Tailoring of bandgap and tolerance factor of (FA,Cs)PbI₃ thin-film solar cells grown by sequential evaporation for improved stability

Perovskite solar cells have attracted significant attention in recent years owing to their high efficiency and a tunable band gap. The vacuum deposition method offers the advantage of being free from external factors like humidity, which can lead to phase degradation of the perovskite material. Formamidinium lead iodide (FAPbI₃) and cesium lead iodide (CsPbI₃) are the suitable perovskite materials for single-junction solar cells. Formamidinium (FA) has suitable bandgap, between 1.48 eV for absorbing solar spectrum and cesium (Cs) has a thermal stability at high temperature. Herein, thermal evaporation method was used to fabricate the perovskite with organic-inorganic mixed. The FAPbI₃ thin films were deposited by varying the ratios of Cs and FA, allowing a comparison between different conditions. The structural stability of halide perovskite is determined by the Goldschmidt tolerance factor. Mixing FA and Cs with large and small cations allows effective tailoring of the tolerance factor. Organic inorganic mixed perovskite obtains stable cubic phase and form uniform grain size leads reduced defect density, increasing the short-circuit current density. Simultaneously, fine bandgap tuning also increased open circuit voltage. The triple cation perovskite was analyzed by X-ray diffraction to confirm that the alpha phase was maintained during long term. The addition of cesium was conducive to formation of perovskite phase and crystallization, which led to a stable photovoltaic performance.