Temperature-dependent ion migration and mobile-ion-induced degradation of perovskite solar cells under illumination

Effects of temperature-dependent ion migration on power reduction of perovskite solar cells (PSCs) under illumination were investigated at temperatures between 15 °C and 45 °C. Mobile-ion concentrations increased with temperature and the power reduction under illumination became faster at high temperatures due to an increase in the number of slow mobile ions and a decrease in the number of fast mobile ions, which was confirmed through ion migration currents. This indicates formation of deep-level defects with low mobility from the fast mobile ions (halide vacancies). Moreover, the transient power responses during maximum-power-point tracking (MPPT) became slower at high temperatures upon illumination, reflecting an increase in the number of slow mobile ions. Furthermore, the light–dark cycling test clarified that the energy yield during 8 h of illumination was reduced by ∼10 rel% per light–dark cycle at 45 °C, which was accompanied by an increase in the concentration of, particularly slow, mobile ions. These results indicate that transient ion-migration analysis for evaluating the concentration of mobile ions (mobile defects) can be effective in understanding the mobile-ion-induced degradation of PSCs.