Graphene Breakthrough has Huge Solar Implications
February 26, 2013
The most recent addition to the long list of the amazing properties of graphene was announced in Nature Physics in a paper authored by ICFO researchers, in collaboration with researchers from MIT, Max Planck Institute for Polymer Research, and Graphenea S.L. The paper demonstrates that graphene is able to convert a single photon that it absorbs into multiple hot electrons, and that the higher photon’s energy, the larger the number of hot electrons created. Since these light-induced electrons can drive currents, hot-electron multiplication is an essential ingredient for light harvesting with very low energy loss. Moreover, the combination of broadband absorption and hot-carrier multiplication enables graphene to efficiently convert light energy from the full solar spectrum into electricity.
A new discovery by researchers at the Institute of Photonic Science (ICFO) has revealed that graphene is even more efficient at converting light into electricity than previously known. Graphene is capable of converting a single photon of light into multiple electrons able to drive electric current. The discovery is an important one for next-generation solar cells, as well as other light-detecting and light-harvesting technologies.
A paradigm shift in the materials industry is likely within the near-future as a variety of unique materials replaces those that we commonly use today, such as plastics. Among these new materials, graphene stands out. The single-atom-thick sheet of pure carbon has an enormous number of potential applications across a variety of fields. Its potential use in high-efficiency, flexible, and transparent solar cells is among the potential applications. Some of the other most discussed applications include: foldable batteries/cellphones/computers, extremely thin computers/displays, desalination and water purification technology, fuel distillation, integrated circuits, single-molecule gas sensors, etc.
“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,” says Frank Koppens, group leader at ICFO.
This ability makes graphene extremely appealing for any technology that requires the conversion of light into electricity, particularly because it allows the development of light detectors with improved efficiency, and should lead to solar cells that are able to capture light energy from all of the solar spectrum with lower loss.
The discovery was made during an experiment that consisted of sending an exact quantity of photons possessing different energies (different colors) onto a monolayer of graphene. “We have seen that high energy photons (e.g. violet) are converted into a larger number of excited electrons than low energy photons (e.g. infrared). 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!” says KJ Tielrooij, a researcher at ICFO.
There are some issues with graphene that need to be resolved before they can be used for ‘direct applications’ though. But once these are resolved, graphene holds a revolutionary potential, especially with regards to technologies 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,” Koppens says in conclusion.