優化背電極薄膜砷化鎵太陽能電池之設計

Abstract

近年來單晶矽太陽能電池透過使用全背電極的概念而在效率上有重大的突破，如此的設計不僅能去除表面電極遮光的因素影響，也能避免過細的電極造成的較大的串聯阻抗損耗，若是能將背電極設計與具有最高單接面轉換效率的砷化鎵材料[1, 2]做結合，有望將效率再進一步提升，然而，由於所需製程十分具有挑戰性，導致目前僅有屈指可數的背電極薄膜砷化鎵太陽能電池實驗結果。因此我們的研究希望使用校正過的Sentaurus TCAD模型優化製程參數，包括厚度、摻雜濃度、週期以及陽極的大小，透過進行電流電壓特性的模擬，找到不同材料品質下趨勢變化的原因與最佳的設計參數。 在我們的研究中顯示背電極薄膜砷化鎵太陽能電池的厚度與摻雜濃度應該設計為1.4 ~1.7微米及5×〖10〗^16 cm^(-3)有較好的表現，除此之外，透過與傳統薄膜砷化鎵太陽能電池的比較，可以發現背電極結構儘管有較短的擴散長度以及較大的暗電流，仍舊由於去除表面電極的短路電流增益而在效率上取得1.5%的領先。另外，在沒有表面電極產生optical shading的背電極太陽能電池中，存在與電極設計相關的electrical shading現象劣化電池效率，透過優化背電極設計的模擬，顯示應將整體週期與陽極大小縮小以減緩陽極上方的複合情形提升效率。

In recent years, breakthroughs of silicon-based solar cells adopt the concepts of back contacts and back junction. The design not only eliminates the shadowing caused by the front contact but also decreases the series resistance. If such a design is applied to GaAs, the material with the highest single-junction power conversion efficiency (PCE)[1, 2], it may be possible to enhance the power conversion efficiency further. However, only few experiments on back-contact GaAs solar cells have been conducted due to fabrication challenges. In this work, we aim to optimize the design parameters by employing a validated Sentaurus TCAD model, including the doping concentration, base thickness, length and pitch of back electrodes. Through current-voltage characteristics, we could determine the best cell performance with different material qualities and their correlation to design parameters. Our study shows that the optimized thickness and doping concentration of back-contact solar cells correspond to 1.4 ~1.7 um and 5×〖10〗^16 cm^(-3) for all material qualities. Compared to conventional GaAs solar cells, the back-contact device have a relatively short diffusion length and large dark current, but PCE is 1~2% higher due to the increment of short-circuit current. Moreover, we show that while the optical shadowing is eliminated in the back-contact design, the electrical shading still affects the cell performance and is sensitive to the electrode length and pitch. Consequently, our simulation shows that both the anode and pitch should be relatively narrow in order to mitigate the recombination loss above the anode region.