Real‐Time Observation of Iodide Ion Migration in Methylammonium Lead Halide Perovskites

Organic–inorganic metal halide perovskites (e.g., CH3NH3PbI3−xClx) emerge as a promising optoelectronic material. However, the Shockley–Queisser limit for the power conversion efficiency (PCE) of perovskite‐based photovoltaic devices is still not reached. Nonradiative recombination pathways may play a significant role and appear as photoluminescence (PL) inactive (or dark) areas on perovskite films. Although these observations are related to the presence of ions/defects, the underlying fundamental physics and detailed microscopic processes, concerning trap/defect status, ion migration, etc., still remain poorly understood. Here correlated wide‐field PL microscopy and impedance spectroscopy are utilized on perovskite films to in situ investigate both the spatial and the temporal evolution of these PL inactive areas under external electric fields. The formation of PL inactive domains is attributed to the migration and accumulation of iodide ions under external fields. Hence, we are able to characterize the kinetic processes and determine the drift velocities of these ions. In addition, it is shown that I2 vapor directly affects the PL quenching of a perovskite film, which provides evidence that the migration/segregation of iodide ions plays an important role in the PL quenching and consequently limits the PCE of organometal halide‐based perovskite photovoltaic devices. Ion migration in organometal halide perovskites is directly observed in real‐time using photoluminescence wide‐field microscopy. A lateral electrode configuration is used to apply an electric field. Accumulating iodide ions quench the photoluminescence of individual crystalline domains starting from the positive electrode moving to the negative, which corresponds to iodide ions moving the opposite way. Impedance measurements confirm the respective migration.