Dispersion Control of Reduced Graphene Oxide and Application to Inverted Organic Solar Cells as a Hole Transport Layer

Abstract

Graphene-based materials have received tremendous attention due to their excellent mechanical and electrical properties. Among them, graphene oxide (GO), which chemically exfoliated from graphite, is possible to solution process in aqueous solutions owing to having oxygen-containing functional groups on its basal plane and edges. Moreover, they have active sites for reduction of GO and covalent functionalization of GO with small organic molecules or inorganic nanomaterials. For these reasons, GO have been researched in many applications such as polymer composites, biomedical, and energy-related materials. In Particular, Reduced graphene oxide (rGO) reduced by functionalized hydrazine from GO is promising alterative material to poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as hole transport layer (HTL) in organic solar cells because it allowed the controllability of electronic properties through versatile functionalization and can improve the long-term stability of solar cells. However, rGO is difficult to apply in inverted organic solar cells because of limited dispersity in alcohol based solvent such as 2-propanol (IPA). The rGO is usually dispersed in dimethylformamide (DMF), which melt the organic active layer. Therefore, more research is required about controlling the dispersibility of rGO while maintaining their electrical properties in order that rGO apply to inverted organic solar cells as a HTL via solution process. In this study, we introduced trifluoromethoxy group on the rGO surface (FMrGO) for controlling the dispersibility of rGO while maintaining their electrical properties. FMrGO is well dispersed in IPA and 2-ethoxyethanol due to the flexible methoxy groups. The structural, optical, and thermal properties of FMrGO was characterized by Infrared spectroscopy, EA, XPS, UV-vis spectroscopy and TGA and their work function was estimated by UPS. Finally, we introduced the FMrGO film as a HTL on the active film through solution process and investigated the effect of FMrGO on device performance and long-term stability of inverted organic solar cells.