Electrical guardrails for charge carriers

Researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have introduced a novel method to enhance the efficiency of graphene in solar cells by using a magnetic field combined with electrical voltages to separate charge carriers. This approach, detailed in the journal Physical Review Applied, could significantly impact the development of solar cells, light sensors, and other graphene-based electronics.

Graphene, a single layer of carbon atoms, is known for its high electron mobility, making it suitable for high-speed electronics. However, its efficiency in converting light to electricity is limited. When graphene absorbs light, it excites electrons and creates positively charged 'holes.' For effective energy conversion, these electrons and holes must be separated and directed to different contacts. Unlike silicon, which uses a bandgap to convert light to electricity, graphene lacks such a gap, necessitating alternative mechanisms.

Prof. Ralf Schützhold, director of the HZDR Institute of Theoretical Physics, explains that one method involves creating tiny edges in graphene to guide charge carriers. However, this method is challenging to scale. The HZDR team proposes using electrical voltages to create potential barriers as 'guardrails' instead of physical edges. By placing narrow metal electrodes on an insulating layer above or below the graphene and applying voltages, potential barriers form. A perpendicular magnetic field then causes electrons and holes to follow circular paths in opposite directions, effectively separating them.

The team calculated the necessary conditions for this mechanism to work, such as the height and width of the barriers and the spacing between electrodes. These factors depend on the magnetic field's strength and the absorbed light's energy. The concept's flexibility allows for adjusting electrode voltages to adapt to different light energies. Moreover, arrays of electrodes could extend this method to larger graphene surfaces without needing to cut or fold the material.

In the long term, this approach could be particularly beneficial for solar cells that harvest infrared radiation. Graphene's lack of a bandgap allows it to absorb infrared photons, which silicon cannot. Other potential applications include sensors for visible and infrared light and defining variable paths for electric charges in graphene-based electronics.

An experimental setup might involve a graphene layer sandwiched between transparent insulating layers with fine metal strips on top, where voltages are applied. The spacing should be in the micrometer range to convert infrared radiation into an electric current. The team suggests a magnetic field of about 0.1 tesla, achievable with permanent magnets, would suffice. Light absorption could be enhanced by stacking multiple graphene layers with transparent insulating layers and mirrors to increase absorption chances. HZDR plans to conduct experiments to test this setup, and Schützhold is optimistic about the outcomes, citing previous successful experiments with graphene edges.

The study, authored by J. Josiek, F. Queisser, S. Winnerl, and R. Schützhold, is titled 'Magnetophotoelectric effect in graphene via tailored potential landscapes' and is published in Physical Review Applied.