Low-Cost and Large-Area Hybrid X-Ray Detectors Combining Direct Perovskite Semiconductor and Indirect Scintillator

Both semiconductors and scintillators have their own advantages in direct and indirect X-ray detection, respectively. However, they are also limited by their intrinsic properties and detection mechanisms. Here, a low-cost and large-area flat X-ray detector is reported by combining a cesium silver bismuth bromide (Cs₂AgBiBr₆) perovskite semiconductor with a ethylenebis-triphenylphosphonium manganese (II) bromide ((C₃₈H₃₄P₂)MnBr₄) scintillator through fast tableting processes. Cs₂AgBiBr₆ and (C₃₈H₃₄P₂)MnBr₄ can attenuate the X-ray photons to induce charge carriers that are collected through the continuous Cs₂AgBiBr₆ grains. (C₃₈H₃₄P₂)MnBr₄ blocks the Cs₂AgBiBr₆ ions migration paths at the grain boundaries to reduce the device dark current/noise and improves the working stability. Most charges generated by (C₃₈H₃₄P₂)MnBr₄ are transferred to the adjacent Cs₂AgBiBr₆, and recombined charges radiate light through scintillation, which will be further absorbed by the surrounding Cs₂AgBiBr₆ perovskite, and further induce collectable charges for indirect X-ray detection, avoiding the unwanted light scattering, self-absorption, or afterglow effects of scintillators. The hybrid X-ray detector displays a high sensitivity of 114 µC Gy_air⁻¹ cm⁻² to 120 keV_p hard X-rays with a lowest detectable dose rate of 0.2 μGy_air s⁻¹, showing 75 times lower detection limit compared to (C₃₈H₃₄P₂)MnBr₄ scintillator, which provides a new path for X-ray flat-panel design.     

Sintered Polycrystalline BiVO4 Pellet for Stable X-Ray Detector with Low Detection Limit

Semiconductors based on Bi element show large attenuation coefficients to X-ray photons and have been recognized as candidates for X-ray detectors. However, the application of stable Bi-based oxide materials to X-ray detectors has been rarely investigated. In this research, the X-ray response of a BiVO₄ pellet has been studied. It has been found that the BiVO₄ pellet has a large resistivity of 1.3 × 10¹² Ω cm, negligible current drift of 6.18 × 10⁻⁸ nA cm⁻¹ s⁻¹ V⁻¹ under electrical bias and mobility lifetime product, µτ, of 1.75 × 10⁻⁴ cm² V⁻¹, which renders the pellet with an X-ray sensitivity of 241.3 µC Gy_air⁻¹ cm⁻² and a detection limit of 62 nGy_air s⁻¹ under 40 KVp X-ray illumination and 40 V bias voltage. The BiVO₄ pellet also shows operational stability under steady X-ray illumination with total dose of 2.01 Gy_air, equal to the dose of 20 000 medical chest X-ray inspections. This research reveals the potential application of BiVO₄ in X-ray detection devices and inspires further research in this area.     

Modifying Surface Termination of CsPbI3 Grain Boundaries by 2D Perovskite Layer for Efficient and Stable Photovoltaics

It is highly desirable for all-inorganic perovskite solar cells (PVSCs) to have reduced nonideal interfacial charge recombination in order to improve the performance. Although the construction of a 2D capping layer on 3D perovskite is an effective way to suppress interfacial nonradiative recombination, it is difficult to apply it to all-inorganic perovskites because of the resistance of Cs⁺ cesium ions in cation exchange reactions. To alleviate this problem, a simple approach using an ultra-thin 2D perovskite to terminate CsPbI₃ grain boundaries (GBs) without damaging the original 3D perovskite is developed. The 2D perovskite at the GBs not only enhances the charge-carrier extraction and transport but also effectively suppresses nonradiative recombination. In addition, because the 2D perovskite can prevent the moisture and oxygen from penetrating into the GBs and at the same time suppress the ion migration, the 2D terminated CsPbI₃ films exhibit significantly improved stability against humidity. Moreover, the devices without encapsulation can retain ≈81% of its initial power conversion efficiency (PCE) after being stored at 40 ± 5% relative humidity for 84 h. The 2D-based champion device exhibits a high PCE of 18.82% with a high open-circuit voltage of 1.16 V.     

Efficient and Stable Inverted Wide-Bandgap Perovskite Solar Cells and Modules Enabled by Hybrid Evaporation-Solution Method

Wide-bandgap perovskite solar cells (WBG-PSCs), when partnered with Si bottom cells in tandem configuration, can provide efficiencies up to 44%; yet, the development of stable, efficient, and scalable WBG-PSCs is required. Here, the utility of the hybrid evaporation-solution method (HESM) is investigated to meet these demanding requirements via its unique advantages including ease of control and reproducibility. A PbI₂/CsBr layer is co-evaporated followed by coating of organic-halide solutions in a green solvent. Bandgaps between 1.55–1.67 eV are systematically screened by varying CsBr and MABr content. Champion efficiencies of 21.06% and 20.35% in cells and 19.83% and 18.73% in mini-modules (16 cm²) for perovskites with 1.64 and 1.67 eV bandgaps are achieved, respectively. Additionally, 18.51%-efficient semi-transparent WBG-PSCs are implemented in 4T perovskite/bifacial silicon configuration, reaching a projected power output of 30.61 mW cm⁻² based on PD IEC TS 60904-1-2 (BiFi200) protocol. Despite similar bandgaps achieved by incorporating Br via MABr solution and/or CsBr evaporation, PSCs having a perovskite layer without MABr addition show significantly higher thermal and moisture stability. This study proves scalable, high-performance, and stable WBG-PSCs are enabled by HESM, hence their use in tandems and in emerging applications such as indoor photovoltaics are now within reach.     

Trade-Off Between Efficiency and Stability of CsPbBr3 Perovskite Quantum Dot-Based Light-Emitting Diodes by Optimized Passivation Ligands for Br/Pb

Although significant progress has been made in improving the external quantum efficiencies (EQEs) of perovskite quantum dot (QD) light-emitting diodes (QLEDs), understanding the degradation mechanism and enhancing stability remain a challenge. Herein,  increasing the content of Br-based passivation ligands is shown to enhance the EQE up to 16.1% by reducing the defects of CsPbBr₃ QDs in a Br-rich environment. However, the operational lifetimes of perovskite QLEDs gradually decrease with the increase of halide content, owing to the intensified ion migration under continuous electric field confirmed by the current behavior of QLEDs and time-of-flight secondary-ion mass spectrometry. Furthermore, a thorough analysis of the relationship between electricity and luminance of QLEDs suggests that a small amount of residue oleic acid ligands could weaken ion migration. Accordingly, a halide- and acid-hybrid (HAH) co-passivation strategy is proposed to optimize the content of Br- and acid-based ligands, and achieve a maximum EQE of 18.6% and an operational lifetime (T₅₀, extrapolated) of 213 h for CsPbBr₃ QLEDs. This approach for passivating QDs combines the high efficiency of Br-based ligands with the improved stability of acid-based ligands. The study elucidates the correlation between ligands and device performance, highlighting the significance of two or even multiple ligands for efficient and stable perovskite QLEDs.     

Variations of Infiltration and Electronic Contact in Mesoscopic Perovskite Solar Cells Revealed by High‐Resolution Multi‐Mapping Techniques

A combination of high‐resolution mapping techniques is developed to probe the homogeneity and defects of mesoscopic perovskite solar cells. Three types of cells using a one‐step infiltration process with methylammonium lead iodide (MAPbI₃) or 5‐ammoniumvaleric acid‐MAPbI₃ solutions, or two‐step process with MAPbI₃ solution are investigated. The correlation between photoluminescence, photocurrent, electroluminescence, and Raman maps gives a detailed understanding of the different infiltration mechanisms, electronic contact at interfaces, and effect on local photocurrent for the cells. The one‐step MAPbI₃ cell has very limited infiltration of the perovskite solution which results in poor device performance. High loading of the mesopores of the TiO₂ and ZrO₂ scaffold is observed when using 5‐ammoniumvaleric acid, but some micrometer‐sized non‐infiltrated areas remain due to dense carbon flakes hindering perovskite infiltration. The two‐step cell has a complex morphology with features having either beneficial or detrimental effects on the local photocurrent. The results not only provide key insights to achieving better infiltration and homogeneity of the perovskite film in mesoporous devices but can also aid further work on planar devices to develop efficient extraction layers. Moreover, this multi‐mapping approach allows the correlation of the local photophysical properties of full perovskite devices, which would be challenging to obtain by other techniques.