Energy-level alignment at interfaces between metals and the organic semiconductor 4,4′-N,N′-dicarbazolyl-biphenyl

We have used ultraviolet photoemission spectroscopy to study the formation of interfaces between the organic semiconductor, 4,4′-N,N′-dicarbazolyl-biphenyl (CBP), and the metals Au, Ag, and Mg. Each interface was studied by depositing the organic on the metal, and by depositing the metal on the organic. The two methods produced inequivalent interfaces, except in the case of Au/CBP. The position of the highest occupied molecular orbital relative to the Fermi level and the magnitude of the interface dipole were measured for each interface. The barrier to electron injection from each metal was estimated using the magnitude of the measured optical gap. An interface dipole, of magnitude nearly independent of the metal work function, was formed when CBP was deposited on a metal surface. The position of the Fermi level within the CBP gap was found to vary strongly with the metal work function.