Using photovoltaic technology to directly harvest energy from sunlight has become an essential component of future global energy production. Although silicon-based inorganic materials continue to dominate the market, the main challenges for inorganic materials-based solar cells are the high cost of production, technical difficulties in the fabrication of large-area cells, and a lack of highly purified silicon. In contrast to inorganic photovoltaic materials, organic/polymer photovoltaic materials are currently gaining popularity due to their significant potential to significantly reduce the cost of solar cells.
An international research team has discovered a physical phenomenon that aids in the conversion of sunlight into electrical energy in 2D materials. The researchers were able to make quasiparticles known as dark Moiré interlayer excitons visible and explain their formation using quantum mechanics. The researchers demonstrate how femtosecond photoemission momentum microscopy, a newly developed experimental technique, provides profound insights at the microscopic level that will be relevant to the development of future technology.
This experiment is groundbreaking because we have discovered the signature of the Moiré potential imprinted on the exciton for the first time, i.e. the impact of the combined properties of the two twisted semiconductor layers. We will investigate this specific effect further in the future to learn more about the properties of the resulting materials.Professor Stefan Mathias
An international research team led by the University of Göttingen has, for the first time, observed the build-up of a physical phenomenon that plays a role in the conversion of sunlight into electrical energy in 2D materials. The scientists succeeded in making quasiparticles — known as dark Moiré interlayer excitons – visible and explaining their formation using quantum mechanics. The researchers show how an experimental technique newly developed in Göttingen, femtosecond photoemission momentum microscopy, provides profound insights at a microscopic level, which will be relevant to the development of future technology. The results were published in Nature.
Atomically thin structures made of two-dimensional semiconductor materials are promising candidates for future components in electronics, optoelectronics, and photovoltaics. Surprisingly, the properties of these semiconductors can be controlled in an unusual way: the atomically thin layers can be stacked on top of each other like Lego bricks. However, there is another important trick: whereas Lego bricks can only be stacked on top – whether directly or twisted at an angle of 90 degrees – the angle of rotation in the semiconductor structure can be varied. This angle of rotation is particularly intriguing for the development of new types of solar cells.
However, while changing this perspective can reveal breakthroughs for new technologies, it also introduces experimental challenges. Because typical experimental approaches have only indirect access to the moiré interlayer excitons, these excitons are commonly referred to as “dark” excitons. “We were able to make these dark excitons visible using femtosecond photoemission momentum microscopy,” says Dr. Marcel Reutzel, junior research group leader at Göttingen University’s Faculty of Physics.
“This enables us to measure the formation of excitons on a millionth of a millionth of a millisecond time scale. Using quantum mechanical theory developed by Professor Ermin Malic’s research group at Marburg, we can describe the dynamics of the formation of these excitons.”
“These findings not only provide a fundamental understanding of the formation of dark Moiré interlayer excitons, but also open up a completely new perspective for scientists to study the optoelectronic properties of new and fascinating materials,” says Professor Stefan Mathias of Göttingen University’s Faculty of Physics. “This experiment is groundbreaking because we have discovered the signature of the Moiré potential imprinted on the exciton for the first time, i.e. the impact of the combined properties of the two twisted semiconductor layers. We will investigate this specific effect further in the future to learn more about the properties of the resulting materials.”
This research was made possible thanks to the German Research Foundation (DFG) who provided Collaborative Research Centre funding for the CRCs “Control of Energy Conversion on Atomic Scales” and “Mathematics of Experiment” in Göttingen, and the CRC “Structure and Dynamics of Internal Interfaces” in Marburg.