Two-dimensional material shows promise for optoelectronics

In the team's experimental setup, electricity
was supplied to a tiny piece of tungsten selenide
(small rectangle at center) through two gold wires
(from top left and right), causing it to emit light
(bright area at center), demonstrating its potential
as an LED material. IMAGE COURTESY OF HUGH
CHURCHILL AND FELICE FRANKE

Team creates LEDs, photovoltaic cells, and light detectors using novel one-molecule-thick material.

A team of MIT researchers has used a novel material that’s just a few atoms thick to create devices that can harness or emit light. This proof-of-concept could lead to ultrathin, lightweight, and flexible photovoltaic cells, light emitting diodes (LEDs), and other optoelectronic devices, they say.

Their report is one of three papers by different groups describing similar results with this material, published in the March 9 issue of Nature Nanotechnology. 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.

The material they used, called tungsten diselenide (WSe2), is part of a class of single-molecule-thick materials under investigation for possible use in new optoelectronic devices — ones that can manipulate the interactions of light and electricity. In these experiments, the MIT researchers were able to use the material to produce diodes, the basic building block of modern electronics.

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.

“It’s known how to make very large-area materials” of this type, Churchill says. While further work will be required, he says, “there’s no reason you wouldn’t be able to do it on an industrial scale.”

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.

In addition to the diodes the team has produced, the team has also used the same methods to make p-type and n-type transistors and other electronic components, Jarillo-Herrero says. Such transistors could have a significant advantage in speed and power consumption because they are so thin, he says.

The research was supported by the U.S. Office of Naval Research, by a Packard fellowship, and by a Pappalardo fellowship, and made use of National Science Foundation-supported facilities.

Source: http://web.mit.edu/newsoffice/2014/two-dimensional-material-shows-promise-for-optoelectronics-0310.html

Major leap towards graphene for solar cells

Surprising result: Graphene retains its properties even when coated with silicon

Graphene has extreme conductivity and is completely transparent while being inexpensive and nontoxic. This makes it a perfect candidate material for transparent contact layers for use in solar cells to conduct electricity without reducing the amount of incoming light  - at least in theory. Whether or not this holds true in a real world setting is questionable as there is no such thing as "ideal" graphene - a free floating, flat honeycomb structure consisting of a single layer of carbon atoms: interactions with adjacent layers can change graphene's properties dramatically. Now, Dr. Marc Gluba and Prof. Dr. Norbert Nickel of the HZB Institute for Silicon Photovoltaics have shown that graphene retains its impressive set of properties when it is coated with a thin silicon film. These findings have paved the way for entirely new possibilities to use in thin-film photovoltaics.

"We examined how graphene's conductive properties change if it is incorporated into a stack of layers similar to a silicon based thin film solar cell and were surprised to find that these properties actually change very little," Marc Gluba explains.

To this end, they grew graphene on a thin copper sheet, next transferred it to a glass substrate, and finally coated it with a thin film of silicon. They examined two different versions that are commonly used in conventional silicon thin-film technologies: one sample contained an amorphous silicon layer, in which the silicon atoms are in a disordered state similar to a hardened molten glas; the other sample contained poly-crystalline silicon to help them observe the effects of a standard crystallization process on graphene's properties.

Even though the morphology of the top layer changed completely as a result of being heated to a temperature of several hundred degrees C, the graphene is still detectable.

"That's something we didn't expect to find, but our results demonstrate that graphene remains graphene even if it is coated with silicon," says Norbert Nickel. Their measurements of carrier mobility using the Hall-effect showed that the mobility of charge carriers within the embedded graphene layer is roughly 30 times greater than that of conventional zinc oxide based contact layers. Says Gluba: "Admittedly, it's been a real challenge connecting this thin contact layer, which is but one atomic layer thick, to external contacts. We're still having to work on that." Adds Nickel: "Our thin film technology colleagues are already pricking up their ears and wanting to incorporate it."

The researchers obtained their measurements on one square centimeter samples, although in practice it is feasible to coat much larger areas than that with graphene.

This work was recently published in Applied Physics Letters Vol. 103, 073102 (2013).
Authors: M. A. Gluba, D. Amkreutz, G. V. Troppenz, J. Rappich, and N. H. Nickel

Source: http://www.helmholtz-berlin.de/pubbin/news_seite?nid=13825&sprache=en&typoid=1