Dye-sensitized solar cells with power conversion efficiencies of up to 6.5% have been fabricated using a cobalt tris-bipyridyl redox mediator with the cis-diisothiocyanato-(2,2′-bipyridyl-4,4′-dicarboxylic acid)-(2,2′-bipyridyl-4,4′-dinonyl) ruthenium(II) (Z907) sensitizer. This represents a significant improvement in efficiency compared with previous reports using ruthenium sensitizers. In situ near-IR transmittance measurements in conjunction with electrochemical impedance spectroscopy have been used to explain the difference in performance between DSCs using Z907 and another benchmark sensitizer cis-diisothiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid) ruthenium(II) bis(tetrabutylammonium) (N719). It is found that the small-perturbation electron diffusion length (L n ) is significantly longer in Z907 cells compared with that in N719 cells, which can explain most of the difference in performance. It is also shown that the longer L n in Z907 cells is caused by inhibited recombination, as opposed to faster transport, and possible reasons for this are discussed. Our methodological approach is especially useful for accurately determining L n when it is shorter than the TiO2 layer thickness, where standard dynamic techniques start to become unreliable.

It is widely believed that the prototypical ruthenium dyes N719 and Z907 are regenerated by iodide with near unity quantum yield following photo-oxidation in dye-sensitized solar cells (DSCs). However, the incident photon-to-current efficiency (IPCE) of DSCs using these dyes decreases with increasing forward bias, limiting power conversion efficiency (η) compared to the hypothetical constant-IPCE case. This phenomenon could arise due to incomplete regeneration, but despite the important implications for cell efficiency, it has received little attention. DSCs employing electrolytes with different iodide concentrations and the Z907 sensitizer have been characterized using complementary photoelectrochemical techniques to test whether the decrease in IPCE is caused by inefficient regeneration. The results strongly suggest that this is the case, even for abnormally high iodide concentrations, where η is reduced by as much as 30% by the effect. Similar results are obtained with the N719 sensitizer. Interestingly, the predicted reduction in photovoltage is partially offset by a change in the electrostatic potential drop across the Helmholtz layer at the TiO2–electroyte interface, which has an estimated microscopic areal capacitance in the range 2.3–9.3 μF cm–2. These findings suggest that it will be important to carefully consider sensitizer regeneration kinetics and interfacial electric fields to further improve the efficiency of DSCs.

Dye-sensitized solar cells (DSCs) employing the [Co(bpy)3](3+/2+) redox mediator have recently attained efficiencies in excess of 12%, increasing the attractiveness of DSCs as an alternative to conventional photovoltaics. Heterogeneous electron transfer from dye-sensitized nanocrystalline TiO2 to [Co(bpy)3](3+) ions in solution, a process known as recombination in the context of DSC operation, is an important loss mechanism in these solar cells. Here, we employ impedance spectroscopy over a range of temperatures to characterize electron storage, transport, and recombination in efficient DSCs based on the [Co(bpy)3](3+/2+) redox mediator, with either the amphiphillic ruthenium sensitizer Z907 or the state-of-the-art organic sensitizer Y123. The temperature dependence of the electron-transport resistance indicates that transport occurs via states at energies lower than commonly assumed for the TiO2 conduction band edge. We show that a non-exponential dependence of capacitance, transport resistance, and recombination resistance on photovoltage can be interpreted as evidence for partial unpinning of the TiO2 energy levels. We also find that the nature of the sensitizing dye determines the predominant recombination route: via the conduction band for Y123 and via band gap states for Z907, which is the main reason for the superior performance of Y123. The different mechanisms appear to arise from changes in electronic coupling between TiO2 donor states and [Co(bpy)3](3+) acceptor states, as opposed to changes in the density of TiO2 states or their energetic matching with the acceptor-state distribution. These findings have implications for modeling heterogeneous electron transfer at dye-sensitized semiconductor-solution interfaces in general and for the optimization of DSCs.

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Replacing the nonyl groups on the solar cell dye Ru(4,4'-carboxylic acid-2,2'-bipyridine)(4,4'-dinonyl-2,2'-bipyridine)(NCS)(2) (Z-907) with amino groups results in a marked decrease in solar cell performance. This is despite the fact that the amino derivative (Z-960) has more favourable light absorption characteristics than Z-907 when used with thick nanocrystalline TiO(2) layers. Electron transfer to the electrolyte from the exposed fluorine-doped tin oxide (FTO) substrate is particularly fast in cells employing the Z-960 dye if a compact TiO(2) blocking layer is not used. The kinetics of electron transfer from the nanocrystalline TiO(2) layer in DSCs employing Z-960 are comparable to those of bare TiO(2) and ca. 2 to 5 times faster than for cells employing Z-907. The faster charge recombination in cells employing Z-960 lowers open-circuit photovoltage and results in very significant charge collection losses that lower short-circuit photocurrent. Voltammetric measurements show that surface modification of FTO electrodes with Z-960 results in slightly more facile charge transfer to acceptor species in triiodide/iodide electrolytes in the dark. A simpler molecule, p-aminobenzoic acid, more dramatically catalyses this charge transfer reaction. Conversely, chemical modification of FTO electrodes with Z-907 or p-toluic acid retards charge transfer kinetics. Similar results are obtained for nanocrystalline TiO(2) electrodes modified with these benzoic acid derivatives. These results strongly imply that surface adsorbed molecules bearing amino groups, including dye molecules, can catalyse charge recombination in dye-sensitized solar cells.