3Zn ([M+]): 752. 0353, found 752.0358.. 23.

(35) 5,15-bis(4-methoxycarbonylphenyl)-10,20-bis(2-thienyl)porphyrinato. zinc(II). (trans-. Zn2S2E). mp ˃ 300 oC; 1H NMR (400 MHz, DMSO-D6) δ = 9.02 (d, J = 4.8 Hz, 4H), 8.78 (d, J = 4.8 Hz, 4H), 8.38 (d, J = 8.4 Hz, 4H), 8.33 (d, J = 8.0 Hz, 4H ), 8.10 (d, J = 4.8 Hz, 2H), 7.95 (d, J = 3.2 Hz, 2H), 7.50 (dd, J = 3.6, 5.4 Hz, 2H), 4.04 (s, 6H); IR (Neat, cm-1): 1699, 1605, 1497, 1270, 1113, 1000, 976, 822, 760, 699; λabs/nm (CH2Cl2): 423, 552, 593; HRMS-FAB calcd for C44H28N4O4S2Zn ([M+]): 804.0843, found 804.0851. 5,10-bis(4-methoxycarbonylphenyl)-15,20-bis(2-thienyl)porphyrinato zinc(II) (cis-Zn2S2E). mp ˃ 300 oC; 1H NMR (400 MHz, DMSO-D6) δ = 9.05 (s, 2H), 9.03 (d, J = 4.8 Hz, 2H), 8.80 (d, J = 4.4 Hz, 2H), 8.78 (s, 2H ), 8.38 (d, J = 8.4 Hz, 4H), 8.33 (d, J = 8.4 Hz, 4H), 8.11 (d, J = 5.2 Hz, 2H), 7.96 (d, J = 8.4 Hz, 2H), 7.56 (dd, J = 3.6, 5.4 Hz, 2H), 4.04 (s, 6H),; IR (Neat, cm-1): 1721, 1605, 1434, 1270, 1174, 1070, 976, 823, 699; λabs/nm (CH2Cl2): 423, 552, 594; HRMSFAB calcd for C44H28N4O4S2Zn ([M+]): 804.0843, found 804.0836. 5,10,15-tris(4-methoxycarbonylphenyl)-20-(2-thienyl)porphyrinato zinc(II) (Zn1S3E). mp ˃ 300 oC; 1H NMR (400 MHz, DMSO-D6) δ = 9.03 (d, J = 4.6 Hz, 2H), 8.80-8.76 (m, 6H), 8.808.76 (m, 6H), 8.33-8.31 (m, 6H), 8.09 (d, J = 5.2 Hz, 1H), 7.95 (d, J = 2.8 Hz, 1H), 7.55 (dd, J = 3.6, 5.2 Hz, 1H), 4.03 (s, 9H); IR (Neat, cm-1): 1719, 1699, 1605, 1433, 1269, 1113, 1010, 980, 848, 731; λabs/nm (CH2Cl2): 422, 551, 590 HRMS-FAB calcd for C48H33N4O6SZn ([M+]): 856.1334, found 856.1335. 5-bis(4-carboxyphenyl)-10,15,20-bis(2-thienyl)porphyrinato zinc(II) (Zn3S1A). mp ˃ 300 o. C; 1H NMR (400 MHz, DMSO-D6) δ = 13.26 (s, 1H) 9.02-8.99 (m, 6H), 8.77 (d, J = 4.4 Hz,. 2H), 8.37 (d, J = 8.0 Hz, 2H), 8.30 (d, J = 8.0 Hz, 2H ), 8.12 (d, J = 5.2 Hz, 3H), 7.95 (d, J =2.0 Hz, 3H), 7.57 (dd, J = 3.1, 5.0 Hz, 3H); IR (Neat, cm-1): 3566, 3324, 3065, 2924, 1708, 1675, 1604, 1421, 1228, 1203, 1170, 1100, 975, 850, 810, 715; λabs/nm (THF), (ε/103 M-1 cm-1): 428 (248), 560 (10.7), 600 (3.0); λem/nm (THF): 618, 660; HRMS-FAB calcd for C39H22N4O2S3Zn ([M+]): 738.0196, found 738.0193. 5,15-bis(4-carboxyphenyl)-10,20-bis(2-thienyl)porphyrinato zinc(II) (trans-Zn2S2A). mp ˃ 300 oC; 1H NMR (400 MHz, DMSO-D6) δ = 13.25 (s, 2H) 9.01 (d, J = 4.4 Hz, 4H), 8.78 (d, J = 4.8 Hz, 4H), 8.37 (d, J = 8.0 Hz, 4H), 8.30 (d, J = 8.0 Hz, 4H ), 8.11 (d, J = 5.2 Hz, 2H), 7.95 (d,. 24.

(36) J = 3.2 Hz, 2H), 7.57 (dd, J = 3.1, 4.9 Hz, 2H); IR (Neat, cm-1): 3512, 3378, 2920, 2851, 1681, 1605, 1421, 1278, 1230, 1045, 994, 809; λabs/nm (THF), (ε/103 M-1 cm-1): 426 (296), 560 (13.2), 600 (4.8); λem/nm (THF): 613, 661; HRMS-FAB calcd for C42H24N4O4S2Zn ([M+]): 776.0530, found 776.0537. 5,10-bis(4-carboxylphenyl)-15,20-bis(2-thienyl)porphyrinato zinc(II) (cis-Zn2S2A). mp ˃ 300 oC; 1H NMR (400 MHz, DMSO-D6) δ = 13.28 (s, 2H), 9.00 (s, 2H), 9.00 (d, J = 4.6 Hz, 2H), 9.78 (d, J = 4.4 Hz, 2H), 8.77 (s, 2H ), 8.37 (d, J = 8.0 Hz, 4H), 8.30 (d, J = 8.0 Hz, 4H), 8.11 (d, J = 5.2 Hz, 2H), 7.95 (d, J = 8.4 Hz, 2H), 7.57 (dd, J = 3.6, 5.0 Hz, 2H); IR (Neat, cm-1): 3550,3367, 2924, 2853, 1687, 1403, 1259, 1174, 1100, 999, 980,822, 791; λabs/nm (THF), (ε/103 M-1 cm-1): 424 (243), 558 (10.7), 600 (3.4); λem/nm (THF): 612, 659 HRMS-FAB calcd for C42H24N4O4S2Zn ([M+]): 776.0530, found 776.0527. 5,10,15-tris(4-carboxylphenyl)-20-(2-thienyl)porphyrinato zinc(II) (Zn1S3A). mp ˃ 300 oC; 1. H NMR (400 MHz, DMSO-D6) δ = 13.26 (s, 3H), 9.023 (d J = 4.4 Hz, 2H), 8.80-8.77 (m, 6H),. 8.37 (d, J = 8.0 Hz, 6H), 8.30 (d, J = 8.0 Hz, 6H), 8.11 (d, J = 5.2 Hz, 1H), 7.95 (d, J = 3.2 Hz, 1H), 7.57 (dd, J = 3.6, 5.5 Hz, 1H); IR (Neat, cm-1): 3550, 3385, 2923, 2849, 1680, 1602, 1403, 1311, 1231, 1174, 995, 791, 765; λabs/nm (THF), (ε/103 M-1 cm-1): 425 (352), 558 (14.5), 600 (4.8); λem/nm (THF): 607, 660; HRMS-FAB calcd for C45H26N4O6SZn ([M+]): 814.0865, found 814.0868. 2.4 Optical Spectroscopy The UV−visible peak positions of the Soret and Q bands and the molar absorption coefficient (ε) of Zn3S1A, trans-Zn2S2A, cis-Zn2S2A and Zn1S3A in THF are listed in Table 4. The UV−visible spectra of the studied zinc porphyrins displayed in Figure 2a show typical metalloporphyrin features. Because of inductive effect from meso-thienyl unit, a slight red shift in Soret and Q bands with the increase number of meso-thienyl groups has observed. Specifically, the Zn3S1A (428, 560, 600 nm) shows a red-shift over Zn1S3A (425, 558, 600 nm), whereas trans-Zn2S2A (426, 560, 600 nm) and cis-Zn2S2A (426, 560, 600 nm) exhibit comparable absorption wavelengths. Noticeably, the extinction coefficients in the Soret band region for the Zn1S3A and trans-Zn2S2A are significantly higher than Zn3S1A and cis-Zn2S2A. The steadystate fluorescence spectra of all zinc porphyrins (Figure 2b) measured in THF by excitation at the 25.

(37) Soret band display a trend similar to the UV−visible spectra with significant red shifting upon increasing the number of meso-thienyl units. Table 4. Optical and electrochemical data of zinc porphyrins. λabs/nma (ε/103 M-1 cm-1) Zn3S1A 428 (248) 560 (10.7) 600 (3.0) trans-Zn2S2A 426 (296) 560 (13.2) 600 (4.8) cis-Zn2S2A 426 (243) 560 (10.7) 600 (3.4) Zn1S3A 425 (352) 558 (14.5) 600 (4.8) Dye. λem/n mb 618 660. EOX /Vc 1.10. E0-0 /eV d 2.04. EOX* /V e -0.94. 613 660. 1.11. 2.05. -0.94. 612 659. 1.12. 2.05. -0.93. 607 660. 1.13. 2.06. -0.93. a. Absorption maximum of zinc porphyrin in THF. bEmission maximum measured in THF by exciting at Soret band. f First oxidation potentials vs. NHE determined using cyclic voltammetry in THF. d E0-0 values were estimated from the intersection of the absorption and emission spectra. eExcited state oxidation potentials approximated from Eox and E0-0.. .. Figure 2. (a) UV‒visible spectra of Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A in THF. Inset displays enlarged spectra for the Q-band absorptions. (b) Fluorescence spectra of Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A.. 26.

(38) To obtain the absorption spectra of the thin films, the TiO2 films with coating thickness of around 1 μm were immersed in THF solutions of zinc porphyrins for 12 h and rinsed with THF to remove non-adsorbed dye. The absorption spectra were recorded by reflectance measurements using an integrating sphere, and the results are shown in Figure 3. The UV−visible absorption spectrum of Zn3S1A/TiO2 (magenta line) does not show any distinguishable change compared to its UV−visible absorption in THF due to weak binding of porphyrin with TiO2, whereas UV−visible absorption spectra of cis-Zn2S2A/TiO2 (red line) and Zn1S3A/TiO2 (black line) shifted to shorter wavelength because of H-type aggregation from face-to-face stacking of porphyrins and tight packing porphyrins on TiO2.22 The absorption spectra of porphyrins/TiO2 demonstrate that the binding behavior of zinc porphyrin depends on the number and position of p-carboxyphenyl groups. When zinc porphyrins have more than two p-carboxyphenyl groups on the meso positions, the absorbance and molecular loading on TiO2 are increased. Although the number of p-carboxyphenyl groups of trans-Zn2S2A is the same as cis-Zn2S2A, the amount of dye loading of trans-Zn2S2A was lower than that of cis-Zn2S2A because of single arm attachment on TiO2 for the former, which is confirmed by the ATR-FTIR spectra (vide infra).. Figure 3. UV‒visible spectra of 3S1A/TiO2, trans-2S2A/TiO2, cis-2S2A/TiO2, 1S3A/TiO2. 2.5 Electrochemical properties and energy levels To determine the first oxidation potential (EOX) of porphyrins, cyclic voltammetry measurements 27.

(39) of zinc porphyrins containing 0.2 M [Bu4N]PF6 as the electrolyte were performed in degassed THF as displayed in Figure 4. The first oxidation couple of zinc porphyrins show reversible redox processes under a scan rate of 500 mV s−1. As listed in Table 4, the 1S3A has the most positive first oxidation potential (1.13 V vs SHE) and the potential of Zn3S1A (1.10 V vs SHE) shows the most cathodic shift. The trend of redox potential is consistent with an overall effect from the electron withdrawing meso-p-carboxyphenyl and electron donating meso-thienyl. From the intersection of the normalized absorption and emission spectra at the Q(0,0) band the zerozero excitation energies, E(0,0), were calculated to be 2.04, 2.05, 2.05, and 2.06 eV for Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A, respectively. From the spectral and electrochemical measurements, the oxidation potential (EOX*) with respect to the first singlet excited state of zinc porphyrins can be obtained and are listed in Table 4. The nearly identical EOX* values of four zinc porphyrins indicate that the driving forces for electron injection from the excited zinc porphyrins to the CB of the TiO2 films might be the same. The oxidation potential of all studied porphyrins dyes is greater than that of I‒/I3‒couple assuring the regeneration of oxidized state.. Figure 4. Cyclic voltammograms of Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A in THF. The excited-state oxidation potentials (EOX*) are obtained from the relation EOX* = Eox1 ‒ E0–0 where Eox1 is the first oxidation potential of a porphyrin dye and E0-0 is the zero-zero excitation energy obtained from the intersection of the absorption and emission spectra. The calculated EOX* of these porphyrins depicted in Table 4 are more negative than the conduction band (–0.50 V vs. NHE) of TiO2, showing the necessary driving force for electron injection from the excited 28.

(40) state of the dye to the CB of TiO2. The systematic energy level diagram of studied porphyrins is shown in Figure 5 demonstrating feasibility of dye regeneration with electrolyte and electron injection to conduction band.23-25. Figure 5. Energy level of Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A porphyrin dyes 2.6 Fluorescence Lifetime Measurements The fluorescence lifetimes of Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A in THF were measured with the time-correlated single photon counting technique, and the results are displayed in Figure 6. The samples in THF were excited at 430 nm (Soret band), and the fluorescence lifetimes were recorded at the maximum of fluorescence Q(0,1) band. All the fluorescence transients were fitted with the single exponential decay function to obtain lifetimes of 0.88 ± 0.02 ns for Zn3S1A, 1.49 ± 0.02 ns for trans-Zn2S2A, 1.40 ± 0.02 ns for cis-Zn2S2A, and 2.00 ± 0.02 ns for Zn1S3A. The excited state lifetimes of the zinc porphyrins decreasing with the increasing number of thienyl substituent group can be rationalized with the heavy atom effect of sulfur atoms, which increases the rate of intersystem crossing (ISC) upon increasing the number of sulfur atoms.. 29.

(41) Figure 6. The ps-fluorescence transients of Zn3S1A, trans-Zn2S2A, cis-ZN2S2A, and Zn1S3A in THF with excitation at 430 nm and monitoring at Q(0,1) band. The black curve indicates the instrument response function (IRF). 2.7 DFT Calculations The ground state structures of Zn3S1A, trans-Zn2S2A, cis-ZN2S2A, and Zn1S3A were optimized using the hybrid B3LYP functional in combination with the standard 6-31G* basis set. Not only the electron density distributions at the HOMO and LUMO orbitals of these porphyrins are similar, but also the energy gaps between LUMO and HOMO are essentially identical (∼2.8 eV). These results are consistent with the relative insensitivity on the absorption spectra for all of the studied porphyrins. As shown in Figure 7, at both the HOMO and LUMO orbitals, the π electron density distributions for the four zinc porphyrins mainly localize at porphyrin conjugated macrocycles. Since the meso-thienyl substituents and meso-p-carboxyphenyl are nearly orthogonal to the mean porphyrin planes, there is diminutive charge delocalization into terminal meso-p-carboxyphenyl moieties at LUMO and LUMO+1 orbitals. Interestingly, the anchoring p-carboxyphenyl possesses nearly exclusive electron density distributions at LUMO+2, suggesting that the electron injection from higher excited states involving LUMO+2 should be more efficient than the lower ones involving LUMO or LUMO+1.. 30.

(42) Figure 7. The molecular orbital diagrams of Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A obtained from DFT calculations. 2.8 ATR-FTIR Spectroscopy We have applied ATR-FTIR spectroscopy as the prime tool to evaluate the number and mode of porphyrinic p-carboxyphenyl attached on TiO2.8,26-29 The ATR-FTIR spectra of zinc porphyrin powder samples are contrasted with the spectra of zinc porphyrin adsorbed on TiO2 and demonstrated in Figure 8. The spectra of Zn3S1A, trans-Zn2S2A, cis-ZN2S2A, and Zn1S3A in powder form exhibit very strong ν(C=O) stretching peaks at 1675, 1681, 1687, and 1680 cm−1, respectively. The ν(C=O) peak at 1675 cm−1 observed in the 3S1A powder disappeared completely in the corresponding spectrum of the 3S1A/TiO2 film, while strong absorptions at 1650 and 1400 cm−1, assigned as νasym(COO¯) and νsym(COO¯), respectively, were observed. The energy difference of the νasym(COO¯) and νsym(COO¯) suggests a bidentate bridging or bidentate chelating p-carboxyphenyl group coordinated to TiO2. A similar pattern can be observed for the ATR-FTIR spectra of cis-Zn2S2A/TiO2, suggesting both the p-carboxyphenyl groups attached on TiO2. On the contrary, the ATR-FTIR spectra of trans-Zn2S2A/TiO2 shows marked increase in νasym(COO¯) and νsym(COO¯) whereas the ν(C=O) stretching peak at 1681 cm−1 is still present with significant absorbance, suggesting the presence of a free unchelated p-. 31.

(43) Figure 8. Contrasted ATR-FTIR spectra of zinc porphyrins in powder form (black line) and zinc porphyrins adsorbed on TiO2 (red line) for (a) Zn3S1A and Zn3S1A/TiO2, (b) trans-Zn2S2A and trans-Zn2S2A/TiO2, (c) cis-Zn2S2A and cis-Zn2S2A/TiO2, (d) Zn1S3A and Zn1S3A/TiO2. ATR-FTIR spectra of porphyrins on TiO2 are normalized for comparison. carboxyphenyl unit. The comparative ATR-FTIR spectrum of Zn1S3A/TiO2 shows more intense absorptions for νasym(COO¯) and νsym(COO¯) than that of ν(C=O), suggesting that out of three pcarboxyphenyl groups one is free and two are involved in attachment with TiO2. Based on the above observation the possible modes of attachment of zinc porphyrins for Zn3S1A, transZn2S2A, cis-Zn2S2A, and Zn1S3A on TiO2 are illustrated in Figure 9. The results of dyeloading and the photovoltaic properties (vide infra) are consistent with those of the ATR-FTIR study, suggesting that the presence of two meso-p-carboxyphenyl at the cis position in cisZn2S2A and Zn1S3A gives stronger attachment to TiO2 and results in better device performances than those of Zn3S1A and trans-Zn2S2A with only a single arm attached to the TiO2.. 32.

(44) Figure 9. Possible modes of attachment of zinc porphyrins on TiO2 are illustrated for Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A. For demonstration purpose, relative sizes of molecules and nanoparticles are not correlated to real dimensions.. 2.9 Dye Loading Measurements To understand the adsorption behavior and quantify the amount of dye loading, we examined the porphyrin densities (Γ) adsorbed on TiO2 surface. The TiO2 electrodes of 10 μm thicknesses were immersed in 0.1 mM zinc porphyrin THF solution containing 0.3 mM CDCA for 3 h at 40 °C. No desorption of cis-Zn2S2A and Zn1S3A was observed when the dye-sensitized TiO2 plates were rinsed with THF, indicating the tight bonding of dyes to the TiO2 surface. On the contrary, Zn3S1A and trans-Zn2S2A were readily desorbed from TiO2 which signifies a weak interaction with TiO2. The porphyrin density (Γ) adsorbed on the rinsed TiO2 film was determined by measuring the absorbance of the zinc porphyrins that were desorbed from the sensitized TiO2 films after being immersed in 0.1 M KOH solution in THF. The saturated Γ values of porphyrins were determined as 83 ± 10, 110 ± 4, 126 ± 5, and 117 ± 1 nmol cm−2 for Zn3S1A, transZn2S2A, cis-Zn2S2A, and Zn1S3A, respectively. The Γ values of these porphyrins are comparatively higher than those of reported meso-aryl-substituted zinc porphyrins.5,13 2.10 Photovoltaic Measurements The photocurrent-voltage (I−V) curves of Zn3S1A, trans-Zn2S2A, cis-ZN2S2A, and Zn1S3A under AM 1.5 solar simulator light and in the dark are shown in Figure 10. Significant differences for the short-circuit current densities (JSC), the open-circuit voltages (VOC) and the power conversion efficiency (η) of the devices were observed. It is noticed that the values of 33.

(45) both JSC and VOC are systematically increased with increasing number of anchoring meso-pcarboxyphenyl and decreasing number of meso-thienyl substituent. The efficiencies of Zn1S3A and cis-Zn2S2A with double arms attached on TiO2 are higher than those of trans-Zn2S2A and Zn3S1A which have only a single arm attached on TiO2. Due to the maximum number of coordinating meso-p-carboxyphenyls and minimum heavy atom effect, Zn1S3A gives the highest efficiency. As demonstrated in Figure 10 and Table 5, the significant difference on VOC values was observed to give the order Zn1S3A > cis-Zn2S2A ≅ trans-Zn2S2A > Zn3S1A. In general, the VOC value is determined by the difference between the Fermi level of electrons on TiO2 and potential level of redox couples in electrolyte. Because the potential level of redox couples in our system is the same, the values of VOC are dependent on the Fermi levels of electrons on TiO2 after electron injection for all the porphyrin systems. Since both Zn1S3A and cis-Zn2S2A devices have greater JSC values than those of the trans-Zn2S2A and Zn3S1A devices, it is expected that the dual-arm anchored devices have more injected electrons on the CB of TiO2 to give higher Fermi levels than those of the single-arm anchored devices, which explains the general trend in the variation of VOC. On the other hand, the much higher Voc and JSC for Zn1S3A than cis-Zn2S2A reflect a better performance for the dye containing less thienyl groups. Under the circumstance that Zn1S3A and cis-Zn2S2A have near identical dye loadings and the same chelating mode, the heavy atom effect appears to decrease the conversion efficiency of cisZn2S2A through, presumably, increasing the rate of ISC, as has been observed from the fluorescence lifetime measurements. On the basis of the I−V curves obtained in the dark (Figure 10b), an increase in the number of meso-p-carboxyphenyl substituent is accompanied with a lower dark current density. Table 5. Photovoltaic properties of zinc porphyrins. Dye. JSC (mA cm-2). VOC (mV). FF. η (%). Zn1S3A. 6.59. 593. 0.77. 3.0. cis-Zn2S2A. 6.08. 546. 0.75. 2.5. trans-Zn2S2A. 4.34. 545. 0.74. 1.8. Zn3S1A. 0.907. 455. 0.48. 0.2. 34.

(46) Figure 10. (a) The I‒V curves of Zn3S1A, trans-Zn2S2A, cis-ZN2S2A, and Zn1S3A sensitized solar cells under solar simulator AM1.5 light. (b) The I‒V curves of zinc porphyrins in the dark. As the dark current density is governed by the impedance of the electron transfer between TiO2 and electrolyte, the adsorbed porphyrin dyes on TiO2 might play a role to increase the interfacial impedance depending on the degree of coverage of sensitizing dyes on the surface of TiO2. According to the results of dye loadings, the Zn1S3A, trans-Zn2S2A, and cis-Zn2S2A give significantly higher molecular loadings on TiO2, which behave like a blocking layer to prevent charge recombination between the electrons on TiO2 and triiodides in the electrolyte. For Zn3S1A, which has a lower surface coverage on TiO2, the triiodides in electrolyte can penetrate closely to the surface of TiO2 to decrease the barrier for the electrons to combine with triiodides and to result in a higher dark current. On the other hand, under applied potential lower than 0.35 V, the dark current of cis-Zn2S2A is smaller than that of Zn1S3A due to the larger amount of dye loading for the former than the latter. However, the dark current of cis-Zn2S2A becomes larger than that of Zn1S3A over 0.35 V, which is consistent with the larger Voc for Zn1S3A than for cis-Zn2S2A. Interestingly, we found that the Voc values are nearly identical for both cisZn2S2A and trans-Zn2S2A devices, even though cis-Zn2S2A has a much higher Jsc value. On the basis of the dark current results shown in Figure 10b, charge recombination is more significant for trans-Zn2S2A than for cis-Zn2S2A. We thus expect that the CB edge of TiO2 might be shifted more upward for trans-Zn2S2A than for cis-Zn2S2A to compensate the loss of the CB electrons of the former due to the severe charge recombination. The very poor performance of the 3S1A device can be understood for its small amount of dye loading and the very high dark current due to significant effect of charge recombination. Figure 11 shows the 35.

(47) spectra of conversion efficiency of incident photon to current (IPCE) of Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A sensitized solar cells at short circuit conditions. Like JSC, the IPCE values of four zinc porphyrins exhibit a strong correlation to the meso substitutions. The IPCEs at 565 nm of zinc porphyrins are 21, 17, 11, and 2.3% for Zn3S1A, trans-Zn2S2A, cis-ZN2S2A, and Zn1S3A, respectively. Since IPCE is dominated by LHE (= 1−10−Abs), for which the absorbance (Abs) is a linear dependence on both the molecular loading on TiO2 and absorption coefficient. The IPCE values at the Soret band and Q band are larger for Zn1S3A and cisZn2S2A, but are much smaller for Zn3S1A. Noticeably, the IPCE values of Zn1S3A and cisZn2S2A are similar to reach 40% at the Soret band, but at the Q band region, Zn1S3A exhibits a larger IPCE than cis-Zn2S2A.. Figure 11. IPCE spectra of Zn3S1A, trans-Zn2S2A, cis-ZN2S2A, and Zn1S3A. According to the absorption spectra of zinc porphyrins/TiO2 shown in Figure 3, the LHE of cisZn2S2A is larger than that of Zn1S3A. However, the IPCE value at the Q band for cis-Zn2S2A is smaller than that for Zn1S3A, possibly due to the smaller electron injection yield at the lowest excited state for cis-Zn2S2A than for Zn1S3A. Moreover, the IPCE values for cis-Zn2S2A at both the Soret band and the Q band are larger than those of trans-Zn2S2A. We expect that, in addition to the larger amount of dye-loading to give larger LHE for cis-Zn2S2A, the possibly lower CB edge of TiO2 upon dye uptake of the cis-Zn2S2A device might lead to larger electron injection rate to give greater electron injection yield for the cis-device than for the trans-device.. 36.