Impact of the Polymorphism and Relativistic Effects on the Electronic Properties of Inorganic Metal Halide Perovskites

Recent finds have revealed in metal halide perovskites the presence of lower local symmetry contributions especially in the cubic phase in detriment to its high symmetry monomorphic structure (Pm-3m). We analyzed the impact of the polymorphic nature in CsBX3 inorganic perovskites (B = Ge, Sn, Pb; X = Cl, Br, I) through first-principle calculations to show how the polymorphism contributes more to the material stability than their monomorphic counterparts. Distinct stability trends can be seen for each halogen and metal series, revealing the role of the (strong) spin–orbit coupling (SOC) on the stability throughout the Ge → Sn → Pb sequence from a set of local motif contributions, such as distortions on the octahedrons, relative tiltings, Cs displacement, and metal off-centering networks. The combination of relativistic quasiparticle correction and SOC provided accurate values of gap energies, showing that the experimental measurement is actually an average from structural local motif contributions. At the same time, given the absence of a prohibited transition, a blue shift in the UV–vis spectra was observed for all chemical compositions from high symmetry structure → polymorphic version. This result revealed that a high suppression of the total optical absorption can be avoided through the replacement of the toxic Pb by greener alternatives, especially CsSnX3 and CsGeX3 (X = Br and I), providing a potential perspective to the market of solar cell devices.