New material allows for ultra-thin solar cells

Researchers have created a new material that may allow for extremely thin, semi-transparent and flexible solar cells.
Scientists at the Vienna University of Technology have managed to create a semiconductor structure consisting of two ultra-thin layers, which appears to be excellently 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.
Research on two-dimensional materials started with graphene, a material made of a single layer of carbon atoms.

"Quite often, two-dimensional crystals have electronic properties that are completely different from those of thicker layers of the same material," said researcher Thomas Mueller.

Mueller's 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," said Mueller.
But a solar cell made only of tungsten diselenide would require countless tiny metal electrodes tightly spaced only a few micrometres 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.
To prevent recombination of electrons and holes, metallic electrodes can be used, through which the charge is sucked away - or a second material is added.

"The holes move inside the tungsten diselenide layer, the electrons, on the other hand, migrate into the molybednium disulphide," said Mueller.
Thus, recombination is suppressed, he explained.