Investigation of polarization effects in organic thin films by surface core-level shifts

Site-Specific Polarization Screening in Organic Thin Films

“…Our findings have implications for transport in (bulk) organic semiconductors, because even chemically homogeneous materials may under certain circumstances be considered to consist of electronically distinct species (with an energy level dispersity up to 0.5 eV), with profound consequences for carrier transfer from molecule to molecule, since carriers may get trapped not only at defects but also at regular lattice sites. The results also make clear why under certain circumstances it may be difficult to observe the influence of intermolecular polarization screening in spectroscopic experiments [12,13].…”

“…4 may also explain the negative outcome of the attempts to measure surface core-level shifts for thin films of organic semiconductors [12,13]: If the crystal structure of the films is such that the polarization screening of the photohole is mainly effected by molecules located in a plane parallel to the surface, surface core-level shifts are expected to be very small. In this sense, the present data reconcile the conflicting evidence with respect to polarization screening in Refs [11][12][13].…”

“…While the principles of polaron-formation are well understood, the new aspect which we report here, namely, the site specificity of the polarization energy in complex unit cells and at surfaces of organic materials, was not experimentally proven so far. In fact, up to now attempts to measure directly the expected influence of local molecular environments on the polarization screening have failed [12,13], for reasons which are currently not understood.…”

Fingerprint of Fractional Charge Transfer at the Metal/Organic Interface

“…To mimic the thin assembly that experiences charge transfer from the substrate, we consider a single F4PEN molecule and add different amounts of electronic charge. The sampling depth of our experiment is around 54 Å, corresponding to the estimated inelastic mean free path of 18 Å [36]. It means that the XPS spectra of the 254 Å nominally thick assembly do not contain contributions from the interface.…”

“…We consider 0.75 electrons as the upper limit of the transferred charge since in this quantification we accumulate also contributions due to other possible sources of (rigid) corelevel shifts: These are changes in the molecular orientation, [41] different charge redistribution, due to the strong electronegativity of the fluorine atoms [31] when comparing surface and bulk environment of the molecules (surface core level shift [36,42]) and imagecharge screening effects due to the capability of the substrate to screen the core-hole generated in the photoemission event. [7] The experimental finding that no appreciable change in the HOMO takes place upon charge transfer (compare the UPS spectra of the thick and thin assemblies in Figure 1) is also supported by our calculations.…”