|[January 14, 2013]
NRL Designs Multi-Junction Solar Cell to Break Efficiency Barrier
WASHINGTON --(Business Wire)--
U.S. Naval Research Laboratory scientists in the Electronics Technology
and Science Division, in collaboration with the Imperial College
London and MicroLink Devices, Inc., Niles, Ill., have proposed a
novel triple-junction solar cell with the potential to break the 50
percent conversion efficiency barrier, which is the current goal in
multi-junction photovoltaic development.
"This research has produced a novel, realistically achievable,
lattice-matched, multi-junction solar cell design with the potential to
break the 50 percent power conversion efficiency mark under concentrated
illumination," said Robert Walters, Ph.D., NRL research
physicist. "At present, the world record triple-junction solar cell
efficiency is 44 percent under concentration and it is generally
accepted that a major technology breakthrough will be required for the
efficiency of these cells to increase much further."
In multi-junction (MJ) solar cells, each junction is
'tuned' to different wavelength bands in the solar spectrum to increase
efficiency. High bandgap semiconductor material is used to absorb the
short wavelength radiation with longer wavelength parts transmitted to
subsequent semiconductors. In theory, an infinite-junction cell could
obtain a maximum power conversion percentage of nearly 87 percent. The
challenge is to develop a semiconductor material system that can attain
a wide range of bandgaps and be grown with high crystalline quality.
By exploring novel semiconductor materials and applying band
structure engineering, via strain-balanced quantum wells, the NRL
research team has produced a design for a MJ solar cell that can achieve
direct band gaps from 0.7 to 1.8 electron volts (eV) with materials that
are all lattice-matched to anindium phosphide (InP) substrate.
"Having all lattice-matched materials with this wide range of band
gaps is the key to breaking the current world record," adds Walters. "It
is well known that materials lattice-matched to InP can achieve band
gaps of about 1.4 eV and below, but no ternary alloy semiconductors
exist with a higher direct band-gap."
The primary innovation enabling this new path to high efficiency
is the identification of InAlAsSb quaternary alloys as a high band gap
material layer that can be grown lattice-matched to InP. Drawing from
their experience with Sb-based compounds for detector and laser
applications, NRL scientists modeled the band structure of InAlAsSb and
showed that this material could potentially achieve a direct band-gap as
high as 1.8eV. With this result, and using a model that includes both
radiative and non-radiative recombination, the NRL scientists created a
solar cell design that is a potential route to over 50 percent power
conversion efficiency under concentrated solar illumination.
Recently awarded a U.S. Department of Energy (DoE), Advanced
Research Projects Agency-Energy (ARPA-E) project, NRL scientists,
working with MicroLink and Rochester Institute of Technology (News - Alert),
Rochester, N.Y., will execute a three year materials and device
development program to realize this new solar cell technology.
Through a highly competitive, peer-reviewed proposal process, ARPA-E
seeks out transformational, breakthrough technologies that show
fundamental technical promise but are too early for private-sector
investment. These projects have the potential to produce game-changing
breakthroughs in energy technology, form the foundation for entirely new
industries, and to have large commercial impacts.
Naval Research Laboratory is the Navy's full-spectrum corporate
laboratory, conducting a broadly based multidisciplinary program of
scientific research and advanced technological development. The
Laboratory, with a total complement of nearly 2,500 personnel, is
located in southwest Washington, D.C., with other major sites at the
Stennis Space Center, Miss., and Monterey, Calif. NRL has served the
Navy and the nation for over 85 years and continues to meet the complex
technological challenges of today's world. For more information, visit
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