Let there be 10 percent light.

Silicon solar cells, which turn sunlight into electricity, have for many years run into two roadblocks: converting light into electric current efficiently and cheaply

For a solar cell to be useful on a large scale, it must convert at least 10 percent of the light energy it receives into electricity But the cost involved in raising efficiency above 10 percent has proved prohibitive. One method, single-crystal silicon, ranks high in efficiency but also high in cost.

So scientists are seeking to make thin-film, amorphous silicon cells -for instance, by spraying silicon compounds onto a plain metal backing. This keeps the cost down, but it often lessens efficiency

However, materials scientist Subhendu Guha of United Solar Systems Corp. in Troy, Mich., says that his company has licked both of those problems at once. His research group has figured out a way to mass-produce thin-film, silicon-alloy solar cells that produce electricity at a stable 10.2 percent efficiency

"We call it a roll-to-roll deposition process," Guha says. "We take a stainless steel roll half a mile long and 14 inches wide, then run it through a machine that deposits the silicon-alloy thin film. This system substantially reduces production costs."

Once cut from the sheet, the resulting solar cell - a 5-millimeter-thick sandwich - has a silicon-alloy surface and two layers of silicon and germanium. Guha says a factory will open next year to roll out solar modules by the mile, priced "substantially below current market levels."

How big a solar cell does one need to light a common lightbulb? "On a good day, the sun delivers about 100 milliwatts of energy per square centimeter onto Earth's surface," Guha says. "That means with 10 percent efficiency, for 100 watts of power, you'll need a panel about a meter square."