Hydrothermal Growth‐Pathway Engineering Enables Suppressed Reverse Compositional Gradients for High‐Efficiency Sb 2 (S,Se) 3 Solar Cells

ABSTRACT Antimony chalcogenide (Sb 2 (S,Se) 3 ) is a promising low‐cost thin‐film photovoltaic absorber, yet hydrothermal deposition is often limited by mismatched S/Se reaction kinetics, which induce reverse compositional and bandgap gradients that impede carrier transport. Herein, ramp‐heating (RH) and hot‐insertion (HI) routes are compared to regulate the hydrothermal film‐formation pathway. Compared with RH, HI promotes a more balanced chalcogen supply by modulating the relative reaction kinetics of sulfur‐ and selenium‐containing species, thereby homogenizing the vertical S/Se distribution and bandgap profile. The balanced reaction pathway favors film densification, more complete grain coalescence, and improved vertical structural uniformity, yielding compact and smooth films with more balanced local bonding. Band‐structure and electrical analyses reveal optimized interfacial energetics, reduced series resistance, enhanced recombination resistance, widened depletion regions, and a defect transition from deep V S(e)1 to shallower Sb S(e)1 states with reduced trap density. Consequently, the power conversion efficiency (PCE) increases from 9.65% to 10.48%, with simultaneous improvements in open‐circuit voltage, short‐circuit current density, and fill factor (FF). These findings identify balanced chalcogen‐supply‐mediated film formation as an effective mechanism for mitigating vertical inhomogeneity in Sb 2 (S,Se) 3 solar cells.