Soluble Reduced Graphene Oxide Sheets Grafted with Polypyridylruthenium-Derivatized Polystyrene Brushes as Light Harvesting Antenna for Photovoltaic Applications

Soluble graphene nanosheets, prepared by grafting polystyrene-based polymer chains from the surface of reduced graphene oxide (RGO), have been functionalized with pendant Ru(II) polypyridine chromophores. <i>N</i>-Hydroxysuccinimide (NHS) derivatized <i>p</i>-vinylbenzoic acid polymer chains were grown from methyl bromoisobutyrate initiation sites on the surface of RGO by atom transfer radical polymerization (ATRP). Deprotection of the resulting NHS polystyrene chains followed by amide coupling with the amine-derivatized Ru(II) polypyridyl complex [Ru(4-CH₂NH₂-4′-CH₃-bpy)(bpy)₂]²⁺ (4-CH₂NH₂-4′-CH₃-bpy = 4-aminomethyl-4′-methyl 2,2′-bipyridine and bpy = 2,2′-bipyridine) afforded the covalently linked RGO-metallopolymer. The hybrid graphene-polymer assembly has been fully characterized with clear evidence for covalent attachment of the metallopolymer brushes to the graphene substrate. On the basis of thermal gravimetric analysis, one polymer strand is grafted to the surface of RGO for every hundred graphene carbons. The covalently linked polymer brushes feature controlled chain lengths of ∼30 repeat units with a small polydispersity index (PDI, ∼ 1.2). Photovoltaic cells based on the derivatized polymers and graphene-polymer assemblies were evaluated. The graphene-polymer assembly in the configuration, ITO/PEDOT:PSS/RGO-PSRu/PC₆₀BM/Al, exhibited enhanced photocurrent and power conversion efficiencies (∼5 fold) relative to devices with the configuration, ITO/PEDOT:PSS/PSRu/PC₆₀BM/Al