The elucidation of intermolecular interactions for the systematic control of nucleation and crystallization is key to produce perovskite photovoltaics with superior performance. In this study, we added a weak coordination solvent acetonitrile (ACN) to an ionic liquid precursor solution and studied the effect of ACN on the coordination environment of Pb2+ in the precursor solution and determined the role of ACN in perovskite nucleation and the crystallization process. The addition of ACN was found to reduce the coordination between the solvent and perovskite solutes (CH3C= OO-···Pb2+), which facilitated the formation of large micelles by sharing halogens. More importantly, the strong hydrogen bond interaction between organic cations and ACN (CH3-NH3+···CH3CN) slowed the growth of perovskite crystals to produce a compact and smooth thin film. MAPbI2.8Br0.2-based solar cells based on intermolecular interaction tuning achieved a high-power conversion efficiency of over 21% with long-term stability for over 1500 h. The results of this study demonstrate that the performance of perovskite photovoltaic devices can effectively be improved by adjusting the chemical environment of various components of the precursor solution.