Gradient conduction band energy engineering driven high-efficiency solution-processed Cu${}_2$ZnSn(S,Se)${}_4$/Zn${}_x$Cd${}_{1-x}$S solar cells

The photovoltaic performance of the environmentally friendly Cu${}_2$ZnSn(S,Se)${}_4$ (CZTSSe) solar cells is lower than its predecessor Cu(In,Ga)Se${}_2$ solar cells. Severe carrier recombination at the CZTSSe/CdS interface is one major reason that results in a large open-circuit voltage loss. Doping zinc into CdS is a feasible strategy to modifying the CdS buffer layer film, but the present methods are not satisfactory. In this study, novel zinc incorporation strategy is developed to deposit a gradient composition ternary Zn${}_x$Cd${}_{1-x}$S buffer layer for optimizing the heterojunction interface. The application of gradient composition Zn${}_x$Cd${}_{1-x}$S buffer layer constructs a gradient conduction band energy configuration in the CZTSSe/buffer layer interface, which highly reduces the interface recombination. The suppressed interface recombination contributes to the enhanced open circuit voltage and device performance. Consequently, the CZTSSe solar cell based on gradient composition Zn${}_x$Cd${}_{1-x}$S buffer layers achieves champion efficiency of 12.35% with V${}_{OC}$ of 504.81 mV, J${}_{SC}$ of 36.90 mA cm${}^{-2}$, and FF of 66.28%. It is worth noting that flammable and the toxic hydrazine solvent are replaced by the safe and low-toxic 2-methoxyethanol, making it more promising for the future commercialization of CZTSSe solar cells. (© 2022 Wiley‐VCH GmbH)