The electronic structure and energy level alignment of porphyrin/metal interfaces studied by ultraviolet photoelectron spectroscopy

Electronic structures of 5, 10,15,20-zinctetraphenylporphyrin (ZnTPP)/metal (Au, Ag, Al, Mg) interfaces prepared in ultrahigh vacuum condition were investigated by ultraviolet photoelectron spectroscopy (UPS). We found that the electronic energy levels of ZnTPP align with the vacuum level of substrate metal with a constant energy shift of the vacuum levels across the interface. These findings cannot be explained by the concepts of Fermi level alignment nor vacuum level alignment. We also found that sample exposure to oxygen induces energy level shift in close relation with change of substrate work function at oxygen exposure. The relation between the observed electronic structure and the electric characteristics of porphyrin/metal interfaces in solar cell is also discussed.