Observing Electron Extraction by Mono layer Graphene Using Time-Resolved Surface Photoresponse Measurements

Graphene is considered a next-generation electrode for indium tin oxide (ITO)-free organic photovoltaic devices (OPVs). However, to date, limited numbers of OPVs containing surface-Modified graphene electrodes perform as well as ITO-based counterparts, and no devices containing a bare graphene electrode have been reported to yield satisfactory rectification characteristics. In this report, we provide experimental data to learn why. Time-resolved surface photoresponse, measurements on templated pentacene-on-graphene films directly reveal that p-doped monolayer graphene efficiently extracts electrons, not holes, from photo-excited pentacene. Accordingly, a graphene/pentacene/MoO3 heterojunction displays a large surface photoresponse and, by inference, efficient dissociation of photogenerated excitons, with graphene serving as an electron extraction layer and MoO3 as a hole extraction layer. In Contrast, a graphene/pentatehe/C-60 heterojunction yields a comparatively insignificant surface photoresponse because both graphene and C-60 act as competing electron extraction layers. The data presented herein:provide experimental insight for future endeavors involving bare graphene as an electrode for organic,photovoltaic devices and strongly suggest that p-doped graphene is best considered a cathode for OPVs.