Perovskite solar cells (PSCs) have been booming for more than a decade. Defect passivation strategies play an important role in updating the efficiency record. Herein, a posttreatment strategy is proposed to passivate the defects in PSCs by quaternary ammonium chloride (QAC) salts. The tetramethylammonium chloride (TC) molecule shows the best achievement among those passivation agents. Enhanced crystallinity and reduced defects of TC-treated perovskite result in promotion of the open-circuit voltage of the device from 1.06 to 1.12 V, thus largely improving the conversion efficiency from 19.8% to 21.7% for the FAMAPbIBr perovskite component. Moreover, the TC-based device retains 93.2% of its initial power conversion efficiency after storage in drying cupboard (relative humidity 20%≈30%) for more than 30 days. By the verification of various characterization techniques (transient resolved photoluminescence, electrochemical impedance spectroscopy, light intensity dependent voltage, and transient absorption spectroscopy), the enhanced performance results from the less recombination retarded by QAC in the treated device compared with the control device. Additionally, X-ray photoelectronic spectroscopy proves the interaction between TC molecule and Pb in perovskite. This system of selection will definitely provide a novel way to search for effective passivators to promote performance of PSCs.