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Bridging the “green” LED gap

Can LEDs outshine fluorescent lighting? Nelson Tansu thinks so. He is engineering nanostructures in the quantum wells of nitride semiconductors in an effort to boost the efficiency of green LED light – and thus the radiative efficiency of the overall white LED.

The incandescent lamp, also known as the everyday lightbulb, has done yeoman’s work in lighting the world’s buildings, flashlights and billboards since its invention in the late 19th century.

It has also come to represent one of the world’s most notorious guzzlers of electric power.

When it is switched on, says the U.S. Department of Energy (DOE), the ordinary lightbulb becomes extraordinarily wasteful, dissipating 95 percent or more of its energy as invisible infrared light or heat.

This is no trivial loss of power. About one-third of the energy generated in the U.S., for example, is used to provide electricity, and about 22 percent of this electrical power is used to light buildings.

Not surprisingly, consumers are beginning to switch to fluorescent lamps, which are more energy-efficient and longer-lasting than lightbulbs. But fluorescent lighting uses mercury vapor, which poses environmental and health concerns.

A safer alternative to the lightbulb’s meager return on investment, says Nelson Tansu, is solid-state lighting, which relies on light-emitting diodes. LEDs are already used in cell phones, car dashboards and the backlighting in the liquid crystal displays in laptops and TVs. They are also found in a growing number of traffic signals and city and highway billboards.

Tansu’s group conducts research in Lehigh’s Smith Family Laboratory for Optical Technologies, which contains state-of-the-art epitaxial growth facilities. The group is also applying semiconductors to solar cells and thermoelectric materials

White LED light, says DOE’s Sandia National Laboratory in its Web site, now achieves twice the luminous efficacy of a lightbulb and one-third that of a fluorescent lamp. In the future, however, LEDs could outpace incandescent lighting by 10 times while doubling the efficacy of fluorescent lighting.

This promise will not become reality, however, until engineers overcome natural limits to the “radiative efficiency” of LED light that occur at the nanoscale.

Tansu and Volkmar Dierolf, an associate professor of physics, have received a three-year grant from DOE to study methods of improving the efficiency of white LEDs. Both are faculty researchers in Lehigh’s Center for Optical Technologies (COT).

“We hope that by greatly improving the efficiency of the electrical power currently used for lighting,” says Tansu, “solid-state lighting can significantly reduce the worldwide demand for energy, while providing reliable and environmentally friendly solutions.”

The DOE award, matched in part by funding from the State of Pennsylvania, is being provided through DOE’s Solid-State Lighting Core Technology Research (CTR) program. The agency’s goal is to develop, by 2025, inexpensive and long-lasting solid-state lighting technologies that achieve 50 percent efficiency while reproducing the spectrum of sunlight.

The competitive CTR grants were awarded last year to five U.S. research groups. Two groups – Lehigh’s and one from the Georgia Institute of Technology – are based at universities. The rest are led by industrial consortiums.

A green light to the future
The light generated by an LED is emitted from a semiconducting material – usually indium gallium nitride (InGaN) or gallium phosphide (GaP) – within the LED.

An InGaN semiconductor emits light in the blue and green portions of the spectrum, and a GaP semiconductor emits light in the red spectrum. a white led must mix these colors in the correct proportion to produce white light. It can achieve this through wavelength conversion or through color mixing, in which multiple LEDs, each one the size of a human hair, are combined in a single lamp to produce white light.

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“We need to engineer the spatial position of both the electron and the hole carriers to increase the chance that they will overlap.” —Nelson Tansu
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