Molecular Orientation Dependent Energy Level Alignment at Organic-Organic Heterojunction Interfaces

Molecular orientation dependent energy level alignments at organic-organic heterojunction (OOH) interfaces have been investigated with synchrotron based high-resolution photoemission spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) measurements. Model systems of the lying-down 3,4,9, 10-perylene-tetracarboxylic-dianhydride (PTCDA) films on both standing-up and lying-down copper hexadecafluorophthalocyanine (F16CuPc) and copper(II) phthalocyanine (CuPc) thin films have been used to illustrate the molecular orientation dependent interface properties. The formation of different interface dipoles at the heterojunction interfaces is strongly influenced by the orientation dependent ionization potentials of the underlying F16CuPC or CuPc thin films. This is attributed to the intrinsic surface dipoles induced in the standing-up F16CuPC (CuPc) film due to the polar intermolecular C-F (C-H) bonds formed at the interface. In situ NEXAFS measurements reveal that the too in-temperature deposition of PTCDA layers does not alter the molecular orientation of the underlying lying-down or standing-up F16CuPC thin films. We also demonstrate that the binding energies of both the C Is core level and the highest-occupied-molecular-orbital (HOMO) of PTCDA on the lying-down F16CuPC thin film is 0.3 eV higher than those on the standing-up F16CuPC thin film. This shows that it is possible to manipulate the energy level alignment at OOH inter-faces by choosing the appropriate molecular orientation. In contrast, the HOMO positions of PTCDA on both lying-down and standing-up CuPc films are almost identical. This Suggests that ail orientation independent Fermi-level pinning occurs at the PTCDA/CuPc interfaces involving interfacial charge transfer.