Will nanowires provide a breakthrough for solar power efficiency?

Wires 1/10,000th the diameter of a human hair can absorb more of the sun's power than previously thought possible, a new study in Nature Photonics suggests.

Although still years away from production, nanowire solar cells could push the conversion efficiency of the sun's energy past the so-called Shockley-Queisser limit, which for decades has served as a fixed ceiling in solar energy research. 

Such a breakthrough would be significant because the sun's power is wildly abundant, but diffuse, and difficult to harvest. Even increasing the limit by a few percent would go a long way in making solar a more viable alternative to fossil fuels.

Today's photovoltaics recover less than a third of the sun's power, but the unique light-absorbing characteristics of nanoscopic structures could "have a major impact on the development of solar cells, exploitation of nanowire solar rays and perhaps the extraction of energy at the international level," according to scientists at the Niels Bohr Institute in Denmark and the École Polytechnique Fédérale de Lausanne in Switzerland, who wrote the study.

The scientists found that the nanowire concentrates the light 15 times more than normal sunlight intensity.

 For years, scientists have sought to construct solar cells that exceed the Shockley-Queisser limit. 

"People are constantly trying to work around this limit, and there are a number of ways people are thinking of doing it," said Jeremy Munday, a University of Maryland professor whose research focuses on energy harvesting and photonics but who is not affiliated with the study.

The trick is that thermodynamic laws govern the balance between the number of particles entering a solar cell and the number of particles exiting, Mr. Munday explains.

But nanoscopic structures behave differently from macroscopic ones, and have the potential to get more out of the sun. Because a nanowire's diameter is smaller than the wavelength of light, it acts as a kind of funnel, naturally focusing the sun's rays. 

"They cause the light to bend and become concentrated in the nanowire," added Munday.