Anisotropic carrier diffusion in single MAPbI grains correlates to their twin domains

Polycrystalline thin films and single crystals of hybrid perovskites - a material group successfully used for photovoltaic and optoelectronic applications - reportedly display heterogeneous charge carrier dynamics often attributed to grain boundaries or crystalline strain. Here, we locally resolved the carrier diffusion in large, isolated methylammonium lead iodide (MAPbI 3 ) grains via spatial- and time-resolved photoluminescence microscopy. We found that the anisotropic carrier dynamics directly correlate with the arrangement of ferroelastic twin domains. Comparing diffusion constants parallel and perpendicular to the domains showed carriers diffuse around 50-60% faster along the parallel direction. Extensive piezoresponse force microscopy experiments on the nature of the domain pattern suggest that the diffusion anisotropy most likely originates from structural and electrical anomalies at ferroelastic domain walls. We believe that the domain walls act as shallow energetic barriers, which delay the transversal diffusion of carriers. Furthermore, we demonstrate a rearrangement of the domains via heat treatment above the cubic-tetragnal phase transition. Together with the previously reported strain engineering via external stress, our findings promise additional routes to tailor the directionality of the charge carrier diffusion in MAPbI 3 -based photovoltaics and optoelectronics as well as other ferroelastic materials for optoelectronic applications. The anisotropic charge carrier diffusion coupled to ferroelastic twin domains in methylammonium lead iodide opens possibilities for further optimization of perovskite solar cells and optoelectronic devices via strain engineering and heat treatments.