A photoelectrochemical solar cell that is based on the dye-sensitization of thin nanocrystalline films of TiO2 (anatase) nanoparticles in contact with a non-aqueous liquid electrolyte is described. The cell, fabricated at NREL, shows a conversion efficiency of ∼9.2% at AM1.5, which approaches the best reported value of 10%-11% by Gratzel at EPFL in Lausanne, Switzerland. Femtosecond (fs) pump-probe spectroscopy has been used to time resolve the injection of electrons into the conduction band of nanocrystalline TiO2 films under ambient conditions following photoexcitation of the adsorbed Ru(II)-complex dye. The measurement indicates an instrument-limited −50 fs upper limit on the electron injection time. We also report the sensitization of nanocrystalline TiO2 by a novel iron-based dye. cis-[FeII(2,2′-bipyridine-4,4′-dicarboxylic acid)2(CN)2], a chromophore with an extremely short-lived, nonemissive excited state. The dye also exhibits a unique “band selective” sensitization through one of its two absorpti...