News: Electrifying Discoveries

Physicists at the University of Regensburg, in collaboration with colleagues from the University of Marburg, the University of Graz, and the Jülich Research Center, have achieved a scientific breakthrough.

For the first time, they were able to directly film and theoretically describe the generation of electrical energy from light. The results have far-reaching implications, particularly for the further development and optimization of sustainable technologies for energy generation using photovoltaics. They provide fundamental insights into the physical processes that occur, for example, in organic solar cells, thereby laying the groundwork for more efficient applications. The study was published in the journal *Physical Review X*, one of the world’s most prestigious physics journals.

Whether in solar cells or during photosynthesis: When light strikes a material, its particles—photons—are absorbed and excite electrons into an excited state. In the process, their spatial distribution changes. At the same time, a bond often forms between the excited, negatively charged electron and the positively charged electron hole it leaves behind. These bound electron-hole pairs, also known as excitons, play a central role in modern optoelectronic materials. Despite their great importance, their internal structure—described by the quantum mechanical wave function—has remained largely inaccessible experimentally until now.

Using a novel method called time-resolved orbital tomography, the researchers have now succeeded for the first time in reconstructing the spatial distribution and temporal evolution of an exciton’s wave function in the first few moments after its formation. The measurements show that the electron-hole pair initially spans about three molecules and shrinks by about 25 percent within the first 400 femtoseconds (i.e., 400 quadrillionths of a second). Marcel Theilen, the lead author of the publication and a doctoral student at the Regensburg Center for Ultrafast Nanoscopy (RUN) for the past year, explains: “To make these ultrafast processes visible, we first generate the excitons with a short visible light pulse, then break them apart again a few femtoseconds later with a high-energy laser pulse, causing the electron to be emitted. From the measured energy and angular distributions, we can then determine the wave function using theoretical models.” By deliberately varying the time interval between the excitation and probing pulses, we obtain individual snapshots that come together to form a kind of movie of the quantum world.

The work was carried out in close collaboration between three research groups that are international leaders in their respective fields. Stefan Tautz and his team at the Jülich Research Center produced the organic semiconductor samples, characterized them, and transported them to Marburg under ultra-high vacuum conditions. There, in Ulrich Höfer’s group, the time-resolved photoemission experiments were conducted—in other words, the “movies” of the exciton were recorded. At the Institute of Physics at the University of Graz, Peter Puschnig’s team developed the theoretical concepts for describing photoemission from excitons, performed the quantum mechanical simulations, and developed an analytical model that enabled the interpretation of the experimental data.

The current publication marks an important milestone in the ERC Synergy Project “Orbital Cinema”. A large part of this project, funded by the EU with 11.3 million euros, is based at the University of Regensburg. It is led there by Professor Ulrich Höfer, who has been an adjunct professor in the Department of Physics at the University of Regensburg since October 2022, and Professor Rupert Huber. Orbital Cinema aims to visualize the dynamics of electrons in materials with unprecedented spatial and temporal resolution.
To this end, Höfer and Huber, along with their colleagues at RUN in Regensburg, have set up a new laboratory where it will be possible not only to observe rapid processes within molecules, but also to control them using intense light fields. In addition, the time resolution of the Marburg experiment is expected to be surpassed by two further orders of magnitude. “In my opinion, the RUN in Regensburg is currently the best place in the world to actually carry out such an ambitious project—one that some colleagues consider to be in the realm of science fiction,” says Höfer, adding: “I am very grateful for the opportunity to continue conducting active research here after my retirement in Marburg and thereby push the boundaries of our knowledge of the nanocosmos a little further.”

Further information: https://orbital-cinema.eu/ and https://run-regensburg.de/ (external link, opens in a new window)

Publication
Observing the spatial and temporal evolution of exciton wave functions in organic semiconductors
Marcel Theilen, Siegfried Kaidisch, Monja Stettner, Sarah Zajusch, Eric Fackelman, Alexa Adamkiewicz, Robert Wallauer, Andreas Windischbacher, Christian S. Kern, Michael G. Ramsey, François C. Bocquet, Serguei Soubatch, F. Stefan Tautz, Ulrich Höfer, and Peter Puschnig
Physical Review X, August 2026
 

https://doi.org/10.1103/3zmg-276c (external link, opens in a new window)