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.

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Solution‐processed metal‐based halide perovskites have taken a dominant position for perovskite optoelectronics including light emission and X‐ray detection; however, the toxicity of the included heavy metals severely restricts their applications for wearable, lightweight, and transient optoelectronic devices. Here, the authors describe investigations of large (4 × 6 × 2 mm3) 3D metal‐free perovskite MDABCO‐NH4I3 (MDBACO = methyl‐N′‐diazabicyclo[2.2.2]octonium) single crystal and its charge recombination and extraction behavior for light emission and X‐ray detection. Unlike conventional 3D metal‐based perovskites, this lightweight and biocompatible perovskite large crystal is processed from aqueous solution at room temperature, and can achieve both an extremely long carrier lifetime up to ≈1.03 µs and the formation of self‐trapped excited states for luminescence. These features contribute to a photoluminescence quantum yield (PLQY) as high as ≈53% at room temperature and an X‐ray sensitivity up to 1997 ± 80 μC Gy cm−2 at 50 V bias (highest among all metal‐free detectors). The ability to tune the perovskite band gap by modulating the structure under high pressure is also demonstrated, which opens up applications for the crystal as colored emitters. These attributes make it a molecular alternative to metal‐based perovskites for biocompatible and transient optoelectronics.

Minimizing surface defect is vital to further improve power conversion efficiency (PCE) and stability of inorganic perovskite solar cells (PSCs). Herein, we designed a passivator trifluoroacetamidine (TFA) to suppress CsPbI 3À x Br x film defects. The amidine group of TFA can strongly chelate onto the perovskite surface to suppress the iodide vacancy, strengthened by additional hydrogen bonds. Moreover, three fluorine atoms allow strong intermolecular connection via intermolecular hydrogen bonds, thus constructing a robust shield against moisture. The TFA-treated PSCs exhibit remarkably suppressed recombination, yielding the record PCEs of 21.35 % and 17.21 % for 0.09 cm 2 and 1.0 cm 2 device areas, both of which are the highest for allinorganic PSCs so far. The device also achieves a PCE of 39.78 % under indoor illumination, the highest for allinorganic indoor photovoltaic devices. Furthermore, TFA greatly improves device ambient stability by preserving 93 % of the initial PCE after 960 h.

X-ray detectors have broad applications in medicine and industry. Although flexible lead-free perovskite films are competitive because of their lightweight and low toxicity, they are less efficient due to low charge transport. Herein, we report low-toxicity, flexible X-ray detectors based on p-type doped MA 3 Bi 2 I 9 (MA = methylammonium) perovskitefilled membranes (PFMs). Strong coordination between dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) and MA 3 Bi 2 I 9 and the establishment of charge-transfer complex (CPX) improved the conductivity by four times. The flexible X-ray detector achieved a high sensitivity of 2065 μC Gy air À 1 cm À 2 and an ultra-low detection limit of 2.71 nGy air s À 1 , which is among the highest values in all environmentally friendly flexible X-ray detectors. Importantly, the PFMs retained excellent charge transport under mechanical stress. All of those make flexible MA 3 Bi 2 I 9 membranes more competitive as next-generation X-ray detection.

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Next-generation wearable electronics requires mechanical robustness. In addition to the previously reported eco-friendliness, low cost, and light weight of molecular perovskites, flexibility is also a desired merit for their practical use. Here we design a flexible X-ray detector based on a novel molecular perovskite, DABCO-CsBr 3 (DABCO = N-N′-diazabicyclo[2.2.2]octonium), which is the missing link between metal-free molecular perovskites A(NH 4 )X 3 (A = divalent organic ammoniums) and conventional metal halide based ABX 3 (B = divalent metal cations) perovskites. DABCO-CsBr 3 inherits its band nature from A(NH 4 )X 3 , while it exhibits a stronger stopping power. DABCO-CsBr 3 shows potential for high-performance ionizing radiation detectors due to low dark current, low ion migration, and an efficient mobility−lifetime (μτ) product. Finally, a molecular-perovskite-based flexible X-ray detector is demonstrated on the basis of the DABCO-CsBr 3 / poly(vinylidene fluoride) composite, with a sensitivity of 106.7 μC Gy air −1 cm −2 . This work enriches the molecular perovskite family and highlights the promise of molecular perovskites for the next-generation eco-friendly and wearable optoelectronic devices.

In the past three years, metal-free perovskites have garnered significant interest as promising candidates for utilization in next-generation wearable electronics. A variety of different molecular structures for these perovskites have been designed for different applications. However, there is still no systematic understanding that can elucidate the relationship between the structural details and properties of perovskites. This would provide a helpful guide for designing a metal-free perovskite with the desired packing structure and properties. Herein, we summarize recently reported structural and functional insights into metal-free perovskites. The underlying design of the molecular structure and its role in the packing structure and resulting properties are explained. In addition, important factors and challenges in the design of a molecular structure that will be useful for future applications are discussed. This information will help enrich the library of potential structures and future applications of metal-free perovskites, which is believed to be much larger than is currently known.