Sprayed CeO2 thin films for electrochromic applications

Cerium dioxide (CeO₂) thin films were prepared by spray pyrolysis using hydrated cerium chloride (CeCl₃·7H₂O) as source compound. The films prepared at substrate temperatures below 300°C were amorphous, while those prepared at optimal conditions (T_s=500°C,s=5 ml/min) were polycrystalline, cubic in structure, preferentially oriented along the (2 0 0) direction and exhibited a transmittance value greater than 80% in the visible range. The cyclic voltammetry study showed that films of CeO₂ deposited on ITO pre-coated glass substrates were capable of charge insertion/extraction when immersed in an electrolyte of propylene carbonate with 1 M LiClO₄.These films also remained fully transparent after Li+ intercalation/deintercalation.     

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.     

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.     

Electrochromic effect in WO3 thin films prepared by spray pyrolysis

Polycrystalline WO₃ thin films were produced by spray pyrolysis on corning glass or ITO coated glass substrates. The optical, electrical and structural properties of these films were studied as a function of preparation conditions in their colored state. The transmittance value decreased from 80% to 5% when the films were colored. This was accompanied by an increase of the reflectivity in the NIR region. In addition, the electrical conductivity varied with heat treatment temperature. The change in electrical conductivity indicates that WO₃ films exhibit a semiconducting behavior in their colored state. Furthermore, a change in the structure of the films from monoclinic to cubic was observed upon coloration. The electrochemical behavior was investigated in 1 M H₂SO₄/H₂O solution using a three-electrode cell.     

Structural, morphological, optical and electrochromic properties of Ti-doped MoO3 thin films

Thin films of molybdenum trioxide (MoO₃) doped with titanium were prepared using a spray pyrolysis technique. Increasing Ti doping concentration was found to hamper the polycrystalline nature of undoped orthorhombic MoO₃ and undergo transformation from polycrystalline to amorphous structure with decrease in grain size. This was also reflected in scanning electron microscopy wherein transformation of thread-like reticulated morphology to spongy structures could be observed at higher Ti concentration (9 at% Ti). With increasing Ti concentration, the charge capacity, coloration efficiency, reversibility and electrochemical stability increased. This improvement could be ascribed to the amorphous spongy morphology of the doped samples that offers easy pathway for intercalation and deintercalation of the ions. Hence, 9 at% Ti-doped MoO₃ can act as an adequate host for electrochromic devices.     

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.     

Preparation and properties of sprayed CuGa0.5In0.5Se2 thin films

CuGa_0.5In_0.5Se₂ thin films were prepared by spray pyrolysis technique at substrate temperatures (T_s) in the range 100–400°C. The films prepared at T_s = 300–350°C were nearly stoichiometric, polycrystalline with a strong preferred (112) orientation. The resistivity of the films varied in the range, 50–1000 Ω cm and the evaluated optical band gap was 1.35 eV.     

Residual charges and infrared absorption in electrochromic WO3 films prepared by hydrogen-introduced sputtering

Electrochromic WO₃ films were prepared by rf−sputtering in atmosphere consisting of Ar/H₂/O₂ mixed gas. The as-sputtered films require several times of injection/extraction of ions (the aging) for obtaining reversible coloration/bleaching. After the aging, there are ions (protons) remaining in the films, namely residual charges. From the results of IR absorption of the as-sputtered and aged films, the residual charges contribute to create OH and HOH bonds. Hydrogen introduced in the films during sputtering is transformed only into OH bonds combining with unstable oxygen in the films. The introduced hydrogen is considered to suppress the growth of grain in WO₃ films from AFM observation.     

Photoelectrical properties of ZnS thin films deposited from aqueous solution using pulsed electrochemical deposition

ZnS is an n-type semiconductor with a wide direct band gap (3.7 eV at room temperature), and it is very suitable as a window layer in heterojunction photovoltaic solar cells. We deposited ZnS thin films on Sn-doped In₂O₃-coated glass substrate using pulsed electrochemical deposition (ECD) from aqueous solutions containing Na₂S₂O₃ and ZnSO₄ with two different compositions, the first group grown from ZnSO₄-rich solution, and the second grown from Na₂S₂O₃-rich solution. We investigated electrical properties of the ZnS thin films and properties of contacts with different metals evaporated on the surfaces. We found that Au and In contacts have Ohmic-like characteristics to ZnS. Furthermore, we observed photoconductivity of the ZnS thin films by means of photoelectrochemical (PEC) measurements. We found that for both the groups of ZnS thin films, the as-deposited film shows weak photosensitivity and after annealing at 300 °C the photosensitivity improved.     

MS2 (M = W, Mo) photosensitive thin films for solar cells

Photosensitive textured WS₂ and MoS₂ films can be obtained by the techniques of reactive sputtering and solid state reaction, as long as the substrates used are each coated with a 10–20 nm Ni layer. When MS₂ (M = W, Mo) layers are deposited onto these substrates and then annealed for half an hour at 1073k in an argon atmosphere, textured films crystallized in the 2H-MS₂ structure are obtained, with their c crystallite axes perpendicular to the plane of the substrate. The films are nearly stoichiometric. The crystallinity enhancement of the films can be attributed to an improvement in the crystallization process related to liquid NiS phases present at the grain boundaries during annealing. Residual phases (Ni_xS_y; Ni;…) are distributed at the grain boundaries and do not strongly disturb the properties of the WS₂ and MoS₂ films. The optical absorption spectra are similar to those of single crystals, and the high photosensitivity of the films is attributed to a grain size enhancement by the NiS phase.     

Chemical bath deposition and electrochromic properties of NiOx films

Nickel oxide (NiO_x) thin films were prepared by the chemical deposition method (solution growth) on two kinds of substrates: (1) glass and (2) glass/SnO₂ : F. Films were thermally treated at 200°C for 10 min in atmosphere. The texture, microstructure and composition were examined by optical microscopy, X-ray diffraction patterns (XRD) and X-ray photoelectron spectroscopy (XPS) analysis of the surface layer. The films exhibited anode electrochromism. The optical properties of the bleached and colored state were examined with transmittance spectroscopy in the visible region and reflectance FTIR spectroscopy. An electrochromic test device (ECTD), consisting of SnO₂/NiO_x/NaOH–H₂O/SnO₂, was assembled and tested by cyclic voltammetry combined with a simultaneous recording of the change of transparency at λ=670 nm. The coloration efficiency was evaluated to be 24.3 cm²/C. The spontaneous ex-situ change of coloration with time of the colored and bleached NiO_x/SnO₂/glass was also examined.     

Preparation of Culn(SxSe1−x)2 thin films by sulfurization and selenization

A CuIn(S_xSe_1−x)₂ alloy thin-film was prepared by selenization of CuInS₂: its composition ratio x can be controlled by the number of selenization cycles implemented. Crystallinity of the films was improved by annealing in vacuum. The resistivity of the film was about 1 Ω cm and increased by one to two orders of magnitude after KCN treatment. An 8.1 % efficiency solar cell was obtained by using this annealed alloy thin-film.     

High-rate dual-target DC magnetron sputter deposition of “blue” electrochromic Mo oxide films

Mo oxide films were prepared by dual-target reactive DC magnetron sputtering under conditions leading to films that were blue in as-deposited state. The deposition rate for this “blue” Mo oxide was as high as 1.5 nm/s, to be compared to 0.85 and 0.1 nm/s for highly transparent Mo oxide films made with dual-target and single-target sputtering, respectively. Good electrochromic properties of the “blue” films evolved after 5 colour/bleach cycles in a Li⁺ conducting electrolyte.     

Electrical and luminescent properties of CuGaSe2 crystals and thin films

CuGaSe₂ thin films with thicknesses of about 2 μm were prepared by flash and single source evaporation onto mica and (1 1 0)-oriented ZnSe substrates in the substrate temperature range 150–450°C. The obtained polycrystalline CuGaSe₂ films had the chalcopyrite structure with the predominant growth direction 2 2 1. Hall effect, conductivity and luminescence measurements have been carried out on CuGaSe₂ thin films and source materials: CuGaSe₂ single crystals grown by Bridgman technique and by chemical vapour transport using I₂ as transport agent. All films and crystals are p-type. Two acceptor levels with ionization energies E_A150–56 meV and E_A2130–150 meV have been identified as due to Ga vacancy and presence of Se atoms on interstitial sites respectively.     

Electrochromic WO3 thin films active in the IR region

Herein, we investigate the electrochromic performances in the infrared (IR) region, in particular in the midwavelength (MW, 3–5 μm) and long wavelength (LW, 8–12 μm) bands, of WO₃ thin films grown by RF-sputtering and pulsed laser deposition. For an optimized voltage window, 200 nm room temperature thin films are the most efficient in the MW band, with the highest contrast in reflection, namely 35%, whereas thicker films, typically 500 nm, are required in the LW band. At 400 °C films show contrasts in reflection lower than 10%, surprisingly associated with a reasonable insertion amount of 0.20. Indeed, no straight correlation between the electrochemical and the optical properties in the IR region was established.     

Defect reduction in electrochemically deposited CdS thin films by annealing in 02

Using the electrochemical deposition method, CdS thin films were deposited from acid solutions (pH = 2.5) containing CdS0₄ and Na₂S₂0₃ on indium-oxide coated glass substrates. These films were annealed in N₂, air, or O₂ atmosphere at 200–500°C for 30 min. Photoluminescence spectra were measured at 77 K. For the films annealed in N₂, the band edge emission became weaker and the luminescence due to defects shifted to longer wavelengths as the annealing temperature was raised above 300°C. However, for the films annealed in air or O₂, the band edge emission was observed strongly irrespective of the annealing temperature and the luminescence due to defects was weak. Thus the O₂ annealing is useful for the defects reduction.     

A note on fast protonic solid state electrochromic device: NiOx/Ta2O5/WO3−x

In the present investigation, the electrochromic properties of a fast protonic solid state device: NiO_x/Ta₂O₅/WO_3−x prepared at room temperature (300 K) is reported. The non-stoichiometric tungsten oxide thin film is prepared by reactive DC magnetron sputtering technique on ITO coated glass; the oxides of tantalum (300 nm) and nickel (100 nm) are prepared by electron beam evaporation. This proton device has a coloration efficiency of 82.4 cm²/C and coloration and bleaching time of 6 and 5 s, respectively, and a transmittance variation of 60%. The work function of WO_3−x thin films by Kelvin probe in uncolored and colored states are 4.73 and 4.30 eV, respectively.     

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.     

The influence of sodium on the properties of CuInS2 thin films and solar cells

The controlled incorporation of sodium into the absorber layer of CuInS₂ solar cells improved cell performance remarkably. Without toxic KCN treatment, conversion efficiencies of over 6% were achieved by sulfurization of sodium-containing precursors. We also investigated the characteristics of the sodium-incorporated CuInS₂ films by intentional addition and diffusion from a soda-lime glass. The ternary compound semiconductor of NaInS₂ was found to form mainly on the surface of each of the CuInS₂ films.     

Electrochromic behavior of WO3 nanotree films prepared by hydrothermal oxidation

Hexagonal structured WO₃ films with tree-like morphology were synthesized on tungsten foils by a hydrothermal method. Each nanotree was composed of several (typically six) nanosheet-shaped “branches”. TEM examination revealed that the nanosheet was a single crystal and its long axis was oriented toward 〈0 0 1〉 direction. The WO₃ nanotree films retain the hexagonal structure after being annealed up to 400 °C for 2 h, while they have a phase transition to monoclinic structure after being annealed at 500 °C for 2 h. Electrochromic (EC) performance of the films was examined in a propylene carbonate solution of 1 M LiClO₄ using an electrochemical workstation and an UV-Vis spectrometer. Due to the large tunnels of hexagonal structure and highly porous surface morphology, a large modulation of optical reflectance of WO₃ nanotree films up to 30% and coloration efficiency of 43.6 cm² C⁻¹ at 500 nm were achieved by annealing the WO₃ nanotree films at 400 °C for 2 h.