Regensburg 2010 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 17: [DS] Organic Electronics and Photovoltaics II (Joint Session DS/CPP/HL/O)

O 17.4: Vortrag

Montag, 22. März 2010, 14:45–15:00, H8

Energy levels and work function of ultra-thin polythiophene films on conductive polymer electrodes — •Johannes Frisch¹, Antje Vollmer², Jürgen P. Rabe¹, and Norbert Koch¹ — ¹Humboldt-Universität zu Berlin, Institut f. Physik, Brook-Taylor-Straße 6, D-12489 Berlin — ²Elektronenspeicherring BESSY II, Albert-Einstein-Str. 15, D-12489 Berlin

In modern organic photovoltaic cells (OPVCs), poly(ethylene- dioxythiophene):poly(styrenesulfonate) (PEDT:PSS) and poly(3-hexylthiophene) (P3HT) are prototypical materials used as transparent anode and as donor polymer, respectively. In this context, it is essential to clarify how the efficiency of OPVCs based on these materials depends on the position of the energy level at the interface. In the present work, the morphology, work function, and energy level alignment of ultra-thin films of P3HT on PEDT:PSS were investigated using photoelectron spectroscopy (XPS, UPS). Ultra-thin films (down to sub-monolayer coverages) were produced by spin coating P3HT films on PEDT:PSS followed by a subsequent washing process. It has been found that the deposition of ultra-thin P3HT films on PEDT:PSS lowers the sample work function (Φ) by 0.4 eV due to doping of P3HT film with positive charges from the PEDT:PSS layer. The valence band edge (VBE) is located only 0.1 eV below the Fermi-level. For thicker P3HT films (ca. 10 nm), Φ further decreases by 0.2 eV and the VBE is observed 0.2 eV below the Fermi-level. These results are discussed within existing concepts of local work function fluctuations and energy level bending at the P3HT/PEDT:PSS interface.