Importance of ITO surface conditions for the interaction with thin CuPc layers

The electronic interaction between the transparent anode material indium tin oxide (ITO) and the common organic hole injection material copper phtalocyanine has been investigated by means of angle-resolved UV photoemission. The study was made on two differently treated ITO substrates, both cleaned in common organic solvents and one of them heated in air after the cleaning. The heat treatment efficiently removes surface carbon-containing contamination, and has also an oxidizing effect. As a result, the ITO work function is increased by 0.4-0.6 eV. This surface preparation has significant impact on the electronic properties of adsorbed CuPc. We find that the Pc HOMO level, important for transport properties, is shifting by 0.5 eV depending on surface conditions. This is interpreted as an effect of Fermi level pinning within the gap of the spin-split Cu derived b1g molecular orbitals.