Band bending and structure dependent HOMO energy at the ZnO(0 0 0 1)-titanyl phthalocyanine interface

We have investigated the initial stages of titanyl phthalocyanine (TiOPc) growth on single crystalline ZnO(0 0 0 1). This organic-semiconductor interface is self-organizing as a 2 × 1 pattern appears in a low energy electron diffraction upon deposition of the molecules. To achieve this pattern, the TiOPc is suggested to adsorb standing with the edge of the molecule along the substrate atomic rows. Photoelectron spectroscopy is used to further analyze the interface; a relatively large upwards band bending amounting to 0.5 eV is found and a splitting of the molecules highest occupied molecular orbital occurs after thermal treatment, indicating that the molecules are lying down.