Methylhydrazine Lone‐Pair Engineering for Polar Lead‐Free Perovskite Enables Self‐Powered X‐Ray Detection

Lead-free A₃Bi₂I₉-type perovskites demonstrate excellent performance in direct X-ray detection owing to their high bulk resistivity and reduced ion migration. However, the reported centrosymmetric A₃Bi₂I₉ can only operate with external voltage, inevitably resulting in energy consumption and bulky monolithic circuits, limiting their further development. Herein, exploiting the methylhydrazine (MHy) cation with 2<i>s</i> ² lone-pair electrons (LPEs), a chiral-polarity perovskite MHy₃Bi₂I₉ are obtained and explored its self-powered X-ray detection properties. Where MHy forms the strong hydrogen bond interaction with the inorganic framework, resulting in the asymmetric Bi₂I₉ unit. Meanwhile, the 2<i>s</i> ² LPEs contribute to generating MHy dipole moments, leading to spontaneous polarization. On the one hand, spontaneous polarization acts as a driving force to realize the X-ray-generated carriers' separation and transport to acquire self-powered detection ability. On the other hand, the reduced noise current and dark current under zero bias further increase the signal-to-noise ratio and lower the detection limit. Notably, the MHy₃Bi₂I₉ single-crystal-based detector displays a considerable sensitivity (106 μC Gy^-1 cm^-2) and an ultralow detection limit (55 nGy s^-1) in self-powered mode. Herein, new insights for constructing polar lead-free perovskite and realizing unprecedented A₃Bi₂I₉-type self-powered X-ray detectors are provided.