Buried CdSe Sacrificial Layer Driving Phase Evolution and Crystallization Kinetics Enables 14.78%-Efficient DMF-Based Kesterite Solar Cells

During selenization, the inherent transport lag of Se from the film surface to the buried Mo/precursor interface causes early-stage Se deficiency in the bottom region, creating a depth-dependent crystallization lag that limits the homogeneity of Cu₂ZnSn(S,Se)₄ (CZTSSe) absorbers. Here, we evaporate an ultrathin CdSe sacrificial layer at the Mo/precursor interface of N,N-dimethylformamide (DMF)-derived CZTSSe absorbers. This layer is rapidly consumed at the onset of selenization, providing an in-situ Se supply at the buried interface and effectively bypassing the intrinsic transport delay from the surface. The locally supplied Se initiates early-stage bottom selenization and promotes the conversion of Cu₂SnSe₃-related intermediates toward CZTSSe while suppressing their persistent accumulation, enabling rear-side nucleation and grain growth to commence substantially earlier than in the conventional surface-dominated process. Meanwhile, the redistributed Cd²⁺ partially substitutes Zn²⁺ sites, reducing defect-assisted recombination and enhancing carrier transport and extraction. The optimized device achieves a champion efficiency of 14.78% with a certified efficiency of 14.30%, establishing a new benchmark for DMF-derived kesterite solar cells. This work demonstrates that prepositioning a sacrificial Se-containing component at the buried interface to enable in situ release of Se offers a viable pathway for coordinating phase evolution and improving multinary chalcogenide absorber quality.