Molecular-Level Realignment in Donor-Acceptor Bilayer Blends on Metals

Molecular bilayers made up of a donor-acceptor blend in contact with the metal and capped with a single-component layer show a tunable energy level alignment at both metal-organic and organic-organic interfaces. To ensure sharp heteromolecular interfaces, a contact layer is formed by a stable blend of pentacene (PEN) and perfluorinated copper phthalocyanine (F16CuPc) on Au(111) and Ag(111) and of perfluoropentacene (PFP) and copper phthalocyanine (CuPc) on Ag(111). Core-level and valence band photoemission reveal that, upon capping with pure F16CuPc, CuPc, and PEN, the electronic states of both contact and capping layers "realign" with respect to the monolayer and the multilayer references. The sign of the shift depends clearly on whether the capping layer is donor-like (PEN, CuPc) or acceptor-like (F16CuPc). As revealed by NEXAFS, the shift in electronic levels of the contact layer upon capping leads to a spectral density variation across the Fermi edge (EF); i.e., it induces molecule/metal charge transfer.