Graphene is a single layer of carbon atoms in which electrons have high mobility. This makes it promising for high-speed electronics, but it is poor at converting light into electricity. Light in graphene excites electrons, leaving positively charged "holes," but the energy cannot be used until electrons and holes are separated and directed to different contacts.
Conventional silicon solar cells rely on a band gap - a range of energies in which there are no allowed states for electrons. Graphene has no such gap. "That is why we have to come up with a different mechanism," says Ralph Schuetzhhold, director of the Institute of Theoretical Physics at HZDR (Helmholtz-Zentrum Dresden-Rossendorf, Germany).
One option is to cut or fold graphene, creating tiny edges. In a magnetic field applied to the material, charges begin to move along circular trajectories, and the edges act as bumpers, directing electrons and holes in opposite directions. But such edges are difficult to create, and the method does not scale well to large areas, which is necessary for practical use.
The HZDR team therefore proposes a new concept: instead of edges, use electrical voltages that create potential barriers. Narrow metal electrodes are placed above or below the graphene on an insulating layer. Applying a voltage creates barriers in the graphene that act as bumpers.
A magnetic field directed perpendicular to the plane of the graphene forces electrons and holes to move along arcs in opposite directions. Combined with the barriers, this produces directed motion: an electron traces an arc, reflects off a barrier and continues along another arc. A negative barrier reflects electrons and lets holes through, while a positive barrier does the opposite. In this way, the charges reach opposite contacts.
In a study published in the journal Physical Review Applied, the scientists calculated the conditions under which the mechanism works. The barriers must be sufficiently high and wide so that electrons do not pass through them. The distance between the electrodes must be matched to the size of the circular trajectories, which depend on the strength of the magnetic field and the energy of the absorbed light.
The flexibility of the concept lies in the ability to tune the voltage on the electrodes. By changing the voltage or switching individual electrodes on and off, charge transport can be adjusted for different light energies. In addition, arrays of electrodes will make it possible to cover large areas of graphene without cuts or folds.
In the long term, the approach could be particularly interesting for solar cells that use infrared radiation. The absence of a band gap is an advantage: unlike silicon, graphene is able to absorb infrared photons whose energies are too small to create electron-hole pairs in conventional silicon cells. Other possible applications are visible- and infrared-light sensors, as well as controlled charge trajectories in graphene electronics.
The first experimental setup might look like a miniature sandwich: a layer of graphene between transparent insulating layers, with thin metal strips on top to which a voltage of a few volts is applied. To convert infrared radiation into current, the distance between the electrodes must be on the order of a micrometer. According to the team's calculations, a magnetic field of about 0.1 tesla, which can be created by permanent magnets, is sufficient.
Light absorption can be enhanced by stacking several layers of graphene with transparent insulators and adding mirrors: light will pass through the active layers several times, increasing the chances of absorption. The necessary components exist, and HZDR plans to build and test the setup. Schuetzhhold is hopeful of success: "Even with graphene edges, experiments confirmed our theoretical prediction - the signal turned out to be even stronger than expected."