Trapping Sunlight With Silicon Nanowires

While there are now silicon photovoltaics that can convert sunlight into electricity at impressive 20 percent efficiencies, the cost of this solar power is prohibitive for large-scale use. Researchers at the Lawrence Berkeley National Laboratory (Berkeley Lab), however, are developing a new approach that could substantially reduce these costs. The key to their success is a new way of trapping sunlight.

“Through the fabrication of thin films from ordered arrays of vertical silicon nanowires, we’ve been able to increase the light-trapping in our solar cells,” said Peidong Yang, a chemist who led this research. “Since the fabrication technique behind this extraordinary light-trapping enhancement is a relatively simple and scalable aqueous chemistry process, we believe our approach represents an economically viable path toward high-efficiency, low-cost thin-film solar cells.”

Yang holds joint appointments with Berkeley Lab’s Materials Sciences Division, and the Chemistry Department of the University of California, Berkeley. He is a leading authority on semiconductor nanowires—one-dimensional strips of materials with widths measuring only one-thousandth that of a human hair and lengths of several microns (one-millionth of a meter) long.

“Typical solar cells are made from very expensive ultrapure single crystal silicon wafers that require about 100 micrometers of thickness to absorb most of the solar light, whereas our radial geometry enables us to effectively trap light with nanowire arrays fabricated from silicon films that are only about eight micrometers thick,” Yang said. “Furthermore, our approach should, in principle, allow us to use metallurgical grade or ‘dirty’ silicon rather than the ultrapure silicon crystals now required, which should cut costs even further.”

While the conversion efficiency of these solar nanowires was only about 5 to 6 percent, Yang said this efficiency was achieved with little effort put into surface passivation, antireflection and other efficiency-increasing modifications.

“With further improvements, most importantly in surface passivation, we think it is possible to push the efficiency to above 10 percent,” Yang said.

Combining a 10 percent or better conversion efficiency with the greatly reduced quantities of starting silicon material and the ability to use metallurgical grade silicon should make the use of silicon nanowires an attractive candidate for large-scale development.

“Our technique can be used in existing solar panel manufacturing processes,” Yang said.

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