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O: Fachverband Oberflächenphysik

O 97: Organic Molecules on Inorganic Substrates: Electronic, Optical and Other Properties II

O 97.6: Vortrag

Freitag, 21. März 2025, 11:45–12:00, H6

Electrons don't wear bowties: The mechanism of Fermi-level pinning and charge for mixed donor-acceptor layers — •Richard Berger1 and Oliver T. Hofmann — TU Graz, Institute of Solid State Physics

Controlling the work function of metal substrates is critical when optimizing the performance of organic electronic devices. A particular powerful method to obtain such control is by employing a monolayer of strong electron donors or electron acceptors, i.e. so-called Fermi-level pinned systems which form charge-transfer-induced dipoles with the underlying substrate However, while the major advantage of this Fermi-level pinned systems is that the effective work function of the interface is (essentially) independent of the underlying substrate, there are only limited ways to fine-tune it. A more flexible way could be to employ a mixture of electron donors and acceptors. However, for such complex systems the underlying charge-transfer mechanism is less clear. In this work, we elucidate this mechanism using machine-learning augmented density functional theory calculations to predict both the atomistic and the electronic structure of the interface using a mixture of F4TCNQ and paraquat as an example. Counterintuitively, we find that the nominally weaker donor overpowers the stronger acceptor, resulting in a dependence of the work function that is effectively determined by the nominal coverage of the donor. At the same time, we find that multiple geometries are energetically very similar, but result in different work function, affecting the reproducibility of different experimental realizations.

Keywords: DFT; machine learning; Fermi-level pinning; electron donor; electron acceptor