An international research team led by the University of Göttingen is making strides in improving cutting-edge technologies like solar cells with a groundbreaking new technique. For the first time, the formation of dark excitons—tiny, challenging-to-detect particles—can now be tracked with unprecedented precision in both time and space. This breakthrough has important implications for the development of future solar cells, LEDs, and detectors. The results are published in Nature Photonics.

Dark excitons are pairs consisting of an electron and the “hole” it leaves behind when it is excited. These particles carry energy but cannot emit light, which is why they are termed “dark.” To visualize an exciton, imagine a balloon (representing the electron) that flies away, leaving behind an empty space (the hole) connected by a Coulomb interaction force. Although these particle states are notoriously difficult to detect, they play a crucial role in atomically thin, two-dimensional structures in special semiconductor compounds.

In a previous study, Professor Stefan Mathias and his team at the University of Göttingen demonstrated how dark excitons are created in extremely short time frames and detailed their dynamics using quantum mechanical theory. Now, in their latest research, the team has developed a novel technique called “Ultrafast Dark-field Momentum Microscopy,” which they used for the first time to track the formation of dark excitons in a material made from tungsten diselenide (WSe₂) and molybdenum disulfide (MoS₂). The team measured the process with astonishing precision, revealing the formation of these excitons in just 55 femtoseconds (55 millionths of a billionth of a second) at a spatial resolution of 480 nanometers.

“This method enabled us to measure the dynamics of charge carriers with incredible accuracy,” said Dr. David Schmitt, the first author of the study and a researcher at Göttingen University’s Faculty of Physics.

The team’s findings provide critical insights into how material properties influence the movement of charge carriers, a key factor in improving the efficiency and quality of technologies like solar cells. Dr. Marcel Reutzel, Junior Research Group Leader in Mathias’ research group, added, “This technique can be used not only for specially designed systems but also for research into new types of materials, allowing for targeted improvements in energy-harvesting devices.”

This novel technique offers a new path forward for the advancement of solar energy technology, as it allows scientists to better understand and control the behavior of charge carriers—key components in the efficiency of solar cells and other optoelectronic devices.

By Impact Lab