New Technique Tracks Dark Excitons for Future Solar Cells

New Technique Tracks Dark Excitons for Future Solar Cells
by Robert Schreiber
Berlin, Germany (SPX) Jan 30, 2025

How can modern technologies, like solar cells, be optimized? An international team of researchers, led by the University of Gottingen, is tackling this question using an innovative new technique. For the first time, the formation of tiny, elusive particles - known as dark excitons - has been precisely tracked in both time and space. These invisible energy carriers are poised to play a crucial role in the development of future solar cells, LEDs, and detectors. The findings were published in Nature Photonics.

Dark excitons are extremely small pairs consisting of an electron and the hole it leaves behind when excited. Despite carrying energy, these particles do not emit light, which is why they are referred to as "dark." One way to think of an exciton is as a balloon (the electron) flying away, leaving behind an empty space (the hole), both still connected by a Coulomb interaction force. Though difficult to detect, dark excitons are especially important in atomically thin, two-dimensional structures found in certain semiconductor materials.

Previously, Professor Stefan Mathias and his research group from the Faculty of Physics at the University of Gottingen demonstrated how dark excitons are created in an incredibly short time span, describing their dynamics using quantum mechanical theory. Now, in their latest study, the team has introduced a groundbreaking method called "Ultrafast Dark-field Momentum Microscopy." For the first time, this technique has allowed them to observe the formation of dark excitons in materials like tungsten diselenide (WSe2) and molybdenum disulphide (MoS2). Astonishingly, the formation process happens in just 55 femtoseconds (0.000000000000055 seconds), measured with a resolution of 480 nanometres (0.00000048 meters).

"This method enabled us to measure the dynamics of charge carriers very precisely," said Dr. David Schmitt, first author and physicist at the University of Gottingen. "The results offer a foundational understanding of how the properties of materials influence the behavior of these charge carriers. In the future, this technique could be used to improve the efficiency and quality of solar cells, for example."

Dr. Marcel Reutzel, Junior Research Group Leader at Gottingen, added, "This technique can be applied not only to specially designed systems, but also to the exploration of new types of materials."

Research Report:Ultrafast nano-imaging of dark excitons