In the quest to develop more efficient and sustainable energy sources, solar cells stand out as a key technology. However, the full potential of solar cells is still yet to be realized. One promising avenue of research focuses on understanding the fundamental particles that carry energy within materials, which could help enhance the performance of solar cells, light-emitting diodes (LEDs), and detectors. A recent study, published in Nature Photonics, reveals an exciting advancement in this area, led by an international team of researchers from the University of Göttingen. This breakthrough offers new insights into the behavior of “dark excitons,” a class of elusive particles that could hold the key to improving the efficiency of future electronic devices.

What Are Dark Excitons?

To understand the significance of this new research, it’s essential to first explain what dark excitons are and why they are important. Excitons are particles formed when an electron in a material absorbs energy and gets excited, creating a “hole” in its original position. The electron and the hole are then bound together by an electrostatic force known as the Coulomb interaction, forming a pair that carries energy. Normally, excitons can emit light as they recombine, which makes them relatively easy to detect.

Dark excitons, however, are a different breed. These particles are made up of an electron and the hole left behind, but crucially, they do not emit light. They are called “dark” because they cannot be directly detected using conventional methods that rely on light emission. This makes them difficult to study, but their importance cannot be overstated. In two-dimensional materials and specially designed semiconductor compounds, dark excitons play a significant role in the movement and storage of energy. Their properties could potentially lead to advancements in technologies like solar cells and LEDs.

A Breakthrough in Tracking Dark Excitons

The challenge in working with dark excitons has always been the difficulty of detecting and tracking their behavior in real-time. The particle’s brief existence and lack of light emission make them nearly invisible to traditional methods. However, a team led by Professor Stefan Mathias from the University of Göttingen’s Faculty of Physics has taken a major step forward in overcoming this challenge.

In a previous publication, Mathias’ team used quantum mechanical theory to describe the creation and dynamics of dark excitons, but the precise tracking of their behavior in time and space remained an elusive goal. In their latest research, they have successfully developed a new technique that enables them to track the formation and movement of dark excitons with unprecedented precision.

The technique is called Ultrafast Dark-field Momentum Microscopy. This advanced method allows researchers to monitor the formation of dark excitons in materials with extremely high time and spatial resolution. Specifically, the team applied this method to a hybrid material made of tungsten diselenide (WSe₂) and molybdenum disulfide (MoS₂), both of which are known for their unique properties in the realm of two-dimensional materials.

The Science Behind Ultrafast Dark-field Momentum Microscopy

Ultrafast Dark-field Momentum Microscopy is an innovative tool that enables scientists to observe the dynamics of charge carriers—such as electrons and excitons—on an incredibly fast timescale. With this method, the research team was able to capture the formation of dark excitons in just 55 femtoseconds (55 millionths of a billionth of a second). This is an extraordinary achievement because it allows scientists to observe the behavior of these particles at a timescale that is faster than the blink of an eye. Additionally, the technique provided a spatial resolution of 480 nanometers, allowing the team to track the movement of these particles with remarkable accuracy.

This breakthrough allows scientists to study how dark excitons form and move through a material. In their study, the researchers focused on two-dimensional materials—thin layers of WSe₂ and MoS₂. These materials are of great interest because of their unique properties, such as strong light-matter interactions and the potential for use in next-generation electronic devices.

What the New Findings Mean for Solar Cells and Other Technologies

The implications of this discovery are far-reaching. By tracking dark excitons, the research team has gained valuable insights into the dynamics of charge carriers within these advanced materials. This understanding could have significant applications in improving the efficiency of solar cells, which rely on the movement of charge carriers to convert light into electricity.

According to Dr. David Schmitt, the first author of the study, the ability to measure the dynamics of charge carriers with such precision offers a deeper understanding of how the properties of materials influence the movement of these carriers. This is crucial for optimizing the performance of solar cells and other optoelectronic devices. Dr. Marcel Reutzel, a Junior Research Group Leader in Mathias’ team, also highlighted the potential of this technique in advancing research into new types of materials. In the future, the ability to track and manipulate dark excitons could lead to the design of materials that maximize energy conversion in solar cells, leading to devices that are both more efficient and more durable.

Beyond solar cells, this new technique could have a broad impact on the development of other electronic devices, such as LEDs, photodetectors, and quantum computers. By better understanding how charge carriers behave in these materials, scientists could unlock new ways to control light and energy at the nanoscale, enabling innovations in fields ranging from energy storage to telecommunications.

Looking Ahead: Future Research and Applications

While this new technique has already provided valuable insights into the formation and movement of dark excitons, there is still much to be explored. The research team at the University of Göttingen is likely to continue investigating the behavior of dark excitons in various materials, with the goal of further optimizing the performance of solar cells and other electronic devices.

One area of future research could involve experimenting with different combinations of two-dimensional materials to discover new ways to control the properties of dark excitons. The ability to engineer these materials at the atomic level opens up endless possibilities for creating new devices with tailored properties. Additionally, researchers may explore how dark excitons can be manipulated or harnessed to enhance the performance of energy-conversion technologies.

Another exciting possibility is the potential integration of these findings into real-world applications. For example, solar cells made from two-dimensional materials could one day be incorporated into flexible, lightweight devices that are both efficient and inexpensive to produce. These could be used for a variety of purposes, from powering everyday electronics to providing energy for remote locations.

Conclusion

The ability to track the formation and movement of dark excitons is a significant leap forward in the field of materials science and could lead to substantial improvements in solar cell efficiency and other optoelectronic technologies. The development of Ultrafast Dark-field Momentum Microscopy has opened up a new frontier in the study of energy carriers at the nanoscale. As researchers continue to refine this technique and explore its applications, we can expect to see new breakthroughs that will help drive the next generation of energy-efficient technologies. The future of solar energy, LEDs, and detectors looks brighter than ever, thanks to the work of these innovative researchers.

Reference: David Schmitt et al, Ultrafast nano-imaging of dark excitons, Nature Photonics (2025). DOI: 10.1038/s41566-024-01568-y