Molecular-Reductant-Induced Control of a Graphene-Organic Interface for Electron Injection

Surface doping of graphene with redox-active molecules is an effective approach to tune its electrical properties, in particular for application as transparent electrodes. Here we present a study and application of surface n-doping of graphene with the molecular reductant (pentamethylcyclopentadienyl)(1,3,5-trimethylbenzene)ruthenium dimer ([RuCp*Mes](2)). Photoemission spectroscopy and carrier-transport measurements are combined to investigate doping-induced changes in the electronic structure of the interface between graphene and phenyldi(pyren-2-yl)phosphine oxide (POPy2), which is a lowelectron-affinity material that has been used as an electron-transport layer (ETL) in organic light-emitting diodes. Photoemission and Hall voltage measurements confirm the n-doping of graphene. Doping with 1-2 nm of [RuCp*Mes](2) reduces the graphene work function by 1.8 eV and the electron injection barrier by more than 1 eV, enhancing electron injection into POPy2 by several orders of magnitude. Graphene/POPy2/Al diodes with doped graphene cathodes exhibit reasonable stability in both nitrogen and air. These results represent a significant step toward the use of graphene as a transparent cathode for organic devices in general and for OLEDs in particular.