Understanding the F 1s NEXAFS Dichroism in Fluorinated Organic Semiconductors

The analysis of NEXAFS spectra acquired at the absorption edge of heteroatoms like fluorine, nitrogen, or oxygen enables the determination of the molecular orientation of individual compounds even in multinary structures. Such an analysis requires detailed knowledge about the nature of the corresponding electronic excitations, especially because 1s-π* and 1s-σ* excitations frequently feature disparate transition dipole moments in planar molecules. Unlike intuitively assumed, the resulting dichroisms of both excitation types in planar systems are, however, not necessarily inverted. Instead, both the structure of the molecules and their alignment on the substrate determine the actual dichroisms. In this study, the NEXAFS signature and dichroisms of thin films of the organic n-type semiconductor perfluoropentacene (PFP) in different crystalline orientations at the carbon and fluorine absorption K-edge are thoroughly analyzed. For lying molecular orientations, an inverted dichroism at the fluorine K-edge comp...