Preparation and properties of electrochromic tungsten oxide film

Tungsten oxide films have been formed by poly-complex. The dip-coating solution was prepared by adding hydrogen peroxide to a tungsten acid solution which was obtained from ion exchange. Amorphous and crystalline films can be obtained at different firing temperature. Optical and electrochemical measurements of these films have been carried out. The XRD, IR, TG-DTA analysis and SEM morphologies observation are also described. The results show that these coatings have good electrochromic properties.     

Comparison of electrochromic amorphous and crystalline tungsten oxide films

A detailed systematic study of the tungsten oxide thin films has been carried out using WO₃ films after they were annealed at progressively increasing temperatures ranging from 350°C to 450°C in oxygen environments. The structural properties of the films were characterized using X-ray diffraction and Raman spectroscopy. The amorphous WO₃ films remain as an amorphous phase up to 385°C and begin to crystallize at 390°C and then are completely crystallized at 450°C. Absorption peaks of the films are found to shift to a higher energy side with increasing annealing temperature up to 385°C and then shift abruptly to a lower energy as the films begin to crystallize at 390°C. Deconvolution of the absorption spectra shows that there are two different polaron transitions in the amorphous WO₃ films.     

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.     

Stress changes in electrochromic thin film electrodes:: Laser beam deflection method (LBDM) as a tool for the analysis of intercalation processes

The mechanical effects of the intercalation processes in electrochromic WO₃ thin films are reported here and discussed. In particular the electrochemical insertion of H, Li and Na ions in WO₃ was studied by means of laser beam deflection method (LBDM). Linear changes of WO₃ stress were observed for small amounts of the inserted charge and linearity was always associated with a reversible mechanical behavior of thin films. An explanation in terms of homogeneity of the insertion is given. An analogous trend was also verified in the absorbance vs. charge curves. As a consequence the constancy of the electrochromic efficiency values for the three different ions was found. In this way it was possible to determine a full reversible behavior for M_xWO₃ in the composition range 0x0.2. The onset of new phases formation when x exceeded previous upper limit, was observed during Li and Na intercalation. Such transitions brought about the loss of optical and mechanical reversibility. During prolonged hydrogen insertion a reversible slope inversion occurred in stress curve so that it was necessary to take into account different possible mechanisms of the WO₃ electrochromic reaction.     

Optical properties of electrochromic iridium oxide and iridium-tantalum oxide thin films in different colouration states

Electrochromic iridium oxide (IrOx) and iridium-tantalum oxide (IrTaOx) thin films were prepared by sputtering. Complex refractive indices were determined for samples deposited on indium-tin oxide covered glass in different colouration states, and for as-deposited samples on sapphire and Corning glass. The refractive index was found to be practically constant for both IrOx (∼1.3) and IrTaOx (∼2). The extinction coefficient was found to vary between the coloured and bleached states with ∼35% for IrOx and ∼55% for IrTaOx at 660 nm. This is believed to be a result of the removal of intraband transitions within the Ir t_2g band during bleaching.     

The electrochromic and photocatalytic properties of electron beam evaporated vanadium-doped tungsten oxide thin films

The electrochromic and photocatalytic properties of vanadium-doped tungsten trioxide thin films prepared at room temperature (300 K) by the electron beam evaporation technique are reported in this paper. The vanadium to tungsten ratio (V/W) in these films are 0.003, 0.019, 0.029 and 0.047. The optical band gap of the vanadium-doped tungsten oxide (WO₃) thin film initially increases from 3.16 to 3.28 eV for V/W ratio 0.003 then decreases to 3.15 eV for V/W ratio 0.047. These vanadium-doped films switch between neutral gray and transparent states. The coloration efficiency (CE) decreases from 82 cm² C⁻¹ (pure WO₃) to 27 cm² C⁻¹ for the film containing V/W ratio 0.047. The photocatalytic activity has enhanced with vanadium doping and maximum activity of 15% (percentage change in optical density of methylene blue due to photo degradation) has been observed for the film containing V/W ratio of 0.019. The Kelvin probe measurements show that the work function of pure WO₃ films is 4.07 eV and vanadium doping initially increases the work function to 4.19 eV for V/W ratio 0.019 and then decreases it to 3.97 eV for film with V/W ratio 0.047.     

Optical and electrochromic properties of oxygen sputtered tungsten oxide (WO3) thin films

Thin films of tungsten oxide (WO₃) were deposited onto glass, ITO coated glass and silicon substrates by pulsed DC magnetron sputtering (in active arc suppression mode) of tungsten metal with pure oxygen as sputter gas. The films were deposited at various oxygen pressures in the range 1.5×10⁻²−5.2×10⁻² mbar. The influence of oxygen sputters gas pressure on the structural, optical and electrochromic properties of the WO₃ thin films has been investigated. All the films grown at various oxygen pressures were found to be amorphous and near stoichiometric. A high refractive index of 2.1 (at λ=550 nm) was obtained for the film deposited at a sputtering pressure of 5.2×10⁻² mbar and it decreases at lower oxygen sputter pressure. The maximum optical band gap of 3.14 eV was obtained for the film deposited at 3.1×10⁻² mbar, and it decreases with increasing sputter pressure. The decrease in band gap and increase in refractive index for the films deposited at 5.2×10⁻² mbar is attributed to the densification of films due to ‘negative ion effects’ in sputter deposition of highly oxygenated targets. The electrochromic studies were performed by protonic intercalation/de-intercalation in the films using 0.5 M HCl dissolved in distilled water as electrolyte. The films deposited at high oxygen pressure are found to exhibit better electrochromic properties with high optical modulation (75%), high coloration efficiency (CE) (141.0 cm²/C) and less switching time at λ=550 nm; the enhanced electrochromism in these films is attributed to their low film density, smaller particle size and larger thickness. However, the faster color/bleach dynamics is these films is ascribed to the large insertion/removal of protons, as evident from the contact potential measurements (CPD) using Kelvin probe. The work function of the films deposited at 1.5 and 5.2×10⁻² mbar are 4.41 and 4.30 eV, respectively.     

Annealing effect on electrochromic properties of tungsten oxide nanowires

The effect of thermal annealing on the electrochromic properties of the tungsten oxide (WO_3−x) nanowires deposited on a transparent conducting substrate by vapor evaporation was investigated. The X-ray diffraction (XRD) indicated that the structures of the nanowries annealed below 500 °C had no significant change. The X-ray photoelectron spectroscopy (XPS) analysis suggested that the O/W ratio and the amount of W⁶⁺ ions in the annealed nanowire films could be increased as increasing annealing temperature. Increased annealing temperature could promote the coloration efficiency and contrast of the nanowire films; however, it could also affect the switching speed of the nanowire films.     

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.     

Electrochromism in nickel oxide and tungsten oxide thin films: Ion intercalation from different electrolytes

Electrochromic (EC) NiO_z and WO_y 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 suitable electrolyte, compatible with both NiO_z and WO_y. To that end we carried out cyclic voltammetry (CV) in potassium hydroxide (KOH), propionic acid, and lithium perchlorate in propylene carbonate (Li-PC). WO_y could be coloured in propionic acid and Li-PC, while NiO_z could be coloured only in KOH. Both films showed best stability in Li-PC, which hence is well suited for further studies of mixed NiO_z and WO_y.     

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.     

A new thin film photochromic material: Oxygen-containing yttrium hydride

In this work we report on photochromism in transparent thin film samples of oxygen-containing yttrium hydride. Exposure to visible and ultraviolet (UV) light at moderate intensity triggers a decrease in the optical transmission of visible and infrared (IR) light. The photo-darkening is colour-neutral. We show that the optical transmission of samples of 500 nm thickness can be reduced by up to 50% after one hour of illumination with light of moderate intensity. The reaction is reversible and samples that are left in the dark return to the initial transparent state. The relaxation time in the dark depends on the temperature of the sample and the duration of the light exposure. The photochromic reaction takes place under ambient conditions in the as-deposited state of the thin-film samples.     

Chemically deposited electrochromic cuprous oxide films for solar light modulation

Thin cuprous oxide electrochromic films on the transparent conductive electrodes were prepared by chemical electroless method. The films cycled in K⁺-based electrolyte revealed typical red-ox peaks with higher intensity compared to those in the Li⁺ and the Na⁺-based electrolytes. The durability of the cuprous oxide due to cycling into LiClO₄ was about 60 cycles. The thermal treatment of the films invoked decrease in red-ox peak intensity, and thus deterioration in the electrochromic properties. The response time of the coloration and bleaching to an abrupt voltage change from −4.5 to +4.5 V and reverse was found to be in the range of about 10 s. The maximum light intensity modulation ability of the films, as the AM1.5 spectrum is taken for an input, was calculated to be about 65%.     

All solid state electrochromic devices on glass and polymeric foils

Tungsten oxide (WO₃), vanadium and nickel-hydroxide (VO_xH_y and NiO_xH_y)-films were evaporated on glass and polymeric substrates covered with indium-tin oxide (ITO). Films of nickel-oxide (NiO_x) were reactively sputtered from a nickel target. In order to obtain electrochromic devices the WO₃ film was used as one electrode and with a polymeric solid state electrolyte (PSSE) glued to each of the other films which served as different counter electrodes. The films for themselves and the complete devices were investigated by optoelectrochemical and other methods. The most stable device was the WO₃–VO_xH_y system which even improved the electrochromic properties after 3×10⁴ cycles.     

Sol–gel deposited nickel oxide films for electrochromic applications

The electrochromic (EC) behavior, the microstructure, and the morphology of sol–gel deposited nickel oxide (NiO_x) coatings were investigated. The films were produced by spin and dip-coating techniques on indium tin oxide (ITO)/glass and Corning glass (2947) substrates.The coating solutions were prepared by reacting nickel(II) 2-ethylhexanoate as the precursor, and isopropanol as the solvent. NiO_x was heat treated at 350 °C for 1 h. The surface morphology, crystal structure, and EC characteristics of the coatings were investigated by scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), atomic force spectroscopy (AFM), X-ray diffractometry (XRD), and cyclic voltammetry (CV).SEM and AFM images revealed that the surface morphology and surface characteristics of the spin- and dip-coated films on both types of substrate were different. XRD spectra revealed that both films were amorphous, either on ITO or Corning glass substrates. CV showed a reversible electrochemical insertion or extraction of the K⁺ ions, cycled in 1 M KOH electrolyte, in both type of film. The crystal structure of the cycled films was found to be XRD amorphous. Spectroelectrochemistry demonstrated that dip-coated films were more stable up to 1000 coloration–bleaching cycles, whereas spin-coated films gradually degraded after 500 cycles.     

Structural and electrochromic properties of tungsten oxide prepared by surfactant-assisted process

By virtue of gemini surfactant template, nanostructured tungsten oxides thin films were prepared from the modified tungsten hexachloride sol-gel techniques. Temperature was varied as it is an important factor for crystallization, surface morphology and microstructure of tungsten oxides, from the studies of X-ray diffractions, scanning electron microscopy and transmission electron microscopy. The mesoporous sample calcined at 300 °C has tri-dimensional vermicular mesopores with nanocrystallites embedded in the pore wall, while such uniform structure would be destroyed by higher calcination temperature of about 400 °C. X-ray photoelectron spectroscopy was used for analyzing the surface-binding states and the stoichiometry for the oxides. Electrochromic characterization was implemented by simultaneous voltametric and spectrophotometric measurements of tungsten oxides/indium tin oxide (ITO) electrodes. The investigation results showed that organized pore-wall nanostructure has strong effects on the electrochemical and chromogenic properties depending on the specific surface area and the impacts from the evolved crystallization.     

Influence of polyethylene glycol template on microstructure and electrochromic properties of tungsten oxide

Electrochemical synthesis of tungsten oxide (WO₃) thin film nanostructures by potentiostatically controlling the surface aggregates formed at the electrode–electrolyte interface, in the presence of a polymeric template (polyethylene glycol 400, PEG) from a plating sol of peroxotungstic acid (PTA) is presented. The nanoparticulate morphology of the WO₃ film changes drastically upon varying PEG content in the precursor sol; from an amorphous structure with randomly distributed pores for a film derived from a PTA sol with PEG:ethanol in a 3:7 volume ratio, to a mesoporous, nanocrystalline material with hybrid structures encompassing spherical grains and nanorod-like shapes with a triclinic modification for a film formed in a sol with PEG:ethanol in a 1:1 volume ratio. This approach highlights the role of the PEG proportion in controlling crystal growth, assembly patterns and pore structure. The film derived from the sol with PEG:ethanol in a 1:1 volume ratio exhibits superior transmission modulation and coloration efficiency as compared to the film obtained from a sol with PEG:ethanol in a 3:7 volume ratio. While the latter film deteriorates rapidly within 35 color-bleach cycles, the former film sustains more than 3500 cycles, without significant degradation. This film also exhibits fast switching between the clear and blue states; these are repercussions of the mesopore structure and the interconnected nanocrystallite phase.     

A complementary electrochromic device based on polyaniline-tethered polyhedral oligomeric silsesquioxane and tungsten oxide

In this paper we report a high-contrast complementary electrochromic device based on polyaniline-tethered polyhedral oligomeric silsesquioxane (POSS-PANI) and tungsten oxide (WO₃). The electrochromic properties, cyclic voltammetry behavior and coloration efficiency of the device are studied. Due to the loosely packed structure of POSS-PANI, it possesses more accessible doping sites and hence gives rise to a significantly higher electrochromic contrast than polyaniline (PANI). Furthermore, the replacement of PANI with POSS-PANI as the complementary layer for WO₃ leads to an enhanced complementary effect, for which the underneath mechanism is also discussed.     

Electrochromic tungsten oxide films: Review of progress 1993–1998

W oxide films are of critical importance for electrochromic device technology, such as for smart windows capable of varying the throughput of visible light and solar energy. This paper reviews the progress that has taken place since 1993 with regard to film deposition, characterization by physical and chemical techniques, optical properties, as well as electrochromic device assembly and performance. The main goal is to provide an easy entrance to the relevant scientific literature.     

Performance of an electrochromic window based on polyaniline, prussian blue and tungsten oxide

In our laboratory various electrochromic windows (ECWs) have been investigated using mainly tungsten oxide (WO₃), polyaniline (PANI) and prussian blue (PB) as electrochromic materials in combination with poly(2-acrylamido-2-methyl-propane-sulphonic acid) (PAMPS) as a solid proton-conducting electrolyte. The ECWs have been characterized by AC-impedance, linear sweep voltammetry and spectroelectrochemical studies in the 290–3300 nm spectral region. The ECWs have the following general multilayered structure: Glass/ITO/EC1/IC/EC2/ITO/Glass, where ITO=indium oxide doped with tin, IC=ionic conductor, EC1 is either PANI or PANI including PB, and EC2 is WO₃. The best of these ECWs has been able to regulate up to 56% (typical 50%) of the transmission of the total solar energy in the 290–3300 nm spectral range. The combination of the two electrochromic materials PANI and PB has been shown to be mutually beneficial in such a way that the colouration of the window is enhanced by the addition of a layer of PB onto PANI, while the adhesion of PB is improved by the presence of PANI. The energy consumption of the ECW is about 0.01 Wh/m² for one complete cycle (−1.8 V/1.2 V). The switching time for 90% colouring/bleaching is typically 10–30 s. A PANI/PB//WO₃ window has been operated for about 50 days (3700 complete cycles) without substantial loss of transmission regulation, though with an increase in switching time (10 min.). Spectra from individual layers in the ECWs have been recorded by making holes in one or two of the electrochromic layers. In this way (the hole method), it has been possible to study the transmission regulation properties for each electrochromic material separately in complete solid state windows. In addition, spectra for complete windows have been simulated by adding contributions from individual electrochromic layers.