starship-design: Nanotech Might Cut Solarcell Costs

["L. Parker" <lparker@cacaphony.net>]

Nanotech Might Cut Solarcell Costs

Berkeley - Apr 1, 2002

University of California, Berkeley, chemists have found a way to make cheap
plastic solar cells flexible enough to paint onto any surface and
potentially able to provide electricity for wearable electronics or other
low-power devices.

The group's first crude solar cells have achieved efficiencies of 1.7
percent, far less than the 10 percent efficiencies of today's standard
commercial photovoltaics. The best solar cells, which are very expensive
semiconductor laminates, convert, at most, 35 percent of the sun's energy
into electricity.

"Our efficiency is not good enough yet by about a factor of 10, but this
technology has the potential to do a lot better," said A. Paul Alivisatos,
professor of chemistry at UC Berkeley and a member of the Materials Science
Division of Lawrence Berkeley National Laboratory. "There is a pretty clear
path for us to take to make this perform much better."

Alivisatos and his co-authors, graduate student Wendy U. Huynh and
post-doctoral fellow Janke J. Dittmer, report their development in the March
29 issue of Science.

"The beauty of this is that you could put solar cells directly on plastic,
which has unlimited flexibility," Dittmer said. "This opens up all sorts of
new applications, like putting solar cells on clothing to power LEDs, radios
or small computer processors."

The solar cell they have created is actually a hybrid, comprised of tiny
nanorods dispersed in an organic polymer or plastic. A layer only 200
nanometers thick is sandwiched between electrodes, and can produce, at
present, about 0.7 volts. The electrode layers and nanorod/polymer layers
could be applied in separate coats, making production fairly easy. And
unlike today's semiconductor-based photovoltaic devices, plastic solar cells
can be manufactured in solution in a beaker without the need for clean rooms
or vacuum chambers.

"Today's high-efficiency solar cells require very sophisticated processing
inside a clean room and complex engineering to make the semiconductor
sandwiches," Alivisatos said. "And because they are baked inside a vacuum
chamber, they have to be made relatively small."

The team's process for making hybrid plastic solar cells involves none of
this.

"We use a much dirtier process that makes it cheap," Huynh said.

The technology takes advantage of recent advances in nanotechnology,
specifically the production of nanocrystals and nanorods pioneered by
Alivisatos and his laboratory colleagues. These are chemically pure clusters
of from 100 to 100,000 atoms with dimensions on the order of a nanometer, or
a billionth of a meter. Because of their small size, they exhibit unusual
and interesting properties governed by quantum mechanics, such as the
absorption of different colors of light depending upon their size.

It was only two years ago that a UC Berkeley team led by Alivisatos found a
way to make nanorods of a reliable size out of cadmium selenide, a
semiconducting material. Conventional semiconductor solar cells are made of
polycrystalline silicon or, in the case of the highest efficiency ones,
crystalline gallium arsenide.

Huynh and Dittmer manufactured nanorods in a beaker containing cadmium
selenide, aiming for rods of a diameter - 7 nanometers - to absorb as much
sunlight as possible. They also aimed for nanorods as long as possible - in
this case, 60 nanometers. They then mixed the nanorods with a plastic
semiconductor, called P3HT - poly-(3-hexylthiophene) - and coated a
transparent electrode with the mixture. The thickness, 200 nanometers - a
thousandth the thickness of a human hair - is a factor of 10 less than the
micron-thickness of semiconductor solar cells. An aluminum coating acting as
the back electrode completed the device.

The nanorods act like wires. When they absorb light of a specific
wavelength, they generate an electron plus an electron hole - a vacancy in
the crystal that moves around just like an electron. The electron travels
the length of the rod until it is collected by the aluminum electrode. The
hole is transferred to the plastic, which is known as a hole-carrier, and
conveyed to the electrode, creating a current.

P3HT and similar plastic semiconductors currently are a hot area of research
in solar cell technology, but by themselves these plastics are lucky to
achieve light-conversion efficiencies of several percent.

"All solar cells using plastic semiconductors have been stuck at two percent
efficiency, but we have that much at the beginning of our research," Huynh
said. "I think we can do so much better than plastic electronics."

"The advantage of hybrid materials consisting of inorganic semiconductors
and organic polymers is that potentially you get the best of both worlds,"
Dittmer added. "Inorganic semiconductors offer excellent, well-established
electronic properties and they are very well suited as solar cell materials.
Polymers offer the advantage of solution processing at room temperature,
which is cheaper and allows for using fully flexible substrates, such as
plastics."

Visiting scientist Keith Barnham, professor of physics at Imperial College,
London, and an expert on high-efficiency solar cells, agreed.

"This is exciting, cheap technology if they can get the efficiency up to 10
percent, which I think they will, in time," Barnham said. "Paul's approach
is a very promising way to get around the problem of the efficiency of
plastic solar cells."

Some of the obvious improvements include better light collection and
concentration, which already are employed in commercial solar cells. But
Alivisatos and his colleagues hope to make significant improvements in the
plastic/nanorod mix, too, ideally packing the nanorods closer together,
perpendicular to the electrodes, using minimal polymer, or even none - the
nanorods would transfer their electrons more directly to the electrode. In
their first-generation solar cells, the nanorods are jumbled up in the
polymer, leading to losses of current via electron-hole recombination and
thus lower efficiency.

They also hope to tune the nanorods to absorb different colors to span the
spectrum of sunlight. An eventual solar cell might have three layers, each
made of nanorods that absorb at different wavelengths.

"For this to really find widespread use, we will have to get up to around 10
percent efficiency," Alivisatos said. "But we think it's very doable."
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