Direct X-ray photoconversion in flexible organic thin film devices operated below 1 V

Real-time single-proton counting with transmissive perovskite nanocrystal scintillators

High-sensitivity radiation detectors for energetic particles are essential for advanced applications in particle physics, astronomy and cancer therapy. Current particle detectors use bulk crystals, and thin-film organic scintillators have low light yields and limited radiation tolerance. Here we present transmissive thin scintillators made from CsPbBr3 nanocrystals, designed for real-time single-proton counting. These perovskite scintillators exhibit exceptional sensitivity, with a high light yield (~100,000 photons per MeV) when subjected to proton beams. This enhanced sensitivity is attributed to radiative emission from biexcitons generated through proton-induced upconversion and impact ionization. These scintillators can detect as few as seven protons per second, a sensitivity level far below the rates encountered in clinical settings. The combination of rapid response (~336 ps) and pronounced ionostability enables diverse applications, including single-proton tracing, patterned irradiation and super-resolution proton imaging. These advancements have the potential to improve proton dosimetry in proton therapy and radiography.

Biocompatible Metal-Free Perovskite Membranes for Wearable X-ray Detectors

Halide perovskites are emerging as promising materials for X-ray detection owing to their compatibility with flexible fabrication, cost-effective solution processing, and exceptional carrier transport behaviors. However, the challenge of removing lead from high-performing perovskites, crucial for wearable electronics, while retaining their superior performance, persists. Here, we present for the first time a highly sensitive and robust flexible X-ray detector utilizing a biocompatible, metal-free perovskite, MDABCO-NH4I3 (MDABCO = methyl-N'-diazabicyclo[2.2.2]octonium). This wearable X-ray detector, based on a MDABCO-NH4I3 thick membrane, exhibits remarkable properties including a large resistivity of 1.13 × 1011 Ω cm, a high mobility-lifetime product (μ-τ) of 1.64 × 10-4 cm2 V-1, and spin Seebeck effect coefficient of 1.9 nV K-1. We achieve a high sensitivity of 6521.6 ± 700 μC Gyair-1 cm-2 and a low detection limit of 77 nGyair s-1, ranking among the highest for biocompatible X-ray detectors. Additionally, the device exhibits effective X-ray imaging at a low dose rate of 1.87 μGyair s-1, which is approximately one-third of the dose rate used in regular medical diagnostics. Crucially, both the MDABCO-NH4I3 thick membrane and the device showcase excellent mechanical robustness. These attributes render the flexible MDABCO-NH4I3 thick membranes highly competitive for next-generation, high-performance, wearable X-ray detection applications.

A detachable interface for stable low-voltage stretchable transistor arrays and high-resolution X-ray imaging

Challenges associated with stretchable optoelectronic devices, such as pixel size, power consumption and stability, severely brock their realization in high-resolution digital imaging. Herein, we develop a universal detachable interface technique that allows uniform, damage-free and reproducible integration of micropatterned stretchable electrodes for pixel-dense intrinsically stretchable organic transistor arrays. Benefiting from the ideal heterocontact and short channel length (2 μm) in our transistors, switching current ratio exceeding 106, device density of 41,000 transistors/cm2, operational voltage down to 5 V and excellent stability are simultaneously achieved. The resultant stretchable transistor-based image sensors exhibit ultrasensitive X-ray detection and high-resolution imaging capability. A megapixel image is demonstrated, which is unprecedented for stretchable direct-conversion X-ray detectors. These results forge a bright future for the stretchable photonic integration toward next-generation visualization equipment.

Toward Low-Voltage and High-Sensitivity Direct X-ray Detectors Based on Thick Bulk Heterojunction Organic Device

Organic semiconducting materials are promising for the fabrication of flexible ionizing radiation detectors for imaging because of their tissue equivalence, simple large-scale processing, and mass production. However, it is challenging to achieve high-sensitivity detection for organic direct detectors prepared by low-cost solution processing because of the compromise between thickness and carrier transport. In this study, high-performance organic direct X-ray detectors were fabricated by building a micrometer-thick bulk heterojunction (BHJ) using poly(3-hexylthiophene-2,5-diyl) (P3HT):(6,6)-phenyl c71 butyric acid methyl ester. A 5 μm BHJ film was fabricated by drop-casting and enhanced crystallization of P3HT using binary solvents and high-boiling-point additives to improve the charge carrier mobility. Furthermore, this organic direct X-ray detector has a sensitivity of >654.26 μC Gyair s-1 and a self-powered response. Because of the architecture of the thick active layer and the energy cascade in this diode detector, it has a very low dark current of 46.26 pA at -2 V. A fast and efficient approach was developed for fabricating thick, highly mobile organic BHJ films for high-performance direct X-ray detectors. It has great potential for application in a new generation of flexible and portable large-area flat-panel detectors.

Perovskite materials in X-ray detection and imaging: recent progress, challenges, and future prospects

Perovskite materials have attracted significant attention as innovative and efficient X-ray detectors owing to their unique properties compared to traditional X-ray detectors. Herein, chronologically, we present an in-depth analysis of X-ray detection technologies employing organic-inorganic hybrids (OIHs), all-inorganic and lead-free perovskite material-based single crystals (SCs), thin/thick films and wafers. Particularly, this review systematically scrutinizes the advancement of the diverse synthesis methods, structural modifications, and device architectures exploited to enhance the radiation sensing performance. In addition, a critical analysis of the crucial factors affecting the performance of the devices is also provided. Our findings revealed that the improvement from single crystallization techniques dominated the film and wafer growth techniques. The probable reason for this is that SC-based devices display a lower trap density, higher resistivity, large carrier mobility and lifetime compared to film- and wafer-based devices. Ultimately, devices with SCs showed outstanding sensitivity and the lowest detectable dose rate (LDDR). These results are superior to some traditional X-ray detectors such as amorphous selenium and CZT. In addition, the limited performance of film-based devices is attributed to the defect formation in the bulk film, surfaces, and grain boundaries. However, wafer-based devices showed the worst performance because of the formation of voids, which impede the movement of charge carriers. We also observed that by performing structural modification, various research groups achieved high-performance devices together with stability. Finally, by fusing the findings from diverse research works, we provide a valuable resource for researchers in the field of X-ray detection, imaging and materials science. Ultimately, this review will serve as a roadmap for directing the difficulties associated with perovskite materials in X-ray detection and imaging, proposing insights into the recent status, challenges, and promising directions for future research.

Flexible and Stretchable Light-Emitting Diodes and Photodetectors for Human-Centric Optoelectronics

Optoelectronic devices with unconventional form factors, such as flexible and stretchable light-emitting or photoresponsive devices, are core elements for the next-generation human-centric optoelectronics. For instance, these deformable devices can be utilized as closely fitted wearable sensors to acquire precise biosignals that are subsequently uploaded to the cloud for immediate examination and diagnosis, and also can be used for vision systems for human-interactive robotics. Their inception was propelled by breakthroughs in novel optoelectronic material technologies and device blueprinting methodologies, endowing flexibility and mechanical resilience to conventional rigid optoelectronic devices. This paper reviews the advancements in such soft optoelectronic device technologies, honing in on various materials, manufacturing techniques, and device design strategies. We will first highlight the general approaches for flexible and stretchable device fabrication, including the appropriate material selection for the substrate, electrodes, and insulation layers. We will then focus on the materials for flexible and stretchable light-emitting diodes, their device integration strategies, and representative application examples. Next, we will move on to the materials for flexible and stretchable photodetectors, highlighting the state-of-the-art materials and device fabrication methods, followed by their representative application examples. At the end, a brief summary will be given, and the potential challenges for further development of functional devices will be discussed as a conclusion.

Tissue Equivalent Curved Organic X-ray Detectors Utilizing High Atomic Number Polythiophene Analogues

Organic semiconductors are a promising material candidate for X-ray detection. However, the low atomic number (Z) of organic semiconductors leads to poor X-ray absorption thus restricting their performance. Herein, the authors propose a new strategy for achieving high-sensitivity performance for X-ray detectors based on organic semiconductors modified with high -Z heteroatoms. X-ray detectors are fabricated with p-type organic semiconductors containing selenium heteroatoms (poly(3-hexyl)selenophene (P3HSe)) in blends with an n-type fullerene derivative ([6,6]-Phenyl C71 butyric acid methyl ester (PC70 BM). When characterized under 70, 100, 150, and 220 kVp X-ray radiation, these heteroatom-containing detectors displayed a superior performance in terms of sensitivity up to 600 ± 11 nC Gy-1  cm-2 with respect to the bismuth oxide (Bi2 O3 ) nanoparticle (NP) sensitized organic detectors. Despite the lower Z of selenium compared to the NPs typically used, the authors identify a more efficient generation of electron-hole pairs, better charge transfer, and charge transport characteristics in heteroatom-incorporated detectors that result in this breakthrough detector performance. The authors also demonstrate flexible X-ray detectors that can be curved to a radius as low as 2 mm with low deviation in X-ray response under 100 repeated bending cycles while maintaining an industry-standard ultra-low dark current of 0.03 ± 0.01 pA mm-2 .

Bi2O3/Gd2O3 Meta-Aerogel with Leaf-Inspired Nanotrap Array Enables Efficient X-Ray Absorption

The increasing utilization of X-rays has generated a growing need for efficient shielding materials. However, the existing Pb-based materials suffer from a narrow X-ray absorbing range, high weight, and rigidity. Inspired by the natural leaf, which can efficiently absorb light through chlorophyll and carotenoids in confined cells, we engineer ultralight and superelastic nanofibrous Bi2O3/Gd2O3 meta-aerogels (BGAs) with X-ray nanotrap arrays by manipulating the 3D confined assembly of 1D Bi2O3 and Gd2O3 nanofibers. The BGAs can synergistically absorb X-ray photons from complementary energy ranges into the nanotraps and induce cyclic collisions with Bi2O3 and Gd2O3 nanofibers, maximizing the effective X-ray attenuation. The meta-aerogel exhibits the integrated performance of efficient X-ray shielding efficiency (60-83%, 16-90 keV), ultralow density (10 mg cm-3), and superelasticity. The production of these meta-aerogels presents an avenue for the development of next-generation X-ray protective materials and the resolution of X-ray imaging systems.

X-ray Detectors Based on Ga2O3 Microwires

X-ray detectors have numerous applications in medical imaging, industrial inspection, and crystal structure analysis. Gallium oxide (Ga₂O₃) shows potential as a material for high-performance X-ray detectors due to its wide bandgap, relatively high mass attenuation coefficient, and resistance to radiation damage. In this study, we present Sn-doped Ga₂O₃ microwire detectors for solar-blind and X-ray detection. The developed detectors exhibit a switching ratio of 1.66 × 10² under X-ray irradiation and can operate stably from room temperature to 623 K, which is one of the highest reported operating temperatures for Ga₂O₃ X-ray detectors to date. These findings offer a promising new direction for the design of Ga₂O₃-based X-ray detectors.

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 .

Realizing nearly-zero dark current and ultrahigh signal-to-noise ratio perovskite X-ray detector and image array by dark-current-shunting strategy

Although perovskite X-ray detectors have revealed promising properties, their dark currents are usually hundreds of times larger than the practical requirements. Here, we report a detector architecture with a unique shunting electrode working as a blanking unit to suppress dark current, and it theoretically can be reduced to zero. We experimentally fabricate the dark-current-shunting X-ray detector, which exhibits a record-low dark current of 51.1 fA at 5 V mm^-1, a detection limit of 7.84 nGy_air s^-1, and a sensitivity of 1.3 × 10⁴ μC Gy_air^-1 cm^-2. The signal-to-noise ratio of our polycrystalline perovskite-based detector is even outperforming many previously reported state-of-the-art single crystal-based X-ray detectors by serval orders of magnitude. Finally, the proof-of-concept X-ray imaging of a 64 × 64 pixels dark-current-shunting detector array is successfully demonstrated. This work provides a device strategy to fundamentally reduce dark current and enhance the signal-to-noise ratio of X-ray detectors and photodetectors in general.

Identification of the Origin of Ultralow Dark Currents in Organic Photodiodes

Organic bulk heterojunction photodiodes (OPDs) attract attention for sensing and imaging. Their detectivity is typically limited by a substantial reverse bias dark current density (J_d ). Recently, using thermal admittance or spectral photocurrent measurements, J_d has been attributed to thermal charge generation mediated by mid-gap states. Here, the temperature dependence of J_d in state-of-the-art OPDs is reported with J_d down to 10^-9  mA cm^-2 at -0.5 V bias. For a variety of donor-acceptor bulk-heterojunction blends it is found that the thermal activation energy of J_d is lower than the effective bandgap of the blends, by ca. 0.3 to 0.5 eV, but higher than expected for mid-gap states. Ultra-sensitive sub-bandgap photocurrent spectroscopy reveals that the minimum photon energy for optical charge generation in OPDs correlates with the dark current thermal activation energy. The dark current in OPDs is attributed to thermal charge generation at the donor-acceptor interface mediated by intra-gap states near the band edges.

Nanocrystalline CaWO4 and ZnWO4 Tungstates for Hybrid Organic-Inorganic X-ray Detectors

Hybrid materials combining an organic matrix and high-Z nanomaterials show potential for applications in radiation detection, allowing unprecedented device architectures and functionality. Herein, novel hybrid organic-inorganic systems were produced using a mixture of tungstate (CaWO4 or ZnWO4) nanoparticles with a P3HT:PCBM blend. The nano-tungstates with a crystallite size of 43 nm for CaWO4 and 30 nm for ZnWO4 were synthesized by the hydrothermal method. Their structure and morphology were characterized by X-ray diffraction and scanning electron microscopy. The hybrid systems were used to fabricate direct conversion X-ray detectors able to operate with zero bias voltage. The detector performance was tested in a wide energy range using monochromatic synchrotron radiation. The addition of nanoparticles with high-Z elements improved the detector response to X-ray radiation compared with that of a pure organic P3HT:PCBM bulk heterojunction cell. The high dynamic range of our detector allows for recording X-ray absorption spectra, including the fine X-ray absorption structure located beyond the absorption edge. The obtained results suggest that nanocrystalline tungstates are promising candidates for application in direct organic-inorganic X-ray detectors.

Spatially nanoconfined N-type polymer semiconductors for stretchable ultrasensitive X-ray detection

Polymer semiconductors are promising candidates for wearable and skin-like X-ray detectors due to their scalable manufacturing, adjustable molecular structures and intrinsic flexibility. Herein, we fabricated an intrinsically stretchable n-type polymer semiconductor through spatial nanoconfinement effect for ultrasensitive X-ray detectors. The design of high-orientation nanofiber structures and dense interpenetrating polymer networks enhanced the electron-transporting efficiency and stability of the polymer semiconductors. The resultant polymer semiconductors exhibited an ultrahigh sensitivity of 1.52 × 10⁴ μC Gy_air^-1 cm^-2, an ultralow detection limit of 37.7 nGy_air s^-1 (comparable to the record-low value of perovskite single crystals), and polymer film X-ray imaging was achieved at a low dose rate of 3.65 μGy_air s^-1 (about 1/12 dose rate of the commercial medical chest X-ray diagnosis). Meanwhile, the hybrid semiconductor films could sustain 100% biaxial stretching strain with minimal degeneracy in photoelectrical performances. These results provide insights into future high-performance, low-cost e-skin photoelectronic detectors and imaging.

Large area polymer semiconductor sub-microwire arrays by coaxial focused electrohydrodynamic jet printing for high-performance OFETs

Large area and highly aligned polymer semiconductor sub-microwires were fabricated using the coaxial focused electrohydrodynamic jet printing technology. As indicated by the results, the sub-microwire arrays have smooth morphology, well reproducibility and controllable with a width of ~110 nm. Analysis shows that the molecular chains inside the sub-microwires mainly exhibited edge-on arrangement and the π-stacking direction (010) of the majority of crystals is parallel to the long axis of the sub-microwires. Sub-microwires based organic field effect transistors showed high mobility with an average of 1.9 cm² V⁻¹ s⁻¹, approximately 5 times higher than that of thin film based organic field effect transistors. In addition, the number of sub-microwires can be conveniently controlled by the printing technique, which can subsequently concisely control the performance of organic field effect transistors. This work demonstrates that sub-microwires fabricated by the coaxial focused electrohydrodynamic jet printing technology create an alternative path for the applications of high-performance organic flexible device.

Here, the authors fabricate large area and highly aligned polymer semiconductor sub-microwires arrays via coaxial focused electrohydrodynamic jet printing technology, achieving high on/off ratio and average mobility that is 5x higher than that of thin film based organic field effect transistors.

Solution-Processed Perovskite/Metal-Oxide Hybrid X-Ray Detector and Array with Decoupled Electronic and Ionic Transport Pathways

Lead halide perovskites possess heavy elements and excellent mobility-lifetime (µτ) product, becoming desirable candidates for X-ray detectors. However, current perovskite photoconduction detectors (PCDs) with vertical geometry, where electronic signals and mobile ions share the same conduction path, are facing with extremely challenging ion-migration issue. Herein, a hybrid X-ray detector device structure, in which perovskite is vertically stacked onto an indium oxide (In₂ O₃ ) transistor with lateral transport geometry is designed, perovskite mainly acts as X-ray sensitizer to activate In₂ O₃ conduction channel, the actual electrical signal is conducted and collected in the lateral metal-oxide device. With the decoupled ionic and electronic transportation, hybrid detectors are insensitive to the ionic motion of perovskite, hence demonstrating no hysteresis and almost no shifting of baseline that are often observed in PCDs, hybrid detectors also exhibit reduced dark current, improved response time, and four times higher photocurrent signals. Finally, array integration of hybrid detectors and preliminary X-ray imaging is realized. The work provides an effective device strategy in addition to the mere material alternations to attain high-performance perovskite-based X-ray detectors and arrays.

Metal-Free PAZE-NH4 X3 ⋅H2 O Perovskite for Flexible Transparent X-ray Detection and Imaging

Metal-free perovskites are of interest for their chemical diversity and eco-friendly properties, and recently have been used for X-ray detection with superior carrier behavior. However, the size and shape complexity of the organic components results in difficulties in evaluating their stability in high-energy radiation. Herein, we introduce multiple hydrogen-bond metal-free PAZE-NH₄ X₃ ⋅H₂ O perovskite, where H₂ O leads to more hydrogen bonds appearing between organic molecules and the perovskite host. As suggested by the theoretical calculations, multiple hydrogen bonds promote stiffness of the lattice, and increase the diffusion barrier to inhibit ionic migration. Then, low trap density, high μτ products and structural flexibility of PAZE-NH₄ Br₃ ⋅H₂ O give a flexible X-ray detector with the highest sensitivity of 3708 μC Gy_air ^-1  cm^-2 , ultra-low detection limit of 0.19 μGy_air ^-1  s^-1 and superior spatial resolution of 5.0 lp mm^-1 .

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.

Applications of Halide Perovskites in X-ray Detection and Imaging

X-ray detection plays an extremely significant function in medical diagnosis, nondestructive testing, safety testing, scientific research, environmental monitoring and other practical applications. However, conventional inorganic semiconductors such as amorphous selenium,...

Flexible Polymer X-ray Detectors with Non-fullerene Acceptors for Enhanced Stability: Toward Printable Tissue Equivalent Devices for Medical Applications

There is growing interest in the development of novel materials and devices capable of ionizing radiation detection for medical applications. Organic semiconductors are promising candidates to meet the demands of modern detectors, such as low manufacturing costs, mechanical flexibility, and a response to radiation equivalent to human tissue. However, organic semiconductors have typically been employed in applications that convert low energy photons into high current densities, for example, solar cells and LEDs, and thus existing design rules must be re-explored for ionizing radiation detection where high energy photons are converted into typically much lower current densities. In this work, we report the optoelectronic and X-ray dosimetric response of a tissue equivalent organic photodetector fabricated with solution-based inks prepared from polymer donor poly(3-hexylthiophene) (P3HT) blended with either a non-fullerene acceptor (5Z,5'Z)-5,5'-((7,7'-(4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one) (o-IDTBR) or a fullerene acceptor, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Indirect detection of X-rays was achieved via coupling of organic photodiodes with a plastic scintillator. Both detectors displayed an excellent response linearity with dose, with sensitivities to 6 MV photons of 263.4 ± 0.6 and 114.2 ± 0.7 pC/cGy recorded for P3HT:PCBM and P3HT:o-IDTBR detectors, respectively. Both detectors also exhibited a fast temporal response, able to resolve individual 3.6 μs pulses from the linear accelerator. Energy dependence measurements highlighted that the photodetectors were highly tissue equivalent, though an under-response in devices compared to water by up to a factor of 2.3 was found for photon energies of 30-200 keV due to the response of the plastic scintillator. The P3HT:o-IDTBR device exhibited a higher stability to radiation, showing just an 18.4% reduction in performance when exposed to radiation doses of up to 10 kGy. The reported devices provide a successful demonstration of stable, printable, flexible, and tissue-equivalent radiation detectors with energy dependence similar to other scintillator-based detectors used in radiotherapy.

Determination of X-ray detection limit and applications in perovskite X-ray detectors

X-ray detection limit and sensitivity are important figure of merits for perovskite X-ray detectors, but literatures lack a valid mathematic expression for determining the lower limit of detection for a perovskite X-ray detector. In this work, we present a thorough analysis and new method for X-ray detection limit determination based on a statistical model that correlates the dark current and the X-ray induced photocurrent with the detection limit. The detection limit can be calculated through the measurement of dark current and sensitivity with an easy-to-follow practice. Alternatively, the detection limit may also be obtained by the measurement of dark current and photocurrent when repeatedly lowering the X-ray dose rate. While the material quality is critical, we show that the device architecture and working mode also have a significant influence on the sensitivity and the detection limit. Our work establishes a fair comparison metrics for material and detector development.

Sb2Se3 film with grain size over 10 µm toward X-ray detection

Direct X-ray detectors are considered as competitive next-generation X-ray detectors because of their high spatial resolution, high sensitivity, and simple device configuration. However, their potential is largely limited by the imperfections of traditional materials, such as the low crystallization temperature of α-Se and the low atomic numbers of α-Si and α-Se. Here, we report the Sb₂Se₃ X-ray thin-film detector with a p-n junction structure, which exhibited a sensitivity of 106.3 µC/(Gy_air·cm²) and response time of < 2.5 ms. This decent performance and the various advantages of Sb₂Se₃, such as the average atomic number of 40.8 and μτ product (μ is the mobility, and τ is the carrier lifetime) of 1.29 × 10^-5 cm²/V, indicate its potential for application in X-ray detection.

Flexible Lead-Free X-ray Detector from Metal-Organic Frameworks

Semiconductive metal-organic frameworks (MOFs) obtained by specific host-guest interactions have attracted a large interest in the last two decades, promising development of next-generation electronic devices. Herein, we designed and presented flexible X-ray detectors using Ni-DABDT (DABDT = 2,5-diamino-1,4-benzenedithiol dihydrochloride) MOFs as the absorbing layer. The π-d coupling interactions of Ni-DABDT throughout the framework implement a conspicuous carrier transportation pathway. The detector that converts X-ray photons directly into carriers manifests an attractive achievement with high detection sensitivity of 98.6 μC Gy_air^-1 cm^-2, with a low detection limit of 7.2 μGy_air s^-1 for the radiation robustness. This work provides insights for next-generation green and high-performance flexible sensor detectors by utilizing MOF materials with the benefits of a designable structure and tunable property, demonstrating a proof-of-concept in wearable X-ray detectors for radiation monitoring and imaging.

Ultrathin and Ultrasensitive Direct X-ray Detector Based on Heterojunction Phototransistors

Most contemporary X-ray detectors adopt device structures with non/low-gain energy conversion, such that a fairly thick X-ray photoconductor or scintillator is required to generate sufficient X-ray-induced charges, and thus numerous merits for thin devices, such as mechanical flexibility and high spatial resolution, have to be compromised. This dilemma is overcome by adopting a new high-gain device concept of a heterojunction X-ray phototransistor. In contrast to conventional detectors, X-ray phototransistors allow both electrical gating and photodoping for effective carrier-density modulation, leading to high photoconductive gain and low noise. As a result, ultrahigh sensitivities of over 10⁵  μC Gy_air ^-1  cm^-2 with low detection limit are achieved by just using an ≈50 nm thin photoconductor. The employment of ultrathin photoconductors also endows the detectors with superior flexibility and high imaging resolution. This concept offers great promise in realizing well-balanced detection performance, mechanical flexibility, integration, and cost for next-generation X-ray detectors.

A High-Sensitivity Flexible Direct X-ray Detector Based on Bi2O3/PDMS Nanocomposite Thin Film

The characteristics of mechanical flexibility, low health risk, and simple processing of polymer nanocomposite materials make them potentially applicable as flexible X-ray detectors. In this study, we report on a high sensitivity, environmentally friendly, and flexible direct X-ray detector using polymer nanocomposite material consisting of bismuth oxide (Bi₂O₃) nanoparticles and polydimethylsiloxane (PDMS). This detector was realized by printing patterned Ag electrodes on the polymer nanocomposite material. The response of PDMS to X-rays was verified for the first time, and the effect of doping different contents of Bi₂O₃ nanoparticles on the performance of the device was tested. The optoelectronic performance of the optimized detector indicated a high sensitivity (203.58 μC Gy_air⁻¹ cm⁻²) to low dose rate (23.90 μGy_air s⁻¹) at a 150 V bias voltage and the X-ray current density (J_X-ray) was 10,000-fold higher than the dark current density (J_dark). The flexible direct X-ray detector could be curled for 10,000 cycles with slight performance degradation. The device exhibited outstanding stability after storage for over one month in air. Finally, this device provides new guidance for the design of high-performance flexible direct X-ray detectors.

Self-powered ultraflexible photonic skin for continuous bio-signal detection via air-operation-stable polymer light-emitting diodes

Ultraflexible optical devices have been used extensively in next-generation wearable electronics owing to their excellent conformability to human skins. Long-term health monitoring also requires the integration of ultraflexible optical devices with an energy-harvesting power source; to make devices self-powered. However, system-level integration of ultraflexible optical sensors with power sources is challenging because of insufficient air operational stability of ultraflexible polymer light-emitting diodes. Here we develop an ultraflexible self-powered organic optical system for photoplethysmogram monitoring by combining air-operation-stable polymer light-emitting diodes, organic solar cells, and organic photodetectors. Adopting an inverted structure and a doped polyethylenimine ethoxylated layer, ultraflexible polymer light-emitting diodes retain 70% of the initial luminance even after 11.3 h of operation under air. Also, integrated optical sensors exhibit a high linearity with the light intensity exponent of 0.98 by polymer light-emitting diode. Such self-powered, ultraflexible photoplethysmogram sensors perform monitoring of blood pulse signals as 77 beats per minute.

Direct detection of 5-MeV protons by flexible organic thin-film devices

The direct detection of 5-MeV protons by flexible organic detectors based on thin films is here demonstrated. The organic devices act as a solid-state detector, in which the energy released by the protons within the active layer of the sensor is converted into an electrical current. These sensors can quantitatively and reliably measure the dose of protons impinging on the sensor both in real time and in integration mode. This study shows how to detect and exploit the energy absorbed both by the organic semiconducting layer and by the plastic substrate, allowing to extrapolate information on the present and past irradiation of the detector. The measured sensitivity, S = (5.15 ± 0.13) pC Gy^-1, and limit of detection, LOD = (30 ± 6) cGy s^-1, of the here proposed detectors assess their efficacy and their potential as proton dosimeters in several fields of application, such as in medical proton therapy.

Solution-Processed Donor-Acceptor Poly(3-hexylthiophene):Phenyl-C61-butyric Acid Methyl Ester Diodes for Low-Voltage α Particle Detection

Diodes fabricated using a blend of poly(3-hexylthiophene) and phenyl-C₆₁-butyric acid methyl ester (6-80 μm thick) as an organic semiconductor component achieved consistent 4 MeV α particle detection. Current-voltage characteristics and current-time measurements were obtained under α irradiation and in its absence. Steady-state and transient (time-of-flight) photoconduction measurements were additionally performed. Low-bias (<20 V) α particle detection gain-efficiency products of order 10^-2 were measured. The α particle detection was achieved reproducibly, reversibly, and repeatably in different devices of varying organic semiconductor layer thicknesses using both the steady-state and time-dependent (dynamic) diode responses. Conductive gain, due to trapped electrons, increased the α particle gain-efficiency product in both forward and reverse bias conditions as well as increasing steady-state photoconduction. The device thickness was optimized to maximize the gain-efficiency product by matching the penetration depth of the α particle, obtained by modeling, to the organic semiconductor layer thickness. Very high confidence α particle detection was achieved (with signal-to-noise ratios exceeding 20) under optimized device dimensions and drive conditions. Hecht function fitting of the gain-efficiency product versus electric field data returns mobility-lifetime products of order 10^-6-10^-7 cm² V^-1. This work demonstrates that solution-processed organic semiconductor diodes are viable for low-voltage α particle detection.

High-Performance X-ray Detector Based on Single-Crystal β-Ga2O3:Mg

X-ray detection plays an important role in medical imaging, scientific research, and security inspection. Recently, the β-Ga₂O₃ single-crystal-based X-ray detector has attracted extensive attention due to its excellent intrinsic properties such as good absorption for X-ray photons, a high breakdown electric field, high stability, and low cost. However, developing a high-performance β-Ga₂O₃-based X-ray detector remains a challenge because of the large dark current and the high oxygen vacancy concentration in the crystals. In this paper, we report a high-performance Mg-doped β-Ga₂O₃ single-crystal-based X-ray detector with a sandwich structure. The reduced dark current enables the detector to have a high sensitivity of 338.9 μC Gy^-1 cm^-2 under 50 keV X-ray irradiation with a dose rate of 69.5 μGy/s. The sensitivity is 16-fold higher than that of the commercial amorphous selenium detector. Furthermore, the reduced oxygen vacancy concentration can improve the response speed (<0.2 s) of the detector. The present studies provide a promising method to obtain the high performances for the X-ray detector based on β-Ga₂O₃ single crystals.

Designing Ultraflexible Perovskite X-Ray Detectors through Interface Engineering

X-ray detectors play a pivotal role in development and advancement of humankind, from far-reaching impact in medicine to furthering the ability to observe distant objects in outer space. While other electronics show the ability to adapt to flexible and lightweight formats, state-of-the-art X-ray detectors rely on materials requiring bulky and fragile configurations, severely limiting their applications. Lead halide perovskites is one of the most rapidly advancing novel materials with success in the field of semiconductor devices. Here, an ultraflexible, lightweight, and highly conformable passively operated thin film perovskite X-ray detector with a sensitivity as high as 9.3 ± 0.5 µC Gy-1 cm-2 at 0 V and a remarkably low limit of detection of 0.58 ± 0.05 μGy s-1 is presented. Various electron and hole transporting layers accessing their individual impact on the detector performance are evaluated. Moreover, it is shown that this ultrathin form-factor allows for fabrication of devices detecting X-rays equivalently from front and back side.

A Study on an Organic Semiconductor-Based Indirect X-ray Detector with Cd-Free QDs for Sensitivity Improvement

In this paper, we studied the optimized conditions for adding inorganic quantum dots (QD) to the P3HT:PC₇₀BM organic active layer to increase the sensitivity of the indirect X-ray detector. Commonly used QDs are composed of hazardous substances with environmental problems, so indium phosphide (InP) QDs were selected as the electron acceptor in this experiment. Among the three different sizes of InP QDs (4, 8, and 12 nm in diameter), the detector with 4 nm InP QDs showed the highest sensitivity, of 2.01 mA/Gy·cm². To further improve the sensitivity, the QDs were fixed to 4 nm in diameter and then the amount of QDs added to the organic active layer was changed from 0 to 5 mg. The highest sensitivity, of 2.26 mA/Gy·cm², was obtained from the detector with a P3HT:PC₇₀BM:InP QDs (1 mg) active layer. In addition, the highest mobility, of 1.69 × 10⁻⁵ cm²/V·s, was obtained from the same detector. Compared to the detector with the pristine P3HT:PC₇₀BM active layer, the detector with a P3HT:PC₇₀BM:InP QDs (1 mg) active layer had sensitivity that was 61.87% higher. The cut-off frequency of the P3HT:PC₇₀BM detector was 21.54 kHz, and that of the P3HT:PC₇₀BM:InP QDs (1 mg) detector was 26.33 kHz, which was improved by 22.24%.

Organic Field-Effect Transistors as Flexible, Tissue-Equivalent Radiation Dosimeters in Medical Applications

Radiation therapy is one of the most prevalent procedures for cancer treatment, but the risks of malignancies induced by peripheral beam in healthy tissues surrounding the target is high. Therefore, being able to accurately measure the exposure dose is a critical aspect of patient care. Here a radiation detector based on an organic field-effect transistor (RAD-OFET) is introduced, an in vivo dosimeter that can be placed directly on a patient's skin to validate in real time the dose being delivered and ensure that for nearby regions an acceptable level of low dose is being received. This device reduces the errors faced by current technologies in approximating the dose profile in a patient's body, is sensitive for doses relevant to radiation treatment procedures, and robust when incorporated into conformal large-area electronics. A model is proposed to describe the operation of RAD-OFETs, based on the interplay between charge photogeneration and trapping.

Boron-Loaded Polymeric Sensor for the Direct Detection of Thermal Neutrons

We report the first demonstration of a solid-state, direct-conversion sensor for thermal neutrons based on a polymer/inorganic nanocomposite. Sensors were fabricated from ultrathick films of poly(triarylamine) (PTAA) semiconducting polymer, with thicknesses up to 100 μm. Boron nanoparticles (NPs) were dispersed throughout the PTAA film to provide the neutron stopping power arising from the high thermal neutron cross section of the isotope ¹⁰B. To maximize the quantum efficiency (QE) of the sensor to thermal neutrons, a high volume fraction of homogeneously dispersed boron nanoparticles was achieved in the thick PTAA film using an optimized processing method. Thick active layers were realized using a high molecular weight of the PTAA so that molecular entanglements provide a high cohesive strength. A nonionic surfactant was used to stabilize the boron dispersion in solvent and hence suppress the formation of agglomerates and associated electrical pathways. Boron nanoparticle loadings of up to ten volume percent were achieved, with thermal neutron quantum efficiency estimates up to 6% resulting. The sensors' neutron responses were characterized under a high flux thermal neutron exposure, showing a linear correlation between the response current and the thermal neutron flux up to ∼10⁷ cm^-2 s^-1. Polymer-based boron nanocomposite sensors offer a new neutron detection technology that uses low-cost, scalable solution processing and provides an alternative to traditional neutron sensors that use rare isotopes, such as ³He.

Influence of gamma-ray irradiation and post-annealing studies on pentacene films: the anisotropic effects on structural and electronic properties

In this work, γ-ray irradiation effects on pentacene thin films are investigated in terms of the change in the crystallinity, and electronic structure as well as chemical states of the film. The pentacene films are γ-irradiated up to 3 kGy and then characterized using synchrotron X-ray diffraction, near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy. We found that γ-ray irradiation creates defects, resulting in a decrease of X-ray diffraction intensity both in the plane normal and in-plane directions. From angle dependent NEXAFS; the transition of C 1s to π* orbital for irradiated samples increases; suggesting that the unoccupied π* states enhance due to defects or radical formation in pentacene thin films. Additionally, the in-plane resistivity shows a decreasing trend of resistance after irradiation. This trend of increase in conductivity is also consistent with C 1s to π transition, which manifests the increase in carrier concentration. Hall effect measurements further confirmed the increase in carrier concentration as a function of dose; however, the mobility of the sample decreases as the dose rate increases due to the defects created. By post-irradiation annealing, the thin film phase diffraction intensity can be recovered. Altogether, the anisotropic studies on pentacene films disclosed that the irradiation leads to defect formation along in-plane and plane normal directions. Overall, these results suggest that pentacene is one of the robust organic electronic materials; whose structure remains mostly intact even after irradiation up to a dose of 3 kGy.

In this work, γ-ray irradiation effects on pentacene thin films are investigated in terms of the change in the crystallinity, and electronic structure as well as chemical states of the film.

Morphology and mobility as tools to control and unprecedentedly enhance X-ray sensitivity in organic thin-films

Organic semiconductor materials exhibit a great potential for the realization of large-area solution-processed devices able to directly detect high-energy radiation. However, only few works investigated on the mechanism of ionizing radiation detection in this class of materials, so far. In this work we investigate the physical processes behind X-ray photoconversion employing bis-(triisopropylsilylethynyl)-pentacene thin-films deposited by bar-assisted meniscus shearing. The thin film coating speed and the use of bis-(triisopropylsilylethynyl)-pentacene:polystyrene blends are explored as tools to control and enhance the detection capability of the devices, by tuning the thin-film morphology and the carrier mobility. The so-obtained detectors reach a record sensitivity of 1.3 · 10⁴ µC/Gy·cm², the highest value reported for organic-based direct X-ray detectors and a very low minimum detectable dose rate of 35 µGy/s. Thus, the employment of organic large-area direct detectors for X-ray radiation in real-life applications can be foreseen.

Though organic semiconductors are attractive for high performance X-ray detection systems, the detection mechanism in organic thin films is not well understood. Here, the authors report the role of morphology and carrier mobility on X-ray sensitivity in detectors with unprecedented performance.

Flexible Inkjet-Printed Triple Cation Perovskite X-ray Detectors

Flexible direct conversion X-ray detectors enable a variety of novel applications in medicine, industry, and science. Hybrid organic-inorganic perovskite semiconductors containing elements of high atomic number combine an efficient X-ray absorption with excellent charge transport properties. Due to their additional cost-effective and low-temperature processability, perovskite semiconductors represent promising candidates to be used as active materials in flexible X-ray detectors. Inspired by the promising results recently reported on X-ray detectors that are based on either triple cation perovskites or inkjet-printed perovskite quantum dots, we here investigate flexible inkjet-printed triple cation perovskite X-ray detectors. The performance of the detectors is evaluated by the X-ray sensitivity, the dark current, and the X-ray stability. Exposed to 70 kVp X-ray radiation, reproducible and highly competitive X-ray sensitivities of up to 59.9 μC/(Gy_aircm²) at low operating voltages of 0.1 V are achieved. Furthermore, a significant dark current reduction is demonstrated in our detectors by replacing spin-coated poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) with sputtered NiO_x hole transport layers. Finally, stable operation of a flexible X-ray detector for a cumulative X-ray exposure of 4 Gy_air is presented, and the applicability of our devices as X-ray imaging detectors is shown. The results of this study represent a proof of concept toward flexible direct conversion X-ray detectors realized by cost-effective and high-throughput digital inkjet printing.

Organic Photodetectors for Next-Generation Wearable Electronics

Next-generation wearable electronics will need to be mechanically flexible and stretchable such that they can be conformally attached onto the human body. Photodetectors that are available in today's market are based on rigid inorganic crystalline materials and they have limited mechanical flexibility. In contrast, photodetectors based on organic polymers and molecules have emerged as promising alternatives due to their inherent mechanical softness, ease of processing, tunable optoelectronic properties, good light sensing performance, and biocompatibility. Here, the recent advances of organic photodetectors in terms of both optoelectronic and mechanical properties are outlined and discussed, and their application in wearable electronics including health monitoring sensors, artificial vision, and self-powering integrated devices are highlighted.

Sensitivity Improvement of Quantum Dot-Blended Hybrid Detector for X-ray Imaging

This study investigated the characteristics of an indirect-type hybrid X-ray detector with a conjugated polymer poly(3-hexylthiophene) (P3HT) and CdSe quantum dot (QD) blended active layer. To improve detection sensitivity, the optimal blending ratio of P3HT:CdSe QDs, ligand exchange effect, and optimal process condition of the active layer were examined. The detector with a P3HT:CdSe QDs = 1:5 blended active layer showed the highest collected charge density (CCD) and highest sensitivity under X-ray irradiation. The replacement of a trioctylphosphine (TOP) ligand by a pyridine ligand effectively assisted the charge transport and reduced the QD aggregation, increasing the detection sensitivity of the detector by 75% after the ligand exchange. To further improve the sensitivity of the proposed detector, the optimized process conditions of the active layer were studied. The sensitivity of the detector with an active layer of about 80 nm thickness formed by a double-coating method showed the highest CCD of 62.5 nA/cm2, and the highest sensitivity of 0.14 mA/Gy∙cm2. Due to additional pyridine treatment between the double-coating processes, the surface roughness of the active layer decreased, and the CCD and sensitivity subsequently increased.

High-Performance X-ray Detection Based on One-Dimensional Inorganic Halide Perovskite CsPbI3

Recently, three-dimensional (3D) halide perovskites were considered as X-ray detection materials because of their high mobility, carrier lifetime, and absorption of X-ray radiation. However, their detection sensitivity and instability at extreme conditions and environments still require optimization. In our present research work, we report using one-dimensional (1D) inorganic halide perovskite CsPbI₃ crystals for stable X-ray detection. Remarkably, an X-ray detector made of CsPbI₃ has a high sensitivity of 2.37 mC·Gy^-1·cm^-2, which is an order of magnitude greater than that of detectors using 3D halide perovskites reported previously. The high-sensitivity X-ray detection of CsPbI₃ crystals is attributed to their high resistivity of 7.4 × 10⁹ Ω·cm and large carrier mobility-lifetime product of 3.63 × 10^-3 cm²·V^-1. Our investigation demonstrates the quite promising applications of X-ray detectors made of the low-dimensional perovskite crystals.

Millimeter-Scale Unipolar Transport in High Sensitivity Organic-Inorganic Semiconductor X-ray Detectors

Hybrid inorganic-in-organic semiconductors are an attractive class of materials for optoelectronic applications. Traditionally, the thicknesses of organic semiconductors are kept below 1 μm due to poor charge transport in such systems. However, recent work suggests that charge carriers in such organic semiconductors can be transported over centimeter length scales opposing this view. In this work, a unipolar X-ray photoconductor based on a bulk heterojunction architecture, consisting of poly(3-hexylthiophene), a C70 derivative, and high atomic number bismuth oxide nanoparticles operating in the 0.1-1 mm thickness regime is demonstrated, having a high sensitivity of ∼160 μC mGy^-1 cm^-3. The high performance enabled by hole drift lengths approaching a millimeter facilitates a device architecture allowing a high fraction of the incident X-rays to be attenuated. An X-ray imager is demonstrated with sufficient resolution for security applications such as portable baggage screening at border crossings and public events and scalable medical applications.

Flexible Sensors—From Materials to Applications

Flexible sensors have the potential to be seamlessly applied to soft and irregularly shaped surfaces such as the human skin or textile fabrics. This benefits conformability dependant applications including smart tattoos, artificial skins and soft robotics. Consequently, materials and structures for innovative flexible sensors, as well as their integration into systems, continue to be in the spotlight of research. This review outlines the current state of flexible sensor technologies and the impact of material developments on this field. Special attention is given to strain, temperature, chemical, light and electropotential sensors, as well as their respective applications.

High sensitivity organic inorganic hybrid X-ray detectors with direct transduction and broadband response

X-ray detectors are critical to healthcare diagnostics, cancer therapy and homeland security, with many potential uses limited by system cost and/or detector dimensions. Current X-ray detector sensitivities are limited by the bulk X-ray attenuation of the materials and consequently necessitate thick crystals (~1 mm–1 cm), resulting in rigid structures, high operational voltages and high cost. Here we present a disruptive, flexible, low cost, broadband, and high sensitivity direct X-ray transduction technology produced by embedding high atomic number bismuth oxide nanoparticles in an organic bulk heterojunction. These hybrid detectors demonstrate sensitivities of 1712 µC mGy⁻¹ cm⁻³ for “soft” X-rays and ~30 and 58 µC mGy⁻¹ cm⁻³ under 6 and 15 MV “hard” X-rays generated from a medical linear accelerator; strongly competing with the current solid state detectors, all achieved at low bias voltages (−10 V) and low power, enabling detector operation powered by coin cell batteries.

X-ray detectors based on low-cost organic semiconductors have inherently low detector sensitivity due to poor X-ray to charge conversion and charge collection. Here, the authors demonstrate a flexible, high-sensitivity X-ray detector based on an organic bulk heterojunction-nanoparticle composite.

Passive radiofrequency x-ray dosimeter tag based on flexible radiation-sensitive oxide field-effect transistor

Distributed x-ray radiation dosimetry is crucial in diverse security areas with significant environmental and human impacts such as nuclear waste management, radiotherapy, or radioprotection devices. We present a fast, real-time dosimetry detection system based on flexible oxide thin-film transistors that show a quantitative shift in threshold voltage of up to 3.4 V/gray upon exposure to ionizing radiation. The transistors use indium-gallium-zinc-oxide as a semiconductor and a multilayer dielectric based on silicon oxide and tantalum oxide. Our measurements demonstrate that the threshold voltage shift is caused by the accumulation of positive ionization charge in the dielectric layer due to high-energy photon absorption in the high-Z dielectric. The high mobility combined with a steep subthreshold slope of the transistor allows for fast, reliable, and ultralow-power readout of the deposited radiation dose. The order-of-magnitude variation in transistor channel impedance upon exposure to radiation makes it possible to use a low-cost, passive radiofrequency identification sensor tag for its readout. In this way, we demonstrate a passive, programmable, wireless sensor that reports in real time the excess of critical radiation doses.