EP2561550A2

EP2561550A2 - Absorbent nanoscale structure of the asymmetric mim type and method of producing such a structure

Info

Publication number: EP2561550A2
Application number: EP11717981A
Authority: EP
Inventors: Stéphane COLLIN; Jean-Luc Pelouard; Fabrice Pardo; Philippe Lalanne; Christophe Sauvan; Anne-Marie Haghiri-Gosnet
Original assignee: Centre National de la Recherche Scientifique CNRS
Current assignee: Centre National de la Recherche Scientifique CNRS
Priority date: 2010-04-23
Filing date: 2011-04-15
Publication date: 2013-02-27
Also published as: FR2959352B1; WO2011131586A2; US20130092211A1; FR2959352A1; WO2011131586A3

Abstract

According to one aspect, the invention relates to an absorbent nanoscale structure (1, 1') of the asymmetric MIM type intended for receiving a broadband incident lightwave, the absorption of which in a given spectral band it is desired to optimize, comprising a dielectric layer (10) which absorbs in said spectral band, which layer is of subwavelength thickness and is placed between a metal grating (11) of subwavelength period and a metal reflector (12). The elements (110, 120) forming the metal grating have at least one dimension (w) suitable for forming, between the metal grating and the metal reflector, beneath the elements of the grating, a plasmon resonator forming a longitudinal Fabry-Perot cavity resonant at a first wavelength of the desired absorption spectral band, and the absorbent layer has, between the metal grating and the metal reflector, at least a first thickness (t_a) suitable for forming at least a first vertical Fabry-Perot cavity resonant at a second wavelength of the desired absorption spectral band.

Description

Asymmetrical MIM type absorbent nanometer structure and method for producing such a structure

STATE OF THE ART

Technical field of the invention

The present invention relates to an asymmetric MIM-type absorbent nanometer structure having broadband light absorption, especially in the visible, and a method for producing such a structure. It applies more particularly to ultra-thin solar cells.

State of the art

It is constant to seek to reduce the thickness of the active (absorbent) layer of solar cells, or photo voltaic, especially to reduce the transit time (time taken by the electrons to reach the electrodes, generally greater than the picosecond ) and thus the recombination of induced photo charges. It is also sought to reduce the thickness of the active layer to reduce the costs associated with the material, both the cost of the material itself and the manufacturing cost generated by the treatment of a greater or lesser amount of material. Moreover, the limitation of the amount of material makes it possible to project the use of rare materials in greater time. When seeking to reduce the thickness of the active layer, one of the difficulties encountered is to achieve a structure in which the light can be confined within the active layer long enough (typically we seek that the photon travels an optical path greater than several times the thickness of the absorbent material) to allow maximum conversion efficiency. A method for confining electromagnetic waves (sunlight) within the active layer is to seek to excite surface plasmon resonances in subwavelength structures.

The article by Atwater et al. ("Plasmonics for improved photovoltaic devices", Nature Materials, 9, 205-213 (2010)] summarizes the recent techniques implemented for the production of photovoltaic devices involving "plasmonics." Plasmonics seeks to take advantage of the resonant interaction obtained under certain conditions between electromagnetic radiation (especially light) and free electrons at the interface between a metal and a dielectric material (air or glass for example). This interaction generates waves of electron density, behaving like waves and called plasmons or surface plasmons. The article describes different types of metal nanostructures that allow the generation of surface plasmons to trap light in very thin semiconductor layers, resulting in a sharp increase in absorption. The article notably describes the use of nanometric particles used as diffusing elements to promote coupling, the use of nanometric particles such as nano antennas, the use of a striated mirror at the rear of a semiconductor layer. conductor for generating surface plasmons at the mirror - semiconductor interface.

Among the references cited by Atwater et al., For example, is the article by Ferry et al. ("Improved red-response in thin film a-Si: H solar ones with soft-imprinted plasmonic black reflectors", Appl Phys Letters 95, 183503 (2009)) which describes a structuring of materials for solar cell type applications. . A structured metal layer on the back of the absorbent layer increases the absorption of longer wavelengths. In this paper, however, broad band absorption is achieved by means of a relatively thick active layer (500 nm). In Linquist et al. (Plasmonic nanocavity arrays for enhanced efficiency in organic photovoltaic cells, Appl Phys Letters 93, 123308 (2008)), a network of nanocavities is formed between a structured metal anode and a cathode (unstructured). This plasmonic structure allows the confinement of the electromagnetic energy and the increase of the absorption at wavelengths higher than 700 nm, due to the existence of surface plasmons between the structured anode and the cathode.

In Le Perchee et al. ("Plasmon-based photosensors comprising a very thin semiconducting region", Appl Phys Letters 94, 181 104 (2009)), an infrared detection system having a very thin active layer is described. As in the solar cell applications, the mechanism of confinement of the light in the semiconductor layer rests on the generation of plasmonic resonances in a horizontal cavity formed by a structure of the MSM (metal-semiconductor-metal) type in which the semiconductor layer is sandwiched between a lower metal mirror and an upper metal network of sublength wave length. It is shown that such a plasmonic resonance can be modeled by a longitudinal Fabry-Perot resonator which verifies the relation where λ is the wavelength of the incident wave), n _eff is the effective index of the plasmonic mode guided in the MSM multilayer structure, L is the width of an element of the network.

The plasmonic resonances evidenced in the aforementioned articles are generated at wavelengths greater than 650-700 nm. This is explained by the very nature of the surface plasmon at the metal-dielectric interface which can not exist at short wavelengths (see for example AV Zayats et al, 'Nano-optics of surface plasmon polaritons', Physics reports 408, 131-314, 2005).

There is therefore a need for the realization of ultra fine structures having increased broadband absorption in the visible band 500 - 800 nm.

The invention has an asymmetric MIM (metal-insulator-metal) type absorbent nanometer structure whose particular geometry makes it possible in particular to generate increased absorption at distributed wavelengths over the entire visible spectrum.

SUMMARY OF THE INVENTION

According to a first aspect, the invention relates to an asymmetric MIM-type absorbent nanometric structure intended to receive an incident broadband light wave whose absorption in a given spectral band of the near-infrared visible range is sought to be optimized, characterized in that it comprises an absorbing dielectric layer in said spectral band, of subwavelength thickness, arranged between a metal network formed of metal elements arranged periodically with a subwavelength period and a metal reflector, and in that :

the elements forming the metal network have at least one dimension (w) adapted to form, between the metal network and the metal reflector, under the elements of the network, a plasmonic resonator forming a longitudinal cavity of the Fabry-Pérot type resonant to a first wavelength of the desired absorption spectral band,

the absorbent layer present between the metal network and the metal reflector at least a first thickness adapted to form at least a first vertical cavity of the Fabry-Pérot type, resonant at a second wavelength of the desired absorption spectral band.

According to a variant, said at least first thickness of the absorbent layer is less than the absorption length of the dielectric material of which it is formed on the desired absorption spectral band. According to a variant, said at least first thickness of the absorbent layer is between substantially once and twice the skin thickness of the metal forming the metal network.

According to a variant, the absorbent layer has a first thickness under the elements of the network and a second thickness under the spaces between the elements of the network, adapted to form a first and a second vertical cavities of the Fabry-Pérot type resonant at two lengths of length. distinct waves of the desired absorption spectral band.

According to a variant, the first and second thicknesses are substantially identical.

According to one variant, the width of the elements of the metal network is adapted to obtain a plasmonic mode of order m = 3.

According to one variant, the structure further comprises a non-absorbing dielectric layer in the desired absorption spectral band, arranged between said absorbing layer and the metal network and / or encapsulating the metal network, making it possible to adjust the thickness between the network metal and the metal reflector.

According to one variant, the period of the metal network is less than half the minimum wavelength of the desired absorption spectral band.

According to a variant, the metal network is one-dimensional, with a period of between 150 and 250 nm, formed of strips with a width of between 80 and 120 nm and a thickness of between 10 and 30 nm.

According to a variant, the metal network is two-dimensional, with a period in any one of the dimensions between 150 and 250 nm, formed of square or rectangular pads with sides of between 80 and 120 nm, of thickness between 10 and 30 nm.

According to a second aspect, the invention relates to a solar cell comprising a nanometric structure according to the first aspect, deposited on a substrate and in which the desired absorption spectral band is in the visible - near infrared.

Alternatively, the solar cell further comprises a transparent conductive layer disposed between the metal reflector and the absorbent layer.

Alternatively, the solar cell further comprises a transparent conductive layer disposed between the absorbent layer and the metal network or on the metal network and the absorbent layer.

According to one variant, the transparent conductive layer is made of ZnO, ITO or SnO. According to a variant, the metal reflector is multilayer, comprising a lower layer for adhesion with the substrate and a top layer of noble metal, for example gold, silver or aluminum.

According to a variant, the metal network is made of noble metal, for example gold, silver or aluminum.

According to one variant, the absorbent layer comprises a type III-V semiconductor material, for example gallium arsenide or indium phosphide.

According to one variant, the absorbent layer comprises one of amorphous silicon, CIGS or cadmium telluride.

According to one variant, the absorbent layer comprises an organic material.

According to one variant, the solar cell comprises an asymmetric MIM type absorbent nanometric structure comprising:

- a silver metal reflector;

an absorbent layer of GaAs with a thickness of less than 50 nm;

a silver metal network with a thickness of less than 30 nm, formed of pads or strips arranged periodically, the width of said pads or strips being between 80 and 120 nm, the linear filling factor being between 0 , 5 and 0.7.

- a silver metal reflector;

an absorbent layer made of GaSb with a thickness of less than 50 nm;

a silver metal network with a thickness of less than 30 nm, formed of studs or strips arranged periodically, with a period of between 270 nm and 330 nm and with a linear filling factor of between 0.5 and 0, 7;

a layer of conductive transparent material arranged on the metal network.

- a silver metal reflector;

an absorbent layer made of CIGS with a thickness of less than 50 nm;

a silver metal network with a thickness of less than 30 nm, formed of pads or strips arranged periodically, with a period of between 500 and 550 nm and with a linear filling factor of between 0.5 and 0.7. ;

a layer of conductive transparent material arranged on the metal network. For example, the layer of conductive transparent material is ZnO: A1, less than 50 nm thick. The applicant has demonstrated an increase in absorption in the visible near-infrared visible spectrum resonances in the layer of conductive transparent material deposited on the metal network.

According to a third aspect, the invention relates to a method of manufacturing a solar cell according to the second aspect comprising:

depositing on the substrate one or more metal layers to form the metal reflector,

depositing the absorbent layer on said metal reflector,

depositing a resin layer and structuring the resin layer to form the elements of the network,

the deposition of the metal forming the metal network and the dissolution of the resin.

According to one variant, the resin is structured by nanoimprinting.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and characteristics of the invention will appear on reading the description, illustrated by the following figures:

1A and 1B, diagrams showing exemplary embodiments of an asymmetrical MIM absorbent structure according to the invention respectively one or two dimensions; Figure 2, theoretical curve of the absorption calculated in a structure of the type of the figure

1A, compared with the solar spectrum;

Figure 3, diagram illustrating the different types of resonators implemented in the example of the structure of Figure 2;

Figures 4, diagram illustrating the dependence of the effective mode index as a function of the wavelength for different types of plasmonic structure;

Figure 5, diagram illustrating the dependence of the energy of excited resonances in the structure as a function of the width of the element of a network; Figure 6, diagram illustrating the dependence of the phase on the reflection of a plane wave in a dielectric on a metal reflector as a function of the wavelength for different types of dielectric;

Figures 7A, 7B, diagrams illustrating the dependence of the energy of excited resonances in the structure as a function of the thickness of the absorbent layer;

Figures 8A, 8B, experimental curves showing absorption versus angle of incidence in a one-dimensional asymmetric MIM structure;

FIGS. 9A, 9B, experimental curves representing the absorption as a function of the angle of incidence in a two-dimensional asymmetric MIM structure, respectively in TM and TE polarization;

10A and 10B, curve illustrating the absorption in a 2-dimensional asymmetric MIM structure according to the invention, respectively as a function of the filling rate, constant period, and as a function of the period, constant filling rate;

Figure 11, a graph showing absorption length in GaAs; FIGS. 12A to 12D, exemplary embodiments of solar cells using an asymmetrical MIM nanometer structure according to the invention;

Figure 13, curves showing the absorption measured in an asymmetric MIM structure comprising an absorbent layer of GaAs for different values of the filling ratio; FIG. 14, a curve representing the calculated absorption in an asymmetric MIM structure comprising an absorbent curve in GaSb;

15, a curve representing the calculated absorption in an asymmetric MIM structure comprising an absorbent curve in CIGS.

DETAILED DESCRIPTION FIGS. 1A and 1B represent two examples 1 and, respectively one-dimensional and two-dimensional, asymmetric MIM-type nanometric structures according to the invention, designed to receive an incident light whose absorption is sought to be maximized in a strip spectral data. An asymmetric MIM type structure (MIM being the abbreviation for Metal / Insulator / Metal) is a multilayer structure comprising at least one layer of dielectric material between a metal network and a metal reflector, the metal network being of sub-length dimensions. wave and reflector having a thickness greater than the skin thickness of the metal (defined as the characteristic distance of the attenuation of an incident wave in the metal), so that it can be considered as semi infinite in a model propagation of waves at the wavelengths considered. This combination of a dielectric layer between a metal array and a semi-infinite reflector makes the structure asymmetrical. Dielectric material includes insulating and semiconductor materials, including doped semiconductors.

It is known to have in this type of structure a propagation of so-called modes

'plasmonics' or surface plasmons, solutions of Maxwell's equations at the metal / dielectric interfaces. The excitation of the various plasmonic modes under the effect of an incident light wave can take place if the resonance and coupling conditions are met, these conditions depending on the geometry of the structure, and particularly those of the metallic network (see for example JA Schuller et al., "Plasmonics for Extreme Light Concentration and Manipulation", Nature Materials 9, 193-204, 2010).

Each of the structures according to the invention comprises a dielectric material layer 10 having a refractive index r, of thickness t _a , arranged between a metal network 11 and a metal reflector 12. In the example of the one-dimensional structure of 1A, the network is formed of strips 110 of width w, thickness t _m , arranged in a period d. In the example of Figure 1B, the network is formed of square studs 120 w side. The period according to one and the other of the dimensions is for example similar, noted d. The dimensions of the period, the width of the bands (or the pads), the thickness of the dielectric layer and the network are sub-wavelength, that is to say less than the wavelength minimum of the desired absorption spectral band. The period d is advantageously less than half the minimum wavelength so as to limit any energy loss by diffraction on the grating whatever the angle of incidence. The layer 10 is moreover absorbent in the desired absorption spectral band. In a preferred application of the invention, especially for application to solar cells, the structure is designed to receive sunlight, and it is sought to make the absorption of the optimal structure between about 500 and 800 nm. The layer is selected from an absorbent material in this spectral band. Advantageously, as will be explained in more detail later, one will choose a high index material (typically greater than 3), which will reduce the thickness of the layer. For example, the dielectric material is a III-V type semiconductor material (having an element in column III and an element in column V of the periodic table of elements), with a gap in the near infrared, for example Gallium arsenide (GaAs) or Indium phosphide (InP). Although having a lower index (typically between 1.5 and 2), organic polymers, for example based on fullerene derivatives, are also promising materials. Other materials such as cadmium telluride, amorphous silicon, microcrystalline silicon or polycrystalline silicon are also contemplated.

The metal network and the metal reflector are for example made of Silver,

Or, or Aluminum, noble metals that do not absorb too much in the visible. According to a preferred variant, the metal network may be made of gold and the metal reflector, protected from the air, in silver, silver being alterable in contact with the atmosphere (sulfurization).

The applicant has shown that for the solar cell application, with a desired absorption band in the visible range between 500 and 800 nm, advantageously, the width of the strips (or pads) of the metal network will be chosen less than 150 nm, advantageously between about 80 and 120 nm, and the period below 250 nm, preferably between about 150 and 250 nm. The thickness of the absorbent layer will be chosen less than 100 nm, and the thickness of the metal network less than 30 nm, advantageously between about 15 and 25 nm. The metal reflector has a thickness greater than the skin thickness of the metal, typically a thickness greater than 50 nm in the case of using gold or silver. More details on the optimization of the dimensions and the choice of materials of the structure will be given later

FIG. 2 presents the results of numerical simulations of a one-dimensional asymmetric MIM plasmonic structure according to the invention (represented in box in FIG. 2), of the type of that represented in FIG. 1A.

In this example, the absorbent layer is Gallium Arsenide (GaAs), the real part of which is about 3.5 (see ED Palik, Handbook of Optical Constants of Solids Academy, Orlando, 1985). Its thickness is 25 nm. The layer is arranged between a silver metal network whose period is 200 nm, and the width of the bands 100 nm. The thickness of the network is 15 nm. The metal reflector is silver.

The model used for these simulations is based on the exact modal method (see for example S. Collin et al., 'Efficient light absorption in metal-semiconductor-metal nanostructures, Appl Phys Letters 85, 194, 2004), with Polarized wave TM (magnetic field parallel to the network bands).

The applicant has demonstrated a remarkable absorption in three spectral bands centered respectively on 560 nm (resonance denoted E), 675 nm (resonance denoted D), 760 nm (resonance denoted C). Curve 21 shows the calculated absorption in the GaAs while curve 22 shows the calculated absorption in the total structure. These two curves are compared with the normalized solar spectrum (AM1.5G, here plotted in photon number / m ² / s / nm ^-1 ). The GaAs is interesting in that it has at room temperature a gap of 1.42 eV well suited for solar photovoltaic application (see for example T. Markwart and L. Castaner, Practical Handbook of Photovoltaics, Elsevier, 2003), the record yield obtained experimentally for a simple junction being 26.1% (maximum theoretical yield 32%). It is observed that the absorption maximum coincides with the maximum emission of the solar spectrum. The small difference (less than 13%) between curves 21 and 22 in the entire visible range shows a very low absorption by the metal lattice throughout the visible range, showing an excellent confinement of light in the GaAs active layer (and low ohmic losses in the metal network). On average, 70%> incident photons are absorbed in the spectral range 500 - 800 nm, and 55% of photons whose energy is greater than the gap (shown by the dotted line 23) are absorbed in the cell. A theoretical conversion efficiency of solar energy of 17% (conversion efficiency of solar energy into electrical energy) is deduced for a cell whose external quantum yield independent of polarization is given by curve 21 of the Figure 2 (assuming that the GaAs layer is composed of a perfect pn junction, the non-radiative recombinations being neglected and the internal quantum yield assumed equal to 1). The calculation of the theoretical yield is described, for example, in G.Araujo et al, 'Limiting efficiencies of GaAs Solar cells', IEEE Transactions on Electron Devices 37, 1402-1405, 1990 or W.Shockley et al,' detailed balance limit of efficiency of Solar Cell Junction, Journal of Applied Physics 32, 510-519, 1961. The applicant has shown that this remarkable absorption in an ultrafine structure (in this example less than 50 nm) can be explained by a combination of resonances whose physical principles are different, and which can therefore be adjusted by adjusting the parameters of the independent structure. It is therefore possible, by adapting the different parameters of the structure, to optimize the position of the wavelengths around which the absorption peaks are centered, in order to obtain the response of the structure for the desired application. .

The multi-resonant structure according to the invention thus makes it possible to solve a paradox which is that, in general, if the travel time of the photon in the structure (that is to say the optical path) is increased, the width is decreased. spectral resonance.

FIG. 3 again shows the absorption spectra in GaAs (curve 31) and total (curve 32) at normal incidence, and the total absorption (dashed curve 33) for an angle of incidence at 30 °. The spectra are plotted in energy here (the energy of the resonance is proportional to the inverse of the resonance wavelength). The nature of the different resonances labeled A-E is shown schematically on the right-hand part of FIG. 3 (diagrams 301 to 305, corresponding respectively to the resonances E, D, C, B and A). The maps of the electromagnetic field are represented by diagrams 306 to 315. The squared modulus of the magnetic field H (curves 306 to 310) highlights the modes of the resonant cavities. For example, the diagram 310 shows a resonance of the first order, while the diagrams 309 and 308 respectively show a resonance of order 2 and of order 3. It can be seen that the order m = 3 is interesting in this exemplary embodiment. because it makes it possible to obtain a resonance at 760 nm, thus in the desired absorption band. Diagrams 306, 307 show the fundamental order for resonances E and D respectively. The squared modulus of the electric field E (curves 311 to 315) highlights the location of the absorption in the cavities.

The applicant has shown that each resonance can be modeled by a Fabry-Perot resonator. The resonance condition of the resonator can be written generally 1 - r _l r ₂ e ^2M = 0 where k = 2π (n + ik _eff _eff) / λ is the wave vector, λ the wavelength,

(n _eff + ik _eff ) the complex effective index of the mode, and ri and r ₂ are the reflection coefficients at the ends of the resonator. By noting φι and φ ₂ the phases induced by these two reflections, the resonance condition is written as 4π / ζ " _ε / λ + φ ₁ + φ ₂ = 2π /? ού p is an integer (p = ± 0, ± 1, ± 2,. ..). For a given wavelength, the size h of the resonator is thus given by the general equation:

_{Α = λ} 2π ^ - (φ _{1 +} φ ₂ ) ₍₁₎

4 n _eff

In the case of a symmetrical classical Fabry-Perot resonator, φι = φ ₂ = 0 in the case of a dielectric surrounded by air, or φι = φ ₂ = ± π in the case of a reflection on a metal of high permittivity, for example silver, aluminum or gold in the infrared. The resonance condition is then simply written: h = - (2)

²ⁿ eff

The applicant has shown that the resonances denoted A, B and C can be described by a plasmonic mode resonance under the fingers (or bands) of metal, in the plane of the solar cell. The MIM structure then plays the role of a Fabry-Perot resonator for a plasmonic wave propagating along the x axis (parallel to the plane of the mirrors) and reflecting at the ends of the elements of the network. The applicant has thus demonstrated a resonant cavity 'horizontal' or 'longitudinal', that is to say parallel to the plane of the network, whose length is given by the width w of the element of the metal network and the index by the effective index n _eff of the propagating mode. The phase change can be neglected as a first approximation at the ends of the resonator, and the wavelengths of the first three resonances approximately follow the equation: λ = (3)

P

with p = 1, 2, 3 for A, B, C respectively.

The plasmonic modes have the particularity of propagating with an effective index greater than that of the dielectric medium (see for example AVZayats et al., 'Nano-optics of surface plasmon polaritons', Physics reports 408, 131-314, 2005). This effect is reinforced by the coupling between several surface plasmons, as in the case of a MIM guide (here Ag-GaAs-Ag). This effect is illustrated in Figure 4, where the applicant has modeled the value of the real part of the effective index as a function of the wavelength. The calculation method is for example described in S.Collin et al, 'Waveguiding in nanoscale metallic apertures', Opt. Express 15, 4310-4320, 2007. Curve 401 is obtained by modeling a semi-infinite multilayer structure, with three interfaces (Air / Silver interface, Silver / GaAs interface, GaAs / Silver interface), with a thickness of 25 nm for the GaAs and 15 nm for Silver. Curve 402 is obtained with a structure having two interfaces (silver / GaAs interface, GaAs / silver interface). The curve 403 is obtained with a structure having only a GaAs / Silver interface. Curve 404 represents the effective index of a mode obtained in a structure having two interfaces Air / GaAs and GaAs / Silver. Curve 401 shows that in the visible, the effective index reaches values of the order of 10, and it remains at a very high level (around 6, twice the GaAs index) in the range. near infrared (1 - 2 μιη). At shorter wavelengths, the effective index collapses and the plasmonic mode disappears. The strong difference reached by the real part of the effective index between the curves 401, 402 on the one hand and 403, 404 on the other hand comes from the existence in the first case of the coupling of two plasmonic modes to the silver interfaces. / GaAs and GaAs / Silver. In the case of the curve 401, the effective index is still slightly higher if one chooses a thin metal thickness since it achieves a 3 ^rd plasmon coupling to the way Air / Silver.

Thus, the Applicant has shown that with a width of the metal grating w = 100 nm, the three resonance peaks A, B, C at 1590 nm, 945 nm and 760 nm respectively, corresponding to the modes m = 1, are obtained. , m = 2, m = 3. The optimization of the parameters of the structure to obtain a resonance corresponding to the mode m = 3 is particularly advantageous since it allows with a small value of the width w of an element of the metallic network ( about 100 nm), a resonance at a wavelength of the visible spectrum.

Figure 5 illustrates the influence of the width w of the elements of the metal network on the energy of the resonance (proportional to the inverse of the wavelength). The curves 501 to 505 represent the energy as a function of the width w for the first 5 plasmonic modes (m = 1 to 5), in an asymmetric MIM plasmonic structure of the type of FIG. 1A (calculation conditions identical to those of FIG. Figure 2). For a given mode, by widening the cavity (w increasing), one shifts towards the low energies and thus the longest wavelengths.

The applicant has demonstrated for the D and E resonances a mechanism different from that shown in the case of resonances A, B and C.

Considering equation (2), it appears that with an index of the order of 3.5 (GaAs index), the smallest resonator at 700 nm has a size of 100 nm and the following order resonates at λ = 350 nm. It is therefore not possible to achieve multiple resonance in a resonator smaller than 100 nm. The applicant has shown that in an asymmetric MIM structure, with an absorbent layer of given index and by choosing the thickness of the layer adequately, it is possible to obtain one or even several resonances in the visible. It appears indeed that the reflection coefficient on an interface between a high-index semiconductor (around 3 or more) and a metal such as silver, aluminum or gold, for example, deviates from its usual values. for wavelengths of 600 nm. This is highlighted in FIG. 6, which represents, as a function of wavelength, the phase at reflection of a plane wave propagating in GaAs on a supposed infinite silver mirror calculated using the Fresnel coefficients. (curve 602). The function giving the phase to the reflection can be approximated by the function (-1 + 2nGaAs kA _g ) where riGaAs is the real part of the index of Gallium Arsenide and kA _g is the imaginary part of the index of Silver (curve 601). For comparison, the curves 604 and 603 respectively represent the reflection phase of a plane wave propagating in a vacuum on a silver mirror and the function (-1 + 2 / kA _g ) which approximates the calculated function of the phase to reflection. Curve 602 shows that at long wavelengths, the phase is close to π (case of the perfect metal). On the other hand, around 600 nm, the phase passes through π / 2. By neglecting the phase change of the wave reflecting on the metallic network (either reflection on the air between the elements of the network, or reflection on the very thin layer of metal under the elements of the network), the equation (1 ) then becomes, at order 0: h = - (4)

Where r ¼ is the index of the absorbing material (eg GaAs).

For an index ¾ = 4, there is therefore a vertical 'Fabry-Perot' cavity (that is to say perpendicular to the plane of the network) between the metal network and the metal reflector which resonates at a wavelength of 640. nm for a thickness of the absorption layer t _a = 20 nm. This resonance exists in the Fabry-Perot fundamental order (order p = 0 in equation (1)), in contrast to the longitudinal cavity evidenced for the plasmonic resonances A, B and C.

It can also be shown that the curve 601 has little variation depending on the metal used and the absorbent material.

As it appears in FIG. 6, when the wavelength moves away from 600 nm, the phase will move substantially away from π / 2, consequently modifying the relation between the resonance wavelength and the thickness of the wavelength. the dielectric layer given by equation (1).

The applicant has shown that in an asymmetric MIM plasmonic structure of the type of FIG. 1A, two resonances of this type (absorption peaks) could be demonstrated. D and E in Figure 3). This effect comes from a first 'vertical' Fabry-Pérot cavity (perpendicular to the network plane) under the network elements (absorption peak E) and a second vertical Fabry-Pérot cavity under the spaces between the elements of the network (absorption peak D), thus forming a 'split' cavity at order 0. The difference between the two resonant wavelengths can be explained at the same thickness of the absorption layer by the different boundary conditions on the upper end of the cavity. By adjusting the thickness of the absorption layer, it is therefore possible to generate two absorption peaks in the visible, at wavelengths less than the resonance of the plasmonic cavity. According to one variant, it is possible to vary the thickness of the absorption layer in an inhomogeneous manner, for example by choosing under the network elements a layer thickness different from that under the spaces between the elements of the network, so that to refine the position of the absorption peaks.

The evolution of the vertical resonance as a function of the thickness t _a of the absorbent layer is illustrated in FIG. 7A for a GaAs layer (same conditions as those of FIG. 2). Curves 701 and 702 represent the energy response of the 'split' vertical Fabry-Perot cavity at order 0. Curves 703 to 705 represent the energy responses respectively for orders 1 to 3 of the vertical Fabry-Perot cavity. .

Curve 7B also represents the calculated curves of energy as a function of the thickness of the absorbent layer, but for lower values of thickness. The curves 701 and 702 of the energy are shown for the vertical Fabry-Perot cavity at order 0 and also the curves 706 to 708 of the energy of the longitudinal plasmonic cavity at orders m = 1 to 3 respectively. It is observed that for the very small thicknesses, the effective incoming index in the plasmonic resonances (resonances A, B, C) decreases as the thickness of the absorbing layer increases, this being linked to a decrease in the coupling between the plasmons of the two Ag / GaAs interfaces. This results in a slight spectral shift of these resonances, which mix with the vertical Fabry-Perot resonances for the lowest energies, near the gap (around 1.4 - 1.6 eV). FIG. 7B shows that in an asymmetric MIM plasmonic structure, by choosing for a material of the given absorption layer its thickness, it is possible to generate in the desired spectral band multiple resonances. In particular, in this example, for a thickness of GaAs layer of about 25 nm, the 5 resonances A, B, C, D, E are obtained, whose 3 resonances C, D, E in the spectral band 500 - 800 nm. Moreover, the applicant has shown that the width of the elements of the network has little influence on the resonance at the fundamental order of the vertical Fabry-Pérot cavity. This appears in particular in Figure 5, where the curves 506, 507 respectively represent the energy of the vertical cavity Fabry-Pérot 'split' as a function of the thickness w. This is remarkable in that it will be possible to influence the wavelengths of the plasmonic resonances by playing on the parameter w without influencing the wavelengths of the 'vertical' resonances. On the contrary, the vertical resonances will be particularly sensitive to the thickness of the absorption layer, while the plasmonic resonances will be less sensitive.

FIGS. 8A and 8B illustrate, by experimental curves, the angular dependence of a one-dimensional asymmetric MIM structure according to the invention (of the type of FIG. 1A).

These curves were obtained with a layer of Si0 ₂ (silicon dioxide) 20 nm thick, deposited on a glass substrate coated with a gold layer. The metal network is made of gold, manufactured by so-called nano-printing technology, described for example in SH Ahn and LJ Guo, High-Speed Roll-to-Roll Nanoimprint Lithography on Flexible Plastic Substrates (Advanced Materials 20, 2044-2049, 2008). The geometrical parameters of the grating (period of 400 nm, width of the elements w = 200 nm, and thickness 20 nm) have been optimized to present two resonant modes between 600 and 1800 nm. Although silicon dioxide is not absorbent in the visible, and therefore not suited to the application of solar cells, these experimental curves make it possible to highlight the geometrical conditions allowing a resonance of a plasmonic mode with an almost perfect absorption, in a wide angular band. Reflection measurements were carried out between 3 ° and 60 °, in TM polarization (magnetic field along the y axis). The excitation of the fundamental mode (m = 1) of the MIM structure shows an almost perfect absorption at λ = 1280 nm (> 98%) whatever the angle of incidence. This insensitivity to the angle of incidence is related to the symmetry of the mode with respect to a plane of symmetry of the structure (also true for m = 3). The MIM nanostructure plays the role of a Fabry-Pérot resonator for the plasmon wave propagating along the x-axis, and reflected at the ends of the elements of the network. Here, the high effective index of the plasmonic mode is due to the very strong coupling between the very fine metal network and the semi-infinite metal reflector. The applicant has shown that the resonance wavelength is mainly determined by the width w of the elements (see FIG. 5) and to a lesser extent by the thickness of the dielectric layer which influences the coupling and therefore the effective index of the mode. It should be noted that here, the layer thickness is not adapted to obtain a resonance of a vertical Fabry-Pérot type cavity between the metal reflector and the metal network. The coupling can also be modified by adjusting the network fill rate, as will be demonstrated in FIG. 10A.

FIGS. 9A and 9B illustrate the angular dependence of a two-dimensional asymmetric MIM structure (of the type of FIG. 1B), respectively in TM and TE polarization. The experimental conditions are the same as those of Figures 8A and 8B. The structure comprises square studs arranged in a two-dimensional periodic structure, with a period in each dimension of 400 nm and a plug width of 250 nm (thickness 20 nm). It is remarkable to note is obtained within 90% absorption in ultra small nanocavities (volume of the order of λ ^3/1000), for both the TE and TM modes. This demonstrates the feasibility of a two-dimensional asymmetric MIM plasmonic structure, which will have a clear advantage in the application to solar cells, since there will be no polarization filtering, and therefore better performance. It should be noted that the vertical Fabry-Pérot resonators highlighted in the multi-resonant structure according to the invention are also insensitive to polarization.

FIGS. 10A and 10B show simulations of the absorption as a function of wavelength in a two-dimensional structure of the type of FIG. 1B, in which the absorbent layer is made of GaAs and has a thickness of 25 nm, The thickness of the metal grating is 20 nm, the default period is 180 nm, and the filling ratio (ratio of the width of the stud per period, measured according to one dimension) is 0.6. Figure 10A shows the results obtained at equal period, by varying the filling ratio (f). Figure 10B shows the results obtained at constant filling rate, by varying the period (Λ). The method of calculation is the RCWA method described for example in P. Lalanne et al, 'Plasmon surface of metallic surfaces perforated by nanohole arrays', Journal of Optics A: Pure and Applied Optics 7, 422-426, 2005.

These curves demonstrate, as in the example of the one-dimensional structure, broadband absorption in the visible with the presence of three absorption peaks (corresponding to the previously described peaks C, D, E). Moreover, if the absorption peaks due to the double resonance of the 'vertical' Fabry-Perot cavity are not very variable as a function of the geometry of the elements of the metal network, a variation of the absorption peak of the plasmonic resonator is observed. , both wavelength and amplitude, related to the size of the resonator (w) and the quality of the coupling in the cavity according to its geometry. In particular, there is an increase in absorption due to plasmonic resonance with the increase of the filling rate presumably due to a better coupling while the absorption due to the 'vertical' resonance under the spaces (resonance D) decreases, presumably due to the decrease of the space between the elements .

The applicant has shown that in this example the best efficiency of a solar cell that would be achieved with this structure is obtained for a filling ratio f = 0.6 and a period d = 180 nm.

Thus, by choosing an asymmetric MIM structure comprising a GaAs layer with a thickness of less than 50 nm, for example between 20 and 30 nm, typically around 25 nm, said GaAs layer being between a metal reflector, for example silver, and a metal network formed of elements of the silver-like bands or pads arranged periodically, FIGS. 7A and 7B show the presence of one or more order 0 vertical resonances in the absorbing layer for lengths of silver. wave included in the near infrared visible spectral band. By choosing the appropriate parameters for the metal network, typically a thickness of less than 30 nm, a width of the elements forming the network of between 80 and 120 nm, and a linear filling factor of between 0.5 and 0.7, it is observed that furthermore, in the visible near infrared spectral band, a horizontal plasmonic resonance of order 3 (see FIG. 5). A multi-resonant structure can thus be obtained having particularly well adapted for the realization of a solar cell because having a broad absorption in the visible near-infrared spectrum, absorption whose physical mechanisms can be explained both by a horizontal plasmonic resonance but also by one or more vertical resonances in the GaAs layer.

Although most of the simulated curves above have been obtained with GaAs as an absorbing dielectric material, it is obvious that other materials are very promising for obtaining a multi-resonant absorbing asymmetric MIM structure as it has. previously defined. In particular, we can search for materials, such as Indium Phosphide (InP) or amorphous Silicon (a-Si: H), which will make it possible to design structures with dielectric layers between 50 and 100 nm, making it easier to obtaining a junction for a solar cell with the current technological means.

Whatever the dielectric material chosen, it will be preferable to choose a thickness of the absorbent layer less than the absorption length of the dielectric material of which it is formed to obtain the desired resonance of a Fabry-Perot cavity between the network. metal and the metal reflector. The absorption length of the material is defined by the depth in the material at which the intensity of an incident light wave of a given wavelength is divided by e. FIG. 11 represents, for example, the length of absorption as a function of the wavelength for GaAs.

Advantageously, the thickness of the absorbent layer is of the order of magnitude of the skin thickness of the metal forming the dielectric network or up to twice the skin thickness, to promote the coupling of the plasmonic modes to the metal interfaces. dielectric / dielectric / metal and obtain high effective mode indices.

Alternatively, the nanoscale structure further comprises a non-absorbing dielectric layer, arranged between the absorbent layer and the metal network to adjust the spacing between the metal network and the metal reflector and thus adjust the resonant wavelength. The dielectric layer may or may not encapsulate the metal network.

Although the results have been presented in one- or two-dimensional structures with elements formed of strips or square studs, the invention is not limited to this type of pattern and other reasons may be envisaged as long as a periodic structure is preserved.

FIGS. 12A to 12D show exemplary embodiments of solar cells 100 obtained with an asymmetric MIM type structure according to the invention.

The ultrafine solar cells MIM can be manufactured on a substrate 101 covered with one or more metal layers 102 forming the metal reflector, itself covered with layers forming the absorbent layer 103. The metal network 104 is disposed on the absorbent layer. In the example of FIG. 12D, a transparent conductive layer 106, for example of the TCO (abbreviation of the transparent transparency conducting oxide) type, is arranged between the metal reflector and the absorbent layer. Alternatively, a transparent conductive layer 105 may also be disposed on the metal network (Figure 12B) or between the metal network and the absorbent layer (Figures 12C, 12D).

The substrate 101 is any, for example formed of glass, plate or sheet metal or plastic.

In the case of a metal reflector made of several layers, the lower layer in contact with the substrate may promote adhesion (for example in chromium or titanium), and the upper layer in contact with the absorber (case of Figures A at C) or with the layer of TCO (case of figure D) will be chosen for its optical properties (preferably a noble metal type Ag, Al, Au, ...) and electrical (lower contact to conduct the current, and contact Schottky or ohmic with the absorber ). These metals can be deposited by vacuum evaporation assisted by electron gun, by vacuum spraying, or by electrolytic growth.

The absorber 103 is for example formed of a direct gap semiconductor material, or behaving like a direct gap semiconductor material, such as for example gallium arsenide (GaAs), indium phosphide (InP), copper and indium selenide (CuInGa (Se, S) 2 or CIGS), Cadmium telluride (CdTe), hydrogenated amorphous silicon (a-Si: H). For example, it consists of a p / p + doped layer, an intrinsic layer i, and an n / n + doped layer, or only two p and n doped layers, or a p or n layer. n and an intrinsic layer forming a Schottky contact with the metal (upper or lower). The layer p (n) can be the bottom layer (top) or the reverse. The absorber may also consist of a heterostructure (different materials forming, for example, the different layers n and p). The absorber can be deposited according to known methods - see for example A. Shah et al., 'Photovoltaic Technology: The Case for Thin-Film Solar Cells', Science, 285, 692-698, 1999 or JJ Schermer et al, Photon confinement in high-efficiency, thin-film III-V solar cells obtained by epitaxial lift-off, Thin Solid Films, 511, 645-653, 2006 for depositing by the so-called lift-off technique.

The metal network can be manufactured by lift-off according to the method comprising the following steps:

depositing a photosensitive or electrosensitive resin on the absorber, then insolation of the resin by UV photolithography or interference lithography, or by electronic lithography.

- development of the resin, dissolution of the insolated parts.

deposition of the metal forming the metal network (by evaporation, by spraying, etc.), the metal is deposited on the absorber at the places where the resin has been insolated,

- Removal (lift-off) by dissolving the resin, only the metal deposited directly on the absorber remains, forming a network according to the pattern insolated in the resin.

According to one variant, the resin may also be structured by nano-printing. In this case, the metal networks are for example made by UV-assisted soft nano-printing. A layer of PMMA resin 200 nm thick is deposited on the metal reflector / absorber assembly, then a thin layer of 10 nm Germanium, and finally a layer of photosensitive liquid resin of 100 to 150 nm thickness used for the nano-printing step. This molding step, or nano-printing, is performed in a press with a silicone mold under very low pressure, and the resin is crosslinked by UV irradiation. The structures obtained are transferred into the germanium layer and the PMMA resin by reactive ion etching. This set of three layers is used to make the metal networks by lift-off: a layer of gold is deposited on the sample, then the PMMA resin is dissolved in a solvent, leaving on the surface only the gold nanostructures.

The transparent conductive layer (105, FIG. 12B) may be deposited on the structure by evaporation, sputtering, or electrolytic growth, for example. The conductive transparent materials used can be ΓΙΤΟ (indium tin oxide), ZnO: Al (zinc oxide doped with aluminum), and SnO 2 (tin dioxide, which can be doped with iron for example).

In another possible embodiment, the transparent conductive dielectric layer is deposited on the absorber, and the metal network is deposited on the transparent conductive dielectric layer (case C and D).

The collection of charges in the cell is made by the metal contact of the bottom (metal reflector) and by the network and / or the transparent conductive layer.

FIG. 13 illustrates first experimental results obtained with a 25 nm GaAs absorbing layer plotted on a 200 nm gold mirror. The metal network is made of gold, made by electronic lithography with a layer of hooked chrome. The metal network comprises a set of square studs arranged periodically with a period of about 200 nm and different values of the linear filling ratio, equal to the size of the pad divided by the period. In FIG. 13, the different curves are obtained from measurements of reflectivity at normal incidence of the structure. Curve 132 represents the measured absorption of a 25 nm layer of GaAs on gold (without the presence of the metal lattice) while layer 131 represents the absorption of GaAs calculated under the same conditions. The curves 131 and 132 are almost superimposed, which proves the quality of the GaAs layer reported. On curves 131, 132, a single peak characteristic of a vertical resonance in GaAs is observed. This intermediate step makes it possible in particular to validate the thickness of the absorbent layer. The curves 133, 134, 135 represent the absorption of the complete structure (reflector - absorbent layer - metal network) for filling factors varying respectively from 0.5 to 0.7. The doubling of the peak related to the vertical resonance is observed at the appearance of a third peak linked to the horizontal plasmon resonance that shifts to red when the fill factor increases. The results of the numerical simulations previously shown are thus verified (see for example FIGS. 10A, 10B).

In addition to GaAs, the applicant has shown remarkable results with other absorbers.

FIGS. 14 and 15 thus represent numerical results obtained respectively with GaSb (Gallium Antimonide) and CIGS.

In FIG. 14, the curve 143 represents the total absorption of an asymmetric MIM structure comprising a stack of several layers including a GaSb layer of 25 nm. The absorbent layer is between a silver metal reflector and a 25 nm thick silver metal grating formed of square studs periodically arranged with a period of 300 nm and a fill factor of 0.56. The structure also comprises a layer of transparent ZnO: Al type conductive material, 50 nm thick, deposited on the metal network. Curve 143 shows a remarkable absorption spectrum in the visible with multi-resonances characterized by the peaks A ', B', C, D '. The applicant has shown the existence of a 3-fold horizontal plasmonic resonance in the absorbing layer at 1100 nm (peak A '). Vertical resonances in the GaSb layer were detected at 740 nm and 900 nm (at 900 nm, the resonance is located between the pads). Another remarkable peak is evidenced in this structure at 520 nm, which the applicant has shown to correspond to a vertical resonance in the ZnO: Al layer. In FIG. 14, curve 142 represents the calculated absorption in FIG. wavelength function only in the GaSb active layer. The comparison between the curves 142 and 143 shows the absorption in the metal network and the ZnO: Al layer. It can thus be verified that in the visible near-infrared spectrum, the absorption of the structure results mainly from the absorption in the layer of GaSb. Curve 141 shows the absorption in GaSb without the presence of the metal lattice. This curve shows a single vertical resonance. The applicant has thus demonstrated in such a structure a theoretical short-circuit current Jsc = 36.7mA / cm ² for the lattice structure against Jsc = 24.5 mA / cm ² for the structure without grating, ie an increase of 50%. The short-circuit current Jsc equal to the theoretical current density calculated for illumination corresponding to the standardized solar spectrum AM1.5G, is characteristic of the performance of a solar cell obtained with such a structure. These results are remarkable for such a thin layer. The applicant has thus shown that an ultra-thin solar cell with very high visible absorption, characterized by multi-resonances between 500 nm and 1000 nm could be obtained thanks to a multilayer structure of the type previously described comprising a layer GaSb and by choosing the characteristic parameters of the structure (mainly the width of the elements of the network, the linear filling factor, the thickness of the absorbent layer and that of the upper layer of transparent conductive material). In particular, the GaSb layer will advantageously be chosen to be less than 50 nm, comprised between a metal reflector advantageously made of silver and a metal network, preferably also made of silver, of thickness less than 30 nm, formed of elements of the type arranged strips or pads. periodically with a period advantageously between 270 and 330 nm and a linear filling factor of between 0.5 and 0.7 preferably. The layer of conductive transparent material is advantageously ZnO: Al, with a thickness of between 40 and 60 nm.

In FIG. 15, the curve 153 represents the total absorption of an asymmetric MIM structure comprising a stack of several layers including a CIGS layer of 45 nm. The absorbent layer is between a silver metal reflector and a silver metal network 20 nm thick formed of square studs periodically arranged with a period of 530 nm and a fill factor of 0.55. The structure also comprises a layer of transparent ZnO: Al type conductive material 50 nm thick deposited on the metal network. Curve 153 shows a remarkable absorption spectrum in the visible with multi-resonances characterized by peaks A ", B", C ", D". The Applicant has shown the existence of a 3-fold horizontal plasmonic resonance in the 1100 nm (peak A) absorbent layer, and a 0-order vertical resonance in the GaSb layer has been demonstrated at 990 nm. nm (peak B "). Wide absorption peaks around 490 nm (D ") and 830 nm (C") corresponding to vertical resonances in the ZnO: A1 layer have also been demonstrated. In FIG. 15, the curve 152 represents the calculated absorption in the CIGS layer for a structure identical to that of the curve 153. By way of comparison, the curve 151 represents the absorption calculated as a function of the wavelength. in an absorbent CIGS layer deposited on molybdenum without the presence of the metal network. The applicant has demonstrated a theoretical short-circuit current of Jsc = 37.7mA / cm ² for the lattice structure (silver reflector - CIGS layer - metal grating - ZnO: A1) against Jsc = 13.2mA / cm ² for the non-lattice structure on molybdenum, an increase of 180%. Here again, the applicant has shown that an ultra-thin solar cell with very high visible absorption, characterized by multi-resonances between 500 nm and 1000 nm, could be obtained thanks to a multilayer structure of the type previously described comprising a layer in CIGS and choosing the characteristic parameters of the structure. In particular, the CIGS layer will advantageously be chosen to be less than 50 nm, comprised between a metal reflector advantageously made of silver and a metal network, also preferably made of silver, of thickness less than 30 nm, formed of elements of the strip or pad type arranged periodically with a period advantageously between 500 and 550 nm and a linear filling factor of between 0.5 and 0.7 preferably. The layer of conductive transparent material is advantageously ZnO: Al, with a thickness of between 40 and 60 nm.

Although described through a certain number of detailed exemplary embodiments, the structure and the method of production of the structure according to the invention comprise various variants, modifications and improvements which will be obvious to those skilled in the art, it being understood that these various variants, modifications and improvements fall within the scope of the invention, as defined by the following claims.

Claims

Absorbent nanometric asymmetric MIM type structure (1,) intended to receive an incident broadband light wave whose absorption in a given spectral band of the near infrared visible range is sought to be optimized, characterized in that it comprises a dielectric layer absorbent (10) in said spectral band, of subwavelength thickness, arranged between a metal network (11) formed of metal elements (110, 120) arranged periodically with a sublongeur wave period, and a metal reflector (12), and in that: the elements (110, 120) forming the metal network have at least one dimension (w) adapted to form, between the metal network and the metal reflector, under the elements of the network, a plasmonic resonator forming a longitudinal cavity of the Fabry-Pérot type resonant at a first wavelength of the desired absorption spectral band, the absorbing layer (10 ) has between the metal network and the metal reflector at least a first thickness (t _a ) adapted to form at least a first vertical cavity Fabry-Perot type, resonant at a second wavelength of the absorption spectral band sought .

Nanoscale structure according to one of the preceding claims, wherein the absorbent layer has a first thickness under the elements of the network and a second thickness under the spaces between the elements of the network, adapted to form a first and a second vertical cavity Fabry type -Pérot resonant at two different wavelengths of the desired absorption spectral band.

The nanoscale structure of claim 4, wherein the first and second thicknesses are substantially the same.

Nano structure according to one of the preceding claims, wherein the width of the elements of the metal network is adapted to obtain a plasmonic mode of order m = 3.

Nanoscale structure according to one of the preceding claims, further comprising a non-absorbing dielectric layer in the desired absorption spectral band, arranged between said absorbent layer and the metal network and / or encapsulating the metal network, for adjusting the thickness between the metal network and the metal reflector.

6. Nano structure according to one of the preceding claims, wherein the period (d) of the metal network is less than half the minimum wavelength of the desired absorption spectral band.

7. A nanoscale structure according to one of the preceding claims, wherein the metal network is one-dimensional, formed of strips (110), or two-dimensional formed of pads (120).

8. The nanoscale structure of claim 7, wherein the width of said strips or said pads is less than 150 nm.

9. Nano structure according to one of the preceding claims, wherein the thickness of the metal elements is less than 30 nm.

10. solar cell (100) comprising a substrate (101) and a nanometric structure according to one of claims 1 to 10 deposited on said substrate (101), and wherein the desired absorption spectral band is in the visible - near infrared.

The solar cell of claim 10, further comprising a transparent conductive layer (106) disposed between the metal reflector (102) and the absorbent layer (103).

12. The solar cell according to one of claims 10 or 11, further comprising a transparent conductive layer (105) disposed between the absorbent layer (103) and the metal network (104) or on the metal network and the absorbent layer.

13. Solar cell according to one of claims 11 or 12 wherein the transparent conductive layer comprises ZnO, ITO or SnO.

14. Solar cell according to one of claims 10 to 13, wherein the metal reflector is multilayer, comprising a lower layer for adhesion with the substrate and a top layer of noble metal, for example gold, silver or aluminum.

15. Solar cell according to one of claims 10 to 14, wherein the metal network is noble metal, for example gold, silver or aluminum.

16. Solar cell according to one of claims 10 to 15, wherein the absorbent layer comprises a material from a type III-V semiconductor, for example gallium arsenide or indium phosphide, amorphous silicon , the CIGS, the

Cadmium telluride or an organic material.

The solar cell of claim 10 wherein the absorbent nanoscale structure comprises:

a silver metal reflector;

an absorbent layer of GaAs with a thickness of less than 50 nm;

a silver metal network having a thickness of less than 30 nm, formed of pads or strips arranged periodically, the width of said pads or strips being between 80 and 120 nm, the linear filling factor being between 0, 5 and 0.7. The solar cell of claim 10 wherein the absorbent nanoscale structure comprises:

a silver metal reflector;

an absorbent layer GaSb of thickness less than 50 nm; a silver metal network with a thickness of less than 30 nm, formed of studs or bands arranged periodically, with a period of between 270 nm and

330 nm and linear filling factor of between 0.5 and 0.7;

a layer of conductive transparent material arranged on the metal network.

The solar cell of claim 10 wherein the absorbent nanoscale structure comprises:

a silver metal reflector;

an absorbent layer of CIGS less than 50 nm thick; a silver metal network with a thickness of less than 30 nm, formed of pads or strips arranged periodically, with a period of between 500 and 550 nm and with a linear filling factor of between 0.5 and 0.7; a layer of conductive transparent material arranged on the metal network.

20. Solar cell according to one of claims 18 or 19, wherein the layer of conductive transparent material is ZnO: A1, less than 50 nm thick.

21. A method of manufacturing a solar cell (100) according to one of claims 10 to 20 comprising:

depositing on the substrate (101) one or more metal layers to form the metal reflector (102),

depositing the absorbent layer (103) on said metal reflector, depositing a layer of resin and structuring the resin layer to form the elements of the network,

22. The method of manufacture of claim 21, wherein the resin is structured by nano printing.