製備氧化鐵半導體薄膜及應用於光電化學電池

Abstract

本研究以多重能隙光電極的目標為出發點，選擇氧化鐵(n type)為吸收低波長光的材料，從研究氧化鐵材料著手，期望製備出高光電轉換效率的薄膜。為求製程簡便，使用溶膠凝膠法製備先驅液，浸鍍、鍛燒成膜，並在先驅液中加入矽離子或鈦離子以提高光電流，從光電流量測結果可以發現摻雜鈦確實對薄膜表現有所助益，矽離子則因離子半徑過大造成氧化鐵晶格扭曲，無法順利形成固態溶液而使光電流降低。 在鍍膜過程中，發現基材前處理對覆膜完整之重要性，因基材和先驅液間親和力不足，需藉有機溶劑對基材進行表面修飾來提高與先驅液的親和力。另一方面，也觀察出不同的電解質與表面型態造成之介面能帶變化對光電流表現的影響。 除了以浸鍍法製備氧化鐵薄膜之外，也利用濺鍍裝置製備薄膜做為對照。在氧氣下濺鍍的薄膜具有p type的光電流，原因是在濺鍍過程中，除了氧化鐵沉積於基材上之外，膜中還夾雜許多的氧分子，這些氧分子抓住電子，留下電洞在價電帶，而讓半導體性質反轉為p。

Compared to silicon, iron oxide can absorb sunlight in the lower wavelength region. It is one of the candidates which can combine with silicon to form ideal multijunction electrode with higher solar-efficiency than the single band-gap silicon electrode. Therefore, the preparation of Fe2O3 thin films and the characterization of their photoelectrochemical properties were carried out in this research. The films were mainly prepared from sol-gels by dip-coating technique, and were compared with that by sputtering technique. It was found that the pretreatment of substrates would be a significant key to the success of smooth film surface (without apparent cracks) from the dip-coating. Due to poor affinity between substrate (ITO glass) and sol-gel, a process (rinsing the substrate in methanol and drying in air at 60 ℃) has been developed in this study, which is sufficient to enhance the affinity. Moreover, we also observed that different electrolytes and morphologies have significant effects on the performance of photocurrents. In order to increase the photocurrent, the effect of doping Si4+ or Ti4+ into the iron oxide film was studied. Either Si4+ or Ti4+ was added into the sol-gel solution and the resulted films were calcined at 700 ℃. It was found that owing to the larger difference between atomic sizes of Si4+ and Fe3+, the addition of Si4+ resulted in the distortion of iron oxide lattice. Therefore, the doping of Si4+ cannot improve the performance of thin films, but the doping of Ti4+ can. Similar to the iron oxide pellets or particles, the films prepared from dip-coating possessed n-type semiconductor property. However, it was found in this research that the films prepared from the oxygen sputtering process possessed p-type property. It suggests that oxygen molecules were inserted between the framework of iron oxide during sputtering. The electrons would be trapped by these O2 molecules, and leaving holes in valence band which led the formation of p-type semiconductor.