Fig. 8. (a) Energy barrier for cations ions (FA+, MA+) diffusion into two different regimes named bulk to surface and surface to vacuum, for pristine-FAPbI3 and MAPbI3 and the mixed cation FA0.5MA0.5PbI3; (b) the energy barrier for iodide ions from the bulk to the vacuum reveals that in FA-rich compositions, I− ions exhibit the lowest energy for migration from the bulk to the surface, but the highest energy from the surface to the vacuum, preventing them from escaping the surface and forming vacancies. Figure reproduced with permission from ref. 89.
