The band gap of formamidinium lead iodide (FAPbl3) perovskites allows broader absorption of the solar spectrum relative to conventional methylammonium lead iodide (MAPbl3). Because the optoelectronic properties of perovskite films are closely related to film quality, deposition of dense and uniform films is crucial for fabricating high-performance perovskite solar cells (PSCs). We report an approach for depositing high-quality FAPbl3 films, involving FAPbl3 crystallization by the direct intramolecular exchange of dimethylsulfoxide (DMSO) molecules intercalated in Pbl2 with formamidinium iodide. This process produces FAPbl3 films with (111)-preferred crystallographic orientation, large-grained dense microstructures, and flat surfaces without residual Pbl2. Using films prepared by this technique, we fabricated FAPbl3-based PSCs with maximum power conversion efficiency greater than 20%.