Depth-Dependent Carrier Transport During Light-Induced Phase Segregation in Wide-Bandgap Perovskites

Light-induced halide phase segregation is a known instability in wide-bandgap perovskite materials and limits their stability in tandem solar cells. Although ion migration is generally considered the driving mechanism, how phase segregation develops across the film thickness and influences carrier transport remains unclear. Here, we resolve the 3D carrier transport behavior of wide-bandgap perovskite films under light-induced phase segregation. By illuminating the films from either the top or bottom surface, we examine how illumination direction affects the spatial evolution of segregation. Phase segregation leads to pronounced electrical heterogeneity within grain interiors and along grain boundaries, accompanied by a depth-dependent asymmetry. Bottom-side illumination results in greater degradation of bulk and grain-boundary transport, indicating stronger disruption of charge-transport pathways near the buried interface. These results clarify how light-induced phase segregation modifies carrier transport across the film thickness and provide insight into the microscopic origins of instability in wide-bandgap perovskites.