Organic and inorganic metal halide perovskite solar cells (PSCs) have been regarded as the next-generation photovoltaic technology. However, the solution processed perovskite films usually have abundant defects, especially at the grain boundaries and surfaces, causing some disadvantages. Herein, 4-(trifluoromethyl)benzylamine (TFMBA) is deposited on the surface of perovskites to form an ultrathin hydrophobic organic molecular layer, passivating the interfacial imperfections. By optimizing the post-treatment conditions, a remarkable power conversion efficiency (PCE) of 20.56% is realized. Impressively, the passivated ultrathin organic molecular layer effectively prohibits the penetration of moisture, resulting in unsealed PSCs retaining 84% of the original PCE after aged 1300 h at 65%−75% relative humidity (RH). SEM and XRD confirm that TFMBA can effectively passivate defects on the surface and grain boundaries, thereby prevent the perovskites from the irreversible decomposition with the immersion of moisture. This suggests that the TFMBA post-treatment contributes to high-quality and stable perovskite films.