Ab initio Molecular Dynamics of Defect Migration in Lead-Halide Perovskite Grain Boundaries

Lead-halide perovskites have raised extraordinary interest within the recent decade. Their fascinating electro(-optical) properties in combination with low cost processability make perovskites the most promising candidate for future photovoltaic technologies as stand-alone photoactive material or in combination with other material classes within tandem configuration. However, weak metal-halide bonds give rise to thermal instabilities, the formation of defects, and may cause a thermally induced decomposition.1,2,3 In addition to point-like defects, thin-film perovskites usually are of strongly polycrystalline nature.4,5 These large structural defects are in strong connection with point-like defects6 and may induce severe stability issues.7 Despite the recent progress in the understanding of the defect chemistry within crystalline perovskites, a detailed atomistic understanding of the migration mechanism of defects at grain boundaries is still missing. Here, we analyze the nature of the migration of iodine interstitials and vacancies within grain boundaries (GB). We carried out Car-Parrinello molecular dynamic simulations to investigate the structure and the migration mechanism of defects within wide-angle GBs of CsPbI3. Our results showed fast iodine migration within the grain boundary with an upper limit of 0.11 eV for the migration barrier. Interestingly, a spontaneous formation of Frenkel pair defects is observed within iodine-rich GBs. Within the accessible timescales, we could rule out defect migration from the GB into the grains.