Huai Zheng

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%.

Dielectric fluid directional spreading under the action of corona discharge

Liquid spreading is a very common nature phenomenon and of significant importance for a broad range of applications. In this study, a dielectric fluid directional spreading phenomenon is presented. Under the action of corona discharge, a dielectric fluid, here a typical silicone directionally spreads along conductive patterns on conductive/nonconductive substrates. Directional spreading behaviors of silicone were experimentally observed on different conductive patterns in detail. Spreading speeds were analyzed at different driving voltages, which induced the corona discharge. The presented phenomenon may be useful to inspire several techniques of manipulating liquid transportation and fabricating micropatterns.

Enhancing the thermal dissipation of a light-converting composite for quantum dot-based white light-emitting diodes through electrospinning nanofibers

Quantum dots (QDs) have been developed as one of the most promising light-converting materials for white light-emitting diodes (LEDs). In current QD-based LED packaging structures, composites of QDs and polymers are used as light-converting layers. However, the ultralow thermal conductivity of such composites seriously hinders the dissipation of QD-generating heat. In this paper, we demonstrate a method to enhance the thermal dissipation of QD-polymer composites through electrospinning polymer nanofibers. QD-polymer films embedded by electrospun nanofibers were prepared. Benefitting from aligned polymer chains in the electrospun nanofibers, the through-panel and in-panel thermal conductivities of the proposed QD-polymer film increased by 39.9% and 423.1%, respectively, compared to traditional QD-polymer film. The proposed and traditional QD-polymer films were both packaged on chip on board (CoB) LEDs for experimental comparison. Compared to traditional QD-polymer film, the luminous flux and luminous efficiency of the LEDs were increased by up to 51.8% and 42.9% by the proposed QD-polymer film under a current of 800 mA, respectively. With an increase in the driving current from 20-800 mA, the correlated color temperature (CCT) variation decreased by 72.7%. The maximum temperatures in the QD-polymer films were reduced from 419 K-411 K under a driving current of 200 mA.

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.

Reduction of Die-Bonding Interface Thermal Resistance for High-Power LEDs Through Embedding Packaging Structure

Thermal management is a key issue for high-power light-emitting diodes (LEDs). In this study, a novel packaging structure was proposed to reduce the die-bonding interface thermal resistance and provided a new idea for LED heat-dissipation. The LED chip was embedded into a square groove at the lead-frame substrate. The gap between the edges of the LED chip and square groove was filled with boron nitride/silicone composite. So the heat generated from the chip could be dissipated from the side surfaces to the substrate. The experimental results show that the heat-dissipation ability of LEDs has been significantly improved by the new embedding packaging structure. The die-bonding interface thermal resistance can be reduced by more than 20%. The junction temperature rise can be reduced by 14%. Due to the fully embedding of LED chip in the square groove, the light intensity distribution is slightly shrunk and the light output power is slightly reduced by about 8%.

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.

Forming desired polymer patterns through spatial-modulated ionic wind

Patterning polymer films is of significant interest for a broad range of applications. In this paper, a method of patterning polymer films is proposed, which is based on the liquid polymer film flow actuated by ionic wind. We placed the mask in ionic wind which can be modulated spatially. Under such conditions of spatial-modulated ionic winds, the liquid polymer presents different flow regimes, the morphologies of which duplicate the masks of different shapes. The patterning mechanism was investigated through numerical simulations. Different masks, such as line strips, serpentine-shaped stripes, and nuts, were adopted to manipulate the polymer-patterns with the presented method. The different polymer patterns were solidified and characterized, which verifies the feasibility of the methods for the formation of different complex polymer patterns.Patterning polymer films is of significant interest for a broad range of applications. In this paper, a method of patterning polymer films is proposed, which is based on the liquid polymer film flow actuated by ionic wind. We placed the mask in ionic wind which can be modulated spatially. Under such conditions of spatial-modulated ionic winds, the liquid polymer presents different flow regimes, the morphologies of which duplicate the masks of different shapes. The patterning mechanism was investigated through numerical simulations. Different masks, such as line strips, serpentine-shaped stripes, and nuts, were adopted to manipulate the polymer-patterns with the presented method. The different polymer patterns were solidified and characterized, which verifies the feasibility of the methods for the formation of different complex polymer patterns.