More efficient solar cells thanks to a revolutionary new study
Energy generation in light-based technologies is based on the ability of materials to convert light into electrical energy and vice versa. Some organic molecular solids have the unique ability to significantly increase the conversion efficiency from solar to electricity, thanks to a process called exciton singlet fission (SEF). In this process two pairs of quasiparticles called excitons are generated by the absorption of a quantum of light (a photon).
The efficiency and speed of the SEF process are dictated by subtle details related to the way in which the molecules arrange themselves in the material. Despite hundreds of studies on the subject, however, there was no way to observe in real time how exactly the molecules move in order to facilitate the SEF event. Understanding this piece of the puzzle is essential for optimizing SEF materials and further increasing their efficiency.
Researchers from the FHI, MPSD and Julius-Maximilians-Universität recently published in Sciences Advances Würzburg were able to trace how molecules in a crystalline material build from pentacene molecules move during the SEF process, using an experimental technique called “femtosecond electron diffraction”. Such a technique can capture snapshots of the atomic structure in real time as the SEF process takes place. To be able to capture these snapshots in pentacene, a material that contains only small and light atoms, the measurements had to achieve exceptional stability and resolution.
Thanks to state-of-the-art theory, the team was able to reveal the molecular motions involved in the initial excitation event and how these motions trigger more complex molecular motions involving many molecules of the crystal. “Our theoretical analysis could solve very complex molecular motions. We could identify a dominant one involving molecules sliding relative to each other, and which can only be triggered through the coupling of electronic excitations to other more localized molecular motions, which then, in turn, couple to this key movement. also observed in experiment ", says Mariana Rossi of MPSD.
These collective atomic motions observed by the team involved in the project could be the key to explaining how the two excitons generated by the SEF process can separate, which is a prerequisite to collect their charges in a solar energy device. "In a nutshell, our hypothesis is that these molecular motions efficiently neutralize the forces that hold the two excitons together soon after they are generated, providing a possible explanation for the origin of the ultrafast time scales linked to fission, and thus facilitate the high efficiency of converting solar energy into electricity, ”said Hélène Seiler, postdoctoral fellow at the FHI in Ralph Ernstorfer's group.
