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 photoexcited 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/pentacene/C60 heterojunction yields a comparatively insignificant surface photoresponse because both graphene and C60 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.
Keywords: built-in voltage; charge extraction; graphene; pentacene; surface photoresponse.