The commercialization of mixed-halide perovskite solar cells (PSCs) is currently impeded by intrinsic instability arising from light-induced ion migration and phase segregation. While various composition engineering strategies have been proposed to mitigate these issues, establishing a quantitative link between microscopic ion transport kinetics and macroscopic device degradation remains a challenge. In this work, we demonstrate how operando Electrochemical Impedance Spectroscopy (EIS) serves as a critical diagnostic bridge to decouple electronic and ionic dynamics, thereby elucidating the impact of material design on photovoltaic performance.

We first investigated the synergetic effect of heat and light on inorganic perovskites. Through temperature-dependent EIS analysis, we utilize an equivalent circuit model comprising bulk/interfacial recombination resistances and a Warburg diffusion element. We reveal that elevated temperatures facilitate ion migration, manifesting as a significant increase in Warburg admittance and a simultaneous collapse in recombination resistance. This analysis quantitatively explains the rapid degradation of power conversion efficiency (PCE) and the formation of iodide-rich domains, establishing a baseline for ion-migration-induced failure.

Building on this mechanism, we apply EIS to validate a "steric control" strategy for stabilizing the perovskite lattice. By systematically tuning the A-site cation size, from DMA-doped mixed-cation compositions, we increase the Goldschmidt tolerance factor to impose steric hindrance against halide motion. Operando EIS results show that increasing the A-site steric bulk significantly suppresses the Warburg admittance under illumination. Notably, Arrhenius analysis of the EIS data allows us to extract the activation energy (E_a) for ion migration, which rises from 0.037 eV in the reference device to 0.199 eV in the sterically optimized device.

This increase in activation energy, quantified solely through impedance measurements, directly correlates with a 54% improvement in operational stability and suppressed photocurrent degradation under continuous illumination. Furthermore, we corroborate these electrical findings with optical characterizations, where "probe-set-probe" photoluminescence mapping visualizes the suppression of phase segregation.

In conclusion, our work highlights that EIS is not merely a characterization tool but a powerful framework for material design. By quantifying the activation energy of ion migration and distinguishing it from carrier recombination processes, EIS provides a predictive metric for assessing the operational stability of perovskite solar cells, guiding the development of robust, phase-stable photovoltaic materials.