Optical excitation of Ru II (2,2′-bipyridyl-4,4′dicarboxylate) 2 (NCS) 2 -sensitized nanocrystalline TiO 2 films results in injection of an electron into the semiconductor. This paper addresses the kinetics of charge recombination which follows this charge separation reaction. These charge recombination kinetics were found to be strongly dependent upon excitation intensity, electrolyte composition, and the application of an electrical bias to the TiO 2 film. For excitation intensities resulting in less than one excited dye molecule/TiO 2 particle, the recombination kinetics were independent of excitation intensity. Increasing the excitation intensity above this level resulted in a rapid acceleration in the charge recombination kinetics. Similarly, for positive electrical potentials applied to the TiO 2 electrode, the recombination kinetics were independent of applied potential. If the applied potential was more negative than a threshold potential V kin , a rapid acceleration of the charge recombination kinetics was again observed, for example from ∼1 ms at +0.1 V vs Ag/AgCl to ∼3 ps at -0.8 V (∼10 8 fold increase in the rate). Moreover, at a constant applied potential the charge recombination kinetics were found to be strongly dependent upon electrolyte composition (up to 10 6 -fold change in rate). This strong dependence upon the electrolyte composition was found to be associated with shifts in the threshold potential V kin . Spectroelectrochemical measurements were used to monitor the shift in the trap/conduction band density of states induced by the electrolyte composition. A direct correlation was observed between the threshold voltage V kin observed from kinetic measurements, and the threshold voltage for electron occupation of conduction band/trap states of the TiO 2 observed from spectroelectrochemical measurements. This direct correlation was observed for a wide range of electrolyte compositions including protic and aprotic solvents and the addition of Li + ions and 4-tert-butylpyridine. We conclude that the charge recombination kinetics in such dye-sensitized films are strongly dependent upon the electron occupation in trap/conduction band states of the TiO 2 film. This occupation may be modulated by variations in light intensity, applied electrical potential, and electrolyte composition. These results are discussed with relevance to the function of dye-sensitized photoelectrochemical devices.

We show for the first time that the photogenerated hole lifetime in TiO 2 is a strong determinant of the ability of TiO 2 to split water. Hole lifetimes were measured using transient absorption spectroscopy over a range of excitation intensities. The lifetimes of the holes were modulated by the use of exogenous scavengers and were also found to vary systematically with the excitation intensity. In all cases the quantum yield of oxygen production is found to be linked to the light intensity used, ranging from below 1 sun equivalent to nearly 1 sun equivalent. We also provide evidence that oxygen production requires four photons for each molecule of oxygen, which is reminiscent of the natural photosynthetic water-splitting mechanism. This in turn suggests a mechanism for oxygen production which requires four-hole chemistry, presumably via three, as yet unidentified intermediates. It is also shown that at excitation densities on the order of 1 sun, nongeminate electron-hole recombination limits the quantum yield significantly.

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 metalto-ligand charge transfer excited states and extensive singlet/triplet mixing, both porphyrins exhibit longlived (>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.

In this paper we focus upon the electron injection dynamics in complete dye-sensitized nanocrystalline metal oxide solar cells (DSSCs). Electron injection dynamics are studied by transient absorption and emission studies of DSSCs and correlated with device photovoltaic performance and charge recombination dynamics. We find that the electron injection dynamics are dependent upon the composition of the redox electrolyte employed in the device. In a device with an electrolyte composition yielding optimum photovoltaic device efficiency, electron injection kinetics exhibit a half time of 150 ps. This half time is 20 times slower than that for control dye-sensitized films covered in inert organic liquids. This retardation is shown to result from the influence of the electrolyte upon the conduction band energetics of the TiO2 electrode. We conclude that optimum DSSC device performance is obtained when the charge separation kinetics are just fast enough to compete successfully with the dye excited-state decay. These conditions allow a high injection yield while minimizing interfacial charge recombination losses, thereby minimizing "kinetic redundancy" in the device. We show furthermore that the nonexponential nature of the injection dynamics can be simulated by a simple inhomogeneous disorder model and discuss the relevance of our findings to the optimization of both dye-sensitized and polymer based photovoltaic devices.