How to manufacture solar panels that are curved in two directions

Abstract

The present invention relates to a method for manufacturing solar panels curved in two directions. A problem arises when solar cells are stacked (attached) to a curved surface (such as a transparent car roof) that is at least locally curved in two directions. While solar cells can bend in one direction (along a cylindrical surface), they bend to a much lesser extent in the other direction. The present invention solves this problem by subdividing a number of solar cells (100) into subgroups (302L, 302R, 304L, 304R, 306L, 306R, 308L, 308R), each associated with a region of the curved surface (302). By selecting these subgroups so that there is little curvature in one direction, the solar cells can be bent vertically. To optimize efficiency, solar cells with an anode and cathode located on one side (opposite the photosensitive surface) are used, allowing the use of flexible foils for interconnecting the solar cells within a subgroup.

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a solar panel curved in two directions, comprising the steps of providing a plurality of solar cells having a photosensitive surface and connecting with a cathode and an anode on opposite sides of the photosensitive surface, and providing a curved surface that is transparent or semi-transparent and curved in two directions. [Background technology]

[0002] In a solar panel, many solar cells are interconnected, usually by using "finger electrodes" to connect the light-sensitive side of one solar cell to the back side of an adjacent solar cell. These interconnected cells are then laminated between two sheets, for example a sheet of polyolefin or one of its copolymers such as EVA (ethylene vinyl acetate) or polyvinyl butyral (PVB), which are then bonded to glass to form a (flat) solar panel.

[0003] It should be noted that lamination in this context is also known as bonding or sealing (using a sealant), and therefore lamination, bonding and sealing are used interchangeably.

[0004] Solar cells are typically thin wafers of semiconductor material. Such solar cells can only be deformed into cylindrical and some other shapes (e.g. conical) in a limited way due to stresses that arise in the solar cell. If these stresses exceed a fracture limit, the solar cell may fracture, rendering it unusable.

[0005] Wafers of semiconductor material in this context include single crystal, polycrystalline and amorphous wafers of silicon, GaAs, InP, etc.

[0006] It should also be noted that "flexible sheet" in this context refers to a sheet of, for example, polyimide or polyester (or more generally, insulating synthetic material) having a conductive layer or pattern thereon (usually copper, silver). When describing a conductive pattern (by etching, machining, or otherwise removing conductive material or selectively adding conductive material to a blank synthetic material), such a sheet is also referred to as a flexible PCB or FCB.

[0007] Producing solar panels that are curved in two dimensions is difficult in that solar cells are very fragile and the stresses that occur when they are curved in two directions are much higher than when they are curved in only one direction. Also, the stresses that occur in the flexible sheet when curved in two directions can lead to wrinkling and folding of the foil.

[0008] It should be noted that adhering the flexible foil and attached solar cells to a curved surface can lead to damage to the cells, the interconnects, or the bonds of the interconnects to the cells.

[0009] One solution to this problem is known from US patent application publication US2014 / 0130848A1, where a standard flat sheet (laminate) containing solar cells is provided and cut into the sheet, which when draped convexly expands, thereby allowing curvature in two directions.

[0010] The drawbacks of this method are that the gaps in the cuts result in low coverage of the solar cell, and the use of finger electrodes reduces the solar cell area available for solar conversion, and although this is not a technical issue, the aesthetic appearance is less than that of a uniformly covered solar panel.

[0011] Another solution to this problem is known from US patent application publication US2017 / 0141253A1, where a group of solar cells are interconnected using finger electrodes and cells as described above, which discloses a method of varying the length of the finger electrodes to disrupt the alignment of the solar cells, after which the solar cells are stacked / bonded to a curved surface.

[0012] The disadvantage of this method is that on curved surfaces, such as the solar roof of a car, the coverage of the solar cells is less than on flat surfaces. Also, the use of finger electrodes reduces the amount of semiconductor area available for solar conversion. Also, arranging the cells before lamination is difficult, because the finger electrodes place them in an irregular matrix. And, although this does not pose a technical problem, the aesthetic appearance is inferior to that of a uniformly covered solar panel.

[0013] It should be noted that, although not widely used due to their high cost, solar cells with both anode and cathode connections on opposite sides of the photosensitive surface are known, and their efficiency is increased because the finger electrodes do not "shadow" part of the photosensitive surface (block light from the photosensitive side). [Prior art documents] [Patent documents]

[0014] [Patent Document 1] US Patent Application Publication No. 2014 / 0130848 [Patent Document 2] US Patent Application Publication No. 2017 / 0141253 Summary of the Invention [Problem to be solved by the invention]

[0015] To solve the problems of the above solutions, the present invention provides a method for overcoming these problems. [Means for solving the problem]

[0016] Therefore, this method Two or more flexible foils having conductive patterns thereon are provided; The solar cells are grouped into subgroups, each of the subgroups being associated with an area on the curved surface and a flexible foil; This method is for each of the subgroups, a soldering step of soldering the solar cells to the flexible foil or gluing the solar cells to the flexible foil with a conductive adhesive; a positioning step of draping two or more subgroups onto a curved surface; and a final lamination step of laminating the subgroups onto a curved surface.

[0017] It is well known that solar cells and flexible foils can tolerate some curvature in one direction, so that they follow (part of) a cylindrical surface, but much less if they are curved in two directions (locally following part of a hyperboloid), especially because the bonding material (lamination material) acting as a filler and the variation in the thickness of the bonding material over the size of the solar cell can "absorb" (even out) some of the curvature. Lamination refers to the maximum lamination thickness (thickness of the initial lamination layers). However, during the lamination process, the thickness may vary locally due to the viscosity of the (uncured and not yet crosslinked) lamination sheets. During the lamination process, the laminated sheets (and the embedded solar cells) are subjected to a pressure of, for example, 1 atmosphere (10 N / cm 2 ) pressure, a temperature between 125°C and 145°C, and a time of, for example, 20 minutes. This allows the EVA layer to be fully crosslinked. Other combinations of time and temperature may be used, with lower temperatures requiring longer times for full crosslinking (curing). It is also possible to cure at lower temperatures and for shorter times, resulting in incomplete (also called partial) crosslinking, leaving the material tacky and still partially viscous. For other laminate materials, other temperatures and cure times can be used. By dividing a number of solar cells into subgroups, each of the subgroups is associated with an area on the curved surface that has a small or negligible curvature in at least one direction, making it possible to form subgroups that can be bonded (laminated) to the curved surface, and thus, when appropriately divided into subgroups, will result in subgroups that have mechanical stresses below the destruction limit of the solar cells and below the limit where the flexible foil will show wrinkles when laminated to the curved surface.

[0018] It should be noted that the bonding material (lamination material) does not need to have a constant thickness between the solar cell and the curved surface, but for efficiency, thermal behavior, and aesthetic reasons it is preferable that the thickness does not vary too much across a single solar cell and across multiple solar cells.

[0019] In the soldering step, the solar cells are electrically and mechanically connected to the flexible foil. This step may involve, for example, soldering of a solder paste at temperatures in excess of 200°C, or bonding with a conductive adhesive such as an epoxy filled with a conductive material such as copper, silver, etc. Curing of such adhesives, usually at temperatures between 100 and 150°C, may take place during lamination.

[0020] In the next positioning step, each of these subgroups is placed (draped) onto the curved surface.

[0021] In a final lamination step, the draped subgroups are laminated onto the curved surface.

[0022] It should be noted that while the use of flexible foils (also known as back contact foils or BCFs) is known (most often polyester / copper foils are used, but polyimide as the carrier and silver as the conductor are known as well), the use of such foils on curved surfaces (curved in two directions) is not known, and without the steps according to the invention flexible foils tend to exhibit wrinkles when folded or draped on or over curved surfaces curved in two directions, making them unsuitable for use under these conditions.

[0023] It should be noted that U.S. Patent Application Publication US2017 / 0141253A1 relies on varying the length of the finger electrodes and does not suggest using flexible foils with solar cells with both anode and cathode connections on one side.

[0024] In one embodiment of the method according to the invention, after the soldering step and before the positioning step, there is a first lamination step, The first lamination step is In each subgroup, laminating the solar cell and associated flexible foil, the lamination being performed at a temperature and for a time such that the lamination results in partial cross-linking; A positioning step of draping the partially crosslinked laminate including the solar cell and the flexible foil onto or into a curved surface; a final lamination step in which the partially cross-linked laminate comprising the solar cell and the flexible foil is laminated to a curved surface, thereby fully cross-linking the laminate applied in the first lamination step.

[0025] Partial lamination of the solar cells and the flexible foil prior to the positioning step facilitates positioning since the solar cells and the flexible foil form an interconnected web, and the (fragile) solar cells are less likely to be damaged during handling and positioning. The advantage of incomplete cross-linking is that the same lamination material can be used for the final lamination, where partial cross-linking is completed. Also, heating a fully cross-linked laminate during the final lamination step can reduce the transparency of the first laminate layer and cause degradation of the encapsulant, such as hardening. This "partial" lamination avoids the problems and risks mentioned above.

[0026] In one embodiment of the method according to the invention, after the soldering step and before the positioning step, a first lamination step is inserted, The first lamination step is laminating the solar cell and the associated flexible foil at a temperature and for a time that results in complete cross-linking; a positioning step of draping the stacked subgroups onto or into a curved surface; and a final lamination step of laminating the laminated subgroups onto a curved surface.

[0027] This embodiment is similar to the previous embodiment with the difference that complete crosslinking occurs in the first lamination step, and although strict control of temperature and time is required to prevent loss of transparency of the laminate material, this may be preferred as the intermediate product (laminated web of solar cells and flexible foil) is less tacky and easier to handle than an incompletely cured (partially cured) laminate sheet, and provides additional protection for the solar cells.

[0028] In another embodiment of the method according to the invention, during the final lamination step between the lamination subgroup and the curved surface, more layers of lamination material (encapsulant) are placed and laminated thereto.

[0029] This embodiment shows the possibility of placing multiple layers of laminate material (for example layers of EVA - ethylene vinyl acetate -) between the solar cell and the curved surface, resulting in more laminate material (encapsulant) that may exhibit local thickness variations, thereby reducing mechanical stresses.

[0030] In yet another embodiment of the method according to the invention, at least during the soldering step, two or more flexible foils are placed in one plane.

[0031] Handling the flexible foil and solar cells in a planar manner allows for manufacturing on standard soldering equipment.

[0032] It should be noted that since not all subgroups need to be manufactured at the same time, the moment at which the soldering step is performed may be different for each subgroup, and not all flexible foils need to be placed on the plane at the same time.

[0033] In yet another embodiment of the method according to the present invention, the method further comprises, before providing the solar cells, estimating the mechanical stresses occurring in the solar cells due to curvature of the curved surface at the location of the solar cells, and based on this estimation, modifying the size of the solar cells such that the mechanical stresses in each solar cell are smaller than the stresses that would cause fracture of the solar cell.

[0034] The size of the subgroups (maximum area) is best determined by iterative testing, starting with an initial size of the solar cell, and simulating what stresses occur when the cell is forced to conform to the curved surface. If the stresses are too high, at what is assumed to be the maximum allowable (e.g. half the destruction limit of the solar cell), reduce the size of the solar panel in the direction where the high stresses occur, etc.

[0035] In yet another embodiment of the method according to the invention, the method further comprises a step of estimating, before providing the solar cell, the mechanical stresses occurring in the solar cell due to the curvature of the curved surface at the location of the solar cell and, based on this estimation, modifying at least locally the thickness of the laminate material so that the stress value occurring in the solar cell is smaller than the stress at which fracture of the solar cell occurs.

[0036] By varying the thickness of the laminate material at least locally, the stresses that occur in the solar cell due to the viscosity of the uncured laminate material can be reduced. A local variation in the thickness of the laminate material can be achieved by locally adding rings, circles, or other shapes before curing the laminate material, building up the 3D shape of the material. It is noted that this solution can be reached using locally added material, but that the encapsulant will do so slightly, so a thicker layer of encapsulant (the laminate material, or the use of several layers) can also be used. 3D shaping of, for example, EVA can be achieved by overlapping one or more sheets on top of each other, where at least one sheet exhibits perforations etc. 3D printing of EVA is also known.

[0037] The invention will be clarified by the use of figures, in which like reference symbols indicate corresponding features. [Brief description of the drawings]

[0038] [Figure 1A] FIG. 1A shows a schematic diagram of a solar cell for use in the present invention, viewed from the photosensitive side. [Figure 1B] FIG. 1B shows a solar cell used in the present invention viewed from the side opposite the photosensitive surface. [Diagram 2] FIG. 2 shows diagrammatically one possible cut-through for a solar panel manufactured according to the present invention. [Figure 3A] FIG. 3A shows a schematic plan view of a solar panel, indicating the areas that form each subgroup. [Figure 3B] FIG. 3B shows a schematic plan view of a solar panel, illustrating the solar cells that form each subgroup. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] FIG. 1A shows a schematic diagram of a solar cell for use in the present invention, viewed from the photosensitive side.

[0040] FIG. 1A shows diagrammatically a solar cell 100 for use in the present invention. The solar cell exposes a photosensitive area 102 and an insulating edge portion 104. It should be noted that this insulating portion (actually an insulating sidewall) occurs only in so-called passivated cells. The advantage is that the cells can be placed opposite each other. To avoid short circuits, the cells without side passivation must be separated. Most commercial cells are cut from large wafers and do not exhibit sidewall passivation, so they must be spaced apart to avoid electrical short circuits. A typical thickness for a solar cell is around 200 μm, although for more flexible solar cells thinner cells, for example 150 μm, are preferred.

[0041] FIG. 1B is a schematic diagram of a solar cell used in the present invention, viewed from the side opposite the photosensitive side.

[0042] Otherwise, notches are provided on the insulating surface 106 to provide contact areas for the anode 108-i and cathode 110-j.

[0043] FIG. 2 shows diagrammatically one possible cut-through for a solar panel manufactured according to the present invention.

[0044] A curved surface 202, for example in the form of a glass roof of an automobile, is bonded to the solar cell 102a with a crosslinked layer 204 of a laminate material, for example EVA (ethylene vinyl acetate) or polyolefin, with its photosensitive side facing the glass.

[0045] The flexible foil comprises a polyester or polyimide film 206 and copper tracks 208. The solar cells are soldered onto the flexible foil, for example by soldering the cells and flexible foil with a solder paste heated to 200° C. or higher, or are glued by using a conductive adhesive (usually a metal-filled epoxy) that cures at temperatures below 150° C., for example. Solar cells 102b and 102c are shown here as belonging to a different subgroup (although it is not shown for these cells that their flexible foil is not the same as that of solar cell 102a).

[0046] It is noted that an additional layer of material can be placed between the flexible foil and the solar cell, either as a solder mask (or a mask for conductive adhesive), or as an aesthetic screen to hide the copper tracks on the flexible foil, or for other purposes. This does not have to be a transparent material. A lamination layer can be added to this layer (to completely seal the flexible foil), but this is not necessary. Also, the flexible foil may include only one layer of insulating material as a carrier, such as polyimide or polyester, with the conductive pattern thereon, or may include additional layers, with or without cutouts, to act as a solder mask, or for other purposes, on either the solar cell side or the opposite side.

[0047] The amount of laminate material (bonding material, encapsulant) between the solar cells may be the result of one laminate layer, but may include multiple layers of laminate material.

[0048] It should also be noted that although an adhesive layer (lamination layer) is required between the curved surface (glass) and the solar cell, a lamination layer covering the flexible foil is not essential to the invention.

[0049] FIG. 3A shows a schematic plan view of a solar panel, indicating the areas that form each subgroup.

[0050] FIG. 3A shows a surface 300 divided into regions 302L, 302R, 304L, 304R, 306L, 306R, and 308. The distribution of curvatures is the result of analysing the maximum stresses that occur when a flexible foil and a solar cell are bonded (laminated) to a curved surface. These stresses can occur in any direction, but in many applications (such as the roof of a car) there is an axis of symmetry (here the x-axis) that simplifies the problem. Then, experimentally (or using another method, e.g. computer-based), a partition of the region is found that leads to acceptable stresses on both the solar cell and the foil. On a related note, it is important to note that stress problems in the solar cell do not necessarily lead to problems in the flexible foil, and vice versa. The size of the solar cell is often much more limited than the (maximum) size of the flexible foil. Thus, moderate curvatures may cause problems in the flexible foil first. However, large localized curvatures may cause problems in the solar cell before problems in the flexible foil.

[0051] FIG. 3B shows a schematic plan view of a solar panel, illustrating the solar cells that form each of the subgroups.

[0052] Figure 3B can be considered as a derivative of Figure 3A, where solar cells are depicted forming subgroups (subgroup boundaries shown with thick lines). As can be seen, even within a subgroup, the orientation of the cells can also be changed, as shown for example in group 304L.

[0053] It should be noted that the definition of subgroup used here is not related to the definition of "group" normally used in solar technology. The standard definition of a group consists of a group of cells, usually part of a series, and in state-of-the-art panels, each group is associated with an optimizer (maximum power point tracking). The definition of a subgroup according to the invention is used to indicate cells that are attached to a (doubly) curved surface without exceeding a given stress level. This means that a subgroup may contain multiple groups, or a group may span multiple subgroups, or any combination thereof. Thus, two subgroups may consist of three groups, or vice versa, but each group may be associated with its own optimizer.

[0054] Furthermore, it should be noted that although the curved surface is usually a transparent or translucent curved surface, it is also possible to use this method on opaque curved surfaces (e.g. metallic curved surfaces), where the light-sensitive side of the solar cell is most removed from the curved surface. In that case, a flexible foil must be placed between the curved surface and the solar cell. However, in most cases this is a less robust solution, since the laminated material is soft and prone to scratches and abrasions.

[0055] It should be noted that the electrical connection between one flex foil and another, or another PCB or FBC, is made by soldering or bonding the other flex foil to the first, or electrical connections via wires may also be used.

[0056] It should be noted that in the context of the present invention, lamination may describe a complete encapsulation, but also the bonding of one part (e.g., a solar cell) to another part (e.g., a curved surface) using a bonding or laminating material as a polyolefin or one of its copolymers such as EVA (Ethylene Vinyl Acetate) or Polyvinyl Butyral (PVB). For example, after the final lamination step defined above, the flexible foil may or may not be covered with a laminating material.

[0057] An exemplary cure cycle would be, for example, forming a sandwich of a sheet of EVA, solar cells, flexible foil, and another sheet of EVA layer in a vacuum oven heated to 140° C., pulling a vacuum for 3.5 minutes, evacuating the sandwich using a silicone membrane at approximately 1 atmosphere of pressure while curing in this state for 17 minutes, removing the pressure, venting for 30 seconds, opening the oven and removing the sandwich. This results in a fully cured (fully crosslinked) sandwich. A partially crosslinked sandwich can be obtained by reducing the cure time to 8 minutes.

[0058] Laminating the aforementioned sandwich to a curved surface, such as the glass roof of a solar car, can be performed, for example, by placing the sandwich including the glass roof, an uncured sheet of EVA, and previously made and cured solar cells and flexible foil into an evacuable bag, drawing a vacuum on the bag (thereby applying a pressure of 1 atmosphere to the contents of the bag), heating the bag to a temperature of 100°C in 40 minutes, gradually heating the bag to a temperature of 130°C with a ramp-up time of 15 minutes, curing at a temperature of 130°C for 20 minutes, and cooling down. Note that the experiment does not show degradation of an already fully crosslinked sandwich. [Explanation of symbols]

[0059] 100 Solar Cells 102 Photosensitive area 102a, 102b, 102c solar cells 106 Insulating Surface 104 Insulated edge part 108 Anode 110 Cathode 202 Curved surface 204 Crosslinked layer 206 Flexible Foil 208 Copper Truck 300 curved surface

Claims

1. 1. A method for manufacturing a solar panel curved in two directions, comprising: providing a plurality of solar cells (100) having a photosensitive surface and connected to an anode (108-i) and a cathode (110-j) on opposite sides of said photosensitive surface; Providing a curved surface (202) that is transparent or semi-transparent and locally curved in at least two directions; Including, The method includes the steps of providing two or more flexible foils (206) having a conductive pattern thereon; said solar cells (100) are grouped into subgroups (302L, 302R, 304L, 304R, 306L, 306R, 308), each of said subgroups being associated with an area on said curved surface and said flexible foil, each of said subgroups being associated with an area on said curved surface having a small or negligible curvature in at least one direction, resulting in a subgroup having a mechanical stress below the fracture limit of said solar cells and below the limit at which wrinkles appear in said flexible foil when laminated against said curved surface; a soldering step, in each of said subgroups, comprising soldering said solar cells (100) to said flexible foil (206), or a bonding step, in each of said subgroups, comprising bonding said solar cells to said flexible foil with a conductive adhesive; a positioning step of draping two or more of said sub-groups onto or within said curved surface; a final lamination step of laminating said subgroups onto said curved surface (202); A method for manufacturing a solar panel curved in two directions, comprising:

2. A method for manufacturing a semiconductor device comprising the steps of: (a) forming a first lamination step after the soldering step or the bonding step and before the positioning step; The first lamination step comprises: In each and every subgroup, the method includes laminating the solar cell and the associated flexible foil, the laminating being carried out at a temperature and for a time that does not result in complete cross-linking; the positioning step includes draping a partially crosslinked laminate, including the solar cell and the flexible foil, onto or into the curved surface; the final lamination step comprises fully cross-linking the laminate applied in the first lamination step by laminating a partially cross-linked laminate comprising the solar cell and the flexible foil to the curved surface. The method of claim 1.

3. A method for manufacturing a semiconductor device comprising the steps of: (a) after the soldering step or the bonding step and before the positioning step, a first lamination step; The first lamination step comprises: In each and every subgroup, the method includes laminating the solar cell and the associated flexible foil, the laminating being carried out at a temperature and for a time that results in complete cross-linking, the positioning step includes draping the stacked subgroups onto or within the curved surface; the final lamination step includes laminating the laminated subgroups to the curved surface. The method of claim 1.

4. The method of claim 1 or 2, wherein during the final lamination step, several layers of laminate material are placed and laminated between the subgroup and the curved surface.

5. The method according to claim 1 or 2, wherein two or more of said flexible foils are laid in one plane at least during the soldering step or the bonding step.

6. Prior to providing the solar cell, Estimating mechanical stresses occurring in the solar cell and in the flexible foil due to curvature of the curved surface at the location of the solar cell; The method of claim 1 or 2, further comprising the step of modifying the size of the solar cells based on this estimation such that the mechanical stress in each solar cell is less than the mechanical stress at which fracture occurs.

7. Prior to providing the solar cell, estimating the mechanical stress occurring at the solar cell due to the curvature of the curved surface at the location of the solar cell; The method according to claim 1 or 2, further comprising a step of at least partially changing the thickness of the laminate material based on this estimation so that the stress value generated in the solar cell is smaller than the stress at which the solar cell fractures.

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