Increasing the extraction efficiency of a light-emitting diode using a pyramid-like phosphor layer

In our research, development of encapsulating material of light emitting diodes (LEDs) is the mainly purpose. In the first part (Chapter 2), optical hybrid materials with high refractive index were synthesis by in-situ production of titanium dioxide (TiO2) directly in a commercial-grade silicone resin via a sol-gel reaction. The optical transparency of the prepared TiO2/silicone hybrid film was investigated by UV-visible spectroscopy. These hybrid films with various TiO2 contents exhibited different refractive indices; the refractive index could reach 1.66 for the hybrid with 30 wt% of the TiO2. In addition, refractive index, thermal stability of the cured hybrid materials was also investigated. In the second part (Chapter 3), novel ZrO2/silicone hybrid materials (ZrO2/AB) useful for the encapsulation of light-emitting diodes (LEDs) are synthesized by an in situ sol-gel reaction of zirconium propoxide directly in a commercial-grade silicone resin by an in situ sol-gel reaction. By the chelation of the acetic acid, the ZrO2/AB hybrids exhibit high transparency owing to the well-dispersed ZrO2 particles in the silicone material, and their refractive index value also increased with increasing weight percentage of the ZrO2 in the hybrid. These high-refractive-index ZrO2/AB hybrids were then used as encapsulating materials to improve the luminous flux of the LEDs. From the results of the luminous flux measurement, the LED encapsulated with the pure silicone material has luminous flux of 3.97 lm. After encapsulated with the ZrO2/AB hybrid, the luminous flux of the LED was enhanced to a value of 4.35, which revealing that the increase in the refractive index of the encapsulating material by the incorporation of the ZrO2 could effectively improved the luminous flux of the LED, and these novel ZrO2/AB hybrids could also be considered as a suitable candidate as the encapsulating material for the LED. In the third part (Chapter 4), ZrO2 particles are synthesized directly in a commercial-grade silicone resin (AB) with the addition of a vinyl monomer (glycidyl methacrylate, GMA) by an in situ sol-gel reaction to obtain the silicone hybrid material (ZrO2/ABG). In addition to use the acetic acid as chelating agent, the addition of the GMA is able to retard the gelation rate of the metal alkoxide compound as well as the growing rate of ZrO2 during the sol-gel reaction, and also enhance the interfacial strength of the inorganic fillers with the silicone matrix by reacting with the silicone resin through a hydrosilation reaction via its vinyl group. The ZrO2/ABG hybrids thus exhibit excellent transparency and high refractive index. These ZrO2/ABG hybrids were then used as encapsulating materials to help the LEDs reach a higher luminous flux. Furthermore, according to the optical theory and the structure of the high-power LEDs, a simple simulation model was developed to estimate the luminous flux of the LEDs encapsulated with the ZrO2/ABG hybrids and the influence of the refractive index of the passivation layer on the luminous flux of the LEDs. Finally, with the help of this simulation model, an optimum combination of the encapsulating material and the passivation layer with appropriate refractive index values was obtained for achieving the highest luminous flux of LED. In the fourth part (Chapter 5), the purpose is to improve thermal mechanical property of the encapsulating materials in a very large temperature difference condition., thus decreasing the influence of the encapsulation on the reliability performance of the LED. The PBA(polybutylacrylate) oliogomer was firstly synthesized by a telomerization using 2-mercaptoethanol as a chain transfer agent. By the absorption to the inorganic particles and the carbonyl groups, the ZrO2 particles were produced on the oligomer chains of the PBA via an in situ sol-gel reaction to obtain the ZrO2-PBA composite particles, and subsequently mixed with a commercial-grade silicone resin to obtain the ZrO2-PBA/silicone hybrid materials (ZrO2-PBA/AB). The well-dispersed ZrO2-PBA domains lead to a high transparency and high refractive index of the ZrO2-PBA/AB hybrids, resulting in a higher luminous flux than the pristine silicone material. In addition to discuss the transmittance, refractive index, thermal resistance, and the water uptake of the ZrO2-PBA/AB hybrids, thermal stress evaluation of the LEDs encapsulated with the ZrO2-PBA/AB hybrids was also carried out in a thermal shock experiment with a temperature change from -35 °C to 125 °C within 15 mins. The results show that these ZrO2-PBA/AB hybrids have a lower thermal stress and exhibit a better mechnical resistance against the thermal shock, and the reliability performance of the LEDs are also greatly improved as encapsulated with the ZrO2-PBA/AB hybrids.

Low-temperature bonding technique for fabrication of high-power GaN-based blue vertical light-emitting diodes

Phosphor temperature in high-power white light-emitting diodes can greatly affect the optical properties, reliability, and lifetime. Accurate estimation of phosphor temperature is the first step for enhancing thermal management inside the package. In this paper, the phosphor temperature was measured by a plug-in method with thermocouple. The thermocouple’s bead was inserted into the phosphor layer, and then the silicone matrix got cured. The phosphor temperature was measured under various input currents. Under 350 mA, the temperature difference between the measured value and the junction temperature was about 17 °C higher than those in the references. The reason for this overestimation was attributed to light energy absorption and conversion of the bead, which was confirmed by thermal simulations.

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

High-power light-emitting diodes (LEDs) in both the AlInGaP (red to amber) and the AlGaInN (blue-green) material systems are now commercially available. These high-power LEDs enable applications wherein high flux is necessary, opening up new markets that previously required a large number of conventional LEDs. Data are presented on high-power AlGaInN LEDs utilizing flip-chip device structures. The high-power flip-chip LED is contained in a package that provides high current and temperature operation, high reliability, and optimized radiation patterns. These LEDs produce record powers of 350 mW (1A dc, 300 K) with low (<4V) forward voltages. The performance of these LEDs is demonstrated in terms of output power, efficiency, and electrical characteristics.

Light emitting diode (LED) is widely used in lighting field. LEDs are revolutionizing the automotive lighting industry, as they are often much more durable, efficient and energy-saving than various traditional car headlight technologies. Blue light emitted from GaN LEDs can be used to stimulate a wider spectrum of emission from the phosphor layer around the LED to create white light. Although LEDs are more efficient than previous lighting technologies, there are still some scientific and engineering challenges need to be overcame, such as with the drive current increases, the efficiency of LEDs is significant decreases. This phenomenon is known as LED droop. Therefore, in order to increase the brightness, it is necessary to use more LED chips, rather than simply increase the drive current. The cost of LED products will increase. At present, the suppression of LED droop is one of the important ways to reduce the cost of LED products. In this work, we focus on suppressing the LED droop and improving the luminous efficiency of LED. To solve above problems, a flip-chip GaN P-N junction model is investigated. The configuration is composed of a three-layer structure. Besides, the P-N junction is analyzed and optimized. The characteristics of proposed LED configuration are investigated by using finite element method (FEM). Research results show that internal quantum efficiency (IQE) drops drastically when current density exceeds 180 A/cm^2, and the luminous efficiency of the device is high when the current density is 100-180 A/cm^2.

Morphological effect of patterned sapphire substrate on efficiency of white-light phosphor LED package

High-power light-emitting diodes (LEDs) have been fabricated through multi-chip integration of small near-ultraviolet (n-UV) LED chips by using a direct flip-chip bonding technique. Based on optical measurements, the absorption of light between adjoining LED chips was not observed when temperature was held constant, although the luminescence characteristics of the LED decreased with increasing package temperature. Large-scale integration LEDs (LSILEDs) with radiant fluxes over 2 W and 121 LED chips in a single package were fabricated. Four types of packages with different chip densities revealed the importance of thermal management through the control of the number and density of the LED chips in order to obtain high radiant flux and high efficiency.

We have demonstrated that a phosphor conversion (PC) white light-emitting diode (LED) light source composed of multi-layered (ML) red, green, and blue (RGB) phosphors with a near-ultraviolet (n-UV) LED as an excitation source shows excellent color rendering, luminous efficacy, and luminous flux. The ML-phosphor structure consists of vertically stacked layers and could significantly reduce cascade excitation loss compared with a mixed RGB PC white LED. A high luminous efficacy of 82 lm/W with a high color-rendering index (CRI) of over 90 was achieved using a ML-RGB PC white LED. The ML-RGB structure also enables a white LED with an ultrahigh CRI of 99 and a high luminous efficacy comparable to that of a high CRI fluorescent lamp. We have also fabricated a high-power ML-PC white LED excited by an n-UV LED. In one package, a high luminous flux of 61 lm at a high CRI of 94 was obtained.

Nonuniform illumination caused by extended source is one of the challenging points for high power Light-emitting diodes (LEDs) especially for LED array applications. A new design method to improve the uniformity of illuminance of extended source is presented. To obtain high uniformity at target plane, a feedback optimization method is introduced including target plane girds, light source girds and target plane and light source girds optimization algorithms. Monte Carlo ray tracing simulation results demonstrate that our feedback optimization algorithms are able to obviously increase the uniformity of illuminance at target plane. The uniformity increases from 0.53 to 0.9, 0.77 and 0.88 by our three different optimization algorithms respectively. This method would be a potential way in improving the uniformity of illuminance at target plane with little light pattern expansion.

The light-emitting diodes (LEDs) are used widely around the world which have long life and are eco-friendly.In the past few decades, LEDs has made major breakthroughs in materials, packaging, and design.LEDs are developing in the direction of miniaturization and integration to meet the different occasions.At the same time, the heating problem of LEDs has become increasingly prominent.The heating power of high-power LEDs accounts for 60%-70% of the input electrical power due to the nonradiative recombination with the multiple quantum well layer and the light confinement within the LEDs packaging, which will cause sharply temperature rising of LEDs [1][2].High temperature will cause luminous flux degradation, wavelength drift, lifetime reduction, and even irreversible damage to the LEDs.Therefore, thermal management and thermal prediction for high-power LEDs are very important which ensure the stable operation of LEDs.At present, the cooling of LEDs is achieved by forced air cooling and liquid cooling.However, forced air cooling is usually bulky and noisy, and the low heat dissipation efficiency of forced air convection limits its application.Therefore, high-performance liquidcoolant-based thermal management technology is a competitive choice for LEDs due to its high heat dissipation efficiency.Spray cooling is a promising liquid-coolant-based thermal management technology for high-heat-flux electronic devices [3][4].It has the advantages of high critical heat flux (CHF), uniform surface temperature distribution, low surface overheating, and less working fluid demand.In the past, a large number of researchers conducted experiments and simulations on it.Although many of studies have shown that spray cooling is an effective way to dissipate heat from electronic devices with high heat flux, there are relatively few researches on spray cooling for LEDs, especially for high-power LEDs [5-6], which are not enough to provide guide for designing and application.In this work, a spray cooling system is proposed and it is used to verify the feasibility, advancement, and effectiveness of spray cooling on high-power LEDs of which the nominal power is 100 W. At the same time, in order to accurately explore the temperature distribution of LEDs, a model predictive inverse method is proposed and carried out.The results show that as the flow rate increases, the LEDs backside substrate temperature and target surface temperature will decrease, and the average heat transfer coefficient of the target surface will increase.The maximum heat transfer coefficient is as high as 26400 W/m 2 K.The Sauter mean diameter is one of the most important parameters of spray cooling.In this work, it is found that under the condition of high pressure drop and high flow rate, the Sauter mean diameter is smaller, which will enhance heat dissipation.At the same time, it is also discovered that as long as the proper thermal management is carried out, the LEDs can work normally when it exceeds 2.5 times of the nominal power.Three-dimensional simulation effectively supplements the experimental results, confirming that the closer to the junction point, the more uneven the temperature distribution.And spray cooling can keep the temperature of the LEDs chip below the temperature of destruction.Moreover, the prediction algorithm enables efficient estimation of the future temperature field of LEDs and early measures to prevent it from damage.According to the research results, spray cooling has been proven as an effective method for the thermal management of highpower LEDs.The design of the spray cooling system and prediction model may aid in solving the dilemma of overheating electronics.

With advances in III-V nitride manufacturing processes, high-power light-emitting diode (LED) chips in the blue and UV wavelengths are now commercially available at reasonable cost and can be used as excitation sources in optical sensing. We describe the use of these high-power blue and UV LEDs for sensitive fluorescence detection, including chip-based flow cytometry, capillary electrophoresis (CE), and single-molecule imaging. By using a blue LED with a focusable power of approximately 40 mW as the excitation source for fluorescent beads, we demonstrate a simple chip-based bead sorter capable of enriching the concentration of green fluorescent beads from 63% to 95%. In CE experiments, we show that a mixture of analyte solution containing 30 nM 6-carboxyrhodamine 6G and 10 nM fluorescein can be separated and detected with excellent signal-to-noise ratio (approximately 17 for 10 nM fluorescein) using the collimated emission from a blue LED; the estimated mass detection limit was approximately 200 zmol for fluorescein. We also demonstrated ultrasensitive fluorescence imaging of single rhodamine 123 molecules and individual lambda-DNA molecules. At a small fraction of the cost of an Ar+ laser, high-power blue and UV LEDs are effective alternatives for lasers and arc lamps in fluorescence applications that demand portability, low cost, and convenience.

We report an enhancement in light emission of a GaN-based blue light-emitting diode (LED) with periodic indium tin oxide (ITO) nanodisc arrays. The periodic nanodisc arrays were elaborately designed by the Bragg diffraction and the maximum transmittance of a model of bilayer film on a GaN epitaxial layer. In the experiment, the periodic ITO nanodisc arrays have been fabricated by polystyrene nanosphere lithography. The electroluminescence intensity of the LEDs with periodic ITO nanodisc arrays is better than that of conventional planar GaN-based LED, and the greatest enhancement could increase the intensity by a factor of 2.08 at an operating current of 20 mA compared with the conventional planar GaN-based LED. The simulation results based on three-dimensional finite difference time-domain method showed that the LED with periodic ITO nanodisc arrays can improve the light-extraction efficiency of LED by enhancing the critical angle of light output. This method would be valuable for the fabrication of high-efficiency planar GaN-based LEDs.

We studied on the effects of patterned indium-tin oxide (ITO) on the electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) in detail. In our experiment, LEDs with varied patterned ITO layers are fabricated. The electrical and optical characteristics are analyzed through the diode equation and 3D electro-thermal simulation, the rate equation and Monte-Carlo ray-tracing simulation, respectively. The analysis results indicate that the light output power (LOP) of the LEDs with patterned ITO achieve an enhancement of 12.7% compared to that of the LEDs with planar ITO. Moreover, the series resistance and ideality factor are the minimum when ITO is planar, and will increase with spacing of ITO pattern decreasing due to the degeneration of current spreading and ohmic contact. However, the light extraction efficiency (LEE) is smaller for LEDs with planar ITO and will increase with spacing decreasing, which is due to the increase of probability of photon redirecting and escaping. These analysis results are helpful to understand the effects of ITO pattern on electrical and optical characteristics and therefore to design an optimal pattern structure.

This paper demonstrates the strong enhancement of light extraction efficiency of light-emitting diodes (LEDs) by a novel three-dimensionally arranged micro-cavity. There are several optimal designed parameters, including chip dimensions, absorption coefficients, the shape of the micro-cavity and package are analyzed on the basis of a Monte-Carlo ray tracing simulation. The most important that studying includes GaN LEDs which are applied to various applications, including traffic signals, backlight system for LCD and outdoor illumination by white light LEDs. The functional of the three-dimensionally arranged micro-cavity is to make the light extraction from LED with high efficiency. The shape of micro-cavities are making like hexagon solids on the top view. The structure were evaluated and simulated by TracePro software respectively. The light extraction efficiency of LED can be greatly improved by three-dimensionally arranged micro-cavity. This study shows that the micro-cavities induced on the surface rather than that inside the LED greatly enhances the light extraction efficiency. This stipulation holds for both sapphire-based and Thin-GaN LEDs. The results indeed identify the attributes of the LED, which make it possible to achieve excellent luminance performance using a GaN-based approach from the LED of "three-dimensionally arranged micro-cavity". This structure was stringent expected to allow a high-efficiency LED, since the illumination systems needed for higher luminance energy can be added independently of the effects.