Method for making a photovoltaic element with a curved shape

Abstract

The invention relates to a method for making a photovoltaic element (1a) with a curved shape, said method comprising the steps of : - Providing a photovoltaic multilayer structure (1) comprising a first metallic ductile foil (2a) as the front surface, a second metallic ductile foil (2b) as the bottom surface and a photovoltaic core (3) disposed between the first metallic ductile foil (2a) and the second metallic ductile foil (2b), - Deforming the photovoltaic multilayer structure (1) to obtain the curved shape. - Removing the first metallic ductile foil (2a) and maintaining the second metallic ductile foil (2b) to obtain the photovoltaic element (1a) with a curved shape.

Description

METHOD FOR MAKING A PHOTOVOLTAIC ELEMENTWITH A CURVED SHAPEFIELD OF THE INVENTION

[0001] The invention relates to a method for making photovoltaic elements with a curved shape, in particular for making photovoltaic slats and photovoltaic tiles, and to the resulting curved shape photovoltaic elements.

[0002] It also relates to the photovoltaic multilayer structure used for making said curved shape photovoltaic elements.BACKGROUND OF THE INVENTION

[0003] In recent years, the market demand for various forms of photovoltaic curved construction products has increased, but the photovoltaic modules packaged in wave-shaped or curved materials are limited and their fabrication requires additional process steps and costs.

[0004] Photovoltaic thin-film technologies such as amorphous silicon, CIGS or CdTe give full play to the technical characteristic of flexible solar by changing the form of these traditional systems.

[0005] As described in documents CN103413847A or CN106906958A, the application of such as flexible thin film technologies into wave-shaped surfaces typically requires the following process steps: (i) providing a first curved wave surface from a construction element as substrate, (ii) laminating the flexible thin solar film to provide mechanical and atmospheric barriers, (iii) physical overlapping of the construction curved wave substrate, the photovoltaic laminate and / or additional layers onto the curved wave substrate surface to produce photovoltaic construction element and (iv) curing the physical overlap of the multilayer structure to form a continuous and uniform surface wave-shaped building component.

[0006] This methodology is widely used to transfer the wave-shape to the photovoltaic element in order to avoid performance losses or critical damages in the photovoltaic devices during permanent deformation processes.

[0007] A more efficient production method would be to laminate the photovoltaic thin film foil on a ductile metallic substrate and then plastically deform the laminate to obtain the desired wave-shaped prefabricate photovoltaic. However during the plastic deformation of the laminate the plastic bending introduces compression and tension which affect the different layers of the laminate. That may introduce stresses which can damage the structure of the photovoltaic film producing performances losses. During plastic bending the peak bending stresses and strains occur at the outside fibers of the cross-section laminate like in the case of elastic bending. Using a single ductile metallic substrate, the photovoltaic thin film foil is just placed on the outside of the cross-section of the laminate and it is then subject to the peak bending stresses and strains and so to potential damage impacting the performances.

[0008] The document EP 0 874 404 discloses a method for manufacturing a solar cell module having a photovoltaic element encapsulated with a resin on a support member. The method comprises the step of forming a bent portion in the photovoltaic element and in the support member. The formation of the bent portion is performed while reducing the working pressure in the normal direction to the surface of the photovoltaic element. The formation of the bent portion can be performed by press molding. In this case, the working pressure is reduced by providing a buffer material between the mold for the press molding and the solar cell module. The formation of the bent portion can also be performed by work with a roller former. In this case, the working pressure is reduced by winding a buffer material around the roll. The buffer material is a sheet of rubber, urethane, foam, nonwoven fabric, polymer resin. In both cases, the buffer material is not part of the photovoltaic multilayer structure.SUMMARY OF THE INVENTION

[0009] The present invention aims to provide another method for making curve shaped photovoltaic elements, and in particular wave-shaped photovoltaic element. The method comprises a step of deforming a photovoltaic prefabricate, and in particular a flat photovoltaic prefabricate, into the desired shape without reducing performance or damaging the photovoltaic material.

[0010] According to the invention, the photovoltaic prefabricate, also called photovoltaic multilayer structure, comprises two metallic ductile foils as the front and bottom surfaces respectively with the photovoltaic thin film core containing the photovoltaic thin film material in between. The external surfaces are under tension or compression depending on the plastic deformation radius orientation. The photovoltaic thin film core, which contains the thin film photovoltaic material, is placed between the two ductile foils. As a result, the stress or strain applied during the deformation over the photovoltaic thin film core is reduced compared to the construction with only one ductile foil. Therefore, the performance losses or damages of the photovoltaic element are avoided or considerably reduced.

[0011] Preferably, the photovoltaic thin film core is placed along or close to the elastic neutral axis of the multilayer structure where the stress or strain remains zero or quasi zero during deformation, so that the photovoltaic element is not subject to any performance losses or damages. Preferably, the photovoltaic thin film core is placed at a distance inferior or equal to 30% of the thickness of the multilayer structure with respect to the elastic neutral axis.

[0012] More specifically, the method for making a photovoltaic element with a curved shape comprises the steps of :- Providing a photovoltaic multilayer structure,- Deforming the photovoltaic multilayer structure to obtain the curved shape, characterized in that the photovoltaic multilayer structure comprises a first metallic ductile foil as the front surface, a second metallic ductile foil as the bottom surface and a photovoltaic core disposed between the first metallic ductile foil and the second metallic ductile foil, and in that after the step of deforming the photovoltaic multilayer structure, there is a step of removing the first metallic ductile foil and maintaining the second metallic ductile foil to obtain the photovoltaic element with a curved shape.

[0013] For the deformation, roll forming is ideal for producing constant profile parts with long lengths and in large quantities, by using high productive converting processes. It is also possible to make holes in-line for the subsequent attachment to the substrate being for example a tile. The transferred waveform can be then applied directly onto the desired construction element through traditional fixing solutions, thus avoiding costly manual alignment and autoclave processes. Oncethe forming process is complete, the front ductile foil is removed so that the photovoltaic material faces outward. This forming approach can be applied to obtain photovoltaic roof tiles and sheets and also photovoltaic blind slat.

[0014] According to preferred embodiments, the method of the invention has one or several of the following features:- the photovoltaic multilayer structure is defined with an elastic neutral axis and a thickness, said photovoltaic core being disposed between the first metallic ductile foil and the second metallic ductile foil at a distance with respect to the elastic neutral axis inferior or equal to 30% of the thickness;- said photovoltaic core is disposed on the elastic neutral axis;- the step of deforming is carried out by rolling;- the first and second metallic ductile foils are made up of material with the same mechanical properties;- the first and second metallic ductile foils are aluminium foils;- the first and second metallic ductile foils have a thickness comprises between 100 pm and 5 mm, the first and second metallic ductile foils having preferably the same thickness;- the photovoltaic core is formed of a stack of layers comprising an insulating layer, a back contact layer, a plurality of junction layers, a front contact layer and at least a transparent insulating and protective front layer covering at least partially the front contact layer;- the photovoltaic core is formed of at least two stacks of said layers in front of each other in order to make two photovoltaic elements with a curved shape during a single deforming step;- holes are provided in the photovoltaic multilayer structure in regions not meant to generate photovoltaic power, during or after the deforming step, said holes being intended to receive fixing means to a substrate or connection means between blind slats.

[0015] The invention also relates to the photovoltaic multilayer structure intended to be used for making the photovoltaic element with a curved shape, characterized in that said photovoltaic multilayer structure comprises a first metallic ductile foil as the front surface, a second metallic ductile foil as the bottom surfaceand a photovoltaic core disposed between the first metallic ductile foil and the second metallic ductile foil.

[0016] The invention also relates to a photovoltaic element with a curved shape, characterized in that the photovoltaic core comprises at least one submodule of solar cells connected in series and at least one sub-module of solar cells connected in parallel, with the solar cells being connected in parallel in the direction of the curved-shape and with the solar cells being connected in series in the direction perpendicular to the direction of the curved-shape.

[0017] According to a preferred embodiment, the photovoltaic element with a curved shape comprises several sub-modules of solar cells connected in series with at least two of the sub-modules of solar cells connected in series being polarized in opposite directions for one array of solar cells connected in series.

[0018] According to another preferred embodiment, the holes are provided in regions not meant to generate photovoltaic power, i.e. in regions without solar cells connected in series or in parallel.

[0019] The invention also relates to a blind slat or a tile characterized in that it is made up of said photovoltaic element with a curved shape obtained with the method aforementioned, the second metallic ductile acting as a substrate for the blind slat and the tile comprising a substrate with the photovoltaic element with a curved shape fixed to the substrate.BRIEF DESCRIPTION OF THE DRAWINGS.

[0020] FIG.1 is a cross-section view in perspective of the photovoltaic multilayer structure according to the invention after the forming step and FIG.5 is a cross-section view of the photovoltaic multilayer structure according to the invention before the forming step.

[0021] FIG.2 is a cross-section view of the arrangement of the monolithic series-connected thin film photovoltaic cells inside the photovoltaic core of the multilayer structure according to the invention.

[0022] FIG.3 is a perspective view of the photovoltaic core with self-shadow zones when the arrangement of the photovoltaic cells is not optimised.

[0023] FIG.4 represents the same perspective view with an optimised arrangement of the photovoltaic cells.

[0024] FIG.6 is a cross-section view of the stack of layers inside the photovoltaic core.

[0025] FIG.7 is a cross-section view of the double stack of layers inside the photovoltaic core according to a variant of FIG.6.

[0026] FIG.8 is a schematic representation of the bending stress repartition(distribution) inside a slab of homogeneous material under elastic deformation. In this situation, the elastic neutral axis is located in the middle of the slab.

[0027] FIG.9 is a cross-section view of the photovoltaic multilayer structure according to the invention after the forming step.

[0028] FIGS.10 and 19 are representations of the roll forming step according to the method of the invention.

[0029] FIG.11 represents the arrangement of FIG.6 after the forming step.

[0030] FIG.12 is a side view of the photovoltaic element with the curved shape according to the invention attached on a tile substrate.

[0031] FIG.13 is the photovoltaic multilayer structure according to the invention comprising the double stack of layers inside the photovoltaic core according to FIG.7.

[0032] FIG.14 represents two shapes of the photovoltaic multilayer structure according to FIG.13 after the forming step. The inversion symmetry is also identified. FIG.15 Illustrates the detachment of two identical photovoltaic elements obtained from the forming step according to FIG.14.

[0033] FIGs.16A to 16C are different layouts of the sub-modules of monolithic series-connected photovoltaic cells inside the photovoltaic core in a mirror configuration.

[0034] FIGs.17A to 17D also schematically represent different layouts of the sub-modules of monolithic series-connected photovoltaic cells inside the photovoltaic core with the corresponding current flow direction and voltage. FIG.17B is a serially connected configuration. FIG.17C is a simple mirror configuration and FIG.17D is a double mirror configuration.

[0035] FIG.18 is a cross-section of FIG.16C where devices are connected in a mirror configuration for a blind slat application.

[0036] FIG.20 is a perspective view of the photovoltaic multilayer structure after the forming step and the perforating step for the holes.

[0037] FIG.21 is a similar view to FIG.20 inside the photovoltaic multilayer structure.

[0038] Legend1 : Photovoltaic multilayer structure a) Curved shape photovoltaic element2: Ductile foil, also called metallic foil a) First ductile foil, also called top foil or front foil b) Second ductile foil, also called bottom foil3: Photovoltaic core4: Arrangement of monolithic series-connected thin film photovoltaic cells5: First interconnection grooves extending through the back contact layer 96: Second interconnection grooves extending through the junction layers 107: Third interconnection grooves extending through the front contact layer 118: Insulating layer9: Back contact layer10: Junction layers11 : Front contact layer12: Transparent insulating protective front layer13: Series-connected photovoltaic cells14: Self-shadow15: Parallel-connected photovoltaic cells16: Maximum tensile stress under elastic deformation17: Maximum compressive stress under elastic deformation18: Stress distribution through the multilayer structure19: Elastic neutral axis where stress is zero20: Roll forming tool: top roll21 : Roll forming tool: bottom roll22: Substrate as a tile substrate23: Inversion center24: Contact electrode25: Connection structure26: Through holesDESCRIPTION OF THE INVENTION

[0039] The invention relates to a method for making a photovoltaic element with a curved shape such as a wave-shape, said method comprising the steps of :- Providing a photovoltaic multilayer structure comprising a first metallic ductile foil as the front surface, a second metallic ductile foil as the bottom surface and a photovoltaic core disposed between the first metallic ductile foil and the second metallic ductile foil,- Deforming the photovoltaic multilayer structure to obtain the curved shape,- Removing the first metallic ductile foil and maintaining the second metallic ductile foil to obtain the photovoltaic element with the curved shape.

[0040] The resulting photovoltaic element with a curved shape can be fixed on a substrate such as a tile or the second ductile foil can directly act as substrate, the photovoltaic element with a curved shape forming, for example, a blind slat.

[0041] The photovoltaic multilayer structure 1 represented in figures 5 and 1 respectively before and after deformation comprises a photovoltaic core 3 disposed between two ductile foils 2. "Ductile" means able to be deformed elastically or plastically. Both foils are metallic foils. For example, it could be aluminium foils. Preferably, both foils are made up of the same material or they are at least made up of materials with the same mechanical properties so as to ensure that the stressstrain relationships are the same during deformation. Thereby, a symmetric structure is kept to minimize the stress over the photovoltaic core. Both foils have a thickness comprises between 0.1 mm and 5 mm. Preferably, both foils have the same thickness. Preferably, the photovoltaic multilayer structure is flat before deformation.

[0042] Preferably, the photovoltaic core 3 is disposed on or close to the neutral axis of the multilayer structure. "Close to" means to a distance inferior or equal to 30% of the thickness of the multilayer structure with respect to the elastic neutral axis. The thickness is defined as the average thickness if it is not constant. The elastic neutral axis is defined as the line in a beam or another member wherein the fibres are neither stretched nor compressed, i.e. wherein the stress remains zero, when subjected to a bending action. It is schematically represented in figure 8, with a monolayer and homogeneous structure 1 submitted to bending in an elastic deformation with the top surface under maximum tension 16, the bottom surfaceunder maximum compression 17 and the elastic neutral axis 19 under zero stress. The elastic neutral axis can be easily determined by the skilled person. For a noncomplex geometry like a geometry with a rectangular or square section, the elastic neutral axis is the axis in the middle of the thickness of the monolayer structure.

[0043] The photovoltaic core contains the photovoltaic material, also called the thin film photovoltaic material. The photovoltaic core 3 represented in figure 6 is formed of a stack of layers comprising an insulating layer 8, a back contact layer 9, a plurality of junction layers 10, a front contact layer 11 and at least a transparent insulating and protective front layer 12 covering at least partially the front contact layer 11 . Typically, the insulating layer 8 can be made of polyamide, PET, aluminum oxide, glass, etc. The back contact layer 9 can be made of molybdenum, aluminum, etc. The front contact layer 11 can be made of transparent conductive oxide such as ITO or ZnO:AI and the transparent insulating and protective front layer can be made of PET or PMMA.

[0044] In a variant, the photovoltaic core comprises two stacks of said layers arranged in front of each other as depicted in figure 7, with the front layer 12 of one stack facing the front layer 12 of the other stack. In another variant, the photovoltaic layer comprises more than two stacks of said layers arranged in front of each other.

[0045] Preferably, the deformation process consists of passing the photovoltaic multilayer structure with the two ductile foils and the photovoltaic core through a roll forming process visible in figures 10 and 19 with the top roll 20, the bottom roll 21 and the multilayer structure 1 in between. It involves the continuous bending of a long strip of sheet metal, typically coiled aluminum laminate according to the invention, into a desired cross-section profile. The strip passes through sets of rolls performing incremental part of the bend until the desired cross-section profile is obtained. Roll forming is ideal but any other deformation process such as stamping could be equally used. The resulting curved photovoltaic multilayer structure is shown in figure 9. Once the forming process is complete, the top foil is separated from the curved multilayer structure so that the thin photovoltaic material faces outward.

[0046] Starting from a multilayer structure with two stacks of layer as in figure 7, it is possible to make curved elements with an inversion center of symmetry 23 (figure 14). In this case, the multilayer structure to be bent can be constructed bycoupling two laminated foils comprising a ductile foil 2 and a photovoltaic core 3. The coupling is done so that the two ductile foils are positioned externally, and the two electrical insulators are positioned in contact with each other in the center of the sandwich. In that way with a unique bending operation, it is possible to obtain two identical shaped curved photovoltaic elements with a photovoltaic 3 and a ductile foil 2 as shown in figure 15.

[0047] According to a preferred variant, the photovoltaic core, and more specifically the insulating layer, is glued with the bottom ductile foil and not with the front foil. The transparent front layer of the photovoltaic core is facing up so that once the forming process is complete only the top foil is separated from the photovoltaic core. The insulating layer of the photovoltaic core remains attached to the bottom foil. The thin photovoltaic material will face outward with the transparent front layer being the external layer.

[0048] During or after the forming step, holes 26 are provided in the curved shape multilayer structure for attaching to the substrate 22 like a tile (figure 12) or for passing cords, wires, etc in the case of blind slats. The holes 26 are preferably provided in regions not meant to generate photovoltaic power, also called nonactive area, i.e. in regions without solar cells monolithically connected. In other words, the holes are provided in regions adjacent to the regions with solar cells monolithically connected. It could be provided in the regions with the contact electrode 24 between two sub-modules of cells connected in series 13 (figures 20 and 21 ). The holes 26 can be perforated during the rolling process leaving the multilayer structure as shown in figures 20 and 21 with the front ductile foil 2. The holes 26 can also be performed with a conventional punching method after the rolling forming process.

[0049] To achieve a useful power level from photovoltaic devices, individual photovoltaic cells must be electrically connected in series in an array typically referred as a photovoltaic module. The total current of the module passes through each individual solar cell, whereas the voltage of the solar cells essentially adds to one another in the series connection.

[0050] The array is made of monolithic series-connected photovoltaic cells with a typical arrangement known from the document WO 2021 / 204358 andrepresented in figure 2 with the reference 4. There are interconnection grooves 5,6,7. They comprise:- first interconnection grooves 5 extending through the back contact layer 9 and which are filled by the junction layer 10;-second interconnection grooves 6 extending through the junction layer 10 and filled by the front contact layer 11 , i.e. the transparent conducting oxide layer;- third interconnection grooves 6 extending through the front contact layer 11 .

[0051] As can be understood by looking at the figure, the first interconnection grooves 5 are substantially parallel and not coincident with respect to the second interconnection grooves 6 which, in turn, are substantially parallel and not coincident with respect to the third interconnection grooves 7. In other words, the grooves 5,6,7 have a certain offset. In this way, the junction layer 10 extends into the first grooves 5 and is in contact with the insulating layer 8, and the front contact layer 11 extends into the second grooves 6 and is in contact with the back contact layer 9. Preferably, the interconnection grooves 5,6,7 are provided by laser scribing. Alternatively, other patterning techniques for forming the interconnection grooves 5,6,7 can be used.

[0052] The high voltages of the typical thin film solar modules with solar cells connected in series may be unsuitable for some applications, especially those ones requiring the application of low voltage directive, covering health and safety risks on electrical equipment. Also the fact that each individual solar cell is passed through by a strong current can be potentially dangerous. To reduce the voltage, the photovoltaic module can be divided in sub-modules with at least one sub-module comprising a plurality of thin film solar cells connected with one another in parallel 15 and at least one sub-module comprising a plurality of thin film solar cells connected with one another in series 13 (figure 4).

[0053] Preferably, the photovoltaic material comprises several sub-modules of solar cells connected in parallel 15 in a direction parallel to the direction at which the wave-shape will be imposed on the multilayer structure, whilst it comprises several sub-modules of cells connected in series (reference 13) along the direction not subject at the plastic deformation. In addition, the solar cells are preferably small size cells with a length in the direction parallel to the direction at which the waveshape will be imposed on the multilayer structure inferior to 5 mm.

[0054] This preferential orientation of the sub-modules has the advantage that the self-shadowing 14 of the solar module due to its wave-shape as schematically illustrated in figure 3 will affect the different cells equally, minimizing the consequences on its performance. Furthermore, the presence of a plurality of small cells allows to reduce the hot spot susceptibility to partial shading of other nature like antennas, bird dropping, smokestacks, trees, etc. compared to a configuration with elongated cells of several centimeters long.

[0055] Each wave of the curved photovoltaic module is comprised of at least one sub-module of a thin film solar cells connected in series monolithically 13 or at least two sub-modules of thin film solar cells connected in series monolithically 13.

[0056] The sub-modules of solar cells connected in series can be arranged according to different layouts depending on the application. For example, the layout of the sub-modules along a wave of the photovoltaic element can be arranged as represented in figures 16A to 16C and 17A to 17D. The layout of figures 16A to 16C and figures 17C and 17D is particularly adapted for blind slats. In figures 16A to 16C and 17C, the sub-modules are respectly arranged as mirror images of one another inside a wave of the photovoltaic element. The mirror image configuration has the effect that the sub-modules are polarized in opposite directions relative to one another. The feature of opposite polarization has the effect that the currents generated in the sub-modules flow in opposite directions to one another. The submodules comprise the series-connected cells 13 with the connection structures 25 and the contact electrode 24 between two sub-modules. The sub-modules displayed in figures 16A and 17C are separated by the regions with the contact electrode 24. The corresponding multilayer structure 1 with both ductile foils 2 is represented in figure 18.

[0057] In figure 16B, the sub-modules are placed adjacent to each other with no separation in between. Both configurations require an electrical connection structure such as an isolation path on which to place an electrically connection ink, paste, coating, or flat ribbon acting as a collecting electrode. In alternative as shown in figure 16C, a single contact area is provided per contact electrode.

[0058] Figure 17B represents another layout without mirror image configuration leading to a higher voltage and low current for large photovoltaic fields. The sub-modules are thus polarized in the same direction relative to one anotherwith the resulting current flowing in one direction as shown in figure 17B in presence of a contact electrode 24 between the sub-modules. Figure 17D represents another layout with a mirror image configuration not between each sub-module but between pairs of sub-modules.

Claims

CLAIMS1. A method for making a photovoltaic element (1a) with a curved shape, said method comprising the steps of :- Providing a photovoltaic multilayer structure (1 ),- Deforming the photovoltaic multilayer structure (1 ) to obtain the curved shape, characterized in that the photovoltaic multilayer structure (1 ) comprises a first metallic ductile foil (2a) as the front surface, a second metallic ductile foil (2b) as the bottom surface and a photovoltaic core (3) disposed between the first metallic ductile foil (2a) and the second metallic ductile foil (2b), and in that after the step of deforming the photovoltaic multilayer structure (1 ), there is a step of removing the first metallic ductile foil (2a) and maintaining the second metallic ductile foil (2b) to obtain the photovoltaic element (1a) with a curved shape.

2. Method according to claim 1 , characterized in that the photovoltaic multilayer structure (1 ) is defined with an elastic neutral axis (19) and a thickness, said photovoltaic core (3) being disposed between the first metallic ductile foil (2a) and the second metallic ductile foil (2b) at a distance with respect to the elastic neutral axis (19) inferior or equal to 30% of the thickness.

3. Method according to claim 1 or 2, characterized in that said photovoltaic core (3) is disposed on the elastic neutral axis (19).

4. Method according to any one of previous claims, characterized in that the step of deforming is carried out by rolling.

5. Method according to any one of previous claims, characterized in that the first and second metallic ductile foils (2a, 2b) are made up of materials with the same mechanical properties.

6. Method according to any one of previous claims, characterized in that the first and second metallic ductile foils (2a, 2b) are aluminium foils.

7. Method according to any one of previous claims, characterized in that the first and second metallic ductile foils (2a, 2b) have a thickness comprises between 100 pm and 5 mm, the first and second metallic ductile foils (2a, 2b) having preferably the same thickness.

8. Method according to any one of previous claims, characterized in that the photovoltaic core (3) is formed of a stack of layers comprising an insulating layer(8), a back contact layer (9), a plurality of junction layers (10), a front contact layer (11 ) and at least a transparent insulating and protective front layer (12) covering at least partially the front contact layer (11 ).

9. Method according to previous claim, characterized in that the photovoltaic core (3) is formed of at least two stacks of said layers in front of each other in order to make two photovoltaic elements (1a) with a curved shape during a single deforming step.

10. Method according to any one of previous claims, characterized in that holes (26) are provided in the photovoltaic multilayer structure (1 ) in regions not meant to generate photovoltaic power, during or after the deforming step, said holes (26) being intended to receive fixing means to a substrate (22) or connection means between blind slats.

11. A photovoltaic multilayer structure (1 ) intended to be used for making a photovoltaic element (1 a) with a curved shape according to any one of previous claims, characterized in that said photovoltaic multilayer structure (1 ) comprises a first metallic ductile foil (2a) as the front surface, a second metallic ductile foil (2b) as the bottom surface and a photovoltaic core (3) disposed between the first metallic ductile foil (2a) and the second metallic ductile foil (2b).

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