Thickness-dependent energy level alignment of rubrene adsorbed on Au(111)

Energy level alignment of rubrene adsorbed on Au(111) was studied by photoemission spectroscopy. After rubrene adsorption, the work function is reduced from 5.24eV for clean Au to 4.31eV, 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 0.4eV for a monolayer of rubrene to 0.9eV for a thick layer.