Formation and metal-to-insulator transition properties of VO2-ZrV2O7 composite films by polymer-assisted deposition

VO₂-ZrV₂O₇ composite films were prepared on silica glass substrates by polymer-assisted deposition using a V-Zr-O solution. The coexistence of ZrV₂O₇ and VO₂ was confirmed by Raman spectroscopy, glance angle X-ray diffraction, and X-ray photoelectron spectroscopy. The composite films with similar thickness of about 95 nm showed decreased particle sizes and significantly enhanced luminous transmittances (from 32.3% at Zr/V=0 to 53.4% at Zr/V=0.12) with increasing Zr/V rations. The enhancement in the luminous transmittance was ascribed to the absorption-edge changes in the composite films. This feature benefits the application of VO₂ to smart windows.     

Thermochromic multilayer films of VO2 and TiO2 with enhanced transmittance

Thermochromic films of VO₂, as well as multilayer films of VO₂ and TiO₂, were made by reactive DC magnetron sputtering. Spectrophotometrically measured transmittance and reflectance were used to determine optical constants pertinent to temperatures below and above a temperature-induced structural change at τ_c≈60 °C. We then used computations to optimize multilayer films for specific applications and, specifically, demonstrated that TiO₂/VO₂/TiO₂ films could display a luminous transmittance significantly higher than that of bare VO₂ films, and that TiO₂/VO₂/TiO₂/VO₂/TiO₂ films could yield a large change of solar transmittance for temperatures above and below τ_c. Our data can serve as starting points for developing novel coatings for windows with superior energy efficiency.     

Thermochromic properties and low emissivity of ZnO:Al/VO2 double-layered films with a lowered phase transition temperature

We studied the optical and semiconductor-metal (S-M) transition properties of ZnO:Al/VO₂/substrate double-layered films that consisted of a ZnO:Al top layer and a VO₂ bottom layer. ZnO:Al and VO₂ films were grown on fused silica substrates by radio frequency magnetron sputtering and polymer-assisted deposition, respectively. The ZnO:Al/VO₂/substrate films displayed low emissivity (0.31-0.32) with integrated luminous transmittance (T_lum>46%) and thermochromic properties (ΔT_sol>4.1%). The low emissivity and thermochromic properties were independently introduced by the transparent conductive ZnO:Al layer and the VO₂ layer. In addition, the S-M transition temperatures for VO₂ shifted to lower temperatures after the ZnO:Al deposition process, which was due to the formation of surface nonstoichiometry—oxygen deficiency that was induced by the ZnO:Al deposition process.     

Temperature-induced metal–semiconductor transition in W-doped VO2 films studied by photoelectron spectroscopy

We present a photoemission study on reactively sputtered W-doped VO₂ films by high-resolution photoelectron spectroscopy. The valence band spectra and core-level lines were analyzed below and above transition temperature on vanadium dioxide films with different tungsten concentrations. It is shown that increase in tungsten content in the film results in decreased transition temperature and smeared metal–semiconductor transition. The centroid and the width of the valence band in the semiconducting state are found to be dependent on tungsten concentration in the film. In the metallic state the valence band width increases and becomes asymmetric demonstrating clearly Fermi edge. The V-2p and O-1s core-level lines exhibit broadening upon going through metal–semiconductor transition which is assigned to the interaction of core-level hole with d band in the final state. Detailed analysis of tungsten 4d core-level line revealed the tungsten valence to be 6+ and 5+.     

VO2 nanostructures based chemiresistors for low power energy consumption hydrogen sensing

Mott-type VO₂ oxide nanobelts are demonstrated to be effective hydrogen gas sensors at room temperature. These nanobelts, synthesized by hydrothermal process and exhibiting the VO₂ (A) crystallographic phase, display room temperature H₂ sensitivity as low as 0.17 ppm. The nanobelts (ultralong belt-like) nanostructures could be an ideal system for fully understanding dimensionally confined transport phenomena in functional oxides and for building functional devices based on individual nanobelts.     

CuInSe2 thin films deposited by UV laser ablation

Application of pulsed laser ablation method for deposition of CuInSe₂ films was studied. The special time–temperature regime was developed. The homogeneous amorphous and polycrystalline CuInSe₂ films were prepared and investigated with XRD, SEM-EDS and optical spectroscopy.     

VO2-based double-layered films for smart windows: Optical design, all-solution preparation and improved properties

An all-solution process was developed to prepare VO₂-based double-layered films containing SiO₂ and TiO₂ antireflection layers. These double-layered films were optimized to improve luminous transmittance (T_lum) and switching efficiency (ΔT_sol). The substrate/VO₂/TiO₂ double-layered structure showed the largest improvement of 21.2% in T_lum (from 40.3% to 61.5%). T_lum could be further improved to the maximum of 84.8% by combining film thickness optimization and antireflection layer deposition. ΔT_sol (usually below 10% for single VO₂ films) could be improved by adjusting the position of antireflection peaks (the highest ΔT_sol was 15.1%). A sample with balanced T_lum and ΔT_sol showed T_lum of about 58% (20 °C) and 54% (90 °C), and ΔT_sol of 10.9%. This work is an important technical breakthrough toward the practical application of VO₂-based smart windows.     

Microstructure evolution of CuInSe2 thin films prepared by single-bath electrodeposition

The composition and the microstructure evolutions of CuInSe₂ thin films under single-bath electrodeposition processes were investigated. It was found that the film composition was mainly determined by the [Se⁴⁺]/[Cu²⁺] ratios in solution, but the film microstructure is strongly dependent on the initial concentrations of Se⁴⁺, Cu²⁺, and In³⁺ precursors. Higher initial concentrations of Cu²⁺ and In³⁺ in solution are beneficial for the fabrication of compact CuInSe₂ thin films with highly crystallized and large grain sized chalcopyrite phase. The microstructure evolution suggests that prior adsorption and reduction of Cu²⁺ ions and the formation of Cu₂Se compound on the substrate can promote the nucleation, growth, and coarsening of CuInSe₂ crystal to form a high quality thin film during the electrodeposition processes.     

Adjustment of the decomposition path for Na2LiAlH6 by TiF3 addition

The decomposition of Na₂LiAlH₆ is studied by in-situ synchrotron diffraction. By addition of TiF₃ and dehydrogenation-rehydrogenation cycling of the samples new decomposition paths are found. Na₃AlH₆ is formed on decomposition in the presence of TiF₃. The additive brings the system closer to equilibrium, and decomposition through Na₃AlH₆ is demonstrated for the first time. The results are in agreement with previously published computational data. For a cycled sample with 10 mol% TiF₃ Na₂LiAlH₆ decomposes fully into Na₃AlH₆ before further decomposition to NaH and Al. This shows clear changes in the kinetics of the system, and may open possibilities of tailoring the decomposition path by the use of additives.     

Optical characterization of In2S3 solar cell buffer layers grown by chemical bath and physical vapor deposition

In this paper, we study the optical properties of indium sulfide thin films to establish the best conditions to obtain a good solar cell buffer layer. The In₂S₃ buffer layers have been prepared by chemical bath deposition (CBD) and thermal evaporation (PVD). Optical behavior differences have been found between CBD and PVD In₂S₃ thin films that have been explained as due to structural, morphological and compositional differences observed in the films prepared by both methods. The resultant refractive index difference has to be attributed to the lower density of the CBD films, which can be related to the presence of oxygen. Its higher refractive index makes PVD film better suited to reduce overall reflectance in a typical CIGS solar cell.     

Spray deposition and compression of TiO2 nanoparticle films for dye-sensitized solar cells on plastic substrates

Spray deposition of powder suspensions followed by room temperature compression was studied as a method for preparing nanostructured TiO₂ films for dye-sensitized solar cells. The structure of the films was analyzed with optical and scanning electron microscopy and the films were applied to dye-sensitized solar cells. Continuous and fast deposition of crack-free 7–14 μm thick films was achieved by heating the substrates during the deposition. Scanning electron microscopy revealed small amount of structural imperfections in the compressed films due to the nature of the deposition method. An energy conversion efficiency of 2.8% was achieved at 100 mW/cm² light intensity.     

CuInTe2 and In-rich telluride chalcopyrites thin films obtained by electrodeposition techniques

In order to obtain single-phase thin films of the system Cu–In–Te with optoelectronic properties adequate for solar cells, electrodeposition techniques were used on substrates of molybdenum supported by glass. Different annealings in Te atmosphere have been done that affect the Te concentration and In/Cu atomic ratio. Single chalcopyrite phase appears in two ranges of composition where the In/Cu atomic ratio varies between 0.21–0.76 and 0.90–3.46, respectively. Morphology, cell lattice parameters and electrical resistance for single-phase samples depend strongly on the composition in the annealed samples. The cell parameters ranges are a=6.141–6.183 Å and c=12.201–12.375 Å.     

MnO2 nanotube and nanowire arrays by electrochemical deposition for supercapacitors

Highly ordered MnO₂ nanotube and nanowire arrays are successfully synthesized via a electrochemical deposition technique using porous alumina templates. The morphologies and microstructures of the MnO₂ nanotube and nanowire arrays are investigated by field emission scanning electron microscopy and transmission electron microscopy. Electrochemical characterization demonstrates that the MnO₂ nanotube array electrode has superior capacitive behaviour to that of the MnO₂ nanowire array electrode. In addition to high specific capacitance, the MnO₂ nanotube array electrode also exhibits good rate capability and good cycling stability, which makes it promising candidate for supercapacitors.     

Thermal-induced changes on the properties of spin-coated P3HT:C60 thin films for solar cell applications

The thermal transition behaviour, optical and structural properties of spin-coated P3HT:C₆₀ blended films with different C₆₀ ratios were investigated using differential scanning calorimetry (DSC), thermo-gravimetric analysis (TGA), ultraviolet-visible (UV-vis) spectroscopy, photoluminescence (PL), Fourier transform infrared absorption (FT-IR) spectroscopy and Raman spectroscopy. DSC analysis showed that the P3HT:C₆₀ blends have quite different thermal characteristics. The absorption spectra of the annealed P3HT:C₆₀ (1:1 wt%) films becomes enhanced and red shifted. This feature is evident in the photoluminescence measurements where the formation of polymer crystallites upon annealing is observed. Raman spectroscopy showed a substantial ordering in the polymer film during annealing. It was found that the performance of a P3HT:C₆₀ (1:1 wt%) device was dramatically improved by annealing.     

Thin film TiO2 photoanodes for water photolysis prepared by dc magnetron sputtering

The TiO₂ thin film photoanodes for hydrogen generation by water photolysis have been prepared by dc reactive magnetron sputtering. Optical monitoring of the plasma emission with the feedback signal I/I₀ controlling the reactive gas admission and reflecting the target oxidation state has been used. X-ray diffraction GID and AFM studies have revealed the systematic evolution of the microstructure with I/I₀, from that of well-crystallized mixture of anatase and rutile at the lowest I/I₀ (oxidized target) to that of amorphous state with finely dispersed rutile grains at the highest I/I₀ (metallic target). It has been demonstrated that the microstructure of TiO₂ affects to a large extent the optical and photoelectrochemical properties of the photoanodes. Optical band gap E_opt and the photocurrent response have been found to increase systematically with a decreasing feedback signal I/I₀. The best efficiency, i.e. the highest IPCE (incident photon-to-current conversion efficiency) has been obtained for the TiO₂ photoanodes sputtered at I/I₀ = 0.09 from the oxidized target.     

Optical and photoelectrical properties of β-In2S3 thin films prepared by two-stage process

β-In₂S₃ films were grown on glass as well as on quartz substrates by rapid heating of metallic indium films in H₂S atmosphere. The effect of sulfurization temperature and time on the growth, structural, electrical and photoelectrical properties of β-In₂S₃ films has been investigated. Highly oriented single-phase β-In₂S₃ films were grown by the sulfurization technique. The morphology and composition of films have been characterized. The optical band gap of β-In₂S₃ is found to vary from 1.9 to 2.5 eV when the sulfurization temperature is varied from 300 to 600 °C or by increasing the sulfurization time. The electrical properties of the thin films have also been studied; they have n-type electrical conductivity. The photoelectrical properties of the β-In₂S₃ films are also found to depend on the sulfurizing temperature. A high photoresponse is obtained for films prepared at a sulfurizing temperature of 600 °C. β-In₂S₃ can be used as an alternative to toxic CdS as a window layer in photovoltaic technology.     

Structural and optical properties of CuInTe2 films prepared by thermal vacuum evaporation from a single source

The CuInTe₂ thin films were prepared by thermal vacuum evaporation from a single source. The effects of heat treatment on both structural an optical properties of CuInTe₂ films were studied. X-ray diffraction studies reveal that the films prepared by the present method showed formation of single phase CuInTe₂ at heating treatment temperature higher than 300°C. The calculated lattice constants for CuInTe₂ films annealed at 300°C were found to be and . The value of the grain size obtained in these films was of the order of 50 nm. The value of optical energy gap (0.97 eV) and the dispersive refractive index n(λ) for CuInTe₂ film annealed at 300°C were evaluated by optical absorption measurement.     

Preparation of Zn doped Cu(In,Ga)Se2 thin films by physical vapor deposition for solar cells

Cu(In,Ga)Se₂ (CIGS) surface was modified with Zn doping using vacuum evaporation. Substrate temperatures and exposure times during the Zn evaporation were changed to control a distribution of Zn in the CIGS films. Diffusion of Zn in the CIGS films was observed at the substrate temperature of over 200°C. The diffusion depth of Zn increases with increasing the exposure time at the substrate temperature of 300°C. Solar cells were fabricated using the Zn doped CIGS films. A distribution of the efficiencies decreases with increasing the exposure time of Zn vapor. The doping of Zn at the film surface improved reproducibility of a high fill factor and efficiency. A solar cell fabricated using the CIGS film modified with Zn doping showed an efficiency of 14.8%.     

Investigation of n-type Cu2O layers prepared by a low cost chemical method for use in photo-voltaic thin film solar cells

A low cost and simple chemical method of boiling copper plates in CuSO₄ solution is used to prepare Cu₂O layers. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), glow discharge optical emission spectroscopy (GDOES) and optical absorption have been used to characterise these layers. It has been found that the layers consist of Cu₂O phase with a thickness of about 1.4 μm for 60 minutes boiling in CuSO₄ solution. The largest grain sizes are in the order of 1 μm and the layers contain cubic Cu₂O phases. The layers are n-type in electrical conduction and the optical band gap observed is 2.2 eV.