Understanding the relationship between ion migration and the anomalous hysteresis in high-efficiency perovskite solar cells: A fresh perspective from halide substitution

Ion migration has recently piqued intensive attention with respect to the emerging perovskite solar cells (PSCs), but exactly how it impacts on cell performance is still elusive. In this paper, we validate a simple model to relate the scan rate-dependent hysteresis of the solar cells and the defect assisted ion migration in perovskite materials by means of halide substitution to form MAPbBr_xI_3-x (x~0-0.6), prepared by a modified two-step method so as to put the systematic study at a high solar cell efficiency level. Concurrent with the substantially increased power conversion efficiency (PCE), significantly reduced hysteresis has also been observed with increasing Br concentration. Bias-dependent kinetic measurements suggest that the hysteresis is caused by the redistribution of mobile ions (ion migration) under external bias and light illumination, which could be suppressed by Br substitution. Our Density Functional Theory study has borne out this notion by showing that the activation energy for I^- (mobile species) migration has been increased from ~0.34 eV in MAPbI₃ to ~0.46 eV in MAPbBr_xI_3-x. This work provides a new approach to fabricating hysteresis-free, high-efficiency PSCs and deepens our understanding of the hysteresis behavior in perovskite materials.