Bandgap-dependent defect properties in mixed-halide perovskites

Defects in wide-bandgap perovskite solar cells critically limit device performance, yet their chemical nature and bandgap-dependent evolution remain insufficiently understood. Here, we investigate the defect properties of mixed-halide wide-bandgap perovskite solar cells with representative bandgaps of 1.67, 1.78, and 1.93 eV by systematically tuning the I/Br ratio while maintaining the same FA/Cs mixed-cation framework. Thermal admittance spectroscopy was employed to quantify the defect energy depth and defect-state density in these devices. The results reveal that the defect activation energy increases monotonically with increasing bandgap, indicating the formation of deeper defect states in Br-rich wide-bandgap perovskites. By using p-phenylenediamine diiodide to passivate iodide-vacancy-related defects, defect-mediated nonradiative recombination is significantly suppressed, leading to prolonged carrier lifetimes and improved device performance. These results suggest that iodide-vacancy-related halide defects are the dominant recombination-active defects in mixed-halide wide-bandgap perovskite solar cells. Notably, although iodide-vacancy passivation reduces defect density and improves device performance, its effectiveness gradually diminishes as the bandgap increases, highlighting the increasing difficulty of defect passivation in Br-rich wide-bandgap perovskites.