Enhancement Of Light Extraction Efficiency Research Articles

High-gravity-assisted scalable synthesis of zirconia nanodispersion for light emitting diodes encapsulation with enhanced light extraction efficiency

Zirconia-based hybrid polymers has become one of the ideal encapsulation material for light emitting diodes (LEDs) with high extraction efficiency due to their proper refractive index to those of semiconductor chips. However, the controlling of the dispersion for zirconia nanoparticles in polymeric hosts to obtain optically transparent hybrid nanocomposites has been one of the major challenges. Herein, we reported the synthesis of zirconia nanodispersion via high-gravity-assisted homogeneous precipitation in an internal circulation rotating packed bed (RPB) reactor followed by two-step modification. An alternative to conventional precipitation in batch stirred tank reactors (STR), the process intensification by high-gravity RPB results in homogeneous micromixing during the nucleation and growth of zirconia particles, which are benefit for continuous and reproducible production of ultrasmall zirconia nanoparticles. The obtained zirconia nanoparticles are purely cubic phase with narrow size-distribution in the range of 3–5 nm according to the X-ray diffraction and transmission electron microscopy characterization, which were similarity to that by the conventional methods. Nevertheless, the average hydrodynamic diameters of zirconia nanoparticles in aqueous solutions obtained by RPB methods were much smaller than those prepared in conventional STR, which enabled easy control and surface modification for highly dispersed nanodispersion in organic solvents and/or polymeric hosts. After two-step surface modification, zirconia nanoparticles with goal-directed structures were obtained, with highly dispersity in various organic solvents (e.g. toluene, trichloromethane, tetrahydrofuran, etc.) and aliphatic epoxy resin, forming transparent hybrid films with tunable refractive indexes. The preliminary applications of these zirconia nanodispersions for LEDs encapsulation were demonstrated. The light extraction efficiency of the LEDs devices packaged with zirconia/epoxy hybrid material increased by 10 percent when the doping content of zirconia nanoparticles was 0.2, compared to the devices encapsulated by original epoxy.

Light extraction efficiency enhancement of flip-chip blue light-emitting diodes by anodic aluminum oxide.

We produced an anodic aluminum oxide (AAO) structure with periodic nanopores on the surface of flip-chip blue light-emitting diodes (FC-BLEDs). The nanopores had diameters ranging from 73 to 85 nm and were separated by distances ranging from approximately 10 to 15 nm. The light extraction efficiency enhancement of the FC-BLEDs subjected to different durations of the second pore-widening process was approximately 1.6–2.9%. The efficiency enhancement may be attributed to the following mechanism: periodic nanopores on the surface of FC-BLEDs reduce the critical angle of total reflection and effective energy transfer from a light emitter into a surface plasmon mode produced by AAO.

High color rendering index and directional emission of white OLEDs using nanorod waveguide channels

This paper presents the enhancement of light extraction and luminance efficiency by embedding nanorod waveguide channels on the external structure of organic light emitting diodes (OLEDs). In accordance with Rayleigh scattering and the Fresnel equations, much of the incident light will be propagated via multiple reflections and then extracted when a ZnO nanorod layer is prepared between the air and the glass backside of the OLED. For a blue OLED with a nanorod layer, the current efficiency and external quantum efficiency (EQE) were enhanced 16.1% and 48.9%, respectively. A nanorod-embedded white OLED (rod-WOLED), fabricated with a photoresist spacer coated between the nanorod layer and the color convention layer, showed a 98.5% enhancement in EQE. Highly stable and directional emission of the white OLED with a color rendering index of 77.5 was obtained, making it suitable for directional illumination applications.

Effect of nitridation temperature on formation and properties of GaN nanowall networks on sapphire (0 0 0 1) grown by laser MBE

Vertical aligned GaN nanowall networks (NWN) have been grown on sapphire (0 0 0 1) substrates using laser assisted molecular beam epitaxy (LMBE) by tuning the sapphire pre-nitridation temperature (200–600 °C). Field emission scanning electron microscopy studies showed the formation of hexagonal shaped vertical GaN NWN with wall width of 8–20 nm on high temperature (600 °C) nitridated sapphire whereas flower shape granular GaN structure with grain size of 50–240 nm was obtained on low temperature (200 °C) nitridated sapphire. The crystalline properties of the GaN NWN have been studied using high resolution X-ray diffraction (HR-XRD). The results show that the full width at half maximum of the GaN(0 0 0 2) X-ray rocking curve has a relatively low value compared to previous reports of hetero-epitaxial grown GaN NWN on sapphire and other substrates. Raman spectroscopy measurements revealed the presence of compressive stress in film whereas tensile stress in GaN NWN structures which is also complimented by HR-XRD analysis. The room temperature photoluminescence spectroscopy measurement for NWN having tip width 8–20 nm showed a 94 meV blue-shift in near band edge emission, even though NWN experienced with tensile stress, which confirm the observation of quantum size effect. The hetero-epitaxial growth of porous GaN NWN with high crystalline quality holds promise for applications in field of nitride based sensors and the enhancement of light extraction efficiency in optoelectronics devices.

Numerical analysis of light extraction enhancement of GaN-based thin-film flip-chip light-emitting diodes with high-refractive-index buckling nanostructures

Abstract In this work, the light extraction efficiency enhancement of GaN-based thin-film flip-chip (TFFC) light-emitting diodes (LEDs) with high-refractive-index (TiO2) buckling nanostructures was studied using the three-dimensional finite difference time domain method. Compared with 2-D photonic crystals, the buckling structures have the advantages of a random directionality and a broad distribution in periodicity, which can effectively extract the guided light propagating in all azimuthal directions over a wide spectrum. Numerical studies revealed that the light extraction efficiency of buckling-structured LEDs reaches 1.1 times that of triangular lattice photonic crystals. The effects of the buckling structure feature sizes and the thickness of the N-GaN layer on the light extraction efficiency for TFFC LEDs were also investigated systematically. With optimized structural parameters, a significant light extraction enhancement of about 2.6 times was achieved for TiO2 buckling-structured TFFC LEDs compared with planar LEDs.

Tapering-induced enhancement of light extraction efficiency of nanowire deep ultraviolet LED by theoretical simulations

A nanowire (NW) structure provides an alternative scheme for deep ultraviolet light emitting diodes (DUV-LEDs) that promises high material quality and better light extraction efficiency (LEE). In this report, we investigate the influence of the tapering angle of closely packed AlGaN NWs, which is found to exist naturally in molecular beam epitaxy (MBE) grown NW structures, on the LEE of NW DUV-LEDs. It is observed that, by having a small tapering angle, the vertical extraction is greatly enhanced for both transverse magnetic (TM) and transverse electric (TE) polarizations. Most notably, the vertical extraction of TM emission increased from 4.8% to 24.3%, which makes the LEE reasonably large to achieve high-performance DUV-LEDs. This is because the breaking of symmetry in the vertical direction changes the propagation of the light significantly to allow more coupling into radiation modes. Finally, we introduce errors to the NW positions to show the advantages of the tapered NW structures can be projected to random closely packed NW arrays. The results obtained in this paper can provide guidelines for designing efficient NW DUV-LEDs.

Enhancing the light-extraction efficiency of AlGaN deep-ultraviolet light-emitting diodes using highly reflective Ni/Mg and Rh as p-type electrodes

Improving the light-extraction efficiency (LEE) is a major issue for the development of deep-ultraviolet (DUV) light-emitting diodes (LEDs). For this improvement, we introduced a transparent p-AlGaN contact layer and a reflective p-type electrode. In this work, we investigated the improvements obtained by replacing conventional Ni/Au p-type electrodes with highly reflective Ni/Mg and Rh electrodes. The external quantum efficiencies (EQEs) of 279 nm DUV LEDs were increased from 4.2 to 6.6% and from 3.4 to 4.5% by introducing Ni/Mg and Rh p-type electrodes, respectively. The LEE enhancement factors for the Ni/Mg and Rh electrodes were 1.6 and 1.4, respectively. These results are explained by the fact that the measured reflectances of the Ni/Mg and Rh electrodes were approximately 80 and 55%, respectively. Moreover, it was concluded that a passivation layer is required for Ni/Mg electrodes to prevent the degradation of the LED properties by the oxidation of Mg.

Omnidirectional Reflector with Total Internal Reflective Interface for Light Extraction Enhancement of Solid‐State Light Source

Light extraction efficiency is key to the performance of solid‐state light sources such as the light‐emitting diode. The reflectance of the bottom mirror has a significant impact on the light extraction efficiency because a large difference in the refractive index between the light source and air leads to multiple internal reflections and optical loss. Therefore, efficiency can be drastically improved with even a slight increase in the bottom mirror reflectance. This paper focuses on wide angular light incidences. An omnidirectional reflector (ODR) is then designed that included a low‐index thick layer (MgF2), a pair of quarter‐wavelength‐thick layers (Ta2O5/MgF2), and a metal layer (Ag) on a solid‐state light source with n = 2.4. The first thick layer works as a perfect reflector for wide angular and broadband incident light since total internal reflection (TIR) occurs at the interface. In the simulations, the angular integrated reflectance for our designed reflector (TI‐ODR) reaches 99.7%. The reflectance is higher than the typical ODR that is composed of a quarter‐wavelength‐thick layer and Ag (98.6%) or an Ag reflector alone (97.9%). The TI‐ODR is then fabricated onto a gallium nitride (GaN)‐based light‐emitting diode and obtained a light extraction efficiency enhancement factor of approximately 1.1.

Enhancement of the light-extraction efficiency of light-emitting diodes with SiO2 photonic crystals

In this paper, we proposed a method of using SiO2 photonic crystals (PhC) to improve the light-extraction efficiency (LEE) of light-emitting diodes (LEDs). Numerical simulations based on the finite-difference time-domain (FDTD) algorithm were performed to reveal the mechanism of using SiO2 PhC to improve the light extraction of LEDs. The effects of several critical parameters, including the depth, the filling factor, and the radius of SiO2 PhC on the enhancement of LEE were investigated. The rigorous coupled-wave analysis (RCWA) method was utilized to verify the variation trends we got from the FDTD simulations. The LEE of the LEDs with normal air-hole PhC structures and the conventional planar LEDs were also studied for comparison. According to our calculations, more than 37% improvement has been achieved in the LEE of the SiO2 PhC LEDs in comparison to that of the conventional planar LEDs. And SiO2 PhC LEDs obtained more light extraction than normal air-hole PhC LEDs. The influences of the SiO2 PhC on the current density and the temperature distributions in the active layer of LEDs have also been discussed in this paper. Results demonstrated no noticeable degradation in electrical and thermal characteristics under high current injections.

Simulation for light extraction efficiency of OLEDs with spheroidal microlenses in hexagonal array

A theoretical model based on ray optics is used to simulate the optical performance of organic light-emitting diodes (OLEDs) with spheroidal microlens arrays (MLAs) in a hexagonal array configuration using the Monte Carlo method. In simulations, ray tracing was performed until 20 reflections occurred from the metal cathode, with 10 consecutive reflections permitted in a single lens pattern. The parameters describing the shape and array of the lens pattern of a MLA are its radius, height, contact angle, and fill factor (FF). Many previous results on how these parameters affect light extraction efficiency (LEE) are inconsistent. In this paper, these contradictory results are discussed and explained by introducing a new parameter. To examine light extraction from an OLED through a MLA, the LEE enhancement is studied considering the effect of absorption by indium tin oxide during multiple reflections from the metal cathode. The device size where LEE enhancement is unchanged with changing lens pattern was identified for a fixed FF; under this condition, the optimal LEE enhancement, 84%, can be obtained using an OLED with a close-packed spheroidal MLA. An ideal maximum LEE enhancement of 120% was achieved with a device with an infinite-sized MLA. The angular intensity distribution of light emitted through a MLA is considered in addition to LEE enhancement for an optimized MLA.

Internal strain induced significant enhancement of deep ultraviolet light extraction efficiency for AlGaN multiple quantum wells grown by MOCVD.

In this work, combined analysis of internal strain effects on optical polarization and internal quantum efficiency (IQE) were conducted for the first time. Deep ultraviolet light extraction efficiency of AlGaN multiple quantum wells (MQWs) have been investigated by means of polarization-dependent photoluminescence (PD-PL) and temperature-dependent photoluminescence (TD-PL). With the increase of compressive internal strain applied to the MQWs by an underlying n-AlGaN layer, the degree of polarization (DOP) of the sample was improved from -0.26 to -0.06 leading to significant enhancement of light extraction efficiency (LEE) as the PL intensity increased by 29.2% even though the internal quantum efficiency declined by 7.7%. The results indicated that proper management of the internal compressive strain in AlGaN MQWs can facilitate the transverse electric (TE) mode and suppress the transverse magnetic (TM) mode which could effectively reduce the total internal reflection (TIR) and absorption. This work threw light upon the promising application of compressively strained MQWs to reduce the wave-guide effect and improve the LEE of deep ultraviolet light emitting diodes (DUV LEDs).

利用截头圆锥形仿生蛾眼结构提高LED光提取效率

利用截头圆锥形仿生蛾眼结构提高LED光提取效率

Enhancing light extraction efficiency of organic light-emitting diodes by embedding tungsten trioxide islands or network structure pattern-transferred from a self-assembled deliquesce cesium chloride mask

Abstract We present a new and simple method to improve internal light out-coupling of organic light emitting diodes (OLEDs) by incorporating tungsten trioxide (WO3) islands or network structure in the devices. These structures were pattern-transferred from a cesium chloride (CsCl) film that self-assembled into the islands or network structure as it deliquesced in air. The size and type of structures could be controlled with the thickness of the CsCl film, the relative humidity, the developing time and the developing temperature. The WO3 patterned structures scattered light in the OLEDs devices to extract light trapped in the waveguide modes. As a result, the current efficiencies, external quantum efficiencies and power efficiencies of the WO3 patterned OLEDs were increased compared with those of a conventional OLED without WO3 layer. Operating voltages of the devices with WO3 layer were significantly decreased because the WO3 layer facelifted the hole injection of the devices.

Highly Efficient Chip-Scale Package LED Based on Surface Patterning

Chip-scale package (CSP) light-emitting diodes (LEDs) are considered as the next generation of highly efficient package LED. However, they suffer from total internal reflection that limits their light extraction efficiency (LEE). In this letter, surface patterning, in terms of microstructures, as a well-controlled surface roughness method is designed and applied on CSP-LED to improve the LEE. Based on simulation results, pyramid and flat top pyramid structures have been designed and optimized. Four types of layouts, including tightly, interlaced, distant and chessboard layout, are also compared. A LEE enhancement of more than 20% is achieved. Furthermore, the four layouts with pyramid structures are fabricated using nano-imprint technology. The experimental results match the simulated results well with only a mismatch of 2.9%. The fabricated layout shows an enhancement of 20.31% when pyramid structures are fully filling the surface. Surface patterned CSP-LED, therefore, exhibits a great potential for high efficient LED.

High refractive index coating of phosphor-in-glass for enhanced light extraction efficiency of white LEDs

The coating of phosphor-in-glass with high refractive index was successfully fabricated for enhanced light extraction efficiency of white LEDs through a multilayer screen-printing and low-temperature sintering procedure. The effects of sintering temperature on the B2O3–SiO2–ZnO glass coating and YAG:Ce phosphor concentration on the phosphor-in-glass coating were investigated. The B2O3–SiO2–ZnO glass matrix possesses low glass transition temperature and appropriate thermal expansion coefficient. With the addition of La2O3 and WO3, high refractive index of 1.82 at 460 nm is acquired. The amorphous glass coating, sintered at 600 °C, yields an optimized transmittance of 50%. Furthermore, negligible thermal degradation is observed in the phosphor-in-glass coating, and the emission peak under the 460-nm excitation is at ~540 nm. When increasing the concentration of phosphor, luminous efficacy and correlated color temperature are both improved, but color rendering index is deteriorated. As the refractive index increases, the light extraction efficiency of white LEDs is enhanced due to the reason that the match of refractive index between the glass matrix and the phosphor decreases the total internal reflection at the interface.

Enhanced light extraction efficiency using self-textured aluminum-doped zinc oxide in organic light-emitting diodes

Abstract We report enhanced light extraction efficiency in organic light-emitting diodes (OLEDs) fabricated on a self-textured aluminum-doped zinc oxide (AZO) anode layer. The self-textured AZO (ST-AZO) layer was fabricated by radio-frequency magnetron sputtering with a short period of thermal treatment without employing any additional etching processes. The green-emitting OLEDs exhibited a maximum power efficiency of 56.1 lm/W with 33.7% external quantum efficiency (EQE). We achieved a 3.24-fold enhancement in power efficiency and 2.55-fold increase in EQE for the OLED fabricated on the ST-AZO anode compared to that fabricated on the ITO anode. Furthermore, a low driving voltage and high current efficiency were obtained simultaneously for the OLED fabricated on the ST-AZO layer compared to that fabricated on the flat ITO anode layer. The ST-AZO layer acted as a random scattering layer that enabled the efficient extraction of generated light and served as the anode layer instead of the commonly used ITO. Our study showed that the ST-AZO layer fabricated by a simple sputtering process effectively improved the optical and electrical properties of the OLED.

Efficiency enhancement of InGaN amber MQWs using nanopillar structures

Abstract We have investigated the use of nanopillar structures on high indium content InGaN amber multiple quantum well (MQW) samples to enhance the emission efficiency. A significant emission enhancement was observed which can be attributed to the enhancement of internal quantum efficiency and light extraction efficiency. The size-dependent strain relaxation effect was characterized by photoluminescence, Raman spectroscopy and time-resolved photoluminescence measurements. In addition, the light extraction efficiency of different MQW samples was studied by finite-different time-domain simulations. Compared to the as-grown sample, the nanopillar amber MQW sample with a diameter of 300 nm has demonstrated an emission enhancement by a factor of 23.8.

Enhanced light extraction efficiency of micro-ring array AlGaN deep ultraviolet light-emitting diodes

An effective approach to overcome inherently poor light extraction efficiency of AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) is presented. We demonstrated the 5 × 5 array micro-ring DUV LED having an inclined sidewall at the outer perimeter and a p-GaN-removed inner circle of the micro-ring, together with MgF2/Al omnidirectional reflectors. The micro-ring array DUV LED shows remarkably higher light output power by 70% than the reference, consistent with the calculated result, as well as comparable turn-on and operational voltages, which are attributed to the effective extraction of strong transverse-magnetic polarized anisotropic emission and the reduction of the absorption loss by the p-GaN contact layer, simultaneously.

Enhanced light extraction efficiency of a InGaN/GaN micro-square array light-emitting diode chip

A InGaN/GaN micro-square array light-emitting diode (LED) chip (micro-chip) has been successfully fabricated by the focused ion beam (FIB) etching technique, which can reduce ohmic contact degradation in the fabrication process of three-dimensional (3D) structure devices. Our results show that the micro-chip exhibits a similar current–voltage performance compared to the corresponding InGaN/GaN planar LED chip (planar-chip). At the driving current of 20 mA, the output power of the micro-chip is improved by 17.8% in comparison to that of the planar-chip. A relatively broad emission and enhanced emission intensity in the perpendicular direction are obtained in angular-resolved EL (AREL) measurements for the micro-chip. Three-dimensional finite difference time domain (FDTD) simulations have also proven enhanced emitted optical energy distribution. The enhancement mechanism is correlated to the increased light extraction efficiency (LEE) of the micro-chip, mainly owing to more photons from the exposed MQWs surfaces that can be efficiently extracted by the micro-square array.

Improved light output of plastic scintillator by a modified self-assembled photonic crystal

In this investigation, we have demonstrated that a modified self-assembled photonic crystal with conformal high refractive index material TiO2 can achieve a great enhancement of light extraction efficiency. A 2.26 fold wavelength- and angle-integrated enhancement ratio can be achieved. The conformal layer increases the number of leaky modes and thus improve the extraction efficiency. The enhancement is attributed to the leaky modes based on the individual microspheres with conformal layer. Their low quality factors with a broadband characteristic are advantageous to the broadband enhancement for the emission spectra of plastic scintillator. Furthermore, the dense conformal layers have excellent combination with the self-assembled microspheres and the whole preparation process cannot destroy the plastic scintillator, which is beneficial to the practical application.