Geometric Artifacts in Time-of-Flight Mobility Measurements of Perovskite Single Crystals

Carrier mobility is an important parameter of metal halide perovskites that determines carrier diffusion length in solar cells, response speed of detectors, and other device characteristics. Time-of-flight (ToF) is widely used to evaluate carrier mobilities in thick perovskite crystals and is often regarded as a direct probe of long-range drift transport. Here we show that extracted ToF mobilities are sensitive to the electrode geometry. The concentration of electric field lines near electrode edges can dramatically shorten the transit time. ToF mobilities can be overestimated by up to 10 times by the distorted electric field in devices with electrode diameters smaller than the crystal thickness. An electrode-diameter-to-crystal-thickness ratio larger than 8 is required to obtain consistent charge transit time and thus the carrier mobility. Using a guard ring biased at the same potential as the electrode, the minimum electrode-diameter-to-crystal-thickness ratio can be reduced to 2.5.