Thin films of crystalline LiXWO3 exhibit large increases in infrared reflectance (2-16 μm) on lithium insertion, in keeping with the Drude model of free electron reflectance. Amorphous Li_xWO₃ and weakly crystalline Li_xV₂O₅ maintain a high degree of infrared transparency for 0<x<0.5. By contrasting the infrared reflectance of a crystalline Li_xWO₃ film against an absorptive substrate, coatings with variable IR emittance can be obtained. An emittance modulation of 0.25-0.82 in the 8-12 μm region has been obtained with crystalline Li_xWO₃ on glass substrates. The design and electrochromic materials requirements for variable emittance coatings are discussed.

Optical and electrochromic properties of Ag-WO₃ and Au-WO₃ cermets with different metal filling factors are presented. The two cermet systems show entirely different optical properties which have been examined in the light of Maxwell-Garnett theory. Under low density lithium ion insertion these systems exhibit normal electrochromic coloration with the formation of tungsten bronze (Li_xW0₃). However, beyond a certain threshold density of lithium ion insertion these systems show an anomalous behavior in the optical properties. This behavior seems to be originating from the irreversible formation of lithium tungstate (Li₂W0₄).

The electrochromic behavior of phosphotungstic acid (PWA), a heteropoly compound was investigated. The electrochemical and in-situ spectroelectrochemical measurements showed that by electrochemical oxidation and reduction, phosphotungstic acid can be bleached and colored, respectively. The colorttion efficiency of PWA in an aqueous solution was found to be 16.8 cm²/C. This is lower than that observed with WO₃ films presumably due to continuous dissolution of the reduced species. The diffusion coefficient of phophotungstate anions [PW₁₂O₄₀^-3] was found to be 2.8 x 10^-5 cm²/s. The fraction of the number of protons per WO₃ molecule involved in the electrochemical reaction was determined ty the EMF measurments, and was found to be >0.2.

Tungsten oxide films of varying stoichiometry were deposited by d.c. magnetron sputtering and their optical and electrochromic (EC) properties were investigated. Spectrophotometric transmittance characteristics of the films on Corning 7059 substrates were used to evaluate optical constants and determine absorption spectra of the samples. Films on conductive transparent layers were used to study the EC behavior of the films. Film stoichiometry was deduced from the effective medium model using optical frequency dielectric constants. Oxygen deficiency in W0₃ leads to an as deposited blue color, the result of a broad absorption band at - 1μm. This broad band has been resolved into three selective absorption bands and an impurity band model is presented as an explanation. It is postulated that lithium insertion/reduction in W0₃ leads to the same color appearance by creating the same absorption bands and that electrochromic performance in the visible region can be optimized by using a film that is slightly reduced as deposited. In this case the impurity bands are already established and less lithium reduction is required to cause a visible change. It is found that electrochromic performance is optimized in films of stoichiometry around WO_2.97 and that as deposited films reduced beyond WO_2.86 are not electro-chromic.

Electrochromic crystalline W0₃ films which exhibit reversible metal-insulator transitions associated with lithium intercalation have been produced by an ion assisted deposition technique at substrate temperatures below 100°C. Electrochemical insertion of lithium in these films has resulted in a near infra-red reflection greater than 50 % .

Tungsten tri-oxide thin films were prepared by deposition from a solution of tungsten chloride, and, for comparison, by the acidification of a solution of sodium tungstate. The mechanism of the formation of HWO₃ is explained in terms of oxidation of WC₁₆ by O₂. followed by chlorination of alcohol and formation of H2 which reduces WO₃ to HWO₃. The electrochromic behavior of these films was investigated by studying their current-voltage behavior, absorbance spectra, response time, and the diffusion coefficient of lithium ions in the film.

In this study we report on our investigation of the microstructure of nickel oxide films produced by e-beam evaporation, sol-gel deposition and, sputtering techniques. We give characteristic cyclic voltammetry, current-voltage relationships, and optical transmission data for films made by each technique. Data is shown for electrodes both uncycled and cycled for 17-20 hrs. We found all samples have at least one phase corresponding to cubic nickel oxide (NaCl structure). Other phases, such as nickel hydroxide, may exist but are not immediately identifiable. The stucture of the films ranges from fine polycrystalline to amorphous. and varies over the surface of the sample. Films that were cycled for 17-20 hours all tended to have improved transmittance, as high as a 20% change. The highest transmission range from bleached to colored was for the evaporated films, which showed ΔT=60%. The sol-gel films showed a large residual coloration in the bleached state after cycling, about a 40% decline was noted. Overall, after cycling the films appeared to be slightly more crystalline. In all films after cycling there were increases in the coloration and bleaching current. Also peak shifts were noted after cycling, the coloration peak tended to shift to higher and the bleaching peak shifted to lower potentials. The overall coloration efficiency (550 nm) for these films ranged from 26-36 cm²/C.

An ITO/W0₃/Mica/IrO_x/ITO electrochromic device has been fabricated using a mica film solid electrolyte and vacuum deposition techniques. The lithium fluorhectorite film was prepared by electrophoresis of a mica sol made from a glass ceramic. Test results from the devices show ~30 points darkening in 30 sec., with bleaching occurring in a slightly shorter time at ± 2.1 v applied.

Multilayered structures were prepared by growing SiO_xN_y and SiOx films upon Al substrates. Different stoichiometries of the oxinitride layers were obtained by a reactive R.F sputtering technique. The infra-red reflectance experiments show that such multilayered structures should be better adapted for application to radiative cooling devices than single layers made with the same materials. This is explained by the presence of strongly absorbing radiative surface and interface modes which appear in the Reststrahlen frequencies because of the multilayered geometry. We performed ATR experiments so as to detect the presence of these modes and to demonstrate their correlation with the reflectance spectra.

Chemically deposited PbS, CuxS and PbS-CuxS thin films possess a range of solar control characteristics suitable for architectural window glazing applications in locations with a generally warm climate or subjected to extreme climates. The possibilities of producing large area coatings with very low capital investment is an inherent attraction of chemical bath deposition technique. This aspect makes the chemically deposited metal chalcogenide films as a low-capital alternative to the commercially available magnetron sputtered metallic coatings in the production of solar control glazings.

Black selective coatings of nickel, chrome and cobalt commercially produced by electrolytic deposition provide an enhanced operational efficiency of solar collectors as compared to those coated with ordinary black paint. High cost of production and elaborative experimental steps of the above process,however, offsets their use with respect to black paint on economic considerations. In view of this, in our laboratory efforts are being continuously made to develop techniques for depositing low cost and large area selective absorber films for photothermal applications of solar energy.

The thermal radiance from the clear sky is weak in the wavelength interval 8-13 μm. A surface material with high emittance in this wavelength interval will cool down to a temperature far below that of the ambience. Cooling powers of 100 Wm^-2 can be obtained at near-ambient temperatures. An appliction based on condensation of water from the atmospere is suggested. Initial tests show that about 400 g m^-2 can be collected in a single night.

We studied power dissipation in illuminated and non-illuminated photovoltaic cells using photothermal and electrothermal measurements as a modulated calorimetric method. We are able to identify, analyze and determine quantitatively the major power loss mechanisms due to their dominance under different bias conditions. To this end we have developed a model which we have applied to data measured under various conditions.

Light trapping plays an important role for solar cells prepared from materials having an indirect bandgap such as crystalline Si. Electronically the performance of a thin Si cell can be superior to that of a thick cell, however, it will absorb IR-light less efficiently than a thick cell. Therefore, in order to be able to profit from the higher electronic performance, one has to make a thin cell which absorbs IR-light equally well as a thick cell. For this purpose we propose a light trapping scheme based on diffraction optics. Rectangular submicron gratings with a period of 0.3 μm and a depth of less than 0.2 μm in SIPOS are shown to give optimum light trapping.

We have achieved the high quality hydrogenated amorphous silicon-germanium (a-SiGe:H) films with high photo-conductivity (σ_ph=10^-4S/cm, under AM-1) and high photo-sensitivity (σ_ph/σ_d=104) by reactive sputtering. In this paper, we present also the electron spin resonance(ESR)-studies of defects are attributed to Si- and Ge-like dangling bonds of a-SiGe:H films.

Cadmium telluride thin films were deposited on ITO, Mo and S.S. substrates by short circuiting the substrates to a cadmium foil immersed in a solution containing CdSO₄ saturated with Te0₂. These films were characterized by x-ray diffraction, scanning electron microscopy and optical absorption measurements. Deposition was carried out by changing the bath temperature. The pH of the deposition bath was around 2. X-ray diffraction data indicate that the crystallinity improves with increase of bath temperature. Optical absorption measurements have yielded a direct bandgap value of 1.47 eV.

The availability of personal computer based systems has enabled the development of a versatile setup for the testing and characterization of solar cells. Such a system acquires the data for the I-V curve which characterizes the cell and outputs the same on a monitor and in graphics plotter. The system described here consists of an IEEE-488 standard interface protocol, programmable powersupply and a programmable digital multimeter, graphics plotter and associated software. The cell parameters were calculated using 5-parameter model written in BASIC language and the power versus voltage curve of the solar cell is computed. Automated data acquisition and analysis is the salient feature of this system.

A processing line fabricating 2 1/2 feet by 5 feet amouphous silicon based monolithic modules is now operational in a limited or batch mode, and is described in this paper. In full operation, this processing line is expected to produce 175,000 modules per year or the equivalent of 10 megawatts (peak). Production cost estimates (fully loaded cost) with the plant in full operation are expected around $1.00 per peak watt. This means that a PV power station using these modules and assuming an equivalent balance-of-system cost in a high insolation region will product electricity for about 12 cents per kilowatt-hour.

A Fresnel lens's imaging performance can be seriously impaired by chromatic dispersion which typically doubles the diameter of the focused sun image in a solar concentrator. This problem can be alleviated by means of a molded diffraction grating whose diffraction-induced dispersion offsets and substantially cancels the lens's intrinsic refractive index dispersion. The grating lines would comprise a second tier of small-scale, Fresnel-type facets superimposed on the lens facets, with a typical grating facet height of about 40 microinches and a facet width ranging from about 1 milliinch at the edge of the lens to around 5 or 10 milliinches near the center. In its primary intended application the grating would function in a core daylighting system to improve the optical performance of a collector which focuses direct sunlight into fiber optic couplers. For this application chromatic dispersion would be reduced by an order of magnitude with only a 2% loss in optical efficiency.

For more than 10 years, the U.S. Department of Energy's Solar Thermal Program has pioneered the development of heliostats, mirrors that track the sun, for solar central-receiver power plants. The field of heliostats is the single most expensive part of such plants, so their cost must be as low as possible for the technology to be commercially successful. Recent efforts have focused on the development of heliostats that use stretched-membrane reflectors in place of the more familiar glass mirrors. In a stretched-membrane heliostat, metal foils are stretched over both sides of a large-diameter metal ring. The reflective surface is a silvered-polymer film glued to the front membrane. A slight vacuum in the space between the two membranes is actively controlled to provide a concave, focused contour to the mirror, and in an emergency this space can be rapidly pressurized to defocus it. Because of their simplicity and lighter weight, stretched-membrane heliostats have the potential to cost significantly less than current glass-mirror designs. The first 50-m² mirror modules, built under contract to Sandia in 1986, demonstrated that the optical performance of membrane heliostats would be at least as good as heliostats using glass mirrors. Insights gained from Sandia's testing and evaluation of the first-generation units were incorporated into the designs of two improved 50-m² mirror modules that were recently completed. Preliminary measurements show significant improvement in optical performance over the first-generation designs, especially in windy conditions.

Surface coatings with strongly angular-dependent transmittance are explored theoretically and experimentally. Theoretical studies are reported for multilayer coatings with more than one uniform metal film and for single-layer coatings with oblique columnar microstructure. The latter coatings are represented by effective medium theories encompassing prolate spheroidal structure elements. Amplitude coefficients for transmission and reflection are derived for the plane spanned by the column axis and the surface normal in uniaxial materials; these coefficients are extensions of well-known Fresnel relations. The transmittance of p-polarized light is angular selective, i.e. is unsymmetric around the surface normal, for absorbing columns. An experimental study of obliquely evaporated metal films showed that well defined inclined columns could be observed in scanning electron micrographs of fractured Cr films, and that the optical properties of such films displayed angular selectivity in qualitative agreement with theory.

Suitability of sputter deposited titanium nitride as an antireflection coating on large area, textured polycrystalline silicon solar cells is examined. The TiN films deposited on solar cells are characterised for their thickness and refractive index using a Gaertner Ellipsometer. Measurements on solar cell out-put characteristics show an enhancement in the short-circuit current in the range of 12 - 25%. It is observed that even for an optimised TiN coating thickness, the number of reflecting grains on the textured surface of the polycrystalline silicon solar cells determine the current enhancement. Spectral response measurements indicate that apart from providing very good optical coupling, TiN film also effectively passivates the solar cell surface.