Nguyen T. Tran

Studies of Phosphor Concentration and Thickness for Phosphor-Based White Light-Emitting-Diodes

The dependence of luminous efficacy on phosphor concentration and thickness for high-power white light-emitting-diode (WLED) lamps is investigated by employing three-dimensional ray-tracing simulations. The simulations show that the brightness or luminous efficacy of WLED lamps highly depends on the combination of phosphor concentration and phosphor thickness (or phosphor-matrix composite volume). The package with lower concentration and higher phosphor thickness has higher luminous efficacy because the light trapping efficiency is lower with the low phosphor concentration. At the correlated color temperature (CCT) value of around 4000 K, ray-tracing simulation and experimental results show 20% and 23% improvement in lumen, respectively, with a 1.8-mm-phosphor package over a 0.8-mm-phosphor package. A package with convex lens can improve the lumen output over flat lens, but this improvement is small, and it requires higher amount of phosphor, up to 25%, to achieve same CCT value.

Light extraction enhanced white light-emitting diodes with multi-layered phosphor configuration

Phosphor-converted white light-emitting diodes (LEDs) with separate red and yellow phosphor layers are investigated under current regulation conditions. This novel packaging scheme of bi-layered phosphors leads to more than 18% increase in luminous flux compared to conventional random mixed phosphor case at the same correlated color temperature. Lower junction temperatures of bi-layered phosphor white LEDs are also observed. This advantage in the thermal characteristics is due to the reduced back reflection of light inside the packages. It is also found that the phosphor conversion efficiency of bi-layered phosphor scheme is higher than that of mixed phosphor case. This is attributed to the enhanced light extraction from the LED packages. In addition, the chromaticity coordinates shifts compared bi-layered phosphor with mixed phosphor white LEDs are almost the same under current regulation conditions. The technology of bi-layered phosphor white LEDs with high efficiency and high color rendering index provides an appropriate solution for general white LED lighting.

Development of High-Performance Optical Silicone for the Packaging of High-Power LEDs

Silicone materials with a relatively high-refractive index have been introduced for the encapsulation of high-power light-emitting diodes (LEDs), and LEDs with relatively short wavelengths. However, most of those existing silicone encapsulants still suffer from thermal and radiation induced degradations and thus lead to reliability issues and a shorten lifetime. A new high-performance silicone has been developed and its performance is compared with other commercial silicone and optical grade epoxy in high-power white LEDs. The new materials had been found to suffer less loss in the lumen output during the aging test and high-temperature/high-humidity test, as well as the Joint Electron Devices Engineering Council (JEDEC) reliability test. It is concluded that this material is excellent for the packaging of high-power white LEDs and high-power colored LEDs, because of its ability in maintaining high-transparency and great radiation/thermal resistance.

Angular CCT Uniformity of Phosphor Converted White LEDs: Effects of Phosphor Materials and Packaging Structures

In this work, we studied the effect of different packaging parameters on the angular homogeneity of correlated color temperature (CCT) in phosphor-in-cup light-emitting diode (LED) packages, including phosphor particle concentration, phosphor-layer surface curvature, and lens surface curvature, using a Monte Carlo ray tracing simulation tool. An optimized packaging geometry with convex phosphor layer was achieved with good CCT uniformity and highest lumen efficiency. Moreover, an approach was developed for the prediction of color uniformity and lumen efficiency when varying the surface curvature of either phosphor layer or clear lens, since the inconsistency of phosphor layer or lens cap happened commonly to the fabricated phosphor conversion white LEDs (pc-WLEDs).

Can Junction Temperature Alone Characterize Thermal Performance of White LED Emitters

Thermal performance of phosphor-based white light emitting diodes (LEDs) under an input current of 350 mA is investigated by finite-element simulation in which the thermal and optical interactions are considered. It is demonstrated that the temperature of the phosphor particles, regardless of phosphor placement, is always higher than the junction temperature. It is concluded that the junction temperature, which characterizes the thermal behavior of monotonic color LED emitters, cannot be used alone for characterizing the thermal behavior of white LED emitters. In fact, the phosphor temperature is critical in determining the lumen performance and reliability of white LED emitters. In addition, the phosphor temperature is effectively reduced by coating the phosphors directly on the chip and maintaining a relatively higher phosphor concentration (above 60 wt.%) in the phosphor-silicone mixture layer.

One-Component, Low-Temperature, and Fast Cure Epoxy Encapsulant With High Refractive Index for LED Applications

The cationic polymerization of diglycidyl ether of bisphenol-A (DGEBA) using a latent cationic thermal initiator, ammonium antimony hexafluoride, and the effect of addition of cycloaliphatic epoxy resin on the polymerization were studied with DSC, TGA, and hardness measurement. It was found that the cationically polymerized DGEBA at 0.5 phr initiator level gives the best compromise of chemical reactivity and optical properties, and therefore is suitable for light emitting diodes (LED) applications as an optically transparent encapsulant. This encapsulant, on one hand, is room temperature stable for at least six months, yet cures fast at low temperature, on the other hand. Moreover, the encapsulant has a high refractive index of 1.6 resulting from the high density of aromatic structure in the cross-linked network. Packaged with this encapsulant, the LED device is shown to exhibit an increased light output as demonstrated by both simulation and experimental measurement. The introduced system provides a new one-component encapsulant that is optically transparent, cures fast at low temperature with no sacrifice of room temperature stability, and has high refractive index at the same time.

Nonmonotonic Phosphor Size Dependence of Luminous Efficacy for Typical White LED Emitters

Phosphor particle size effect on luminous efficacy is studied for three types of phosphor converted white light-emitting diodes (WLEDs) using a Monte Carlo simulation tool. It is demonstrated for the first time that the size effect is nonmonotonic and exhibits minima in the submicrometer size range. In particular, for different LED packages, the lowest luminous efficacy all exists at the dimensionless particle size parameter (πd/λ) around 5.6. Moreover, the simulation results are verified by a theoretical model developed for the estimation of critical particle size. Finally, this study provides important technical implications for phosphor development and resolves one of the most critical issues in WLED manufacturing.

Influence of Die Attach Layer on Thermal Performance of High Power Light Emitting Diodes

In this paper, the influence of the die attach adhesive (DAA) layer on the thermal performance of high power light emitting diodes was first investigated by using finite element analysis, and some key results were verified by the experimental data. Effective thermal management of the studied light emitting diode package can be achieved by selecting a DAA material with a proper thermal conductivity and by manipulating the geometry parameters of the DAA layer, such as the DAA area, and the bond-line thickness. The significance of DAA thermal conductivity to heat dissipation was further demonstrated by an analysis of the bottleneck to heat transfer.

Packaging of Phosphor Based High Power White LEDs: Effects of Phosphor Concentration and Packaging Configuration

Two types of packaged white light emitting diodes in which one has a flat-top (FT) emitting surface and the other is aflat-top-with-lens (FTWL) type are fabricated by using the same leadframe and investigated on their optical properties, such as optical power, luminous efficiency, correlated color temperature (CCT), chromaticity coordinate, and color-rendering index (CRI), as a function of phosphor concentration in silicone encapsulant. It is found out that the optical power, CRI, and CCT decrease steadily as the phosphor ratio increases, while the luminous efficiency increases up to a level and then drops after a certain value of the phosphor ratio for both types of packages. Due to the totally internal reflection (TIR) at the encapsulant-air interface, the FT package shows a 10∼11% power (in mW) reduction compared with the FTWL package at the same phosphor concentration. However, it is demonstrated that the FT package provides a more efficient way of utilizing phosphor than the FTWL package based on the same targeted chromaticity coordinates due to the TIR effect inside, resulting in a reduced phosphor usage with a lumen output only about 3% lower than that of the FTWL package.

LED and Optical Device Packaging and Materials

As for integrated circuit (IC) device packaging, the packaging materials are critical to the LED packaging because the device packaging and assembly yield, and the device reliability and lifetime are determined by the quality of packaging and assembly materials as well as their processing. This presents serious challenges to the development of LED packaging materials, which is exactly the objective of this chapter to review those challenges and to point out the direction of further development.