CsPbBr₃, MAPbBr₃, and FAPbBr₃ Bromide Perovskite Single Crystals: Interband Critical Points under Dry N₂ and Optical Degradation under Humid Air

We investigated the complex dielectric function of bromide perovskite single crystals (MAPbBr₃, FAPbBr₃ and CsPbBr₃) by spectroscopic ellipsometry from 1 to 5 eV in the 183–423 K temperature range and under a dry nitrogen environment. The temperature dependence of all critical points provided a benchmark for the interband electronic transition energies in the three bromide perovskites for a wide spectral range. We found that the number of transitions in CsPbBr₃ depends on the crystallographic phase, with the orthorhombic lattice showing an extra transition with respect to the tetragonal and the cubic phases. Through density functional theory calculations, we identified the changes in the electronic structure that accompany the tetragonal-to-orthorhombic transition, leading to splitting of the complex dielectric function along the a and b crystallographic directions. We thereon used a similar experimental approach to study the robustness of bromide perovskites under humid air. We found that FAPbBr₃ and CsPbBr₃ remain unchanged up to 383 K, whereas MAPbBr₃ is irreversibly modified at 363 K. The degradation of MAPbBr₃ impacts its optical transitions and introduces a pregap absorption tail. Together with this, a dramatic damage of the surface is observed with the formation of reverse truncated pyramids accompanied by local worsening of the carrier extraction barrier in the remaining flat sample surface.