Nanostructured graded-index antireflection layer formation on GaN for enhancing light extraction from light-emitting diodes

We describe the fabrication and characterization of a randomly etched gallium nitride (GaN) surface for enhancing light extraction from light-emitting diodes. Our technique uses silica spheres as nano-targets in a sputter-etch process and produces a fine-grained surface with features around 35 nm. The textured surface layer acts as a graded refractive index layer with antireflection properties. Measurements show that photoluminescence intensity from such treated surfaces on a GaN LED wafer increases 2.2?times over that from pristine surfaces. These findings are also supported by computer modelling studies described here.     

Simulation of the light extraction efficiency of nanostructure light-emitting diodes

The light extraction efficiencies have been calculated for various InGaN/GaN multiple quantum well nanostructure light-emitting diodes including nanopillar,nanorough of P-GaN surface,coreshell and nano-interlayer structure.From the calculated results we can see that the light extraction efficiency is remarkably improved in the nanostructures,especially those with an InGaN or AlGaN nano-interlayer.With a 420-nm luminescence wavelength,the light extraction efficiency can reach as high as 65% for the InGaN or AlGaN nano-interlayer structure with appropriate In or Al content while only 26% for the planar structure.     

Improved light extraction of GaN-based light-emitting diodes with surface-patterned ITO

The surface patterning of the indium tin oxide (ITO) transparent current layer has been investigated to improve the light extraction efficiency of GaN-based light-emitting diodes (LEDs). LEDs with periodic micro-hexagon patterned ITO have been fabricated utilizing standard lithography techniques and inductively coupled plasma (ICP) technology. The luminance intensity of the LED chips with patterned ITO following 160 s ICP etching was enhanced by about 50% compared to the LED chips with unpatterned ITO. Detailed processing parameters are provided. scanning electron microscopy (SEM) and atomic force microscopy (AFM) are used to examine the micro-structures. The results indicate that the surface-patterned ITO technique could have potential applications in high-power GaN-based LEDs.     

Enhancing light extraction of GaN-based blue light-emitting diodes by a tuned nanopillar arrayEnhancing light extraction of GaN-based blue light-emitting diodes by a tuned nanopillar array

Surface patterning of p-GaN to improve the light extraction efficiency of GaN-based blue light-emitting diodes （LEDs） has been investigated. Periodic nanopillar arrays on p-GaN have been fabricated by polystyrene （PS） nanosphere lithog- raphy; the diameter of the nanopillars can be tuned to optimize the electrical and optical properties of the LEDs. The electroluminescence intensity of the nanopillar-patterned LEDs is better than that of conventional LEDs; the greatest en- hancement increased the intensity by a factor of 1.41 at a 20 mA injection current. The enhancements can be explained by a model of bilayer film on a GaN substrate. This method may serve as a practical approach to improve the efficiency of light extraction from LEDs.     

Numerical study on the wavelength-dependence of light extraction efficiency in AlGaN-based ultraviolet light-emitting diodes

Wavelength-dependence of light extraction efficiency (LEE) in AlGaN-based ultraviolet (UV) light-emitting diode (LED) structures is numerically studied based on three-dimensional finite-difference time-domain methods. Due to strong UV light absorption in the p-GaN contact layer, LEE of the UV vertical LED structures remains to be only 6–7 % for the transverse-electric mode and 2–3 % for the transverse-magnetic mode, respectively. The effective LEE of UV LEDs is calculated by considering the optical polarization-dependent LEE, and is found to increase from 4 to 5.5 % as the wavelength increases from 260 to 360 nm. It is shown that the wavelength-dependence of LEE can partially explain the decrease in external quantum efficiency with decreasing wavelengths in AlGaN-based UV LEDs.     

Modelling gratings on either side of the substrate for light extraction in light-emitting diodes

We present a three-dimensional model based on rigorous coupled wave analysis (RCWA) which allows us to study the influence of periodic structures on light extraction in light-emitting diodes. The gratings can be placed both at the interface between the emitting layers and the substrate, where they coherently interact with the dipole field, or at the interface between the substrate and air, where multiple incoherent reflections in the thick substrate contribute to the overall extraction. Even in the case of a grating at the interface between the substrate and air, these multiple incoherent reflections in the substrate still contribute to the overall extraction for large devices, an effect which has been mostly ignored in literature FDTD simulations.     

Functional imprinting structures on GaN-based light-emitting diodes for light pattern modulation and light extraction efficiency enhancement

In this study, we demonstrated a simple method that can be used to simultaneously modulate the far field pattern and enhance the light extraction of GaN-based light-emitting diodes (LEDs). In this method, microstructures were imprinted on a reliable spin-on-glass surface on top of a transparent conductive layer. The far-field pattern was modulated using microoptical structures, and at the same time, the light extraction was enhanced owing to the small refractive index difference and surface roughness. There was no decrease in the electrical performance of these devices. The peak intensity shifted from 0 to 22° in one-dimensional (1D) asymmetrically blazed periodic structures, and a flattened distribution with a uniform intensity within a span of 110° was observed in two-dimensional (2D) symmetrically periodic structures. This method achieved 13 and 40% light enhancements for 1D and 2D structures, respectively.     

Enhancing light extraction of GaN-based blue light-emitting diodes by a tuned nanopillar array

Surface patterning of p-GaN to improve the light extraction efficiency of GaN-based blue light-emitting diodes(LEDs) has been investigated. Periodic nanopillar arrays on p-GaN have been fabricated by polystyrene(PS) nanosphere lithography; the diameter of the nanopillars can be tuned to optimize the electrical and optical properties of the LEDs. The electroluminescence intensity of the nanopillar-patterned LEDs is better than that of conventional LEDs; the greatest enhancement increased the intensity by a factor of 1.41 at a 20 mA injection current. The enhancements can be explained by a model of bilayer film on a GaN substrate. This method may serve as a practical approach to improve the efficiency of light extraction from LEDs.     

空穴注入层对蓝色有机电致发光器件性能的影响

以DPVBi为发光层,NPB为空穴传输层,在阳极ITO和NPB之间分别插入不同的空穴注入层CuPc和PEDOT:PSS,制备了两种结构的蓝色有机电致发光器件(OLEDs):ITO/CuPc/NPB/DPVBi/BCP/Alq3/Al和ITO/PEDOT:PSS/NPB/DPVBi/BCP/Alq3/Al,研究了不同空穴注入材料对蓝色OLEDs发光性能的影响,并与没有空穴注入层的器件进行了比较.其中CuPc分别采用旋涂和真空蒸镀两种丁艺,比较了不同成膜工艺对器件发光特性的影响.结果表明:加入空穴注入层的器件比没有空穴注入层器件性能要好,其中插入水溶性CuPc的器件,其发光亮度和效率虽然比蒸镀CuPc器件要低,但比插入PEDOT:PSS 器件发光性能要好.又由于水溶性CuPc采用旋涂工艺成膜,与传统CuPc相比,制备工艺简单,所以为一种不错的空穴注入材料.     

Influence of organic capping layers on the performance of transparent organic light-emitting diodes

We investigate a set of transparent organic LEDs (TOLEDs) with different organic capping layer (OC) thicknesses to understand the capping layer effect. We find that thickness variation of the OC strongly influences the emission properties of TOLEDs and exhibits different trends for top or bottom emission. The external quantum efficiency for the top side can be enhanced by a factor of 63%, but that of the bottom side only by 4% compared to a reference device without an OC. Additionally, we demonstrate that the introduction of the OC is an effective method to control the bottom-to-top emission ratio within a measured range from 2.87 to 6.05.     

Multi-stacked organic light-emitting diodes using zinc oxide nanoparticle interfacial layers

Stacked organic light-emitting diodes (SOLEDs) with 30-nm nanoparticle (NP) interfacial layers were investigated. Zinc oxide (ZnO) was used as an interfacial layer between two green polymer (GP) layers. SOLEDs with NP interfacial layers had higher device efficiency than did a single-unit device due to the high probability of exciton recombination that originated from the Auger electron-assisted energy up-conversion process. Although the current density and luminance of SOLEDs with ZnO NP interfacial layers were smaller than those of the reference device, the efficiency was doubled because of the big band alignment difference and the large band gap between GP and ZnO NP interfacial layers, which induced more radiative-exciton recombination.     

基于空穴注入层摩擦定向的偏振蓝光聚合物电致发光器件

研究了利用摩擦空穴注入层3,4-乙撑二氧噻吩:聚苯乙烯磺酸(PEDOT:PSS)作为定向层实现聚芴(PFO)薄膜的偏振电致发光,蓝光的色坐标为(0.20,0.21).从聚合物薄膜的紫外可见吸收和光致发光偏振特性,研究了不同定向层摩擦强度、退火温度以及退火时间下PFO薄膜的二向色性,并证明退火温度是决定器件偏振性能的关键因素.当摩擦强度为25 mm退火温度和时间分别为200℃和30 min时,得到较好的偏振性能,器件的电致发光偏振率约为3. 关键词： 偏振发光 摩擦定向 聚合物电致发光 空穴注入层     

Realization of high-performance blue organic light-emitting diodes using multi-emissive layers

High-performance blue organic light-emitting diodes （OLEDs） are developed. A concept of using multiple-emissive layer （EML） configuration is adopted. In this letter, bis（2-methyl-8-quinolinolate）-4- （phenylphenolato）A1 （BAlq） and 9,10-di（naphtha-2-yl）anthracene （ADN）, which serve n- and p-type EMLs, respectively, are used to evaluate and demonstrate the multi-EML concept for blue OLEDs. The thickness effect of individual EMLs and the number of EMLs, e.g., triple and quadruple EML components, on the power efficiency of blue OLEDs are systematically investigated. To illustrate the point, the total thickness of the emissive region in different blue OLEDs are kept contact at 30 nm for comparison. The power efficiency of blue OLEDs with a quadruple EML structure of BAlq/ADN/BAlq/ADN is about 40% higher than that of blue OLEDs having a single EML unit. The Commission Internationale deL＇eclairage color coordinates of multi-EML OLEDs have values that represent the average of blue emissions from individual EMLs of BAlq and ADN.     

Fine-tuning the thicknesses of organic layers to realize high-efficiency and long-lifetime blue organic light-emitting diodes

By using p-bis(p-N,N-diphenyl-aminostyryl)benzene doped 2-tert-butyl-9,10-bis-β-naphthyl)-anthracene as an emitting layer,we fabricate a high-efficiency and long-lifetime blue organic light emitting diode with a maximum external quantum efficiency of 6.19% and a stable lifetime at a high initial current density of 0.0375 A/cm2.We demonstrate that the change in the thicknesses of organic layers affects the operating voltage and luminous efficiency greater than the lifetime.The lifetime being independent of thickness is beneficial in achieving high-quality full-colour display devices and white lighting sources with multi-emitters.     

Two-color light-emitting diodes with polarization-sensitive high extraction efficiency based on graphene

In the present study, graphene photonic crystals are employed to enhance the light extraction efficiency(LEE) of two-color, red and blue, light-emitting diode(LED). The transmission characteristics of one-dimensional(1D) Fibonacci graphene photonic crystal LED(FGPC-LED) are investigated by using the transfer matrix method and the scaling study is presented. We analyzed the influence of period, thickness, and permittivity in the structure to enhance the LEE. The transmission spectrum of 1D FGPC has been optimized in detail. In addition, the effects of the angle of incidence and the state of polarization are investigated. As the main result, we found the optimum values of relevant parameters to enhance the extraction of red and blue light from an LED as well as provide perfect omnidirectional and high peak transmission filters for the TE and TM modes.     

Enhanced light extraction of GaN-based light-emitting diodes with periodic textured SiO_2 on Al-doped ZnO transparent conductive layer

We report an effective enhancement in light extraction of Ga N-based light-emitting diodes(LEDs) with an Al-doped Zn O(AZO) transparent conductive layer by incorporating a top regular textured SiO_2 layer. The 2 inch transparent throughpore anodic aluminum oxide(AAO) membrane was fabricated and used as the etching mask. The periodic pore with a pitch of about 410 nm was successfully transferred to the surface of the SiO_2 layer without any etching damages to the AZO layer and the electrodes. The light output power was enhanced by 19% at 20 m A and 56% at 100 m A compared to that of the planar LEDs without a patterned surface. This approach offers a technique to fabricate a low-cost and large-area regular pattern on the LED chip for achieving enhanced light extraction without an obvious increase of the forward voltage.     

Effects of different buffer layers on the electro-luminescence performances in white organic light-emitting diodes

The effects of different hole injection materials as the buffer layer on the electro-luminescence (EL) performances of white organic light-emitting diodes (WOLEDs) are investigated in detail. It is found that the EL performances and electric properties were strongly dependent on the structure of the used hole injection materials with different thicknesses, which directly affected the injection and transport properties in devices, and thus the EL efficiency and lifetime. It can be seen that a hybrid buffer layer of 5 nm aluminum fluoride (AlF₃)/15 nm 4,4′,4″-tris(3-methylphenylphenylamino) (m-MTDATA) as the hole injection buffer layer shows the best EL performances in efficiency and lifetime, showing a promising hole injection material in WOLEDs. The mechanisms behind the enhanced performance of the hybrid buffer layer in WOLEDs are discussed based on X-ray photoelectron spectroscopy (XPS) measurement.     

Soluble polymer layers as an anode buffer for organic light-emitting diodes

A polymeric layer for an anode buffer is a promising organic light-emitting diode (OLED) component providing some merits such as mechanical film robustness, high temperature stability, smoothened surface and fabrication convenience. In this study, the comparison between some polymeric anode buffer systems for small molecule-based OLEDs was carried out. The device with BFE buffer, one of the well-known conjugated polymers with a triphenylamine moiety, shows the best performances which are further improved by annealing BFE layer at a high temperature of 250 °C.     

Enhanced performances of AlGaInP-based light-emitting diodes with Schottky current blocking layers

A new epitaxial structure of AlGaInP-based light-emitting diode(LED) with a 0.5-μm GaP window layer was fabricated. In addition, indium tin oxide(ITO) and localized Cr deposition beneath the p-pad electrode were used as the current spreading layer and the Schottky current blocking layer(CBL), respectively. The results indicated that ITO and the Schottky CBL improve the total light extraction efficiency by relieving the current density crowding beneath the p-pad electrode. At the current of 20 mA, the light output power of the novel LED was 40% and 19% higher than those of the traditional LED and the new epitaxial LED without CBL. It was also found that the novel LED with ITO and CBL shows better thermal characteristics.     

Properties of CoPt ferromagnetic layers for application in spin light-emitting diodes

The magnetic properties of Co₄₅Pt₅₅ films deposited by electron-beam evaporation in vacuum have been studied. The measurements of the Faraday and Kerr magnetooptical effects confirm the presence of the easy-magnetization axis perpendicular to the Co₄₅Pt₅₅ surface. It is shown that the perpendicular magnetic anisotropy and the residual magnetization are retained at 300 K for a long time. The magnetic characteristics of the Co₄₅Pt₅₅ layer surface have been studied by magnetic force microscopy, and “circular” mobile magnetic structures have been detected. The spin light-emitting diodes based on In(Ga)As/GaAs heteronanostructures with Co₄₅Pt₅₅ contact layers were fabricated. These diodes emit circularly-polarized light in the absence of an external magnetic field.