Enhanced Electroluminescence Intensity Research Articles

Unveiling the potential: Gallium-doped zinc oxide/silver-nanoparticles/glass structure for superior anode performance in polymer light-emitting diodes

This study examines the effectiveness of gallium-doped zinc oxide (GZO) combined with silver nanoparticles (Ag-NPs) on glass substrates as anodes for polymer light-emitting diodes (PLEDs). The proposed GZO/Ag-NPs/glass structure achieved superior electrical and optical properties, with a peak luminance of 5834 cd/m2 and an average current efficiency of 1.91 cd/A, significantly outperforming the conventional GZO/Ag-NPs/GZO/glass anode. The GZO/Ag-NPs/glass anode enhanced electroluminescence intensity by 379 % compared to the GZO/glass anode, while the GZO/Ag-NPs/GZO/glass anode showed a 196 % enhancement. This highlights the potential for improved PLED performance through enhanced anode conductivity and localized surface plasmon resonance effects.

Understanding the Luminescence Characteristics of Ultraviolet InGaN/AlGaN Multiple Quantum Wells with Different In Gradients

The electroluminescence (EL) properties of InGaN/AlGaN ultraviolet light-emitting multiple quantum wells (MQWs) with identical average In content but different In gradients (In content increases linearly, along the growth direction) are investigated numerically. It is found that the luminescence efficiency is improved, and the EL spectral peak wavelength becomes longer for the MQW sample with a larger In gradient. Since the influence of In gradient is different for the conduction and valence bands in InGaN layers, the distribution of electrons and holes in QWs may be changed, leading to a redshift of EL spectra. In particular, when the In gradient increases, the overlap integral of electron-hole wavefunction in InGaN QWs increases, resulting in a higher radiative recombination rate and an enhanced EL intensity.

Strain mediated light emission using heterojunctions of all-inorganic mixed-halide perovskite nanocrystals via piezo-phototronic effect

Utilizing the superior luminescence properties and high color purity of inorganic perovskite nanocrystals (NCs), we report the fabrication of color-saturated CsPbBr 3−x I x (x = 0–3)/ZnO heterojunctions based white light emitting diodes (LEDs) on a flexible platform. Here, bandgap engineered mixed halide perovskite NCs have been prepared by solution phase synthesis technique that was spin coated on sputtered ZnO film, acting as an electron injector layer as well as an efficient luminescent material. Broad (~ 350–700 nm) electroluminescence (EL) emission features covering the near UV and entire visible region of the electromagnetic spectrum have been successfully achieved at an ambient (humidity ~ 50–55%) condition with comparatively lower forward bias (V th ~ 2 V). Finally, we demonstrate the significant enhancement (up to ~ 75%) of EL intensity by applying an external stress owing to the piezoelectric properties of ZnO films. The modulation of energy band alignment in CsPbBr 3 /ZnO heterojunction due to the local strain induced positive piezo-potential in ZnO leads to an enhanced radiative recombination efficiency in CsPbBr 3 domain, resulting in superior EL emission characteristics. The combinatorial fascinating optical properties of inorganic perovskites and piezo-phototronic effect of ZnO provide a promising platform to fabricate highly luminescent LEDs that may find perspective applications in high-resolution flexible display devices and wearable electronics. • Band gap engineered mixed halide inorganic perovskite NCs (CsPbBr 3−x I x (x = 0–3)) by solution phase synthesis technique. • Perovskite/ZnO p-n junction based moisture resistant broad white LEDs under fairly lower forward bias of ~ 2V. • Successful tuning of EL emission by increasing the I - /Br - ratio of mixed halide perovskites through anion exchange reaction. • Enhancement of EL intensity up to ~ 75% by applying an external stress owing to the piezoelectric properties of ZnO films.

High-efficiency green micro-LEDs with GaN tunnel junctions grown hybrid by PA-MBE and MOCVD

We fabricated p-i-n tunnel junction (TJ) contacts for hole injection on c-plane green micro-light-emitting diodes (micro-LEDs) by a hybrid growth approach using plasma-assisted molecular beam epitaxy (PA-MBE) and metal–organic chemical vapor deposition (MOCVD). The TJ was formed by an MBE-grown ultra-thin unintentionally doped InGaN polarization layer and an n ++ / n + - GaN layer on the activated p ++ - GaN layer prepared by MOCVD. This hybrid growth approach allowed for the realization of a steep doping interface and ultrathin depletion width for efficient inter-band tunneling. Compared to standard micro-LEDs, the TJ micro-LEDs showed a reduced device resistance, enhanced electroluminescence intensity, and a reduced efficiency droop. The size-independent J-V characteristics indicate that TJ could serve as an excellent current spreading layer. All these results demonstrated that hybrid TJ contacts contributed to the realization of high-performance micro-LEDs with long emission wavelengths.

Understanding Temporal Evolution of Electroluminescence Intensity in Lead Sulfide (PbS) Colloidal Quantum Dot Infrared Light-Emitting Diodes

We, for the first time, report a temporal evolution of the electroluminescence (EL) intensity in lead sulfide (PbS) colloidal quantum dot (CQD) infrared light-emitting diodes. The EL intensity was varied during infrared light emission, and its origin is attributed to competition between the achievement of charge balance associated with interfacial charging at the PbS/ZnO CQD interface and the electric-field induced luminescence quenching. The effect of multi-carrier emission on the enhanced EL intensity is discussed relating to shifting in the wavelength at the peak EL intensity.

Ag nanoparticle surface-plasmon-resonance-enhanced electroluminescence from semipolar n-ZnO/p-GaN heterojunction light-emitting diodes

Ag nanoparticles (NPs) were used to demonstrate the surface-plasmon-resonance (SPR)-enhanced electroluminescence (EL) from semipolar n-ZnO/p-GaN heterojunction light-emitting diodes (LEDs). Ag NPs were synthesized with an average diameter of 30 nm, which was well matched to the optimal calculation result obtained by the finite difference time domain (FDTD) method. The photoluminescence intensities from the top excitation of GaN and ZnO were 70 and 40% enhanced by coupling Ag NPs, respectively. The EL intensity was increased 20% by lighting the semipolar n-ZnO/p-GaN heterojunction LEDs coupled with Ag NPs.

Enhanced Electroluminescence From Si Quantum Dots-Based Light-Emitting Devices With Si Nanowire Structures and Hydrogen Passivation

Enhanced electroluminescence (EL) has been achieved from the light-emitting devices containing Si quantum dots/SiO2 multilayers deposited on Si nanowire arrays because of the good antireflection characteristics of Si nanowire structures, which improves the light extraction efficiency. However, it is found that the EL is first enhanced with increasing the depth of the Si nanowires and then reduced to further increase the depth, though it exhibits the lowest reflectance (∼3%), which may be due to the increased surface defect states after long time etching, as revealed by electron spin resonance measurements. It is demonstrated that the posthydrogen plasma annealing treatments can passivate the surface defect states which, in turn, improve device performance. The best device shows the turn-on voltage as low as 3.5 V, and the EL intensity of devices on Si nanowire arrays is enhanced 16-fold, compared with that of flat one.

Effect of SiO2 layers on electroluminescence from Si nanocrystal/SiO2 superlattices prepared using argon ion beam assisted sputtering

This study investigated the electroluminescence (EL) properties of Si-rich oxide (SRO)/SiO2 superlattices light emitting devices (LEDs). Each SiO2 layer of the superlattices was prepared by using argon ion beam assisted sputtering (IBAS). Transmission electron microscopy revealed that the treatment of Ar ion beams on the SiO2 layers did not affect the size or distribution of the Si nanocrystals in the SRO layers, but enhanced the thin-film quality of the SiO2 and formed a clear SiO2/SRO interface. The refractive index of SiO2 was increased by IBAS because of an increase in the density of SiO2. The EL efficiency was doubled for the IBAS device compared with that of a reference device. According to the retention property, the enhanced EL intensity of the IBAS device was ascribed to lower the charge loss rate through enhancing injection barrier of SiO2. The mechanism of the EL enhancement of the IBAS LED was discussed.

Hybrid electrode based on carbon nanotube and graphene for ultraviolet light-emitting diodes

We report on a transparent current-spreading electrode that combines carbon nanotubes (CNTs) and graphene for UV-LEDs. We compared the device performance and long-term stability of this CNT–graphene hybrid electrode (CGE) with those of the silver nanowire–graphene electrode (SGE) reported previously. Both types of electrode offered excellent UV transmittance and reduced sheet resistance through the bridging effect for grain boundaries and defects in graphene by nanotubes or nanowires. UV-LEDs using such electrodes offered reduced forward voltage and enhanced electroluminescence intensity, but the device with the CGE showed excellent stability whereas the SGE degraded with time.

Optimization of dressed-photon–phonon-assisted annealing for fabricating GaP light-emitting diodes

Using the two-level two-state model, we analyzed the characteristics of enhanced electroluminescence intensity from a GaP LED fabricated by dressed-photon–phonon-assisted annealing. In this model, we utilized the fact that the adiabatic potential barrier of the electronic excited level in configuration space is lower than that of the ground level. It was confirmed by experiments that, in an actual excited level, the barrier was reduced to 0.48 eV. From this finding, it was shown that the spatial distribution of Zn atoms can be changed by means of current injection and light irradiation even at room temperature. In addition, we showed that a structure that is suitable for light emission via DPPs is formed by means of a transition between a low-barrier excited level and a high-barrier ground level, due to stimulated emission. Also, regarding the optimized conditions for maximizing the effect of DPP-assisted annealing, it was found that the optimum ratio of the number of injected electrons to the number of irradiated photons is close to 1, and this was confirmed experimentally.

Effect of scattering from localized surface plasmon resonance on improving the luminescence efficiency of silicon nitride light-emitting devices

We demonstrate the localized surface plasmon resonance enhanced silicon-rich silicon nitride based light-emitting devices by embedding the silver nanostructures between the silicon substrate and luminescent matrix. An about 30 times enhancement of external quantum efficiency is achieved by these inserted silver nanostructures. We attribute this distinct enhancement mainly to the improved back-scattering and carrier injection by the addition of silver nanostructures. The coupling between localized surface plasmons and excitons as well as the increase of light extraction via the surface roughening of ITO electrode also contributes to the enhancement of electroluminescence intensity and its efficiency.

Enhanced luminescence related to transparent conductive oxide in ZnS-based EL device fabricated by screen printing method

This experiment is focused to characterize electroluminescence (EL) properties related to transparent conductive oxide (TCO) like In2O3 or SnO2 being dispersedly inserted into the two different layered structures fabricated by screen printing method. X-ray diffraction and FE-SEM measurement were carried out to characterize the material properties of the ZnS-based EL devices depending on the layered structures and the material constitutions. The results show that the enhanced EL intensity with the TCO insertion is approximately 1.2 times greater than TCO-free sample under the 1-layered structure while in no significant improvement of EL intensity is observed under the 2-layered EL structure. But for the EL intensity comparison under the device structure, 2-layered EL structure having a separate dielectric layer is 1.4 times brighter than the 1-layered structure.

Laterally-current-injected light-emitting diodes based on nanocrystalline-Si/SiO_2 superlattice

Laterally electrically-pumped Si light-emitting diodes (LEDs) based on truncated nanocrystalline-Si (nc-Si)/SiO2 quantum wells are fabricated with complementary-metal-semiconductor-oxide (CMOS) process. Visible electroluminescence (EL) can be observed under a reverse bias larger than ~6 V. The light emission would probably originate from the spontaneous hot-carrier relaxations within the conduction and the valance bands when the device is sufficiently reverse-biased. The EL spectral profile is found to be modulated by varying structure parameters of the interdigitated finger electrodes. Up to ~20 times EL intensity enhancement is achieved as compared to vertical-current-injection LED prepared using the same material system. Based on the lateral-current-injection scheme, a Si/SiO2 MQW LED with Fabry-Perot (FP) microcavity and an on-chip waveguided LED that emits at 1.55-µm are proposed.

Evolution of electroluminescence from multiple Si-implanted silicon nitride films with thermal annealing

Influence of thermal annealing on electroluminescence (EL) from multiple-Si-implanted silicon nitride films has been investigated. A reduced injection current and an enhanced EL intensity have been obtained simultaneously by increasing the annealing temperature, which results in a higher EL quantum efficiency. In addition, four emission bands are identified, and their peak energies, intensities, and full widths at half maxima are found to change with annealing temperature. A model is proposed to illustrate the carrier transport, the mechanisms of the four emission bands, and the evolution of the EL bands with annealing as well. The two major bands and the minor ultraviolet band are explained in terms of the recombination of the injected electrons in either the silicon dangling-bond (≡Si0) states or the nitride conduction band with the injected holes in either the band tail states above the nitride valence band or the valence band itself, while the minor near infrared band is attributed to the Si nanocrystals formed in the thin film.

Light-emitting properties of GaAs/InGaAs quantum wells with a GaAs barrier δ-doped with Mn atoms

The structural, electric, and luminescence properties of quantum-confined InGaAs/GaAs heterostructures, modified by δ-Mn doping of the GaAs barrier, have been investigated. The structures were prepared by combination of metal-organic chemical vapor deposition and laser sputtering of solid targets. The structures had a high crystal quality and pronounced electroluminescence intensity. The Mn content in the δ-doped layer, providing enhanced electroluminescence intensity and reduced operating currents of light-emitting diodes, was experimentally determined.

Enhanced electroluminescence intensity of InGaN∕GaN multi-quantum-wells based on Mg-doped GaN annealed in O2

In Ga N ∕ Ga N multi-quantum-well blue (461±4nm) light emitting diodes with higher electroluminescence intensity are obtained by postgrowth thermal annealing at 720°C in O2-ambient. Based on our first-principle total-energy calculations, we conclude that besides dissociating the Mg–H complex by forming H2O, annealing in O2 has another positive effect on the activation of acceptor Mg in GaN. Mg can be further activated by the formation of an impurity band above the valence band maximum of host GaN from the passivated MgGa–ON complex. Our calculated ionization energy for acceptor Mg in the passivated system is about 30meV shallower than that in pure GaN, in good agreement with previous experimental measurement. Our model can explain that the enhanced electroluminescence intensity of InGaN∕GaN MQWs based on Mg-doped p-type GaN is due to a decrease in the ionization energy of Mg acceptor with the presence of oxygen.

Enhanced electroluminescence efficiency of oxidized amorphous silicon nitride light-emitting devices by modulating Si∕N ratio

The authors had reported green-yellow electroluminescence (EL) from N-rich oxidized amorphous silicon nitride (a-SiN:O) light-emitting devices (LEDs) in a previous work. In this work, a significantly enhanced EL intensity was obtained in the LED by employing Si-rich a-SiN:O instead of N-rich a-SiN:O as luminescent active layer. Moreover, the Si-rich a-SiN:O devices also exhibit lower turn-on voltage and the external quantum efficiency is found to be three times higher than that of the N-rich a-SiN:O devices. The electrical characteristics analyses reveal that the injection barrier for Si-rich a-SiN:O devices is reduced by 30% compared to that of N-rich a-SiN:O devices, which results in a remarkably enhanced carrier-injection efficiency and gives rise to the notable improved performances of the LEDs.

GaN-based light-emitting diodes prepared on vicinal sapphire substrates

Thin InGaN epitaxial layers and GaN-based light-emitting diodes (LEDs) on conventional and vicinal cut sapphire substrates are prepared. It is found that indium atoms are distributed much more uniformly in the samples prepared on vicinal cut sapphire substrates. It is also found that stronger electroluminescence intensity can be achieved without the band-filling effect of localised states from the LEDs with vicinal cut sapphire substrate. With 20 mA current injection, it is found that 44% electroluminescence intensity enhancement can be achieved by using the 1deg tilted sapphire substrate

Comparison of the Performance of InGaN∕AlGaN MQW LEDs Grown on c-Plane and a-Plane Sapphire Substrates

We report a comparative study on the performance of InGaN/AlGaN multiple quantum well (MQW) light emitting diodes (LEDs) fabricated on c- and a-plane sapphire substrates, respectively. It was found that the LEDs grown on a-plane sapphire substrates exhibited enhanced electroluminescence intensity, decreased double crystal X-ray diffraction linewidth, reduced etching pit density, and smaller ideality factor compared to those deposited on c-plane sapphire substrates. The improved LED characteristics are attributed to threading dislocation density decrement inside the LEDs due to the reduced mismatch between LED structure and a-plane sapphire substrate.

Temperature dependence of electroluminescence from silicon p-i-n light-emitting diodes

The temperature dependence of electroluminescence from silicon p-i-n light-emitting diodes with a layer of β-FeSi2 particles inserted in intrinsic silicon was investigated. Anomalous blueshift of the peak energy and enhanced electroluminescence intensity of the silicon band-edge emission were observed at temperatures from 50to200K. The electroluminescence intensity was enhanced due to longer diffusion paths of the injected electrons at elevated temperature, as well as thermal escape of the electrons from the β-FeSi2 particles. The low peak energy compared to that from bulk silicon at low temperature is due to the bound electron-hole pairs induced by the strain potential at the interface between silicon and β-FeSi2 particles. The blueshift of the peak is ascribed to the transition of bound electron-hole pairs into free excitons at elevated temperature. Room temperature electroluminescence from such a silicon light-emitting diode can be obtained at a low current density of 0.3A∕cm2.