Comparative studies of “all sol–gel” electrochromic windows employing various counter-electrodes

Electrochromic (EC) “smart” windows for buildings represent an effective way to modulate the intensity of incoming solar radiation. While it is accepted that WO₃ films represent the best option for the working electrode, the choice of the best counter-electrode is still debatable. Optical properties of counter-electrodes such as Ce, Fe, V and Sn oxides are presented. Electrochromic windows were made with a sol–gel WO₃ active colouring film (150°C), Ce, Fe, V oxide counter-electrodes and a sol–gel organic–inorganic hybrid (Li⁺ormolyte) ion conductor. The electrochromic responses of these devices predicted from the charge capacities, photopic transmittances and coloration efficiencies of individual films are compared with measured values.     

Sol–gel derived nanocrystalline CeO2–TiO2 coatings for electrochromic windows

Mixed CeO₂–TiO₂ coatings synthesized by sol–gel spin coating process using mixed organic–inorganic Ti(OC₃H₇)₄ and CeCl₃·7H₂O precursors with different Ce/Ti mole ratios were investigated by a wide range of characterization techniques. The attempts were directed towards achieving coatings with high transparency in the visible region and good electrochemical properties. Elucidation of the structural and optical features of the films yielded information on the aspects relevant to their usage in transmissive electrochromic devices. The films have been found to exhibit properties for counter electrode in electrochromic smart windows in which they are able to retain their transparency under charge insertion, high enough for practical uses. The high optical modulation and fastest switching for WO₃ film in the device configuration with the Ce/Ti (1:1) film is interpreted in terms of conducive microstructural changes induced by addition of TiO₂ in an amount equivalent to CeO₂.     

Kinetics and thermodynamic behavior of WO3 and WO3:P thin films

 There is a considerable interest in the research and development of materials and devices, that can be used for optical switching of large-scale glazings. Several potential switching technologies are available for glazings, including those based on electrochromic, thermochromic and photochromic phenomena. One of the most promising technologies for optical switching devices is electrochromism (EC). In order to improve the electrochromic properties of tungsten oxide, we have investigated the effect of phosphorous insertion on the electrochromic behavior of oxide films prepared by the sol–gel process.The kinetics and thermodynamics of electrochemical intercalation of lithium into Li_xWO₃ and Li_xWO₃:P films prepared by the sol–gel process were investigated. The standard Gibbs energy for lithium intercalation was calculated. The chemical diffusion coefficients, D, of lithium intercalation into oxide, were measured by galvanostatic intermittent titration technique (GITT), as functions of the depth of lithium intercalation.     

Electrochromism of sol–gel derived niobium oxide films

Niobium oxide films are promising cathodic electrochromics that in many aspects can compete with the more frequently studied WO₃ films. The films reported herein were prepared using the sol–gel route from a NbCl₅ precursor. The electrochromic properties were pronounced for crystalline films heat-treated at 500°C and exhibited transmittance changes between coloured and bleached states of 60% in the ultraviolet (UV) and 80% in the visible (VIS) and near-infrared (NIR) regions. Improved bleaching and more reversible electrochromism of thick niobium oxide films (thickness (d)>250 nm) were obtained by lithiation.Electrochromic (EC) devices were also prepared by assembling niobium oxide and lithiated niobium oxide films of different thicknesses with a hybrid inorganic/organic Li⁺ ionic conductor (organically modified electrolyte-ormolyte) and a molybdenum and antimony doped tin oxide (SnO₂ : Sb(7%) : Mo(10%) counter electrode films. The EC devices exhibited adequate colouring/bleaching kinetics (<2 min) and colouring/bleaching changes up to 40–50%.     

Electrochromic properties of heat-treated thin films of CeO2–TiO2–ZrO2 prepared by sol–gel route

CeO₂–TiO₂–ZrO₂ thin films were prepared using the sol–gel process and deposited on glass and ITO-coated glass substrates via dip-coating technique. The samples were heat treated between 100 and 500 °C. The heat treatment effects on the electrochromic performances of the films were determined by means of cyclic voltammetry measurements. The structural behavior of the film was characterized by atomic force microscopy and X-ray diffraction. Refractive index, extinction coefficient, and thickness of the films were determined in the 350–1000 nm wavelength, using nkd spectrophotometry analysis.Heat treatment temperature affects the electrochromic, optical, and structural properties of the film. The charge density of the samples increased from 8.8 to 14.8 mC/cm², with increasing heat-treatment temperatures from 100 to 500 °C. It was determined that the highest ratio between anodic and cathodic charge takes place with increase of temperature up to 500 °C.     

Electrically activated thin film optical coatings as functional layers in electrochromic devices

This study presents results on technology and characterization of molybdenum oxide, tungsten oxide and mixed oxide films based on Mo and W. These films were deposited by low-temperature carbonyl CVD process at atmospheric pressure and by simplified sol–gel method using spinning and spraying approaches. The obtained films were structurally and optically investigated. The films show good optical quality with optical transmittance of about 70% in the visible spectral range. Cyclic voltammograms as well as the transmittance modulation at different wavelengths in the visible spectral range were measured to characterize the electrochromic behaviour of the films. The colour efficiencies of the optimized films are in the order of 110–115 cm²/C, in case of spray deposited WO₃-sol–gel films—130 cm²/C.     

Optical properties of sol–gel dip-coated Ta2O5 films for electrochromic applications

Amorphous Ta₂O₅ films were prepared by sol–gel dip process on different substrates. The dip-coating technique was used to prepare amorphous Ta₂O₅ films by hydrolysis and condensation of tantalum ethoxide, Ta(OC₂H₅)₅, precursor. Stable coating solutions were prepared using acetic acid as a chelating ligand and catalyzer. Single layer and multi-layered Ta₂O₅ films were fabricated at a dipping rate of 107 mm/min. The microstructure, stoichiometry and optical properties of these films were investigated as a function of the film thickness. Room temperature CV measurements clearly revealed a protonic conductor behavior for Ta₂O₅ films. Optical properties such as refractive index, extinction coefficient and optical band gap value of the Ta₂O₅ films were calculated from optical transmittance measurements. It was found that the refractive index and extinction coefficient values were affected by the thickness of the coatings. The refractive index at a wavelength of 550 nm increased from 1.70 to 1.72 with increasing film thickness. The optical band gap value (3.75±0.12 eV) of the coating was unaffected by the film thickness. These results indicate that sol–gel-deposited Ta₂O₅ films have a promising application as proton conductors in electrochromic devices.     

Effect of citric acid on properties of CeO2 films for electrochromic windows

An alcohol based sol–gel process involving cerium chloride heptahydrate and citric acid in different mole ratios has been employed for the deposition of CeO₂ films. The structural, electrochemical, and optical properties of the films have been investigated using a wide range of techniques. Differential thermal analysis has shown the onset of crystallization of CeO₂ at 389 °C. The addition of an additive (citric acid) to the precursor sol has led to homogeneity and also a reduced ion storage capacity in the films. This observation emphasizes on the use of optimum content of the citric acid such that the films are suitable in terms of transparency as well as uniformity characteristics and also exhibit good electrochemical response. As is evidenced by the SEM study, the degree of polycrystalline grain formation in the citric acid derived films is observed to be less. The XPS results have confirmed the presence of Ce⁴⁺ state in the films. The optically passive behavior of the films is affirmed by their negligible transmission modulation upon Li ion insertion and extraction. A higher proportion of citric acid has also resulted in a reduced porosity and diminished crystallite size of the cerianite phase. The effect of the CeO₂ films on the switching kinetics of the tungsten oxide (WO₃) films has revealed an increase in the coloration time of the latter with the diminished crystallite size of CeO₂ nanograins in the former.     

Effect of TiO2 mixtures on the optical, structural and electrochromic properties of Nb2O5 thin films

Metal oxide films are important for various optical devices and especially for solar energy materials. TiO₂-mixed Nb₂O₅ thin films have been produced by sol–gel dip-coating method. Several parameters such as heat treatment, thickness, and mixture percentages are studied for the effect of the optical, structural and electrochromic properties of the materials. Optical parameters of the films were calculated through transmission and reflection measurement by a refractive index, extinction coefficient and thickness analyzer. Structural, electrochromic and surface analyses of the films were done by X-ray diffractometer, potentiostat/galvanostat and atomic force microscope systems.     

Characterization by resonance Raman spectroscopy of sol–gel TiO2 films sensitized by the Ru(PPh3)2(dcbipy)Cl2 complex for solar cells application

The development of TiO₂ photoelectrochemical cells is largely dependent on the elaboration of efficient dyes molecules to sensitize the nanocrystalline titania surfaces. In the present work, a new system functioning on this basis is proposed: an Ru(II) complex bearing two triphenylphosphine (PPh₃) and one 2,2′-bipyridyl-4,4′-dicarboxylate (dcbipy) chromophoric ligands, deposited on TiO₂ prepared through a simple sol–gel process is tested in a wet regenerative photoelectrochemical cell. The grafting of the complex on TiO₂ surface is characterized using Raman spectroscopy. The measurement of the full-width at half-maximum (FWHM) of the TiO₂ lower frequency E_g Raman line allowed to compare the crystallite sizes of the sol–gel films (13–14 nm) with those of nanocrystalline anatase films (7–8 nm). Very intense Raman bands were observed (ex situ and in situ) for the Ru(PPh₃)₂(dcbipy)Cl₂ complex chemisorbed on TiO₂. The most important vibrations were unambiguously assigned to the PPh₃ or to dcbipy ligands. Altering the potential applied to the electrode for a given laser excitation energy can selectively enhance the vibrational modes of PPh₃. A reversible shift of the dcbipy Raman lines was also observed. The electronic absorption spectrum and the electrochemical data are taken into account in order to explain the obtained results for the chemisorbed dye on both nanocrystalline and sol–gel films.     

WO3−x nanowires based electrochromic devices

A simple method was developed to fabricate tungsten oxide (WO_3−x) nanowires based electrochromic devices. The WO_3−x nanowires are grown directly from tungsten oxide powders in a tube furnace. The WO_3−x nanowires have diameters ranging from 30 to 70 nm and lengths up to several micrometers. The WO_3−x nanowires based device has short bleach-coloration transition time and can be grown on a large scale directly onto an ITO-coated glass that makes it potential in many electrochromic applications. The structure, morphology, and composition of the WO_3−x nanowires were characterized using the scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and energy-dispersive spectrometer. The optical and electrochromic performance of the nanowires layer under lithium intercalation was studied in detail by UV–VIS–NIR spectroscope and cyclic voltameter.     

Integrated low-emittance–electrochromic devices incorporating ZnS/Ag/ZnS coatings as transparent conductors

We have prepared and tested integrated low-emittance–electrochromic devices using ZnS/Ag/ZnS coatings as transparent electrodes and WO₃ films as electrochromic layers. These devices exhibit adequate coloration and can withstand more than 1000 bleaching-coloration cycles, provided that the metal layer is protected from the liquid electrolyte by a combination of thick dielectric films (ZnS/WO₃). We have also predicted the optimum configuration of the WO₃/ZnS/Ag/ZnS/Glass stack that maximizes transmission in the visible. Integration of low emittance and electrochromic films into one device could improve the performance and reduce the cost of electrochromic windows.     

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.     

Wet chemical preparation and characterization of electrochromic WO3

Thin films of electrochromic WO₃ were prepared via wet chemical deposition. Precursor solutions containing WOCl₄ in isopropanol were used and films were deposited by spin coating. Various techniques were used for characterization of the films such as Rutherford backscattering spectroscopy (RBS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), UV-VIS spectroscopy and electrochemical methods. Composition, structural characteristics and electrochromic properties were studied as function of the curing temperature, in the range 80–500°C.     

Optical filters from SiO2 and TiO2 multi-layers using sol–gel spin coating method

Sol–gel spin coating process is used to produce optical filters from SiO₂ and TiO₂ multi-layers. By coating the films symmetrically on both sides of the glass substrates, we designed two types of three-layer anti-reflective (AR) filters for the near–infrared region, and a nine-layer reflective filter for the near–UV region. We develop a simple theoretical model for these filters, which incorporates sol–gel film densification during the coating process, and fit it to the experimental data to extract properties of the individual layers in the coatings.     

Electrochromic properties of nanocomposite WO3 films

A new nanocomposite WO₃ (NWO) film-based electrochromic layer was fabricated by a spray and electroplating technique in sequence. An indium–tin oxide (ITO) nanoparticle layer was employed as a permanent template to generate the particular nanostructure. The structure and morphology of the NWO film were characterized. The optical and electrochromic properties of the NWO films under lithium intercalation are described and compared to the regular WO₃ film. The NWO films showed an improved cycling life and an improved contrast with compatible bleach-coloration transition time, owing to the larger reactive surface area. The nanocomposite WO₃ film-based electrochromic device (NWO-ECD) was also successfully fabricated. Most importantly, the NWO film can be prepared on a large scale directly onto a transparent conductive substrate, which demonstrates its potential for many electrochromic applications, especially, smart windows, sunroof and displays.     

Characterization and electrochromic properties of CuxNi1−xO films prepared by sol–gel dip-coating

Cu_xNi_1−xO electrochromic thin films were prepared by sol–gel dip coating and characterized by XRD, UV–vis absorption and electrochromic test. XRD results show that the structure of the Cu_x Ni_1−xO thin films is still in cubic NiO structure. UV–vis absorption spectra show that the absorption edges of the Cu_xNi_1−xO films can be tuned from 335 nm (x = 0) to 550 nm (x = 0.3), and the transmittance of the colored films decrease as the content of Cu increases. Cu_xNi_1−xO films show good electrochromic behavior, both the coloring and bleaching time for a Cu_0.2Ni_0.8O film were less than 1 s, with a variation of transmittance up to 75% at the wavelength of 632.8 nm.     

Electrochromic characterization of sol–gel deposited coatings

Electrochromic devices have increasing application in large-area display devices, switchable mirrors and smart windows. A variety of vacuum deposition technologies have been used to make electrochromic devices. The sol–gel process offers an alternative approach to the synthesis of low-cost, high-quality electrochromic device layers. This paper discusses the developments in sol–gel-deposited electrochromic films and a prototype all sol–gel device with switching from 80% to 50% (550 nm). The sol–gel process involves the formation of oxide networks upon hydrolysis–condensation of alkoxide precursors. In this study, we cover sol–gel-deposited oxides of WO₃, TiO₂, Nb₂O₅, V₂O₅, Ni(OH)₂, Co(OH)₂ and CuO_x. The coloration reaction voltammetry and spectral optical properties are presented for selected films.     

Improved electrochromic films of NiOx and WOxPy obtained by spray pyrolysis

Electrochromic films of NiO_x and WO_xP_y were produced by the spray pyrolysis technique. The nickel-oxide-based coatings were obtained from both an alcoholic solution of nickel nitrate and aqueous solution of the mixture nickel nitrate/cobalt nitrate. Coatings obtained from alcoholic solutions showed a noticeable contrast of optical transmittance from fully bleached to colored state. X-ray diffraction analysis showed a slight crystallization in NiO_x after electrochemical treatment: one diffraction peak for as-deposited films turned to three diffraction peaks for electrochemical treated samples. Coatings obtained from aqueous solution of mixture nickel nitrate/cobalt nitrate showed an optimized electrochromic behavior at a Ni:Co proportion of 90:10. At this condition an optical contrast of 50% is found. X-ray diffraction showed that these samples comprised a phase mixture of Co₃O₄ and NiO.WO_xP_y samples were obtained from polytungsten gel in which H₃PO₄ was added. We found that for 8.3 at% of P:W, the electrochromism was optimized. Pyrolytic coatings of WO_xP_y show superior behavior than those of WO_x obtained by spray pyrolysis, both in optical contrast and durability.     

Intercalation studies on electron beam evaporated MoO3 films for electrochemical devices

Now-a-days a large number of extensive research has been focused on electrochromic oxide thin films, owing to their potential applications in smart windows, low cost materials in filters, low cost electrochemical devices and also in solar cell windows. Among the varieties of electrochromic transition metal oxides, the molybdenum oxide (MoO₃) and tungsten oxide (WO₃), form a group of predominant ionic solids that exhibit electrochromic effect. The electrochromic response of these materials are aesthetically superior to many other electrochromic materials, because WO₃ and MoO₃ absorb light more intensely and uniformly. In the present case, we have discussed about the electrochromic behaviour of electron beam evaporated MoO₃ films. Moreover, the MoO₃ film can also be used as a potential electro-active material for high energy density secondary lithium ion batteries; because it exhibits two-dimensional van der Waals bonded layered structure in orthorhombic phase. The films were prepared by evaporating the palletized MoO₃ powder under the vacuum of the order of 1 × 10⁻⁵ mbar. The electrochemical behaviour of the films was studied by intercalating/deintercalating the K⁺ ions from KCl electrolyte solutions using three electrode electrochemical cell by the cyclic-voltammetry technique. The studies were carried out for different scanning rates. The films have changed their colour as dark blue in the colouration process and returns to the original colour while the bleaching process. The diffusion coefficient values (D) of the intercalated/deintercalated films were calculated by Randle's Servcik equation. The optical transparency of the coloured and bleached films was studied by the UV–Vis–NIR spectrophotometer. The change in bonding assignment of the intercalated MoO₃ films was studied by FTIR spectroscopic analysis. A feasible study on the effect of substrate temperatures and annealing temperatures on optical density (OD) and colouration efficiency of the films were discussed and explored their performance for the low cost electrochemical devices.