Regulating grain growth via Li2SnS3 interphase in kesterite solar cells with certified efficiencies exceeding 15%

The selenization reaction process is the key step in determining the quality of Cu 2 ZnSn(S,Se) 4 thin films. Imbalanced migration kinetics of metal ions during selenization led to high concentrations of deep-level defects, resulting in dramatic open-circuit voltage loss. In this work, we reported a Li 2 SnS 3 interphase strategy to modify cation migration paths and balance Zn 2+ /Sn 4+ migration differences. The Li 2 SnS 3 interphase selectively encapsulates the Cu 2 Sn(S,Se) 3 intermediate grains, serving as the rate-determining layer for ion migration. The Zn 2+ /Sn 4+ migration barrier difference in the interphase decreases from 0.41 eV in Cu 2 Sn(S,Se) 3 to 0.21 eV in Li 2 SnS 3 , which promotes the formation of larger, uniform, high-crystallinity grains. As a result, device efficiency improves from 13.86% to 15.45% (certified at 15.04%), and open-circuit voltage reaches 602 mV at a bandgap of 1.10 eV.