Charge Injection across Self-Assembly Monolayers in Organic Field Effect Transistors: Odd-Even Effects

Alkanethiol monolayers self-assembled on the Au source and drain electrodes have been used to improve the response of organic field effect transistors by different groups in the past. Here, we investigate the role of self-assembly monolayers of chain length n, where n ranges between 3 and 18, in modulating the charge injection in pentacene field effect transistors. The charge carrier mobility μ exhibits large fluctuations correlated with odd-even n. For n < 8, μ increases by one order of magnitude due to the decrease of the hole injection barrier and the improved molecular order at the organic-metallic interface. For n > 8, μ decays exponentially with an inverse decay length β=0.6 Å-1. As the morphology in the channel is invariant, we account for both the electron structure and the molecular disorder at the interface by introducing a length scale of injection as a function of the chain length. Charge injection across the interface occurs by through-bond tunneling of holes mediated by the alkanethiol layer. At short chain length, decrease of both interfacial morphological disorder and pentacene-surface interactions dominates over the increasing thickness of the alkanethiol monolayer. At long chain length, the charge injection across the alkanethiol monolayer completely governs the transistor response. The odd-even effect is ascribed to the anisotropic coupling between the alkanethiol terminal sigma bond and the HOMO level of ordered pentacene molecules. Our results show that the pentacene transistor is a sensitive gauge for probing charge transport across single monolayers, and that its response is sensitive to sub-molecular details of the interface. This work was supported by the EU-Integrated Project NAIMO (No NMP4-CT-2004-500355).