Heteroleptic Ruthenium(II) Complexes with 2,2′-Bipyridines Having Carbonitriles as Anchoring Groups for ZnO Surfaces: Syntheses, Physicochemical Properties, and Applications in Organic Solar Cells

Abstract

Heteroleptic ruthenium (II) complexes were used for sensitizing ZnO surfaces in organic solar cells (OSCs) as mediators with photoactive layers. The complexes [Ru(4,4′-X2-bpy)(Mebpy-CN)2]2+ (with X = -CH3, -OCH3 and -N(CH3)2; bpy = 2,2′-bipyridine; Mebpy-CN = 4-methyl-2,2′-bipyridine-4′-carbonitrile) were synthesized and studied by analytical and spectroscopical techniques. Spectroscopic, photophysical, and electrochemical properties were tuned by changing the electron-donating ability of the -X substituents at the 4,4′-positions of the bpy ring and rationalized by quantum mechanical calculations. These complexes were attached through nitrile groups to ZnO as interfacial layer in an OSC device with a PBDB-T:ITIC photoactive layer. This modified inorganic electron transport layer generates enhancement in photoconversion of the solar cells, reaching up to a 23% increase with respect to the unsensitized OSCs. The introduction of these dyes suppresses some degradative reactions of the nonfullerene acceptor due to the photocatalytic activity of zinc oxide, which was maintained stable for about 11 months. Improving OSC efficiencies and stabilities can thus be achieved by a judicious combination of new inorganic and organic materials.