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.     

Thermochromic glazing of windows with better luminous solar transmittance

Anti-reflection coating on a thermochromic glazing VO₂ film has been studied by RF sputtering technique and it is proposed as a possible solution to enhance luminous solar transmittance of the glazing film. SiO₂ anti-reflection coating raised the luminous transmittance of the thermochromic film significantly both at low and high temperatures. In addition, it was found that thermochromism was maintained in the AR coated thermochromic film. More importantly, the thermochromism of VO₂ film showed greater distinctiveness after application of AR coating, while the transition temperature of the thermochromism, 70°C was not affected by the AR coating.     

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.     

Single-material TiO2 double-layer antireflection coatings

This paper demonstrates that a double-layer antireflection (DLAR) coating can be fabricated using a single material, titanium dioxide (TiO₂). The optical properties of the top and bottom TiO₂ layers were controlled by varying the deposition and sintering conditions, resulting in a range of refractive indices, n=1.73–2.63 at 600 nm. Weighted average reflectances of 6.5% (measured) and 7.0% (calculated) were achieved for TiO₂ DLAR coatings in air and under glass, respectively. When implemented in a high-efficiency silicon solar cell, a short-circuit current density increase of ΔJ_sc=2.5 mA/cm² can be expected for an optimised TiO₂ DLAR coating when compared to a commercial TiO₂ single-layer antireflection coating.     

Comparison of microstructures and thermal insulation capability of plasma sprayed nanostructured and traditional YPSZ coatings

Abstract

Microstructural features and thermal insulation capability of plasma sprayed nanostructured and traditional yttria partly stabilised zirconia (YPSZ: ZrO₂–8Y₂O₃) coatings were investigated. Both nanostructured YPSZ coating and traditional YPSZ coating were mainly composed of non-transformable t-ZrO₂ phases. The detailed microstructures of the nanostructured YPSZ coating were observed using FESEM, which presented three types of microstructures: columnar grains, equiaxed grains and nanosized zirconia particles embedded in the so called matrix formed by melted powders. However, the traditional YPSZ coating only contained columnar and equiaxed grains. The average porosities of nanostructured and traditional YPSZ coatings are 12·5 and 9·7% respectively. Compared with the traditional YPSZ coating, the nanostructured coating contained finer microcracks. The nanostructured YPSZ coating has higher thermal insulation capability than the traditional YPSZ coating. For YPSZ coating of 200 μm in thickness, the temperature drop ΔT (thermal insulation capability) of the nanostructured coating at 1350°C increased by 27% compared with that of the traditional coating.     

Optical and electrochemical properties of CeO2 and CeO2---TiO2 coatings

Thin solid films of CeO₂ and mixed CeO₂---TiO₂ were prepared by the sol-gel route via the dip-coating technique. Particulate sols of ceria were made from inorganic ((NH₄)₂Ce(NO₃)₆) precursor which were used for preparation of CeO₂ thin solid films while CeO₂---TiO₂ coatings have been made by using mixed organic-inorganic (Ti(OiPr)₄ and CeCl₃·7H₂O) precursors. The solar transmission values (T_s) of both coatings are in the range 0.6–0.8 and depend on coating thickness. Cyclic voltammetric (CV) measurements show that the CeO₂/LiOH system exhibits higher overall electrochemical reversibility when compared to the CeO₂---TiO₂/LiOH system. The CeO₂/LiOH system is also less sensitive with regard to the coating thickness. Coulometric measurements show that CeO₂ exhibits a larger storage capability which was determined as a function of the coating thickness. “In situ” UV-VIS spectroelectrochemical measurements which have been performed on CeO₂ and CeO₂---TiO₂ coatings revealed that both types of samples exhibit electrochromic effect in the spectral range 500 < λ < 330 nm but remain unchanged in the visible spectral range.     

Characterisation and stability of low-emittance multiple coatings for glazing applications

We present experimental studies on the deposition and characterisation of ZnS/Ag/ZnS low-e coatings carried out to assess their optical performance and stability. Theoretical predictions of the coatings optical properties were combined with measured emissivity () values to derive a figure of merit (η) defined as the ratio of luminous transmittance over emissivity. A coating consisting of 37.0 nm ZnS/21.5 nm Ag/37.0 nm ZnS was found to maximise η. Long-term experiments on the produced ZnS/Ag/ZnS coatings, under solar irradiation have revealed that a moderate vacuum (10⁻²–10⁻³ mbar) is required to avoid degradation of the film properties. Four-layer ZnS/Al/Ag/ZnS and five-layer ZnS/Ag/ZnS/Ag/ZnS coatings were produced and tested for thermal stability. They were both found to withstand heating at 300°C for 3 h. The four-layer structures however, present low transmittance in the visible, due to absorption by the Al layer.     

Computations of the optical properties of metal/insulator-multilayers for solar thermal applications

A computer program has been developed to calculate the optical properties of metal/ insulator-multilayer systems from complex refractive indices of corresponding bulk materials. In addition, interdiffusion in multilayer structures could be detected by fitting experimental reflectance spectra. Among M/Al₂O₃-systems, M = (Ta, Mo, W, orPt), optimum multilayer structures were evaluated as selective absorbers for solar thermal applications at various operating temperatures. The influence of the kind of metal, the modulation number, and the layer thicknesses on the solar thermal performance were studied. At temperatures of T_B ≈ 1100 K and 100-fold sunlight concentration Mo/Al₂0₃- and W/Al₂O₃-multilayers with a modulation number of 6.5 and metal layer thicknesses of a few nm exhibited an absorptance of merit up to A_m ≈ 0.8. Lower temperatures favoured Pt-systems (A_m ≈ 0.94 at 400 K) and higher temperatures Ta-systems (A_m ≈ 0.72 at 1400 K).     

Optical properties of sol–gel deposited Al2O3 films

Highly transparent, uniform and corrosion resistant Al₂O₃ films were prepared on stainless-steel and quartz substrates by the sol–gel process from stable coating solutions using aluminum-sec-butoxide, Al(OBu^s)₃ as precursor, acetylacetone, AcAcH as chelating agent and nitric acid, HNO₃, as catalyzer. Films up to 1000 nm thick were prepared by multiple spin coating deposition, and were characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), optical spectroscopy and micro Vickers hardness test. XRD of the film heat treated at 400°C showed that they had an amorphous structure. XPS confirmed that they were stoichiometric Al₂O₃. The refractive index (n) and extinction coefficient (k) were found to be n=1.56±0.01 and k=0.003±0.0002 at 600 nm, respectively. The surface microhardness and corrosion resistance investigations showed that Al₂O₃ films improved the surface properties of stainless-steel substrates.     

A study of zenith luminance on Madrid cloudless skies

The zenith luminance (L_z) has been measured for cloudless skies at the IDMP station in Madrid. Mean values obtained at every 15 min have been fitted vs. solar elevation (α). A 5th degree polynomial connecting log (L_z) against (α) gives the best correlation in terms of the correlation coefficient (r=0.9358). A possible explanation of the dependence of the zenith luminance on solar elevation at low values is given.An exponential model obtained by other authors (Vázquez and Bernabeu, 1997) for a nearby temporary station in the same city does not account for the qualitative dependence of L_z vs. α for α<≈15°, and also overestimates experimental values for high solar elevations.The effect of turbidity on L_z is studied categorizing available data in three specified groups of T_v, the visual or luminous/illuminance turbidity. Best fits with 5th degree polynomials connecting log (L_z) against (α) are given for: T_v<3, 3<T_v<5, and T_v>5.     

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.     

Spectral selectivity of black and green painted surfaces

Black, green and mixed paints were prepared from organically modified siloxane resin. The solar absorptance (a_s) of prepared black paint/metal coatings was 0.90 with corresponding thermal emittance (e_T) 0.20, whereas green paint coatings did not reach satisfactory solar absorptance (a_s<0.8O). To improve the absorptance of the green coating, the black paint was admixed into the green paint. Optical properties of the prepared coatings were determined by the help of Kubelka–Munk formalism.     

Solar photoassisted catalytic decomposition of the chlorinated hydrocarbons trichloroethylene and trichloromethane

Two chlorinated hydrocarbons (trichloroethylene and chloroform), in dilute aqueous solution (50 ppm), were catalytically degraded to HCl and CO₂ in the presence of solar-illuminated undoped n-type titanium dioxide (anatase) (band gap = 3.0 eV ≈ 350 nm). The initial rate of dechlorination was essentially constant over much of the day. The TiO₂ catalyst was filtered and reused several times to check for activity loss; no singificant change in the activity of the catalyst was observed.     

Optical properties of higher and lower refractive index composites in solar selective coatings

The focus of attention in this study was the choice of material for optically solar selective coatings on the basis of their optical constants. A computer programme which calculates the optical constants, solar absorptance at air mass (AM)-2, α, and thermal emittance at 300 K, , of the 200-nm-thick selective coating on the assumption of both the Maxwell Garnett and Bruggeman theories for the metallic volume fraction below and above 0.3 respectively, was used to design the structure of the composite films. Two systems of composite thin films of metal and dielectric were investigated experimentally, fabricated by RF and DC sputter coater and were verified with computer simulations. One system consist of lower refractive index composites such as Ni : SiO₂ and the other of higher refractive index composites such as V : Al₂O₃ in the spectral range of 0.3–20 μm. These films were fabricated on infrared reflective substrates such as nickel plated copper or aluminium. Results of the copper substrates are being presented here. For comparison and verification, tungsten, cobalt and chromium based composites, having different refractive indices, were also investigated which validated the concept of the choice of material in selective coatings. It was observed that high refractive index composites have lower reflective properties by choosing suitable metallic volume fraction in dielectric and antireflection coating. The higher value of the imaginary part of refractive index, k, is responsible for higher absorption by a factor α_λ=4πk/λ. Solar absorptance of 0.98 and 0.96 was achieved by simulation and experimental findings with less than 0.05 thermal emittance for 200 nm thick composites of V : Al₂O₃. It results that higher values of both n and k of the material are more suitable in solar selective coatings.     

Spectrally selective solar absorbers in different non-black colours

Silicon paints were prepared from yellow, ochre, dark ochre, green and blue pigments. To improve the solar absorptance, a_s, of these coatings, an existing black paint was admixed in different ratios. The optical properties of the mixed paints thus formed are expressed in terms of the Kubelka–Munk absorption and scattering coefficients in the spectral region 400–17000 nm. The scattering coefficient obtained for all paints was essentially equal. In the visible region the absorption coefficient follows the spectral characteristics of each respective colour. In the infrared absorption at 9000 nm and above 12000 nm are seen in all cases which result in a thickness-dependent increase of the thermal emittance, e_T, of the coating. The metric chroma (C_ab^*) and lightness (L^*) in CIELAB colour space were calculated for wide-angle observer in average daylight conditions. A range of non-black spectrally selective solar absorber surfaces with a_s>0.8 and e_T<0.3 have been prepared with L^*<45 and C_ab^*<10.     

High performance sputtered Ni : SiO2 composite solar absorber surfaces

A solar selective absorber can be prepared by dispersing suitably sized metallic particles in an insulating host matrix. Absorption is assisted by controlling optical interference within the composite film. Graded index thin films of metallic nickel in quartz (Ni : SiO₂) were made by co-sputtering with metal volume fractions ranging from 10–90% from top (anti reflecting coating) to bottom (base layer) of the structure, to minimise optical interference peaks. The films are 100–170 nm thick with an additional 70 nm anti-reflection (AR) coating. Coatings of different thickness, metal volume fraction and compositional gradient were investigated. Substrates were Al and Cu and films were deposited on either the bare substrate or substrates coated with evaporated nickel. The influence of substrate choice on the optical properties was studied. Films with solar absorptance, α, in the range 0.90–0.96 and thermal emittance, ,=0.03–0.14 were achieved. The dependence of these properties on thickness, film composition and gradient and substrate were determined. A computer programme which calculates the solar absorptance and thermal emittance based on the assumptions of both Maxwell Garnett and Bruggeman theories for metal fill factors below and above 0.3, respectively, was used to design the structure of the composite films. The theoretical results are not presented here.     

Optical filters from SiO2 and TiO2 multi-layers using sol–gel spin coating method

Sol–gel spin coating process is used to produce optical filters from SiO₂ and TiO₂ multi-layers. By coating the films symmetrically on both sides of the glass substrates, we designed two types of three-layer anti-reflective (AR) filters for the near–infrared region, and a nine-layer reflective filter for the near–UV region. We develop a simple theoretical model for these filters, which incorporates sol–gel film densification during the coating process, and fit it to the experimental data to extract properties of the individual layers in the coatings.     

Optical properties of electrochromic all-solid-state devices

We have investigated the optical properties of an all-thin-film electrochromic device, with a thin film of ZrO₂ acting as an ion conductor. The device also employed electrochromic layers of amorphous or crystalline WO₃ and NiV_xO_yH_z. Transmission (T) and reflection (R) spectra were recorded in the wavelength range 300–2500 nm at different intercalation levels, both for single films and complete devices. The results show that T decreases significantly upon intercalation in the WO₃ thin films as well as in the devices. The reflectance only shows minor changes.     

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.     

Design and simulation of antireflection coating systems for optoelectronic devices: Application to silicon solar cells

Reflectance calculation for various single-, double- and triple-layer Antireflection coatings (ARCs) on silicon substrate are presented. A calculation program is developed to determine the optimum thickness and the refractive index of each layer at a single wavelength for optoelectronic applications and through the visible spectrum for photovoltaic applications.Ta₂O₅, ZnS, Al₂O₃ single layer, MgF₂/Zns double layer and MgF₂/Al₂O₃/ZnS triple layer ARC systems are deposited on silicon substrate using electron beam and thermal evaporation as deposition techniques. The reflectance as a function of the wavelength of AR coating systems on silicon substrate is measured. All curves show good accordance between the theoretical and the experimental reflectance. As application in the photovoltaic field, a ZnS single-layer AR coating is evaporated on concentrator silicon solar cells. Spectral response and current–voltage characteristics are measured before and after ZnS ARC deposition to estimate the improvement of the cell performances. Short-circuit current and cell efficiency are increased by about 31% and 29.4%, respectively.