Hydrothermal deposition of antimony selenosulfide (Sb₂(S,Se₃)) has enabled solar cell applications to surpass the 10% efficiency threshold. This deposition process involves the reaction of three precursor materials: Sb, S, and Se. However, this process generates an unfavourable gradient of Se and S anions in the Sb₂(S,Se)₃ film, which limits further efficiency improvements. Herein, we demonstrate how NH₄F can be used as an additive to regulate the band gradient of the Sb₂(S,Se)₃ and modify the surface of the CdS electron-transporting layer. On the one hand, NH₄F inhibits the decomposition of Na₂S₂O₃ and selenourea, which optimizes the deposition process and allows for adjustment of the Se/S ratio and their distribution in the Sb₂(S,Se)₃ film. On the other hand, hydrolysis of NH₄F induces dissolution and redeposition of CdS, thereby effectively improving the morphology and crystallinity of the CdS substrate. Finally, the dual effect of NH4F enables improved surface morphology and energy alignment of the Sb₂(S,Se)₃ film, thus yielding a maximum efficiency of 10.28%, a 12% improvement over the control device. This study demonstrates an effective strategy for simultaneously modifying a sulfide-based substrate and regulating the element distribution during the deposition of a metal chalcogenide film for optoelectronic device applications.