以鈦板為可撓基材之染料敏化太陽能電池:光物理和光電化學研究

Abstract

本論文分為三大部分，主要探討以鈦版為光電極基材之可撓式背面照光染料敏化太陽能電池。 第一部分為改善此系統之元件效能及增加其長期穩定性之基礎探討。首先，本研究針對此系統之重要製程參數進行基礎研究探討：沉積白金對電極濺鍍時間、製備二氧化鈦/鈦版基材光電極燒結溫度、鈦板基材厚度及電解液組成。在針對各條件最佳化後，其元件光電轉換效率可達5.95%。同時，本研究發現背照式染料敏化太陽能電池之長期穩定性優於傳統正面照光染料敏化太陽能電池，主要由於電解液中的碘會吸收紫外光，紫外光減少將抑制染料分子於二氧化鈦表面脫附發生。相較於一般正照式系統，本研究中之背照式系統在經過連續五百小時照光後仍保有優良穩定性。 然而，此系統主要缺點為二氧化鈦/鈦版基材於高溫燒結後所產生之非晶相金屬氧化物及背面照光所導致較低入射光強度。經由製備二氧化鈦/鈦版基材光電極過程中，高溫燒結所產生於鈦板基材表面之非晶相金屬氧化物將會阻礙電子傳輸，導致元件效能下降。而由於鈦板基材不透光的特性，太陽光必須由白金對電極入射，以致此系統之入射光強 度比起一般正照式染料敏化太陽能電池低，導致激發染料電子量下降並減少光電流產生。有鑑於此，第二部分將針對前述兩項缺點提出解決方法。為了減少高溫製程中所產生之非晶相金屬氧化物，本研究利用不含膠黏劑之二氧化鈦漿料，製備二氧化鈦/鈦版基材光電極，並對此光電極進行較低溫度燒結，以降低非晶相金屬氧化物生成。本研究結果發現含有有機膠黏劑之二氧化鈦漿料並不適用低溫燒結製程，因為低溫燒結無法完全去除此漿料中之有機膠黏劑，使其殘留於二氧化鈦薄膜阻礙電子傳輸。本研究提出使用不含有機膠黏劑之二氧化鈦漿料於較低溫製程中(350 oC)，所製備光電極之元件光電轉換效率可達到4.34%，近乎與使用含有機膠黏劑之二氧化鈦漿料於較高溫製程中(450 oC)，所製備光電極所製備之元件效能(4.33%)。結果顯示使用不含膠黏劑之二氧化鈦漿料於低溫製程來製備元件將有效減少製備成本及能量消耗。為了有效提高經由背面入射光之強度，本研究提出具有網狀結構之白金對電極增加此系統之背面入射光強度。在白金濺鍍電流40 mA 及濺鍍時間5 s 條件下，網狀白金對電極具有高達99%之穿透度，高於一般非網狀白金對電極92%之穿透度。即便是網狀白金對電極對於電解液之氧化還原對催化能力稍不及一般非網狀白金對電極，但其高穿透度之優點使其組成之元件擁有較優良之效能表現，其光電轉換效率可達4.77%。 第三部分中，本研究引入具有高傳導電子速度之二氧化鈦奈米管，來彌補傳統二氧化鈦奈米粒子於粒子間眾多晶界而造成之低傳導電子速率。由於電子於一維結構之二氧化鈦奈米管中具有較高傳輸能力及較低再結合發生機會，此結構在近幾年來已被廣泛探討，但其低表面積之缺點限制其應用範圍。本研究提出兩項方法來彌補此結構低表面積之缺點。首先，本研究提出以二氧化鈦奈米管為底層，二氧化鈦奈米粒子為頂層之複合結構，結合二氧化鈦奈米管及二氧化鈦奈米粒子優點於光電極。二氧化鈦奈米管不但能加速電子傳輸，且能增加二氧化鈦奈米粒子與鈦版間接觸，而二氧化鈦奈米粒子能提供高染料吸附表面積，進一步提升電子激發量。以此複合結構光電極所製備之元件效能可達到6.68%的光電轉換效率，高於以二氧化鈦奈米粒子製備光電極之元件效能(5.55%)。再者，本研究提出將二氧化鈦奈米粒子利用化學法填充到二氧化鈦奈米管之內層及外層，使得此複合膜具有高表面積及高電子傳遞速度的特性。以此複合結構光電極所製備之元件效能可達到6.45%的光電轉換效率，高於以二氧化鈦奈米粒子製備光電極之元件效能(4.21%)。另一方面，許多研究學者利用增加二氧化鈦奈米管長度來增加其表面積，進而提高其染料吸附量；然而，其開環電壓卻會因管長過長所導致電子再結合而急遽下降。有鑑於上述缺點，本研究首先探討二氧化鈦奈米管之管長來彌補其表面積較小之缺點，並進一步利用氧化釔修飾二氧化鈦奈米管表面來彌補其開環電壓下降。此修飾使二氧化鈦之傳導帶向負方向移動，並能降低電子再結合發生機會，進一步改善電池效能表現。在相同管長條件下，經由氧化釔修飾二氧化鈦奈米管製備之元件光電轉換效率可達6.52%，高於以二氧化鈦奈米管所製備之元件光電轉換效率(5.35%)。 對於可撓式染料敏化太陽能電池，不僅是如前幾章提到的可撓式光電極重要，可撓式對電極也是不可或缺的要素。傳統上，研究學者以陽極蝕刻法於鈦板表面製備二氧化鈦奈米管並應用於光電極。然而，本研究中最後一章節利用具有二氧化鈦奈米管印記之鈦板作為對電極基材，此二氧化鈦奈米管印記於超音波震盪移除鈦板上之二氧化鈦奈米管後形成於鈦板上。利用此具有印記之鈦板比起一般無印記之鈦板有較高白金濺鍍表面積，對於電解液中氧化還原反應有較高催化能力。以此具有印記之鈦板所製備之元件光電轉換效率可達9.35%，高於以無印記之鈦板所製備之元件光電轉換效率(7.81%)。

There are three parts in this dissertation aim to investigate the Ti foil-based photoanodes for back-illuminated dye-sensitized solar cells (DSSCs). In the first part, the fundamental researches in this system was made to improve the cell performance and the durability. The effects of Pt sputtering periods on the counter electrode, sintering temperatures for TiO2/Ti foil based photoanodes, thickness of Ti foils, and the composition of the electrolyte were investigated. A solar-to-electricity conversion efficiency (η) of 5.95% was obtained after optimizations of these parameters. Meanwhile, the improved durability was found for back-illuminated DSSCs, due to the absorption of UV-rays by the iodine in the electrolyte. The back-illuminated DSSC shows a good durability even under continuous illumination of 100 mW/cm2 light intensity least for 500 h. The main challenges in this system lie on the formation of amorphous metal oxide between the TiO2 semiconductor layer and the Ti foil with sintering process to hinder the transfer of electrons and the lower incident light caused by back illumination, resulting in less dye-excitation and reduction of the photocurrent density. In the second part, strategies to solve the challenges of formation of amorphous metal oxide and back illumination are proposed. A binder-free TiO2 paste was used for the TiO2/Ti foil based photoanode with a low temperature sintering process to minimize the formation of amorphous metal oxides. It cannot be realized with a TiO2 paste with binder, because the residual of the organic binder in the TiO2 films of the photoanodes fabricated in a low sintering temperature would act as obstacles for electron transportation and reduce cell performance. It was evidenced that the DSSC with the TiO2 film fabricated with a binder-free TiO2 paste sintered at a relatively low temperature shows competitive performance (350 oC, η = 4.34%) to the cell with a TiO2 paste with bindersintered at higher temperature (450 oC, η = 4.33%). Therefore, the energy consumption can be reduced with this low temperature sintering process. Net-like Pt counter electrodes were made to increase the incident light illuminated from the back side of the cell. Higher transmittance of an average value of 99% was obtained for net-like Pt counter electrodes, with compared to that of 92% for a bare one, under Pt sputtering current of 40 mA for 5 s. Even the catalytic ability of the Pt layer is worse for net-like Pt counter electrodes caused by less Pt deposition, their higher transmittances lead to the better performance for the pertinent DSSC (η = 4.77%). Other than applying TiO2 nanoparticles (TNPs) as the semiconductor layer for the studies mentioned above, the other structure with higher electron transferring rate was introduced in the last part to compensate for the lower electron transportation in TNPs caused by numerous boundaries between TNPs. One-dimensional TiO2 nanotubes (TNTs) were widely used in the recent years because it exhibits better electron transportation and can reduce the loss of electrons by recombination. However, its smaller surface area as compared to that of TNPs limits its application. Two methods were proposed to solve this problem. One is to combine the TNTs with TNPs, and there are two ways applied in this dissertation, i.e., fabrication of a layer-by-layer structure and infiltration of TNPs into TNTs. For the layer-by-layer structure, the underlayer TNTs improves the electron transportation and increase contact points between Ti foil and TNPs, while TNPs overlayer provides larger surface area for dye adsorption. The pertinent DSSC exhibited a η of 6.68% with compared to that of the cell with only TNPs (η = 5.55%). For the TNTs infiltrated with TNPs, this composite film possesses higher surface area and higher electrons transfer rate, and η of 6.45% and 4.21% were obtained for the DSSC with the composite film and bare TNTs on the photoanodes, respectively. The other method to solve the problem of smaller surface area of TNTs is to increase their length, but the open-circuit voltage will decrease at the same time. A core-shell structure was applied with a yttrium oxide thin film on the surface of TNTs tocompensate for the lower open-circuit voltage with longer TNTs made to increase the surface area. The η of the pertinent DSSC was improved to 6.52% with compared to that of 5.35% for the DSSC with bare TNTs. For a flexible DSSC, not only a flexible photoanode but a flexible counter electrode is indispensable. Traditionally, the Ti foils are anodized for making TNTs and applied in a photoanode. However, in the last chapter in the last part, the imprints of TNTs were utilized by first fabricating TNTs on Ti foils by anodization and removing TNTs completely from Ti foils by ultrasonically vibration. The resulting Ti foils with TNTs imprints was applied as the substrate of counter electrodes with highly active surface area for Pt sputtering. These counter electrodes exhibit better catalytic ability for I-/I3- redox reaction. An extremely high η of 9.35% was obtained for the DSSC with a Pt-sputtered Ti foil with TNT imprints on its surface as the counter electrode, which is much higher than that for the cell with bare Ti foil as the substrate of its counter electrode (7.81%).