  The interface formation between copper phthalocyanine (CuPc) and two
representative metal substrates, i.e., Au and Co, was investigated by the
combination of ultraviolet photoelectron spectroscopy and inverse photoelectron
spectroscopy. The occupied and unoccupied molecular orbitals and thus the
transport band gap of CuPc are highly influenced by film thickness, i.e.,
molecule-substrate distance. Due to the image charge potential given by the
metallic substrates the transport band gap of CuPc "opens" from $(1.4 \pm 0.3)$
eV for 1 nm thickness to $(2.2 \pm 0.3)$ eV, and saturates at this value above
10 nm CuPc thickness. The interface dipoles with values of 1.2 eV and 1.0 eV
for Au and Co substrates, respectively, predominantly depend on the metal
substrate work functions. X-ray photoelectron spectroscopy measurements using
synchrotron radiation provide detailed information on the interaction between
CuPc and the two metal substrates. While charge transfer from the Au or Co
substrate to the Cu metal center is present only at sub-monolayer coverages,
the authors observe a net charge transfer from the molecule to the Co substrate
for films in the nm range. Consequently, the Fermi level is shifted as in the
case of a p-type doping of the molecule. This is, however, a competing
phenomenon to the energy band shifts due to the image charge potential.
