Material scientists have been working for years to create ultra-thin, flexible solar cells, and they’ve nearly brought that dream to fruition. Incredibly thin layers of photoactive crystal tungsten diselenide have been combined with molybdenum disulfide in order to create a semi-transparent material that could revolutionize solar technology. This material is so light, 300 square meters weighs only one gram. The research was led by Thomas Mueller of the Vienna University of Technology and the results were published in the journal Nano Letters.

When the researchers say “ultra-thin,” they mean it. Each layer within the material is only a few atoms thick. (For reference, a strand of hair is approximately 300,000 atoms thick.) The king of all ultra-thin materials is graphene, which is an incredibly strong monolayer of carbon atoms arranged in a honeycomb pattern. Graphene’s strength is partially due to the absence of a bandgap, which prevents it from being used as a semiconductor. While graphene itself is out, it has been used as inspiration for the development of other thin materials that have conductive properties.

“Quite often, two-dimensional crystals have electronic properties that are completely different from those of thicker layers of the same material,” Mueller said in a press release.

Mueller’s team had previously developed tungsten diselenide: two layers of selenium atoms surrounding a sheet of tungsten atoms. The material has a bandgap which gives it conductive properties, but it is still durable, transparent, flexible, and best of all, inexpensive to produce. Prior research has shown that it is an order of magnitude more sensitive than graphene.

“We had already been able to show that tungsten diselenide can be used to turn light into electric energy and vice versa,” he explained.

One downfall to using only tungsten deselenide was that it required electrodes to be tightly packed into the material to reduce recombination. After light hits a photoactive material, electrons are displaced and a positively-charged hole remains where they were. This separation contributes to the current. If the electrons were to return and recombine with the hole, the current is diminished. In this latest study, Mueller’s lab solved this problem by adding molybdenium disulphide, which is a layer of molybdenum atoms in between two layers of sulfur atoms. This material does not require the use of electrodes to suppress recombination, and instead does it by retaining the electrons in a different layer.

“The holes move inside the tungsten diselenide layer, the electrons, on the other hand, migrate into the molybednium disulphide,” Mueller added. “One of the greatest challenges was to stack the two materials, creating an atomically flat structure. If there are any molecules between the two layers, so that there is no direct contact, the solar cell will not work.”

While this material does let some light through, much of the light is converted into electricity. Functional solar cells made out of this material could be used on windows to generate energy for the building without compromising the aesthetic or obscuring the view.