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

Kesterite Cu₂ ZnSnS₄ 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 Cu₂ ZnSnS₄ 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 Cu₂ ZnSnS₄ 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.