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Illumination & Displays

Thermally stable white LEDs

Novel glass phosphors outperform silicone phosphors for next-generation solid-state lighting.
10 July 2014, SPIE Newsroom. DOI: 10.1117/2.1201407.005527

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

Purchase SPIE Field Guide to IlluminationThe 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 CIEx and CIEy 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 CIEx, 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.


Wood-Hi Cheng, Li-Yin Chen, Wei-Chih Cheng
Department of Photonics
National Sun Yat-sen University
Kaohsiung, Taiwan

Wood-Hi Cheng is a professor. His research interests are laser module packaging, reliability study of LEDs, and broadband fiber amplifiers. He is an SPIE Fellow and also a fellow of both the Optical Society (OSA) and the Institute of Electrical and Electronics Engineers.

Li-Yin Chen is an assistant professor. Her research interests include optical and electrical characterization of organic/inorganic semiconductors, organic/inorganic optoelectronic and electronic devices, and physics of liquid crystalline semiconductors. She holds a PhD from National Taiwan University, Taiwan.

Wei-Chih Cheng is a doctoral student. His research interests are the packaging and applications of high-power LED modules. He received an MS degree in electro-optical engineering, in 2006, from National Kaohsiung First University of Science and Technology, Taiwan, in 2006.


References:
1. Y. C. Hsu, Y. K. Lin, M. H. Chen, C. C. Tsai, J. H. Kuang, S. B. Huang, H. L. Hu, Y. I. Su, W. H. Cheng, Failure mechanisms associated with lens shape of high-power LED modules in aging test, IEEE Trans. Electron. Devices 55, p. 689-694, 2008.
2. C. C. Tsai, M. H. Chen, Y. C. Huang, Y. C. Hsu, Y. T. Lo, Y. J. Lin, J. H. Kuang, et al., Decay mechanisms of radiation pattern and optical spectrum of high-power LED modules in aging test, IEEE J. Sel. Top. Quant. Electron. 15, p. 1156-1162, 2009.
3. J. Wang, C. C. Tsai, W. C. Cheng, M. H. Chen, C. H. Chung, W. H. Cheng, High thermal stability of phosphor-converted white-light-emitting diodes employing Ce:YAG doped glass, IEEE J. Sel. Top. Quant. Electron. 17, p. 741746, 2011.
4. T. Yanagisawa, T. Kojima, Long-term accelerated current operation of white light-emitting diodes, J. Luminesc. 114, p. 39-42, 2005.
5. Trace Pro User Guide, Trace Pro 3.2.2, Lambda Research Corp., 1995.
6. J. P. Holman, Heat Transfer, McGraw-Hill, 2000.
7. C. C. Tsai, W. C. Cheng, J. K. Chang, L. Y. Chen, J. H. Chen, Y. C. Hsu, W. H. Cheng, Ultra-high thermal-stable glass phosphor layer for phosphor-converted white light-emitting diodes, J. Display Technol. 9(6), p. 427-432, 2013.
8. W. H. Cheng, C. C. Tsai, J. Wang, Lumen degradation and chromaticity shift in glass and silicone based high-power phosphor-converted white-emitting diodes under thermal tests, Proc. SPIE 8123, p. 81230F, 2011. doi:10.1117/12.892546