Lead-free halide double perovskites (HDPs) have sparked broad interest in developing “green” photodetectors; however, self-powered X-ray detectors in this family remain elusive. Here, by exploiting the chirality-induced polar photovoltaic effect in a chiral-polar 2D HDP, (R-MPA)4AgBiI8 (1, R-MPA = R-β-methylphenethylammonium), we successfully realized self-powered X-ray detection. The significant spontaneous electric polarization in 1 gives it a large polar photovoltage of 0.36 V, which drives the separation and transport of X-ray-generated carriers, thus acquiring the capability of self-powered detection. Consequently, X-ray detectors based on high-quality single crystals of 1 exhibit a high sensitivity of 46.3 μC Gy–1 cm–2 and an ultralow detection limit of 85 nGy s–1 at zero bias. The sensitivity can be further increased to 949.6 μC Gy–1 cm–2 at 50 V bias, outperforming all current 2D HDP detectors. Our work is the first to demonstrate self-powered X-ray detection in single-phase lead-free HDPs, enlightening future design of “green” self-powered radiation detectors.

Lead halide perovskites have made great advance in direct X‐ray detection, however the presence of toxic lead and the requirement of high working voltages severely limit their applicability and operational stability. Thus, exploring “green” lead‐free hybrid perovskites capable of detecting X‐rays at zero bias is crucial but remains toughly challenging. Here, utilizing chiral R/S‐1‐phenylpropylamine (R/S‐PPA) cations, a pair of 0D chiral‐polar perovskites, (R/S‐PPA)2BiI5 (1R/1S) are constructed. Their intrinsic spontaneous electric polarization induces a large bulk photovoltage of 0.63 V, which acts as a driving force to separate and transport photogenerated carriers, thus endowing them with the capability of self‐driven detection. Consequently, self‐driven X‐ray detectors with a low detection limit of 270 nGy s−1 are successfully constructed based on high‐quality, inch‐sized single crystals of 1R. Notably, they show suppressed baseline drift under the self‐driven mode, exhibiting superior operational stability. This study realizes self‐driven X‐ray detection in a single‐phase lead‐free hybrid perovskite by exploiting the intrinsic bulk photovoltaic effect, which sheds light on future explorations of lead‐free hybrid perovskites toward “green” self‐driven radiation detectors with high performance.