Metal-to-Acceptor Charge Transfer through a Molecular Spacer Layer

We investigate how the thermodynamic equilibrium is reached when a strong electron acceptor molecule (hexaaza-triphenylene-hexacarbonitrile, HATCN) is deposited on Ag(111) precovered with a physisorbed tris(8-hydroxyquinolinato)aluminum (Alq3) monolayer. Photoemission measurements reveal that equilibrium is achieved via a charge transfer from the metal to the HATCN layer through the Alq3 monolayer. The work function increase observed upon HATCN adsorption is explained by the formation of an interface dipole resulting from the metal-to-acceptor charge transfer and an electric field induced reorientation of the Alq3 molecule dipoles within the interlayer.