Observation of First-Order, Marcus Normal, Interfacial Electron Transfer at Dye-Sensitized TiO2 Interfaces

Abstract

Dye-sensitized solar cells combine the robustness of nanocrystalline TiO2 with molecular dyes or “sensitizers” that absorb broadly across the visible. The surface anchored sensitizer undergoes a photo-induced electron transfer to the TiO2 acceptor states and is subsequently regenerated by a redox mediator present in the external electrolyte. Efficient light-to-energy conversion requires that the injected electron be transported to the external circuit prior to recombination with the oxidized redox mediator. Despite the importance of this unwanted recombination reaction the mechanism remains elusive. This is due in part to complex kinetics that usually cannot be described with first- or second order kinetic models, but satisfactory fitting instead require sums of exponentials or stretched exponential functions. The non-exponential kinetics are likely due to transport of the injected electron and/or the electron acceptor prior to recombination. Here we report charge recombination studies of charge recombination to a family of oxidized amines that provide compelling evidence for first-order kinetics with abstracted rate constants that show a driving force dependence. The recombination reaction between nanocrystalline anatase TiO2 and a series of symmetrically substituted triphenylamine (TPA) redox mediators that possessed electron donating or withdrawing groups at the para-position of the phenyl rings was quantified on nanosecond and longer time scales. The groups utilized afforded an ~0.5 V change in TPA+/0 reduction potential. The nanocrystalline TiO2 thin films were sensitized by a ruthenium polypyridyl complex bearing either a carboxylic or a phosphonic acid functional group for surface anchoring. Pulsed light excitation of the film resulted in excited-state injection to yield TiO2(e-)|S+. Sensitizer regeneration and the subsequent charge recombination reaction TiO2(e-) + TPA+ → TiO2 + TPA were then monitored spectroscopically. Concentration dependent data revealed that recombination could be described by a distribution of first-order rate constants that became strictly first-order when the driving force was larger. The reaction rate increased with more positive TPA+/0 reduction potentials, consistent with electron transfer in the Marcus normal region. The recombination reaction was monitored as a function of temperature, and an Arrhenius analysis provided activation energies. Interestingly, the same activation barrier was abstracted for all the TPA derivatives, suggesting that a common rate limiting step exists for all these reactions. A model for recombination consistent with both the thermodynamics and activational parameters will be presented.