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 TiO 2 photoanodes in aqueous solution. Owing to the good thermal stability of imidazolium, [DMPI] 2 −Ru adsorbed on TiO 2 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 − /I 3 − electrolytes, solars cell based on [DMPI] 2 −Ru‐sensitized TiO 2 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 TiO 2 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.