LED efficiency puzzle solved by UC Santa Barbara theorists
Barbara, Calif., April 19, 2011) -- Researchers at the University of
California, Santa Barbara, say they’ve figured out the cause of a
problem that’s made light-emitting diodes (LEDs) impractical for general
lighting purposes. Their work will help engineers develop a new
generation of high-performance, energy-efficient lighting that could
replace incandescent and fluorescent bulbs.
“Identifying the root cause of the problem is an indispensable first
step toward devising solutions,” says Chris Van de Walle, a professor
in the Materials Department at UC Santa Barbara who heads the research
group that carried out the work.
Van de Walle and his colleagues are working to improve the
performance of nitride-based LEDs, which are efficient, non-toxic and
long-lasting light sources. They investigated a phenomenon referred to
as “droop”—the drop in efficiency that occurs in these LEDs when they’re
operating at the high powers required to illuminate a room. The cause
of this decline has been the subject of considerable debate, but the UC
Santa Barbara researchers say they’ve figured out the mechanism
responsible for the effect by performing quantum-mechanical
LED droop, they conclude, can be attributed to Auger recombination, a
process that occurs in semiconductors, in which three charge-carriers
interact without giving off light. The researchers also discovered that
indirect Auger effects, which involve a scattering mechanism, are
significant—a finding that accounts for the discrepancy between the
observed degree of droop and that predicted by other theoretical
studies, which only accounted for direct Auger processes.
In nitride LEDs, “These indirect processes form the dominant
contribution to the Auger recombination rate,” says Emmanouil Kioupakis,
a postdoctoral researcher at UC Santa Barbara and lead author of a
paper published online April 19 in Applied Physics Letters. The other
authors are Van de Walle, Patrick Rinke, now with the Fritz Haber
Institute in Germany, and Kris Delaney, a project scientist at UC Santa
LED droop can’t be eliminated because Auger effects are intrinsic,
but it could be minimized, the researchers say, by using thicker quantum
wells in LEDs or growing devices along non-polar or semi-polar growth
directions in order to keep carrier density low.
“With Auger recombination now established as the culprit, we can
focus on creative approaches to suppress or circumvent this loss
mechanism,” Van de Walle says.
The work was supported by the
Center for Energy Efficient Materials, an Energy Frontier Research
Center funded by the U.S. Department of Energy, and by UC Santa
Barbara’s Solid State Lighting and Energy Center.
Computational resources were provided by the U.S. Department of
Energy’s National Energy Research Scientific Computing Center at
Lawrence Berkeley National Laboratory, the California NanoSystems
Institute’s Computing Facility at UC Santa Barbara, and the National
Science Foundation-funded TeraGrid.
Note to editors: Emmanouil Kioupakis can be contacted via email at kioup [at] engineering [dot] ucsb [dot] edu.
“Indirect Auger recombination as a cause of efficiency droop in nitride light-emitting diodes” doi: 10.1063/1.3570656
The Center for Energy Efficient Materials (CEEM)
is one of 46 Energy Frontier Research Centers funded by the U.S.
Department of Energy to address critical energy challenges through
technological advancements. The Center was launched in August 2009 and
focuses on fundamental research in three key areas: photovoltaics,
thermoelectric, and solid-state lighting. It is led by UC Santa Barbara
in partnership with researchers at the National Renewable Energy
Laboratory, the Los Alamos National Laboratory, the University of
California, Santa Cruz, and Harvard University. For more information,
The Solid State Lighting and Energy Center (SSLEC) is focused
on new semiconductor-based technologies for energy-efficient lighting
and displays, power electronics, and solar energy conversion. The
objective of the SSLEC is to provide a forum for its members—key
industry partners and faculty and student researchers at UC Santa
Barbara—to work in collaboration and across scientific disciplines to
address the most challenging problems in these important and timely
areas of research. For more information, visit: sslec.ucsb.edu.