Green LEDs for Efficient Lighting
A new approach to fabricating light-emitting diodes (LEDs) could be used to increase their efficiency by 20 percent while yielding higher-quality light than conventional LEDs. Researchers at the National Renewable Energy Laboratory (NREL) in Golden, CO, have demonstrated the approach by making a yellow-green LED that could soon be combined with other colored LEDs to yield white light. The new LED could help replace current, inefficient methods of generating white light.
LEDs, devices that emit photons when an electrical charge is applied to them, are more efficient and last longer than incandescent lightbulbs. By varying the composition of the semiconductor LEDs, materials scientists can coax the devices into emitting different colors. At the minimum, producing white light requires combining red, blue, and green, but so far, only red- and blue-light-emitting diodes are well developed. To produce green light, LED manufacturers typically apply one or more phosphor materials to blue LEDs. The phospors convert high energy blue spectrum light into lower-energy light through a process that reduces overall luminosity by approximately 20 percent.
To eliminate this loss of efficiency, researchers have tried to develop efficient green LEDs that don’t require phosphors. But a major stumbling block is that the different known semiconductor materials that can be combined to emit green light, typically indium and gallium nitride, have different-sized crystal lattice structures. For semiconductors to work efficiently, each layer of the device has to have a similarly sized lattice structure as the layer above or below it.
To get around the lattice-size mismatch, NREL researchers used a fabrication method that they had previously developed for building highly efficient multi-junction solar cells. Their method relies on using additional layers of other semiconducting materials with intermediate-sized lattice structures that bridge the gap between the disparate-sized semiconductors. “If you try to do it in one shot, the whole thing will be defective,” says Angelo Mascarenhas, team leader for solid state spectroscopy in the Center for Basic Sciences at NREL. “You have to grow a sequence of layers in a step-wise fashion.”
Applying the same concepts to LEDs, Mascarenhas and colleagues combined aluminum gallium indium phosphide and gallium arsenide, two well-developed semiconductor materials, which yielded yellow-green. If they can now develop a blue-green LED, they can combine the two near-green diodes with existing red and blue LEDs that would yield high-quality white light with a color rendering index (CRI) above 90. This would be much better than the ratings in the 70s that conventional LEDs usually get. (Sunlight is the standard, with a CRI of 100.)
Eugene Fitzgerald, a professor of materials science and engineering at MIT, however, says developing a high-efficiency blue-green LED is much more difficult than the yellow-green LED they just formed. Yellow-green LEDs use arsenide-phosphides, materials that are much more developed for use as LEDs than nitride-based diodes that are required for blue-green light emission. “The material science on the nitride side is still really primitive when it comes to mass production, whereas arsenide-phosphide diodes can be scaled very readily,” he says.
Fitzgerald developed a yellow-green LED several years ago using layering techniques similar to those recently used at NREL. He is now trying to push arsenide-phosphides further to yield a pure green LED for a three-colored light rather pursuing the four-colored approach that Mascarenhas and colleagues are pursuing.
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