Phosphor-Coated LED Converts Blue Light to White

Michael A. Greenwood

Coating LEDs with phosphors to create a whiter color is nothing new. But a variety of techniques that are used to do this are hamstrung by efficiency issues.

One conventional approach, for example, suffers from a high percentage of the emitted light being reflected back toward the chip, where it is dispersed and wasted. Other coating approaches are more effective, but they, too, experience notable losses resulting from quantum conversion and total internal reflection.

Researchers Steven C. Allen and Andrew J. Steckl of the University of Cincinnati have devised a phosphor-based LED that appears to overcome many of these problems and moves closer to the efficiency goals established by the Optoelectronics Industry Development Association (OIDA) in Washington.

An organic white phosphor-covered LED illuminates a poster. Reprinted with permission of Applied Physics Letters.

Their approach involved physically separating the phosphor from the LED (as opposed to directly coating the surface of the LED) to reduce absorption. They also used a semitransparent phosphor rather than a diffuse phosphor layer, which allowed light to pass through with minimal deflection.

Their design, known as enhanced light extraction by internal reflection, consisted of a Cree 460-nm LED die mounted onto an aluminum base and an enhanced reflector film. The blue LED was situated at the center of a hemispherical poly(methyl methacrylate) shell with an interior phosphor coating. Several varieties of phosphors were applied to gauge their effectiveness.

The devices were tested at drive currents ranging from 1 to 350 mA. The investigators found that an organic phosphor allowed the blue light to be converted into white light with an efficiency of 0.99. OIDA’s goal is to achieve conversion efficiencies of 0.855. Conventional phosphor-based LEDs currently achieve conversion efficiencies of <0.40.

Organic phosphors, however, have an issue with longevity. They lose their stability with repeated use, making them impractical for many solid-state lighting applications. The investigators said that they are experimenting with an inorganic composite phosphor, which allows light to pass through and meets the maximum efficiency requirements for low scattering.

Applied Physics Letters, April 10, 2008, Vol. 92, pp. 143309.