Fluorescent microspheres create white-light LEDs

Most white-light light-emitting diodes (LEDs) are fabricated by coating phosphors onto blue LEDs; however, the patent situation related to phosphors is becoming more competitive.

Feb 1st, 2008

Most white-light light-emitting diodes (LEDs) are fabricated by coating phosphors onto blue LEDs (see www.laserfocusworld.com/articles/212470); however, the patent situation related to phosphors is becoming more competitive. And, white-light LEDs produced by combining red, green, and blue diodes are more expensive and often require bulky optics. Researchers at the University of Hong Kong (Hong Kong, China) have developed a simple, alternate approach that produces white-light LEDs by coating a blue LED with layers of green and red fluorescent microspheres.

Microspheres with internally incorporated fluorescent dyes are commercially available with diameters from tens of nanometers to micrometers. Layers of red and green microspheres in a polystyrene matrix were found to self-assemble in a low-cost evaporative coating process onto a gallium nitride blue LED with peak emission at 470 nm. By varying the concentration of the microsphere solution and the number of applied layers, spectral conversion of the red and green microspheres on the blue source can create both cool (bluish-white) and warm (yellowish-white) white-light LEDs, with 27.3 and 26.7 lm/W, respectively, at a 20 mA drive current, comparing favorably to white-light LEDs based on phosphor coatings. Contact Anthony Hoi Wai Choi at hwchoi@eee.hku.hk.

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