Study of n-Type Fe2O3 and p-Type CuO Thin Films for Two Energy Gap Tandem Solar Cell Application
|關鍵字:||多重能隙;氧化鐵;氧化銅;光/暗電導率;太陽能電池;multi-bandgap;Fe2O3;CuO;photo/dark conductivity ratio;solar cell||公開日期:||2005||摘要:||本研究為符合能源需求與經濟效益的觀點，設計出以矽為基底的多重能隙太陽能電池。由多重能隙、材料價格與穩定性等考量，n型氧化鐵(α-Fe2O3)與p型氧化銅(CuO)為特性符合的高能隙材料，因此，以製備出n-Fe2O3/p-CuO|n-Si/p-Si元件為目標，來增進光電轉換效率；且藉由製備氧化鐵與氧化銅薄膜，分析應用於太陽能電池材料的可行性。
以磁控射頻濺鍍法製備氧化鐵薄膜的實驗中，發現使用含氧氣的電漿氣體，造成薄膜內氧氣吸附，氧化鐵薄膜為p型，在經由高溫後處理後，回復為n型半導體。氧化鐵薄膜中摻雜鈦原子，高溫處理後，暗電導率以在氧氣電將氣體下製備0.2at%Ti-Fe2O3的6.38×10-7(ohm-1．cm-1)最高，且光/暗電導率比值達3.21。變換電漿氣體組成，經後處理之1at％Ti- Fe2O3 ，以Ar/O2(90/10)電漿氣體製備，有最高的光/暗電導率比。
在溶膠凝膠法旋轉塗佈製備氧化銅薄膜實驗裡，證實旋塗多層氧化銅薄膜，為可靠的厚膜製備程序。氧化銅薄膜在受光後，易將光能轉換成熱能，造成光/暗電導率比值僅1.1。變換前趨液溶劑組成，造成晶粒大小與排列緻密度變化，使用小分子的MEA較大分子DEA晶粒緻密，溶液裡添加水有幫助結成較大晶粒。摻雜鋰原子，晶粒隨摻雜濃度上升而下降，暗電導率由純氧化銅5.04×10-4上升2.4倍至2.05 at% Li-CuO 1.33×10-3(ohm-1．cm-1)。各種製程的氧化銅薄膜，光/暗電導率比皆僅約1.1。
In this study, we have designed a silicon based multi-junction solar cell to answer to energy requirements and economical benefits. To fit in the design parameters (multi-band gap、cost、stability), n type α-Fe2O3 and p type CuO were chosen as the suitable high energy gap materials. Therefore, the preparation of n-Fe2O3/p-CuO|n-Si/p-Si device was the expected ultimate goal; and the preparation of Fe2O3 and CuO thin films with high photoelectrical efficiency was the initial major target of this research. From the preparation of Fe2O3 thin films by applying the method of RF magnetron sputter-deposition, it was found that p type samples were made in oxygen plasma gas. After post annealing in 600℃, the samples became n type. When these n-type Fe2O3 films were doped with different amounts of Ti, the one with 0.2at％ Ti had the highest photo/dark conductivity ratio (i.e., 3.21) and the highest dark conductivity (i.e., 6.38×10-7 ohm-1．cm-1). Moreover, when the films were doped with 1at% Ti and prepared in different plasma gases (Ar, O2, or mixture of Ar and O2), the one from the plasma gas composition Ar/O2 (90/10) had the highest photo/dark conductivity ratio (i.e., 3.12). CuO thin films were spin-coated on the substrates from the sol-gel prepared in this research. The thicker films can be from the multi-layers coating. However, it was found that CuO films were easier to convert photon energy to heat after illumination, which led low photo/dark ratio (i.e., about 1.1). Changing the composition of sol-gel solution would result in different grain sizes and crystal densities. When CuO films doped with Li, the grain size went down with increasing Li doping concentration. The dark conductivity was raised from 5.04×10-4 (of pure CuO) to 1.33×10-3 ohm-1cm-1 (of CuO doped with 2.05 at%Li). Nevertheless, the photo/dark ratios of all the CuO films prepared from this study were just around 1.1. The energy gaps of α-Fe2O3 and CuO films prepared in this research were around 1.85eV and 1.68eV respectively, which met the high energy gap requirement expected from the design of silicon based multi-junction solar cell. However, all the photo/dark ratios were too low, which were not sufficient in application. Therefore, more efforts need to be put in the future, in order to achieve the ultimate goal of n-Fe2O3/p-CuO|n-Si/p-Si device.
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