Surface treatment with moisture‐tolerant molecules is an effective approach for improving the long‐term stability of perovskite solar cells (PSCs), but may bring drawbacks to carrier dynamics and power conversion efficiency (PCE). Although PSCs with both high stability and PCE have been realized by specially designed molecules recently, the underlying mechanism boosting the superior PCE is still not clear, limiting the further optimization of the photovoltaic performances. Herein, by monitoring carrier dynamics in tetra‐ethyl ammonium iodine (TEAI)‐treated MAPbI₃ using femtosecond transient reflectivity technique with the rationally selected pump‐probe wavelengths, the electron and hole diffusivities are found being 7 and 3 times faster than those in the pristine MAPbI₃, respectively. The differentially enhanced electron and hole diffusivity and the associated charge separation might originate from TEAI, which can effectively passivate both the negatively and positively charged defects. This study indicates that quaternary ammonium halide might be an efficient tool for improving both moisture stability and PCE of perovskites due to its hydrophobic organic cations ready to anchor along the surface and anions with a deeper penetration.