The pursuit of stability in halide perovskites: the monovalent cation and the key for surface and bulk self-healing

Abstract

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.