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EETE MAR 2015

materials Graphene polymer speeds electron transport By R. Colin Johnson Graphene is considered by many as the successor to silicon because its electron mobility can be over 10X that of silicon plus it solves many of the problems with scaling silicon. However, graphene’s lack of the bandgap it needs to create transistors has slowed its development. Now researchers propose coating it with a conductive polymer to produce organic electronics that rival silicon at a fraction of the price. Researchers at the Umeå University (Sweden) in collaboration with Stanford University and its Synchrontron Radiation Lightsource at SLAC (formerly the Stanford Linear Acceleration Center) created prototypes of the new hybrid graphene/ polymer material with remarkable results. Besides greatly accelerating the polymer, making it into a high-mobility semiconductor, the hybrid material maintained its Diagram of why experiments at the Stanford Synchrotron Radiation Lightsource showed a thicker film (top) had significantly higher electron mobility than thinner polymer films (bottom) by allowing more crystal-to-crystal pathways for conduction. (Source: Umeå University) flexibility and seemed to work both for planar electronics and vertically oriented conduction for light-emitting diodes (LEDs) and solar cells. “The polymer film itself is flexible and the graphene layer is also flexible by itself. Our experimental glass and silicon substrates are rigid, so our overall stack was rigid, but we can make the same graphene/polymer thin layers on a flexible substrate and then the whole stack is flexible, so it can be applied to make flexible devices,” professor David Barbero at Umeå University told EETimes as leader of the international research team that performed the experiments at Stanford’s SLAC. Graphene is difficult to deposit across a whole wafer, but Barbero’s team found a way to easily grow the graphene monolayer on metal, then transfer it to nearly any substrate--silicon and glass for the experiments--but also to other polymers for flexible applications. “A large-area single layer graphene was synthesized on a copper foil by chemical vapor deposition (CVD) using a vertical quartz tube. The graphene layer was then spin-coated with a polymer layer (poly-methyl meth-acrylate, PMMA), and the copper was etched in ammonium persulfate, and the copper residues removed by wet cleaning. Finally, the graphene was floated in deionized water and transferred to a silicon or glass substrate, dried and the PMMA was dissolved to leave a clean monolayer of graphene,” Barbero told us. “The next step was to deposit the semiconducting polymer on top of the graphene. This was done by spin-coating from a dilute solution until dry, and resulted in a uniform thin film of well-defined thickness.” When characterizing the material, the researchers made the remarkable finding that the electron mobility properties of the hybrid material were enhanced by depositing the semiconducting polymer as a slightly thicker film, which is just the opposite of other thin films. In fact, by making the polymer semiconductor about 50 nanometers thick, its electron mobility was likewise boosted about 50 times over the same polymer film deposited at 10 nanometers thick. The researchers speculated that the thicker film gave the randomly oriented crystals it more crystal-to-crystal pathways than a thinner film (unlike single-crystal films which whose crystals are side-byside). Besides finding that the performance was even better than growing graphene on silicon, another notable finding was that conduction in the axis vertical to the surface was just as good, making the transparent material a good candidate for optical devices such as inexpensive photovoltaic (PV) solar cells and LEDs. “The results show that there is potential for opto-electronic applications, and it would indeed be nice to build a more efficient photovoltaic solar cell based on a graphene monolayer, which would moreover make the device be lightweight, superthin and flexible,” Barbero told us. Since the initial discovery, the researchers have been experimenting with different semiconducting polymers atop graphene with the goal of exceeding the performance of standard silicon chips as well as improving the performance of all sorts of plastic photovoltaic and photonic devices. Semiconducting polymers atop a graphene monolayer increase their electron mobility dramatically. (Source: Umeå University) 12 Electronic Engineering Times Europe March 2015 www.electronics-eetimes.com


EETE MAR 2015
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