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.
