Patent ID: 12224370

DETAILED DESCRIPTION OF THE INVENTION

FIG.1Aschematically shows a solar cell used in the invention, seen from the photosensitive side.

FIG.1Aschematically shows a solar cell100used in the invention. The solar cell shows a photosensitive area102and an insulating edge portion104. It is noted that this insulated portion (actually an insulating side wall) only occurs at so-called passivated cells. An advantage is that it enables the cells to be placed against each other. Cells without passivation on the sides must be separated to avoid shorts. Most commercially available cells are cut from a large wafer and do not show a passivation of the side walls, so they need to be spaced from each other to avoid electrical shorting.

A typical thickness for a solar cell is in the order of 200 μm, but for more flexible solar cells thinner cells, for example with a thickness of 150 μm, are preferred.

FIG.1Bschematically shows a solar cell used in the invention, seen from the side opposite to the photosensitive side.

On an otherwise insulating surface106cutouts are provided that provide contact areas to anode108-iand cathode110-j.

FIG.2schematically shows one possible cut-through of a solar panel fabricated according to the invention.

A curved surface202in the form of, e.g., the glass roof of a car, is bonded with a cross-linked layer204of lamination material, for example EVA (Ethylene Vinyl Acetate) or a polyolefin to solar cells102a. The photosensitive side is facing the glass.

The flexible foil comprises a polyester or polyimide film206and copper tracks208. The solar cell is soldered on a flexible foil, either by soldering the cells and the flexible foil with solder paste that is heated to, for example, 200° C. or more, or by using electrically conductive adhesive (typically a metal filled epoxy) that is cured at a temperature of, for example, less than 150° C.

Solar cells102band102care here depicted as belonging to another subgroup (the flexible foil—although not depicted for these cells—is not the same as the flexible foil of cell102a.

It is noted that between the flexible foil and the solar cells a further layer of material can be placed, either as a solder mask (or a mask for electrically conductive adhesive), or as an esthetic screen to obscure the copper tracks on the flexible foil, or for any other purpose. This need not be a transparent material. A lamination layer may be added on this layer (thereby completely encapsulating the flexible foil), but is not necessary.

Also the flexible foil may comprise only one layer of insulating material as a carrier, such as polyimide or polyester, with a conductive pattern thereon, or it may comprise further layers with or without cut-outs to act as a solder mask, or for other purposes, the further layers either on the side of the solar cells or on the opposite side.

The amount of lamination material (bonding material, encapsulant) between the solar cell may be the result of one lamination layer but may comprise several layer of lamination material.

It is further noted that a bonding layer (a lamination layer) between curved surface (glass) and solar cells is necessary, but a lamination layer covering the flexible foil is not essential to the invention.

FIG.3Aschematically shows a planar view of solar panel, showing the areas forming each subgroup.

FIG.3Ashows a curved surface300that is divided in areas302L,302R,304L,304R,306L,306R and308.

The distribution of the curved surface is the result of an analysis of the maximum stresses occurring when flexible foil and solar cells are adhered (laminated) to the curved surface. These stresses can occur in any direction, but in many applications (such as a car roof) there is an axis of symmetry (here the x-axis) simplifying the problem. Empirically (or using another method, for example computer based) a division of areas is then found that result in acceptable stresses for both the solar cells and the foils.
It is noted that, although related, a stress problem in a solar cell need not result in a problem in the flexible foil, and vice versa: the size of a solar cell is often much more limited than the (maximum) size of a flexible foil. Therefore, at modest curvatures a problem may first occur in the flexible foil. However, at a large local curvature may result in a problem in the solar cells before a problem in the flexible foil occurs.

FIG.3Bschematically shows a planar view of solar panel, showing the solar cells forming each subgroup.

FIG.3Bcan be thought to be derived fromFIG.3A. Here the solar cells are depicted that form the subgroups (borders of subgroups indicated by thicker lines). As can be seen also the orientation of the cells can be changed, even within a subgroup, as shown in for example group304L.

It is remarked that the definition of subgroup used here has no relation to the definition of “group” usually used in solar technology. The standard definition of group comprises a group of cells, typically being part of a string, and in state of the art panels each group is associated with an optimizer (a Maximum Power Point Tracker). The definition of subgroup according to the invention is used to denote cells that are attached to a (doubly) curved surface without exceeding predetermined stress levels. This implies that one subgroup may comprise more than one group, or one group may extend over more than one subgroup, or any combination thereof. It is thus possible that two subgroups comprise three groups or vice versa, while each group may be associated with its own optimizer.

It is further remarked that typically the curved surface is a transparent or translucent curved surface, but it is also possible to use the method with a non-transparent curved surface (for example a metal curved surface) and that the photosensitive side of the solar cells are most removed from the curved surface. In that case the flexible foils need to be placed between the curved surface and the solar cells However, this is in most cases a less robust solution as the lamination material is softer and more prone to scratches and abrasion.

It is also remarked that electrical connections between one flexible foil to another flexible foil, or to other PCB's or FBC's, can be made by soldering or bonding another flexible foil (preferably formed as a strip with several lines forming the electrical connections) on the first flexible foil. Also electrical connections via wires can be used.

It is noted that, in the context of this invention, laminating may describe a total encapsulation, but may also describe bonding one part (for example the solar cells) to another (for example the curved surface) using a bonding or lamination material, such as of a polyolefin or one of its copolymers, such as EVA (ethylene vinyl acetate), or polyvinylbutyral (PVB). For example, after the final lamination step defined earlier, the flexible foil may or may not be covered with lamination material.

An exemplary curing cycle is, for example,form a sandwich of a sheet of EVA, the solar cells, the flexible foil and another sheet of EVA layer in a vacuum oven that is heated to 140° C.,evacuate for 3½ minutes,while evacuated press on the sandwich with a pressure of approximately 1 atmosphere using a silicon membrane,cure in this condition for 17 minutes,remove pressure,ventilate for 30 seconds,open the oven and remove the sandwich.
This results in a fully cured (fully crosslinked) sandwich. A partial crosslinked sandwich is obtained by reducing the time for curing to 8 minutes.

Laminating the before described sandwich to a curved surface, such as the glass roof of a solar car, is done by, for example:Place the glass roof, an uncured sheet of EVA and the previously made and cured sandwich comprising solar cells and flexible foil, in an evacuable bag,evacuate the bag (thereby applying a pressure of one atmosphere to the contents of the bag),heat the bag to a temperature of 100° C. during a time of 40 minutes,gradually heat the bag further to a temperature of 130° C. in a ramp-up time of 15 minutes,cure for 20 minutes at the temperature of 130° C. andcool down.
It is noted that experiments show no degradation of the already fully crosslinked sandwich.