There are a large number of ionic defects on the surface and grain boundaries of perovskite films, which are detrimental to the performance of perovskite solar cells (PSCs). Passivating these defects is an effective approach to suppress non-radiative recombination and to improve the efficiency and stability of the perovskite solar cells. Here, an aromatic organic molecule, 4-bromo-benzonitrile (PBBN), is used on (FAPbI3)0.95(MAPbBr3)0.05-based perovskite films for surface and grain-boundary defect passivation. Theoretical calculation and experimental results indicate that the Pb-I antisite, Pb-cluster, and I vacancy defects can be effectively passivated by the cyano group (-CN) of PBBN, resulting in significantly suppressed nonradiative recombination. As a result, the power conversion efficiency (PCE) of the (FAPbI3)0.95(MAPbBr3)0.05 planar perovskite solar cells based on the passivated perovskite films is improved from 20.09% to 22.28% with reduced hysteresis. Moreover, PBBN treatment also enhances the ambient stability of PSCs. The unencapsulated devices can retain 90% of the initial efficiency after 500 h storage under ambient atmosphere.