An Insulating Al₂O₃ Overlayer Prevents Lateral Hole Hopping Across Dye-Sensitized TiO₂ Surfaces

Abstract

Three chromophores of the general form [Ru(bpy′) 2 (4,4′-(PO 3 H 2 ) 2 -2,2′bipyridine)] 2+ , where bpy′ is 4,4′-(C(CH 3 ) 3 ) 2 -2,2′-bipyridine (Ru(dtb) 2 P); 4,4′-(CH 3 O) 2 -2,2′-bipyridine (Ru(OMe) 2 P), and 2,2′-bipyridine (RuP) were anchored to mesoporous thin films of TiO 2 nanocrystallites at saturation surface coverages to investigate lateral self-exchange Ru III/II intermolecular hole hopping in 0.1 M LiClO 4 / CH 3 CN 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: TiO 2 |Ru(OMe) 2 P (k et = 1.4 × 10 6 s −1 ) > TiO 2 | RuP (7.1 × 10 5 s −1 ) > TiO 2 |Ru(dtb) 2 P (6.5 × 10 4 s −1 ). Analysis of the anisotropy data with Monte Carlo simulations provided hole hopping rate constants for TiO 2 |RuP and TiO 2 |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 TiO 2 (e − )|Ru III → TiO 2 |Ru II charge recombination rate constants. The atomic layer deposition of an ∼10 Å layer of Al 2 O 3 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.

Dual-Sensitizer Photoanode for Bromide Oxidation

“…The anisotropy decays were modeled with the previously described Monte Carlo simulations. 45,46 An intermolecular energy transfer rate constant of 4 × 10 7 s −1 was found that corresponds to ∼25 ns per energy transfer, consistent with reports of related ruthenium complexes (Figure 3B). 47 Therefore, in the absence of excited-state electron transfer, energy transfer occurs an average of 26 times within the excited-state lifetime.…”

Perspectives on Dye Sensitization of Nanocrystalline Mesoporous Thin Films

“…Temperature-dependent kinetic studies made as a function of the surface coverage support the conclusion that rapid hole hopping promotes charge recombination . Taken together, these findings indicate that unwanted charge recombination can be inhibited through control of lateral hole hopping, an unexpected finding that may be further exploited in future research …”

“…75 Taken together, these findings indicate that unwanted charge recombination can be inhibited through control of lateral hole hopping, an unexpected finding that may be further exploited in future research. 76 2.3.3. Recombination to Acceptor−Bridge−Donor (A−B− D) Sensitizers.…”

Dye-sensitized solar cells strike back

“…Polarized light generates an anisotropic population of interfacial states whose timedependent reactivity clearly demonstrates that hole hopping followed excited-state injection under a variety of experimental conditions. [109][110][111] Monte Carlo simulations revealed that an oxidized sensitizer could circumnavigate an entire anatase nanocrystal by hole-hopping before charge recombination occurred. 110 The discovery that hole-hopping rates were directly correlated with charge recombination kinetics represents an important finding.…”