Applying a two-step procedure and solvent engineering, we fabricated a stable tin-based perovskite, formamidinium tin triiodide (FASnI3), solar cell for lead-free photovoltaic applications. The first step was deposition of a SnI2 layer with the solvent dimethyl sulfoxide; the second step was application of a cosolvent system containing hexafluoro-2-propanol (HFP), isopropyl alcohol (IPA), and chlorobenzene (CB) in a 5:5:2 ratio to deposit the FAI layer on the SnI2 layer. The traditional IPA solvent prevented the formation of a stable FASnI3 layer such that a stable device could not be fabricated. HFP was hence used to form hydrogen bonds with IPA and FAI to retard the crystal growth of FASnI3; CB served as the antisolvent. Ethylenediammonium dihypophosphite in the first step was an effective reducing agent to increase the efficiency of power conversion from ∼5% to ∼7% with great reproducibility and stability over 4000 h.