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Researchers with Berkeley Lab and the University of
California, Berkeley, have developed the first ultra-thin solar cells
comprised entirely of inorganic nanocrystals and spin-cast from
solution. These dual nanocrystal solar cells are as cheap and easy to
make as solar cells made from organic polymers and offer the added
advantage of being stable in air because they contain no organic
materials.
"Our colloidal inorganic nanocrystals share all of
the primary advantages of organics -- scalable and controlled
synthesis, an ability to be processed in solution, and a decreased
sensitivity to substitutional doping – while retaining the broadband
absorption and superior transport properties of traditional
photovoltaic semiconductors," said Ilan Gur, a researcher in Berkeley
Lab's Materials Sciences Division and fourth-year graduate student in
UC Berkeley's Department of Materials Science and Engineering.
Gur is the principal author of a paper appearing in
the October 21 issue of the journal Science that announces this new
development. He is a doctoral candidate in the research group of Paul
Alivisatos, director of Berkeley Lab's Materials Sciences Division,
and the Chancellor's Professor of Chemistry and Materials Science at
UC Berkeley. Alivisatos is a leading authority on nanocrystals and a
co-author of the Science paper. Other co-authors are Berkeley Lab's
Neil A. Fromer and UC Berkeley's Michael Geier.
In this paper, the researchers describe a technique
whereby rod-shaped nanometer-sized crystals of two semiconductors,
cadmium-selenide (CdSe) and cadmium-telluride (CdTe), were synthesized
separately and then dissolved in solution and spin-cast onto a
conductive glass substrate. The resulting films, which were about
1,000 times thinner than a human hair, displayed efficiencies for
converting sunlight to electricity of about 3 percent. This is
comparable to the conversion efficiencies of the best organic solar
cells, but still substantially lower than conventional silicon solar
cell thin films.
"We obviously still have a long way to go in terms
of energy conversion efficiency," said Gur, "but our dual nanocrystal
solar cells are ultra-thin and solution-processed, which means they
retain the cost-reduction potential that has made organic cells so
attractive vis-a-vis their conventional semiconductor counterparts."
As every consumer in this country is painfully
aware, the costs of fossil fuels are rising. From escalating prices at
gas pumps, to melting polar ice caps, the message is loud and clear:
Alternative energy sources must be found. Solar energy is in many ways
an ideal choice. As a source it is plentiful – the sun shines
approximately 1,000 watts of energy per square meter of the planet's
surface every day – and would last the lifetime of our planet. It
would add no pollutants to the atmosphere, contribute nothing to
global climate change, and is free. The cost comes in when solar
energy is converted to electrical power.
Most commercial solar cells today are made from
silicon. Like many conventional semiconductors, silicon offers
excellent, well-established electronic properties. However, the use of
silicon or other conventional semiconductors in photovoltaic devices
has to date been limited by the high cost of production -- even the
fabrication of the simplest semiconductor cell is a complex process
that has to take place under exactly controlled conditions, such as
high vacuum and temperatures between 400 and 1,400 degrees Celsius.
When it was discovered, back in 1977, that a certain group of "conjugated"
organic polymers could be made to conduct electricity, there was
immediate interest in using these materials in photovoltaic devices.
While it was shown that plastic solar cells could be made in bulk
quantities for a few cents each, the efficiency by which these devices
converted light into electricity has always been poor compared to the
power conversion efficiencies of cells made from semiconductors. In
2002, Alivisatos and members of his research group announced a
breakthrough in which they were able to fashion hybrid solar cells out
of organic polymers and CdSe. While these hybrids offer some of the
best features of semiconductor and plastic solar cells, they remain
sensitive to air because they contain organics.
"A solar cell that relies exclusively on colloidal nanocrystals has
been anticipated theoretically in recent years," said Alivisatos. "We've
now demonstrated such a device and have presented a mechanism for its
operation."
Unlike conventional semiconductor solar cells, in which an
electrical current flows between layers of n-type and p-type
semiconductor films, with these new inorganic nanocrystal solar cells,
current flows due to a pair of molecules that serve as donors and
receptors of electrical charges, also known as a donor-acceptor
heterojunction. This is the same mechanism by which current flows in
plastic solar cells.
"Because our inorganic nanocrystal solar cells appear to work
primarily based on the donor-acceptor heterojunction model that is
typical of organic systems, they help us to better understand the
specific material properties needed to make such devices," said Gur. "This
work also elucidates some key similarities between polymer and
nanocrystal films."
The CdSe and CdTe films are electrical insulators in the dark but
when exposed to sunlight undergo a dramatic rise in electrical
conductivity, as much as three orders of magnitude. Sintering the
nanocrystals was found to significantly enhance the performance of
these films. Unlike plastic solar cells, whose performance
deteriorates over time, aging seems to improve the performance of
these inorganic nanocrystal solar cells.
"The next step is for us to better characterize and further develop
our prototypical system, as there is still a great deal we don't fully
understand," said Gur. "After that, we have a lot of directions that
we'd like to pursue, such as introducing variations in the system
architecture and our choice of semiconductor materials." According to
the Energy Foundation, if the available residential and commercial
rooftops in this country were to be coated with solar cell thin films,
they could furnish an estimated 710,000 megawatts of electricity
across the United States, which is more than three-quarters of all the
electricity that this country is currently able to generate. Because
of its favorable sunlight levels, California is considered a prime
candidate for this technology. |
