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.     

Electrical and optical properties of highly conducting CdO:F thin film deposited by sol–gel dip coating technique

Highly conducting fluorine-doped cadmium oxide (CdO:F) thin films were deposited by sol–gel dip coating technique on glass and Si substrates. F concentration in the films was varied from 2.0% to 13.8% as determined from energy dispersive X-ray analysis. X-ray diffraction pattern showed that the films were polycrystalline in nature. The optimum F concentration for obtaining maximum conductivity was found to be 9.7%. The corresponding electrical conductivity was found to be 1.088×10⁴ S/cm and mobility 60.41 V/cm². Analysis of UV-VIS-NIR spectrum of the film with F concentration 9.7% showed a direct band gap energy of 2.3 eV.     

Investigation of sol–gel derived thin films of titanium dioxide doped with vanadium oxide

Thin films of titanium dioxide and titanium–vanadium oxide were obtained by a sol–gel method. The coatings are uniform, smooth with very good optical properties. The solutions of both kinds are stable for more than a year. Structure and vibrational properties were studied with the help of X-ray diffraction (XRD) analysis and infrared spectroscopy (IR). The refractive indices and film thicknesses were measured by an ellipsometer at a wavelength of 638.2 nm, as a function of annealing temperature. The optical properties were investigated by ultraviolet–visible (UV–VIS) spectroscopy.     

Synthesis and characterization of aluminum-doped CdO thin films by sol–gel process

Aluminum-doped cadmium oxide (CdO:Al) thin films are deposited on glass substrates by the sol–gel dip-coating method, taking cadmium acetate dihydrate as the precursor material. Aluminum nitrate has been taken as a source of Al-dopant. XRD pattern reveals the good crystallinity of CdO thin films. SEM micrograph showed the presence of faceted crystallites. Optical study shows 40–85% transparency with a bandgap value lying in the range 2.76–2.52 eV, depending upon the Al content in the films. Optimum percentage of Al was 5.22 for a maximum room temperature conductivity of 2.81×10³ (Ω cm)⁻¹. Hall measurement confirmed that the material is of n-type, with mobility and carrier concentrations lying in the range 413–14.7 cm²/V s, and 3.4×10¹⁹–8.11×10²⁰ cm⁻³, when percentage of Al varies in the range 1.32–7.24.     

Titanium dioxide films for photovoltaic cells derived from a sol–gel process

We present a sol–gel process which is used to prepare thin titanium dioxide nanoparticle layers by spin-coating or doctor-blading techniques. After being sensitized with a dye, the films are used for the transport of electrons in photoelectrochemical solar cells. The titanium dioxide particles have diameters of about 100 nm. A comparison with screen-printed layers consisting of particles with a typical size of 25 nm shows that the power conversion efficiencies of the cells reach identical levels. Thus, we can show that a large internal surface of the nanocrystalline layers, which is one prerequisite for a good solar cell performance, can only be achieved by adjusting the respective size of the nanoparticles. The coating and heating processes, and the obtained crystal structure of titanium dioxide also play a major part in the preparation of highly efficient devices.     

Silica antireflective films on glass produced by the sol–gel method

Up to now antireflective silica thin films deposited on glass do not have enough strength and display poor adhesion to the substrate. Three methods to obtain silica antireflective films on glass were surveyed to identify the best way to produce the antireflective effect on glass by introducing a certain degree of porosity. Porous silica layers, obtained from the polymeric and colloidal methods, permit a considerable reduction of these light reflections compared with uncoated glasses in all the cases studied, but the degree of reduction as well as the adhesive properties is different depending on the method used to achieve the precursor solution. The AM1.5 solar transmittance increased from 0.915 for the bare slide up to 0.970 for the best-made sample corresponding to the Triton-doped silica.     

Solar photocatalytic degradation of gaseous formaldehyde by sol–gel TiO2 thin film for enhancement of indoor air quality

Solar photocatalytic degradation of formaldehyde in the gaseous phase has been investigated. The tested photoreactor is made of a borosilicate glass tube with the inner surface coated with a sol–gel TiO₂ thin film. In a pseudo-first-order Langmuir–Hinshelwood (L–H) model, the maximum reaction rate constant obtained is 0.148 min⁻¹ under an exposure to sunlight with solar UVA irradiance of 1.56 mW/cm². The solar photolysis effect is found to be negligible. It is also found that the sol–gel TiO₂ thin film has a lower apparent photonic efficiency of solar photocatalysis than a Degussa P25 TiO₂ coating. However, for the photonic efficiency taking into account the absorbed and scattered photons only and, in other words, excluding the transmitted photons, the thin film has a higher value. Based on a total of 28 measured data, an empirical-correlation equation has been developed to express the reactant residue with respect to the solar UVA irradiance and exposure time. A reasonable agreement between the correlation and experimental data is obtained. The findings of this investigation can be applied to design optimization of a honeycomb photoreactor made up of TiO₂-coated glass tubes or polygonal cells.     

Electrical characterization and Poole–Frenkel effect in sol–gel derived ZnO:Al thin films

Electrical properties of ZnO:Al thin films, prepared by sol–gel dip-coating technique were studied in the range of 0.32% to 1.62% Al concentrations in the films. Room temperature electrical conductivity was found in the range of 0.08 to 1.39 S/cm for different aluminium concentrations in the films. I–E characteristics of the films at a constant temperature showed non-linearity, while non-linearity becomes more and more pronounced with increase in temperature and this could be explained by Poole–Frenkel model of thermionic emission. Presence of adsorbed oxygen and excess Al atoms at grain boundaries is assumed to be the cause of this effect. These atoms produce defect levels, which trapped electrons and created a potential barrier across the grain boundaries. In the presence of an external field, the barrier height was attenuated, resulting in the thermionic emission of electrons from the trapped level to the conduction band. The trapped potentials (φ_t) were calculated for different doping concentrations in the films. The thermoelectric power (TEP) measurement confirmed the n-type nature of the films and the room temperature Seebeck coefficient was found to be −91.65 μV/K.     

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.     

Anti-reflection (AR) coatings made by sol–gel processes: A review

Traditionally, various vacuum-based processes, depending on material systems and properties, and chemical etching process have been used for producing different types of anti-reflection (AR) coating on different substrate materials. In this paper, the development of sol–gel derived AR coating on different substrates for various applications in the past 40 years are reviewed. These coatings possess good uniformity in thickness and properties which have met requirements for various applications. The major approaches to fabricate AR coating and their characteristics have been discussed. This paper outlines the major solution coating processes and design principles of AR coatings. Major fabrication processes used in AR coating technologies have been compared. Different solution chemistries developed for producing different materials for AR coating preparation have been extensively reviewed. The optical performance of different types of sol–gel-derived AR coatings have been summarized and comparison to the commercial AR coating produced by traditional technologies have been discussed. The sol–gel AR coating has been shown to possess comparable or superior performance to those produced by vacuum-based processes.     

Optical properties of mixed V/Ce oxide films prepared via inorganic sol–gel route

New mixed V/Ce films at 55, 38, and 32 at% of V were prepared via inorganic sol–gel route by dip-coating technique. The absorption edge of prepared films shifts towards higher wavelengths at higher concentration of added vanadium pentoxide. The indirect-allowed band gap (E_G) also changes in dependence of added vanadium oxide from 2.8 up to 2.3 eV.     

Optical and electrochemical characteristics of sol–gel deposited iron oxide films

Homogenous, crack free iron oxide films are prepared by the sol–gel spin coating technique from a solution of iron iso-propoxide and isopropanol. The films were characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV-visible (UV–Vis) spectroscopy and cyclic voltammetry (CV). XRD of the films showed that they had an amorphous structure. The optical constants refractive index (n) and extinction coefficient (k) were measured by scanning spectrometer in the wavelength range of 390–990 nm. The n and k values were found n =2.3±0.01 and k =0.2±0.002 at 650 nm. The electrochemical behavior investigated in 0.5 M LiClO₄ propylene carbonate (PC) electrolyte-CV examinations showed good rechargeability of the Li⁺/e⁻ insertion extraction process beyond 300 cycles. Spectroelectrochemistry showed that these films exhibit weak cathodic coloration in the spectral range of 350–800 nm.     

Effect of humidity on structure and electrochromic properties of sol–gel-derived tungsten oxide films

The influence of varying relative humidity (RH55 and 75%) during thin film deposition from an oxalato-acetylated peroxotungstic acid sol by dip coating, on the microstructure and electrochromic properties of pristine tungsten oxide (WO₃) films obtained upon annealing is presented. The films fabricated under a relative humidity of 55% are amorphous whereas the ones cast under a substantially humid atmosphere (RH75%) are characterized by interconnected nanocrystallites with a triclinic phase and a nanoporous surface morphology as well. Upon lithium insertion, larger integrated values of transmission modulation and coloration efficiency are observed over the photopic and solar regions, for the films prepared under a RH75% as compared to that observed for the films deposited under a RH of 55%. Functional improvements are due to the larger surface area of nanocrystallites and a porous microstructure, a consequence of a higher degree of hydration and hydroxylation in the former films in contrast to the non-porous and a rather featureless structure of the latter films. Faster switching kinetics between the clear and blue states, a greater current density for lithium intercalation, a higher diffusion coefficient for lithium and a superior cycling stability, again shown by the film fabricated under a 75% RH confirm that the WO₃ film microstructure is most conducive for a more facile ion insertion–extraction process, which hints at its potential for electrochromic window applications.     

Preparation of a TiMEMO nanocomposite by the sol–gel method and its application in coloured thickness insensitive spectrally selective (TISS) coatings

An organic–inorganic nanocomposite was prepared via sol–gel processing from 3-(trimethoxysilyl)propyl methacrylate (MAPTMS) and titanium(IV) isopropoxide (TIP) precursors (TiMEMO) in the form of a viscous resin, and used as a binder for the preparation of coloured thickness insensitive spectrally selective (TISS) paints and corresponding solar absorber coatings. The spectral selectivity of TiMEMO-based TISS paints was optimized by varying the concentrations of binder and different pigments: black, coloured (red, green and blue) and aluminium flakes, the latter imparting low thermal emittance, which was correlated to the presence of titanium in the TiMEMO sol–gel host. The formation and the ensuing structure of the sol–gel TiMEMO hybrid was studied in detail and the nanocomposite structure of the TiMEMO binder formed was assessed from infrared and ²⁹Si NMR measurements, which confirmed the formation of Ti–O–Si linkages established after the hydrolysed precursors condensed into a compliant resinous material. XRD measurements provided additional information about the existence of small coherent domains of silsesquioxane units in the sol–gel host. The abrasion resistance of the non-pigmented TiMEMO binder deposited in thin film form on a PMMA substrate was assessed by the Taber test, and its hardness compared with other resin binders which have been used for making TISS paint coatings. The surface properties of the non-pigmented TiMEMO binder and the ensuing TISS paint coatings were determined from contact angle measurements. The results showed that the water contact angles of non-pigmented TiMEMO binder increased from 70° to 125–135° for the corresponding pigmented TISS paint coatings, inferring the influence of surface roughness on surface energy in the presence of pigments. SEM measurements revealed a striking similarity in the surface morphology of the TISS paint coatings with some other surfaces exhibiting the Lotus effect.     

Sol–gel derived CuCoMnOx spinel coatings for solar absorbers: Structural and optical properties

A novel black coloured coating with the composition CuCoMnO_x was prepared using sol–gel synthesis. The coatings were deposited using the dip-coating technique from alcoholic sols based on Mn-acetate and Co- and Cu-chloride precursors. Thermogravimetric analysis showed that xerogels become crystalline at 316°C while X-ray diffraction analysis revealed that the coatings and powders correspond predominantly to CuCoMnO_x spinels. Rutherford back scattering (RBS) and transmission electron microscopic (TEM) studies combined with energy dispersive X-ray spectroscopy (EDXS) measurements confirmed that Cu, Mn and Co are present in the films in stoichiometric ratios close to that in the initial sols. IR spectroscopy has been employed to study the formation of sols by following the changes in the vibrational bands of the acetate groups during both thermal hydrolysis and the ageing of sols to xerogels. It was found that ageing of xerogels was accompanied by the formation of −COO⁻ bridging units, which at 250°C are no longer visible in the IR spectra but substituted by the vibrational modes characteristic for CuCoMnO_x. The solar absorptance (a_s) and thermal emittance (e_T) of the coatings when deposited on an Al-substrate are a_s=0.9 and e_T=0.05, which rank deposited black sol–gel CuCoMnO_x spinels among the promising candidates for spectrally selective absorber coatings for solar collectors and solar facades.     

Optical transmittance and photoconductivity studies on ZnO : Al thin films prepared by the sol–gel technique

Polycrystalline ZnO : Al thin films have been prepared by the (Sol–gel) chemical deposition method. The ZnO : Al thin films are very transparent (90%) in the near UV, VIS and IR regions. The films are oriented along the c-axis ([0 0 2] direction) in the hexagonal structure. It is known that pure ZnO thin films are not chemically stable in corrosive media, but aluminium stabilizes the ZnO system and increases its electrical conductivity. Finally, the ZnO : Al thin films are reasonably stable under storage in air and in reactive atmospheres like O₂, H₂O, H₂ or in weak acids. Dark- and photo-conductivity of the ZnO : Al films are very high (1–100 Ω⁻¹ cm⁻¹), so that they can be used as transparent conductors in solar cells or in electrochromic devices.     

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.     

Towards electrochromic stability in sol–gel-derived tungsten oxide films: cyclic voltammetric and spectrophotometric investigations

Hitherto unexplored irreversible changes during initial coloration/bleaching cycles for sol–gel-derived tungsten oxide (WO₃) films have been investigated using cyclic voltammetric and spectrophotometric techniques. Non-ideal features appearing in the initial five anodic (deintercalation) cycles in the voltammogram with simultaneous decreased optical transmission of the bleached films have been explained in terms of possible stoichiometric variations affecting the coloration efficiency (CE) of the films and the associated mechanisms. Electrochromic stability attained thereafter manifests in retraceable voltammograms and almost invariant value of the CE.     

Development and investigation of sol–gel solutions for the formation of TiO2 coatings

We show a technological approach for the sol–gel processing of stabilized xerogel colloidal titanium oxide films. Glycerol was used as a drying control additive agent. Glycerol helped in stabilizing the solution. The thermal transformation of a xerogel film was studied by differential thermal analysis (DTA) and powder X-ray diffraction. The optical index of the annealed coatings was evaluated using UV–VIS–NIR spectrophotometry. The results showed that a nanocrystalline titania anatase film of high optical quality (n=2.34 at 600 nm) can be obtained by the sol–gel process.     

Titanium–tin oxide protective films on a black cobalt photothermal absorber

We report the effect of covering an electrodeposited black cobalt absorber film with a Ti : Sn oxide film at various atomic ratios prepared by the sol–gel dip process. The resulting composite was characterized in its optical, structural and morphological properties. After thermal treatment at 400°C, the uncoated black cobalt film is oxidized and Co₃O₄ is formed. Samples covered with Ti : Sn films and thermally treated at the same temperature suffered lower oxidation maintaining in great extent the original metallic cobalt structure. The optical properties of the resulting material were affected by the presence of the Ti : Sn coating, and the best protective film obtained was a transparent Ti : Sn (8 : 2) atomic ratio oxide film, with a 210 nm thickness. This composite system exhibits an absorptance value of 0.91 and an emittance value of 0.34 after a 100 h, 400°C thermal treatment. A photothermal material composed of a layer of black cobalt and a protective oxide film coating seems then a promising solar energy absorber capable of withstanding high operating temperatures (400°C).