Graphene's solar cell potential revealed by photon absorption

The discovery - made by researchers at the Institute of Photonic Science (ICFO), in collaboration with Massachusetts Institute of Technology, Max Planck Institute for Polymer Research, and Graphenea S.L. Donostia-San Sebastian - demonstrates that graphene is able to convert a single photon that it absorbs into multiple electrons that could drive electric current.

The discovery makes graphene an important alternative material for light detection and harvesting technologies, which are currently based on conventional semiconductors like silicon.

‘In most materials, one absorbed photon generates one electron, but in the case of graphene, we have seen that one absorbed photon is able to produce many excited electrons, and therefore generate larger electrical signals’ said Frank Koppens, group leader at ICFO.

This feature is claimed to make graphene an ideal building block for any device that relies on converting light into electricity. In particular, it enables efficient light detectors and potentially solar cells that can harvest light energy from the full solar spectrum with lower loss.

The experiment, published in Nature Physics,consisted of sending a known number of photons with different energies (different colors) onto a monolayer of graphene.

‘We have seen that high energy photons are converted into a larger number of excited electrons than low energy photons. The observed relation between the photon energy and the number of generated excited electrons shows that graphene converts light into electricity with very high efficiency. Even though it was already speculated that graphene holds potential for light-to-electricity conversion, it now turns out that it is even more suitable than expected,’ explained KJ Tielrooij, a researcher at ICFO.

Although there are some issues for direct applications, such as graphene’s low absorption, graphene holds the potential to cause radical changes in many technologies that are currently based on conventional semiconductors.

‘It was known that graphene is able to absorb a very large spectrum of light colors. However now we know that once the material has absorbed light, the energy conversion efficiency is very high. Our next challenge will be to find ways of extracting the electrical current and enhance the absorption of graphene. Then we will be able to design graphene devices that detect light more efficiently and could potentially even lead to more efficient solar cells,’ said Koppens in a statement.