The use of graphene as a transparent conducting film has been hotly pursued of late, in large part because it offers a potentially cheaper alternative to indium tin oxide (ITO) where a bottleneck of supply seems to be looming.
It has not been clear whether photovoltaic manufacturers have taken any interest in graphene as an alternative for transparent conducting films. This lack of interest may in part be the result of there being little research into whether graphene maintains its attractive characteristic of high carrier mobility when used in conjunction with silicon.
Now researchers at the Helmholtz Zentrum Berlin (HZB) Institute in Germany have shown that graphene does not lose its impressive conductivity characteristics even when mated with silicon.
"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 Marc Gluba of the HZB Institute for Silicon Photovoltaics in a press release.
The research, which was published in the journal Applied Physics Letters (“Embedded graphene for large-area silicon-based devices”), used the method of growing the graphene by chemical vapor deposition on a copper sheet and then transferring it to a glass substrate. This was then covered with a thin film of silicon.
The researchers experimented with two different forms of silicon commonly used in thin-film technologies: amorphous silicon and polycrystalline silicon. In both cases, despite completely different morphology of the silicon, 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," said Norbert Nickel, another researcher on the project, in a press release.
In their measurements, the researchers determined that the carrier mobility of the graphene layer was roughly 30 times greater than that of conventional zinc oxide-based contact layers.
Although the researchers concede that connecting the graphene-based contact layer to external contacts is difficult, it has garnered the interest of their thin-film technology colleagues. "Our thin film technology colleagues are already pricking up their ears and wanting to incorporate it,” Nickel adds.
Illustration: Marc A. Gluba/HZB
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.