Fabricataion of dye sensitized solar cells with enhanced energy conversion efficiency

FABRICATION OF DYE SENSITIZED SOLAR CELLS WITH ENHANCED ENERGY CONVERSION EFFICIENCY ZHANG WEI NATIONAL UNIVERSITY OF SINGAPORE 2011 FABRICATION OF DYE SENSITIZED SOLAR CELLS WITH ENHANCED ENERGY CONVERSION EFFICIENCY ZHANG WEI (M. Sci.) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL & BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 ACKNOWLEDGEMENTS Firstly, I would like to express my greatest appreciation to my supervisor, Professor Liu Bin, for her guidance, support and encouragement throughout my entire Ph.D study. Her meticulous attentions to details, incisive but constructive criticisms and insightful comments have helped me shape the direction of this thesis to the form presented here. Her dedication and enthusiasm for scientific research, her knowledge which is both broad-based and focused, and her stories on the successful integration of ideas across different disciplines, have always been a source of inspiration. I am also thankful to her for her strong support in other aspects of life than research. I deeply appreciate my parents and my wife Xu Qiao. Their love and encouragement light up many lonely moments in my life as a graduate student away from home and have been the source of courage when I was down. I would like to express my sincere thanks to all my friends and colleagues in the research group. Their support, friendship and encouragement made my Ph.D study a journey of happiness. I am also thankful to laboratory and professional officers in the department for technical services rendered in this thesis study. Without their assistance, this work could not have been completed on time. Special acknowledgement is also given to the National University of Singapore for financial support. Last, but not least, I am grateful to every individual who has helped me in one way or another during my Ph. D study. I TABLE OF CONTENTS ACKNOWLEDGEMENTS I TABLE OF CONTENTS II SUMMARY V LIST OF TABLES IX LIST OF FIGURES X LIST OF SCHEMES XV LIST OF ABBREVIATIONS XVI Chapter Chapter INTRODUCTION 1.1 Background 1.2 Objectives and Scopes LITERATURE REVIEW 2.1 Photovoltaics – A brief history 2.2 Dye sensitized solar cells (DSSCs) 11 2.2.1 Basic principles of DSSCs 12 2.2.2 Charge transfer and transport dynamics 13 2.2.3 Basic parameters to evaluate the performance of 14 DSSCs 2.2.4 Characterization techniques of DSSCs 2.2.5 Comparison of DSSCs with other photovoltaic devices 2.3 Recent Progress in DSSCs II 15 17 18 2.3.1 Development of new photoelectrodes 18 2.3.2 Development of new sensitizers 22 Chapter 2.3.3 Development of new hole transporting materials 26 2.3.4 Development of new counter electrode materials 33 2.4 References 34 FABRICATION OF TiO NANOROD PHOTOELECTRODE 43 FOR DYE SENSITIZED SOLAR CELL APPLICATION Chapter 3.1 Introduction 43 3.2 Experimental section 46 3.3 Results and discussion 49 3.4 Conclusions 56 3.5 References 57 FACILE CONSTRUCTION OF NANOFIBROUS ZnO 60 PHOTOELECTRODE FOR DYE SENSITIZED SOLAR CELL APPLICATION Chapter 4.1 Introduction 60 4.2 Experimental section 61 4.3 Results and discussion 63 4.4 Conclusions 70 4.5 References 70 A TRIPHENYLAMINE BASED CONJUGATED 73 POLYMER WITH DONOR-π-ACCEPTOR ARCHITECTURE AS ORGANIC SENSITIZER FOR DYE SENSITIZED SOLA CELLS 5.1 Introduction 73 III Chapter 5.2 Experimental section 75 5.3 Results and discussion 79 5.4 Conclusions 83 5.5 References 84 HIGH PERFORMANCE SOLID-STATE ORGANIC DYE 87 SENSITIZED SOLAR CELLS WITH P3HT AS HOLE TRANSPORTING MATERIAL Chapter 6.1 Introduction 87 6.2 Experimental section 89 6.3 Results and discussion 92 6.4 Conclusions 104 6.5 References 104 ANATASE MESOPOROUS TiO NANOFIBERS WITH 109 HIGH SURFACE AREA FOR SOLID-STATE DYE-SENSITIZED SOLAR CELLS 7.1 Introduction 109 7.2 Experimental section 111 7.3 Results and discussion 115 7.4 Conclusions 126 7.5 References 126 Chapter CONCLUSIONS AND OUTLOOK 131 Appendix LIST OF PUBLICATIONS 134 IV SUMMARY Exploiting new technologies that power the world efficiently and cleanly in the future is critically important due to the depleted petroleum resources and public environmental concerns. Dye sensitized solar cells (DSSCs) represent a cheap and clean technology that harnesses solar energy efficiently and have been intensively studied. How to further decrease the production cost meanwhile enhance device performance becomes the bottleneck for large scale application and commercialization of DSSCs. The thesis focuses on the development of new materials (photoelectrode material, dye sensitizer and hole transporting material) with the motivation to further enhance energy conversion efficiency of DSSCs. The thesis is divided into eight chapters. Chapter outlines motivation and scope of the work. Chapter surveys the current literature. Major findings of the study are discussed in Chapters through 7, with conclusions and outlooks summarized in Chapter 8. The appendix contains a publication list. In chapter 3, a cost-effective and scalable method to prepare high-quality TiO nanofibers is developed based on electrospinning technique using environmentally friendly poly(ethylene oxide) (PEO) as the matrix polymer. Compared to conventional matrix polymers, PEO can be easily removed at a low calcination temperature (400 °C), which allows the TiO nanofibers to be maintained in pure anatase phase with high crystallinity during calcination. This is of high importance for the application of TiO nanofibers in DSSCs as only the anatase phase crystals were reported to show good photovoltaic performance. Various characterization results reveal that the synthesized V TiO nanofibers have well-controlled diameters, uniform morphology, pure anatase phase and high crystallinity. In addition, the TiO grain size in the synthesized nanofibers could be easily tuned by changing the preparation conditions. To demonstrate the application of these TiO nanofibers, DSSCs were fabricated and the best devices have shown an energy conversion efficiency (η) of 6.44% under 100 mW cm−2 AM 1.5G illumination, which represents one of the most efficient DSSCs using TiO nanofibers or nanorods as the photoelectrode. Based on the electrospinning technique developed in chapter 3, chapter describes a facile method to prepare nanofibrous ZnO photoelectrodes with tunable thicknesses and good adhesion to fluorine-doped tin dioxide (FTO) substrate. As compared to the method describe in chapter 3, the method in chapter avoids the paste and film making procedures, which further reduces the fabrication cost of DSSCs. The best device has an η of 3.02% under 100 mW cm−2 AM1.5G illumination, which is greatly improved as compared to previous reports adopting ZnO photoelectrodes with a similar structure. These two chapters together demonstrate electrospinning technique as a powerful tool for the fabrication of photoelectrodes in DSSCs. Chapter reports the design and synthesis of a new orgainc sensitizer based on conjugated polymer with a unique donor (D)-π bridge-acceptor (A) structure (triphenylamine based electron donating backbone as donor, cyanoacetic acid based electron accepting side chain as acceptor and conjugated thiophene units as π bridge). As compared to conventional ruthinium dye sensitizers, polymer dye sensitizer has the advantages of low cost (independence of rare matal), easy design and synthesis, high VI molar absorptivity, and tunable optoelectronic properties. An η of 3.39% is obtained under 100 mW cm−2 AM 1.5G illumination, which represents the highest efficiency for polymer dye sensitized DSSCs reported so far. These features show good promise of conjugated polymers as sensitizers for DSSC application. Chapter describes the fabrication of solid-state dye-sensitized solar cells (SDSCs) using poly(3-hexylthiophene) (P3HT) as hole transporting material. Through optimization of device fabrication, an η up to 3.85% is achieved under 100 mW cm−2 AM1.5G illumination, which is one of the most efficient SDSCs using polymeric hole transporting material. More importantly, this work represents the first systematic study of charge transport and recombination in SDSCs using conjugated polymer as the hole transporting material, which sheds light on understanding the operation of highly efficient solid-state devices. Combining the photoelectrode preparation technique (chapter and chapter 4) with advantages of organic dye as sensitizer (chapter 5) and P3HT as hole transporting material (chapter 6), Chapter describe the development of a new type of SDSCs employing electrospun mesoporous TiO nanofibers (NFs) as photoelectrode, organic dye D131 as the sensitizer and P3HT as the hole transporting material. As compared to the regular electrospun TiO NFs, mesoporous TiO NFs have high surface area, resulting in greatly improved dye loading amount and light harvesting ability. Accordingly, an η of 1.82% is obtained under 100 mW cm−2 AM1.5G illumination for mesoporous TiO NF-based devices, which is 3-fold higher than that for regular TiO NF-based devices fabricated under the same conditions (η = 0.42%). In addition, VII mesopores on TiO NF surfaces have negligible effect on charge transport and collection. Initial aging test proves good stability of the fabricated devices, which indicates the prospect of mesoporous TiO NFs as photoelectrode material for SDSC application. VIII Chapter fabricated under the same conditions. Detailed device fabrication procedures are described in the experimental section. The TiO film thickness was kept at 1.8 μm and the active cell area was 0.10 cm2. Figure 7.4 (a) The configuration of a typical device employing mesoporous TiO nanofibers as the photoelectrode, D131 as the sensitizer and P3HT as the hole transporting material. (b) The chemical structure of D131. (c) Energy band diagram of each component in the device. Figure 7.4c displays the energy band diagram for each component in the device. The detailed determination of the energy levels for D131 and P3HT has been shown in 120 Chapter the chapter (Figure 6.2 and Figure 6.3). The lowest unoccupied molecular orbital (LUMO) level of D131 (–2.90 eV) is well above the conduction band edge of TiO (~ –4.00 eV vs vacuum level).[34] Therefore, electron injection from the LUMO of D131 into the conduction band of TiO is energetically feasible. Meanwhile, the highest occupied molecular orbital (HOMO) of D131 (–5.24 eV) is lower than that of P3HT (–5.10 eV), ensuring the hole conduction from D131 to P3HT. However, the electron transfer pathway from the LUMO of P3HT to the conduction band of TiO should be blocked by D131, as the LUMO level of D131 is ~ 0.30 eV higher than that of P3HT (–3.18 eV). This energy band diagram indicates that the function of P3HT in the present SDSC is largely HTM, even though it can absorb a fraction of incident light that passes through the D131 sensitized TiO film. Figure 7.5a shows the typical photocurrent density-voltage curves obtained under irradiation of 100 mW cm−2. All data are repeated in three independent devices. For devices based on regular NFs, a short circuit photocurrent density (J sc ) of 0.973 mA cm-2, an open circuit photovoltage (V oc ) of 0.857 V and a fill factor (FF) of 0.50 were achieved, resulting in an energy conversion efficiency (η) of 0.42%. The device performance was greatly improved for mesoporous NFs-based devices, which gave J sc of 3.979 mA cm-2, V oc of 0.915 V, FF of 0.50 and η of 1.82%. The improved energy conversion efficiency is thus mainly attributed to ~ 3-fold increase in J sc as compared to that for regular NF based devices. To understand the difference in J sc , dye-desorption experiment was first performed. As shown in the last column of the inset of Figure 7.5a, the dye loading for mesoporous NF film is 8.25 × 10-8 mol cm-2, 121 Chapter which is ~ 3.3-fold more relative to that for regular NF film (1.93× 10-8 mol cm-2). Figure 7.5 Typical photocurrent density-voltage curves (a) and IPCE spectra (b) of SDSCs based on mesoporous and regular NF photoelectrodes. The improved J sc value for mesoporous NFs-based devices is further verified by the incident photon-to-electron conversion efficiency (IPCE) spectra, as shown in 122 Chapter Figure 7.5b. The maximum IPCE values obtained at 410 nm are 61% and 19% for the mesoporous and regular NF-based devices, respectively, suggesting that light harvesting is significantly improved for mesoporous NF-based devices as compared to that for regular NFs. Figure 7.6 IMPS (a) and IMVS (b) of SDSCs based on mesoporous and regular NF photoelectrodes. To obtain deep insights into the effect of different NF structures on device performance, intensity modulated photocurrent spectroscopy (IMPS) and intensity 123 Chapter modulated photovoltage spectroscopy (IMVS) have also been employed to investigate the charge transport and recombination characteristics in mesoporous and regular NF-based devices. In these measurements, frequency-dependent photocurrent or photovoltage responses of a typical device to modulated incident light were recorded. From the IMPS measurements (Figure 7.6a), the transport time (τ d ) of injected electrons through TiO film can be calculated from the equation τ d = 1/(2πf d, ),[35] where f d, is the characteristic frequency at the minimum of the IMPS imaginary component. Thus, τ d values are estimated to be 1.05 and 0.88 ms in mesoporous and regular NF films, respectively. In addition, from the equation D n = d2/(2.35τ d ),[36] where d is the thickness of the photoelectrode (~ 1.8 μm), the electron diffusion coefficients (D n ) in mesoporous and regular NF films are calculated to be 1.31 × 10-5 and 1.57× 10-5 cm2 s-1, respectively. Although the mesoporous NF film has a much higher pore volume (0.272 cm3 g-1) as compared to that for regular NF film (0.0649 cm3 g-1), which could result in a more tortuous propagation of electrons and a more frequent electron trapping and detraping events in TiO layer,[37,38] the only slightly smaller D n for mesoporous NF films indicates that the retardation of electron transportation caused by mesopores is not very serious in this case. This could be probably due to the TiCl post-treatment step in device fabrication, which partially fills the pores in mesoporous NFs and to some extent offsets the original difference in NF structures. A similar phenomenon has also been observed by Kim et. al.[39] From the IMVS measurements (Figure 7.6b), the recombination lifetime (τ n ) was calculated using the equation τ n = 1/(2πf n, ),[35] where f n, is the characteristic 124 Chapter frequency at the minimum of the IMVS imaginary component. Thus, τ n values are estimated to be 6.20 and 4.60 ms for mesoporous and regular NF films, respectively. In general, the longer lifetime indicates a slower recombination rate between photoelectrons in the conduction band of TiO and the hole conductor P3HT. This result is consistent with the improved V oc for mesoporous NF-based device (0.915 V) as compared to that for regular NF-based device (0.857 V).[14] Furthermore, the charge collection efficiency, η cc , described by the equation η cc = – τ d /τ n ,[40] is calculated to be 83.0% and 80.8% for mesoporous and regular NF films, respectively. Considering the obtained dye-desorption and IPCE results, the similar charge collection efficiency for mesoporous and regular NF films further confirms that the significant difference in device photocurrent is mainly due to different dye adsorption in these two TiO NF films. To evaluate the long-term device stability, a typical device stored in a glove-box for ~6 months was retested, which gave J sc of 3.887 mA cm-2, V oc of 0.918 V, FF of 0.48 and η of 1.73%. As compared to the typical performance of a freshly prepared cell (Figure 5a), the aged cell sustained ~ 94% of its initial device efficiency, demonstrating good stability of the fabricated devices. Future aging test should be performed under more stringent conditions (AM 1.5G full sunlight soaking for 1000 h at 60 °C), which needs special sealing technique for SDSCs as the exposure of P3HT to the extrinsic oxygen or water will affect the intrinsic device-stability.[41, 42] 7.4 Conclusions 125 Chapter In summary, we prepared two types of TiO nanofibers in pure anatase phase by electrospinning technique. One is regular NFs with smooth surfaces, and the other is mesoporous NFs with uniform pore size distribution. A high BET surface area of 112 m2 g-1 was obtained for mesoporous NF films due to the existence of mesopores on the nanofiber surface. Solid-state dye-sensitized solar cells were fabricated based on these nanofibers using organic indoline dye D131 as the sensitizer and P3HT as the hole transporting material. As compared to the low J sc (0.973 mA cm-2) and η (0.42%) for regular NF-based devices, a three-fold increase in J sc (3.979 mA cm-2) was obtained for mesoporous NF-based devices, yielding an energy conversion efficiency of 1.82%. The increase in J sc was mainly attributed to the greatly improved dye adsorption for mesoporous NFs. In addition, IMPS and IMVS analysis reveals that the mesopores on NF surfaces have very minor effects on charge transport and collection relative to that for regular NFs. The initial aging test proved good stability of the devices. 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For the devices based on nanofibrous ZnO photoelectrodes, an energy conversion efficiency (η) of 3.02% has been achieved under 100 mW cm−2 AM 1.5G illumination, which is greatly improved as compared to the previous reports adopting photoelectrodes with a similar structure. For the devices based on regular TiO nanofibrous photoelectrodes, an η of 6.44% has been achieved, which represents one of the most efficient DSSCs using TiO nanofibers or nanorods as the photoelectrode. Meanwhile, for the devices based on mesoporous TiO nanofibrous photoelectrodes, an η of 1.82% has been obtained in solid-state dye-sensitized solar cells, which is 3-fold higher than that for regular TiO nanofibers-based devices fabricated under the same conditions (η = 0.42%). These results demonstrate that electrospinning technique is a powerful tool for the fabrication of photoelectrodes in DSSCs. (2) A novel orgainc sensitizer based on conjugated polymer with unique D-π-A structure has been designed and synthesized. An η of 3.39% was obtained under 100 mW cm−2 AM 1.5G illumination, which represents the highest efficiency for polymer dye 131 Chapter sensitized DSSCs reported so far. These features show good promise of conjugated polymers as sensitizers for DSSC application. (3) Efficient solid-state dye-sensitized solar cells (SDSCs) have been successfully fabricated by using organic dye as sensitizer and poly(3-hexylthiophene) (P3HT) as hole transporting material. Through optimization of device fabrication, an η up to 3.85% was achieved under 100 mW cm-2 AM1.5G illumination, which is one of the most efficient SDSCs using polymeric hole transporting material. These results show good promise of conjugated polymers as efficient hole transporting material for low-cost SDSC application. (4) Charge transport and recombination in SDSCs using conjugated polymer as the hole transporting material were systematically studied for the first time based on TiO -D131/P3HT model system. The limiting factor for energy conversion efficiency was found to be the relatively large hole transport impedance in P3HT as compared to electron transport impedance in TiO . This study sheds light on understanding the operation of highly efficient solid-state devices and provides basis for further improving device performance. 8.2 Outlook (1) As the large hole transport impedance of P3HT is the major limiting factor for the energy conversion efficiency of P3HT-based SDSCs (chapter 6), further enhancement of device performance could be achieved by improving the hole conductivity of P3HT via chemical modification or physical doping, provided that its penetration into mesoporous TiO films is not affected. 132 Chapter (2) To improve the performance of ZnO-based DSSCs (chapter 4), future work should be focused on designing new dye sensitizers that are suitable for ZnO photoelectrode. This is due to the poor chemical stability of ZnO in acidic dye solution and the formation of Zn2+/dye complexes which could block the injection of electrons from the dye molecules to the semiconducting electrodes. (3) To further improve the performance of mesoporous TiO NF-based device (chapter 7), future work should be focused on reducing the diameter of present electrospun mesoporous TiO NFs by fine-tune the composition of electrospinning gel and experimental conditions, which could further increase the surface area of mesoporous TiO NFs and improve dye loading amount. 133 Appendix LIST OF PUBLICATIONS 1. Wei Zhang, Rui Zhu, Xizhe Liu, Bin Liu*, Seeram Ramakrishna*. Facile Construction of Nanofibrous ZnO Photoelectrode for Dye-sensitized Solar Cell Applications. Appl. Phys. Lett. 2009, 95, 043304-7. (Top 20 most downloaded article in August 2009) 2. Wei Zhang, Zhen Fang, Mingjuan Su, Mark Saeys, Bin Liu*. A Triphenylamine based Conjugated Polymer with Donor-Acceptor Architecture as Organic Sensitizer for Dye-sensitized Solar Cells. Macromol. Rapid Commun. 2009, 30, 1533-1537. (most accessed article of March 2009-Feburary 2010) 3. Wei Zhang, Rui Zhu, Lin Ke, Xizhe Liu, Bin Liu*, Seeram Ramakrishna*. Anatase Mesoporous TiO Nanofibers with High Surface Area for Solid-state Dye-sensitized Solar Cells. Small. 2010, 6, 2176-2182. 4. Wei Zhang, Yueming Cheng, Xiong Yin, Bin Liu*. Solid-state Dye-sensitized Solar Cells with Conjugated Polymer as Hole-transporting Materials. Macromol. Chem. Phys. 2010, 212, 15-23. (has been featured on materials views) 5. Xizhe Liu, Wei Zhang, Satoshi Uchida, Liping Cai, Bin Liu*, Seeram Ramakrishna*. An Efficient Organic Dye-sensitized Solar Cell with in-situ Polymerized Poly(3,4-ethylenedioxythiophene) as Hole Transporting Material. Adv. Mater. 2010, 22, E150–E155. (top most downloaded article of the month of April-May 2010) 134 Appendix 6. Wei Zhang, Rui Zhu, Feng Li, Qing Wang*, Bin Liu*. High Performance Solid-state Organic Dye Sensitized Solar Cells with P3HT as Hole Transporter. J. Phys. Chem. C 2011, 115, 7038-7043. 7. Wei Zhang, Rui Zhu, Bin Liu*, Seeram Ramakrishna. Low-cost Fabrication of TiO Nanorod Photoelectrode for Dye Sensitized Solar Cell Application. Aust. J. Chem. 2011, 64, 1282-1287. 135 [...]... junction solar cell with 6% efficiency, which is a milestone of photovoltaic technology.[3] Within a year, a thin-film heterojunction solar cell based on Cu 2 S/CdS also achieved 6% efficiency. [4] A year later, a 6% GaAs pn junction solar cell was reported by RCA Lab in the US.[5] With in a year, Hoffman Electronics (USA) offered commercial Si photovoltaic cells with 2% efficient at $1500/W The efficiency. .. (d) the variation of ZnO film thickness with different eletrospinning time Figure 4.2 XRD pattern (a) and SAED pattern (b) of the calcined ZnO nanofibers Figure 4.3 Typical photocurrent density-voltage curves of DSSCs made of ZnO nanofibrous photoelectrodes with a film thickness of 1.5 μm (a), 3.2 μm (b) and 5.0 μm (c), without Zn(OAc) 2 solution treatment; (d) film thickness of 5.0 μm with Zn(OAc) 2... economic reach of wider markets Figure 2.1 Historical trends of cost per watt for solar cells and volume of production.[15] 10 Chapter 2 It is estimated that covering 0.1% of the Earth’s surface with photovoltaic devices with an efficiency of 10% would satisfy the global energy consumption per year![16] Commercial photovoltaic devices with 10% efficiency have been widely available Great efforts are directed... cost per watt of silicon solar cells has dropped significantly over the past decade, these devices are still expensive to compete with conventional grid electricity It is an urgent task to develop much cheaper photovoltaic devices with reasonable efficiency for widespread application of photovoltaic technology In this context, a new type of photovoltaic devices called dye sensitized solar cells (DSSCs)... further enhance the energy conversion efficiency of DSSCs Major efforts have been placed on the development of new photoelectrode material, new sensitizer and new hole transporting materials through cost-effective methodologies As energy conversion efficiency of devices is the key parameter to evaluate the performance of these new materials, efforts have also been devoted to the optimization of device fabrication...LIST OF TABLES Table 2.1 Performance of photovoltaic and photoelectrochemical solar cells Table 2.2 Some representative CP sensitizers Table 3.1 BET results for nanofibers with different TiO 2 grain size Table 3.2 Device characteristics of DSSCs made of TiO 2 nanorods with different TiO 2 grain size: (A) 9.9 nm; (B) 17.5 nm; and (C) 18.7 nm Table 6.1 The performance of D131 sensitized SDSCs... the 1980s The first thin-film solar cell with over 10% efficiency was produced in 1980 based on Cu 2 S/CdS ARCO Solar was the first company to provide photovoltaic modules with over 1 MW per year (1982) In 1985, researchers of the University of New South Wales (Australia) fabricated a Si solar cell with more than 20% efficiency under standard sunlight.[11] In 1986, ARCO Solar produced the first commercial... of volatile liquid electrolyte containing I-/I 3 - redox couple Problems such as evaporation and leakage of liquid electrolyte, corrosion of Pt counter electrode by I-/I 3 -, and degradation of dye molecules in electrolyte largely compromise device stability.[12] These disadvantages have led to the rapid development of solid-state dye- sensitized solar cells (SDSCs) by replacing liquid electrolyte with. .. ruthenium dyes with organic dye molecules owing to their many advantages.[19,20] Firstly and most importantly, organic dyes are relatively cheaper as compared to ruthenium dyes because there is no limitation of resources such as precious noble metals Secondly, organic dyes generally have much higher absorption coefficients than that of ruthenium dyes and the light absorption band of organic dyes can... the maximal power point, respectively (4) Energy conversion efficiency (η) The energy conversion efficiency is defined as the ratio of P m to the incident radiation power (P in ) on the solar cell surface: 14 Chapter 2 η= Pm I sc ⋅ Voc ⋅ FF = Pin Pin (2.2) As η is a function of I sc , V oc and FF, improvement of the DSSC performance is achieved by optimization of three parameters η is also dependent . FABRICATION OF DYE SENSITIZED SOLAR CELLS WITH ENHANCED ENERGY CONVERSION EFFICIENCY ZHANG WEI NATIONAL UNIVERSITY OF SINGAPORE 2011 FABRICATION OF DYE SENSITIZED. SENSITIZED SOLAR CELLS WITH ENHANCED ENERGY CONVERSION EFFICIENCY ZHANG WEI (M. Sci.) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL. curves of DSSCs made of ZnO nanofibrous photoelectrodes with a film thickness of 1.5 μm (a), 3.2 μm (b) and 5.0 μm (c), without Zn(OAc) 2 solution treatment; (d) film thickness of 5.0 μm with