Possible breakthrough using graphene for solar cells

Graphene retains its exceptional conductivity and transparency when coated with silicon film
October 14, 2013

Graphene was deposited onto a glass substrate. The ultrathin layer is only one atomic layer thick (0.3 Angström, or 0.03 nanometers), but 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. (Credit: Marc A. Gluba/HZB)

Researchers at the Helmholtz-Zentrum Berlin (HZB) Institute for Silicon Photovoltaics have shown that graphene retains its impressive set of properties when it is coated with a thin silicon film.

These findings may allow for entirely new possibilities to use graphene in thin-film photovoltaics.

Graphene has extreme conductivity and is completely transparent while being inexpensive and nontoxic. This would makes it a perfect candidate material for transparent contact layers for use in solar cells to conduct electricity without reducing the amount of incoming light.

To test this possibility, the researchers grew graphene on a thin copper sheet,  transferred it to a glass substrate, and 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 glass; the other sample contained polycrystalline 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 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 HZB researcher Dr. Norbert 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.

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.

No forecasts of possible dates of commercial products are available.

Abstract of Applied Physics Letters paper

Macroscopic graphene films buried below amorphous and crystalline silicon capping layers are studied by Raman backscattering spectroscopy and Hall-effect measurements. The graphene films are grown by chemical vapor deposition on copper foil and transferred to glass substrates. Uncapped films possess charge-carrier mobilities of 2030 cm2/Vs at hole concentrations of 3.6 × 1012 cm−2. Graphene withstands the deposition and subsequent crystallization of silicon capping layers. However, the crystallinity of the silicon cap has large influence on the field-induced doping of graphene. Temperature dependent Hall-effect measurements reveal that the mobility of embedded graphene is limited by charged-impurity and phonon-assisted scattering.