Theoretically expected, it was now experimentally proven by scientists for the first time that the fascinating new material graphene is also highly efficient at converting light into electricity–which makes it an ideal candidate to boost the sensitivity of imaging sensors and also to increase the maximum conversion efficiency of photovoltaic cells.
(Photo : Mitchell Ong, Stanford School )
This illustration shows lithium atoms (in red) adsorbed to a layer of graphene to create electricity when the graphene is bent, squeezed or twisted.
Current materials used for these applications include silicon and gallium arsenide, but they just generate a single electron for each photon absorbed. Since a photon contains more energy than one electron can carry, much of the energy contained in the incoming light is lost as heat. Graphene on the other hand can generate multiple electrons from absorbing one photon, according to theoretical research that was now confirmed in the lab as described this week in Nature Physics.
Previous work had inspired hope that graphene had this property, says Frank Koppens, a group leader at the Institute of Photonic Sciences in Spain, who led the research. To conduct the experiment, the researchers used two ultrafast light pulses. The first sent a known amount of energy into a single layer of graphene. The second served as a probe that counted the electrons the first one generated.
Koppens said he is “reasonably confident” that the group can enhance the performance of light sensors like those used in cameras, night vision goggles, and certain medical sensors quite soon–after all, his lab is already working on a prototype device to demonstrate the new found capability of graphene.
A second but more difficult application would be solar cells. The material could help to increase the theoretical efficiency limit to about 60%, about twice as much as the 30% limit possible with today’s silicon cells, which currently reach about 20% in the field and 25% in the lab. But Koppens cautions that key engineering challenges stand in the way of that, which includes figuring out how to extract power from a system at all.
The new paper illustrates a “very important concept,” since future devices will depend on an understanding of the physical processes that occur when graphene absorbs light, says says he and colleagues have a still-unpublished paper that describes a similar result. Demonstrating this property in graphene opens a promising new field of research, he says.
Graphene was already exciting as a photovoltaic material because of its unique optical properties, says Andrea Ferrari, a professor of nanotechnology at the University of Cambridge in the U.K. who was not involved in this research. The material “can work with every possible wavelength you can think of,” he says. “There is no other material in the world with this behavior.” It is also flexible, robust, relatively cheap, and easily integrated with other materials. The new research “adds a third layer of interest to graphene for optics,” he says.
Among Koppens’s collaborators were MIT physics professor Leonid Levitov and Justin Chien Wen Song, a graduate student in Levitov’s lab, who helped Koppens interpret the data through theoretical modeling.