Promotion of electrochromism in spray-deposited molybdenum oxide-doped iridium oxide thin films

Electrochromic molybdenum oxide-doped iridium oxide thin films were prepared by using a pneumatic spray pyrolysis technique onto fluorine-doped tin oxide (FTO) coated conducting glass substrates. An aqueous solution of 0.01 M ammonium molybdate was mixed with 0.01 M iridium trichloride in different volume proportions and the resultant solution was used as a precursor for spraying. An aqueous electrolyte (0.5 N H₂SO₄) was used to study electrochromic properties of thin films using cyclic voltammetry (CV), chronoamperometry (CA) and spectrophotometry. During the potential scan the iridium oxide electrode switches between coloured and bleached state due to Ir⁺⁴–Ir⁺³ intervalency charge transfers. The optical density difference (ΔOD)_λ=630 nm and colouration efficiency was maximum for 2% molybdenum oxide-doped sample. Moreover, loss in charge density during extended cycling is less than undoped and other doped (>2%) samples.     

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.     

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.     

Optical properties of chemical vapor deposited thin films of molybdenum and tungsten based metal oxides

Thin films of tungsten oxide, molybdenum oxide and mixed MoO₃–WO₃ oxides were obtained by atmospheric pressure chemical vapor deposition (CVD). All the films were prepared using identical technological parameters and through investigation of the optical properties of as deposited and annealed at 400°C a comparative study is reported. Raman, IR and VIS spectrophotometry and spectral ellipsometry methods were used. The mixed MoO₃–WO₃ films have higher optical absorption with maxima at a closer position with respect to the human eye sensitivity peak at 2.5 eV. The observed electrochromic effect is better expressed in the mixed films; the electrical charge inserted is higher.     

Electrochemical and optical properties of sputter deposited Ir–Ta and Ir oxide thin films

Addition of tantalum oxide has been investigated in order to improve the electrochromic properties of iridium oxide. Films of iridium–tantalum oxide and iridium oxide have been prepared and studied with regard to their optical and electrochemical properties. It can be seen that the addition of tantalum decreases the optical absorption coefficient and increases the ion diffusion coefficient. The change of properties is thought to be a result of the dilution of colouring iridium oxide with the better ion conducting tantalum oxide.     

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.     

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.     

Optically passive counter electrodes for electrochromic devices: transition metal–cerium oxide thin films

Films of W–Ce oxide and Ti–Ce oxide were made by reactive DC magnetron sputtering. Li⁺ intercalation/deintercalation showed that the electrochromic coloration efficiency depended on the Ce/W and Ce/Ti ratio, and at proper values of these ratios the films remained transparent irrespective of their ion content. Ti–Ce oxide combines this property with good electrochemical cycling durability, and this material can serve as a counter electrode in transparent electrochromic devices.     

Optical properties of tungsten and titanium oxide thin films prepared by plasma sputter deposition

Tungsten oxide and titanium oxide thin films were prepared by RF reactive magnetron sputter deposition. The stationary and rotating substrate holders were applied to analyze the rotating effect. The optical properties and thicknesses of oxide films were determined by a proposed optical model and the measured transmittance spectra. The dispersed refractive indices of thin films have a wide range distribution in different sputtering conditions. In the situation of rotating substrate holder, the refractive index was lower than that of the stationary substrate holder. Also, amorphous TiO₂ structure can be prepared by using rotating substrate holder. The transmittance spectrum of crystalline TiO₂ reveals that the textured structure on the film surface affects the transmittance characteristic.     

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.     

Optical properties of oxygenated amorphous cadmium telluride thin films

Oxygenated cadmium telluride (CdTe:O) thin films are prepared by radio-frequency (RF) sputtering from a polycrystalline CdTe target in an atmosphere composed of a mixture of argon, nitrogen and oxygen gases. X-ray diffraction analysis showed that as-deposited films are amorphous when deposited in the presence of nitrogen. X-ray photoelectron spectroscopy analysis revealed that the layers do not contain any nitrogen while the chemical composition of the films depends strongly on the partial pressure of N₂ used during sputtering. finally, optical transmission measurements in the IR-Visible region showed that the optical constants of the samples vary considerably with their oxygen contents. X-ray reflectometry studies enabled us to correlate some of these variations with the density of the layers.     

Ion transport in electrochromic nickel oxide thin films

Dynamic process of ion injection into nickel oxide film in both KOH and LiClO₄–PC electrolyte were studied using electrochemical measurement. Cottrell and Butler–Volmer current–time relations were applied to obtain ion diffusion coefficient D and exchange current I_ex by fitting the experimental data. Results showed that the film displays complex transport behavior which depends on the deposition rate of the film, the environment and sequence of heat treatment, the adopted electrolyte. The change of the number of the oxygen and nickel vacancies are suggested to be responsible for the phenomena.     

Studies on electrochromism of spray pyrolyzed cobalt oxide thin films

Cobalt oxide thin films were prepared by spray pyrolysis technique on to fluorine doped tin oxide (F.T.O.) coated glass substrates. The electrochromic cell was formed by using these films as working electrodes and the electrochromic characteristics were determined by using cyclic voltammetry and chronoamperometry. The films exhibited anodic electrochromism; changing colour from grey to pale yellow.     

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%.     

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.     

Design and fabrication of electrochromic light modulators

The battery-like structure of electrochromic devices warrants the application of a stoichiometric oxidation-reduction model to interpret and to optimize their operation. Of particular significance is the variation of the potential of electrochromic materials with composition, the bleached state absorption coefficients, their electrochemical stability limits and their coloration functionality versus Faradaic charge. Once these input parameters are known, the optical transmission spectrum of electrochromic light modulating devices can be predicted as a function of terminal voltage given certain assumptions about the thickness and redox state of the electrochromic and counter electrode materials on device assembly. The latter is not readily determined directly, but may be derived from properties of the two terminal device. Furthermore, the electro-optical characteristics of the two terminal device can be used to determine the source of any deviation from its optimal dynamic range.     

Electrochromic thin films prepared by sol–gel process

New methods are shown for lower temperature preparation of amorphous tungsten oxide thin film and preparation of crystalline iridium oxide thin film by sol–gel process using metal chloride as the starting materials and ethanol as a solvent. These electrochromic materials were combined with gel solid electrolyte, and preparation of fully solid-state electrochromic display (ECD) was made. The transmittance of the ECD could be made to change by 35% by applying a voltage of 3 V for 0.2 sec.     

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.     

Electrochemical investigations on spray deposited tin oxide thin films

Tin oxide (SnO₂) thin films were prepared by a simple and inexpensive spray pyrolysis technique from an aqueous solution at various substrate temperatures viz. 300, 400 and 500 °C, and their electrochemical studies have been carried out. The thin films have been optically and electrochemically characterized by means of transmittance, cyclic voltammetry and chronoamperometry. The mechanism of reduction and oxidation reactions that took place during the potential cycling is presented. The samples deposited at 500 °C exhibit better performance in terms of coloration efficiency, reversibility, contrast ratio and response time.     

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.