Determination of frontier orbital alignment and band bending at an organic semiconductor heterointerface by combined x-ray and ultraviolet photoemission measurements

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 thick Alq₃ substrate layer. After each growth step the sample was characterized in situ by x-ray and ultraviolet photoemission spectroscopy. The offset of the HOMO maxima at the interface was determined to be -0.13 eV from Alq₃ to 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 from Alq₃ to TPD. These values are consistent with previous assumptions that this interface represents a higher barrier for electron injection from Alq₃ to TPD than for hole injection from TPD to Alq₃.