Understanding the Stability of MAPbBr3 versus MAPbI3: Suppression of Methylammonium Migration and Reduction of Halide Migration

Solar cells based on metal halide perovskites often show excellent efficiency but poor stability. This degradation of perovskite devices has been associated with the migration of mobile ions. MAPbBr(3) perovskite materials are significantly more stable under ambient conditions than MAPbI(3) perovskite materials. In this work, we use transient ion drift to quantify the key characteristics of ion migration in MAPbBr(3) perovskite solar cells. We then proceed to compare them with those of MAPbI(3) perovskite solar cells. We find that in MAPbBr(3), bromide migration is the main process at play and that contrary to the case of MAPbI(3), there is no evidence for methylammonium migration. Quantitatively, we find a reduced activation energy, a reduced diffusion coefficient, and a reduced concentration for halide ions in MAPbBr(3) compared to MAPbI(3). Understanding this difference in mobile ion migration is a crucial step in understanding the enhanced stability of MAPbBr(3) versus MAPbI(3).