二氧化鈦奈米管應用於鈦基材之染敏太陽能電池

Abstract

本論文主要分為三議題，在第三章和第四章中，不僅將二氧化鈦奈米管應用於以鈦板為光電極基材之可撓式背面照光染料敏化太陽能電池，更於第五章中應用二氧化鈦奈米管於以鈦板為對電極基材之可撓式正面照光染料敏化太陽能電池。其主要目的在工作電極及對電極中引進新材料並提升染敏太陽能電池之效能。 在第三章和第四章探將二氧化鈦奈米管應用於以鈦板為光電極基材之可撓式背面照光染料敏化太陽能電池。此時的速率決定步驟在於工作電極的部分，在固定對電極下，工作電極就是我們的變因；因此改變工作電極參數即對整體效率有所改變。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率，此結構在近幾年來被廣泛探討，但相較於二氧化鈦奈米粒子具有較低表面積之缺點而限制其應用範圍。因此，在第三章中首次採用雙壁二氧化鈦奈米管做為染敏之光陽極的新材料，利用此一維和雙壁的結構的來加速電子傳輸和增加其表面積讓染料吸附；而此雙壁二氧化鈦奈米管的成長機制也在此研究中提出。以雙壁二氧化鈦奈米管光電極所製備之元件效能可達到6.85%的光電轉換效率，高於以一般二氧化鈦奈米管製備光電極之染敏元件效能(4.63%)。 除了在上述所提及的二氧化鈦奈米管光電極，二氧化鈦奈米管也應用於以二氧化鈦奈米粒子光電極之染敏太陽能電池於第四章中。二氧化鈦奈米粒子的隨機分布使其於粒子間有晶界效應，而降低其電子傳輸速率及增加再結合的機率。在此章節中，利用具有二氧化鈦奈米管印記之鈦板作為光電極基材讓二氧化鈦奈米粒子沉積於其上，其中，利用超音波震盪移除鈦板上之二氧化鈦奈米管後即可形成具有二氧化鈦奈米管印記的鈦板。由於此結構可增加二氧化鈦奈米粒子和有印記之鈦板基材間的電子接觸，所以可降低其電子再結合的機率，進而增加電子壽命及電子收集效率。此外，利用具有印記之鈦板比起一般無印記之鈦板有較高的表面積使更多二氧化鈦奈米粒子沉積於其上，進而增加其染料吸附量。以此具有印記之鈦板所製備之元件光電轉換效率可達7.05%，高於以無印記之一般鈦板所製備之元件光電轉換效率(3.96%)。 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，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。因此在第五章中，引進poly (ethylenedioxythiophene) (PEDOT) 和二氧化鈦奈米管的複合新結構並應用於染敏太陽能電池之可撓式對電極中。在做對電極的研究時，此時的速率決定步驟在於對電極的部分，在固定工作電極下，對電極就是我們的變因；因此改變對電極參數即對整體效率有所改變。在此複合結構中，二氧化鈦奈米管扮演了重要角色，包括：相較於一般平坦的PEDOT有較高的活性催化面積、由於其一維結構可加速電子在對電極中的傳輸、由於二氧化鈦奈米管的支撐可防止PEDOT在此厚的膜厚下不會垮，並進而縮短對電極和電解質的距離而加速電子的傳導。同時，在此章節也最適化PEDOT在二氧化鈦奈米管上的電量。以PEDOT 和二氧化鈦奈米管的複合結構所製備之對電極其光電轉換效率可高達8.82%，高於以一般平坦的PEDOT所製備之元件光電轉換效率(6.50%)，甚至也高於以白金所製備之元件效率(8.24%)。所以此PEDOT 和二氧化鈦奈米管的複合結構具有潛力取代以白金製備之染敏太陽能電池的對電極材料，可降低成本並提高染敏太陽能電池之效能。

There are three main topics covered in this thesis, namely, applying TiO2 nanotubes (TNTs) to not only the Ti–based working electrodes (WEs) for back–illuminated dye–sensitized solar cells (DSSCs) as discussed in Chapter 3 and 4, but also the Ti–based counter electrodes (CEs) for front–illuminated DSSCs, studied in Chapter 5. The main purpose of this dissertation is to investigate the new approaches in the WEs and CEs and improve the cell performance of Ti–based DSSCs.Application of TNTs to the WEs of Ti–based DSSCs is studied in Chapter 3 and 4. In these chapters, the rate determining step is the WE of the DSSCs. Constant the CE and only differ in the WE; therefore, once change the parameters of the WE, it is prone to influence the performance of the DSSC. One–dimensional 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 limits its application as compared to that of TiO2 particles (TNPs). Therefore, a new strategy was adopted in Chapter 3, in which double–wall TiO2 nanotubes (DWTNTs) were used as the photoanode of DSSCs for the first time, in order to to obtain the advantages of fast electron transfer and high surface area for dye adsorption due to their one–dimensional double–wall tubular morphology. In addition, the comprehensive growth mechanism of the DWTNTs was also studied. The DSSC with DWTNTs as WE exhibited the best η of 6.85% with compared to that of the cell with bare TNTs (η = 4.63%).TNTs applied to not only the TNTs–based photoanodes as studied above, but also the TNPs–based photoanode were investigated in Chapter 4. TNPs are usually distributed randomly and have grain boundary effects, leading to limit the electron transport through the particles as well as increasing the probability of recombination. In this chapter, 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 imprinted–Ti foils were applied as the working substrates for photoanodes of DSSCs with the coating of TNPs on them. Due to the enhancement of electrical contact between the TNPs and the imprinted–Ti foils, the probabilities of recombination can be reduced so as to prolong the electron lifetime (τe) and enhance the charge collection efficiency (ηcc). Moreover, it shows higher surface area for dye adsorption for the DSSC with imprinted–Ti foils as working substrate than that of the cell with bare Ti foils. The η of the pertinent DSSC was improved to 7.05% with compared to that 3.96% for the cell with TNPs–coated bare Ti foil.Traditionally, the Ti foils are anodized for preparing TNTs and applied in a photoanode of DSSCs. However, not only a flexible WE but a flexible CE is indispensable to obtain a flexible DSSC. In Chapter 5, a new strategy is presented to fabricate a flexible CE with the hybrid structure of PEDOT and TNTs (PEDOT/TNTs) on Ti foils. In this chapter, the rate determining step is the CE of the DSSCs. Constant the WE and only differ in the CE; therefore, once change the parameters of the CE, it is prone to influence the performance of the DSSC. TNTs in PEDOT/TNTs plays an important role of providing larger active surface area compared to flat PEDOT, improving charge transfer at CEs due to its one–dimensional structures, also avoiding the collapse of thicker PEDOT films with TNTs supporting, and shortening the distance between CE and the electrolyte. The optimization of amount of PEDOT deposited onto to TNTs was also studied at the same time. A high η of 8.82% was obtained for the DSSC with PEDOT/TNTs as CE, which is much better than flat PEDOT (η= 6.50%) and even also Pt(η= 8.24%). It is expected that the efficient and economical film of PEDOT/TNTs can be a potential candidate for replacing the expensive Pt film on the CE of DSSCs.