• Energy Research

Electrochromism in Nickel Oxide Films Containing Mg, Al, Si, V, Zr, Nb, Ag, or Ta, Solar Energy Mater. Solar Cells

Electrochromic (EC) NiOz and WOy thin films were prepared by sputtering and were used in a feasibility study aimed at investigating mixtures of these two oxides. The object was to identify a su ...

Suspended particle devices (SPDs) are able to rapidly switch from a dark bluish-black state to a clear greyish appearance when an AC electric field is applied. Two-flux and four-flux models were used to derive refractive indices and extinction coefficients, as well as scattering and absorption coefficients, of the particle-containing active layer. These entities were used in model calculations to predict direct, total and diffuse components of transmittance and reflectance, along with color appearance and haze, as a function of the thickness of the active layer. An optimum thickness for optical contrast of the SPD was determined in this way and was found to be in the range of 200–300 nm. The devices exhibit significant haze particularly in reflection. We are grateful to the Prometeo program of the Ecuador government and to Senescyt (Secretaría Nacional de Educación Superior, Ciencia, Tecnología e Innovación). This work has been supported by Ministerio de Economía y Competitividad of Spain (Grant nos. TEC2013-47342-C2-2-R and TEC2013-50138-EXP) and the R&D Program SINFOTON S2013/MIT-2790 of the Comunidad de Madrid, and from the European Research Council under the European Community's Seventh Framework Program (FP7/2007-2013)/ERC Grant agreement no. 267234/GRINDOOR). Publicado

Polyethylene oxide is a frequently used component in polymer electrolytes developed for applications in electrochromic devices. The transmittance variation may occur as a result of either proton or lithium ion intercalation into the electrochromic films. Impedance spectroscopy data in the low-frequency space-charge relaxation regime can be used to obtain estimates of ion concentrations and ion diffusion coefficients in ion-conducting materials. We apply this method to literature data for pure polyethylene oxide where the residual conductivity is believed to be due to protons. The obtained diffusion coefficient is found to be in the order of, or higher than, reported lithium ion diffusion coefficients in low molecular weight polyethylene oxide. Hence it is likely that proton intercalation will be of importance for electrochromic devices, provided there is a significant amount of protons present.

Electrochromic smart windows are able to vary their throughput of radiant energy by low-voltage electrical pulses. This function is caused by reversible shuttling of electrons and charge balancing ions between an electrochromic thin film and a transparent counter electrode. The ion transport takes place via a solid electrolyte. Charge transport is evoked by a voltage applied between transparent electrical conductors surrounding the electrochromic film/electrolyte/counter electrode stack. This review summarizes recent progress concerning (i) calculated optical properties of crystalline WO3, (ii) electrochromic properties of heavily disordered W oxide and oxyfluoride films produced by reactive magnetron bias sputtering, (iii) novel transparent reactively sputter deposited Zr-Ce oxide counter electrodes, and (iv) a new proton-conducting antimonic-acid-based polymer electrolyte. Special in-depth presentations are given on elastic light scattering from W-oxide-based films and of electronic bandstructure effects affecting opto-chronopotentiometry data in Zr-Ce oxide. The review also contains some new device data for an electrochromic smart window capable of very high optical transmittance.

Abstract Thermochromic films of VO2, as well as multilayer films of VO2 and TiO2, were made by reactive DC magnetron sputtering. Spectrophotometrically measured transmittance and reflectance were used to determine optical constants pertinent to temperatures below and above a temperature-induced structural change at τc≈60 °C. We then used computations to optimize multilayer films for specific applications and, specifically, demonstrated that TiO2/VO2/TiO2 films could display a luminous transmittance significantly higher than that of bare VO2 films, and that TiO2/VO2/TiO2/VO2/TiO2 films could yield a large change of solar transmittance for temperatures above and below τc. Our data can serve as starting points for developing novel coatings for windows with superior energy efficiency.

Abstract Tungsten oxide is a widely used electrochromic material for smart windows. In order to study the charge carriers involved in the electrochromic process, it is important to characterize the electrical transport in tungsten oxide. Substoichiometric amorphous tungsten oxide films were prepared by DC-magnetron sputtering. The films were electrochemically intercalated with lithium. The Li/W intercalation ratios for the tungsten oxide films were in the range 0.15–0.53. Temperature dependent resistivity measurements were performed in the temperature range 77–300 K for samples at different lithium intercalation levels. It was found that the data are consistent with the variable range hopping model.

In this paper, we show that electrochemical quasi-steady state potential curves of disordered tungsten, titanium, cerium and iridium oxides exhibit fine structure in striking agreement with the density of electronic states, as obtained from ab initio calculations for the crystalline counterparts. The ability to probe the electronic structure by our electrochemical technique is restricted to disordered non-metallic materials. It is highly probable that the probed electronic states are required to be localized, in order for the technique to give a good picture of their density. The electrochemical density of states is often smaller than the computed one due to kinetic effects, i.e. very slow relaxations of the charge carriers. By combining electrochemical and electrical conductivity measurements for the case of tungsten oxide, we have verified the localized character of the probed states and estimated their localization radius.

Abstract Polymer electrolytes containing poly(ethylene imine) (PEI) and lithium bis(trifluoromethylsulfonyl) imide (LiTFSI) can serve as model electrolytes for electrochromic devices. Such electrolytes were characterized by differential scanning calorimetry, conductivity, and viscosity measurements. The glass transition temperature (Tg) and viscosity of the PEI–LiTFSI electrolytes have minima at a [N]:[Li] ratio of 100:1. Both Tg and viscosity increased at high salt concentrations. The temperature dependences of ionic conductivity and viscosity followed an Arrhenius equation with parameters depending only weakly on the salt concentration. The fluid behavior of the electrolytes could be reconciled with the Bingham plastic model with parameters being functions of salt concentration.