Graphene efficiently converts light into electricity
Converting light into electricity, as happens in solar cells and light detectors, can probably be realised more efficiently with graphene. Most materials convert a single light particle into a single electron, but in graphene a single light particle can be converted into several hot electrons. As these hot electrons can generate current this mechanism of charge multiplication is an essential ingredient for obtaining energy from light with a minimal loss of energy. Klaas-Jan Tielrooij, who performed his PhD research at FOM Institute AMOLF, has published about this in Nature Physics.
Graphene, a single atomic layer of carbon atoms, is an important candidate for new opto- electronic applications. The researchers have now demonstrated that the combination of the broadband absorption of graphene and charge multiplication ensures that graphene can efficiently convert sunlight from the entire solar spectrum into electricity. Up until now it had not been clear how efficiently and by which mechanism graphene converts photons into electrons.
Double pulse
Studying the conversion of photons (light particles) into electrons in graphene is particularly challenging, as the process of converting photos into electrons and the subsequent charge multiplication takes place at a timescale of femtoseconds (10-15 s, a million billionth of a second). The researchers therefore used a pump-probe technique in which use is made of two short successive pulses with an extremely high time resolution.
Tielrooij and his colleagues illuminated a single layer of graphene with a precisely known number of absorbed photons of a known photon energy (colour). Very shortly afterwards a Terahertz pulse was used to observe the number of hot electrons produced (electrons with a higher energy than in the equilibrium state). This technique revealed that for an equal number of absorbed photons, photons with high photon energy (e.g. violet) resulted in more hot electrons than photons with a low photon energy (e.g. infrared). This is the result of charge multiplication. In the mechanism of charge multiplication an initially excited electron loses its energy by transferring it to other electrons that therefore obtain more energy (above the Fermi level in the equilibrium state), as a result of which a large number of hot electrons are produced.
Klaas-Jan Tielrooij carried out his research at the Institut de Ciències Fotóniques in Barcelona and at FOM Institute AMOLF. The collaboration also involved researchers from the Massachusetts Institute of Technology in Cambridge, Massachusetts (US), the Max Planck Institute for Polymer Research in Mainz (Germany) and Graphenea S.L. in San Sebastian (Spain). Tielrooij's research abroad was made possible with a Rubicon grant from NWO. In 2011, Tielrooij won the FOM Physics Thesis Award.
Source
Press release of NWO
Reference
K.J. Tielrooij, J.C.W. Song, S.A. Jensen, A. Centeno, A. Pesquera, A. Zurutuza Elorza, M. Bonn, L.S. Levitov & F.H.L. Koppens, 'Photoexcitation cascade and multiple hot-carrier generation in graphene', Nature Physics 2013.