Electron injection and recombination in dye sensitized nanocrystalline titanium dioxide films: A comparison of ruthenium bipyridyl and porphyrin sensitizer dyes

This paper is concerned with the parameters influencing the interfacial ele ctron transfer kinetics, and therefore the sensitizing efficiency, for diff erent sensitizer dyes adsorbed to nanocrystalline titanium dioxide films. W e consider three sensitizer dyes: Ru(2,2'-bipyridyl-4,4'-dicarboxylate)(2)- cis-(Ru(dcbpy)(2)(NCS)(2)) and zinc and Free base tetracarboxyphenyl porphy rins (ZnTCPP & H2TCPP). These dyes were selected as they exhibit large diff erences in their oxidation potentials and photophysics, while retaining sim ilar carboxylate groups for binding to the TiO2 surface. For example, where as the photophysics of Ru(dcbpy)(2)(NCS)(2) in solution is dominated by ult rafast (<100 fs) relaxation processes Co nonemissive excited states associa ted with metal-to-ligand charge transfer excited states and extensive singl et/triplet mixing, both porphyrins exhibit long-lived (>1 ns) pi* singlet e xcited states and only weak singlet/triplet mixing. The ground and excited- state oxidation potentials also differ by up to 600 mV between these differ ent dyes. Remarkably, we find that the large differences in these dyes' pho tophysics and redox chemistry have rather little influence upon the interfa cial electron transfer kinetics observed following adsorption of these dyes to the nanocrystalline TiO2 films. The kinetics of electron injection into the TiO2 conduction band following pulsed optical excitation of the adsorb ed sensitizer dyes are found to be indistinguishable for all three sensitiz er dyes. For all three dyes, the kinetics are ultrafast and multiexponentia l, requiring a minimum of three time constants ranging from <100 fs to simi lar to 10 ps Similarly, the recombination kinetics were also found to be hi ghly nonexponential and only weakly sensitive to the identity of the sensit izer dye. We conclude that the multiexponential nature of the injection rec ombination kinetics are not associated with properties of the sensitizer dy e, but rather with heterogeneities/trap states associated with the TiO2 Fil m. We further conclude that the large difference between the rate electron injection and recombination observed for all three dyes is not associated w ith specific characteristics of the sensitizer dyes but rather results from electron trapping within defect/surface states of the TiO2 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 int erfacial electron transfer kinetics between these dyes and discuss alternat ive mechanisms influencing the sensitizing efficiencies of these dyes.