Effect of Thermal Stress on the Ion Density and Mobility Distribution in Perovskite Solar Cells

Ion migration is a key phenomenon that influences the performance and stability of metal halide perovskite solar cells (PSCs). In this work, we systematically study how ion mobility evolves under thermal stress at 85 °C, using capacitance-frequency (<i>C-f</i>) spectroscopy measured in the dark. Aided by drift-diffusion simulations, we demonstrate that the measured <i>C-f</i> spectra cannot be reproduced using a single anion density with a unique ion mobility, particularly in the low-frequency domain where nonideal behavior emerges. A double-Gaussian distribution of anion mobilities provides a substantially better fit even for fresh devices and remains necessary after prolonged aging under thermal stress. This evolution suggests that heat stress not only influences the redistribution of mobile ions but also induces morphology-related changes, such as interface modifications and interlayer degradation, which collectively alter the electrochemical response of the device. Furthermore, the experimental hysteresis index (HI) for fresh and aged devices cannot be captured by assuming a single ion mobility, indicating that a distribution of ion mobilities is necessary to fully describe hysteresis behavior. These findings elucidate the complex ion dynamics under thermal stress and point toward the presence of distinct ionic populations contributing to device behavior, with implications for improving PSC stability.