|[May 03, 2012]
Conquering LED Efficiency Droop: Overcoming mysterious phenomenon could lead to affordable LED lighting
WASHINGTON --(Business Wire)--
Like a coffee enthusiast who struggles to get a buzz from that third cup
of morning joe, light-emitting diodes (LEDs) seem to reach a point where
more electricity no longer imparts the same kick and productivity
levels-off. Now a team of researchers from California and Japan has
devised a new design for green and blue LEDs that avoids much of this
vexing efficiency droop. The findings will be presented at the
Conference on Lasers and Electro-Optics (CLEO:
2012), taking place May 6-11 in San Jose, Calif.
Schematic of the transparent LED packaging design (left). A working blue LED using the design (right). (Photo: Yuji Zhao, University of California Santa Barbara)
By changing the orientation of the crystal structure in semiconductor
films, the team created LEDs with high efficiency and extremely low
droop. Droop, which is a dramatic drop in efficiency at high currents,
is one of the main problems limiting the growth of the solid-state
Blue and green LEDs provide the essential hues that blend with other
wavelengths to give us the familiar white light we expect from household
bulbs. These highly prized colors, however, have been among the hardest
to engineer and the "droopiest" of the LEDs.
"We believe this technology could be a big breakthrough and has the
potential to change the future of lighting," says Yuji Zhao, a graduate
student at the Solid State Lighting and Energy Center at the University
of California Santa Barbara (UCSB) and one of the lead researchers on
LEDs are many times more energy efficient than heat-spewing incandescent
bulbs, but, due to droop, lose a significant fraction of that efficiency
at the high current levels required for typical household lighting. No
one i exactly sure what causes droop, although different research
groups have offered several competing explanations. Despite the
uncertainty, the UC Santa Barbara-based research team has developed a
promising technique to minimize droop by making changes to the way the
LEDs are manufactured.
LED chips are made from layers of doped semiconductors sandwiched
together. When a voltage is applied across the layers, electrons and
holes (an absence of electrons) migrate toward an area of the LED called
the active layer, where they combine, begetting a photon in the process.
In most commercially available blue LEDs, the crystals that make up the
semiconductor layers are grown in a flat orientation called the c-plane.
This traditional orientation of the crystals, however, may create
electrical fields that interfere with the reunion of the electrons and
The UCSB researchers' LEDs have non-traditional, tilted crystal
orientations that lessen the effect of the field, and exhibit some of
the lowest reported measures of droop. Using this approach the team was
also able to fabricate LED chips that are smaller than standard
commercial LEDs, which could cut down on manufacturing costs.
Further work remains to be done. "The biggest problem right now is the
relatively high cost of the gallium nitride [GaN] bulk substrates," says
Zhao, referring to the thin sheets of crystalline material on which the
group's LEDs are grown. "At UCSB we are also developing methods to mass
produce high quality GaN bulk substrates. I have confidence in this and
I think it's just a matter of time before [the cost of GaN] will no
longer be an obstacle."
CLEO: 2012 presentation JTh4J.2, "Semipolar (2021) Blue and Green InGaN
Light-Emitting Diodes (News - Alert)," by Yuji Zhao et al. is at 5 p.m. on Thursday,
May 10 in the San Jose Convention Center.
EDITOR'S NOTE: High-resolution images are available upon request.
Contact Angela Stark, email@example.com.
A Press Room for credentialed press and analysts will be located on-site
in the San Jose Convention Center, May 6 - May 11. Media interested in
attending the conference should register on the CLEO
website or contact Angela Stark at 202.416.1443, firstname.lastname@example.org.
With a distinguished history as the industry's leading event on laser
science, the Conference on Lasers and Electro-Optics (CLEO) is where
laser technology was first introduced. CLEO unites the field of lasers
and electro-optics by bringing together all aspects of laser technology,
with content stemming from basic research to industry application. CLEO:
Expo showcases the latest products and applications from more than 300
participating companies from around the world, providing hands-on
demonstrations of the latest market innovations and applications. The
Expo also offers valuable on-floor programming, including Market Focus
and the Technology Transfer program.
Sponsored by the American Physical Society's (APS) Laser Science
Division, the Institute of Electronic Engineers (IEEE (News - Alert)) Photonics Society
and the Optical Society (OSA), CLEO provides the full range of critical
developments in the field, showcasing the most significant milestones
from laboratory to marketplace. With an unparalleled breadth and depth
of coverage, CLEO connects all of the critical vertical markets in
lasers and electro-optics. For more information, visit the conference's
website at www.cleoconference.org.
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