Hydrogen Diffusion in Hybrid Perovskites from Exchange NMR

Ion migration is an important phenomenon affecting the performance of hybrid perovskite solar cells. It is particularly challenging, however, to disentangle the contribution of H⁺ diffusion from that of other ions, and the atomic-scale mechanism remains unclear. Here, we use ²H exchange NMR to prove that ²H⁺ ions exchange between MA⁺ cations on the time scale of seconds for both MAPbI₃ and FA_0.7MA_0.3PbI₃ perovskites. We do this by exploiting ¹⁵N-enriched MA⁺ to label the cations by their ¹⁵N spin state. The exchange rates and activation energy are then calculated by performing experiments as functions of mixing time and temperature. By comparing the measured exchange rates to previously measured bulk H⁺ diffusivities, we demonstrate that, after dissociating, H⁺ ions travel through the lattice before associating to another cation rather than hopping between adjacent cations.