Monoammonium Modified Dion-Jacobson Quasi-2D Perovskite for High Efficiency Pure-Blue Light Emitting Diodes.

Quasi-2D perovskites exhibit impressive optoelectronic properties and hold significant promise for future light-emitting devices. However, the efficiency of perovskite light-emitting diodes (PeLEDs) is seriously limited by defect-induced nonradiative recombination and imbalanced charge injection. Here, the defect states are passivated and charge injection balance is effectively improved by introducing the additive cyclohexanemethylammonium (CHMA) to bromide-based Dion-Jacobson (D-J) structure quasi-2D perovskite emission layer. CHMA participates in the crystallization of perovskite, leading to high quality film composed of compact and well-contacted grains with enhanced hole transportation and less defects. As a result, the corresponding PeLEDs exhibit stable pure blue emission at 466 nm with a maximum external quantum efficiency (EQE) of 9.22%. According to current knowledge, this represents the highest EQE reported for pure-blue PeLEDs based on quasi-2D bromide perovskite thin films. These findings underscore the potential of quasi-2D perovskites for advanced light-emitting devices and pave the way for further advancements in PeLEDs.

Bilayer phosphine oxide modification toward efficient and large-area pure-blue perovskite quantum dot light-emitting diodes.

Blue emissive halide perovskite light-emitting diodes (LEDs) are gaining increasing attention. Reducing defects in halide perovskites to improve the performance of the resulting LEDs is a main research direction, but there are limited passivation methods for achieving efficient and spectrally-stable pure-blue LEDs based on mixed-halide perovskites. In this work, double modification layers containing phosphine oxides, i.e., diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide (TSPO1) and 2,7-bis(diphenylphosphoryl)-9,9'-spirobifluorene (SPPO13), are developed to passivate mixed-halide perovskite quantum dot (QD) films. The comprehensive spectroscopic and structural characterization results indicate the presence of strong interactions between TSPO1/SPPO13 and the QDs. Besides, the combination of the bilayer exhibits a synergistic hole-blocking effect, improving the charge balance of the LEDs. LEDs based on the QD/TSPO1/SPPO13 films deliver stable electroluminesence at 469 nm and present a maximum external quantum efficiency (EQE) and luminance of 4.87% and 560 cd m-2, respectively. Benefiting from the uniform QD/TSPO1/SPPO13 film over a large area, LEDs with an area of 64 mm2 show a maximum EQE of 3.91%, which represents the first efficient large-area mixed-halide perovskite LED with stable pure-blue emission. This work provides a method to improve the perovskite QDs-based film quality and optoelectronic properties, and is a step toward the fabrication of highly-efficient large-area blue perovskite LEDs.

Long-range transport and ultrafast interfacial charge transfer in perovskite/monolayer semiconductor heterostructure for enhanced light absorption and photocarrier lifetime.

Atomically thin two-dimensional transition metal dichalcogenides (TMDs) have shown great potential for optoelectronic applications, including photodetectors, phototransistors, and spintronic devices. However, the applications of TMD-based optoelectronic devices are severely restricted by their weak light absorption and short exciton lifetime due to their atomically thin nature and strong excitonic effect. To simultaneously enhance the light absorption and photocarrier lifetime of monolayer semiconductors, here, we report 3D/2D perovskite/TMD type II heterostructures by coupling solution processed highly smooth and ligand free CsPbBr3 film with MoS2 and WS2 monolayers. By time-resolved spectroscopy, we show interfacial hole transfer from MoS2 (WS2) to the perovskite layer occurs in an ultrafast time scale (100 and 350 fs) and interfacial electron transfer from ultrathin CsPbBr3 to MoS2 (WS2) in ∼3 (9) ps, forming a long-lived charge separation with a lifetime of >20 ns. With increasing CsPbBr3 thickness, the electron transfer rate from CsPbBr3 to TMD is slower, but the efficiency remains to be near-unity due to coupled long-range diffusion and ultrafast interfacial electron transfer. This study indicates that coupling solution processed lead halide perovskites with strong light absorption and long carrier diffusion length to monolayer semiconductors to form a type II heterostructure is a promising strategy to simultaneously enhance the light harvesting capability and photocarrier lifetime of monolayer semiconductors.

Efficient light-emitting diodes based on oriented perovskite nanoplatelets.

Solution-processed planar perovskite light-emitting diodes (LEDs) promise high-performance and cost-effective electroluminescent devices ideal for large-area display and lighting applications. Exploiting emission layers with high ratios of horizontal transition dipole moments (TDMs) is expected to boost the photon outcoupling of planar LEDs. However, LEDs based on anisotropic perovskite nanoemitters remain to be inefficient (external quantum efficiency, EQE <5%) due to the difficulties of simultaneously controlling the orientations of TDMs, achieving high photoluminescence quantum yields (PLQYs) and realizing charge balance in the films of assembled nanostructures. Here, we demonstrate efficient electroluminescence from an in situ grown perovskite film composed of a monolayer of face-on oriented nanoplatelets. The ratio of horizontal TDMs of the perovskite nanoplatelet film is ~84%, which leads to a light-outcoupling efficiency of ~31%, substantially higher than that of isotropic emitters (~23%). In consequence, LEDs with a peak EQE of 23.6% are achieved, representing highly efficient planar perovskite LEDs.

Mixed Halide Perovskite Films by Vapor Anion Exchange for Spectrally Stable Blue Stimulated Emission.

Solution-processed all-inorganic CsPbX3 perovskites exhibit outstanding optoelectronic properties and are being considered as a promising optical gain medium, with impressive performance in the green and red region. However, the development of CsPbX3 for blue emission is still lagging far behind, owing to difficulties in thin films synthesis and spectral instability subject to light irradiation. Here, a facile vapor anion exchange (VAE) method that enables preparation of blue-emitting perovskite films with both excellent surface morphology and good photo-stability is reported. The mixed-Br/Cl quasi-2D perovskite films show spectrally stable pure blue emission (471 nm) under continuous-wave laser irradiation with power density as high as 81 W cm-2 . Furthermore, optically pumped blue amplified spontaneous emission (ASE) is realized based on the mixed-Br/Cl perovskite films. By changing the duration of VAE treatment, the ASE peak can be tuned from 537 nm down to 475 nm. This work not only presents a facile method to prepare high quality mixed halide Cs-based perovskite films, but also pave the way for further exploration of stable blue perovskite lasing.

Near-Unity-Efficiency Energy Transfer from Perovskite to Monolayer Semiconductor through Long-Range Migration and Asymmetric Interfacial Transfer.

van der Waals heterostructures combining perovskites of strong light absorption with atomically thin two-dimensional (2D) transition-metal dichalcogenides (TMDs) hold great potential for light-harvesting and optoelectronic applications. However, current research studies integrating TMDs with low-dimensional perovskite nanomaterials generally suffer from poor carrier/energy transport and harnessing, stemming from poor interfacial interaction due to the nanostructured nature and ligands on surface/interface. To overcome the limitations, here, we report prototypical three-dimensional (3D)/2D perovskite/TMD heterostructures by combing highly smooth and ligand-free CsPbBr3 film with a WSe2 monolayer. We show that the energy transfer at interface occurs through asymmetric two-step charge-transfer process, with ultrafast hole transfer in ∼200 fs and subsequent electron transfer in ∼10 ps, driven by the asymmetric type I band alignment. The energy migration and transfer from CsPbBr3 film to WSe2 can be well described by a one-dimensional diffusion model with a carrier diffusion length of ∼500 nm in CsPbBr3 film. Thanks to the long-range carrier migration and ultrafast interfacial transfer, highly efficient (>90%) energy transfer to WSe2 can be achieved with CsPbBr3 film as thick as ∼180 nm, which can capture most of the light above its band gap. The efficient light and energy harvesting in perovskite/TMD 3D/2D heterostructures suggest great promise in optoelectronic and photonic devices.

Coexistence of light-induced photoluminescence enhancement and quenching in CH3NH3PbBr3 perovskite films.

Lead halide perovskites are promising semiconductors for various optoelectronic devices working in a wide photo-excitation density regime. However, photo-induced instability, attributed to illumination-activated mobile ions, has been an obstacle to their application. Herein, we use the time evolution of photoluminescence (PL) to investigate the light illumination effects of CH3NH3PbBr3 perovskite films under relatively high excitation (up to 4.5 W cm-2). We demonstrate that continuous illumination can lead to both PL enhancement and PL quenching simultaneously, with their weight ratios depending on the excitation density. The experimental data can be well described and interpreted by considering the coexistence of and competition between the photo-induced annihilation and the formation of long-living filled trap states. Our study may provide in-depth insight into the photo-induced instability of perovskite films and help to improve the performance of perovskite-based optoelectronic devices.

Achieving long carrier lifetime and high optical gain in all-inorganic CsPbBr3 perovskite films via top and bottom surface modification.

Solution-processed all-inorganic CsPbX3(X = Br, I, and Cl) perovskites are proven to be promising materials for various optoelectronic applications. However, CsPbX3 films as optical gain media were confronted with unsatisfactory surface coverage and inferior photoluminescence performance when compared with their colloidal nanocrystal counterparts. Herein, we demonstrate a strategy for improving the optical properties via modification of both top and bottom surfaces of CsPbBr3 films. The treated perovskite films show ultra-smooth morphology and a carrier lifetime of 44 ns, more than one order of magnitude longer than the untreated one. Meanwhile, a mixed polymer layer on the top of the perovskite film could combine surface passivation with symmetric waveguide effects, leading to an outstanding net gain coefficient of 694 cm-1. These merits predict the great potential of all-inorganic perovskite films to support high efficiency charge transport or stimulated emission.

R-phycoerythrin proteins@ZIF-8 composite thin films for mercury ion detection.

Mercury, as one of the most prevalent toxic metals released by various natural and anthropogenic processes, causes severe pollution of soil and groundwater. In this work, R-phycoerythrin (R-PE) proteins encapsulated into ZIF-8 composite thin films were prepared via a solid-confinement conversion process and applied as fluorescent sensors for mercury ion detection. The R-PE proteins encapsulated into ZIF-8 exhibit dual color emissions including green (518 nm) and red (602, 650 nm) fluorescence, while the original orange emission (578 nm) of pure R-PE is significantly suppressed. R-PE@ZIF-8 presents excellent selectivity and sensitivity for mercury detection in a large pH range without buffer solution. Under the optimal conditions, there is a good linear relationship between the fluorescence quenching efficiencies of R-PE@ZIF-8 and logarithmic concentrations of mercury ions in the range of 0.001-50 µM with the detection limit (LOD) of 6.7 nM much lower than the guideline value given by the World Health Organization. Furthermore, multi-peak detection of R-PE@ZIF-8 improves the detection accuracy of Hg2+ concentration.

Embedded Two-Dimensional Perovskite Nanoplatelets with Air-Stable Luminescence.

Two-dimensional (2D) perovskites represent a class of promising nanostructures for optoelectronic applications owing to their giant oscillator strength transition of excitons and high luminescence. However, major challenges lie in the surface ligand engineering and ambient stability. Here, we show that air-stable quasi-2D CsPbBr3 nanoplatelets can be formed in the matrix of Cs4PbBr6 nanosheets by reducing the thickness of Cs4PbBr6 to ∼7.6 nm, the scale comparable to the exciton Bohr radius of CsPbBr3. The 2D behavior of excitons is evidenced by the linear increase of the radiative lifetime with increasing temperature. Moreover, the wide-bandgap Cs4PbBr6 plays roles of surface passivation and protection, which leads to good photoluminescence properties without the photobleaching effect and with ambient stability for over 1 month. Our work demonstrates a unique quasi-2D heterostructure of perovskite nanomaterials, which may either serve as a workbench for studying the exciton recombination dynamics or find application in high-performance optoelectronic devices.

Understanding the Role of Lithium Doping in Reducing Nonradiative Loss in Lead Halide Perovskites.

Adding alkali metal into lead halide perovskites has recently been demonstrated as an effective strategy for reducing nonradiative loss. However, the suggested role of the alkali metal is usually limited to surface passivation, and the semiconductor doping effect is rarely discussed. Here, the mechanism of lithium doping in the photocarrier recombination in solution-processed methylammonium lead halide films is investigated by photoluminescence and photoelectron spectroscopies. It is demonstrated that lithium doping weakens the electron-phonon coupling and acts as donor in perovskites, which provide solid evidence that lithium enters the lattice rather than just in the surface region. The n-type doping creates free electrons to fill the trap states in both the bulk and surface regions, leading to suppressed trapping of photocarriers and reduces nonradiative recombination.

Bright Tail States in Blue-Emitting Ultrasmall Perovskite Quantum Dots.

All-inorganic lead halide perovskite quantum dots (CsPbBr3 QDs) are attracting significant research interests because of their highly efficient light-emitting performance combined with tunable emission wavelength facilely realized by ion exchange. However, blue emission from perovskite QDs with strong quantum confinement is rarely reported and suffers from lower luminescence efficiency. Here we report blue-emitting ultrasmall (∼3 nm) CsPbBr3 QDs with photoluminescence (PL) quantum yield as high as 68%. Using time-resolved and steady-state PL spectroscopy, we elucidate the mechanism of the highly efficient PL as recombination of excitons localized in radiative band tail states. Through analyzing the spectral-dependent PL lifetime and the PL line shape, we obtain a large band tail width of ∼80 meV and a high density of state of ∼1020 cm-3. The relaxation of photocarriers into the radiative tail states suppresses the capture by nonradiative centers. Our results provide solid evidence for the positive role of band tail states in the optical properties of lead halide perovskites, which can be further tailored for high-performance optoelectronic devices.

Effects of Organic Cation Length on Exciton Recombination in Two-Dimensional Layered Lead Iodide Hybrid Perovskite Crystals.

In recent years, 2D layered organic-inorganic lead halide perovskites have attracted considerable attention due to the distinctive quantum confinement effects as well as prominent excitonic luminescence. Herein, we show that the recombination dynamics and photoluminescence (PL) of the 2D layered perovskites can be tuned by the organic cation length. 2D lead iodide perovskite crystals with increased length of the organic chains reveal blue-shifted PL as well as enhanced relative internal quantum efficiency. Furthermore, we provide experimental evidence that the formation of face-sharing [PbI6]4- octahedron in perovskites with long alkyls induces additional confinement for the excitons, leading to 1D-like recombination. As a result, the PL spectra show enhanced inhomogeneous broadening at low temperature. Our work provides physical understanding of the role of organic cation in the optical properties of 2D layered perovskites, and would benefit the improvement of luminescence efficiency of such materials.

Development of a Liquid Chip Technique to Simultaneously Detect Spring Viremia of Carp Virus, Infectious Hematopoietic Necrosis Virus, and Viral Hemorrhagic Septicemia of Salmonids.

A liquid chip technique was developed to detect spring viremia of carp virus (SVCV), infectious hematopoietic necrosis virus (IHNV), and viral hemorrhagic septicemia virus (VHSV) of salmonids simultaneously. Sequences of the G gene of SVCV, N gene of IHNV, and G gene of VHSV were used to design SVCV-, IHNV-, and VHSV-specific primers, which were labeled with biotin and subjected to amination modification. They were then coupled with fluorescence-coded microspheres and used for hybridization with reverse-transcription PCR products of SVCV, IHNV, and VHSV. A BD FACSArray was used to detect fluorescence signal in the reaction system. This assay system had a high sensitivity to SVCV, VHSV, and IHNV, with LODs of 10, 10, and 100 pg/µL, respectively. Moreover, the assay was specific for the detection of SVCV, IHNV, and VHSV and was not susceptible to cross-detection of other viruses, including pike fry rhabdovirus, hirame rhabdovirus, infectious pancreatic necrosis virus, viral nervous necrosis virus, yellowtail ascites virus, grass carp reovirus, red sea bream iridovirus, and koi herpesvirus. The liquid chip assay technique established in this study provides a novel, convenient, and rapid approach for the detection of SVCV, IHNV, and VHSV.