Graphene

Print edition : December 13, 2013

Graphene was deposited onto a glass substrate. The ultra-thin layer is but one atomic layer thick although charge carriers are able to move about freely within this layer. This property is retained even if the graphene layer is covered with amorphous or polycrystalline silicon. Photo: Marc A. Gluba/HZA

GRAPHENE has extreme conductivity and is completely transparent while being inexpensive and non-toxic. This makes it a perfect candidate material for use in solar cells because it can 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, Marc Gluba and Norbert Nickel of the HZB Institute for Silicon Photovoltaics, Germany, have shown that graphene retains its impressive set of properties when it is coated with a thin silicon film.

The scientists examined how graphene’s conductive properties changed when it was incorporated into a stack of layers similar to a silicon-based thin-film solar cell and found that these properties actually changed very little.

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 were in a disordered state similar to a hardened molten glass; the other sample contained polycrystalline silicon to help them observe the effects of a standard crystallisation 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 Celsius, the graphene was 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 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. “Admittedly,” says Gluba, “it’s been a real challenge connecting this thin contact layer, which is but one atomic layer thick, to external contacts. We still have to work on that.” This work was recently published in the journal Applied Physics Letters.

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