Thermally stable white LEDs

White light-emitting-diodes (WLEDs) perform well in many settings where solid-state light fittings with long lifetime, low power consumption, and high efficiency are required. However, high-power WLEDs suffer from high heat flux during operation. Improvements in chip structure and fabrication have solved the problem of heat degradation of chip materials, and now the challenge of high-power WLEDs is the reliability of their packaging.^1–3

The most common LED packaging material is silicone, a polymer-based material, which has cerium-doped yttrium aluminum garnet (Ce³⁺:YAG) phosphors blended into it to emit white light. These operate as color conversion elements, which transfer parts of the blue light emitted from a gallium nitride (GaN) LED chip to yellow light. Unfortunately, the extremely high heat flux radiated from the GaN chip induces bond breakage in silicone.⁴ The sub-band defects consequently make silicone yellowish and decrease transmittance of silicone. The optical performance of WLED modules such as lumen and chromaticity thus deteriorates after a period of operation.^5–7 To address this, we have developed novel glass-based phosphors with high thermal stability.

We sintered a uniform mixture of Ce³⁺:YAG phosphors and glass powders (SiO₂-Na₂O₃-Al₂O₃-Ca₂O) at 700°C for 30min to form glass phosphors. Figure 1 shows the glass phosphor samples (thickness = 0.5mm) with different concentrations of Ce³⁺:YAG ranging from 0 to 5wt%. The bright yellow appearance of the glass phosphors results from their high photoluminescence quantum yield, which is up to 68.7% for 4wt% glass phosphors.

Figure 1. Glass phosphors (GPs) with 0 to 5wt% cerium-doped yttrium aluminum garnet (Ce³⁺:YAG) concentrations.

The emission spectra and the 1931 International Commission on Illumination (1931-CIE) color coordinates of the WLEDs using glass phosphors are shown in Figures 2 and 3, respectively. We attribute the 465nm band to the emission of the blue LED and the broad emission band around 560nm to fluorescence from the glass phosphors. We can satisfy the color requirements of commercial WLEDs—that the 1931-CIE color coordinates CIE_x and CIE_y are both 0.33—by using the 4wt% glass phosphor as the wavelength converter.

Figure 2. Emission spectra of white LEDs (WLEDs) utilizing glass phosphors as wavelength converters. cps: Counts per second.

Figure 3. 1931 International Commission on Illumination (1931-CIE) chromaticity coordinates of the resulting WLEDs.

We prepared both glass phosphors (GPs) and silicone phosphors (SPs) to compare their thermal stability. Silicone phosphors were fabricated by baking the mixture of silicone and Ce³⁺:YAG phosphor crystals at 150°C. We chose the concentration of phosphor crystals in those samples to maintain CIE_{x, y} = 0.33±0.005. We measured the luminosity, chromaticity, transmittance, and optical spectrum of the GPs and SPs before and after thermal aging to evaluate their thermal stability. For the GPs, we ran the thermal aging test at 150, 250, 350, and 450°C for 1008 hours. For the SPs, we ran the same test at only 150 and 250°C for 1008 hours.

Figure 4 shows that the lumen loss, chromaticity shift, and transmittance loss of GPs and (inset) SPs after thermal aging are dramatically lower for GPs than SPs. The lumen losses of SPs were 4.6 and 19.1 times higher than those of GPs at 150 and 250°C, respectively. The chromaticity shifts of SPs were 298 and 1408 times higher than those of GPs at 150 and 250°C, respectively. The transmittance losses of the SPs were 4.4 and 15.6 times higher than those of the GPs at 150 and 250°C, respectively. The optical performance of GPs changes only slightly under even severe (450°C) thermal test conditions. These results all demonstrate that GPs are much more thermally stable than SPs, which we ascribe to the GPs' high glass transition temperature (567°C).

Figure 4. Lumen loss (top), chromaticity shift (middle), and transmittance loss (bottom) of GPs and (insets) SPs versus aging temperatures.⁸

In summary, we have demonstrated that GPs for WLEDs show ultra-high thermal stability. The lumen loss, chromaticity shift, and transmittance loss of the glass phosphors under thermal aging are dramatically less than those of SPs, and they exhibit far superior thermal stability. Our GPs may be a useful encapsulating material for WLEDs where highly reliable white solid-state lighting modules are required and where silicone simply cannot stand the heat, humidity, or other deteriorating factors. Our next step is to develop such high-power WLEDs.