Determining the Ion Mobility in Perovskite Solar Cells from Impedance Spectroscopy

Mobile ions in metal halide perovskites are found to degrade perovskite solar cells (PSCs). Therefore, characterizing their density and mobility is crucial for improving the long-term performance of PSCs. We introduce a formula to determine the mobility directly from impedance spectroscopy. The validity of the formula is confirmed through extensive drift-diffusion simulations varying 38 parameters including mobilities, ion densities, layer thicknesses, and trap densities. All in all, these simulations describe a wide variety of PSCs. Alternative formulas from the literature are also tested but are found to be suboptimal. After validation, we experimentally determine the ion mobility of a methylammonium lead iodide PSC to be 4 × 10^-10 m² V^-1 s^-1. This new formula, which depends on the low-frequency feature of the impedance spectrum, facilitates the precise and straightforward determination of the ion mobility in PSCs.