Exploring the Limits of Three-Dimensional Perovskites: The Case of FAPb1–xSnxBr3

Metal halide perovskites based on formamidinium (FA) show interesting properties compared to methylammonium (MA) compounds. The larger molecular size of FA compared to that of MA leads, however, to a structural instability in FAPbI3, placing FA at the border of tolerance factor for three-dimensional (3D) lead iodide perovskites. Because of the huge interest in mixed Sn/Pb perovskites, here we investigate FAPb1–xSnxBr3 perovskites in a full compositional range (0 ≤ x ≤ 1). We find a non-monotonic band gap evolution with increasing Sn content, which, through first-principles computational analyses, we relate to a distorted structure that dynamically averages to a cubic phase, as determined by X-ray diffraction. The large FA cation induces an instantaneous structure made by partly decoupled SnBr3 units, which leads to the observed band gap opening. FASnBr3 thus likely represents a limit compound of 3D perovskites. Intermediate FAPb1–xSnxBr3 compositions maintain a band gap of ∼1.8 eV up to 85% Sn content, making them interesting candidates for applications in tandem devices.