Numerical Study to Improve the Back Interface Contact of CZTSSe Solar Cells Using Oxygen-Doped Mo(Se1–x, Ox)2

Kesterite-based Cu2ZnSn(S, Se)4 (CZTSSe) thin-film solar cells have attracted great interest in the field of photovoltaic technology due to their earth-abundant and environment-benign constituents. However, the efficiency of CZTSSe solar cells reaches a bottleneck stage of 12.6%, which has been improved from the aspects of the absorption layer and interface contact. Herein, the effect of oxygen-doped molybdenum selenide (Mo(Se1–x, Ox)2) on the back interface contact of the CZTSSe solar cells has been investigated using SCAPS software, revealing that oxygen doping could regulate the band gap of Mo(Se1–x, Ox)2, thus greatly optimizing the band arrangement at both the Mo/Mo(Se1–x, Ox)2 interface and the Mo(Se1–x, Ox)2/CZTSSe interface. These numerical results illustrate that the Mo(Se1–x, Ox)2 layer with a higher carrier concentration and appropriate band gap (0.9 eV) can optimize the band alignment of the back interface and effectively promote solar cell performance from the energy conversion efficiency of 9.93% with an oxygen-free MoSe2 layer up to 19.27% with oxygen-doped Mo(Se1–x, Ox)2. These encouraging results demonstrate that oxygen-doped Mo(Se1–x, Ox)2 is an essential route to improve the back interface contact of CZTSSe solar cells, which may also be generally applicable to other thin-film solar cells.