Doping may be a no-no for athletes, but researchers in
the University
of Florida’s physics department
say it was key in getting unprecedented power conversion efficiency from a new
graphene solar cell created in their laboratory.
Graphene solar cells are one of industry’s great hopes
for cheaper, durable solar power cells in the future. But previous attempts to
use graphene, a single-atom-thick honeycomb lattice of carbon atoms, in solar
cells have only managed power conversion efficiencies ranging up to 2.9%. The
UF team was able to achieve a record breaking 8.6% efficiency with their device
by chemically treating, or doping, the graphene with
trifluoromethanesulfonyl-amide, or TFSA. Their results are published online in Nano Letters.
“The dopant makes the graphene film more conductive and
increases the electric field potential inside the cell,” said Xiaochang Miao, a
graduate student in the physics department. That makes it more efficient at converting
sunlight into electricity. And unlike other dopants that have been tried in the
past, TFSA is stable—its effects are long lasting.
The solar cell that Miao and her coworkers created in
the laboratory looks like a 5-mm-square window framed in gold. The window, a
wafer of silicon coated with a monolayer of graphene, is where the magic
happens.
Graphene and silicon, when they come together, form what
is called a Schottky junction—a one-way street for electrons that when
illuminated with light, acts as the power conversion zone for an entire class
of solar cells. Schottky junctions are commonly formed by layering a metal on
top of a semiconductor. But researchers at the UF Nanoscience Institute for
Medical and Engineering Technologies discovered in 2011 that graphene, a
semi-metal, made a suitable substitute for metal in creating the junction.
“Graphene, unlike conventional metals, is transparent
and flexible, so it has great potential to be an important component in the
kind of solar cells we hope to see incorporated into building exteriors and
other materials in the future,” said Arthur Hebard, distinguished professor of
physics at UF and co-author on the paper. “Showing that its power-converting
capabilities can be enhanced by such a simple, inexpensive treatment bodes well
for its future.”
The researchers said that if graphene solar cells reach
10% power conversion efficiency they could be a contender in the market place,
if production costs are kept low enough.
The prototype solar cell created in the UF lab was built
on a rigid base of silicon, which is not considered an economical material for
mass production. But Hebard said that he sees real possibilities for combining
the use of doped graphene with less expensive, more flexible substrates like
the polymer sheets currently under development in research laboratories around
the world.
