Design of hypotoxic lead-free perovskites, e.g. Bismuth(Bi)-based perovskites, is much beneficial for commercialization of perovskite X-ray detectors due to their strong radiation absorption. Nevertheless, the design principles governing the selection of A-site cations for achieving high-performance X-ray detectors remain elusive. Here, seven molecules (methylamine MA, amine NH₃, dimethylbiguanide DGA, phenylethylamine PEA, 4-fluorophenethylamine p-FPEA, 1,3-propanediamine PDA, and 1,4-butanediamine BDA) and calculated their dipole moments and interaction strength with metal halide (BiI₃) are selected. The first-principles calculations and related spectroscopy measurements confirm that organic molecules (DGA) with large dipole moments can have strong interactions with perovskite octahedron and improve the carrier transport between the organic and inorganic clusters. Consequently, zero-dimensional single crystal (SC) (DGA)BiI₅∙H₂O is synthesized. The (DGA)BiI₅∙H₂O SCs demonstrate an exceptional carrier mobility-lifetime product of 6.55 × 10^-3cm² V^-1, resulting in the high sensitivity of 5879.4 µCGy_air ^-1cm^-2, featuring a low detection limit (4.7 nGy_air s^-1) and remarkable X-ray irradiation stability even after 100 days of aging at a high electric field (100 V mm^-1). Furthermore, the (DGA)BiI₅∙H₂O SCs for imaging, achieving a notable spatial resolution of 5.5 lp mm^-1 are applied. This investigation establishes a pathway for systematically screening A-site cations to design low-dimensional SCs for high-performance X-ray detection.

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