We investigate the effect of a thin alumina coating of nanocrystalline TiO 2 films on recombination dynamics of dye-sensitized solar cells. Both coated and uncoated cells were measured by a combination of techniques: transient absorption spectroscopy, electrochemical impedance spectroscopy, and open-circuit voltage decay. It is found that the alumina barrier reduces the recombination of photoinjected electrons to both dye cations and the oxidized redox couple. It is proposed that this observed retardation can be attributed primarily to two effects: almost complete passivation of surface trap states in TiO 2 that are able to inject electrons to acceptor species, and slowing down by a factor of 3-4 the rate of interfacial charge transfer from conduction-band INTRODUCTIONNew concepts for photovoltaic energy conversion are increasingly focusing at the intersection between organic and inorganic materials. Such devices are based upon nanometerscale interpenetrating networks of electron and hole transporting materials 1-3 rather than employing the long diffusion length, high-purity crystals, as used in conventional silicon solar cells. Dye-sensitized solar cells (DSSCs) are so far the most efficient devices based on this concept. DSSCs base their ability to convert photon energy to electricity on fast conversion of photogenerated excitons to energetic charge carriers spatially separated in distinct nanoscale domains or phases. The energy associated with photoexcitation of the sensitizer dye is employed to inject electrons into the conduction band of nanocrystalline TiO 2 and the holes into the redox electrolyte, generally the I − / I 3 − redox couple in an organic solvent. Under illumination, the Fermi level of electrons in the TiO 2 is raised 4 relative to the chemical potential of the redox couple in the electrolyte, resulting in a large thermodynamic driving force (ca. 0.8 eV) for electron transfer from the TiO 2 to the redox couple. Slow kinetics for this charge recombination process are therefore essential to allow the charge carriers enough time to be collected into their respective contacts. 5 Recently, the concept of a "kinetic barrier" for interfacial charge recombination processes in DSSCs has been further demonstrated by the use of ultrathin ͑ഛ1 nm͒ conformal alumina ͑Al 2 O 3 ͒ layers deposited in situ onto the surface on the nanocrystalline metal oxide films, leading to a significant increase in DSSC energy conversion efficiencies, as reported by several groups. [6][7][8][9][10][11][12] Palomares et al. 11 investigated the electron recombination process at the interface between TiO 2 and the oxidized dye, and have shown slow kinetics for the electron transfer process when the TiO 2 nanocrystalline particles were coated with Al 2 O 3 . However, direct evidence for slow recombination kinetics between the electrons in the TiO 2 and the oxidized species in the electrolyte ͑I 3 − ͒ were not addressed. Due to the strong interest in this approach, 12,13 we decided to investigate further the origin of the effic...

This paper analyses the use of impedance spectroscopy as a characterization tool applied to thermoelectric materials. The impedance function of the thermoelectric system under adiabatic conditions and Peltier mode operation is calculated by solving the heat equation in the frequency domain. The analysis, focused on the complex plane, provides the required equivalent circuit elements to interpret the impedance measurements. Using this approach, all the relevant thermoelectric parameters and thermal properties can be potentially extracted at a given temperature from the impedance spectra, i.e., the Seebeck coefficient, electrical resistivity, thermal conductivity, figure of merit (zT), specific heat, and thermal diffusivity. This can be done without the need of measuring temperature differences. To validate the models described, impedance measurements have been carried out in single thermoelectric elements and modules, showing an excellent agreement with the theory. The simple nature of the measurements in conjunction with the advantage of obtaining all the important thermoelectric parameters opens up the possibility of establishing impedance spectroscopy as a very useful characterization method for the thermoelectric field.

The electrochemical behaviour of nanoporous TiO(2) in a room temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide (EMITFSI), was investigated by cyclic voltammetry (CV) and impedance spectroscopy. Exponentially rising currents in voltammetry were attributed to the charging/discharging of electrons in the TiO(2) film and a charge transfer mechanism. The main features of the voltammetry and impedance followed the same trends in the ionic liquid as in other organic solvents and also in aqueous electrolytes. In the presence of lithium ions, the onset potential of the charge accumulation increased due to the change of the initial position of the TiO(2) conduction band. The results show that substitution of organic solvents contained in solar cells, supercapacitors or other electrochemical devices is in general feasible, though requires some adjustment in the electrolyte composition for optimal performance.

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We discuss the interpretation of usually broad oxidation peaks observed in electronically conducting polymers, in terms of the statistical distributions functions of polarons and bipolarons. The analysis is based on examining the chemical capacitance, that relates the change of concentration to a modification of the chemical potential of a given species, for different statistical models. We first review the standard models for single energy species that provide a nernstian dependence, and the limitations of these models are discussed. A new model that assumes a Gaussian distribution of energies related to molecular geometry fluctuations is suggested, and this model shows excellent agreement with the results of electrochemical oxidation of polypyrrole in quasiequilibrium conditions. From a fit of the data, it is found that the density of conjugated chain segments in polypyrrole, Ns approximately 10(21) cm(-3), shows a Gaussian distribution of half width sigma approximately 170 meV, tentatively attributed to bipolaron formation energies.