FIELD OF THE INVENTION

One subject of the invention is a photovoltaic module encapsulant based on an ethylene/alkyl acrylate copolymer in which a crosslinking agent (peroxides or isocyanates or any other component having a crosslinking function) is present only in very small amounts. The present invention also relates to a photovoltaic module, or to the use of this encapsulant composition in such a module, comprising, besides the encapsulant layer, at least one adjacent layer forming a “frontsheet” or “backsheet”, more generally these three successive layers: “frontsheet”, encapsulant and “backsheet”.

Global warming, related to the greenhouse gases given off by fossil fuels, has led to the development of alternative energy solutions which do not emit such gases during the operation thereof, such as, for example, photovoltaic modules. A photovoltaic module comprises a “photovoltaic cell”, this cell being capable of converting light energy into electricity.

There are many types of photovoltaic panel structures.

A conventional photovoltaic cell has been represented inFIG. 1; this photovoltaic cell10comprises individual cells12, one individual cell containing a photovoltaic sensor14, generally based on silicon treated in order to obtain photoelectric properties, in contact with electron collectors16placed above (upper collectors) and below (lower collectors) the photovoltaic sensor. The upper collectors16of an individual cell are connected to the lower collectors16of another individual cell12via conducting bars18, generally consisting of an alloy of metals. All these individual cells12are connected to one another, in series and/or in parallel, in order to form the photovoltaic cell10. When the photovoltaic cell10is placed under a light source, it delivers a direct electric current which can be recovered at the terminals19of the cell10.

With reference toFIG. 2, the photovoltaic module20comprises the photovoltaic cell10ofFIG. 1encased in an “encapsulant”, the latter being composed of an upper part22and of a lower part23. An upper protective layer24(known under the term “frontsheet”, used hereinafter) and a layer which protects the back of the module (known under the term “backsheet”, also used hereinafter)26are positioned on either side of the encapsulated cell.

Impact and moisture protection of the photovoltaic cell10is provided by the upper protective layer24, generally made of glass.

The backsheet26, for example a multilayer film based on fluoropolymer and on polyethylene terephthalate, contributes to the moisture protection of the photovoltaic module20and to the electrical insulation of the individual cells12in order to prevent any contact with the external environment.

The encapsulant22must perfectly adopt the shape of the space existing between the photovoltaic cell10and the protective layers24and26in order to avoid the presence of air, which would limit the efficiency of the photovoltaic module. The encapsulant22must also prevent contact of the individual cells12with atmospheric oxygen and water, in order to limit the corrosion thereof. The upper portion of the encapsulant22is between the cell10and the upper protective layer24. The lower portion of the encapsulant22is between the cell10and the backsheet26.

In the presence of solar radiation, heating occurs inside the solar module and temperatures of 80° C. (or more) may be reached, which necessitates that the layers be perfectly bonded to one another throughout the life cycle of the module.

PRIOR ART

Currently, the majority of the photovoltaic encapsulation market corresponds to formulations based on an EVA to which a peroxide, a silane and various functional additives are added.

EVA exhibits many qualities and properties advantageous for this application. This is because it confers mainly very good properties of transparency, of mechanical strength and of resistance to aging and generally excellent thermomechanical and mechanical properties. Furthermore, this thermoplastic is relatively inexpensive, so that its use for this application has become virtually inescapable.

Nevertheless, the type of encapsulant based on EVA, with peroxide and silane, has one major disadvantage.

Specifically, when the environmental conditions have deteriorated, that is to say when the EVA encapsulant ages under hot and damp conditions (DHT (damp heat test): 85° C./85% RH (relative humidity)), this component is subject to hydrolysis which brings about the appearance of acetic acid, a source of yellowing of the encapsulant and of corrosion of the metal connections of the photovoltaic module.

A person skilled in the art might envisage the replacement of the EVA by an ethylene/alkyl acrylate copolymer but this solution, although admittedly avoiding the specific problems related to EVA when the latter is present in a difficult environment, does not make it possible to obtain a correct photovoltaic module. This is because, during the lamination, numerous bubbles are formed during the crosslinking due to the peroxide, present in significant amounts.

Furthermore, document WO 2006/095911 provides a solution by the use of a formulation based on an ethylene/alkyl acrylate copolymer, the melting point of which (T in ° C.), obtained according to the standard JIS K 7121, would correspond to the following formula: −3.0X+125>T>−3.0X+109, the component X representing the molar content of the polar comonomer (acrylate). Furthermore, this document provides for the combining of this copolymer with a silane in order to introduce the adhesive properties on the glass.
However, such a formulation would not make it possible to obtain an encapsulant which is effective over the long term. This is because the silane exhibits the disadvantage of not making possible a good level of adhesion to the glass when it is not chemically bonded to the polymer.

Thus, an encapsulation solution based on an alternative component to EVA but exhibiting properties which are just as advantageous, while eliminating the risk of giving off acetic acid, is currently being sought.

This solution should furthermore enable the use of silane, making possible in particular the attachment to the walls of the frontsheet, that is to say to a component made of glass-ceramic or synthetic glass (conventionally PMMA).

BRIEF DESCRIPTION OF THE INVENTION

It has been found, by the applicant company, after various experiments, that a composition based on an ethylene/alkyl acrylate copolymer and on silane could, with a very low content of crosslinking agent(s), exhibit highly satisfactory thermomechanical properties and physicochemical characteristics.

Thus, the present invention relates to a photovoltaic module encapsulant intended to encase a photovoltaic cell, comprising:an ethylene/alkyl acrylate copolymer, the melt flow index, MFI, of said copolymer being between 1 g/10 min and 40 g/10 min;a silane, representing between 0.1% and 0.5% of the weight of said composition;

characterized in that it additionally comprises a crosslinking agent representing between 0.1% and 0.5% of the weight of the composition and in that said copolymer represents at least 99% of the weight of said composition.

The applicant company has in fact discovered that there was a very specific advantage in carrying out an “incomplete crosslinking”. The crosslinking agent, preferably selected from the monoperoxycarbonate family and more specifically consisting of Luperox® TBEC or (OO-tert-butyl O-(2-ethylhexyl) monoperoxycarbonate), present in a very small amount, enables the grafting of the silane to the copolymer and sufficiently crosslinks the latter in order to render it creep resistant without having to achieve a significant gel level. This very small content of peroxide avoids any problem of appearance of bubbles in the lamination step.

This “incomplete crosslinking” has the advantage of being able to be carried out in the lamination step, as carried out currently, but also starting from the extrusion of the encapsulant film thus then enabling a faster lamination.

The composition according to the invention first exhibits the following advantages:the impossibility of the appearance of acetic acid, more generally of any acid, during its use, whatever the environmental conditions;the maintenance of excellent adhesive properties of the composition throughout the lifetime of the composition, in particular in its use as encapsulant of a photovoltaic module;improving the optical properties during aging in a difficult environment;the maintenance of excellent thermomechanical properties and of physicochemical characteristics, at least as satisfactory as the current solution based on EVA (crosslinking agent and silane).

Other characteristics and distinctive features of the primary mixture of the invention are presented below:advantageously, the crosslinking agent belongs to the monoperoxycarbonate family, preferably consists of OO-tert-butyl O-(2-ethylhexyl) monoperoxycarbonate;preferably, for the abovesaid copolymer, the weight content of ethylene is between 50% and 85%, preferably between 60% and 84%, and the weight content of alkyl acrylate is between 15% and 50%, preferably between 16% and 40%;advantageously, the silane consists of a vinyl silane or (meth)acrylic silanes;according to a nonlimiting specification of the invention, the silane consists of methacryloxypropylsilane;advantageously, the crosslinking agent represents less than 0.3% of the weight of the composition;preferably the above said copolymer has a melt flow index, MFI, between 2 g/10 min and 10 g/10 min;according to one possibility offered by the invention, the composition consists solely of the abovesaid copolymer, the abovesaid crosslinking agent and the abovesaid silane;according to another possibility offered by the invention, the composition additionally comprises additives intended to confer additional specific properties, in particular plasticizers, adhesion promoters, UV stabilizers and absorbers, antioxidants, flame retardants and/or fillers.

The invention also relates to the use of the encapsulant as described above in a photovoltaic module.

Finally, the invention relates to a photovoltaic module comprising a structure consisting of a combination of at least one encapsulant and a frontsheet or backsheet, characterized in that the encapsulant is as described above.

