Graphene solar cells are now one step closer to reality, thanks to a new discovery made by researchers at the HZB Institute for Silicon Photovoltaics. The discovery: the many impressive properties of graphene, such as extreme conductivity and “complete” transparency for example, are apparently completely unaffected by the application of thin silicon film. The discovery means that thin-film photovoltaics which utilize graphene’s many great qualities could be just off the horizon.
Graphene is considered by many researchers to be a “near perfect” candidate material for the transparent contact layers used in solar cells — thanks to the material’s ability “to conduct electricity, without reducing the amount of incoming light.” That’s what’s been theorized anyway — until the material is tested in real-world environments, there are unknowns. This new research now brings the day when graphene can be be tested for this purpose, in real-world conditions, that much closer.
Graphene was deposited onto a glass substrate. The ultrathin layer is but one atomic layer thick (0.3 Angström, or 0.03 nanometers), 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.
Image Credit: Marc A. Gluba/HZB
“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,” explains researcher Marc Gluba.
The press release from the Helmholtz-Zentrum Berlin für Materialien und Energie provides details on the research:
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 are in a disordered state similar to a hardened molten glas; 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 being heated to a temperature of several hundred degrees C, 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,” states researcher Norbert Nickel. “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.”
Gluba adds: “Admittedly, it’s been a real challenge connecting this thin contact layer, which is but one atomic layer thick, to external contacts. We’re still having to work on that.”
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