In this study, we employ a MACl post-treatment to enhance the quality of MA₃Sb₂I₉ perovskite thin films fabricated through antisolvent processing. This treatment facilitated the formation of Cl-Sb bond interactions between MACl and the perovskite thin films, effectively passivating the I^- vacancies and grain boundary defects on the MA₃Sb₂I₉ thin-film surface. This process not only improved the surface morphology and crystallinity of the thin film but also reduced the defect states density of the surface, thereby enhancing the efficiency of carrier extraction and transport. Consequently, the sensitivity of self-powered photodetectors based on the optimized thin-film preparation increased from 3.89 × 10⁷ Jones to 5.72 × 10⁸ Jones, representing an improvement by one order of magnitude. Furthermore, the rise and fall times were shortened from 37/76 ms to 31/37 ms, respectively, indicating an enhancement in the response speed of the devices.