Improved Light Extraction Efficiency Research Articles

36‐3: Improvement of Light Extraction Efficiency of Laser‐Phosphor Light Source

We investigated the laser‐phosphor light source by using inorganic phosphor wheel. We experimentally confirmed the light extraction efficiency of the inorganic phosphor wheel which is 8% higher than conventional phosphor wheel. In addition, we explain about the cause of improvement of the efficiency by showing fluorescence emission model.

Controlled synthesis of polystyrene-assisted tin-doped indium oxide nanowire networks

Abstract

Study on sapphire microstructure processing technology based on wet etching

Aiming at the problem that sapphire surface roughness is quite large after wet etching in sapphire microstructure processing technology, we optimize the wet etching process parameters, study on the influences of concentration and temperature of etching solution and etching time on the sapphire surface roughness and etching rate, choose different process parameters for the experiment and test and analyze the sapphire results after wet etching. Aiming at test results, we also optimize the process parameters and do experiment. Experimental results show that, after optimizing the parameters of etching solution, surface roughness of etched sapphire is 0.39 nm, effectively with reduced surface roughness, improved light extraction efficiency and meeting the production requirements of high-precision optical pressure sensor.

Dielectric nanoparticles for the enhancement of OLED light extraction efficiency

This work reports the use of dielectric nanoparticles placed at glass substrate in the improvement of light extraction efficiency of organic light emitting diode (OLED). The nanoparticles will act as scattering medium for the light trapped in the waveguiding modes of the device. The scattering efficiency of dielectric nanoparticles has been calculated by Mie Theory. The finite difference time domain (FDTD) analysis and simulation estimate the effect of dielectric nanoparticles on the light extraction efficiency of OLED. The efficiency depends upon the diameter, interparticle separation and refractive index of dielectric nanoparticles. It is shown that the dielectric nanoparticles layer can enhance the light extraction efficiency by a factor of 1.7.

Suspended p–n junction InGaN/GaN multiple quantum wells device with bottom silver reflector

In this study, we propose, fabricate, and characterize suspended p–n junction InGaN/GaN multiple quantum wells (MQWs) device with a bottom silver reflector for diverse applications. The suspended device was achieved through a double-side process on the GaN-on-silicon platform and tested by using a micro-reflectance/transmittance setup. When the suspended device operates under the light-emitting diode mode, a fraction of the emitted light reflected by the bottom silver reflector can enter into the air through the top escape cone, leading to improved light extraction efficiency. When the suspended device operates under the photodiode mode, part of the transmitted light can be reflected back by the bottom silver reflector to enhance the photovoltaic effect. The experimental results demonstrate that such suspended device can be used as both transmitter and receiver for visible light communication. And the detecting features even when the suspended structure is under turn-on state, demonstrating a potential realization of light emission and photodetection simultaneously.

Simulation and Experimental Study on Anti-reflection Characteristics of Nano-patterned Si Structures for Si Quantum Dot-Based Light-Emitting Devices.

Surface-textured structure is currently an interesting topic since it can efficiently reduce the optical losses in advanced optoelectronic devices via light management. In this work, we built a model in finite-difference time-domain (FDTD) solutions by setting the simulation parameters based on the morphology of the Si nanostructures and compared with the experimental results in order to study the anti-reflection behaviors of the present nano-patterned structures. It is found that the reflectance is gradually reduced by increasing the depth of Si nanostructures which is in well agreement with the experimental observations. The reflectance can be lower than 10 % in the light range from 400 to 850 nm for Si nano-patterned structures with a depth of 150 nm despite the quite low aspect ratio, which can be understood as the formation of gradually changed index layer and the scattering effect of Si nano-patterned structures. By depositing the Si quantum dots/SiO2 multilayers on nano-patterned Si substrate, the reflectance can be further suppressed and the luminescence intensity centered at 820 nm from Si quantum dots is enhanced by 6.6-fold compared with that of flat one, which can be attributed to the improved light extraction efficiency. However, the further etch time causes the reduction of luminescence intensity from Si quantum dots which may ascribe to the serious surface recombination of carriers.

Improved light extraction efficiency of GaN-based flip-chip light-emitting diodes with an antireflective interface layer

GaN-based flip-chip light-emitting diodes (FC-LEDs) grown on nanopatterned sapphire substrates (NPSS) are fabricated using self-assembled SiO2 nanospheres as masks during inductively coupled plasma etching. By controlling the pattern spacing, epitaxial GaN can be grown from the top or bottom of patterns to obtain two different GaN/substrate interfaces. The optoelectronic characteristics of FC-LED chips with different GaN/sapphire interfaces are studied. The FC-LED with an antireflective interface layer consisting of a NPSS with GaN in the pattern spacings demonstrates better optical properties than the FC-LED with an interface embedded with air voids. Our study indicates that the two types of FC-LEDs grown on NPSS show higher crystal quality and improved electrical and optical characteristics compared with those of FC-LEDs grown on conventional planar sapphire substrates.

Influences of the p-GaN Growth Temperature on the Optoelectronic Performances of GaN-Based Blue Light-Emitting Diodes

We investigate the influence of p-(Al)GaN growth temperature ( $T_{g}$ ) on the optoelectronic performances of InGaN/GaN multiple-quantum-well (MQW) blue light-emitting diodes (LEDs). We systematically combine various characterization techniques, such as current–voltage, current-light output power, capacitance–voltage ( $C$ – $V$ ) under forward and reverse biases, photocurrent and electroreflectance (ER) spectroscopies, temperature-dependent electroluminescence, and the internal quantum efficiency (IQE). From the experimental analyses, it is shown that increasing $T_{g}$ induces: 1) the reduced optical loss by the improved crystal quality of the p-GaN layer (improved light extraction efficiency) and 2) the decreased IQE due to the Mg diffusion from the p-(Al)GaN layer to the MQW region. This paper demonstrates that the fine control of the Mg diffusion from the p-GaN layer to the InGaN/GaN MQW region is the key factor for achieving highly efficient blue LEDs. Moreover, a method of estimating the Mg diffusion length is proposed for the first time by analyzing both the $C$ – $V$ curves and the ER spectra under reverse biases.

Improvement of Light Extraction Efficiency in Flip-Chip Light Emitting Diodes on SiC Substrate via Transparent Haze Films with Morphology-Controlled Collapsed Alumina Nanorods.

We demonstrate GaN-based flip-chip light emitting diodes (FC-LEDs) on SiC substrate achieving high extraction efficiency by simply attaching the optically transparent haze films consisting of collapsed alumina nanorods. Through controlled etching time of alumina nanorods, we obtain four types of films that have different morphologies with different optical transmittance and haze properties. We show that the light output power of the FC-LEDs with film, which has 95.6% transmittance and 62.7% haze, increases by 20.4% in comparison to the bare LEDs. The angular radiation pattern of the LEDs also follows the Lambertian emission pattern without deteriorating the electrical properties of the device. The improvement of light extraction is mainly attributed to the reduced total internal reflection (TIR) via efficient out-coupling of guided light from SiC substrate to air by collapsed alumina nanorod structures in the film. The high transparency of film and reduced Fresnel reflection via graded refractive index transition between the film and SiC substrate also contribute to the extraction enhancement of the device. We systematically investigate the influence of haze film's geometrical or optical properties on the extraction efficiency of FC-LEDs, and this study will provide a novel approach to enhance the performance of various optoelectronic devices.

Highly efficient and reliable high power InGaN/GaN LEDs with 3D patterned step‐like ITO and wavy sidewalls

Nitride‐based high power LEDs with finger‐like SiO2 current blocking layer (CBL), three‐dimensional (3D) patterned step‐like ITO double layers and wavy sidewalls were fabricated. The finger‐like SiO2 CBL beneath finger‐like p‐electrode was designed to prevent current crowding effect, thereby facilitating uniform current spreading over the entire chip. In addition, 3D patterned step‐like ITO double layers, including alternating 230 nm thick patterned upper step ITO layer and 100 nm thick lower step ITO layer, were formed by combining photolithography and aqua regia etchant. We showed that the top light extraction efficiency of high power LEDs can be significantly enhanced by taking 3D patterned step‐like ITO. The light output power of high power LEDs with 3D patterned step‐like ITO double layers is 13.9% higher than that of LEDs with smooth ITO layer. High‐power LEDs with wavy sidewalls was fabricated by an optimized mask design in conjunction with dry etching process based on Cl2/BCl3 to improve light extraction efficiency at the horizontal direction. We demonstrated that light output power of high power LEDs with wavy sidewalls can be improved by 11% as compared to LEDs with flat sidewalls.

Enhancement of light extraction efficiency of vertical LED with patterned graphene as current spreading layer

In this paper we report an optimised pattern of surface textured graphene current spreading layer (CSL) for the enhancement of light extraction efficiency (LEE) in InGaN/GaN vertical light emitting diodes (V-LEDs). It is found that by texturing graphene surface LEE improves drastically. This improvement is attributed to better current spreading of graphene and increased random and multiple scattering of light through textured surfaces. Simulation results illustrate that V-LEDs with surface textured (hexagonal pattern) ITO as CSL shows threefold improvement in light extraction efficiency compared to V-LEDs with no surface texturing on ITO CSL. Further, LEE of V-LEDs having patterned graphene CSL is compared with that for indium tin oxide (ITO) CSL. V-LEDs with optimised hexagonal patterning on graphene CSL shows 13.42% enhancement of LEE compared to that of LED with hexagonal patterning on ITO surface.

랜덤 택스쳐 필름을 이용한 유기 발광 소자의 광추출 효율 향상에 관한 연구

An improvement of light-extraction efficiency of organic light-emitting diodes was studied by using random-textured films (RTF). Device was made in a structure of RTF/glass/ITO/TPD//LiF/Al. RTF mold was made by spreading PDMS solution on a sandpaper. By pressing this mold on the glass substrate pre-coated with ZPU material, the RTF was obtained. From this study, there was an improvement of external quantum efficiency by about 30% in the device with the random-textured film (RTF 40) compared to that of the reference one.

Enhanced light extraction efficiency of integrated LEDs devices 7with the taper holes microstructures arrays

Abstract A kind of polydimethylsiloxane (PDMS) film with taper holes microstructures arrays (THMA) was fabricated and coated on the integrated LEDs device. The integrated LEDs device with THMA was then measured along with a comparative series of tests of the integrated LEDs device encapsulated flatly in the following categories: electroluminescence spectra, luminous flux, radiant flux and light extraction efficiency. The experimental results demonstrated that light extraction efficiency of integrated LEDs devices could be enhanced by THMA. Furthermore, the improvement of light extraction efficiency is up to 23.41–24.43% under different injection currents.

Improved light extraction efficiency of GaN-based vertical LEDs using hierarchical micro/subwavelength structures

We demonstrate the hierarchical micro- and subwavelength-structures on the surface of GaN based green vertical light-emitting diodes (VLEDs) to enhance the light extraction efficiency. Hierarchical structures (HSs), which were fabricated by simple two-step process consisting of pattern transfer of reflowed photoresist for micro structures (MSs) and self-masked dry etching method for subwavelength structures (SWSs), play an important role in enhancing the light extraction efficiency by reducing both the total internal reflection and the Fresnel reflection. HSs lead to an enhancement of optical output power of ∼200% compared to the conventional VLEDs with flat surface as a result of reflection suppression and transmittance enhancement, without any serious degradation of electrical characteristics and a change of peak emission wavelength as function of injection current. Furthermore, optical simulations by ray-tracing and the rigorous coupled-wave analysis methods provide design guidelines for MSs and SWSs, respectively.

Multiple emissive layers white organic light emitting device with nanoplatforms patterning structure for improved current efficiency and color balance

In this paper, white organic light-emitting device (WOLED) with the structure of ITO/NPB/EML (blue)/CBP/EML (red)/CBP/EML (blue)/TPBi/LiF/Al was selected as reference. Then, the reference device was modified by nanoplatforms patterning structure at the TPBi/LiF or NPB/EML interface. The electroluminescence performance of above two kinds of devices was carefully investigated. The results indicate that WOLED with nanoplatforms at TPBi/LiF interface exhibits an enhanced current efficiency relative to reference device, and the enhancement factor is 1.52, which is due to improved light extraction efficiency. When nanoplatforms are located at the NPB/EML interface, WOLED shows more significantly improved current efficiency relative to reference device, and the enhancement factor is 1.80. Here, the improvement in current efficiency is attributed to increased light extraction efficiency and broadened carrier recombination zone. In addition, it is also found that WOLED with nanoplatforms located at the NPB/EML interface exhibits remarkably strengthened red light intensity in electroluminescence spectrum and good color balance, in which the intensity ratio of red-light to blue-light (Ired/Iblue) is 0.68, higher than that of reference WOLED (Ired/Iblue=0.5). The implanting process of nanoplatforms patterning structure introduced in this paper is very simple and low-cost, which can be scaled up to large area WOLED manufacturing.

Development of Highly Efficient Light Extraction Technology with Nano-structure for OLED Devices

Novel nano-structure for the improved light extraction efficiency of reduced evanescent mode OLED devices (REM OLED) was developed. The nano-structure was designed to have high transmittance especially over critical angle in order to achieve good combination with the light distribution pattern of the REM OLED devices. In this system, the wide angular substrate mode was efficiently converted into the external mode, and high efficacy green OLED with EQE of 54 % was obtained. Relative light extraction efficiency to a hemisphere lens system was 89 %.

Enhancement of TE polarized light extraction efficiency in nanoscale (AlN)m /(GaN)n (m>n) superlattice substitution for Al-rich AlGaN disorder alloy: ultra-thin GaN layer modulation

The problem of achieving high light extraction efficiency in Al-rich AlxGaN is of paramount importance for the realization of AlGaN-based deep ultraviolet (DUV) optoelectronic devices. To solve this problem, we investigate the microscopic mechanism of valence band inversion and light polarization, a crucial factor for enhancing light extraction efficiency, in Al-rich AlxGaN alloy using the Heyd–Scuseria–Ernzerhof hybrid functional, local-density approximation with 1/2 occupation, and the Perdew–Burke–Ernzerhof functional, in which the spin–orbit coupling effect is included. We find that the microscopic Ga-atom distribution can effectively modulate the valence band structure of Al-rich AlxGaN. Moreover, we prove that the valence band arrangement in the decreasing order of heavy hole, light hole, and crystal-field split-off hole can be realized by using nanoscale (AlN)m/(GaN)n (m>n) superlattice (SL) substituting for Al-rich AlxGaN disorder alloy as the active layer of optoelectronic devices due to the ultra-thin GaN layer modulation. The valence band maximum, i.e., the heavy hole band, has px- and py-like characteristics and is highly localized in the SL structure, which leads to the desired transverse electric (TE) polarized (E⊥c) light emission with improved light extraction efficiency in the DUV spectral region. Some important band-structure parameters and electron/hole effective masses are also given. The physical origin for the valence band inversion and TE polarization in (AlN)m/(GaN)n SL is analyzed in depth.

Improvement of the light extraction efficiency of GaN-based LEDs using rolled-up nanotube arrays.

In this paper, we have investigated the effect of rolled-up nanotubes on the light extraction efficiency of GaN-based LEDs using two-dimensional finite element method simulation. The light extraction involves two successive steps, including the coupling from the light source to the tube and the subsequent emission from the tube to the air. Significantly enhanced light extraction efficiency is observed for both TE and TM waves by optimizing the nanotube geometry and dimension as well as the separation between the nanotube and light source. We have further shown that densely packed nanotube arrays can be integrated with GaN-based LEDs to achieve unequivocal improvement of light extraction efficiency over a large surface area. With recent advances in rolled-up micro- and nanotubes, it is expected that this study can offer a potentially flexible, low cost approach to enhance the light extraction of various LED devices.

Improved light extraction efficiency and leakage characteristics in light-emitting diodes by nanorod arrays formed on hexagonal pits

We report the enhancement of light extraction efficiency (LEE) and electrical performance in GaN-based green light-emitting diodes (LEDs) using ZnO nanorods formed on the etched surface of p-GaN. Green LEDs with hybrid ZnO nanorod structures grown on the hexagonally etched topmost layer of the LEDs, show an improvement in electroluminescence intensity that is 3.5 times higher than LEDs without any other surface treatments. The improvement in LEE in LEDs with nanohybrid structures was confirmed by finite-difference time-domain simulation analysis. Besides LEE enhancement, the surface etching effects on the reduction of leakage current of fabricated LEDs were also investigated.

Enhanced optical output performance in InGaN/GaN light-emitting diode embedded with SiO₂nanoparticles.

We demonstrated the InGaN/GaN-based light-emitting diodes (LEDs) with SiO₂ nanoparticles embedded in nanopillar GaN template. With the SiO₂ nanoparticles placed between the GaN nanopillars, subsequent overgrowth of GaN layer started only on the exposed tips of the nanopillars and rapidly switched to the lateral growth mode. This resulted in a high quality GaN layer "sitting" on the nanopillars and the layer of pores formed over the SiO₂ nanoparticles. For multi-quantum-well LEDs grown on top of such template, ~3 fold increase in optical output was observed compared to reference samples. The effect is attributed mainly to the improved light extraction efficiency due to additional scattering in the nanopillars-SiO₂-pores portion of the structure, also to the increased internal quantum efficiency caused by a decreased dislocation density and relaxed strain due to the GaN nanopillars.