WO2013030482A1 - Reflector device for rear face of optical devices - Google Patents

Abstract

A reflector device comprises an array (26) of patterns (28), which is periodic along at least one predetermined direction (Y), and is interposed between a first and a second different media (14, 30), the first medium (14) being a semiconducting material. Furthermore: ■ the refractive index n₂ of the patterns (28) is lower than the refractive index n₁ of the first medium (14); ■ the refractive index n₃ of the second medium (30) is lower than or equal to the refractive index n₂ of the patterns (28); ■ the array (26) is defined by the relations: P = λ_reg/n₃, where P is the period of the array (26) in the predetermined direction (Y), and λ_reg is a wavelength of adjustment of the array (26) which is less than the wavelength corresponding to the forbidden band of the semiconducting material of the first medium (14); 0.2 ≤ W/P ≤ 0.9, where W is the width of the patterns (28) of the array (26) in the predetermined direction (Y); and h₁ ≤ n₂ x λ_reg/2, where h₁ is the thickness of the patterns (28) of the array (26).

Description

 REFLECTIVE DEVICE FOR REAR SIDE OF OPTICAL DEVICES FIELD OF THE INVENTION

 The present invention belongs to the field of optical reflection and is particularly applicable in optical devices such as photovoltaic cells and photonic detectors.

STATE OF THE ART

Briefly, a photovoltaic cell comprises a slice of semiconductor material, usually based on monocrystalline silicon, capable of generating electron / hole pairs under the effect of an incident light flux. This wafer comprises metal electrodes on each of its front and rear faces for collecting the electric current thus produced. The "front" face usually designates the face of the cell receiving the incident luminous flux, and the "rear" face is the face opposite to the front face.

There is currently a production of photovoltaic cells that have increasingly thin silicon wafers in order to, on the one hand, achieve a saving in active material, and on the other hand, to reduce the average distance of travel of photo charges -Generated in the silicon towards the collection electrodes, and thus to limit the volume recombinations of the charges. This reduction in the thickness of the silicon wafer, however, implies a decrease in the overall optical absorption efficiency, since the wafer is all the more transparent to the incident light flux that it is thin. This transparency is however not uniform on the light spectrum and occurs mainly in an infrared spectral range close to the wavelength of the band gap.

FIG. 1 is a plot of the optical absorption of three monocrystalline silicon layers with a thickness of 50 microns, 100 micrometers and 150 microns, respectively. The absorption corresponds to a single passage of light through the silicon layer. As can be seen, the optical absorption drops for wavelengths greater than about 800 nanometers, namely for near-infrared wavelengths. Wavelengths below 800 nanometers remain well absorbed. This phenomenon is explained in particular by the depth of power penetration of wavelengths in monocrystalline silicon, which is equal to 10 micrometers at 800 nanometers, and drops rapidly beyond, ranging for example from 45 micrometers to 900 nanometers. Without any particular measure, part of the incident flux therefore passes through a thin photovoltaic cell without being absorbed and is therefore lost. A logical solution to compensate for this loss is to choose a metal electrode made on the entire back face taking advantage of the reflective properties of the metals, the electrode thus reflecting the portion of the luminous flux having passed through the cell towards the silicon wafer .

However, such a solution is not satisfactory for thin slices. First, there is a problem as to the mechanical strength of a thin wafer during the various stages of manufacture of the cell. In particular, a full plate metallization carried out on the rear face of the wafer, for example aluminum, generates a mechanical stress on the silicon wafer during the annealing steps used to form an ohmic contact between the wafer and the metallization. deposited on the back side. However, this constraint is even higher than the silicon wafer is thin. In addition, for a slice made of monocrystalline silicon, this mechanical stress is reflected on the silicon crystal, which greatly increases the risk of breakage of it. This is also why it is not used full plate electrode for thin silicon wafers but rather a partial metallization of the back side in the form of metal grid.

Then, a full plate metallization of the rear face increases the sensitivity of the cell to surface defects, in particular to the effects of surface recombination of the charges between the silicon and the metal of the electrode. Finally, a metal is not a perfect reflector and the electrode therefore absorbs a part of the radiation, a phenomenon that is difficult or impossible to remedy because of surface plasmons between the silicon and the metal that are generated by the presence of corrugations on the metal. In addition, a metal is more strongly absorbent in the infrared. However, the absorption of infrared causes a heating of the electrode, heating which is reflected on the silicon wafer, which degrades the quantum efficiency of it.

For all these reasons, a partial metallization of the rear face is carried out, usually by depositing on the back side of the wafer one or more layers of transparent dielectric passivation materials, for example silica, alumina or nitride. silicon, through which are formed openings for effecting the metallization of the electrode on the rear face. However, the portions of the rear face, which include only the passivation layer or layers, have a transparency disadvantageous and in particular do not substantially reflect the near infrared. The portion of luminous flux passing through the thin wafer at these portions is lost. Many alternative solutions to full-plate metallization have been proposed, in particular the formation of a Bragg mirror or photonic crystals on the rear face. However, these optical structures are bulky, require delicate technological manufacturing processes, and are incompatible with the manufacture of electrodes which requires deep annealing and etching in order to make ohmic metal / silicon alloy contacts.

It has also been proposed metallized diffraction gratings on the back face of the silicon wafer in order to redirect part of the flux in the wafer, but the redirection is only very partial and valid only in a narrow range of wavelengths defined by the geometric characteristics of the network. In addition, in the absence of special protection measures, a metallized diffraction grating very poorly supports the anneals used to manufacture the electrodes and is even partially destroyed. Thus, the alternative solutions, based on conventional concepts of optics, are not satisfactory since they do not allow to simply obtain a rear face that is reflective over a wide spectral range, at least on the portions of the back side not provided with metallization.

More recently, it has been discovered that only optical structures made of dielectric material, with high refractive index contrast, can block the passage of light in certain spectral bands by coupling light to so-called "slow" Bloch modes. ". This blocking power depends on the dimensioning of the structure and the refractive indices in the presence, without well defined design laws have so far been defined. For example, reference can be made to the works of Mateus et al described in WO2005 / 089098 and the document "Ultrabroadband mirror using low index cladded subwavelength grating", IEEE Photonics Technology Letters, vol.16, p. 518, 2004.

Referring to Figure 2, which shows Figure 1 of WO2005 / 089098, the latter describes a reflective optical structure 10 with high reflectivity comprising a substrate 12 made of silicon, on which rests a layer 14 of low-index material. refraction in Si0 ₂ , and a periodic grating of "sub-wavelength" patterns, that is to say at least one of its geometrical characteristics is smaller than the wavelength range with which structure is intended to interact. The network is formed on the low-index layer 14 and consists of parallel strips 16, identical, of rectangular section, regularly spaced and made of high refractive index material, such as polysilicon. The space 18 separating the strips 16, as well as the space above them, is filled with a material of low refractive index, namely air. The structural design parameters 10 are the refractive indices of the various elements 12, 14, 16, 18, the period A of the periodic grating strips 16, the thickness t _g _ strips 16, the fill factor FF network, defined as the ratio of the width / _g of the strips 16 over the period A, and the thickness _ ti of the low index layer 14.

FIG. 3, which reproduces FIG. 2A of document WO2005 / 089098, is a plot of the reflectivity as a function of the wavelength of a structure of period Λ = 700 nm, of thickness t _g = 0.46 μιη, of form factor FF = 0.75, and of thickness t _L greater than 100 nm. The refractive index of the silicon substrate 12 is equal to 3.48, the refractive index of the low index layer 14 is equal to 1.47, the refractive index of the air 18 is equal to 1 , and the refractive index of the bands 16 is equal to 3.48. The reflectivity is here considered for incident radiation on the strip network 16 as illustrated by the arrow 20. As can be seen, the structure 10 behaves as a near-perfect mirror for a wide range of centered wavelengths on the value 1.5 μιη.

In view of the reflectivity response of the structure 10, a simple idea would be to use this type of structure on the rear face of a thin silicon wafer of a photovoltaic cell in order to reflect the portion of luminous flux that has passed through the wafer. of silicon. In addition, it is necessary to adapt the structure 10 so that its reflectivity plateau is positioned below the cutoff wavelength of the silicon, which is not immediate in the absence of a design law well defined, the application of a modified structure for this purpose simply did not work.

Indeed, in order to reflect incident radiation on the strips 16, the medium 18 in which they must be immersed must necessarily be of lower refractive index. It is thus observed that, by replacing the air with silicon, the reflective behavior of the structure is very strongly degraded, its reflectivity being notably highly modulated and less than 45% due to a pronounced diffraction phenomenon. In addition, it should be noted that this type of structure has a behavior that differs very significantly in the direction of light. In particular, an incident radiation on the silicon substrate 12 of the structure 10, as illustrated by the arrow 22, simply does not have a pronounced reflective character, the structure being rather similar to a diffraction grating.

There is therefore a need for a structure capable of efficiently reflecting, over a wide spectral range, incident light through a material whose refractive index is greater than the refractive index of a material or a medium arranged on the opposite side of the structure.

SUMMARY OF THE INVENTION

The first object of the present invention is therefore to provide such a structure having a high reflectivity over a wide range of wavelengths.

For this purpose, the subject of the invention is a reflector device comprising a pattern network, periodic in at least one predetermined direction, interposed between first and second different media, and in contact with said media (14, 30), the first medium being of semiconductor material, wherein:

du premier milieu; ^■ the refractive index n ₂ of the patterns is less than the refractive index

the first medium;

^■ the refractive index n ₃ of the second medium is less than or equal to the index of refraction n ₂ of the reasons;

■ the network is defined by relationships:

o is the period of the network in the predetermined direction, and λ

 is a wavelength setting the network less than the wavelength corresponding to the forbidden band of the semiconductor material of the first medium;

o is the width of the patterns of the network according to the direction

 predetermined; and

 o is the thickness of the patterns of the network.

En d'autres termes, la structure du type précité présente un fort pouvoir réflecteur sur une large gamme de longueurs d'onde pour un flux lumineux traversant le premier milieu, par exemple un matériau semi-conducteur, tel que du silicium, c'est-à-dire un matériau qui présente un fort indice de réfraction.

In other words, the structure of the aforementioned type has a high reflectivity over a wide range of wavelengths for a luminous flux passing through the first medium, for example a semiconductor material, such as silicon, it is that is, a material that has a high refractive index.

According to one embodiment, the ratio is greater than or equal to 0.4 and lower or

equal to 0.7, which optimizes both the network function and the reflection function. According to one embodiment, the thickness of the patterns of the grating is greater than or equal to less than 0.05.

reflectivity, the network becoming more transparent at this wavelength as the thickness of the patterns decreases. More particularly, the thickness of the patterns is chosen even higher than the contrast between the indices r and n ₂ is low, which allows to obtain a constant or equivalent result for different thicknesses and indices.

inférieur ou égal à 0,75 permet notamment d'obtenir une réflectivité maximale proche de 1. According to one embodiment, the material of the first medium is, for example, crystalline silicon, and the refractive index n ₂ of the patterns of the grating is less than or equal to 0.75 times the refractive index r of the first medium, and preferably less than or equal to 0.5 times the refractive index of the first medium. In particular, it is observed that the higher the refractive index contrast, the better the reflectivity of the structure, and the greater the spectral range of operation. Refractive index contrast

less than or equal to 0.75 makes it possible to obtain a maximum reflectivity close to 1.

According to one embodiment, the rear face of the network comprises an intermediate layer, in particular a passivation layer, of refractive index n _2, which is between the refractive index n ₂ of the patterns and the refractive index n _3. medium and thickness less than or equal to the thickness ¾ of the patterns. In particular, the intermediate layer consists of SiOx, in particular Si0 _2, Al ₂ 03 SiN, SiN _4, Zr0 _2, Ti0 _2, B ₂ 0 _3, of a polymer such as e.g. PMMA (polymethyl methacrylate), a transparent conductive oxide of low refractive index, especially ZnO, Sn0 ₂ or ΓΙΤΟ, or a conductive transparent resin. As is known, the passivation layer, usually made of dielectric material or slightly transparent conductor, is generally necessary to ensure good performance of certain devices, and in particular the operation of a photovoltaic cell. The invention therefore also applies to photovoltaic cells having such a passivation layer. According to one embodiment, the front face of the network comprises a layer made of a material different from the material of the first medium, the different material having a refractive index η of between 0.8 times and 1.2 times the index of refraction r of the material of the first medium, and greater than the index of refraction n ₂ of the patterns of the network. For example, the material of the first medium is crystalline silicon and the different material is amorphous silicon or a III-V type semiconductor material. In some technologies, the active function, for example, photovoltaics of a photovoltaic cell, requires the stacking of several layers, for example of different doping. The reflector device according to the invention therefore applies to such devices. According to one embodiment, the periodic grating has at least two directions according to which it has a periodicity, in particular at least two perpendicular directions, so that the grating is insensitive to the polarization of light.

According to one embodiment, the first medium is for example made of silicon, and the units consist of SiOx, in particular SiO ₂ , Al ₂ O ₃ , Si, SiN ₄ , ZrO ₂ , TiO ₂ , B ₂ 0 ₃ , a polymer such as for example PMMA (polymethyl methacrylate), a transparent conductive oxide of low refractive index, in particular ZnO, SnO ₂ or ΓΙΤΟ, or a transparent resin conductive. According to one embodiment, the medium covering the periodic network is air. Alternatively, the medium covering the periodic network is an encapsulation layer, especially ethylene-vinyl acetate, polychlorinated biphenyl, or an epoxy resin. A second object of the invention is to provide a reflective structure on the rear face of a photovoltaic cell, and more particularly a non-necessarily metallic structure having a high reflectivity over a wide range of wavelengths shorter than the wavelength. breaking of the cell, and this for a luminous flux that passes through the photovoltaic cell. For this purpose, the invention also relates to a photovoltaic device comprising a slice of semiconductor material for absorbing a luminous flux, said slice having a front face receiving the incident luminous flux, a rear face opposite to the front face, and an electrode partially covering the rear face, in which:

du premier milieu ; ^■ the refractive index n ₂ of the patterns is less than the refractive index

the first medium;

^■ the refractive index n ₃ of the second medium is less than or equal to the index of refraction n ₂ of the reasons;

^■ the network is defined by relationships:

est une longueur d'onde de réglage du réseau inférieure à la longueur d'onde correspondant à la bande interdite du matériau semi-conducteur de la tranche ; o is the period of the network in the predetermined direction, and

is a wavelength setting the network less than the wavelength corresponding to the forbidden band of the semiconductor material of the wafer;

W is the width of the patterns of the network according to the direction

 predetermined; and

 is the thickness of the patterns of the network.

In other words, a reflector device of the aforementioned type is formed on at least a portion of the rear face of the wafer not covered by the electrode, said wafer constituting the first medium of the reflector device.

According to one embodiment, the slice of absorbent material has a drop in its optical absorption in a predetermined range of wavelengths less than the wavelength corresponding to the forbidden band of the semiconductor material, and the length of setting wave of the network is between

is the lower bound of said range. In particular, the adjustment wavelength is substantially equal to the lower limit of said range. this allows

to obtain a high reflection in the wavelength range for which the silicon wafer is poorly absorbent.

According to one embodiment, the refractive index n ₂ of the patterns of the grating is less than or equal to 0.75 times the refractive index r of the first medium, and preferably less than or equal to 0.5 times the index. of refraction r of the first medium. The invention also relates to the use of a reflector of the aforementioned type for reflecting a light flux incident on the first medium in a wavelength range greater than or equal to the wavelength of adjustment

Selon un mode de réalisation, la tranche est constituée de silicium, et la borne inférieure est comprise entre 750 nanomètres et 800 nanomètres.

According to one embodiment, the wafer is made of silicon, and the lower limit is between 750 nanometers and 800 nanometers.

According to another embodiment, the wafer is made of germanium, and the lower limit is greater than 1 micrometer.

The invention also relates to a method of manufacturing a photovoltaic device comprising a wafer of semiconductor material based on silicon for the absorption of a luminous flux, said wafer having a front face receiving the incident luminous flux, a rear face opposite to the front face, and an electrode partially covering the rear face.

According to the invention, this method consists of:

du matériau semiconducteur, et réalisé dans l'épaisseur d'au moins une partie de la face arrière non recouverte par l'électrode ; et forming a pattern network, periodic in at least one predetermined direction, of refractive index n ₂ less than the refractive index

semiconductor material, and made in the thickness of at least a portion of the rear face not covered by the electrode; and

covering at least the rear face portion comprising the grating of a medium of refractive index n ₃ less than or equal to the refractive index n ₂ of the patterns of the network and covering; the network being defined by relations: is the period of the network in the predetermined direction, is

a wavelength setting the network less than the wavelength corresponding to the forbidden band of the semiconductor absorbent material, and n ₃ is the refractive index of the medium covering the network;

is the width of the patterns of the network according to the direction

 predetermined; and

 is the thickness of the patterns of the network.

Le matériau semi-conducteur absorbant est du silicium cristallin, et la tranche a une épaisseur inférieure à 150 micromètres.

The absorbent semiconductor material is crystalline silicon, and the wafer has a thickness of less than 150 microns.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be better understood on reading the following description, given solely by way of example, and made with reference to the accompanying drawings, in which identical references designate identical or similar elements, and in which:

^■ Figure 1 is an optical absorption route of thin silicon wafers as a function of incident wavelength thereon;

^■ Figure 2 is a schematic perspective view of a reflective sub-wave length according to the prior art structure;

^■ Figure 3 is a plot of the reflectivity of the structure of Figure 2 as a function of the incident wavelength thereon;

^■ figure 4 is a schematic bottom view of a device according to a first embodiment of the invention illustrating an arranged wave length sub-network on the rear face of a photovoltaic cell;

^■ Figure 5 is a schematic sectional view along the plane AA of Figure 4;

^■ Figure 6 is a schematic sectional detail view of the device of Figure 4;

^■ Figure 7 is a plot illustrating the shape of the reflectivity obtained by wave length sub-structures of a device according to the invention depending on the incident wavelength on such structures;

^■ Figure 8 a schematic detail view in section of a device according to a second embodiment of the invention;

^■ Figures 9a and 9b are detail diagrammatic views in section of a device according to a third embodiment of the invention;

^■ Figure 10 details a sectional schematic view of a device according to a fourth embodiment of the invention;

^■ Figure 11 a schematic detailed view in section of a device according to a fifth embodiment of the invention;

^■ Figure 12 is a schematic top view of a periodic array according to a sixth embodiment of the invention;

^■ Figure 13 is a schematic top view of a periodic array according to a seventh embodiment of the invention; and

^■ Figures 14 to 18 are plots of reflectivity and transmission of embodiments of the invention depending on the incident wavelength on subwavelength networks. DETAILED DESCRIPTION OF THE INVENTION

It will now be described by way of example and with reference to FIGS. 4 to 6 a first embodiment of a photovoltaic device 10 according to the invention.

The device 10 comprises a photovoltaic cell 12. The cell 12 comprises a wafer 14 of refractive index absorbing material ι, in particular monocrystalline silicon or polycrystalline silicon which may comprise a germanium fraction less than 10%.

This wafer has a front face 16, intended to be irradiated by incident radiation 18 to be converted into current, and a rear face 20 opposite to the front face 16. The thickness of the wafer 14 is between 5 micrometers and 150 micrometers, and thus allows a portion of incident radiation 18 to pass, in particular wavelengths greater than about 800 nanometers as illustrated in FIG.

The cell 12 also comprises an array of metal electrodes 22 formed on and / or in the front face 16 for collecting the current generated in the wafer 14 by the absorption of the incident radiation, said electrodes being in ohmic contact with one or more PN junctions (not shown) made in the wafer 14. Similarly, the cell 12 comprises an array of metal electrodes 24 formed on and / or in the rear face 20 for the application of a potential to said rear face 20, said electrode array 24 only partially covering the rear face 20. The elements just described are conventional and therefore will not be described in more detail later. It will be remembered that the invention applies to any type of photovoltaic cell whose rear face 20 is not completely covered by metal. According to the invention, a periodic grating 26 of patterns 28, of refractive index n ₂ , is produced in the thickness of the portions of the rear face 20 between the electrodes 24 of the rear face. In particular, the grating 26 is constituted by parallel strips 28, identical, of rectangular section, regularly spaced, and arranged in the same direction X. The grating 26 is therefore periodic in the direction Y perpendicular to the direction X.

The device 10 finally comprises a medium 30 of refractive index n ₃ in contact with the periodic network 26. The geometrical characteristics of the grating 26 and the refractive indexes n ₁ , n ₂ , and n ₃ satisfy the following relationships:

P is the period of the grating 26 along the Y direction;

 is the setting wavelength of the network, less than the wavelength corresponding to the forbidden band of the absorbent material constituting the wafer 14; W is the width of the strips 28 along the Y direction; and

h ₁ is the thickness of the strips 28.

The periodic grating 26 thus has a high reflectivity in a wide range of wavelengths, in particular a width range greater than 200 nanometers, as schematically illustrated in FIG. 7.

The period P of the network 26 makes it possible to adjust the position of the range of high reflectivity wavelengths, and more particularly the lower limit of this range.

manière équivalente la période P du réseau 26 est choisie inférieure ou égale à . La

borne inférieure de la gamme de réflectivité élevée est notamment choisie entre 300

If a high reflectivity is desired for wavelengths greater than the value, the adjustment wavelength is chosen to be less than or equal to, or

equivalent way the period P of the network 26 is chosen less than or equal to. The

lower bound of the high reflectivity range is especially chosen between 300

nanometers and 1000 nanometers.

As previously described, there is a rapid drop in the optical absorption of wafer 14 from a wavelength of about 800 nanometers, as

. Notamment, la borne inférieure est réglée entre 750 nanomètres et 800 nanomètres. Notamment, en réglant la période P du réseau 26 sur une valeur inférieure comprise entre le réseau 26 se comporte

FIG. 1. The high reflectivity range of the grating 26 is thus set to coincide at least partially, and advantageously completely with the weak optical absorption range of the wafer 14, that is to say that the grating 26 is set to match the values

. In particular, the lower limit is set between 750 nanometers and 800 nanometers. In particular, by adjusting the period P of the network 26 to a lower value between the network 26 behaves

then as a reflector that returns to the wafer 14 the radiation having passed through it.

The refractive indices r, n ₂ , and n ₃ , and more particularly the index contrasts of the relations (1) and (2), the ratio, and the thickness of the relation (5) allow

to them to adjust the shape of the reflectivity of the grating 26. In particular, since a sufficient contrast exists between the refractive indices r and n ₂ of the wafer 14 and the patterns 28, namely for a wafer 14 in particular based on silicon a contrast less than or equal to 0.75, and preferably less than or equal to 0.5

selon la relation (4), préférentiellement un rapport compris entre 0,4 et 0,7 pour optimiser à la fois la fonction de réseau et la fonction de réflexion, et une épaisseur comprise entre , permettent d'obtenir un plateau de réflectivité supérieur à 0,9

to improve reflectivity and to broaden the range of reflection, a report

according to relation (4), preferably a ratio of between 0.4 and 0.7 to optimize both the network function and the reflection function, and a thickness between, make it possible to obtain a reflectivity plateau greater than 0.9

over a wavelength range of greater than 200 nanometers. The network 26 therefore behaves as a near-perfect mirror plane over a wide range of wavelengths greater than the wavelength.

Preferably, the thickness of the patterns 28 is chosen the higher the index contrast is low. Indeed, the lower the index difference, the greater the thickness

of the network is advantageously important to obtain an equivalent result. For example, for nanometers, and for

nanometers.

The units 28 are advantageously constituted by SiOx, in particular SiO ₂ , Al ₂ O ₃ , SiN, SiN ₄ , ZrO ₂ , TiO ₂ , B ₂ O ₃ , a polymer such as, for example PMMA (polymethyl methacrylate), a transparent conductive oxide of low refractive index, especially ZnO, Sn0 ₂ or ΓΙΤΟ, or a conductive transparent resin. Materials with a high melting point, such as Si0 ₂ , Al ₂ O ₃ , and TiO ₂ , are preferred because they are known in the manufacture of silicon-based components. and that the patterns 28 made of these materials undergo substantially no degradation during final anneals usually used in the manufacture of photovoltaic cells. Advantageously, the medium 30 is air with a refractive index close to 1. Similarly, the units 30 may be filled with air, the network 26 then consisting of a trench network.

In a variant, the photovoltaic cell 12 with its grating is encapsulated in a low refractive index protection material constituting the medium 30, such as for example EVA (ethylene-vinyl acetate), PCB (polychlorinated biphenyl), a epoxy resin, or PMMA, which are transparent and have a refractive index of less than 1.5.

In the photovoltaic cell 12 just described, the absorption wafer 14 consists of a single material and the network is in direct contact with the medium 30. In some applications, the cell 12 has a more complex structure at the level of its back side.

In a first variant of the invention, illustrated by the view in detail of FIG. 8, a thin film 40 of refractive index material, different from the absorbent material

of the wafer 14, is deposited on the rear face 20 of the wafer 14 and the network 26 is formed in this additional layer 40. The layer 40 is for example planar or in line with the network 26 as illustrated in FIG. a thickness above

patterns 26 lower than for example, and has a refractive index t which differs from

less than 20% of the refractive index r ^ of the absorbent material of the wafer 14. For example, the layer 40 is made of amorphous silicon or a type III-V semiconductor. Such a layer 40 does not substantially modify the reflectivity of the grating 26 as described above and makes it possible, for example, to constitute active elements, in particular photodiodes, on this face of the wafer 14.

In a second variant of the invention, illustrated by the view in detail of FIGS. 9a and 9b, an intermediate layer, in particular a passivation layer 42, of refractive index n ₂ , covers at least the portions of the rear face 20 Without metallization, and advantageously all of the rear face 20. As is known per se, the passivation layer 42 improves the performance of the photovoltaic cell and consists of a transparent dielectric or slightly conductive material. The refractive index n ₂ of the layer 42 is chosen between the refractive index n ₂ of the patterns 28 and the refractive index n ₃ of the medium 30, and the thickness h ₂ of the passivation layer 42 is smaller than the thickness of the patterns 28, so that the layer 42 does not substantially modify the reflectivity of the grating 26 as described above. The passivation layer 42 may be flat, as shown in FIG. 9a, or conform to the shape of the patterns 28 of the grating 26, the passivation layer 42 defining in the latter case an array of patterns, as illustrated in FIG. Figure 9b.

It is observed that the reflectivities of the grating 26 for these two embodiments of the layer 42 are substantially identical. Simple production methods of the patterns 28 and the layer 42 can thus be used by depositing the same thickness of material on the rear face 20 in which trenches are formed. The layer 42 may also be made of the same material as that of the patterns 28.

For example, the layer 42 consists of SiOx, in particular SiO ₂ , Al ₂ O ₃ , Si, SiN ₄ , ZrO ₂ , TiO ₂ , B ₂ O ₃ , a polymer such as for example PMMA (polymethyl methacrylate), a transparent conductive oxide of low refractive index, in particular ZnO, SnO ₂ or ΓΙΤΟ, or a transparent conductive resin.

In what has been described above, the patterns 28 of the grating 26 have a rectangular section. Other sections are possible for the patterns 28, such as for example a trapezoidal section as illustrated in FIG. 10, the width W being that at mid-height of the patterns 28, or a semicircular section as illustrated in FIG. Figure 11, the width W being the diameter of the semicircles. In what has been described previously, the periodic grating 26 is one-dimensional, that is to say it has only one direction of periodicity, namely the Y direction perpendicular to the strips 28. Two-dimensional gratings, c that is to say comprising at least two periodicity axes, are also possible, for example to make the device independent of the polarization of the incident light.

For example, the pattern pattern includes bands forming closed concentric contours, such as concentric squares as shown in FIG. 12, or concentric circles. In a variant, the network is a matrix of square, rectangular or circular islands, as illustrated in FIG. 13. The relationships described above then apply at each periodicity, the characteristics of the network possibly being identical or different according to the directions of periodicity. Examples of reflectivity and periodic network transmission 26 will be described as a function of the incident wavelength on a device according to the invention comprising an absorption slice 14 of crystalline silicon of refractive index r equal to at 3.55 and less than 150 micrometers thick, networks being set to define a lower limit substantially equal to 800 nanometers.

Figure 14 is a reflectivity plot of a periodic array 26 of rectangular section strips 28. The patterns 28 of the grating 26 are in Si0 ₂ , of refractive index n ₂ equal to

1.5, the period P of the network is equal to 700 nanometers and the medium 30 is air of refractive index n ₃ equal to 1. The ratio is equal to 0.55, and the thickness of the patterns 28 is

equal to 250 nanometers.

The reflectivity in solid line corresponds to a grating 26 directly in contact with the air, whereas the dotted line reflectivity corresponds to a grating 26 covered with a passivation layer 42 made of Si0 ₂ with a thickness h ₂ equal to 100 nanometers. In the view of FIG. 14, it can be observed in particular that the passivation layer 42 has the effect of reinforcing the reflectivity plateau of the grating 26.

Figure 15 is a plot of the reflectivity and transmission of a periodic array 26 of strips 28 of rectangular section. The patterns 28 of the grating 26 are made of A1 ₂ 0 ₃ , of refractive index n ₂ equal to 1.8, the period P of the grating is equal to 575 nanometers, and the medium 30 is a transparent encapsulation material. refractive index n ₃ equal to

1.45. The ratio is equal to 0.55, and the thickness of the patterns 28 is equal to 250

nanometers. The grating 26 is also covered with a passivation layer 42 at A1 ₂ 0 ₃ with a thickness h ₂ of 50 nanometers.

As can be seen, a reflectivity plateau close to 1 is obtained over a wide range of wavelengths, even in the presence of the encapsulation material which is usually used in photovoltaic modules to protect the cells. In addition, it is noted that the transmission increases rapidly for medium and far infrared, that is to say beyond the wavelength A _c corresponding to the bandgap of the absorption wafer 14. Thus, radiation, which is not absorbed by the latter, does not have the effect of heating the network 26 and the layers of passivation material and encapsulation, and thus to heat back the absorption slice 14. Note also that for light from the medium 30, the structure is not reflective but passes on average 85% of the radiation to the medium 14 over the entire spectral range. This is particularly advantageous for example for bifacial photovoltaic cells which recover light also by their rear face.

FIG. 16 is a plot of reflectivity and transmission of reflectivity and transmission of a periodic grating 26 of rectangular section strips 28. The patterns 28 of the grating 26 are made of SiN, of refractive index n ₂ equal to 2, the period P of the grating is equal to 530 nanometers, and the medium 30 is a transparent encapsulation material of refractive index n ₃ equal to 1.45. The ratio is equal to 0.6, and the thickness of

patterns 28 is equal to 270 nanometers. The grating 26 is also covered with a passivation layer 42 of SiN having a thickness h ₂ of 50 nanometers.

As can be seen, even in the presence of the encapsulation material, a reflectivity plateau close to 1 is obtained over the entire range of wavelengths in which the absorption of quantum absorption is low. .

FIG. 17 is a plot of the reflectivity of a periodic grating 26 of trapezoidal section strips 28. The patterns 28 of the grating 26 are made of Si0 ₂ , the period P of the grating is equal to 700 nanometers and the medium 30 is air. The report is equal to

 0.62, the thickness of the patterns 28 is equal to 250 nanometers, and the network 26 is directly in contact with the air.

FIG. 18 is a plot of the reflectivity of a periodic grating 26 of strips 28 with semicircular section. The patterns 28 of the grating 26 are made of Si0 ₂ , the period P of the grating is equal to 700 nanometers, and the medium 30 is air. The report is equal to

 0.7, the thickness of the patterns 28 is equal to 250 nanometers, and the network 26 is directly in contact with the air. Of course, the embodiments described above are not mutually exclusive of each other and can be combined.

Silicon-based photovoltaic cells have been described. Of course, the invention also applies to other types of photovoltaic devices, and more generally photonic devices, for example infrared detectors. Likewise, the invention applies to devices comprising an absorbing semiconductor material different from silicon, for example an absorbent material in CdTe, CdHgTe, InSb, Ge, or GaP. The detectors to which the invention applies comprise, as in the photovoltaic device embodiments described above, a semiconductor substrate whose front face, which receives the light, comprises the active detection element, and one face rear which comprises the reflector device according to the invention. In particular, the photovoltaic device comprises an absorption slice made of germanium and the lower limit of the high reflection range of the grating is greater than 1 micrometer.

Similarly, mathematical relationships involving refractive indices have been described. It will be understood that in the case of complex refractive indices, the relationships described above apply to the real parts of the indices, the imaginary parts being smaller than their respective real parts.

Claims

 1. reflector device comprising a network (26) of patterns (28), periodic in at least one predetermined direction (Y), interposed between a first and a second different media (14, 30) and in contact with said media (14, 30); ), the first medium (14) being of semiconductor material, wherein: ^■ the refractive index n ₂ of the units (28) is lower than the refractive index i of the first medium (14); ^■ the refractive index n ₃ of the second medium (30) is less than or equal to the refractive index n ₂ of the units (28); ^■ the network (26) is defined by the relations: o is the period of the network (26) according to the direction

predetermined (Y), and is a network setting wavelength (26)

 less than the wavelength corresponding to the forbidden band of the semiconductor material of the first medium (14); o is the width of the patterns (28) of the grating (26) according to the

 predetermined direction (Y); and  o is the thickness of the patterns (28) of the grating (26).

Reflector device according to Claim 1, characterized in that the ratio

is greater than or equal to 0.4 and less than or equal to 0.7. 3. Reflective device according to one of claims 1 and 2, characterized in that the thickness of the patterns of the network is greater than or equal to

4. Reflector device according to any one of the preceding claims, characterized in that the refractive index n ₂ of the patterns of the grating is less than or equal to 0.75 times the refractive index n ₁ of the first medium, and of preferably less than or equal to 0.5 times the refractive index n ₁ of the first medium. 5. reflector device according to any one of the preceding claims, characterized in that the rear face (20) of the network (26) comprises an intermediate layer (42), in particular a passivation layer, refractive index n ₂ included between the refractive index n ₂ of the patterns (28) and the refractive index n ₃ of the medium (30) and of thickness less than or equal to the thickness of the patterns (28). 6. reflector device according to any one of the preceding claims, characterized in that the intermediate layer (42) consists of SiOx, including Si0 ₂ , Al ₂ 0 ₃ , Si, SiN ₄ , Zr0 ₂ , TiO ₂ , B ₂ 0 ₃ , a polymer such as for example PMMA (polymethyl methacrylate), a transparent conductive oxide of low refractive index, in particular ZnO, SnO ₂ or ΓΙΤΟ, or a transparent conductive resin. Reflective device according to any one of the preceding claims, characterized in that the front face of the grating (26) comprises a layer (40) made of a material different from the material of the first medium (14), the material being different ( 40) having a refractive index n between 0.8 times and 1.2 times the refractive index of the material of the first medium (14), and greater than the index of refraction n ₂ of the patterns (28) of the grating . 8. Reflector device according to claim 7, characterized in that the material of the first medium (14) is crystalline silicon and the different material (40) is amorphous silicon or a III-V type semiconductor material. 9. reflector device according to any one of the preceding claims, characterized in that the periodic array (26) has at least two directions in which it has a periodicity, in particular at least two perpendicular directions. Reflection device according to any one of the preceding claims, characterized in that the first medium (14) is made of silicon, and in that the units (28) consist of SiOx, in particular SiO ₂ , Al ₂ O ₃ , SiN, SiN ₄ , ZrO ₂ , TiO ₂ , B ₂ 0 ₃ , a polymer such as, for example, PMMA (polymethyl methacrylate), a transparent conductive oxide of low refractive index , in particular ZnO, Sn0 ₂ or ΓΙΤΟ, or a conductive transparent resin. 11. reflector device according to any one of the preceding claims, characterized in that the medium (30) covering the periodic array (26) is air. 12. reflector device according to any one of claims 1 to 10, characterized in that the medium (30) covering the periodic array (26) comprises an encapsulation layer, in particular ethylene vinyl acetate, polychlorinated biphenyl, in an epoxy resin, or polymethyl methacrylate. 13. A photovoltaic device (10) comprising a wafer (14) of semiconductor material for absorbing a luminous flux, said wafer (14) having a front face (16) receiving the incident luminous flux (18), a rear face (20) opposite to the front face (16), and an electrode (24) partially covering the rear face (20), characterized in that it comprises a reflector device according to any preceding claim on at least one part of the rear face (20) not covered by the electrode (24), said wafer (14) constituting the first medium of the reflector device. Photovoltaic device (10) according to claim 13, characterized in that the wafer (14) of absorbent material has a drop in its optical absorption in a predetermined range of wavelengths shorter than the wavelength corresponding to the forbidden band of the semiconductor material, and in that the adjusting wavelength of the grating (26) of the reflecting device is included

 between is the lower bound of said range.

Photovoltaic device (10) according to claim 14, characterized in that the adjustment wavelength is substantially equal to the lower limit of

said range. 16. Photovoltaic device (10) according to claim 14 or 15, characterized in that the wafer (14) is made of silicon, and in that the lower limit is between 750 nanometers and 800 nanometers. 17. Photovoltaic device (10) according to claim 14 or 15, characterized in that the wafer (14) is made of germanium, and in that the lower limit is greater than 1 micrometer. 18. A method of manufacturing a photovoltaic device comprising a slice of semiconductor material for absorbing a luminous flux, said slice having a front face receiving the incident luminous flux, a rear face opposite to the front face, and an electrode partially covering the rear face, characterized in that it consists: forming a pattern network, periodic in at least one predetermined direction, of refractive index n ₂ less than the refractive index

semiconductor material constituting the wafer, and made in the thickness of at least a portion of the rear face not covered by the electrode; and covering at least the rear face portion comprising the grating of a medium of refractive index n ₃ less than or equal to the refractive index n ₂ of the patterns of the network and covering;  the network being defined by relations: is the period of the network in the predetermined direction,

is a wavelength of adjustment of the network less than the wavelength corresponding to the forbidden band of the constituent semiconductor absorbent material of the wafer, and n ₃ is the index of refraction of the medium covering the network; is the width of the patterns of the network according to the direction

 predetermined; and  is the thickness of the patterns of the network.

19. A method of manufacturing a photovoltaic device according to claim 18, characterized in that the absorbent semiconductor material is crystalline silicon, and in that the wafer (14) has a thickness of less than 150 micrometers.