Interfacial electronic structures in an organic light-emitting diode

Abstract

Direct measurements of electronic properties have been made for the metal/organic and organic/organic interfaces in a multilayer organic light-emitting diode (LED) using ultraviolet photoemission spectroscopy. The device configuration considered is indium–tin oxide (ITO)/copper phthalocyanine (CuPc)/N,N′-bis-(1-naphyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamne (NPB)/8-hydroxyquinoline aluminum (Alq)/Mg. For the material interfaces considered here, our result indicates that the traditional concept of vacuum-level alignment, though not valid for metal/organic interfaces, still holds at organic/organic interfaces. This implies that little charge transfer occurs at the interfaces due to the small interaction between organic molecules. The largest band offsets are observed between the lowest unoccupied state levels of the organic molecules. Based on the directly measured energy-level alignments, a model is proposed to explain the improved efficiency of multilayer organic LEDs, as compared to those with a single organic layer structure.