Correlating Carrier Type with Frontier Molecular Orbital Energy Levels in Organic Thin Film Transistors of Functionalized Acene Derivatives

We investigate the relationship between the charge carrier type in organic thin film transistors (OTFTs) and molecular energy levels. We examine a series of functionalized acenes that collectively have their HOMOs range from -4.9 eV to -5.6 eV and LUMOs range from -2.8 eV to -3.7 eV, as measured by cyclic voltammetry. Placed together, these 20 molecules allow us to chart the transition from OTFTs that display only hole transport, to ambipolar, to solely electron transport. Specifically, we note that for octadecyltrimethoxysilane (OTS) treated substrates, with top contact gold electrodes, electron injection and transport occurs when the LUMO < -3.15 eV, while hole injection and transport ceases when the HOMO < -5.6 eV. Ambipolar transport prevails when molecules have HOMO/ LUMO levels within the aforementioned range. This is seen across channel lengths ranging from 50-150 microm and using only gold as electrodes. This empirical plot is the first time such a detailed study has been made on the onset of charge injection and transport for a class of organic semiconductors. It provides guidelines for future molecular design.