Band Edge Engineering for the Improvement of Open-Circuit Voltage: Ag-Based Selenized Cu2ZnSn(SSe)4 Surface Regulated by Lithium

Open-circuit voltage (V-oc) of kesterite Cu2ZnSn(SSe)(4) (CZTSSe) solar cells is severely stalemated by the pinning of fermi energy level due to the excessive p-type CuZn acceptor near the buffer/absorber interface. Although the formation of CuZn can be suppressed by Ag incorporation, the high formation energy of p-type AgZn 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 (Ag2ZnSnSe4) 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 LiZn, 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.