Control of Electron Injection Barrier by Electron Doping of Metal Phthalocyanines

Metal phthalocyanine (MPc, with M = Fe, Co, Ni, Cu, Zn) thin-films grown on Au(110) have been electron-doped by exposition to alkali metal (potassium). Progressive occupation of the former lowest unoccupied molecular orbital (LUMO) up to the filling of the 2-fold degenerate LUMO with 2 e g symmetry has been observed in photoemission spectra for the NiPc, CuPc, and ZnPc films. The evolution of the electron filling induces a different process for FePc and CoPc, whose empty electronic states localized on the central metal atoms are involved in the electron donation process by the alkali metal. Electron filling is accompanied by a sudden decrease of the work function mainly due to a variation of the surface dipole. The molecular films remain semiconducting, with the hole injection barrier strongly reduced at saturation of the electron filling. The selective influence of the electron donation process on the molecular states (ligands, metal-related) is discussed.