Advances in solar electric technology.

Scientists have proposed a way to control the distribution of contaminants in silicon, potentially opening up the use of cheaper, "dirtier" starting materials for making solar cells. In a study published in the September Nature Materials, the researchers predict that the strategy could lower production costs of solar cells.

Up until now, pure silicon was required for solar cells or the result was a significant loss of efficiency. Up until recently there had been enough pure stock for the electronics industry, but the needs of the growing photovoltaic industry—which uses silicon for more than 90 percent of its solar cells—caused overall demand to exceed supply in 2004. This triggered a drastic price increase in pure silicon, dealing a blow to the solar cell makers.

Researchers turned to Lawrence Berkeley's synchrotron, a circular accelerator approximately 65 meters in diameter. The machine generates X rays intense enough to identify within silicon samples individual metal clusters on the order of tens of nanometers in diameter. The researchers mapped the distribution of the clusters and used a sophisticated technique for measuring how far charges traveled in the samples, an indicator of the material's efficiency in converting sunlight into electricity.

They found that silicon hosting larger but fewer numbers of clusters performed better than did samples with smaller but many more clusters. They tested this result by heating samples and then cooling them at different rates, which enabled the researchers to control the distribution of the metal. Weber's team found that silicon with micrometer-size clusters, spaced hundreds of micrometers apart, was four times as efficient as silicon with more-finely-distributed, nanosize clusters.

The result could be much cheaoer solar cells that do not have to go through the rigorous purification process that has been necessary up until now.