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.