Smart Lighting: New LED Drops the “Droop”
Band diagram of conventional GaInN/GaN
active region and new polarization-matched GaInN/GaInN
active region of a light-emitting diode.
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Researchers use streamlined polarization to boost
performance of LEDs
Researchers at Rensselaer Polytechnic Institute have
developed and demonstrated a new type of light emitting diode
(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.
The new device achieves a notable reduction in “efficiency
droop,” a well-known phenomenon that provokes LEDs to be most
efficient when receiving low-density currents of electricity,
but then to lose efficiency as higher density currents of
electricity are fed into the device. The cause of this 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 university’s National
Science Foundation-funded Smart Lighting Engineering Research
Center.
“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
re-designed 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 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 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 healthcare,
transportation systems, digital displays, and computer
networking.
Along with Schubert, 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; along with
Samsung Electro-Mechanics researchers Min Ho Kim, Hun Jae
Chung, Sukho Yoon, Cheolsoo Sone, and Yongjo Park.
Funding for the project was contributed by Samsung
Electro-Mechanics, the U.S. National Science Foundation, the
Rensselaer Smart Lighting Engineering Research Center, Sandia
National Laboratories, Rochester Institute of Technology, U.S.
Department of Energy, U.S. Department of Defense, Magnolia
Optics, Crystal IS, Troy Research Corporation, and New York
state.
For more information on Rensselaer’s Future Chips
Constellation, visit: http://www.rpi.edu/futurechips/index.htm.
For more information on Rensselaer’s Smart Lighting Center,
visit: smartlighting.rpi.edu.
Published
January 12,
2009 |
Contact: Michael Mullaney
Phone: (518) 276-6161
E-mail: mullam@rpi.edu |
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