Revisiting the Role of Graphene Quantum Dots in Ternary Organic Solar Cells: Insights into the Nanostructure Reconstruction and Effective Förster Resonance Energy Transfer

Recent studies have introduced the graphene quantum dot (GQD) as a third material for the bulk-heterojunction polymer:fullerene solar cell (PSC) to improve light conversion efficiency. Although exciton generation/dissociation and carrier transport in the GQD-incorporated light-absorbing layer are strongly influenced by the ternary component, detailed analysis on the role of GQD in the light-absorbing layer is lacking. In this study, a perspective on origin of improved photovoltaic performance of GQD-incorporated PSC is provided. The Förster resonance energy transfer (FRET) from GQD to polymer:fullerene and reorganization of the ternary-component film are analyzed. The GQD chemical nature difference after controlling nitrogen functionality affects the quantum yield (QY) and surface energy. Because the GQD is distributed in the fullerene-rich domain, actual improvements in the FRET to polymer-rich phase are not great, despite the improved QY and red-shifted photoluminescence. However, changes in the surface energy affect the degree of crystallinity of polymer domains and nanophase separation in polymer:fullerene films. The intense FRET from GQD to fullerene and favorable changes in the nanostructure lead to the enhancing the power conversion efficiency of GQD-containing ternary PSC.