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DS: Fachverband Dünne Schichten

DS 46: Organic Thin Films III

DS 46.4: Vortrag

Freitag, 26. März 2010, 14:45–15:00, H8

Energy level alignment at sexithiophene-fullerene heterojunctions from photoelectron spectroscopy: differentiating between intrinsic and extrinsic energy shifts — •Raphael Schlesinger¹, Hendrik Glowatzki¹, Johannes Frisch¹, Ralf-Peter Blum¹, Patrick Amsalem¹, Antje Vollmer², Jürgen P. Rabe¹, and Norbert Koch¹ — ¹Humboldt-Universität zu Berlin, Institut für Physik, 12489 Berlin, Germany — ²HZB-Bessy II Albert-Einstein-Straße 15, 12489 Berlin, Germany

Ultraviolet photoelectron spectroscopy (UPS) was used to study the electronic structure of heterojunctions of sexithiophene (6T) and C₆₀ on Au, Ag, and PEDT:PSS. Our results suggest that a monolayer of 6T on PEDT:PSS is lying flat, while molecules in multilayers stand vertically upright. The highest occupied molecular orbital (HOMO) of the 6T monolayer is less than 0.2 eV below the Fermi-level, while for multilayer 6T films the HOMO is pinned at 0.4 eV below the Fermi-level. This difference in binding energy of the frontier 6T levels is attributed to a charge-exchange reaction between 6T and PEDT:PSS, which is confined to the 6T monolayer only. The energy level alignment at the C₆₀/6T interface is found to follow the vacuum level alignment model. We will further highlight that effects often assigned to band bending in organic molecule films may well be induced by the UPS measurement itself. 6T films on PEDT:PSS exhibited a non-reversible shift of the energy levels/work function to higher binding energies by up to ca. 0.5 eV as a function of illumination with ultraviolet light (UV) that is commonly used to excite photoelectrons in UPS experiments.