Perovskite solar cells have emerged as a promising next-generation electrical power generating tool. However, imperfections in perovskite films are one of the crucial factors preventing the commercialization of perovskite solar cells. Passivation has proven to be an effective strategy to reduce the density of defect states in perovskite crystals and inhibit ion migration. Although significant work on chloride ion and N,Ndimethylformamide (DMF) has shown that the additives are able to passivate different types of trap defects, systematic studies on the effects of DMF and HCl on perovskite crystallization when used in conjunction with each other are elusive. Here, we systematically investigated the synergistic effect of DMF and hydrochloric acid (HCl) on methylammonium (MA+)-based perovskite films with the two-step spin-coating method. As a Lewis base, DMF coordinates well with Pb2+ to facilitate a decrease in the number of defects, thereby improving the carrier separation and transport, while HCl improves the overall perovskite film morphology. Addition of 20 μL HCl/20 μL DMF to 10 mL of methylammonium iodide/isopropyl alcohol solution afforded ca. 500 nm thick perovskite films with no observable defects within the grains. The process allowed fabrication of devices with an active area of 0.16 cm2, which produced power conversion efficiencies up to 18.37% with minimal hysteresis.