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.     

Structural and electrochemical studies of WO3 films deposited by pulsed spray pyrolysis

Transparent conductive and WO₃ electrochromic thin films were deposited by spray pyrolysis technique. The films were deposited using solutions of WCl₆ in dimethylformamide on SnO₂:F (FTO) substrates with different sheet resistances. Noticeable effects of substrate on structural, morphological and optical properties of the WO₃ films and on its electrochromic behavior are presented and discussed. Hexagonal and monoclinic WO₃ structures were obtained on amorphous glass substrates; also the monoclinic structure on polycrystalline FTO substrates was obtained. Cyclic structural changes during the colored and blanched states were found from XRD and electron diffraction result analysis: The hydrogen tungsten bronze in the tetragonal phase after the hydrogen extraction change to the original WO₃ monoclinic phase.     

Microcrystalline-silicon thin films prepared by chemical transport deposition

We have investigated on the production of microcrystalline-silicon (μc-Si) films from solid Si sources by the chemical transport deposition, and could obtain photo-sensitive μc-Si films. The crystallinity and photo-sensitivity of μc-Si films are improved by increasing hydrogen pressure and the highest photo-sensitivity of 50 times is obtained at 200 Pa. The high density of atomic hydrogen probably causes the defect passivation in the high-pressure conditions. The distance between the Si target and the substrate is also important to improve the film properties, and a shorter distance is effective for higher deposition rate, crystallinity and photo-sensitivity.     

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.     

Proton diffusion in polyethylene oxide: Relevance to electrochromic device design

Polyethylene oxide is a frequently used component in polymer electrolytes developed for applications in electrochromic devices. The transmittance variation may occur as a result of either proton or lithium ion intercalation into the electrochromic films. Impedance spectroscopy data in the low-frequency space-charge relaxation regime can be used to obtain estimates of ion concentrations and ion diffusion coefficients in ion-conducting materials. We apply this method to literature data for pure polyethylene oxide where the residual conductivity is believed to be due to protons. The obtained diffusion coefficient is found to be in the order of, or higher than, reported lithium ion diffusion coefficients in low molecular weight polyethylene oxide. Hence it is likely that proton intercalation will be of importance for electrochromic devices, provided there is a significant amount of protons present.     

Total SiH4/H2 pressure effect on microcrystalline silicon thin films growth and structure

The effect of the total SiH₄/H₂ gas pressure (1–10 Torr) on the growth rate, the film crystallinity and the nature of hydrogen bonding of microcrystalline silicon thin films deposited by 13.56 MHz plasma-enhanced chemical vapour deposition (PECVD) was investigated under well-controlled discharge conditions. The deposition rate presents an optimum for 2.5 Torr, which does not follow the trend of silane consumption that increases with pressure and is attributed to an increase in plasma density. The film crystallinity increases with pressure from 1–2.5 Torr and then remains almost the same, whereas the films deposited at 1 Torr are highly stressed. On the other hand, hydrogen bonding is also drastically affected.     

Electrochemical and electrochromic response of poly(thiophene-3-acetic acid) films

Thiophene-3-acetic acid has been polymerized in chloroform by a chemical method using FeCl₃ as oxidant. The films were prepared casting the solubilized polymer on ITO electrodes and studied using cyclic voltammetry, chronoamperometry and spectroelectrochemistry. During the potential sweep, an electrochromic process is observed in which the film color changes from red to black. High electrochromic efficiency was observed for more than 600 cycles, although it decreases to 73% of the initial value. Until 264 cycles, the electrochromic efficiency at 750 nm is stable and the value is 242 cm² C⁻¹.     

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.     

Electrochromic characterization of Co(OH)2 thin film prepared by sol–gel process

Electrochemical, spectroscopic and structural measurements were used to characterize the electrochromic behavior and stability of sol–gel deposited Co(OH)₂ thin films. These films were prepared from polymeric solutions containing cobalt methoxyethoxide precursor by spin coating technique. The as-deposited films are amorphous and show crystalline structure after heat treatment at 450°C. Sol–gel-deposited cobalt hydroxide films show reversible electrochromic response in 1 M LiClO₄/ propylene carbonate solution beyond 500 cycles. The structural and chemical properties of the films were investigated by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. Spectral transmittance change was T_p=29.9–60.2% for cobalt hydroxide films. It is argued that reversible lithium insertion capacity, good cyclic reversibility of Co(OH)₂ films make them suitable as counterelectrode layers in the electrochromic devices.     

Synthesis of derivatives of polythiophene and their application in an electrochromic device

Poly(3-methylthiophene) (P3MT) and poly(3-octylthiophene) (P3OT) are conducting polymers with interesting optoelectronic properties and a wide variety of optoelectronic applications. In this work, 3-octylthiophene was synthesized through a Grignard reactive and was analyzed by nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FT-IR). The morphology of electrosynthesized P3OT and P3MT thin films was analyzed by atomic force microscopy and different film morphologies were found for different electrodeposition technique and the solution temperature. Optical absorbance spectra of the two types of films confirm the bipolaron band structure and a starting π–π^* interband transition around 2 eV. Electrochromic performance of the polymers with both liquid and polymeric electrolytes showed a high switch potential for P3OT that for P3MT films. The kinetic optical response of a P3MT electrochromic device shows a good color contrast and a capacitive current characteristic under a square electrical potential.     

Thin film of CeO2-SiO2: a new ion storage layer for smart windows

Thin solid films of CeO₂-SiO₂, used as a counter-electrode layer in electrochromic devices, were prepared by the sol–gel dip coating, using an aqueous-based process. The influence of the SiO₂ addition on electrochemistry of the CeO₂ oxide coatings was determined by chronoamperometric measurements. The films exhibit a larger charge storage capacity, which was determined as a function of the coatings thickness. The peak occurrence in the chronoamperometric curve during the deintercalation of lithium ions in the cerium/silicon films is analyzed in terms of trapping energy levels for Li⁺ ions into the film.

In situ UV–Vis spectroelectrochemical measurements of the CeO₂-SiO₂ coatings indicated that the films remained transparent in the visible spectral range during the intercalation process. Powders were characterized by X-ray diffraction after the same thermal treatment of the films, indicating a decrease of crystallinity with the doping. The feasibility for use of these electrodes as ion storage for electrochromic devices was investigated.     

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.     

Comparison between direct small molecular weight alcohols fuel cells’ and hydrogen fuel cell’s parameters at low and high temperature. Thermodynamic study

In this paper, direct alcohol fuel cells’ thermodynamics parameters were calculated by means of analytical formulas using thermodynamic data. The effect of temperature on reversible efficiency, electromotive force and specific energy for fuel cells supplied with alcohols having from one to five carbons was studied in the range between 298.15 and 1300 K.All parameters were also compared with those of hydrogen fuel cell. It was found that reversible efficiency, electromotive force and specific energy are a function of the amount of carbon atoms in the fuel and a function of temperature. In addition, alcohol’s structure influences fuel cells’ parameters too.We have found that there is a competition between hydrogen, ethanol and methanol at standard conditions, while methanol, ethanol, propanol isomers, 2-methylpropan-1-ol and butan-2-ol, as classified, appear more indicated than hydrogen at more elevated temperature.Therefore, the approach presented here can be considered as sufficient enough for a primary choice of an alcohol that could be used in fuel cells in the future.     

NiOxHy薄膜电致变色性能的研究

利用直流磁控溅射法，在Ｏ２＋Ｈ２的气氛下制备了ＮｉＯｘＨｙ薄膜，研究了不同氢气含量对薄膜的初始沉积态、漂白态和着色态透光性能的影响，含氢量为８０％时，薄膜的初始沉积态的平均可见光透射比最高。含氢量为６０％时，薄膜的电致变色能力最佳。用获得的ＮｉＯｘＨｙ薄膜制备的反射型全固态电致变色器件的控光范围可达７７％。对ＮｉＯｘＨｙ薄膜三种状态的红外吸收光谱分析表明，ＮｉＯｘＨｙ薄膜的变色机理可用：Ｎｉ（ＯＨ）２（漂白态）＝ＮｉＯＯＨ（着色态）＋Ｈ＋＋ｅ－表示。     

Improved hydrogen storage properties in Mg-based thin films by tailoring structures

We prepared Mg-based thin films by magnetron sputtering and presented a comparative and systematic study in their structural, optical and electrical characteristics. We built a thin film model to investigate their hydrogen absorption and desorption kinetics in ambient air, as well as chemical and electrical switching behaviors by analyzing transmittance and resistance data. The remarkably enhanced kinetics was achieved by preparing the sandwich-like structured film. The Pd–Mg–Pd film was found to exhibit better gasochromic, chemochromic and electrochromic properties, which could be attributed to the enhanced cooperation effect and more extended Mg–Pd interfaces. The structural effect of kinetics in thin films shed light on how to further improve the hydrogen storage performance in bulk Mg-based materials.     

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.     

Hydrothermal synthesis of electrode materials pyrochlore tungsten trioxide film

Hydrothermal synthesis methods have been successfully used to prepare new transition-metal oxides for cathodes in electrochemical devices such as lithium batteries and electrochromic windows. The tungsten oxides were the first studied, but the method has been extended to the oxides of molybdenum, vanadium and manganese. Sodium tungsten oxide films with the pyrochlore structure have been prepared on gold/alumina and indium-doped tin oxide substrates. These films reversibly and rapidly intercalate lithium and hydrogen ions.     

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.     

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.     

Characterization of gas sensing behavior of multi walled carbon nanotube polyaniline composite films

In this paper, we report multiwall carbon nanotube (MWNT) doped polyaniline (PANI) composite thin films for hydrogen gas sensing applications. PANI was synthesized by in- situ chemical oxidative polymerization of aniline using ammonium persulfate in acidic medium.This emeraldine salt form of PANI was converted into emeraldine PANI base and doped with MWNT (4 wt%) in presence of champhor sulfonic acid (CSA) by solution mixing method. The MWNT/PANI composite films were deposited onto ITO coated glass substrate using spin cast technique. The gas sensitivity of these composite films was evaluated by measuring the change in electrical resistance of composite films in presence of hydrogen gas for different pressures at room tempeature. It is observed that the MWNT/PANI composite film shows a higher sensitivity in comparison to pure PANI and it decreases on increasing hydrogen gas pressure. The composite films have also been characterized by X-ray diffraction (XRD) analysis and Atomic force microscopy (AFM).