Graphene has extreme conductivity and is completely transparent while being inexpensive and nontoxic, which makes it perfect for transparent contact layers for use in solar cells to conduct electricity without reducing the amount of incoming light.
That is theory, but the real world remains questionable because there is no such thing as “ideal” grapheme, which is a free floating, flat honeycomb structure consisting of a single layer of carbon atoms: Interactions with adjacent layers can change graphene’s properties dramatically.
“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,” said Dr. Marc Gluba of the HZB Institute for Silicon Photovoltaics in Berlin, Germany.
To this end, researchers 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 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 glass; 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 heating it to a temperature of several hundred degrees centigrade, 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,” said Prof. Dr. Norbert Nickel of the HZB Institute for Silicon Photovoltaics.
Their measurements of carrier mobility using the Hall-effect showed 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, it’s been a real challenge connecting this thin contact layer, which is but one atomic layer thick, to external contacts,” Gluba said. “We’re still having to work on that.”
“Our thin film technology colleagues are already pricking up their ears and wanting to incorporate it,” Nickel said. 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.