Orientation-dependent ionization potential of CuPc and energy level alignment at C60/CuPc interface

The electronic structure evolution of interfaces of fullerene (C-60) with copper phthalocyanine (CuPc) on highly oriented pyrolytic graphite (HOPG) and on native silicon oxide has been investigated with ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. The LUMO edge of C-60 was found to be pinned at the interface with CuPc on SiO2. A substantial difference in the electron affinity of CuPc on the two substrates was observed as the orientation of CuPc is lying flat on HOPG and standing up on SiO2. The ionization potential and electron affinity of C-60 were not affected by the orientation of CuPc due to the spherical symmetry of C-60 molecules. We observed band bending in C-60 on the standing-up orientation of CuPc molecules, while the energy levels of C-60 on the flat-lying orientation of CuPc molecules were observed to be flat. The observation points to a dependence of photoexcited charge transfer on the relative molecular orientation at the interface.