'Ion-freeze' efficiency in perovskite solar cells: time scales for ion immobilization

It is essential to shed light on the complex multiscale dynamics of mobile ions in perovskite solar cells for developing efficiency testing protocols, gaining a deeper understanding of the degradation mechanisms, improving long-term stability, and, ultimately, driving the commercialization of this technology. This study provides characteristic times for inhibiting the presence of mobile ions in efficiency measurements, enabling a comparison between 'ion-freeze' and steady-state efficiencies to quantify ion-induced degradation losses. Using current transient analysis during current-voltage measurements for a wide range of sweep rates, we achieve precise time-scale mapping of the complex ionic landscape of perovskites, spanning from ion immobilization (at time scales much shorter than the diffusion rate) to the equilibrium situation, encompassing the ubiquitous hysteresis effects for which we define a scan-rate range of occurrence. We corroborate the theory by using Impedance Spectroscopy. Our study provides valuable information about the intricate ionic dynamics in the context of classical current-voltage measurements, supporting the integration of these complementary experiments into the foundational characterization practices of the perovskite photovoltaic community.