Role of charge transfer, dipole-dipole interactions, and electrostatics in Fermi-level pinning at a molecular heterojunction on a metal surface

Recently, Niederhausen et al. [Phys. Rev. B 86, 081411(R) (2012)] have reported on the energy level alignment of C-60 adsorbed on a bilayer alpha-sexithiophene (6T) film on Ag(111). The possibility of charge transfer from the metal to the C-60 through the bilayer 6T as discussed by the authors may have a strong impact on understanding the energy level alignment (ELA) at organic-organic (O-O) heterojunctions grown on electrodes. In this paper, we aim at a comprehensive picture of the ELA at O-O interfaces on a metal. We carry out a detailed investigation of the same pair of materials on Ag(111) as employed previously, however, with varying 6T interlayer thickness. The results allow unambiguous identification of integer charge transfer towards a fraction of the C-60 molecules as the mechanism leading to the formation of interface dipoles. Varying the 6T interlayer thickness also reveals the dependence of the observed features on the C-60-metal distance. This dependence is quantitatively addressed by electrostatic considerations involving a metal-to-overlayer charge transfer. From this, we demonstrate the important role of dipole-dipole interaction potentials in the molecular layer and electric fields resulting from interface dipole formation for the energy level alignment. These findings provide a deeper understanding of the fundamental mechanisms that establish electronic equilibrium at molecular heterojunctions and will aid the prediction of an accurate energy level alignment at device relevant heterojunctions, e. g. in organic opto-electronic devices. DOI: 10.1103/PhysRevB.87.035440