One of the most important technologies for sustainable energy is photovoltaics, which converts light into power. We have known since the time of Max Planck and Albert Einstein that both light and electricity are made up of tiny quantized packets called photons and elementary charges, which are represented by electrons and holes.
Some substances generate more free charges from photons than would be predicted. Researchers have now captured an image of this process using an ultrafast film. The journal Nature has published their findings.
Better solar cells thanks to exciton splitting
Normally, only two free charges in the material get the energy of a single photon in a solar cell. A few molecules, such as pentacene, are an exception and demonstrate the conversion of one photon into four charges. Exciton fission, also known as excitation doubling, could be very beneficial for high-efficiency photovoltaics, specifically to replace the prevalent silicon-based technology.
Researchers from the Technical University of Berlin, the Fritz Haber Institute of the Max Planck Society, and the Julius-Maximilians-Universität Würzburg have now resolved a long-standing disagreement about the mechanism of the process by capturing an ultrafast movie of the photon-to-electricity conversion process.
“When pentacene is excited by light, the charges in the material rapidly react,” explains Prof. Ralph Ernstorfer, a senior author of the study. “It was an open and highly disputed question whether an absorbed photon excites two electrons and holes directly or initially just one electron-hole pair, which subsequently shares its energy with another charge pair.” Ernstorfer is head of a Max Planck research group at the Fritz Haber Institute and Professor of Experimental Physics at the Technical University of Berlin.
Snapshots of one billionth of a millionth of a second
Time- and angle-resolved photoemission spectroscopy, a state-of-the-art method for observing the dynamics of electrons on the femtosecond time scale—a billionth of a millionth of a second—was employed by the researchers to answer this mystery. They were able to take the first pictures of the transient energised electrons using our ultrafast electron movie camera.
“Seeing these charge carrier pairs was crucial to decipher the process,” says Alexander Neef, from the Fritz Haber Institute and the first author of the study. “An excited electron-hole pair not only has a specific energy but also adapts distinct patterns, which are called orbitals. To understand the process of singlet fission it is such essential to identify the orbital shapes of the charge carriers and how these change over time.”
Crucial for the use of organic semiconductors
The researchers were able to deconstruct the dynamics of the excited charge carriers for the first time based on their orbital features using the images from the ultrafast electron movie.
“We can now say with certainty that only one electron-hole pair is excited immediately after photon excitation and identified the mechanism of the free charge carrier-doubling process,” adds Alexander Neef.
“Resolving this initial step in exciton fission is essential to successfully implement this class of organic semiconductors in innovative photovoltaic applications and, thus, to further boost the conversion efficiency of today’s solar-cells,” states Prof. Jens Pflaum, whose group at the University of Würzburg has provided the high quality molecular crystals for this study.
Such a promotion will have huge impacts as solar energy and its generation by these third-generation cells will be a major energy source of the future.
