Cation substitution-induced band gap and stability engineering of formamidinium-based perovskite films prepared in ambient conditions

An intermediate complex approach coupled with a preheating strategy are used to grow mixed-cation FA_1-xCs_xPbI₃ perovskite films in ambient air. This was achieved through an effective PbI₂ conversion and the elimination of the detrimental moisture effect, which was ensured by the formation of an intermediate complex along with the rapid solvent evaporation induced by the preheating of the substrate and/or precursor solutions. Moreover, it is found that the control of [CsI]/[FAI] ratio clearly influences the crystallization of the α-perovskite phase and could effectively inhibit the growth of non-perovskite crystallites, which stabilizes the photoactive α-phase. X-ray diffraction analyses indicate that a change in crystal orientation from (100) to (111) direction occurs for 35% CsI added in the perovskite precursor, inducing a decrease in the band gap energy. Moreover, the stability is significantly improved in the (111) direction, in agreement with computational results obtained in literature.