The high efficiency of precursor-to-perovskite conversion is one of the core factors in boosting the performance of perovskite solar cells (PSCs). Herein, we report a liquid/liquid interfacial synthesis of terpyridine-zinc(ii) (ZnTPY) coordination nanosheets (CONASHs), which can enhance PbI2-based perovskite crystallization and thus the efficiency of PSCs. Ultrasonically fragmented ZnTPY CONASHs, rich in uncoordinated terpyridine moieties, caused multidentate chelation with PbI2, resulting in a well-dispersed ZnTPY:PbI2 complex. This metal-organic complex could act as a heterogeneous nucleation seed and facilitate the formation of a PbI2-to-[PbI6−x]4− coordinated octahedral framework in a precursor solution of perovskite, thereby reducing the crystallization barrier of tetragonal CH3NH3PbI3 perovskites and achieving a complete precursor-to-perovskite conversion. Consequently, ZnTPY-capped perovskite crystals had a long photoluminescence (PL) lifetime, which is attributed to the more passivated trap states associated with ZnTPY. The corresponding PSCs exhibited an optimal power conversion efficiency of 19.8% compared with 17.9% of the controlled device. The results prove that the performance of PSC families can be improved by dispersing PbI2 in a solution with CONASHs for enhancing the PbI2-to-perovskite conversion.