X-ray detectors are extensively utilized, including in medical diagnosis, scientific research, and security screening. So far, X-ray detectors have been developed mainly on the basis of metal-based semiconductors. Recently, in addition to traditional Si, Cd(Zn)Te and Ge, crystals based on metal halide perovskites have emerged as a new generation of semiconductors for radiation detection due to their high-Z elements Pb, Bi, and Br. [1-3] However, the requirements for practical wearable materials to be lightweight, economically inexpensive, and environmentally friendly motivate the exploration for nontoxic, low-cost, and simple organic compounds. [4] Lightweight semiconductors based on conjugated molecules or polymers have been demonstrated in a proof-of-principle manner for direct X-ray detection, including 4-hydroxycyanobenzene (4HCB), 1,8-naphthaleneimide (NTI), 1,5-dinitronaphthalene, and rubrene. [5-7] However, the fabrication of large-scale crystals with exceptionally Metal-free halide perovskites, as a specific category of the perovskite family, have recently emerged as novel semiconductors for organic ferroelectrics and promise the wide chemical diversity of the ABX 3 perovskite structure with mechanical flexibility, light weight, and eco-friendly processing. However, after the initial discovery 17 years ago, there has been no experimental information about their charge transport properties and only one brief mention of their optoelectronic properties. Here, growth of large single crystals of metalfree halide perovskite DABCO-NH 4 Br 3 (DABCO = N-N′-diazabicyclo[2.2.2] octonium) is reported together with characterization of their instrinsic optical and electronic properties and demonstration, of metal-free halide perovskite optoelectronics. The results reveal that the crystals have an unusually large semigap of ≈16 eV and a specific band nature with the valence band maximum and the conduction band minimum mainly dominated by the halide and DABCO 2+ , respectively. The unusually large semigap rationalizes extremely long lifetimes approaching the millisecond regime, leading to very high charge diffusion lengths (tens of µm). The crystals also exhibit high X-ray attenuation as well as being lightweight. All these properties translate to high-performance X-ray imaging with sensitivity up to 173 µC Gy air −1 cm −2. This makes metal-free perovskites novel candidates for the next generation of optoelectronics.