Ultralow Ion Migration and X‐Ray Response from Strong Dipole Moments in (Br‐EA) ₂ PbBr ₄ Perovskite Single Crystals

2D hybrid perovskites composed of quantum-well structures demonstrate immense potential in optoelectronics because of their unique combination of environmental stability and optoelectrical properties. However, improving electrical properties via quantum-well engineering results in severe ion migration and inferior chemical stability. In this study, a novel strategy to avoid the above trade-off between electrical properties and stability for modifying the electric field profile in perovskites through molecular design is proposed. To this end, ethylammonium (EA) cations are substituted with 2-bromoethylamine (Br-EA) cations and fabricate pure 2D perovskite (Br-EA)₂PbBr₄ single crystals. The molecular dipole moments of the Br-EA cations and induced electric field impart several novel features to the X-ray response, including a ferro-response current, reverse current, and super-high V_oc of 95 V. The resulting X-ray detectors achieve a sensitivity of 540.7 µC Gy_air ^-1 cm^-2 with an extra 38.1% gained from local dipoles, an impressive limit of detection of 9.8 nGy_air s^-1, and an ultralow dark current drift of 3.69 × 10^-8 nA cm^-1 s^-1 V^-1 at -100 V. This study presents valuable insights into the novel effects of dipole moments of A-site cations on the properties of perovskite materials and inspires further exploration of their potential applications.