Ion Migration in Mesoscopic Perovskite Solar Cells: Effects on Electroluminescence, Open Circuit Voltage, and Photovoltaic Quantum Efficiency

Perovskite solar cells (PSCs) commonly suffer from drastic changes in performance over time, dominated by the redistribution of mobile ionic defects. Common characterization techniques for solar cells, such as electroluminescence (EL) imaging, are compromised by transient ionic effects, which makes understanding them important for the device optimization process. This study looks at the shape of EL transients in carbon‐based triple mesoscopic PSCs (CPSCs), which have emerged as a potential solution for increased stability and scalability. The slow changes in EL are explained in terms of the migration of two ionic species with different mobilities, and increased ionic densities over time. The effects of applying a forward bias in the dark and illuminating at open circuit for several hours are found to be similar, both pointing toward an increased mobile ion density which causes current collection efficiency losses without decreasing the radiative recombination yield. Current losses are found to be highly dependent on the ionic distribution, as demonstrated with measurements and simulations of the external quantum efficiency (EQE). The findings explain the commonly observed shape of the EQE in CPSCs and help to further understand the effects of ion migration. Furthermore, this study establishes an effective way to analyze ion‐dominated current losses by measuring low temperature EQE spectra with different preconditioning voltages, which enables to directly compare the effect of different ionic distributions.