Photovoltaic module comprising a concentration optic with subwavelength patterns and solar generator for satellite comprising said module

Abstract

A photovoltaic module comprises at least one photovoltaic cell and one concentration optic device, to be illuminated by a light flux emitting at at least one illumination wavelength belonging to a band of wavelengths defined by a minimum wavelength and a maximum wavelength, the band of wavelengths being that of the solar radiation of the order of [380 nm-1600 nm]. The concentration optic device is a monolithic component and comprises at least one diffractive structure comprising subwavelength patterns, defined in a structured material; the patterns having at least one dimension less than or equal to the average illumination wavelength divided by the refractive index of the structured material; the patterns being separated from one another by subwavelength distances, defined between centres of adjacent patterns; the concentration optic device ensuring at least one focusing function and one diffraction function. A solar panel comprising the photovoltaic module is also provided.

Claims

1. A photovoltaic module comprising: at least one photovoltaic cell; and at least one concentration optic device configured to be illuminated by a light flux emitting at at least one illumination wavelength belonging to a band of wavelengths defined by a minimum wavelength and a maximum wavelength, said band of wavelengths being of solar radiation on an order of 380 nm to 1600 nm, wherein said at least one concentration optic device is a monolithic component having a first surface and a second surface, said at least one concentration optic device comprising at least one diffractive structure on said first surface and a focusing structure on said first surface, each surface operating in transmission, said at least one diffractive structure having one level subwavelength patterns with at least one dimension less than or equal to an average illumination wavelength divided by a refractive index of said at least one concentration optic device, said subwavelength patterns being separated by subwavelength distances between centers of adjacent subwavelength patterns, wherein said one level subwavelength patterns codes a variation of effective index in the plane of said first surface, and said subwavelength distances are periodic, and wherein the focusing structure comprises a second diffractive structure comprising subwavelength patterns.

2. The photovoltaic module of claim 1, wherein the concentration optic device comprises a diffractive structure comprising the subwavelength patterns and a refractive optic.

3. The photovoltaic module of claim 1, wherein the concentration optic device comprises diffractive structures on the second surface.

4. The photovoltaic module of claim 2, wherein said refractive optic comprises at least one curved surface refractive lens.

5. The photovoltaic module of claim 2, wherein said refractive optic comprises at least one Fresnel lens having a set of microprisms having a length of approximately a few hundred microns.

6. The photovoltaic module of claim 2, wherein the refractive optic comprises the same material as the subwavelength patterns.

7. The photovoltaic module of claim 1, wherein said at least one photovoltaic cell comprises at least one photosensitive material being sensitive to said band of wavelengths.

8. The photovoltaic module of claim 1, wherein said at least one photovoltaic cell comprises one of: a vertical multijunctional structure having a stack of materials, each of said materials being at least sensitive in a subband of wavelengths of said band of wavelengths; and a horizontal multijunctional structure having a set of individual adjacent photovoltaic subcells in a plane comprising materials sensitive in subbands of wavelengths of said band of wavelengths.

9. The photovoltaic module of claim 8, further comprising photovoltaic subcells having different sizes relative to one another.

10. The photovoltaic module of claim 1, wherein said at least one photovoltaic cell comprises a horizontal multijunctional structure having a set of individual adjacent photovoltaic subcells in a plane comprising materials sensitive in subbands of wavelengths of said band of wavelengths.

11. The photovoltaic module of claim 1, wherein said at least one photovoltaic cell comprises subsets of individual photovoltaic cells in different planes.

12. The photovoltaic module of claim 1, wherein the subwavelength patterns comprise pillars or holes.

13. The photovoltaic module of claim 1, wherein said at least one concentration device comprises a diffractive structure with patterns of subwavelength dimensions having a longitudinal aberration correction function and/or a lateral spectral separation function.

14. The photovoltaic module of claim 1, wherein said at least one concentration optic device comprises silica, glass, a silicon nitride type material, polydimethylsiloxane, poly(methyl methacrylate), or polycarbonate.

15. The photovoltaic module of claim 1, wherein the dimensions of the subwavelength patterns are on an order of 80 nm to 250 nm in width or diameter, and 1 to 2 μm in height.

16. The photovoltaic module of claim 1, wherein the at least one photovoltaic cell comprises a set of photovoltaic cells.

17. A photovoltaic module comprising: at least one photovoltaic cell; and at least one concentration optic device configured to be illuminated by a light flux emitting at at least one illumination wavelength belonging to a band of wavelengths defined by a minimum wavelength and a maximum wavelength, said band of wavelengths being of solar radiation on an order of 380 nm to 1600 nm, wherein said at least one concentration optic device is a monolithic component having a first surface and a second surface, said at least one concentration optic device comprising at least one diffractive structure on said first surface and a focusing structure on said first surface, each surface operating in transmission, said at least one diffractive structure having one level subwavelength patterns with at least one dimension less than or equal to an average illumination wavelength divided by a refractive index of said at least one concentration optic device, said subwavelength patterns being separated by subwavelength distances between centers of adjacent subwavelength patterns, wherein said at least one concentration device comprises a diffractive structure with patterns of subwavelength dimensions having a longitudinal aberration correction function and/or a lateral spectral separation function, and wherein said at least one concentration device comprises a central zone and outer zones, each zone being made up of patterns of pillars having round or square sections and/or hole types with round or square section, the dimensions of said patterns varying from one zone to the other so as to code a variation of an effective index and therefore a phase variation.

18. The photovoltaic module of claim 16, wherein the subwavelength patterns have at least one dimension that is less than said average illumination wavelength divided by the refractive index of the concentration optic device.

19. The photovoltaic module of claim 16, wherein the concentration optic device comprises a set of microlenses, each microlens being coupled to a diffractive structure comprising patterns of subwavelength dimensions.

20. The photovoltaic module of claim 16, wherein the concentration optic device comprises a set of Fresnel lenses with microprisms, each Fresnel lens being coupled to a diffractive structure comprising patterns of subwavelength dimensions.

21. The photovoltaic module of claim 1, wherein the at least one diffractive structure comprises two diffractive structures comprising subwavelength patterns, one of the two diffractive structures providing a focusing function.

22. A solar panel for a satellite comprising at least one photovoltaic module according to claim 1.

23. The photovoltaic module of claim 17, wherein the patterns of pillars have lateral dimensions less than or equal to 350 nm, a spacing between centers of two consecutive zones being approximately 350 nm, and heights of the patterns being between approximately 1 micron and 10 microns.

24. A photovoltaic module comprising: at least one photovoltaic cell; and at least one concentration optic device configured to be illuminated by a light flux emitting at at least one illumination wavelength belonging to a band of wavelengths defined by a minimum wavelength and a maximum wavelength, said band of wavelengths being of solar radiation on an order of 380 nm to 1600 nm, wherein said at least one concentration optic device is a monolithic component having a first surface and a second surface, said at least one concentration optic device comprising at least one diffractive structure on said first surface and a focusing structure on said first surface, each surface operating in transmission, said at least one diffractive structure having one level subwavelength patterns with at least one dimension less than or equal to an average illumination wavelength divided by a refractive index of said at least one concentration optic device, said subwavelength patterns being separated by subwavelength distances between centers of adjacent subwavelength patterns, wherein said one level subwavelength patterns codes a variation of effective index in the plane of said first surface, and said subwavelength distances are periodic, and wherein the at least one diffractive structure comprises two diffractive structures comprising subwavelength patterns, one of the two diffractive structures providing a focusing function.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood and other advantages will become apparent on reading the following description, given in a nonlimiting manner and through the figures in which:

(2) FIG. 1 schematically represents a photovoltaic module of the known art comprising a photovoltaic cell and a concentration optic;

(3) FIGS. 2a and 2b respectively illustrate a photovoltaic cell comprising a multijunction structure of stacked materials and a module incorporating said cell;

(4) FIG. 3 illustrates different optical concentrator solutions operating in transmission, proposed in the prior art and incorporated in photovoltaic modules;

(5) FIG. 4 illustrates a Fresnel lens structure comprising microprisms;

(6) FIGS. 5a and 5b illustrate the chromatic effect of a diffractive Fresnel lens, on the efficiency of said lens for two different positions of the lens relative to the cell;

(7) FIG. 6 illustrates different optical concentrator solutions operating in reflection, proposed in the prior art and incorporated in photovoltaic modules;

(8) FIGS. 7a, 7b and 7c illustrate the geometrical constraints linked to the angular dimension of the sun θs and the acceptance angle α of the device, according to three different configurations;

(9) FIGS. 8a, 8b, 8c illustrate a photovoltaic module configuration for space application proposed in the prior art;

(10) FIGS. 9a and 9b illustrate examples of spatio-chromatic separation architecture according to the known art;

(11) FIG. 10 schematically represents a photovoltaic module according to the invention;

(12) FIG. 11 schematically represents a variant of the invention comprising a set of cells CV with vertical multijunctions;

(13) FIG. 12 schematically represents a variant of the invention comprising a set of cells CV comprising individual subcells arranged laterally;

(14) FIGS. 13a and 13b illustrate a simplified representation of the shadowing effect and of the drop in diffraction efficiency as a function of the deflection angle in the case of a diffraction optic comprising a microprism grating;

(15) FIG. 14 illustrates the diffraction efficiency in the scalar domain of a lens as a function of the ratio of the illumination wavelength to the design wavelength;

(16) FIG. 15 illustrates the spectral behaviour of subwavelength components compared to that of a conventional blazed diffractive lens;

(17) FIG. 16 illustrates the AM1.5 type solar spectrum corresponding to the solar spectrum after passing through the atmosphere, the sun forming an angle of approximately 48° relative to its position at the zenith;

(18) FIG. 17 illustrates an exemplary concentration optic device comprising a subwavelength grating of distributor type;

(19) FIG. 18 illustrates an exemplary concentration optic device used in a variant for the photovoltaic module of the invention;

(20) FIG. 19 illustrates a second inventive variant photovoltaic module of the invention incorporating a focusing function in a subwavelength diffractive structure;

(21) FIGS. 20a and 20b illustrate an exemplary concentration optic device used in the invention including a spectral separation function;

(22) FIGS. 21a and 21b illustrate an exemplary concentration optic device used in the invention including a longitudinal aberration correction function;

(23) FIG. 22 illustrates an example of variation of effective index with the width of the patterns, used in the exemplary concentration optic device illustrated in FIGS. 21a and 21b;

(24) FIGS. 23a, 23b and 23c illustrate a concentration optic device comprising a diffractive structure ensuring a focusing function and a diffractive structure ensuring a spectral separation function;

(25) FIG. 24 illustrates a concentration optic device comprising a single diffractive structure ensuring a focusing function and a spectral separation function.

DETAILED DESCRIPTION

(26) The subject of the present invention is a photovoltaic module comprising at least one photovoltaic cell and one concentration optic device comprising a structure comprising subwavelength patterns, defined in a so-called structured material, of which at least one dimension of said patterns is less than said average illumination wavelength (λ.sub.c) divided by the refractive index of said structured material, said concentration optic device ensuring at least a focusing function and a diffraction function.

(27) The focusing function is ensured by a refractive optic of lens type associated with a diffractive structure comprising subwavelength patterns, defined in a so-called structured material of which at least one dimension of said patterns is less than said average illumination wavelength over the illumination spectrum (λ.sub.c) divided by the refractive index of said structured material.

(28) FIG. 10 schematically represents a variant photovoltaic module of the present invention CPV.sub.o, comprising a photovoltaic cell CV.sub.o and a concentration optic device, also called concentrator, comprising the diffractive structure Smi.sub.λ comprising subwavelength patterns, associated with a refractive structure OR.sub.o having a curved surface. The solar flux FLs is picked up by the photovoltaic cell made up of vertical multijunctions.

(29) FIG. 11 schematically represents a variant photovoltaic module of the present invention comprising a set of photovoltaic cells, coupled to a set of concentration optic devices, said cells advantageously comprising vertical multijunction structures previously described and consisting, for example, of three materials sensitive in different bands of wavelengths.

(30) More specifically, each cell CVi comprises a stack of three materials sensitive in different bands of wavelengths, M1, M2 and M3, respectively sensitive in the bands of wavelengths Δλ.sub.1, Δλ.sub.2 and Δλ.sub.3. The concentration optic device comprises a set of microlenses L.sub.1, L.sub.2, L.sub.3, L.sub.4 and a set of structures of subwavelength patterns Sm.sub.1λ, Sm.sub.2λ, Sm.sub.3λ, Sm.sub.4λ.

(31) FIG. 12 schematically represents a variant photovoltaic module of the present invention comprising a set of photovoltaic cells coupled to a set of concentration optic devices, each cell CVi comprising a set of single junction subcells CVij arranged laterally, each of the three subcells represented comprising a material sensitive in a band of wavelengths different to those of the other subcells belonging to the same cell. More specifically, each cell CVi comprises a set of three subcells (three subcells are represented: CVi.sub.1, CVi.sub.2, CVi.sub.3) comprising three materials that are photosensitive in different bands of wavelengths, M1, M2 and M3, respectively sensitive in the bands of wavelengths Δλ.sub.1, Δλ.sub.2 and Δλ.sub.3. The concentration optic device comprises a set of microlenses L.sub.1, L.sub.2, L.sub.3, L.sub.4 and a set of structures associated with subwavelength patterns Sm.sub.1λ, Sm.sub.2λ, Sm.sub.3λ, Sm.sub.4λ.

(32) The benefits, performance levels and functionalities of the subwavelength optics are detailed hereinbelow. Generally, the subwavelength optics constitute a family of optics consisting of binary structures (produced using a single masking level) of a size smaller than the wavelength etched into a dielectric material whose principle is based on the synthesis of artificial material. Typically, starting from a periodic structure of a period smaller than the average illumination wavelength of the spectrum divided by the index of the structured material, it is possible to synthesize any effective index distribution, and therefore any phase distribution, by controlling the size of the microstructures. The possible phase distributions include that of a diffractive component such as a diffraction grating or a diffraction lens, also called diffractive Fresnel lens (based on the law of diffraction, and on phase jumps of 2pi or of a few modulo 2pi, unlike the Fresnel lenses with microprisms, whose principle is based on the law of refraction). These structures have properties that are particularly suitable for effective compact optic applications, notably with high aperture and/or wide spectral band.

(33) Furthermore, given the local technological mastery of the structures sizes, it is possible to control the diffracted light energy directed in different directions according to the wavelength, targeting the different photosensitive cells, sensitive to different regions of the spectrum. More particularly, it is possible to increase the light flux arriving on certain photosensitive cells which would be less effective, so as to obtain, for the different junctions, almost identical efficiencies and avoid having the lowest junction efficiency limit the current for all the junctions as a result of the series assembly. Similarly, the design of the component makes it possible to adapt the surface area of the junctions (different cell widths) to balance the efficiency of the different junctions relative to one another for an optimum series assembly.

(34) From a technological point of view, this family of optics has the advantage of having a binary profile: it requires only a single lithography step, without mask alignment unlike the multilevel optics, which reduces their final efficiency. The profiles are compatible with volume manufacturing means such as nano imprint, moulding or hot embossing, and many other techniques, the master being itself able to be manufactured with the means of the semiconductor industry (ICP etching and electronic lithography).

(35) These technologies advantageously use materials that are light and transparent in the visible such as polydimethylsiloxane PDMS, poly(methyl methacrylate) PMMA, polycarbonate PC.

(36) At the performance level, these optics exhibit a capacity to address even optics with high numerical aperture as described in the articles by: Ph. Lalanne, S. Astilean, P. Chavel, E. Cambarnd H. Launois, “Blazed.Math.binary subwavelength gratings with efficiencies larger than those of conventional échelette gratings”. Opt. Lett. 23. 1081.Math.1083 (1998) and by M S. L Lee, Ph Lalanne, J C. Rodier, P Chavel, E Cambril, Y Chen, “Imaging with blazed.Math.binary diffractive elements”, Opt. A: Pure Appl. Opt. Vol. 4, S119 (2002).

(37) They are of particular interest for the compactness of the devices, which is difficult in the case of the conventional diffractive optics, as FIGS. 13a and 13b show by respectively illustrating a simplified representation of the shadowing effect by ray tracing and the drop in diffraction efficiency as a function of the deflection angle of a diffraction grating (equivalent to the half-aperture angle of a lens). The dotted line shows the theoretical drop in diffraction efficiency of a blazed grating etched in glass (n=1.5).

(38) In reality, the drop in diffraction efficiency of the conventional diffractive lenses with high apertures is all the greater experimentally because it is difficult to produce the jumps of 2π with an abrupt transition.

(39) Furthermore, the subwavelength optics exhibit unique dispersion properties, which makes it possible to have them work effectively over a wide spectral band. Typically by structuring an AsGa material, a dispersion of the refractive index makes it possible to achieve a variation Δn=0.36 over the 8-12 μm band compared to a value of 0.02 for a non-structured material. By virtue of this property of dispersion of the effective index, it is possible to design diffractive optics whose efficiency is maintained over a wide band, as described in the article by C. Sauvan, Ph. Lalanne, M-S. L. Lee, “Broadband blazing with artificial dielectrics”, Opt. Lett. 29, 1593-1595 (2004), in the patent application US 2007/0103782, in the article by C. Ribot, PhD Thesis from University of Paris Sud (2008) or in that of M-S. L. Lee, S. Bansropun, O. Huet, S. Cassette, B. Loiseaux, A. P. Wood, C. Sauvan and P. Lalanne, “Sub-wavelength structures for broadband diffractive optics”, ICO 2005, 0602-34 (2005).

(40) This property is of great interest for the solar applications for which the spectrum is typically very wide [380 nm-1600 nm].

(41) For example, by designing a conventional blazed diffractive lens with λ.sub.0=800 nm, the spectral interval then lies between 0.5λ.sub.0 and 2λ.sub.0 as illustrated in FIG. 14 which gives the diffraction efficiency in the scalar domain of this lens as a function of the ratio of the illumination wavelength to the design wavelength λ/λ.sub.0. Even though it is possible to adjust the design wavelength to balance the spectral losses, the drop in efficiency remains significant.

(42) By way of comparison, FIG. 15 illustrates the spectral behaviour of subwavelength components compared to that of a conventional blazed diffractive lens. It emerges that the gain over the band is significant. More specifically, this figure illustrates the behaviour of the diffraction efficiency of a conventional blazed diffractive optic (dotted lines, curve C.sub.15a) and of a subwavelength optic (continuous lines, curve C.sub.15b) as a function of the illumination wavelength (λ.sub.0 being the design wavelength), in the scalar domain.

(43) Also, with these optics, the capacity to control the phase by controlling the size of the structures makes it possible not only to synthesize an optic optimized to one or more wavelengths, but also to adjust or distribute, on demand, the intensity of the different orders of diffraction, which is useful in the case of spectral separation, where the light intensity received is not the same within the spectrum.

(44) An example of AM1.5 type solar spectrum is given in FIG. 16, the AM1.5 solar spectrum corresponding to the solar spectrum after passing through the atmosphere, the sun forming an angle of approximately 48° relative to its position at zenith.

(45) An exemplary application of these components is that of the diffractive function in a refractive/diffractive achromatic system, so as to not only achromatize the focal length (diffractive function), but also to achromatize the diffraction efficiency. Thus, compared to a pure refractive solution (refractive Fresnel lens with microprisms approach), a better efficiency is obtained that is linked to a folding of the longitudinal chromatism of the focal length without suffering losses of the wideband diffraction efficiency of a conventional diffractive lens.

(46) Another example in the use of these optics is the spectral separation. They can be used as blazed grating, using the order 0 for the infrared and the order 1 for the visible and the near IR, as in the approach proposed by the Liège space centre (CSL) and described in the article by C. Michel et al., “Study of solar concentrator for space, based on a diffractive/refractive combination”, “Renewable Energy and the Environment, OSA, SM2A2 (2012).

(47) One of the benefits of the solution of the present invention thus lies in the spectrally wider diffraction efficiency without affecting the efficiency at the design wavelength (that is to say having the best efficiency) or that at the average wavelength, even in the presence of a compact system with a greater deflection angle of the order 1 (typically in the case of a×10 concentration, and a focal distance of 5 mm), which is not the case with a conventional blazed grating.

(48) In a variant of the invention, the concentration optic device can comprise a subwavelength grating of “distributor” type, for which the choice of the sizes of its constituent subwavelength structures makes it possible to engineer the angles and orders of diffraction to distribute/allocate the energy spatially and spectrally over laterally arranged single-junction cells.

(49) In a variant of the invention, the laterally arranged single-junction cells may or may not be at the same level horizontally.

(50) In a variant of the invention, the laterally arranged single-junction cells may or may not be of the same size.

(51) Studies have shown the capacity of the subwavelength components of “distributor” type to laterally separate different orders of diffraction with controlled efficiencies for a beam splitter application as described in the article by G. Bloom, Ch. Larat, E. Lallier, M-SL. Lee-Bouhours, B. Loiseaux, J-P. Huignard, “Design and optimization of a high efficiency array generator in the mid-IR with binary subwavelength grooves”, Appl. Opt. 50, 701-709 (2011) and the thesis by Guillaume Bloom: “Combinaison cohérente de lasers à cascade quantique (Coherent Combination of Quantum Cascaded Lasers)” upheld on 14 Feb. 2012, Paris University XI ORSAY. FIG. 17 illustrates an exemplary subwavelength grating of “distributor” type making it possible to create a number of orders of diffraction with controlled energies. In the case of the documents cited, the aim of the grating was to produce diffracted orders with almost equal energies, but this concept applies also to producing orders of diffraction with the desired energy distribution according to the orders of diffraction.

(52) The solution of the present invention is of particular interest for space applications. In effect, the device of the invention is a static system, not requiring any step of deployment of the concentrators (detailed previously in a prior art solution) and remains compatible with a tracking system for satellite (typically of the order of 3° for a focal length of 5 mm, for a concentration rate of 12). By proposing a very compact solution, it is also possible to reduce the volume of the panels and simplify the solution and the test means on the ground to be implemented before the launch of the satellite.

(53) Moreover, the device of the invention is based on the use of subwavelength optics that can be produced in light materials making it possible to synthesize efficient planar optics with shorter focal length.

(54) The solution can also be combined with a spectral separation technique, to increase the cell conversion efficiency.

(55) The applicant has thus solved the following problems: to have a compact system, therefore a short focusing distance, typically less than one centimetre to a few millimetres for example or less, and components of little bulk; to combine a focusing function and a chromatism correction function; to ensure a good efficiency of the concentrator over the entire spectrum addressed, by proposing association of a focusing function with very short focal length with a diffractive function with numerical aperture smaller than that of the refractive component.

(56) The different functions provided by the various elements of the architecture are: a compactness by virtue of the use of microlenses and of subwavelength optics; a better concentration optical efficiency on the photovoltaic cell through the refractive-diffractive combination, which allows for a better longitudinal achromatization by compensation of the difference between f(λ.sub.m) and f(λ.sub.M). By way of example, an initial difference of ˜350 μm approximately between f(λ.sub.m=350 nm) and f(λ.sub.M=1400 nm), for an open system with f/D=1, and f=5 mm, is brought to almost 0 between these two wavelengths by virtue of the longitudinal achromatization. More specifically, within the interval [λ.sub.m; λ.sub.M], the variation of the focal length is reduced by at least a factor 3; a better spectral efficiency by exploiting the wideband properties of the subwavelength optics.

(57) According to a variant of the invention, and this to obtain an even more compact system, the concentration optic device can comprise a refractive Fresnel lens-type system with microprisms, combined with a subwavelength diffractive lens, as illustrated in FIG. 18.

(58) According to this example, the hybrid refractive/diffractive concentrator thus comprises a refractive component consisting of microprisms and a diffractive component consisting of subwavelength structures.

(59) Advantageously, this concentration solution can be combined with a spectral separation technique, whose function can also be integrated on the diffractive face at the same time as the longitudinal chromatism correction function. The principle consists in laterally separating the different focusing points as a function of the wavelength of the solar spectrum using a grating function, integrated on the diffractive component, using single junctions of material distributed laterally and sensitive in different bands of wavelengths, as described previously. The advantage of the subwavelength structure for the implementation of the spectral separation function is the possibility of adjusting the energy directed onto each of the laterally distributed junctions and allowing for substantially different junction surface areas, for a better efficiency of the series assembly, while maintaining a good diffraction efficiency over the entire spectrum.

(60) According to a variant of the invention, the photovoltaic module can comprise a concentration optic device ensuring a focusing function, as illustrated in FIG. 19, a set of cells CVi, that can comprise subsets of cells CVij (CVi.sub.1, CVi.sub.2, CVi.sub.3 represented) such as those described in the first variant of the invention.

(61) According to a variant of the invention, the photovoltaic module can comprise a concentration optic device ensuring a focusing function and a spectral separation function, as illustrated in FIGS. 20a and 20b. The concentration optic device synthesizes the focusing function and the lateral spectral separation function, thus making it possible to obtain a very compact system.

(62) The optic device exhibits a concentration rate C (that can be equal to 12, C=12), defined as the diameter of the concentration device divided by the width of the cell.

(63) By assuming the use of Nb.sub.cell of individual subcells coupled to the concentration optic device, sensitive to different regions of the solar spectrum, juxtaposed alongside one another in batches, and of cell width, the period of the grating is given by the relationship

(64) $\Lambda =\frac{{\lambda }_{\mathrm{Max}}-{\lambda }_{\min }}{2\sin \left(\mathrm{atan}\left(\frac{{\mathrm{Nb}}_{\mathrm{cell}}.Math.D}{2f.Math.C}\right)\right)}.$

(65) For a component designed in its order 1 of diffraction, the period depends only on the ratio f/D and on the concentration rate and on the number of spectral “pathways” or individual subcells per concentrator.

(66) The table below gives examples of structures, assuming

(67) Nb.sub.cell=3 and provides period values expressed in microns, that can use an open silica lens with f/D=1 (focal length/diameter), for example with a diameter of approximately 5 mm.

(68) TABLE-US-00001 C f/D Λ 9 1 3 9 0.5 1.6 12 1 4 12 0.5 2.1 20 1 6.7 20 0.5 3.4

(69) The patterns of the diffractive structure exhibit widths of between 0 and 240 nm and heights of 2 microns.

(70) According to a variant of the invention, the concentration optic device used in the present invention can also ensure a longitudinal aberration correction function.

(71) Such a component is illustrated in FIGS. 21a and 21b and has a central zone Z.sub.c and outer zones; only the outermost, referenced Z.sub.N, is referenced in the figures.

(72) The height of the patterns h.sub.2′ is defined below

(73) $h_2^{\prime }=\frac{{\lambda }_0}{n_{\mathrm{Max}}\left({\lambda }_0\right)-n_{\min }\left({\lambda }_0\right)},$
in which n.sub.Max(λ.sub.0) and n.sub.min (λ.sub.0) correspond to the effective indexes for a diffractive structure consisting of subwavelength patterns for a component operating in its order 1 of diffraction.

(74) Typically, n.sub.Max(λ.sub.0)−n.sub.min(λ.sub.0) can be equal to approximately 0.5 for a silica-type material (n˜1.5), and 1 for a silicon nitride (n˜2).

(75) These values are calculated by a method for solving the diffraction problem in an infinite periodic structure, called Fourier modal method or RCWA (Rigorous Coupled-Wave Analysis). More particularly, the effective index in the periodic structure is given by the effective index of the Bloch mode being propagated in the subwavelength structure.

(76) By way of example, FIG. 22 gives the effective indexes of a periodic subwavelength structure consisting of pillars of square section, of 350 nm period as a function of the width of the pillars, for pillars etched in silicon nitride or in silica, for an illumination at 800 nm. The curve C.sub.22a relates to the silicon nitride, the curve C.sub.22b relating to the silica.

(77) The number of zones is given by:

(78) $N=\mathrm{floor}\left(\frac{D^2}{8{\mathrm{fd}}_2.Math.{\lambda }_0}\right),$

(79) where “floor” is the “staircase” function, that is to say that floor (x) corresponds to the integer less than or equal to x.

(80) The radius of the central zone is equal to:
r.sub.1=√{square root over (2.fd.sub.2.λ.sub.0)}.

(81) The width of the zone n is given by n, lying between 2 and N (the maximum number of zones):
width.sup.n=r.sub.n−r.sub.n−1=√{square root over (2.fd.sub.2.λ.sub.0)}.(√{square root over (n)}−√{square root over (n−1)}),
or, for the last zone
width.sup.N=r.sub.N−r.sub.N−1=√{square root over (2.fd.sub.2.λ.sub.0)}.(√{square root over (N)}−√{square root over (N−1)}),

(82) where fd.sub.2 represents the focal length of the diffractive component and depends on the constringences v.sub.1 and v.sub.2, respectively of the refractive and diffractive component which constitute the lens sought of focal length f which constitutes the concentrator.

(83) The constringences v.sub.1 and v.sub.2 depend essentially on the material constituting the diffractive component and on the wavelengths chosen by definition over the band considered, here λ.sub.min=400 nm, λ.sub.Max==1400 nm:

(84) $v_2=\frac{{\lambda }_0}{{\lambda }_{\min }-{\lambda }_{M\mathrm{ax}}},{\mathrm{fd}}_2=-f.Math.\frac{v_1-v_2}{v_2},v_1=\frac{n\left({\lambda }_0\right)-1}{n\left({\lambda }_{\min }\right)-n\left({\lambda }_{\mathrm{Max}}\right)}$

(85) TABLE-US-00002 λ λ.sub.min = 400 nm λ.sub.0 = 800 nm λ.sub.Max= = 1400 nm 400 nm-1400 nm n(λ.sub.min) n(λ.sub.0) n(λ.sub.Max) ν.sub.1 ν.sub.2 Silica 1.4701 1.4533 1.4458 18.6543 −0.8000 Silicon 2.0739 2.0102 1.9924 12.3924 −0.8000 nitride

(86) The refractive lens of the concentrator is, for its part, defined by its focal length fd.sub.1 given by:

(87) ${\mathrm{fd}}_1=f.Math.\frac{v_1-v_2}{v_1}.$

(88) By way of example, by considering an open silica lens with f/D=1 (focal length/diameter), of diameter approximately 5 mm, used for a×12 concentration rate. The diffractive lens making it possible to reduce the longitudinal chromatic aberration can consist of different diffractive Fresnel zones of a typical height of approximately 2 μm for a use in its order 1 of diffraction. The central zone is a disc of radius 441 μm and the outermost zone is a ring 39 μm wide.

(89) The patterns of the diffractive structure exhibit widths of between 0 and 240 nm and heights of 2 microns.

(90) According to a variant of the invention, the concentration optic device can comprise two diffractive structures on each of its faces, one of the diffractive structures making it possible to ensure a focusing function and one diffractive structure ensuring a spectral separation function.

(91) FIGS. 23a, 23b and 23c thus illustrate an exemplary configuration consisting of a focusing diffractive lens located on one face and a diffraction grating for the spectral separation located on the second face.

(92) FIG. 23a gives a simplified view of a cross section of the component across its diameter with a first diffractive structure S.sub.D1 for the focusing and a second diffractive structure S.sub.D2 for the spectral separation.

(93) FIG. 23b gives a plan view of the component, the grey levels representing a variation of effective index coded by virtue of the subwavelength patterns. The darker regions correspond to higher effective index levels. The lighter zones correspond to lower effective index levels.

(94) FIG. 23c gives a bottom view of the component, the grey levels representing a variation of effective index coded by virtue of the subwavelength patterns.

(95) In this example, the concentrator consists of a diffractive lens and of a diffraction grating. These two elements are located on each of the faces of a single component which serves both as concentrator and as spectral separator. Each function is produced using subwavelength patterns. The calculation of the diffractive lens, serving as focusing function, is different from that of the lens used to reduce the longitudinal chromatic aberrations, by its focal length. A typical focal length is 5 mm for a focusing function. Based on the preceding equations, and considering an open lens of silica with f/D=1 (focal length/diameter), of approximately 5 mm diameter, the diffractive lens making it possible to produce the focusing function is made up of different diffractive Fresnel zones of approximately 2 μm typical height for a use in its order 1 of diffraction. The central zone is a disc of radius 89 μm and the outermost zone is a ring 1.6 μm wide. The different zones are made up of subwavelength patterns.

(96) The patterns of the diffractive structure exhibit widths of between 0 and 240 nm and heights of 2 microns.

(97) The use of an all-diffractive subwavelength optic on its own can make it possible to produce the focusing function, despite a high aperture f/1, in a very compact system, typically 5 mm, or typically 16 times more compact than the device presented previously by the company Entech.

(98) According to a variant of the invention, the concentration device can comprise a single diffractive structure in which the two functions can be combined and implemented on a single face of the component, called bottom face, facing the photovoltaic cell, the opposite so called top face being planar. FIG. 24 thus illustrates the bottom view of such a concentration device. The grey levels represent a variation of effective index coded by virtue of the subwavelength patterns. The darker regions correspond to higher effective index levels. The lighter regions correspond to lower effective index levels (it should be noted that, for greater clarity in this figure and the preceding ones, these figures are not to scale, not showing the exact number of rings, this number in this case being higher).