Microcrystals as amplifiers to learn about the facet- and point de-fect-dependent lability and stabilization of hybrid perovskite semi-conductors against humidity and light

Shape-controlled methyl ammonium lead bromide microcrystals are used to unravel the facet-selective differences in the reactivity of hybrid perovskite semiconductors against chemical and physical triggers. The microstructure of thin films made of hybrid perovskites is highly dynamic because it is prone to small deviations in external conditions, e.g. the presence of traces of water. During operation in photovoltaics, even the exposure to light or the infusion of heat can induce unpredictable changes. Because such films consist of irregular microcrystalline domains fused together, it is very hard to study the mentioned processes directly. The investigation of isolated microcrys-tals as models can solve this problem. Herein, two types of well-defined CH3NH3PbBr3 microcrystals are compared to each other, one with cubic shape terminated by (100) facets, the other with rhombododecahedral shape and exclusively (110) facets. Those microcrys-tals amplify the sensitivity of hybrid perovskites against humidity, against irradiation or to heat. The surfaces of the cubic particles are thermodynamically more stable, but their reactivity is much higher. Although chemically equivalent, rhombodocecahedral microcrystals are much more stable. However, it is demonstrated that cubic microcrystals are significantly less labile, when a small fraction of CH3NH3+ is substituted by Cs+.