Inside Rensselaer
* Greenlighting a Greener World
Christian Wetzel, professor of physics and the Wellfleet Professor of Future Chips, is working to develop a high-performance green LED in an effort to make a true white LED.
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Greenlighting a Green World
Just a few years ago, most conversations Christian Wetzel had about his research began with a quick explanation of LEDs.

More recently, however, he’s noticed that the mention of LEDs — light-emitting diodes — no longer prompts puzzled looks. He rarely has to delve into the elevator pitch about LEDs needing only a fraction of the energy required by conventional light bulbs, or mention that LEDs contain none of the toxic heavy metals used in the newer compact fluorescent light bulbs. He no longer has to sell the idea that LEDs are incredibly durable and long-lived.

The virtues of sustainability and efficiency are now so engrained in the public consciousness, Wetzel said, that he can usually skip over the nuts and bolts of solid-state lighting and instead launch right into explaining his work on developing a high-performance, low-cost green LED.

“Going green means different things to different people. For most, it means being more conscious about the environmental and global impacts of one’s actions. For companies, going green also means making a profit by selling equipment and services that allow one’s customers to be more efficient and reduce costs,” said Wetzel, associate professor of physics and the Wellfleet Professor of Future Chips at Rensselaer.

“I’m doing both of those, but I’m also trying to make an LED that literally shines green light.”

The color of light produced by an LED depends on the type of semiconductor material it contains, and the holy grail of solid-state lighting, Wetzel said, is a true white LED. The white LEDs commonly used in novelty lighting applications, such as key chains, auto headlights, and grocery freezers, are actually blue LEDs coated with yellow phosphorus — which adds a step to the manufacturing process and also results in a faux-white illumination with a noticeable bluish tint.

The key to true white LEDs, Wetzel said, is all about green.

“We have high-performance red LEDs, we have high-performance blue LEDs, and if we paired them with a high-performance green LED we would be able to produce every color visible to the human eye — including true white,” he said. “Every computer monitor and television produces its picture by using red, blue, and green. That means developing a high-performance green LED would likely lead to a new generation of high-performance, energy-efficient display devices. The problem, however, is that green LEDs are much more difficult to create than I, or anyone else, imagined.”

Simple preliminary attempts to create green LEDs, by merely adding more indium (In) to the gallium nitride (GaN) materials that composed blue LEDs, were unsuccessful. The resulting green LEDs just weren’t strong or bright enough to stand toe-to-toe with red or blue. Wetzel and his research group have been working to tweak precisely how to add more indium, and how to grow the structure more carefully into a device, with the goal of boosting the strength and light output of green LEDs. They’re endeavoring, he said, to “close the green gap.”

Wetzel has been working on this problem for several years, and in recent months received an additional $1.8 million in funding from the U.S. Department of Energy.

For more information on Wetzel’s work, visit http://green.rpi.edu.
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Inside Rensselaer
Volume 3, Number 8, September 11, 2009
©2009 Rensselaer Polytechnic Institute
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