Quantitative Multiscale Diffusion Framework for Halide Perovskites Establishes Stability-Hysteresis-Diffusion Relationship

Anomalous properties such as operational instability and photocurrent hysteresis in metal halide perovskite (MHP)-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 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 diffusion coefficients of GBs (<i>D_GB</i>) and volume (<i>D_V</i>) diffusions, such that MHPs (such as MAPbI₃) with a larger <i>D_V</i> also possess a smaller <i>D_GB</i>. Importantly, this relationship explains some of the most conflicting observations in the literature, namely that MHPs with improved stability (linked to <i>D_V</i>) typically exhibit reduced hysteresis. This nontrivial relation between volume and GB halide diffusivities is observed across a wide range of MHP systems, including MA- and FA-based iodide and bromide perovskites and is valid when GB passivation approaches are used. The quantitative elucidation of multiscale halide diffusion in polycrystalline MHPs provides an important path toward addressing outstanding issues of stability and hysteresis in MHP device technologies, as will be discussed in this presentation.