Fast Hole Surface Conduction Observed for Indoline Sensitizer Dyes Immobilized at Fluorine-Doped Tin Oxide-TiO2 Surfaces

The indoline dyes D102, D131, D149, and D205 have been characterized when adsorbed on fluorine-doped tin oxide (FTO) and TiO2 electrode surfaces. Adsorption from 50:50 acetonitrile tert-butanol onto fluorine-doped tin oxide (FTO) allows approximate Langmuirian binding constants of 6.5 x 10(4), 2.0 x 10(3), 2.0 x 10(4), and 1.5 x 10(4) mol(-1) dm(3), respectively, to be determined. Voltammetric data obtained in acetonitrile/0.1 M NBu4PF6 indicate reversible one-electron oxidation at E-mid = 0.94, 0.91, 0.88, and 0.88 V vs Ag/AgCl(3 M KCl), respectively, with dye aggregation (at high coverage) causing additional peak features at more positive potentials. Slow chemical degradation processes and electron transfer catalysis for iodide oxidation were observed for all four oxidized indolinium cations. When adsorbed onto TiO2 nanoparticle films (ca. 9 nm particle diameter and ca. 3 mu m thickness on FTO), reversible voltammetric responses with E-mid = 1.08, 1.16, 0.92, and 0.95 V vs Ag/AgCl(3 M KCl), respectively, suggest exceptionally fast hole hopping diffusion (with D-app > 5 x 10(-9) m(2) s(-1)) for adsorbed layers of all four indoline dyes, presumably due to pi-pi stacking in surface aggregates. Slow dye degradation is shown to affect charge transport via electron hopping. Spectroelectrochemical data for the adsorbed indoline dyes on FTO-TiO2 revealed a red-shift of absorption peaks after oxidation and the presence of a strong charge transfer band in the near-IR region. The implications of the indoline dye reactivity and fast hole mobility for solar cell devices are discussed.