Enhanced Se‐incorporation and Dynamic Vertical Cation Distribution Enables Stage‐specific Crystallization Optimization for High Efficiency Cu ₂ ZnSn(S,Se) ₄ Solar Cells

ABSTRACT Insufficient Se‐incorporation and undesirable cation‐redistribution limit the crystallization quality of Cu 2 ZnSn(S,Se) 4 (CZTSSe) films in selenization. By introducing cracks in precursor film, which provides pathway for mass transport, enhanced Se‐incorporation and favorable cation‐redistribution are simultaneously achieved for the crystallization of CZTSSe film. The dynamic vertical cation distribution causes different compositions of the surface layer in different crystallization stages, leading to a stage‐specific crystallization optimization of the layer. While the Cu‐poor and Zn‐rich composition in the initial crystallization stage suppresses the formation of Sn Zn deep defects, the elevated Cu composition appears in later crystallization stage facilitates rapid growth of CZTSSe grains and suppression of insufficient‐diffusion caused defects. Furthermore, the enhanced Se‐incorporation in later stage of crystallization promotes the formation of high valence Cu 2+ and Sn 4+ in bulk of the film. Such process enhances bulk crystallization by forming liquid CuSe and suppresses formation of Sn Zn deep defects by reducing low valence Sn 2+ . The morphology modified crystallization of CZTSSe film yields solar cell with efficiency of 13.1% (without alloying in absorber). This work elucidates the intricate interplay among precursor morphology, Se‐incorporation, cation‐redistribution, and crystallization of CZTSSe films, providing new insight for the crystallization optimization of CZTSSe films.