Electrochromic systems with their reversible optical switching ability offer important and interesting application possibilities. Work was carried out in our laboratory towards the fabrication of an all-solid electrochromic system. Lithiation studies of two transition metal oxides (W03 and V205) were done for their use as base electrochromic layer and counter electrode layer in the solid system. Also a thermally evaporated layer of LiBO2 was studied for its application as ion conducting layer. The optimum results from these studies were used for the fabrication of an all-solid electrochromic prototype. The results on the electrochromic performance of this system are presented. Several hundred coloration and bleaching cycles of such systems have been carried out without any change in its performance. All results indicate the high suitability of such a system for "smart" window application.
A prototype of smart window has been built using tungsten and iridium oxides as complementary electrochromic electrodes and proton conducting polymer electrolytes obtained by dissolving phosphoric acid into basic polymers. The main properties of the individual layers are first described. Then, the performances and limitations of a complete cell are discussed in terms of optical efficiency, reponse time, memory effect and cyclability.
Different anhydrous potassium ion conducting polymer electrolytes have been studied : (POE)nKTFSI (trifluorosulfonimide), plasticized (POE)nKTFSI, (PEM)nKTFSI and (POE-Acetal)nKTFSI. DSC and conductivity measurements have been used to characterize these electrolytes. Several solutions to avoid cristallization have been examined. The best conductivity has been obtained with (POE-Acetal)nKTFSI. This electrolyte exhibits good transmission in the VIS-IR region. The response time of cells using these electrolytes and Prussian blue and KXWO3 as electrodes has been measured and compared.
We report on the fabrication and use of new ion storage layers as counter electrodes for completely solid state electrochromic windows. The ion storage layers are transparent composites of a room temperature ion-conducting polymer and an electroactive material based upon the polyorganodisulfide/dithiolate redox couple. Solid state devices consisting of molybdenum doped W03, a polymer ion conductor layer and the new ion storage layers colored rapidly from a pale yellow to a deep blue-green, upon application of 1 .2 V d.c. The photopic transmittance changed from 6 1 to 9% , and the solar transmittance from 45 to 5% during the coloration process, for a typical example. Because the counter electrode does not change optical properties as a function of redox state, problems with color imbalance can be avoided completely. The large number of polyorganodisulfides that may be used for this application allows for flexibility in device design and properties, and the use of solid state components ensures improved stability of windows incorporating this technology.
Electrochromic tungsten oxyfluoride thin films were made by reactive magnetron sputtering of W in Ar+O2+CF4. A moderate CF4 fraction led to deposition at enhanced rates and films with fairly neutral colour in their dark state. Transparent vanadium oxyfluoride films were made analogously; they were annealing posttreated in Ar+02+CF4. By fluorination it was possible to avoid the short-wavelength absorption that otherwise limits the usefulness of V205 in electrochromic smart windows. A transparent, adhesive, viscoelastic polymer electrolyte laminate was developed. It has good voltammetric cycling capability (106 times), good optical transmittance, and high conductivity, ranging from 10-6 S/cm (-10°C) to 10-3 S/cm (80°C).
This paper gives the experimental evidences to establish a new electrochromism for the nickel oxide films deposited by rf reactive sputtering. ESCA results indicate that all the films in as-deposited, coloured and bleached states are nonstoichiometric nickel oxide with different oxidation states and there is no evidence to support the exitance of nickel hydroxide in the three states. SIMS has been used to analyze the injected Li+ concentration in these films. The relative content of Li in films is 0 for as-deposited, 1.3 for coloured and 4.0 for bleached. Li+ implantation has been used to initially bleach the dark as-deposited films. The results indicate that the as-deposited films can be bleached by Li+ implantation. A new electrochromic reaction model for the reactively sputtered nickel oxide films has been proposed.
New transition metal oxyhydroxides containing one or two metals such as W, Nb and Ti have been recently synthetized by "soft chemistry". All these materials have a crystal structure that allows further ionic insertion reactions. Moreover, the reduction of W6+, Nb5+ and Ti4+ during electrochemical insertion may lead to a colored material. For these reasons, the electrochromic properties of thin films of some of these oxyhydroxides, obtained by vacuum evaporation from bulk materials, have been investigated. This presentation will describe the method of preparation of thin films obtained from H2Ti3O7, HNbWO6, H20 and from mixtures of these two metal oxyhydroxides. Their composition are determined. The electrochromic properties in aqueous acid electrolyte and in lithium electrolyte will be presented and compared to those of amorphous W03 films. It will be shown that the coloration efficiency of these films may be improved by coevaporating mixtures of two oxyhydroxides. Moreover, the presence of Ti and Nb in the films has been shown to be directly connected to the nature of the oxyhydroxides
Optical multichannel analysis (OMA) was used to follow the kinetics of color changes in electrochromic polyaniline (PANI) films when they are polarized in an electrolytic medium at different potentials; these color changes which are related to the transition from one polymer form to another one are very fast. For the setting of electrochromic devices, PANI must be used in acid electrolytic media. In these media, the color change mechanisms are essentially related to the chains protonation/deprotonation. The study is now extended to different pH media (neutral and alkaline) in order to obtain a better understanding of the chemical mechanisms of passage from one form to another one, and particularly of the oxidation of phenyl in quinone rings. OMA allowed us to also obtain an apparent value of the films absorption coefficient. In the investigated range (300/800 nm), several absorption phenomena characteristic of the different polymer forms can be displayed. The range in which two different forms coexist can then be characterized by the deconvolution of the optical spectra. Finally, the electrochromism in polymer is characterized by the change of optical density (Delta) DO directly obtained from the OMA experimental spectra.
By combining a processable electronically conductive polymer [poly(3-octylthiophene) (P3OT), polypyrrole (PPy)], a solid polymer electrolyte [(PEO)8LiClO4], and a metal oxide (V2O5), a solid state electrochromic device is constructed. The polymer films are fabricated by spin coating from solution (P3OT) and template polymerization (PPy). The metal oxide is electrochemically doped with Li+ and the electrodes are mounted in a sandwich structure with a thin film of polymer electrolyte in between. As the applied cell potential is changed, the optical absorption of the cell is changed. Owing to the difference in columbic capacity between the different materials, the optical changes of the cell are due to optical changes of the polymer only. This means that, instead of having to adapt often contradictory optical changes in two electrochromic materials for the desired application, one can use a polymer with a proper optical signature, letting the band gap determine the electro-optical behavior of the cell.
Heavy-metal hexacyanoferrate stability during voltammetric cycling on the FeIII(CN)63-/FeII(CN)64- is explained by the nature of the ions entering or leaving the film. In contact with a KCl aqueous electrolyte, as many cycling can be performed as more K+ ions are intercalated in the lattice. This is concluded from the relation between the stability of the iron(III), indium(III), and copper(II) compounds, and the nature of the ions leaving or entering the film during a voltammetric cycle, as shown by the mirage technique.
The electrochromic properties of all solid state devices (ASSDs) are strongly defined by thin film materials used as well as the method of deposition. Various thin film materials deposited by evaporation and sputtering are described serving as electrode, reflector, electrolyte, storage medium, or electrochromic film in ASSD. The impact of process parameters upon the device functionality is shown. In addition, the long-term stability of ASSDs for the different thin film systems is reported.
In this work, optical and electrochemical properties of electrochromic NiOx films are studied. Quartz crystal microbalance study reveals that OH- insertion/extraction occurs during coloration and bleach reaction of NiOx film in an organic electrolyte. Electrochromic glazings consisting of WO3/PEO-based Li+ electrolyte/NiOx shows promising solar control properties. The possibility toward NIR-reflectance control is exemplified.
Application of smart windows and absorbers demand electrochromic (EC) devices with long term stability and a large scale production technology. The paper presents recent results on preparation of rigid and flexible EC devices with 0.3 X 0.3 m2 active area in a three layer arrangement (polyaniline/polymeric electrolyte/tungsten trioxide). The main items and risks of processing an EC element are discussed. It is shown, that highly conductive, chemical resistant electrodes (sheet resistance 5 (Omega) /sq., transparency 85%) on flexible PMMA and PC substrates can be prepared by low temperature sputtering of indium tin oxide (ITO). Deposition apparatus and parameters are described. Well known standard techniques for the synthesis of EC films like polyaniline and tungsten trioxide are adapted for large surfaces: polyaniline and tungsten trioxide based EC films on ITO glass have been prepared with chemical and electrochemical preparation techniques. Electrode geometry plays an important role for the homogeneity of the grown film. We succeed in minimizing tolerances in optical density over 0.3 X 0.3 m2 down to 2%. The solid polymer electrolyte essentially determines the performance of the EC device. High transmittance, proper conductivity, and strong adhesion, are the main attributes.
Due to the fallibility of mechanical shading devices such as shutters or blinds, thin films with variable transmittance, which can be applied directly to a glazing surface, are being investigated for so-called `smart windows' or for applications together with transparent insulating materials. In the films discussed here, the transmittance is changed by varying the scattering properties, which depend on changes in the effective refractive index and/or size of microstructures. This may be achieved by orientation and relaxation of birefringent liquid crystal molecules within polymer-encapsulated droplets by applying an electric field, by reversible separation and mixing of two polymers with temperature variation, or by exploiting the Christiansen effect. In this paper, the effect of varying experimentally accessible parameters on the transmittance change is first explored theoretically by applying Mie and multiple scattering theories. These results are complemented by measurements of the spectral dependence of the transmittance and reflectance of the first two types of switching samples, i.e., polymer-dispersed liquid crystal (PDLC) films and thermotropic layers of polymer blends. Changes of almost 40% in the visual and solar direct-hemispherical transmittance were measured for the range of sample thicknesses investigated here.
Optical, electrical, and structural properties of electrochromic, amorphous, or polycrystalline WO3 thin films prepared by thermal vaporization or chemical vapor deposition (CVD) have been studied for samples in colored and bleached states. During the electrical coloration process, a shift of the absorption peak toward higher energies and an increase in the conductivity were observed for all samples. In the same time for the polycrystalline samples a structural change from monoclinic to cubic structure appeared and an increase in the reflectivity in the infrared wavelength range. The crystal structure of colored films changed back to the initial state when the films were electrically bleached, however, the optical and electrical properties were not absolutely reversible. It is possible that a fraction of hydrogen cations still remained inside bleached films and did not contribute to the coloration. An x ray diffraction study during the oxidation in air of a colored sample as a function of time indicates the possibility of following the kinetics of the electrochromic process from the shift of the XRD peaks.
Because of their dense structure, electrochromic sputtered tungsten oxide thin films often show low coloration efficiency, low ion mobility, and the tendency to incorporate lithium ions irreversibly. An interesting possibility to modify the structure is the admixture of other non- absorbing metal oxides. Recent results from Japan indicate improved electrochromic behavior of evaporated WO3 - TiO2 mixed films. In order to verify this effect for different deposition techniques, we examined thin films of WO3 - TiO2 produced by the sol-gel method and dc magnetron sputtering. Both types of films were characterized optically and electrochemically in organic LiClO4 solutions. The cycling stability as a function of titania concentration was recorded. Improved electrochromic properties were found for both sputtered and sol-gel films.
Thin films produced by dip-coating from tungsten alkoxide solutions are of interest for large area electrochromic (switchable) window coatings. The window systems consist, in part, of a layer of tungsten oxide (WO3) on a layer of indium tin oxide (ITO) on glass substrate. Sol-gel processing has several advantages over other preparation techniques. However, there is the possibility of hydrocarbon residue within the films. Such carbon may restrict the electrochromic performance of the films. Rutherford backscattering spectroscopy (RBS) has been used for determining elemental composition and depth profiles in these film structures. The spectra confirm that sol-gel samples contain a substantial level of a light element, such as carbon. Auger electron spectroscopy supports the estimate obtained for the carbon content. The residual carbon can, however, be burnt out by firing at > 500 degree(s)C. Under certain conditions, a sub-layering is seen in the depth profiles.
The degradation of the electrochromic film of a-WO3 was investigated from the equilibrium state of different driving voltage and the time dependent emf was also obtained in an open or short circuit of zero volts. Based on the mechanism of activation and the principal of chemical reaction kinetics, the correct definition of electrochromic memory was made and a relation of memory was obtained. It was also found that at the initial time of natural bleaching, the change rate of proton concentration in the film was also obtained. According to the mechanism of electrochromic memory, it was indicated that in the open circuit case, the theoretical reaction of the change rate of electric potential was in good agrement with the experimental results, and furthermore, the reaction constant was determined with the experimental data. In the short circuit case, there exist two effects on degradation, the short circuit current caused by the backward emf, and the oxidation of the colored film. The experimental data shows that, in the short circuit case, the degradation strongly depends on the short circuit current and the effect of chemical reaction can be neglected.
Thin films of nickel oxide obtained by reactive sputtering and deposited either at room temperature or at 250 degree(s)C presented electrochromic properties in basic aqueous electrolytes. The structure of the films is characteristic of a polycrystalline material with preferential orientation, by which it was possible to follow the changes in lattice parameters with electrochemical intercalation, growth temperature, and annealing processes. Photocurrent and electroreflectance measurements were performed in order to detect the variations in the band gap energy and in the electronic transitions as a function of the electrochromic state. The results are consistent with the presence of impurity levels due to nickel vacancies. The extracted values of the band gap energies increased for the clearer states, followed by a decrease in the carrier's recombination rates.
The electrochromic properties of transition metal oxides are a consequence of an ion/electron intercalation process. In nickel oxide electrodes, the electrochemical/electrochromic reaction has been pointed out to be a proton/electron intercalation process. However, recent works showed that the mechanism of the reaction could be much more complex. In this work, we studied the electrochemical reactions of rf sputtered thin nickel oxide films in aqueous electrolytes containing different cations. An electrochemical quartz crystal microbalance has been used in order to follow the mass changes during the experiments. Also, the mechanical stress changes have been measured in situ by an optical technique. The results show that the incorporation of the cations in the film has to be considered; possible reaction mechanisms are discussed in this work.
Electrochromic nickel oxide based thin film coatings have been prepared on transparent conducting electrodes using anodic electrodeposition and colloidal precipitation techniques. Coatings were deposited from solutions containing nickel sulphate, ammonium hydroxide, and a solution stabilizer, polyoxyethylene sorbitan monolaurate (a nonionic surfactant). The films were cycled between their colored and bleached states using a 3-electrode cell containing 1 M KOH electrolyte. The dependence of the optical and electrochromic properties of the coatings on bath concentration and deposition time was investigated. Films were analyzed using Fourier transform spectrophotometry, UV/VIS/NIR spectrophotometry and cyclic voltammetry. The best films produced by anodic electrodeposition on indium tin oxide coated glass substrates have solar transmittance of 0.82 and 0.22 in the bleached and colored state, respectively. Corresponding optimum values for films produced by colloidal precipitation are 0.82 and 0.47. An O-H stretch at 3647 cm-1 corresponding to Ni(OH)2 was found for both anodic and colloidal deposited films in the reduced state. In the oxidized state a stretch was identified at 564 cm-1 characteristic of NiOOH.
Ni-O thin films have been produced onto ITO coated glass using the dc magnetron sputtering technique. The colored and bleached states of Ni-O films have been obtained by applying alternatively a positive 0.8 V potential and a negative 1.0 V potential to the sample (WE). Normal transmittance of Ni-O films in the colored and bleached states have been measured in the wavelength range of 0.35 - 2.5 micrometers by spectrophotometer.
Electrochromic tungsten oxyfluoride thin films were made by reactive magnetron sputtering of W in Ar+O2+CF4. A moderate CF4 fraction led to deposition at enhanced rates and films with fairly neutral color in their dark state. Transparent vanadium oxyfluoride films were made analogously; they were annealing post-treated in Ar+O2+CF4. By fluorination it was possible to avoid the short-wavelength absorption that otherwise limits the usefulness of V2O5 in electrochromic smart windows. A transparent, adhesive, viscoelastic polymer electrolyte laminate was developed. It has good voltammetric cycling capability (106 times), good optical transmittance, and high conductivity, ranging from 10-6 S/cm (-10 degree(s)C) to 10-3 S/cm (80 degree(s)C).
Solid-state electrochromic (EC) half-cells (ITO/EC/Ta2O5/ITO) have been fabricated, where the EC layer is WO3 or NiO film. Each layer was prepared by rf sputtering in an atmosphere of Ar-O2 mixture. The conduction mechanism in Ta2O5 electrolyte films was discussed from the measurement of ac conductivity. Both ionic and electronic conductions were found in the Ta2O5 films which were prepared in the oxygen content region less than 20%. In order to obtain pure ionic conduction in the Ta2O5 electrolyte films, they should be prepared in Ar-O2 mixture with the oxygen content region more than 20%. Coloration efficiency (42 cm2/C) of the half-cells with WO3 were a little smaller than that (60 cm2/C) of corresponding cells with liquid electrolyte. On the other hand, cells with NiO indicated only a low coloration efficiency (about 8 cm2/C). When Ta2O5 film was prepared in the atmosphere of Ar-O2 and moisture, the coloration efficiency increased a little. In the self-bleaching test under the open- circuit condition, the optical density (OD) of the cell with NiO film fell off, down to a half of the initial OD at 50 hours after stopping the coloration.
Thin solid films of CeO2 and mixed CeO2-TiO2 were prepared by the sol-gel route via the dip-coating technique. Particulate sols of ceria were used to make thin solid films with an efficiency of 0.024 micrometers /dipping while the efficiency of the CeO2-TiO2 coating process is about 0.05 micrometers /dipping. The solar transmission values (Ts) of both coatings are in the range 0.6 - 0.8 and depend on coating thickness. Cyclic voltammetric (CV) measurements show that the CeO2/LiOH system exhibits higher overall electrochemical reversibility when compared to the CeO2-TiO2/LiOH system. The CeO2/LiOH system is also less sensitive with regard to the coating thickness. Coulometric measurements show that CeO2 exhibits a larger storage capability which was determined as a function of the coating thickness.
Viscosity and ionic conductivity were carried out on gel electrolytes made of highly amorphous polymer, poly(methylmethacrylate), LiClO4 salt, and a solvent of high boiling temperature, propylene carbonate. In the composition range 0 weight % - 35 weight % of PMMA in the gel, the conductivity decreases very slightly as the amount of PMMA increases. On the other hand, a large increase of viscosity is observed. This result is interpreted on the basis that, from a microscopic point of view, the gel may be seen as a free liquid electrolyte encaged in a polymer matrix. For amounts of PMMA larger than 35 weight % the conductivity decreases more rapidly. These gels are transparent in the visible region, thermally stable in a large domain of temperatures, i.e., -65 degree(s)C, +160 degree(s)C and electrochemically stable, i.e., (Delta) V equals 4.5 V. They are good candidates as `solid electrolytes' for the realization of solid state electrochromic `smart' windows.
A novel basic proton conducting polymer (so-called proton-vacancy conducting polymer) has been synthesized. It is based on the copolymerization via the sol-gel process of sulfonamide- containing groups, partially deprotonated, and an internal plasticizer (POE segments). All the organic groups are attached to trialkoxysilanes which, through the hydrolysis-condensation process, lead to a silica-based backbone. The flexible films obtained are homogeneous and transparent. Maximum conductivities are observed with approximately equals 15% of the sulfonamide groups deprotonated by the organic base and approximately equals 10% of 2000 Mw POE plasticizer. The conductivity values are 2 10-7Ω-1-cm-1 at 30 degree(s)C and 10-5 Ω -1cm-1 at about 84 degree(s)C. The polymers are thermically stable up to 220 degree(s)C and their electrochemical stability range is close to 2 V.
Thermochromic VO2 thin films were prepared by electron beam depositing 99.99% pure metallic vanadium onto the glass substrates followed by annealing post-treatment. The treatment was performed in oxygen pressure ranging from 1 X 10-3 to 1.5 Torr during the time from 2 to 10 h at a temperature of 723 K. Structural and optical properties of the films were measured as a function of both the annealing time and oxygen pressure. The metal-semiconductor transition of the VO2 films annealed in 0.3 Torr of gaseous oxygen, for 7 h was observed at Tc approximately equals 345 K. At the temperature below Tc these films are infrared-transparent semiconductors having monoclinic structure. At the temperature above Tc they become metallic structures exhibiting a tetragonal crystalline lattice and their optical transmittance at a wavelength of 1.2 micrometers has decreased to a value as low a 30%.
A systematic study of the synthesis of modified PEO-based ionically conducting polymer electrolyte systems for utilization in electrochromic windows is reported. Copolymerization of ethylene oxide and various epoxy-functionalized monomers have produced polymers incorporating sidechains. These bulky functionalities inhibit crystallinity, and therefore, give rise to ionic conductivity levels which are two to three orders of magnitude higher than those of PEO at room temperature and below.
Colloidal particles of WO3 and IrO2 are synthesized and dispersed within a gelatinous perfluorinated ionomer matrix. Experimental procedures are established in order to obtain percolation between the electrochromic particles. Colloidal particle sizes are measured by quasi elastic light scattering. Electrochemical properties of the mixed colloid electrodes are determined by cyclic voltammetry and impedance spectroscopy. Preliminary optical tests are performed in order to measure transmission and contrast of electrochromic half cells with a mixed colloid electrode, and also a sputtered oxide electrode.