電泳沉積石墨烯導電膜作為染敏太陽電池對電極之研究

Electrophoretic Deposition of Graphene-Based Thin Films as Counter Electrodes for Dye-Sensitized Solar Cells

本研究以化學還原法合成石墨烯片（Graphene platelets），以天然石墨為原料，採用改良式哈莫法（modified Hummers method）製程，增加預氧化程序，並避免使用硝酸鈉，使得製程更為安全與環保。X-ray繞射分析證實石墨烯成品的層間距離增加；原子力顯微鏡照片推估石墨烯的平均厚度為13.782nm，約為20層天然石墨的層間距離。拉曼分析指出聯胺還原使石墨烯材料中的缺陷與錯位明顯減少，然而無論還原與否，結構的拉曼2D峰值均遠低於天然石墨，顯示成品的層間堆疊不夠規律整齊。合成的石墨烯具有良好的導電性，但可見光透過率較差。於透明導電玻璃基板，以直流電泳在石墨烯懸浮水溶液中沉積石墨烯薄膜。探討不同電泳參數對於薄膜電性的影響，結果指出較高操作電壓與較長沉積時間，可以得到較佳導電性的薄膜。當操作電壓10V，沉積時間60秒時，石墨烯薄膜導電性最好，其片電阻為280.1±4.5Ω/，具有載子濃度為4.34´1017cm-3；遷移率為569.5cm2V-1s-1。電子能譜分析顯示，直流電泳沉積於鍍膜的同時，可將原石墨烯表面的含氧官能基還原成碳環結構。取導電性最佳的鍍膜產品做為染敏太陽電池的對電極，以此所組裝的元件，進行光伏測試，並與傳統的白金對電極比較，結果顯示其DSSC元件的光電轉換效率僅為白金電極所製作元件的20%，如果於石墨烯表面濺鍍一薄層白金，可以大幅提升對電極的活性，此一現象與電化學阻抗分析結果相符。 In the present study, a modified Hummer’s method was employed to oxidize natural graphite to graphite oxide. After ultrasonic exfoliation and post-treatment through hydrazine reduction, the as-derived reduced graphene oxide possessed several desirable properties, including an ultra-large surface area, electrical conductivity, optical transparency, and substantial electro-catalytic activity. Although sodium nitrate is an effective oxidant in the traditional Hummer’s method, it was obsoleted in the present study due to its ecofriendly synthesis route. The findings of the X-ray diffraction analysis of the as-synthesized samples revealed that the distance between graphene layers was enlarged due to intercalation. On the basis of atomic force microscopy results, the thickness of the as-prepared multilayered graphene was estimated to be 20 layers of the graphitic plane with a sheet resistance of 280.1 ± 4.5 Ω/, a carrier concentration of 4.34 ´ 1017 , and an electric mobility of 9.5 cm2V -1 s -1 . The as-synthesized graphene was then dispersed in acidic water and coated on an ITO substrate through electrophoretic deposition(EPD) , serving as a counter electrode for dye-sensitized solar cells. The photovoltaic characterization indicated that under AM 1.5 G illumination, the cell prepared using the EPD-graphene counter electrode showed insufficient regeneration activity, leading to a low energy conversion efficiency of 0.380%, which was only 20% of the conversion efficiency of a standard platinum film. When a thin layer of sputtered platinum was applied, the performance of the Pt-decorated EPD-graphene counter electrode was substantially enhanced.