Run Hu

Design of a novel freeform lens for LED uniform illumination and conformal phosphor coating

A conformal phosphor coating can realize a phosphor layer with uniform thickness, which could enhance the angular color uniformity (ACU) of light-emitting diode (LED) packaging. In this study, a novel freeform lens was designed for simultaneous realization of LED uniform illumination and conformal phosphor coating. The detailed algorithm of the design method, which involves an extended light source and double refractions, was presented. The packaging configuration of the LED modules and the modeling of the light-conversion process were also presented. Monte Carlo ray-tracing simulations were conducted to validate the design method by comparisons with a conventional freeform lens. It is demonstrated that for the LED module with the present freeform lens, the illumination uniformity and ACU was 0.89 and 0.9283, respectively. The present freeform lens can realize equivalent illumination uniformity, but the angular color uniformity can be enhanced by 282.3% when compared with the conventional freeform lens.

Study on the Optical Properties of Conformal Coating Light-Emitting Diode by Monte Carlo Simulation

The light extraction efficiency (LEE), correlated color temperature (CCT), and angular color uniformity (ACU) of the conformal coating phosphor silicone gel were investigated by Monte Carlo ray-tracing simulations. Relative to the conventional dispensing method, the conformal coating method can make the light-emitting diode (LED) module obtain better optical properties. It was found that the thickness of the phosphor layer in conformal coating was a key factor determining the optical performance. The thickness could be optimized for good optical properties. The optimal thickness is around 50-70 μm when taking LEE, CCT, and ACU into consideration.

Conformal phosphor coating using capillary microchannel for controlling color deviation of phosphor-converted white light-emitting diodes

We demonstrated a conformal phosphor coating method for phosphor-converted white light-emitting diodes (LEDs) using capillary microchannel formed by a fixture. The mixture of the phosphor particles and the silicone spontaneously flows in capillary microchannel and was pinned at the edge of the fixture due to the surface tension effect. The present coating method was applied to different packaging types with both the conventional chip and the vertical injection chip. Experimental results show this method can efficiently improve the angular color uniformity (ACU). Compared with those samples packaged by conventional dispensing coating, angular color correlated temperature (CCT) deviation of the test samples by the present method can reduce from 1500K to 200K for the average CCT 5200K from -90° to +90 view angles, and reduce from 5000K to 1000K for the average CCT 9150K. Additionally, this method can prevent the packaging non-consistency that the average CCT varies from package to package due to the deviation of phosphor-silicone mixture volume. In the experiments, even when the phosphor-silicone mixture volume varies from 0.3 µl to 0.5 µl, the deviation of the average CCT is less than 80K.

Quantum Dots-Converted Light-Emitting Diodes Packaging for Lighting and Display: Status and Perspectives

Recent years, semiconductor quantum dots (QDs) have attracted tremendous attentions for their unique characteristics for solid-state lighting (SSL) and thin-film display applications. The pure and tunable spectra of QDs make it possible to simultaneously achieve excellent color-rendering properties and high luminous efficiency (LE) when combining colloidal QDs with light-emitting diodes (LEDs). Due to its solution-based synthetic route, QDs are impractical for fabrication of LED. QDs have to be incorporated into polymer matrix, and the mixture is dispensed into the LED mold or placed onto the LED to fabricate the QD–LEDs, which is known as the packaging process. In this process, the compatibility of QDs’ surface ligands with the polymer matrix should be ensured, otherwise the poor compatibility can lead to agglomeration or surface damage of QDs. Besides, combination of QDs–polymer with LED chip is a key step that converts part of blue light into other wavelengths (WLs) of light, so as to generate white light in the end. Since QD-LEDs consist of three or more kinds of QDs, the spectra distribution should be optimized to achieve a high color-rendering ability. This requires both theoretical spectra optimization and experimental validation. In addition, to prolong the reliability and lifetime of QD-LEDs, QDs have to be protected from oxygen and moisture penetration. And the heat generation inside the package should be well controlled because high temperature results in QDs’ thermal quenching, consequently deteriorates QD-LEDs’ performance greatly. Overall, QD-LEDs’ packaging and applications present the abovementioned technical challenges. A profound and comprehensive understanding of these problems enables the advancements of QD-LEDs’ packaging processes and designs. In this review, we summarized the recent progress in the packaging of QD-LEDs. The wide applications of QD-LEDs in lighting and display were overviewed, followed by the challenges and the corresponding progresses for the QD-LEDs’ packaging. This is a domain in which significant progress has been achieved in the last decade, and reporting on these advances will facilitate state-of-the-art QD-LEDs’ packaging and application technologies. [DOI: 10.1115/1.4033143]

Hotspot Location Shift in the High-Power Phosphor-Converted White Light-Emitting Diode Packages

Thermal management of high-power light-emitting diodes (LEDs) plays an important role in determining their optical properties, reliability, and lifetime. In this paper, we present a method to study the temperature field of phosphor-converted LED packages by combining the Monte Carlo optical simulation and finite element simulation together. The temperature field, including the heat generation in both LED chip and phosphor layer, are presented and analyzed. It was found that temperature increased with the increase in phosphor concentration and the hotspot location in remote phosphor-coating packages shifted with the changes in phosphor concentration, while there was no shift in direct phosphor-coating packages. It was concluded that the hotspot location in the high-power phosphor-converted white LED packages depended on phosphor concentrations as well as packaging methods.

Local heating realization by reverse thermal cloak

Transformation thermodynamics, as one of the important branches among the extensions of transformation optics, has attracted plentiful attentions and interests recently. The result of transformation thermodynamics, or called as “thermal cloak”, can realize isothermal region and hide objects from heat. In this paper, we presented the concept of “reverse thermal cloak” to correspond to the thermal cloak and made a simple engineering definition to identify them. By full-wave simulations, we verified that the reverse thermal cloak can concentrate heat and realize local heating. The performance of local heating depends on the anisotropic dispersion of the cloaking layers thermal conductivity. Three-dimensional finite element simulations demonstrated that the reverse thermal cloak can be used to heat up objects. Besides pre-engineered metamaterials, such reverse thermal cloak can even be realized with homogenous materials by alternating spoke-like structure or Hashin coated-sphere structure.

Study on phosphor sedimentation effect in white light-emitting diode packages by modeling multi-layer phosphors with the modified Kubelka-Munk theory

In this study, we studied the phosphor sedimentation effect in white phosphor-converted light-emitting diode packages by modeling the multi-layer phosphors with gradient concentrations. The essence of phosphor sedimentation can attribute to the variation of phosphor concentrations. By modifying the Kubelka-Munk theory, we built a multi-layer phosphor model with considering the light scattering, light absorption, and light conversion process simultaneously. With a brief review of Kubelka-Munk theory, multi-layer phosphors were modeled on the basis of single-layer phosphor model. The phosphor sedimentation effect was characterized by modeling multi-layer phosphors with gradient concentrations, whereas keeping the total amount of phosphors at the same level. It is found from the five calculation cases that phosphor sedimentation will cause the drop of light extraction efficiency (LEE) by 13.04%. Furthermore, the phosphor layer with inverse-gradient concentrations will enhance the LEE 16.56%. To figure out th...

Comprehensive Study on the Transmitted and Reflected Light Through the Phosphor Layer in Light-Emitting Diode Packages

In this study, we modeled the transmitted and reflected light of a phosphor layer in light-emitting diode packages by coupling the revised Kubelka-Munk and Mie-Lorenz theories. Through analyzing the transmitted and reflected light separately, it is found that the transmitted and reflected blue light vary monotonically with the changes of phosphor particle size, concentration and thickness. While the trends of the transmitted and reflected yellow light are non-monotonic, which are influenced by the interactions among the phosphor particle size, concentration and thickness. The light extraction efficiency through a phosphor layer was discussed. The reasons behind these phenomena were also presented.

Modeling the Light Extraction Efficiency of Bi-Layer Phosphors in White LEDs

In this letter, we modeled the bi-layer phosphors in light-emitting diode packages and optimized the concentration gradient to enhance the light extraction efficiency (LEE). The modeling processes were presented in detail. Based on our bi-layer model, we changed the concentration from 0.05 to 0.4 g/cm3 in both the upper and lower phosphor layers. It was found that when the concentration gradient between the upper layer and lower layer increases from 0.05:0.4 to 0.4:0.05, the LEE would be enhanced as much as 17.63%. The trends were validated by experiments. Some potential ways were proposed to realize this kind of phosphor configuration where the concentration of the upper layer was larger than that of the lower layer.

Effects of current crowding on light extraction efficiency of conventional GaN-based light-emitting diodes.

Current crowding effects (CCEs) on light extraction efficiency (LEE) of conventional GaN-based light-emitting diodes (LEDs) are analyzed through Monte Carlo ray-tracing simulation. The non-uniform radiative power distribution of the active layer of the Monte Carlo model is obtained based on the current spreading theory and rate equation. The simulation results illustrate that CCE around n-pad (n-CCE) has little effect on LEE, while CCE around p-pad (p-CCE) results in a notable LEE droop due to the significant absorption of photons emitted under p-pad. LEE droop is alleviated by a SiO₂ current blocking layer (CBL) and reflective p-pad. Compared to the conventional LEDs without CBL, the simulated LEE of LEDs with CBL at 20 A/cm² and 70 A/cm² is enhanced by 7.7% and 19.0%, respectively. It is further enhanced by 7.6% and 11.4% after employing a reflective p-pad due to decreased absorption. These enhancements are in accordance with the experimental results. Output power of LEDs with CBL is enhanced by 8.7% and 18.2% at 20 A/cm² and 70 A/cm², respectively. And the reflective p-pad results in a further enhancement of 8.9% and 12.7%.