染料修飾二氧化鈦電極、膠態高分子電解質及無鉑對電極之染料敏化太陽電池研究

Abstract

本論文主要探討各材料的改進或系統改變對染料敏化太陽電池光電轉換行為影響，同時也針對元件效率及穩定性進行探討。 本文的第一部份為針對染料敏化太陽電池進行最適化探討，藉由高分子分子量控制，製備不同孔徑分布之TiO2工作電極。其中以P2P1分布之TiO2工作電極有最佳效率表現，在電極含有光散射粒子條件下，其光電轉換效率可達9.04 %。並藉由雷射光物理分析及利用交流阻抗分析，發現P2P1-TiO2電極有最低之界面電子轉移阻抗及最長之電子壽命。 另一方面，在低溫製備TiO2工作電極製程方面，本實驗室首先提出將奈米碳管導入低溫製備之TiO2電極中並應用於染料敏化太陽電池。藉由控制奈米碳管分布(0.1 wt%)及Ti-前趨物比例 (TTIP/TiO2=0.08)，可製備出光電轉換效率達5.02%之低溫染料敏化太陽電池。 同時，我們亦針對Miyasaka教授團隊所開發之低溫TiO2漿料來製備低溫塑膠基材之染料敏化太陽電池，並以中央大學吳春桂教授實驗室所開發之SJW-E1染料進行最適化研究，同時針對TiOx緻密層、各種共吸附劑結構及元件穩定性進行探討。最佳化後之低溫塑膠染料敏化電池效率於100 mW/cm2光強度下可達6.31%，並發現以SJW-E1為染料之元件在光照穩定性測試下，比N719有更好的穩定性表現。 本文的第二部分乃針對中研院化學所林建村教授實驗室所合成之有機染料進行染料敏化太陽電池元件測試及光電化學性質探討。最佳元件表現可達6.15 % (相對於N3系統為7.86 %)。研究發現，當在第三oligothiophene接有arylamibes之染料，元件會有較高的開環電位。同時進行雷射光物理及交流阻抗分析推論在第三oligothiophene接有arylamibes之染料可以抑制於TiO2上的電子與氧化態的染料與I3-進行再結合反應。同時理論計算結果發現，在第三oligothiophene接有arylamibes之染料在在電子激發態時有較高之電荷傳遞效率。 在第五章中，將探討光譜互補式之染料共敏化行為於塑膠基材之低溫染料敏化太陽電池應用。研究發現，兩共敏化系統分別為N719/FL 與black dye/FL系統之染料敏化太陽電池之元件光電效能表現比使用單一種類染料的太陽電池來的高，並分別可達到5.10%與3.78%。然而在FL /Chl-e6共敏化系統便沒有電流加成現象，元件效率表現只介於兩種染料單一存在之間。並由交流阻抗分析此三種共敏化系統發現，N719/FL 與black dye/FL系統共敏化後，元件的TiO2特徵頻率與單一染料條件維持相同，甚至往更低頻位移。然而FL /Chl-e6共敏化系統之元件TiO2特徵頻率甚至比單一FL染料高，意味著FL /Chl-e6共敏化後因染料聚集現象使得電子於TiO2中的電子壽命更低。 本文的第三部分為探討膠態高分子電解質之製備及應用於染料敏化太陽電池。此部分將分別討論兩種膠態高分子電解質系統。首先歸納在電解質的有機溶劑之各項物化性質中，donor number為影響染料敏化太陽電池表現之重要影響因素。同時，在PVDF-HFP膠態高分子電解質中，以有機碘鹽(TBAI)為氧化還原對之元件表現，比無機碘(LiI)之表現來的佳。在含有5 wt% 的PVDF-HFP條件下，可得與液態電解質相仿之效率表現。並且在含有0.8 M TBAI與0.12 M I2時有最佳電流表現。進而導入二氧化矽奈米粒子於PVDF-HFP電解質中，由於降低PVDF-HFP之結晶度，明顯增加了離子傳輸路徑，因此提升了離子擴散係數，並使得元件有更好的光電流表現。在100 mW/cm2的光照下，最佳光電流可達14.04 mA/cm2，開環電位0.71 V，光電轉換效率為5.97 %。 另外，以同時聚合法方式製備膠態高分子電解質研究中，將低黏度之反應溶液導入奈米孔洞之TiO2電極中，再進行聚合反應使電解質固化，期能解決高分子電解質與工作電極間之界面阻力以提升光電轉換電流。研究發現，以B4Br作為交聯劑所製備之膠態電解質由於具有微相分離現象，使得有較高之導離度及元件電流表現。藉由將具可反應性官能基結構的分子與染料共吸附，除了可以作為共吸附劑降低染料之聚集現象外，並可在電解質的聚合過程中與TiO2電極界面間形成化學鍵結，可有效降低界面阻抗使得光電流從7.72 mA/cm2提升至10.00 mA/cm2。為了降低TiO2電極內之離子傳遞阻力，本研究藉由導入不同含量之均一粒徑PMMA於TiO2電極中，經燒結後製備含有350 nm孔洞之TiO2電極。經實驗發現在PMMA/TiO2比例為3.75時有一最佳結果，使元件之光電轉換效率從3.61%有效地提升至5.81 %。 最後在本文的第四部分，一系列之poly(3,4-alkylenedioxythiophene)導電高分子以電化學聚合法製備於FTO玻璃上，並將之作為染料敏化太陽電池之對電極進行探討。實驗發現元件以PProDOT-Et2作為對電極有最佳之效率表現為7.88 %與對白金對電極表現相當(7.77 %)。元件之FF很明顯的受到PProDOT-Et2電聚合電量之不同而改變，並且當電聚合電量高於80 mC/cm2時，因高分子薄膜之聚集現象，降低的氧化還原的活性面積，使得元件之光電流及光電轉換效率明顯下降。將PProDOT-Et2導電高分子薄膜用於低溫塑膠染料敏化太陽電池亦有類似的趨勢表現，其最佳光電轉效率在100 mW/cm2光強度下為5.20 %，相對於以白金對電極之染料敏化太陽電池為5.11 %。

The main purpose of this thesis is to investigate the behaviors of new approaches in electrodes (working and counter), sensitizers and gel polymer electrolytes for dye-sensitized solar cells (DSSCs) and discussing the influences on the cell performance and stability of DSSCs. In the first part of this thesis (Chapter 2 and 3), the optimization of solar energy conversion efficiency of DSSCs was investigated by the tuning of TiO2 photoelectrode’s morphology. Double-layered TiO2 photoelectrodes were designed by the coating of TiO2 suspension incorporated with low and high molecular weight poly(ethylene glycol) as a binder. Among four types of TiO2 electrodes, the P2P1 showed the highest efficiency under the conditions of identical film thickness and constant irradiation. This can be explained by the larger pore size and higher surface area of P2P1 TiO2 electrode than the other materials and these two factors assist for the facile transport of I3-/I- ion couple through the TiO2 matrix. The best efficiency (h) of 9.04% for a solar cell was obtained by introducing the light scattering particles to the TiO2 electrode measured under AM 1.5G. As for the part of low-temperature fabricated DSSC, the TiO2 film with the TTIP/TiO2 molar ratio of 0.08 has the best conduction. Meanwhile, the charge transport resistance at the TiO2/dye/electrolyte interface increased as a function of the MWCNT concentration, ranged from 0.1 to 0.5 wt%, due to a decrease in the surface area for dye adsorption. The DSSC with the TiO2 containing 0.1 wt% of MWCNT resulted in a JSC of 9.08 mA/cm2 and a cell conversion efficiency of 5.02 %. On the other hand, TiO2 film prepared by using binder-free TiO2 paste which developed by Prof. Miyasaka’s group was also used in plastic DSSC to optimal the SJW-E1 dye which synthesized by Prof. Wu’s group. The effects of TiOx buffer layer and co-adsorbents as well as long-term stability of plastic DSSCs were investigated. The TiOx buffer layer not only benefited the adhesion between TiO2 thin film and ITO/PEN substrate but also reduced the electron recombination, resulting in the improvement of the FF and conversion efficiency of cells. The optimized solar cell based on SJW-E1 showed a high efficiency of 6.31 % at 100 mW/cm2 (AM 1.5G), and SJW-E1 based solar cell showed a better stability than that of N719 based after 500 h light soaking test. In the second part of this thesis (Chapter 4), the co-sensitization of dyes for the complementary in the spectral characteristics in plastic DSSCs was investigated. Two co-sensitization systems for the plastic DSSCs, including N719/FL and black dye/FL showed enhanced photovoltaic performances compared with that of each dye individually. The optimal conversion efficiencies of N719/FL and black dye/FL DSSCs reached 5.10 % and 3.78 %, respectively, which were higher than that of individual sensitizers. However, for the system co-sensitized with FL and Chl-e6, the cell performances only lay in between that of each dye. From the EIS analysis, the characteristic frequencies (C.F.) at TiO2/dye/electrolyte interface for N719/FL and black dye/FL are kept the same or lower than that of individual dyes. While for the FL /Chl-e6 co-sensitized DSSCs, the C.F. were higher than that based on only FL, indicating that they had shorter electron lifetime in the TiO2 electrode after co-sensitization. In the third part of this thesis (Chapter 5), two kinds of gel polymer electrolytes were developed and used in DSSCs. At the beginning, it was found that the donor number of solvent in electrolyte is the one of the key factors that effect the photovoltaic performance of DSSC. Meanwhile, the quasi-solid state DSSCs were fabricated with polyvinyidene fluoride-co-hexafluoro propylene (PVDF-HFP) in methoxy propionitrile (MPN) as gel polymer electrolyte (GPE), tetrabutylammonium iodide/iodine as redox couple, 4-TBP as additive and nano-silica as fillers. The energy conversion efficiency of the cell with 5 wt% PVDF-HFP is comparable to that one obtained in liquid electrolyte system. Solar cell containing PVDF-HFP with 0.8 M of TBAI and 0.12 M of I2 shows maximum photocurrent. Moreover, the addition of 1wt% nano-silica is found to improve the at-rest durability and the performance of the solar cell. A photocurrent of 14.04 mA/cm2, a VOC of 0.71 V and an overall conversion efficiency of 5.97 % under 100 mW/cm2 irradiation was observed for the best performance of a solar cell in this work. On the other hand, the ionic conductivities and performances of DSSCs of GPEs prepared by in situ polymerization with different cross-linkers were investigated. The poly(imidazole-co-butylmethacrylate)-based GPE containing the B4Br cross-linker showed a higher ionic conductivity, due to the formation of micro-phase separation that resulted in an increase of ion transport paths in the GPE. Moreover, a co-adsorbent, (4-pyridylthio) acetic acid, co-adsorbed with N3 dye on the TiO2 electrode not only reduced dye aggregation, but also reacted with the cross-linkers in the GPE at the TiO2/GPE interface after gelling, thus the value of JSC significantly increased from 7.72 to 10.00 mA/cm2. In addition, in order to reduce the ionic diffusion resistance within the TiO2 electrode, incorporation of monodispersed PMMA in the TiO2 paste was considered. With the optimal volume ratio of PMMA/TiO2 (v/v = 3.75), the micro-porous TiO2 electrode exhibited larger pores (ca. 350 nm) uniformly distributed after sintering, and the ionic diffusion resistance within the TiO2 film could significantly be reduced. The cell conversion efficiency increased from 3.61 to 5.81% under illumination of 100 mW/cm2, an improvement of ca. 55 %. In the fourth part of this thesis (Chapter 6), a series of poly(3,4-alkylenedioxythiophene) counter electrodes prepared by electrochemical polymerization on the fluorine-doped tin oxide (FTO) glass substrate were incorporated in the platinum-free DSSCs. Cells fabricated with a PProDOT-Et2 counter electrode showed a higher conversion efficiency of 7.88 % compared to cells fabricated with PEDOT (3.93 %), PProDOT (7.08 %), and sputtered-Pt (7.77 %) electrodes. The FF was strongly dependent on the deposition charge capacity of the PProDOT-Et2 layer, but the aggregation of PProDOT-Et2 in higher deposition capacities (> 80 mC/cm2) resulted in decreases in JSC and the cell conversion efficiency. Incorporating the best ProDOT-Et2 film (40 mC/cm2) as the counter electrode in plastic DSSC was compared and showed similar tendency as mentioned above. The cell fabricated with a PProDOT-Et2 counter electrode showed a higher conversion efficiency of 5.20 % compared with that fabricated with sputtered-Pt (5.11%) electrodes under the illumination of 100 mW/cm2 (AM 1.5G).