Dresden 2011 – wissenschaftliches Programm

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DS: Fachverband Dünne Schichten

DS 64: Organic Thin Films III

DS 64.6: Vortrag

Freitag, 18. März 2011, 15:15–15:30, GER 38

UV-induced sample degradation can mimic band bending during photoemission of organic thin films — •Raphael Schlesinger¹, Johannes Frisch¹, Antje Vollmer², and Norbert Koch¹ — ¹Humboldt-Universität zu Berlin, Institut für Physik, 12489 Berlin, Germany — ²HZB-BESSY II, Albert-Einstein-Str. 15, 12489 Berlin, Germany

Photoemission spectroscopy (PES) has evolved to a widely used method for characterizing conjugated organic materials' electronic structure at interfaces. Noteworthy, energy level shifts ascribed to band bending in thickness-dependent studies of organic thin films are often not unequivocally reproducible. We show for prototypical thiophene-based molecular and polymer materials that such energy shifts may in fact be induced by the PES measurement itself. The valence spectra of thin films on PEDT:PSS and Au exhibit irreversibly rigid shifts by up to 0.4 eV to higher binding energies, without notable changes of the spectral shape. This shift is a monotonous function of ultraviolet (UV) light irradiation time and photon flux. Photo-electron induced defects may be created in the sample that either lead to gap states, which shift the Fermi-level, or to the build-up of space charges. The degradation proceeds at a lower rate if a high-conductivity substrate is used for the organic materials, indicating that photo-electron induced degradation is enhanced by subtle sample charging.