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Silicon Valleys

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Cheap, abundant silicon, the stuff computer chips are made of, is also the material of choice for solar cells, devices that transform sunlight into electricity. Silicon, however, converts at most only about 25 percent of the sunlight that hits it; increasing that percentage would go a long way toward making solar cells more competitive with other sources of power. Thanks to a happy accident, a group of Harvard physicists may have found a way to do just that.

About two years ago, Tsing-Hua Her, Eric Mazur, and Claudia Wu were using a laser to try to make pieces of silicon react with various types of gases inside a chamber. The physicists were hoping to find better ways to etch circuit patterns onto computer chips. But they discovered that their process created rows of perfect spikes on the surface of the silicon. "It's really amazing," says Her, "The spikes are very well organized."

The spike-covered silicon couldn't be used to make computer chips, so the physicists decided to test it for other potential applications. Wu and her colleagues soon found that spiky silicon absorbs nearly all the light that falls on it. The spikes recapture light that isn't initially absorbed or that has bounced off other spikes. The physicists aren't exactly sure how the light-absorbing spikes form but speculate that a chemical reaction might occur between the silicon surface melted by the laser and chlorine or fluorine gas. Conditions have to be just right: with a longer or weaker laser pulse, or gases other than fluorine or chlorine, the spikes fail to form.

The spiky silicon generates about 60 percent more electric current than does flat silicon, at least for some colors of light. Because of the powerful laser needed, the process of making the spiked silicon is still a little slow and expensive. But as the Harvard team makes fabrication more reliable, their silicon could become the standard for solar cells of the future.

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