Patent Publication Number: US-2012037213-A1

Title: Backsheet for a photovoltaic module

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application Ser. No. 61/372,464, filed Aug. 11, 2010, which is herein incorporated by reference. 
    
    
     BACKGROUND 
     1. Field The present disclosure relates to an encapsulation. More particularly, the present disclosure relates to a backsheet for a photovoltaic module. 
     2. Description of Related Art 
     Solar energy has gained much research attention for being a seemingly inexhaustible energy source. For such purpose, photovoltaic modules that convert solar energy directly into electrical energy are developed. 
     In general, the photovoltaic module mechanically supports the solar cells, and protects the solar cells against environmental degradation. The photovoltaic module generally comprises a rigid and transparent protective front panel such as glass, and a rear panel or sheet, which is typically called a backsheet. The front panel and backsheet encapsulate the solar cell(s) and provide protection from environmental damage. 
     A known backsheet comprising a weather-resistant layer, a moisture-barrier layer and an insulating layer is disclosed in the prior art. In this technology, a layer of polyethylene terephthalate (PET) is adopted as the insulating layer. However, when a polyethylene terephthalate layer is employed in the backsheet, a tie layer is further required to insure sufficient adhesion with the encapsulant which bonds the backsheet to the solar cell. In addition, polyethylene terephthalate undergoes an orientation process in order to produce a sheet useful for the fabrication of the photovoltaic module, and renders the backsheet expensive. In view of the above, there exists in this art a need of an improved backsheet, which would have a lower cost and provide an excellent electrical insulation. 
     SUMMARY 
     According to one aspect of the present disclosure, a backsheet for a photovoltaic member is provided. The backsheet includes a protective layer and a layer of polymeric foam. The polymeric foam has a bulk density of about 0.01 g/cm 3  to about 0.6 g/cm 3  and is derived from an olefin monomer. The protective layer and the polymeric foam layer are respectively situated on two opposite surfaces of the backsheet. 
     According to another aspect of the present disclosure, a photovoltaic module is provided. The photovoltaic module includes a backsheet described above and a photovoltaic member for converting light into electricity. The photovoltaic member is positioned at the side of the polymeric foam layer of the backsheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure can be more fully understood by reading the following detailed description of the embodiments, with reference made to the accompanying drawings as follows: 
         FIG. 1  is a cross-sectional view schematically illustrating a backsheet according to one embodiment of the present disclosure; 
         FIG. 2  is a cross-sectional view schematically illustrating a backsheet according to another embodiment of the present disclosure; and 
         FIG. 3  is a cross-sectional view schematically illustrating a photovoltaic module according to one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the present embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawings. 
       FIG. 1  is a cross-sectional view schematically illustrating a backsheet  100  according to one embodiment of the present disclosure. The backsheet  100  is operable to protect a photovoltaic member (not shown in  FIG. 1 ), which converts sunlight into electricity. As depicted in  FIG. 1 , the backsheet  100  comprises a layer of polymeric foam  110  and a protective layer  120 . The polymeric foam layer  110  and the protective layer  120  are respectively situated on the opposite surfaces of the backsheet  100 , and the polymeric foam layer  110  is for connecting to the photovoltaic member. 
     The polymeric foam  110  has a predetermined pore volume, and the bulk density of the polymeric foam  110  is about 0.01 g/cm 3  to about 0.6 g/cm 3 . The layer of the polymeric foam  110  is for connecting to the photovoltaic member, and requires a property of electrical insulation to prevent an electric current generated by the photovoltaic member from leakage via the backsheet  100 . The insulating property of the polymeric foam  110  depends on the pore volume therein. In particular, the insulating property of the polymeric foam  110  may be enhanced when the ratio of the pore volume to the bulk volume (hereinafter referring to “porosity”) increases. Furthermore, the higher the porosity, the lower the bulk density of the polymeric foam  110 . However, when the bulk density of the polymeric foam  110  is less than a certain value, for example 0.01 g/cm 3 , the mechanical strength of the polymeric foam  110  becomes too weak and is difficult to be applied in the backsheet  100 . In contrast, when the bulk density of the polymeric foam  110  is greater than a certain value, for example 0.6 g/cm 3 , the porosity of the polymeric foam  110  is too low, meaning more materials are used and the cost savings become negated. In one specific example, the bulk density of the polymeric foam ranges from about 0.2 g/cm 3  to about 0.46 g/cm 3 . 
     In one embodiment, the polymeric foam  110  may comprise a plurality of “closed cell”. The term “closed cell” refers to a structure existed in the polymeric foam, in which a void space is enclosed and surrounded by the structure of the closed cell. In this case, the polymeric foam  110  may provide a desired electrical insulation and mechanical strength. However, the present disclosure is not limited thereto, and a polymeric foam having a structure of open cells may be employed in the present disclosure as well. 
     In another embodiment, the polymeric foam  110  comprises a cross-linked structure for increasing the mechanical strength of the polymeric foam. In particular, the cross-linked structure in the polymeric foam  110  may be formed by illuminating an electron beam (E-beam) to the polymeric foam  110 . 
     The polymeric foam  110 , also requires a property of non-hydrolysis since the backsheet  100  is usually installed outdoors. The polymeric foam  110  derived from an olefin monomer may provide both desired properties of insulation and non-hydrolysis. Suitable materials for the polymeric foam  110  include, but are not limited to, polyethylene, copolymer of ethylene and propylene, copolymer of ethylene and 1-butene, copolymer of ethylene and 1-hexene, copolymer of ethylene and 1-octene, copolymer of ethylene and vinylacetate, copolymer of ethylene and methylacrylate, copolymer of ethylene and ethylacrylate, copolymer of ethylene and acrylic acid, and copolymer of ethylene and maleic anhydride. 
     The pore in the polymeric foam  110  may be formed by any method known in the art. In one embodiment, the pore in the polymeric foam  110  may be generated by a physical blowing agent, which does not chemically react with the polymeric material. Suitable physical blowing agents include, but are not limited to, water, nitrogen gas, carbon dioxide, and gaseous hydrocarbons such as pentane. In another embodiment, the pore in the polymeric foam  110  may be formed by a chemical blowing agent, which chemically react within the polymeric matrix and thus generating gas that causes the foaming process. Suitable chemical blowing agents may be sodium bicarbonate or azobisformamide, for example. 
     In some embodiments, the polymeric foam layer  110  may comprise a filler or additive. In one example, the filler or additive added in the polymeric foam layer  110  may be a desiccating agent for absorbing moisture that leaks into the photovoltaic module. The desiccating agent, for example, may be a zeolite having a pore size of at least 3 Angstroms. 
     The insulating property of the polymeric foam  110  also depends on the thickness of the polymeric foam layer  110 . In one embodiment, the thickness of the polymeric foam layer  110  is greater than 0.05 mm, specifically greater than 0.1 mm, more specifically greater than 0.2 mm. In some examples, the thickness of the polymeric foam layer  110  is about 0.2 mm to about 6.4 mm. 
     Typically, a conventional backsheet is adhered to a photovoltaic member by an additional ethylene-vinyl acetate copolymer (EVA) film. For the purpose of firm bonding, a tie layer for connecting to the EVA film is needed in the prior art because the insulating layer such as PET can not provide sufficient adhesion with the EVA film. In one embodiment of the present disclosure, the backsheet  100  has a layer of polymeric foam  110  disposed on an outmost surface thereof and may provide excellent adhesion with the EVA film. Therefore, the tie layer used in the prior art is no longer required. Moreover, the layer of polymeric foam  110  may simultaneously provide a function of insulation, and therefore a polyethylene terephthalate (PET) layer may also be eliminated. Furthermore, the layer of polymeric foam  110  disclosed herein eventually needs a less amount of material compared to a solid layer. Therefore, the backsheet disclosed herein is cost-effective. 
     The protective layer  120  is disposed on an outmost surface that is opposite to the layer of polymeric foam  110 , and provides a function of weather resistance. Usually, the protective layer  120  is directly exposed to the ambient environment. The protective layer may be made of a material such as metal, polymer, inorganic composition or a combination thereof. In some embodiments, protective layer  120  may be made from PE, PC, fluorinated polymer or Nylon. 
     An additional moisture-barrier layer is not required when the backsheet  100  disclosed herein is applied in a photovoltaic module which is not so sensitive to moisture, for example, polycrystalline silicon photovoltaic modules. In these applications, the backsheet  100  may lack for a moisture-barrier layer. In one example, the backsheet  100  may further comprise a first adhesive layer  130  disposed between the protective layer  120  and the polymeric foam layer  110 , as depicted in  FIG. 1 . The first adhesive layer may be made from a copolymer of ethylene and acrylic acid, or a copolymer of ethylene and maleic anhydride. In some examples, the thickness of the first adhesive layer  130  is about 0.01 mm to about 0.5 mm. 
     Optionally, the backsheet  100  may further comprise a moisture-barrier layer  140  disposed between the polymeric foam layer  110  and the protective layer  120 , as depicted in  FIG. 2 . When the backsheet  100  disclosed herein is applied in a photovoltaic module which is sensitive to moisture, such as an amorphous silicon photovoltaic module, the moisture-barrier layer  140  may provide a sufficient resistance to moisture. In these examples, the moisture-barrier layer  140  may comprise a layer of aluminum. 
     In one embodiment, backsheet  100  may further comprise a second adhesive layer  150  disposed between the moisture-barrier layer  140  and the protective layer  120 . The second adhesive layer  150  may comprise a copolymer of ethylene and acrylic acid or a copolymer of ethylene and maleic anhydride, for example. In addition, the second adhesive layer  150  may be about 0.01 mm to about 0.5 mm in thickness. 
     In another embodiment, backsheet  100  may further comprise a third adhesive layer  160  disposed between the moisture-barrier layer  140  and the polymeric foam layer  110 . The third adhesive layer  160  may be the same as or different from the second adhesive layer  150 . 
     In some embodiment, the moisture-barrier layer  140  may be in direct contact with the polymeric foam layer  110 , without the third adhesive layer  160  disposed therebetween. Similarly, the moisture-barrier layer  140  may directly contact the protective layer  120 , without the second adhesive layer  150  disposed therebetween. 
     According to another aspect of the present disclosure, a photovoltaic module is provides.  FIG. 3  is a cross-sectional view schematically illustrating a photovoltaic module  300  according to one embodiment of the present disclosure. Referring to  FIG. 3 , the photovoltaic module  300  comprises a backsheet  100  and a photovoltaic member  200 . The backsheet  100  may be any embodiment described hereinbefore. For instance, the backsheet  100  may comprise a layer of polymeric foam  110 , a protective layer  120 , a moisture-barrier layer  140 , and a third adhesive layer  160 . The photovoltaic member  200  capable of converting light into electricity is disposed at the side of the polymeric foam layer  110  of the backsheet  100 . The protective layer  120  is positioned at the outmost surface of the photovoltaic module  300 . 
     In one embodiment, the photovoltaic member  200  comprises a front transparent substrate  210 , a transparent conductive oxide layer  220 , a photoelectric conversion layer  230  and a back electrode  240 , as depicted in  FIG. 3 . 
     The front transparent substrate  210  is disposed on the outmost side of the photovoltaic member  200  in order to protect the photovoltaic module  300  from environmental degradation. In general, the front transparent substrate  210  may be made of glass, and light may be transmitted into the photovoltaic member  200  through the transparent substrate  210 . 
     The transparent conductive oxide layer  220  is disposed on the front transparent substrate  210 . In some examples, the transparent conductive oxide layer  220  may comprise zinc oxide (ZnO), fluorine doped tin dioxide (SnO 2 :F), or indium tin oxide (ITO). In other examples, the transparent conductive oxide layer  220  has a textured structure (not shown) on the interface between the transparent conductive oxide layer  220  and the photoelectric conversion layer  230  for trapping light that is transmitted into the photovoltaic member  200 . 
     The photoelectric conversion layer  230  for converting light into electricity is disposed between the transparent conductive oxide layer  220  and the back electrode  240 . It should be noted that in the present disclosure the term “photoelectric conversion layer” comprises all layers that is needed to absorb the light and convert it into electricity. Various thin film semiconductor materials may be employed in the photoelectric conversion layer  230 . Suitable materials include, but are not limited to, amorphous silicon (a-Si:H), polycrystalline silicon, signal crystalline silicon, amorphous silicon carbide (a-SiC), and amorphous silicon-germanium (a-SiGe). In the amorphous silicon embodiment, the photoelectric conversion layer  220  may comprise a p-doped amorphous silicon layer, an intrinsic amorphous silicon layer, and an n-doped amorphous silicon layer (also known as “p-i-n structure”). Further, a plurality of repetitive p-i-n layers (“pin-pin-pin” or “pin-pin-pin-pin”) may sequentially be formed as well. In other examples, the photoelectric conversion layer  230  may comprise GaAs, GIGS, or CdTe. 
     The back electrode  240  is disposed on the photoelectric conversion layer  230 , and in contact with the photoelectric conversion layer  230 . In some examples, the back electrode  240  may be made of silver, aluminum, copper, chromium, nickel or transparent conductive oxide, depending on the needs. The electricity generated by the photoelectric conversion layer  230  may be transmitted to an external loading device through the back electrode  240  and the transparent conductive oxide layer  220 . 
     In examples, the photovoltaic module  300  may further comprise a sealing layer  250  which is sandwiched between the polymeric foam layer  110  and the back electrode  240 . The sealing layer directly contacts the back electrode  240  and the polymeric foam layer  110  of the backsheet  100 , and therefore bonds the photovoltaic member  200  and the backsheet  100  together. In some examples, the sealing layer  250  is a layer of ethylene-vinyl acetate copolymer. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present disclosure without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the present disclosure cover modifications and variations of this disclosure provided they fall within the scope of the following claims.