Quantification of ion migration in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> perovskite solar cells by transient capacitance measurements

Ion migration in halide perovskite films leads to device degradation and impedes large scale commercial applications. We use transient ion-drift measurements to quantify activation energy, diffusion coefficient, and concentration of mobile ions in methylammonium lead triiodide (MAPbI₃) perovskite solar cells, and find that their properties change close to the tetragonal-to-orthorhombic phase transition temperature. We identify three migrating ion species which we attribute to the migration of iodide (I⁻) and methylammonium (MA⁺). We find that the concentration of mobile MA⁺ ions is one order of magnitude higher than the one of mobile I⁻ ions, and that the diffusion coefficient of mobile MA⁺ ions is three orders of magnitude lower than the one for mobile I− ions in our samples. This quantification of mobile ions in MAPbI₃ will lead to a better understanding of ion migration and its role in operation and degradation of perovskite solar cells.