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Updated 3:00 PM August 7, 2008




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Freeing light shines promise on energy-efficient lighting

The latest bright idea in energy-efficient lighting for homes and offices uses big science in nano-small packages to dim the future of Edison's light bulb.

In the August issue of Nature Photonics scientists at U-M and Princeton University announce a discovery that pushes more appealing white light from organic light-emitting devices (OLEDs).

The innovation in the paper "Enhanced Light Out-Coupling of Organic Light-Emitting Devices Using Embedded Low-Index Grids" describes a way to deliver significantly more bright light from a watt than incandescent bulbs.

"Our demonstration here shows that OLEDs are a very exciting technology for use in interior illumination," says Stephen Forrest, professor of electrical engineering and physics and vice president for research. "We hope that white emitting OLEDs will play a major role in the world of energy conservation."

Forrest and co-author Yiru Sun, visiting U-M from Princeton University, have wrestled with a classic problem in the new generation of lighting called white organic light-emitting devices, or WOLED: freeing the light generated, but mostly trapped, inside the device.

Incandescent light bulbs give off light as a by-product of heat. The light is appealing, but inefficient, putting out 15 lumens of light for every watt or electricity.

The best fluorescent tube lights put out 90 lumens of light per watt, but the light can be harsh, the fixtures are expensive and the tubes lose their efficiency with age. They also rely on many environmentally unfriendly substances such as mercury.

WOLEDs generate white light by using electricity to send an electron into nanometer-thick layers of organic materials that serve as semiconductors. These carbon-based materials are dyes, the ones used in photographic prints and car paint, so they are inexpensive and can be put on plastic sheets or metal foils, further reducing costs.

The excited electron in these layers casts bright white light. The bad news, Forrest says, has been that about 60 percent of it is trapped inside the layers.

The Nature Photonics paper describes a tandem system of organic grids and micro lenses that guide the light out of the thin layers and into the air. The grids refract the trapped light, bouncing it into a layer of dome-shaped lenses that then pull the light out.

This process — all of which is packed into a lighting sandwich roughly the thickness of a sheet of paper — was shown to emit approximately 70 lumens from a single watt of power.

More light out translates into energy savings as lighting now accounts for 22 percent of U.S. electricity consumption.

Reducing the amount of coal-generated electricity and finding more efficient ways to power appliances and lighting is one of the focuses of the Michigan Memorial Phoenix Energy Institute, and the WOLED work is one example of how science can open new doors in conservation, says Gary Was, institute director.

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