A series
of cis-[Ru(LL)(dcbH2)(NCS)2] compounds, where dcbH2 = 2,2′-bipyridine-4,4′-dicarboxylic
acid and LL = 1,10-phenanthroline (Ru(phen)), 4,7-dipyrrole-1,10-phenanthroline
(Ru(pyr)), 4,7-diindole-1,10-phenanthroline (Ru(ind)), or 4,7-dicarbazole-1,10-phenanthroline
(Ru(cbz)), was investigated for application as sensitizers in mesoporous
TiO2 dye-sensitized solar cells (DSSCs). A systematic increase
in the number of rings of the aromatic substituents at the 4,7-positions
of the 1,10-phenanthroline allowed tuning of the molecular size of
the sensitizers and the energy stored in the excited state while maintaining
the same ground-state Ru3+/2+ reduction potentials. These
small structural changes had a significant influence on the rates
and/or efficiencies of electron injection, back-electron transfer,
recombination to oxidized mediators, lateral self-exchange electron
transfer, and regeneration through iodide oxidation that were reflected
in distinct photoelectrochemical performance of full operating DSSCs.
The global efficiencies, open-circuit voltages, and short-circuit
current densities of the DSSCs consistently followed the trend Ru(pyr) <
Ru(ind) < Ru(phen) < Ru(cbz), and the most optimal performance
of Ru(cbz) was ascribed to dramatically slower recombination to the
oxidized redox mediators. Transient photovoltage and transient absorption
experiments both revealed significantly slower recombination as the
size of the aromatic substituents increased with Ru(cbz) providing
the most promising behavior for application in dye sensitization.