Defect Control for 12.5% Efficiency Cu <sub>2</sub>ZnSnSe <sub>4</sub> Kesterite Thin-Film Solar Cells by Engineering of Local Chemical Environment

Kesterite-based Cu2ZnSn(S,Se)4 quaternary semiconductor solid solutions are emerging as promising materials for low-cost, relatively benign, high-efficiency thin-film photovoltaics. However, the current state-of-the-art Cu2ZnSn(S,Se)4 devices suffer from cation-disordering defects and defect clusters, which generally result in severe band-tailing, considerable interface and bulk recombination, and ultimately unsatisfactory performance. Herein, we report critical growth conditions for obtaining high-quality Cu2ZnSnSe4 absorber layers with the formation of detrimental intrinsic defects largely suppressed. By controlling the oxidation degree of metal elements and modifying the local chemical composition, we essentially modify the local chemical environment during kesterite thin-film growth, thereby effectively suppressing detrimental intrinsic defects. This modification also activates desirable shallow acceptor Cu vacancies, resulting in increased hole mobility and thereby bulk conductivity of Cu2ZnSnSe4 by two orders of magnitude. As a result, we demonstrate 12.5% efficiency devices with greatly reduced band-tailing and enhanced carrier lifetime. These encouraging results demonstrate not only an essential route to overcome the long-standing challenge of defect control in kesterite semiconductors, but also more generally provide an exemplary strategy to improve the electronic quality for a wider range of multinary compound semiconductors.