White LEDs Approximate Daylight
Daniel S. Burgess
In pursuit of alternative material systems for white LEDs, scientists at National Institute for Materials Science in Tsukuba, at Sharp Corp. in Mihara and at Fujikura Ltd. in Sakura, all in Japan, are investigating phosphors of α-SiAlON containing lithium and doped with Eu2+ for use with 460-nm InGaN LEDs. They suggest that the warm white emitters point the way to a substitute to white LEDs based on the YAG:Ce3+ phosphor for applications such as LCD backlights, flashlights and automotive interior illumination.
SiAlON-based phosphors enable white LEDs with various color temperatures. Courtesy of Rong-Jun Xie and Fujikura Ltd.
Rong-Jun Xie, a senior researcher at the institute, explained that there is a commercial impetus to develop replacements for YAG:Ce, which is patented by Nichia Corp. of Tokyo. Beyond increasing competition, however, another goal is to improve the performance of white LEDs.
For example, Xie noted, LEDs that employ YAG:Ce alone cannot produce a warm white because they rely on the mixing of blue and yellow light produced by down-conversion. Although the addition of another phosphor can compensate for the lack of red in the output spectrum to achieve a more natural white, this decreases the luminous efficacy, he said. Moreover, thermal quenching can be a problem with some compositions of YAG:Ce, resulting in variations in the chromaticity of the LEDs.
Proposed alternatives in the form of yellow phosphors such as Sr2SiO4:Eu2+ display similar thermal issues, he said. As a result, the scientists have focused on those based on the alpha polymorph of SiAlON, an alloy that offers thermal and chemical stability. Initially, they employed calcium-containing α-SiAlON as the host for Eu2+. But they found that LEDs using this orange-yellow phosphor produced too warm a white.
Now they have blueshifted the emission by 15 to 30 nm by replacing the calcium with lithium. They synthesize the phosphor by heating a mixture of α-Si3N4, AlN, Li2CO3 and Eu2O3 at 1700 °C in nitrogen for two hours. Commercially available InGaN LEDs serve as the 460-nm primary light sources.
Using an intensified multichannel spectrophotometer from Otsuka Electronics Co. Ltd. of Hirakata, Japan, the scientists characterized the performance of devices incorporating various concentrations of the phosphor. Driven at a bias of 20 mA and operated at room temperature in the tests, the LEDs demonstrated luminous efficiencies of 40 to 44 lm/W, correlated color temperatures of 4000 to 8000 K and color rendering indexes of 63 to 74.
Although the ability to achieve color temperatures approximating daylight is promising, Xie noted that the color rendering index must be improved for wider application. One technique the researchers are exploring is to employ a mixture of several SiAlON-based phosphors in a device. They also are investigating potential nitride materials for triple-phosphor white LEDs, which have the potential to offer higher color rendering indices.
Applied Physics Letters, March 6, 2006, 101104.
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