Jingcao Chu

Optical Performance Enhancement of Quantum Dot-Based Light-Emitting Diodes Through an Optimized Remote Structure

Quantum dot (QD)-based light-emitting diodes (LEDs) always show low efficiency and unstable performance at high driving currents due to the saturation effect of QDs. In this paper, a novel remote packaging structure was proposed to reduce the saturation effect and enhance optical performances of QD-based LEDs. In the proposed packaging structure, a crater lens and an air gap between the QD-polymer film and the lens were introduced. The comparison experiments between the proposed structure and the current structures, a fully filling structure, and an air-gap structure with a spherical lens were conducted. The luminous flux increasing rates of current structures were far lower than that of the proposed packaging structure at high driving currents. Consequently, compared with the fully filling structure and the air-gap structure with the spherical lens, the proposed packaging structure has increased the luminous flux by 70.5% and 50.1% at the driving current of 800 mA. In terms of the correlated color temperature stability with the driving current, the proposed packaging structure shows the best performance. In addition, the angular color uniformity has been greatly improved by the proposed packaging structure.

Fabrication of adjustable-morphology lens based on electrohydrodynamic for high-power light-emitting diodes

Advanced lens fabricating techniques are urgently required for the development of light-emitting diode (LED) packaging. In this study, a method of fabricating LED lenses was proposed based on electrohydrodynamics (EHD). In this method of fabricating lenses, the normal stress on the air–liquid interface was induced and adjusted by an electric field applied around liquid lens material. By controlling this interface stress, the lenses could be tailored into the desired morphologies. Three kinds of typical lens morphologies, namely cone, ellipse and asymmetric, were fabricated by varying the electric field type. Moreover, the detailed characteristics of each of the morphologies were significantly and accurately regulated. As a result of varying the lens morphology, the light intensity distributions (LIDs) of the LED modules changed obviously. The strongest light intensity increased up to 39% and its occurring view angle varied from 0° to 48°. Meanwhile, the light efficiency of the lenses fabricated in the presented method was examined. The results showed that the light efficiency sacrifice was less than 2%.

Fabrication of Lens With Large View Angle Through Droplet Evaporation for High-Power Light-Emitting Diodes

A simple and low-cost method for fabricating lens with large view angle was proposed based on droplet evaporation for high-power light-emitting diodes (LEDs). In such a method, an immiscible and vaporizable droplet was deposited on the top surface of a liquid silicone lens. By applying a heat load, the silicone was cured and the droplet was evaporated simultaneously. Then, lenses with craters were fabricated for large view-angle emission of LEDs. In experiments, the diameter and depth of the crater on the lens surface were controlled by adjusting the volume of droplet. As a result of the variation of carter size, light intensity distributions (LIDs) of LED modules were obviously modulated. Compared with traditional spherical cap lens, the view angles of the maximum light intensity of the fabricated lens varied from 0° to ±54°. Meanwhile, the maximum light intensity increased by 17%.

A novel cooling method for LED filament bulb using ionic wind

Thermal dissipation for Light emitting diode (LED) filament bulb is a very important issue. The heat dissipation ability of LED filament bulb is limited by its complex structure. In this study, a novel cooling method for LED filament bulb based on ionic wind was presented. The bulb was embedded with needle to ring electrodes as ionic wind generator. Once there was ionic wind produced in the bulb, the heat convection between the LED filaments and the air is enhanced. The flow status of ionic wind in the bulb was analyzed by Computational Fluid Dynamics (CFD) method. The LED junction temperature of the filaments was obtained by forward voltage method. The experimental results showed that the LED junction temperature was dramatically reduced. The LED junction temperature can be reduced by more than 30 V. The cooling solution proposed in this study can help to improve the product quality and broaden the application future of LED filament bulb.

Thermal dissipation enhancement of LED filament bulb by ionic wind

The aim of this paper is to study the heat dissipation enhancement effect of LED filament bulb by using ionic wind. A LED filament bulb with 6 filaments is used for studying. There are two holes drilled at the end of the bulb and the ionic wind blows into the inner space of the bulb through the holes that will strengthen the heat convection of the filaments. The ionic wind generator includes the needle-net electrode, the flow channel and a high voltage DC power supply. The temperature of the filament is measured by an infrared thermal image instrument. According to the experiment results, the drop of filament surface temperature can reach to about 15°C with the corona discharge volatage higher than 9kV.

Enhancing angular color uniformity of white light-emitting diodes by cone-type phosphor layer geometry

Angular color uniformity (ACU) is an important optical parameter for phosphor-converted white light-emitting diodes (LEDs). The phosphor layer geometry usually presents the spherical-cap shape by the conventional dispensing coating method, thus yellow ring phenomenon appears in the radiation pattern and leads to poor ACU. In this paper, a kind of cone-type phosphor layer geometry was proposed to enhance the ACU of white LEDs. Optical simulations were performed by the Monte Carlo ray-tracing method. Results show that the proposed cone-type phosphor layer geometry has significant positive effect on enhancing the ACU of white LEDs. Compared to the conventional phosphor layer geometry of spherical cap with the same volume, the ACU is increased by more than 19% by the cone-type phosphor layer geometry. With the same correlated color temperature (CCT) and phosphor concentration, the ACU is increased by more than 20% and the phosphor layer volume is reduced by more than 4.5% to save material cost.

Study on conformal phosphor coating for phosphor converted white LEDs through ionic wind patterning

The morphology of phosphor layer in white phosphor converted white LEDs is very important since it plays a vital role in optical performance such as correlated color temperature (CCT) and angle color uniformity (ACU). Owing to its outstanding optical performance, conformal phosphor coating process has attracted plenty of attention since it was proposed. In this article, a new method through ionic wind patterning for realizing conformal phosphor coating was proposed. The principle of such a patterning method was based on the interaction between ionic wind and liquid encapsulation. Masks were applied to control the ion flow for obtaining the desired patterning. As a result, compact and uniform phosphor encapsulation layer was successfully fabricated, its geometry was presented and its optical performance enhancement was demonstrated by experiments. Compared with the traditional dispensing coating process, the angular CCT deviation of the proposed structure is reduced from 1630K to 366K, when the averaged CCT is ~5400K.

Simulation and design of a hot-film air flow sensor with sapphire as substrate

A finite element model for evaluating the performance of the MEMS air flow sensor with sapphire as substrate was established. The temperature distribution of the MEMS sensor chip with respect to different air flow was calculated by COMSOL Multiphysics. The trenches etched in the substrate were used to reduce the heat transfer from the heater to the temperature measuring thermistors within the substrate. The influences of the width and depth of the trenches on the output of the sensor chip were analyzed. The simulation results shows the potential of using sapphire substrate for processing the MEMS air flow sensor.

Optical performance enhancement of light-emitting diodes with quantum dots through new remote type structure

Nowadays colloidal quantum dots (QDs) have been developed as a promising light-converting material for light-emitting diodes (LEDs). However, QDs-based LEDs always show saturation effect at high forward current. In this paper, a new remote packaging structure was proposed to reduce the saturation effect and utilize the QDs fully. The comparison of optical performance between proposed remote structure and current structures, flat structure and air-gap with regular lens was conducted through experiments. The luminous efficiencies of air-gap structure with regular lens and proposed remote structure are over 22% higher than flat structure at the driving current of 20mA. With the current increasing from 20 to 800mA, the luminous efficiencies of flat structure and air-gap structure with regular lens decrease by 66.5% and 61.5%, respectively. While the proposed remote structure shows the luminous efficiency decrease of 37.7%. Therefore, at the driving current of 800mA, compared to current structures, the proposed remote structure increases the luminous flux by 70.5% and 50.1%, respectively. The experimental results demonstrate that the new packaging structure have large advantages in terms of the luminous efficiency and the stability.

Numerical and experimental study of lens manufacturing based on electrohydrodynamics for light-emitting diodes

In this study, a new lens fabricating method based on electrohydrodynamics (EHD) was proposed and demonstrated for light-emitting diodes (LEDs). In such a lens fabricating method, the normal stress on the air-liquid interface was induced and adjusted by the electric field applied around liquid lens material. Through controlling this interface stress, the lens could be tailored into desired morphologies. A numerical model for the silicone lens fabricating process has been built. Simulation results show that three kinds of desired lens morphologies such as cone, ellipse and asymmetry can be achieved by plate-plate, ring-plate and nonaxisymmetrical cylinder-plate electrode pairs, respectively. After that, experiments were conducted to verify simulation results. Cone, ellipse and asymmetry lenses were obtained by experiments. The characteristics in detail of each of morphologies were significantly and accurately regulated. The maximum relative height variation reaches 29.6%, 16.7%, and 7.4% for the cone, ellipse and asymmetry lenses, respectively. And fabricated lenses based on EHD have high surface quality with average roughness less than 5nm.