Influence of OH groups on charge transport across organic-organic interfaces: A systematic approach employing an "ideal" device
The charge transport across a pentacene/SAM interface has been studied by scanning tunnelling spectroscopy (STS) as a function of temperature and film thickness in order to obtain information on the transport mechanisms and in particular on the importance of interfacial OH-groups on n-transport in organic semiconductors. The current-voltage (I-V) characteristics of pentacene thin films deposited on a mercaptoundecanol self-assembled monolayer (SAM) on Au(111) reveal an asymmetric behaviour. At positive sample bias the onset currents shift towards higher voltages for decreasing temperatures, whereas such changes are not seen at negative bias. For lower temperatures, the variation of current onset with layer thickness is absent. These observations are explained by OH-groups at the SAM-surface effectively acting as charge traps. When electrons are caught in these traps at the organic-organic interface, charge transport is severely affected. Imaging of the SAM after loading the traps suggests that the attachment of electrons to the OH-groups exposed at the organic surface is a reversible process.
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