應用於染料敏化太陽能電池之鈮摻雜二氧化鈦奈米粒的製備及其光電特性研究

Abstract

本篇論文專注在二氧化鈦的光電特性上，利用過渡金屬來摻雜二氧化鈦，使其擁有不同的光電性質。我們利用溶膠-凝膠法及水熱法合成出具有鈮摻雜之二氧化鈦奈米粒，從掃描式電子顯微鏡下可以發現，經水熱處理後的二氧化鈦奈米粒大約為20至30奈米，且鈮元素為均勻分佈。而鈮的含量變化從0.5 mol %至3 mol %，可以由X光光電子能譜儀來證實鈮已摻雜進入二氧化鈦晶格之中，另外晶相上只有些微受到影響，主要仍為銳鈦礦相。我們將其應用於染料敏化太陽能電池之光陽極上，鈮摻雜之二氧化鈦奈米粒具有向下位移的導帶能階，使開路電壓下降但短路電流獲得改善，同時也將同族的鉭元素以同樣實驗手法摻雜，發現在導帶能階上的改變趨勢和鈮相同，皆為隨著濃度提昇而向下位移。為了證實摻雜型二氧化鈦奈米粒具有修飾導帶能階的作用，我們也嘗試了使用不同的鈮之前軀物及水熱方式來製備鈮摻雜二氧化鈦奈米粒，其結果幾乎和酸性水熱環境下相符的。材料電性分析上，我們搭配Mott-Schottky先針對單純材料進行電性的分析，其材料本身導帶能階已有修飾的現象產生，而完整元件狀態下進行交流阻抗分析，可以證實鈮的摻雜確實使導帶能階位移；另外我們也針對使用鈮摻雜二氧化鈦奈米粒進行IMPS及IMVS分析，和交流阻抗分析所獲得的結果一致，並顯示鈮摻雜之後較不易和電解液產生再結合反應，擁有較長的電子生命期。

In this thesis, we studied the fabrication and characterization of new TiO2 photoanode materials with controllable conduction band shift by doping transition metals into TiO2 structures. We prepared the Nb-doped TiO2 nanoparticles by a sol-gel method followed by a hydrothermal treatment. The homogenous distribution of Nb in the TiO2 nanoparticles with size of ~20-30 nm is confirmed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results of XPS show that Nb has successfully doped into the TiO2 lattice. However, the crystallinity of TiO2 was affected by Nb-doping slightly and but shows the anatase phase. The Nb-doped TiO2 nanoparticles were applied into the DSSC device as photoanode materials. After doping, the conduction band shifted downward causing a lower Voc, higher Jsc and higher efficiency compare to the un-doped TiO2. Moreover, Ta-doped TiO2 nanoparticles have also been prepared under the same condition. The shift of the conduction band shows the same behavior as the Nb-doped TiO2. In order to confirm the behavior that the metallic-doping plays a functional role on DSSC by tailoring the electronic level, we uesd different Nb precursors to prepare the Nb-doped nanoparticles, the results are almost the same as that of the Nb-TiO2 prepared by acidity hydrothermal method. According to the Mott-Schottky plots, the flat-band potentials of the materials were modified by the metallic-doping. Electrochemical impedance spectroscopy (EIS) has also been applied to understand the charge transport kinetics with the Nb-doping. The conduction band shift downward with the increasing amounts of Nb-doping in TiO2. Furthermore, the intensity-modulated photoelectric (IMPS/IMVS) analysis show the similar results to those of EIS with longer electron lifetimes than those of un-doped TiO2.