  Interface dipole determines the electronic energy alignment in donor/acceptor
interfaces and plays an important role in organic photovoltaics. Here we
present a study combining first principles density functional theory (DFT) with
ultraviolet photoemission spectroscopy (UPS) and time-of-flight secondary ion
mass spectrometry (TOF-SIMS) to investigate the interface dipole, energy level
alignment, and structural properties at the interface between CuPc and C60. DFT
finds a sizable interface dipole for the face-on orientation, in quantitative
agreement with the UPS measurement, and rules out charge transfer as the origin
of the interface dipole. Using TOF-SIMS we show that the interfacial morphology
for the bilayer CuPc/C60 film is characterized by molecular intermixing,
containing both the face-on and the edge-on orientation. The complementary
experimental and theoretical results provide both insight into the origin of
the interface dipole and direct evidence for the effect of interfacial
morphology on the interface dipole.
