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

We investigate the role of self-assembly monolayers in modulating the response of organic field-effect transistors. Alkanethiol monolayers of chain length n are self-assembled on the source and drainelectrodes of pentacene field-effect transistors. The charge carrier mobility μ exhibits large fluctuationscorrelated with odd-even n. For n hole injection barrier and the improved molecular order at the organic-metallic interface. For n g 8, μdecays exponentially with an inverse decay length ) 0.6 Å-1. Our results show that (i) charge injectionacross the interface occurs by through-bond tunneling of holes mediated by the alkanethiol layer; (ii) in thelong-chain regime, the charge injection across the alkanethiol monolayer completely governs the transistorresponse; (iii) the transistor is a sensitive gauge for probing charge transport across single monolayers.The odd-even effect is ascribed to the anisotropic coupling between the alkanethiol terminal σ bond andthe HOMO level of ordered pentacene molecules.