Sn Substitution by Ge: Strategies to Overcome the Open-Circuit Voltage Deficit of Kesterite Solar Cells

Current state-of-the-art Cu2ZnSn(S,Se)4 kesterite solar cells are limited by low open-circuit voltages (VOC). In order to evaluate to what extent the substitution of Sn by Ge is able to result in higher VOC values, this article focuses on Cu2ZnGeSe4 “CZGSe” devices. To reveal their full potential, different strategies are explored that, in particular, aim at the optimization of the CZGSe/buffer heterojunction. Here, employing hard X-ray photoelectron spectroscopy, it is evidenced that only a combination of different surface treatments is able to remove all detrimental secondary phases. Further improvements are achieved by establishing a solar cell heat treatment in air. A systematic study of the impact of different annealing temperatures and durations determines the best heat treatment parameters to be 60 min at 200 °C. Also, Zn(O,S,OH) as a more transparent alternative to the heavy-metal compound CdS buffer layer has been realized. Combining all of the strategies, solar cells with 8.5 and 7.5% total area efficiency have been prepared, which is a record for Sn-free kesterite solar cells and any kesterite solar cell with a Zn(O,S,OH) buffer, respectively. Beyond these records, this work clearly confirms the emerging trend that Ge-for-Sn substitution is a successful strategy to improve the VOC of kesterite solar cells.