Electrical guardrails for charge carriers
HZDR team proposes a new approach to harness graphene for solar cells
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graphene is an ultrathin, highly conductive material that holds great promise for future electronic applications. Yet its ability to convert light into electricity is limited. Researchers at the Helmholtz-Zentrum Dresden- Rossendorf (HZDR) have proposed a new approach that combines a magnetic field with carefully controlled electrical voltages to separate the charge carriers generated when graphene absorbs light. As the team explains in the journal Physical Review Applied (DOI: 10.1103/k1y8-vr4y), the concept could open up new possibilities for solar cells, light sensors and graphene-based electronics.
A magnetic field perpendicular to the graphene sheet curves the paths of electrons and ‘holes’ (blue and orange). Voltages applied to the electrodes, shown in gold, create potential barriers that act as guardrails.
Copyright: B. Schröder/HZDR
Graphene is a single layer of carbon atoms in which electrons can move with high mobility, making it promising for high-speed electronic components. Yet it is not very efficient at converting light into electricity. While light absorbed by graphene does excite electrons, leaving behind positively charged carriers called ‘holes,’ the absorbed light energy cannot be harnessed effectively unless electrons and holes are separated and directed to different contacts.
Conventional solar cells use semiconductors such as silicon, whose bandgap plays a key role in converting light into electrical energy. Graphene does not have such a gap. “That’s why we have to come up with a different mechanism,” says Prof. Ralf Schützhold, director of the HZDR Institute of Theoretical Physics. One option is to cut or fold the graphene to create tiny edges. When light is absorbed by the material, creating charge carriers, a magnetic field applied to the graphene forces these carriers into circular paths. The edges act like guardrails, guiding the charge carriers in opposite directions – electrons and holes drift apart and are separated.
Voltage instead of edges
Creating such edges is challenging, however, and the approach is difficult to scale up to larger surfaces – a prerequisite for practical applications. That is why the HZDR team is proposing a new concept: “Instead of an edge, we propose using electrical voltages, generating potential barriers, as guardrails,” explains Schützhold. Specifically, the plan is to place narrow metal electrodes onto an insulating layer above or below the graphene. Applying voltages to the electrodes creates potential barriers in the graphene. Crucially, a magnetic field applied perpendicular to the graphene plane causes electrons and holes to follow circular paths in opposite directions. Together with the potential barriers, this produces directed motion: An electron traces an arc, is reflected at the barrier, and continues along another arc. A negative potential barrier reflects electrons and lets holes pass, while a positive barrier has the opposite effect. This way, electrons and holes are guided to opposite contacts.
In their study, the team calculated the conditions under which the mechanism should work. For instance, the barriers must be sufficiently high and wide to prevent electrons from simply passing through. The spacing between the electrodes must also be matched to the circular trajectories, whose size depends on both the strength of the magnetic field and the energy of the absorbed light. One advantage of the concept is its flexibility. “We can adjust the voltage at the electrodes,” Schützhold emphasizes. By changing the voltages or switching individual electrodes on and off, it should be possible to adapt the charge transport to different light energies. Moreover, arrays of electrodes could extend the concept to larger graphene surfaces without the need to cut or fold the material.
Prospects for infrared light harvesting
In the long term, this approach could prove particularly interesting for solar cells that utilize infrared radiation. The absence of a bandgap offers an opportunity here: Unlike silicon, graphene can also absorb infrared photons whose energy is too low to generate electron-hole pairs in conventional silicon solar cells. Other applications could include sensors for visible light or infrared radiation. Similarly, the electrodes could be used to define variable paths for electric charges – a prospect for graphene-based electronics.
A first experimental setup might look like a tiny sandwich: a layer of graphene between transparent insulating layers, with fine metal strips across the top, to which voltages in the volt range are applied. To convert infrared radiation into an electric current, the spacing should be in the micrometer range. According to the team's calculations, a magnetic field of around 0.1 tesla, which could be generated using permanent magnets, should be sufficient. Light absorption could be increased by stacking several graphene layers separated by transparent insulating layers and adding mirrors. The light would pass through the active layers several times, increasing the chances of absorption. The required components are available, and HZDR is planning experiments to assemble and test them. Schützhold is optimistic: “Even with graphene edges, the experiments confirmed our theoretical prediction – the signal was actually stronger than expected. That’s why I hope our new approach will also work.”
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