NL2008841C2 - Multilayer backsheet for photovoltaic modules. - Google Patents

Description

MULTILAYER BACKSHEET FOR PHOTOVOLTAIC MODULES

The present invention relates to a multilayer backsheet for photovoltaic modules, a process for the production of the backsheet, and its use in a 5 process for the production of photovoltaic modules.

Background of the invention

Photovoltaic modules typically comprise photovoltaic cells arranged in an 10 essentially flat photovoltaic module. Photovoltaic cells can typically be categorized into two types based on the light absorbing material used, namely bulk or wafer-based photovoltaic cells and thin film photovoltaic cells.

Typically the cells are combined in a certain pattern, and are interconnected to create a single power output. The modules are typically 15 enclosed in a matrix of polymeric materials. The photovoltaic cells typically comprise a doped semiconductor material, which converts incoming light into electric energy. Commonly used materials include monocrystalline silicon (c-Si), poly- or multi-crystalline silicon (poly-Si or mc-Si) to form the more traditional wafer-based photovoltaic cells. Alternatively, thin film photovoltaic 20 cells are formed from materials that include amorphous silicon (a-Si), microcrystalline silicon ([mu]c-Si), cadmium telluride (CdTe), copper indium selenide (CulnSe2 or CIS), copper indium/gallium diselenide (CulnxGa(i-X)Se2 or CIGS), light absorbing dyes, and organic semiconductors.

Photovoltaic modules derived from wafer- based photovoltaic cells 25 often comprise a series of self-supporting wafers that are soldered together. The wafers generally have a thickness of between about 160 and about 240 pm, commonly known as photovoltaic cell layer. The layer typically further comprises electrical wirings such as cross ribbons connecting the individual cell units and bus bars having one end connected to the cells and the other 30 exiting the module. More recent developments incorporate the contact elements or ribbons into the cell, e.g. back contact or rear contact photovoltaic modules, thereby no longer arequiring ribbons to connect the front of one cell to the back of a next cell.

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The photovoltaic cell layer is usually wedged between layers of polymeric encapsulants and outer protective layers to form a weather resistant module. Subject to the outdoor application, the photovoltaic modules have to be durably resistant to the different weathering conditions, including variations 5 in humidity and temperature, exposure to UV and other radiation, and exposure to chemicals and/or (micro)biological growth related with the outdoor exposure; migration of ions; oxidation, mechanical load though exposure to wind and snow; and resilience against mechanical impacts, such as for instance through hail, 10 In general, a photovoltaic cell module comprises, starting from the light incident side to the back side, an incident layer or front sheet; a front encapsulant layer; the photovoltaic cell layer; a back encapsulant layer, and a backing layer or backsheet.

The role of the front sheet is to protect the photovoltaic module against 15 mechanical impact and weathering while allowing light to pass to the active layer. Typical front sheets are made from a glass pane, usually low-iron tempered glass with a thickness of about 3.2 to 4 mm, or from transparent polymers such as PMMA, or transparent multilayer composites. The front sheet is typically connected to the photovoltaic cell layer by means of a 20 transparent encapsulant, typically a polymer layer that can act as a heat melt adhesive. The backside of the photovoltaic cell layer is typically attached to a second encapsulant layer, followed by the backsheet as the rear protective layer of the module.

Frontsheet and backsheet have to provide barrier properties versus 25 humidity; migration of ions, mechanical strength; cut-through resistance; weathering resistance and/or electrical insulation properties as set out before.

Backsheets are typically composed from several different film layers, typically comprising at least three layers, and are prepared from different polymeric materials. Often backsheets comprise fluoropolymer layers, which 30 so far required the use of organic solvent-based adhesives at the fluoropolymer film/polyethylene terephthalate film interface to bind the fluoropolymer to the structurally stronger backsheet layer. Not only is this adhesive a cumbersome material due to the use of solvents, but it also 3 increases process complexity . Furthermore, the adhesive layer typically is the weakest part of the laminate with respect to outdoor wheatherability.

US-A- 2012/0028060 for instance discloses a multilayer backsheet comprising inside layers made of partly aromatic polyesters in combination 5 with at least one outside adhesion promoter layer made of a block co-polyesteramide.

WO-A- 2010/101582 discloses a laminated multilayer backsheet film comprising of partly aromatic polyester layer and several different functional layers, for use with an EVA encapsulant sheet as the combined backsheet for 10 solar cells.

WO-A-2011/044417 discloses a coextruded film prepared by extrusion lamination at a temperature of 270°C or higher from a fluoropolymer film and a stretched polyester film, which are bonded together by an ethylene copolymer hot melt material.

15 The current layup process for photovoltaic modules comprises layering the front sheet first face down, with the glass pane acting as a rigid base for the other layers during lamination, and adding the encapsulant film, followed by the photovoltaic components, followed again by the second encapsulant film, and finally, a backsheet film as disclosed in the above publications.

20 The completed module line up is placed under reduced pressure, and then heated, whereby the photovoltaic cells and connectors and ribbons, where applicable, and perforated cells, such as back-contact cells,,are firmly embedded in the two encapsulant films which melt, and crosslink where a crosslinking system is provided.

25 This process thus typically involves the handling of at least three different films, i.e. front and back encapsulant film, followed by the backsheet film.

An alternative module line up includes a first rigid layer being typically a first glass layer, a first thin gauge encapsulant layer a back contact cSi cell, a 30 second perforated encapsulant layer, a conductive patterned layer between the second encapsulant layer and the backsheet an adhesive layer connecting the conductive patterned layer to the rigid backsheet layer. The back contact cells are now connected by conductive adhesive dots that are 4 connecting the BCC thought the holes of the encapsulant layer to the conductive patterned layer.

While many different multi-layer films have been proposed in the literature, the difference in properties between the layers usually requires the 5 use of additional tie coats or adhesive layers, making the films as such overly complex, and hence costly. Furthermore, the complexity of such films also typically results in handling issues, which require additional steps and protection materials during processing and preparation of photovoltaic modules.

10 The term ’’encapsulant” herein refers to a material comprising polymeric components that melts and thereby encapsulates the functional components of a photovoltaic cell during the lamination process.

Suitable EVA materials are typically composed of from one or more EVA copolymers, and a crosslinking system. Preferably the EVA comprises 15 more than 18% of vinyl acetate.

In view of the foregoing, it should be apparent that there exists a need for a low-cost and highly stable backsheet including an encapsulant layer, which can be prepared, and applied using a relatively simple and inexpensive manufacturing process, such that the photovoltaic modules have adequate 20 stability, durability and performance.

Applicants have now surprisingly found that the complexity of the process can be improved significantly by reducing the number of films in the lay-up procedure, by combining the second encapsulant and the backsheet into a single, coextruded film. Yet further, the specific line-up permits to 25 reduce the amount of waste materials as well as deletes an additional lay-up step, while also removing the need for solvent based adhesives.

Yet further, applicants found that by combining the backsheet with the encapsulant layers, any crimp or shrinking of the encapsulant sheet during the lamination is strongly reduced due to the dimensional support by the 30 backsheet.

Yet further, by a specific choice for a transparent top encapsulant layer, the issue with mix-up between front and back-encapsulant during the lamination process is mooted.

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Accordingly, the present invention relates to a multilayer backsheet film for the production of photovoltaic cells, comprising: a first transparent upward facing outer encapsulant layer (a); a second encapsulant layer adjacent to the first layer (b); and optionally, a further downward facing encapsulant layer (c) 5 and(d) a polymer film. The term “upward facing” as used herein refers to the side of the film that will be layered into a photovoltaic module facing the light incident side.

The term “downward facing” as used herein accordingly refers to the side of the film that will be layered into a photovoltaic module facing the side 10 that is facing the backside of the photovoltaic module. However, the film or sheet according to the invention does not require to prepared in a manner whereby an upward facing side is facing upward.

In a further aspect, the present invention also relates to the process for the preparation of a coextruded multilayer backsheet film for photovoltaic 15 cells, comprising: (i) providing a first encapsulant material (a), and (ii) providing a second encapsulant material (b); and optionally, (iii) providing a third encapsulant material (c), and (iv) providing a polymer film (d) optionally comprising an adhesion promoting layer, and (v) coextruding the materials (a), (b) and , where applicable (c) onto the film (d) such that layer (a) is adhered 20 to layer (b), and layer (b) is adhered to one side of the film (d) or to one side of layer (c) and layer (c) to film (d), where applicable, wherein material (a) is transparent.

In yet a further aspect, the present invention relates to a process for the preparation of a photovoltaic module comprising adhering the film according 25 to the invention to the backside of a photovoltaic cell layer.

In yet a further aspect the present invention relates to a photovoltaic module comprising a film according to the invention.

Short Description of the Figures

The following figures serve to illustrate the invention: 30 Fig. 1 discloses a first preferred embodiment of the subject invention. Herein disclosed is a multilayer film (1) comprising a first, transparent encapsulant layer (11) adhering to a second, opaque encapsulant layer (12), the later adhering to a film (13).

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Fig. 2 discloses a first preferred embodiment of the subject invention. Herein disclosed is a multilayer film (1) comprising a first, transparent encapsulant layer (11) adhering to a second, opaque encapsulant layer (12), the later adhering to an adhesion promoting layer (13), the latter again adhering to film 5 (14).

Detailed Description of the Invention

There are no specific restrictions on the thickness of individual layers used in the laminated film described herein. Thickness varies according to specific application.

10 The layer(s) (a) preferably are transparent. This allows to encapsulate solar cells without the any loss of active surface due to creep of opaque material onto the front of the solar cell.

Each layer (a), (b) and/or (c) described herein may be may a single layer, or multiple layers of the same or similar material.

15 Layer (b) and/or (c) preferably are opaque, more preferably a diffuse reflective layer.

The opaque layers (b) and/or (c) preferably comprises pigments comprising coated inert titanium oxide and/or mica. Layer (b) preferably permits to reflect at least 90% of light in the wavelength of from 400 to 880 nm, as determined 20 by ASTM standard “E 903—Test Method for Solar Absorptance, Reflectance, and Transmittance of Materials Using Integrating Spheres.

The present invention also preferably relates to a film according to any one of the previous claims, wherein the content ratio of a constituent unit derived from vinyl acetate in the ethylene vinyl acetate co-polymer employed 25 in layer (a) is preferably 25% by mass or more.

The overall film thickness preferably is in the range of from 150 to 1000 pm. The encapsulant layers (a) to (c) preferably have a combined thickness of from 150 to 600 pm.

The one or more outer polymer layers (a) preferably have a thickness 30 of from 50 to 250 pm. The thickness of the layer will be advantageously be chosen sufficiently high to embed photovoltaic cell and any ribbons, where present during the lamination process.

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For classical photovoltaic cells with ribbon connectors, the one or more outer layer preferably has a thickness of from 160 to 250 pm, more preferably of from 170 to 210 pm.

The one or more inner layer (b) preferably has a thickness of from 10 to 5 250 ?m. The thickness of the layer (b) depends on the polymer material employed, as well as on the function and type of photovoltaic cell.

The one or more outer layer (c) preferably have a thickness of from 50 to 250 pm. The thickness of the layer (c) is typically similar or equal to the thickness of layer (a). The thickness of the layers will be advantageously be 10 chosen sufficiently high to embed photovoltaic cell and any ribbons, where present during the lamination process.The one or more outer layer (c) preferably have a thickness of from 75 to 250 pm. The thickness of the layer (c) is typically similar or equal to the thickness of layer (a).. The thickness of the layers will be advantageously be chosen sufficiently high to embed 15 photovoltaic cell and any ribbons, where present during the lamination process.

In a preferred embodiment, layer (a) and layer (c) comprise ethylene vinyl acetate copolymer, whereas layer (b) comprises a different polymer, either an ethylene vinyl acetate copolymer with a different composition, e.g.

20 with a lower vinyl acetate monomer content, or one or more polymers of a different composition, such as preferably a polyolefin, or a block copolymer.

In this embodiment, which is particularly useful for back contact cells, the thickness of layer (a) and (c) is in the range of from may preferably range of from 35 to 250 pm, more preferably in the range of from 40 to 180 pm.

25 In a particularly preferred embodiment, layer (a) is transparent, and comprises comprise ethylene vinyl acetate copolymer, with a thickness of from 35 to 250 pm; layer (b) is transparent or opaque, and does not comprise an ethylene vinyl acetate copolymer, and has a thickness of from 15 tot 90 pm; and layer (c) is a transparent, or preferably opaque layer comprising an 30 ethylene vinyl acetate copolymer, and has a thickness in the range of from 35 to 250 pm.

In a preferred embodiment, outside layers (a) and (c) comprise EVA copolymers, and intermediate layer (b) comprises a recycled EVA copolymer material. Preferably all three layers may be transparent.

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The term “recycled” material relates to material that has been employed in a film formation and subsequently has been trimmed off, , or film ends or other unused parts of the film.

Such films can typically not be used again for the same purpose, but 5 are usually considered as “waste” materials since the active silane composition at the surface is usually not sufficiently high to ensure sufficient bonding with the surface of the front or backsheet, and/or the components to be encapsulated.

In a further preferred embodiment, at least one of layers (b) and (c) is a 10 white pigmented layer acting as a diffuse reflector. In a yet a further preferred embodiment, layer (b) may be a polyolefin layer, either white pigmented, or transparent.

In yet a further preferred embodiment, layer (c) comprises recycled EVA material, and is a diffuse reflector. Unless stated otherwise, all 15 percentages, parts, ratios, etc., are by weight. When an amount, concentration, or other value or parameter is given as either a range, preferred range or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range 20 limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range.

25 When the term "about" is used in describing a value or an end-point of a range, the disclosure should be understood to include the specific value or end- point referred to.

As used herein, the terms "comprises," "comprising," "includes," "including," "containing," "characterized by," "has," "having" or any other 30 variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

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Further, unless expressly stated to the contrary, "or" refers to an inclusive or and not to an exclusive or.

The transitional phrase "consisting essentially of limits the scope of a claim to the specified materials or steps and those that do not materially affect 5 the basic and novel characteristic(s) of the claimed invention.

Where applicants have defined an invention or a portion thereof with an open-ended term such as "comprising," it should be readily understood that unless otherwise stated the description should be interpreted to also describe such an invention using the term "consisting essentially of”.

10 Use of "a" or "an" are employed to describe elements and components of the invention. This is merely for convenience and to give a general sense of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise.

15 In describing certain polymers it should be understood that sometimes applicants are referring to the polymers by the monomers used to produce them or the amounts of the monomers used to produce the polymers. While such a description may not include the specific nomenclature used to describe the final polymer or may not contain product-by-process terminology, any 20 such reference to monomers and amounts should be interpreted to mean that the polymer comprises those monomers (i.e. copolymerized units of those monomers) or that amount of the monomers, and the corresponding polymers and compositions thereof.

In describing and/or claiming this invention, the term "copolymer" is 25 used to refer to polymers formed by copolymerization of two or more monomers. Such copolymers include dipolymers, terpolymers or higher order copolymers.

The layers (a), (b) and (c) advantageously may comprise one or more ethylene/vinyl acetate copolymers (EVA copolymer, or EVA herein),

30 comprising copolymerized units of ethylene and vinyl acetate. The EVA

copolymer may have a melt flow rate (MFR) in the range of from 0.1 to 1000 g/10 minutes, preferably of from 0.3 to 300 g/10 minutes, yet more preferably of from 0.5 to 50 g/10 minutes, as determined in accordance with ASTM D1238 at 190°C and 2.16 kg.

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The EVA copolymer may be a single EVA copolymer or a mixture of two or more different EVA copolymers. By different EVA copolymer is meant that the copolymers having different comonomer ratios, and/or the weight average molecular weight and/or molecular weight distribution may differ.

5 Accordingly the EVA copolymer may also comprise copolymers that have the same comonomer ratios, but different MFR due to having different molecular weight distribution. Preferably, where layer(a) and layer (b) comprise EVA copolymers, these EVA copolymers have different composition to avoid mixing of the layers during the lamination process.

10 In a preferred embodiment, the EVA copolymers advantageously comprise further monomers other than ethylene and vinyl acetate, such as alkyl acrylates, whereby the alkyl moiety of the alkyl acrylate may contain 1 -6 or 1 -A carbon atoms, and may be selected from methyl groups, ethyl groups, and branched or unbranched propyl, butyl, pentyl, and hexyl groups.

15 Exemplary alkyl acrylates include, but are not limited to, methyl acrylate, ethyl acrylate, /-butyl acrylate, and n-butyl acrylate. The polarity of the alkyl acrylate comonomer may be manipulated by changing the relative amount and identity of the alkyl group present in the comonomer. Similarly, a Ci-Cö alkyl methacrylate comonomer may be used as a comonomer. Such 20 comonomers include methyl methacrylate, ethyl methacrylate, i-butyl methacrylate, and n-butyl methacrylate.

The EVA compositions used in the materials according to the invention may further comprise one or more other optional polymers, such as, for example, polyolefins including ethylene homopolymers, propylene 25 homopolymers, additional ethylene copolymers, and propylene copolymers; ethylene (meth)acrylic copolymers. The optional polymers may be present in an amount of up to about 25 wt%, based on the total weight of the EVA copolymer, provided that the inclusion of such optional polymers does not adversely affect the desirable performance characteristics of the EVA 30 copolymer, such as the transparency, melt flow index, pigment dispersion and/or adhesion properties.

The EVA copolymers used herein may also contain other additives known within the art. The additives may include processing aids, flow enhancing additives, lubricants, dyes, flame retardants, impact modifiers, 11 nucleating agents, anti-blocking agents such as silica, thermal stabilizers, UV absorbers, UV stabilizers, dispersants, surfactants, chelating agents, coupling agents, reinforcement additives, such as glass fiber, fillers and the like.

Generally, additives that may reduce the optical clarity of the EVA 5 copolymer, such as reinforcement additives and fillers, are preferably present in layer(s) (b) and below,,as set out below.

Layer (b) may alternatively comprise a poly methyl metacrylate n-butylacrylate block copolymer, as disclosed in WO2012057079, and commercially available as “Kurarity” from Kuraray Corp.

10 A further preferred embodiment comprises a polyolefin in layer (b), preferably a polyethylene or polypropylene, such as an LDPE type. The benefit of this layer is the high barrier properties, as well as the fact that the crimp due to annealing of the EVA layer(s) is further reduced.

Polyolefins, such as polyethylene and polypropylene suitable for the 15 layer (b) include high density polyethylene, medium density polyethylene, low density polyethylene, linear low density polyethylene, metallocene-derived low density polyethylene, and polypropylene copolymer. Low density polyethylene, and polypropylene copolymers having a suitably high MFI and a melt temperature at in the range of from 135 to 155°C.

20 Layer (b) may also comprise polymers selected from poly(meth)acrylates, polyepoxides, polyurethanes, functionalized polyolefins, e.g. on the basis of EPM or EPDM rubbers, silicones and/or ionomers, nd/or combinations thereof.

Suitable polyolefin copolymer materials include ethylene-Ci to C4 alkyl 25 (meth)acrylate copolymers, for example, ethylene-methyl methacrylate copolymers, ethylene-methyl acrylate copolymers, ethylene-ethyl methacrylate copolymers, ethylene-ethyl acrylate copolymers, ethylene-propyl methacrylate copolymers, ethylene-propyl acrylate copolymers, ethylene-butyl methacrylate copolymers, ethylene-butyl acrylate copolymers, and mixtures of 30 two or more copolymers thereof, wherein copolymer units resulting from ethylene account for 50% to 99%; preferably 70% to 95%, by total weight of each copolymer; ethylene-methacrylic acid copolymers, ethylene-acrylic acid copolymers and blends thereof, wherein copolymer units resulting from ethylene account for 50 to 99%, preferably 70 to 95%, by total weight of each 12 copolymer; ethylene-maleic anhydride copolymers, wherein copolymer units resulted from ethylene account for 50 to 99%, preferably 70 to 95%, by total weight of the copolymer; polybasic polymers formed by ethylene with at least two co-monomers selected from Ci to C4 alkyl methacrylate, C1 to C4 alkyl 5 acrylate, ethylene-methacrylic acid, ethylene-acrylic acid and ethylene-maleic anhydride, non-restrictive examples of which include, for example, terpolymers of ethylene-methyl aerylatemethacrylie acid, wherein copolymer units resulting from methyl acrylate account for 2 to 30 % by weight and copolymer units resulting from methacrylic acid account for 1 to 30 % by 10 weight, terpolymers of ethylene-butyl acrylatemethacrylic acid, wherein copolymer units resulting from butyl acrylate account for 2 to 30 % by weight and copolymer units resulting from methacrylic acid account for 1 to 30 % by weight, terpolymers of ethylene-propyl methacrylateacrylic acid, wherein copolymer units resulting from propyl methacrylate account 15 for 2 to 30 % 15 by weight and copolymer units resulting from acrylic acid account for 1 to 30 % by weight, terpolymers of ethylene-methyl acrylate-acrylic acid, wherein copolymer units resulting from methyl acrylate account for 2 to 30 % by weight and copolymer units resulted from acrylic acid account for 1 to 30 % by weight, terpolymers of ethylene-methyl acrylate-maleic anhydride, wherein copolymer 20 units resulting from methyl acrylate account for 2 to 30 % by weight and copolymer units resulting from maleic anhydride account for 0.2 to 10 % by weight, terpolymers of ethylene-butyl acrylate-maleic anhydride, wherein copolymer units resulting from butyl acrylate account for 2 to 30 % by weight and copolymer units resulted from maleic anhydride account for 0.2 to10 % by 25 weight, and terpolymers of ethylene-acrylic acid-maleic anhydride, wherein copolymer units resulting from acrylic acid account for 2 to 30 % by weight and copolymer units resulting from maleic anhydride account for 0.2 to 10 % by weight; copolymers formed by ethylene and glycidyl methacrylate with at least one co-monomer selected from C1 to C4 alkyl methacrylate, C1 to C4 30 alkyl acrylate, ethylene-methacrylic acid, ethylene-acrylic acid, and ethylene-maleic anhydride, non-restrictive examples of which include, for example, terpolymers of ethylenebutyl acrylate-glycidyl methacrylate, wherein copolymer units resulting from butyl acrylate account for 2 to 30 % by weight 13 and copolymer units resulting from glycidyl methacrylate account for 1 to 15 % by weight; and blends of two or more above-described materials.

Preferably, the material employed in layer (b) has a higher MFI at the same temperature than the material employed in layer (a) ad/or (c).

5 Thermal stabilizers can be used and have been widely disclosed within the art. Any known thermal stabilizer may find utility within the compositions useful in the invention. Preferable general classes of thermal stabilizers include, but are not limited to, phenolic antioxidants, alkylated monophenols, alkylthiomethylphenols, hydroquinones, alkylated hydroquinones, tocopherols, 10 hydroxylated thiodiphenyl ethers, alkylidenebisphenols, O-, N- and S-benzyl compounds, hydroxybenzylated malonates, aromatic hydroxybenzyl compounds, triazine compounds, aminic antioxidants, aryl amines, diaryl amines, polyaryl amines, acylaminophenols, oxamides, metal deactivators, phosphites, phosphonites, benzylphosphonates, ascorbic acid (vitamin C), 15 compounds that destroy peroxide, hydroxylamines, nitrones, thiosynergists, benzofuranones, indolinones, and the like and mixtures thereof. The EVA copolymer may contain any effective amount of thermal stabilizers. Use of a thermal stabilizer is optional and in some instances is not preferred. When thermal stabilizers are used, the EVA copolymer contains at least about 0.05 20 wt%, and up to about 10 wt%, more preferably up to about 5 wt%, and most preferably up to about 1 wt%, of thermal stabilizers, based on the total weight of the EVA copolymer.

UV absorbers may preferably be used and have also been widely disclosed within the art. Any known UV absorber may find utility within the 25 present invention, provided it is compatible with the film system and does not adversely affect properties or processability. Preferable general classes of UV absorbers include, but are not limited to, benzotriazoles, hydroxybenzo-phenones, hydroxyphenyl triazines, esters of substituted and unsubstituted benzoic acids, and the like and mixtures thereof. The EVA copolymer may 30 contain any effective amount of UV absorbers. Use of a UV absorber is optional and in some instances is not preferred. When UV absorbers are utilized, the EVA copolymer contains at least about 0.05 wt%, and up to about 10 wt%, more preferably up to about 5 wt%, and most preferably up to about 1 wt%, of UV absorbers, based on the total weight of the EVA copolymer.

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Particularly preferred are hindered amine light stabilizers (HALS), wich are widely disclosed within the art. Generally, hindered amine light stabilizers are disclosed to be secondary, tertiary, acetylated, N-hydrocarbyloxy substituted, hydroxy substituted, or other substituted cyclic amines which are 5 characterized by a substantial amount of steric hindrance, generally derived from aliphatic substitution on the carbon atoms adjacent to the amine function. The EVA copolymer may preferably contain any effective amount of hindered amine light stabilizers. Use of hindered amine light stabilizers is optional and in some instances is not preferred.

10 When hindered amine light stabilizers are used, the EVA copolymer contains at least about 0.05 wt%, and up to about 10 wt%, more preferably up to about 5 wt%, and most preferably, up to about 1 wt%, of hindered amine light stabilizers, based on the total weight of the EVA copolymer.

The film, according to the present invention further preferably 15 comprises one or more organic peroxides, which enables to crosslink the ethylene-vinyl acetate copolymer, thereby increasing the adhesion strength, humidity resistance and penetration resistance, while maintaining a high transparency, if so desired.

Any organic peroxides that are decomposed at a temperature of at 20 least 110°C to generate radicals may advantageously be employed as the above-mentioned organic peroxide.

The organic peroxide or combination of peroxides are generally selected in the consideration of film-forming temperature, conditions for preparing the composition, curing (bonding) temperature, heat resistance of 25 body to be bonded and storage stability.

According to a preferred embodiment of the subject invention, the peroxide is chosen such that it does essentially no decompose the resin processing temperature, in particular during coextrusion and/or a further extrusion and pelletizing step, while is only activated at the solar cell formation 30 temperature.

“Essentially not decomposing” according to the present invention refers to a half-life of at least 0.1 to 1 hours at the coextrusion temperature.

Examples of the organic peroxides include 2,5-dimethylhexane-2,5-dihydroperoxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, 3-di-tert- 15 butylperoxide, dicumyl peroxide, 2,5-dimethyl-2,5-di(2-ethylhaxanoylperoxy)hexane, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne, tert-butylcumylperoxide, [alpha],[alpha]'-bis(tert-butylperoxyisopropyl)benzene, [alpha],[alpha]'-bis(tert-butylperoxy)diisopropylbenzene, n-butyl-4,4-bis(tert-5 butylperoxy)butane, 2,2-bis(tert-butylperoxy)butane, 1,1 -bis(tert- butylperoxy)cyclohexane, 1,1 -bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butylperoxybenzoate, benzoyl peroxide, and 1,1-di (tert-hexylperoxy)- 3,3,5-trimethylcyclohexane. Of these, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, and 1,1-di (tert-hexylperoxy)-3,3,5-10 trimethylcyclohexane are particularly preferred.

The content of the organic peroxide in the film layers is preferably in the range of 0.1 to 5 parts by weight, more preferably in the range of 0.2 to 1.8 parts by weight based on 100 parts by weight of ethylene-vinyl acetate copolymer.

15 The film of the present invention may further contain crosslinking auxiliary agents, if so required. Cross-linking auxiliary agents herein are understood as compounds providing at least one, preferably several radically polymerizable functional groups. The crosslinking auxiliary agent increases the gel fraction of ethylene-vinyl acetate copolymer, thereby improving the 20 durability and mechanical properties of the encapsulant. Crosslinking auxiliary agents are typically employed in an amount of 10 parts by weight or less, preferably in the range of 0.1 to 5.0 parts by weight, based on 100 parts by weight of ethylene-vinyl acetate copolymer. Examples of the cross-linking auxiliary agents comprise tri-functional cross-linking auxiliary agents such as 25 triallyl cyanurate and triallyl isocyanurate, and mono- or di-functional crosslinking auxiliary agents of (meth)acryl esters. Among these compounds, triallyl cyanurate and triallyl isocyanurate are particularly preferred.

Silane coupling agents may be added to the EVA copolymer to improve its adhesive strength. Useful illustrative silane coupling agents include 30 [gamma]-chloropropylmethoxysilane, vinylmethoxysilane, vinyltriethoxysilane, vinyltris([beta]- methoxyethoxy)silane,[gamma]-vinylbenzylpropylmethoxy-silane, N-[beta]-(N- vinylbenzylaminoethyl)-[gamma]-aminopropyl-trimethoxysilane, [gamma]- methacryloxypropyltrimethoxysilane, [gamma]-methacryloxypropyltriethoxysilane, vinyltriacetoxysilane, [gamma]- 16 methacryloxypropyltrimethoxysilane, [gamma]- glycidoxypropyltrimethoxysilane, [gamma]-glycidoxypropyltriethoxysilane, [beta]-(3,4- epoxycyclohexyl)ethylthmethoxysilane, vinylthchlorosilane, [gamma]mercaptopropylmethoxysilane, [gamma]-aminopropyltriethoxysilane, 5 N-[beta]-(aminoethyl)-[gamma]- aminopropyltrinethoxysilane, and/or mixtures of two or more thereof.

The silane coupling agents are preferably incorporated in the encapsulant layer at a level of about 0.01 to about 5 wt%, or more preferably about 0.05 to about 1 wt%, based on the total weight of the EVA copolymer.

10 A preferred embodiment of the present invention resides in a film wherein layer a) is transparent, while layer (b) is an opaque, preferably white, reflective layer acting as a diffuse reflector.

This allows to overcome a particular problem with monolayer diffuse reflector loaded polymers as back encapsulants, which tend to overflow onto 15 the cell upside during the lamination process. This is particularly the case where polymer encapsulants with similar melt flow and melt temperatures are employed as transparent front encapsulant, and a white reflective back encapsulant. The thus formed cells, tend to have a lower effective surface.

Preferably, the film according to the invention claim, wherein layer (b) 20 and/or (c) comprise diffuse reflective pigments, has a reflection efficiency of at least 90% for light with a wavelength in the range of from 400 to 800 nm, more preferably at least 92%, and yet more preferably at least 95%, as determined according to ASTM standard ‘E 903—Test Method for Solar Absorptance, Reflectance, and Transmittance of Materials Using Integrating Spheres.

25 Film (d) preferably comprises a substrate material selected from polyesters or fluorine-containing polymers. This may be a single layer, or preferably multiple layers of polyester and a single layer or multiple layers of fluorine-containing polymer, for example, a laminated film of two or multiple layers of polyester and a layer of a fluorine containing polymer.

30 Accordingly, the film substrate may advantageously be selected from (i) partly aromatic polyesters, (ii) fluorine-containing polymers; (iii) polyesters or fluorine-containing polymers with a coat of metal or metal oxide/non-metal oxide on the surface; or (iv) a laminated film made from two or more materials found above. The polyester preferably is a partly aromatic polyester. This 17 polyester preferably comprises polymers selected from the group consisting of polymeric C2 to Cö alkylene phthalates, polymeric C2 to Cq alkylene naphthalates, and mixtures or blends thereof, such as polyethylene terephthalate (PET), polytrimethylene terephthalate, polybutylene 5 terephthalate, polyhexylene terephthalate, polyethylene o-phthalate, polytrimethylene o-phthalate, polybutylene o-phthalate, and polyhexylene o-phthalate, preferably polyethylene terephthalate; polymeric C2 to Cö alkylene naphthalates, preferably polymeric C2 to C4 alkylene naphthalates, such as polyethylene naphthalate, polytrimethylene naphthalate, and polybutylene 10 naphthalate; and copolymers and blends of two or more above materials.

Suitable polyester substrates may be formed by film-casting and then treating by biaxial orientation to further improve mechanical strength and gas barrier properties. Such films are known for their good mechanical, dielectric, and gas barrier properties, but also thermal stability.

15 .The fluorine-containing polymers may be any suitable fluorine- containing polymer known in the art, including polymers of fluoroethylene; vinylidene fluoride; chlorotrifluoroethylene; tetrafluoroethylene; and copolymers of any of the above with other non-fluorinated, partially or fully fluorinated monomers, such as ethylene, propylene, fluoroethylene, ethylene 20 fluoride, vinylidene fluoride, chlorotrifluoroethylene, hexafluoropropylene, tetrafluoroethylene, perfluoroalkoxyvinyl ether, and perfluoropropylene.

The fluorine-containing polymer substrate may be a single opr a multiple layer, e.g. a laminated film of double-layer or multi-layer fluorine-containing polymer. The total thickness of the fluorine-containing polymer 25 substrate layer is preferably in the range of from 10 to 350 pm, more preferably in the range of from 15 to 300 pm, and most preferably in the range of from 20 to 250 pm.

Additional layers may be present, such as metal or metal oxide layers, which may be laminated or deposited by a suitable process, such as chemical 30 or physical vapour deposition.

An adhesion promoting layer may optionally be arranged for better adhesion of layers (b) or (c), and film (d). The presence of this layer is required where the layer (b) or (c) and film (d) do not enter into any spontaneous adhesive connection.

18

The thickness of the adhesion layer preferably is in the range of from 5 to 400 pm, preferably 40 to 200 pm.

In order to increase the bonding strength between the substrate and the bonding layer, the film surface may be surface-treated. Suitable surface 5 treatment methods include corona or plasma treatment, flame treatment, or primer treatment.

Suitable primers include imine and amine primers, which may be applied as required, and at a thickness as required, provided that the primer does not adversely affect the bonding strength between the film substrate and 10 the adhesion promoting layer. Typical thicknesses for each primer layer are in the range of from than 0.1 to 1.5 pm.

When surface treatment methods such as corona treatment or flame treatment are used, the adhesion promoting layer, or layer (b) may be directly placed on the film surface, i.e. without an additional primer coat or layer.

15 The back sheet comprising the EVA copolymers may have a smooth or rough surface on one or both sides. Preferably, the sheet has rough surfaces on both sides to facilitate de-aeration during the lamination process. Rough surfaces can advantageously be created by mechanically embossing or by melt fracture during extrusion of the sheets followed by quenching so that 20 surface roughness is retained during handling. The surface pattern can be applied to the sheet through well-known, common art processes. For example, the extruded sheet may be passed over a specially prepared surface of a die roll positioned in close proximity to the exit of the extruder die. This imparts the desired surface characteristics to one side of the molten polymer exiting 25 the die. When the surface of such a die roll has minute peaks and valleys, it will impart a rough surface to the side of the polymer sheet that passes over the roll, and the rough surface will generally conform respectively to the valleys and peaks of the roll surface.

The multilayer sheets according to the invention may be produced by 30 any suitable process, for example, through dipcoating, solution casting, compression molding, injection molding, lamination, melt extrusion casting, blown film processes, extrusion coating, tandem extrusion coating, or by any other procedures that are known to those of skill in the art. Preferably, though 19 the sheet is formed by melt coextrusion casting, melt extrusion coating, blown film or sheet processes, or tandem melt extrusion coating processes.

Layer (c) is most preferably placed between at least one surface of layer (b) and an inner surface of film (d), and may advantageously be coated 5 onto film (d) prior to coextrusion of (a) and (b) onto the film (c).

Alternatively, in particular where a polyolefin layer is applied, the encapsulant layers and the adhesion promoting layer, which may then also serve as barrier layer, may be coextruded onto the film (d).

Two or more extruders may be used for bonding the layers (a), (b) and 10 (c) onto each other, and onto film (d) by extrusion coating. Preferably, polymers of similar or identical melt flow index (MFI) should be employed to avoid distortion and annealing effects upon heating during the solar cell encapsulation process. Where required, different extrusion temperatures may be used for the different layers, as long as the MFI at that temperature is 15 within a range of from 2 above or below the MFI of a different layer.

The following, non-limiting examples are provided to illustrate the invention.

Example 1 A first and a third EVA layer (a) and (c), fully formulated and suitable as 20 encapsulant of type 33% VA with an MFI of ~ 45 g/10’ at 190°C at 2.16 kg, were coextruded at a low temperature of about 100°C onto a hydrolytically stabilised PET backsheet with a primer layer.

At this temperature the MFI of the individual EVA resin was recorded at ~ 2.7 g/10’.

25 A polyolefin material (b) (Low Density of Exxon type LD650) with an MFI of ~ 22 g/10’ at 190°C at 2.16 kg, was coextruded in the (b) layer in between (a) and (c) at a temperature of ~ 120°C. At this temperature the MFI of the individual resin was recorded at - 3g/10’

The layers (a) and (c) were approximately 180 pm thick. Layer (b) was 30 approximately 90 pm. The film when leaving the die was at a temperature of 105°C. Hence no premature cure of the EVA layers of the film occurred, nor melt fracture occurred and the film was particular thermally stable and not shrink sensitive at all during the lamination process. The EVA layer (c) was also pigmented with a diffuse reflector of type TiC>2 in one execution. This was 20 found to not influence the ease of processing the film where the melt temperature of both resins is substantially different (EVA melting at about 63°C, whereas LDPE melts at about 105°C). This co-extrusion was realized on a conventional feed-block and die hardware, without the use of a 5 multimanifold. The thus prepared backsheet was employed in a lay-up of a solar cell, whereby the complete multilayer backsheet was placed on the backside of the cell. The thus obtained cells showed no flow of a white pigmented layer on top of the front of the cells, and passed accelerated humidity and heat exposure tests.

10 The example above clearly shows the advantages of the process and materials of the present invention.

Although several specific embodiments of the present invention have been described in the detailed description above, this description is not intended to limit the invention to the particular form or embodiments disclosed 15 herein since they are to be recognised as illustrative rather than restrictive, and it will be obvious to those skilled in the art that the invention is not limited to the examples.

Claims (16)

A multi-layer back-contact film for the production of photovoltaic cells, comprising: a first transparent and upwardly directed, outer, enveloping layer (a), a second enveloping layer located adjacent to the first layer (b); any additional, downward-facing, enveloping layer (c); and (d) a polymer film. The back-contact contact film of claim 1, wherein the polymer film comprises a bi-oriented and partially aromatic polyester, as well as an adhesion promoting layer that improves the adhesion of the polyester film to the envelope layers (b) or (c), as applicable. The film of claim 1 or claim 2, further comprising a protective layer (e) provided with a fluoropolymer, wherein (e) is directly adhered to layer (d) along the distal side of layer (c) ). 4. A film according to any one of claims 1-3, wherein at least one of the layers (b) or (c) is opaque. The film of claim 4, wherein layer (b) and / or (c) comprises or comprises diffusely reflecting pigments, and exhibits or exhibits an efficiency of at least 75% with respect to the reflection of light with a wavelength located in the range of 400-800 nm, as determined in accordance with the ASTM standard E 903. The film of claim 5, wherein the pigments preferably comprise coated inert titanium oxide and / or mica. The film of any one of the preceding claims, wherein one or more layers (a), (b), and / or (c) comprise or comprise an ethylene vinyl acetate copolymer. 8. A film according to any one of the preceding claims, wherein the content ratio of a component unit derived from vinyl acetate in the ethylene vinyl acetate copolymer used in the layers (a), (b), and / or (c), 18% or more on a weight basis. 9. The film of any one of the preceding claims, wherein layer (b) comprises an ethylene vinyl acetate copolymer that is different from the ethylene vinyl acetate copolymer used in (a); a block copolymer such as a hydrogenated polystyrene block copolymer with butadiene, isoprene and / or butylene / ethylene copolymers (SIS, SBS, and / or SEBS); a polymethacrylate polyacrylate block copolymer, a polyolefin, or an olefin copolymer with copolymerizable functionalized monomers such as methacrylic acid (ionomer); and optionally functionalized polyolefin, a polyurethane, and / or a silicone polymer. 10. Method for the production of a co-extruded multi-layer back-contact film for photovoltaic cells, comprising: (i) providing a first wrapping material (a), and (ii) providing a second wrapping material (b), and optionally (iii) providing a third wrapping material (c), and (iv) providing a polymer film (d) optionally comprising an adhesion promoting layer, and (v) co-extruding the materials (a) , (b) on the film (d), so that layer (a) is adhered to layer (b), layer (b) is adhered to one side of the film (d) or one side of layer (c), and layer (c) on the film (d), as applicable. The method of claim 10, wherein the temperature difference between the materials exiting the extruders is in the range of 5 to 60 ° C, and preferably is in the range of 10 to 50 ° C. Method according to claim 10 or claim 11, wherein the difference in the melt index of materials (a), (b) is in the range of 0 to 5, and this at the extrusion temperature. A method for the production of a photovoltaic module, including the adhesion of the film 10 according to any of claims 1-9 on the reverse side of a layer of photovoltaic cells. A photovoltaic module, comprising a laminated foil as claimed in any one of claims 1-9. 15 The photovoltaic module according to claim 14, further comprising a front sheet and a front envelope comprising an envelope layer with an ethylene vinyl acetate copolymer, as well as a photovoltaic cell embedded in the front envelope layer and layer (a). 20 The photovoltaic module according to claim 15, wherein the front sheet comprises one or more glass plates with a thickness that is between 1.6 and 4 mm.