We illustrate the results of ab initio molecular dynamics simulations coupled to first principles electronic structure calculations on the effect of light absorption on the electronic and dynamical properties of lead-halide perovskites. The role of the organic cation dynamics and of ion/defect migration is analyzed in relation to photoinduced structural transformations and solar cell operation. Iodine defects, such as vacancies and interstitials, undergo a light-induced dynamical transformation leading to their mutual annihilation, accounting for the observed enhanced photoluminescence quantum yield following light irradiation. We also show how most of perovskites unusual properties in terms of defects and trapping dynamics can be explained by the close similarity between the perovskite properties and the photochemistry of iodine, both for 3D and 2D materials. Along with the unusual defect chemistry, the role of large polarons in screening charge carriers from recombination is finally illustrated based on quantitative models that allow us to estimate the extent and size of polaron distortion in various perovskites from first principles. The combination of unusual defect chemistry and of polaron screening of the charge carriers largely contributes to the outstanding optoelectronic properties of lead-halide perovskites.
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