薄膜砷化鎵太陽能電池使用選擇性濾光膜之光電特性

Abstract

在2013Alta Device公司成功的利用磊晶層剝離法，有效的抑制往背面基板的自發輻射而成功的製造出了轉換效率28.8%的薄膜型太陽能電池，打破了停滯多年的效率紀錄[1]。在本研究裡主要的目標，能在薄膜砷化鎵太陽能電池正面上加入一選擇性濾光膜，抑制往正面的自發輻射復合，便能使得暗電流之產生能有效的降低，進而提升薄膜砷化鎵太陽能電池之開路電壓。在薄膜砷化鎵太陽能電池部份，由於磊晶層剝離法會使得磊晶層長時間浸泡在蝕刻液中，若沒有適當的保護，會導致太陽能電池特性變差且製程時間冗長，因此我們使用濕蝕刻的方式來完成基板的移除，並且在背面電鍍鎳作為我們磊晶層的載具，完成了良好的可撓性且四吋晶圓的1/4的薄膜砷化鎵太陽能電池的製作。在選擇性濾光膜製作的部分，我們設計並且蒸鍍二氧化鈦(TiO2)與二氧化矽(SiO2)兩種介電質材料交互堆疊12對，完成截止波長795nm、858nm以及915nm 的選擇性濾光膜。從我們的實驗結果來看，當薄膜砷化鎵太陽能電池在表面結構加上截止波長858nm~915nm的選擇性濾光膜，其與正面沒有選擇性濾光膜相比會有約0.8~3.3mV的開路電壓提升。此外我們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各種表面結構下薄膜砷化鎵太陽能電池開路電壓的增益[2]。根據我們模擬的結果，當薄膜砷化鎵太陽能電池的品質接近理想時，使用截止波長為860nm的選擇性濾光膜，開路電壓能有42.6mV最大的提升。我們認為，在如今高材料品質的太陽能電池當中，若想繼續追求更高的效率極限，使用光譜選擇性濾光膜的光調制或許是未來可以嘗試的目標。

In 2013, Alta device, Inc. has successfully demonstrated thin-film GaAs single-junction solar cells with record power conversion efficiency (PCE) of 28.8% by using an epitaxial lift-off (ELO) technique to eliminate the backward spontaneous emission into the substrate[1]. In this work, we aim to further suppressing the spontaneous emission to the front side by adding a selective filter membrane on the thin-film GaAs solar cells. It can effectively reduce the dark current, and thus enhance the open-circuit voltage (Voc) of GaAs thin film solar cells. Regarding the fabrication of thin film GaAs solar cells using the ELO method, the epitaxial layers need to be soaked in the etching solution. Without proper protection, the process likely deteriorates the solar cell performance and lengthens the process time. Therefore, we use a wet chemical etch method to remove the substrate completely, followed by electroplating of Nickle on the back as the carrier. We can successfully manufacture flexible GaAs thin-film solar cells in one quarter of a four-inch wafer. Regarding the fabrication of selective filters, we design and deposit 12 pairs of titanium dioxide (TiO2) and silicon dioxide (SiO2) dielectric stacks, targeting at cutoff wavelengths of 795nm, 858nm, and 915nm. According to the experimental results, the net Voc of the thin-film cells with the 858nm and 915nm cutoffs increased by 0.8 and 3.3mV, respectively, compared to the devices before the deposition of the selective filters. Furthermore, we have successfully developed an optical model that combines a rigorous couple wave analysis (RCWA) and the photon recycling calculation developed by NREL to quantify the Voc enhancement for thin-film GaAs solar cells with different top structures[2]. According to our simulation results, the thin-film device with nearly ideal material quality exhibits a maximal Voc enhancement 42.6mV by introducing a selective filter with the 860nm cutoff. We conclude that light management using spectrally selective optical filter may be vital to pursuit the efficiency limit of high-quality thin-film GaAs solar cells in the near future.