Sun and Sand: Dirty silicon could supply solar
power
Aimee Cunningham
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.
Silicon is the second most abundant element in Earth's crust, but
nature's primary sources of silicon—sand and quartz—are tainted with
metals. Converting silicon from these sources into superpure
crystals is an expensive and time-consuming process.
While there had been enough pure stock for the electronics
industry, 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, notes Eicke R. Weber, a materials
scientist at the University of California, Berkeley and the Lawrence
Berkeley (Calif.) National Laboratory. This triggered a drastic
price increase in pure silicon, dealing a blow to the solar cell
makers.
Silicon stock that is less pure and therefore less expensive is
available, says Weber. But the increased amounts of iron, copper,
and other metal contaminants in these stocks reduce solar cell
efficiency. Clusters of these metal atoms attract the solar cell's
charge-carrying electrons, reducing the amount of current that the
cell can generate.
Weber and his colleagues set out to see whether they could
minimize the toll taken by these clusters without having to get rid
of them.
To do this, they 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. Weber's team 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.
The researchers 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.
"Without changing the total metal content—only changing the way
it is distributed—we get a drastic change in the electrical
property," says Weber. "If it is possible to concentrate the metals
in a few big clusters, in principle, one can make good solar cells
out of dirty starting material."
"It's excellent work," says Bhushan Sopori, an electrical
engineer at the Department of Energy's National Renewable Energy
Laboratory in Golden, Colo. But he cautions that "you often do not
have as much control [over metal impurities] as you think" when
growing silicon crystals.
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