Interfacial electronic structures in an organic light-emitting diode

Direct measurements of electronic properties have been made for the metal/o rganic and organic/organic interfaces in a multilayer organic light-emittin g diode (LED) using ultraviolet photoemission spectroscopy. The device conf iguration 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-hydro xyquinoline aluminum (Alq)/Mg. For the material interfaces considered here, our result indicates that the traditional concept of vacuum-level alignmen t, though not valid for metal/organic interfaces, still holds at organic/or ganic interfaces. This implies that little charge transfer occurs at the in terfaces due to the small interaction between organic molecules. The larges t band offsets are observed between the lowest unoccupied state levels of t he organic molecules. Based on the directly measured energy-level alignment s, a model is proposed to explain the improved efficiency of multilayer org anic LEDs, as compared to those with a single organic layer structure. (C) 1999 American Institute of Physics. [S0003-6951(99)04705-1].