Ultrafast snapshots reveal the first moments on the way from light to electricity

Physicists at the University of Graz, collaborating with researchers from Marburg University and Forschungszentrum Jülich in Germany, have made a significant scientific breakthrough. For the first time, they have captured and described the formation of electrical energy from light through high-speed imaging and theoretical analysis.

This achievement holds great potential for enhancing sustainable energy production, particularly in photovoltaic systems such as organic solar cells. The study, published in the journal Physical Review X, provides essential insights into the fundamental processes occurring within these materials.

When light interacts with a surface, its photons are absorbed by the material, energizing electrons and forming excitons—electron-hole pairs that play a crucial role in modern optoelectronic materials. Previous studies, however, had not thoroughly examined the internal quantum-mechanical properties of excitons. In this latest research, the scientists succeeded in experimentally reconstructing the spatial distribution and temporal evolution of an exciton's wave function during its brief existence.

Peter Puschnig, a professor at the University of Graz, explained that the measurements revealed the electron-hole pair extending across roughly three molecules before shrinking by around 25% within 400 femtoseconds—the smallest unit of time in the study.

To observe this rapid process, the researchers employed a technique called Photoemission Orbital Tomography (POT). They first excited the exciton with an ultrashort light pulse and then ejected the electrons from the electron-hole pair using a second high-energy laser pulse. By measuring the energy and direction of the ejected electrons, the team could infer the exciton's quantum-mechanical state.

By altering the time delay between the initial excitation and the subsequent laser pulse, they created a series of snapshots that could be compiled into a video of the quantum world.

The experiments were conducted in collaboration with three leading research groups, each contributing crucial components to the project. Stefan Tautz and his team at Forschungszentrum Jülich prepared and characterized the organic semiconductor samples, which were then transported to Marburg under ultra-high vacuum conditions to preserve their quality.

Monja Stettner, a Ph.D. student, prepared the samples as part of her dissertation work and assisted with the experiments in Marburg. Ulrich Höfer's group at Marburg University performed the high-precision photoemission experiments and analyzed the measurement data. Meanwhile, Puschnig's team at the University of Graz developed the theoretical framework for describing photoemission from excitons and created a model that enabled the interpretation of the experimental data.

The research technique, Photoemission Orbital Tomography (POT), was developed in Puschnig's group and has since been refined in collaboration with Tautz's team at Forschungszentrum Jülich, establishing it as a valuable tool for investigating electronic states in organic materials. This project is part of the EU initiative, Orbital Cinema, which aims to visualize electron dynamics in materials with exceptional spatial and temporal resolution.

Looking ahead, the researchers plan to study the separation of electrons and holes in donor-acceptor systems, a process that significantly influences the efficiency of converting light into electrical current—a key factor in the development of future organic photovoltaics.