利用金屬輔助蝕刻法製造單晶矽薄膜於太陽能電池之應用

Abstract

隨著現今石化能源面臨短缺的危機與全球暖化問題，綠色再生能源受到很高的關注，在可再生能源中，因為太陽光相對穏定，因此太陽能電池也變得越來重要。在現今的太陽能產業中以矽太陽能電池為主，然而，矽太陽能電池的成本與石油發電比較下仍然很貴，在製作矽太陽能模組的成本裏，矽基板的成本佔矽太陽能電池模組成本的一半，導致矽太陽能電池的成本提高，減少矽材料的使用能有效降低矽太陽電池的成本。在本篇論文中利用低成本的金屬輔助蝕刻法(metal assisted etching)來製造單晶矽薄膜，利用微影技術(lithography)於單晶矽片的表面形成微米孔洞的光阻圖形，接著透過金屬輔助蝕刻法進行縱向蝕刻以及橫向蝕刻，在縱向蝕刻中，銀粒子會通過微米洞向下蝕刻至特定的深度，而橫向蝕刻可以把把微米洞結構底部往水平方向蝕刻，直至微米洞結構與矽基板分開，經過縱向以及橫向蝕刻過程後，可以利用把薄膜轉移到玻璃基板上，剩餘的矽基板可以回收。 在波長400nm到900nm之下，單晶矽薄膜的光吸收率大於89%，我們利用XRD檢測證明單晶矽薄膜與矽晶片有相同晶格特性，在布拉格角度(Bragg Angle) 69o之下有峰值，代表晶格方向為(100)。而我們利用微波電導衰退(Microwave photoconductive decay)及霍爾效應(hall effect)來量測單單晶矽薄膜的載子生命週期(carrier lifetime)及載子遷移率(carrier mobility)，單晶矽薄膜的載子生命週期為25μs，載子遷移率為240cm2/Vs，因此載子傳輸距離(carrier diffusion length)為125μm，大於單晶矽薄膜之厚度，因此適合於製作太陽能電池。最後，我們利用此單晶矽薄膜與有機材料PEDOT:PSS製成有機無機太陽能電池，不需要利用高温退火等高温製程，達到低成本，低温的製程的需求，元件之能量轉換效率(power conversion efficiency)為1.87%，短路電流12.76mA，開路電壓0.37V。

Global warming and fossil fuels crisis are two important issues in the world, so alternative renewable energy attracts much attention. Among them, the renewable solar energy is much potential because sun light is relatively stable and clean. Currently, crystalline Si solar cells dominate the photovoltaic market, but the cost of Si solar cells is still high compared to fossil fuels. Because half of the production cost of Si solar cell modules comes from Si wafer, reducing the Si thickness constitutes a strong leverage for cost reduction. In this work, we introduce a low cost method to fabricate the crystalline Si thin foils by vertical and horizontal muti-step metal-assisted etching. Including vertical etching forms deep trenches where the depth can be controlled by the etching time, and horizontal etching removes the Si materials from the Si substrate. The Si thin foil can be transferred to glass substrate, the remaining Si substrate can be recycled. The optical absorption of Si thin foil was over 89% at wavelength between 400 and 900nm. From XRD spectrum, the Si thin foils have a peak near 69o, which corresponding to Si (100) plane. In addition, the carrier lifetime and carrier mobility of crystalline Si thin foils are 25 μs, and 240 cm2/Vs, resulting in a carrier diffusion length of 125 μm. The carrier diffusion length is much longer than the thickness of the thin foils, so the Si thin foils are suitable to fabricating photovoltaic devices. Finally, we use the conducting polymer, poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), to fabricate hybrid solar cells on the Si thin foils. The fabrication process does not require high temperature annealing. The power conversion efficiency reached 1.87%, where the short circuit current density was 12.76 mA/cm2, and the open circuit voltage was 0.37 V.