Metal‐Free Halide Perovskite Single Crystals with Very Long Charge Lifetimes for Efficient X‐ray Imaging

Abstract

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.

Metal‐Free Organic Halide Perovskite: A New Class for Next Optoelectronic Generation Devices

“…Our previous work and other density of states calculations indicate that the main contribution of the CBM comes from DABCO 2+ and the valence band maximum mainly originates from the p orbitals of the halide as described above. [18] We also showed long charge lifetimes/long charge diffusion lengths, comparable to those from typical Pb-based halide perovskite crystals. Many more studies of charge transport are needed for the metal-free perovskites together with the role of traps.…”

“…It was largely this fact that led us to investigate some of these important properties. [18] Refs. [16,18,[27][28][29] all calculated band structures for a variety of ANH 4 X 3 compounds and for a variety of reasons (including to predict/understand ferroelectric, mechanical, and nonlinear optical (NLO) properties).…”

“…A-F) Reproduced with permission. [18] Copyright 2020, Wiley-VCH. G) (Computed static d norm (color bar) and dielectric constant (as a percentage of the LiNbO 3 value, y-axis) as functions of the computed bandgap (x-axis, eV) of the metal-free perovskites.…”

“…We have previously demonstrated a mass loss at ≈250 °C for DABCO-NH 4 Br 3 -based perovskite. [18] Such thermal stability indicates their nonmetallic nature. In ref.…”

“…[1] While halide perovskites have also been known for a long time, it is only since 2009, when a class of organic-inorganic understanding of optically active non-centrosymmetric, ferroelectric, and mechanical properties, [16,17] and, most recently, their intrinsic charge-transport and optoelectronic properties. [18] The challenge of structural versatility for functional application still remains. This paper mainly summarizes research progress in metal-free perovskites and presents some prospects to stimulate interest for optoelectronic and energy-related areas.…”