- 學位論文
多種不同氧化鈦奈米材料於染料敏化太陽能電池的應用
Development of Various TiO2 Nanostructures as Photoanodes for Dye-sensitized Solar Cell Applications
摘要
由於石油能源的短缺議題日趨重要，發展廉價的替代能源如太陽能電池已成為重要的趨勢。染料敏化太陽能電池 (Dye-sensitized solar cells，DSSCs) 因製備簡易、成本低廉以及不錯的光電轉換效率，因此成為具有相當競爭的替代能源。而染料敏化太陽能電池的基本工作原理就是吸收太陽光透過與被染料敏化的半導體材料作用而轉換成電力輸出。本論文的主要目的是探討調控不同形貌的一維氧化鈦奈米材料於染料敏化太陽能電池的應用以期望達到具有較好的染料吸附率、較快的電子傳遞速度以及較高的光電轉換效率。本論文的主要工作分成三部分：(1) 製備二氧化鈦奈米管陣列於透明導電玻璃(fluorine-doped tin oxide，FTO)、(2) 製備具高比表面積的棒狀二氧化鈦奈米材料 (3) 製備不同形貌之二氧化鈦奈米材料及其染料敏化太陽能電池的應用。 首先，利用陽極氧化處理法製備膜厚為1.3 µm 鈦薄膜沉積於透明導電基板上，再利用氧化方式製備氧化鈦奈米管陣列作為光陽極；在經過450 oC的鍛燒之後利用XRD檢測得知其氧化鈦奈米管陣列相態為銳鈦礦。之後，再將氧化鈦奈米管陣列利用四氯化鈦作後處理並比較其光電轉換效率發現：經過四氯化鈦後處理的氧化鈦奈米管陣列電池其光電轉換效率(2.59 %)較未處理的電池的效率(1.90 %) 提高 36%。但此效率值仍偏低推斷可能是由於氧化鈦奈米管陣列的長度過短，因此可以增加氧化鈦奈米管陣列的長度以增加電池效率。進一步地在電壓為60V下以及不同陽極氧化時間 (2、4、6小時) 於鈦板上成長不同厚度的氧化鈦奈米管陣列並轉移到FTO基板上。之後，利用二次陽極氧化處理方式製備獨立雙開口式(open-ended)二氧化鈦奈米管陣列以及一端封閉(close-ended)的奈米管陣列結構；先前製備的二氧化鈦奈米管利用XRD、SEM以及TEM來鑑定其結構與相態。而被剝落的一端封閉氧化鈦奈米管陣列也製備成一端封閉開口向下、一端封閉開口向上以及雙開口式的氧化鈦奈米管陣列於FTO基板上。以陽極氧化時間為6小時的氧化鈦奈米陣列為例，其對應於一端封閉開口向下、一端封閉開口向上以及雙開口式之電池光電轉換效率分別為6.91 %、 6.47 % 以及7.69 %。 此外，利用以異丙醇為溶劑之溶熱法製備具高比表面積一維棒狀氧化鈦奈米材料，並探討實驗參數包含異丙醇鈦的濃度、乙酸以及異丙醇含量對於最終產物的影響，並利用BET、XRD、SEM以及TEM等方式分析所製備的一維棒狀二氧化鈦奈米材料之結構特性。與DP-25所製備的電池效率 (6.43%) 比較，一維棒狀二氧化鈦所製備的電池具有9.21 %之光電轉換效率，其效率增加原因推測是因為一維棒狀二氧化鈦具有較高的染料吸附率、較佳的光捕捉效率以及較快的電子傳遞速度。進一步探討不同醇類的溶劑(如乙醇、丙醇、異丙醇、丁醇、第三丁醇以及苯甲醇) 、溶熱反應時間以及溫度對於最終氧化鈦奈米材料形貌的影響；而其產物之形貌、結晶特性以及尺寸大小係利用TEM、 SEM、 XRD以及HR-TEM 等技術來分析。以不同醇類為溶劑所製備之氧化鈦奈米材料其電池轉換效率分列如下： 乙醇(方塊狀)：8.16 %、丙醇 (長方形)：8.39 %、丁醇 (啞鈴狀)：7.70 %、第三丁醇以及苯甲醇(呈細小棒狀團聚)：7.95以及6.17 %。在這些不同氧化鈦形貌當中，以異丙醇為溶劑所製備的氧化鈦棒狀奈米材料所製備的電池具有最高的光電轉換效率(9.21 %)，推測是由於較高的染料吸附量、較佳的光捕捉效率以及較快的電子傳遞速度。
並列摘要
The development of cost-effective renewable energy sources such as solar energy is one of the alternatives for the rapid depletion of fossil fuels. Dye-sensitized solar cells are the most prominent candidates for the easy fabrication, cost-effective and the conversion of solar energy into efficient electricity. Dye-sensitized solar cells (DSSCs) harvest solar energy and convert it into electricity through a suitable dye sensitized semiconductor photoanode material. The aim of this study was to develop one-dimensional TiO2 nanostructured materials by tuning the morphology of TiO2 to obtain an excellent photoanode with better dye loading, fast electron transport and good light harvesting capacity. These facilitate the further enhancement in the photovoltaic performance of DSSCs. The main objectives of this work were: (i) The fabrication of TiO2 nanotube arrays on transparent conducting substrates for front-side illumination DSSCs, (ii) Preparation of rod shaped TiO2 nanocrystals with high surface area for better photovoltaic performance and (iii) Preparation of different morphologies of TiO2 nanocrystals and its photovoltaic performance in DSSCs. Firstly, TiO2 nanotube arrays were grown by anodizing the titanium (Ti) film deposited on transparent conducting FTO (fluorine doped tin oxide) glass and used as a photoanode for the fabrication of DSSCs. A TiO2 nanotube arrays with a thickness of ~ 2 µm was obtained by anodizing 1.36 µm thick Ti film. X-ray diffraction (XRD) investigation revealed the formation of anatase phase TiO2 after annealing at 450 °C. Further, the anodized TiO2 nanotubes were subjected to TiCl4 post treatment. The photoelectric conversion efficiency of the device fabricated using TiCl4 treated TiO2 nanotube was found to be 2.59 % which was ~ 36 % enhancement than the untreated TiO2 nanotubes (1.90 %). The efficiency of DSSCs in this study is relatively lower, which may be attributed to the shorter TiO2 nanotubes. The efficiency can be further increased by increasing the film thickness. Therefore, TiO2 nanotubes were grown on Ti foil using the anodization process at 60 V for various anodization times such as 2, 4 and 6 h and transferred onto FTO substrate to perform front-side illumination on DSSCs. Subsequently, the close ended and open ended freestanding TiO2 nanotubes were obtained via a second anodization steps. The structural and morphological analyzes were done using XRD, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The detached close ended TiO2 nanotubes were affixed as close ended-downside & close ended-upside and open ended nanotubes as such on FTO glasses. The cells fabricated using 6 h anodized TiO2 nanotube photoelectrodes attained an excellent photoelectric efficiencies of 6.91 %, 6.47 % and 7.69 % for close ended-downside, close ended-upside and open ended, respectively. In addition, one dimensional rod shaped TiO2 nanostructures with high surface area were synthesized by a simple non-aqueous solvothermal process using isopropyl alcohol solvent. In this study, the process parameters such as the concentration of Titanium (IV) isopropoxide (TTIP) and acetic acid were varied. The hydrothermal process was also carried out to study the solvent effect on the morphology of TiO2. The prepared TiO2 nanocrystals were characterized by BET (Brunauer Emmett and Teller) surface area, XRD, SEM and TEM analysis. The device fabricated using a TiO2 nanorod was compared with commercial (DP-25) TiO2 nanoparticles based cell. The optimal conversion efficiency (9.21 %) of the DSSC fabricated from TiO2 nanorods was significantly higher than that of the DP-25 TiO2 nanoparticles based cell (6.43 %) due to high dye loading, good light harvesting and fast electron transport.