Sensitizing TiO2 nanoparticles with Ru(II) polypyridyl complexes containing anchoring nitrile groups for applications in solar cells

In this work, five dyes-based on Ru(II) complexes with 2,2́- bipyridyl ligands substituted at the 4,4́-positions with a different number of nitrile anchoring groups and distinct −X electronic donor units were tested as TiO₂ electrode sensitizers in solar cells (SCs). All dyes were PF₆^- salts: three complexes of formulae [Ru(4,4́-(X)₂-bpy)₂(Mebpy-CN)]²⁺ with one nitrile group and X =  − CH_3, –OCH_, −N(CH₃)₂; and two of formulae [Ru(4,4′-(X)₂-bpy)(Mebpy-CN)₂]²⁺, with two nitrile units and X = –OCH₃, −N(CH₃)₂. In all cases, bpy = 2,2′-bipyridine and Mebpy-CN = 4-methyl-2,2′-bipyridine-4′-carbonitrile. After characterizing the electrodes morphologically and optically, they were employed in DSSCs (dye-sensitized SCs) using Pt/FTO as counter-electrode (CE) and a KI/I₂ in CH₃CN as liquid electrolyte. Current density–voltage (J-V) and impedance spectroscopy (IS) measurements highlighted that the SC with the highest efficiency (η) is the one that contains − N(CH₃)₂ as donor units and two nitrile groups as anchoring entities. It could be concluded that higher electronic delocalization that occurs between the bpys substituted with strong electronic donor groups and the metal orbitals impose excellent properties to improve efficiency, which further improves if two anchoring groups are employed.