A variety of optical coatings are discussed in the context of solar energy utilization. Well known coatings such as heat mirrors, selective absorbers, and reflective films are covered briefly. Emphasis is placed on the materials limitations and design choices for various lesser known optical coatings and materials. Physical and optical properties are detailed for protective antireflection films, fluorescent concentrator materials, holographic films, cold mirrors, radiative cooling surfaces, and optical switching films including electrochromic, thermochromic, photochromic, and liquid crystal types. For many of these materials research is only now being considered, and various design and durability issues must be addressed.

Transparent heat mirrors can be constructed from single- or multi-layer coatings. For single-layer mirrors wide-band-gap semiconductors are the best available materials. The required fundamental semiconductor properties are reviewed and experimental data of binary and ternary compounds discussed.

A technique of reactive d.c. magnetron sputtering with r.f. substrate bias has been evolved to give metal oxide films which exhibit heat reflecting properties while remaining highly transparent. Films of indiumtin, indium and cadmiumtin oxide have been deposited onto plastic sheet at room temperature at rates of greater than 0.5μm min.-1 Preliminary assessments of durability with accelerated weathering with exposure to high U. V. levels and high humidities have Given very encouraging results. The properties achieved with a sinale coating of about 300 nm of oxide to a 50μm thick P.E.T. sheet are visible transmittances of over 70% with heat emissivities lower than 0.3. These properties are commensurate with them providina an energy and cost effective addition to new and existing windows.

Multi-layer coatings of metal-dielectric type were prepared on a poly (ethylene terephthalate) (PET) film using chemical and physical preparation techniques. Hydrolysis of tetrabutyl titanate (TBT) followed by condensation gave rise to well-controlled transparent dielectric layers with relatively high refractive index and a silver layer was prepared by vacuum evaporation. Changes of chemical coating condition gave various types of optical characteristics. This film reduces energy dissipation from a single windowpane by around 35%.

We report on transparent and heat-reflecting Indium-Tin-Oxide (ITO) films prepared by well controlled reactive electron-beam evaporation of In₂0₃+ 9 mol. % SnO₂ onto glass in - 5x 10_-4 Torr of 02. Typical results for 0.3 pm thick films deposited on substrates at 300°C were: visible light absorptance < 2 %, thermal infrared reflectance > 90 %, and dc resistivity 3x 10-4 Qcm. Similar properties were found for substrate temperatures down to 150°C, whereas a rapid deterioration took place at still lower temperatures. After antireflection coating with -0.1 pm of evaporated MgF₂, the visible transmittance became - 1% larger than that of the uncoated glass, while the infrared reflectance increased by a few percent only. The ITO+ MgF2 coatings show much weaker iridescence than the bare ITO films.

Methods of depositing thin films on glass using the vacuum coating technic have been developed to impede the transfer of heat through glass thus reducing the energy costs for room heating or air conditioning. Heat reflecting so-called low emissivity coatings permit a maximum amount of daylight to pass through, but then block the heat that is generated when light strikes an object (greenhouse effect). They are composed of metals like silver or copper sandwiched in selected oxide films or they are transparent semi-conducting monofilms. Double glazed insulating units with coated glass achieve k-values in the order of magnitude 1,8 to 1,5 Watts per squaremeters and degree Kelvin. Maximum available trans-mittance values at lambda = 550 nm are 85% (single pane), maximum reflectance values are 93% measured at lambda = 8 wa. The corresponding emissivities are around 0,1. The investigated low-e films are stable within 1% concerning transmittance and sheet resistance changes when exposed to elevated temperatures in air of up to 150°C. Solar control films used to keep out sunheat are sputtered in a reactive gasatmosphere on. the base of titanium, stainless steel or chromium. Reflectance values of 32% are achieved at a transmission of e.g. 8%. The shading coefficient b is about 0,27. Large-scale production equipment for sputter deposition of the cited films is introduced.

Thin films. We're considering their importance and use in great quantity. But how do you get thin films?- especially in great quantity, big sizes, high quality and still cheap? "Thin" , where coatings are concerned is often defined as less than one mil or sg. "Thick" films are more than a mil or so. For optical coatings we're often in the tenth micron (1000 A, 0.004 mil) range or less.

Calculations of the transmittance and reflectance between 0.35 pm and 10 pm of semitransparent films of TiN, ZrN and HfN have been performed. The calculations are based on recently reported optical constants. They show that these compounds can be used as transparent heat-mirrors. These materials show considerable higher emittance than the noble-metals but comparable or higher visible transmittance. It is also shown that the transmittance can be increased by the technique of induced transmission.

A number of highly doped oxides - in particular Sn0₂ and In₂0₃ - have been reported to show high infrared reflectivity while being transparent to visible radiation. Their optical behaviour is based on metal-like properties i.e. high concentration and mobility of the conduction electrons and a semiconductor bandgap of about 3 eV or higher. Thin films of these oxides can be adapted for many applications and a few examples will be described. The performance of these filters strongly depends on the doping and preparation conditions. Coatings prepared by spray pyrolysis show excellent electrical and optical properties, the upper limits of which are being discussed on the basis of defect mechanisms in solids.

The agglomeration of Ni, Cr, Pt, and Rh films heated in air is discussed. Films of Pt and Rh require -1000°C before agglomeration occurs, whereas films of Cr and Ni require from 300°C to 400°C. This suggests that selective surfaces employing Pt or Rh thermal infrared reflective layers may have a long lifetime at very high temperatures. One such Pt selective surface is briefly described.

A unique approach to producing silver solar reflectors is to fire films onto substrates from metallo-organic resinates. This approach has been investigated and results are presented. The solar reflectance has been found to be a function of the type of glass and of the film thickness. Films with the optimum combination have properties that compare favorably with materials currently used. Data are presented for accelerated corrosion tests which show that films prepared by metallo-organic technology are significantly improved compared to those produced by conventional wet chemical processing.

Two methods of protecting second surface silvered glass mirrors from environmental degradation have been evaluated. One method employed silver mirrors overcoated with Al, Ni, 304 stainless steel, Cr, or an Al/Cu alloy that were prepared by ion-plating. The other method used conventional wet process silver/copper mirrors protected with a thin electroless nickel coating. These mirrors were compared with conventional paint backed silver/ copper mirrors after exposure. to elevated temperatures and water vapor. The electroless nickel coated mirrors showed consistently more resistance to these stresses than either the conventional or ion-plated mirrors suggesting that they may prove more durable for field service.*

Epoxy, urethane, anerobic, and acrylic adhesives are being investigated for laminating thin glass mirrors to aluminized steel sheet. The adhesive must withstand bending shear loads as well as stresses resulting from differential thermal expansion over the 20-year life of the solar collector. Following screening tests with one foot square laminates, panels two feet square were laminated and subjected to accelerated environmental testing while flexed into a parabolic shape. This paper discusses the effects of the processing variables and the results obtained with the various adhesives.

This selective absorber consists of a number of thin layers on a supporting substrate, including a carbonaceous absorbing layer, a catalyst layer, and a metallic infrared reflecting layer. The catalyst layer serves to catalyze the pyrolysis of a carbon containing gaseous compound to form the carbonaceous layer. Under proper conditions, the layer is black, strongly bonded to the surface, and very thin.

Development of new selective absorbers for high temperatures above 350-400°C and the application in solar farm and solar tower plants. Extensive theoretical work has been per-formed in order to examine the existing potential concerning the available materials and the possible coating methods. Concerning the materials new chemicals will be developed and their integration into high temperature photothermal stacks studied. The new materials include the rare earth metals, transition metals and the elements of the 8th group of the periodic system. The coating methods are chemical vapor deposition (CVD), techniques of catalysis, electroplating, heterogeneous reactions between a gas and a solid phase and topochemic reactions. The characterization of these selective absorber coatings will be carried out by measuring the thermal optical data (∝ , ∈)by X-ray examination, determination of the mechanical compatibility and chemical analysis.

Mo/A1₂0₃, composite coatings are shown to be useful in the fabrication of stable, effi-cient, selective solar absorbers. One such absorber deposited at 400°C showed no decrease in its solar absorptivity after 33 days in 500°C air. It was found that a higher substrate temperature during deposition can induce a surprisingly large surface roughness in these co-evaporated Mo/Al₂0₃ composites. This effect, when combined with grading the cermet composition, was used to produce solar absorptivities greater than 0.99 with a calculated thermal emissivity of 0.20 at 500°C, but it also resulted in a decrease in thermal stability in air. Regardless of the degree of surface roughness, all absorbers were stable in vacuum, to 750°C.

The developments of solar-selective absorption systems for the practical use of solar energy applications have been investigated. For the higher temperature use, 700 % 800°K, ZrCx/Zr films' with excellent heat-resistant characteristics were deposited on the stainless steel(SUS-304) pipes, 30 mm in diameter and 4500 mm in length, by reactive r-f sputtering of pure Zr cylindrical targets. The obtained collector surfaces exhibit the optical properties of a r‘, 0.93 (in visible wave length region) and E ti 0.25 (at A = 3 % 4 μm) and heat-resistant durability at ti 900°K. An absorption tube is to be settled in an evacuated glass tube-type collector, coated with ITO selective transparent film, constructed in an Al parabolic plane mirror system for the solar - thermal electric power generator in the "Sunshine project". For the lower temperature use of the solar thermal energy, below 400 ~450°K, the Cr-CrOx composite films deposited on a thin Alfoil of 40 pm thick have been developed. Cr-CrOx composite films were deposited having a graded composition by reactively evaporating Cr metals in an oxygen atmosphere of oxygen partial pressure%of 10^-2 % 10^-3 Pa with a roll-coating system. The obtained optical characteristics are a % 0.9 in visible wave length region and c (ε,' 0.1 at λ = 6 ~ 10 μm. The products showing stable operating characteristics are capable to apply on the heating and cooling of buildings and other general uses.

Thin Ni-C and NiN_x films have been produced by reactive sputtering of Ni in an argon/ methane and an argon/nitrogen gas mixture, respectively. The properties of these films are discussed. Simple models are presented for the description of the optical properties. Both Ni-C and NiN_x films exhibit an excellent spectral selectivity.

A new selective surface has been developed on nickel which combines high solar absorptivity (∝s 0.97) with emissivity at 100°C of 0.08. The surface treatment has been applied to thin nickel foil, resulting in a product capable of application to metallic or non metallic (such as concrete, glass, or plastic) substrates. Results show the durability of the surface to be at least equal to existing selective surfaces. Information on adhesives suitable for applying the foil to non-concentating solar collecting systems is given.

Research and development effort seeking to develop low cost, high performance solar selective paint has been conducted under government support for approximately eight years. Other investigators have spent many years in bringing various types of selective paint to the market place. This report is intended primarily to focus on the DOE programs that the Los Alamos National Laboratory has the responsibility of monitoring.

The possibility to use oxidized stainless steel as the absorbing surface in solar thermal collectors is evaluated. In this evaluation the influence of the Fe₂0₃, Cr₂0₃ and a mixed oxide obtained in chemical oxidation as well as the steel base, are studied separately. Conventional optical theory is used for the calculation of the resulting tandem reflectance which aiveg good agreement with related experimental curves. We can understand the high selectivity for the chemical oxide as the combined effect of the moderate reflectance of the ferritic stainless steel, a n-value as small as 1.4 and a sufficiently strong tail in k into the NIR-region for the oxide-layer.

Authors are relating some works carried out in the "Departement d'Heliophysique" and concerned with hot absorbing selective surfaces, rough surfaced, realized either by electrolytic way or chemical way, the common principal element of both being constituted by nickel. Then, electrochemical deposits are obtained directly by only one mode, that is the way for nickel-copper layers. Under chemical process, the operation includes two stages, deposition followed by chemical etching. Every samples are characterized by measuring optothermal properties (monochromatic absorptivity in the wavelength band 0.25 μm - 2.5 μm, total directional emissivity as a function of temperature) and using physico-chemical analysis via various methods : X-Rays,electron microscopy, Auger spectroscopy, Energy Disper-sive X-Rays analysis. The aim of such a study is double : - to make out interesting properties for some of these deposits (for example : a = 0.95, = 0.20) in the view of thermal conversion of solar energy in the mean temperature range (100°C < T < 200°C), - to study roughness influence upon the evolution of optical properties. In what concerns the first point, as foreseen application requiring sufficiently stable materials, the samples have been tested under temperature levels up to 200°C during a few thousands of hours, so that one can have some ideas upon ageing phenomena.

Solar absorbing Black Chrome coating have been exposed to high temperatures (350-400°C) under high solar fluxes (0.4 to 2.0 MW/mL) to test for their stability under actual operating conditions. Field tests at the White Snds Solar Furnace have shown higher stability than expected from oven tested samples. Laboratory studies utilizing spectrally selective concentrated solar simulated radiation have indicated that the cause of the higher stability under solar irradiation is photo-stimulated desorption of oxygen bearing species at the absorber surface and resultant reduced oxidation of the absorber.

Electrodeposited black chrome solar selective films degrade rapidly when heated to temperatures above 300°C in air. The application of sol-gel protective coatings to the black chrome films has been investigated as a possible means to improve the oxidation resistance of black chrome at high temperatures. The sol-gel coating process consists of applying an alcoholic solution containing polymeric glass precursors. After the coating is fired for about one-half hour at a moderate temperature (-450°C), a glass layer is obtained. Because of the wide range of sol-gel processing parameters, initial studies have concentrated on determining the effect of the following variables on the thermal stability of sol-gel coated black chrome: (1) sol-gel composition; (2) firing temperature; (3) firing atmosphere; (4) sol-gel coating thickness; (5) pre-aging of the black chrome films. Of the compositions studied, only Si0₂/B₂0₃ and Si0₂/B203/Al₂0₃/Na₂0/Ba0 resulted in improved thermal stability. Of the other processing parameters, the firing temperature had the most significant effect on the resulting black chrome thermal stability, with 500°C firing being optimum. For the best combination of process variables studied to date, the solar absorptance of a sol-gel coated sample decreased from 0.97 to 0.95 after 100 hrs at 400°C, while, for an uncoated control sample, the solar absorptance decreased to 0.89. For most of the solgel coated films, continued aging at 400°C resulted in continued protection of the black chrome film.*

Sputtered Si:H is a very promising material for use in thin-film solar cells, solar selective absorbers and optical coatings for the near infrared region. Optical property-composition relationships have been determined for Si:H coatings having wide ranges of H content and Si-H bonding. The dependence of the optical absorption edge, optical band gap and refractive index at 2 μm wavelength on H content and Si-H bonding is described. Microstructural and topographical features of the films that influence their absorption and scattering characteristics are discussed. Composition and bonding diagrams used to select deposition conditions for the desired optical properties are also presented. Finally, multilayer Si:H/Si0₂ all-dielectric laser mirrors with reflectances greater than 99% at 1.315, 2.7 and 3.8 μm are described to illustrate the application of these coatings.

Oxide coatings for solar cells deposited from organic solutions were studied. Among them, Al₂0₃ layer was found to decrease the surface recombination velocity of silicon by about one tenth. Thus new constitution was designed by inserting thin Al₂0₃ film between silicon substrate and AR coating. IR absorption and C-V characteristics indicated that the reduced surface recombination velocity corresponded to the decrease of surface state density by terminating action of H or OH. Instead of Ti0₂, Ta₂0₅ layer was introduced to improve the absorption of short wavelength. As the effect of Al₂O₃ layer the efficiency increased by about 5 %.