Slow Heating and Rapid Cooling in Ar + S Atmosphere Enables >800 mV for Sb ₂ S ₃ Thin‐Film Photovoltaics Toward Artificial Light Pollution Energy Conversion

High‐voltage output in photovoltaics is crucial for enabling an efficient building‐integrated photovoltaic (BIPV) system that integrates hydrogen production, which is a promising strategy for creating green‐energy houses and reducing CO 2 emissions. This work aims to develop an optimized annealing process in an Ar + S atmosphere for preparing S‐rich, low‐defect Sb 2 S 3 thin films for high‐performance thin‐film solar cells. By controlling the heating and cooling rates, the slow heating and rapid cooling (SHRC) process proved optimal, yielding a champion device with a power conversion efficiency of 7.20%, along with an open‐circuit voltage ( V oc ) of 0.803 V, a short‐circuit current density of 15.17 mA/cm 2 , and a fill factor of 59.09%. Notably, we achieved an excellent V oc of 0.807 V, which, to the best of our knowledge, surpasses most previously published V oc values for single‐junction Sb 2 S 3 solar cells. Importantly, these SHRC‐annealed Sb 2 S 3 solar cells maintain V oc > 0.5 V even under monochromatic LED illumination at a low light intensity of 0.5 mW/cm 2 . These results demonstrate that an Ar + S atmosphere coupled with a carefully designed annealing profile provides a pathway toward high‐voltage‐output Sb 2 S 3 thin‐film solar cells, enabling BIPV systems to harvest low‐intensity “waste” light, such as urban nighttime illumination, for hydrogen production via water splitting.