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)2P); 4,4′-(CH3O)2-2,2′-bipyridine (Ru(OMe)2P), and 2,2′-bipyridine (RuP) were anchored to mesoporous thin films of TiO2 nanocrystallites at saturation surface coverages to investigate lateral self-exchange RuIII/II intermolecular hole hopping in 0.1 M LiClO4/CH3CN electrolytes. Hole hopping was initiated by a potential step 500 mV positive of the E1/2 (RuIII/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 kR extracted from the potential step data revealed self-exchange rate constants that followed the trend: TiO2|Ru(OMe)2P (ket = 1.4 × 106 s–1) > TiO2|RuP (7.1 × 105 s–1) > TiO2|Ru(dtb)2P (6.5 × 104 s–1). Analysis of the anisotropy data with Monte Carlo simulations provided hole hopping rate constants for TiO2|RuP and TiO2|Ru(dtb)2P 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–)|RuIII → TiO2|RuII charge recombination rate constants. The atomic layer deposition of an ∼10 Å 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.