|Abstract: ||本論文分為兩個主要部分，第一部分是以二氧化鈦 (TiO2) 粉體製備染料敏化太陽能電池(dye-sensitized solar cells, DSSC) 的光陽極 (photoanode) 並最佳化電池效能，第二部分為使用含矽氧烷基之化合物作為共吸附物以提高 DSSC 的光電轉換效率。|
本研究第一部分以 Degussa P25 粉體製備TiO2 薄膜光陽極，討論製程條件對所得薄膜結構以及 DSSC 光電轉換效率的影響。光陽極為 (TiO2 薄膜/緻密層/fluorine-doped tin oxide (FTO) 導電玻璃) 的結構，製備程序為先以四異丙烷氧化鈦 (titanium tetraisopropoxide, TTIP)/乙二醇甲醚 (2-methoxyethanol, 2-ME) 之混合液於 FTO 導電玻璃上製得緻密層，再於其上以聚乙二醇 (Polyethylene glycol, PEG20k, 分子量 = 20,000)/P25粉體之混合漿料製作TiO2薄膜可得之。使用以 TTIP/2-ME (w/w) = 0.2 製備之緻密層搭配以 PEG20k/TiO2 (w/w) = 0.35 製備之TiO2薄膜所得光陽極組裝 DSSC，可達到 5.35±0.08 % 的最高光電轉換效率。另外，本研究以分子量更高的PEG200k (分子量 = 200,000) 取代PEG20k增加漿料黏度，可改善TiO2 薄膜製程的穩定度。TiO2 薄膜的微結構可由製備過程中 PEG20k 或 PEG200k 的添加比例調整，TiO2 的表面積增加可讓其吸附更多染料，提高 DSSC 的短路電流 (short-circuit current, Jsc)，但其開路電壓 (open-circuit voltage, Voc) 則會因電子/電洞再結合位置 (recombination sites) 數量增加而降低。使用 PEG200k/TiO2 (w/w) = 0.25 製備TiO2 薄膜光陽極之 DSSC 具有最高光電轉換效率為5.51±0.17 %。
本研究第二部分使用四乙氧基矽烷 (tetraethoxysilane, TEOS) 或苯基三乙氧基矽烷(phenyltriethoxysilane, PTEOS) 作為染料的共吸附劑，討論其對 DSSC 效能的影響。根據 DSSC 之 i-V 行為與電化學阻抗分析的結果，使用TEOS 或 PTEOS 共吸附劑可增加電子由染料注入TiO2 的效率而提高 Jsc，且可降低暗電流 (dark current) 並使 Voc 上升，進而提高 DSSC 的光電轉換效率。使用 0.25 mM TEOS 或 0.5 mM PTEOS 與染料進行共吸附製備光陽極，所得 DSSC 之光電轉換效率可從 4.55±0.13 % 分別提高至 4.97±0.05 % 或 5.10±0.07 %，效率增加的幅度分別為9.2 % 或 12.5 %。
This thesis containing two major parts, the first part is preparation and optimization of P25-based TiO2 photoanode for its application in a dye-sensitized solar cell (DSSC), and the second part is study on the effects of siloxane derivatives as coadsorbents on the performance of a DSSC.
The structure of TiO2 photoanode plays a pivotal role in the performance of a DSSC. In the first part of this thesis, the influences of the preparation parameters of P25-based TiO2 photoanode and blocking layer on the energy conversion efficiency of a DSSC were investigated. The blocking layer, which sandwiched between the fluorine-doped tin oxide (FTO) glass and the TiO2 film, was derived from the mixture solution of titanium tetraisopropoxide (TTIP) and 2-methoxylethanol (2-ME). The DSSC with blocking layer synthesized using TTIP/2-ME ratio of 1/5 (w/w) exhibited superior energy conversion efficiency, which suggested that such blocking layer effectively reduce the electrolyte oxidation on the FTO surface and therefore improve the DSSC performance. The porosity of a TiO2 photoanode can be adjusted by the polyethylene glycol (PEG20k) content employed during the preparation process. When the average thickness of the TiO2 layer was approximately 20 micrometer, the performances of DSSCs employing photoanodes prepared using different PEG20k/TiO2 ratios were evaluated, and the best energy conversion efficiency of 5.35±0.08 % was achieved with the PEG20k/TiO2 (w/w) of 0.35. PEG200k was used to replace PEG20k in order to increase the viscosity of the TiO2 paste. The viscosity of the paste and the porosity of the TiO2 photoanode were adjusted by the PEG200k content employed during the preparation process. The performances of DSSCs employing photoanodes prepared using different PEG200k/TiO2 ratios were evaluated. With the increasing of PEG200k content in the TiO2 paste, the amount of dye absorption increases and resulted in an enhancement in short-circuit current (Jsc) but decreases in open-circuit voltage (Voc) of the DSSC. The best energy conversion efficiency of 5.51±0.17 % was achieved with the DSSC using the TiO2 photoanode prepared from a paste with PEG200k/TiO2 (w/w) ratio of 0.25.
In the second part of the thesis, tetraethoxysilane (TEOS) or phenyltriethoxysilane (PTEOS) was added in a dye solution as the coadsorbent and its effects on the photovoltaic properties of the resulting DSSC were investigated. It was found that the coadsorption of siloxane derivatives can hinder the formation of dye aggregates and improve the electron injection yield and thus increasing Jsc. This has also led to a rise in Voc, which is attributed to the decrease of charge recombination probability. Electrochemical impedance spectra data indicate that the electron lifetime was improved by the coadsorption of siloxane derivatives, which accounting for the significant improvement of Voc. The overall conversion efficiency was improved to 4.97±0.05 % and 5.10±0.07 % from 4.55±0.13 % upon addition of 0.25 mM TEOS and 0.5 mM PTEOS to the dye solution for TiO2 sensitization, respective.