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

Injection of charge carriers into the LUMO and HOMO levels of copper phthal ocyanine (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 g ap (E-gsp) Of CuPc polymorphs as well as the band edge alignment with respe ct to the Fermi level of the Au(1 1 1) substrate. We find E-gsp values sign ificantly lower than the optical band gap. This indicates the presence of i ntermolecular interactions, giving rise to in-gap states that can play a pr edominant role in controlling injection as well as transport properties. Th e lowest value of E-gsp is found for the crystalline phases of CuPc obtaine d by thermal annealing, whereas the largest band gap and inferior charge tr ansport properties are found for samples grown on substrates at room temper ature. (C) 2001 Elsevier Science B.V. All rights reserved.