A Bright Future For Optoelectronics: A Diode Made From a 2D Material Facilitates Novel Solar Cells
A special type of diode made from a crystalline material whose layers are just three atoms thick has been successfully realized for the first time. The superior properties of such ultra-thin crystals open up previously unimagined possibilities for solar cells, photodiodes and light-emitting diodes. The paper, now published in Nature Nanotechnology, not only documents the actual functionality of a so-called p-n diode made of tungsten diselenide, it also demonstrates its usefulness for numerous applications. These findings, obtained through a FWF project, thus constitute significant progress on the future path to 2D optoelectronics.
Electronic devices require semiconductors. These are usually made from crystalline silicon. The state of the art here is the use of three-dimensional crystals. But these not only combine low flexibility with high weight – they are also expensive to manufacture. Alternative approaches – organic semiconductors and thin-film technologies – result, in turn, in materials with inferior quality and durability. Two-dimensional (2D) crystals – crystalline material layers with a thickness of just one or a few atoms – offer a better chance of success. They can be produced economically on a large scale and are flexible, yet still exhibit all the advantages of crystalline materials. Now a team from the Institute of Photonics at the Vienna University of Technology has succeeded in producing the first diode with a p-n junction from such 2D crystals – thus laying the foundation for radical changes in optoelectronics.
A Gap in The Result
The starting material used for this by the team working with Prof. Thomas Mueller was tungsten diselenide (WSe2). It has one major advantage over graphene, the most well-known 2D crystalline material at present, as Prof. Mueller explains: "Tungsten diselenide has a band gap – so electrons require a certain energy to cross over to the conduction band. Graphene can't easily provide this basic requirement for many electronic components." To ensure that WSe2 was actually present in the form of a 2D layer for the team's further work, it was mechanically "peeled" from three-dimensional crystals in such a way that layers having a thickness of just 0.7 nanometers were created. As Prof. Mueller explains: "We subsequently used complex procedures to check whether we had indeed succeeded in realizing 2D crystals, as only such thin layers exhibit the required properties." Spectroscopic analyses, optical contrast measurements and atomic force microscopy confirmed that the researchers had achieved the desired result. The monolayer WSe2 was then placed between two electrodes and the electrical characteristics were measured. This unambiguously confirmed its function as a p-n diode: it was possible to inject both positive (p, holes) and negative (n, electrons) charges, with current flow exclusively in one direction, as is usual in diodes.
"WSe2 in monolayer crystalline form is theoretically an ideal starting material for p-n diodes and optoelectronics – but no one had ever proven it before. We have now done just that. We measured an efficiency of 0.5 percent in converting light to electrical energy," says Prof. Mueller, explaining the first demonstration worldwide of the photovoltaic characteristics of a 2D crystalline material. The high transparency, at 95 percent, means it can even be used simultaneously as window glass and as a solar cell. However, it is also possible to stack several such ultra-thin layers one on top of another to increase the efficiency to as much as 10 percent – of course at the expense of transparency.
The material's functionality as a photodiode was also proven, achieving a sensitivity one order of magnitude higher than that of graphene. These properties are further enhanced by the ability to convert electrical energy to light.
Overall, the results of this FWF project offer impressive proof that WSe2 possesses superior optoelectronic properties that create new possibilities for solar cells, photodiodes and light-emitting diodes.
Original publication: A. Pospischil, M. M. Furchi, und T. Mueller, Solar-energy conversion and light emission in an atomic monolayer p-n diode, Nature Nanotechnology (2014), dx.doi.org/10.1038/nnano.2014.14
Prof. Thomas Mueller
Vienna University of Technology
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Vienna, March 10th, 2014