Slim and sturdy solar cells: nanocrystals offer path to electricity.

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."