Graphene, the two-dimensional lattice of carbon atoms, may be the wonder material du jour, but ultrathin layers of other elements are also proving to be an exciting area of research. One-atom-thick sheets of germanium and tin have shown potential as semiconductors and a topological insulators respectively, and now ultrathin layers of tungsten and selenium have been used to create a diode that could be used in ultrathin, flexible, semi-transparent solar cells.

Although graphene-based solar cells have graced our pages in the past, the electronic states in graphene are "not very practical for creating photovoltaics," according to Thomas Mueller from the Vienna University of Technology. That's why he and his team went looking elsewhere and sandwiched a layer of tungsten atoms between two layers of selenium atoms. The resulting tungsten diselenide (WSe2) monolayer was found to absorb light, similar to graphene, but was also able to use the light to generate electrical power.

The tungsten diselenide material is so thin that 95 percent of light hitting it passes straight through, with a tenth of the remaining five percent that is absorbed by the material converted into electrical power. Although this is a small amount overall, the researchers point out that this equates to a quite high internal conversion efficiency and that greater amounts of electricity can be generated by stacking multiple layers on top of each other.

However, the high transparency of the material can also be seen as a benefit, with the potential for solar cell layers to be placed on windows to generate electricity while letting light into the building. The material is also flexible, opening up the potential for it to be used in flexible solar cells and displays.

The results of the Vienna University of Technology team's experiments appear in the journal Nature Nanotechnology.

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