Light Extraction Efficiency Of LEDs Research Articles

Design of photonic crystals for light‐emitting diodes

AbstractPhotonic crystals (PCs) can greatly enhance the optoelectronic performance of light‐emitting diodes (LEDs) due to their distinctive color, photonic band gap, etc. Therefore, many scholars have conducted extensive research based on the high light extraction efficiency, good monochromaticity, and other excellent optoelectronic properties of PC LEDs. This review discusses the main principles of photonic crystals to improve the optoelectronic performance of LEDs and summarizes 12 structural applications of photonic crystal LEDs, such as PC slabs, Bragg grating, backside reflectors, surface PC, embedded PC, dual PC, PC beads, CPC, PC thin films, LIPC, defective PC, and composite architectures with other materials that boost LED optoelectronic qualities. In summary, it is found that photonic crystals can not only greatly improve the light extraction efficiency of LEDs but also improve other optoelectronic properties such as luminescent color and directional radiation angle, and reduce the manufacturing cost of LEDs. Photonic crystal LEDs are expected to be a strong candidate for future lighting technology. Finally, the prospects and challenges of PC LEDs are summarized.

The dependence of light extraction improvement on optimized surface microstructure for AlGaN-based UVC-LEDs considering TM-polarized emission

In order to improve the light extraction of AlGaN-based short wavelength ultraviolet light emitting diodes (DUC-LEDs), a type of microstructure with high aspect ratio is introduced and optimized on the AlN substrate surface. And, particle swarm optimization (PSO) algorithm is used to inverse design of the surface microstructure to maximize the light extraction efficiency (LEE). Considering that the propagation characteristics of TM-polarized light are different from that of TE-polarized light, the optical field distribution and LEE is analyzed for the UVC-LEDs with different TE-polarized component when the optimized surface microstructure is applied. Furthermore, the preparation process tolerance of the high aspect ratio structure is discussed by calculating the LED's LEE when the structural deviation occurs or morphology changes. Simulation results show that, by using the optimized surface microstructure based on parabola cone array, the LEDs' LEE is increased from 4.4% to 8.7% and from 0.4% to 3.7% for TE-polarized and TM-polarized emission, respectively. In addition, it is demonstrated that the light extraction improvement by the surface microstructure has a good tolerance to the structural deviation and morphology. The results are significant for improving light extraction and realizing high efficient short wavelength AlGaN-based UVC-LEDs by designing surface microstructures.

Impact of roughening density on the light extraction efficiency of thin-film flip-chip ultraviolet LEDs grown on SiC.

Discovering ways to increase the LED light extraction efficiency (LEE) should help create the largest performance improvement in the power of UV AlGaN LEDs. Employing surface roughening to increase the LEE of typical AlGaN UV LEDs is challenging and not well understood, yet it can be achieved easily in AlGaN LEDs grown on SiC. We fabricate thin-film UV LEDs (~294-310 nm) grown on SiC-with reflective contacts and roughened emission surface-to study and optimize KOH roughening of N-face AlN on the LEE as a function of roughened AlN pyramid size and KOH solution temperature. The LEE increased the most (2X) when the average AlN pyramid base diagonals (d) were comparable to the electroluminescence (EL) wavelength in the AlN layer (d ~λEL; 42-52 pyramids/µm2), but the LEE enhancement diminished when d was much larger than λEL (d ~5.5λEL; 2-3 pyramids/µm2). The UV LEDs had a 10 nm p-GaN contact layer, and the forward voltage was ~6 V at ~8 A/cm2, with a voltage efficiency (VE) of ~70%. The VE of the LEDs did not change after KOH roughening. This work suggests important implications to increase the LEE of AlGaN 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

光栅形状对发光二极管光提取效率的影响

光栅形状对发光二极管光提取效率的影响

Bridging the "green gap" of LEDs: giant light output enhancement and directional control of LEDs via embedded nano-void photonic crystals.

Green LEDs do not show the same level of performance as their blue and red cousins, greatly hindering the solid-state lighting development, which is the so-called "green gap". In this work, nano-void photonic crystals (NVPCs) were fabricated to embed within the GaN/InGaN green LEDs by using epitaxial lateral overgrowth (ELO) and nano-sphere lithography techniques. The NVPCs act as an efficient scattering back-reflector to outcouple the guided and downward photons, which not only boost the light extraction efficiency of LEDs with an enhancement of 78% but also collimate the view angle of LEDs from 131.5° to 114.0°. This could be because of the highly scattering nature of NVPCs which reduce the interference giving rise to Fabry-Perot resonance. Moreover, due to the threading dislocation suppression and strain relief by the NVPCs, the internal quantum efficiency was increased by 25% and droop behavior was reduced from 37.4% to 25.9%. The enhancement of light output power can be achieved as high as 151% at a driving current of 350 mA. Giant light output enhancement and directional control via NVPCs point the way towards a promising avenue of solid-state lighting.

Large-scale high quality glass microlens arrays fabricated by laser enhanced wet etching.

Large-scale high quality microlens arrays (MLAs) play an important role in enhancing the imaging quality of CCD and CMOS as well as the light extraction efficiency of LEDs and OLEDs. To meet the requirement in MLAs' wide application areas, a rapid fabrication method to fabricate large-scale MLAs with high quality, high fill factor and high uniformity is needed, especially on the glass substrate. In this paper, we present a simple and cost-efficient approach to the development of both concave and convex large-scale microlens arrays (MLAs) by using femtosecond laser wet etching method and replication technique. A large-scale high quality square-shaped microlens array with 512 × 512 units was fabricated.The unit size is 20 × 20 μm² on the whole scale of 1 × 1 cm². Its perfect uniformity and optical performance are demonstrated.

Impact of microstructures by femtosecond laser direct writing on the LED efficiency

We report on fabricating microstructures on the surfaces of LED chips by femtosecond laser direct writing. The changed trend of LED light extraction efficiency with the variation of the laser power and pulse was studied. It is found that there is an optimal surface treating condition (laser power and pulse number) in fabricating the microstructures on the surface of the LEDs to make the LED luminous flux maximum. In our experiment, the luminous efficiency of LED is close to the optimum enhancement value when the LED was surface structured with a laser power of 0.48 μW, three laser pulses at one point and the point spacing of 5 μm. In this condition, it can be improved by 23.53%.

Development of a cost effective surface-patterned transparent conductive coating as top-contact of light emitting diodes

Sol-gel process has been used to form indium zinc oxide films using an optimized combination of zinc to indium concentration in the precursor solutions. Different structures, like one (1D) and two-dimensional (2D) gratings and diffractive optical elements (DOEs) in the form of Fresnel lens are fabricated on the film surface of proposed top metal contact of LED by imprint soft lithography technique. These structures can enhance the LED's light extraction efficiency (LEE) or can shape the output beam pattern, respectively. Several characterizations are done to analyze the material and structural properties of the films. The presence of 1D and 2D gratings as well as DOEs is confirmed from field emission scanning electron and atomic force microscopes analyses. Although, X-ray diffraction shows amorphous nature of the film, but transmission electron microscopy study shows that it is nano crystalline in nature having fine particles (∼8 nm) of hexagonal ZnO. Shrinkage behaviour of gratings as a function of curing temperature is explained by Fourier transform infra-red spectra and thermo gravimetric-differential thermal analysis. The visible transmission and sheet resistance of the sample are found comparable to tin doped indium oxide (ITO). Therefore, the film can compete as low cost substitute of ITO as top metal contact of LEDs.

Local nanotip arrays sculptured by atomic force microscopy to enhance the light-output efficiency of GaN-based light-emitting diode structures

In this work, local nanotip arrays on GaN-based light-emitting (LED) structures were fabricated through nano-oxidation using an atomic force microscope (AFM). The photoluminescence (PL) intensity of the InGaN/GaN multiple quantum wells (MQWs) active layer and the light extraction efficiency of the LED structure were enhanced by forming this nanotips structure to serve as a graded-refractive index layer, which is further validated by the finite-difference time-domain analysis. The PL emission peak of the MQWs active layer has a blue-shift phenomenon that is caused by a partial reduction of the strain on the InGaN well. It is expected that our approach opens a promising route for simultaneously enhancing both the internal quantum efficiency and the light extraction efficiency of GaN-based LEDs. The proposed AFM-based method will be of importance for local patterning the light emitting components for optoelectronic applications.

Optimizing Design of LEDs with Photonic Crystal and Planer Micro Cavity for High Extraction Efficiency

LED structures with the introducing of surface PC and planer micro cavity was described to improve the light extraction efficiency of LEDs. The distributions of guided modes propagated in LED were analyzed, and the modal extraction lengths were determined using high-resolution angle-resolved measurements in order to research the interaction of each guided mode with PCs. The impact of structure parameters design on modal extraction length was learned. Result shows that PC extraction will not interfere with micro cavity effects, and can be used together to enhance LEE.

Fabrication and Optimum Design of Lens on Luminous Uniformity of High-Power LED

The paper demonstrates the effect of lens inner structure on radiation pattern and light extraction efficiency of LED and the application to artificial light in compact greenhouse. A commercial software package of Trace Pro and one-factor at-a-time method are used to simulate the lens with different inner structure. The optimum parameters of lens inner structure design for the maximum light extraction efficiency and the best uniform luminous are described by the corner radius of curvature, lens width, and lens height, respectively. In comparison with a commercial artificial light of LED matrix used in agriculture, the high-power LED module with proposed lens inner structure exhibit good improvement in uniformity of illumination and light extraction. This study may provide a practical guideline for design and fabrication of a high-performance lens used in various compact agricultural applications.

薄膜厚度对GZO透明导电膜及其LED器件性能的影响

GZO transparent conductive films were prepared by RF magnetron sputtering.The influence of thickness on GZO films was studied by several methods,including atomic force microscopy,XRD,Hall Effect tester and UV-Vis spectrophotometer.LEDs with GZO transparent conductive layers were also fabricated.These results show that,as the thickness increases,the crystal quality is improved and the resistivity of the film is reduced.When the thickness reached 500 nm,the lowest resistivity of 2.79 × 10-4 Ω.cm and the highest transmittance of 97.9% at 460 nm wavelength were obtained.However,the variation of film thickness has little effect on the forward voltage of LED.But it is an important factor on improving the light extraction efficiency of LEDs.

Effects of packaging structure on optical performances of phosphor converted light emitting diodes

Effects of tilt angles of reflective cup structure and phosphor surface geometries on light extraction efficiency and angular color uniformity (ACU) of phosphor converted light emitting diodes (pcLED) are investigated by Monte Carlo ray-tracing simulations. It is found that tilt angles of reflective cup and phosphor surface geometries affect the light extraction efficiency and the ACU distinctly. When the tilt angle varied from 60° to 15°, the light extraction efficiency of LED can achieve the improvements of 13.87%, 18.25% and 14.79% respectively, when the phosphor surface geometry is concave, flat and convex, respectively. It is also found the variety law of phosphor concentrations with the change of tilt angles and phosphor surface geometries to maintain a fixed correlated color temperature (CCT).

微纳光学在LED芯片中应用研究的综述

LED is considered to as the most possible new solid source to general lighting field because of its advantages,such as high effectiveness,small volume,long expectancy,etc.But the light extraction efficiency of LED chip is too low.The processing techniques about varieties of optical micro-nano structures of LED's extension surface is introduced such as surface roughening on the LED chips,double-layer texturing structure,two-dimensional photonic crystal pattern structure,two crossed grating structures,etc.The external quantum of LED chips is enhanced by various kinds of processing technologies,thus the light extraction efficiency of LED is improved.

Fabrication and theoretical analysis of GaN-based vertical light-emitting diodes with SiO2 current blocking layer

We report the fabrication and characteristics of 1 × 1 mm2 blue vertical light-emitting diodes (VLEDs) with a SiO2 current blocking layer (CBL), together with their theoretical analysis in terms of electrical, optical and thermal properties. The SiO2 CBL improves the total light extraction efficiency by relieving the current crowding under the metal n-electrode. Additionally, the total light extraction efficiency of LEDs with SiO2 CBL remains almost the same independent of injection current, whereas that of conventional VLEDs is gradually decreased with injection current. The optical output power (Pout) for the VLED with SiO2 CBL is increased by about 10% at 350 mA in a wafer level without serious degradation in operating voltage resulting from the insertion of SiO2 CBL, thus resulting in a high Pout of 670 mW at 950 mA. The simulated light–current–voltage characteristics of the fabricated devices were reasonably consistent with the measured results.

GaN-based LEDs with a high light extraction composite surface structure fabricated by a modified YAG laser lift-off technology and the patterned sapphire substrates

Vertical structure LEDs have been fabricated with a novel light extraction composite surface structure composed of a micron grating and nano-structure. The composite surface structure was generated by using a modified YAG laser lift-off technique, separating the wafers from cone-shaped patterned sapphire substrates. LEDs thus fabricated showed the light output power increase about 1.7–2.5 times when compared with conventional vertical structure LEDs grown on plane sapphire substrates. A three-dimensional finite difference time domain method was used to simulate this new kind of LED device. It was determined that nano-structures in composite surface patterns play a key role in the improvement of light extraction efficiency of LEDs.

GaN基倒装焊LED芯片的光提取效率模拟与分析

According to Monte-Carlo ray tracing method,this paper simulated the GaN-based flip-chip LED light extraction efficiency(LEE),compared the differences between the status of sapphire with substrate and without it,with one side roughening and double sides roughening,with and without buffer layer,and made further selection and optimization about the structure and the size of surface roughing unit.The results showed that both the thicker substrate and introduction of AlN buffer layer were favorable for the increase of LEE,the LEE for the chip with double surfaces roughened sapphire substrate was significantly better than the one with single surface roughened sapphire substrate,the structure and size of the surface roughing unit have great influences on the LEE,the LEE was relatively higher when the size of micro roughened structure was comparable to its one-to-one distance.

LED梯度折射率封装结构的蒙特卡罗模拟

Light propagation in nanoparticle-loaded encapsulants is simulated based on the method of Monte Carlo,referring to the multilayer graded refractive index structure for LED encapsulants.And the influence of scattering coefficient on transmittance is analyzed.The results show that the transmittance decreases with scattering coefficient.For a given number of layers,the encapsulants will yield maximum transmittance if the refractive index value of each layer is optimized.The nanoparticle-loaded encapsulant consists of multiple layers with refractive-index values that gradually decrease,transmittance will be higher than in traditional non-graded encapsulants,and will improve light extraction efficiency of LED.

Design Optimization of Photonic Crystal Structure for Improved Light Extraction of GaN LED

We performed a theoretical analysis on improved light extraction efficiency of LEDs with photonic crystals (PCs). The light propagation and extraction of PC LEDs were simulated using the finite-difference time-domain method for various PC LED structures. LEDs with both top and bottom PCs and PC LEDs grown on patterned substrates were considered to maximize light extraction efficiency. The design parameters of the PC were varied, and optimized values were obtained. A disordered PC was simulated, and we showed that the light extraction efficiency of a disordered PC was nearly equivalent to that of an ordered PC with the same pattern periodicity. This result revealed that the increased light extraction of PC LEDs was mainly due to scattering. Moreover, by comparing the enhancement of PC LEDs with different shapes of air holes, we showed that only the density of holes and the area occupied by holes play important roles in light extraction. The shapes of the holes have no strong effect on the enhancement of light extraction.