Tungsten diselenide optoelectronics advanced by three research groups

MIT, TU Vienna, and UW researchers have made recent advances on PV cells, LEDs, and other optoelectronic components made possible using ultrathin tungsten diselenide.

MIT research

One of three papers from different groups describing similar results with the dichalcogenide material tungsten diselenide (WSe₂) has been published in the March 9 advance online edition of Nature Nanotechnology. The published research from the Massachusetts Institute of Technology (MIT; Cambridge, MA) describes how the material, just a few atoms thick, can create devices that can harness or emit light. This proof-of-concept could lead to ultrathin, lightweight, and flexible photovoltaic (PV) cells, light emitting diodes (LEDs), and other optoelectronic devices, they say. The MIT research was carried out by Pablo Jarillo-Herrero, the Mitsui Career Development Associate Professor of Physics, graduate students Britton Baugher and Yafang Yang, and postdoc Hugh Churchill.

Typically, diodes (which allow electrons to flow in only one direction) are made by "doping," which is a process of injecting other atoms into the crystal structure of a host material. By using different materials for this irreversible process, it is possible to make either of the two basic kinds of semiconducting materials, p-type or n-type. But with the new material, either p-type or n-type functions can be obtained just by bringing the vanishingly thin film into very close proximity with an adjacent metal electrode, and tuning the voltage in this electrode from positive to negative. That means the material can easily and instantly be switched from one type to the other, which is rarely the case with conventional semiconductors.

In their experiments, the MIT team produced a device with a sheet of WSe₂ material that was electrically doped half n-type and half p-type, creating a working diode that has properties "very close to the ideal," Jarillo-Herrero says. By making diodes, it is possible to produce all three basic optoelectronic devices--photodetectors, photovoltaic cells, and LEDs; the MIT team has demonstrated all three, Jarillo-Herrero says. While these are proof-of-concept devices, and not designed for scaling up, the successful demonstration could point the way toward a wide range of potential uses, he says.

In principle, Jarillo-Herrero says, because this material can be engineered to produce different values of a key property called bandgap, it should be possible to make LEDs that produce any color—something that is difficult to do with conventional materials. And because the material is so thin, transparent, and lightweight, devices such as solar cells or displays could potentially be built into building or vehicle windows, or even incorporated into clothing, he says. While selenium is not as abundant as silicon or other promising materials for electronics, the thinness of these sheets is a big advantage, Churchill points out: "It's thousands or tens of thousands of times thinner" than conventional diode materials, "so you'd use thousands of times less material" to make devices of a given size.

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TU Vienna research

In their Nature Nanotechnology paper, Vienna University of Technology (TU Vienna; Vienna, Austria) researchers have created a diode made of tungsten diselenide. Experiments show that this material may be used to create ultrathin flexible solar cells and even flexible displays could become possible.

The layer is so thin that 95% of the light just passes through—but a tenth of the remaining 5%, which are absorbed by the material, are converted into electrical power. Therefore, the internal efficiency is quite high. A larger portion of the incident light can be used if several of the ultrathin layers are stacked on top of each other—but sometimes the high transparency can be a useful side effect. "We are envisioning solar cell layers on glass facades, which let part of the light into the building while at the same time creating electricity," says Thomas Mueller.

UW research

University of Washington (UW; Seattle, WA) scientists have built the thinnest-known LED that can be used as a source of light energy in electronics. The LED is based off of two-dimensional, flexible semiconductors, making it possible to stack or use in much smaller and more diverse applications than current technology allows.

"We are able to make the thinnest-possible LEDs, only three atoms thick yet mechanically strong. Such thin and foldable LEDs are critical for future portable and integrated electronic devices," said Xiaodong Xu, a UW assistant professor in materials science and engineering and in physics.

The UW's LED is made from flat sheets of the molecular semiconductor known as tungsten diselenide, a member of a group of two-dimensional materials that have been recently identified as the thinnest-known semiconductors. Researchers use regular adhesive tape to extract a single sheet of this material from thick, layered pieces in a method inspired by the 2010 Nobel Prize in Physics awarded to the University of Manchester for isolating one-atom-thick flakes of carbon, called graphene, from a piece of graphite.

SOURCES: MIT, TU Vienna, and UW; http://web.mit.edu/newsoffice/2014/two-dimensional-material-shows-promise-for-optoelectronics-0310.html, https://www.tuwien.ac.at/en/news/news_detail/article/8679/, and http://www.washington.edu/news/2014/03/10/scientists-build-thinnest-possible-leds-to-be-stronger-more-energy-efficient/?utm_source=rss&utm_medium=rss&utm_campaign=scientists-build-thinnest-possible-leds-to-be-stronger-more-energy-efficient, respectively