Electron Injection and Recombination in Dye Sensitized Nanocrystalline Titanium Dioxide Films: A Comparison of Ruthenium Bipyridyl and Porphyrin Sensitizer Dyes

Abstract

This paper is concerned with the parameters influencing the interfacial electron transfer kinetics, and therefore the sensitizing efficiency, for different sensitizer dyes adsorbed to nanocrystalline titanium dioxide films. We consider three sensitizer dyes: Ru(2,2‘-bipyridyl-4,4‘-dicarboxylate) 2 -cis- (NCS) 2 (Ru(dcbpy) 2 (NCS) 2 ) and zinc and free base tetracarboxyphenyl porphyrins (ZnTCPP & H 2 TCPP). These dyes were selected as they exhibit large differences in their oxidation potentials and photophysics, while retaining similar carboxylate groups for binding to the TiO 2 surface. For example, whereas the photophysics of Ru(dcbpy) 2 (NCS) 2 in solution is dominated by ultrafast (<100 fs) relaxation processes to nonemissive excited states associated with metal-to-ligand charge transfer excited states and extensive singlet/triplet mixing, both porphyrins exhibit long-lived (>1 ns) π* singlet excited states and only weak singlet/triplet mixing. The ground and excited-state oxidation potentials also differ by up to 600 mV between these different dyes. Remarkably, we find that the large differences in these dyes' photophysics and redox chemistry have rather little influence upon the interfacial electron transfer kinetics observed following adsorption of these dyes to the nanocrystalline TiO 2 films. The kinetics of electron injection into the TiO 2 conduction band following pulsed optical excitation of the adsorbed sensitizer dyes are found to be indistinguishable for all three sensitizer dyes. For all three dyes, the kinetics are ultrafast and multiexponential, requiring a minimum of three time constants ranging from <100 fs to ∼10 ps. Similarly, the recombination kinetics were also found to be highly nonexponential and only weakly sensitive to the identity of the sensitizer dye. We conclude that the multiexponential nature of the injection/recombination kinetics are not associated with properties of the sensitizer dye, but rather with heterogeneities/trap states associated with the TiO 2 film. We further conclude that the large difference between the rate of electron injection and recombination observed for all three dyes is not associated with specific characteristics of the sensitizer dyes but rather results from electron trapping within defect/surface states of the TiO 2 film. Finally, we conclude that the higher sensitizing efficiency reported for Ru(dcbpy) 2 (NCS) 2 compared to ZnTCPP cannot be attributed to differences in the interfacial electron transfer kinetics between these dyes and discuss alternative mechanisms influencing the sensitizing efficiencies of these dyes.