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Sheets of stapled semiconductors could make ultra thin solar cells

Researchers combine tungsten diselenide with molybdenum disulphide to create 'designer' optoelectronic material

Researchers at the Vienna University of Technology have used two ultra-thin layers to create a new semiconductor structure suited for photovoltaic energy conversion.

Several months ago, Marco Furchi, Thomas Mueller, and Andreas Pospischil (pictured l-r) produced an ultra-thin layer of the photoactive crystal tungsten diselenide. Now, they have combined this semiconductor with another layer made of molybdenum disulphide, creating a material that shows potential for a new kind of solar cell technology, they say,  that is extremely thin, semi-transparent, and flexible.

Two layers with different functions

Tungsten diselenide is a semiconductor which consists of three atomic layers. One layer of tungsten is sandwiched between two layers of selenium atoms. (The image shows the two semiconductor layers in the middle, connected to electrodes on either side).

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", says Mueller. Thus, recombination is suppressed.

This is only possible if the energies of the electrons in both layers are tuned exactly the right way. In the experiment, this can be done using electrostatic fields. Florian Libisch and Joachim Burgdörfer (TU Vienna) provided computer simulations to calculate how the energy of the electrons changes in both materials and which voltage leads to an optimum yield of electrical power.

Tightly packed layers

"One of the greatest challenges was to stack the two materials, creating an atomically flat structure", says Thomas Mueller. "If there are any molecules between the two layers, so that there is no direct contact, the solar cell will not work." Eventually, this feat was accomplished by heating both layers in vacuum and stacking it in ambient atmosphere. Water between the two layers was removed by heating the layer structure once again.

Part of the incoming light passes right through the material. The rest is absorbed and converted into electric energy. The material could be used for glass fronts, letting most of the light in, but still creating electricity. As it only consists of a few atomic layers, it is extremely light weight (300 square meters weigh only one gram), and very flexible. Now the team is working on stacking more than two layers - this will reduce transparency, but increase the electrical power.

2D materials

Ultra-thin 2D materials, which consist only of one or a few atomic layers are a hot topic. Research on such materials started with graphene, which is made of a single layer of carbon atoms. Mueller and his team  applied their knowledge gained in handling, analysing and improving ultra-thin layers of graphene to other ultra-thin materials to do this work. The team was the first to combine two different ultra-thin semiconductor layers and study their optoelectronic properties.

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