Abstract
We propose a new model of defect formation and ion migration at the surfaces/grain boundaries of lead-halide perovskites, based on ab initio calculations. Inspired by the spread of experimentally measured activation energies for ion migration in similar lead-iodide perovskites, we define an effective defect formation energy weighed upon surface and bulk contributions. We thus link the large variation in measured activation energies for ion migration to the different defect formation energies of polycrystalline thin films with different grains size. Defect formation is facilitated at surfaces; thus, smaller grains exhibit an averagely lower activation energy to ion migration than larger grains. We also account for the increased ion migration observed under light. Overall, our findings point at surface passivation as a major direction to stabilize lead-halide perovskites against formation of defects, limiting in turn the the associated decomposition reactions.