Ultrawide‐Bandgap Rb ₂ AgI ₃ Single‐Crystals: Dual‐Functional Platform for Polarization‐Sensitive UVB Photodetection and High‐Sensitivity X‐Ray Imaging

Lead‐free ultrawide‐bandgap semiconductors are crucial for next‐generation optoelectronics, yet achieving materials that concurrently enable polarization‐resolved ultraviolet detection and high‐energy radiation sensing remains a grand challenge. Here, we report the synthesis of high‐quality Rb 2 AgI 3 perovskite single‐crystals via a customized chemical vapor deposition method. This material uniquely integrates an ultrawide bandgap (≈4.03 eV), high‐Z constituents for efficient X‐ray attenuation, and a pronounced anisotropic structure. The resulting photodetector exhibits exceptional dual‐functional performance. For ultraviolet‐B detection, it achieves a high responsivity (0.29 A/W), ultrafast speed (42.4/54 µs). Critically, the detector exhibits unprecedented wavelength‐switchable polarization sensitivity, with a degree of linear polarization (DLP) reversing from −0.68 at 295 nm to 0.72 at 305 nm, a first for all‐inorganic perovskites. This phenomenon is harnessed for proof‐of‐concept demonstrations in dual‐wavelength information encoding and image contrast enhancement using a convolutional neural network (Recognition accuracy >97%). Concurrently, the device serves as a high‐sensitivity X‐ray detector, demonstrating a superior sensitivity of 5.3 × 10 4 µC Gy air −1 cm −2 , an ultralow detection limit of 27 nGy air s −1 , and exceptional operational stability, validated by high‐resolution biological imaging. This work establishes Rb 2 AgI 3 as a versatile, high‐stability material platform, fundamentally redefining the frontiers of multifunctional optoelectronics for next‐generation high‐energy radiation sensing and polarization‐resolved optoelectronics.