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 (Alq(3))/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 Alq(3) 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 Alq(3) 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 Alq(3) to TPD. These values are consistent with previous assumptions that this interface represents a higher barrier for electron injection from Alq(3) to TPD than for hole injection from TPD to Alq(3). (C) 1998 American Institute of Physics, [S0003-6951 (98)03434-2].