Decoupling Framework and Composition Engineering via Sequential Selenium Source Switching for Efficient Sb 2 (S,Se) 3 Thin Film Solar Cells

Antimony selenosulfide (Sb 2 (S,Se) 3 ) has recently attracted intense interest as an earth‐abundant, non‐toxic, and bandgap‐tunable photovoltaic absorber. However, conventional one‐step deposition routes typically produce films with inadequate crystallinity and inhomogeneous elemental distribution through the film thickness, which severely hamper charge transport and limit device performance. Herein, we introduce an innovative selenium‐source switching driven two‐step sequential deposition (SDE) strategy that decouples framework formation from compositional engineering to overcome these limitations. First, a selenourea‐based precursor is used to deposit a thick Sb 2 (S,Se) 3 film serving as a primary light‐harvesting scaffold. Then, a sodium selenosulfate‐based precursor solution is applied for secondary deposition. Owing to the high reactivity of this solution system, selenium species readily penetrate into the as‐deposited film, inducing extensive ion diffusion and recrystallization throughout the bulk Sb 2 (S,Se) 3 material. This SDE strategy simultaneously optimizes the vertical Se/S grading, tailors the bandgap profile, and eliminates bulk deep‐level traps and surface recombination centers. Consequently, the SDE‐Sb 2 (S,Se) 3 device achieves a remarkable efficiency of 10.90%, placing it among the highest efficiencies for this material system. This strategy not only resolves the intrinsic bottlenecks of one‐step techniques but also provides a generalizable route for synthesizing high‐quality multicomponent chalcogenide films and offers new insights into ion‐exchange kinetics during sequential deposition.