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.     

Optical and electrochemical properties of V2O5:Ta Sol–Gel thin films

Vanadium and tantalum-doped vanadium pentoxide, V₂O₅ and V₂O₅:Ta thin films (2.5 and 5 mol% of Ta) were prepared using sol–gel dip-coating technique.The coating solutions were prepared by reacting vanadium (V) oxytripropoxide and tantalum ethoxide (V) as precursors using anhydrous isopropyl alcohol as solvent.The films were deposited on a transparent glass substrate with ITO conducting film by dip-coating technique, with a withdrawal of 20 cm/min from the vanadium–tantalum solution and heat treated at 300 °C for 1 h. The resulting films were characterized by cyclic voltammetry, optical spectroscopy and by X-ray diffraction analysis (XRD). XRD data show that the films thermally treated at 300 °C were crystalline.A charge density of 70 mC/cm² was obtained for the film with 5 mol% of Ta, with an excellent stability up to 1500 cycles.     

Preparation of In2O3:F thin films grown by spray pyrolysis technique

Transparent conducting fluorine doped indium oxide (In₂O₃:F) thin films have been deposited on Corning 7059 glass substrates by the spray pyrolysis technique. The structural, electrical, and optical properties of these films were investigated as a function of substrate temperature. The X-ray diffraction pattern of the films deposited at lower substrate temperature (T_s=300 °C) showed no peaks of In₂O₃:F. In the useful range for deposition (i.e. 425–600 °C), the orientation of the films was predominantly [400]. For the 4500 Å thick In₂O₃:F deposited with an F content of 10-wt%, the minimum sheet resistance was 120 Ω and average transmission in the visible wavelength rang (400–700 nm) was 88%.     

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.     

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.     

Pulsed laser deposition of W–V–O composite films: Preparation, characterization and gasochromic studies

Modified tungsten oxide films by vanadium oxide provide neutrally coloring electrochromic electrodes for smart windows technology. In this study W–V–O mixed oxide films were fabricated by Nd:YAG pulsed laser deposition (PLD), λ=1064 nm, from mixed pressed powders of (WO₃)_1−x(V₂O₅)_x, x=0, 0.09, 0.17, 0.23, 0.29 and 0.33, at 13.3 Pa oxygen partial pressure and 200 °C temperature on glass substrates. X-ray photoelectron spectroscopy (XPS) revealed V⁵⁺, V⁴⁺, W⁶⁺ and W⁵⁺ surface oxide states, where the ratio of W⁵⁺/W⁶⁺ enhances by the amount of vanadium in the films. Surface morphology was studied by scanning electron microscope (SEM) and optical properties by transmission-reflection spectra. Results showed that films with a low amount of vanadium oxide have better porosity and higher optical band gaps. The gasochromic response to hydrogen gas exposure was found better for x=0.09 in the sense of both deeper and faster coloring. Weak responses of samples with more vanadium oxide were attributed to higher amounts of W⁵⁺ in the films and also to lower porosity.     

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.     

Ellipsometric spectroscopy study of cobalt oxide thin films deposited by sol–gel

Due to their unique optical properties, solar selective coatings enhance the thermal efficiency of solar photothermal converters. Hence it seems to be interesting to study the optical properties of promising materials as solar selective coatings. In an earlier work, it was demonstrated that sol–gel deposited cobalt oxide thin films possess suitable optical properties as selective coatings. In this work, cobalt oxide thin films were prepared by same technique and their optical properties were analyzed as a function of the dipping time of the substrate in the sol, using the spectroscopy ellipsometry, atomic force microscopy and X-ray photoelectron spectroscopy techniques. The optical constants (n and k) for these films, in the 200–800 nm range, are reported as a function of the dipping time. The fitting of ellipsometric data, I_s and I_c, for the glass substrate and the cobalt oxide thin film, as modeled with the Lorentz and Tauc–Lorentz dispersion relations, indicated that the film microstructure resembles a multilayer stack with voids. From these results, the Co₃O₄ and void percentages in the film were estimated. Both, thin film thickness and void/Co₃O₄ percentage ratio, were determined to be strongly dependent on the immersion time. Furthermore, the total thickness of a multilayered film was found to be the sum of thickness of each individual layer.     

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 copper diffusion on photovoltaic characteristics of CuGaSe2–GaAs cells

CuGaSe₂–GaAs heterojunctions were fabricated by fast evaporation of polycrystalline CuGaSe₂ from a single source on n-type GaAs substrates. The best CuGaSe₂–GaAs photocell (without an antireflective coating) exhibited an efficiency of 11.5%, J_sc=32 mA/cm², V_oc=610 mV and FF=0.60. The spectral distribution of photosensitivity of CuGaSe₂–GaAs junctions extends from 400 to 900 nm. The CuGaSe₂ films were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. XRD analysis indicated that the thin films were strongly oriented along the (1 1 2) plane. SEM studies of CuGaSe₂ films showed nearly stoichiometric composition with grain size about 1–2 μm. The energy dispersive X-ray spectroscopy (EDX) analysis of Cu concentration distribution in n-type GaAs showed that Cu diffused from the film into n-type GaAs during the growth process resulting in formation of the latent p–n homojunction in substrate. The diffusion coefficient of Cu in GaAs at growth temperature (520°C) estimated from EDX measurements was 6×10⁻⁸ cm²/s.     

Dye-sensitized sputtered titanium oxide films for photovoltaic applications: influence of the O2/Ar gas flow ratio during the deposition

Titanium oxide films were prepared by reactive DC magnetron sputtering onto SnO₂:F coated glass substrates. The O₂/Ar gas flow ratio was kept at a constant value Γ during the deposition, and a series of films were deposited with 0.050<Γ<0.072. Structural studies were performed by X-ray diffraction and transmission electron microscopy; the structure displayed penniform features with a clear dependence on Γ. Charge transport in the films was evaluated by use of time-resolved photocurrents; a diffusion model was fitted to the experimental data and two different transport mechanisms were proposed depending on the film stoichiometry. Dye sensitization in cis-dithiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium (II) was performed to evaluate incident photon-to-current conversion efficiency and solar cell properties of the films. These parameters showed a clear dependence on Γ. Optical measurements gave evidence for the presence of polaron absorption for the film deposited at Γ=0.050.     

Optical characterization of tandem absorber/reflector systems based on titanium oxide–carbon coatings

The use of chemical methods to produce coatings with good spectrally selective properties is described. Sol–gel and dipping techniques were combined to fabricate thickness-sensitive solar-selective absorbing coatings based on titanium oxide combined with carbon black (TiO₂/CB) and carbon nanotubes (TiO₂/CNT) on top of stainless-steel substrates. The low to high reflectance transition (λ_T) observed in the tandem system based on TiO₂/CB films was smooth and red shifted when compared to the sharp transition observed in systems based on mesoporous TiO₂/CNT films. Mesoporosity of the oxide was controlled by the use of polymeric surfactants with different molecular weights, causing a significant blue shift in λ_T as the molecular weight increases. These films were characterized by X-ray diffraction and UV–vis light absorption spectroscopy.     

The electro-optical properties of amorphous indium tin oxide films prepared at room temperature by pulsed laser deposition

The electrical and optical properties of pulsed laser deposited amorphous indium tin oxide films at room temperature are discussed. The films were grown from indium oxide (In₂O₃) targets of different tin (Sn) doping content (0, 5 and 10 wt%) at different oxygen pressures (P_O2) ranging from 1×10⁻³ to 5×10⁻² Torr. The electrical and optical properties of the films were examined by Hall measurements and optical spectrophotometry. It was found that high conductivity amorphous films could be prepared at room temperature irrespective of the Sn doping content. The properties of these films deposited from 0, 5, 10 wt% Sn-doped In₂O₃ targets show a similar response to changes in P_O2. The maximal conductivity of (4.0, 2.1 and 1.8)×10³ S/cm and optical transmittance (visible) higher than 90% were obtained at P_O2 region of (1–1.5)×10⁻² Torr. An undoped In₂O₃ film produced the highest conductivity of 4×10³ S/cm in these studies.     

Electrodeposited cuprous oxide on indium tin oxide for solar applications

Semiconducting cuprous oxide films were prepared by electrodeposition onto commercial conducting glass coated with indium tin oxide deposited by spraying technique. The cuprous oxide (Cu₂O) films were deposited using a galvanostatic method from an alkaline CuSO₄ bath containing lactic acid and sodium hydroxide at a temperature of 60°C. The film's thickness was about 4–6 μm. This paper includes discussion for Cu₂O films fabrication, scanning electron microscopy and X-ray diffractometry studies, optical properties and experimental results of solar cells. The values of the open circuit voltage V_oc of 340 mV and the short circuit current density I_sc of 245 μA/cm² for ITO/Cu₂O solar cell were obtained by depositing graphite paste on the rear of the Cu₂O layer. It should be stressed that these cells exhibited photovoltaic properties after heat treatment of the films for 3 h at 130°C. An electrodeposited layer of Cu₂O offers wider possibilites for application and production of low cost cells, both in metal–semiconductor and hetero-junction cell structures, hence the need to improve the photovoltaic properties of the cells.     

Optical and structural properties of – thin films

In this study, the sol–gel spin-coating method has been used to make Ta₂O₅–CeO₂ thin films. These films have been prepared in various composition ratios to observe changes in their optical and structural properties. Reflectance and transmittance spectra were collected in the spectral range of 300–1000 nm and were accurately fit using the Tauc–Lorentz model. Film thicknesses, refractive indices, absorption coefficients, and optical band gaps were extracted from the theoretical fit. The highest refractive index value was found at 5% CeO₂ doping. The structure of the films was characterized by X-ray diffractometry and Fourier transform infrared spectrometry, while the surface morphology was examined through atomic force microscopy.     

CdO/Cu2O solar cells by chemical deposition

CdO and Cu₂O thin films have been grown on glass substrates by chemical deposition method. Optical transmittances of the CdO and Cu₂O thin films have been measured as 60–70% and 3–8%, respectively in 400–900 nm range at room temperature. Bandgaps of the CdO and Cu₂O thin films were calculated as 2.3 and 2.1 eV respectively from the optical transmission curves. The X-ray diffraction spectra showed that films are polycrystalline. Their resistivity, as measured by Van der Pauw method yielded 10⁻²–10⁻³ Ω cm for CdO and approximately 10³ Ω cm for Cu₂O. CdO/Cu₂O solar cells were made by using CdO and Cu₂O thin films. Open circuit voltages and short circuit currents of these solar cells were measured by silver paste contacts and were found to be between 1–8 mV and 1–4 μA.     

Initial results of CdS/CuInTe2 heterojunction formed by flash evaporation

The CuInTe₂ thin films is one of the most attractive semiconductors for solar cells applications, since its direct band gap energy (Eg≈1 eV) is suitable as an absorber in photovoltaic conversion. In this letter the CuInTe₂ thin films are prepared by flash evaporation technique. X-ray diffraction measurements on the as-deposited CuInTe₂ film showed that these films consist mainly of the chalcopyrite phase. The junction formation in the n-CdS/p-CuInTe₂ cell has been investigated using current–voltage (I–V) and capacitance-voltage (C–V) measurements.     

Application of 1/f current noise for quality and age monitoring of electrochromic devices

This is a continuation of an earlier study on 1/f noise in electrochromic (EC) devices undergoing discharge via a resistor. The EC devices comprised films of W oxide and Ni–V oxide joined by a polymer electrolyte, and with this three-layer stack positioned between transparent conducting In₂O₃:Sn films backed by polyester foils. We also investigated “symmetrical” devices with two identical films of W oxide or Ni–V oxide. The power spectral density S_i at fixed frequency scaled with current (I) as S_iI². Color/bleach cycling for about 2500 times degraded the optical properties and homogeneity of the EC devices and increased the 1/f noise intensity by a factor of four, which confirms the earlier assumption that 1/f noise has a good potential to serve as quality and aging assessment for EC devices. Studies of “symmetrical” devices proved that the noise was mainly associated with the Ni oxide, and measurements on individual parts of an EC device indicated that the 1/f noise originated from localized areas.     

Effect of TiO2 mixtures on the optical, structural and electrochromic properties of Nb2O5 thin films

Metal oxide films are important for various optical devices and especially for solar energy materials. TiO₂-mixed Nb₂O₅ thin films have been produced by sol–gel dip-coating method. Several parameters such as heat treatment, thickness, and mixture percentages are studied for the effect of the optical, structural and electrochromic properties of the materials. Optical parameters of the films were calculated through transmission and reflection measurement by a refractive index, extinction coefficient and thickness analyzer. Structural, electrochromic and surface analyses of the films were done by X-ray diffractometer, potentiostat/galvanostat and atomic force microscope systems.     

Electrochromic characterization of sol–gel deposited coatings

Electrochromic devices have increasing application in large-area display devices, switchable mirrors and smart windows. A variety of vacuum deposition technologies have been used to make electrochromic devices. The sol–gel process offers an alternative approach to the synthesis of low-cost, high-quality electrochromic device layers. This paper discusses the developments in sol–gel-deposited electrochromic films and a prototype all sol–gel device with switching from 80% to 50% (550 nm). The sol–gel process involves the formation of oxide networks upon hydrolysis–condensation of alkoxide precursors. In this study, we cover sol–gel-deposited oxides of WO₃, TiO₂, Nb₂O₅, V₂O₅, Ni(OH)₂, Co(OH)₂ and CuO_x. The coloration reaction voltammetry and spectral optical properties are presented for selected films.