Tailoring Multi‐Phenyl Ring Cation for Stable Scalable Hybrid Bismuth Iodide Amorphous Film: Enabling Record Sensitivity and High‐Performance X‐Ray Array Imaging

Abstract The 329‐type bismuth (Bi)‐based metal halide (MH) polycrystalline films have potential to be applied in the new generation of X‐ray imaging technology owing to high X‐ray absorption coefficients and excellent detection properties. However, the mutually independent [Bi₂X₉]³⁻ units and numerous grain boundaries in the material lead to low carrier transport and collection capabilities, severe ion migration, large dark currents, and poor response uniformity. Here, a new multi‐phenyl ring methyltriphenylphosphonium (MTP) is designed to optimize the energy band structure. For the first time, the coupling between the A‐site cation and [Bi₂X₉]³⁻ is realized, making it the main contributor to the conduction band minimum (CBM), getting rid of dilemma that carrier transport is confined to [Bi₂X₉]³⁻. Further, the preparation of MTP₃Bi₂I₉ amorphous large‐area wafer is achieved by melt‐quenching; the steric hindrance effect improves stability, increases ion migration energy, and promotes response uniformity (14%). Moreover, the amorphous structure takes advantage of A‐site cation participation in the conductivity, achieving a record sensitivity (7601 µC Gy⁻¹ cm⁻²) and low dark current (≈0.11 nA) in the field of amorphous X‐ray detection, and features low‐temperature large‐area preparation. Ultimately, designing amorphous array imaging devices that exhibit excellent response uniformity and potential imaging capabilities is succeeded here.