Strategies for Optimizing the Performance of Cyclometalated Ruthenium Sensitizers for Dye-Sensitized Solar Cells

Pursuant to our goal of optimizing the performance of cyclometalated Ru sensitizers in the dye-sensitized solar cell (DSSC), the physicochemical properties of a series of tris-heteroleptic Ru-II complexes are reported. Each of these complexes contains a metal ligated by: (i) a bidentate 2,2'-bipyridine-4,4'-dicarboxylic acid (dcbpy) ligand to anchor the dye to the TiO2 surface; (ii) a cyclometalating ligand - with electron-withdrawing groups to ensure a sufficiently high oxidation potential for dye regeneration in the DSSC; and (iii) a 2,2'-bipyridine (bpy) ligand. UV/Vis and electrochemical data reveal that each complex exhibits broad metal-to-ligand charge transfer (MLCT) bands of significant intensity (epsilon = 1.0-2.3 x 10(4)M(-1)cm(-1)) in the visible region, and ground- and excited-state redox potentials that are appropriate for sensitizing TiO2. Analysis of the dyes in the DSSC highlights the sensitivity of cell performance to the oxidation potential for each of the dyes, which has important implications in the development of cyclometalated Ru sensitizers.