Grain Size Influences Activation Energy and Migration Pathways in MAPbBr₃ Perovskite Solar Cells

Ion migration in perovskite layers can significantly reduce the long-term stability of the devices. While perovskite composition engineering has proven an interesting tool to mitigate ion migration, many optoelectronic devices require a specific bandgap and thus require a specific perovskite composition. Here, we look at the effect of grain size to mitigate ion migration. We find that in MAPbBr₃ solar cells prepared with grain sizes varying from 2 to 11 μm the activation energy for bromide ion migration increases from 0.17 to 0.28 eV. Moreover, we observe the appearance of a second bromide ion migration pathway for the devices with largest grain size, which we attribute to ion migration mediated by the bulk of the perovskite, as opposed to ion migration mediated by the grain boundaries. Together, these results suggest the beneficial nature of grain engineering for reduction of ion migration in perovskite solar cells.