25 March 2013

Nanowire Based Solar Cell Increases Shockley-Queisser Efficiency Limit

The 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.
Credit: Niels Bohr Institute
The development of a nanowire based solar cell that increases sunlight concentration to a a factor of 15 raises the standard efficiency limit of solar cells known as the Shockley-Queisser limit.

One of the most popular and common source of renewable and sustainable energy is the Sun. Solar energy is not dependent on weather conditions such as wind power or need to be near a power source such as geothermal or hydroelectric energy producers.

Solar energy is produced by solar panels or solar cells. These cells, also known as photovoltaic cells, convert sunlight to electrical energy.

The focus on solar cell technology is raising the efficiency of the solar cell to convert solar energy to electrical energy. In solar cell production, the value that is used to gauge the efficiency of the solar cell is the Shockley-Quesser limit. This limit refers to the maximum theoretical efficiency of a solar cell using a p-n junction to collect power from the cell.

A p-n junction refers to the boundary of two semiconductors; the p-type and the n-type. The p-type semiconductor contains excess holes while the n-type contains excess free electrons.

The Shockley-Queisser limit puts the maximum solar cell efficiency at around 33.7%. This means that at most, only 33.7% of sunlight can be converted into elecrical energy. Currently, silicon based photovoltaic cells have an efficiency of 22%.

Raising the Shockley-Queisser Limit

Scientists from the Nano-Science Center at the Niels Bohr Institut, Denmark and the Ecole Polytechnique Fédérale de Lausanne, Switzerland, have shown that a single nanowire can concentrate the sunlight up to 15 times of the normal sun light intensity. The results are surprising and the potential for developing a new type of highly efficient solar cells is great.

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

Video: Shockley Queisser Limit

The research groups have during recent years studied how to develop and improve the quality of the nanowire crystals, which is a cylindrical structure with a diameter of about 10,000 part 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.

It turns out that the nanowires 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 it can cause resonances in the intensity of light in and around nanowires. Thus, the resonances can give a concentrated sunlight, where the energy is converted, which can be used to give a higher conversion effeciency of the sun's energy, says Peter 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 efficiency limit - the so-called "Shockley-Queisser Limit" - is a limit, which for many years has been a landmark for solar cells efficiency among researchers, but now it seems that it may be increased.

Nanowire crystals are used as the solar cells. The image (left) shows a SEM (Scaning 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).
Credit: Niels Bohr Institute

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 Peter 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.


University of Copenhagen - Niels Bohr Institute
Nature Photonics
École Polytechnique Fédérale de Lausanne
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