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Use of a lead–tin mixed perovskite is generally considered an effective method to broaden the absorption wavelength of perovskite thin films. However, the preparation of lead–tin mixed perovskites is a major challenge due to the multivalent state of tin and stability in the atmosphere. This study attempted to replace the organic cation and metal elements of perovskites with a relatively thermal stable formamidinium (FA⁺) and a more environmentally friendly tin element. MA_0.5FA_0.5Pb_0.8Sn_0.2I₃ lead–tin mixed perovskite thin films were prepared with the one-step spin-coating method. By adjusting the dimethylformamide (DMF):dimethyl sulfoxide (DMSO) concentration ratio of the lead–tin mixed perovskite precursor solution, the surface morphologies, crystallinity, and light-absorbing properties of the films were changed during synthesis to optimize the lead–tin mixed perovskite films as a light-absorbing layer of the inverted perovskite solar cells. The quality of the prepared lead–tin mixed perovskite film was the highest when the ratio of DMF:DMSO = 1:4. The power-conversion efficiency of the perovskite solar cell prepared with the film was 8.05%.