Tailoring Multi-Phenyl Ring Cation for Stable Scalable Hybrid Bismuth Iodide Amorphous Film: Enabling Record Sensitivity and High-Performance X-Ray Array Imaging.

The 329-type bismuth (Bi)-based metal halide (MH) polycrystalline films have potential to be applied in the new generation of X-ray imaging technology owing to high X-ray absorption coefficients and excellent detection properties. However, the mutually independent [Bi2X9]3- units and numerous grain boundaries in the material lead to low carrier transport and collection capabilities, severe ion migration, large dark currents, and poor response uniformity. Here, a new multi-phenyl ring methyltriphenylphosphonium (MTP) is designed to optimize the energy band structure. For the first time, the coupling between the A-site cation and [Bi2X9]3- is realized, making it the main contributor to the conduction band minimum (CBM), getting rid of dilemma that carrier transport is confined to [Bi2X9]3-. Further, the preparation of MTP3Bi2I9 amorphous large-area wafer is achieved by melt-quenching; the steric hindrance effect improves stability, increases ion migration energy, and promotes response uniformity (14%). Moreover, the amorphous structure takes advantage of A-site cation participation in the conductivity, achieving a record sensitivity (7601 µC Gy-1 cm-2) and low dark current (≈0.11 nA) in the field of amorphous X-ray detection, and features low-temperature large-area preparation. Ultimately, designing amorphous array imaging devices that exhibit excellent response uniformity and potential imaging capabilities is succeeded here.

Metal Halide Nanocrystals@Silica Aerogel Composite with Enhanced Dispersion Stability and Light Output for Efficient X-Ray Imaging in Harsh Environment.

Metal halide nanocrystals (MHNCs) embedded in a polymer matrix as flexible X-ray detector screens is an effective strategy with the advantages of low cost, facile preparation, and large area flexibility. However, MHNCs easily aggregate during preparation, recombination, under mechanical force, storage, or high operating temperature. Meanwhile, it shows an unmatched refractive index with polymer, resulting in low light yield. The related stability and properties of the device remain a huge unrevealed challenge. Herein, a composite screen (CZBM@AG-PS) by integrating MHNCs (Cs2ZnBr4: Mn2+ as an example) into silica aerogel (AG) and embedded in polystyrene (PS) is successfully developed. Further characterization points to the high porosity AG template that can effectively improve the dispersion of MHNCs in polymer detector screens, essentially decreasing nonradiative transition, Rayleigh scattering, and performance aging induced by aggregation in harsh environments. Furthermore, the higher light output and lower optical crosstalk are also achieved by a novel light propagation path based on the MHNCs/AG and AG/PS interfaces. Finally, the optimized CZBM@AG-PS screen shows much enhanced light yield, spatial resolution, and temperature stability. Significantly, the strategy is proven universal by the performance tests of other MHNCs embedded composite films for ultra-stable and efficient X-ray imaging.

Novel PHA Organic Spacer Increases Interlayer Interactions for High Efficiency in 2D Ruddlesden-Popper CsPbI3 Solar Cells.

The two-dimensional (2D) Ruddlesden-Popper (RP) CsPbI3 with hydrophobic organic spacers can significantly improve the environmental and phase stability of photovoltaic devices by suppressing ion migration and inducing steric hindrance. However, due to the multiple-quantum-well structure, these spacer cations lead to weak interactions in 2D RP CsPbI3, which seriously affect the carrier transport. Here, a novel N-H-group-rich phenylhydrazine spacer, namely, PHA, was developed for 2D RP CsPbI3 perovskite solar cells (PSCs). A series of characterizations confirm that the 2D perovskites using PHA spacers enhanced the N-H···I hydrogen-bonding interaction between the organic spacer cations and the [PbI6]4- inorganic layer and accelerated the crystallization rate of the perovskite film, which was beneficial to the preparation of high-quality films with preferred vertical orientation, large grain size, and dense morphology. Meanwhile, the trap state density of the as-prepared 2D RP perovskite films is significantly reduced to enable efficient charge carrier transport. As a result, the (PHA)2Cs4Pb5I16 PSCs achieved a performance of 16.23% with good environmental stability. This work provides a simple organic spacer selection scheme to realize interaction optimization in 2D RP CsPbI3 to develop efficient and stable PSCs.

Low-Trap-Density CsPbX3 Film for High-Efficiency Indoor Photovoltaics.

The continuous advancement of the Internet of Things (IoT) and photovoltaic technology has promoted the development of indoor photovoltaics (IPVs) that powers wireless devices. Nowadays, the CsPbX3 perovskite has received widespread attention because of its high power conversion efficiency (PCE) in an indoor environment and suitable band gap for IPVs. In this work, we regulated the thickness of the photoactive layer (to optimize the carrier transport process without affecting indoor absorption) and bromine substitution (to adjust the band gap and improve the quality of the film) to reduce trap-assisted carrier recombination. A CsPbI2.7Br0.3 perovskite cell with excellent performance was obtained, which is superior to c-Si cells in a low-light environment. The optimized device achieved PCE values of 32.69 and 33.11% under a 1000 lux fluorescent lamp and white light-emitting diode (WLED) illumination. The J-V hysteresis of the device is also effectively suppressed. Moreover, it has a steady-state output power of 7.76 µW (0.09 cm2, and can be enhanced by enlarging the areas), which can meet the consumption of many small wireless devices. It is worth noting that the optimized device has excellent applicability to be used in a complex indoor environment.

Strategies for Improving the Stability of Tin-Based Perovskite (ASnX3) Solar Cells.

Although lead-based perovskite solar cells (PSCs) are highly efficient, the toxicity of lead (Pb) limits its large-scale commercialization. As such, there is an urgent need to find alternatives. Many studies have examined tin-based PSCs. However, pure tin-based perovskites are easily oxidized in the air or just in glovebox with an ultrasmall amount of oxygen. Such a characteristic makes their performance and stability less ideal compared with those of lead-based perovskites. Herein, how to address the instability of tin-based perovskites is introduced in detail. First, the crystalline structure, optical properties, and sources of instability of tin-based perovskites are summarized. Next, the preparation methods of tin-based perovskite are discussed. Then, various measures for solving the instability problem are explained using four strategies: additive engineering, deoxidizer, partial substitution, and reduced dimensions. Finally, the challenges and prospects are laid out to help researchers develop highly efficient and stable tin-based perovskites in the future.

Ultrastable Laurionite Spontaneously Encapsulates Reduced-dimensional Lead Halide Perovskites.

Reduced dimensional lead halide perovskites (RDPs) have attracted great research interest in diverse optical and optoelectronic fields. However, their poor stability is one of the most challenging obstacles prohibiting them from practical applications. Here, we reveal that ultrastable laurionite-type Pb(OH)Br can spontaneously encapsulate the RDPs in their formation solution without introducing any additional chemicals, forming RDP@Pb(OH)Br core-shell microparticles. Interestingly, the number of the perovskite layers within the RDPs can be conveniently and precisely controlled by varying the amount of CsBr introduced into the reaction solution. A single RDP@Pb(OH)Br core-shell microparticle composed of RDP nanocrystals with different numbers of perovskite layers can be also prepared, showing different colors under different light excitations. More interestingly, barcoded RDP@Pb(OH)Br microparticles with different parts emitting different lights can also be prepared. The morphology of the emitting microstructures can be conveniently manipulated. The RDP@Pb(OH)Br microparticles demonstrate outstanding environmental, chemical, thermal, and optical stability, as well as strong resistance to anion exchange processes. This study not only deepens our understanding of the reaction processes in the extensively used saturation recrystallization method but also points out that it is highly possible to dramatically improve the performance of the optoelectronic devices through manipulating the spontaneous formation process of Pb(OH)Br.

Multilevel Static-Dynamic Anticounterfeiting Based on Stimuli-Responsive Luminescence in a Niobate Structure.

Anticounterfeiting is a highly required technique to protect the product and the consumer rights in the modern society. The conventional luminescent anticounterfeiting is based on downconversion luminescence excited by an ultraviolet light, which is easy to be faked. In this work, we realized six luminescent modes in a niobate-based structure (LiNbO3:RE3+, RE3+ = Pr3+, Tm3+, Er3+, Yb3+), in which photostimulated luminescence of LiNbO3:Pr3+, and upconversion luminescence color evolution of LiNbO3:Er3+ were first presented. Based on the above luminescent modes of LiNbO3:RE3+, multilevel anticounterfeiting devices were developed. By employing mechanoluminescence and persistent luminescence, we achieved dual-mode anticounterfeiting that could display the luminescent patterns without any direct irradiation. In addition, another dual-mode anticounterfeiting based on photostimulated luminescence and upconversion luminescence excited by a near-infrared light was realized, which could display the anticounterfeiting patterns in both static and dynamic states. To obtain an even higher anticounterfeiting level, downconversion luminescence, thermoluminescence, photostimulated luminescence, and upconversion luminescence were simultaneously applied in a food trademark. This four-mode anticounterfeiting trademark could not only show a static-dynamic luminescence that is hard to be faked but also allow consumers to distinguish the food freshness. The presented multilevel anticounterfeiting strategies could be employed to resolve the counterfeit issues in various fields.

Spy Must Be Spotted: A Multistimuli-Responsive Luminescent Material for Dynamic Multimodal Anticounterfeiting and Encryption.

The development of luminescent materials for anticounterfeiting and encryption is of great importance. Herein, we develop a multistimuli-responsive luminescent material, Na2CaGe2O6:Pb2+/Er3+, and use it to print luminescent images. The photoluminescence and upconversion luminescence of these images show different patterns and colors under different stimuli. The photostimulated luminescence (PSL) of the printed images causes dynamic changes in appearance and is accordingly applied for dynamic multimodal anticounterfeiting on banknotes. The PSL of these luminescent images is also applied in a virtual war scenario to demonstrate that the dynamic PSL-encrypted information in the fabricated image is sufficiently safe even in extreme cases and that spies will be detected. These results can inspire us with more creative security designs based on this luminescent material.

An energy self-compensating phosphosilicate material applied to temperature sensors.

For years, researchers have been exploring effective methods of sustaining the emission intensity of phosphors with increasing temperature by suppressing emission loss. In this work, we developed a multi-cationic site and lattice-distorted phosphosilicate phosphor, Ca8Al2P6SiO28:Ce, Eu. To obtain luminous-self-healing properties, we attempted to change the energy depths and density distributions of the traps to achieve self-suppression of emission loss by energy compensation from the traps or energy transfer between Ce3+ and Eu2+/Eu3+. The temperature-dependent emission spectra indicate that the luminescence of Ce3+ presents similar change trends in both single and co-doped samples. Meanwhile, the change trends of the Eu2+/Eu3+ emission intensities show obvious differences. Combined with the thermoluminescence curves, decay times, temperature-dependent fluorescence characteristics and cathodoluminescence spectra, we speculate that the traps play an important role in the luminescence of Ce3+ due to the smaller energy difference of the Ce3+ excited states and the conduction band. The abnormal luminescence of Eu2+/Eu3+ mainly results from the energy transfer of Ce3+ to Eu2+/Eu3+. For this phenomenon, a high thermal sensitive fluorescence intensity ratio is obtained in a broad temperature range, which implies that this material can be applied in temperature sensors.

Nonequivalent Substitution and Charge-Induced Emitter-Migration Design of Tuning Spectral and Duration Properties of NaCa2GeO4F:Mn(2+) Persistent Luminescent Phosphor.

We combine nonequivalent substitution and charge-induced emitter-migration approaches and design an efficient method to optionally tune the spectral and duration properties of NaCa2GeO4F:Mn(2+) phosphor. A series of representative codopants have been investigated in detail and classified into two categories: RA (RA = Li(+), Al(3+), N(3-), Ga(3+), B(3+)) and RB (RB = Mg(2+), F(-), Bi(3+), Zn(2+), Cd(2+), Sc(3+), Tm(3+)). Results reveal that the nonequivalent substitution of RA codopants would induce foreign negative defects and stabilize Mn(2+) emitters at octahedral Na/Ca sites for red emission. In constrast, the RB codopants would generate foreign positive defects and make Mn(2+) emitters migrate to tetrahedral Ge(4+) sites for green-yellow emission. At the same time, the RA codopants are in favor of the generation of intrinsic positive traps with shallow trap depth and thus efficiently improve the duration properties of phosphors. On the basis of the experimental results, a possible nonequivalent substitution and charge-induced emitter-migration model has been proposed, and we can optionally tune the spectral (568 ↔ 627 nm) and the duration (minutes to more than 6 h) properties according to this model.

Morphology Controllable Synthesis of ScF3:Er3+, Yb3+ Nano/Sub-Microncrystals by Hydrothermal/Solvothermal Process.

In this paper, red phosphors Yb3+-Er3+ co-doped ScF3 nano/microcrystals were successfully prepared by a facile hydrothermal/solvothermal route using the sodium dodecyl benzene sulfonate (SDBS) as the surfactant. The structure, morphologies and up-conversion (UC) photoluminescence properties of the as-prepared products were well characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) spectra, respectively. The SEM images show that the obtained samples are the uniform cubic and cuboid crystals. With the increase of the surfactant SDBS or the change in the solvent types, the sample change their size from nanometer to submicron. Upon the 980 nm laser diode excitation, the ScF3:Era+, Yb3+ nanocrystals exhibit red emission which can be assigned to the characteristic 4F9/2/4I15/2 transition of Er3+. In order to understand the emission mechanisms of ScF3:ErS+, Yb3+ nanocrystals, the dependence of UC luminescence intensity on the 980 nm excitation power was measured, suggesting that the UC phenomenon results from a two-photon process. Meanwhile, the emission intensities of the YbS+-Er3+ codoped ScF3 nano/sub-micro crystals with different solution composition show an obvious change under the 980 nm laser excitation. Therefore, the phosphors Yb3+-Er3+ co-doped ScF3 possibly have a potential application in the biological applications.

Warm white light generation from a single phase Dy3+ doped Mg2Al4Si5O18 phosphor for white UV-LEDs.

A series of Mg2-xAl4Si5O18:xDy(3+) (0 ≤x≤ 0.18) samples were synthesized, for the first time, by a solid state method both in a reducing atmosphere and in air. XRD, diffuse reflectance spectra, excitation spectra, emission spectra, decay times and thermal quenching were used to investigate the structure, photoluminescence, energy transfer and thermal properties. The results show that Mg2Al4Si5O18:Dy(3+) can efficiently absorb UV light and emit violet-blue light in the range of 400 to 500 nm from oxygen vacancies in the host as well as blue light (∼480 nm) and yellow light (∼576 nm) from the f-f transitions of Dy(3+). The emission intensities of the samples obtained under a reducing atmosphere are far superior to those of the samples obtained in air due to an efficient energy transition from oxygen vacancies in the host to Dy(3+). An analysis of the thermal quenching shows that the phosphor Mg2Al4Si5O18:Dy(3+) has excellent thermal properties. The emission intensities of typical samples synthesized in a reducing atmosphere and in air at 250 °C are 70% and 81% of their initial intensities at 20 °C, respectively. In addition, the emission colors of all of the samples are located in the white light region and the optimal chromaticity coordinates and Correlated Color Temperature are (x = 0.34, y = 0.33) and 5129 K, respectively. Therefore, these white Mg2Al4Si5O18:Dy(3+) phosphors could serve as promising candidates for white-light UV-LEDs.

Enhanced photoluminescence and thermal properties of size mismatch in Sr(2.97-x-y)Eu0.03Mg(x)Ba(y)SiO5 for high-power white light-emitting diodes.

In this Study, Mg(2+) and Ba(2+) act to enhance the maximum emission of Sr2.97SiO5:0.03Eu(2+) significantly and redshift the emission band to the orange-red region in Sr(2.97-x-y)Mg(x)Ba(y)SiO5:0.03Eu(2+). Size mismatch between the host and the doped cations tunes the photoluminescence spectra shift systematically. A slight blue shift when increasing the amount of Mg(2+) occurs in the Sr(2.97-x)Eu0.03Mg(x)SiO5 lattices, and a rapid red shift occurs when Ba(2+) is codoped in the Sr(2.57-y)Eu0.03Mg0.4Ba(y)SiO5 lattices. The emission spectra were tuned from 585 to 601 nm by changing the concentration of Ba(2+). Accordingly, we propose the underlying mechanisms of the changes in the photoluminescence properties by adjusting the cation composition of phosphors. The influence of the size mismatch on the thermal quenching is also observed. This mechanism could be widely applied to oxide materials and could be useful in tuning the photoluminescence properties, which are sensitive to local coordination environment. The emission bands of Sr(2.97-x-y)Eu0.03Mg(x)Ba(y)SiO5 show the blue shift with increasing temperature, which could be described in terms of back tunneling of the excited electrons from the low-energy excited state to the high-energy excited state. Thus, the Sr(2.97-x-y)Eu0.03Mg(x)Ba(y)SiO5 phosphors could have potential applications in the daylight LEDs or warm white LEDs.