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New LED Drops the 'Droop'

Photonics.com
Jan 2009
TROY, N.Y., Jan. 15, 2009 – Researchers at Rensselaer Polytechnic Institute (RPI) have developed and demonstrated a new type of LED with significantly improved lighting performance and energy efficiency.

The new polarization-matched LED, developed in collaboration with Samsung
Electro-Mechanics, exhibits an 18 percent increase in light output and a 22 percent increase in wall-plug efficiency, which essentially measures the amount of electricity the LED converts into light.

LEDs.jpg


Photo courtesy of E. Fred Schubert, RPI.

The new device achieves a notable reduction in “efficiency droop,” a well-known phenomenon that causes LEDs to be most efficient when receiving low-density currents of electricity but to lose efficiency as higher-density currents are fed into the device. The cause of the droop is not yet fully understood, but studies have shown that electron leakage is likely a large part of the problem.

“This droop is under the spotlight since today’s high-brightness LEDs are operated at current densities far beyond where efficiency peaks,” said project leader E. Fred Schubert, Wellfleet Senior Constellation Professor of Future Chips at Rensselaer and head of the institute’s Smart Lighting Engineering Research Center, which is funded by the National Science Foundation.

“This challenge has been a stumbling block because reducing the current densities to values where LEDs are more efficient is unacceptable. Our new LED, however, which has a radically redesigned active region, namely a polarization-matched active region, tackles this issue and brings LEDs closer to being able to operate efficiently at high current densities,” Schubert said.

Results of the study are explained in a paper published online this week by Applied Physics Letters.

Focusing on the active region of LEDs where the light is generated, Schubert’s team discovered that the region contained materials with mismatched polarization. The polarization mismatch likely causes electron leakage and, therefore, a loss of efficiency, Schubert said.

The researchers discovered that the polarization mismatch can be strongly reduced by introducing a new quantum-barrier design. They replaced the conventional gallium indium nitride/gallium nitride (GaInN/GaN) layer of the LED active region, and replaced it with gallium indium nitride/gallium indium nitride (GaInN/GaInN). This substitution allows the layers of the active region to have a better matched polarization, and, in turn, to reduce both electron leakage and efficiency droop.

The benefits seen by testing the new GaInN/GaInN LED were consistent with theoretical simulations showing polarization matching reducing electron leakage and efficiency droop.

Schubert expects that a new wave of lighting devices based on LEDs and solid-state lighting will supplant the common light bulb in coming years, leading to vast environmental, energy and cost benefits as well as innovations in health care, transportation systems, digital displays and computer networking.

Co-authors on the paper include Rensselaer physics, Future Chips and electrical engineering graduate students Jiuru Xu, Martin F. Schubert and Ahmed N. Noemaun; Rensselaer Future Chips research assistant Di Zhu; Jong Kyu Kim, research assistant professor of electrical, computer and systems engineering at Rensselaer; and Samsung Electro-Mechanics researchers Min Ho Kim, Hun Jae Chung, Sukho Yoon, Cheolsoo Sone and Yongjo Park.

Funding was provided by Samsung Electro-Mechanics, the US National Science Foundation, the Rensselaer Smart Lighting Engineering Research Center, Sandia National Laboratories, Rochester Institute of Technology, US Department of Energy, US Department of Defense, Magnolia Optics, Crystal IS, Troy Research Corp and New York state.

For more information, visit: www.rpi.edu



GLOSSARY
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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