Saturday, 20 July 2013

World thinnest light absorber

Scientists break record for
thinnest light-absorber
These four wafers contain the
thinnest light-absorber layer ever
built. Credit: Mark Shwartz, Stanford
Stanford University scientists have
created the thinnest, most efficient
absorber of visible light on record.
The nanosize structure, thousands of
times thinner than an ordinary sheet
of paper, could lower the cost and
improve the efficiency of solar cells,
according to the scientists. Their
results are published in the current
online edition of the journal Nano
Letters .
"Achieving complete absorption of
visible light with a minimal amount
of material is highly desirable for
many applications, including solar
energy conversion to fuel and
electricity," said Stacey Bent, a
professor of chemical engineering at
Stanford and a member of the
research team. "Our results show
that it is possible for an extremely
thin layer of material to absorb
almost 100 percent of incident light
of a specific wavelength."
Thinner solar cells require less
material and therefore cost less. The
challenge for researchers is to reduce
the thickness of the cell without
compromising its ability to absorb
and convert sunlight into clean
For the study, the Stanford team
created thin wafers dotted with
trillions of round particles of gold.
Each gold nanodot was about 14
nanometers tall and 17 nanometers
Visible spectrum
An ideal solar cell would be able to
absorb the entire visible light
spectrum, from violet light waves 400
nanometers long to red waves 700
nanometers in length, as well as
invisible ultraviolet and infrared light .
In the experiment, postdoctoral
scholar Carl Hagglund and his
colleagues were able to tune the gold
nanodots to absorb one light from
one spot on the spectrum: reddish-
orange light waves about 600
nanometers long.
"Much like a guitar string, which has
a resonance frequency that changes
when you tune it, metal particles
have a resonance frequency that can
be fine-tuned to absorb a particular
wavelength of light," said Hagglund,
lead author of the study. "We tuned
the optical properties of our system
to maximize the light absorption ."
This is a cross-section of the record-
thin absorber layer showing three
gold nanodots, each about 14x17
nanometers in size and coated with
tin sulfide. Credit: Carl Hagglund,
Stanford Unibversity
The gold nanodot-filled wafers were
fabricated at a nearby Hitachi facility
using a technique called block-
copolymer lithography. Each wafer
contained about 520 billion nanodots
per square inch. Under the
microscope, the hexagonal array of
particles was reminiscent of a
Hagglund's team added a thin-film
coating on top of the wafers using a
process called atomic layer
deposition. "It's a very attractive
technique, because you can coat the
particles uniformly and control the
thickness of the film down to the
atomic level, " he said. "That allowed
us to tune the system simply by
changing the thickness of the coating
around the dots. People have built
arrays like this, but they haven't
tuned them to the optimal conditions
for light absorption. That's one novel
aspect of our work."
Record results
The results were record-setting. "The
coated wafers absorbed 99 percent
of the reddish-orange light,"
Hagglund said. "We also achieved 93
percent absorption in the gold
nanodots themselves. The volume of
each dot is equivalent to a layer of
gold just 1.6 nanometers thick,
making it the thinnest absorber of
visible light on record – about 1,000
times thinner than commercially
available thin film solar cell
The previous record-holder required
an absorber layer three times thicker
to reach total light absorption, he
added. "So we've substantially
pushed the limits of what can be
achieved for light harvesting by
optimizing these ultrathin, nano-
engineered systems," Hagglund said.
The next step for the Stanford team
is to demonstrate that the technology
can be used in actual solar cells.
"We are now looking at building
structures using ultrathin
semiconductor materials that can
absorb sunlight," said Bent, co-
director of the Stanford Center on
Nanostructuring for Efficient Energy
Conversion (CNEEC). "These
prototypes will then be tested to see
how efficiently we can achieve solar
energy conversion ."
In the experiment, the researchers
applied three types of coatings – tin
sulfide, zinc oxide and aluminum
oxide – on different nanodot arrays.
"None of these coatings are light-
absorbing," Hagglund said. "But it
has been shown theoretically that if
you apply a semiconductor coating,
you can shift the absorption from the
metal particles to the semiconductor
materials. That would create more
long-lived energetic charge carriers
that could be channeled into some
useful process, like making an
electrical current or synthesizing
Final goal
The ultimate goal, Bent added, is to
develop improved solar cells and
solar fuel devices by confining the
absorption of sunlight to the smallest
amount of material possible. "This
provides a benefit in minimizing the
material necessary to build the
device, of course," she said. "But the
expectation is that it will also allow
for higher efficiencies, because by
design, the charge carriers will be
produced very close to where they
are desired – that is, near where
they will be collected to produce an
electrical current or to drive a
chemical reaction."
The scientists are also considering
nanodot arrays made of less
expensive metals. "We chose gold
because it was more chemically
stable for our experiment," Hagglund
said. "Although the cost of the gold
was virtually negligible, silver is
cheaper and better from an optical
point of view if you want to make a
good solar cell. Our device represents
an orders-of-magnitude reduction in
thickness. This suggests that we can
eventually reduce the thickness of
solar cells quite a lot."
Provided by Stanford University

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