The alignment of the highest occupied molecular orbitals (HOMO) at the tris (8-hydroxy quinoline) aluminum(Alq3)/N,N′-di-(3-methylphenyl)-N,N′diphenyl-4,4′-diaminobiphenyl (TPD) heterojunction, used in organic light-emitting diodes (OLED), was determined by growing a TPD layer in several steps on a thickAlq3substrate layer. After each growth step the sample was characterizedin situby x-ray and ultraviolet photoemission spectroscopy. The offset of the HOMO maxima at the interface was determined to be −0.13 eV fromAlq3to TPD. By including the known HOMO–lowest occupied molecular orbital (LUMO) gaps for both molecules into the evaluation, the offset of the LUMO minima was determined to be −0.33 eV fromAlq3to TPD. These values are consistent with previous assumptions that this interface represents a higher barrier for electron injection fromAlq3to TPD than for hole injection from TPD toAlq3.©1998 American Institute of Physics.