A single nanowire that uses 10,000 times less material can capture 15 times more light and produce energy with incredible efficiency at a much lower cost. The technology could provide the basis for a new generation of highly efficient solar cells or for powering microchips and quantum computers. Despite their size, nanowire crystals have tremendous potential for energy production. Extremely thin filaments, they concentrate the sun’s rays into a very small area in the crystal by up to a factor of 15. Because the diameter of a nanowire crystal is smaller than the wavelength of light coming from the sun, it can cause resonances in the intensity of light in and around nanowires. When equipped with the right electronic properties, the nanowire becomes a tiny solar cell, transforming sunlight into electric current. Scientists from the Nano-Science Center at Niels Bohr Institute and at Ecole Polytechnique Fédérale de Lausanne have built a nanowire solar cell out of gallium arsenide — a material better at converting light into power than silicon. The solar cell collects up to 12 times more light than conventional flat ones. 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). Courtesy of Niels Bohr Institute. The design increases the typical efficiency limit — the so-called Shockley-Queisser limit — which has for many years been a landmark for solar cell efficiency among researchers. “It’s exciting as a researcher to move the theoretical limits, as we know,” said Peter Krogstrup, who just completed his doctorate at Niels Bohr. “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.” The nanowire, standing vertically, acts like a very efficient light tunnel, absorbing light as though it were 12 times bigger; i.e., it has a greater field of vision than expected, the investigators say. The prototype is nearly 10 percent more efficient at transforming light into power than allowed, in theory, for conventional single material solar panels. The efficiency could be increased further by optimizing the dimensions of the nanowire, improving the quality of the gallium arsenide and using better electrical contacts to extract the current. This figure shows that the sun’s rays are drawn into a nanowire, which stands on a substrate. At a given wavelength, the sunlight is concentrated up to 15 times. Consequently, there is great potential in using nanowires in the development of future solar cells. Courtesy of Niels Bohr Institute. In practice, arrays of the nanowires could attain 33 percent efficiency, whereas commercial, flat solar panels are currently only 20 percent efficient, the study reports. In addition, the arrays could use at least 10,000 times less gallium arsenide, reducing the cost to only $10 per square meter instead of $100,000. “It will take some years before production of solar cells consisting of nanowires becomes a reality,” Krogstrup said. The findings were reported in Nature Photonics (doi: 10.1038/nphoton.2013.32). For more information, visit: www.epfl.ch or www.nbi.ku.dk/english