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

Ion migration and dark current suppression in quasi-2D perovskite-based X-ray detectors

Cesium-based lead-free double perovskite materials (Cs2AgBiBr6) have garnered significant attention in the X-ray detection field due to their environment friendly characteristics. However, their substantial ion migration properties lead to large dark currents and detection limits in Cs2AgBiBr6-based X-ray detectors, restricting the detection performance of the device. In terms of process technology, ultrasonic spraying is more suitable than a spin-coating method for fabricating large-area, micron-scale perovskite thick films, with higher cost-effectiveness, which is crucial for X-ray detection. This work introduces a BA+ (BA+ = CH3CH2CH2CH2NH3 +, n-butyl) source into the precursor solution and employs ultrasonic spraying to fabricate quasi-two-dimensional structured polycrystalline (BA)2Cs9Ag5Bi5Br31 perovskite thick films, developing a low-cost, eco-friendly X-ray detector with low dark current density and low detection limit. Characterization results reveal that the ion migration activation energy of (BA)2Cs9Ag5Bi5Br31 reaches 419 meV, approximately 17% higher than that of traditional three-dimensional perovskites, effectively suppressing perovskite ion migration and subsequently reducing the dark current. The (BA)2Cs9Ag5Bi5Br31-based X-ray detectors exhibit high resistivity (about 1.75 × 1010 Ω cm), low dark current density (66 nA cm-2), minimal dark current drift (0.016 pA cm-1 s-1 V-1), and detection limit (138 nGyair s-1), holding considerable promise for applications in low-noise, low-dose X-ray detection.

Arising 2D Perovskites for Ionizing Radiation Detection

2D perovskites have greatly improved moisture stability owing to the large organic cations embedded in the inorganic octahedral structure, which also suppresses the ions migration and reduces the dark current. The suppression of ions migration by 2D perovskites effectively suppresses excessive device noise and baseline drift and shows excellent potential in the direct X-ray detection field. In addition, 2D perovskites have gradually emerged with many unique properties, such as anisotropy, tunable bandgap, high photoluminescence quantum yield, and wide range exciton binding energy, which continuously promote the development of 2D perovskites in ionizing radiation detection. This review aims to systematically summarize the advances and progress of 2D halide perovskite semiconductor and scintillator ionizing radiation detectors, including reported alpha (α) particle, beta (β) particle, neutron, X-ray, and gamma (γ) ray detection. The unique structural features of 2D perovskites and their advantages in X-ray detection are discussed. Development directions are also proposed to overcome the limitations of 2D halide perovskite radiation detectors.

High-Performance Hard X-Ray Imaging Detector Using Facet-Dependent Bismuth Vanadate

Bismuth vanadate (BiVO4) exhibits large absorption efficiency for hard X-rays, which endows it with a robust capacity to attenuate X-ray radiation across a broad energy range. The anisotropic properties of BiVO4 allow for the manipulation of their physical and chemical characteristics through crystallographic orientation and exposed facets. In this study, the issue of heavy recombination caused by sluggish electron transport in BiVO4 is successfully addressed by enhancing the abundance of the (040) crystal face ratio using a Co2+ crystal face exposure agent. The facet-dependent modifications exhibit excellent and balanced intrinsic charge transport properties, and finely optimize both the sensitivity and detection limit of BiVO4 X-ray detectors. As a result, ultra-stable BiVO4 metal oxide X-ray detectors demonstrate a high sensitivity of 3164 µC Gyair -1 cm-2 and a low detection limit of 20.76 nGyair s-1 under 110 kVp hard X-rays, establishing a new benchmark for X-ray detectors based on polycrystalline Bi-halides and metal oxides. These findings highlight the significance of crystal orientation in optimizing materials for X-ray detection, setting a new sensitivity record for X-ray detectors based on polycrystalline Bi-halides and metal oxides, which paves the way for the development of advanced, low-dose, and highly stable imaging systems specifically for hard X-rays.

High-Performance Flat-Panel Perovskite X-ray Detectors Enabled by Defect Passivation in Ruddlesden-Popper Perovskites

Halide perovskites have emerged as promising candidates in X-ray detection due to their strong X-ray absorption and excellent optoelectronic properties. The development of sensitive and stable flat-panel X-ray detectors with high resolution is crucial for practical applications. In this paper, we introduce a novel flat-panel X-ray detector that integrates quasi-two-dimensional (2D) Ruddlesden-Popper (RP) perovskite with a pixeled thin film transistor (TFT) backplane. We incorporate 2,5-dibromopyrimidine (DBPM) as an additive to passivate the Lewis acid defects in the quasi-2D RP perovskite. This modification results in suppressed ion migration, improved optoelectronic performance, and enhanced operational stability of the device. Impressively, the activation energy of the RP perovskite increases from 0.96 to 1.35 eV with the DBPM additive. As a result, X-ray detectors exhibit a high sensitivity of ∼13,600 μC Gyair-1 cm-2, a low detection limit of 6.56 nGyair s-1, and excellent operational stability. Moreover, the flat-panel detectors demonstrate a high spatial resolution of 3.7 line pairs per millimeter and excellent X-ray imaging properties under a remarkably low X-ray dose of ∼50 μGyair, which is just half of the X-ray dose typically used in commercial equipment. This study opens new avenues for the development of flat-panel perovskite X-ray detectors with significant potential for various applications.

Molecular Design of Layered Hybrid Silver Bismuth Bromine Single Crystal for Ultra-Stable X-Ray Detection With Record Sensitivity

Nowadays, weak interlayer coupling and unclear mechanism in layered hybrid silver bismuth bromine (LH-AgBiBr) are the main reasons for limiting its further enhanced X-ray detection sensitivity and stability. Herein, the design rules for LH-AgBiBr and its influence on X-ray detection performance are reported for the first time. Although shortening amine size can enhance interlayer coupling, its detection performance is severely hampered by its easier defect formation caused by enlarged micro strain. In contrast, an appropriate divalent amine design endows the material with improved interlayer coupling and released micro strain, which benefits crystal stability and mechanical hardness. Another contribution is to increase material density and dielectric constant; thus, enhancing X-ray absorption and carrier transport. Consequently, the optimized parallel device based on BDA2 AgBiBr8 achieves a record sensitivity of 2638 µC Gyair -1 cm-2 and an ultra-low detection limit of 7.4 nGyair s-1 , outperforming other reported LH-AgBiBr X-ray detectors. Moreover, the unencapsulated device displays remarkable anti-moisture, anti-thermal (>150 °C), and anti-radiation (>1000 Gyair ) endurance. Eventually, high-resolution hard X-ray imaging is demonstrated by linear detector arrays under a benign dose rate (1.63 µGyair s-1 ) and low external bias (5 V). Hence, these findings provide guidelines for future materials design and device optimization.

Halide Perovskites and Their Derivatives for Efficient, High-Resolution Direct Radiation Detection: Design Strategies and Applications

The past decade has witnessed a rapid rise in the performance of optoelectronic devices based on lead-halide perovskites (LHPs). The large mobility-lifetime products and defect tolerance of these materials, essential for optoelectronics, also make them well-suited for radiation detectors, especially given the heavy elements present, which is essential for strong X-ray and γ-ray attenuation. Over the past decade, LHP thick films, wafers, and single crystals have given rise to direct radiation detectors that have outperformed incumbent technologies in terms of sensitivity (reported values up to 3.5 × 106 µC Gyair -1 cm-2 ), limit of detection (directly measured values down to 1.5 nGyair s-1 ), along with competitive energy and imaging resolution at room temperature. At the same time, lead-free perovskite-inspired materials (e.g., methylammonium bismuth iodide), which have underperformed in solar cells, have recently matched and, in some areas (e.g., in polarization stability), surpassed the performance of LHP detectors. These advances open up opportunities to achieve devices for safer medical imaging, as well as more effective non-invasive analysis for security, nuclear safety, or product inspection applications. Herein, the principles behind the rapid rises in performance of LHP and perovskite-inspired material detectors, and how their properties and performance link with critical applications in non-invasive diagnostics are discussed. The key strategies to engineer the performance of these materials, and the important challenges to overcome to commercialize these new technologies are also discussed.

Additive-Induced Synergies of Ion Migration Inhibition and Defect Passivation toward Sensitive Perovskite X-ray Detectors

Two-dimensional (2D) Ruddlesden-Popper (RP) metal halide perovskites have emerged as a promising material for X-ray detection. However, defects and ion migration generated nonradiative recombination and high dark current could cause severe performance degradation, which hinders their application. Herein, rubrene was added to the precursor solution of BA₂MA₃Pb₄I₁₃ to modulate the performance of the 2D RP perovskite X-ray detectors. The cation-π interaction between rubrene and perovskite could passivate the defects and inhibit the ion migration, resulting in improved performance and stability. The detectors made with rubrene exhibited a sensitivity of 354.30 μC·Gy_air^-1 cm^-2 and a detection limit of 112.85 nGy_air s^-1. This work highlights the synergistic effect of rubrene in defect passivation and ion migration inhibition, providing a facile approach toward sensitive perovskite X-ray detectors.