Single crystals of halide perovskites represent a benchmark for understanding their bulk properties. We have studied the dielectric function of lead bromide perovskite single crystals (MAPbBr3, CsPbBr3 and for the first time FAPbBr3) by spectroscopic ellipsometry in the spectral range 1-5 eV while gradually varying the temperature from 183K to 440K. An extremely low absorption coefficient in the sub-bandgap region was found in all three types of crystals, testifying the high optical quality of the bulk. We extracted the bandgap values through critical point analysis showing that Tauc-based values are systematically underestimated. The two structural phase transitions occurring in the explored temperature range, i.e. orthorhombic to tetragonal and tetragonal to cubic, show markedly distinct optical behaviours, with the former having a discontinuous character. The cross-correlation of optical data with DFT calculations evidences the role of octahedral tilting to tailor the value of the bandgap in the three perovskites at a given temperature, whereas differences in the thermal expansion affect the slope of the bandgap trend as a function of the temperature.

We find significant differences between degradation and healing at the surface or in the bulk for each of the different APbBr₃ single crystals (A = CH₃NH₃ ⁺, methylammonium (MA); HC(NH₂)₂ ⁺, formamidinium (FA); and cesium, Cs⁺). Using 1- and 2-photon microscopy and photobleaching we conclude that kinetics dominate the surface and thermodynamics the bulk stability. Fluorescence-lifetime imaging microscopy, as well as results from several other methods, relate the (damaged) state of the halide perovskite (HaP) after photobleaching to its modified optical and electronic properties. The A cation type strongly influences both the kinetics and the thermodynamics of recovery and degradation: FA heals best the bulk material with faster self-healing; Cs⁺ protects the surface best, being the least volatile of the A cations and possibly through O-passivation; MA passivates defects via methylamine from photo-dissociation, which binds to Pb²⁺. DFT simulations provide insight into the passivating role of MA, and also indicate the importance of the Br₃ ⁻ defect as well as predicts its stability. The occurrence and rate of self-healing are suggested to explain the low effective defect density in the HaPs and through this, their excellent performance. These results rationalize the use of mixed A-cation materials for optimizing both solar cell stability and overall performance of HaP-based devices, and provide a basis for designing new HaP variants.

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.