A new combination of solar cells cracked a U.S. Department of Energy's efficiency goal, and could find use in the space program or eventual earthbound solar power plants, officials say.

Researchers at Sandia National Laboratories stacked a gallium arsenide solar cell made by Varian Associates Inc. of Palo Alto, Calif., on a silicon cell from Stanford University to achieve a 31 percent efficiency rating. It can generate 31 watts of electricity for every 100 watts of sunlight it collects.DOE had set a 30 percent efficiency goal, and previous attempts at stacking yielded 20 percent, said Sandia's Dan Arvizu.

The gallium arsenide cell is best at converting light on the blue segment of the spectrum, while the silicon chip is better on red. Each cell is about 27 percent efficient. Researchers may almost have reached the limit of efficiency for single solar cells, so stacking is the most promising way to achieve high efficiency, said Jan Werthen, Varian's manager of solar research.

"The bottom line is how much is the electricity on a cents-per-kilowatt-hour basis," said Robert Annan, head of DOE's photovoltaic energy technology division. Some coal- and oil-fired power systems cost 30 cents per kwh, and Sandia's new chip might generate at 12 cents within a decade, Annan said.

Similar costs might also be possible with a cell developed by the Solar Energy Research Institute and Arco Solar, Annan said. SERI and Arco reduced the cost of production, while Sandia increased efficiency, he said.