Molecular Control of Recombination Dynamics in Dye-Sensitized Nanocrystalline TiO2 Films: Free Energy vs Distance Dependence

Abstract

In this paper we address the dependence of the charge recombination dynamics in dye-sensitized, nanocrystalline TiO₂ films upon the properties of the sensitizer dye employed. In particular we focus upon dependence of the charge recombination kinetics upon the dye oxidation potential E⁰(D⁺/D), determined electrochemically, and the spatial separation r of the dye cation HOMO orbital from the metal oxide surface, determined by semiempirical calculations. Our studies employed a series of ruthenium bipyridyl dyes in addition to porphyrin and phthalocyanine dyes. A strong correlation is observed between the recombination dynamics and the spatial separation r, with variation in r by 3 Å resulting in a more than 10-fold change in the recombination half-time t_50%. This correlation is found to be in agreement with electron tunneling theory, t_50% ∝ exp(−βr) with β = 0.95 ± 0.2 Å^-1. In contrast, the recombination dynamics were found to be relatively insensitive to variations in E⁰(D⁺/D), indicative of the recombination reaction lying near the peak of the Marcus free energy curve, ΔG ∼ λ, and with λ ∼ 0.8 eV. A correlation is also observed between the recombination half-time and the temporal shape of the kinetics, with faster recombination dynamics being more dispersive (less monoexponential). Comparison with numerical Monte Carlo type simulations suggests this correlation is attributed to a shift from fast recombination dynamics primarily limited by dispersive electron transport within the metal oxide film to slower dynamics primarily limited by the interfacial electron-transfer reaction. We conclude that the primary factor controlling the charge recombination dynamics in dye-sensitized, nanocrystalline TiO₂ films is the spatial separation of the dye cation from the electrode surface. In particular, we show that for the Ru(dcbpy)₂NCS₂ dye series, the use of X = NCS rather than X = CN results in a 2 Å shift in the dye cation HOMO orbital away from the electrode surface, causing a 7-fold retardation of the recombination dynamics, resulting in the remarkably slow recombination dynamics observed for this sensitizer dye.