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.