Nanocrystal Discovery Promises Cheap Solar Cells

Nov 9, 2005 12:37 PM

A scientific milestone achieved at the Lawrence Berkeley National Laboratory is expected to bring us one step closer to a future in which the rooftops of homes and commercial buildings can be laminated with inexpensive, ultrathin films of nanosized semiconductors. It is hoped that the solar cells manufactured using this nanocrystal technology will efficiently convert sunlight into electrical power and provide virtually all of our electricity needs.

Researchers with the U.S. Department of Energy’s Berkeley Lab and the University of California, Berkeley, have developed the first ultrathin 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.

Ilan Gur, a researcher in Berkeley Lab’s Materials Sciences Division, is the principal author of a paper in Science that describes an inexpensive process for mass-producing solar cell thin films from semiconductors.

“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 coauthor of the Science paper. Other coauthors 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 1000 times thinner than a human hair, displayed efficiencies for converting sunlight to electricity of about 3%. 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 ultrathin 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.”

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°C and 1400°C.

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