Unusual polarization-insensitive negative photoresponse of chiral double perovskite ( <i>R</i>/ <i>S</i>-MBA) ₄AgBiI ₈

Traditional perovskite single crystals typically exhibit polarization-dependent photoresponse characteristics due to crystallographic anisotropy, particularly showing sensitivity to linearly polarized light (LP), 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 (<em>R</em>/<em>S</em>-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.