Double-decker solar cell.

Researchers have created a two-layer solar cell that's the most efficient yet among cells made of organic materials. It's essentially a two-solar cell sandwich with a transparent film in the middle. The top layer picks up the higher-energy photons from the blue part of the spectrum. Most of the lower-energy photons make it unscathed through both the first cell and the film. The bottom layer is optimized for absorbing photons in the lower-energy, red part of the spectrum.

Together, the layers absorb more than 6 percent of light's energy. That's a record for organic solar cells, say Alan Heeger of the University of California, Santa Barbara and his colleagues in the July 13 Science.

Heeger says that his team's is the first double-layer organic solar cell that can be manufactured by painting polymers on top of each other. Costs could be far less than those of conventional silicon-crystal photovoltaics.

On the other hand, compared with silicon crystals, polymers have an irregular structure that increases electrical resistance and limits efficiency (SN: 5/26/07, p. 328). However, Heeger says, the cell's microscopic structure can be improved. "There's a lot of headroom for increasing the efficiency," he says.--D.C.