Energy level alignment in N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/hexadecafluoro copper phthalocyanine (F16CuPc)/Au and NPB/CuPc/Au heterojunction

The interfacial electronic structures of N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/hexadecafluoro copper phthalocyanine (F16CuPc)/Au and NPB/copper phthalocyanine (CuPc)/Au were investigated by in situ X-ray and ultraviolet photoelectron spectroscopy to study the hole-injection barrier depending on the ionization energy of hole-injection layer materials. Although the measured ionization energy of F16CuPc (6.30 eV) was much higher than that of CuPc (5.15 eV), the difference in the barrier heights of the two different films was relatively marginal (0.27 eV) due to the formation of interface dipole caused by the charge redistribution. We confirmed that the interface dipole of the buffer layer (CuPc and F16CuPc) pushed down the core levels as well as the valence levels of the top organic layer (NPB) in the NPB/F16CuPc/Au and NPB/CuPc/Au heterojunction.