Fluorination of copper phthalocyanines: Electronic structure and interface properties

We compare the electronic structure of differently fluorinated copper phthalocyanines (CuPC, CuPCF4, and CuPCF16) using x-ray photoemission spectroscopy and valence-band ultraviolet photoemission spectroscopy. Whereas the ionization potential (IP) is increased by more than 1 eV as a function of the degree of fluorination, further electronic properties such as the optical gap or the composition of the highest occupied molecular orbital and lowest unoccupied molecular orbital remain nearly unchanged. This fact renders these compounds an ideal tool for the investigation of the influence of the IP on the interface properties. At the interface to gold, besides interface dipoles we observe both downward and upward band bending. These phenomena depend clearly on the IP of the phthalocyanines.