Graphene, a sheet of carbon atoms arranged in a honeycomb-like lattice just one atom thick, has a number of unique electronic and mechanical properties. These come thanks to the fact that electrons whiz through graphene at extremely high speeds, behaving like "Dirac" particles with little resistance. Graphene is also transparent to light and thanks to its Dirac electrons, can absorb light of any colour.

Although researchers have made solar cells from graphene before now, the power conversion efficiencies of these devices was quite low at around 1.9%. In the new cells made by Sefaattin Tongay and colleagues, a graphene sheet doped with the organic compound (trifluoromethanesulphonyl)amide, or TFSA, was placed atop a silicon wafer to make a graphene/silicon Schottky junction.

Such photovoltaic devices work by producing electron-holes pairs when exposed to sunlight. The electrons and holes are then separated by the Schotky interface and collected by electrodes contacted onto the oppositely charged graphene and silicon. The current produced by the flowing electrons and holes allows the device to generate power.

Readjusting charges

Doping graphene with TFSA changes the Fermi level of the graphene, something that has the effect of readjusting the charges at the graphene/silicon junction, explains Tongay. This increases the strength of the electric field across the interface and allows electrons and holes to be collected more efficiently, ultimately leading to an increase in the amount power generated.

“The final device is robust, cheap and easy to make,” he told nanotechweb.org. “What is more, transferring graphene onto silicon causes minimal disturbance at the graphene surface, so the interface remains pristine.” A clean interface is important because any disorder in this area acts as a trap for separated charges, so reducing their lifetime, which means that they cannot be collected as efficiently.

The team, which reports its work in Nano Letters, is now busy trying to push the record power efficiency of its graphene solar cells even higher. “We are trying out different organic coating materials that have potentially higher doping effects and coming up with varied device structures,” revealed Tongay. “We hope to publish new results on this new work in the very near future.”