Highly polar groups and supramolecular order enable charge injection and long-range conductivity in organic materials without extended π-systems

Abstract

Electronic conductivity in organic materials is well-established. Both semiconductive and metallic behavior is observed in (quasi) 0-, 1-, 2- and 3-dimensional carbon-based materials and is applied in a wide range of commercial devices. Despite their large structural variety, these materials commonly have an extended π-system, formed by a double-digit number of conjugated sp2-hybridized carbon atoms, which is responsible for the conductivity. Here, we present a class of organic molecular materials that, despite the absence of an extended π-system, show a distinct direct current conductivity in quasi-1D supramolecular stacks of small organic molecules. Long-range conductivity takes place by removal of an electron from the highest occupied molecular orbital, i.e. oxidation, followed by charge transfer between neighboring molecules. Kelvin probe force microscopy (KPFM) on thin-film devices shows that the resulting transport band only becomes energetically accessible for charge injection from the contacts thanks to interfacial dipoles that shift the relevant molecular orbital levels by up to 2 eV. Long-range order in the form of fibrillar supramolecular polymers with lengths exceeding several micrometers enhances the macroscopic conductivity but is not essential. While current densities are moderate, this work provides a compelling explanation for the existing body of knowledge on non-π-conjugated conductivity and suggests a new way to endow organic materials with conductivity.