Electron transfer dynamics in nanocrystalline titanium dioxide solar cellssensitized with ruthenium or osmium polypyridyl complexes

The electron transfer dynamics in solar cells that utilize sensitized nanoc rystalline titanium dioxide photoelectrodes and the iodide/triiodide redox couple have been studied on a nanosecond time scale. The ruthenium and osmi um bipyridyl complexes Ru(H2L')(2)(CN)(2), Os(H2L)(2)(CN)(2), Ru(H2L')(2)(N CS)(2), and Os(H2L')(2)(NCS)(2), when H2L' is 4,4'-dicarboxylic acid 2,2'-b ipyridine, inject electrons into the semiconductor with a rate constant > 1 0(8) s(-1). The effects of excitation intensity, temperature, and applied p otential on the recombination reaction were analyzed using a second-order k inetics model. The rates of charge recombination decrease with increasing d riving force to the oxidized sensitizer, indicating that charge recombinati on occurs in the Marcus inverted region. The electronic coupling factors be tween the oxidized sensitizer and the injected electrons in TiO2 and the re organization energies for the recombination reaction vary significantly for the different metal complexes. The charge recombination rates are well des cribed by semiclassical electron transfer theory with reorganization energi es of 0.55-1.18 eV. Solar cells sensitized with Ru(H2L')(2)(CN)(2), Os(H2L' )(2)-(CN)(2), and Ru(H2L')(2)(NCS)(2) have favorable photoelectrochemical c haracteristics, and iodide is oxidized efficiently. in contrast, iodide oxi dation limits the efficiency of cells based on sensitization of TiO2 with O s(H2L')(2)(NCS)(2). The observation that charge recombination occurs in the Marcus inverted region has important implications for the design of molecu lar sensitizers in nanocrystalline solar cells operated under our experimen tal conditions.