Charge-carrier injection into CuPc thin films: A scanning tunneling microscopy study

Injection of charge carriers into the LUMO and HOMO levels of copper phthalocyanine (CuPc) is investigated by scanning tunneling microscopy (STM) and spectroscopy. The threshold for injection of charge carriers emitted by the tip of the STM allows us to determine the so-called single-particle band gap (Egsp) of CuPc polymorphs as well as the band edge alignment with respect to the Fermi level of the Au(1 1 1) substrate. We find Egsp values significantly lower than the optical band gap. This indicates the presence of intermolecular interactions, giving rise to in-gap states that can play a predominant role in controlling injection as well as transport properties. The lowest value of Egsp is found for the crystalline phases of CuPc obtained by thermal annealing, whereas the largest band gap and inferior charge-transport properties are found for samples grown on substrates at room temperature.