選擇性濾光膜應用於單接面砷化鎵電池之特性研究

Abstract

根據理論效率極限的計算模型，單接面砷化鎵太陽能電池最高的效率極限在AM1.5G太陽光頻譜照射下可達33.5%，然而，即使在電性上已能製程趨近於完美，其效率水平仍就遠低於理論極限，也因此，可以輕易的推斷這段效率上的差距其實主要是來自於光性所造成的。近年來，Alta Devic這家公司藉由消除往背面基板的自發輻射而成功的製造出了在轉換效率28.8%的薄膜型太陽能電池，而這項因素正是關係到了太陽能電池電性的表現，特別是其開路電壓的提升，更是被大家公認為通往超高效率的道路上最後要走的一段。 在我們的實驗中，我們嘗試利用嚴格偶合波分析法( RCWA )及美國在生能源局( NREL )的光子回收模型來量化藉由降低自發輻射暗電流的生成所達成的開路電壓提升。我們所設計的表面結構為藉由二氧化鈦與二氧化矽兩種不同折射率材料交互堆疊所形成的選擇性濾光薄膜，其特色是可以依照我們的設計有不同的截止波長。根據我們的模擬結果，當設計的濾光膜截止波長在840nm時除了會有最大的開路電壓，相較於不做任何處理的原始電池( Bare cell )有著36.4mV的電壓增益外，還有優越的抗反射效果來維持高短路電流密度。另外在我們的實驗結果上也顯示用這樣的方法來降低自發輻射暗電流的生成，不僅能維持高短路電流密度亦同時能提升開路電壓。

In detailed balance model, the efficiency of single-junction solar cells can be potentially as high as 33.5% under AM 1.5G illumination. However the best state-of-the-art devices are still far lower than those figures, even the electronic quality is nearly perfect. Therefore the efficiency gap should stem from the light management inside solar cells. Recently, Alta device, Inc. has successfully fabricated a thin-film GaAs single junction solar cell with conversion efficiency of 28.8%, under 1 sun illumination, which aggregates the loss of backward emission into substrate[1]. This factor can be highly relevant to the cell’s performance, especially open-circuit voltage (Voc), and maximizing Voc is generally considered as the last mile to approach ultra-high efficiency limit. In this work, we try to quantify the Voc enhancement in GaAs solar cells by reducing emission loss. The simulation tools are RCWA simulation and photon recycling model NREL developed recently. The top structures we simulate here are different cutoff wavelength thin film selective filter of alternate TiO2 and SiO2. After our calculation, the cutoff wavelength of 840 nm can make the biggest Voc enhancement 36.4meV compared with bare one, and the structure also has excellent anti-reflection ability for maintaining high Jsc. Our results also show that using this way to enhance Voc is especially suitable for cells with ordinary material quality. Therefore, the requests of ideal top structures for solar cells’ use are not only near-perfect anti-reflection, but the ability to minimize the emission loss.