Ruthenium(II) complexes bearing a naphthalimide fragment: a modular dye platform for the dye-sensitized solar cell

Cycloruthenated complexes of the type [Ru(II)(N^N)2(C^N)](+) (N^N = substituted 2,2'-bipyridine; C^N = substituted 3-(2'-pyridyl)-1,8-naphthalimide ligand) are shown to generate high power conversion efficiencies (PCEs) in the dye-sensitized solar cell (DSSC). It is shown that substitution of the pyridine ring of the C^N ligand with conjugated groups can enhance molar absorption extinction coefficients, while the electron density imparted on the metal center is alleviated by the 1,8-naphthalimide fragment. This latter feature maintains a Ru(III)/Ru(II) redox couple more positive than 0.8 V versus NHE, thereby accommodating regeneration of the oxidized dye by an iodide-based redox mediator. This dye platform can consequently be modulated at various sites to enhance light absorption and suppress recombination between the redox mediator and the TiO2 surface without compromising dye regeneration, thereby maintaining device PCEs as high as 7%. We also introduce a new phosphine-based coadsorbent, bis(2-ethylhexyl)phosphinic acid (BEPA), which is significantly easier to synthesize than the widely used bis(3,3-dimethylbutyl)phosphinic acid (DINHOP) while also facilitating high dye loading.