NREL finds a way to give LEDs the green light

Light bulbs that last 100 years and fill rooms with brilliant ambiance may become a reality sooner rather than later, thanks to a National Renewable Energy Laboratory discovery.

NREL scientists found a way to generate a tricky combination of green and red that may just prove to be the biggest boost for illumination since Edison's light bulb.

Green isn't just a symbol of environmentalism, it is a real color, and a desperately needed one for researchers looking for a way to light homes, streets and buildings at a fraction of today's costs.

LEDs — light-emitting diodes — are the promise of the future because unlike tungsten bulbs or compact fluorescent bulbs, they deliver most of their energy as light, rather than heat. An extra plus is that they don't contain dangerous mercury.

The era of LEDs is fast approaching. The U.S. Department of Energy expects to phase out tungsten bulbs in four years and compact-fluorescents in 10 years. That will leave LEDs with virtually 100 percent of the market.

To make an LED that appears white, researchers minimally need the colors red, green and blue. The white light from the sun is really all the colors of the rainbow. Without at least red, blue and green from the spectrum, no lighting device will be practical for home or office use.

Red proved easy to generate, and about 15 years ago, Japanese scientists found a way to generate blue, thus providing two of the key colors from the spectrum of white light.

But green has been elusive. In fact, the $10 LEDs that people can buy now are made to look white by aiming the blue light at a phosphor, which then emits green. It works OK, but the clunky process saps a big chunk of the efficiency from the light.

NREL Jumps into LED Research via Solar Cells

Along came NREL, a world leader in designing solar cells, but a neophyte in the lighting realm.

NREL scientist Angelo Mascarenhas, who holds patents in solar-cell technology, realized that an LED is just the reverse of a solar cell. One takes electricity and turns it into light; the other takes sunlight and turns it into electricity.

"We'd been working with solar cells for 30 years," Mascarenhas said. "Could we find some device where we could just reverse the process of making solar cells?"

Indeed, Mascarenhas found it. NREL had won major scientific awards with its inverted metamorphic solar cells, in which the cells are built by combining layers of different lattice sizes to optimally capture solar energy. In fact, an NREL-produced IMM cell set a world record by converting 40 percent of absorbed sunlight into electricity.

Along the way, "We had already developed some of the know-how to capture sunlight in this green spectral region," Mascarenhas said. They hadn't reached there, because solar cells don't need a green, but they had begun to understand the challenges of getting to a green.

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