ITVI20120133A1 - APPLICATION OF THE BACKSHEET ENCAPSTER FOR PHOTOVOLTAIC MODULES USING CELLS CONTACT REAR - Google Patents

Description

DESCRIPTION

â € œAPPLICATION OF THE BACKSHEET ENCAPSULANT FOR PHOTOVOLTAIC MODULES USING REAR CONTACTED CELLSâ €

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of photovoltaic modules. In particular, the present invention relates to the field of backcontact-backsheets used in photovoltaic modules. Even more particularly, the present invention relates to a multilayer system to be applied to the surface of a backcontact-backsheet during the assembly of a photovoltaic module and to a process for producing such a multilayer system.

STATE OF THE TECHNIQUE

Solar cells are used to convert sunlight into electricity by means of the photovoltaic effect. Solar cells therefore represent one of the most promising alternative energy sources to replace fossil fuels. The solar cells are formed from semiconductor materials and are assembled to form the so-called photovoltaic modules which, in turn, are grouped to form the photovoltaic systems typically installed on the roofs of homes or the like.

To form the photovoltaic modules, groups of solar cells, grouped in series through suitable electrical conductors called "ribbons", are typically encapsulated by means of an encapsulating material such as polyvinyl acetate with a high content of ethylene vinyl acetate, commonly called ÈVA. The encapsulating material enclosing the solar cells is then inserted between a surface layer and a base or back-sheet in order to complete the photovoltaic module. The surface layer, or main surface of the module, typically made of glass, covers the surface of the module exposed to the sun and allows sunlight to reach the cells. The back sheet, on the other hand, performs a variety of tasks. It ensures protection for the encapsulating material and for the solar cells from environmental agents, while preventing oxidation of the electrical connections. In particular, the back-sheet prevents humidity, oxygen and other factors related to atmospheric conditions from damaging the encapsulating material, the cells and the electrical connections. The back sheet also provides electrical insulation for the cells and corresponding electrical circuits. Furthermore, the back-sheet must have high opacity for aesthetic purposes and high reflectivity in the part oriented towards the sun for functional purposes.

Traditional photovoltaic modules are made from a plurality of layers. The lower layer is constituted by the backsheet, on which is placed a first layer of encapsulant comprising ÈVA. The cells are then deposited on the first encapsulant layer comprising ÈVA. The cells are arranged on the encapsulant to form strings, where the cells are connected in series with each other by means of ribbons. Adjacent strings are then connected by two or more connections called "busbars". A second layer of encapsulant is then placed on the cells and, on top of this, a glass. The electrical connection in photovoltaic modules including traditional solar cells takes place both on the front and on the rear side of the cell. The electrical connection to the electrode placed on the front side of the cell, that is the side exposed to light radiation, is traditionally made by means of a series of essentially parallel thread-like contacts called "fingers", connected by means of two or more ribbons arranged transversely with respect to to the longitudinal direction of the fingers. This way of contacting the front electrode, also called "H-patterning", creates shading problems on the surface exposed to light radiation due to the presence of the fingers and ribbons, which shield the light incident on the front surface of the cell. Traditional electrical contacts therefore lead to a reduction in the efficiency of solar cells and modules.

The cells are also quite spaced from each other as the ribbons must be allowed to find the space to be folded in such a way as to pass from the upper side of a cell to the lower side of the adjacent cell. Furthermore, the need to make a connection between a string of cells and the adjacent string implies the need to find the space for the passage of the busbars.

In order to overcome these problems, solar cells with back contact or back contact cells have been devised. This family includes the following types of solar cells: Interdigitated Back Contact (IBC) cells, Emitter Wrap Through (EWP) cells, Metallization Wrap Through (MWT) cells. The cells contacted at the rear are advantageous in that they allow the contact with both electrodes of the cell to be transferred to the rear side of the cell, that is, to the side not exposed to light radiation. This reduces the effect of shading and, therefore, of reduction of the effective surface of the cell exposed to radiation, due to the presence of ohmic contacts on the external surface of the cell.

A further advantage deriving from the use of posteriorly contacted cells lies in the fact that the module assembly process can be highly automated, eliminating at the same time a phase that is the syringing of the cells. Furthermore, since it is not necessary to provide connections by means of ribbon and busbar, the thicknesses of the encapsulant can be reduced with advantages in terms of light transparency and costs.

A type of cell contacted at the rear that is particularly efficient and convenient to create is given by MWT cells. Figures 1a and 1b show, respectively, the front surface (i.e. surface exposed to light radiation) and the rear surface of an MWT cell. Figure 2 instead shows a cross section of an MWT cell.

An MWT cell is made starting from a doped wafer in order to obtain a junction between a zone 662 with free charges of a given polarity and a zone 664 with free charges of the opposite polarity. For example, zone 662 can be doped with n-type impurities and zone 664 doped with p-type impurities. The plane of the seam is generally parallel to the front and back surfaces of the wafer.

As shown in Figures 1b and 2, the electrode corresponding to the zone 664 is contacted by means of the electrode 640 formed directly on the rear side of the cell. On the other hand, the electrode corresponding to the zone 662 is contacted by means of the contact 620, which comprises several portions. As shown in Figures 1a and 2, the portion 622 of the contact 620 with the electrode 662 is formed on the front side of the cell and comprises the metal semiconductor junction by which the charge present in the electrode 662 can be collected. This charge is then transferred on the rear side of the cell to the rear portion 624 of the contact 620. The charge transfer occurs through the portion 626 of the contact 620, formed inside a through hole made through the wafer thickness. In this way, the presence of ribbons on the front surface of the cell is avoided, with a consequent increase in efficiency compared to the traditional architecture of a solar cell.

Figure 1b shows that the cell 600 shown in the figure comprises, on the rear side of the cell 600, 15 contacts 640 to the electrode of a given polarity and 16 contacts 624 to the electrode of the opposite polarity. Therefore in total there are 31 ohmic contacts on the rear surface of the cell 600.

The cells contacted later pose new technological problems regarding the design and structure of the modules intended to accommodate them. For example, the backsheet must be designed to support a connection circuit on which the connections to both electrodes (base and emitter) of the cell are carried. One of the solutions developed in order to deal with this problem is the so-called backcontact-backsheet, an evolution of the traditional backsheet in which the connection circuit is made directly on the surface of the backsheet facing the cell. The structure of the backcontactbacksheet will be described more fully in the following. Figure 3 shows the structure of a photovoltaic module comprising solar cells contacted at the rear. The rear-contacted solar cell 600 is placed between a layer of upper encapsulating material 450 and a layer of lower encapsulating material 400. The cell 600 and layers 400 and 450 of encapsulating material are then enclosed between a surface layer 800 typically made of glass or in a transparent and anti-reflective material and the back-sheet 200, which can be a backcontact-backsheet. Figure 3 shows the tracks 220 of conductive material constituting the connection circuit to the electrodes of the solar cells. If the backsheet 200 is a backcontact-backshhet, the link circuit is formed directly on the surface of the underlying substrate and firmly attached to it. The connection circuit is used to ensure electrical contact with both electrodes, ie with the base and the emitter, of the solar cell 600. In particular, the conductive material tracks have pads 222, which they mark the points of the connection circuit to be put in electrical connection with a contact to one of the electrodes formed on the surface of the cell 600.

The assembly of a photovoltaic module of the type illustrated in figure 3 typically takes place according to the procedure described below.

The lower encapsulating material layer 400 to be placed between the cell 600 and the backsheet or backcontact-backsheet 200 is perforated so that the holes made in the lower encapsulating material layer 400 correspond to the areas in which the pads 222 are arranged for contact with the electrodes, once the module has been completed. The perforated encapsulating material layer 400 is then placed on top of the backsheet or backcontact-backsheet 200 so that the holes in the lower encapsulating material layer 400 correspond to or are aligned with the pads 222, so as to leave the pads 222 exposed towards the 'external.

A lump or drop of electrically conductive material is then deposited on the pads 222 of the conductive tracks of the connection circuit formed on the surface of the backsheet or backcontact-backsheet 200. The surface of the pads 222 is left exposed by the holes in the layer of lower encapsulating material 400. The conductive material deposited on the pads 222 may for example comprise a conductive paste of the type known as "Electro Conductive Adhesive" (ECA).

Subsequently, the cells 600 to be incorporated into the module are placed on the lower encapsulating material layer 400 so that each element of contact with the electrodes formed on the rear surface of the cell comes into contact with a lump of conductive paste applied on one of the pads 222 and exposed to contact with the cells 600 through one of the holes made in the lower encapsulating material layer 400. The upper encapsulating material layer 450 is then placed on the upper surface of the cell 600, opposite the rear surface in contact with the conductive paste applied on the pads 222. Finally, a layer of transparent and anti-reflective material 800 is placed over the layer of upper encapsulating material 450.

Once the structure has been prepared as just described, it is turned upside down and then laminated under vacuum at a temperature between 145 ° and 165 ° for a time ranging from 8 to 18 minutes.

Figure 4a shows the structure of the module before the lamination process. The components of the module, stacked as described above, are individually distinguishable. In particular, figure 4a shows a stack comprising, starting from the bottom and going upwards of the figure, the backsheet or backcontact-backsheet 200 with the conductive pads 222 on which the conductive paste 300 has been affixed, the layer of encapsulating material lower 400, cells 600, upper encapsulating material layer 450 and surface layer 800.

Figure 4b schematically shows the structure of the module after the lamination process. In the first stage of lamination, the structure is placed in a vacuum chamber from which the air is evacuated by means of pumps. A pressure is then applied to the structure to compact the different layers of which the module structure is composed, while maintaining the vacuum in the area where the module is located. The entire cycle preferably has a total duration of less than 18 minutes. The cycle preferably takes place at a temperature ranging from 140 ° to 165 °.

The lamination produces the hardening of the conductive paste 300 through its polymerization, thus causing the fixing of the cells 600 to the back-sheet 200. Furthermore, the lamination process also has the task of melting and subsequently polymerizing the layers of upper encapsulating material 450 and lower 400. In this way, the encapsulating material of the lower layer 400, by melting, fills the voids between the conductive paste 300, the backsheet or backcontact-backsheet 200 and the rear surface of the cells 600. Furthermore, by means of the polymerization, the upper encapsulating material layer 450 also exerts an adhesive action between the surface layer 800 and the outer surface of the cells 600 in contact with the upper encapsulating material layer 450. Similarly, the lower encapsulating material layer 400 subsequently exerts polymerization also an adhesive action between the rear surface of the cells 600 and the back-sheet 200.

The previous description highlights the technical and engineering difficulties that the production of a photovoltaic module entails.

First of all, drilling a plurality of holes in a sheet of encapsulating material such as VA is not a trivial task, especially if the holes must have predetermined positions and the tolerance on positioning error must be reduced to about 0.1 mm on a dimension overall of 1700 mm x 1000 mm. The tolerance is so low since each hole in the lower encapsulating material layer 400 must correspond to one of the contact pads 222 of the connection circuit. Therefore, the pattern of the holes in the lower encapsulating material layer 400 must exactly match the pattern according to which the pads 222 were made.

It must then be considered that the sheet of encapsulating material will have the dimensions of the module, that is typically 1, 7 m x 1 m. These considerable dimensions make the operation of opening the holes even more difficult.

Furthermore, the encapsulating layer typically has no mechanical consistency and has high thermal expansion / shrinkage coefficients so its dimensional stability is limited. For example, when subjected to a tensile stress, a sheet or lamina of encapsulating material tends to extend viscoelastic. This characteristic of the encapsulating material makes the drilling operation even more difficult.

The number of holes to be opened is then very high. As previously described with reference to Figure 1b, 16 contacts at one electrode and 15 contacts at the other electrode are generally formed on the rear surface of an MWT cell. So 31 contacts in total. A photovoltaic module typically contains 60 or 72 cells. Therefore the number of holes to be made on the layer of encapsulating material will be equal to 1860 and 2232 in the case, respectively, of a module with 60 and 72 cells. The operation of opening the holes is therefore quite long and laborious, thus contributing to the increase in production costs. An extremely delicate operation in the production of a photovoltaic module is then the alignment between the lower layer of encapsulating material 400 perforated and the underlying back-sheet or backcontact-backsheet 200, on whose conductive pads 222 the paste must be applied conductive. Each hole in the lower encapsulating material layer 400 must in fact correspond exactly to the respective conductive pad 222. The margin of error in the alignment between the lower encapsulating material layer 400 and the back-sheet or backcontact-backsheet 200 is minimum, in as a phase shift between the holes in the layer of encapsulating material and the conductive pads 222 involves the covering of the conductive pad 222 by the layer of encapsulating material 400 and, therefore, the lack of contact between one of the electrodes of the cell 600 and the backsheet or backcontactbacksheet 200. Even worse, a phase shift between the holes in the encapsulating layer 400 and the conductive pads 222 could cause the shedding of the conductive paste from the conductive pads 222 to unwanted areas of the surface of the back-sheet or backcontac-backsheet 200 exposed to the cells during successive phases of the assembly of the photovoltaic module.

While being able to align exactly the holes of the lower encapsulating material layer 400 with the conductive pads 222 on the backsheet or backcontact-backsheet 200, a fundamental problem arises during the subsequent assembly phases of the module. In fact, the position of the lower encapsulating material layer 400 with respect to the backsheet or backcontact-backsheet 200 must remain unchanged in the subsequent stages of the assembly process which include: deposition of the cells 600, deposition of the upper encapsulating material layer 450, deposition of the layer surface 800 and, finally, lamination.

It may happen that the structure to be assembled must be transported from one point to another of the production line when passing from one of the assembly phases mentioned above to the next. The lower encapsulating material layer 400 may also shift relative to the backsheet 200 when the upper encapsulating material layer 450 or the surface layer 800 are added to the structure as a result of the stresses received by the structure during the deposition process. Furthermore, it should be noted that during lamination the layer of encapsulating material 400 can deform by expanding or contracting in response to the increase in temperature. Therefore, keeping the position of the lower encapsulating material layer 400 fixed with respect to the backsheet or backcontact-backsheet 200 is a non-trivial task that a manufacturer of photovoltaic modules cannot avoid by resorting to the means made available by the state of the art.

OBJECT OF THIS INVENTION

In view of the aforementioned problems and the disadvantages related to the traditional assembly procedure of photovoltaic modules, an object of the present invention is to provide a component for photovoltaic modules to be applied on the surface of a backcontact-backsheet for photovoltaic modules and a method for producing this component that allow to overcome said problems.

In particular, one of the objects of the present invention is to provide a multilayer structure comprising a layer of dielectric material and a layer of encapsulating material to be applied to the surface of a backcontactbacksheet for photovoltaic modules in such a way as to allow a practical, economical and accurate of a photovoltaic module.

A further object of the present invention is to provide a multilayer structure and a method for making it thanks to which an excellent degree of stability of the lower encapsulating material layer can be obtained during the various assembly phases of the photovoltaic module, so that the layer of encapsulating material cannot move when mounting the module.

A further object of the present invention is to provide a multilayer structure and a method for making it thanks to which it is possible to obtain greater precision in the final and intermediate structure of the photovoltaic module, thus allowing a high degree of process repeatability and reducing to minimum waste of resources due to the production of defective products.

A further object of the present invention is to provide a multilayer structure and a method for making it thanks to which the risks of unwanted contamination and spreading of the conductive adhesive paste can be avoided during the assembly process of the photovoltaic module.

A further object of the present invention is to provide a multilayer structure and a method for making it thanks to which the awkward and complex step of drilling the lower encapsulating material layer during the assembly of the photovoltaic module can be avoided and facilitated thanks to the use of a method industrial and repeatable.

The critical steps of drilling the lower encapsulating material layer and aligning the encapsulating material layer with the backcontact-backsheet are normally the responsibility of the PV module manufacturer. Therefore, a further object of the present invention is to provide a multilayer structure to be applied to the surface of a backcontact-backsheet and capable of relieving a solar module manufacturer from the tasks of drilling and aligning the layer of encapsulating material, thus simplifying and economizing the solar module assembly and / or production process.

BRIEF DESCRIPTION OF THE INVENTION

According to the present invention, a multilayer structure is provided which can be applied on the surface of a backcontact-backsheet for a photovoltaic module based on the new and inventive concept that the assembly process of a photovoltaic module can be simplified, speeded up and made more precise. if the lower encapsulating material layer is supplied together with a pretreated dielectric material layer so as to firmly fix the encapsulating material layer to one of its surfaces.

The present invention is also based on the new and inventive concept that to the surface of the dielectric material opposite to the surface on which the layer of encapsulating material is fixed, a layer of thermo-adhesive resin can be applied in such a way as to allow excellent adhesion between the layer of dielectric material and the surface of the backcontact-backsheet to which the multilayer structure is applied.

Preferred embodiments of the present invention are provided by the dependent claims and the following description.

The present invention also provides a method for producing a multilayer structure adapted to be applied on the surface of a backcontact-backsheet for a photovoltaic module as defined in claim 14 and in the following description.

BRIEF DESCRIPTION OF THE FIGURES

Further characteristics and advantages of the present invention will become clearer from the following description of the embodiments of the device according to the present invention represented in the drawing tables. In the drawing tables, identical and / or similar and / or corresponding parts are identified by the same reference numbers or letters, in particular, in the figures:

Figure 1a shows a top view of the outer or front surface of a solar cell of the MWT type;

Figure 1b shows a top view of the inner or rear surface of a solar cell of the MWT type;

Figure 2 shows a cross section of a solar cell of the MWT type;

Figure 3 shows an exploded view of a portion of a photovoltaic module;

Figure 4a shows the structure of a photovoltaic module prior to the lamination process;

Figure 4b shows the structure of a photovoltaic module following the lamination process;

Figure 5 shows a cross section of a conventional backcontactbacksheet;

Figure 6 shows a cross section of an intermediate step of the realization of a multilayer structure according to an embodiment of the present invention; Figure 7 shows a cross section of a system comprising a multilayer structure according to an embodiment of the present invention and a portion of the backcontact-backsheet on the surface of which the multilayer structure is applied;

Figure 8 shows a cross section of a system comprising a solar cell, a multilayer structure according to an embodiment of the present invention and the backcontact-backsheet on the surface of which the multilayer structure is applied;

Figure 9 shows a cross section of a system comprising a multilayer structure according to an embodiment of the present invention and a portion of the backcontact-backsheet on the surface of which the multilayer structure is applied, after the system has been subjected to a lamination process .

DETAILED DESCRIPTION

In the following, the present invention will be described with reference to particular embodiments as shown in the attached figures. However, the present invention is not limited to the particular embodiments described in the following detailed description and shown in the figures but, rather, the described embodiments simply show several aspects of the present invention whose scope is defined by the claims.

Further modifications and variations of the present invention will be clear to the skilled person. Accordingly, the present description is to be regarded as including all modifications and / or variations of the present invention, the purpose of which is defined by the claims.

Figure 5 schematically shows a backcontactbacksheet 200 for a photovoltaic module commonly used in the state of the art. The air side of the photovoltaic module is the one placed at the bottom, under the backcontact-backsheet 200 shown in figure 5.

The backcontact-backsheet 200 comprises an insulating complex or substrate 210 exposed to the air side of the photovoltaic module.

The insulating substrate 210 comprises a first insulating layer 212, an intermediate layer 214 and a second insulating layer 216.

The first insulating layer 212 has a surface exposed towards the air side of the photovoltaic module and is used as a barrier against humidity, UV rays, oxygen and external agents that could penetrate into the module and damage some of the component parts or cause degradation of the polyurethane or polyester adhesive yellowing it.

The internal surface of the first insulating layer 212 opposite the surface exposed to the air-side is then in contact with an intermediate layer 214, which acts as a barrier against humidity and water vapor. The intermediate layer is typically made of aluminum, preferably with a thickness between 8 and 25 Î1⁄4m.

The internal surface of the intermediate layer 214, opposite the surface in contact with the first insulating layer212, is then in contact with the second insulating layer 216, which acts as an electrical insulator and as a further barrier.

A layer of conductive material 220 is then applied to the internal surface of the second insulating layer 216 opposite the intermediate layer 214. This layer of conductive material 220 applied to the surface of the second rear insulating layer 216 will subsequently be treated so as to form a pattern generally comprising elongated elements such as tracks, tracks, paths, etc. This pattern forms the connection circuit with the electrodes of the solar cells. Not being formed continuously on the inner surface of the second insulating layer 216, the conductive layer 220 generally leaves exposed some portions of the inner surface of the second rear insulating layer 216 on which it is applied.

The connection circuit obtained in the layer of conductive material 220 can be formed by means of one of the techniques commonly used for the production of printed circuits. For example, the connection circuit can be formed in the conductive layer 220 by means of an optical lithographic technique, in which a photoresist layer is uniformly applied to the surface of the conductive layer 220, exposed according to the pattern to be reproduced and then developed. The surface comprising the developed photoresist is then subjected to a chemical attack (etching) which produces the desired pattern. Finally, the residual photoresist is removed. Alternatively, the connection circuit can be formed on the layer of conductive material by means of an ablation process, to be obtained by mechanical means such as a cutter or by evaporation using for example a laser.

The conductive layer 220 then comprises pads 222 formed in predetermined positions of the pattern of which the conductive circuit in the conductive layer 220 is composed. The pads 222 will be placed in electrical contact with the ohmic contacts formed on the surface of the electrodes of the solar cells through a lump of conductive material. The pads 222 therefore ensure electrical contact with the solar cells mounted in the photovoltaic module and are formed in positions corresponding to the positions of the ohmic contacts 624 and 640 formed on the rear surface of the solar cell 600 and shown in Figures 1b and 2.

Again with reference to Figure 5, the backcontactbacksheet 200 then comprises a layer of dielectric material 240 deposited on the internal side of the backcontactbacksheet 200, ie on the side opposite the air side and facing the cells. Such a layer of dielectric material 240 is generally formed, for example by means of a screen printing process, in such a way that the dielectric material completely covers the portion of the surface of the second rear insulating layer 216 left exposed by the conductive layer 220 applied above it. The dielectric material thus has the function of electrically insulating two adjacent tracks or elements but disconnected from each other of the pattern formed by the conductive layer 220. Furthermore, the layer of dielectric material 220 has the function of neutralizing any surface currents that they could flow on the surface of the substrate 210 on which the conductive material layer 220 and a portion of the dielectric material layer 240 are attached.

The layer of dielectric material 240 is then deposited so as to leave partially exposed the layer of conductive material 220. More specifically, the layer of dielectric material 240 has openings 242 located in correspondence with the pads 222 of the conductive layer 220 shown in figures from 3 to 5 and used to contact the electrodes of the photovoltaic cells. Thus the dielectric layer 240 leaves the conductive pads 222 exposed towards the internal side of the backcontact-backsheet 200, ie towards the side facing the cells.

Finally, the portions of the conductive layer 220 left exposed by the layer of dielectric material 240 are covered by a protective layer 260 so as to prevent the exposed surface of the layer of conductive material 220 from oxidizing, corroding, scratching or damaging in general. The protective layer 260 may comprise an organic material, which is generally deposited by screen printing. Alternatively, the exposed portions of the conductive layer 220 can be protected by a metallic layer applied from the origin to the exposed surface. For example, if the layer of conductive material 220 comprises copper, it can be protected by a thin layer of nickel deposited on the surface of the layer of conductive material 220 comprising copper. If layer 220 were aluminum, it could be coated with copper and then nickel or tin. The choice of aluminum would be competitive with copper for a reason of lower costs for the same electrical resistivity.

The back-sheet 200 shown in Figure 5 has the drawbacks described above, such that the subsequent assembly of the photovoltaic module is slow, expensive and not very precise. This is largely due to the fact that a layer of encapsulating material placed between the cells and the backcontact-backsheet must first be perforated and then kept perfectly aligned with the conductive pads 222 of the backcontact-backsheet during all stages of assembly.

In order to overcome the drawbacks associated with backcontactbacksheets known in the state of the art, the multilayer structure 1000 is proposed, the constituent layers of which are shown in figure 6. The multilayer structure according to an embodiment of the present invention shown in figure 6 is to be applied to the surface of a backcontact-backsheet. The backcontact-backsheet to the surface of which the structure 1000 is applied comprises, like that shown in figure 5, an insulating substrate 210 and an electrical conductive layer 220 fixed to the internal surface of the substrate 210 formed as a connection circuit to the electrodes of the cells solar 600. Unlike the backcontact-backsheet shown in figure 5, the backcontact-backsheet to which the structure 1000 is applied does not include the dielectric material layer 240. Therefore, the surface of the backcontact-backsheet on which the structure is applied 1000 exposes the entire connection circuit obtained in the layer of conductive material 220. The surface of the layer of conductive material 220 exposed is then covered with a protective layer 260, as shown in Figure 5 and as described with reference to the same figure. The protective layer 260 is made in an innovative way through the structure 1000.

The structure 1000 shown in Figure 6 comprises a layer of dielectric material 240 which has an internal or upper surface 242 facing the inside of the photovoltaic module and a lower or external surface 244 facing the backcontact-backsheet on which the structure 1000 will be applied. .

The dielectric material layer 240 comprises an inextensible thin film. According to an embodiment of the present invention, the dielectric material layer 240 comprises a polymer. According to particular embodiments of the present invention, the dielectric material layer 240 comprises polyethylene terephthalate (PET), polypropylene (PP) or polyimide (PI) or other polymers having mechanical stability and dielectric strength characteristics. Preferably, the dielectric material layer 240 may further comprise bi-oriented PET or bi-oriented PP. According to an embodiment of the present invention, the layer of dielectric material 240 has a thickness of between 20 and 40 Î1⁄4m. Preferably, the layer of dielectric material 240 has a thickness comprised between 23 and 36 µm.

To the lower surface 244 of the layer of dielectric material 240 is fixed a layer of thermoadhesive material 270 which guarantees the adhesion of the structure 1000 to the surface of the underlying backcontact-backsheet and is able to conform according to the different heights presented by the upper face of the backcontact-backsheet and fill in all the gaps present. In particular, the layer of thermo-adhesive material 270 will adhere to the surface of the layer of conductive material 220 as well as to the portions of the surface of the backcontact-backsheet left free by the layer of conductive material 220. The layer of thermo-adhesive material 270 comprises an inner or upper surface 274 facing the dielectric material layer 240 and an outer or lower surface 272 opposite the inner surface 274 and facing the backcontactbacksheet.

According to some embodiments of the present invention, the layer of thermoadhesive material 270 comprises a resin. According to particular embodiments of the present invention, the layer of thermoadhesive material 270 can comprise a thermosetting resin or a thermoplastic resin. According to a particular embodiment of the present invention, the layer of thermoadhesive material 270 comprises a resin selected from epoxy, epoxyphenolic, or copolyester, or polyurethane or ionomeric polyolefins. According to a particular embodiment of the present invention, the layer of thermoadhesive material 270 comprises a resin whose melting temperature is between 60 ° and 160 °. Preferably, the resin of the thermo-adhesive material layer 270 is not tacky when handled cold.

The layer of thermo-adhesive material 270 has the function of fixing the structure 1000 to the surface of the backcontactbacksheet on which it is applied. In particular, the layer of thermo-adhesive material 270 ensures the adhesion between the layer of dielectric material 240 and the layer of conductive material 220 in which the connection circuit is formed.

The structure 1000 then comprises a layer of encapsulating material 280 comprising in turn a lower or outer surface 284 and an upper or inner surface 282. The layer of encapsulating material 280 is applied to the layer of dielectric material 240 so that the lower surface 284 of the encapsulating material layer 280 faces the upper surface 242 of the dielectric material layer 240. According to an embodiment of the present invention, the encapsulating material layer 280 comprises ÃVA. According to other embodiments of the present invention, the layer of encapsulating material 280 comprises silicones, or ionomer resins, or thermo polyurethanes, or polyolefins, or thermopolyolefins. According to an embodiment of the present invention, the layer of encapsulating material 280 has a thickness of between 100 and 500 Î1⁄4m.

The adhesion between the dielectric material layer 240 and the encapsulating material layer 280 is provided by a coating layer 250 applied to the inner surface 242 of the dielectric material layer 240. As will be described later, according to an embodiment the coating layer 250 is the result of a chemical or physical surface treatment to which the upper surface 242 of the dielectric material layer 240 is subjected. The coating layer 250 comprises an upper surface 252, to which the lower surface is attached 284 of the layer of encapsulating material 280 and a lower surface 254, firmly adhering to the upper surface 242 of the layer of dielectric material 240.

As shown in Figure 7, the multilayer structure 1000 has a plurality of through holes 286. The through holes 286 are made in predetermined positions each corresponding to a predetermined position on the connection circuit formed in the layer of conductive material 220 of the backcontact-backsheet on which surface the multilayer structure 1000 must be applied. More specifically, the holes 286 are such that, once the multilayer structure 1000 has been aligned with the backcontact-backsheet, the position of each hole 286 corresponds to the position of one of the ohmic contacts 624 and 640 formed on the back surface of the cells. Furthermore, when the multilayer structure 1000 has been aligned and applied to the backcontact-backsheet, the position of each through hole 286 will form one of the contact points on the layer of conductive material 220 indicated with the reference number 222 in figures 7 to 9. In this way, when the multilayer structure 1000 is applied to the backsheet-backcontact for which it is made, the holes 286 of the multilayer structure 1000 leave the contact points 222 on the conductive material layer 220 exposed. the surface of the layer of conductive material 220, as well as the surface of the contact points 222, can be covered by the protective layer 260 shown in Figure 5 but not in Figures 6 to 9.

The contact points 222 on the backcontact-backsheet can thus be put into electrical contact through the holes 286 with the ohmic contacts 640 or 624 on the rear surface of the cell 600. Such ohmic contacts 640 and 624 can for example be formed, respectively, on the positive electrode (contact p) and on the negative electrode (contact n) of the photovoltaic cell 600, as also shown in Figure 8.

The multilayer structure 1000 shown in Figures 6 to 9 provides a pre-perforated structure having a mechanical consistency easily applicable to the surface of a traditional backcontact-backsheet. Not having to center this structure with the points 222 that according to the prior art would have been previously formed, the uncertainty of centering of the structure is reduced. The 1000 structure thus makes it possible to speed up, simplify and make more precise the assembly operations of a photovoltaic module, thanks to the fact that the punching with the backcontact-backsheet of the layer of encapsulating material placed between the cells and the backcontact-backsheet are not most requested to the manufacturer of photovoltaic modules. Also, the alignment of the 1000 structure with the underlying backcontactbacksheet is much less critical than the alignment of a single sheet of encapsulant with a backcontactbacksheet on which the connecting pads have been defined. In fact, the structure 1000 must be aligned with the backcontact-backsheet so that the through holes 286 correspond to the tracks of the connecting circuit and not to the preformed pads 222 shown for example in Figure 3. The margin of error in the alignment is therefore very wider than in the case in which the connection pads 222 are preformed in the connection circuit as openings in the dielectric layer deposited on the surface of the backcontact-backsheet, according to the requirements of the prior art and as shown in Figures 3 and 5.

The realization of the multilayer structure 1000 takes place according to the procedure described below.

Initially, a layer of dielectric material 240 is chosen. The layer of thermo-adhesive material 270 is then applied to the lower surface 244 of the layer of dielectric material 240. The application can take place by means of coextrusion, hot lamination or coating or again by coupling by means of adhesive. According to a further embodiment of the present invention not shown in the figures, the layer of thermo-adhesive material 270 can be fixed to the layer of dielectric material 240 by means of an intermediate adhesive interposed between the upper surface 274 of the layer of thermo-adhesive material 270 and the surface bottom 244 of the dielectric material layer 240.

The manufacturing method of the multilayer structure 1000 then comprises a chemical or physical surface treatment of the upper surface 242 of the layer of dielectric material 240 facing the interior of the photovoltaic module. The surface treatment is carried out in order to ensure a stable adhesion with the layer of encapsulating material 280 to be applied over the layer of dielectric material 240.

The surface treatment can be such as to ensure adhesion between the layer of dielectric material 240 and the layer of encapsulating material 280 strong enough to allow the subsequent processing steps of the structure 1000 to be carried out easily and the assembly of the photovoltaic module in which the structure it must be contained. However, the surface treatment at the upper surface 242 of the dielectric material layer 240 does not necessarily have to ensure the final adhesion between the dielectric material layer 240 and the encapsulating material layer 280, i.e. the adhesion force present between the two layers at completed solar module. This definitive adhesion can in fact be achieved in the phase in which the solar module is assembled, for example during the lamination process described above. In particular, the surface treatment can be such that the adhesion force (peeling force) between the dielectric material layer 240 and the encapsulating material layer 280 is between 0.3 and 1 N / cm prior to the assembly of the multilayer structure. 1000 in the photovoltaic module in which it must be contained.

Similarly, according to an embodiment of the present invention, the thermoadhesive material layer 270 is applied to the outer surface 244 of the dielectric material layer 240 by means of an intermediate adhesive layer. The intermediate adhesive layer is such as to ensure a strong enough adhesion to allow the subsequent processing phases of the 1000 structure and the assembly of the photovoltaic module in which the structure must be contained to be carried out easily. However, the intermediate adhesive layer does not necessarily have to ensure the definitive adhesion between the layer of dielectric material 240 and the layer of thermo-adhesive material 270, that is, the adhesion force present between the two layers when the solar module is completed. This definitive adhesion can in fact be achieved in the phase in which the solar module is assembled, for example during the lamination process described above.

According to some embodiments of the present invention, the physical or chemical treatment to which the upper surface 242 of the dielectric material layer 240 is subjected comprises a corona or plasma treatment. According to other embodiments of the present invention, the physical or chemical treatment comprises applying a primer layer to the upper surface 242 of the dielectric material layer 240. According to a particular embodiment of the present invention, the primer layer comprises polyester , copolyester, or an acrylic polymer.

After subjecting the upper surface 242 of the dielectric material layer 240 to the chemical or physical treatment, the encapsulating material layer 280 is applied to the dielectric material layer 240. The presence of the surface treatment performed on the surface 242 of the dielectric material layer 240 ensures the adhesion between the two layers 240 and 280. The layer of encapsulating material is thus stably fixed to the layer of dielectric material 240 below.

Once the thermoadhesive material layer 270, the dielectric material layer 240 and the encapsulating material layer 280 have been fixed to each other, the multilayer structure 1000 is perforated so as to obtain a plurality of through holes 286 in predetermined positions. As previously mentioned, the through holes 286 are made in such a way as to allow the connection between the contacts 624 and 640 on the rear surface of the cells that will be assembled in the photovoltaic module and the connection circuit formed on the internal surface of the backcontact-backsheet. The through holes 286 are then drilled in the multilayer structure 1000 so as to reproduce the reason according to which the ohmic contacts 624 and 640 are formed on the rear surface of the cells to be assembled in the module.

The layer of dielectric material 240 placed to support the layer of encapsulating material 280 causes the structure 1000 to have the mechanical consistency that the single sheet of encapsulating material would lack. Since the 1000 structure has mechanical consistency, drilling a plurality of through holes in it is much easier than in a single sheet of encapsulating material.

According to an embodiment of the present invention, the drilling of the structure 1000 takes place by through punching. Punching offers the advantage of repeatability, as it is possible to use a mold comprising a matrix in which a plurality of punches are arranged in such a way as to reproduce a predetermined pattern. In particular, the plurality of punches can be arranged so as to reproduce the reason according to which the holes 286 to be drilled in the structure 1000 must be arranged. This matrix may for example comprise 31 punches arranged to each correspond to the position of one of the ohmic contacts 624 and 640 on the rear surface of a cell 600 shown in FIG. 1b. The mold with the matrix can then be translated by a predetermined distance and direction in order to drill the holes on the entire surface of the 1000 multilayer structure.

According to other embodiments of the present invention, the drilling of the multilayer structure 1000 takes place by laser contouring or milling.

Once perforated, the 1000 multilayer structure can be marketed as a single component.

According to a further embodiment of the present invention, the perforated multilayer structure 1000 can be applied to the surface of the backcontact-backsheet, aligned and fixed to the backcontact-backsheet, as shown in figure 9. The fixing of the multilayer structure 1000 to the underlying backcontact-backsheet it can take place, for example, by means of a hot lamination process, or by means of selective laser heating in predetermined points of the system, or even by the use of a thermo-adhesive 270 which has weak cold adhesion properties. The heating of the system causes the partial melting of the thermoadhesive resin layer 270 placed between the dielectric material layer 270 and the surface of the backcontact-backsheet. The layer of thermo-adhesive resin 270, melting, causes the fixing of the structure 1000 to the backcontact-backsheet. Furthermore, the thermoadhesive material 270 of which the layer 270 is composed penetrates after melting into the interstices between adjacent tracks of the connecting circuit of the layer of conductive material 220, as shown in Figure 9. In this way the layer 270 performs the function of electrical insulator which in the backcontactbacksheet according to the known art shown in figure 5 is carried out by the dielectric layer 240.

According to one embodiment, the thermoadhesive resin layer 270 is such as to ensure a non-definitive fixing between the multilayer structure 1000 and the backcontactbacksheet. In particular, the thermoadhesive resin layer 270, once melted and returned to the solid state, fixes the dielectric layer 240 to the layer of conductive material 220 in such a way as to allow the execution of the subsequent assembly steps of the module. However, according to this embodiment, the thermoadhesive resin layer 270 does not ensure the final fixing, that is, the adhesion force present between the dielectric layer 240 and the conductive material layer 220 when the solar module is completed. This definitive adhesion can be achieved in the phase in which the solar module is assembled, for example during the lamination process described above, similarly to what has been discussed above regarding the adhesion between the dielectric layer 240 and the encapsulating material layer 280 .

According to the embodiment of Figure 9, an integrated system is thus provided in which the lower encapsulating material layer 400 is included in the system and integrally fixed thereto. In this way, the manufacturer of photovoltaic modules no longer has to pose the problem of having to drill the lower encapsulating material layer before mounting, so that the holes on the encapsulant layer reproduce the same pattern as the pads on the connecting circuit of the layer. of conductive material. The need to align the encapsulating material layer to the backcontact-backsheet is also eliminated thanks to the system shown in Figure 9. Furthermore, the solar module manufacturer can speed up, simplify and economize production while improving quality. In fact, thanks to the system formed by the backcontact-backsheet and the multilayer structure 1000 shown in Figure 9, the risks associated with the displacement of the layer of encapsulating material with respect to the backcontact-backsheet are eliminated during all stages of the assembly.

According to a further embodiment of the present invention, once the multilayer structure 1000 has been aligned and fixed to the backcontact-backsheet as described above, a plurality of conductive elements are deposited on the surface of the connection circuit formed in the conductive layer 220 through the 286 holes in the multilayer structure 1000. Each conductive element is deposited in a predetermined position of the connection circuit. In particular, each conductive element is deposited at one of the contact points 222 with the ohmic contacts of the cells 600. The conductive elements are deposited on the contact points 222 through the holes 286 preferably as a thin layer. The conductive elements have the main function of avoiding oxidation and / or damage to the surface of the conductive pads 222 of the layer of conductive material 220 exposed through the holes 286. Furthermore, the preferably thin layer formed by the conductive elements on the surface of the pads 222 constitutes an interface to the ohmic contact of the cell which will facilitate the subsequent deposition of the conductive adhesive 300. According to an embodiment of the present invention, the conductive elements comprise a conductive adhesive paste such as ECA.

Once the 1000 multilayer structure has been fixed to the backcontact-backsheet, it is possible to proceed to the following phases of the assembly of the photovoltaic module. In particular, as shown in Figure 8, the conductive paste 300 is deposited through the holes 286 on the contact points 222 of the connection circuit formed in the conductive layer 220. The solar cells 600 are then placed on the layer of encapsulating material 280 so that the ohmic contacts 624 and 640 are in contact with the conductive paste 300, as previously described. The subsequent assembly steps also follow the procedure described above.

The present invention therefore provides a perforated multilayer structure 1000 to be used together with a backcontactbacksheet for photovoltaic modules comprising rear-contacted cells.

The 1000 multilayer structure has numerous advantages that allow to speed up, economize and make the assembly of photovoltaic modules more precise.

Since the structure 1000 has mechanical consistency due to the presence of the inextensible layer of dielectric material 240, it is relatively easy to drill or punch.

The structure 1000 avoids the use of the dielectric material layer 240 deposited on the surface of the backcontact-backsheet shown in Figure 5. The dielectric layer 240 typically comprises a polymeric material, which requires specific hardening, drying and polymerization processes. Furthermore, the layer of dielectric material shown in Figure 5 of the backcontact-backsheet according to the prior art covers the entire connection circuit, except for the connection pads 222 which correspond to openings in the dielectric layer 240 having a diameter of approximately 2, 5-3, 5 mm. The operation of aligning the encapsulating layer 280 perforated with the openings of the dielectric layer is therefore critical in the case of backcontact-backsheet according to the prior art, since the holes of the encapsulating layer must correspond exactly to the openings in the dielectric layer and to conductive pitches 222.

The structure 1000 comprises both the encapsulating material layer 280 and the dielectric material layer 240. Both layers are pierced together prior to application to the backcontact-backsheet. Therefore, thanks to the 1000 multilayer structure, there will be no problem of aligning the encapsulant with the dielectric openings that leave the pads open. The multilayer structure 1000 must still be aligned with the backcontact-backsheet, but in such a way that each through hole 286 of the structure 1000 corresponds to a track of the connection circuit and not exactly to a pad 222. Therefore the alignment operation of the structure 1000 with the backcontactbacksheet is much less critical than aligning the perforated single sheet of encapsulating material with the backcontactbacksheet coated with the dielectric layer 240.

In addition, the 1000 structure can be attached to the backcontactbacksheet and be marketed together with the backcontactbacksheet as a single product to be supplied to the PV module manufacturer.

A further variant of the process described provides that, once the backcontact-backsheet set with the structure 1000 has been assembled, a thin layer of conductive adhesive is deposited through the openings 286 on the conductive surface 220. This deposition allows to avoid oxidation of the exposed surface of the layer of conductive material 220 and has an interface towards the ohmic contact of the cell which will facilitate the subsequent deposition of the conductive adhesive 300.

Although the present invention has been described with reference to the embodiments described above, it is clear to the person skilled in the art that it is possible to make various modifications, variations and improvements of the present invention in light of the teaching described above and in the The scope of the attached claims without departing from the object and scope of protection of the invention.

In addition to this, those fields which are considered to be known by those skilled in the art have not been described in order to avoid unnecessarily overshadowing the described invention. Accordingly, the invention is not limited to the embodiments described above, but is limited solely by the scope of the appended claims.

Claims (28)

CLAIMS 1. Multilayer structure (1000) suitable to be applied on the surface of a backcontact-backsheet for a photovoltaic module comprising rear-contacted solar cells, said structure comprising: a layer of dielectric material (240), said layer of dielectric material (240) comprising an external surface (244) exposed towards the outside of said structure and an internal surface (242) opposite to said external surface, having said layer of dielectric material (240) a plurality of through holes, a layer of encapsulating material (280) coupled to said layer of dielectric material (240) so as to be stably fixed to said inner surface (242) of said layer of dielectric material (240), having said layer of encapsulating material (280) a plurality of through holes, a layer of thermo-adhesive material (270) coupled to said outer surface (244) of said layer of dielectric material (240) in such a way as to allow stable adhesion between said layer of dielectric material (240) and said surface of said backcontact-backsheet, having said layer of thermo-adhesive material (270) a plurality of through holes, said multilayer structure being such that: each of said through holes of said layer of encapsulating material (280) is made in a position corresponding to the position of one of said through holes of said layer of dielectric material (240) and each of said through holes of said layer of thermo-adhesive material (270) is made in a position corresponding to the position of one of said through holes of said layer of dielectric material (240) and to the position of one of said through holes of said layer for encapsulating material (280) in such a way that said multilayer structure has a plurality of through holes (286). Multilayer structure according to claim 1 wherein said layer of encapsulating material (280) comprises ÃVA, or silicones, or ionomers, or thermo polyurethanes, or polyolefins, or thermopolyolefins. Multilayer structure according to one of claims 1 and 2 wherein said layer of encapsulating material (280) has a thickness of between 100 and 500 Î1⁄4m. Multilayer structure according to one of claims 1 to 3 wherein said dielectric material layer (240) comprises an inextensible film. Multilayer structure according to one of claims 1 to 4 wherein said dielectric material layer (240) comprises polyethylene terephthalate (PET), or polypropylene (PP), or polyimide (PI). 6. Multilayer structure according to one of claims 1 to 5 wherein said layer of dielectric material (240) has a thickness comprised between 23 and 36 Î1⁄4m. 7. Multilayer structure according to one of claims 1 to 6 wherein said inner surface (242) of said layer of dielectric material (240) has a chemical or physical treatment such as to allow stable adhesion between said layer of dielectric material (240) and said layer of encapsulating material (280). 8. Multilayer structure according to claim 7 wherein said surface treatment comprises a corona or plasma treatment. 9. Multilayer structure according to one of claims 7 and 8 wherein said surface treatment comprises applying a primer layer, said primer layer comprising polyester, copolyester, or an acrylic polymer. 10. Multilayer structure according to one of claims 1 to 9 wherein said layer of thermoadhesive material (270) comprises a resin selected from epoxy, epoxyphenolic, copolyester, polyurethane or ionomer polyolefin resins. 11. Multilayer structure according to claim 10 wherein said resin is thermoplastic or thermosetting, said resin having a melting temperature between 60 ° C and 160 ° C, said resin being non-sticky if handled cold. 12. Backcontact-backsheet for a photovoltaic module comprising rear-contacted solar cells, said backcontact-backsheet comprising a surface, said surface of said backcontactbacksheet comprising a layer of electrical conductive material (220) formed as a connection circuit to the electrodes of said solar cells ( 600) and firmly adhering to said surface of said backcontact-backsheet, being a multilayer structure (1000) according to one of claims 1 to 1 1 firmly fixed to said surface of said backcontact-backsheet. Backcontact-backsheet according to claim 12 further comprising a plurality of conductive elements on the surface of said connecting circuit formed in said layer of conductive material (220), each of said conductive elements being deposited in a predetermined position of said connecting circuit through one of said through holes (286) of said multilayer structure (1000) with antioxidant protection function of the surface of said layer of conductive material (220). 14. Method for the production of a multilayer structure suitable for being applied on the surface of a backcontact-backsheet for a photovoltaic module comprising rear-contacted solar cells, said method comprising: choice of a layer of dielectric material (240), said layer of dielectric material (240) comprising an external surface (244) exposed towards the exterior of said structure and an internal surface (242) opposite to said external surface, application of a layer of encapsulating material (280) on said inner surface (242) of said layer of dielectric material (240), said application being performed in such a way that said layer of encapsulating material (280) is stably attached to said layer of dielectric material ( 240), application of a layer of thermo-adhesive material (270) to said outer surface (244) of said dielectric material layer (240) in such a way as to allow stable adhesion between said layer of dielectric material (240) and said surface of said back contact. backsheet, said application being performed in such a way that said layer of thermo-adhesive material (270) is stably fixed to said layer of dielectric material (240), drilling of said multilayer structure so as to open a plurality of through holes (286) in predetermined positions in said multilayer structure. A method according to claim 14 further comprising a physical or chemical treatment of said inner surface (242) of said layer of dielectric material (240) so as to allow a stable adhesion between said layer of dielectric material (240) and said layer of encapsulating material (280), said treatment being performed before said application of said layer of encapsulating material (280) on said inner surface (242) of said layer of dielectric material (240). A method according to claim 15 wherein said treatment comprises corona or plasma treatment. A method according to one of claims 15 and 16 wherein said surface treatment comprises the application of a primer layer, said primer layer comprising polyester, copolyester, or an acrylic polymer. Method according to one of claims 14 to 17 wherein said layer of thermo-adhesive material (270) is applied to said outer surface (244) of said layer of dielectric material (240) by coextrusion or hot rolling. Method according to one of claims 14 to 17 wherein said layer of thermo-adhesive material (270) is applied to said outer surface (244) of said layer of dielectric material (240) by means of an intermediate adhesive layer. Method according to one of claims 14 to 19 wherein said drilling of said multilayer structure takes place by punching said multilayer structure. 21. A method according to claim 20 wherein said punching is performed by means of a matrix composed of punches arranged so as to reproduce a predetermined pattern. Method according to one of claims 14 to 19 wherein said drilling of said multilayer structure is performed by laser contouring or milling. Method according to one of claims 14 to 22 wherein the definitive and stable connection over time of said layer of encapsulating material (280) to said layer of dielectric material (240) takes place during a lamination of said completely assembled photovoltaic module. 24. The method of claim 23 wherein said lamination occurs during a vacuum cycle. Method according to one of claims 23 and 24 wherein said lamination takes place at temperatures between 140 ° C and 165 ° C. Method according to one of claims 14 to 25 further comprising fixing said multilayer structure (1000) to said surface of said backcontactbacksheet, said surface of said backcontact-backsheet comprising a layer of electrical conductive material (220) formed as a circuit connection to the electrodes of said solar cells (600) and firmly adhering to said surface of said backcontact-backsheet, said fixing being performed after said drilling of said multilayer structure. 27. Method according to claim 26 wherein the definitive and stable connection over time between said backcontact-backsheet and said multilayer structure occurs during a lamination of said completely assembled photovoltaic module. Method according to one of claims 26 and 27 further comprising a deposition of a plurality of conductive elements on the surface of said connecting circuit formed in said conductive layer (220), each of said conductive elements being deposited in a predetermined position of said circuit connection through one of said through holes (286) of said multilayer structure, said deposition being performed after said fixing of said multilayer structure to said backcontact-backsheet, said deposition being performed with the function of antioxidant protection of the surface of said layer of material electrical conductive (220).