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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.

Strain effects on vacancies, electronic states and stability have been disclosed for perovskite structured materials (ABX3) including metal oxides and organic-inorganic hybrids critical for energy storage and generation. Ion substitution has been pursued as an attractive solution to alter the crystallization or induced electronic of organic-inorganic hybrid halide lead perovskite. However, the disorganized structure of perovskite result from inappropriate substitution may cause unwanted phase transition and photo-stability. Herein, we introduce crystallization of restriction ABX3composition with moderate zinc (Zn) substitution to obtain the ordered and stable CH3NH3(Zn:Pb)I3-xClxcrystal, which is achieved viareleasing lattice strain during an appropriate lattice constriction within BX6octahedron. We obtained an unprecedented efficiency of 20.06% under simulated sunlight with facilitating binary metal perovskite of CH3NH3(1Zn:100Pb)I3-xClx. Our results are important to realize scale up and practical applications of efficient planar perovskite solar cells.