Grain Boundaries Passivation to Attain High Efficient Cu ₂ ZnSn(S _x , Se _1‐x ) ₄ Solar Cells

Abstract Suppressing the carrier recombination at microscopic regions such as grain boundaries (GBs) is crucial for the development of high‐performance polycrystalline inorganic thin‐film solar cells. However, the unique chemical environment of GBs makes their passivation a significant challenge. Here, a hydrothermal Li 2 S treatment strategy is proposed that utilizes the reactive GBs and controllable dopant ions to anchor GBs defects under additional pressure assistance, reducing carrier recombination loss at GBs of Cu 2 ZnSn(S,Se) 4 (CZTSSe) polycrystalline thin film. Unlike conventional Li₂S doping during film growth, this postsynthesis strategy simultaneously optimizes GB and surface electrical properties. Cathodoluminescence confirms suppressed nonradiative recombination at GBs, driven by preferential Li⁺/S 2 ⁻ migration during hydrothermal reactions. This GB‐healing strategy effectively passivates defect states and enhances band bending at GBs, facilitating carrier separation. Furthermore, the hydrothermal Li 2 S process effectively reduces the interfacial transport barrier, facilitating the carrier transport at the interface of the p‐n heterojunction. Ultimately, the CZTSSe solar cell treated with hydrothermal Li 2 S achieved an impressive efficiency of 13.2%, offering an effective strategy for the passivation of GBs in the formed polycrystalline thin‐film materials beyond the traditional doping method.