Interfacial electronic structure of N,N'-bis(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine/copper phthalocyanine:C60 composite/Au studied by ultraviolet photoemission spectroscopy

The interfacial electronic structures of N,N-'-bis(1-naphthyl)-N,N-'-diphenyl-1,1(')-biphenyl-4,4(')-diamine (NPB)/copper phthalocyanine (CuPc)/Au, NPB/C-60/Au, and NPB/CuPc:C-60 composite/Au were investigated by in situ ultraviolet photoelectron spectroscopy to understand the highly efficient hole injection in organic light-emitting diode. The hole-injection barrier of CuPc:C-60/Au was 0.52 eV, while those of CuPc/Au and C-60/Au were 0.96 and 1.62 eV, respectively. The lowered injection barrier is attributed to the smaller interface dipole of CuPc:C-60 compared to that of pristine CuPc. This small interface dipole pulled up the highest occupied molecular orbital of CuPc in composite, which results in the decreased hole-injection barrier. (c) 2007 American Institute of Physics.