Light-emitting Diode Chip Research Articles

Particle based investigation of self-heating effect of phosphor particles in phosphor converted light emitting diodes

Since 2003, self-heating effect of phosphor which reduces the efficacy and reliability of phosphor conversion (Pc) light emitting diodes (LEDs), has been a growing research area of opto-thermal Pc-LED modeling. However, few studies have focused on the particles based nature of the phosphor self-heating. Based on a new approach, accurate Monte Carlo ray tracing simulations are performed in regions of interest over discretized control volumes where only a single phosphor particle is exposed to the light radiation. Governed by the Mie scattering effects, heat generation values of phosphor particles showed strong dependency on their optical and geometrical properties. Additionally, based on the emissive behavior of the LED, space above the LED can be divided into two irradiance levels; near and far scale regions. In near scale region, where irradiance levels are above 0.1 W/mm2, phosphor particles exhibited significant self-heating in milli-Watt scale values. Derived self-heating values are imported to simplified thermal models where phosphor particles showed a temperature rise in excess of 100 °C more than LED chip which can lead to considerable conversion efficiency loss and a reduction in lifetime. Higher temperatures are expected in higher irradiance levels where matrix material carbonization can also occur causing fire hazards.

Enhanced Directional Light Extraction from Patterned Rare‐Earth Phosphor Films

AbstractThe combination of light‐emitting diodes (LEDs) and rare earth (RE) phosphors as color‐converting layers comprises the basis of solid‐state lighting. Indeed, most LED lamps include a photoluminescent coating made of phosphor material, i.e., crystalline matrix suitably doped with RE elements, to produce white light from a blue or ultraviolet LED chip. Transparent phosphor‐based films constitute starting materials for new refined emitters that allow different photonic designs to be implemented. Among the different photonic strategies typically employed to tune or enhance emission, surface texturing has proved its versatility and feasibility in a wide range of materials and devices. However, most of the nanofabrication techniques cannot be applied to RE phosphors directly because of their chemical stability or because of their cost. The first monolithic patterned structure of down‐shifting nanophosphors with square arrays of nanoholes with different lattice parameters is reported in this study. It is shown that a low‐cost soft‐nanolithography procedure can be applied to red‐emitting nanophosphors (GdVO4:Eu3+ nanocrystals) to tune their emission properties, attaining a twofold directional enhancement of the emitted light at predesigned emission wavelengths in specific directions.

Color Tuning of Biomass-Derived Carbon Nanodots by Reaction Temperature Toward White Light-Emitting Diodes

In this work, carbon nanodots (CNDs) were synthesized from extract of mango leaves. Sphere nanodots were formed rapidly by one-step microwave heating. The photoluminescence (PL) of the CNDs was found greatly dependent on the reaction temperature. The emission peak position of the CNDs changed from 550[Formula: see text]nm to 430[Formula: see text]nm when the heating temperature increased from 120[Formula: see text]C to 150[Formula: see text]C. Particularly, the CNDs synthesized at 130[Formula: see text]C showed multi-band emission at 411[Formula: see text]nm, 480[Formula: see text]nm and 530[Formula: see text]nm, providing emitting color from blue to yellow. Moreover, the free chlorophyll molecules in the solution added red fluorescence at 670[Formula: see text]nm, and the integrated emitting color of the CNDs solution was close to white. Coated on a commercial 365[Formula: see text]nm light-emitting diode (LED) chip, the CNDs showed greenish white light with CIE coordinates of (0.37, 0.44). This work provided a one-pot, rapid and green method to obtain multi-emissive CNDs toward white LEDs.

Color‐Tunable 3D InGaN/GaN Multi‐Quantum‐Well Light‐Emitting‐Diode Based on Microfacet Emission and Programmable Driving Power Supply

AbstractColor‐tunable InGaN/GaN multi‐quantum‐well (MQW) light‐emitting diodes (LEDs) are reported based on GaN microfacet structure directly grown on c‐plane patterned sapphire substrate by metal organic vapor phase epitaxy (MOVPE) through promoting 3D growth. By adjusting GaN growth temperature and pattern arrangement, a GaN microfacet with almost pure {101} semipolar facets is obtained. The multifacetted InGaN/GaN MQW LED chip evolves three distinct emission peaks around 630, 530, and 450 nm in electroluminescence (EL) as injection current increases from 1 to 100 mA. The EL behavior originates from locally different facets of the complex 3D structure: MQWs grown on c‐planes and semipolar facets, respectively, which is confirmed by cathodoluminescence characterization in a scanning transmission electron microscope (STEM‐CL). Considering the dependence of emission wavelength and intensity on injection currents, a programmable power supply is designed to drive the LED. The specific color of the LED is tuned by time‐shared driving of the currents based on three channels with controllable magnitudes and duty cycle from the power supply, covering red, yellow, green, cyan, blue, and purple. Furthermore, white LEDs with high color rendering index (CRI) up to 96.1 and correlated color temperature (CCT) between 4000 and 10 000 K are achieved.

Influence of beam homogenization on bond strength of adhesives to dentin

ObjectiveThis study evaluated the effect of beam homogeneity on the microtensile bond strength (μTBS) of two adhesive resins to dentin. MethodsOne polywave light-emitting-diode (LED) LCU (Bluephase Style, Ivoclar Vivadent AG) was used with two different light guides: a regular tip (RT, 1010 mW/cm2 emittance) and a homogenizer tip (HT, 946 mW/cm2 emittance). The emission spectra and beam profiles were measured from both light guides. Extracted third molars were prepared for μTBS evaluation using two adhesive systems: Excite F (EXF) and Adhese Universal (ADU). Bond strength was calculated for each specimen (n = 10) at locations that correlated with the output of the two LED chips emitting blue (455 nm) and the one chip that emitted violet light (409 nm) after 24-hs and after one-year water-storage. The μTBS was analyzed using a four-way analysis of variance (factors: adhesive system, light guide, LED wavelength, and storage time) and post-hoc Tukey test (α = 0.05). ResultsEXF always delivered a higher μTBS than ADU (p < 0.0001), with the μTBS of ADU being about 20% lower than EXF. The light guide (p = 0.0259) and storage time (p = 0.0009) significantly influenced the μTBS. The LED wavelengths had no influence on the μTBS (p > 0.05). SignificanceHomogeneity of the emitted light beam was associated with higher 24-h μTBS to dentin, regardless of the adhesive tested. Also, differences in the composition of adhesives can affect their compatibility with restorative composites and their ability to maintain bonding over one year.

Optimizing the efficiency of gallium nitride‐based light‐emitting diodes from contact area of current spreading to electrode

AbstractA nonuniform current spreading in the current spreader layer greatly reduced the internal quantum efficiency (IQE) of the light‐emitting diodes (LED). The effects of the current spreading layer on the electrode contact area toward the IQE in a vertical design of gallium nitride (GaN)‐based LED chip is analytically analyzed. The contact area was varied by changing the value of the electrode's width from 2 to 12 μm. Efficiency droop and current density at peak IQE are analyzed based on contact area. The width of 2 μm requires 1.6 μAm−2 current density to achieve peak efficiency and produces a droop of 0.2150. The width of 12 μm requires 9.6 μAm−2 current density to achieve peak efficiency and produces 0.0557 droop. The increase in contact area increases the current density needed to achieve peak IQE while decreases efficiency droop. The optimal spreader contact width of this vertical LED design is 6 μm.

Double core shell structured Al@Al2O3@SiO2 filled epoxy composites for thermal management application

Polymer composites are preferred as thermal conductive interface materials because of their light weight, low cost, and ease of processing. However, polymers are subjected to low thermal conductivity. The enhancement of their thermal conductivity is often at the cost of deteriorating dielectric properties, resulting in a substantial increase in the dielectric constant, dielectric loss, and conductivity. In order to address this problem, we developed a double core shell structured Al@Al2O3@SiO2 filled epoxy composite to enhance the thermal conductivity as well as keep the low dielectric constant and dielectric loss. The thermal conductivity of the composite with a loading of 40 wt. % fillers can be enhanced by three times that of the pure epoxy. Compared with the composite loaded with the unmodified aluminum particles, the double core shell Al@Al2O3@SiO2 is more effective in curbing the trend of the steep rise of the dielectric constant, dielectric loss, and conductivity of the composites. The potential use of the obtained composites for the thermal management of power light emitting diodes (LEDs) and chips has also been demonstrated by a practical LED chip and the analysis of finite element simulation.

Ultra-high color rendering warm-white light-emitting diodes based on an efficient green-emitting garnet phosphor for solid-state lighting

High light-quality and low color temperature are two crucial factors for a comfortable healthy illumination. Phosphor-converted white light-emitting diodes (LEDs) could be readily realized by combining near-UV (380–420 nm) LED chips with tri-color phosphors, and thus near-UV-excitable inorganic phosphors with efficient visible luminescence are highly demanded for fabricating high-performance near-UV-pumped white LEDs. Herein, we demonstrated a novel highly efficient green-emitting phosphor based on Ce3+/Tb3+ ions co-activated Ca2GdHf2Al3O12 (abbreviated as: CGHAO) garnet compound, which enabled the realization of ultra-high color rendering warm-white LEDs. Notably, the CGHAO:Ce3+,Tb3+ phosphors could be efficiently excited by near-UV light around 408 nm and exhibited intense green emissions around 543 nm with high internal quantum efficiency of 82.7% and external quantum efficiency of 60.6%. The energy transfer from Ce3+ to Tb3+ ions was studied in detail, and the composition of CGHAO:Ce3+,Tb3+ phosphors was optimized. Utilizing the optimal CGHAO:0.04Ce3+,0.4Tb3+ green phosphor as color converter, a prototype near-UV-pumped white LED device was fabricated, and upon 120 mA driving current it demonstrated bright warm-white light with excellent CIE chromaticity coordinates of (0.391, 0.360), low correlated color temperature of 3575 K, ultra-high color rendering index (Ra = 94.4, R12 = 89.0, and R9 = 80.6) and good luminous efficacy of 27.40 lm·W−1. This work offers a new strategy for designing efficient LED phosphors for high-color-quality solid-state lighting.

Anisotropy and Mechanistic Elucidation of Wet‐Chemical Gallium Nitride Etching at the Atomic Level

Etching of gallium nitride is a key step in the production of blue and white light‐emitting diodes (LEDs). Etching in aqueous KOH solution creates a rough surface on the LED chip to facilitate outcoupling of the photons generated, drastically increasing the resulting LED's efficiency. Compared with the common technique of dry etching, wet‐chemical etching using aqueous KOH solution has significant advantages, e.g., lower complexity and cost and less remaining surface damage. An in‐depth analysis of the molecular etch reaction by characterization of the reaction products is reported. The mechanism identified explains the cause of anisotropic etching, which leads to the formation of hexagonal pyramids. The concept of hydroxide repulsion by protruding NH and NH2 groups established in the literature is adapted and further developed. The susceptibility of several polar, semipolar, and nonpolar crystal facets may also be explained, as well as the commonly observed increase in average pyramid size over etch time.

Enhancement of the optical properties of CsPbBr3 perovskite nanocrystals using three different solvents.

Green-emitting CsPbBr3 perovskite nanocrystals were synthesized by the modified hot-injection method using three different solvents. The produced nanocrystals showed a narrow green emission band centered at 515-520 nm with full width at half-maximum (FWHM) values of approximately 18-20 nm. The highest photoluminescence quantum yield (PLQY) was obtained for the nanocrystal sample synthesized using a paraffin liquid solvent, with a value of 70.1% under excitation at 450 nm. The CsPbBr3 nanocrystals film light-emitting diodes (LED) chip module showed a luminous efficacy of 40.7lm/Wrad. The white LED (WLED) with green CsPbBr3 and red CsPbI3 nanocrystal films emitted bluish-white light with a high color rendering index of 89, and the luminous efficacy of the WLED reached 16.3lm/Wrad.

Quality safety monitoring of LED chips using deep learning-based vision inspection methods

The surface quality safety inspection of a light-emitting diodes (LED) chip is an indispensable step in the LED production process. The traditional vision inspection algorithm extracts defect features by artificial feature extraction. This method has become increasingly difficult as chips have become more miniaturized, and it is difficult to meet the quality safety requirements. Recently, machine learning-based vision detection methods, especially the models of deep convolution neural network (CNN), have made significant breakthroughs in this field, and their performances greatly exceed the traditional model based on manual features. In this paper, a parallel deep convolution model parallel spatial pyramid pooling network (PSPP-net) for LED chip surface quality detection is proposed. Specifically, this model aims at utilizing the advantages of the spatial pyramid pooling (SPP-net) model to online and offline extract two groups of depth-based CNN features through offline training and online recognition of two depth-based CNN data conversion streams. Then, the features are mixed and intersected in the GPU to form 1024-dimensional image feature vectors. Finally, softmax regression is adopted for defect classification and recognition. The problem of off-line feature training extraction and on-line defect recognition in surface quality safety inspection of LED chips is solved.

Optical-spatial-summing-based NOMA with fine-grained power allocation for VLC-enabled IoT applications.

In this Letter, we propose and demonstrate a practical optical-spatial-summing-based non-orthogonal multiple access (OSS-NOMA) technique for visible light communication (VLC) systems. This technique is innovative in adopting OSS in that the transmitter of OSS-NOMA VLC can be built upon commercial illuminating light emitting diodes (LEDs), free of LEDs' harmful nonlinearity. Unlike conventional NOMA VLC using analog components such as digital-to-analog converters and bias-T in the transmitter side, OSS-NOMA exploits only digital control signals to drive a LED array in forming optical power superposition for NOMA signals. We demonstrate that by simply switching different amounts of LED chips on or off, the proposed OSS-NOMA transmitter can deliver a fine-grained power allocation ratio ranging from 0.01 to one for two users. The implemented OSS-NOMA VLC prototype leveraging commercial components can achieve low bit error rates of ≤3.1×10-3 for two users at a data rate of 800 kbps, confirming the promising potential of this novel OSS-NOMA VLC for Internet of Things (IoT) applications.

GEOMETRICAL ANALYSIS OF LIGHT-EMITTING DIODE FOR ENHANCING EXTRACTION EFFICIENCY

A non-uniform current spreading in the current spreader can greatly reduce the efficiency of the light-emitting diode (LED). The effects of the electrode contact area to the spreading layer towards extraction efficiency of LED chips is analysed in analytical simulations. Length of current spreading and light extraction efficiency is analysed for variation of contact area. The contact area value is varied by changing the shape of the electrode and the value of width of contact area. The increase in contact area decreases light extraction efficiency as more light are absorbed by the bottom electrode surface. The effective current spreading length for Indium Tin Oxide (ITO) of thickness 300nm is 36.44µm. The 6 strips ‘fork’ design is the most optimum. The design has the most area for photons produced in active region to escape without reducing the area cover with current density. This enables the chip to has more extraction efficiency with more uniform current spreading.

Scattering Nanoparticles-Induced Reflection Effect for Enhancing Optical Efficiency of Inverted Quantum Dots-Light-Emitting Diodes Combined With the Centrifugation Technique

AbstractInverted packaging structure is a promising alternative for thermal isolation between light-emitting diode (LED) chips and quantum dot (QD) converters with effective heat dissipation. However, serious reflection loss occurs at the lead frame owing to the inverted bonding of LED chips. In this study, the scattering nanoparticles-induced reflection effect has been developed to enhance the optical efficiency of inverted QD-LEDs combined with the centrifugation technique. The strong back-scattered effect of boron nitride (BN) nanoparticles with a thin columnar structure is chosen for reflection enhancement according to the ray-tracing and finite different time-domain simulations. Furthermore, a centrifugation technique is introduced to control the equilibrium geometry of the BN-incorporating reflector (BNR) by changing the centrifugal speed. Results indicate that the luminous flux of inverted QD/BNR-LEDs using the optimized concave BNR structure largely increases by 82.8% compared with reference inverted QD-LEDs. The great enhancement is attributed to the light concentrated effect of the concave geometry and the strong diffusion reflection ability of BN scattering nanoparticles. Consequently, the smart design on reflection properties of inverted QD-LEDs is critical for achieving high optical performances.

Simple Synthesis of CuInS2/ZnS Core/Shell Quantum Dots for White Light-Emitting Diodes.

In this study, the CuInS2/ZnS core/shell quantum dots (QDs) were prepared via simple and environmentally friendly solvothermal synthesis and were used as phosphors for white light-emitting diodes (WLEDs). The surface defect of the CuInS2 core QDs were passivated by the ZnS shell by forming CuInS2/ZnS core/shell QDs. By adjusting the Cu/In ratio and the nucleation temperature, the photoluminescence (PL) peak of the CuInS2 QDs was tunable in a range of 651–775 nm. After coating the ZnS layer and modifying oleic acid ligands, the PL quantum yield increased to 85.06%. The CuInS2/ZnS QD powder thermal stability results showed that the PL intensity of the QDs remained 91% at 100°C for 10 min. High color rendering index values (CRI, 90) and correlated color temperature of 4360 K for the efficient WLEDs were fabricated using CuInS2/ZnS QDs and (Ba,Sr)2SiO4:Eu2+ as color converters in combination with a blue GaN light-emitting diode chip.

A Numerical and Experimental Study of a Novel Heat Sink Design for Natural Convection Cooling of LED Grow Lights

Light-emitting diode (LED) grow lights are increasingly used in large-scale indoor farming to provide controlled light intensity and spectrum to maximize photosynthesis at various growth stages of plants. As well as converting electricity into light, the LED chips generate heat, so the boards must be properly cooled to maintain the high efficiency and reliability of the LED chips. Currently, LED grow lights are cooled by forced convection air cooling, the fans of which are often the points of failure and also consumers of a significant amount of power. Natural convection cooling is promising as it does not require any moving parts, but one major design challenge is to improve its relatively low heat transfer rate. This paper presents a novel heat sink design for natural convection cooling of LED grow lights. The new design consists of a large rectangular fin array with openings in the base transverse to the fins to increase air flow, and hence the heat transfer. Numerical simulations and experimental testing of a prototype LED grow light with the new heat sink showed that openings achieved their intended purpose. It was found that the new heat sink can transfer the necessary heat flux within the safe operating temperature range of LED chips, which is adequate for cooling LED grow lights.

Bright cyan-to-green color-tunable emissions from Ce3+/Tb3+ co-activated garnet phosphors for high-color-quality solid-state lighting

High-efficiency near-UV excitable inorganic phosphors with tunable emissions are highly demanded for white light-emitting diodes (LEDs) and displays. In this paper, a series of color-tunable cyan-green-emitting Ca2Lu0.99-xZr2(AlO4)3:1%Ce3+,xTb3+ (CLZA:1%Ce3+,xTb3+) phosphors was synthesized via a high-temperature solid-state reaction method. Their crystal structure, morphology, emission and excitation spectra, energy transfer properties, CIE color coordinates, internal quantum efficiency and temperature-dependent spectra were systematically researched. Impressively, these CLZA:1%Ce3+,xTb3+ phosphors showed an intense excitation band in the 370–475 nm wavelength range with a peak at 408 nm corresponding to the 4f→5d transitions of Ce3+ ions. Owing to the energy transfer from Ce3+ to Tb3+ ions, bright cyan-to-green color-tunable emissions were obtained in CLZA:1%Ce3+,xTb3+ phosphors with an increase of the Tb3+ doping concentration, and the corresponding CIE color coordinates changed from (0.156, 0.249) to (0.341, 0.533) when the Tb3+ doping concentration increased from x = 0 to 90%. The internal quantum efficiency reached up to 69%. Moreover, temperature-dependent spectra demonstrated good thermal stability and color stability of the as-prepared Ce3+/Tb3+ doped Ca2LuZr2(AlO4)3 phosphors. In addition, a white LED device was fabricated by coating the blend of the as-prepared cyan-emitting CLZA:1%Ce3+,10%Tb3+ phosphor and commercial red phosphor CaAlSiN3:Eu2+ on a 400 nm LED chip, which exhibited bright warm-white light with CIE coordinates of (0.425, 0.376), low correlated color temperature of 2,975 K and high color rendering index value of Ra = 83 and R9 = 89. These results revealed that the CLZA:Ce3+,Tb3+ phosphors could be excellent candidates for the application of near-UV-excited warm-white LEDs.

An Investigation Into the Optothermal Behavior of a High Power Red Light Emitting Diode: Impact of an Optical Path

Abstract Monochromatic light emitting diodes (LEDs) are getting more attention day by day for a very wide range of applications such as general lighting, signage, automotive lighting, display, water purification, and skin imaging. While they are taking place in a large number of applications, thermal challenges associated with the operation of LEDs have become a significant issue to address since their performance is greatly affected by thermal conditions. Thus, this study focuses on identifying thermal, optical, and electrical characteristics of an AlGaInP-based red LED considering the impact of the LED dome on the chip performance. The junction temperature measurement results obtained with forward voltage change method were validated with thermal imaging method (TIM) and computational models. It was observed that the LED dome may critically affect the thermal, optical, and electrical behaviors of the LED chip. In fact, a 3.7% increase in junction temperature and a 6.1% drop in optical conversion efficiency were found at the normal operation of the red LED after the LED dome was removed. The results were also compared with a blue LED, and lower junction temperatures were measured for the red LED at each driving current. The difference in junction temperature became even more noticeable at higher driving currents. Results have shown a good agreement between three different methods with a maximum variation of 6.9%.

Thermal Analysis and Optimization of Light-Emitting Diodes Filament Lamp

Abstract Due to the high efficiency, light-emitting diodes (LED) filament lamps have become more and more popular alternative to the incandescent lamp. However, the heat generated by the LED chips is traditionally dissipated by relying on the natural convection within lamps, resulting in poor heat dissipation performance for LED filament lamps. A numerical simulation model of the typical LED filament lamp was established to simulate and analyze the heat dissipation and airflow phenomenon of LED filament lamps in this study. In addition, increasing lamp sizes, increasing phosphor diameters, and using finned phosphor layers were considered as optimization measures to improve the heat dissipation performance of LED filament lamps. When these optimization measures are applied, chip junction temperatures are reduced. A reduction of 6.9 °C is seen when the lamp radius is increased from 25 mm to 31 mm. When the phosphor diameter is increased to 4 mm from 2 mm, the junction temperature is reduced by 17.2 °C. Integration of a finned phosphor layer where there are 12 fins at a height of 1 mm and thickness of 0.2 mm in the layer decreased the junction temperature by 10.9 °C. These optimization results provide technical support for the design and manufacture of LED filament lamps, and thermal analysis results provide theoretical support for the promotion of these optimization methods.

Ultrapure and highly efficient green light emitting devices based on ligand-modified CsPbBr3 quantum dots

All inorganic CsPbBr 3 perovskite quantum dots (QDs) have been recognized as promising optical materials to fabricate green light emission devices because of their excellent optical performance. However, regular CsPbBr 3 QDs with an oleic acid (OA) ligand show poor stability, which limits their practical application. We replaced the OA ligand in CsPbBr 3 QDs with a 2-hexyldecanoic acid (DA) ligand and, in the synthesis, found that the new material has better optical properties than regular CsPbBr 3 QDs ( CsPbBr 3 - OA QDs). Due to the strong binding energy between the DA ligand and QDs, the ligand-modified CsPbBr 3 QDs ( CsPbBr 3 - DA QDs) show a high photoluminescence quantum yield (PLQY) of 96%, while the PLQY of CsPbBr 3 - OA QDs is 84%. Subsequently, the CsPbBr 3 QDs coated on the blue light-emitting diode (LED) chips as green phosphors are demonstrated. The color conversion from blue to pure green is achieved by adding the CsPbBr 3 - OA QDs solution up to 60 μL, while the pure green emission devices only need 18 μL CsPbBr 3 - DA QDs solution under the same concentration. The ultrapure, highly efficient green light-emitting devices based on CsPbBr 3 - DA QDs exhibit a luminous efficiency of 43.6 lm/W with a CIE (0.2086, 0.7635) under a 15.3 mA driving current. In addition, the green emission wavelength of the devices based on CsPbBr 3 - DA QDs almost has no shift, even under a high injection current. These results highlight the promise of DA ligand-modified CsPbBr 3 QDs for light-emitting devices and enrich the application field of ligand-modified CsPbBr 3 QDs.