Correlation Between Charge Recombination and Lateral Hole-Hopping Kinetics in a Series of cis-Ru(phen')(dcb)(NCS)2 Dye-Sensitized Solar Cells.

Four complexes of the general form cis-Ru(phen')(dcb)(NCS)2, where dcb is 4,4'-(CO2H)2-2,2'-bipyridine and phen' is 1,10-phenanthroline (phen), 4,7-(C6H5)2-phen (Ph2-phen), 4,7-(CH3)2-phen (Me2-phen), or 3,4,7,8-(CH3)4-phen (Me4-phen), were anchored to mesoporous TiO2 thin films for applications as sensitizers in dye-sensitized solar cells (DSSCs). The compounds displayed metal based reductions Eo(RuIII/II) = 1.01 ± 0.05 V vs NHE and were potent reductants competent of excited-state electron transfer to TiO2 with yields ϕinj ≥ 0.75 in acetonitrile electrolytes. Average charge recombination rate constants, kcr, abstracted from nanosecond transient absorption measurements, and the apparent diffusion coefficients for lateral hole-hopping, abstracted from chronabsorptometry measurements, showed the same sensitizer dependency: Ru(Me4-phen) > Ru(Ph2-phen) > Ru(Me2-phen) ≈ Ru(phen). When used in operational solar cells, Ru(Ph2-phen) was most optimal with an efficiency of (6.6 ± 0.5)% in ionic liquids under 1 sun illumination. The superior performance of Ru(Ph2-phen) was traced to a higher injection yield and more efficient regeneration due to an unusually small sensitivity of kcr to the number of injected electrons.