Unusual polarization-insensitive negative photoresponse of chiral double perovskite (R/S-MBA)₄AgBiI₈

Traditional perovskite single crystals typically exhibit polarization-dependent photoresponse characteristics due to crystallographic anisotropy, particularly showing sensitivity to linearly polarized light (LPL), and to left/right circularly polarized light (CPL) in chiral perovskites. However, achieving complete linear- and circular-polarization insensitivity in single perovskite crystals with intrinsic structural anisotropy remains an unresolved challenge. This study presents the observation of an unusual linear/circular-polarization-insensitive negative photoresponse in two-dimensional (2D) structured chiral lead-free double perovskite (R/S-MBA)₄AgBiI₈ single crystals. These crystals were prepared by a method combining cyclic heating and slow cooling processes. They have a defect density as low as 8.01 × 10⁹ cm⁻³ and a chiroptical anisotropy factor g_CD of 1.5 × 10⁻³. Mechanistic investigations through density functional theory (DFT) calculation, temperature dependent fluorescence spectroscopy and Raman spectroscopy attribute this anomaly to strong exciton self-trapping effects (STE), where photo-generated excitons form stable localized trapped states, dominating charge transport and effectively masking the polarization sensitivity. This discovery provides new insights into the structure–property relationship of perovskite materials and suggests potential pathways for designing novel polarization-insensitive optoelectronic devices.