Two-Step Cooling Strategy for Synergistic Control of Cu_Zn and Sn_Zn Defects Enabling 12.87% Efficiency (Ag,Cu)₂ZnSn(S,Se)₄ Solar Cells

Abundant intrinsic defects and defect clusters in Cu2ZnSn(S,Se)(4) (CZTSSe) solar cells lead to severe nonradiative recombination and limited photoelectric performance. Therefore, developing effective method to suppress the detrimental defects is the key to achieve high-efficiency solar cell. Herein, a convenient two-step cooling strategy in selenization process is reported to suppress the Cu-Zn and Sn-Zn defects and defect clusters synergistically. The results show that rapid cooling during section from selenization temperature to turning temperature can inhibit the volatilization of Sn and restrain the corresponding Sn-related defects, while slow cooling during the subsequent temperature section can reduce the degree of Cu-Zn disorder. Benefitting from the synergistic effect of two-step cooling, a significantly lowered concentration of Sn-Zn and Cu-Zn defect and their defect clusters [2Cu(Zn)+Sn-Zn] in absorber is observed, meanwhile, a reduced band tailing effect and promoted carrier collection efficiency of the photovoltaic device is obtained. Finally, a device with improved open-circuit voltage (V-oc) of 505.5 mV and efficiency of 12.87% is achieved. This study demonstrates the impact of cooling process on defects controlling for the first time and provides a simple and effective new strategy for intrinsic defect control, which may be universal in other inorganic thin film solar cells.