An Insulating Al2o3 Overlayer Prevents Lateral Hole Hopping Across Dye-Sensitized Tio2 Surfaces

Three chromophores of the general form [Ru(bpy')(2)(4,4'-(PO3H2)2-2,2'-bipyridine)](2+), where bpy' is 4,4'-(C(CH3)(3))2-2,2'-bipyridine (Ru(dtb)(2)P); 4,4'-(CH3O)2-2,2'-bipyridine (Ru(OMe)(2)P), and 2,2'-bipyridine (RuP) were anchored to mesoporous thin films of TiO2 nanocrystallites at saturation surface coverages to investigate lateral self-exchange Ru-III/(II) intermolecular hole hopping in 0.1 M LiClO4/CH3CN electrolytes. Hole hopping was initiated by a potential step 500 mV positive of the E-1/2 (Ru-III/(II)) potential or by pulsed laser (532 nm, 8 ns fwhm) excitation and monitored by visible absorption chronoabsorptometry and time-resolved absorption anisotropy measurements, respectively. The hole hopping rate constant k(R) extracted from the potential step data revealed self-exchange rate constants that followed the trend: TiO2[Ru(OMe)(2)P (k(et) = 1.4 x 10(6) s(-1)) > TiO2 vertical bar RuP (7.1 X 10(5) s(-1)) > TiO2 vertical bar Ru(dtb)(2)P (6.5 x 10(4) s(-1)). Analysis of the anisotropy data with Monte Carlo simulations provided hole hopping rate constants for TiO2 vertical bar RuP and TiO2 vertical bar Ru(dtb)(2)P that were within experimental error the same as that measured with the potential step. The hole hopping rate constants were found to trend with the TiO2(e(-))vertical bar Ru-III -> TiO2 vertical bar Ru-II charge recombination rate constants. The atomic layer deposition of an similar to 10 angstrom layer of Al2O3 on top of the dye-sensitized films was found to prevent hole hopping by both initiation methods even though the chromophore surface coverage exceeded the percolation threshold and excited-state injection was efficient. The dramatic hole hopping turnoff was attributed to a larger outer-sphere reorganization energy for self-exchange due to the restricted access of electrolyte to the redox active chromophores. The implications of these findings for solar energy conversion applications are discussed.