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Nanowire solar cells raise efficiency limit

Mon, 03/25/2013 - 8:35am

Nanowire crystals are used as the solar cells. The image (left) shows a SEM (scanning electron microscope) image of GaAs nanowire crystal grown on a Silicon substrate. A TEM (transmission electron microscope) image (middle) shows a single nanowire. Further zooming in on the crystal structure, using STEM (scanning transmission electron microscope) imaging, shows the actual atomic columns (right). Image: Niels Bohr InstituteScientists from the Nano-Science Center at the Niels Bohr Institute, Denmark, and the Ecole Polytechnique Fédérale de Lausanne, Switzerland, have shown that a single nanowire can concentrate sunlight up to 15 times normal intensity.

Due to the unique physical light absorption properties of nanowires, the limit of how much energy we can utilize from the sun's rays is higher than previously believed. These results demonstrate the great potential of development of nanowire-based solar cells, says Peter Krogstrup on the surprising discovery that is described in the journal Nature Photonics.

The research groups have during recent years studied how to develop and improve the quality of nanowire crystals, which are cylindrical structures with a diameter of just 10,000th of a human hair. The nanowires are predicted to have great potential in the development not only of solar cells, but also of future quantum computers and other electronic products.

The nanowires, they have discovered, naturally concentrate the sun's rays into a very small area in the crystal by up to a factor 15. Because the diameter of a nanowire crystal is smaller than the wavelength of the light coming from the sun, resonances are created in the intensity of light in and around nanowires. These resonances produce a concentrated sunlight, where the energy is converted and can be used to give a higher conversion efficiency of the sun's energy, says Krogstrup, who with this discovery contributes to that the research in solar cell technology based on nanowires get a real boost.

New efficiency limit

The typical solar cell efficiency limit―the so-called "Shockley-Queisser Limit"―is a limit which for many years has been a landmark for solar cell efficiency among researchers. Now it seems that it may be increased.

“It's exciting as a researcher to move the theoretical limits, as we know. Although it does not sound like much, that the limit is moved by only a few percent, it will have a major impact on the development of solar cells, exploitation of nanowire solar rays and perhaps the extraction of energy at international level. However, it will take some years years before production of solar cells consisting of nanowires becomes a reality,” says Krogstrup, who just completed his PhD at the Niels Bohr Institute, University of Copenhagen.

The research is conducted in collaboration with the Laboratory des Matériaux Semiconducteurs, Ecole Polytechnique Fédérale de Lausanne, the Foundation and the company SunFlake A / S. Their scientific findings work support results published in the journal Science in January. Here, a group of researchers from Lund, showed that the sun’s rays was sucked into the nanowires due to the high amount of power that their solar cell produced.

Source: Niels Bohr Institute

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