Work function tuning of directly grown graphene via ultraviolet-ozone treatment for electrode application in organic photovoltaic devices

Graphene is an efficient electrode material for flexible optoelectronic devices because of its high transmittance and electrical conductivity. A graphene film directly grown using plasma-enhanced chemical vapor deposition is suitable for electrode applications because it can be prepared on a glass substrate at relatively low temperatures without a transfer process from a metal catalyst. However, to obtain efficient optoelectronic devices using graphene electrodes, surface modification is necessary. Herein, we tuned the work function of a graphene electrode using ultraviolet (UV)-O3 treatment. The power conversion efficiency of organic photovoltaic devices was significantly increased by the UV-O3 treatment of the graphene anode. To investigate the origin of the improved device performance, the interface between the graphene anode and tetraphenyldibenzoperiflanthene (DBP) donor layer was analyzed via in situ ultraviolet photoelectron spectroscopy. The energy offset between the Fermi level of graphene and the highest occupied molecular orbital level of DBP was significantly reduced because of the increased work function. The results indicate that the electronic structure modification of gra-phene is of significant importance for fabricating efficient optoelectronic devices.