Bias-Dependent Normal and Inverted J- v Hysteresis in Perovskite Solar Cells

Perovskite solar cells (PSCs) typically exhibit hysteresis in current density-voltage (J-V) measurements. The most common type of J-V hysteresis in PSCs is normal hysteresis, in which the performance in the reverse scan is better than that in the forward scan. However, inverted hysteresis also exists, in which the reverse scan performance is worse than in the forward scan; this hysteresis, however, is significantly less well studied. In this work, we show that the hysteresis decreases when the sweep rate is decreased only in cases involving a small bias range, and it does not decrease with a large bias range. Under large forward bias and slowing sweep rate, we observe enhanced normal hysteresis or inverted hysteresis in PSCs. Moreover, the degree of normal and inverted hysteresis can be adjusted by varying the bias. Here, we hypothesize that the tunable hysteresis is derived from the different distribution of ionic defects (V_I and V_MA) at the electron (hole) transport layer/perovskite interface due to ionic movement in the perovskite layer under the different bias scanning conditions. This conclusion is confirmed using Kelvin probe force microscopy with different bias voltages and scanning rates, which shows surface potential hysteresis based on ionic-migration-related Fermi level shifting in perovskite films and agrees with the tunable J-V hysteresis hypothesis. Moreover, the increased time response in the milliseconds region in open-circuit voltage decay after J-V scanning further corroborates the mechanism of ionic migration under bias. Our work provides new insights into the ionic movement hypothesis for the J-V hysteresis in PSCs.