Metal halide perovskites and derivatives exhibit a high sensitivity and low detection limit as direct X‐ray detectors. Inorganic 2D bismuth halide perovskites are promising for X‐ray detections, but have not been reported. Moreover, the quantitative relationship between the structural dimensionality of A3B2X9 perovskites and their compositions has never been investigated, and the underlying mechanism is unclear. Here, the key structural descriptors for 2D A3B2X9 perovskite derivatives are reported: i) octahedral factor μ, 0.377 < μ < 0895; ii) tolerance factor t, 0.8 < t < 1.06; iii) (rA‐0.55)/t < 1.48 Å. Accordingly, a new 2D A3B2X9 perovskite derivative, Rb3Bi2I9, with high X‐ray attenuation coefficients is found. The assembled X‐ray detector exhibits a high μτ product of 2.51 × 10−3 cm2 V−1, good sensitivity for 159.7 μC Gyair−1 cm−2, and a record low detection limit of 8.32 nGyair s−1 among all direct and indirect perovskite X‐ray detectors. The device also exhibits good stability toward external bias and continuous gamma ray radiations (480 000 Gy). This work provides crystal structural insights to rationally design 2D perovskites for new types of radiation detectors.

Thanks to its strong X-ray absorption and large carrier diffusion length, perovskites have demonstrated excellent performance for X-ray detection. Combination of perovskite with thin-film transistor (TFT) arrays to construct flat-panel X-ray imager (FPXI) is required for X-ray imaging, yet this is rarely reported. Solution processing of perovskite thick film onto TFT can enable the electronic connection, however the amounts of pinholes inevitably form during the solvent evaporation and result in a porous film with deteriorated performance and stability. Here a novel strategy is raised to achieve high-quality perovskite thick films for TFT integration via soft-pressing and in situ polymerization of multi-functional binder (TMTA). The combined process largely eliminates the pinholes, improves the surface smoothness, passivates grains boundaries, reduces ionic migration, and improves stability. Accordingly, a compact and smooth MAPbI 3 thick film integrating with TFT arrays is prepared for flat-panel X-ray imaging. The largest film (28 × 28 cm 2 ) is obtained with the state-of-theart performance (ratio of sensitivity to noise current: 1.41 × 10 11 µC Gy −1 A −1 ) among polycrystalline films. It is hoped that the work provides guidance for fabricating compact perovskite thick films and push perovskite FPXI one step further for low-dose X-ray imaging.

Organic-inorganic hybrid perovskites have demonstrated excellent performance in converting X-ray photons to electrical signals with high detectivity and sensitivity. Solution-processed large-area perovskite polycrystalline thick film is promising for scalable X-ray flat panel detection imaging. However, ionic migration is severe due to the numerous grain boundaries and pinholes in the thick films, resulting in a large dark current and serious baseline drift. Herein, a new strategy is proposed to suppress the ion migration by inserting 2D Ruddlesden-Popper layer into the 3D perovskite film. The quasi-2D perovskite thick films exhibit lower defects density, suppressed ion migration, and higher thermal stability compared to their 3D counterpart. Based on these advantages, the quasi-2D perovskite-based X-ray detector shows a sensitivity of 10 860 µC Gy air −1 cm −2 with a stable dark current and photocurrent response. Impressively, the extra-low detection limit of 69 nGy air s −1 is the lowest in all those reported polycrystalline film-based detectors. The quasi-2D film is believed to be a very promising choice for digital flat detectors for sensitive radiation detection and low-dose dosimeter.