太陽能電池轉換效率改善之研究

Abstract

在本篇論文中，我們針對提升太陽能電池轉換效率做一系列之研究與探討，主要內容包含了使用四氟化碳電漿處理結合硝酸氧化方式鈍化p型矽基板表面上的缺陷並使得開路電壓有著顯著的提升。從研究中顯示開路電壓VOC對擴散飽和電流J01的些微變動相當敏感。因此，表面載子復合的問題使用我們所提出的兩階段鈍化方式能有效的抑止。在太陽能電池中只要控制好少數載子的生命週期自然開路電壓、短路電流與電池的最終轉換效率皆會有效的增進。而且四氟化碳電漿處理與堆疊氮化矽抗反射層都只用電漿輔助器相沉積系統，因此從工業界的觀點來看，我們所提出的鈍化技術也可以融入目前的傳統製程中且使的製程能更便宜電池效率也更加提升。接下來為了改善晶片的品質，我們成功的使用高溫磷擴散方式吸附金屬雜質純化p型矽基板內部的金屬點缺陷並使得開路電壓、短路電流與有效轉換效率有著顯著的提升。從研究中顯示開路電壓VOC對晶片的少數載子生命週期呈現正相關的關係。因此，金屬雜質所造成的載子復合的問題使用我們所提出的高溫磷擴散吸附金屬雜質方式能有效的抑止。在太陽能電池中只要控制好少數載子的生命週期自然開路電壓、短路電流與電池的最終轉換效率皆會有效的增進。而且晶片雜質吸附純化所使用的爐管也是標準設備之一，從工業界的觀點來看，我們所提出的晶片雜質吸附純化技術也可以融入目前的傳統製程中且能實現更緊密且高平均效率的太陽能電池能源分佈，對矽晶圓的利用率必須也跟著有效率的提高。 改善了表面與晶片的品質後，針對表面射極層的鈍化效果，我們提出以氧化後蝕刻的方式完成選擇性射極的結構。並且在二次質譜儀的分析下挑出照光區射極層與金屬下接觸區濃度的最佳阻抗匹配。使用選擇性射極的結構，不論是在照光區射極層擴散飽和電流的抑止、短波長響應的吸收、開路電壓的增加、光電轉換效率的突破連在金屬下的光電流的收集都有著大幅度的成效。這一切都再次說明當進入高效率太陽能電池的階段時，每一個光電轉換的損失都代表著重要的地位。因此，我們提出以氧化後蝕刻的方式完成此結構不但能符合目前工業界同軸製程，並且比其他還需要精準對位系統的方式來的簡易、有效率且低成本。最後完成了晶片的表面鈍化、本體雜質純化與選擇性射極層結構，我們在P型矽基板使用傳統網印製程搭配選擇性射極與背面氧化鋁/氮化矽堆疊結構在開路電壓與短路電流展現優異的電性結果。片電阻值為100/sq的表面射極層結構在載子復合的貢獻上展現好的抑制效果。20/90奈米的氧化鋁/氮化矽堆疊層在元件背面除了以負的固定電荷方式提供了一個高密度的電場，對晶圓表面的缺陷密度有著良好的鈍化能力外並且對長波段的光提供了內部反射的路徑，因此不論在開路電壓或是短路電流都有顯著的幫助。這個製程對現在的傳統製程來說便宜、快速且能符合產線的同軸製程。

In this dissertation, surface passivation of solar cells has been investigated using CF4 plasma treatment on low temperature oxides to enhance the open-circuit voltage of the solar cells. Low temperature oxides grown by nitric acid solution are treated with CF4 plasma. Solar cells undergoing this scheme show an improved performance, including low saturation current density and good quantum efficiency at short wavelengths. Experimental results demonstrate that CF4 plasma pretreatment on low temperature oxides can significantly improve the open-circuit voltage, short-circuit current, and fill factor for silicon wafer based solar cells. This technique is very promising for in-line solar cell manufacturing. In order to improve the quality of wafer, the efficacy of higher-temperature gettering processes in reducing impurity concentrations is assessed by QSSPC measurement. Compared to a baseline phosphorus diffusion, 850℃、900℃ and 950℃ diffusions result in a larger reduction in impurity precipitation. By different gettering process, the minority carrier lifetimes show that the greater reduction in precipitated metals is associated with a strong increase. So we and makes the open-circuit voltage, short circuit current and the effective conversion efficiency has significantly improved. Therefore, in the case of electrode screen-printing, a selective-emitter cell who has a low contact resistance due to heavy doping underneath the metal grid, improved front-surface passivation of the lightly doped region between the grid, and reduced recombination under the metal contact have been fabricated by oxidation etchback process successfully. According to SIMS data, we choice the optimize concentration in lightly doped region between the grid and heavy doping underneath the metal grid. The selective-emitter structure is not only increased short-wavelength response absorption, but also suppressed the diffusion current in emitter region. The minority carrier lifetime、open-circuit voltage and conversion efficiency are increasing. Finally, the open-circuit voltage (VOC) of an industrial screen-printed p-Si solar cell was successfully enhanced via improved rear surface passivation using Al2O3/SiNx stack layers. The carrier recombination at the surface can be effectively suppressed with a light-doped emitter of 100 /sq and Al2O3/SiNx stack layers. The 20/90-nm Al2O3/SiNx stack layers, passivated on the rear side, not only improved the minority carrier lifetime but also enhanced rear internal reflection. Both JSC and VOC were increased for silicon wafer–based solar cells. This process may be cheaper and easier to implement from an industrial standpoint.