Engineering of Heterogeneous Redox Environment Enables Balanced Phase Evolution and Nucleation for 14.4% Efficiency CZTSSe Solar Cell

ABSTRACT Currently, the absorber of CZTSSe solar cells face challenges such as multilayer crystal structure and high bulk defect density, mainly due to their high‐speed surface grain growth and high‐temperature decomposition. Herein, an effective strategy is proposed to simultaneous promote crystal quality and suppress defect formation by manipulating the heterogeneous redox environment in the CZTS precursor, which balances the crystallization process and CZTSSe phase evolution during absorber growth. With several Cu 2+ /Sn 2+ layers being inserted into the Cu + /Sn 4+ precursor, the reduced local surface energy of the precursor enlarges the CZTSSe critical nuclei and decreases its selenization rate, which promote the crystals with more uniform vertical morphology and enlarged lateral size. Concurrently, the extension of phase formation window increases the diffusion of selenium components to the absorber bulk, weakens the decomposition of the CZTSSe film, and thus suppresses the formation of harmful defects Cu Sn and [2Cu Zn + Sn Zn ]. Consequently, the CZTSSe absorber fabricated using this strategy exhibits a monolayer large‐grain structure, achieving an order of magnitude reduction in the defect density, a power conversion efficiency with 14.4% is attained.