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James Tour, a chemistry professor at Rice University, says that this is “certainly the easiest method I’ve seen for making [graphene thin films] over large areas.” He thinks that the process could easily be converted into a larger, commercial-scale manufacturing technique. “It’s very amenable for rapid production,” he says. “It’s not going to take much to get these things produced … and cover large areas.”

Chhowalla and his colleagues control the thickness of the film by changing the suspension’s volume. A volume of 20 milliliters results in a film that is mostly one to two nanometers in thickness, while an 80-milliliter suspension results in films that are mainly three to five nanometers thick. The thinner films are 95 percent transparent. The researchers have used the films as the transparent electrodes in organic solar cells. They have also made transistors by placing their films on a silicon substrate and depositing gold electrodes on them.

The graphene films need a lot more work. Right now, the transistors do not carry as much current as those made from individual graphene flakes, which, the researchers speculate, is because of overlapping flakes in their films. For high-quality transistors, they will need to make single-layer graphene films with no overlap. They also need to improve the conductivity of their film: indium tin oxide is still hundreds of times more conductive. Organic solar cells with indium tin oxide electrodes are between 3 percent and 5 percent efficient. “With graphene thin-film electrodes, we get 0.1 percent,” Chhowalla says, “but these are proof-of-concept devices and of course will improve with time.”

Tour believes that the film holds more promise for organic solar cells than for transistors. Many researchers are also studying carbon nanotube films as a way to replace indium tin oxide coatings on solar cells. But Tour says that graphene would be “possibly easier than using carbon nanotubes because of the greater availability of the material.” The industry might also find it easier to adopt graphene because of the concerns that some people have about the effects of carbon nanotubes on the environment.

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Credit: Manish Chhowalla, Rutgers University

Tagged: Computing, Materials, solar cells, transistors, graphene, carbon, thin films

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