Halide Segregation in Wide-Bandgap Quasi-2D Perovskites under Rapid Thermal Cycling

Wide-bandgap (WBG) perovskites are susceptible to ion migration and phase separation under external stressors. Reduced-dimensional perovskites (RDPs) have emerged as alternatives to 3D perovskites for improving stability. However, the influence of spacer cations on the thermal cycling stability of WBG RDPs remains poorly understood. Here, we investigate the effect of two representative organic spacers, butylammonium (BA) and 1,4-phenylenedimethylammonium (PDMA), on the stability of WBG RDPs (n = 4, ∼1.75 eV) under light illumination and thermal cycling using in situ grazing-incidence X-ray wide-angle scattering (GIWAXS) and photoluminescence (PL). In situ GIWAXS and PL reveal that RDPs with BA⁺ undergo phase separation and pronounced structural degradation under combined stress, particularly along the in-plane direction, whereas RDPs with PDMA²⁺ demonstrate better stability. These results indicate that bulky organic cations influence the thermal and structural stabilities of WBG RDPs, providing design guidelines for stable perovskite absorbers in tandem solar cells.