Bulk and Surface Damage and Self-Repair in Bromide Perovskites: The A cation as a Double-Edged Sword

Photobleaching, combined with several microscopy and spectroscopy approaches, was used in order to examine degradation and healing in the bulk and surface of three APbBr₃ single crystals (A=methylammonium (MA); formamidinium (FA); or Cs). Significant differences were observed, depending on the location and the type of A cation: at the surface, self-healing was impeded by the escape of vaporization products. A passivation effect largely compensated for this in the case of the Cs system, but not for the other cations. In the bulk, the FA cation led to the fastest post-damage healing process whereas the Cs system showed the slowest healing rate. The MA system in some cases showed improvement under bleaching. In order to explain the observed bulk phenomena, DFT calculations were performed. The difference in the rate of self-healing in each of the systems was correlated to the A-cation’s effect on Br₃^--defect stability, whereas the unique attributes of the MA system were associated with a possible defect passivation effect of photo-dissociated methylamine. These results rationalize the use of mixed A-cation materials for optimizing both solar cell stability and overall performance of HaP-based devices.