High-Temperature Stable FAPbBr3 Single Crystals for Sensitive X-ray and Visible Light Detection toward Space

PEAI Surface Treatment for Low Ion Migration and High-Performance FAPbBr3 Single-Crystal X-ray Detectors

Organometal halide perovskite single crystals (SCs) are the most promising candidates for the next generation of radiation detection materials. However, surface defects severely affect their detection performance and limit further applications. Here, we identified the surface defect types of FAPbBr3 SCs and employed phenethylammonium iodide (PEAI) solution to treat the crystal surface and to investigate their effects on ion migration, photoelectric performance, and X-ray detection performance. Our experimental results demonstrated that the surface defects, such as the metallic Pb and Br vacancies, can be effectively passivated by both the PEAI and the two-dimensional (2D) PEA2PbI4 layers. The PEAI layer can elongate the carrier lifetime, lower the trap density, and suppress ion migration in FAPbBr3 SCs. The 2D PEA2PbI4 layer can form a dense and full surface coverage, suppress ion migration, and lower the dark current of the SCs. The X-ray sensitivity of the PEAI-passivated FAPbBr3 SC detectors is 227.93 μCGyair-1 cm-2, which is an order of magnitude higher than that of the pristine FAPbBr3 SC detectors. This work demonstrates that surface treatment plays a critical role in the crystal quality and the X-ray detection performance of SCs.

Characterization of Cl-doped two-dimensional (PEA)2PbBr4 perovskite single crystals for fast neutron and gamma ray detection

In this paper, a high-quality Cl-doped two-dimensional halide perovskite (PEA)2Pb(Br0.95Cl0.05)4 crystal was prepared using a seed-induced volatile solvent method. On optimizing the Cl- doping concentration, we found that 5% Cl-doping results in (PEA)2PbBr4 with the highest optical and photon yield. Based on the Cl-doped (PEA)2PbBr4 single crystal, the response characterization of the (PEA)2Pb(Br0.95Cl0.05)4 crystal in the mixed field of neutrons and gamma rays (n/γ) has been verified. Using the time-of-flight method and the linear relationship between integral charge and neutron yield, it was proved that (PEA)2Pb(Br0.95Cl0.05)4 crystal can be used for n/γ screening. The time difference between the fast neutron released by a single nuclear reaction and the γ photon arriving at the detector was 130 ns, and the arrival time of the γ photon is earlier than that of the fast neutron. This work has a broad application prospect in the study of nuclear reaction kinetics, the monitoring of the neutron yield of fusion devices and the total energy released by nuclear reactions.

Lead-Free Bismuth-Based Perovskite X-ray Detector with High Sensitivity and Low Detection Limit

Bismuth-based halide perovskites have shown great potential for direct X-ray detection, attributable to their nontoxicity and advantages in detection sensitivity and spatial resolution. However, the practical application of such materials still faces the critical challenge of combining both high sensitivity and low detection limits. Here, we report a new type of zero-dimensional (0D) perovskite (HIS)BiI5 (1, where HIS2+ = histamine) with high sensitivity and a low detection limit. Structurally, the strong N-H···I hydrogen bonds between HIS2+ cations and inorganic frameworks enhance the rigidity of the structure and diminish the intermolecular distance between adjacent inorganic [Bi2I10]4- dimers. By virtue of such structural merits, single crystal 1 exhibits excellent physical properties perpendicular to both the (001) and (010) faces. Perpendicular to the (010) face, 1 exhibited a high electrical resistivity (2.31 × 1011 Ω cm) and a large carrier mobility-lifetime product (μτ) (2.81 × 10-4 cm2 V-1) under X-ray illumination. Benefiting from these superior physical properties, it demonstrates an excellent X-ray detection capability with a sensitivity of approximately 103 μC Gyair-1 cm-2 and a detection limit of 36 nGyair s-1 in both directions perpendicular to the (001) and (010) crystal faces. These results provide a promising candidate material for the development of new, lead-free, high-performance X-ray detectors.

Interlamellar-Spacing Engineering of Stable and Toxicity-Reduced 2D Perovskite Single Crystal for High-Resolution X-ray Imaging

Two-dimensional (2D) lead halide perovskites are excellent candidates for X-ray detection due to their high resistivity, high ion migration barrier, and large X-ray absorption coefficients. However, the high toxicity and long interlamellar distance of the 2D perovskites limit their wide application in high sensitivity X-ray detection. Herein, we demonstrate stable and toxicity-reduced 2D perovskite single crystals (SCs) realized by interlamellar-spacing engineering via a distortion self-balancing strategy. The engineered low-toxicity 2D SC detectors achieve high stability, large mobility-lifetime product, and therefore high-performance X-ray detection. Specifically, the detectors exhibit a record high sensitivity of 13488 μC Gy1- cm-2, a low detection limit of 8.23 nGy s-1, as well as a high spatial resolution of 8.56 lp mm-1 in X-ray imaging, all of which are far better than those of the high-toxicity 2D lead-based perovskite detectors. These advances provide a new technical solution for the low-cost fabrication of low-toxicity, scalable X-ray detectors.

van der Waals Integration of Large-Area Monocrystalline 3D Perovskite Thin Films on Arbitrary Semiconductor Substrates for Heterojunctions

Perovskite monocrystalline films are regarded as desirable candidates for the integration of high-performance optoelectronics due to their unique photophysical properties. However, the heterogeneous integration of a perovskite monocrystalline film with other semiconductors is fundamentally limited by the lattice mismatch, which hinders direct epitaxy. Herein, the van der Waals (vdW) integration strategy for 3D perovskites is developed, where perovskite monocrystalline films are epitaxially grown on the mother substrate, followed by its peeling off and transferring to arbitrary semiconductors, forming monocrystalline heterojunctions. The as-achieved CsPbBr3-Nb-doped SrTiO3 (Nb:STO) vdW p-n heterojunction exhibited comparable performance to their directly epitaxial counterpart, demonstrating the feasibility of vdW integration for 3D perovskites. Furthermore, the vdW integration could be extended to silicon substrates, rendering the CsPbBr3-n-Si and CsPbCl3-p-Si p-n heterojunction with apparent rectification behaviors and photoresponse. The vdW integration significantly enriches the selections of semiconductors hybridizing with perovskites and provides opportunities for monocrystalline perovskite optoelectronics with complex configurations and multiple functionalities.

Metal-Halide Perovskite Submicrometer-Thick Films for Ultra-Stable Self-Powered Direct X-Ray Detectors

Metal-halide perovskites are revolutionizing the world of X-ray detectors, due to the development of sensitive, fast, and cost-effective devices. Self-powered operation, ensuring portability and low power consumption, has also been recently demonstrated in both bulk materials and thin films. However, the signal stability and repeatability under continuous X-ray exposure has only been tested up to a few hours, often reporting degradation of the detection performance. Here it is shown that self-powered direct X-ray detectors, fabricated starting from a FAPbBr3 submicrometer-thick film deposition onto a mesoporous TiO2 scaffold, can withstand a 26-day uninterrupted X-ray exposure with negligible signal loss, demonstrating ultra-high operational stability and excellent repeatability. No structural modification is observed after irradiation with a total ionizing dose of almost 200 Gy, revealing an unexpectedly high radiation hardness for a metal-halide perovskite thin film. In addition, trap-assisted photoconductive gain enabled the device to achieve a record bulk sensitivity of 7.28 C Gy-1 cm-3 at 0 V, an unprecedented value in the field of thin-film-based photoconductors and photodiodes for "hard" X-rays. Finally, prototypal validation under the X-ray beam produced by a medical linear accelerator for cancer treatment is also introduced.

Surface Engineering of Perovskite Single Crystals by Atomic Layer Deposited Tin Oxide for Optical Communication

Perovskite single crystals with excellent physical properties have broad prospects in the field of optoelectronics. However, the presence of dangling bonds, surface dislocations, and chemical impurities results in high surface defect density and sensitivity to humidity. Unfortunately, there are relatively few surface engineering strategies for single perovskite single crystals. We present a strategy utilizing atomic layer deposited SnOx to passivate surface defects in perovskite single crystals. The photodetector prepared based on the modified FAPbBr3 single crystals exhibits a low dark current of 1.89 × 10-9 A at a 5 V bias, close to 4 times lower with respect to the pristine device, a high detectivity of 2.3 × 1010 jones, and a fast response time of 27 μs. Moreover, the photodetectors feature long-term operational stability because the presence of a dense SnOx capping layer hinders the ingress of moisture and diffusion of ions. We further demonstrate the promise of our perovskite single crystal detectors for real-time subaqueous optical communication.

Surface Microstructure Engineering in MAPbBr3 Microsheets for Performance-Enhanced Photodetectors

Metal halide-perovskite-based photodetectors have recently emerged as a class of promising optoelectronic devices in various fields. Meanwhile, nano/microstructuring perovskite-based photodetectors are a facile integration with complementary metal-oxide semiconductors for miniaturized imaging systems. However, there are still challenges to be overcome in reducing the losses caused by light reflection on the surface of microstructural perovskites. In this work, surface microstructure engineering is employed in MAPbBr3 microsheets for reducing light reflection and improving light absorption, resulting in high-performance perovskite photodetectors. MAPbBr3 microsheets, which possess different surface morphologies of flat, upright hemisphere arrays and inverted hemisphere arrays (IHAs), are fabricated by a simple microstructure template-assisted space confinement process. The light absorption capacity of IHA MAPbBr3 is significantly higher than that of the other two structures. Hence, IHA photodetectors with excellent figures of merit, including low dark current, decent responsivity, and fast speed, are achieved. Furthermore, the noise of the IHA photodetectors is only ∼10-13 A/H z , which results in the superior sensitivity for weak light detection with a specific detectivity up to 1011 Jones. Our results demonstrate that surface engineering is a simple, low-cost, yet effective approach to improve the performance of nano-/micro-optoelectronic devices.

Dimensional and Optoelectronic Tuning of Lead-free Perovskite Cs3 Bi2 I9-n Brn Single Crystals for Enhanced Hard X-ray Detection

Inorganic Bi-based perovskites have shown great potential in X-ray detection for their large absorption to X-rays, diverse low-dimensional structures, and eco-friendliness without toxic metals. However, they suffer from poor carrier transport properties compared to Pb-based perovskites. Here, we propose a mixed-halogen strategy to tune the structural dimensions and optoelectronic properties of Cs3 Bi2 I9-n Brn (0≤n≤9). Ten centimeter-sized single crystals are successfully grown by the Bridgman technique. Upon doping bromine to zero-dimensional Cs3 Bi2 I9 , the crystal transforms into a two-dimensional structure as the bromine content reaches Cs3 Bi2 I8 Br. Correspondingly, the optoelectronic properties are adjusted. Among these crystals, Cs3 Bi2 I8 Br exhibits negligible ion migration, moderate resistivity, and the best carrier transport capability. The sensitivities in 100 keV hard X-ray detection are 1.33×104 and 1.74×104  μC Gyair -1  cm-2 at room temperature and 75 °C, respectively, which are the highest among all reported bismuth perovskites. Moreover, the lowest detection limit of 28.6 nGyair  s-1 and ultralow dark current drift of 9.12×10-9  nA cm-1  s-1  V-1 are obtained owing to the high ionic activation energy. Our work demonstrates that Br incorporation is an effective strategy to enhance the X-ray detection performance by tuning the dimensional and optoelectronic properties.

Ultrasensitive and Robust CsPbBr3 Single-Crystal X-ray Detectors Based on Interface Engineering

Halide lead perovskites have shown great development in recent years for ionizing radiation detection. However, the bias-induced interfacial electrochemical reaction between the perovskite and electrode severely deteriorates detector performance. We report that BCP strongly interacts with Al and constructs a stable Al-BCP chelating interface, resulting in the suppression of a detrimental electrochemical reaction. The fabricated Au/Al/BCP/C60/CsPbBr3/Au detector shows a low dark current of 3 nA with a stable baseline at an extremely high bias of 100 V (∼100 V mm-1). The superior high-bias stability enables a high sensitivity of 7.3 × 104 μC Gyair-1 cm-2 at 100 V. Meanwhile, a low detection limit of 15 nGyair s-1 at 40 V is achieved due to the reduced noise. The outstanding performance of our device exceeds that of most advanced detectors based on CsPbBr3 single crystals. Besides, X-ray imaging with 1 mm spatial resolution is well demonstrated at a low dose rate of 200 nGyair s-1. The interfacial chelating strategy overcomes the technical limitation of bias-induced instability of perovskite radiation detectors and can be anticipated to operate under an extremely high electrical field.

An interfacial toughening strategy for high stability 2D/3D perovskite X-ray detectors with a carbon nanotube thin film electrode

Single-crystalline metal halide perovskite materials hold great promise for developing next-generation low-dose X-ray detection. To bring this new technology into reality, it is important to improve the durability of perovksite detectors by suppressing the well-known corrosion and ion diffusion problems at the perovskite/electrode interface. For imaging application, it is also imperative to develop new assembling approaches to realise non-planar interconnection between thick perovskite crystals and thin-film transistor (TFT) backplanes. Herein, a flexible and mechanically robust carbon nanotube (CNT) film was proposed to replace noble metal electrodes. The proposed CNT film, whose binder contains a carboxyl group, can form solid contact with a phenethylamine-based two-dimensional (2D) perovskite via amide coupling, thus toughening the perovskite-electrode interface. The resulting CNT/2D-3D perovskite detector shows an applaudable low dark current, high sensitivity, a low dose detection limit and excellent stability, retaining 98% of its initial sensitivity after storage for three months. Moreover, the flexible CNT films are also beneficial for making non-planar interconnection between thick perovskite crystals and TFT backplanes. The proposed flexible CNT thin film electrode thus provides a facile route towards realising a low-dose, high-resolution and highly stable perovskite X-ray detector.

Degradation and Self-Healing of FAPbBr3 Perovskite under Soft-X-Ray Irradiation

The extensive use of perovskites as light absorbers calls for a deeper understanding of the interaction of these materials with light. Here, the evolution of the chemical and optoelectronic properties of formamidinium lead tri-bromide (FAPbBr3 ) films is tracked under the soft X-ray beam of a high-brilliance synchrotron source by photoemission spectroscopy and micro-photoluminescence. Two contrasting processes are at play during the irradiation. The degradation of the material manifests with the formation of Pb0 metallic clusters, loss of gaseous Br2 , decrease and shift of the photoluminescence emission. The recovery of the photoluminescence signal for prolonged beam exposure times is ascribed to self-healing of FAPbBr3 , thanks to the re-oxidation of Pb0 and migration of FA+ and Br- ions. This scenario is validated on FAPbBr3 films treated by Ar+ ion sputtering. The degradation/self-healing effect, which is previously reported for irradiation up to the ultraviolet regime, has the potential of extending the lifetime of X-ray detectors based on perovskites.

X-Site Substituted 2D Cs2 Pb(SCN)2 Br2 Perovskites for X-Ray Detection

2D Ruddlesden-Popper (RP) perovskites have been intensively investigated due to their superior stability and outstanding optoelectrical properties. However, investigations on 2D RP perovskites are mainly focused on A-site substituted perovskites and few reports are on X-site substituted perovskites especially in X-ray detection field. Here, X-site substituted 2D RP perovskite Cs2 Pb(SCN)2 Br2 polycrystalline wafers are prepared and systematically studied for X-ray detection. The obtained wafers show a large resistivity of 2.0 × 1010 Ω cm, a high ion activation energy of 0.75 eV, a small current drift of 2.39 × 10-6 nA cm-1 s-1 V-1 , and charge carrier mobility-lifetime product under X-ray as high as 1.29 × 10-4 cm2 V-1 . These merits enable Cs2 Pb(SCN)2 Br2 wafer detectors with a sensitivity of 216.3 µC Gyair -1 cm-2 , a limit of detection of 42.4 nGyair s-1 , and good imaging ability with high spatial resolution of 1.08 lp mm-1 . In addition, Cs2 Pb(SCN)2 Br2 wafer detectors demonstrate excellent operational stability under high working field up to 2100 V cm-1 after continuous X-ray irradiation with a total dose of 45.2 Gyair . The promising features such as short octahedral spacing and weak ion migration will open up a new perspective and opportunity for SCN-based 2D perovskites in X-ray detection.

Interlayer-Spacing Engineering of Lead-Free Perovskite Single Crystal for High-Performance X-Ray Imaging

Lead-free A₃ Bi₂ I₉ -type perovskites are demonstrated as a class of promising semiconductors for high-performance X-ray detection due to their high bulk resistivity and strong X-ray absorption, as well as reduced ion migration. However, due to their long interlamellar distance along their c-axis, their limited carrier transport along the vertical direction is a bottleneck for their detection sensitivity. Herein, a new A-site cation of aminoguanidinium (AG) with all-NH₂ terminals is designed to shorten the interlayer spacing by forming more and stronger NH···I hydrogen bonds. The prepared large AG₃ Bi₂ I₉ single crystals (SCs) render shorter interlamellar distance for a larger mobility-lifetime product of 7.94 × 10^-3  cm²  V^-1 , which is three times higher than the value measured on the best MA₃ Bi₂ I₉ SC (2.87 × 10^-3  cm²  V^-1 ). Therefore, the X-ray detectors fabricated on the AG₃ Bi₂ I₉ SC exhibit high sensitivity of 5791 uC Gy^-1  cm^-2 , a low detection limit of 2.6 nGy s^-1, and a short response time of 690 µs, all of which are far better than those of the state-of-the-art MA₃ Bi₂ I₉ SC detectors. The combination of high sensitivity and high stability enables astonishingly high spatial resolution (8.7 lp mm^-1 ) X-ray imaging. This work will facilitate the development of low-cost and high-performance lead-free X-ray detectors.

Improving the Performance and High-Field Stability of FAPbBr3 Single Crystals in X-Ray Detection with Chenodeoxycholic Acid Additive

Organometal halide perovskite single crystals are one of the most promising radiation detection materials due to their unique advantages of high absorption coefficient, long carrier diffusion length, and low defect density. However, the severe ion migration in perovskites deteriorates the X-ray detection performance under longtime and high-field operating conditions. This work reports an effective additive of chenodeoxycholic acid (CDCA), which can suppress the ion migration and improve the performance and the operational stability of FAPbBr₃ single crystals (SCs) in X-ray detection significantl. The CDCA molecules in precursors effectively suppress the decomposition of FA ions, resulting in a better crystal orientation and stoichiometry. The trace amounts of CDCA residues in FAPbBr₃ SCs improve the thermal stability and effectively suppress the ion migration. The resulting detector shows an impressive X-ray sensitivity up to 21 386.88 µC Gy_air ^-1 cm^-2 under -500 V and a detection limit of 15.23 nGy_air s^-1 . The response current of the detector at 225 V cm^-1 field is barely changed under the 7200 s irradiation with a dose rate of 1.949 mGy_air s^-1 . This work provides insights for the additive selection and improving the operational stability of perovskite single crystals for commercial applications.

Ultrahigh-Flux X-ray Detection by a Solution-Grown Perovskite CsPbBr3 Single-Crystal Semiconductor Detector

Solution-processed perovskites are promising for hard X-ray and gamma-ray detection, but there are limited reports on their performance under extremely intense X-rays. Here, a solution-grown all-inorganic perovskite CsPbBr₃ single-crystal semiconductor detector capable of operating at ultrahigh X-ray flux of 10¹⁰ photons s^-1 mm^-2 is reported. High-quality solution-grown CsPbBr₃ single crystals are fabricated into detectors with a Schottky diode structure of eutectic gallium indium/CsPbBr₃ /Au. A high reverse-bias voltage of 1000 V (435 V mm^{- 1} ) can be applied with a small and stable dark current of ≈60-70 nA (≈9-10 nA mm^{- 2} ), which enables a high sensitivity larger than 10 000 µC Gy_air ^-1 cm^{- 2} and a simultaneous low detection limit of 22 nGy_air s^{- 1} . The CsPbBr₃ semiconductor detector shows an excellent photocurrent linearity and reproducibility under 58.61 keV synchrotron X-rays with flux from 10⁶ to 10¹⁰ photons s^{- 1} mm^{- 2} . Defect characterization by thermally stimulated current spectroscopy shows a similar low defect density of a synchrotron X-ray and a lab X-ray irradiated device. Solid-state nuclear magnetic resonance spectroscopy suggests that the excellent performance of the solution-grown CsPbBr₃ single crystal may be associated with its good short-range order, comparable to the spectrometer-grade melt-grown CsPbBr₃ .

Recent Development of Halide Perovskite Materials and Devices for Ionizing Radiation Detection

Ionizing radiation such as X-rays and γ-rays has been extensively studied and used in various fields such as medical imaging, radiographic nondestructive testing, nuclear defense, homeland security, and scientific research. Therefore, the detection of such high-energy radiation with high-sensitivity and low-cost-based materials and devices is highly important and desirable. Halide perovskites have emerged as promising candidates for radiation detection due to the large light absorption coefficient, large resistivity, low leakage current, high mobility, and simplicity in synthesis and processing as compared with commercial silicon (Si) and amorphous selenium (a-Se). In this review, we provide an extensive overview of current progress in terms of materials development and corresponding device architectures for radiation detection. We discuss the properties of a plethora of reported compounds involving organic-inorganic hybrid, all-inorganic, all-organic perovskite and antiperovskite structures, as well as the continuous breakthroughs in device architectures, performance, and environmental stability. We focus on the critical advancements of the field in the past few years and we provide valuable insight for the development of next-generation materials and devices for radiation detection and imaging applications.

A-Site Cation Engineering of Ruddlesden-Popper Perovskites for Stable, Sensitive, and Portable Direct Conversion X-ray Imaging Detectors

Perovskite flat-panel X-ray detectors are promising products for realizing low-dose medical imaging, a nondestructive test, and security inspection. However, the perovskite X-ray imager still faces intractable problems such as severe baseline drift, a low signal-to-noise ratio, and rapid performance degradation, which were involved by the notorious intrinsic ion migration of the perovskite functional layer. In this work, sensitive, stable, and portable pixel quasi-two-dimensional (2D) Ruddlesden-Popper (RP) perovskite X-ray imagers were obtained by an advanced solvent-free laminated fabrication approach. A-Site cation engineering of RP perovskites provides a hint for solving the trade-off between stability and detection performance, resulting in a stable pixel X-ray imager that shows a sensitivity of ∼7000 μC Gy_air^-1 cm^-2, a detection limit of 7.8 nGy_air s^-1, and good 2D multipixel X-ray imaging. This work demonstrates both a high-performance, stable X-ray imager and its robust fabrication, paving the way for adopting a RP perovskite imager as novel flat-panel X-ray detectors.

Organic-Inorganic Lead Halide Perovskite Single Crystal: From Synthesis to Applications

Organic-inorganic lead halide perovskite is widely used in the photoelectric field due to its excellent photoelectric characteristics. Among them, perovskite single crystals have attracted much attention due to its lower trap density and better carrier transport capacity than their corresponding polycrystalline materials. Owing to these characteristics, perovskite single crystals have been widely used in solar cells, photodetectors, light-emitting diode (LED), and so on, which have greater potential than polycrystals in a series of optoelectronic applications. However, the fabrication of single-crystal devices is limited by size, thickness, and interface problems, which makes the development of single-crystal devices inferior to polycrystalline devices, which also limits their future development. Here, several representative optoelectronic applications of perovskite single crystals are introduced, and some existing problems and challenges are discussed. Finally, we outlook the growth mechanism of single crystals and further the prospects of perovskite single crystals in the further field of microelectronics.

Vapor-Deposited Amino Coupling of Hybrid Perovskite Single Crystals and Silicon Wafers toward Highly Efficient Multiwavelength Photodetection

The complementary integration of perovskite single crystals (PSCs) and silicon-based circuitry provides a feasible way to combine their superiority toward efficient multiwavelength photodetection and imaging readout; however, it suffers from distinct lattice mismatch as well as the ambiguous coupling interface effect. Herein, we develop a vacuum-assisted vapor deposition strategy to realize an ultrauniform aminosiloxane interface-modified silicon wafer, which enables the monolithic epitaxial growth of PSCs with the highest mechanical coupling strength up to 340,000 N m^-2 achieved so far. According to the molecular coupling engineering development with different aminosiloxanes, we achieve a highly efficient multiwavelength-responsive integrated photodetector, possessing specific photodetectivity values of 4.36 × 10¹² jones and 4.55 × 10¹¹ jones within the visible and NIR regions, respectively, as well as the lowest X-ray detection limit of 42.6 nGy_air s^-1. Moreover, a particularly wide -3dB cut-off frequency of 6350 Hz as well as a 120 dB linear dynamic range (LDR) also endows the integrated device with excellent dynamic photodetection capability. This work provides an efficacious approach in the integration technology for PSC-based optoelectronic applications.

Improved Crystallization Quality of FAPbBr3 Single Crystals by a Seeded Solution Method

Solution-grown hybrid perovskite, FAPbBr₃, has attracted great attentions recently due to its inspiring optoelectronic properties and low-cost preparation method. However, challenges of solution growth for FAPbBr₃ bulk crystals remain yet, such as uncontrollable crystalline morphologies, irregular shapes, and limited crystal sizes, which are attributed to the dense crystallization nucleus. In this work, we investigate the effects of growth conditions and seed behaviors on the crystallization quality and the yield of FAPbBr₃ single crystals. First, the spontaneous nucleation is tailored by optimizing the precursor concentration and heating rate. Furthermore, the seeded crystals are introduced to solve the issues related to the morphology and the yield of single crystals. Based on the above-mentioned investigations, an optimized growth method, a seeded solution method, under a heating rate of 0.1 °C/h is proposed, and centimeter-scale FAPbBr₃ single crystals with a very narrow FWHM of high-resolution X-ray diffraction rocking curves and a high yield of ∼100% of single crystals are obtained. The resulting FAPbBr₃ single crystal exhibits a bulk resistivity of 3.42 × 10⁹ Ω·cm and a superior I_ON/I_OFF ratio over 10⁴ under 405 nm light at a bias of 10 V. Finally, the pulse height spectra with an energy resolution of ∼21.4% are also achieved based on an AZO/FAPbBr₃/Au detector, illuminated using an uncollimated ²⁴¹Am@5.49 MeV α-particle source at room temperature. This modified seeded solution method shows great potential in preparing high-quality and high-yield perovskite single crystals.

Regulating interface Schottky barriers toward a high-performance self-powered imaging photodetector

Two-dimensional (2D) layered organic-inorganic hybrid perovskites have attracted wide attention in high-performance optoelectronic applications due to their good stability and excellent optoelectronic properties. Here, a high-performance self-powered photodetector is realized based on an asymmetrical metal-semiconductor-metal (MSM) device structure (Pt-(PEA)2PbI4 SC-Ag), which introduces a strong built-in electric field by regulating interface Schottky barriers. Benefitting from excellent built-in electrical potential, the photodetector shows attractive photovoltaic properties without any power supply, including high photo-responsivity (114.07 mA W-1), fast response time (1.2 μs/582 μs) and high detectivity (4.56 × 1012 Jones). Furthermore, it exhibits high-fidelity imaging capability at zero bias voltage. In addition, the photodetectors show excellent stability by maintaining 99.4% of the initial responsivity in air after 84 days. This work enables a significant advance in perovskite SC photodetectors for developing stable and high-performance devices.

Regulating the Intermolecular Hydrogen Bond to Realize Directional Dimension Reduction of Lead Iodide Perovskite toward Low-Dimensional Photovoltaics

A low-dimensional organic amine lead halide perovskite is an attractive semiconductor material that has potential application prospects in photovoltaics, light-emitting diodes, detectors, X-ray imaging, and other fields. It has been reported that the photoelectric properties of low-dimensional perovskite can be controlled by adjusting the chain length of organic ammonium, the ratio of precursor components, and van der Waals interaction between amine molecules. Herein, we report the successful synthesis of low-dimensional perovskite (PdEA)PbI₄ (PdEA = piperidine ethylammonium) and (MlEA)PbI₄ (MlEA = morpholine ethylammonium) single crystals by regulating the intermolecular hydrogen bond of organic ammonium ligands. The two-dimensional (2D) layered structure (PdEA)PbI₄ single crystal with a fluorescence reflection peak at 563 nm was produced by the reaction of PdEA with PbO in a concentrated hydroiodic acid aqueous solution. Differently, the (MlEA)PbI₄ single crystal prepared by replacing MlEA with PdEA presents a one-dimensional (1D) rod structure, and its fluorescence reflection peak is located at 531 nm. The optical bandgaps of (PdEA)PbI₄ and (MlEA)PbI₄ perovskite films were about 2.16 and 2.33 eV, respectively. Low-dimensional perovskite solar cells with 2D (PdEA)PbI₄ and 1D (MlEA)PbI₄ of perovskite films yielded efficiencies of 1.18 and 1.52%, respectively.

Achieving dual-color imaging by dual-band perovskite photodetectors coupled with algorithms

Dual-color or multispectral imaging based on conventional optical imaging techniques is suffering from the bottleneck of complex manufacturing and time consumption caused by multiple imaging. Herein, we develop a dual-color computational imaging system combining a vertically stacked dual-channel dual-band perovskite photodetectors (PDs) and the advanced Fourier imaging algorithm. Significantly, our imaging system bypasses the complex fabrication process of high-density dual-band PD arrays and is enabled to capture two high-resolution spectral images at the same time. Based on the experiments and simulations, we confirm that the spectral overlap of dual-band PDs will cause detrimental effect for color identification, and optimizing the bandwidth spectrum is beneficial for achieving much better spectral imaging. Moreover, we have further improved the imaging quality by increasing the sampling rate and suppressing current fluctuations. We suggest that these results provide important interesting insights for the development of advanced imaging systems, including IR imaging, THz imaging, multispectral/hyperspectral imaging, etc.

Electrode Spacing as a Determinant of the Output Performance of Planar-Type Photodetectors Based on Methylammonium Lead Bromide Perovskite Single Crystals

Methylammonium lead bromide is a very perspective hybrid semiconductor material, suitable for high-sensitive, filter-free photodetection of electromagnetic radiation. Herein, we studied the effect of electrode spacing on the output performance and stability of planar-type photodetectors based on high-quality MAPbBr₃ single crystals. Such crystals, as large as 4.5×4.5×1.2 mm³ were synthesized via the inverse temperature crystallization method and were further used for the fabrication of planar Au/MAPbBr₃/Au photodetectors with variable electrode spacing (in the range between 125 and 25 μm). We report that the electrode spacing has a profound impact on photocurrent densities and key detector parameters (responsivity R, external quantum efficiency EQE, and specific detectivity D*). In the studied fivefold electrode spacing, the photocurrent density increased over 4 times, with decreasing active area of the devices. This effect is attributed to intrinsic photocurrent amplification. Based on the transient photocurrent measurements and calculated key parameters, we determined the device sample with the best output performance. The champion sample with an electrode spacing of 50 μm exhibited great detection ability, especially for a low light intensity of 200 nWcm^-2, for which we calculated the R of 19.55 A W^-1, EQE of 4253%, and D* of 3.42 × 10¹² Jones (cm Hz^1/2 W^-1). Moreover, the functional stability of this device showed a minimal reduction of photodetection ability after 2000 cycles, which makes it very promising for the next generation of optoelectronic devices.

Searching for High-Quality Halide Perovskite Single Crystals toward X-ray Detection

Metal halide perovskite materials, which combine outstanding physical properties, large absorption coefficient, tailored composition, and low-cost solution-processing, have aroused wide attention for use in various optoelectronic devices. Recently, perovskite single crystals have been rapidly outpacing traditional semiconductor materials in the field of radiation detection. As a prerequisite, achieving high-quality single crystals under controllable solution-phase growth must be tackled to fulfill their full potential as a new paradigm in this stagnated field. This Perspective summarizes the advances in X-ray detectors based on lead halide perovskite single crystals, presenting a comprehensive picture of the relationship among composition engineering, synthesis, and device properties. Additionally, we share our thoughts on several outstanding challenges of perovskite single crystals as promising X-ray detectors and propose possible approaches to the unresolved issues. We anticipate that this Perspective can open up new opportunities to improve their optoelectronic properties, which confers fascinating photonics applications with above and beyond state-of-the-art performance.

Physical Properties of Candidate X-Ray Detector Material Rb4Ag2BiBr9

Recently, metal halide perovskites have emerged as promising semiconductor candidates for sensitive X-ray photon detection due to their suitable bandgap energies, excellent charge transport properties, and low material cost afforded by their low-temperature solution-processing preparation. Here, we report an improved methodology for single crystal (SC) growth, thermal and electrical properties of a two-dimensional (2D) layered halide material Rb₄Ag₂BiBr₉, which has been identified as a potential candidate for X-ray radiation detection applications. The measured heat capacity for Rb₄Ag₂BiBr₉ implies that there are no structural phase transitions upon cooling. Temperature dependence of thermal transport measurements further suggest remarkably low thermal conductivities of Rb₄Ag₂BiBr₉ that are comparable to the lowest reported in literature. The bulk crystal resistivity is determined to be 2.59×10⁹ Ω·cm from the current-voltage (I-V) curve. Density of trap states are estimated to be ~10¹⁰ cm^-3 using the space-charge-limited-current (SCLC) measurements. The fabricated Rb₄Ag₂BiBr₉-based X-ray detector shows good operational stability with no apparent current drift, which may be ascribed to the 2D crystal structure of Rb₄Ag₂BiBr₉. Finally, by varying the X-ray tube current to change the corresponding dose rate, the Rb₄Ag₂BiBr₉ X-ray detector sensitivity is determined to be 222.03 uCGy^-1cm^-2 (at an electric field of E = 24 V/mm).

Nucleation Engineering in Sprayed MA3Bi2I9 Films for Direct-Conversion X-ray Detectors

Metal halide perovskite and its derivatives show great promise in X-ray detection. However, large-scale fabrication of high-quality thick perovskite films is still full of challenges due to the complicated crystal nucleation process that always introduces lots of cracks or pinholes in the final perovskite film. Here, a MA₃Bi₂I₉ film was fabricated by the cost-effective, scalable spraying process, and MACl was used as an additive to effectively tune the crystallization process. As a result, a dense MA₃Bi₂I₉ film constituted by large grains was obtained, which has a high carrier mobility of ∼1 cm² V^-1 s^-1 and a large activation energy (E_a) for ion migration of 0.91 eV. Thanks to the outstanding optoelectronic characteristics, X-ray detectors with a configuration of ITO/MA₃Bi₂I₉/Au show a sensitivity of 35 μC Gy_air^-1 cm^-2 and a limit of detection (LoD) of 0.14 μGy_airs^-1, which is outstanding compared with commercial α-Se detectors.

Inch-Sized Thin Metal Halide Perovskite Single-Crystal Wafers for Sensitive X-Ray Detection

Metal halide perovskite single crystals are a promising candidate for X-ray detection due to their large atomic number and high carrier mobility and lifetime. However, it is still challenging to grow large-area and thin single crystals directly onto substrates to meet real-world applications. In this work, millimeter-thick and inch-sized methylammonium lead tribromide (MAPbBr3) single-crystal wafers are grown directly on indium tin oxide (ITO) substrates through controlling the distance between solution surface and substrates. The single-crystal wafers are polished and treated with O3 to achieve smooth surface, lower trap density, and better electrical properties. X-ray detectors with a high sensitivity of 632 µC Gyair -1 cm-2 under -5 V and 525 µC Gyair -1 cm-2 under -1 V bias can be achieved. This work provides an effective way to fabricate substrate-integrated, large-area, and thickness-controlled perovskite single-crystal X-ray detectors, which is instructive for developing imaging application based on perovskite single crystals.

A lead-free halide hybrid perovskite (TMHD)BiCl5 for ultraviolet photodetection

Lead halide hybrid perovskites with a wide bandgap (e.g., CH3NH3PbCl3) have gained tremendous attention in the field of ultraviolet (UV) photodetection due to their brilliant optoelectronic activity.

Highly Sensitive Ultraviolet and Visible Wavelength Sensor Composed of Two Identical Perovskite Nanofilm Photodetectors

This work reports the design of a wavelength sensor composed of two identical perovskite (FA_0.85 Cs_0.15 PbI₃ ) photodetectors (PDs) that are capable of discriminating incident wavelength in a quantitative way. Due to strong wavelength-dependent absorption coefficient, the penetration depth of the photons in the FA_0.85 Cs_0.15 PbI₃ nanofilms increases with the increasing wavelength, leading to a gradual decrease of photo-generated current for PD1, but an increase of photocurrent in PD2, according to the theoretical simulation of Technology Computer Aided Design. This special evolution of photo-generated current as a function of wavelength facilitates the quantitative determination of the wavelength since the current ratio of both PDs monotonously decreases with the increase of wavelength from 265 to 810 nm. The average absolute error and the average relative error are estimated to be 7.6 nm and 1.68%, respectively, which are much better than other semiconductors materials-based wavelength sensors previously reported. It is believed that the present perovskite film-based wavelength sensor will have potential application in the future color/spectrum optoelectronic devices.

Stable Perovskite Solar Cells with Bulk-Mixed Electron Transport Layer by Multifunctional Defect Passivation

Multifarious electron transport layers (ETLs), especially fullerene derivatives, have been applied in organic-inorganic hybrid perovskite (OIHP) devices owing to their superior optoelectronic properties. However, a PCBM Lewis acid molecule can only passivate the iodine-rich defect sites, which cannot solve the problem of uncoordinated Pb²⁺ and water oxygen erosion due to the high volatility of halide I^- and the hydrophilicity of organic cation MA⁺. Herein, we introduce a Lewis base, TBA-Azo with an electron-donating Azo moiety, and hydrophobic long alkyl chains into the PCBM layer to form a multifunctional bulk-mixed electron transport layer (MBE). PCBM of MBE can combine with iodine-rich trap sites at the surface and grain boundaries of perovskite. TBA-Azo molecules of MBE can passivate uncoordinated Pb²⁺ by forming Lewis adducts and isolate water/oxygen at the perovskite surface with hydrophobic alkyl chains. It results in a decrease of trap densities with 1 order of magnitude, effectively inhibiting both bimolecular and trap-induced recombination and thus elongating the carrier lifetime. The passivation of MBE can effectively improve the open-circuit voltage from 1.05 to 1.10 V. Furthermore, three long carbon chain structures of TBA-Azo in MBE can improve the water-resistant ability of OIHP devices, which can maintain 90% of the original PCE after 500 h at the humidity of 50 ± 10%. We believe that the MBE with multifunctional defect passivation provides a strategy for simultaneously achieving high-performance and high-stability OIHP optoelectronic devices.

Photodetectors with High Responsivity by Thickness Tunable Mixed Halide Perovskite Nanosheets

Chemical transformation of typically "nonlayered" phases into two-dimensional structures remains a formidable task. Among the thickness tunable CsPbX₃ (X = Br, Br/I, I) nanosheets (NSs), CsPbBr_1.5I_1.5 NSs with a thickness of ∼4.9 nm have structural stability superior to ∼6.8 nm CsPbI₃ NSs and better hole mobility than ∼3.7 nm CsPbBr₃ NSs. Moving beyond the much-explored CsPbBr₃ photodetectors, we demonstrate a sharp improvement of the photodetection of CsPbBr_1.5I_1.5 NS devices by thickening the NSs to ∼6.1 nm through combining 8-carbon and 18-carbon ligand surfactants. Thereby, the responsivity increases up to one of the highest values of 3313 A W^-1 at 1.5 V (and 3946 A W^-1 at 2 V) with detectivity of 1.6 × 10¹¹ Jones at 1.5 V, due to the increase in carrier mobility up to 7.9 × 10^-4 cm² V^-1 s^-1. The better device performance of the NSs than 8.6-13.9 nm nanocubes (NCs) is due to their planarity which facilitates in-plane mobilization of the charge carriers.

Solvent Free Laminated Fabrication of Lead Halide Perovskites for Sensitive and Stable X-ray Detection

The halide perovskite X-ray detector can meet the urgent needs of low-dose medical imaging by X-rays. However, there is still a pressing challenge in lacking robust methods for large-scale fabrication of high-quality perovskite films with tunable thickness. Here we report a laminated fabrication of polycrystalline MAPbI₃ by using solvent-free liquid perovskite molten-salt (PMS), that offers reduced toxic issue, scalable fabrication, and highly tunability in film thickness. Nylon membrane was chosen as a scaffold for the infiltration of PMS, which simultaneously acts as a physical barrier to suppress the ionic migration in the MAPbI₃-nylon composite (denoted as MAPbI₃-LLP). The enhanced material properties result in good stability and high performance of X-ray detectors that show low detection limit and high sensitivity. Additionally, single gamma-ray photon detection was realized by MAPbI₃-LLP detectors. The promising performance characteristics of such polycrystalline detectors can accelerate the adoption of polycrystalline perovskites in X-ray imaging and gamma-ray detection.

Incorporating Guanidinium as Perovskitizer-Cation of Two-Dimensional Metal Halide for Crystal-Array Photodetectors

Two-dimensional (2D) hybrid perovskites are recently emerging as a potential family of semiconductors for versatile optoelectronic applications. Currently, the "perovskitizer" moieties are rigidly limited to small-size cations, while few 2D metal-halides containing guanidinium cations inside perovskite cages have been studied for photodetection. Herein, we present a new 2D hybrid perovskite, (i-BA)₂ (G)Pb₂ I₇ (where G is guanidinium and i-BA is isobutylammonium), which adopts a bilayered framework of {GPb₂ I₇ }. Single-crystal structure analyses disclose that G cations act as the perovskitizer, confined in the flexible perovskite cages formed by the distorted PbI₆ octahedra. Such inorganic sheets are crucial to the superior semiconducting properties and optical bandgap, as verified by the density functional theory calculation. Furthermore, its planar crystal-array photodetector shows fascinating photoelectric performance, including a quite low dark current (∼4.6×10^-11  A), a large current switching ratio (∼1.0×10³ ), and a notable photo-responsivity of ∼0.72 A W^-1 , suggesting great potential of (i-BA)₂ (G)Pb₂ I₇ for photodetection.