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

ABSTRACT

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 Cu ₂ ZnSn(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 Cu ₂ SnSe ₃ ‐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 Cd ²⁺ partially substitutes Zn ²⁺ 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.