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Coating traps more light in solar cells

09 Jul 2008

Solar cells could be made thinner and more efficient thanks to a light-trapping coating designed by researchers in Germany.

Researchers from the University of Jena have simulated a coating that can be applied to any solar cell to boost the amount of light that it can trap. According to the group, the layer can be applied easily to the top of solar panels of any size and could be a step towards fabricating thinner photovoltaics (Optics Express 16 9332).

"We have theorized that the efficiency of a crystalline silicon solar cell could be increased from 28.7% to 32.8% with our coating," Stephan Fahr, a researcher from the Photonics Group at Jena, told optics.org. "A key advantage is that to obtain this increase, nothing needs to be changed in the design of the solar cell, the filter can simply be applied as an add-on to any existing cell. Enhancing the absorption also means that new solar cells could be fabricated thinner for the same performance."

As light enters a typical solar cell, it is refracted by a textured surface interface. This light then reflects off of the back side of the device and is either emitted through the front or kept inside the absorber by total internal reflection. The coating predicted by the group simply changes the probability by which photons are kept inside the absorber by encouraging total internal reflection.

"The coating enhances the absorption of direct sunlight by steering the path of photons in the solar cell," explained Fahr. "By acting as an angle and wavelength selective filter, the coating increases the average path length and absorption at long wavelengths without altering the solar cell performance at short wavelengths."

The group's approach explored the threshold wavelength between perfect transmission and perfect reflection of a solar cell. The exact wavelength depends on the thickness of the absorber, the absorption coefficient and the spectra of the direct sunlight and of the diffuse scattered light from the sky.

"For a 10 µm thick monocrystalline silicon solar cell we found the threshold wavelength to be around 870 nm," commented Fahr. "This means that for wavelengths longer than 870 nm the filter should be perfectly reflecting for angles of incidence larger than 2.5 °."

Whilst the team has so far only calculated the performance of the coating theoretically, it aims to verify its findings in the near future. "We hope to apply such filters on top of various solar cells that are available to the experimental partners of our consortium: the Institute of Physical Electronics and the Institute of Energy Research in Germany," concluded Fahr. "We will also be extending the concept to tandem solar cells."

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