FR3023062A1 - SILICON HETEROJUNCTION PHOTOVOLTAIC CELL AND METHOD OF MANUFACTURING SUCH CELL - Google Patents

Abstract

The invention relates to a silicon heterojunction photovoltaic cell (1), comprising successively: a substrate (10) of doped crystalline silicon, a layer (11) of doped amorphous or microcrystalline silicon, an electrode (12) at least partially transparent to solar radiation, comprising an electrically percolating network of metallic nanowires, - a dielectric layer (13) at least partially transparent to solar radiation encapsulating said network of metal nanowires, - at least one pad (14a, 14b) ensuring an electrical contact (13) with the electrode (12).

Description

The present invention relates to a silicon heterojunction photovoltaic cell and a method for manufacturing such a cell. BACKGROUND OF THE INVENTION The present invention relates to a silicon heterojunction photovoltaic cell and a method for manufacturing such a cell. BACKGROUND OF THE INVENTION In a photovoltaic cell with silicon heterojunction (often referred to by the acronym SHJ of the Anglo-Saxon term "Silicon HeteroJunction solar cell"), the internal electric field essential to the photovoltaic effect is created by a layer doped hydrogenated amorphous silicon (conventionally noted a-Si: H) deposited on a doped crystalline silicon substrate of the opposite type (conventionally noted c-Si), in contrast to a conventional homojunction structure in which the internal electric field is obtained by a junction p-doped crystalline silicon / n-doped crystalline silicon The doped amorphous silicon layer of opposite type to that of the substrate forms the emitter of the photovoltaic cell. Similarly the rear repulsive field (or before in an inverted emitter cell configuration) can also be advantageously formed by the deposition of a hydrogenated amorphous silicon doped layer of the same type as the crystalline substrate. The realization of the heterojunction and the rear repulsive field (or before according to the emitter configuration) from amorphous silicon, which can be deposited at a low temperature, makes it possible to minimize the thermal budget imposed on the crystalline silicon substrate.

In such a cell, it is necessary to cover the surface of the amorphous silicon layers with an electrically conductive layer to allow the transport of electrical charges to the metal contacts used to collect the electric current generated by the cell. Amorphous silicon is indeed not sufficiently conductive to ensure this function.

Said electrically conductive layer must also be as transparent as possible to let as much light into the cell as possible both on the front face and on the back face if the cell is bifacial. On the other hand, it is generally sought to obtain a layer having a refractive index adapted to act as an antireflection layer.

Finally, this layer advantageously has an adequate output work to maximize the collection of photogenerated charges.

Currently, this function is provided by conductive transparent oxides (OTC) and in particular by indium tin doped oxide (ITO), which has excellent optical properties and electric. However, because of the high cost of the latter, alternative solutions are being studied, including the use of electrically percolating networks of metal nanowires [1], [2]. The networks of metallic nanowires have a high conductivity (resistance per square of less than 30 Ω / 1) for a transparency of the order of 90%. These properties are close to, or even superior to, those of transparent conductive oxides.

However, their integration with a photovoltaic cell poses the following problems. On the one hand, the adhesion of the metal nanowires to the substrate is poor, resulting in easy tearing of said nanowires. On the other hand, these nanowires being in the form of an entanglement of son separated by air and not a continuous layer, they do not allow to play the role of antireflection layer like an OTC, nor to protect the layer of a-Si: H which degrades during prolonged exposure to air. To remedy these problems, one solution is to use an OTC layer to encapsulate the nanowire network [3]. This hybrid structure makes it possible to decouple the optimization of the optical and electrical properties of the electrode, the network of metal nanowires providing electrical conduction while allowing light to pass through and the OTC layer acting as an antireflection layer. The matrix covering the nanowires (OTC) being conductive, the contact during the metallization (by screen printing of a conductive ink, evaporation of metal contacts, electrochemical deposition ...) is direct. However, such a cell remains dependent on the use of transparent conductive oxides. BRIEF DESCRIPTION OF THE INVENTION An object of the invention is to design a silicon heterojunction photovoltaic cell making it possible to completely overcome the use of conductive transparent oxides while ensuring the antireflection function and the protection of the coating layer. amorphous silicon. According to the invention, there is provided a silicon heterojunction photovoltaic cell, comprising successively: a doped crystalline silicon substrate, a doped doped amorphous or microcrystalline silicon layer, an at least partially transparent solar radiation electrode, comprising: an electrically percolating network of metal nanowires, a dielectric layer at least partially transparent to solar radiation encapsulating said network of metal nanowires, at least one pad providing electrical contact with the electrode. By "nanowire" is meant in the present text a wire having a diameter of between 20 nm and 200 nm, preferably between 40 and 150 nm. "Electrically percolating network" is understood to mean a network in which the metal nanowires are arranged - randomly or not - with respect to each other so as to allow some recovery of the nanowires and thus allow the passage of the current. This thus ensures the electrical conduction property necessary for the electrode. In addition, spaces within the network are formed between adjacent nanowires, allowing the passage of light and also in general the penetration of the material of the dielectric layer. Advantageously, the dielectric layer has a refractive index between the refractive index of air and the refractive index of silicon. According to one embodiment, the dielectric layer comprises at least one of the following materials: SiO 2, SiN, Al 2 O 3, MgF 2.

According to one embodiment, the dielectric layer comprises a stack of at least a first layer extending from the network of metal nanowires and a second layer extending over the first layer, the first layer having a refractive index greater than that of the second layer. Preferably, the material of the metal nanowires comprises silver, copper and / or gold. According to one embodiment, the silicon layer is n-doped and the dielectric layer has a positive fixed charge density. According to another embodiment, the silicon layer is p-doped and the dielectric layer has a negative fixed charge density.

Advantageously, the electrical resistance of the electrode is less than or equal to 20 Ω / λ. Advantageously, the transmission factor of the network of metal nanowires for a wavelength of 600 nm is greater than 90%. According to one embodiment, the network of metal nanowires is locally denser in each region of the electrode in contact with a respective electrical contact pad. In this case, the transmission factor of each denser region of the network of metal nanowires for a wavelength of 600 nm is advantageously between 10 and 20% while the transmission factor of the network of metal nanowires in the other regions the electrode for a wavelength of 600 nm is greater than 90%. Another object of the invention relates to a method of manufacturing a photovoltaic cell as described above. This method comprises the following steps: - forming, on a doped crystalline silicon substrate, a layer of doped amorphous or microcrystalline silicon, - forming, on said silicon layer, an electrically percolating network of 10 metal nanowires, said network forming an electrode at least partially transparent to solar radiation, - deposition, on said electrode, of a dielectric layer at least partially transparent to solar radiation so as to encapsulate said network of metal nanowires, 15 - formation of at least one stud ensuring electrical contact with the electrode. According to one embodiment, the method comprises the localized densification of the network of metal nanowires in each region of the electrode in contact with a respective pad. Said localized densification advantageously comprises the formation of a mask 20 on the network of metal nanowires, said mask having openings facing each respective region to be densified, and then a deposit of additional metal nanowires through the openings of said mask. According to one embodiment of the invention, the dielectric layer is formed on the entire surface of the electrode and then a localized ablation of said layer is applied so as to form openings intended for the passage of each contact pad. respective electric. Alternatively, before forming the dielectric layer, a mask is formed on the regions of the electrode intended to be in contact with a respective pad, and then the dielectric layer is formed through said mask. According to another variant, each pad is formed on the electrode and then the dielectric layer is formed around said pads. BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings, in which: FIG. 1A is a schematic sectional view of a photovoltaic cell according to a mode embodiment of the invention, - Figure 1B is a schematic sectional view of a photovoltaic cell according to another embodiment of the invention, - Figures 2A to 2C are schematic sectional views illustrating successive steps of method of manufacturing a photovoltaic cell according to an embodiment of the invention, - Figures 3A to 3D are schematic sectional views illustrating successive steps of the method of manufacturing a photovoltaic cell according to an embodiment of the invention. 4A to 4D are diagrammatic sectional views illustrating successive steps in the method of manufacturing a photovoltaic cell that according to one embodiment of the invention, - Figures 5A to 5D are schematic sectional views illustrating successive steps of the method of manufacturing a photovoltaic cell according to one embodiment of the invention.

For reasons of readability of the figures, they are not necessarily made to scale. In addition, only one side of the cell has been shown. In the majority of cases, the cell also comprises at the rear face at least one amorphous silicon layer and a metallization. Preferably, the cell may be symmetrical.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION FIG. 1A illustrates a silicon heterojunction photovoltaic cell according to a first embodiment of the invention. Said cell 1 comprises a doped silicon substrate 10 covered with at least one layer 11 of hydrogenated amorphous or microcrystalline silicon (in the following text reference will be made to an amorphous silicon layer but the description also applies to the case where a microcrystalline silicon layer is used in place of the amorphous silicon layer). The hydrogenated amorphous silicon layer may be doped with a type of doping opposite to that of the substrate, so as to form the heterojunction. It then forms the transmitter side of the cell. It can also be of a doping of identical type to that of the substrate in the case where we represent in FIG. 1A the repulsive field. Where appropriate, the doped hydrogenated silicon layer of the opposite type will be disposed on the other side of the cell so as to form the emitter side. Although the various layers are shown planar in FIGS. 1A to 5D, they may in fact have a textured surface such as is used to reduce the reflectivity of the surface of the cell and improve the optical confinement. The layer 11 of amorphous silicon is covered with an electrode 12 formed of a network of metal nanowires. The metal nanowires are advantageously silver nanowires, but copper or gold nanowires can also be envisaged. For a review on the properties of said nanowires and their methods of obtaining, we can refer to [4]. Said network is encapsulated in a layer 13 made of a dielectric material at least partially transparent to solar radiation, that is to say having a transmission factor greater than 90% for a wavelength of 600 nm. By "encapsulated" is meant that the layer 13, which is conformably deposited on the network of nanowires, substantially marries the relief of said network, the material being deposited on the nanowires and between them. According to the thickness of deposited dielectric material and according to the organization of said nanowires within the network (that is to say that all the nanowires are well plated parallel to the silicon layer or that some nanowires point out of the network) and their diameter, there may be cases where the nanowires are completely covered with the dielectric material and other cases where some nanowires are flush with the surface of the dielectric layer. Optionally, the dielectric material also extends through the entanglement of the nanowires of the network. In the latter case, the dielectric material can form a continuous solid medium between the nanowires of the network. The dielectric layer 13 may extend, in the opposite direction to the substrate 10, beyond the surface of the nanowire array. Thus, the dielectric layer 13 comprises a lower portion whose thickness is equal to that of the network of metal nanowires and an upper portion which extends over the first portion. The total thickness of the dielectric layer is typically between 50 and 150 nm. The dielectric layer 13 thus makes it possible to enhance the adhesion of the nanowires to the amorphous silicon layer 11 and to protect said layer 11 by avoiding its contact with the air which could damage it. Furthermore, the layer 13 provides the antireflection function, the antireflection effect depending on the thickness of said layer 13. For this purpose, the layer 13 advantageously comprises a dielectric material whose refractive index is between that of the air (which is of the order of 1) and that of silicon (which is of the order of 4). The layer 13 may in particular comprise one of the following materials: SiO 2, SiN, Al 2 O 3, MgF 2. Moreover, the material of the layer 13 may advantageously be chosen as a function of the doping type of the amorphous silicon layer 11 in order to promote the selectivity of the contacts. Thus, a dielectric material having a positive fixed charge density (for example SiO 2) is preferably chosen for an n-doped amorphous silicon layer, while a dielectric material having a negative fixed charge density (for example SiN and Al 2 O 3) is preferably chosen for a p-doped amorphous silicon layer.

The cell 1 also comprises electrical contact pads 14a, 14b, here represented in number of two, but it goes without saying that the number and the arrangement of said pads are defined by those skilled in the art during the design of the cell. The term "pad" covers any form of electrical contacts, including lines.

The pads 14a, 14b provide electrical contact with the network of metal nanowires which forms the electrode 12, and thus extend through the upper portion of the dielectric layer 13 which is electrically insulating, to the surface of the network . Said pads 14a, 14b are metallic, for example silver, copper, or nickel.

Figure 1B illustrates a variant of the photovoltaic cell of Figure 1A. The structure of the cell is similar to that of FIG. 1A, except with regard to the dielectric layer 13. In fact, while the layer 13 of the cell of FIG. 1A consists of the same dielectric material over its entire length. thickness, the layer 13 of the cell of FIG. 1B comprises a stack of two different dielectric materials. This stack comprises a first layer 13a extending from the surface of the amorphous silicon layer 11 through the network of metal nanowires and a second layer 13b extending over the first layer. The first layer is typically made by a conformal deposit, according to the relief of the nanowires.

Particularly advantageously, the first layer 13a has a refractive index greater than that of the second layer 13b. Thus, this stack provides a refractive index gradient between ambient air and silicon, which improves the antireflection properties. For example, the first layer 13a may be made of SiN or SiO 2 and the second layer 13b may be made of Al 2 O 3.

A photovoltaic module can be formed from at least one photovoltaic cell as described above. Various embodiments of the method of manufacturing such a cell will now be described. The network of metal nanowires is deposited on the amorphous silicon layer so as to form an electrically percolating network. The density of the nanowires is furthermore chosen to ensure the electrical conductivity and the transparency required for the electrode. As an indication, for a transmission factor of 90%, the density of nanowires is 45 mg / m 2 and the electrical resistance is 25 Ω / 1. There are various techniques for depositing such a network of nanowires, among which: spraying ("spray" according to the English terminology), the chemical bath, spin-coating, dip-coating. For example, for spray deposition, the metal nanowires are placed in solution and said solution is projected onto the amorphous silicon layer by means of a head which sweeps the surface of said layer. A number of passes are thus made until the desired density for the network is achieved. As an indication, for a solution flow rate of 0.5 to 1 ml / min projected by a head moved at a speed of 10 to 50 mm / s, is carried out between 2 and 6 passages and a network of nanowires is obtained. silver having an electrical resistance of the order of 20 Ω / and a transmission factor for a wavelength of 600 nm which is greater than 90%. The network of nanowires may have a substantially uniform density over the entire surface of the amorphous silicon layer.

However, according to an advantageous embodiment of the invention, the network is locally denser in regions intended to be in contact with the contact pads. In particular, as will be seen below, this difference in density makes it possible to form the contact pads by electrochemical deposition. Obtaining a locally higher density in certain regions of the network implies, once the network has reached the required density for the regions situated between the pads, the installation of a mask whose openings correspond to the placement of the studs, and further deposition of the nanowires until the required density is obtained in the exposed regions through the mask. In such a case, the density of the two network regions can be characterized by their transmission factor: as indicated above, the regions extending between the pads have a transmission factor greater than 90% for a wavelength of 600 nm while the denser regions intended to be in contact with the pads have a transmission factor of between 0 and 20% and preferably between 0 and 10%.

As an indication, to densify the network obtained by the process whose parameters are given above, the projection is continued through a mask, with the same solution flow rate of 0.5 to 1 ml / min and the same head speed of 10 to 50 mm / s, so as to achieve the order of 2 to 5 additional passages. The local electrical resistance of the network of nanowires is then less than 10 Ω / and its local transmission factor is of the order of 10 to 20% for a wavelength of 600 nm. Regarding the dielectric layer, it is formed by any suitable deposition technique that is to say allowing a low temperature deposition, so as not to damage the amorphous silicon layers themselves deposited at low temperature (by example: evaporation, plasma enhanced chemical vapor deposition, sputtering). As an indication, a layer of SiO 2 may be deposited on a thickness of the order of 80 to 120 nm, by evaporation, the deposition rate being of a few λ / s and the applied voltage being of the order of 5 to 20 kV.

According to one embodiment, the dielectric layer is formed continuously over the entire surface of the electrode, then openings are formed to allow the passage of the contact pads to the network of metal nanowires. The creation of these openings can be achieved by localized ablation, for example by means of a laser. Such laser ablation is substantially facilitated by the metallic character of the nanowire array which is interposed between the portion of the dielectric layer to be removed and the amorphous silicon layer. Indeed, it is then possible to use a laser spallation technique, which consists of focusing the laser beam at the metal / dielectric interface. The heating of the metal at this interface causes the ejection of the dielectric layer above the focusing point. The openings are created in the dielectric layer without damaging the underlying amorphous silicon layer. On the other hand, in the conventional case of the stack of a dielectric layer and of a transparent conductive oxide layer, the laser beam, not being totally absorbed by the transparent conductive oxide, damages the silicon layer. underlying amorphous [5] by localized heating thereof. According to another embodiment, said openings are formed directly during the formation of the dielectric layer, by depositing said layer through a mask which protects the regions of the network of nanowires intended to be in contact with the pads.

According to another embodiment, the dielectric layer is formed after the pads, so as to extend around said pads. The contact pads may be made by any suitable metallization technique, in particular by screen printing of a metallic ink, by evaporation of metal contacts, or by electrochemical deposition.

Figures 2A to 2C schematically illustrate a first embodiment of the method of manufacturing a photovoltaic cell. With reference to FIG. 2A, on the layer 11 of amorphous silicon covering the substrate 10 is formed the network of metal nanowires forming the transparent electrode 12. As indicated above, this network can be deposited by spray, chemical bath, spin-coating or dip-coating. With reference to FIG. 2B, a mask 16 is placed that is generally placed in contact with the surface of the network of nanowires so as to protect the regions thereof that are intended to be in contact with the contact pads. during the deposition of dielectric material. The dielectric layer 13 is deposited so as to encapsulate the network of metal nanowires. Thus, in the layer 13, there are formed openings 13a, 13b for the passage of the contact pads which will be formed later. With reference to FIG. 2C, the contact pads 14a, 14b are formed by metallization of the openings 13a, 13b formed in the dielectric layer 13. The contact pads are therefore in direct contact with the metal nanowires. The metallization may comprise a metallic ink screen, evaporation of metal contacts or electrochemical deposition. Figures 3A to 3D illustrate schematically a second embodiment of the method of manufacturing a photovoltaic cell. With reference to FIG. 3A, on the layer 11 of amorphous silicon covering the substrate 10, the network of metallic nanowires forming the transparent electrode 12 is formed. As indicated above, this network can be deposited by spray, chemical bath, spin-coating or dip-coating.

With reference to FIG. 3B, the dielectric layer 13 is deposited on the entire surface of the electrode 12, so as to encapsulate the network of metal nanowires and to extend beyond said network in the direction opposite to the substrate 10. With reference to FIG. 3C, openings 13a, 13b are formed in said layer 13 for the passage of the contact pads which will be formed later, these openings extending through a portion of the layer 13 at least until the surface of the network of metal nanowires in the direction opposite to the substrate 10. The formation of said openings 13a, 13b is achieved by localized ablation, for example by means of a laser. With reference to FIG. 3D, the contact pads 14a, 14b are formed by metallization of the openings 13a, 13b formed in the dielectric layer 13. The contact pads are therefore in direct contact with the metal nanowires. As indicated above, the metallization may comprise a metallic ink screen, evaporation of metal contacts or electrochemical deposition. Figures 4A to 4D schematically illustrate a third embodiment of the method of manufacturing a photovoltaic cell. With reference to FIG. 4A, on the layer 11 of amorphous silicon covering the substrate 10 is formed the network of metal nanowires forming the transparent electrode 12. As indicated above, this network can be deposited by spray, chemical bath, spin-coating or dip-coating. In this step, the density of the nanowire array is substantially homogeneous over the entire surface of the electrode. With reference to FIG. 4B, the network of metal nanowires is locally densified. For this purpose, a mask 15 having openings 15a, 15b corresponding to the regions 12a, 12b of the network of nanowires which it is desired to densify is placed opposite the grating already deposited, generally directly on the latter. The nanowires are then deposited through the mask 15, so as to increase the density of nanowires only in the regions 12a, 12b.

With reference to FIG. 4C, the dielectric layer 13 is deposited on the entire surface of the electrode 12, so as to encapsulate the network of metal nanowires and to extend beyond said network in the direction opposite to the substrate 10. With reference to FIG. 4D, openings 13a, 13b are created in said layer 13 for the passage of the contact pads which will be formed later, these openings extending through a portion of the layer 13 to the surface the metal nanowires network opposite to the substrate 10. The formation of said openings 13a, 13b is achieved by localized ablation, for example by laser spallation. Then, the contact pads 14a, 14b are formed by metallization of the openings 13a, 13b formed in the dielectric layer 13, the contact pads being in direct contact with the metal nanowires. This step, which is similar to the step illustrated in FIGS. 2C and 3D, has not been illustrated again here. Figures 5A to 5D schematically illustrate a fourth embodiment of the method of manufacturing a photovoltaic cell.

With reference to FIG. 5A, on the layer 11 of amorphous silicon covering the substrate 10 is formed the network of metal nanowires forming the transparent electrode 12. As indicated above, this network can be deposited by spray, chemical bath, spin-coating or dip-coating. In this step, the density of the nanowire array is substantially homogeneous over the entire surface of the electrode.

With reference to FIG. 5B, the network of metal nanowires is locally densified. For this purpose, a mask 15 having openings 15a, 15b corresponding to the regions 12a, 12b of the network of nanowires which it is desired to densify is placed opposite the already deposited network. The nanowires are then deposited through the mask 15, so as to increase the density of nanowires only in the regions 12a, 12b.

With reference to FIG. 5C, the dielectric layer 13 is deposited on the entire surface of the electrode 12, so as to encapsulate the network of metal nanowires and to extend beyond said network in the direction opposite to the substrate 10. Openings 13a, 13b are also formed for the contact pads, the creation of these openings can be made by deposition through a mask (as described with reference to FIG. 2B) or by localized ablation, in particular by laser spallation ( as described with reference to Figures 3C and 4D). With reference to FIG. 5D, the contact pads 14a, 14b are made to grow by electrochemical deposition in the openings 13a, 13b. Electrochemical deposition is already practiced on conductive transparent oxide layers on conventional photovoltaic cells. An electrochemical deposit requires that the regions on which the metal is to be deposited are sufficiently electrically conductive, whereas the regions on which no metal is to be deposited are not electrically conductive. For this, we can deposit a metal bonding layer for electrochemically growth of the pad. Returning to the fourth embodiment of the invention, it is noted that one can take advantage of the higher density of the regions 12a, 12b of the network to implement an electrochemical deposit by avoiding the deposition of the bonding layer and the masking layer required for electrochemical deposition on an OTC layer. Indeed, on the structure shown in FIG. 5C, the densified regions 12a, 12b of the metal nanowire network act as a tie layer while the dielectric layer 13 defining the openings 13a, 13b plays the role of the masking, and thus allow, in the step illustrated in Figure 5D, to proceed directly to an electrochemical deposition. An advantage of producing the contact pads by electrochemical deposition, compared with the traditional screen printing, is to form contact pads with a high aspect ratio, that is to say thin metal lines (thus generating a lower shading and therefore a larger photogenerated current) and thick (consequently very current-conducting). The embodiments described above are presented for illustrative purposes only and those skilled in the art can implement variants of the various steps without departing from the scope of the present invention.

REFERENCES [1] T.G. Chen et al, Flexible nanowire meshes for high-efficiency organic microtextured-silicon hybrid photovoltaics, Appl. Mater. Interfaces 2012, 4, 6857-6864 [2] S. Xie et al, Large size, high-uniformity, random silver nanowire networks as transparent electrodes for crystalline silicon wafer solar cells, Optics Express 2013, 21, S3, A355-A362 [ 3] M. Huang et al, Hybrid silver nanoparticle and transparent conductive oxide structure for silicon solar cell applications, Physica Status Solidi RRL 2014, 8, 5, 399-403 [4] D. Langley et al, Flexible Transparent Conductive Materials Based on silver nanowire networks: a review, Nanotechnology 24 (2013) 452001 [5] US 2012/0291844

Claims (17)

REVENDICATIONS1. Silicon heterojunction photovoltaic cell (1), comprising successively: - a substrate (10) of doped crystalline silicon, - a layer (11) of doped amorphous or microcrystalline silicon, - an electrode (12) at least partially transparent to solar radiation , comprising an electrically percolating network of metallic nanowires, - a dielectric layer (13) at least partially transparent to solar radiation encapsulating said network of metal nanowires, - at least one pad (14a, 14b) providing an electrical contact (13) with the electrode (12). 2. Photovoltaic cell according to claim 1, wherein the dielectric layer (13) has a refractive index between the refractive index of air and the refractive index of silicon. Photovoltaic cell according to one of claims 1 or 2, wherein the dielectric layer (13) comprises at least one of the following materials: SiO 2, SiN, Al 2 O 3, MgF 2. Photovoltaic cell according to one of claims 1 to 3, wherein the dielectric layer (13) comprises a stack of at least a first layer (13a) extending from the network of metal nanowires and a second layer (13b). ) extending over the first layer, the first layer (13a) having a refractive index greater than that of the second layer (13b). 5. Photovoltaic cell according to one of claims 1 to 4, wherein the material of the metal nanowires comprises silver, copper and / or gold. 6. Photovoltaic cell according to one of claims 1 to 5, wherein the layer (11) of silicon is n-doped and the dielectric layer (13) has a positive fixed charge density. Photovoltaic cell according to one of claims 1 to 5, wherein the layer (11) of silicon is p-doped and the dielectric layer (13) has a negative fixed charge density. 8. Photovoltaic cell according to one of claims 1 to 7, wherein the electrical resistance of the electrode (12) is less than or equal to 20 Ω /. 9. Photovoltaic cell according to one of claims 1 to 10, wherein the transmission factor of the network of metal nanowires for a wavelength of 600 nm is greater than 90%. 10. Photovoltaic cell according to one of claims 1 to 9, wherein the network of metal nanowires is locally denser in each region (12a, 12b) of the electrode (12) in contact with an electrical contact pad (14a). , 14b). The photovoltaic cell according to claim 11, wherein the transmission factor of each denser region (12a, 12b) of the metal nanowire array for a wavelength of 600 nm is between 10 and 20% while the factor transmission of the network of metal nanowires in the other regions of the electrode for a wavelength of 600 nm is greater than 90%. 12. A method for manufacturing a silicon heterojunction photovoltaic cell (1), comprising the steps of: forming, on a substrate (10) of doped crystalline silicon, a layer (11) of doped amorphous silicon or microcrystalline silicon - forming, on said silicon layer (11), an electrically percolating network of metal nanowires, said array forming an electrode (12) at least partially transparent to solar radiation, - depositing, on said electrode (12), a dielectric layer (13) at least partially transparent to solar radiation so as to encapsulate said network of metal nanowires, - forming at least one pad (14a, 14b) providing electrical contact with the electrode (12). 30 13. The method of claim 12, comprising the localized densification of the network of metal nanowires in each region (12a, 12b) of the electrode (12) in contact with a pad (14a, 14b) respectively. 35 14. The method of claim 13 wherein said localized densification comprises the formation of a mask (15) on the network of metal nanowires, said mask having openings (15a, 15b) facing each respective region (12a, 12b). to densify, then a deposit of additional metal nanowires through the openings of said mask. 15. Method according to one of claims 12 to 14, wherein the dielectric layer (13) is formed on the entire surface of the electrode (12) and then localized ablation of said layer is carried out so as to form openings (13a, 13b) for the passage of each respective electrical contact pad (14a, 14b). Method according to one of claims 12 to 14, wherein before forming the dielectric layer (13), a mask (16) is formed on the regions (12a, 12b) of the electrode intended to be in contact with each other. with a respective stud (14a, 14b), and then forming the dielectric layer (13) through said mask (16). 17. Method according to one of claims 12 to 14, wherein each pad (14a, 14b) is formed on the electrode (12) and then forming the dielectric layer (13) around said pads.