Slow CH3NH3+ Diffusion in CH3NH3PbI3 under Light Measured by Solid-State NMR and Tracer Diffusion

We investigate methylammonium (MA) transport in MA lead iodide under illumination and show this, as in the dark, to be measurable but negligible when compared with the major carriers. H-1 and C-13 nuclear magnetic resonance (NMR) spectra show constant linewidths as a function of temperature, indicating the absence of significant MA diffusion. C-13 tracer-exchange experiments reveal two distinct diffusion processes, one attributed to bulk MA transport and the other most probably due to higher dimensional defects. The former process has a diffusion coefficient that is consistent with the upper limit extracted from NMR measurements. Derived bulk conductivities for MA cations are orders of magnitude below the experimental ionic conductivity, corroborating the picture of pure iodine transport under illumination, as it was previously experimentally shown only for the dark situation.