Energy level alignment of rubrene adsorbed on Au(111) was studied by photoemission spectroscopy. After rubrene adsorption, the work function is reduced from for clean Au to , suggesting the invalidity of vacuum level alignment and the presence of a strong interfacial dipole. The frontier molecular orbital energies of rubrene are modified by electrode surface polarization in the submonolayer regime. As a consequence, the hole injection barrier is thickness dependent and varies from about for a monolayer of rubrene to for a thick layer.
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