Substrate effects on the electronic properties of an organic/organic heterojunction

The electronic structures of copper-phthalocyanine/tris(8-hydroxyquinoline) aluminum (CuPcAl q3) heterojunctions on Mg and indium tin oxide (ITO) substrates have been studied by photoemission spectroscopy. While the typical vacuum energy level lineup occurred at the CuPcAl q3 junction on ITO, the same junction formed on Mg displayed vastly different electronic structures, showing a 0.5 eV band bending associated with the formation of a space charge layer. The substrate effects were explained by the proximity of the Mg's Fermi level to the lowest unoccupied molecular orbital of CuPc, resulting in spontaneous charge transfer. The results show the feasibility of tuning the electronic properties of an organic heterojunction via the Fermi level of the substrate. © 2006 American Institute of Physics.