Electroluminescence Brightness Research Articles

Luminescence in Zinc Selenide Doped with Sn and (Sn, Dy) Phosphors

AbstractZinc selenide doped with Sn and (Sn, Dy) phosphors has been prepared by firing the samples in an atmosphere of nitrogen gas. The voltage and frequency dependence of electrolyte brightness has been studied. Voltage dependence of electroluminescence (EL) brightness reveals an acceleration collision mechanism in the Schottky barrier at the metal–semiconductor interfaces. EL and photoluminescence (PL) spectra and thermoluminescence (TL) glow curves of these phosphors have also been recorded to understand the nature and mechanism involved in the luminescence process. The trapping parameters are calculated for the glow curves of these phosphors.

Silver nanoparticle-induced enhancement of light extraction in two-dimensional light-emitting diodes.

Monolayer transition metal dichalcogenides (TMDCs) with direct bandgaps are considered promising candidates for building light-emitting diodes (LEDs). One crucial indicator of their performance is the brightness of electroluminescence (EL). In this study, we fabricate WS2-based LEDs that make full use of the assistance of effective transient-mode charge injection. By introducing self-assembled silver nanoparticles (NPs) on top of the LED, the extraction efficiency is significantly improved, with a 2.9-fold EL enhancement observed in the experiment. Full-wave simulations further confirm that the improvement comes from the scattering capability of silver NPs, with results qualitatively fitting the experiment. This approach, with its compatibility with van der Waals heterostructures, can be further promoted to enhance the brightness of 2D monolayer TMDC-based LEDs.

H2O treatment-induced uniform NiOX interfacial layer boosting brightness and light-emitting efficiency of blue perovskite electroluminescence

The reported NiO x interfacial layers in blue perovskite light-emitting diodes (PeLEDs) usually require high-temperature annealing and complex interface modification. Herein, we report a kind of uniform NiO x anode interfacial layer induced by H 2 O treatment, which effectively enhances the brightness and light-emitting efficiency of blue PeLEDs simultaneously. Compared to the as-prepared NiO x anode interfacial layer, H 2 O treatment renders uniform and pinhole-free NiO x morphology. The solution-processed perovskite blue emissive layer prepared atop the H 2 O-treated NiO x interfacial layer demonstrates enhanced photoluminescent property and superior morphology with low trap density. The blue PeLEDs employing H 2 O-treated NiO x as anode interfacial layer show a maximum luminance of 9052 cd/m 2 and a maximum external quantum efficiency (EQE) of 1.80%, whereas the control device based on the as-prepared NiO x anode interfacial layer merely exhibits a maximum luminance of 3850 cd/m 2 and an EQE of 0.88%, leading to about 135% and 104% increase in brightness and efficiency, respectively. The PeLEDs emit pure blue light with emission peak located at 482 nm and demonstrate superior spectral stability under different driving voltages and operating time. • High-brightness blue PeLEDs were fabricated using NiO x interfacial layer. • H 2 O-treated NiO x interfacial layer induces to form high-quality blue perovskite emissive layer. • The blue PeLEDs attain a maximum luminance of 9052 cd/m 2 and a maximum EQE of 1.80%. • The PeLEDs emit stable blue light with emission peak at 482 nm.

Spectral-Luminescent and Electroluminescent Properties of Charge-transfer Systems Based On Electron-donating Diphenylamine Derivatives and Acceptors of Dibenzothiophene Sulfone and Phenanthridine.

Spectral characteristics and luminescence under the photo- and electro-excitation of substituted dibenzthiophene sulfone and phenanthridine were studied in this paper. Diphenylamines are substituents introduced in the 2nd and 7th positions (linear configuration) or the 3rd and 6th positions (angular configuration) of dibenzthiophene sulfone or phenanthridine. All molecules show delayed fluorescence, both in solutions and films produced by thermal vacuum deposition. The value of the energy gap between the S1 and T1 states has been estimated and is shown to depend not only on the spatial arrangement of the fragments among themselves (linear or angular), but also on the nature of the substituent in diphenylamine. The highest electroluminescence brightness was found for the molecules, in which triplet levels are involved, both through the process of triplet-triplet annihilation and through thermally activated delayed fluorescence.

Shungite Carbon Nanoparticles as Modifiers of Zns:Cu Phosphor, Based on Analysis of the EPR Spectral Lines of Mn<sup>+2</sup>

Water dispersions of shungite carbon (ShC) nanoparticles were used for modifying the particle surface of ZnS:Cu, commercial electrophosphor. The EPR spectrum of ZnS:Cu powder has parameters consistent with the paramagnetic centers of Mn+2. Modifying the phodphor surface with ShC nanoparticles results in a non-monotonic change in line width, amplitude and integral intensity caused by nanoparticle concentration, which correlates with variations in the brightness of electroluminescence. Variations in the parameters of spectral lines with nanoparticle concentration and UHF saturation power are interpreted in terms of modification and creation of additional dislocations in the subsurface layer of phosphor with new physico-chemical properties.

Single microwire based smart color-switchable light-emitting diode

Achieving high-brightness and high-efficiency low-dimensional light sources remains a challenging issue due to the lack of high electrical conductance electrodes, as well as the efficient charge injection into the luminescent layer. Herein, by employing chemical vapor deposition methods, individual ZnO microwires with heavily Ga-impurity (GZO MWs) have been successfully synthesized, and then directly assembled on p-GaN substrate to form a heterostructure light-emitting device. In the device, the single GZO MW, which possessing metallic behavior, can function as an outstanding conductance electrode, whilst an efficient luminescence layer. Electroluminescence (EL) characteristics of the single GZO MW-diode operated under forward bias were performed, yielding bright, blue/green lighting with the main peaks positioned at 450 nm and spectral line width of 90 nm. While it also can emit strong broadband, stable and monochromatic white emission under reverse bias. The dual-color-illuminating can be switched repeatedly by reversing the bias polarity. The bias-polarity-switched light source enables independent control over the EL illuminating-color and brightness. The fabrication of single MW based color-switchable light-emitting diode can afford a promising scheme toward the miniaturized, and smart light sources, which hold tremendous power in optical communication, optoelectronic devices and photoelectricity encoders.

Suppression of Electric Field-Induced Segregation in Sky-Blue Perovskite Light-Emitting Electrochemical Cells.

Inexpensive perovskite light-emitting devices fabricated by a simple wet chemical approach have recently demonstrated very prospective characteristics such as narrowband emission, low turn-on bias, high brightness, and high external quantum efficiency of electroluminescence, and have presented a good alternative to well-established technology of epitaxially grown III-V semiconducting alloys. Engineering of highly efficient perovskite light-emitting devices emitting green, red, and near-infrared light has been demonstrated in numerous reports and has faced no major fundamental limitations. On the contrary, the devices emitting blue light, in particular, based on 3D mixed-halide perovskites, suffer from electric field-induced phase separation (segregation). This crystal lattice defect-mediated phenomenon results in an undesirable color change of electroluminescence. Here we report a novel approach towards the suppression of the segregation in single-layer perovskite light-emitting electrochemical cells. Co-crystallization of direct band gap CsPb(Cl,Br) and indirect band gap CsPb(Cl,Br) phases in the presence of poly(ethylene oxide) during a thin film deposition affords passivation of surface defect states and an increase in the density of photoexcited charge carriers in CsPb(Cl,Br) grains. Furthermore, the hexahalide phase prevents the dissociation of the emissive grains in the strong electric field during the device operation. Entirely resistant to 5.7 × 10 V·m electric field-driven segregation light-emitting electrochemical cell exhibits stable emission at wavelength 479 nm with maximum external quantum efficiency 0.7%, maximum brightness 47 cd·m, and turn-on bias of 2.5 V.

One-dimensional micro-scale patterned conjugated polymer structures in bilayer architecture and light emitting diode application

To maximize luminescent brightness in organic light-emitting diode (OLED) systems, charge transport dynamics of emissive layer is needed to be controlled. Herein, we pattern aligned micrometer scale conjugated polymer structures using a solution based processing method. Super yellow (SY)-polyethylene oxide (PEO) nanofibers were electrospun using simple uniaxial electrospinning method. A hybrid bilayer of SY-PEO fiber and poly (9,9-dioctylfluorene) derivative PFO film was formed by the sequential deposition of the electrospun fiber and spin-coated film. The maximum brightness of hybrid LED devices was increased over 1.7 fold to that of film-film bilayer device possessing same average film thickness. Besides, optical properties such as absorption/luminescence of SY were modulated by the rapid crystallization and solvent evaporation during electrospinning process compared to the spin-coated film. The electroluminescence enhancement and color control was attributed to balanced charge transport and enhanced Förster resonance energy transfer (FRET). A patterned hybrid bilayer was formed by the combination of the SY-PEO fiber and PFO thin film via solution based sequential deposition methods, including electrospinning and spin-coating. Thanks to the molecular packing and increment of conjugation length in electrospun SY-PEO nanofibers, such nanofibers exhibited distinct optical properties in comparison with SY thin film. Micro-dimensional alignment of conjugated polymer fiber-film bilayer device exhibited superior electroluminescent brightness besides color tunability of constructed LED device. Such enhancements were attributed to balanced charge transport and enhanced Förster resonance energy transfer in fiber-film hybrid emissive bilayer. • Aligned conjugated polymer fibers by electrospinning process. • Electrospun super yellow -polyethylene oxide nanofibers. • One-dimensional conjugated polymer microstructures in fiber-film hybrid bilayer architecture. • Utilization of conjugated polymer nanofibers in light emitting devices.

Superinjection in Diamond p - i - n Diodes: Bright Single-Photon Electroluminescence of Color Centers Beyond the Doping Limit

Color centers in diamond are considered one of the most promising platforms for electrically driven single-photon sources operating under ambient conditions. Efficient electrical excitation of these emitters is challenging, due to the extremely high activation energy of donors in diamond, which limits the density of free electrons. It may possible to overcome this doping limit, though. The authors predict and numerically demonstrate a superinjection effect in homojunction diamond $p\ensuremath{-}i\ensuremath{-}n$ diodes that allows a 3000-fold increase in free-electron density, thereby enhancing the brightness of single-photon electroluminescence by three orders of magnitude.

Enhanced electroluminescence performance of all-inorganic quantum dot light-emitting diodes: A promising candidate for hole transport layer of Cu-doped NiO nanocrystals

Abstract

Enhanced DC-Operated Electroluminescence of Forwardly Aligned p/MQW/n InGaN Nanorod LEDs via DC Offset-AC Dielectrophoresis.

We introduce an orientation-controlled alignment process of p-GaN/InGaN multiquantum-well/n-GaN (p/MQW/n InGaN) nanorod light-emitting diodes (LEDs) by applying the direct current (DC) offset-alternating current (AC) or pulsed DC electric fields across interdigitated metal electrodes. The as-forwardly aligned p/MQW/n InGaN nanorod LEDs by a pulsed DC dielectrophoresis (DEP) assembly process improve the electroluminescence (EL) intensities by 1.8 times compared to the conventional AC DEP assembly process under DC electric field operation and exhibit an enhanced applied current and EL brightness in the current-voltage and EL intensity-voltage curves, which can be directly used as the fundamental data to construct DC-operated nanorod LED devices, such as LED areal surface lightings, scalable lightings (micrometers to inches) and formable surface lightings. The enhancement in the applied current, the improved EL intensity, and the increased number of forwardly aligned p/MQW/n InGaN nanorods in panchromatic cathodoluminescence images confirm the considerable enhancement of forwardly aligned one-dimensional nanorod LEDs between two opposite electrodes using DC offset-AC or a pulsed DC electric field DEP assembly process. These DC offset-AC or pulsed DC electric field DEP assembly processes suggest that designing for these types of interactions could yield new ways to control the orientation of asymmetric p/MQW/n InGaN diode-type LED nanorods with a relatively low aspect ratio.

Composite electrode with gate-tunable work function for optoelectronic devices

The work function (WF) of Al:LiF composite electrode is first reported to be tunable by electrostatic field effect via the polarization of LiF on Al atom layer. This observation was demonstrated in Kelvin probe force microscope measurement. The optimized Al:LiF composite electrode was employed as organic light emitting diode (OLED) cathode to show the electroluminescent brightness increased from 0 cd m−2 to >8000 cd m−2 and sub second temporal response in an electrolyte gated OLEDs, superior to LiF/Al and Al electrodes. These results are plausibly ascribed to the augmented electron injection with the concurrence of the upward shift of cathode WF induced by gating potential, leading to the improved electron/hole injection balance for efficient electroluminescence.

Ultrathin Quantum Dot Display Integrated with Wearable Electronics.

An ultrathin skin-attachable display is a critical component for an information output port in next-generation wearable electronics. In this regard, quantum dot (QD) light-emitting diodes (QLEDs) offer unique and attractive characteristics for future displays, including high color purity with narrow bandwidths, high electroluminescence (EL) brightness at low operating voltages, and easy processability. Here, ultrathin QLED displays that utilize a passive matrix to address individual pixels are reported. The ultrathin thickness (≈5.5 µm) of the QLED display enables its conformal contact with the wearer's skin and prevents its failure under vigorous mechanical deformation. QDs with relatively thick shells are employed to improve EL characteristics (brightness up to 44 719 cd m-2 at 9 V, which is the record highest among wearable LEDs reported to date) by suppressing the nonradiative recombination. Various patterns, including letters, numbers, and symbols can be successfully visualized on the skin-mounted QLED display. Furthermore, the combination of the ultrathin QLED display with flexible driving circuits and wearable sensors results in a fully integrated QLED display that can directly show sensor data.

Synthesis and luminescent properties of iridium(III) ionic binuclear complexes with 1,4-bis[2-(2-pyridyl)benzimidazolato]butane as a bridging ligand

New ionic binuclear complexes of iridium(III) containing 1,4-bis[2-(2-pyridyl)benzimidazolato]butane as a bridging ligand were synthesized. These compounds exhibit intensive photo- and electroluminescence of yellow-green, green-yellow, and pink colors. The maximal electroluminescence brightness was 4565 cd/m2.

Gate-Tunable Electron Injection Based Organic Light-Emitting Diodes for Low-Cost and Low-Voltage Active Matrix Displays.

Low-cost and low-voltage active matrix displays were fabricated by simply patterning gate electrode arrays on a polymer electrolyte (PE)-coated polymer light-emitting diode (PLED). Structurally, a PE capacitor seamlessly stacked on a PLED by sharing a common Al:LiF composite electrode (PEC|PLED). This monolithic integrated organic optoelectronic device was characterized and interpreted as the tunable work function (surface potential) because of the perturbation of accumulated ions on Al:LiF composite electrode by PEC charging and discharging. The modulation of electron injection by the PEC resulted in increases in the electroluminescent brightness, from <100 cd m-2 to >8000 cd m-2, and the external quantum efficiency from <0.025% to 2.4%.

Effect of magnesium on the performance characteristics of ZnS:Cu,Mn phosphors

We have studied the effect of magnesium added to the starting mixture as MgS or MgCl2 on the performance characteristics of synthesized ZnS:Cu,Mn phosphor powders. It has been shown that the incorporation of certain amounts of magnesium into the ZnS:Cu,Mn phosphor leads to an increase in photo- and electroluminescence brightness and is favorable for Mn incorporation into the structure of the phosphor. In particular, additional doping of a dc electroluminescent ZnS:Cu,Mn phosphor with 30 mol % Mg ensured not only a shift of its luminescence spectrum to shorter wavelengths, thereby extending the color range of light sources, but also a twofold increase in its brightness.

Enhanced Optical and Electrical Properties of Polymer-Assisted All-Inorganic Perovskites for Light-Emitting Diodes.

Highly bright light-emitting diodes based on solution-processed all-inorganic perovskite thin film are demonstrated. The cesium lead bromide (CsPbBr3 ) created using a new poly(ethylene oxide)-additive spin-coating method exhibits photoluminescence quantum yield up to 60% and excellent uniformity of electrical current distribution. Using the smooth CsPbBr3 films as emitting layers, green perovskite-based light-emitting diodes (PeLEDs) exhibit electroluminescent brightness and efficiency above 53 000 cd m-2 and 4%: a new benchmark of device performance for all-inorganic PeLEDs.

Bright blue electroluminescence of diamond/CeF3 composite films

Diamond/CeF3 composite film (DCCF) electroluminescence devices was fabricated on highly doping n-type silicon substrates by using plasma chemical vapor deposition techniques and vacuum electron beam evaporation techniques. The films were characterized by field emission scanning electron microscopy (FE-SEM), Energy dispersive spectrum (EDS), X-ray diffraction (XRD) and electroluminescence (EL) spectrum. The EL spectrum of DCCFs shows that a strong broad peaks are located at 440 nm, and its EL brightness of the device can reach 10 cd/m2 at a forward applied voltage of 120 V. The blue light emitting peak is assigned to the transitions from the excited 5d state to 4f (2F5/2) and 4f (2F7/2) states of Ce3+ ions, respectively.

Structural, morphological and electroluminescence studies of Zno:Co nanophosphor

The nanoparticles of zinc oxide (ZnO) doped with various concentrations of cobalt (Co) were synthesized by chemical precipitation method in the presence of capping agent polyvinylpyrrolidone (PVP). The effect of doping concentration on structural and morphological properties has been studied by X-ray diffraction (XRD) and scanning electron microscope (SEM). Cell volume, bond length, texture coefficient, lattice constants and dislocation density are also studied. Here, we also compared the interplaner spacing and relative peak intensities from their standard values with different angles. Crystallite sizes have been calculated by Debye–Scherrer’s formula whose values are decreasing with increase in cobalt content up to 3 %. It has been seen that the growth orientation of the prepared ZnO nanorods was (101). The XRD analysis also ensures that ZnO has a hexagonal (wurtzite) crystal structure. The electroluminescence (EL) cells were prepared by placing pure and cobalt-doped ZnO nanoparticles between ITO-coated conducting glass plate and aluminium foil. Alternating voltage of various frequencies was applied, and EL brightness at different voltages was measured and corresponding current was also recorded. The voltage dependence of electroluminescence (EL) brightness of the ZnO:Co shows exponential increase. The linear voltage–current characteristic indicates ohmic nature. The EL brightness at a particular voltage is found to increase by increasing Co doping, but for higher percentage of Co the EL brightness is reduced. It is also seen that Co does not influence the threshold voltage. The brightness is also affected by increasing the frequency of AC signal.

Study of structural, morphological, optical and electroluminescent properties of undoped ZnO nanorods grown by a simple chemical precipitation

AbstractIn this work, zinc oxide (ZnO) nanorods were obtained by a simple chemical precipitation method in the presence of capping agent: polyvinyl pyrrolidone (PVP) at room temperature. X-ray diffraction (XRD) result indicates that the synthesized undoped ZnO nanorods have hexagonal wurtzite structure without any impurities. It has been observed that the growth direction of the prepared ZnO nanorods is [1 0 1]. XRD analysis revealed that the nanorods have the crystallite size of 49 nm. Crystallite size is calculated by Debye-Scherrer formula and lattice strain is calculated by Williomson-Hall equation. Cell volume, Lorentz factor, Lorentz polarization factor, bond length, texture coefficient, lattice constants and dislocation density have also been studied. We also compared the interplanar spacings and relative peak intensities with their standard values at different angles. The scanning electron microscope (SEM) images confirmed the size and shape of these nanorods. It has been found that the diameter of the nanorods ranges from 1.52 μm to 1.61 μm and the length is about 4.89 μm. It has also been observed that at room temperature ultraviolet visible (UV-Vis) absorption band is around 355 nm (blue shifted as compared to the bulk). The average particle size has also been calculated by mathematical model of effective mass approximation equation, using UV-Vis absorption peak. Finally, the bandgap has been calculated using UV-absorption peak. Electroluminescence (EL) studies show that emission of light is possible at very small threshold voltage and it increases rapidly with increasing applied voltage. It is seen that smaller ZnO nanoparticles give higher electroluminescence brightness starting at lower threshold voltage. The brightness is also affected by increasing the frequency of AC signal.