High-Performance Ruthenium Sensitizers Containing Imidazolium Counterions for Efficient Dye Sensitization in Water

A new type of water-soluble ruthenium sensitizers incorporating imidazolium counterions, denoted [DMPI]₂ -Ru and [DMHI]₂ -Ru, has been developed, which can be efficiently adsorbed onto TiO₂ photoanodes in aqueous solution. Owing to the good thermal stability of imidazolium, [DMPI]₂ -Ru adsorbed on TiO₂ has a higher decomposition temperature than N719 dye [di(tetrabutylammonium) cis-di(thiocyanato)bis(2,2'-bipyridine-4,4'-dicarboxylato)ruthenium(II)]. When using organic solvent-based I^- /I₃^- electrolytes, solars cell based on [DMPI]₂ -Ru-sensitized TiO₂ in water show high power conversion efficiencies (PCE) of up to 10.2 %, which is higher than that of N719 (9.9 %) under the common conditions for dye sensitization in organic solvent. [DMHI]₂ -Ru, with poorer water solubility than [DMPI]₂ -Ru, gives a smaller dye-adsorption amount on TiO₂ and thus a lower PCE of 9.4 %. From the viewpoint of safety and environmental impact, the fabrication of dye-sensitized solar cells (DSSCs) by using water as solvent is undoubtedly a preferable strategy. Although the [DMPI]₂ -Ru-based device fabricated by using water as the solvent for both the dye-sensitization process and the electrolyte gives a relatively low efficiency, it provides a promising approach for the practical application of DSSCs.