The electronic structure of porphyrin/metal interfaces studied by UV photoemission spectroscopy

The electronic structures of the interface between porphyrin and metal (Mg, Al, Ag and Au) were studied by UV photoemission spectroscopy (UPS). The samples were 5,10,15,20-tetraphenylporphynatozinc(ZnTPP), 5,10,15,20-tetra(4-pyridyl) porphyrin(H2T(4-Py)P) and 5,10,15,20-tetra-phenylporphyrin(H2TPP) which are known as organic semiconductors. We found that the energy levels of these porphyrins relative to the Fermi level of the substrate metals could be expressed as linear functions of the work function of metals with a shift of the vacuum level at interface (Δ). This indicates that the energy levels of these porphyrins are fixed to the vacuum level of substrate metal with an interfacial dipole layer. We also found that the magnitude of Δ is constant at ZnTPP/metal interfaces, but depends on the work function of the metal at H2T(4-Py)P and H2TPP/metal interfaces. This work function dependence of Δ might be explained by the existence of interface state.