Segmented Control of Selenization Environment for High‐Quality Cu ₂ ZnSn(S,Se) ₄ Films Toward Efficient Kesterite Solar Cells

Abstract High‐crystalline‐quality absorbers with fewer defects are crucial for further improvement of open‐circuit voltage ( V _OC ) and efficiency of Cu ₂ ZnSn(S,Se) ₄ (CZTSSe) solar cells. However, the preparation of high‐quality CZTSSe absorbers remains challenging due to the uncontrollability of the selenization reaction and the complexity of the required selenization environment for film growth. Herein, a novel segmented control strategy for the selenization environment, specifically targeting the evaporation area of Se, to regulate the selenization reactions and improve the absorber quality is proposed. The large evaporation area of Se in the initial stage of the selenization provides a great evaporation and diffusion flux for Se, which facilitates rapid phase transition reactions and enables the attainment of a single‐layer thin film. The reduced evaporation area of Se in the later stage creates a soft‐selenization environment for grain growth, effectively suppressing the loss of Sn and promoting element homogenization. Consequently, the mitigation of Sn‐related deep‐level defects on the surface and in the bulk induced by element imbalance is simultaneously achieved. This leads to a significant improvement in nonradiative recombination suppression and carrier collection enhancement, thereby enhancing the V _OC . As a result, the CZTSSe device delivers an impressive efficiency of 13.77% with a low V _OC deficit.