Graphene is highly promising as an electrode for flexible optoelectronic devices due to its excellent conductivity and transparency. However, it is necessary to tailor its work function with a charge injection layer in order to obtain favorable energy level alignment for efficient charge injection. An adequate charge injection layer can only be chosen with the understanding of the interfacial electronic structure between a charge transport layer and an electrode. In this study, we investigated the energy level alignment of N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB)/hexaazatriphenylene hexacarbonitrile (HATCN)/graphene using in situ ultraviolet and X-ray photoelectron spectroscopy measurements. The effective work function of graphene was significantly increased by 0.94 eV by the HAT-CN hole injection layer HIL) due to the interface dipole formation. In addition, the charge generation barrier (CGB) between NPB and HAT-CN, which plays a decisive role in charge injection efficiency with a charge generation HIL, was measured to be 0.66 eV. This CGB on graphene is the same as the CGBs on other electrodes, and smaller than that of the widely-used MoO3 HIL. Therefore, HAT-CN could be a promising HIL for efficient flexible organic light- emitting diodes with a graphene anode. (C) 2016 Elsevier B. V. All rights reserved.