Perfecting large-area perovskite film coating technology remains a key challenge to commercialize perovskite solar cells. Herein, nontoxic dimethyl sulfoxide (DMSO) is recommended as the only solvent to fabricate formamidinium (FA)–cesium lead halide perovskite by heat-assisted blade coating. DMSO effectively promotes the formation of α-phase crystals and improves the crystallinity. In addition, high substrate temperature is found to improve the film compactness, preferred facet orientation, and desired phase transition. Fast coating mode is adopted to inhibit ion migration and reduce the hysteresis of solar cells. Without any additives or surface treatments, the resulting perovskite film with chemical formula FACsPbIBr shows smooth surface, dense grains, and excellent crystallinity. The corresponding solar cells achieve efficiencies of 20.2% and 17.1% with active areas of 0.1 and 1.0 cm, respectively. The unencapsulated devices maintain ≈100% and 70% of their initial efficiencies after 500 h of storage in air with relative humidity of about 25% and heating at 85 °C, respectively.