Band Edge Engineering for the Improvement of Open‐Circuit Voltage: Ag‐Based Selenized Cu 2 ZnSn(SSe) 4 Surface Regulated by Lithium

Open‐circuit voltage ( V oc ) of kesterite Cu 2 ZnSn(SSe) 4 (CZTSSe) solar cells is severely stalemated by the pinning of fermi energy level due to the excessive p‐type Cu Zn acceptor near the buffer/absorber interface. Although the formation of Cu Zn can be suppressed by Ag incorporation, the high formation energy of p‐type Ag Zn defects results in the expected weak n‐type surface difficult to be maintained. Based on the doping limit rule, it is found that Ag‐based selenized kesterite (Ag 2 ZnSnSe 4 ) facilitating the formation of n‐type defects by lowering the conduction band is conducive to the stable weak n‐type surface rather than suppressing the formation of p‐type defects by lowering the valence band. Furthermore, Li post‐treatment makes part of strong n‐type region into the expected weak n‐type due to the low formation energy of p‐type Li Zn , which is greatly convenient for experimental implementation. This study presents that Ag‐based selenized CZTSSe surface combined with Li post‐treatment is a feasible way to overcome V oc ‐ deficit of kesterite solar cells and highlights that band edge engineering is a promising way for designing an expected n‐ or p‐type characteristic of chalcogenide semiconductors by extrinsic doping.