Step potential analysis of cobalt oxide-based electrochromic devices

The characterization of electrochemical behavior of electrochromic intercalation device based on cobalt oxide thin film was carried out using the step potential excitation method. A method based on generating plots of current density as a function of passed charge has been applied for characterization of electrochromic cobalt oxide thin films using an aqueous KOH electrolyte. The device resistance and the intercalation capacity of the material are calculated. Dynamic built-in potential estimated from step potential experiment and plots of the built-in potential as function of the passed charge, V_bi(ΔQ), are generated for intercalation process. The intercalation efficiency curve is obtained to confirm the nature of energy distribution of intercalation sites in electrochromic cobalt oxide.     

Sb–Cu–Li electrochromic mirrors

Switchable mirrors offer significant advantages over traditional electrochromic devices for control of incident light in architectural and aerospace applications due to their large dynamic ranges in both transmission and reflection in the visible and near infrared regimes. Here we describe construction and spectroscopic characterization of a complete electrochromic mirror device consisting of an antimony–copper alloy (40 at% Cu) active electrode coupled with an optically passive vanadium oxide counter electrode and a crosslinked polymer gel electrolyte. Transmittance and reflectance spectra in the visible–near IR (300–2500 nm) in both mirror and transparent states are reported. The photopic transmittance of the complete device varied from less than 3% to more than 20% during cycling, requiring about 40 min for complete switching in each direction. At the same time, the photopic reflectance varied from 40% to 25%. The crosslinked polymer improves the stability of the mirror electrode relative to that in a liquid electrolyte.     

All solid-state electrochromic devices with gelatin-based electrolyte

6×8 cm² electrochromic devices (ECDs) with the configuration K-glass/EC-layer/electrolyte/ion-storage (IS) layer/K-glass, have been assembled using Nb₂O₅:Mo EC layers, a (CeO₂)_0.81–TiO₂ IS-layer and a new gelatin electrolyte containing Li⁺ ions. The structure of the electrolyte is X-ray amorphous. Its ionic conductivity passed by a maximum of 1.5×10⁻⁵ S/cm for a lithium concentration of 0.3 g/15 ml. The value increases with temperature and follows an Arrhenius law with an activation energy of 49.5 kJ/mol. All solid-state devices show a reversible gray coloration, a long-term stability of more than 25,000 switching cycles (±2.0 V/90 s), a transmission change at 550 nm between 60% (bleached state) and 40% (colored state) corresponding to a change of the optical density (ΔOD=0.15) with a coloration efficiency increasing from 10 cm²/C (initial cycle) to 23 cm²/C (25,000th cycle).     

Studies on electrochromic properties of nickel oxide thin films prepared by spray pyrolysis technique

Electrochromic nickel oxide thin films were prepared by using a simple and inexpensive spray pyrolysis technique (SPT) onto fluorine-doped tin oxide (FTO) coated glass substrates from nickel chloride solution. Transparent NiO-thin films were obtained at a substrate temperature 350°C. The films were cubic NiO with preferred orientation in the (1 1 1) direction. Infrared spectroscopy results show presence of free hydroxyl ion and water in nickel oxide thin films. The electrochromic properties of the thin films were studied in an aqueous alkaline electrolyte (0.1 M KOH) using cyclic voltammetry (CV), chronoamperometry (CA) and spectrophotometry. The films exhibit anodic electrochromism, changing colour from transparent to black. The colouration efficiency at 630 nm was calculated to be 37 cm²/C.     

Thickness-dependent electrochromic properties of solution thermolyzed tungsten oxide thin films

Tungsten oxide (WO₃) thin films are prepared by using a simple and inexpensive solution thermolysis technique. Thin film samples of different thickness are obtained by varying quantity of ammonium tungstate solution sprayed onto the preheated conducting glass substrate. A simple three-electrode cell has been formed to study the electrochemical and electrochromic properties. The electrochemical parameters of the cell such as anodic peak current, anodic peak potential, threshold voltage, amount of H⁺ ions intercalated into and deintercalated out of the WO₃ samples are calculated. The effect of film thickness on these parameters are studied. The extent of electrochromism and reversibility of the colouration/bleaching processes of various WO₃ samples are described. The colouration efficiencies at 633 nm are calculated. The maximum colouration efficiency obtained for thicker film, is 56 cm²/C. The samples were found to be stable in 0.05N H₂SO₄ electrolyte up to 1×10³ colour/bleach cycles.     

Improved electrochromic devices with an inorganic solid electrolyte protective layer

The durability problem of electrochromic devices (ECD) in Li ion-conducting electrolytes may be due to the degradation of the ECD electrode matrix and irreversibly reacting Li ions during cycling. With this hypothesis, we investigated the performance of WO₃ film with an inorganic solid electrolyte, lithium phosphorous oxynitride (LiPON), protective layer and of the ECD composed of WO₃ and V₂O₅ with a LiPON protective layer prepared by RF magnetron sputtering. WO₃ and ECD (glass/ITO/V₂O₅/LiPON/electrolyte/LiPON/WO₃/ITO/glass) with a protective layer not only showed improved durability with continuous potential cycling, but also demonstrated a good memory effect under voltage-off, good response times and high coloration efficiency (CE). Our results demonstrated that LiPON layers are electrochemically stable, enhance the electrochromic properties in Li ion-conducting electrolytes, and can be used as a protective layer for ECD.     

Influence of deposition temperature on electrochromic properties of sputtered W03 thin films

The influence of deposition temperature on the electrochromic properties of reactively sputtered WO₃ films has been investigated. A set of W0₃ films produced in the temperature range of 47–400°C is electrochemically cycled with a constant current supply, which provides an easy control of the charge injected into the films. Optical absorption after coloration, and, hence, the coloration efficiency of the films, significantly increases with increasing deposition temperature (up to 400°C) in the wavelength range of ∼ 1000–2500 nm. Both optical absorption and coloration efficiency decrease at the highest temperatures studied (350–400°C) over most of the visible spectrum (∼ 500–800 nm). Optical modulation across the solar spectrum remains fairly constant for charge injection of 10 mC/cm², and the modulation is quite satisfactory for all films for electrochromic device applications. The films produced at higher substrate temperatures show smaller modulation of the visible spectrum, compared with the films sputtered at lower substrate temperatures. This could be more suitable for some applications of the solar control systems. The resistance of the films during the electrochemical process is found to increase with increasing substrate temperature. This causes difficulty in charge injection, requiring significantly larger voltages in techniques such as a cyclic voltammetry for the films deposited at higher temperatures. As a result the good electrochromic performance of the films deposited at higher temperatures can be masked by experimental conditions.     

Design, production and characterisation of an all solid state electrochromic medium size device

An electrochromic device based on a five layer coating deposited on a glass sheet was produced. The layers were all obtained by physical vapour deposition in the same vacuum environment, using a commercial apparatus capable of producing thin films on large area substrates. Small (50×50 mm²) and medium (300×300 mm²) size specimens were prepared for preliminary study, to establish the electrochemical and optical performances of the device. Electrochemical measurements were performed on each active material, as well as on the whole specimens, in order to relate electrochemical properties to process conditions. Also, luminous and solar parameters were obtained from (near) normal incidence spectrophotometry in both the bleached and coloured states, to confirm the values used in the design phase of the electrochromic device. One of the specimens was also submitted to variable angle photometric and radiometric measurements, using an integrating sphere. This data was then used to evaluate the room luminance distribution and energy loads using a building simulation code.     

Sprayed CeO2 thin films for electrochromic applications

Cerium dioxide (CeO₂) thin films were prepared by spray pyrolysis using hydrated cerium chloride (CeCl₃·7H₂O) as source compound. The films prepared at substrate temperatures below 300°C were amorphous, while those prepared at optimal conditions (T_s=500°C,s=5 ml/min) were polycrystalline, cubic in structure, preferentially oriented along the (2 0 0) direction and exhibited a transmittance value greater than 80% in the visible range. The cyclic voltammetry study showed that films of CeO₂ deposited on ITO pre-coated glass substrates were capable of charge insertion/extraction when immersed in an electrolyte of propylene carbonate with 1 M LiClO₄.These films also remained fully transparent after Li+ intercalation/deintercalation.     

High-rate dual-target DC magnetron sputter deposition of “blue” electrochromic Mo oxide films

Mo oxide films were prepared by dual-target reactive DC magnetron sputtering under conditions leading to films that were blue in as-deposited state. The deposition rate for this “blue” Mo oxide was as high as 1.5 nm/s, to be compared to 0.85 and 0.1 nm/s for highly transparent Mo oxide films made with dual-target and single-target sputtering, respectively. Good electrochromic properties of the “blue” films evolved after 5 colour/bleach cycles in a Li⁺ conducting electrolyte.     

A comparison of absorptance and reflectance modulation variable transmission electrochromic devices

A comparison is made of the optical properties and electrochromic performance of two types of electrochromic device for use as variable transmission glazing. The devices employ respectively amorphous tungsten oxide and crystalline tungsten oxide as the active electrochromic layer. Both devices exhibit pronounced transmission modulation. Some measure of reflectance modulation is observable for the crystalline tungsten oxide device.     

Development and characterisation of electrochromic devices on polymeric substrates

The development of full solid-state electrochromic (EC) devices on polymeric substrate is underway within a CEC BRITE-EURAM project (Project “FREDOPS”, BE-4137) carried out by four industries, two universities and two research centers from Belgium, Denmark, France, and Italy. The specific goal of this project is to develop a Fast Response Electrochromic Device On Polymeric Substrate (FREDOPS); in order to satisfy the required range of specifications in terms of fast response, long term performance and high contrast ratio, several systems based on different materials have been tested. The full cells consist of an electrochromic material layer and a counter electrode, inserted between two PET/ITO layers and separated by a polymeric electrolyte. Different types of polymeric electrolytes, counter electrodes and electrochromic layers have been developed, studied and checked. Full devices have been assembled using different combinations. Voltammetric and spectrophotometric measurements have been executed to check the electrochromic behaviour of the developed layers in half and full cells. Comparison of the electrochromic performances of different materials based cells has led to the rejection of several solutions due to poor performance and incompatibilities between layers. Considering that the electrochromic devices are finalised for different uses (window, sunroof,…), some performance specifications for each application are defined. A testing bench for cycling and ageing was developed. The present paper discusses these results in order to indicate the best performance.     

Sol–gel electrochromic coatings and devices: A review

A brief updated review is made on sol–gel-derived electrochromic films (some of which used as ion storage films) of different chemical systems. Performances of selected films measured in electrochemical cells or in devices are discussed and the degradation problems experienced by different authors enumerated.     

Sol-gel deposited electrochromic devices

This paper describes the current status of fabrication of electrochromic devices based on sol-gel deposited electrochromic and counter-electrode thin films. Sol-gel film deposition is being developed by Sustainable Technologies Australia as a technique for fabrication of switchable architectural glazings, which have the potential to lead to significant energy reductions in commercial buildings, especially in warm and hot climates. The paper describes the performance of these devices, with particular emphasis on the performance of sol-gel deposited electrodes, including estimates of the energy performance of the devices.     

The influence of the intercalate species on the quasi-static electrochromic behavior of tungsten-oxide-based devices

The influence of the intercalate species on the quasi-static electrochromic behavior of tungsten-oxide-based devices is investigated. Two different electrolytes are used in the devices: an aqueous sulfuric acid solution, from which it is assumed that intercalation of hydrogen occurs; and a solution of lithium perchlorate in propylene carbonate, from which it is assumed that intercalation of lithium occurs. Experiments are performed in which a step-current of small magnitude is imposed through the device, and the corresponding time-dependence of the electrical potential and optical transmission are measured simultaneously. The behavior of the optical efficiency is relatively insensitive to the nature of the intercalate species, but the device potential is appreciably more sensitive to lithium intercalation than to hydrogen intercalation. The disparity in electrical behavior is likely due to increased strain effects and/or a diminished availability of sites associated with the larger lithium intercalate. It is shown that the electrical and optical behavior of the two types of devices may be related by a single linear scaling relation, indicating that the fundamental processes involved in the operation of the devices are similar.     

Improved cyclability by tungsten addition in electrochromic NiO thin films

Aiming at improving the electrochromic properties of NiO thin films, well-known as anodic counter electrodes, the effect of tungsten addition was investigated. Ni–W–O thin films were deposited by pulsed laser deposition in optimized conditions, namely a 10⁻¹ mbar oxygen pressure and a room temperature substrate. The presence of W led to a progressive film amorphization. An increase in cyclability for Ni–W–O (5%_t WO₃) electrode, cycled in KOH 1 M electrolyte, was associated with a limited dissolution of the oxidized phases with tungsten addition. HRTEM investigations of cycled films revealed that the stabilization is correlated to the existence of an α(II) hydroxide phase.     

Elucidation of the electrochromic mechanism of nanostructured iron oxides films

Nanostructured hematite thin films were electrochemically cycled in an aqueous solution of LiOH. Through optical, structural, morphological, and magnetic measurements, the coloration mechanism of electrochromic iron oxide thin films was elucidated. The conditions for double or single electrochromic behavior are given in this work. During the electrochemical cycling, it was found that topotactic transformations of hexagonal crystal structures are favored; i.e. α-Fe₂O₃ to Fe(OH)₂ and subsequently to δ-FeOOH. These topotactic redox reactions are responsible for color changes of iron oxide films.     

Optical indices of lithiated electrochromic oxides

Optical indices have been determined for thin films of several electrochromic oxide materials. One of the most important materials in electrochromic devices, WO₃, was thoroughly characterized for a range of electrochromic states by sequential injection of Li ions. Another promising material, Li_0.5Ni_0.5O, was also studied in detail. Less detailed results are presented for three other common lithium-intercalating electrochromic electrode materials: V₂O₅, LiCoO₂, and CeO₂–TiO₂. The films were grown by sputtering, pulsed laser deposition (PLD) and sol–gel techniques. Measurements were made using a combination of variable-angle spectroscopic ellipsometry and spectroradiometry. The optical constants were then extracted using physical and spectral models appropriate to each material. Optical indices of the underlying transparent conductors, determined in separate studies, were fixed in the models of this work. The optical models frequently agree well with independent physical measurements of film structure, particularly surface roughness by atomic force microscopy. Inhomogeneity due to surface roughness, gradient composition, and phase separation are common in both the transparent conductors and electrochromics, resulting sometimes in particularly complex models for these materials. Complete sets of data are presented over the entire solar spectrum for a range of colored states. These data are suitable for prediction of additional optical properties such as oblique transmittance and design of complete electrochromic devices.     

IR electrochromic WO3 thin films: From optimization to devices

This paper reports enhanced electrochromic properties in the infrared region, so-called IR, and in particular, in the middle wavelength (MW: 3–5 μm) and long wavelength (LW: 8–12 μm), of radio frequency sputtered (RFS) WO₃ thin films, thanks to a careful optimization of the deposition conditions, namely the duration of deposition (<240 min), the substrate nature (FTO or Au), and the chamber pressure (15 and 45 mT). Significant modulations in reflectance (as high as 73% in the MW) and in the apparent temperature (up to 35 °C) between the inserted state and the deinserted one, for WO₃ thin films cycled in H₃PO₄ liquid electrolyte, are reported. Such performances correspond to a variation in emissivity of at least 40% as required for military applications. Finally, coupling both modelling and experimental approaches, first trends on the incorporation of the WO₃ single layer in full electrochromic devices (ECDs) are discussed considering mainly an all-solid-state device configuration.     

Stress in photochromic and electrochromic effects on tungsten oxide film

Optical absorbance at 632.8 nm and the stress generated in tungsten oxide film due to photochromic and electrochromic effects were measured. WO₃ thin films were deposited by reactive sputtering and the absorbance was obtained by measuring the optical transmittance of a laser beam through the film. The stress was calculated by measuring the substrate curvature and using the Stoney equation for multilayered films, since two layers are deposited onto a substrate for the electrochromism studies. The optical absorbance and the stress in the tungsten oxide film, as a function of UV irradiation time in photochromism and of inserted charge in electrochromism, are showed and discussed. In both effects the stresses generated were rendered as due to cation insertions into the film: H⁺ protons in photochromism and Li⁺ ions in electrochromism. The accuracy of the Stoney equation used for the stress calculation was also discussed.