The electrochromic properties of sputtered nickel oxide films

Electrochromic nickel oxide films were prepared by reactive RF sputtering from a nickel target in an oxygen atmosphere. The films were deposited as a compact 40 nm layer of trivalent nickel oxide, Ni₂O₃. Reduction and oxidation of the films in 1 M KOH resulted in bleaching and coloration, respectively. Voltammetry indicated that the eventual electrochromic reaction involved the β-Ni(OH)₂/β-NiOOH couple. In situ visible spectra showed electrochromic modulation of the transmittance throughout the visible range with a peak change in transmittance of about 60% at a wavelength of 500 nm. In situ spectra in the near-infrared region indicated improved electrochromic switching in this region; the sputtered nickel oxide film exhibited about a 30% change in transmittance in comparison to less than 10% for a similar electroprecipitated nickel hydroxide film. The sputtered nickel oxide films exhibited durable electrochromic switching for over 2500 coloration/bleaching cycles, a significant improvement over the less than 500 switching cycles exhibited by electroprecipitated nickel hydroxide films.     

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.     

Effects of deposition temperature on characteristics of Ga-doped ZnO film prepared by highly efficient cylindrical rotating magnetron sputtering for organic solar cells

We report the characteristics of Ga-doped zinc oxide (GZO) films prepared by a highly efficient cylindrical rotating magnetron sputtering (CRMS) system as a function of substrate temperature for use as a transparent conducting electrode in bulk hetero-junction organic solar cells (OSCs). Using a rotating cylindrical GZO target, low sheet resistance (∼11.67 Ω/square) and highly transparent (90%) GZO films were deposited with high usage (∼80%) of the cylindrical GZO target. High usage of the cylindrical GZO target in the CRMS system indicates that CRMS is a promising deposition technique to prepare cost-efficient GZO electrodes for low cost OSCs. Resistivity and optical transmittance of the CRMS-grown GZO film were mainly affected by substrate temperature because the grain size and activation of the Ga dopant were critically dependent on the substrate temperature. In addition, the performance of OSC fabricated on GZO electrode sputtered at 230 °C (11.67 Ω/square) is better than OSC fabricated on as-deposited GZO electrode (29.20 Ω/square). OSCs fabricated on the GZO electrode sputtered at 230 °C showed an open circuit voltage of 0.558 V, short circuit current of 8.987 m A/cm², fill factor of 0.628 and power conversion efficiency of 3.149%.     

Selective absorbers based on AlCuFe thin films

AlCuFe films of different thicknesses were ion beam sputter deposited onto substrates held at temperatures of 480–490°C. Composition of the films was measured with elastic recoil detection and found to be Al₆₂Cu₂₅Fe₁₃. X-ray diffraction of AlCuFe films ≈ 200 nm thick showed that these films consist of a mixture of quasicrystalline and crystalline phases. Selective absorbers were produced by deposition of sandwich systems of 50 nm Al₂O₃/10–12 nm AlCuFe/70 nm Al₂O₃ on a copper film on Al₂O₃ coated Si-wafers. A solar absorptance of 90% was achieved, while the near normal directional emittance measured at room temperature is about 2.5%. Optical constants and DC-resistivity data of the AlCuFe films are reported.     

Ultra-thin nanocrystalline lanthanum strontium cobalt ferrite (La0.6Sr0.4Co0.8Fe0.2O3−δ) films synthesis by RF-sputtering and temperature-dependent conductivity studies

Nanocrystalline lanthanum strontium cobalt ferrite (LSCF) ultra-thin films with high in-plane electrical conductivity have been deposited by RF sputtering from composite targets. The films, with nominal thickness of 54 nm, are crystalline when annealed or deposited at temperatures above 450 °C. Effects of annealing temperature, annealing time, and substrate temperature on crystallization, microstructure, and room temperature lateral electrical conductivity have been systematically studied. No interfacial reaction products between the LSCF and single crystalline yttria-stabilized zirconia (YSZ) were observed from X-ray diffraction studies upon annealing until 750 °C. In-plane electrical conductivity as high as 580 S cm⁻¹ at 650 °C has been observed for LSCF thin films deposited on single crystalline YSZ substrates and sputtered nanocrystalline YSZ thin films; while activation energy for conductivity were determined to be 0.15 eV and 0.10 eV for the former and latter films, respectively, in 650–400 °C range. The high in-plane electrical conductivity for the nanocrystalline LSCF ultra-thin films is likely attributed to their low level of porosity. Micro-solid oxide fuels cells using 15 nm thick LSCF films as cathodes and sub-100 nm yttria-doped zirconia thin film electrolytes have been fabricated successfully and demonstrated to achieve peak power density of 60 mW cm⁻² at 500 °C. Our results demonstrate that RF sputtering provides a low-temperature synthesis route for realizing ultra-thin nanocrystalline LSCF films as cathodes for intermediate- or low-temperature solid oxide fuel cells.     

Comparative studies of “all sol–gel” electrochromic windows employing various counter-electrodes

Electrochromic (EC) “smart” windows for buildings represent an effective way to modulate the intensity of incoming solar radiation. While it is accepted that WO₃ films represent the best option for the working electrode, the choice of the best counter-electrode is still debatable. Optical properties of counter-electrodes such as Ce, Fe, V and Sn oxides are presented. Electrochromic windows were made with a sol–gel WO₃ active colouring film (150°C), Ce, Fe, V oxide counter-electrodes and a sol–gel organic–inorganic hybrid (Li⁺ormolyte) ion conductor. The electrochromic responses of these devices predicted from the charge capacities, photopic transmittances and coloration efficiencies of individual films are compared with measured values.     

Enhanced photoactivity of visible light responsive W incorporated FeVO4 photoanode for solar water splitting

Herein, we report the preparation, characterization and investigation of previously unexplored W incorporated iron vanadate (FeVO₄) electrodes for solar light driven water oxidation in photoelectrochemical cell. The W incorporated FeVO₄ films on F-doped SnO₂ substrates have been prepared by layer-by-layer deposition of metal–organic precursor and subsequent thermal decomposition at 550 °C in air. The synthesized films with a band gap of about 2.06 eV are responsive to visible light up to wavelength of ∼600 nm, i.e. being able to harvest ∼45% of the solar spectrum. The W incorporated FeVO₄ photoanodes are active materials for photoelectrochemical water oxidation and, yield a significantly enhanced (2.5 fold higher) photocurrent in comparison to pristine FeVO₄ photoanodes. This improvement can be attributed to increased n-type conductivity by W⁶⁺ ion doping in the FeVO₄ lattice. The incident photon to current conversion efficiency achieved with developed photoanodes is as high as 6.5% at 400 nm.     

RF sputter deposition of the high-quality intrinsic and n-type ZnO window layers for Cu(In,Ga)Se2-based solar cell applications

Transparent ZnO films were prepared by rf magnetron sputtering, and their electrical, optical, and structural properties were investigated under various sputtering conditions. Aluminum-doped n-type(n-ZnO) and undoped intrinsic-ZnO (i-ZnO) layers were deposited on a glass substrate by incorporating different targets in the same reaction chamber. The n-ZnO films were strongly affected by argon ambient pressure and substrate temperature, and films deposited at 2 mTorr and 100°C showed superior properties in resistivity, transmission, and figure of merit (FOM). The sheet resistance of ZnO film was less dependent on film thickness when the substrate was heated during deposition. These positive effects of elevated substrate temperature are presumably attributed to the rearrangement of the sputtered atoms by the heat energy. Also, the films are electrically uniform through the 5 cm×5 cm substrate. The maximum deviation in sheet resistance is less than 10%. All of the films showed strong (0 0 2) diffraction peak near 2θ =34°. The undoped i-ZnO films deposited in the mixture of argon and oxygen gases showed high transmission properties in the visible range, independent of the Ar/O₂ ratio, while resistivity rose with increased oxygen partial pressure. The Cu(In,Ga)Se₂ solar cells, incorporating bi-layer ZnO films (n-ZnO/i-ZnO) as window layer, were finally fabricated. The fabricated solar cells showed 14.48% solar efficiency under AM 1.5 conditions (100 mW/cm²).     

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.     

Optical and electrochemical characteristics of niobium oxide films prepared by sol-gel process and magnetron sputtering A comparison

Electrochromic niobia (Nb₂0₅) coatings were prepared by the sot-gel spin-coating and d.c. magnetron sputtering techniques. Parameters were investigated for the process fabrication of sol-gel spin coated Nb₂0₅ films exhibiting high coloration efficiency comparable with that d.c. magnetron sputtered niobia films. X-ray diffraction studies (XRD) showed that the sot-gel deposited and magnetron sputtered films heat treated at temperatures below 450°C, were amorphous, whereas those heat treated at higher temperatures were slightly crystalline. X-ray photoelectron spectroscopy (XPS) studies showed that the stoichiometry of the films was Nb₂0₅. The refractive index and electrochromic coloration were found to depend on the preparation technique. Both films showed low absorption and high transparency in the visible range. We found that the n, k values of the sot-gel deposited films to be lower than for the sputtered films. The n and k values were n = 1.82 and k = 3 × 10⁻³, and n = 2.28 and K = 4 × 10⁻³ at 530 urn for sot-gel deposited and sputtered films, respectively. The electrochemical behavior and structural changes were investigated in 1 M LiC10₄/propylene carbonate solution. Using the electrochemical measurements and X-ray photoelectron spectroscopy, the probable electrode reaction with the lithiation and delithiation is Nb₂O₅ + x Li⁺ + x e⁻ ↔ Li_xNb₂0₅. Cyclic voltametric (CV) measurements showed that both Nb₂0₅ films exhibits electrochemical reversibility beyond 1200 cycles without change in performance. “In situ” optical measurement revealed that those films exhibit an electrochromic effect in the spectral range 300 < λ < 2100 nm but remain unchanged in the infrared spectral range. The change in visible transmittance was 40% for 250 nm thick electrodes. Spectroelectrochemical measurements showed that spin coated films were essentially electrochemically equivalent to those prepared by d.c. magnetron sputter deposition.     

Enhancement of photoelectrochemical response by aligned nanorods in ZnO thin films

ZnO thin films are deposited in pure Ar and mixed Ar and N₂ gas ambient at various substrate temperatures by rf sputtering ZnO targets. We find that the deposition in pure Ar ambient leads to polycrystalline ZnO thin films. However, the presence of N₂ in the deposition ambient promotes the formation of aligned nanorods at temperatures above 300 °C. ZnO films with aligned nanorods deposited at 500 °C exhibit significantly enhanced photoelectrochemical response, compared to polycrystalline ZnO thin films grown at the same temperature. Our results suggest that aligned nanostructures may offer potential advantages for improving the efficiency of photoelectrochemical water-splitting for H₂ production.     

Preparation conditions of sputtered electrochromic WO3 films and their infrared absorption spectra

Tungsten oxide films were prepared by rf sputtering in an argon-oxygen atmosphere from W and WO₃ targets. To bring about reversible electrochromic (EC) characteristics, as-deposited films require an aging process (i.e. cycles of injection/ejection of charge carriers). The infrared absorption band at around 3300 cm⁻¹ increases during the aging process, and it is assigned as OH stretching vibrations of absorbed water.By coloration after aging, the 3300 cm⁻¹ band decreases, and a new band appears at 2400 cm ⁻¹. The latter band is considered to be to the stretching mode of radicals incorporated in the WO₃ matrix. At low coloration levels, the 2400 cm⁻¹ band increases slightly with injected charge, and a coloration mechanism other than the usual double injection model may be considered.The coloration efficiency depends on the preparation conditions. Its maximum value is the same for films prepared from W and WO₃ targets, and is 60 cm²/C at a wavelength of 600 nm. When a tungsten target is used, the substrate temperature is low and the deposition rate is high compared with a WO₃ target.     

Fabrication and characterization of amorphous In–Zn–O/SiOx/n-Si heterojunction solar cells

Amorphous In–Zn–O (a-IZO) films were deposited on SiO_x covered n-type Si substrates by using pulsed laser deposition (PLD) technique to form a-IZO/SiO_x/n-Si heterojunction solar cells. The a-IZO films grown at 150 °C with various laser power (250–500 mJ/pulse) exhibit low resistivity (2–3 × 10⁻³ Ω cm) and high transparency (∼80%) in the visible wavelength range. The highest conversion efficiency of the fabricated a-IZO/SiO_x/n-Si solar cells is 2.2% under 100 mW/cm² illumination (AM1.5 condition). The open-circuit voltage, short-circuit current density and fill factor of the best device are 0.24 V, 28.4 mA/cm² and 33.6%, respectively.     

The temperature behavior of smart windows under direct solar radiation

The purpose of this paper was to investigate the variation in temperature of electrochromic devices under direct solar radiation and to compare the results with double-glazed glass. The devices consisted of a V₂O₅ layer as an ion storage film and a WO₃ layer as an electrochromic layer. The V₂O₅ and WO₃ films were prepared by thermal and electron beam evaporation, respectively. The optical properties and structures of these films were investigated. Both the ion storage film and the electrochromic layer were amorphous. The optical absorption was caused by a direct-forbidden transition in V₂O₅ and by an indirect-allowed transition in WO₃. The maximum temperatures under solar radiation were measured for colored and bleached devices, double glass and air, they were found to be approximately 63, 63, 53 and 36 °C, respectively. The rates of increasing temperature to the incident power density for colored, bleached devices and double glass were 0.051, 0.049 and 0.041 °C/(W/m²), respectively.     

Influence of sputtering conditions on the solar and luminous optical properties of amorphous LixWoy thin films

Thin films of amorphous tungsten oxide were deposited by sputtering onto glass substrates coated by conductive indium–tin oxide. The films were sputtered at different oxygen-to-argon flow ratios with different pressure and power. Elastic recoil detection analysis determined the density and the stoichiometry. X-ray diffraction measurements showed that the films were amorphous. The films were electrochemically intercalated with lithium ions. At several intercalation levels of each film, the optical reflectance and transmittance were measured in the wavelength range 0.3–2.5 μm. We study the effect of various sputtering conditions on the coloration efficiency of the films and on the luminous and solar optical properties. The O₂/Ar ratio and the sputter pressure determine to a large extent the optical absorption. As-deposited sputtered tungsten oxide with sufficiently little oxygen exhibits an absorption peak similar to the case of lithium intercalation.     

Effect of heat treatment on physical properties of CdO films deposited by sol–gel method

CdO film has been deposited by sol–gel spin coating method on the glass substrate and then the film has been annealed at 400, 500, 600 °C for 1 h. Effect of annealing temperature on the structural and optical properties of the film has been investigated. The crystal structure and orientation of the as-grown and annealed CdO films have been investigated by X-ray diffraction method. Annealed CdO films are polycrystalline with (111) preferential orientation. The information on strain and grain size is obtained from the full width-at-half-maximum (FWHM) of the diffraction peaks. Texture coefficient and lattice constant have been calculated. The surface morphology of the films has been analyzed. The optical band gap value decreased with increasing the annealing temperatures.     

Brush electrodeposited CdSexTe1−x thin films and their properties

CdSe_xTe_1−x thin films were brush plated on titanium and conducting glass substrates from the precursors at different substrate temperatures in the range of 30-80 °C. X-ray diffraction studies indicated the films to possess hexagonal structure irrespective of composition. The strain and dislocation density decrease with increase of substrate temperature. The crystallite size increased from 30 to 100 nm as the substrate temperature increased. The resistivity of the films decreased with increase of substrate temperature. The carrier density and mobility increased with substrate temperature. Optical band gap of the films varied in the range of 1.45-1.72 eV. Higher photosensitivity was obtained compared to earlier reports.     

The kinetic behaviour of ion transport in WO3 based films produced by sputter and sol–gel deposition:: Part I. The simulation model

This paper develops a new simulation model for charge injection and extraction in electrochromic WO₃ films under conditions of constant current during the charge injection process. The model is applied to the constant current coloration and bleaching of electrochromic films, and values for the diffusion coefficient and other model parameters have been obtained by fitting the model to experimental data. Application of the model to coloration and bleaching of electrochromic devices is discussed, in particular the use of the model in the design of electrochromic switching schemes.     

Growth, structural and optical properties of copper indium diselenide thin films deposited by thermal evaporation method

Copper indium diselenide (CuInSe₂) compound was synthesized by reacting its constituent’s elements copper, indium and selenium in near stoichiometric proportions (i.e. 1:1:2 with 5% excess selenium) in an evacuated quartz ampoule. Synthesized pulverized compound material was used as an evaporant material to deposit thin films of CuInSe₂ onto organically cleaned sodalime glass substrates, held at different temperatures (300-573 K), by means of single source thermal evaporation method. The phase structure and the composition of chemical constituents present in the synthesized compound and thin films have been investigated using X-ray diffraction and energy dispersive X-ray analysis, respectively. The investigations show that CuInSe₂ thin films grown above 423 K are single phase, having preferred orientation of grains along the (112) direction, and having near stoichiometric composition of elements. The surface morphology of CuInSe₂ films, deposited at different substrate temperatures, has been studied using the atomic force microscopy to estimate its surface roughness. An analysis of the transmission spectra of CuInSe₂ films, recorded in the wavelength range of 500-1500 nm, revealed that the optical absorption coefficient and the energy band gap for CuInSe₂ films, deposited at different substrate temperatures, are ∼10⁴ cm⁻¹ and 1.01-1.06 eV, respectively. The transmission spectrum was analyzed using iterative method to calculate the refractive index and the extinction coefficient of CuInSe₂ thin film deposited at 523 K. The Hall effect measurements and the temperature dependence of the electrical conductivity of CuInSe₂ thin films, deposited at different substrate temperatures, revealed that the films had electrical resistivity in the range of 0.15-20 ohm cm, and the activation energy 82-42 meV, both being influenced by the substrate temperature.     

Printed Co3O4 film as an electrocatalyst for hydrogen production by a monolithic photovoltaic-electrolysis system

In a monolithic photovoltaic-electrolysis system, photovoltaic and catalytic layers are generally constructed on both sides of a conducting substrate. For the realization of this architecture it is critical to fabricate the catalytic layer at sufficiently low temperature in order not to deteriorate the photovoltaic photoanode that is already installed on the reverse side of the catalytic layer. In this study we demonstrate successful fabrication of Co₃O₄ electrocatalyst films at low temperature (∼50^°C) on a stainless steel substrate by a paste coating method using Nafion as a binder. The Co₃O₄ films were found to be catalytically efficient and stable in water splitting reaction in an alkaline aqueous solution. More importantly, the Co₃O₄ films casted at low temperature (∼50^°C) revealed the highest hydrogen production rate in the electrolytic cell compared to the films prepared at higher temperatures (e.g. 150 and 300^°C), which would be very beneficial in the construction of a monolithic photovoltaic-electrolysis system.