Electrochromic materials have been intensively studied for applications of various switchable optical systems. These materials exhibit adjustable optical absorption upon reversible oxidation/reduction processes. Since a reversible oxidation/reduction phenomenon is provided by electrically-driven electrochemical reactions, these materials are known as electrochromics. There are many publications including proceedings, books, and review articles written on electrochromic (EC) materials and their applications. This paper focuses on conventional and some new electrochromic devices (ECD), their specifications, and applications.

Electrochromic tungsten oxide thin films produced by dip-coating from a sol-gel solution are of interest for large area electrochromic window applications. The influence of the sol-gel formulation and the subsequent processing of the film required to produce uniform WO₃ films are discussed together with the effects of the dipping and processing parameters on the structure, optical properties, and electrochemical behavior of the films. The electrochemical behavior of the films has been studied using cyclic voltammetry. Electrolyte solutions, with different cations for insertion into the WO₃ layers, have been used in this work, and the resultant coloration of the films studied using spectrophotometry. Both colored and uncolored films have been studied using Rutherford backscattering spectrometry (RBS) and scanning electron microscopy (SEM). The sol-gel processing steps are shown to have a significant influence on the film microstructure and therefore the electrochemical coloration behavior of the films, as well as the lifetime of the film under repeated cycling. Results are shown illustrating the coloration behavior of the films, and the transmittance of the films over the visible and near-infrared spectra. WO₃ films approximately 0.15 micrometers thick are highly transparent and color quite uniformly, although the process is not completely reversible. There is evidence from the auger electron spectroscopy (AES) and RBS data that there is residual carbon in the films after conventional processing. Some progress has been made toward examination of the effect of this carbon on both coloration efficiency and long-term switching life of prototype devices.

Mirage detection is an interesting technique used to monitor the flux of counter-ion between an electrochromic film and a liquid electrolyte during voltammetric experiments. The mirage effect occurs when a light probe, such as a laser beam, passing through a layer of variable refractive index is deflected. It was first discovered as the result of a temperature gradient, and called `photothermal mirage effect.' It can also result from a concentration gradient due to a flux of electroactive species or counter-ions. This later case allows investigation directly in the direction of counter-ion flux between an electrolyte solution and a thin film of a material, i.e., to know whether the counter-ion exits or enters the film. The mechanism of transfer between the electrode and the solution can then be directly evidenced. This paper presents three examples of experiments, issued from the laboratory, illustrating the possibilities of the technique for studying electrochromic materials.

Thin layers of mixed oxides CeO₂ - TiO₂ and CeO₂ - ZrO₂ with molar ratio 1:1 have been prepared by sol-gel process using the dip-coating technique. The precursor sols consist of a mixture of nitrate ammonium cerium salt [Ce(NH₄)₂ (NO₃)₆], zirconium propoxide or tetraisopropyl orthotitanate, and ethanol. The thickness of the multilayer films is typically 300 nm after densification at 450 degree(s)C. Their use as transparent H⁺ and Li⁺ ions storage electrode (counter electrode) for electrochromic devices is studied by electrochemical voltammetry, impedance, and optical spectroscopies. They typically have an optical transmission of 80% in the visible range, do not color after ions insertion, and show good electrochemical stability. These layers also have been tested in complete, all solid state cell having the configuration glass/ITO/EC/electrolyte/CeO₂ - TiO₂/ITO/glass. The electrochromic layer (EC) was either WO₃ or polytoluidine. The protonic electrolyte was a cellulose-polyacetate polymer and the lithium electrolyte was PEO-LiN(SO₂CF₃)₂. Their electrochemical, optical, and lifetime performances are reported.

The results concerning the optical, electrochemical, and mechanical properties of the reversible electrochemical intercalation of cations H⁺ and Li⁺ on nonstoichiometric, anhydrous dc sputtered NiO_x films deposited onto thin (0.15 mm) ITO- covered glass plates are presented. It is shown that these properties greatly depend on the optical state of the samples prior to any electrochemical intercalation procedure and that the intercalation of cations induces a compressive mechanical stress, which is reversible both for lithium and for hydrogen cations, and is greater in the first case. Electrochromic efficiencies also are discussed for both cases.

Thin films of Ni(OH)_x prepared by an alternately dipping deposition (ADD) technique onto ITO substrates present electrochromic efficiencies of 35 cm²/c and optical density variation around 30% on bleaching/coloring cycles in 0.1 M KOH solution. The dependence of the electrochromic performance on the stoichiometry, structure, and morphology of the studied material was investigated by means of variations on those properties induced by heat treatments. The ADD technique also allowed the growth of samples in a powder form that could be analyzed easily by TGA, DSC, and XRD. The results point out that changes in stoichiometry are followed by changes in structure and that the presence of OH radicals in this material is essential for the observation of electrochromic contrast.

This paper reports on the preparation, electrical, and optical analysis of electrodes and prototype electrochromic devices using a solid polymer ion conductor. For these devices electrodes were developed consisting of cobalt-doped nickel oxide, manganese-nickel oxide, and niobium oxide. Optical and voltammetric data was obtained for each electrode. Solid polymer electrolytes were synthesized from modified amorphous poly(ethylene oxide) [a-PEO] complexed with a metal silicate. Electrochromic devices were made using cobalt-doped nickel oxide/niobium oxide, and cobalt-doped nickel oxide/manganese-nickel electrode laminations. Optical spectra as a function of voltage was obtained for each device. The best cobalt-doped nickel oxide/a-PEO/manganese-nickel oxide device showed photopic transmittance to be Tp(bleached) equals 0.76 and Tp(colored) equals 0.44. The corresponding integrated solar transmittance was Ts(bleached) equals 0.64, Ts(colored) equals 0.46. The best cobalt- doped nickel oxide/a-PEO/niobium oxide device had photopic transmittance of Tp(bleached) equals 0.65 and Tp(colored) equals 0.16. The corresponding integrated solar transmittance was Ts(bleached) equals 0.45 and Ts(colored) equals 0.15. Of the two devices, the nickel/niobium oxide device had the best combination of electrical and optical properties. Better device properties are expected with improvements in the solid polymer electrolyte and lamination process.

The electrochromic reaction of cobalt oxide thin films prepared by electrochemical deposition was studied. Chronopotentiometric and potentiodynamic experiments associated with transmittance measurements were carried out. The mass and structure changes due to the electrochromic reaction were followed using a quartz crystal microbalance and stress experiments, respectively. The stoichiometry of the anodically deposited virgin film is discussed.

Angular selective films have been reactively deposited onto an oblique glass substrate by vacuum arc evaporation. The deposition beam is magnetically filtered before striking the glass. The structure and optical properties of these films are discussed for targets of aluminum, chromium, and titanium at various partial pressures of oxygen and various final thicknesses. The most promising films are cermets, with aluminum giving superior angular selectivity for the deposition conditions studied. The angle of incidence dependence of solar and luminous transmittance is analyzed for some Al/Al₂O₃ films and their performance is compared to simple isotropic solar control films.

Angle dependent light control glass (LCG) is a new glass that scatters only incident lights from particular angles and transmits incident lights from all other angles. This unique light control function of LCG is caused by the microstructure in the specific polymer film sandwiched by a pair of glasses. The optical performance and properties described in this paper support that the principle of an angle dependent light control function can be explained mainly by diffraction due to the microstructure like a stack of various transmission volume phase gratings.

Films of SnO_xF_y and SnN_xF_y were made by reactive rf magnetron sputtering at rates up to on the order of magnitude 50 nm/min. They have refractive indices of on the order of magnitude 1.7 and are useful for antireflecting metal-oxide-coated substrates. Spectral optical data, showing the antireflection property, are given for SnO_x/SnO_xF_y and SnO_x/SnN_xF_y tandem layers.

There are several different kinds of coatings, both active and passive, for the control of the radiation flux through a window system. These entail coatings for the reduction of thermal heat losses in cold climates, and coatings for the reduction of not only the thermal part of the solar spectrum but also the visible part. Considerable effort is devoted to the study of optically switching films, for which the optical properties depend on certain conditions. When these coatings and films are to be used in the windows of buildings, scattering is usually unwanted, while in some applications strong scattering is a characteristic feature of the film. In either case it is important to be able to measure the scattered transmittance accurately. In this paper it is demonstrated that large errors can occur when diffuse transmittance is measured with an ordinary integrating sphere. A model is presented for the calculation of the true transmittance value from the measured signal. The separation into the diffuse and specular components is always instrument dependent. These components can vary considerably depending on the size and geometry of the sphere ports and on how the measurement is performed. By using two different modes of operation this difficulty can be reduced.

Plastic foils containing nonabsorbing pigments can display a high reflectance of solar radiation combined with a high transmittance in the atmospheric window region in the thermal infrared. Such foils can be applied as selective covers enabling radiative cooling of an underlying material even in direct sunlight. Extensive calculations were performed of the optical properties of nonabsorbing foils pigmented with various oxides and sulphides. The calculations were carried out by the four flux theory using input calculations for single spheres by the Lorenz-Mie theory. The optical properties of the foils were optimized for radiative cooling applications with respect to particle radius, pigment volume fraction, thickness of the foil, and refractive index of the particles. In particular, ZnS is a suitable pigment material because of its high refractive index and low infrared absorption. It should be feasible to achieve a solar reflectance of 0.9 in combination with an infrared transmittance of 0.8 to 0.85 in the atmospheric window region by foils made of a transparent matrix material pigmented with ZnS. Initial experimental studies have been performed on pigmented polyethylene foils.

TiNxOy films were deposited by activated reactive evaporation (ARE) on copper to produce a tandem solar absorber with a low emittance of 0.042 at 153 degree(s)C. The influence of oxygen and nitrogen partial pressures and the plasma current in the ARE-process were investigated. The optical selectivity of the absorber is determined mainly by the thickness and oxygen content of the coating. Increasing the oxygen partial pressure leads to higher oxygen content, lower emittance, and dielectric-like properties, i.e., small imaginary parts of the refractive index and small dispersion. Other preparation parameters are shown to have minor importance on the selective properties. The effective cut-off wavelength of the transition from high short wavelength absorptance to low long wavelength emittance can be shifted between 0.5 and 2 micrometers by changing either the thickness from 17 to 110 nm or the nitrogen to oxygen pressure ratio in the reactive gas from 1 to 200. The slope of the transition is affected by the pressure ratio.

Organic waste detoxification requires cleavage of carbon bonds. Such reactions can be photo- driven by light that is energetic enough to disrupt such bonds. Alternately, light can be used to activate catalyst materials, which in turn can break organic bonds. In either case, photons with wavelengths less than 400 nm are required. Because the terrestrial solar resource below 400 nm is so small (roughly 3% of the available spectrum), highly efficient optical concentrators are needed that can withstand outdoor service conditions. In the past, optical elements for solar application have been designed to prevent ultraviolet (UV) radiation from reaching the reflective layer to avoid the potentially harmful effects of such light on the collector materials themselves. This effectively forfeits the UV part of the spectrum in return for some measure of protection against optical degradation. To optimize the cost/performance benefit of photochemical reaction systems, optical materials must be developed that are not only highly efficient but also inherently stable against the radiation they are designed to concentrate. The requirements of UV optical elements in terms of appropriate spectral bands and level of reflectance are established based upon the needs of photochemical applications. Relevant literature on UV reflector materials is reviewed which, along with discussions with industrial contacts, allows the establishment of a database of currently available materials. Although a number of related technologies exist that require UV reflectors, to date little attention has been paid to achieving outdoor durability required for solar applications.

Dichromated gelatin layers facilitate the design and fabrication of large format (1 m²) holographic optical elements (HOE) that exhibit high optical quality and diffraction efficiency. In this paper we present the results achieved in the development and fabrication of such layers and elucidate upon their applicability as holographic solar concentrators. The objective of this report is the presentation of the experience gained in the design and manufacturing of large format spectrally selective solar concentrators. The holographic lens diffracts the white sunlight into various spectral ranges outfitted with solar cells that have appropriately selected band gaps. The purpose of the holographic concentrator is the spectral and spatial separation of the incident solar radiation in order to achieve an improved overall conversion efficiency. A number of manufacturing techniques were especially developed for the design, optimization and fabrication of the specialized holographic concentrators. The HOEs needed for the construction of the integrated collector optics are: lenses and/or lens arrays and phase-correction plates. The HOES are designed by means of computer programs that facilitate the optimization of the recording geometry and provide information for the correction procedures needed for optimized performance. The HOEs are recorded in dichromated gelatin films (DCG) and are subsequently subjected to chemical and thermal treatment processes in order to promote the desired characteristics and suppress the undesired properties. These procedures guarantee the realization of HOEs with high diffraction efficiencies and low scattering losses. The optimized holographic process: exposure, development and after-treatment, was described explicitly in previous publications. The emphasis is placed on the development of a novel copying technique for the batch reproduction and manufacturing of large format holographic lenses for solar concentrators.

The development of electrodeposited selective nickel-cobalt black coating for solar thermal energy conversion is described. Optimized electrolyte composition and operating conditions are standardized by use of Hull cell. Ammonium acetate has been used as a complexing agent to produce quality black coating with good optical properties. The present system produces black coatings in the current density range from 4 to 8 A/dm². Optimized coating possesses solar absorptance (varies direct as) of 0.96 and thermal emittance ((Epsilon) ) of 0.11.

The transduction and conversion of radiant energy into work in a quantum process are dependant on the luminescent properties of the materials involved. Materials with photoluminescent efficiencies greater than 0.1% are likely candidates for solar cells and solar converters. The luminescent optical properties of a material are directly related to the output device parameters. The chemical potential of the incoming light is a function of the photon energy and incident radiance. The amount of work per particle, or voltage, that can be extracted by a solar converter is related to chemical potential of the excitation, which can be inferred from the photoluminescence efficiency at ambient temperature. A discussion is made as to the use and optical properties of materials such as Si and GaAs, FeS₂, and biological and organic dyes as efficient solar quantum converter materials. Proper choice of absorber thickness as to maximize the luminescent output observed is shown to optimize solar converter performance.

The close spaced vapor transport (CSVT) is an efficient and cost-effective technique that allows the growth of polycrystalline as well as epitaxial thin layers of semiconductors. It has been applied to II-VI materials, especially to zinc and cadmium chalcogenides. A summary table including the deposition parameters, i.e., the nature of the ambient gas, the temperature of the source, the temperature difference between source and substrate, and the values of the growth rates measured on various substrates is presented for ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe. Experimental results concerning the growth of ZnSe on GaAs substrates are also reported. The CSVT system uses an Ar atmosphere and the working temperature is ca. 825 degree(s)C. The temperatures of source and substrate are measured during deposition and growth rates of the ZnSe films are studied as a function of the reciprocal temperature of the substrate surface for GaAs and quartz (inert) substrates. The measured values of the growth rate are compared to the theoretical ones given by the reaction-limited model and the diffusion-limited model. The validity of the models is discussed in terms of the nature of the molecules participating in the transport.

Photoelectrochemical (PEC) etching of n-InP is studied as a method to engrave relief microstructures. Experiments of PEC were performed with holographic exposures ((lambda) equals 0.4579 micrometers ) and homogeneous white light on n-InP. The triangular profile characteristic of holographic patterns recorded parallel to the <011> direction appeared even when the sample was etched using homogeneous white light. In this case deep random microstructures were obtained that present interesting anti-reflecting properties that may be useful in solar cells applications.

Studies on slurry painted CdSe_xTe_1-x photoactive films prepared from CdSe and CdTe synthesized by a low temperature wet process are reported. The films on Ti sub- strates, heat treated at 550°C in argon were polycrystalline with the bandgap increasing and room temperature conductivity decreasing with the x value. In poly- sulphide electrolyte, the films with a composition CdSe_0.5Te_0.5 gave a V_oc of 425 mV, J_sc of 8.5 mA cm^-2, FF of 0.54 and η of 3.25% at an illumination of 60 mW cm^-2.

Pure and indium incorporated titanium dioxide thin films were prepared using the spray/CVD technique. The incorporation of foreign atoms in the oxide film affects both the photovoltaic and photoelectrochemical characteristics of the heterojunction n-Si/TiO₂. The presence of In in the titanium oxide matrix up to a film thickness of 100 nm had no effect on the transmittance of the oxide in the visible region. The conductivity and bandgap energy were found to increase with In-incorporation. The increased conductivity of the indium-containing oxide films is reflected in the improved photovoltaic properties of the prepared n-Si/TiO₂- In solar cells. The photoelectrochemical properties of the prepared photoanodes revealed that the charge transfer step at the oxide/electrolyte interface leads to the deterioration of the device quality. A model for the effect of indium incorporation on the band structure of the TiO₂ semiconducting film was suggested.

Fabrication of high efficiency (approximately equals 25%) micro-groove ((mu) g) silicon solar cell (Fig. 1) has been reported. In this cell structure, the p-n junction to collect photo-carriers are parallel to the illuminated surface but the carrier movement normal to the (111) junction planes contribute to the photo-current only. A 2-D device model is therefore proposed for analysis of this cell. The optical generation rate (OGR) is obtained by a ray-tracing algorithm (RTA). By Fourier transform, a 2-D OGR formula is developed on the basis of the numerical data from the RTA. With appropriate boundary conditions and the 2-D OGR formula, the relevant transport equations are solved. Expressions for minority carrier and photo-current densities have been derived and, to evaluate the cell performance parameters, a computer code has been developed. Computer simulation shows a maximum efficiency of 25.5% at AMO, 135.3 mW/cm² and 26.4% at AM1.5, 100 mW/cm², which are well within reach of the present VLSI technology. The theoretical results obtained from the code are compared with the experimental values. Effects of cell thickness, base doping level, junction depth, and surface recombination velocities on cell performance parameters have been studied. The efficiency is found to maximize at about 100 micrometers cell thickness and at a junction depth of about 0.1 micrometers . It is found that the (mu) g-cell efficiency is less sensitive to change in junction depth. To test the proposed 2-D model, the boundary conditions and the current density expressions are compared with those of a planar cell of the same thickness. As groove angle 0 (54.75 deg for (mu) g-cells) tends to zero, all the boundary conditions and the current density expressions reduce to the corresponding ones for the planar structure.

Thermoelectric power of spray deposited transparent conducting Zinc oxide thin films has been determined from room temperature up to 200 degree(s)C with reference to pure lead, and the thickness as well as temperature dependence of its various parameters have been studied. The Fermi levels were determined using a nondegenerate semiconduction model. The carrier scattering index, the activation energy, and the temperature coefficient of activation energy have also been obtained at various thicknesses and temperatures.

Recent research progress on an electrically-tunable, variable reflectivity, completely inorganic thin film electrochromic window is discussed. Some of the properties of window cells composed of rf diode sputter-deposited electrochromic layers of cathodically-coloring tungsten oxide and of anodically-coloring lithium cobalt oxide are presented and discussed. It is highly probable that what has been learned regarding the production of such research window cells by rf diode sputtering can be transferred readily to determining the conditions needed to fabricate variable-reflectivity electrochromic windows by one or more production-worthy processes.

The fundamental electronic properties (including optical) of a solid are determined by the character of the interaction between its ionic and electronic subsystems. The state of the ionic subsystem can be changed persistently and reversibly by an external stimulus (current, light). The pertinent ion-controlled phenomena in the solids (solid electrolytes or mixed conductors) causes the ions to be inserted, extracted, transferred, or absorbed or transforms the states of the ions by redox reactions in the bulk or on the surface of the solid or on the interface of a (laminar or planar) solid ionic heterosystem. Several functions can be accomplished: (1) the optical response function of the solid or heterosystem can be controlled (variable optics) by ionic processes (redox reactions); (2) the external stimulus (current, light) or absorbed surface molecules, atoms or ions from a gas or liquid-like environment can be detected by a corresponding optical response (physical or chemical sensing); (3) the ion insertion, extraction, transfer, and storage in a solid ionic heterosystem implies energy conversion and accumulation. Redox reactions create color centers and are related to chromogenics (chemi-, thermo-, photo-, cathodo-, and electro- chromic phenomena). Solid ionic systems are sensitive to the chemical composition of the environment. Solid ionic heterosystems (electrode- electrolyte) form a three-phase interface together with the environment. A laminar heterosystem can be used for `smart windows,' large scale memory displays with small energy consumption, reversible electrochromic photography, and rechargeable solar batteries. A combination of laminar and planar heterosystems can be used for integrated optics and for special opto-electrochemical `smart sensors.' Solid state ionics constitutes the scientific and technological base for the creation and design of devices for the above mentioned applications.