This post was written by Keith Dawson for UBM Tech’s community Web site All LED Lighting, sponsored by Philips Lumileds. It is archived here because the All LED Lighting site has gone dark. This material is Copyright 2013-2015 by UBM Americas.


Glass Phosphors for Thermal Stability

Researchers have produced glass phosphors and demonstrated their radically improved stability against lumen and transmittance losses and shifts in chromaticity, compared to traditional silicon-based phosphors.

Emission spectra of white LEDs utilizing glass phosphors as wavelength converters. (cps = counts per second.)
(Source: W-H Cheng et al.)

In typical designs for phosphor-converted white LEDs, the silicon packaging material in which phosphors are carried closely overlays LED chips. This material can suffer over time from the heat generated at the LED junction. Heat breaks down bonds in the silicone, which can become yellowed and more opaque, reducing the amount of light that can get through and shifting the color.

The researchers from the National Sun Yat-sen University in Taiwan produced glass phosphors (GPs) that exhibit better thermal stability. A sintering process at 700°C com­bined cerium-doped yttrium alum­inum garnet (Ce3+:YAG) phosphors with powdered glass (SiO2-Na2O3-Al2O3-Ca2O). The researchers used various concentra­tions of the powdered phosphors, from 1% to 5%, and demonstrated that the resultant material interacts with 465-nm blue LED light to produce a spectrum usable as a white LED source (see Figure 1).

Factor by which losses and color shift are smaller for glass-based phosphors than for silicon-based phosphors
degradation 150°C 250°C
lumen losses 4.6 19.1
transmittance losses 4.4 15.6
chromaticity shifts 298 1,408

In order to evaluate the thermal stability of the new phosphors, the researchers thermally aged samples of GPs and silicone phosphors (SPs) for 1,008 hours. They measured the luminosity, chromaticity, transmittance, and optical spectrum of both phosphors before and after the aging process. For SPs they ran the test at 150°C and 250°C. The GPs they additionally heated to 350°C and 450°C.

The table compares the samples heated to 150°C and 250°C. The GPs are radically more stable under high temperatures, especially when it comes to color shifts. The researchers conclude:

Our GPs may be a useful encapsulating material for [white LEDs] 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.

I don't know to what extent these researchers are privy to the proprietary methods LED makers, and phosphor makers, may be using to improve thermal stability in their products. The simple method they used to make SPs may be years behind the state of the art. Does anyone here know what sorts of techniques are behind the obvious improvements in thermal stability that white LEDs have exhibited over the last half dozen years?

— Keith Dawson Circle me on Google+ Follow me on Twitter Visit my LinkedIn page, Editor-in-Chief, All LED Lighting