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'-(CO 2 H) 2 -2,2'-bipyridine and phen' is 1,10-phenanthroline (phen), 4,7-(C 6 H 5 ) 2 -phen (Ph 2 -phen), 4,7-(CH 3 ) 2 -phen (Me 2 -phen), or 3,4,7,8-(CH 3 ) 4 -phen (Me 4 -phen), were anchored to mesoporous TiO 2 hin films for applications as sensitizers in dye-sensitized solar cells (DSSCs). The compounds displayed metal based reductions E o (Ru III/II ) = 1.01 ± 0.05 V vs NHE and were potent reductants competent of excited-state electron transfer to TiO 2 with yields ϕ inj ≥ 0.75 in acetonitrile electrolytes. Average charge recombination rate constants, k cr , 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(Me 4 -phen) > Ru(Ph 2 -phen) > Ru(Me 2 -phen) ≈ Ru(phen). When used in operational solar cells, Ru(Ph 2 -phen) was most optimal with an efficiency of (6.6 ± 0.5)% in ionic liquids under 1 sun illumination. The superior performance of Ru(Ph 2 -phen) was traced to a higher injection yield and more efficient regeneration due to an unusually small sensitivity of k cr o the number of injected electrons.