In this paper we focus upon the electron injection dynamics in complete dye sensitized nanocrystalline titanium dioxide solar cells (DSSCs) employing the ruthenium bipyridyl sensitizer dye N719. Electron injection dynamics and quantum yields are studied by time resolved single photon counting and the results are correlated with device performance. In typical DSSC devices, electron injection kinetics were found to proceed from the N719 triplet state with an half time of 200 ± 60 ps and quantum yield of 84 ± 5 %. We find that these injection dynamics are independent of presence of iodide / triiodide redox couple and of the pH of the peptisation step used in the synthesis of the TiO 2 nanoparticles. They are furthermore found to be only weakly dependent upon the application of electrical bias to the device. In contrast, we find these dynamics to be strongly dependent upon the concentration of t-butyl pyridine (tBP) and lithium cations in the electrolyte. This dependence is correlated with shifts of the TiO 2 conduction band energetics as a function of tBP and Li + concentration, from which we conclude that a 100 meV shift in band edge results in approximately a two fold retardation of injection dynamics.We find that electron injection quantum yield determined from these transient emission data as a function of tBP and Li + concentration shows a linear correlation with device short circuit density J sc . We thus conclude that the relative energetics of the dye excited state versus the titanium dioxide acceptor states is a key determinant of the dynamics of electron injection in DSSC, and that variations in these energetics, and therefore in the kinetics and efficiency of electron injection, impact directly upon device photovoltaic efficiency. Finally we discuss these results in terms of singlet versus triplet electron injection pathways and the concept of minimisation of kinetic redundancy.

Injection efficiency, η inj , and diffusion length, L, in dye-sensitized solar cells have been derived from the spectral response (incident photon to current efficiency, IPCE) of the cells under front side or backside illumination. Values of L from IPCE are found to be ∼2 times shorter than the values of L derived from the normal small perturbation transient method. IPCE-derived values of L (2 to more than 40 µm) and η inj (63-90%) are found to correlate with the photocurrent (and indirectly with the photovoltage) of the different cells indicating the extent to which each factor limits the cell efficiency. IPCE spectra varied with light intensity, so that diffusion lengths derived from both methods show similar trends, e.g., L from IPCE is found to increase 3 times when the light intensity is increased 10 times up to approximately 0.1 sun where L tends to plateau or peak. The values for η inj derived from the spectral response are shown to be in quantitative agreement with those determined from picosecond transient emission spectroscopy. To illustrate the utility of this method, L and η inj were measured on cells with and without the TiCl 4 chemical bath treatment. The results show that the increase in photocurrent after the TiCl 4 treatment is due to around a 2-fold increase in L despite a 3-fold reduction in the electron diffusion coefficient. The increased L can be explained by a factor of 10 decrease in electron recombination rate.

The optimization of interfacial charge transfer is crucial to the design of dye-sensitized solar cells. In this paper we address the dynamics of the charge separation and recombination in liquid-electrolyte and solid-state cells employing a series of amphiphilic ruthenium dyes with varying hydrocarbon chain lengths, acting as an insulating barrier for electron-hole recombination. Dynamics of electron injection, monitored by time-resolved emission spectroscopy, and of charge recombination and regeneration, monitored by transient optical absorption spectroscopy, are correlated with device performance. We find that increasing dye alkyl chain length results in slower charge recombination dynamics to both the dye cation and the redox electrolyte or solid-state hole conductor (spiro-OMeTAD). These slower recombination dynamics are however paralleled by reduced rates for both electron injection into the TiO2 electrode and dye regeneration by the I-/I3- redox couple or spiro-OMeTAD. Kinetic competition between electron recombination with dye cations and dye ground state regeneration by the iodide electrolyte is found to be a key factor for liquid electrolyte cells, with optimum device performance being obtained when the dye regeneration is just fast enough to compete with electron-hole recombination. These results are discussed in terms of the minimization of kinetic redundancy in solid-state and liquid-electrolyte dye-sensitized photovoltaic devices.

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