Electroluminescence Enhancement Research Articles

Hole injection and electroluminescence enhancement by Ag periodic nanorods array on indium tin oxide electrode in OLED

Periodic silver nanorods array have been used to improve the hole injection and the electroluminescence of organic LEDs (OLED). The nanorods have been fabricated on the indium tin oxide anode by electron beam lithography technique with a precise control of the nanoparticles parameters. The μ-OLEDs fabricated on top of the nanorods show higher-current density and lower turn-on voltages in comparison with the reference device. Moreover, the results emphasise the existence of the localised surface plasmon resonance enhancement effect as well as an important improvement of the hole injection from the anode.

Performance enhancement of blue light-emitting diodes with InGaN/GaN multi-quantum wells grown on Si substrates by inserting thin AlGaN interlayers

We have grown blue light-emitting diodes (LEDs) having InGaN/GaN multi-quantum wells (MQWs) with thin AlyGa1−yN (0 < y < 0.3) interlayers on Si(111) substrates. It was found by high-resolution transmission electron microscopy observations and three-dimensional atom probe analysis that 1-nm-thick interlayers with an AlN mole fraction of less than y = 0.3 were continuously formed between GaN barriers and InGaN wells, and that the AlN mole fraction up to y = 0.15 could be consistently controlled. The external quantum efficiency of the blue LED was enhanced in the low-current-density region (≤45 A/cm2) but reduced in the high-current-density region by the insertion of the thin Al0.15Ga0.85N interlayers in the MQWs. We also found that reductions in both forward voltage and wavelength shift with current were achieved by inserting the interlayers even though the inserted AlGaN layers had potential higher than that of the GaN barriers. The obtained peak wall-plug efficiency was 83% at room temperature. We suggest that the enhanced electroluminescence (EL) performance was caused by the introduction of polarization-induced hole carriers in the InGaN wells on the side adjacent to the thin AlGaN/InGaN interface and efficient electron carrier transport through multiple wells. This model is supported by temperature-dependent EL properties and band-diagram simulations. We also found that inserting the interlayers brought about a reduction in the Shockley-Read-Hall nonradiative recombination component, corresponding to the shrinkage of V-defects. This is another conceivable reason for the observed performance enhancement.

Efficiency of Blue Organic Light-emitting Diodes Enhanced by Employing an Exciton Feedback Layer**Supported by the National Natural Science Foundation of China under Grant No 60906022, the Natural Science Foundation of Tianjin under Grant No 10JCYBJC01100, the Scientific Developing Foundation of Tianjin Education Commission under Grant No 2011ZD02, and the Key Science and Technology Support Program of Tianjin under Grant No

We report that a novel exciton feedback effect is observed by introducing the bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (BAlq) inserted between the emitting layer (EML) and the electron transporting layer in blue organic light emitting diodes. As an exciton feedback layer (EFL), the BAlq does not act as a traditional hole blocking effect. The design of this kind of device structure can greatly reduce excitons' quenching due to accumulated space charge at the exciton formation interface. Meanwhile, the non-radiative energy transfer from EFL to the EML can also be utilized to enhance the excitons' formation, which is confirmed by the test of photolumimescent transient lifetime decay and electroluminescence enhancement of these devices. Accordingly, the optimal device presents the improved performances with the maximum current efficiency of 4.2 cd/A and the luminance of 24600 cd/m2, which are about 1.45 times and 1.75 times higher than those of device A (control device) without the EFL, respectively. Simultaneously, the device shows an excellent color stability with a tiny offset of the CIE coordinates (Δx = ±0.003, Δy = ±0.004) and a relatively lower efficiency roll-off of 26.2% under the driving voltage varying from 3 V to 10 V.

The role of graphene formed on silver nanowire transparent conductive electrode in ultra-violet light emitting diodes.

This paper reports a highly reliable transparent conductive electrode (TCE) that integrates silver nanowires (AgNWs) and high-quality graphene as a protecting layer. Graphene with minimized defects and large graphene domains has been successfully obtained through a facile two-step growth approach. Ultraviolet light emitting diodes (UV-LEDs) were fabricated with AgNWs or hybrid electrodes where AgNWs were combined with two-step grown graphene (A-2GE) or conventional one-step grown graphene (A-1GE). The device performance and reliability of the UV-LEDs with three different electrodes were compared. The A-2GE offered high figure of merit owing to the excellent UV transmittance and reduced sheet resistance. As a consequence, the UV-LEDs made with A-2GE demonstrated reduced forward voltage, enhanced electroluminescence (EL) intensity, and alleviated efficiency droop. The effects of joule heating and UV light illumination on the electrode stability were also studied. The present findings prove superior performance of the A-2GE under high current injection and continuous operation of UV LED, compared to other electrodes. From our observation, the A-2GE would be a reliable TCE for high power UV-LEDs.

Color-Tunable and Highly Luminous N(3-)-Doped Ba2-xCaxSiO4-δN2/3δ:Eu(2+) (0.0 ≤ x ≤ 1.0) Phosphors for White NUV-LED.

In the search for well-defined phosphor materials for white NUV-LEDs, the highly enhanced luminous efficacy by N(3-) doping as well as color tunability via Ca substitution has been successfully obtained in Ba2-xCaxSiO4-δN2/3δ:Eu(2+) (x = 0.0, 0.5, 0.8, 1.0) phosphors. With increasing Ca-substitution rate, the crystal structures of the phosphor materials are changed from the primitive orthorhombic structure to the hexagonal one, so that the CIE coordinates move from bluish-green (at Ca = 0.0), to blue (at Ca = 0.5), and finally to near white region (at Ca = 0.8 and 1.0) in these materials. In combination with the results from X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy, the elemental distribution of the phosphor materials found from secondary-ion mass spectrometry (SIMS) directly indicates that the N(3-) ions are partially substituted for O(2-) ions into the crystal lattice of alkaline-earth orthosilicates and thus critically improves the color-tunable photoluminescence (PL) and electroluminescence (EL) efficiency of the phosphor materials for white NUV-LEDs. The newly found "the N(3-) doping and color-tunable effect" on large PL and EL enhancement may provide a platform in the discovery of new efficient phosphors for solid state lighting.

Effects of Doping and Electrode Contacts on Performance of Organic Light-Emitting Transistors Based on Pentacene and Tris(8-hydroxyquinoline)aluminum

Heterojunction-based organic light-emitting field-effect transistors (OLEFETs) with a top-contact, long-channel geometry were fabricated and comparatively characterized. The neutral cluster beam deposition (NCBD) method was used to successively deposit two layers of p-type pentacene and n-type tris(8-hydroxyquinoline)aluminum (Alq3). For doped OLEFETs, 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) was used as a highly fluorescent dye dopant to enhance the light-emission efficiency and change the emission color. OLEFETs revealed fine device characteristics based on deposition of highly crystalline active layers. The combination of the highly fluorescent DCM doping and asymmetric electrode configuration (Au and Li:Al or LiF/Al) exhibited efficient energy transfer and enhanced electroluminescence (EL) emission. The operating light-emission mechanisms were discussed based on EL photos acquired using a charge-coupled device (CCD) camera.

Hydrothermal synthesis of Y2O3:Eu3+ nanorods and its growth mechanism and luminescence properties

Homogeneous Y2O3:Eu3+ nanorods with the lengths of several micrometres were successfully synthesised on a large scale by using a urea-assisted hydrothermal method and a post-calcining process. In this study, the influences of urea content and NaOH concentration on the oriented growth, photoluminescence (PL) and electroluminescence (EL) intensity enhancement of Y2O3:Eu3+ were investigated. As a precipitant for isotropic growth, urea can counteract the effect of NaOH on oriented growth along the c-axis during hydrothermal treatment. The Y2O3:Eu3+ powders exhibited a strong red emission centred at 613 nm under either 245 nm UV excitation or the direct current high electric field. The PL intensity of the Y2O3:Eu3+ phosphor prepared with 0.3 g of urea reached 141 % that of the sample prepared under the same conditions but without urea. The strategy for controlling the oriented growth, PL and EL enhancement of Y2O3:Eu3+ can be extended to the synthesis of other inorganic nano/micromaterials.

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.

Effect of SiO2 Spacer-Layer Thickness on Localized Surface Plasmon-Enhanced ZnO Nanorod Array LEDs.

Localized surface plasmon (LSP)-enhanced ultraviolet LEDs have been constructed via spin-coating Ag nanoparticles onto ZnO/SiO2 core/shell nanorod array/p-GaN heterostructures. Different from the previous reports where the dielectric spacer-layer thickness was determined only through photoluminescence (PL) characterization, the SiO2 shell thickness in this work is also optimized by actual electroluminescence (EL) measurements to maximize the enhancement. It is interesting to find that the enhancement ratios derived from PL and EL measurements demonstrate different thickness dependences on SiO2 shell: an optimal 3.5-fold PL enhancement was obtained at the SiO2 thickness of 16 nm, while an "abnormal" 7-fold EL enhancement was achieved at the thickness of 12 nm. Time-resolved spectroscopy studies, as well as theoretical estimations and numerical simulations, reveal that the higher-ratio EL enhancement stems from joint contributions, both internal-quantum-efficiency improvement induced by exciton-LSP coupling and light-extraction-efficiency improvement aroused by photon-LSP coupling.

On-Demand Coupling of Electrically Generated Excitons with Surface Plasmons via Voltage-Controlled Emission Zone Position.

The ability to confine and manipulate light below the diffraction limit is a major goal of future multifunctional optoelectronic/plasmonic systems. Here, we demonstrate the design and realization of a tunable and localized electrical source of excitons coupled to surface plasmons based on a polymer light-emitting field-effect transistor (LEFET). Gold nanorods that are integrated into the channel support localized surface plasmons and serve as nanoantennas for enhanced electroluminescence. By precise spatial control of the near-infrared emission zone in the LEFET via the applied voltages the near-field coupling between electrically generated excitons and the nanorods can be turned on or off as visualized by a change of electroluminescence intensity. Numerical calculations and spectroscopic measurements corroborate significant local electroluminescence enhancement due to the high local density of photonic states in the vicinity of the gold nanorods. Importantly, the integration of plasmonic nanostructures hardly influences the electrical performance of the LEFETs, thus, highlighting their mutual compatibility in novel active plasmonic devices.

Plasmonic enhancement in electroluminescence from light-emitting electrochemical cells incorporating gold nanourchins

Enhanced EL from localized surface plasmon resonance in light-emitting electrochemical cells incorporating gold nanourchins.

Surface plasmon-enhanced quantum dot light-emitting diodes by incorporating gold nanoparticles.

Surface plasmon-enhanced electroluminescence (EL) has been demonstrated by incorporating gold (Au) nanoparticles (NPs) in quantum dot light-emitting diode (QLED). Time-resolved photoluminescence (TRPL) spectroscopy reveals that the EL enhancement is ascribed to the near-field enhancement through an effective coupling between excitons of the quantum dot emitters and localized surface plasmons around Au NPs. It is found that the size of Au NPs and the distance between the Au NPs and the emissive layer have significant effects on the performance of QLED. The enhancement can be maximized as the SP resonance wavelength of Au NPs matches well with the PL emission wavelength of the QD film and the distance between Au NPs and the emissive layer maintains 15 nm. The photoluminance (PL) and EL intensity can be enhanced by 4.4 and 1.7 folds with the incorporation of Au NPs. The maximum current efficiency of 4.56 cd/A can be achieved for the resulting QLEDs by incorprating Au NPs with an enhancement factor of 2.0. In addition, the enhancement ratio of 2.2 can be achieved for the lifetime of resulting QLED.

Broadband localized surface-plasmon-enhanced green light-emitting diodes by silver nanocone array

Green light-emitting diodes (LEDs) with silver nanocone-shaped structures embedded in p-GaN have been demonstrated with the surface plasmon (SP) enhancement effect. The resonance frequency has been broadened and the strength of coupling has been considerably enhanced. Compared with the LED without Ag nanocones, the integrated photoluminescence (PL) intensity of the SP-enhanced LED was improved by ∼275%, and the electroluminescence (EL) enhancement ratio at a different wavelength was evaluated at an injection current of 50 mA/mm2. At the same time, a reduction in the recombination lifetime indicated an increased internal quantum efficiency of LEDs. The results of simulation using nanocones as well as nanorods indicate good correlation with the experimental observation of the broadening effect. This structure is promising for converting incident photons into the localized surface plasmon (LSP) mode, to enhance the emission of LEDs within a broad wavelength range.

Enhancement of electroluminescence from embedded Si quantum dots/SiO2multilayers film by localized-surface-plasmon and surface roughening.

In this paper, we prepared a novel structure to enhance the electroluminescence intensity from Si quantum dots/SiO2multilayers. An amorphous Si/SiO2 multilayer film was fabricated by plasma-enhanced chemical vapor deposition on a Pt nanoparticle (NP)-coated Si nanopillar array substrate. By thermal annealing, an embedded Si quantum dot (QDs)/SiO2 multilayer film was obtained. The result shows that electroluminescence intensity was significantly enhanced. And, the turn-on voltage of the luminescent device was reduced to 3 V. The enhancement of the light emission is due to the resonance coupling between the localized-surface-plasmon (LSP) of Pt NPs and the band-gap emission of Si QDs/SiO2 multilayers. The other factors were the improved absorption of excitation light and the increase of light extraction ratio by surface roughening structures. These excellent characteristics are promising for silicon-based light-emitting applications.

Electric-field-induced strong enhancement of electroluminescence in multilayer molybdenum disulfide.

The layered transition metal dichalcogenides have attracted considerable interest for their unique electronic and optical properties. While the monolayer MoS2 exhibits a direct bandgap, the multilayer MoS2 is an indirect bandgap semiconductor and generally optically inactive. Here we report electric-field-induced strong electroluminescence in multilayer MoS2. We show that GaN–Al2O3–MoS2 and GaN–Al2O3–MoS2–Al2O3-graphene vertical heterojunctions can be created with excellent rectification behaviour. Electroluminescence studies demonstrate prominent direct bandgap excitonic emission in multilayer MoS2 over the entire vertical junction area. Importantly, the electroluminescence efficiency observed in multilayer MoS2 is comparable to or higher than that in monolayers. This strong electroluminescence can be attributed to electric-field-induced carrier redistribution from the lowest energy points (indirect bandgap) to higher energy points (direct bandgap) in k-space. The electric-field-induced electroluminescence is general for other layered materials including WSe2 and can open up a new pathway towards transition metal dichalcogenide-based optoelectronic devices.

Enhanced ultraviolet emission from Au/Ag-nanoparticles@MgO/ZnO heterostructure light-emitting diodes: A combined effect of exciton- and photon- localized surface plasmon couplings.

Localized surface plasmon (LSP)-enhanced ultraviolet light-emitting diodes (LEDs) based on a Au/MgO/ZnO metal/insulator/semi- conductor heterostructure were fabricated by embedding Ag nanoparticles (Ag-NPs) into MgO dielectric layer. A ~6-fold electroluminescence (EL) enhancement was achieved from the Ag-NPs decorated device. Time-resolved spectroscopy studies, as well as analogue simulation and theoretical estimation based on experimental data, reveal that the internal quantum efficiency and light extraction efficiency of the heterojunction LED are increased ~3-fold and ~2-fold, respectively, as a result of the introduction of Ag LSPs. This result indicates that the observed EL enhancement originates from a combined effect of both exciton-LSP coupling and photon-LSP coupling.

Electroluminescence enhancement in ultraviolet organic light‐emitting diode with graded hole‐injection and ‐transporting structure

Electroluminescent intensity and external quantum efficiency (EQE) in ultraviolet organic light‐emitting diodes (UV OLEDs) have been remarkably enhanced by using a graded hole‐injection and ‐transporting (HIT) structure of MoO3/N,N ′‐bis(naphthalen‐1‐yl)‐N,N ′‐bis(phenyl)‐benzidine/MoO3/4,4′‐bis(carbazol‐9‐yl)biphenyl (CBP). The graded‐HIT based UV OLED shows superior short‐wavelength emis‐ sion with spectral peak of ∼410 nm, maximum electroluminescent intensity of 2.2 mW/cm2 at 215 mA/cm2 and an EQE of 0.72% at 5.5 mA/cm2. Impedance spectroscopy is employed to clarify the enhanced hole‐injection and ‐transporting capacity of the graded‐HIT structure. Our results provide a simple and effective approach for constructing efficient UV OLEDs. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Dual enhancement of light extraction efficiency of flip-chip light-emitting diodes with multiple beveled SiC surface and porous ZnO nanoparticle layer coating

A porous ZnO nanoparticle layer coating composed of columnar ZnO nanoparticle piles and a multiple-beveled substrate was used to enhance the light extraction efficiency of GaN-based flip-chip light-emitting diodes (FC-LEDs), which were grown on high-purity SiC substrates. The SiC substrate was multiple-beveled by fabricating an ‘X’ pattern on the face of it, followed by a deposition of a porous ZnO nanoparticle layer on the ‘X’-patterned surface. A porous ZnO nanoparticle layer was fabricated with gas phase cluster beam deposition in a glancing incidence. The incident angular-resolved light transmission of the ZnO nanostructure beyond the critical angle of total internal reflection (TIR) was greatly enhanced. The light output power of the LED was improved by nearly 60% compared to the original planar GaN-based LED on an SiC substrate (FC-SLED), which contained a significant enhancement supplemental to the 18% electroluminescence (EL) enhancement realized with the ‘X’-pattern beveling. We demonstrated that a dual enhancement of light extraction efficiency was achieved by using the hierarchical surface consisting of microscale textures (the multiple-beveled surfaces) and nanoscale structures (the ZnO nanoparticle layers).

Spectra of surface plasmon polariton enhanced electroluminescence from electroformed Al-Al2O3-Ag diodes

Narrow band-pass filters have been used to measure the spectral distribution of electroluminescent photons with energies between 1.8 eV and 3.0 eV from electroformed Al-Al2O3-Ag diodes with anodic Al2O3 thicknesses between 12 nm and 18 nm. Electroforming of metal-insulator-metal (MIM) diodes is a non-destructive dielectric breakdown that results in a conducting channel in the insulator and changes the initial high resistance of the MIM diode to a low resistance state. It is a critical step in the development of resistive-switching memories that utilize MIM diodes as the active element. Electroforming of Al-Al2O3-Ag diodes in vacuum results in voltage-controlled negative resistance (VCNR) in the current-voltage (I-V) characteristics. Electroluminescence (EL) and electron emission into vacuum (EM) develop simultaneously with the current increase that results in VCNR in the I-V characteristics. EL is due to recombination of electrons injected at the Al-Al2O3 interface with radiative defect centers in Al2O3. Measurements of EL photons between 1.8 eV and 3.0 eV using a wide band-pass filter showed that EL intensity is exponentially dependent on Al2O3 thickness for Al-Al2O3-Ag diodes between 12 nm and 20 nm thick. Enhanced El intensity in the thinnest diodes is attributed to an increase in the spontaneous emission rate of recombination centers due to high electromagnetic fields generated in Al2O3 when EL photons interact with electrons in Ag or Al to form surface plasmon polaritons at the Al2O3-Ag or Al2O3-Al interface. El intensity is a maximum at 2.0–2.2 eV for Al-Al2O3-Ag diodes with Al2O3 thicknesses between 12 nm and 18 nm. EL in diodes with 12 nm or 14 nm of Al2O3 is enhanced by factors of 8–10 over EL from a diode with 18 nm of Al2O3. The extent of EL enhancement in the thinnest diodes can vary significantly between samples. A narrow band of recombination centers was found in one Al-Al2O3-Ag diode with 12 nm of Al2O3; it had EL intensity 100 times greater at 2.15 eV than the diode with 18 nm of Al2O3. EL intensity for photons with energies greater than 2.6 eV is nearly the same for all diodes.

Enhanced waveguide-type ultraviolet electroluminescence from ZnO/MgZnO core/shell nanorod array light-emitting diodes via coupling with Ag nanoparticles localized surface plasmons.

Localized surface plasmon (LSP) enhanced waveguide-type ultraviolet light-emitting diodes (LEDs) were fabricated by sputtering Ag nanoparticles (Ag-NPs) onto ZnO/MgZnO core/shell nanorod array (CS-NRA)/p-GaN heterostructures. A ∼9-fold enhancement of ZnO ultraviolet electroluminescence (EL) was demonstrated by the Ag-NPs decorated LED compared with the device without Ag-NPs. Angle-dependent EL measurements, as well as finite-difference time-domain simulations of the EL intensity spatial distribution, confirmed the waveguide-type EL transmission mode along the NR's axial direction. The increased spontaneous emission rate observed in time-resolved spectroscopy suggested that the ZnO EL enhancement was attributed to LSP-exciton/polariton coupling. However, a direct coupling is very difficult to achieve between Ag-LSPs and electron-hole pairs in the active region due to their "remote" separation. Thereby, two possible models involving the dynamic process of interactions among excitons, photons, and LSPs, were established to understand the selective enhancement of ZnO EL.