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

• Published in: Applied physics letters 1998-08, Vol.73 (8), p.1026-1028
• Main Authors: Schlaf, R., Parkinson, B. A., Lee, P. A., Nebesny, K. W., Armstrong, N. R.
• Format: Article
• Language: English
• ISSN: 0003-6951; 1077-3118
• DOI: 10.1063/1.122073

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.