A method is provided for preparing a thin film of perovskite material having an adjustable bandgap. The method forms a thin film of material having the formula BX2, where anionic part X is a halide, and where the cation B is lead (Pb), tin (Sn), or germanium (Ge). A solution is formed of materials with the formulas A1X and A2X, where cation A1 is formamidinium, and where cation A2 is an organic cation having a larger size larger than a methylammonium cation. The method deposits the solution over the BX2 thin film, and forms a perovskite material having the formula A11-YA2yBX3. For example, the A2 cation may be an ammonium cation such as ethylammonium, guanidinium, dimethylammonium, acetamidinium, or substituted derivatives of the above-mentioned ammonium cations. In one aspect, the perovskite material A1BX3 may be formamidinium iodide (FAI), and A2BX3 may be ethylammonium iodide (EtAI). Tandem solar cells are also provided.