Week of Oct. 22, 2005; Vol. 168, No. 17

Slim and Sturdy Solar Cells: Nanocrystals offer path to electricity

Aimee Cunningham

Engineers have for years been developing solar cells made of inexpensive
plastic, but the devices have limitations. For instance, the cells\'
short lifetimes when exposed to sunlight have prevented these inventions
from getting beyond the prototype phase. Now, researchers describe a
solar cell made of thin films of inorganic nanocrystals that have
several of the advantages of plastic but avoid some of its shortcomings.

Researchers first turned to semiconducting plastics to devise solar
cells because such materials are cheaper to produce than silicon, the
main ingredient of conventional solar cells, and are more flexible.
Plastic formulations also open the possibility of printing solar cells
onto various surfaces, much as ink is printed on a newspaper.

Semiconducting plastics, however, break down over time, says Ilan Gur, a
graduate student and materials science engineer at Lawrence Berkeley
(Calif.) National Laboratory and the University of California, Berkeley.
So, he and his colleagues decided to make a similar solar cell out of
durable inorganic materials instead.

The researchers designed their inorganic cell around the same
electron-donor-and-acceptor system used in plastic solar cells. The team
deposited a thin layer of electron-rich cadmium telluride nanocrystals
atop electrically conductive glass and then applied a thin layer of
electron-hungry cadmium selenide nanocrystals. Together, the two layers
are about 200 nanometers thick.

The scientists topped off their stack with electrically conductive
aluminum. When the solar cell is illuminated, electrons generate a
current as they move through the nanocrystal layers to the aluminum
contact.

Conventional silicon solar cells convert between 10 and 15 percent of
the sun\'s energy into electricity. The prototype inorganic nanocrystal
solar cells have efficiencies of only about 3 percent, the group reports
in the Oct. 21 Science . Gur notes that this efficiency is \"in the same
ballpark\" as that of today\'s plastic models. With improvement, he
says, the nanocrystal approach could become \"a general strategy to
cheaply produce solar cells.\"

The research \"points to a potentially promising direction,\" says
Moungi G. Bawendi, a physical chemist at the Massachusetts Institute of
Technology. He notes, however, that it has a high-temperature processing
step that would make the solar cells difficult to incorporate into some
materials.

\"It opens the door for printing solar cells made with inorganic
materials,\" says Michael McGehee, a materials scientist at Stanford
University. While the efficiency isn\'t good enough yet, he notes, \"I
think we should take every reasonable approach we can think of because
it\'s so important to find one that works.\"

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References:

Gur, I., et al . 2005. Air-stable all-inorganic nanocrystal solar cells
processed from solution. Science 310(Oct. 21):462-465. Abstract
available at
[[http://www.sciencemag.org/cgi/content/abstract/310/5747/462]{.underline}](http://www.sciencemag.org/cgi/content/abstract/310/5747/462)
.

Further Readings:

Goho, A. 2003. Plastic chips: New materials boost organic electronics.
Science News 164(Aug. 30):133. Available at
[[http://www.sciencenews.org/articles/20030830/fob5.asp]{.underline}](http://www.sciencenews.org/articles/20030830/fob5.asp)
.

Sources:

Ilan Gur

Materials Science Division

Lawrence Berkeley National Laboratory

Berkeley, CA 94720

Moungi G. Bawendi

Department of Chemistry

Massachusetts Institute of Technology

Cambridge, MA 02139

Michael McGehee

Department of Materials Science and Engineering Stanford University
Stanford, CA 94305