The bricks of the future for photovoltaics

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image: Artistic representation showing twisted layers of tungsten diselenide (top) and molybdenum disulfide (bottom). Following excitation by light, a multitude of optically “dark” excitons are formed between the layers. These “dark” excitons are electron-hole pairs linked by Coulomb interaction (light and dark spheres connected by field lines), which cannot be observed directly in visible light. One of the most interesting quasiparticles is the “moiré interlayer exciton” – shown in the middle of the image – in which the hole is located in one shell and the electron in the other. The formation of these excitons on the femtosecond time scale and the influence of the moiré potential (illustrated by peaks and valleys in the layers) were investigated in the current study using femtosecond photoemission pulse microscopy and the theory of quantum mechanics.
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Credit: Brad Baxley, Part to Whole, LLC

An international research team led by the University of Göttingen has, for the first time, observed the accumulation of a physical phenomenon that plays a role in the conversion of sunlight into electrical energy in 2D materials. Scientists have managed to make quasiparticles — known as dark moiré interlayer excitons — visible and explain their formation using quantum mechanics. The researchers show how an experimental technique newly developed in Göttingen, femtosecond photoemission pulse microscopy, provides deep insights at the microscopic level, which will be relevant for the development of future technologies. 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. Interestingly, the properties of these semiconductors can be controlled in unusual ways: like Lego bricks, the atomically thin layers can be stacked on top of each other. However, there is another important trick: while Lego bricks can only be stacked on top – either straight or twisted at a 90 degree angle – the angle of rotation in the semiconductor structure can be amended. It is precisely this angle of rotation that is of interest for the production of new types of solar cells. However, while changing this angle can reveal breakthroughs for new technologies, it also leads to experimental challenges. In fact, typical experimental approaches only have indirect access to moiré interlayer excitons, hence these excitons are commonly referred to as “dark” excitons. “With the help of femtosecond photoemission pulse microscopy, we have actually managed to make these dark excitons visible,” says Dr. Marcel Reutzel, Junior Research Group Leader at the Faculty of Physics at the University of Göttingen. “This allows us to measure how excitons form on a time scale of a millionth of a millionth of a millisecond. We can describe the dynamics of the formation of these excitons using the quantum mechanical theory developed by the research group of Professor Ermin Malic in Marburg.”

“These results not only give us fundamental insight into the formation of dark moiré interlayer excitons, but also open up a whole new perspective for scientists to study the optoelectronic properties of fascinating new materials,” says Professor Stefan Mathias, director of studies at the faculty of physics at the University of Göttingen. “This experiment is revolutionary because, for the first time, we have detected the signature of the Moiré potential imprinted on the exciton, that is to say the impact of the combined properties of the two twisted semiconductor layers. In the future , we will investigate this specific effect in more detail to learn more about the properties of the resulting materials.”

This research was made possible by the German Research Foundation (DFG) which funded the CRC “Control of Energy Conversion on Atomic Scales” and “Mathematics of Experiment” in Göttingen, and the CRC “Structure and Dynamics of Internal Interfaces ” in Marburg.

Original release: Schmitt et al. “Formation of moiré intercalary excitons in space and time”, Nature 2022. DOI: 10.1038/s41586-022-04977-7

Contact:

Professor Stefan Mathias

University of Goettingen

Faculty of Physics – I. Institute of Physics

Friedrich Hund Weg 1, 37077 Goettingen, Germany

Tel: +49 (0)551 39-27601

Email: [email protected]

https://www.uni-goettingen.de/en/580823.html


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