NRL Designs Multi-Junction Solar Cell to Break Efficiency Barrier

  NRL Designs Multi-Junction Solar Cell to Break Efficiency Barrier

Business Wire

WASHINGTON -- January 14, 2013

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
an indium 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

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, Rochester, N.Y., will execute a three year
materials and device development program to realize this new solar cell

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

The U.S. 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 the NRL homepage or join the conversation on Twitter,
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U.S. Naval Research Laboratory
Daniel Parry, 202-767-2541
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