A redox-flow electrochromic window.

A low-cost electrochromic (EC) window based on a redox-flow system that does not require expensive transparent conductive oxide (TCO) substrates is introduced and demonstrated for the first time. An aqueous I3­/I­ redox electrolyte is used in place of a TCO to oxidize/reduce a molecular layer of an EC triphenylamine derivative that is anchored to a mesoporous TiO2 scaffold on the inner faces of a double-paned window. The redox electrolyte is electrochemically oxidized/reduced in an external two-compartment cell and circulated through the window cavity using an inexpensive peristaltic pump, resulting in coloration or decoloration of the window due to reaction of the redox solution with the triphenylamine derivative. The absorption characteristics, coloration/decoloration times, and cycling stability of the prototype EC window are evaluated, and prospects for further development are discussed.

Efficient dye-sensitized solar cells using a tetramethylthiourea redox mediator.

An organic redox couple tetramethylthiourea/tetramethylformaminium disulfide (TMTU/TMFDS(2+) ) is evaluated in dye-sensitized solar cells in conjunction with a series of indoline and ruthenium-based dyes. Of these, devices with indoline dye D205 show the best performance, with an optimized power conversion efficiency of 7.6 % under AM 1.5G 1 sun illumination. Charge collection and injection are highly efficient in all TMTU-based DSCs studied. Regeneration of indoline dyes is highly efficient, whereas regeneration of ruthenium dyes by TMTU is less efficient, accounting for their inferior performance. Impedance spectroscopy results reveal that using an optimized TMTU/TMFDS(2+) electrolyte solution, the TiO2 conduction band edge is 300-400 meV lower than when an optimized I3 (-) /I(-) electrolyte is used. The would-be loss in open-circuit voltage caused by the downward conduction band shift is mostly compensated by approximately the 200 meV lower redox level of the TMTU/TMFDS(2+) electrolyte and up to 1000 times slower recombination rates. This makes TMTU/TMFDS(2+) a promising redox couple in the development of highly efficient solar energy conversion devices.

Heterogeneous electron transfer from dye-sensitized nanocrystalline TiO2 to [Co(bpy)3]3+: insights gained from impedance spectroscopy.

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.

Conformal growth of nanocrystalline CdX (X = S, Se) on mesoscopic NiO and their photoelectrochemical properties.

Semiconductor-sensitized NiO photocathodes have been fabricated by successive ionic-layer adsorption and reaction (SILAR) deposition of CdS, CdSe and cascaded CdS/CdSe onto mesoscopic NiO films. Detailed morphological and structural characterization reveals that the growth of CdS and CdSe on mesoscopic NiO electrodes results in the formation of crystalline and conformal layers under ambient conditions. With a polysulfide redox electrolyte and a Pt counter electrode, CdX (X = S and Se)-sensitized p-NiO solar cells operating in a photocathodic mode are unambiguously demonstrated when NiO blocking layers are used, which are critical to prevent anodic photocurrent due to electron injection from CdX into the SnO2:F substrate. To decrease the recombination rate, a CdS barrier layer was deposited between NiO and a CdSe sensitizer which results in much enhanced cell performance. Front and rear spectral incident photon-to-current efficiency (IPCE) measurements were used to investigate charge collection and separation in the cells. The measurements indicate that charge collection in this system is limited by a short hole diffusion length.

Patterned 3-dimensional metal grid electrodes as alternative electron collectors in dye-sensitized solar cells.

We describe the application of 3-dimensional metal grid electrodes (3D-MGEs) as electron collectors in dye-sensitized solar cells (DSCs) as a replacement for fluorinated tin oxide (FTO) electrodes. Requirements, structure, advantages, and limitations of the metal grid electrodes are discussed. Solar conversion efficiencies of 6.2% have been achieved in 3D-MGE based solar cells, comparable to that fabricated on FTO (7.1%). The charge transport properties and collection efficiencies in these novel solar cells have been studied using electrochemical impedance spectroscopy.

The influence of dye structure on charge recombination in dye-sensitized solar cells.

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.

Synthesis and properties of [Pt(4-CO(2)CH(3)-py)(2)(mnt)]: comparison of pyridyl and bipyridyl-based dyes for solar cells.

The dye complexes [Pt(4-CO(2)R-py)(2)(mnt)] (R = H (3a), CH(3) (3b)) and the precursor complexes [Pt(4-CO(2)R-py)(2)Cl(2)] (2a, 2b) (py = pyridyl) were synthesised, characterised by electrochemical, spectroscopic, spectroelectrochemical (UV-vis-nIR and in situ EPR) and hybrid DFT computational methods and attached to a TiO(2) substrate to determine charge recombination kinetics. The results were compared to the bipyridyl analogues [Pt{X,X'-(CO(2)R)-2,2'-bipyridyl}(mnt)], (X = 3 or 4). The electronic characteristics of the bis-pyridyl complex were found to be different to the bipyridyl complexes making the former harder to reduce, shifting the lowest-energy absorption band to higher energy and showing separate degenerate LUMO orbitals on the two pyridine rings. The latter point determines that the di-reduced pyridyl complex remains EPR active, unlike the bipyridyl analogue. Complex 3a attached to nanocrystalline TiO(2) shows a long charge recombination lifetime in comparison with the analogous complex with the ubiquitous 4,4'-(CO(2)H)(2)-bipyridyl ligand, suggesting that pyridyl complexes may possess some advantage over bipyridyl complexes in dye-sensitised solar cells.

Dye-sensitized solar cells based on oriented TiO2 nanotube arrays: transport, trapping, and transfer of electrons.

Dye-sensitized solar cells fabricated using ordered arrays of titania nanotubes (tube lengths 5, 10, and 20 microm) grown on titanium have been characterized by a range of experimental methods. The collection efficiency for photoinjected electrons in the cells is close to 100% under short circuit conditions, even for a 20 microm thick nanotube array. Transport, trapping, and back transfer of electrons in the nanotube cells have been studied in detail by a range of complementary experimental techniques. Analysis of the experimental results has shown that the electron diffusion length (which depends on the diffusion coefficient and lifetime of the photoinjected electrons) is of the order of 100 microm in the titania nanotube cells. This is consistent with the observation that the collection efficiency for electrons is close to 100%, even for the thickest (20 microm) nanotube films used in the study. The study revealed a substantial discrepancy between the shapes of the electron trap distributions measured experimentally using charge extraction techniques and those inferred indirectly from transient current and voltage measurements. The discrepancy is resolved by introduction of a numerical factor to account for non-ideal thermodynamic behavior of free electrons in the nanostructured titania.