FR3094569A1 - PHOTOVOLTAIC CELL - Google Patents

Abstract

Photovoltaic cell The invention relates to a photovoltaic cell (10) comprising a stack of layers, the layers being superimposed along a stacking direction, the stack comprising: - an active layer, - a mirror, and - a layer intermediate interposed between the active layer and the mirror, the intermediate layer being made of a first material and having a set of nanostructures (14) arranged in a network, at least one nanostructure (14) being devoid of a plane of symmetry not parallel to the intermediate layer having an intersection reduced to a point with the stacking direction, said at least one nanostructure (14) being made of a second material, the first material being distinct from the second material. Figure for the abstract: Figure 2

Description

PHOTOVOLTAIC CELL

The present invention relates to a photovoltaic cell.

In the field of photovoltaic cells having a thickness of less than 500 nanometers (nm), the majority of sunlight passes through the active layer without being absorbed.

As a result, the absorption over the spectral range of sensitivity of the cell is very low. Therefore, it is desirable to use optical trapping techniques to increase absorption.

In particular, such optical trapping can be obtained by the use of metallic nanostructures adapted to generate surface plasmons. A surface plasmon corresponds to the excitation of conduction electrons at the interface between a metallic material and a dielectric material.

Examples of such structures are detailed in the article by Harry A. Atwater entitled “ Plasmonics for improved photovoltaic devices ” published in the journal Nature Materials of March 2010 (volume 9).

However, the yield of such structures is still too low.

There is therefore a need for a photovoltaic cell having improved efficiency.

To this end, the description describes a photovoltaic cell comprising a stack of layers, the layers being superimposed along a stacking direction, the stack comprising an active layer, a mirror, and an intermediate layer interposed between the active layer and the mirror, the intermediate layer being made of a first material and having a set of nanostructures arranged in a network, at least one nanostructure being devoid of a plane of symmetry not parallel to the intermediate layer having an intersection reduced to a point with the stacking direction, said at least one nanostructure being made of a second material, the first material being distinct from the second material.

• ladite au moins une nanostructure comporte au plus un plan de symétrie comportant la direction d’empilement.
• le premier matériau présente un premier indice optique, le deuxième matériau présente un deuxième indice optique, le rapport entre le premier indice optique et le deuxième indice optique étant compris entre 1,5 et 3.
• le premier matériau et le deuxième matériau sont choisis dans un groupe constitué des paires suivantes : {Ag, TiO₂}, {SiO₂, AlInP}, {SiO₂, Au}, {TiO₂, Au}, {SiO₂/Cu}, {TiO₂, AlInP}, {SiO₂, AlAsSb} et {Si₃N₄/InAlAs}.
• chaque nanostructure est agencée selon des lignes parallèles.
• chaque nanostructure est agencée au sommet d’un hexagone ou au centre de l’hexagone, les hexagones formés par les nanostructures partageant au moins un côté.
• chaque nanostructure présente une forme choisie dans le groupe constitué d’un cylindre, d’un cylindre à base triangulaire, d’une pyramide, d’une pyramide à base carrée et d’une pyramide à base triangulaire.
• un pas est défini pour le réseau, le pas étant compris entre 500 nanomètres et 700 nanomètres.
• il est défini une hauteur pour chaque nanostructure comme étant la dimension le long de la direction d’empilement et une longueur pour chaque nanostructure comme étant la dimension le long d’une direction perpendiculaire à la direction d’empilement, le rapport entre la hauteur et la dimension étant inférieur ou égal à 60%, de préférence supérieur ou égal à 40%.
• un pas est défini pour le réseau, le rapport entre la dimension et le pas étant supérieur ou égal à 80%.

According to particular embodiments, the photovoltaic cell has one or more of the following characteristics, taken separately or according to all the technically possible combinations:

• said at least one nanostructure comprises at most one plane of symmetry comprising the stacking direction.
• the first material has a first optical index, the second material has a second optical index, the ratio between the first optical index and the second optical index being between 1.5 and 3.
• the first material and the second material are chosen from a group consisting of the following pairs: {Ag, TiO ₂ }, {SiO ₂ , AlInP}, {SiO ₂ , Au}, {TiO ₂ , Au}, {SiO ₂ /Cu }, {TiO ₂ , AlInP}, {SiO ₂ , AlAsSb} and {Si ₃ N ₄ /InAlAs}.
• each nanostructure is arranged along parallel lines.
• each nanostructure is arranged at the top of a hexagon or at the center of the hexagon, the hexagons formed by the nanostructures sharing at least one side.
• each nanostructure has a shape selected from the group consisting of a cylinder, a triangular-based cylinder, a pyramid, a square-based pyramid and a triangular-based pyramid.
• a pitch is defined for the grating, the pitch being between 500 nanometers and 700 nanometers.
• a height is defined for each nanostructure as being the dimension along the stacking direction and a length for each nanostructure as being the dimension along a direction perpendicular to the stacking direction, the ratio between the height and the dimension being less than or equal to 60%, preferably greater than or equal to 40%.
• a step is defined for the network, the ratio between the dimension and the step being greater than or equal to 80%.

Characteristics and advantages of the invention will appear on reading the following description, given solely by way of non-limiting example, and made with reference to the appended drawings, in which:

- FIG. 1 is a schematic representation of an example of a photovoltaic cell comprising a stack of layers including an intermediate layer;

- FIG. 2 is a top view of an example of an intermediate layer;

- FIG. 3 is a top view of another example of an intermediate layer;

- FIG. 4 is a perspective diagram of an example of a nanostructure forming part of the intermediate layer of the cell of FIG. 1;

- FIG. 5 is a perspective diagram of yet another example of a nanostructure forming part of the intermediate layer of the cell of FIG. 1;

- FIG. 6 is a perspective diagram of another example of a nanostructure forming part of the intermediate layer of the cell of FIG. 1;

- FIG. 7 is a perspective diagram of yet another example of a nanostructure forming part of the intermediate layer of the cell of FIG. 1;

- FIG. 8 is a graph showing the variation of the absorption of a first example of cell over the operating spectral band of the cell;

- FIG. 9 is a graph showing the variation of the absorption of two reference cells over the spectral band of operation of the cells;

- FIG. 10 is a graph showing the variation of the absorption of a second example of cell over the operating spectral band of the cell;

- FIG. 11 is a graph showing the variation of the absorption of a third example of cell over the operating spectral band of the cell;

- FIG. 12 is a graph showing the variation of the absorption of a fourth example of cell over the operating spectral band of the cell;

- FIG. 13 is a graph showing the variation of the absorption of a fifth example of cell over the operating spectral band of the cell, and

- FIG. 14 is a graph showing the variation of the absorption of a sixth example of cell over the operating spectral band of the cell.

A photovoltaic cell 10 is shown schematically in Figure 1.

A photovoltaic cell is an element capable of converting incident solar energy into electrical energy.

Cell 10 is, for example, a III-V semiconductor cell.

A "III - V" type semiconductor is a composite semiconductor made from one or more elements from column III of the periodic table of elements (boron, aluminium, gallium, indium, etc.) and one or more column V elements or pnictogens (nitrogen, phosphorus, arsenic, antimony, etc.).

In the present case, the cell 10 is a cell made with semiconductors such as GaAs, InP or InGaAs.

Alternatively, cell 10 is a CIGS thin film cell or CdTe.

The cell 10 comprises a set 12 of layers.

The layers of set 12 are planar layers.

The layers are superimposed along a stacking direction. The stacking direction is represented by an axis Z in FIG. 1 and is denoted stacking direction Z in the remainder of the description.

According to the example shown in Figure 1, the set of layers has seven layers.

The seven layers of set 12 are now described from top to bottom, with the topmost layer being the layer that first interacts with incident light.

The first layer C1 is an antireflection layer.

The first layer C1 has a first thickness e1.

By definition, the thickness of a layer is the dimension of a layer along the stacking direction Z.

For example, the first thickness e1 is between 25 nm and 125 nm.

A quantity X is between two values A and B when the quantity X is greater than or equal to A and less than or equal to B.

According to the case represented, the first thickness e1 is equal to 90 nm.

The first layer C1 is made of a first material M1.

According to a particular example, the first material M1 is MgF ₂ .

As a variant, the first material M1 is SiO ₂ , Si ₃ N ₄ .

The first material M1 has a first optical index.

The first optical index is a low index.

By the expression 'low index' is meant a material whose optical index is less than or equal to 2.0.

The second layer C2 is a layer also serving as antireflection, that is to say a layer which reduces the proportion of light incident on the assembly 12 and reflected by the assembly 12.

The second layer C2 has a second thickness e2.

For example, the second thickness e2 is between 30 nm and 50 nm.

According to the case represented, the second thickness e2 is equal to 40 nm.

The second layer C2 is made of a second material M2.

According to a particular example, the second material M2 is ZnS.

Alternatively, the second material M2 is TiO ₂ or Si ₃ N ₄ .

The second material M2 has a second optical index.

The second optical index is a strong index.

By the expression “high index”, it is understood a material having an optical index greater than or equal to 2.0.

Preferably, the second optical index is less than or equal to 3.0.

The third layer C3 serves as a passivation layer.

The third layer C3 also serves selective contacts to collect electrons and holes in the solar cell.

The third layer C3 has a third thickness e3.

For example, the third thickness e3 is between 5 nm and 15 nm.

According to the case represented, the third thickness e3 is equal to 10 nm.

The third layer C3 is made of a third material M3.

According to a particular example, the third material M3 is AlInP.

Alternatively, the third material M3 is AlAsSb or InAlAs.

The fourth layer C4 is an active layer.

The fourth layer C4 is often referred to as an absorber.

The fourth layer C4 has a fourth thickness e4.

The fourth thickness e4 is less than or equal to 100 nm.

In particular, the fourth thickness e4 is between 40 nm and 60 nm.

According to the case represented, the fourth thickness e4 is equal to 50 nm.

The fourth layer C4 is made of a fourth material M4.

According to a particular example, the fourth material M4 is InGaAs.

As a variant, other III-V semiconductor materials can be envisaged, such as InGaAsP, GaAs or InGaP.

The fifth layer C5 has the same role as the third layer C3.

The fifth layer C5 is similar to the third layer C3. The same remarks as for the third layer C3 are also valid for the fifth layer C5 and are not repeated here.

The sixth layer C6 is an intermediate layer.

It should be noted that the sixth layer C6 is inserted between the fourth layer C4, that is to say the active layer, and the seventh layer C7 described later which plays the role of a mirror.

The sixth layer C6 is capable of generating the optical trapping effect observed in the fourth layer C4.

The sixth layer C6 has a sixth thickness e6.

For example, the sixth thickness e6 is between 50 nm and 800 nm.

As visible in FIG. 2, the sixth layer C6 comprises a base layer presenting a set of nanostructures 14.

The base layer is made of a base material MB.

The base material MB and the nanostructure material MN are distinct.

The base material MB has a base optical index MB denoted nB.

The MB base material is a high index material.

Preferably, the optical index nB of the base material MB is less than or equal to 4.0.

In particular, the base material MB is a non-absorbing wide-gap semiconductor.

By the expression “large gap”, it is understood a gap energy greater than or equal to 2 eV.

According to a variant, the base material MB is also absorbent and active.

A nanostructure is, by definition, a structure whose each dimension is strictly less than 1 micrometer.

The nanostructures 14 are arranged in a network.

A lattice is a regular set of elements.

In particular, according to the example described, the nanostructures 14 are arranged along parallel lines, the nanostructures of a line being equidistant from each other.

The distance between each line is called the lattice pitch P.

The grating pitch P is between 500 nm and 700 nm.

Preferably, the pitch P of the grating is between 550 nm and 750 nm.

According to another example, the network is a hexagonal network.

Advantageously, the hexagonal lattice is the lattice shown in Figure 3.

In such a case, each nanostructure 14 is arranged at the top of a hexagon or at the center of the hexagon, the hexagons formed by the nanostructures 14 sharing at least one side.

According to the example described with reference to FIG. 2, each nanostructure 14 is identical.

Each nanostructure 14 has an asymmetric shape .

By definition, it is understood that a shape is asymmetrical when the shape has at most one plane of symmetry including the stacking direction Z.

Preferably, the asymmetric shape comprises at most one plane of symmetry comprising the stacking direction Z.

For convenience, it is possible at this stage to introduce a first transverse direction marked by an X axis in Figure 1 (and orthogonal to the sheet) as well as a second transverse direction marked by a Y axis in Figure 1 and orthogonal to the first direction X. Each of the transverse directions X and Y is orthogonal to the stacking direction Z.

In the example described in figure 2, each nanostructure 14 comprises a plane of symmetry comprising the stacking direction Z.

More precisely, the plane of symmetry is the plane comprising the stacking direction Z and forming an angle of 45° with each of the transverse directions X and Y.

Several shapes are possible with reference to Figures 2 to 7.

Figures 2 to 5 propose a nanostructure 14 having the shape of a cylinder.

In particular, it is proposed in Figures 2 and 3 a cylinder with a square base.

In the case of Figure 4, the base of the cylinder is a stepped shape.

More specifically, in the example shown, the staircase having a first side extending along the first transverse direction X and a second side extending along the second transverse direction Y.

The first side and the second side of the staircase have the same dimension.

The first side is connected to the second side by a series of floors, four in this case.

A floor is a set with one side parallel to the second transverse direction Y and one side parallel to the first transverse direction X.

Both sides of a story are equal, so each side is equal to the dimension of either the first side or the second side divided by the number of stories.

The base of the cylinder can have other shapes not shown, such as an "L".

Figure 5 corresponds to the case of a pyramid.

In the proposed case, the pyramid has a triangular base and more specifically an isosceles right triangle.

Alternatively, the pyramid has a square base.

Another example of the shape of nanostructure 14 is proposed in figure 6.

In such a case, the nanostructure 14 is a superposition of stairs as described with reference to FIG. 4, the stairs having a decreasing size along the stacking direction.

In the example of figure 4, the nanostructure 14 comprises a superposition of three staircases.

The ratio of the sides of a stair to the sides of a stair that are in contact with it is between 50% and 75%, typically equal to 66%.

In each of the cases proposed, the shape of the nanostructure 14 has one face contained in the plane comprising the first transverse direction X and the stacking direction Z and another face contained in the plane comprising the second transverse direction Y and the direction d. stacking Z.

For each proposed shape, a first dimension is defined along the first transverse direction X, denoted d1, a second dimension along the second transverse direction Y, denoted d2 and a third dimension along the stacking direction Z and denoted height h.

It should be noted that, in each of the cases illustrated, the first dimension d1 and the second dimension d2 are equal.

For the sake of simplification, the value of the first dimension d1 and of the second dimension d2 will be called length d.

However, this is not mandatory, especially in the case of L, the first dimension d1 and the second dimension d2 differ.

In such a case, the length d is taken equal to the geometric mean of the two dimensions d1 and d2.

The length d is between 400 nm and 680 nm.

Preferably, the length d is between 400 nm and 600 nm.

The height h is between 100 nm and 400 nm.

According to one example, the ratio between the height and the length, said first ratio R1 is less than or equal to 60%.

The first ratio R1 is also greater than or equal to 20%.

According to certain particular cases, the first ratio R1 is greater than or equal to 40%.

In addition, the ratio between the length d and the pitch P, called second ratio R2, is greater than or equal to 80%.

The nanostructures 14 are made of a material with MN nanostructures.

The MN nanostructure material is a dielectric material.

The MN nanostructure material has an optical index of nMN nanostructures.

The MN nanostructure material is a low index material.

Preferably, the optical index of nMN nanostructures is greater than or equal to 1.1.

The ratio between the basic optical index nB and the optical index of nanostructures nMN, known as the fourth ratio R4, is between 1.5 and 3.0.

Several specific examples of materials are possible.

According to a first example, the base material MB is TiO ₂ and the nanostructure material MN is Ag.

As a variant, the pair for the base material MB and the nanostructure material MN is chosen from the following pairs: SiO ₂ /Au, TiO ₂ /Au, TiO ₂ /Cu and SiO ₂ /Cu.

More generally, according to the first example, the pair chosen is a pair of SiO ₂ or TiO ₂ with a metallic element.

According to a second example, the base material MB is SiO ₂ and the nanostructure material MN is AlInP.

As a variant, the pair for the base material MB and the nanostructure material MN is chosen from the following pairs: TiO ₂ /AlInP, SiO ₂ /AlAsSb and Si ₃ N ₄ /InAlAs .

More generally, according to the second example, the pair chosen is a pair of SiO ₂ or TiO ₂ or Si ₃ N ₄ with a III-V semiconductor element, in particular a ternary element.

In summary, the pair for the base material MB and the nanostructure material MN are chosen from a group consisting of the following pairs: {Ag, TiO ₂ }, {SiO ₂ , AlInP}, {SiO ₂ , Au}, {TiO ₂ , Au}, {SiO ₂ /Cu}, {TiO ₂ , AlInP}, {SiO ₂ , AlAsSb} and {Si ₃ N ₄ /InAlAs}.

The seventh layer C7 is a mirror.

In this case, the seventh layer C7 is a plane mirror.

The seventh layer C7 has a seventh thickness e7.

For example, the seventh thickness e7 is between 100 nm and 3 μm.

The seventh layer C7 is made of a seventh material M7.

The seventh M7 material is a metal.

According to a particular example, the seventh material M7 is Ag.

Alternatively, the seventh material M7 is Au, Cu or Al.

According to a variant, the seventh layer C7 has a spacer.

Preferably, the sum of the seven thicknesses is less than 500 nanometers.

The operation of layer stacking is described in the following.

The light incident on the cell 10 passes through the first layer C1 and the second layer C2 which ensures that the part transmitted to the other layers is maximized.

The interaction of the transmitted light with the nanostructures 14 of the sixth layer C6 makes it possible to create a light trap at the level of the active layer C4.

The light escaping towards the seventh layer C7 is reflected to be trapped in the active layer C4.

The asymmetry introduced into the nanostructures 14 makes it possible to increase the number of resonant modes.

Such an effect is obtained as soon as at least one asymmetric nanostructure 14 is present. This effect is reinforced by a greater number of asymmetric nanostructures, for example a number greater than or equal to 50% of the total number of nanostructures 14, advantageously greater than or equal to 75% of the total number of nanostructures 14.

Such an increase results in an increase in cell uptake 10.

This appears on studying the results of Figures 8 to 14 resulting from simulations carried out by the applicant.

The simulations were performed using a code called RETICOLO developed by the Institut d'Optique.

Each of the figures 8 to 14 represent the evolution of the absorption of a layer with the wavelength on a spectral band going from 0 to 1.8 µm.

In the case of Figure 8, the simulated cell stack is the stack of Figure 1 with the nanostructures 14 of Figure 2 with d = 400 nm, p = 600 nm, h = 100 nm, the base material MB which is TiO ₂ and the material of the nanostructures which is Ag.

The values of dimension d, lattice pitch p and height h were optimized during the simulation to obtain the best absorption.

In figure 8, curve A6 is the absorption curve of all the layers of the cell while curve A4 is the absorption curve of the active layer C4.

Curve A4 should be compared to curves A1ref and A2ref in figure 9.

Curve A1ref corresponds to the absorption of the active layer of a second reference cell presenting a stack similar to that of figure 1 but without mirror (layer C7) and without nanostructure in the sixth layer C6. The light makes a simple passage in the cell.

Curve A2ref corresponds to the absorption of the active layer of a second reference cell presenting a stack similar to that of figure 1 but without nanostructure in the sixth layer C6. Due to the presence of the mirror, the light makes a double passage in the cell.

As expected, the A2ref curve presents a higher absorption than that of the A1ref curve.

However, the A2ref curve is much lower than the A4 curve, especially for wavelengths greater than or equal to 600 nm.

As specific examples, at 600 nm, the absorption of the stack of figure 1 is 30% higher than that of the second reference cell, while at 1 μm, the absorption of the stack of Figure 1 is 20% higher than that of the second reference cell.

This implies that the presence of the nanostructures 14 makes it possible to increase the absorption of the cell and therefore the yield.

The same observations are possible in the cases of FIGS. 10 to 14. These figures correspond to FIGS. 8 for different stacks.

In the case of Figure 10, the simulated cell stack is the stack of Figure 1 with the nanostructures 14 of Figure 5 with d = 600 nm, P = 700 nm, h = 113 nm, the base material MB which is TiO ₂ and the material of the nanostructures which is Ag.

It should be noted that due to the structural proximity of the nanostructures of FIG. 4 and of FIG. 5, the simulation of FIG. 10 would also be valid for the stack of FIG. 1 with the nanostructures 14 of FIG. 4 with d = 600 nm, P = 700 nm, h = 113 nm, the base material MB which is TiO ₂ and the material of the nanostructures which is Ag.

In the case of Figure 11, the simulated cell stack is the stack of Figure 1 with the nanostructures 14 of Figure 7 with d = 500 nm, P = 500 nm, h = 200 nm, the base material MB which is TiO ₂ and the material of the nanostructures 14 which is Ag.

It should be noted that due to the structural proximity of the nanostructures 14 of FIG. 6 and of FIG. 7, the simulation of FIG. 11 would also be valid for the stack of FIG. 1 with the nanostructures 14 of FIG. 7 with d=500 nm, P=500 nm, h=200 nm, the base material MB which is TiO ₂ and the material of the nanostructures which is Ag.

In the case of Figure 12, the simulated cell stack is the stack of Figure 1 with the nanostructures 14 of Figure 2 with d = 515 nm, P = 560 nm, h = 230 nm, the base material MB which is AlInP and the material of the nanostructures 14 which is SiO ₂ .

In the case of Figure 13, the simulated cell stack is the stack of Figure 1 with the nanostructures 14 of Figure 5 with d = 515 nm, P = 560 nm, h = 230 nm, the base material MB which is TiO ₂ and the material of the nanostructures 14 which is Ag.

In the case of Figure 14, the simulated cell stack is the stack of Figure 1 with the nanostructures 14 of Figure 7 with d = 680 nm, P = 680 nm, h = 390 nm, the base material MB which is TiO ₂ and the material of the nanostructures 14 which is Ag.

In each case, absorption is higher.

Thus, such a stack which is not a metallic structure makes it possible to limit the parasitic absorption due to plasmon resonances by the use of asymmetric nanostructures 14.

This makes it possible to obtain a stack of layers making it possible to absorb 80% of the incident light power in the absorber.

Furthermore, the absorber has good absorption between 300 nm and 1.5 µm.

The efficiency of cell 10 is therefore increased.

Claims (10)

Photovoltaic cell (10) comprising a stack (12) of layers (C1, C2, C3, C4, C5, C6, C7), the layers (C1, C2, C3, C4, C5, C6, C7) being superposed along of a stacking direction (Z), the stack (12) comprising:
- an active layer (C4),
- a mirror (C7), and
- an intermediate layer (C6) inserted between the active layer (C4) and the mirror (C7), the intermediate layer (C6) being made of a first material (MB) and having a set of nanostructures (14) arranged in a network, at least one nanostructure (14) being devoid of a plane of symmetry not parallel to the intermediate layer (C6) having an intersection reduced to one point with the stacking direction (Z), said at least one nanostructure (14) being made of a second material (MN), the first material (MB) being distinct from the second material (MN). Cell according to Claim 1, in which the said at least one nanostructure (14) comprises at most one plane of symmetry comprising the stacking direction (Z). Cell according to Claim 1 or 2, in which the first material (MB) has a first optical index (nMB), the second material (MN) has a second optical index (nMN), the ratio between the first optical index (nMB) and the second optical index (nMN) being between 1.5 and 3. Cell according to any one of Claims 1 to 3, in which the first material (MB) and the second material (MN) are chosen from a group consisting of the following pairs: {Ag, TiO ₂ }, {SiO ₂ , AlInP} , {SiO ₂ , Au}, {TiO ₂ , Au}, {SiO ₂ /Cu}, {TiO ₂ , AlInP}, {SiO ₂ , AlAsSb} and {Si ₃ N ₄ /InAlAs}. Cell according to any one of claims 1 to 4, in which each nanostructure (14) is arranged in parallel lines. Cell according to any one of claims 1 to 5, in which each nanostructure (14) is arranged at the top of a hexagon or in the center of the hexagon, the hexagons formed by the nanostructures (14) sharing at least one side. A cell according to any one of claims 1 to 6, wherein each nanostructure (14) has a shape selected from the group consisting of a cylinder, a cylinder with a triangular base, a pyramid, a pyramid with square base and a pyramid with a triangular base. Cell according to any one of Claims 1 to 7, in which a pitch (P) is defined for the grating, the pitch (P) being between 500 nanometers and 700 nanometers. Cell according to any one of Claims 1 to 8, in which a height (h) is defined for each nanostructure (14) as being the dimension along the stacking direction (Z) and a length (d) for each nanostructure (14) as being the dimension along a direction perpendicular to the stacking direction (Z), the ratio between the height (h) and the dimension (d) being less than or equal to 60%, preferably greater than or equal to 40%. Cell according to Claim 9, in which a pitch (P) is defined for the grating, the ratio between the dimension (d) and the pitch (P) being greater than or equal to 80%.