Free-bromine-induced anomalous bulk photovoltaic effect in chiral metal halides for self-driven circularly polarized light detection

Chiral polar metal halides can generate spontaneous polarization, endowing them with potential for self-driven circularly polarized light (CPL) detection. However, most chiral metal halides crystallize in non-polar space groups and cannot generate spontaneous polarization, making efficient self-driven CPL detection difficult. In this work, for the first time, we propose a strategy to induce spontaneous polarization into non-polar chiral halides by introducing free halogens into the halide lattice via a solvothermal method. We find that this strategy not only breaks the intrinsic symmetry of non-polar R/S-MPZPbBr₄ and transforms it into R/S-MPZPbBr₄·0.5Br₂ with the polar space group C₂, but also induces larger octahedral distortion, which enhances the circular dichroism anisotropy factor (g_CD) by 8 times. Benefiting from spontaneous polarization and the enhanced g_CD, R/S-MPZPbBr₄·0.5Br₂ exhibits an anomalous photovoltaic effect of 3.8 V under zero bias and enables efficient self-driven CPL detection, with an anisotropic response factor (g_res) as high as 0.61. Our work opens a new pathway for designing chiral metal halides for self-driven CPL detection.