  The volume of research into organo-lead hailde perovskites is increasing
rapidly, with perovskite solar cell efficiencies reaching as high as 22
percent. There is considerable evidence that mobile ions in the perovskite
strongly influence the properties of the solar cell, with the majority of
studies carried out on whole cells under bias. Here we use muon spin relaxation
to directly probe iodide diffusion in methyl ammonium lead iodide (MAPI). This
is the first time that has been used to detect iodide diffusion in any material
and the results provide valuable insight into the movement of ions in lead
halide perovskites. The experiment was carried out in the dark with no external
biases applied and allowed us to calculate a diffusion coefficient of 1.6
x10-14 cm2/s for iodide in MAPI at 300 K.
