Photovoltaic Device with over 5% Efficiency Based on an n-Type Ag2ZnSnSe4 Absorber
The kesterite material Cu2ZnSn(S,Se)4 (CZTSSe) is an attractive earth-abundant semiconductor for photovoltaics. However, the power conversion efficiency is limited by a large density of I–II antisite defects, which cause severe band tailing and open-circuit voltage loss. Ag2ZnSnSe4 (AZTSe) is a promising alternative to CZTSSe with a substantially lower I–II antisite defect density and smaller band tailing. AZTSe is weakly n-type, and this study reports for the first time on how the carrier density is impacted by stoichiometry. This study presents the first-ever photovoltaic device based on AZTSe, which exhibits an efficiency of 5.2%, which is the highest value reported for an n-type thin-film absorber. Due to the weakly n-type nature of the absorber, a new architecture is employed (SnO:F/AZTSe/MoO3/ITO) to replace conventional contacts and buffer materials. Using this platform, it is shown that the band tailing parameter in AZTSe more closely resembles that of CIGSe than CZTSSe, underscoring the strong promise of this absorber. In demonstrating the ability to collect photogenerated carriers from AZTSe, this study paves the way for novel thin-film heterojunction architectures where light absorption in the n-type device layer can supplement absorption in the p-type layer as opposed to producing a net optical loss.