Charge-carrier injection into polymorphic forms of CuPc: A scanning tunneling microscopy study

In this work injection of charge carriers into the empty and filled states of copper phthalocyanine (CuPc) thin films is investigated by scanning tunneling microscopy and spectroscopy. Measurements of the threshold energy for electrons and holes from the tip of the scanning electron microscope allows us to determine the so-called single-particle band gap (E(gsp)) of different polymorphic phases present on CuPc thin films. We find that the value of E(gsp) depends on the crystal structure and packing density of the material, E(gsp) being in the range of 0.18 to 1.1 eV, which is much smaller than the optical band gap of ~1.7 eV. The lowest values of E(gsp) are found for the crystalline phases of CuPc, obtained by thermal annealing, whereas the largest values and inferior charge-transport properties are found for unannealed samples where the packing density of the CuPc molecules and long-range crystallographic order appear to be lowest. This discrepancy 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 of charge carriers into this material relevant, for applications in organic electroluminescent devices for example.
Keywords: Mueller, Riess