Passivation of Deep‐Level Defects through Chemical Environment Regulation for Efficient CZTSSe Solar Cells Based on Active Selenium Adsorption–Desorption Process

Abstract Insufficient selenization and uneven distribution of elements caused by the poor diffusion and reaction activity of selenium clusters is one of the main issues limiting the efficiency of Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells. Here, this work designs a simple and feasible strategy to improve the activity of selenium (Se) by implementing high‐temperature treatment on graphite boxes loaded with Se pellets. The rapid adsorption/desorption characteristics of graphite on active gaseous small‐molecule selenium have successfully introduced hyperactive Se₄, Se₃, and Se₂ into the selenization process. The results indicate that the adsorbed non‐toxic gaseous active Se₃ and Se₄ can quickly and uniformly diffuse into the precursor film at low temperatures, thereby inducing nucleation and grain growth at both surface and back interface simultaneously, which inhibits the upward migration and aggregation of cations, especially Cu, and promotes the homogenization of elements. The overall relatively Cu‐poor chemical environment suppresses the formation of Cu_Zn defects and [2Cu_Zn+Sn_Zn] defect clusters, and also promotes the generation of favorable V_Cu. The band tail states and non‐radiative recombination are then optimized. Finally, the CZTSSe solar cells achieve a power conversion efficiency (PCE) of 14.5%, with V_OC/V_OC^SQ of 67% being one of the highest in the literature.