Chemical Engineering Journal, Vol.372, 624-630, 2019
Graphene quantum dots with nitrogen and oxygen derived from simultaneous reaction of solvent as exfoliant and dopant
Graphene quantum dots (GQDs) are promising materials for optoelectronic devices because their band-gap, derived from quantum confinement and edge effects, can be easily tuned via their size or surface/edge states. In this paper, a novel approach to synthesize nitrogen-and oxygen-doped GQDs (NO-GQDs) is presented. Nitrogen and oxygen are mainly bound at the GQD edges, resulting in high crystallinity and good electrical properties. A simple solvothermal reaction using N-methyl-2-pyrrolidone (NMP), whose surface energy is similar to that of graphite as a raw material, can simultaneously exfoliate, cut, and finally transform the graphite into the GQDs with heteroatoms derived from the decomposed NMP solution. The synthesized NO-GQDs have a less defective and more selectively edge-functionalized structure compared to other reported GQDs. The electrical properties of NO-GQDs are investigated using them as the additive of hole-transporting materials (HTMs) in an optoelectronic device such as perovskite solar cells (PeSCs). Compared with PEDOT:PSS, a mixture of NO-GQDs and PEDOT:PSS shows a 36.2% increase in the power conversion efficiency (PCE) (maximum PCE: 11.47%) and good device stability. Therefore, it is believed that the improvement of photovoltaics is solely attributed from NO-GQDs which act as a positive role of faster hole transfer. We could confirm that the NO-GQDs facilitate hole-extraction from a photoactive layer and guarantee the more stable operation of PeSCs.