Improvement Of Luminescence Efficiency Research Articles

Recent progress in high-performance thermally activated delayed fluorescence exciplexes based on multiple reverse intersystem crossing channels

In the field of organic light-emitting diodes (OLEDs), exciplex materials with thermally activated delayed fluorescence (TADF) characteristics have garnered significant attention in recent years owing to their potential for high fluorescence efficiency, primarily achieved through the reverse intersystem crossing (RISC) process. By converting non-radiative triplet states to radiative singlet states, the RISC process can effectively regulate exciton behavior, resulting in the harvest of triplet excitons and the improvement of luminescence efficiency. Recently, multi-RISC strategies have been developed to further enhance the performance of TADF exciplex materials and devices. In this paper, we review these research progress in this area. We classify molecular design strategies according to the composition of exciplexes, discuss the design principles and energy-harvesting mechanism of high-performance TADF exciplexes based on multi-RISC strategies, and prospect their further research and development. The progressive endeavors summarized in this review may inspire further research on material design and device fabrication, and promote the development of OLED applications.

Theoretical perspective for structural isomerism effect on photoelectric properties of organic room temperature phosphorescence molecules

Purely organic room temperature phosphorescence (RTP) materials have garnered extensive attentions in anti-counterfeiting and encryption, information display, biological imaging and organic light emitting diodes (OLEDs) due to the features of long lifetimes, low toxicity and good biocompatibility. Nevertheless, both the species and amounts of the efficient RTP molecules are far from meeting the requirements for practical applications, and the quantitative relationship between molecular structures and optoelectronic properties needs full elucidated. Herein, the photophysical properties and properties of three isomers (o-BA, m-BA and p-BA) are explored based on the first-principles calculations coupled with thermal vibration correlation function (TVCF) method and kinetic Monte Carlo simulations. The substituent effects on the molecular structures, intermolecular interactions, transition properties, Huang-Rhys factors and reorganization energies, excited state dynamics as well as transfer integrals and charge carrier mobilities are investigated in detail to clarify the structure-property relationships. Results show that o-BA achieves fast radiative rate on account of impressive transition dipole moment and oscillator strength, but equally large non-radiative decay rate leads to low RTP efficiency. Promisingly, the highest holes mobility is determined for o-BA due to the largest transfer integrals of holes originating from the strong intermolecular interactions. In addition, compared with o-BA and p-BA, m-BA could achieve fast intersystem crossing (ISC) rate with large spin orbit coupling constant and low non-radiative decay rate with small geometric structure changes. Relatively balanced holes and electrons mobilities of m-BA have been demonstrated, which contributes to the recombination of excitons and the improvement of luminescence efficiency. For p-BA, the strong intermolecular hydrogen bonding restricts the free rotation of single bonds and promotes the molecular coplanarity, which contributes to efficient emission. Furthermore, the charge mobility is calculated and the temperature dependence is investigated, the hopping mechanisms for studied isomers are illustrated. This work reveals the relationship among molecular structures, crystal packing modes, photophysical properties and charge transport properties of isomers, and provides an insight for molecular design and property prediction.

The role of gold metallic particles on improving green and NIR emissions of Ho3+ ions in non-conventional SeO2 based glass ceramics

This study is devoted to investigate the role of Au0 particles on the improvement of luminescence efficiency of Ho3+ ions in a non-conventional lead boroselenate glass ceramic. Characterization by different spectroscopic techniques indicated that the samples consist of [SeO3]2− and [SeO4]2− structural groups. Among them, the fraction of [SeO3]2− groups exhibited an increasing trend with increase of Au2O3 content. These studies have further revealed that a part of Au3+ ions reduced in to gold nano particles during ceramicization process. Optical absorption (OA) spectra of these samples exhibited a wide absorption band with gradual growth with the Au2O3 concentration. This band is attributed to the surface plasmon resonance (SPR) of gold nanoparticles. Additionally, the OA spectra exhibited several characteristic bands of Ho3+ ions in the spectral region of 450–2000 nm. Photoluminescence (PL) spectra of (Ho free samples) containing gold particles exhibited a broad band in the green to red spectral region, identified due to the transition of electrons from sp → d energy bands of gold nanoparticles. PL spectra of co-doped samples exhibited the emission bands in the green, orange and red regions with enhanced intensities as Au2O3 content is increased. OA and PL spectra were characterized using Judd-Ofelt theory and various radiative parameters were estimated. The impact of Au0 particles on the PL efficiency of holmium ions is comprehensively discussed using kinetic rate equations. The crucial role of gold particles as sensitizers in enhancing the luminescence efficiency especially the green emission is quantitatively explained.

Improvement of Luminescence Efficiency and Stability of CsPbBr2 Quantum Dot Films with Microlens Array Structure

Improvement of Luminescence Efficiency and Stability of CsPbBr2 Quantum Dot Films with Microlens Array Structure

Significant improvement of luminescence efficiency and thermal stability of NaZr2(PO4)3:Eu2+ phosphors induced by Na+-Sc3+ substitution

In this work, the luminescence efficiency and thermal stability of NaZr2(PO4)3:Eu2+ phosphors were significantly improved through Na+-Sc3+ ions substitution for Zr4+ ions. The evolutions of crystal structure and luminescent performances induced by Na+-Sc3+ ions substitution were investigated in details for Na1+xZr2-xScx(PO4)3:Eu2+ phosphors. The extra Na+ ions occupied the interstitial sites in NaZr2(PO4)3 lattice accompanied with the replacement of Zr4+ ions by Sc3+ ions, resulting in the increase of crystal structure density of materials. As a result, the displacement (ΔR) between the ground state and excited state of Eu2+ activators in the configuration coordinate diagram was decreased because of the increased crystal structure density induced by Na+-Sc3+ ions substitution, then leading to the significant improvement of luminescence efficiency and thermal stability. The presented study revealed that crystal structure density might be used a useful proxy in screening higher performance phosphors.

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.

Improving the quantum efficiency of the lanthanide-organic framework [Eu2(MELL)(H2O)6] by heating: A simple strategy to produce efficient luminescent devices

Luminescent materials have been widely studied due to the increasing number of applications in catalysis, biosensors and electro-optical devices. In this sense, the improvement of luminescent efficiency has been sought and several strategies have been proposed. In this work, experimental and theoretical approaches were used to achieve and understand the improvement of luminescent efficiency of lanthanide organic frameworks through a heating process. In this study, [Eu2(MELL)(H2O)6] (EuMELL) was synthesized and characterized by scanning electron and fluorescence microscopies, thermogravimetric analysis, powder X-ray diffraction and photoluminescence techniques. In parallel, theoretical simulations of the material in solid phase were carried out using semiempirical approaches. The comparison between experimental and theoretical results indicated that the more accurate structure was calculated using the Sparkle/PM3 model. The temperature effects on the structure as well as the photophysical properties were evaluated by measurements in situ heating and compared to theoretical simulations using the Sparkle/PM3 geometry. The excellent agreement between the computational and experimental results in this study opens up a series of possibilities for studying other systems, particularly when structural details are not easily available. Moreover, that the controlled heating contributes to improve its the quantum efficiency by approximately 45%, suggesting that this approach can be a valuable tool for technological applications.

Energy transfer induced improvement of luminescent efficiency and thermal stability in phosphate phosphor.

Ce3+ and Eu2+/Tb3+/Mn2+ ions codoped Ca6BaP4O17 (CBPO) phosphors have been prepared via a high-temperature solid state reaction. The structural refinement indicates that the as-prepared phosphors crystallize in monoclinic phase (C2/m) and there are two Ca sites and one Ba site in host lattice. The doping ions are determined to occupy Ca sites and the emission of Ce3+ and Eu2+ ions at different Ca sites were identified and discussed. Since bright blue and yellow emissions were observed from Ce3+and Eu2+ ions monodoped CBPO under n-UV excitation, respectively. They were codoped into the CBPO for designing energy transfer from Ce3+ to Eu2+ to improve the luminescence efficiency of Eu2+. In addition, Tb3+ ions were added into the CBPO:Ce3+ system for realizing highly efficient green emission. The energy transfer mechanisms from Ce3+ to Eu2+/Tb3+ ions were discussed. Interestingly, the incorporation of Mn2+ ions into the CBPO:Ce3+ system enhanced the blue emission of Ce3+ ions due to the modification of crystal lattice. Finally, the thermal stability of CBPO:Ce3+, Eu2+/Tb3+/Mn2+ phosphors were investigated systematically and corresponding mechanisms were proposed. Based on these results, the as-prepared CBPO:Ce3+, Eu2+/Tb3+/Mn2+ phosphors can act as potential blue, yellow, green, and emission-tunable phosphors for n-UV based white LEDs.

Role of ZnS shell on stability, cytotoxicity, and photocytotoxicity of water-soluble CdSe semiconductor quantum dots surface modified with glutathione

The present study examines the stability and cytotoxicity of two quantum dots (QDs) systems in cell culture medium in the presence and absence of a thin layer of a ZnS shell. The two systems were built from a core, CdSe, and surface modified with glutathione (GSH), named CdSe∼GSH and CdSe/ZnS∼GSH. CdSe/ZnS∼GSH QDs exhibited a high photostability with a pronounced enhancement in photoluminescence in cell culture medium. Both systems showed insignificant reduction in cell viability of HFB-4 and MCF-7 cell lines in the dark. Following 60 min of low laser power exposure (irradiance of 10 mW cm−2), CdSe∼GSH QDs showed a remarkable decrease in cell viability, which may result from the detachment of GSH molecules, whereas CdSe/ZnS∼GSH QDs showed an insignificant decrease either immediately after irradiation or even 2 h post-exposure, which can be attributed to the high affinity between ZnS and GSH coatings. This study demonstrated that a thin layer of ZnS shell played a crucial role in the stability of CdSe/ZnS∼GSH QDs in cell culture medium with an improvement in luminescence efficiency, whereas surface modification with GSH molecules in the presence of ZnS showed no significant cytotoxic effects before or after photoirradiation, which makes this system attractive for several biomedical applications.

Influence of Rapid Thermal Annealing on Carrier Dynamics in GaInNAs/GaAs Multiple Quantum Wells

The excitation intensity and time-resolved photoluminescence spectroscopy are used to investigate the impact of annealing on the carrier dynamics in the Ga0.66In0.34N0.013As0.987/GaAs multiple quantum well structure grown by metalorganic chemical vapor deposition. The measurement of excitation intensity photoluminescence (PL), performed for as-grown and annealed samples at different temperatures, indicates that the localized potential has come down slightly after annealing but does not alter the fact that PL emission at low temperature is dominated by localized exciton recombination. In contrast, free carrier recombination is magnified by post-grown annealing at room temperature. Our results show that the decay times are 0.587 and 0.327ns at 10K for the as-grown and annealed samples, and radiative decay times also shorten significantly after annealing at all temperatures. Hence the improvement of luminescence efficiency after annealing is caused by the reduction of localization and enhancement of radiative recombination rate. The reduction of the density of nonradiative centers is demonstrated indirectly after annealing.

Enhancement of photo- and electro-luminescence of GaN-based LED structure grown on a nanometer-scaled patterned sapphire substrate

In this study, a 2 in. sized a highly periodic nanometer-scaled patterned sapphire substrate (NPSS) was fabricated using nanoimprint lithography (NIL) and inductively coupled plasma etching to improve the light-extraction efficiency of GaN-based light-emitting diodes (LEDs). A blue LED structure was grown on the nanometer-scale patterned sapphire substrates, and the photoluminescence (PL) and electroluminescence (EL) were measured to confirm the effectiveness of the nanometer-scaled patterns on sapphire. An improvement in luminescence efficiency was observed when NPSS was applied; 2 times stronger PL intensity and 2.8 times stronger EL intensity than the LED structure grown on the unpatterned sapphire wafers were measured. These results show highly periodic nanometer-scaled patterns create multi-photon scattering and effectively enhance the light-extraction efficiency of LEDs.

Luminescence Properties of Eu(III) Complex/Polyvinylpyrrolidone Electrospun Composite Nanofibers

Herein, we report that an innovative type of 1D nanomaterials, electrospun composite nanofibers (diameters ∼175 ± 25 nm) containing a complex of Eu(TTA)3phen in a matrix of polyvinylpyrrolidone (PVP), was prepared and characterized, and their luminescent properties were evaluated. The study revealed that, when the content of Eu(TTA)3phen was low (i.e., less than 23 wt %), the Eu(TTA)3phen was predominantly distributed in the composite nanofibers as molecular clusters and/or nanoscaled particles with sizes smaller than 10 nm; this substantially enhanced intermolecular interactions between individual Eu(TTA)3phen molecules and the neighboring chain segments of PVP. The Judd−Ofelt theory was used to study the effect of the dispersion of Eu(TTA)3phen on the local chemical environment of Eu3+ ions. The composite nanofibers demonstrated a significant improvement of luminescent efficiency compared with neat Eu(TTA)3phen.

Red Organic Light-Emitting Diode Using the Heteroleptic Tris-Cyclometalated Iridium Complex

A novel red phosphorescent heteroleptic tris-cyclometalated iridium complex, Ir(ppy)2(dpq) based on 2-phenylpyridine (ppy) and 2,4-diphenylquinoline (dpq) ligand was synthesized and characterized for application in organic light-emitting diodes (OLEDs). The heteroleptic tris-cyclometalated iridium complex leads to a significant improvement in luminescence efficiency by cascade exciton transfer and the avoidance of triplet-triplet (T-T) annihilation. A ppy ligand with triplet state energies intermediate between that of the host and the emitting ligand can be successfully employed as “sensitizing ligand” allowing for efficient energy transfer from the host to the emitting ligand. A maximum luminance efficiency of ηc = 14.06 cd/A was achieved at a current density of J = 20.94 mA/cm2. At a higher current density of J = 100 mA/cm2, an efficiency of ηc = 10.77 cd/A was obtained.

Micro-Raman study on the improvement of luminescence efficiency of GaAsN alloys

We have used in situ micro-Raman spectroscopy to clarify the mechanism of the luminescence efficiency improvement of GaAsN alloys by laser irradiation. Raman peak intensity of GaAs-like longitudinal optical (LO) mode phonon was observed to increase with the laser irradiation time. With increasing laser power density, the Raman intensity of GaAs-like LO mode phonon was found to increase more rapidly and to be saturated in a shorter time. Changes in the Raman intensity can be expressed by the same equation used for the luminescence efficiency improvement. Time constants of changes in the Raman intensity were almost in agreement with those of the luminescence efficiency improvement. This indicates that the increase in Raman intensity of GaAs-like LO phonon is closely related to the luminescence efficiency improvement.

Electroluminescence from single monolayers of nanocrystals in molecular organic devices.

The integration of organic and inorganic materials at the nanometre scale into hybrid optoelectronic structures enables active devices that combine the diversity of organic materials with the high-performance electronic and optical properties of inorganic nanocrystals. The optimization of such hybrid devices ultimately depends upon the precise positioning of the functionally distinct materials. Previous studies have already emphasized that this is a challenge, owing to the lack of well-developed nanometre-scale fabrication techniques. Here we demonstrate a hybrid light-emitting diode (LED) that combines the ease of processability of organic materials with the narrow-band, efficient luminescence of colloidal quantum dots (QDs). To isolate the luminescence processes from charge conduction, we fabricate a quantum-dot LED (QD-LED) that contains only a single monolayer of QDs, sandwiched between two organic thin films. This is achieved by a method that uses material phase segregation between the QD aliphatic capping groups and the aromatic organic materials. In our devices, where QDs function exclusively as lumophores, we observe a 25-fold improvement in luminescence efficiency (1.6 cd A(-1) at 2,000 cd m(-2)) over the best previous QD-LED results. The reproducibility and precision of our phase-segregation approach suggests that this technique could be widely applicable to the fabrication of other hybrid organic/inorganic devices.

Improvement of luminescence efficiency by electrical annealing in single-layer organic light-emitting diodes based on a conjugated dendrimer

In this paper, we study the effects of electrical annealing at different voltages on the performance of organic light-emitting diodes. The light-emitting diodes studied here are single-layer devices based on a conjugated dendrimer doped with 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole as the emissive layer. We find that these devices can be annealed electrically by applying a voltage. This process reduces the turn-on voltage and enhances the brightness and efficiency. We obtained an external electroluminescence quantum efficiency of 0.07% photon/electron and a brightness of 2900 cd m-2 after 12.4 V electrical annealing, which are about 6 times and 9 times higher than un-annealing devices, respectively. The improved luminance and efficiency are attributed to the presence of a space charge field near the electrodes caused by charging of traps.

Improvement of luminescence efficiency and photostability in polymer thin films

The photodegradation mechanism and luminescence efficiency of a series of thin polymer films prepared under a variety of conditions was studied. The conjugated polymer, poly( m-phenylene-co-2,5-dioctoxy- p-phenylenevinylene) (PmPV), is shown by infra-red spectroscopy to degrade via the chain scission of the carbon double bond along the polymer backbone. This causes a reduction in conjugation length and a blue shift in its absorption and photoluminescence (PL) spectra. To reduce photodegradation effects, films were prepared using argon (Ar) gas and were investigated in air and an oxygen free environment. The initial PL intensity increased by over 70% for Ar treated films. The PL decay in air was bi-exponential in nature, with a sharp initial decay linked to atmospheric oxygen, and a longer second decay linked to oxygen embedded in the film. The increase in both PL efficiency and degradation lifetime, coupled with device encapsulation, should significantly improve the performance of electroluminescent devices.

GaN系青色・紫外LEDの発光メカニズムと発光効率の改善策

GaN系青色・紫外LEDの発光メカニズムと発光効率の改善策

Photoluminescence study of GaN/InGaN multiquantum well structures at room temperature

The room temperature photoluminescence (PL) of InGaN-based multiple quantum well (MQW) structures was studied. The PL intensity was significantly dependent on the well width and depth. The maximum PL intensity from an MQW with In0.15Ga0.85N wells was produced for a 2nm well; the intensity decreased significantly for thinner and thicker ones. Also, the PL intensity increased exponentially with the energy band gap difference between well and barrier layers. A deep well was effective for the improvement of luminescence efficiency.

Passivation of GaAs surfaces*

Exposure to atomic hydrogen lowers the decomposition temperature of GaAs. Simultaneous exposure of GaAs to atomic hydrogen and atomic nitrogen above 500°C results in a layer rich in GaN. The degree of passivation was monitored by photoluminescence. A fourfold improvement in luminescence efficiency was obtained by nitridization, while a factor-of-ten improvement can be obtained with the more complicated technique of generating an (AlGa)As skin.