To date, it is urgent to produce perovskite films with comparative or even better morphologies in an open-air environment. Unfortunately, a substantial number of trap states on the grain surface, especially the grain boundaries (GBs) of a perovskite layer, can bring about significant deterioration in the performance of PSCs. Trap-induced carrier recombination directly exerts a detrimental influence on the carrier collection efficiency and electronic properties of a perovskite active film. Herein, 4(5)-iodoimidazole (4II), a small organic molecule agent, was introduced to passivate the surface and bulk traps of the active film, which resulted in a controlled morphology, improved carrier extraction and suppressed ion migration for the devices fabricated in a relatively humid and O2-containing environment. Conductive atomic force microscopy (C-AFM) and Kelvin probe force microscopy (KPFM) measurements were applied to study trap passivation and suppression of ion migration across the GBs of perovskite films. The results manifest that the -CN group preferably bonds with the less-coordinated Pb2+ and the -NH- group favorably forms hydrogen bonds with the uncoordinated I-. As a result, the champion device delivered a significantly boosted power conversion efficiency from 17.22% to 20.95%, with an improved fill factor (FF) from 70.54% to 80.40%, and improved ambient stability of the unencapsulated device. This study may probe research insight into the design of passivators with synergistic effects for morphology control and reduction of carrier recombination loss for equally efficient perovskite photovoltaics fabricated in ambient air.