DE3002704A1 - SELECTIVE SOLAR RADIATION ABSORBER AND METHOD FOR PRODUCING THE SAME - Google Patents

Description

PATENT ADVOCATE DR. HERMANN O. TM. CIFHt - DIPTOM PHYSICIST D-8000 MUNICH 19 - FLOGGENS Γ RA S SE 17 - 1 E LE F O N: 08 9/1 7 70 61

January 24, 1980 D / M E 1296-D

Applicant:

Exxon Research and Engineering Comp.

Florham Park, N.Y. / UNITED STATES

Selective solar radiation absorber and method of manufacture

same

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Phone: Telex: Telegram: Postal check: Banks: Dcutvdic blink Munich 5215145 Zeus Cables: Munich Chaste bench Munich (089) 177061 Fax via: Zeus Palent, Konlo-No .: Munich Account No. ¹ Mf) iy> (089) 177062 (089) 376063 Munich 94854-807 Loo. No. 423.11343

PATENT ADVOCATE DR. HERMANN C. ΓΗ. DIEHL · D I PLOMPH YS I KE R D-GOOO MUNICH 19 - FLOGGENSTRASSE 17 - T E LE F O N: 0 8 9/17 70

"January 24, 1980 D / M

E 1296-D

Applicant:

Exxon Research and Engineering Comp. Florham Park, N.Y. / UNITED STATES

Selective solar radiation absorber and method of manufacturing the same

The invention is concerned with selective solar radiation absorbers according to the preamble of the main claim and method for producing the same.

The invention lies generally in the field of controlled influencing of the optical properties of solid surfaces. The invention is particularly concerned with improvements in absorbers for Solar energy conversion carriers, which are able to capture incident light effectively and at the same time To minimize radiation losses. This objective falls under the term selectively absorbing surfaces. It is known that surfaces of this type have a higher degree of efficiency than non-selectively absor-

Phone: Telex: Telegram: Postal check: liankcn: Deutsche Bank Munich 5215145 Zeus Cables: Munich Cuddle bench Munich (0X9) 1771) 61 TcIcIUx via: Zcuspiitenl, Konlo-No .: Munich Loo. No. 6946339 (OS ¹ »17 71) 62 (089) 376063 Munich 94854-807 Loo. No. 423.11343

biting surfaces.

Solar thermal energy converter panels usually contain one or two layers of glazing, one Light absorber connected to a heat collector system using a fluid such as water or air or forms part of such and insulation, which is the absorber connected to the heat collector system the only crucial component. However, since the absorber works at temperatures that are around 50 to 150 ° are above the temperature of the surrounding air, the other components are used to avoid heat loss added. The details of these components will of course depend on the particular application for which which the panels are constructed.

The use of such collectors in the home is generally related

1. The swimming pool heating

2. Space or hot water heating. 3. Solar cooling

The performance data required for these three types of collectors are very different. So amounts typically the temperature difference between the absorber and the outside air during operation Application according to 1 less than 20 C, for the application according to 2 about 30 to 60 C, for the application according to 3 around 55 to 80 C. For the collectors in the lower temperature range, the heat losses almost occur exclusively through thermal conduction and convection.

In the case of collectors for the higher temperature range, however, the hot absorber can be a remarkable one Radiate amount of heat into the air. The amount and the wavelength with which this radiation occurs vary with the absorber temperature, but for materials at temperatures of less than 100 C the radiated temperature Total energy is at wavelengths that are greater than about 4 xim.

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On the other hand, almost all of the solar radiation energy hits on the earth with wavelengths that are less than about 2 / µm. Due to the separation in wavelengths of these two energies it is beneficial to use a material that maximizes the collection of solar energy, in which it has a 'reflectivity R' which is preferably less than about 0.1 for wavelengths less than about 2 / µm and reflectivity gene that is greater than about 0.8 for wavelengths greater than about 4 µm. That means the emission of heat radiation at longer wavelengths is kept to a minimum and that the material should have an emissivity of 0.2 or less (the Emissivity e is defined as e = 1 - R) for Wavelengths greater than about 4 µm.

For absorbers that work at temperatures above 100 ° C, the thermal emission spectrum is partially superimposed on the spectrum of the incoming solar radiation. The choice of the optimal wavelength Λ Τ, at which a transition between the reflective to the absorbing properties takes place, is therefore more critical and shifted in the direction of shorter wavelengths, where Λ ^T furthermore depends on the required degree of sunlight concentration that is needed to to increase the absorber temperature to values above 100 ° C.

A number of techniques have been proposed for making selectively absorbent (or reflective) Surfaces. In this regard, reference is made, for example, to the following literature: Vol. 1, Journal of Energy, pp. 100-107 (1977); Volume 13, Journal of Vacuum Science Technology, pp. 127-134 (1976); and

B.O. Seraphin et al, "Photothermal Energy Conversion and Optical Properties of Solids "in Optical Properties of Solids - New Developments, B.O. Seraphin (ed.), North Holland (Amsterdam) (1976).

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For example, it is possible to tailor the optical properties of surfaces by adding a Applying semiconductor layer to a highly reflective substrate. This is sometimes called a "tandem stack" or "Tandem stack" means «Reference is made in this regard to Volume 19" Solar Energy ", pages 429-432 (1976). The transition between absorbent and reflective Surface is determined by the wavelength at which the semiconductor becomes transparent. The semiconductor can also replaced by a composite material such as a MgO / Au ceramic-metal mixture film (cermet) will. In this regard, reference is made to Volume 29, "Applied Physics Letters", pages 478-480 (1976). The semiconductor can be applied by various methods, such as chemical vapor deposition, Vacuum vapor deposition or as fine grains in one transparent high temperature binder. The tandem stack can also be set up the other way around by adding a Host surface, which reflects infrared light and transmits visible light, over an absorbing one Layer used. It is in this regard on "Technology Review" pages 63-71 (October / November 1976) and Volume 30 "Applied Physics Letters" pages 380-381 (1977) referenced.

Another technique used is called "multilayer interference filter stack" (multilayer interference stacks) whose effect is based on an interference between alternating metal and dielectric layers. For broadband selectivity, the layer thicknesses must must be adhered to very precisely. In this regard, reference is made to the following literature by way of example: Proceedings of Symposium on Materials Science Aspects of Thin Film Systems for Solar Energy Conversion, Tucson, Arizona, pp. 135-ff (May 1974);

H.S. Gurev et al "Chemical Vapor Deposition", 5th International Conference, J.M. Blocher (ed.), The Electrochemical Society, Princeton, N.J, pp. 667-668 (1975)? and Volume 4, "Applied Optics" pp. 917-925 (1965).

Another class consists of coatings known as "metallic blacks" will. They are typically applied by electroplating to shiny metal substrates, but they can also can be generated by vapor deposition or sputtering. Reference is made in this regard to Volume 1, Journal of Energy, pages 100-107 (1977). It finally exists also a class of materials that have large-scale or extensive surface roughness, such as it can be formed, for example, by a growth of dendrites or by etching, so that a surface arises, which is spectrally selective, i.e. which absorbs light to a great extent that is normal to it and has a low emissivity for angles away from normal. Such materials are used in conjunction with a material that is slightly spectrally selective. An example of this is dendritically grown tungsten. Reference is made in this regard to Volume 26, Applied Physics Letters "pages 557-559 (1975) referenced. To other types of surface structuring lead films with graded index (see e.g. Volume 16, "Applied Optics? pages 1438-1443, (1977)), corrugated 'or ribbed surfaces (see e.g. Volume 22, Optics Communications, pp. 421-426, (1977)) and a concept using a structure similar to that of Butterfly's eye is (see e.g. Volume 244, 'Nature "pages 281-282, h 973). Many of the absorbers discussed with a structured surface have, however, as already by L. Melamed et al in Volume 1, "journal of Energy '} page 100-107 (1977), noted that the weak point was a strong dependence of the absorption capacity on the angle of incidence the incident radiation.

The present invention is based on the object of an effective selective solar radiation absorber to accomplish. According to the invention, this object is achieved by the subject matter of the main claim.

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Preferred developments are described in the subclaims. The invention further aims to Creation of a process for the production of selective solar radiation absorbers. This task will solved according to the invention by the subject matter of the claim »Preferred developments are in the subclaims described.

The selective solar radiation absorber according to the invention contains a protected interface with a structured stepped surface design, which is characterized by cavities or depressions which have an average depth D and an average distance S, the ratio D / S being at least 7 a wavelength range over which the reflectivity of the incident radiation is less than about 5%, the range being approximately determined by the relationship 2OS - Π - 6D. Any method capable of producing such structured stepped surfaces is suitable for practicing the invention.

The stepped, textured surface of the present invention exhibits exceptionally low reflectivity, which corresponds to a high absorption, over a substantial area of the incoming solar radiation . In addition, a high reflectivity (about 0.8) in the infrared range (over about 4 / µm) is obtained at 100 ° C) ο It therefore shows collectors which structured stepped surface of the invention use a greater efficiency than those which do not - the extremely low reflectivity obtained is also substantially independent the angle of incidence of solar radiation.

-Al-

The accompanying drawings serve to further explain the invention.

Fig. 1 shows in a diagram the radiation spectrum of the sun and dependent on the absorber temperature Thermal emission curves, with intensity and wavelength plotted as coordinates are.

Fig. 2A shows in cross section an example of one according to the invention graded, structured interface.

Fig. 2B shows part of the interface in a greatly enlarged Scale.

Fig. 3 shows the dependency of the reflectivity on the characteristics of the gradation and structuring of the interface, with the reflectivity as coordinates and the wavelength are plotted.

4 shows the dependence of the calculated reflectivity of an air / glass-gold / gold graded interface on the depth of the gradations and the angle of incidence, the reflectivity being plotted as coordinates in percent and the wavelength in μτα.

Fig. 5 shows the reflection spectrum of a particle track etched Quartz glass that has a layer of tellurium, with the reflectivity and the Wavelength are plotted.

Fig. 6A shows the gradient of a composition of a gold-quartz film formed on a quartz substrate 5, wherein the volume percent and the depth are plotted as coordinates.

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Fig. 6B shows the reflectivity of the film of Fig. 6A, taking as coordinates the reflectivity and the wavelength are plotted.

In Fig. 1, curve 10 shows the radiation spectrum of the sun, curve 11 the spectrum of thermal emission, i.e. blackbody radiation at 100 ° C, curve 12 at 220 ° C, and curve 13 at 30 ° C. One recognises, that the spectrum of thermal emission is partially superimposed with the energy spectrum of sunlight. With As the absorber temperature increases, the thermal emission spectrum moves towards shorter wavelengths shifted so that the amount by which there is an overlap increases.

In the course of the present application is with the designations "Structure" and "structures" the formation of a surface in a horizontal direction, i.e. running parallel to it Direction is meant, while the terms “gradation”, “graduated” or “step depth” refer to the structuring is understood in the direction perpendicular to this. A significant amount of previous work in the field selectively reflective surfaces dealt with either structuring or grading of the surfaces. In contrast, when practicing the present invention, exceptionally low levels are obtained Reflectivity values (less than about 5%) over a significant range of wavelengths by both the structure as well as the gradation of the interface on which the solar radiation is incident can be determined.

The wavelength range is approximately 20 S STi! "- ⁶ D, where D and S relate to the formation of the surface, which means that the surface is characterized by cavities (or protrusions) of an average depth D and an average distance S.

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A D / S ratio of at least about 7 provides an area of low reflectivity and sufficient spectral latitude to accommodate the solar spectrum and is therefore preferred. The depth D should preferably be chosen such that the wavelength of the transition between the reflecting and absorbing state occurs at approximately ? \ T.

It is known that normal window glass has a reflectivity of about 4% per interface. One On the other hand, quartz surface treated according to the present invention exhibits a reflectivity of 2% per interface in the visible spectral range for a reduction of approximately 50%. Obviously increase one such a low reflectivity and a correspondingly high transmittance would be effective in one use with a solar collector system.

In Figures 2 A and 2 B is an inventive structured and graded interface 21 is shown. The interface separates a highly transparent material 20, on which the solar radiation 23 is incident, and a highly reflective material 22 from each other. The relative Thicknesses of the layers 20 and 22 are irrelevant, apart from the fact that they are used for their manufacture will depend on the method used and that if both layers 20 and 22 are very thin, the use a substrate 24 serving as a holder may be preferred. The interface 21 is formed by cavities or Depressions with a mean depth D and mean distances S are characterized.

The optically transparent cover material 20, which in the Used in practicing the invention has an index of refraction generally between 1.3 and 2.0 and a thickness ranging from about D to 1000 µm.

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The optically transparent material is a material

such as a dielectric or a semiconductor, which is substantially transparent in the thermal infrared range and a smooth surface to air
which has a reflectivity of less than about 5% with respect to the spectrum of sunlight. The preferred thickness depends on the method that
is used to produce the layer. The maximum thickness is limited by the absorption properties of the material in the wavelength range in which it radiates energy. For a material with a working range of about 100 ° C, the thermal emission band is between about 4 and 20 ^ im. The material used in layer 20 can be any of inorganic

Oxide glasses, any of the silicone rubber encapsulating materials commonly used in the electronics and solar cell industries, or any of a number of polymers used in the plastics industry
are known or any of other materials which, at practical thicknesses, are substantially transparent to the thermal emission band. For example, 0.17mm (5 mils) thick polyethylene transmits 80% of the infrared radiation and reflects less than 5%.

The optically reflective lower layer 22 used for
Infrared has a high reflectivity that
is greater than about 0.8 _; can be a metal film,
eg made of aluminum _f nickel or tungsten, which on the

Back surface of the optically transparent layer 20, or any of those in the plastics industry known conductive polymer or part of a solid metal on which the optically transparent Layer is deposited. The underlayer must be sufficiently thick to be substantially completely opaque or completely opaque. to be opaque and reflect the infrared.

BATH ORIGINAL

The typical formation of the interface can be achieved by a Diffusion of two layers into one another or by creating a rough surface in one layer or in the other layer by a variety of different measures, such as by Sputter etching, reactive sputtering, particle track etching, thermal evaporation by a low gas Printing or replication and subsequent deposition of the second layer on the first Layer.

Particle track etching is preferred because it Generally speaking, it is difficult to otherwise be so finely graded in the desired interface Generate morphology. Particle track etching can be used on a wide variety of materials will.

When investigating the damage in the area of the traces, the high-energy particles in Left behind solid bodies, it was found that the trace of damage left when passing through a high-energy Ions created by a dielectric material can be etched out. It will be related to this for example on R.L. Fleischer et al, Nuclear Tracks in Solids, university of California Press referenced 1975. Suitable dielectric materials for use in the present invention are those that have a resistivity greater than approximately

3
2 χ 10 JZ. cm. Examples of such materials are given in Table 1-1 on page 16 of the above article. They include insulators such as silicate minerals, alkali halides, insulating glasses and polymers ( J? = 10 to 10 Ji-cm),

J 3

bad insulators like MoS ₂ {ß = 2 χ 10 to 2 χ

10 _ /? _ -Cm) and semiconductors such as V ₉ O ₁ .

3 4 ^ j

Glass ( JP = 2 10 to 2 χ 10 JZ- cm). "

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By suitable adjustment in the density of the used for bombarding a dielectric surface Ions and through modifications in the etching technique can be a highly controlled surface design reach. This surface finish can have a low reflectivity surface cause. Such low reflectivity surfaces can be used in a variety of areas of application with benefit, including applications that allow permeability for energy in the near infrared range, in the range of the visible or optical spectrum and in the ultraviolet Require wavelength range in and / or through a transparent material. For etching of particle tracks Two process steps are necessary for a surface. The first step is that you exposing a surface to a collimated beam of high energy ions. The ions can split nuclei be or any of a number of elements known to those skilled in the art of particle track etching are known. Examples of sources of nuclear fission particles include

? ^ "λ y ^ R 0 ^, 0"? ΔΔ

U, U, Th, PU and cosmic rays.

Examples of other elements conventionally used in particle track etching and of the energies that are responsible for different materials are required are described in the aforementioned Fleischer article on p. 18 - 20. the Elements range from hydrogen to iron and uranium fission particles.

The particles penetrate a certain distance into the surface, leaving a trace of in their trajectory

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left damaged material. By applying a suitable chemical etchant to the surface exposing, conical bores or cavities or depressions are created that are centered around the respective traces of destruction.

The depth to which the particles penetrate the surface depend on the type of material and the energy of the high-energy radiation. The nature of the material is determined by its absorption coefficient for particle radiation, whereby, as is well known, with increasing absorption coefficient the depth of penetration decreases. Regarding the energy of the high-energy particles, there is all It is common knowledge that the greater the particle energy, the greater the depth of penetration into the material. To the Creation of an effective surface with low reflectivity in the ultraviolet, in the optical and in the near infrared range (approximately 0.1-2 jam) the penetration depth must be greater than the depth to which the surface will be etched (less than about 1 / in). To the particle track frenzy one needs such high-energy particles that this condition is easily met. Specific Values can be found in the article by L.C. Northcliffe et al "Range and Stopping Power Tables of Heavy. Ions ", Nuclear Data Tables A7, pp. 233-463 (1970). One obtains, for example, for 3 MeV Particles of 160 penetrate polymers to a depth of approximately 4 µm for polyethylene and about 5 yum for polyterephthalate (Mylar). The depth of penetration seems relatively insensitive 5 compared to the composition of the polymers.

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The ratio of etching depth to base diameter is determined by the sample material, the etchant and determined by the etching conditions »General guidelines for the choice of etching agents and etching conditions are well known for low concentrations of such traces of damage »If However, you want to get a distance between the individual tracks in which the pitch S of the an overlap of the conical depressions obtained is sufficiently small, is required one has a considerably greater density of traces of damage than is usually the case and it is in some cases it is necessary to modify the etching agents and the etching conditions in order to obtain the best results receives. In all cases, the etching times or the etchant strengths must be drastically reduced or concentrations are reduced, since you need considerably smaller wells than for conventional single particle trace detection.

For example, Table 2-2 by Fleischer et al shows a recipe for the etching process of exposed Given quartz glass, in which an immersion in 28 mol% HF for a period of one minute is required at room temperature. For the purposes of the present invention, this is done Etching too fast - A suitable etching, however, finds when using 10 mol% HF over a period of 90 seconds at room temperature. Etching with 2.5 wt.% NH.HF "in water over a A period of 6 minutes at room temperature is equivalent and somewhat safer.

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It should be noted that when the concentration of the HF solution is decreased to 1 mol%, none Particle orbit etching is observed more. (It is known that for some materials the ratio the Bahri etching speed to the etching speed in solid material of the etching strength or the medicament concentration depends. For this very weak etch the ratio is probably up to 1 ■). The conical depressions, which are produced by the HF etching, have a diameter of about 120 nm and depths of about 150 nm. This corresponds to a track / solid material etching ratio of 3.5.

It would be preferable if this ratio were significantly larger. If the ratio is larger, D for a given value of S increases and thus also the wavelength range of the window with low reflectivity. However, a ratio of 3.5 is as good as it was achieved in quartz (Fleischer et al Table 2-3). Much steeper i.e. more pointed cones (larger ratios) can be achieved in plastics, but here, as in the case of quartz, the etching rate must be reduced in order to do so manageable etching times are achieved. The concentration of the etchant and the temperature can can typically be decreased somewhat without decreasing the etch ratio, but there are limits to the depend on the material, the additives in the formulation and the impurities in the material. For example, the polycarbonates LEXAN (trademarks of General Electric, Schenectady, New York) and CR-39 (available from Homelite Corp. Wilmington, Delaware) nominally the same composition, however, they are characterized by different

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drive manufactured. In the case of particle track etching with parameters corresponding to the state of the art LEXAN material shows an irregular surface while CR-39 material has cleaner, sharper indentations having.

The distance between the wells is determined by the density of the particles with which the Samples were shot at. When the surface is used as a low-reflectivity coating for applications to be used in the field of solar energy, the particle density is sufficient

10-2 12-2

from about 10 cm to 4 χ 10 cm, so that the Distances between the pits, which determine S, are sufficiently smaller than the shortest wavelength of the light to be absorbed (approx. 0.1

The depth to which the pits are etched is then determined by the cone angle (that of the Depends on the material, the etchant and the etching conditions), whereby the depth is independent of the etching time, because as soon as the individual cones overlap, the surface remains constant. The surface design is expediently measured by taking transmission electron microscope images a shadowed acetate impression of the surface.

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ΛΤΓ

-2α-

If a surface with a low reflectivity is used for solar technology applications, where λ max = 2 pm and ^ min = 0.3 / Ίη, the depressions are preferably at least a factor of 7 deeper than they are wide at their base (this corresponds to a path - against solid material etching ratio of approx. 15) so that a cone angle of less than approx. 4 ° is created. This condition can easily be achieved with most plastics and minerals. The cone angles in quartz and other oxide glasses are somewhat larger, which significantly reduces the width of the spectral range ^ f \ mxx \ to λ max of the window with low reflectivity.

In order to generate a surface that absorbs high temperature solar radiation energy, a strongly networked one is preferably used Plastic used. Examples are polyacrylonitrile, polyvinylidene chloride and rayon. The plastic becomes one exposed to a collimated beam of high energy ions. The total flow to create conical depressions 0.1 m in diameter is about 10 cm-2, which, as mentioned above, is the necessary requirement that the bases of the wells overlap and no significant flat unetched surface areas remain. The ions should penetrate deeper than the etchant so that the depressions have pointed lower ends. 4 MeV Ions with a penetration depth of about 5 µm are suitable.

The heavily damaged areas created in the trajectory of the ions are etched considerably faster than the undamaged areas Plastic, so that you can etch conical depressions, the depth of which is several times the base diameter is as described above.

If required, the plastic can be converted into a material that is stable at high temperatures, by carbonization via slow heating, as is the case in the production of "high modulus" carbon fiber This technique is well known and educates not part of this invention.

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The general procedure is as follows:

First, it is heated in oxygen in order to achieve the

To further cross-link molecular structure. Heating in inert gas takes place at around 250 ° C. When reaching around 2000 ° C the material is essentially pure carbon. After during the heating process a noticeable glass formation and shrinkage takes place if the plastic should be as thin how to be practically manageable, so that one obtains a slight diffusion and only low tension. A suitable one Thickness range is between about 0.0254 mm and 0.254 mm.

If the surface is to be used at a temperature that could burn it, it can It may be desirable to let the carbon react with silicon vapor, so that a refractory SiC layer arises. This approach can be used with any of a number of materials, including any one of a number of materials React to achieve a refractory carbon compound, SiC being a good example. Care must only be taken that the reacted layer does not become so thick that the structuring of the Surface clogs. A thickness of the order of 10 nm would be sufficient for this. After processing, the surface is attached to a suitable substrate with a high temperature stable adhesive will.

With a view to a long service life of the surface it is preferred to ensure that the interfacial energy between the two layers, i.e. between the optically transparent and the reflective layer is considerably smaller than the surface free energy the two layers separated, which has the advantage that the two layers not only protect the interface, but also stabilize. A stabilization is necessary, since the

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Roughening of the surface, the surface of which is considerably enlarged and without such stabilization the Surface tension would try to smooth the interface.

In the curve of Fig. 3, in which the reflectivity and the wavelength are plotted as coordinates, the wavelength range is shown over which the reflectivity is graded as a result of the structured Surface design according to the present invention drops to a very low level. The desired wavelength λ „at 6 D is determined by the expected

The operating temperature of the collector is determined and depends on the concentration ratio used, if a Concentrator is used. Typically, "V" ranges between about 1-3 yam. The wavelength ^ at 20 S is the minimum limit length of the solar radiation spectrum and is around 350 nm.

Found extremely low reflectivity that is shown in Fig. 3, is essentially independent of the angle of incidence of the solar radiation on the absorber. This can be seen from FIG. 4, in which the reflectivity and the wavelength are plotted as coordinates.

4 shows calculated values for the reflectivity of an air / glass-Gcld / gold structured, stepped interface as a function of both the step depth (in a range of 0.1-0.3 μm) and the angle of incidence of the incident solar radiation (perpendicular and 57 ° from the vertical). The calculations are carried out by means of a computer model of the system, which contains a dielectric layer, an interface and a metallic layer, the interface being replaced as a model by a hundred thin layers, which are mixtures of metal and dielectric, the metal content in percent by volume increases linearly with the depth of the boundary layer.

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7 04

The dielectric constants of the dielectric and metal components are taken from the literature for the specific dielectrics and metals used. the Dielectric constants of the boundary layer are calculated from the dielectric constants of the components, using a mean approximation as described in Volume 45 "Thin Solid Films" pages 19 to 29 (1977) is described. The reflectivity of this assembly composed of films is determined by a iterative technique in which the reflections from each of the 101 interfaces are summed up under careful consideration of multiple reflections. The number of boundary layers chosen is large enough in order to be able to assume that the calculation result essentially corresponds to the reflectivity of a smooth Level is equivalent. The topology of the surface is used in the choice of the approximation in the calculation of the dielectric constant the Dielektikunu-Hetall mixtures within the level taken into account.

The anti-reflective properties of the graded according to the invention structured surfaces are an obvious advantage for any material that needs to collect light. Many photo elements, in particular, have high reflectivity if their surfaces are not treated are. For example, smooth silicon surfaces have a reflectivity of approximately 36%. If the If the surface is macroscopically roughened, the reflectivity can be reduced to about 10%. If anti-reflective films are added, the reflectivity can be further reduced to only a few percent, but with a precisely applied anti-reflective film adding to the final cost of the device Book hits. A structure for which the use of thin film photo elements is contemplated includes a glass plate that forms a substrate. about this: will be a A transparent conductor is deposited as a contact and then a thin film is made of a semiconductor material applied along with the rest of the device.

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The efficiency can clearly be increased if the interface between the transparent substrate and the transparent conductor and / or the interface between the transparent conductor and the semiconductor are structured and tiered in the manner described above. Such a surface design can be achieved by putting a thin layer of dielectric and then putting the clear conductor on top a flat glass substrate is vapor-deposited. The semiconductor layer can then be deposited over the transparent conductor layer will. Evaporation of the dielectric layer and the transparent conductor when appropriate Conditions can produce the desired surface design according to the invention. It is generally known, that thermal evaporation of materials through a residual gas atmosphere for the deposition of a layer of fine particles and the desired rough surface, area supplies. In this regard, for example, on volume 38 "Thin Solid Films" pages 109 to 115 (1976) referenced. Such a surface is stabilized by means of a transparent overlying one Layer achieved in the manner described above. Alternatively, other techniques such as For example, particle track etching can be used to produce the desired surface design. Man can therefore have the desired surface shape at the interface between the glass and the transparent Produce conductors and / or at the interface between the transparent conductor and the semiconductor.

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Examples: Example 1
A 1 µm thick Te layer was applied to microscope slide glasses in about 10 seconds with one press

-4

evaporated from about 1.33 χ 1o Pa, so that the desired Surface shape was obtained. The substrates were during the evaporation processes at one temperature

held at about 120 ° C. Then the slides were Glasses covered with polystyrene by immersing them in a solution of polystyrene in INHIBISOL (trademark from Penetone, Tenafly, NoJ.). An uncoated slide glass had an absorbency for the solar spectrum of about 0.95 or a reflectivity of about 5% and a thermal emissivity at 100 C at 0.39. The coated slide glass had an absorbance of about 0.9. The emissivity of these films was relatively large due to the small thickness of the Te ~ layer. She was in Infrared partially transparent.

Example 2

A Te layer about 1 µm thick was evaporated onto aluminum foil substrates in about 10 seconds at a pressure of about 4 10 Pa in order to achieve the desired surface shape. The substrates were kept at a temperature of about 170 ° C. during the evaporation process. The substrates were then coated with polystyrene to a thickness of about 420 nm by immersing them in a solution of polystyrene in INHIBISOL. The uncoated substrate had a solar spectrum absorbance of about 0.95 and a reflectance of about 5%, respectively

5 thermal emissivity at 100 ° C of 0.23.

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Various coated substrates have been used measured. They all had absorptivities of 0.93 and emissivities ranging from 0.16 to 0.25 on. The uncoated film was very brittle. It would have easily wiped off the slide can be. The coated film was smooth and sufficiently strong. By gently rubbing it was they did not attack.

Example 3

In this example, a dielectric layer took over the function of the holder. It was a quartz glass slide with a thickness of about 3.17 mm. This was used to etch a particle path in which it was mixed with 63 MeV Cu ions in a particle density It was then etched in 2.5 wt% ammonium difluoride in water at room temperature for about 6 minutes, and an approximately 1 jam layer of T 'was applied to the etched surface of the slide evaporated in a good vacuum (room vacuum of about 1.33 × 10 Pa). During the evaporation, the slide was kept at a temperature of about 140 ^° C. The reflectivity of the interface was below 3% at about 500 nm and increased Values of over 20% at 2.5 the slide did not have high reflectivity in this area. There are actually no materials that are 3.17mm thick in this area that are transparent. In practice the dielectric layer would be very much thinner. The spectral reflectivity of this

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Slide is shown in FIG. The curve 51 shows the reflectivity of the smooth surface of Te, curve 50 shows the reflectivity of the structured surface (when looking through the glass onto the Glass / TE interface looks, as well as including the Ref lexionsveritiogens the air / ^ las boundary area of about 4%) while curve 52 finally shows the lack of scatter due to the fine structuring. This is completely different from, for example, surfaces made of dendritic tungsten, which no directional component with respect to the reflected light but reflect diffusely overall.

Example 4

In this example, the substrate was a microscope slide on which Au-SiO ₂ was applied together by cathode sputtering, a sputtering target being used which consisted of a SiO ₂ disc with a diameter of 15.24 cm, on which one Gold disc with a diameter of 6.35 cm was applied, the center of which was located approximately at a distance of 7.62 cm from the center of the SiO ₂ disc. The slide was placed directly under the gold. In this position the substrate received a film with approximately 50 volume percent gold. After creating a layer with a thickness of 100 nm, the slide was slowly shifted in the direction of the far side of the SiO disk, this being done until material with a thickness of a further 100 nm was deposited, the composition of which only contained 10% by volume of gold “(See Fig. 6 A). The resulting film exhibited selective behavior. Its spectral reflectance is shown in Fig. 6B. He showed an absorption

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BATH ORIGINAL

E 1296-D

like for a solar spectrum with air mass 2 of 0.91 and a thermal emissivity (T = 100 C) of 0.34. The emissivity was increased as the stage only contained 50 vol.% Gold on the bottom. A stage that extends up to 100% gold would have a remarkably lower emissivity.

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Claims (16)

Claims 1 J Selective solar radiation absorber characterized by at least one stepped, structured interface (21) in which the depressions or the projections have a mean depth or height D and mean distances S, D and S are chosen so that for the interested Range of wavelengths ? \ Of the incident radiation for the purpose of achieving a reflectivity of less than approx. 5% the relationship 2OS - Λ ^ 6D is applicable. 2. Selective solar radiation absorber according to claim 1, characterized in that the interface (21) is protected. 3. Selective solar radiation absorber according to claim or 2, characterized in that in the interface the ratio D / S is at least about 7. 4. Selective solar radiation absorber according to one of the preceding claims, characterized in that that the interface (21) has a first layer on which the solar radiation impinges and which at least over the wavelengths of black body radiation at a certain temperature is essential is transparent and has a reflectivity of less than about 5% in the spectrum of solar radiation, of a second layer (22) which is essentially reflective over the wavelengths of the same radiation is. 5. Selective solar radiation absorber according to claim 4, characterized in that the interface energy is smaller in the interface (21) is separated as the surface free energy of the two layers. [naked 6. Selective solar radiation absorber according to one of the The preceding claims, characterized / in that 6D is between approximately 1 to 3 µm. 7. Selective solar radiation absorber according to one of the preceding claims, characterized in that that 2OS is approximately 350 nm. 8. Selective solar radiation absorber, marked by at least one protected, stepped structured interface, which is defined by depressions is characterized by a mean depth D and mean distances S, the boundary surface being one Specifies the wavelength range over which the reflectivity for the incident radiation is below about 5% and the wavelength range by the relationship 2OS ^ ? \ ^ 6D is fixed, where A is the observed wavelength of the incident radiation. 9. Selective solar radiation absorber according to claim 8, characterized in that the Wavelength at 6D is in the range of about 1 to 3 ^ / m. 10. Selective solar radiation absorber according to claim 7 or 8, characterized in that the wavelength at 2OS is approximately 350 nm. 11. A method for producing a selective solar radiation absorber, in particular according to one of the preceding claims, characterized in that at least one stepped structured interface is provided with depressions or projections which have a mean depth or height D and mean distances S, wherein D and S are chosen so that the relationship 2OS - A ^ 6D applies to the wavelength range of interest of the incident radiation in order to achieve a reflectivity of less than approx. 5%. 030032/0706
BATH ORIGINAL ". '. si 12. The method according to claim 11, characterized marked that the interface is provided with a protection. 13. Process for the production of a selective solar radiation absorber / characterized in that at least one protected, graded, structured interface formed by depressions of a medium depth D and medium distances S is characterized, the interface having a wavelength range over which the reflectivity of the incident radiation is less than approximately 5% and the wavelength range by the relationship 2OS - ?) ^ 6D is determined, where f \ means the observed wavelength of the incident radiation. 14. The method according to any one of claims 11 to 13, characterized in that in the Interface a ratio of D / S of at least about 7 is produced. 15. The method according to any one of the preceding claims 11 to 14, characterized in that the interface between a first layer and a second layer is formed, the first layer at least over the wavelengths of the black body radiation is transparent at a certain operating temperature and has a reflectivity of less than about 5% in the Has solar radiation spectrum »and the second layer being substantially over the wavelength of the same Reflects radiation. 16. The method according to any one of claims 11 to 15, characterized in that the depressions or projections are etched by particle track formed by bombarding a dielectric material with particles of high energy so that through Damage defined traces arise and this 030032/0706 'BADORiGlNAL 1 Traces of damage to the formation of depressions with a medium depth D and medium distances S to be etched. 030032/0706