A bright future for optoelectronics

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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 LEDs.

The related research paper, published in Nature Nanotechnology, not only documents the functionality of a p-n diode made of tungsten diselenide, it also demonstrates its usefulness for numerous applications.

These findings, obtained through an Austrian Science Fund FWF project, constitute significant progress on the path to 2D optoelectronics.

Electronic devices require semiconductors which are usually made from crystalline silicon whose three-dimensional crystals not only combine low flexibility with weight but 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 and can be produced economically on a large scale as well as being flexible yet exhibiting 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, laying the foundation for radical changes in optoelectronics.

A gap in the result

The starting material used by Prof. Thomas Mueller's team was tungsten diselenide (WSe2).

It has one major advantage over graphene, the most well-known 2D crystalline material at present, as Prof. Mueller explained: "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 WSe2 was present in a 2D layer, it was mechanically ‘peeled' from three-dimensional crystals in such a way layers with a thickness of just 0.7 nanometers were created.

As Prof. Mueller explained: "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 researchers had achieved the desired result.

The monolayer WSe2 was placed between two electrodes and the electrical characteristics measured. This unambiguously confirmed its function as a p-n diode: it was possible to inject both positive (p, hole) and negative (n, electron) charges, with current flow exclusively in one direction, as is usual in diodes.

Thin success

"WSe2 in monolayer crystalline form is theoretically an ideal starting material for p-n diodes and optoelectronics," said Prof. Mueller, "but no one had ever proven it. We have done just that. We measured an efficiency of 0.5% in converting light to electrical energy."

The high transparency, at 95%, 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 – 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 the FWF project offer impressive proof WSe2 possesses superior optoelectronic properties that create new possibilities for solar cells, photodiodes and LEDs.