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Anomalous properties such as operational instability and photocurrent hysteresis in perovskite-based devices present a major obstacle to their future commercialization. Halide ion/defect migration has been widely accepted as one of the main mechanisms behind these limiting properties, but a definitive explanation of this relationship has remained elusive. Here, we present a quantitative multi-scale diffusion framework that fully describes halide diffusion in polycrystalline metal halide perovskites (MHPs). By using time-of-flight secondary ion mass spectroscopy (ToF-SIMS) technique we could simultaneously monitor both the fast grain boundary (GB) diffusivity and three to four orders of magnitude slower volume/bulk diffusivity. Our framework reveals an inverse relationship between the activation energies of GB (EGB) and volume (EV) diffusions, such that MHPs (such as MAPbI3) with a larger EV also possess a smaller EGB. Importantly, this relationship explains some of the most conflicting observations in the literature, namely that MHPs with improved stability typically exhibit reduced hysteresis, thanks to the simultaneous existence of small volume and large GB halide diffusivities, respectively, pointing us to propose a model of grain boundary “strength”. This nontrivial relation between volume and GB halide diffusivities is derived from a wide range of MHP systems, including MA- and FA-based iodide and bromide perovskites. Even when GB passivation approaches are used, GB diffusivity increases reducing hysteresis at the expense of volume diffusion, which enhances stability. The quantitative elucidation of multiscale halide diffusion in polycrystalline MHPs provides an important path toward addressing these outstanding issues.
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