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

TL;DR

This study investigates how different monovalent cations affect the surface and bulk stability, degradation, and self-healing of halide perovskites, revealing the roles of thermodynamics and kinetics in their stability.

Contribution

It provides new insights into how A-cation types influence degradation and self-healing mechanisms in halide perovskites, guiding improved material design.

Findings

FA heals bulk faster than others

Cs+ best protects surface stability

MA passivates defects via methylamine

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 not only provide insight into the latter conclusion, but also show the importance and stability of the Br3- defect. 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.