Ultrathin solar cells reach nearly 20 percent efficiency

Researchers at the French Centre de Nanosciences et de Nanotechnologies (C2N) in collaboration with researchers at German Fraunhofer ISE and others have reached a solar cell efficiency »of nearly 20 percent« thanks to an ultrathin absorbing layer made of 205 nm-thick gallium arsenide (GaAs) on a nanostructured back mirror. Researchers of the team led by Stéphane Collin used nanoimprint lithography to directly emboss a sol-gel derived film of titanium dioxide, an inexpensive, rapid and scalable technique.
The solar cells fabricated from GaAs required at least 1 micrometer-thick layers of semiconductor material, or even 40 µm or more in the case of silicon. »Thinning the absorber automatically reduces absorption of sunlight and conversion efficiency«, say the scientists at ISE. A flat mirror at the backside of the cell can help and lead to double-pass absorption, but no more. Yet, a stronger thickness reduction would enable material savings of scarce materials and industrial throughput improvements due to shorter deposition times. But previous attempts of light trapping have been greatly limited in performance by the optical and electrical losses.
Frank Dimroth, Head of the Department »III-V Photovoltaics and Concentrator Technology« at Fraunhofer ISE expects to be able »to even further improve the efficiency of ultrathin solar cells«. The work »demonstrates that this architecture should enable 25 percent efficiency in the short term.« Even if the limits are still unknown, researchers are convinced that the thickness could be further reduced without efficiency loss, at least by a factor of two. GaAs solar cells are still commercially limited to space application due to their cost. However, researchers are already working to extend this concept for large-scale photovoltaics made of e.g. CdTe, CIGS or silicon materials, says the institute.
© PHOTON

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