Researchers at École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have for the first time observed and measured graphene converting a single photon into multiple electrons in a photovoltaic device. This work should buoy hopes that graphene can serve as a material for photovoltaics with very high energy-conversion efficiencies.
The discovery builds on work conducted last year by the Barcelona-based Institute of Photonic Science (ICFO). ICFO scientists were able to indirectly show that graphene is capable of converting one photon into multiple electrons. In that research, the team excited the graphene by exposing it to photons of different energies (colors). They then used a pulse of terahertz radiation to measure the resulting hot-electron distribution. They determined that a higher photon energy (violet) resulted in higher numbers of hot electrons than a lower photon energy (infrared).
In this most recent EPFL work, the researchers had to devise a way to measure the conversion process, which occurs on a femto-second scale (10-15 seconds). That’s far faster than any conventional method for detecting electron movement.
The team turned to a new technique called ultrafast time- and angle-resolved photoemission spectroscopy” (trARPES). The measurements themselves, the results of which were published in the journal Nano Letters, took place at the Rutherford Appleton Laboratory at Oxford University.
The graphene was placed in an ultra-high vacuum chamber where the material was then hit with an ultrafast “pump” pulse of laser light. The laser light excites the electrons in the graphene bringing them to a higher energy state . In this heightened state, the graphene is then hit with a time-delayed, “probe” pulse that serves to take a snapshot of the energy each electron has at that moment. By doing this numerous times, the researchers create a kind of stop-motion movie of the conversion process.
“This indicates that a photovoltaic device using doped graphene could show significant efficiency in converting light to electricity,” said Marco Grioni of EPFL in a press release.
While nanomaterials in photovoltaics have held out the promise of converting a single photon into multiple electrons in research dating back to 2004, there have been skeptics as to whether this ability will actually lead to higher conversion efficiencies.
Eran Rabani, a researcher at Tel Aviv University, back in 2011 declared that he was not so convinced by the research on electron multiplication.
“Our theory shows that current predictions to increase efficiencies won't work,” Rabani said in a press release at the time. “The increase in efficiencies cannot be achieved yet through Multiexciton Generation, a process by which several charge carriers (electrons and holes) are generated from one photon.”
This skepticism may account for why so much energy has been devoted to measuring and characterizing the generation of multiple electrons from a single photon.
But if multiple electron generation can—as some hope—boost conversion efficiency to 60 percent from what was thought to be a 32 percent limit, then proving that the event indeed occurs is well worth it.