From Stars, More Sunlight

ORLANDO, FLORIDA--Star-shaped plastics may soon help the sun shine a bit brighter. In recent years, researchers have been testing whether so-called dendrimers--polymers grown into spherical particles--can be used as everything from drug carriers to catalysts. Now, they've also been shown to come in handy for boosting the efficiency of solar cells, a team reported here on 7 April at the semiannual meeting of the American Chemical Society.

Most commercial solar cells use inorganic semiconductors to convert sunlight into a steady stream of electrons that can serve as a power source. But in 1991, Michael Grätzel of the Swiss Federal Institute of Technology in Lausanne devised a cheaper solar cell. It uses a rhodamine-based dye to absorb sunlight, which excites electrons in the dye and gives them enough energy to hop around. The electrons quickly jump to a network of titanium dioxide particles, which ferries them to an electrode that is connected to an external circuit, allowing them to power lights, toasters, or other electrical equipment. Unfortunately, most of these Grätzel cells convert only a measly 6% of the energy in incoming sunlight to usable electrons.

Dendrimers might give them a boost. Unlike chainlike plastics, dendrimers have a large surface area, perfect for attaching numerous light-harvesting groups, reasoned chemist Jean Fréchet of the University of California, Berkeley. For their study, Fréchet's group created dendrimers shrouded with coumarin 2 dyes, which readily absorb ultraviolet light. They then added these to the mix in a conventional Grätzel cell. When they turned on a test lamp, the electrical efficiency of the device jumped by a couple of percent, he says.

"It's a promising approach," says Mary Ann Fox, a chemist and the chancellor of North Carolina State University in Raleigh, whose research group is pursuing a similar strategy. Dendrimers currently require complex synthetic steps to produce, typically making them expensive and therefore of questionable commercial value. But Fréchet notes that if studies with dendrimers reveal promising light-absorbing compounds, researchers may be able to duplicate much of the effect with conventional and cheaper polymers that are easier to exploit commercially.

Related sites
Jean Fréchet's home page
Michael Grätzel's home page
More about solar cells