All‐Inorganic <100>‐Oriented 2D Perovskite with High Equivalent Atomic Number for Efficient X‐Ray Absorption and Detection

Abstract 2D perovskites are widely recognized as promising candidates for X‐ray detection. However, most 2D perovskites are organic‐inorganic hybrids with low equivalent atomic numbers, limiting their X‐ray absorption capacity, while all‐inorganic 2D perovskites typically exhibit <110> orientation, suffering from severe structural distortion and poor carrier transport. To address these challenges, an all‐inorganic sulfur‐halide mixed perovskite, BiCuSCl 2 , is proposed featuring a 2D <100>‐oriented structure, and further enhance its performance via BiOCl heteroepitaxial passivation. The all‐inorganic composition endows BiCuSCl 2 with a high equivalent atomic number across the entire X‐ray energy range, while the <100> orientation minimizes structural distortion. Meanwhile, the epitaxial BiOCl layer effectively passivates grain boundary defects and suppresses ionic migration. As a result, the BiCuSCl 2 wafer detector achieves a significantly improved µτ product (from 1.98 × 10 −3 to 7.76 × 10 −3 cm 2 V −1 ) and high resistivity (1.12 × 10 11 Ω cm). The optimized device exhibits an ultralow dark current density (6.32 × 10 −11 A cm −2 ) and a sensitivity of 219 µC Gy −1 cm −2 under identical bias. The realized specific detectivity (ratio of sensitivity to noise current density, 3.47 × 10 12 µC Gy −1 A −1 ) is among the highest all lead‐free perovskite X‐ray detectors.