Optimization of metal-based multilayers for transparent heat mirrors

Multilayer calculations by conventional matrix methods for dielectricum/noble metal/dielectricum systems on glass have been performed. The aim has been to maximize the integrated parameters for solar transmission, T_sol, and transmission of visible light, T_eye We have found that Ag is superior for maximizing the transmittance over the narrow visible wavelength range owing to the low internal absorptance. When solar energy transmission is considered multilayers based on Au are the best choice for metal film thicknesses above 15 nm. For thinner films, Ag is still to be preferred.     

Durability of porous silica antireflection coatings for solar collector cover plates

The preparation of porous silica antireflection films on glass and plastic substrates is described briefly, and it is shown to be possible to obtain solar reflectance values of 2 per cent per sheet by the use of such films. There is some increase in this reflectance value during exposure tests, but in the outdoor trials described here the average year-long increase is less than 0.6 per cent. This increase is attributed to adsorption of airborne contaminants partially filling the pores of the film, leading to an increase in the refractive index and hence in the reflectance value.     

Solution growth, characterization and applications of zinc sulphide thin films

Zinc sulphide (ZnS) thin films were successfully deposited on glass substrates under varying deposition conditions using the electroless or solution growth technique. The film properties investigated include their transmittance/reflectance/absorbance spectra, bandgap, optical constants, and thicknesses. Films grown under certain parametric conditions were found to exhibit high transmittance ( 64–98%), low absorbance, and low reflectance in the ultraviolet (uv)/visible/near infrared (nir) regions up to 1.00 μm. Those obtained under other conditions exhibited high transmittance ( 78–98%) and low absorbance ( 0.01–0.1) in the uv/visible regions but low transmittance ( 30–37 and high absorbance ( 0.56) in the nir region. These characteristics revealed their suitability for various solar device applications. Bandgap range E_g 3.7–3.8 eV and thickness range t 0.07 – 0.73 μm were obtained.     

Silica low-reflection coatings for collector covers, by a dip-coating process

A procedure for the deposition of antireflection coatings onto collector cover plates, by means of a dip-coating process using an ethanol-based colloidal silical sol, is described. By use of this process the solar transmittance of such cover plates can be increased by an increment 0.05 thus eliminating over 60 per cent of the reflectance losses. This increased transmittance increases the daily heat output for a single-glazed flat plate collector by a factor of approximately 1.1, and for double-glazed units by a factor of approximately 1.2. These antireflection films can be deposited on glass and on some types of plastic substrate. The abrasion resistance of the deposits on glass can be improved greatly by baking to 500°C. Outdoor weathering tests have shown the films probably have adequate stability for use in solar collector applications.     

Chemical vapour deposited SnO2:Sb heat mirror coatings for cylindrical solar collectors

A chemical vapour deposition technique for growth of SnO₂:Sb heat mirror coatings on the inner walls of long cover glass tubes for cylidrical solar collectors is reported. The best performance of the tin-oxide film is obtained for those films grown from a source of SnCl₂ + 1 mol% Sb on Corning glass tubes at 520°C. These films, supported on 2 mm glass substrates, have a solar transmittance of 0.85 and an infrared reflectance of 0.8. The heat mirror coatings are observed to increase the stagnation temperature of the absorber in anevacuated tubular collector from 142 to 161°C under incident optical flux of 1150 W/m²     

玻璃衬底和硅衬底沉积TZO透明导电薄膜的对比研究

利用直流磁控溅射法在室温水冷玻璃衬底和硅片衬底上制备出掺钛氧化锌(ZnO∶Ti)透明导电薄膜。SEM和XRD研究结果表明,两种衬底上的ZnO∶Ti薄膜均为为六角纤锌矿结构的多晶薄膜,且具有c轴择优取向。讨论了衬底对掺钛氧化锌透明导电薄膜光学、电学性能的影响。当玻璃衬底薄膜厚度为568nm时,薄膜电阻率达到最小值1.64×10-4Ω.cm,硅衬底上薄膜厚度为641nm时有最小电阻率2.69×10-4Ω.cm。两种衬底所制备薄膜都具有良好的附着性能,玻璃衬底薄膜样品在波长为500~800nm的可见光中平均透过率都超过了91%,硅衬底上薄膜样品的折射率约为2.05,ZnO∶Ti薄膜可以用作薄膜太阳电池的透明电极。     

Solar filters based on iron oxides used as efficient windows for energy savings

The application of windows coated with solar filters is intended to improve the interior comfort of buildings by blocking the transmittance of heat while allowing visible light to pass through. In this report we test the physical properties and thermal behavior of solar filters based on iron oxides. These filters consisted of a thin film of FeO and Fe₂O₃. This film was deposited on soda-lime glass substrates of 600 × 300 mm by rf magnetron sputtering using a high-purity Fe target (99.99%) in an argon plasma. Since the sputtering conditions and the dimensions of the equipment used facilitated the oxidation of iron, we subjected small samples of 45 × 22 mm to a heating reduction treatment in a hydrogen and nitrogen atmosphere for 10 min at 400 °C. This treatment allowed us to obtain the desired oxidation state of iron within these thin films. The solar filters had a transmittance of 30.2% for the visible light spectrum and 39.9% for the near-infrared spectrum. Furthermore, the reflectance of these filters was determined to be 17.5% and 19% for visible and near-infrared light, respectively. A simulation of thermal behavior in houses at different locations in México was performed using Energy-10 software. These data were used to determine energy savings attributed to the use of when iron-based solar filters are used on windows. These solar filters resulted in a 9–16% reduction in the amount of energy required for air conditioning (heating and cooling). In addition, a simulation was made at Mexicali BC using the commercial glass Reflectasol^®AP Tintex of Vitro Co., which concluded that the FeO-based solar filters performed better than the other tested.     

磁控溅射电致变色非晶态氧化钨薄膜

利用平面磁控反应溅射在具有透明导电膜的玻璃基片上沉积氧化钨膜层。Ｘ射线衍射分析结果表明，基片在室温状态下得到的膜呈非晶态。以０．２Ｎ浓度的ＨＣｌ为电解液，用电化学方法研究了Ｈ＋注入及抽出后氧化钨膜光学性能的变化及这种变化与膜的制备参数之间的关系。获得了沉积氧化钨膜近于最佳的工艺条件。在纯氧气氛下，溅射功率密度１．２Ｗ／ｃｍ２，溅射气体压强１．３Ｐａ时，制备的非晶态氧化钨膜，在５０次电化学循环后，漂白态与着色态的可见光透射率之差约为０．５７，其电化学循环的变色寿命也长。光电子能谱（ＸＰＳ）分析表明，Ｈ＋注入后着色态膜内出现了Ｗ５＋、Ｗ４＋。对电致变色机理也作了讨论。     

薄膜厚度对ZnO:Ti透明导电薄膜光电性能的影响

利用直流磁控溅射法在室温水冷玻璃衬底上制备出可见光透过率高、电阻率低的掺钛氧化锌(ZnO∶Ti)透明导电薄膜。SEM和XRD研究结果表明,ZnO∶Ti薄膜为六角纤锌矿结构的多晶薄膜,且具有c轴择优取向。讨论了薄膜厚度对掺钛氧化锌透明导电薄膜光学、电学性能的影响。当薄膜厚度为835nm时,薄膜具有最低电阻率3.34×10-4Ω.cm。所制备薄膜附着性能良好,在波长为500～800nm的可见光中平均透过率均超过91%,ZnO∶Ti薄膜可用作薄膜太阳电池和液晶显示器的透明电极。     

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.     

ITO/metal/ITO multilayer structures based on Ag and Cu metal films for high-performance transparent electrodes

Transparent conductive indium tin oxide (ITO)/metal/ITO multilayer electrodes have been prepared by sputtering at room temperature. Ag and Cu thin films with thickness ranging from 5 to 35 nm have been used as intermediate metal layer, between ITO coatings of about 30 nm thickness. Evolution of the optical and electrical characteristics of the multilayers as a function of each metal film thickness has been analyzed. High-quality transparent electrodes have been obtained, with sheet resistance below 6 Ω/sq for Ag film thickness above 10 nm or Cu film thickness above 16 nm. These multilayers also showed high transmittance in the visible spectral range, above 90% by discounting the glass substrate, with a maximum that is located at lower wavelength for Ag-based electrodes than for the Cu-based ones. After heating at 350 °C in flowing nitrogen, some improvement in the optoelectronical characteristics of the multilayer electrodes has been achieved that is related to the structural improvement of the ITO components.     

Structural and optical properties of textured ZnO:Al films on glass substrates prepared by in-line rf magnetron sputtering

Textured ZnO:Al films with excellent light scattering properties as a front electrode of silicon thin film solar cells were prepared on glass substrates by an in-line rf magnetron sputtering, followed by a wet-etching process to modify the surface morphologies of the films. Deposition parameters and wet etching conditions of the films were controlled precisely to obtain the optimized surface features. All as-deposited films show a strong preferred orientation in the [0 0 1] direction under our experimental conditions. The microstructure of the films was significantly affected by working pressure and film compactness was reduced with increasing working pressure, while the effect of a substrate temperature on the microstructure is less pronounced. A low resistivity of 4.25×10⁻⁴ Ω cm and high optical transmittance of above 80% in a visible range were obtained in the films deposited at 1.5 mTorr and 100 °C. After wet etching process, the surface morphologies of the films were changed dramatically depending on the microstructure and film compactness of the initial films. By controlling the surface feature, the haze factor and angular resolved distribution of the textured ZnO:Al films were improved remarkably when compared with commercial SnO₂:F films. The textured ZnO:Al and SnO₂:F films were applied as substrates for a silicon thin film solar cells with tandem structure of a-Si:H/μc-Si:H. Compared with the solar cells with the SnO₂:F films, a significant enhancement in the short-circuit current density of the μc-Si:H bottom cell was achieved, which is due to the improved light scattering on the highly textured ZnO:Al film surfaces in the long wavelength above 600 nm.     

Effect of substrate temperature on the structure and optical properties of CdTe thin film

The present work deals with the preparation, structure and optical characterization of cadmium telluride (CdTe) thin films. These films are formed by vacuum evaporation on the well-cleaned glass substrates. The compositional analyses are made by energy dispersive analysis by X-ray. The thicknesses of the samples are measured by multiple beam interferometry. The samples are prepared at different substrate temperatures. The X-ray diffraction has been employed to study the structure of the film. The structures of the samples are found to be crystalline and the crystallite size increases with the increase of substrate temperature. The d-spacing and lattice parameters of the samples are calculated and the results are also discussed. Optical characteristics of the CdTe samples have been analyzed using spectrophotometer in the wavelength range of 400–800 nm. The transmittance is found to decrease with the increase of film thickness. The transmittance falls steeply with decreasing wavelength. It reveals that CdTe films are having considerable absorption throughout the wavelength region (400–800 nm). The optical band gap energy has been evaluated from the plot of α² vs. hν. Two possible direct transitions have been observed for all the CdTe films in visible region. The observed allowed transition may be attributed due to the spin orbit splitting of the valence band.     

Properties, performance and current status of the laminated electrochromic glass of Gesimat

In the last 10 years Gesimat has developed a large-area electrochromic glazing with an advanced polymer electrolyte and tungsten oxide and Prussian Blue as complementary electrochromic layers. This electrochromic glass has a switching range between 75% and 8% visible transmittance and between 56% and 6% solar transmittance. The polymer electrolyte is based on polyvinyl butyral (PVB), a polymer in use as an interlayer for laminated safety glass since more than 60 years. The electrochromic films are deposited by a new large-area electrodeposition process. For lamination the standard methods of laminated safety glass production can be used.     

Synthesis and characterization of rough electrochromic phosphotungstic acid films obtained by spray-gel process

Rough electrochromic phosphotungstic acid (PWA) films were fabricated by spraying a gel of phosphotungstate anions with a molar ratio of P:W=1:12 onto glass substrates. The substrates were coated with transparent and electrically conducting SnO₂:F at 300°C. Analysis by X-ray photoelectron spectroscopy determined that the P:W molar ratio in the films was approximately 1:14. Infrared spectroscopy and X-ray diffraction showed that the film is a form of polycrystalline phosphotungstic acid. SEM micrographs showed that the films have a rough morphology based on fiber-shape bridges. Optoelectrochemical measurements demonstrated pronounced electrochromism in the PWA films upon H⁺ intercalation/deintercalation with a high diffuse reflectance (R_d) and transmittance (T_d). We found for as-deposited films that R_d/total reflectance (R_t) and T_d/total transmittance (T_t) at 550 nm was around 0.83 and 0.68, respectively. This ratio decreases at bleached state to 0.74 and 0.41 for R_d/R_t and T_d/T_t, respectively.     

Structural, electrical and optical properties of annealed Al/Sb multiayer films

The multilayer Al/Sb thin films were deposited on quartz glass substrates using magnetron sputtering method and annealed at high temperature to obtain AlSb films. The experimental conditions were optimized to obtain the AlSb single phase. XRD measurements indicate that high-temperature annealing is necessary to form AlSb and is helpful to the grain growth of AlSb polycrystalline. The average grain size of AlSb polycrystalline increases obviously with the increase in annealing temperature when higher than 500 °C. The electrical measurements show that the prepared AlSb films are p-type semiconductors with the conductivity activation energy of 0.21 and 0.01 eV. The optical band gap for a typical AlSb film is 1.76 eV. The obvious photovoltaic effect has been observed in TCO/CdS/AlSb/ZnTe:Cu/Au devices, which demonstrated the potential of AlSb film as the absorber layer in thin film solar cells.     

Synthesis of cobalt–silicon oxide thin films

Silicon–cobalt oxide thin films were prepared by the dipping sol–gel process. Samples with different number of dipping–annealing cycles were prepared. Some data regarding the precursor sol are given from small angle X-ray scattering characterization. Composition, structure, surface morphology and optical properties are obtained from X-ray diffraction, reflectance, transmittance, FTIR, scanning electron microscopy and EDX spectroscopy measurements. The silicon–cobalt oxide thin films prepared in this work are mostly amorphous. They have a high absorption coefficient in the visible and infrared regions. A refractive index from 2.15 to 1.79 (at 1200 nm wavelength), and a band gap between 3.73 and 3.68 eV with increasing film thickness were measured in the films. Sol–gel prepared Si–Co oxide thin films could be well suited for use in photothermal solar collectors.     

Fabrication of tin oxide film by sol–gel method for photovoltaic solar cell system

Transparent conducting tin(IV) oxide thin films have been developed with a sol–gel method, which is a low-cost process for the electrode materials of solar cell substrate. The precursor solution was made of tin isopropoxide dissolved in isopropyl alcohol. The hydrolysis rate was controlled by the addition of triethanolamine. Dipping and spin-coating technique were applied to coat tin oxide on borosilicate glass. The resistivity of the thin film was lower than 0.01 Ω-cm and the transmittance was higher than 90% in a visible range.     

CdSe thin films as solar control coatings

CdSe thin films deposited by a physical vapour deposition method were investigated as solar control coatings on architectural glazings. The optical transmittance and the near-normal specular reflectance in the range 0.40−2.40 μm and spectral distribution of reflected and transmitted intesities in the same range showed that CdSe thin films have solar control characteristics comparable to commercially available metallic coatings and other materials such as PbS and Cu_ξS films. The solar control characteristics of CdSe films were found to be dependent on film parameters, including deposition rate and deposition temperature.     

Silvered-PMMA reflectors

Metallized, flexible polymeric reflector materials are much lighter and potentially less expensive than the conventional glass/metal mirrors often used in solar thermal concentrators such as heliostats and parabolic dishes and troughs. Unweathered silvered-PMMA reflectors have a solar reflectance at least as high as glass reflectors, but their environmental durability needs to be demonstrated. ECP-305, a silvered-PMMA film available commercially from the 3M Company and developed in collaboration with the National Renewable Energy Laboratory, is the current state of the art. Important progress has been made in overcoming the three primary mechanisms causing ECP-305 to lose reflectance. These mechanisms are: (1) photon-induced silver corrosion, (2) surface soiling, and (3) a form of delamination called tunneling. Given the progress in resolving these performance lifetime issues, silvered-PMMA films should meet the reflectance and durability goals.