Fatigue stability of CH₃NH₃PbI₃ based perovskite solar cells in day/night cycling

The remarkable power conversion efficiency of perovskite solar cells (PSCs) is overshadowed by concerns about their stability, and its degradation mechanism remains elusive. PSCs are reported to suffer long-term degradation under real working conditions. In this work, we systematically studied the degradation mechanism of PSCs with various device configurations (planar, meso and inverted device structure) by continuous 12-h day/night cycling tests. The fatigue phenomenon, defined in our previous work, was observed both in planar and meso devices. A relationship between the fatigue instability and device-physics of PSCs is established. Through a comparative analysis of results from day/night cycling tests, bulk/interfacial morphology analysis, and transient photocurrent/photovoltage decay measurements, we identify the fatigue behavior in the day/night cycling tests by a cyclic ion movement mechanism, where ions migrate towards the electrode interfaces under illumination and move back to the bulk in the dark. This cyclic migration of ions generates defects in bulk perovskite without destroying the crystal structure. The present study offers a new approach to evaluate the stability of PSCs, contributing to better understanding of degradation mechanisms that are critically important for applications of this novel photovoltaic technology.