Device Postannealing Enabling over 12% Efficient Solution‐Processed Cu2ZnSnS4 Solar Cells with Cd2+ Substitution

Abstract Kesterite Cu 2 ZnSnS 4 is a promising photovoltaic material containing low‐cost, earth‐abundant, and stable semiconductor elements. However, the highest power conversion efficiency of thin‐film solar cells based on Cu 2 ZnSnS 4 is only about 11% due to low open‐circuit voltage and fill factor mainly caused by antisite defects and unfavorable heterojunction interface. In this work, a postannealing procedure is proposed to complete a Cd‐alloyed Cu 2 ZnSnS 4 device. The postannealing to complete the device significantly enhances the performance of the indium tin oxide and promotes the moderate interdiffusion of elements between the layers in the device. As a result of the diffusion of Cu, Zn, In, and Sn, the interfacial electron and hole densities are improved, leading to the achievement of a suitable band alignment for carrier transport. The postannealing also reduces the interface traps and deep‐level defects, contributing to decreased nonradiative recombination. Therefore, the open‐circuit voltage and fill factor are both improved, and an efficiency over 12% for pure sulfide‐based kesterite thin‐film solar cells is obtained.