利用埋入式次波長奈米結構提升非晶矽太陽能電池光捕捉效率

Abstract

隨著矽原料的短缺及降低成本的的考量，矽薄膜太陽能電池日益受到重視，但也由於材料厚度的減少，吸收效率隨之下降，其中更以紅外波段影響甚大，因此如何有效的增加光在矽材料中的光學路徑，實為矽薄膜太陽能電池之重要課題。 傳統上的光捕捉結構主要是製做於元件的背面，只能藉由繞射或散射增加通過主動層而還沒有被完全吸收的光線之光學路徑長，但這樣的光捕捉結構往往就沒有抗反射效果。在本論文中，我們試圖利用一個結構應用於非晶矽太陽能電池之上，能有效增加非晶矽太陽能電池元件的光電流以及光電轉換效率。我們採用旋塗法搭配奈微米球微影術這種大面積、便宜又快速的方法，加上非等向性蝕刻達到控制次波長形貌的效果，在事先沈積在玻璃透明基板上的氮化矽薄膜蝕刻出蛋型次波長奈米結構，再於其上利用真空濺鍍系統、高密度電漿化學氣相沈積系統、以及電子槍金屬蒸鍍系統依序沈積透明導電層、非晶矽主動層、和金屬背反射層，而製作出位於入光面且埋入非晶矽主動層的奈米結構，來同時達到抗反射和光捕捉的效果。 我們可經由積分球吸收頻譜的量測而明確的看見結構所帶來的優異的抗反射和光捕捉效果，甚至在光線大角度入射元件時，奈米結構依然可以提供更勝於傳統多層膜抗反射層的表現。短路電流密度可有效的大幅增加，進而將元件光電轉換效率由原本的5.36%提升至8.38%。最後我們還利用三維的嚴格耦合波分析(rigorous coupled wave analysis)模擬軟體進行結構的最佳化，進一步預測在製程可行的範圍內，不同的結構寬度及高度對元件吸收及產生之光電流密度的模擬，可由此得知往後實際實驗的方向，並減少製程上不必要的測試與時間。

Light trapping in amorphous silicon thin film solar cells has been an intensive study owing to the low absorption coefficient in near-infrared. We demonstrate a frontal pre-patterned substrate (PPS) on amorphous silicon solar cells, utilizing scalable colloidal lithography, to serve both functions of anti-reflection at short wavelength and light trapping effect at long wavelength. We measured the absorption spectrum by an integrating sphere at normal incidence and the external quantum efficiency (EQE) of three type structures. The power conversion efficiency of the pre-patterned cell is measured 8.38%, which shows 56.34% and 8.83% enhancement compared to the reference cell with a flat substrate and the commercialized Asahi U-type substrate, respectively. The increased efficiency is mainly resulted from the enhanced short-circuit current density (from 12.89 mA/cm2 to 19.77 mA/cm2). Moreover, the angle-resolved absorption spectroscopy shows superior optical coupling to the absorber layer at large angles of incidence (AOIs), which guarantees sufficient light harvesting for the entire day. We also present a design optimization of frontal pre-patterned substrate with broadband antireflective subwavelength structures based on the theoretical calculation using a three-dimensional rigorous coupled wave analysis (RCWA) method. The best structure on our pre-patterned substrate is 500 nm bottom width and 400~450 nm height of SiNx nipple pattern.