Impacts of Molecular Orientation on the Hole Injection Barrier Reduction: CuPc/HAT-CN/Graphene

The molecular orientation affected by the interaction between a substrate and deposited molecules plays an important role in device performance. It is known that the molecular orientation influences not only the charge transport property but also its electronic structure. Therefore, the combined study of morphology and electronic structure is of high importance for device application. As a transparent electrode, graphene has many promising advantages. However, graphene itself does not have an adequate work function for either an anode or a cathode, and thus the insertion of a charge injection layer is necessary for it to be used as an electrode. In this study, the hole injection barrier (HIB) reduction was investigated at the interface of copper phthalocyanine (CuPc)/graphene with the insertion of a hexaazatriphenylene hexacarbonitrile (HAT-CN) layer between them. The insertion of the HAT-CN layer roughens the originally flat graphene surface and it weakens the pi-interaction between CuPc and of graphene. This induces face-on and edge-on mixed orientations of CuPc, while CuPc on bare graphene shows merely a face-on orientation. As a result, the HIB is reduced by the contribution of edge-on CuPc having lower ionization energy (0.37 eV) along with the high work function of the HAT-CN layer (0.26 eV).