Dresden 2009 – wissenschaftliches Programm
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SYOP: Symposium Organic Photovoltaics: From Single Molecules to Devices
SYOP 4: Organic Photovoltaics: from Single Molecules to Devices
SYOP 4.46: Poster
Donnerstag, 26. März 2009, 16:30–19:00, P1A
Model studies on the injection of charge carriers into organic semiconductors: The case of pentacene single crystals grown on self-assembled monolayers (SAMs) — Zhihong Wang1, Asif Bashir1, Daniel Käfer1, Alexander Birkner1, Gregor Witte2, and •Christof Wöll1 — 1Physik. Chemie, Ruhr-Universität Bochum — 2AG Molek. Festkörperphysik, Philipps-Universität Marburg
SEM and STM micrographs reveal that the deposition of the organic semiconductor (OSC) pentacene on Au-supported SAMs with different termination yields similar morphologies, namely multilayer islands containing upright-oriented molecules. Charge transport across the SAM / within the OSC was studied using the STM-tip as a second electrode. For films on alkanethiol SAMs the I-V characteristics show not only a pronounced asymmetry, but also a strong dependence on temperature and film thickness [1]. An explanation was provided by numerical simulations which found a dominance of p(n)-transport at neg. (pos.) bias. A different behaviour is found for films on a mercaptoundecanol SAM. Here, stable images could only be obtained for neg. substrate bias; for pos. bias the tip was found to penetrate the OSC-island resulting in a complete removal during scanning. We explain this unexpected behaviour by electron trapping at OH-groups (SAM) and by quenching of n-transport in the OSC. This is in accord with the observation that in pentacene devices n-type transport is only found in absence of OH groups [2]. Possible microscopic mechanisms for the trapping of charges at the OH-groups will be discussed.
[1] Ruppel et al., J.Appl.Phys. 102, 033708 (2007), [2] Chua et al., Nature 434, 194 (2005)