An Insulating AlO Overlayer Prevents Lateral Hole Hopping Across Dye-Sensitized TiO Surfaces.

Three chromophores of the general form [Ru(bpy')(4,4'-(POH)-2,2'-bipyridine)], where bpy' is 4,4'-(C(CH))-2,2'-bipyridine (Ru(dtb)P); 4,4'-(CHO)-2,2'-bipyridine (Ru(OMe)P), and 2,2'-bipyridine (RuP) were anchored to mesoporous thin films of TiO nanocrystallites at saturation surface coverages to investigate lateral self-exchange Ru intermolecular hole hopping in 0.1 M LiClO/CHCN electrolytes. Hole hopping was initiated by a potential step 500 mV positive of the (Ru) 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 extracted from the potential step data revealed self-exchange rate constants that followed the trend: TiO|Ru(OMe)P ( = 1.4 × 10 s) > TiO|RuP (7.1 × 10 s) > TiO|Ru(dtb)P (6.5 × 10 s). Analysis of the anisotropy data with Monte Carlo simulations provided hole hopping rate constants for TiO|RuP and TiO|Ru(dtb)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 TiO(e)|Ru → TiO|Ru charge recombination rate constants. The atomic layer deposition of an ∼10 Å layer of AlO 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.