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Material makes it possible to create ultra-thin solar cells

Posted: 06 Aug 2014  Print Version  Bookmark and Share

Keywords:Mueller  Vienna University of Technology  graphene  diselenide 

Researchers at the Vienna University of Technology created a structure that could pave the way for ultra-thin solar cells. They successfully combined two semiconductor materials, which consist of only three atomic layers each.

Extremely thin, semi-transparent, flexible solar cells could soon become a reality. At the VUT, Thomas Mueller, Marco Furchi, and Andreas Pospischil have managed to create a semiconductor structure consisting of two ultra-thin layers, which appears to be suited for photovoltaic energy conversion.

Several months ago, the team had already produced an ultra-thin layer of the photoactive crystal tungsten diselenide. Now, this semiconductor has successfully been combined with another layer made of molybdenum disulphide, creating a designer-material that may be used in future low-cost solar cells. With this advance, the researchers hope to establish a new kind of solar cell technology.

 Solar cell's layer system

The solar cell's layer system: two semiconductor layers in the middle, connected to electrodes on either side.

Ultra-thin materials, which consist only of one or a few atomic layers, are currently a hot topic in materials science today. Research on two-dimensional materials started with graphene, a material made of a single layer of carbon atoms. Like other research groups all over the world, Mueller and his team acquired the necessary know-how to handle, analyse, and improve ultra-thin layers by working with graphene. This know-how has now been applied to other ultra-thin materials.

"Quite often, two-dimensional crystals have electronic properties that are completely different from those of thicker layers of the same material," says Thomas Mueller. His team was the first to combine two different ultra-thin semiconductor layers and study their optoelectronic properties.

Tungsten diselenide is a semiconductor which consists of three atomic layers. One layer of tungsten is sandwiched between two layers of selenium atoms. "We had already been able to show that tungsten diselenide can be used to turn light into electric energy and vice versa," says Mueller. But a solar cell made only of tungsten diselenide would require countless tiny metal electrodes tightly spaced only a few micrometers apart. If the material is combined with molybdenium disulphide, which also consists of three atomic layers, this problem is circumvented. The heterostructure can now be used to build large-area solar cells.

When light shines on a photoactive material, single electrons are removed from their original position. A positively charged hole remains, where the electron used to be. Both the electron and the hole can move freely in the material, but they only contribute to the electrical current when they are kept apart so that they cannot recombine.

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