Biaxial Ferroelectric Crystals of 2D Metal‐Halide Perovskite Enable Multidirectional Bulk Photovoltaics for Self‐Driven X‐Ray Detection

Abstract 2D metal halide perovskite ferroelectrics, which enable bulk photovoltaic effects (BPVEs), have shown great potentials toward self‐driven X‐ray detection. Despite extensive studies on this ferroelectric class, their BPVE‐active responses are mainly confined in the uniaxial polarization direction, that limit their practical applications. Here, multidirectional BPVEs are successfully achieved in biaxial perovskite ferroelectric crystals, ( n ‐hexylammonium) 2 CsPb 2 Br 7 ( 1 ), allowing for self‐driven X‐ray detection. Compound 1 shows biaxial ferroelectricity with spontaneous polarization ( P s ) of 2.71 and 2.68 µC cm −2 along its b ‐ and c ‐axes directions, respectively. This merit endows notable BPVEs with the open‐circuit voltages of 0.72 and 0.66 V under X‐ray irradiation. Combining with its large carrier mobility‐lifetime product ( µτ = 3.23 × 10 −3 cm 2 V −1 ), single crystals of 1 demonstrate fascinating X‐ray detection performances at zero bias, including high sensitivity (2 × 10 2 µC Gy −1 cm −2 ) and low detection limit (≈51.2 nGy s −1 ). These attributes are solidly observed along both b ‐ and c ‐axes directions, which facilitate the assembling of its crystal‐based devices. This work suggests the potential of ferroelectric materials for future smart optoelectronic device applications.