Hysteresis and Its Correlation to Ionic Defects in Perovskite Solar Cells

Ion migration has been reported to be one of the main reasons for hysteresis in the current-voltage (J-V) characteristics of perovskite solar cells. We investigate the interplay between ionic conduction and hysteresis types by studying Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.9Br_0.1)₃ triple-cation perovskite solar cells through a combination of impedance spectroscopy (IS) and sweep-rate-dependent J-V curves. By comparing polycrystalline devices to single-crystal MAPbI₃ devices, we separate two defects, β and γ, both originating from long-range ionic conduction in the bulk. Defect β is associated with a dielectric relaxation, while the migration of γ is influenced by the perovskite/hole transport layer interface. These conduction types are the causes of different types of hysteresis in J-V curves. The accumulation of ionic defects at the transport layer is the dominant cause for observing tunnel-diode-like characteristics in the J-V curves. By comparing devices with interface modifications at the electron and hole transport layers, we discuss the species and polarity of involved defects.