The UCSB Solid State Lighting and Energy Center research team, led by engineering professors Shuji Nakamura and Steven DenBaars, presented a new energy-efficient design for LED manufacturing at the Conference on Lasers and Electro-Optics in San Jose last week.

Though much more energy efficient than incandescent bulbs, LEDs experience a drop in efficiency, referred to as the “droop,” under high electrical currents like those found in household settings. The new design rearranges the orientation of the crystal structures inside of the bulb to avoid much of this loss.

According to graduate student Yuji Zhao, a member of the research team, the goal of the project is to combat the wasteful nature of current lighting systems. “Right now we are using lighting sources that are not very efficient,” Zhao said. “A lot of energy goes to heat instead of lighting. We’re trying to develop LED-based lighting sources to replace those conventional sources.” Zhao said that while good progress has been made in LED lighting sources over the past 10 years, efficiency remains a looming issue. Most commercial LEDs generate blue and green light, which are the closest to the plain white light preferred in household settings. These two colors of light, however, also tend to be associated with the great- est droop.

According to team member Chih-Chien Pan, there is a current-depen- dant droop — whereby efficiency is lost under a high current — and a temperature-dependant droop, whereby it is lost with increasing heat.

“We need to improve efficiency in current-dependant droop or temperature-dependant droop,” Pan said. “For the current-dependant droop, the so-called ‘efficient droop,’ we can reduce it by reducing the defects or the carrier overflow.”

Zhao said the team has also been working on reducing the temperature-dependant droop and now has a demonstrable prototype.

“We have developed a device that shows very good droop perfor- mance, which means we have high efficiency under a high current,” Zhao said.

According to Zhao, when an electrical current runs through the layers of semiconductors contained in an LED, electrons and holes combine to generate photons — the particles that make up light. One proposed explanation for the droop is the orientation of these semiconductors, which may generate electrical fields that interrupt the light production process.

When the team altered the orientation of the semiconductors, they noticed a dramatic increase in efficiency, though Zhao said they are still unsure about why the new method works. While the development is a breakthrough in solid state lighting, Zhao said there is still much progress to be made.

“It is something related to the material and the physics,” Zhao said. “We are trying to explore the ‘why.’”

The team in charge of the project consists of Zhao, Pan, Shinichi Tanaka, engineering professors James S. Speck, DenBaars and Nakamura. They work not only on designs of LEDs, but also on the production process, including the acquiring of correct materials, manufacturing and packing.