Reducing the metal work function beyond pauli pushback: A computational investigation of tetrathiafulvalene and viologen on coinage metal surfaces

Application of (sub)monolayers of organic molecules on metal electrodes in order to tune the effective work function has become a field of significant interest. Due to its low ionization potential and quinoidal structure, viologen (1H,1′H-[4,4′]bipyridinylidene) is proposed as a particularly potent work function reducing molecule. In the present contribution, its interaction with Au(111), Ag(111), and Cu(111) is compared to that of the prototypical electron donor tetrathiafulvalene (TTF) using density functional theory based band-structure calculations. The work function modification in both systems is found to be determined by a subtle interplay between effects due to adsorption induced geometric distortions and the donation of electrons from the respective molecular HOMO to the metal. The interfacial charge transfer is investigated in real space as well as in terms of changes in the occupation of the molecular orbitals. Overall, viologen is found to be an excellent choice for decreasing the substrate work function. On gold, a reduction by up to -1.6 eV is predicted, resulting in the viologen covered Au surface having a work function equivalent to that of pristine magnesium.