# Patent

## Title
Encapsulating material for solar cell and solar cell module

## Abstract

An encapsulating material for solar cell of the invention contains an ethylene/α-olefin copolymer, an organic peroxide, at least one crosslinking aid selected from a group consisting of divinyl aromatic compounds, cyanurates, diallyl compounds, triallyl compounds, oximes and maleimides, and a (meth)acrylate-based monomer. The content of the (meth)acrylate-based monomer in the encapsulating material for solar cell is in a range of 0.1 parts by weight to 5.0 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer, and the content of the crosslinking aid in the encapsulating material for solar cell is in a range of 0.1 parts by weight to 3 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer.

## Background
TECHNICAL FIELD
The present invention relates to an encapsulating material for solar cell and a solar cell module.
BACKGROUND ART
In response to the increasing seriousness of global environmental issues, energy issues and the like, a solar cell is attracting attention as clean energy-generating means with no concern over depletion. In a case in which a solar cell is used outdoors such as on the roof of a building, it is usual to use the solar cell in a solar cell module form.
Generally, the solar cell module is manufactured in the following order. First, a crystalline solar cell element (hereinafter also referred to as a power generation element or a cell in some cases which indicates the same thing) formed of polycrystalline silicon or monocrystalline silicon, or a thin film-type solar cell element obtained by forming an extremely thin (several micrometers) film of amorphous silicon or crystal silicon on a glass substrate or the like is manufactured. Next, to obtain a crystalline solar cell module, a protective sheet (a transparent surface protective member) for the solar cell module, an encapsulating material for solar cell, the crystalline solar cell element, an encapsulating material for solar cell, and a protective sheet (a back surface protective member) for the solar cell module are sequentially laminated. Meanwhile, to obtain a thin film-based solar cell module, a thin film-type solar cell element, an encapsulating material for solar cell, and a protective sheet (a back surface protective member) for the solar cell module are sequentially laminated. After that, the solar cell module is manufactured using a lamination method or the like in which the above-described laminate is suctioned in a vacuum, heated and pressed. The solar cell module manufactured in the above-described manner is weather resistant and is also suitable for outdoor use such as on the roof of a building.
An ethylene/vinyl acetate (EVA) copolymer film is widely used as an encapsulating material for solar cell due to its excellent transparency, flexibility, adhesiveness and the like.
Patent Document 1 (Japanese Examined Patent Publication No. 6-35575) describes a composition for encapsulation which encapsulates an electronic material obtained by mixing an organic peroxide or a photosensitizer and a silane coupling agent with EVA as a composition for encapsulation. Furthermore, Patent Document 1 discloses that (meth)acrylic acid ester and/or an allyl group-containing compound may be mixed.
In addition, Patent Document 2 (Japanese Unexamined Patent Publication No. 2007-123488) discloses an adhesive sheet for solar cell containing an organic peroxide and a polyfunctional monomer having a total of four or more of any one or both of an acryloyl group and a methacryloyl group in EVA.
In contrast to what has been described above, recently, there have been active studies regarding polyolefin-based materials, and there is another proposal of a resin composition for an encapsulating material for solar cell for which an ethylene/α-olefin copolymer having an excellent balance between stiffness and crosslinking characteristics and extrusion moldability is used (for example, refer to Patent Document 3).
RELATED DOCUMENT
Patent Document


    [Patent Document 1] Japanese Examined Patent Publication No. 6-35575
    [Patent Document 2] Japanese Unexamined Patent Publication No. 2007-123488
    [Patent Document 3] Japanese Unexamined Patent Publication No. 2010-258439


DISCLOSURE OF THE INVENTION
In accordance with the recent distribution of solar power generation, the size of a power generation system has been increasing, such as a mega solar power generation system, and also there is a movement of increasing the system voltage to decrease the transmission loss. In a solar cell module, an increase in the system voltage increases the potential difference between a frame and a cell. That is, generally, when the frame of the solar cell module is grounded, and the system voltage of a solar cell array reaches a voltage in a range of 600 V to 1000 V, the potential difference between the frame and the cell serves as the system voltage (in a range of 600 V to 1000 V) in a module in which the voltage becomes highest, and daytime power generation is maintained in a state in which a high voltage is applied. In addition, glass has a lower electric resistance compared with an encapsulating material for solar cell, and a high voltage is generated a glass plate and the cell through the frame. That is, in a state of daytime power generation, in modules connected in series, the potential difference between the cell and the module and the potential difference between the cell and the surface of a glass plate increase sequentially from the ground side, and the potential difference on the high voltage side of the system voltage is rarely changed in a portion with the largest potential difference. As a solar cell module used in the above-described state, there is a report regarding an example of a module in which a crystalline power generation element is used in which a potential induced degradation (PID) phenomenon in which the output is significantly decreased and the characteristics deteriorate occurs. As a result, there has been a demand for the improvement of the electrical characteristics of an encapsulating material for solar cell in direct contact with a solar cell element to solve the above-described problem.
However, according to the present inventors' studies, in a case in which the EVA composition described in Patent Document 1 or 2 was used as a constituent material of an encapsulating material for solar cell, there was a concern over the possibility of a component being generated by the decomposition of EVA such as acetic acid gas affecting a solar cell element. Furthermore, the EVA composition contained a large amount of a polar group, and thus had insufficient electrical characteristics. In addition, a resin composition for an encapsulating material for solar cell composed of the ethylene/α-olefin copolymer described in Patent Document 3 had insufficient electrical characteristics.
The invention has been made in consideration of the above-described problems of the related art, and an object of the invention is to provide an encapsulating material for solar cell having excellent electrical characteristics.
As a result of intensive studies for achieving the above-described object, the inventors found that, when an organic peroxide, at least one crosslinking aid selected from a group consisting of divinyl aromatic compounds, cyanurates, diallyl compounds, triallyl compounds, oximes and maleimides, and a (meth)acrylate-based monomer are added to an encapsulating material for solar cell containing an ethylene/α-olefin copolymer as a main component, an encapsulating material for solar cell having excellent electrical characteristics can be obtained. Furthermore, the inventors found that, when the content of an aluminum element contained in the ethylene/α-olefin copolymer is adjusted within a specific range, the electrical characteristics become superior, and completed the invention.
That is, according to the invention, encapsulating materials for solar cell described below are provided.
[1] An encapsulating material for solar cell containing an ethylene/α-olefin copolymer, an organic peroxide, at least one crosslinking aid selected from a group consisting of divinyl aromatic compounds, cyanurates, diallyl compounds, triallyl compounds, oximes and maleimides, and a (meth)acrylate-based monomer, in which
a content of the (meth)acrylate-based monomer in the encapsulating material for solar cell is in a range of 0.1 parts by weight to 5.0 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer, and
a content of the crosslinking aid in the encapsulating material for solar cell is in a range of 0.1 parts by weight to 3 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer.
[2] The encapsulating material for solar cell according to the above [1], in which a content of an aluminum element in the ethylene/α-olefin copolymer is in a range of 10 ppm to 500 ppm.
[3] The encapsulating material for solar cell according to the above [1] or [2], in which a volume resistivity of an encapsulating layer formed by crosslinking the encapsulating material for solar cell, which is based on JIS K6911 and measured at a temperature of 100° C. with an applied voltage of 500 V, is in a range of 1.0×1015 Ω·cm to 1.0×1018 Ω·cm.
[4] The encapsulating material for solar cell according to any one of the above [1] to [3], in which a molecular weight of the (meth)acrylate-based monomer is equal to or more than 150.
[5] The encapsulating material for solar cell according to any one of the above [1] to [4], in which the ethylene/α-olefin copolymer satisfies the following requirements a1) to a4):
a1) a content ratio of a structural unit derived from ethylene is in a range of 80 mol % to 90 mol %, and a content ratio of a structural unit derived from an α-olefin having 3 to 20 carbon atoms is in a range of 10 mol % to 20 mol %;
a2) MFR, which is based on ASTM D1238 and measured under conditions of a temperature of 190° C. and a load of 2.16 kg, is in a range of 0.1 g/10 minutes to 50 g/10 minutes;
a3) a density, which is measured on the basis of ASTM D1505, is in a range of 0.865 g/cm3 to 0.884 g/cm3; and
a4) a shore A hardness, which is measured on the basis of ASTM D2240, is in a range of 60 to 85.
[6] The encapsulating material for solar cell according to the above [5], in which MFR of the ethylene/α-olefin copolymer, which is based on ASTM D1238 and measured under conditions of a temperature of 190° C. and a load of 2.16 kg, is in a range of 10 g/10 minutes to 50 g/10 minutes.
[7] The encapsulating material for solar cell according to any one of the above [1] to [6], in which a one-minute half-life temperature of the organic peroxide is in a range of 100° C. to 170° C., and
a content of the organic peroxide in the encapsulating material for solar cell is in a range of 0.1 parts by weight to 3.0 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer.
[8] The encapsulating material for solar cell according to any one of the above [1] to [7], in which the ethylene/α-olefin copolymer is polymerized in the presence of a catalyst for olefin polymerization containing at least one compound selected from a group consisting of metallocene compounds, organic aluminiumoxy compounds and organic aluminum compounds.
[9] The encapsulating material for solar cell according to any one of the above [1] to [8], further containing an ethylenic unsaturated silane compound,
in which a content of the ethylenic unsaturated silane compound in the encapsulating material for solar cell is in a range of 0.1 parts by weight to 5 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer.
[10] The encapsulating material for solar cell according to any one of the above [1] to [9], further containing at least one of additives selected from a group consisting of an ultraviolet absorber, a hindered amine-based light stabilizer and a heat-resistant stabilizer,
in which a content of the above-described additives in the encapsulating material for solar cell is in a range of 0.005 parts by weight to 5 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer.
[11] The encapsulating material for solar cell according to any one of the above [1] to [10] obtained by melting and kneading the ethylene/α-olefin copolymer, the organic peroxide, the (meth)acrylate-based monomer, and the crosslinking aid, and then molding by extrusion the mixture into a sheet shape.
[12] The encapsulating material for solar cell according to any one of the above [1] to [11] having a sheet shape.
[13] A solar cell module including:
a transparent surface protective member,
a back surface protective member,
a solar cell element, and
an encapsulating layer which is formed by crosslinking the encapsulating material for solar cell according to any one of the above [1] to [12] and encapsulates the solar cell element between the transparent surface protective member and the back surface protective member.
[14] A solar cell module including:
a transparent surface protective member,
a back surface protective member,
a solar cell element, and
an encapsulating layer encapsulating the solar cell element between the transparent surface protective member and the back surface protective member,
in which the encapsulating layer is a crosslinked resin layer in which a (meth)acrylate-based monomer is grafted in a range of 0.1 parts by weight to 5.0 parts by weight, and at least one kind of crosslinking aid selected from a group consisting of divinyl aromatic compounds, cyanurates, diallyl compounds, triallyl compounds, oximes and maleimides is grafted in a range of 0.1 parts by weight to 3 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer.
According to the invention, it is possible to provide an encapsulating material for solar cell having excellent electrical characteristics.
According to the invention, when the above-described encapsulating material for solar cell is used, a variety of the above-described characteristics are excellent, and, even when the temperature increases during the use of a solar cell module, it is possible to avoid a trouble such as the deformation of the encapsulating material. In addition, it is possible to provide a solar cell module that does not impair the appearance of a solar cell and is excellent in terms of economic aspects such as costs.
Furthermore, when the above-described encapsulating material for solar cell is used, it is possible to provide a solar cell module capable of significantly suppressing the occurrence of PID even when a state in which a high voltage is applied between a frame and a cell is maintained.



BRIEF DESCRIPTION OF THE DRAWINGS
The above-described object, other objects, characteristics and advantages will be further clarified using preferable embodiments described below and the following drawings accompanied by the embodiments.
FIG. 1 is a cross-sectional view schematically illustrating an embodiment of a solar cell module of the invention.
FIG. 2 is a plan view schematically illustrating a configuration example of a light-incident surface and a back surface of a solar cell element.



DESCRIPTION OF EMBODIMENTS
Hereinafter, the embodiments of the invention will be described using the drawings. Further, in all the drawings, the same components will be given the same reference numerals and will not be described again as appropriate.
In addition, in the present specification, “a (meth)acrylate” refers to “an acrylate” or “a methacrylate”.
In addition, “A to B” indicates “equal to and more than A and equal to and less than B” unless particularly otherwise described.
1. With Regard to an Encapsulating Material for Solar Cell
An encapsulating material for solar cell of the embodiment contains an ethylene/α-olefin copolymer, an organic peroxide, at least one crosslinking aid selected from a group consisting of divinyl aromatic compounds, cyanurates, diallyl compounds, triallyl compounds, oximes and maleimides, and a (meth)acrylate-based monomer.
(Ethylene/α-Olefin Copolymer)
The ethylene/α-olefin copolymer used in the encapsulating material for solar cell of the embodiment is obtained by copolymerizing ethylene and an α-olefin having 3 to 20 carbon atoms. As the α-olefin, generally, it is possible to solely use an α-olefin having 3 to 20 carbon atoms, or to use a combination of two α-olefins having 3 to 20 carbon atoms. Among the α-olefins, an α-olefin having 10 or less carbon atoms is preferred, and an α-olefin having 3 to 8 carbon atoms is particularly preferred. Specific examples of the α-olefin include propylene, 1-butene, 1-pentene, 1-hexene, 3-methyl-1-butene, 3,3-dimethyl-1-butene, 4-methyl-1-pentene, 1-octene, 1-decene, 1-dodecene, or the like. Among the above-described α-olefins, propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene and 1-octene are preferred in terms of easy procurement. Meanwhile, the ethylene/α-olefin copolymer may be a random copolymer or a block copolymer, but is preferably a random copolymer from the viewpoint of flexibility.
Furthermore, the ethylene/α-olefin copolymer of the embodiment may be a copolymer composed of ethylene, an α-olefin having 3 to 20 carbon atoms and a unconjugated polyene. The α-olefin is the same as described above, and examples of the unconjugated polyene include 5-ethylidene-2-norbornene (ENB), 5-vinyl-2-norbornene (VNB), dicyclopentadiene (DCPD) and the like. The unconjugated polyene can be solely used, or a combination of two or more unconjugated polyene can be used.
The ethylene/α-olefin copolymer of the embodiment preferably further satisfies the following requirements a1) to a5).
(Requirement a1)
The content ratio of a structural unit, which is contained in the ethylene/α-olefin copolymer and is derived from ethylene, is preferably in a range of 80 mol % to 90 mol %, more preferably in a range of 80 mol % to 88 mol %, still more preferably in a range of 82 mol % to 88 mol %, and particularly preferably in a range of 82 mol % to 87 mol %. The content ratio of a structural unit, which is contained in the ethylene/α-olefin copolymer and is derived from the α-olefin having 3 to 20 carbon atoms (hereinafter, also referred to as “α-olefin unit”), is preferably in a range of 10 mol % to 20 mol %, more preferably in a range of 12 mol % to 20 mol %, still more preferably in a range of 12 mol % to 18 mol %, and particularly preferably in a range of 13 mol % to 18 mol %.
When the content ratio of the α-olefin unit contained in the ethylene/α-olefin copolymer is set to equal to or more than 10 mol %, high transparency can be obtained. In addition, it is possible to easily carry out extrusion-molding at a low temperature, for example, extrusion-molding at 130° C. or lower is possible. Therefore, even in a case in which the organic peroxide is forcibly inserted into the ethylene/α-olefin copolymer, it is possible to suppress the progress of a crosslinking reaction in an extruder, and to prevent the generation of a gel-form foreign substance in a sheet of the encapsulating material for solar cell and the deterioration of the appearance of the sheet. In addition, since appropriate flexibility can be obtained, it is possible to prevent the occurrence of cracking in the solar cell element or the chipping of the thin film electrode during the lamination molding of the solar cell module.
When the content ratio of the α-olefin unit contained in the ethylene/α-olefin copolymer is set to equal to or less than 20 mol %, the crystallization rate of the ethylene/α-olefin copolymer becomes appropriate, and therefore a sheet extruded from an extruder does not become sticky, the sheet is easily peeled from a first cooling roll, and it is possible to efficiently obtain a sheet-shaped sheet of the encapsulating material for solar cell. In addition, since the sheet does not become sticky, blocking can be prevented, and the feeding property of the sheet is favorable. In addition, it is also possible to prevent the degradation of the heat resistance.
(Requirement a2)
The melt flow rate (MFR) of the ethylene/α-olefin copolymer, which is based on ASTM D1238 and measured under conditions of 190° C. and a load of 2.16 kg, is generally in a range of 0.1 g/10 minutes to 50 g/10 minutes, preferably in a range of 2 g/10 minutes to 50 g/10 minutes, more preferably in a range of 10 g/10 minutes to 50 g/10 minutes, still more preferably in a range of 10 g/10 minutes to 40 g/10 minutes, particularly in a range of 12 g/10 minutes to 27 g/10 minutes, and most preferably in a range of 15 g/10 minutes to 25 g/10 minutes. MFR of the ethylene/α-olefin copolymer can be adjusted by adjusting the polymerization temperature and the polymerization pressure during a polymerization reaction described below, the molar ratio between the monomer concentration and the hydrogen concentration in ethylene and the α-olefin in a polymerization system, and the like.
When MFR is within a range of 0.1 g/10 minutes to 10 g/10 minutes, it is possible to manufacture a sheet through calendar molding. When MFR is within a range of 0.1 g/10 minutes to 10 g/10 minutes, the fluidity of the resin composition containing the ethylene/α-olefin copolymer is low, and therefore it is possible to prevent a lamination apparatus from being contaminated by the molten resin extracted when the sheet is laminated with the cell element, which is preferable.
Furthermore, when MFR is equal to or more than 2 g/10 minutes, the fluidity of the encapsulating material for solar cell improves, and it is also possible to produce the encapsulating material through sheet extrusion molding. When MFR is equal to or more than 10 g/10 minutes, in a case in which a sheet is manufactured through extrusion molding, the fluidity of the resin composition containing the ethylene/α-olefin copolymer is improved, and it is possible to improve the productivity during sheet extrusion molding.
In addition, when MFR is set to equal to or less than 50 g/10 minutes, the molecular weight is increased so that it is possible to suppress the adhesion to a roll surface of a chilled roll or the like, and therefore peeling is not required, and a sheet having a uniform thickness can be molded. Furthermore, since the resin composition becomes “stiff”, it is possible to easily mold a sheet having a thickness of equal to or more than 0.3 mm. In addition, since the crosslinking characteristic (particularly, crosslinking rate) is improved during the lamination molding of the solar cell module, the crosslinkable resin is sufficiently crosslinked so that the degradation of the heat resistance can be suppressed.
When MFR is equal to or less than 27 g/10 minutes, furthermore, it is possible to suppress drawdown during the sheet molding, and thus it is possible to mold a sheet having a wide width, to further improve the crosslinking characteristic and the heat resistance, and to obtain the most favorable sheet of the encapsulating material for solar cell.
(Requirement a3)
The density of the ethylene/α-olefin copolymer, which is measured on the basis of ASTM D1505, is preferably in a range of 0.865 g/cm3 to 0.884 g/cm3, more preferably in a range of 0.866 g/cm3 to 0.883 g/cm3, still more preferably in a range of 0.866 g/cm3 to 0.880 g/cm3, and particularly preferably in a range of 0.867 g/cm3 to 0.880 g/cm3. The density of the ethylene/α-olefin copolymer can be adjusted using the balance between the content ratio of the ethylene unit and the content ratio of the α-olefin unit. That is, when the content ratio of the ethylene unit is increased, the crystallinity increases, and the ethylene/α-olefin copolymer having a high density can be obtained. On the other hand, when the content ratio of the ethylene unit is decreased, the crystallinity decreases, and the ethylene/α-olefin copolymer having a low density can be obtained.
When the density of the ethylene/α-olefin copolymer is equal to or less than 0.884 g/cm3, the crystallinity becomes low, and it is possible to increase the transparency. Furthermore, extrusion-molding at a low temperature becomes easy, and, for example, it is possible to carry out extrusion-molding at 130° C. or lower. Therefore, even when the organic peroxide is forcibly inserted into the ethylene/α-olefin copolymer, it is possible to prevent the progress of a crosslinking reaction in an extruder, to suppress the generation of a gel-form foreign substance in the sheet of the encapsulating material for solar cell, and to suppress the deterioration of the appearance of the sheet. In addition, since the flexibility is high, it is possible to prevent the occurrence of cracking in a cell which is a solar cell element or the chipping of the thin film electrode during the lamination molding of the solar cell module.
On the other hand, when the density of the ethylene/α-olefin copolymer is equal to or more than 0.865 g/cm3, it is possible to increase the crystallization rate of the ethylene/α-olefin copolymer, and therefore a sheet extruded from an extruder does not become sticky, the sheet is easily peeled from a first cooling roll, and it is possible to easily obtain a sheet of the encapsulating material for solar cell. In addition, since the sheet does not become sticky, it is possible to suppress the occurrence of blocking and improve the feeding property of the sheet. In addition, since the sheet is sufficiently crosslinked, it is possible to suppress the degradation of the heat resistance.
(Requirement a4)
The shore A hardness of the ethylene/α-olefin copolymer, which is measured on the basis of ASTM D2240, is preferably in a range of 60 to 85, more preferably in a range of 62 to 83, still more preferably in a range of 62 to 80, and particularly preferably in a range of 65 to 80. The shore A hardness of the ethylene/α-olefin copolymer can be adjusted by adjusting the content ratio or density of the ethylene unit in the ethylene/α-olefin copolymer within the above-described numeric range. That is, the shore A hardness becomes great in the ethylene/α-olefin copolymer having a high content ratio and a high density of the ethylene unit. On the other hand, the shore A hardness becomes low in the ethylene/α-olefin copolymer having a low content ratio of the ethylene unit and a low density.
When the shore A hardness is equal to or more than 60, the ethylene/α-olefin copolymer does not easily become sticky, and is capable of suppressing blocking. In addition, it is also possible to improve the feeding property of the sheet when the encapsulating material for solar cell is processed in a sheet shape, and it is also possible to suppress a decrease in the heat resistance.
On the other hand, when the shore A hardness is equal to or less than 85, the crystallinity becomes low, and it is possible to increase the transparency. Furthermore, since the flexibility is high, it is possible to prevent the occurrence of cracking in a cell which is the solar cell element or the chipping of the thin film electrode during the lamination molding of the solar cell module.
(Requirement a5)
The content (residue amount) of an aluminum element (hereinafter, also expressed as “Al”) contained in the ethylene/α-olefin copolymer is preferably in a range of 10 ppm to 500 ppm, more preferably in a range of 20 ppm to 400 ppm, and still more preferably in a range of 20 ppm to 300 ppm. The content of Al is dependent on the concentration of an organic aluminumoxy compound or an organic aluminum compound being added during the polymerization process of the ethylene/α-olefin copolymer.
In a case in which the content of Al is equal to or more than 10 ppm, the organic aluminumoxy compound or the organic aluminum compound added in the polymerization process of the ethylene/α-olefin copolymer can be added at a concentration at which the activity of a metallocene compound is sufficiently developed, and therefore it becomes unnecessary to add a compound which reacts with the metallocene compound so as to form an ion pair. In a case in which the compound forming the ion pair is added, there is a case in which the compound forming the ion pair remains in the ethylene/α-olefin copolymer such that the electrical characteristics are degraded (for example, there is a tendency of electrical characteristics degrading at a high temperature of approximately 100° C.), which can be prevented.
In addition, a decrease in the content of Al requires a decalcification treatment using an acid or an alkali, and the acid or the alkali remaining in the obtained ethylene/α-olefin copolymer tends to corrode the electrode. In addition, the decalcification treatment increases the cost of the ethylene/α-olefin copolymer. However, when the content of Al is equal to or more than 10 ppm, the decalcification treatment becomes unnecessary.
In addition, when the content of Al is equal to or less than 500 ppm, it is possible to prevent the progress of the crosslinking reaction in an extruder, and therefore it is possible to prevent the generation of a gel-form foreign substance in a sheet of the encapsulating material for solar cell and the deterioration of the appearance of the sheet.
As a method for controlling the content of the above-described aluminum element contained in the ethylene/α-olefin copolymer, for example, it is possible to control the content of the aluminum element contained in the ethylene/α-olefin copolymer by adjusting the concentrations of the organic aluminumoxy compound (II-1) and the organic aluminum compound (II-2) described in the below-described method for manufacturing the ethylene/α-olefin copolymer in a manufacturing step or the polymerization activity of the metallocene compound among the conditions for manufacturing the ethylene/α-olefin copolymer.
Furthermore, the encapsulating material for solar cell of the embodiment preferably further satisfies the following requirements.
(B Value)
The B value of the ethylene/α-olefin copolymer, which is obtained from the 13C-NMR spectrum and the following equation (1), is preferably in a range of 0.9 to 1.5, more preferably in a range of 0.9 to 1.3, still more preferably in a range of 0.95 to 1.3, particularly preferably in a range of 0.95 to 1.2, and most preferably in a range of 1.0 to 1.2. The B value can be adjusted by changing a polymerization catalyst when the ethylene/α-olefin copolymer is polymerized. More specifically, the ethylene/α-olefin copolymer having a B value in the above-described numeric range can be obtained using the metallocene compound described below.

B value=[POE]/(2×[PO]×[PE])  (1)

(In the equation (1), [PE] represents the proportion (molar fraction) of the structural unit derived from ethylene in the ethylene/α-olefin copolymer, [PO] represents the proportion (molar fraction) of the structural unit derived from an α-olefin having 3 to 20 carbon atoms in the ethylene/α-olefin copolymer, and [POE] represents the proportion (molar fraction) of α-olefin-ethylene chains in all dyad chains.)
The B value is an index indicating the distribution state of the ethylene unit and the α-olefin unit in the ethylene/α-olefin copolymer, and can be obtained based on the report by J. C. Randall (Macromolecules, 15, 353 (1982), J. Ray (Macromolecules, 10, 773 (1977))).
As the B value increases, the block chain of the ethylene unit or the α-olefin copolymer becomes shorter, and a high B value indicates that the distribution of the ethylene unit and the α-olefin unit is uniform and the composition distribution of copolymer rubber is narrow. Meanwhile, when the B value is equal to or more than 0.9, it is possible to decrease the composition distribution of the ethylene/α-olefin copolymer. Particularly, since the block chain of the ethylene unit becomes small, and extrusion-molding at a low temperature becomes easy, for example, it is possible to carry out extrusion-molding at 130° C. or lower. Therefore, even in a case in which the organic peroxide is forcibly inserted into the ethylene/α-olefin copolymer, it is possible to suppress the progress of a crosslinking reaction in an extruder, and to prevent the generation of a gel-form foreign substance in a sheet of the encapsulating material for solar cell and the deterioration of the appearance of the sheet.
(Tαβ/Tαα)
The intensity ratio (Tαβ/Tαα) of Tαβ to Tαα of the ethylene/α-olefin copolymer in the 13C-NMR spectrum is preferably equal to or less than 1.5, more preferably equal to or less than 1.2, particularly preferably equal to or less than 1.0, and most preferably less than 0.7. Tαβ/Tαα can be adjusted by changing a polymerization catalyst when the ethylene/α-olefin copolymer is polymerized. More specifically, the ethylene/α-olefin copolymer having a Tαβ/Tαα in the above-described numeric range can be obtained using the metallocene compound described below.
Tαα and Tαβ in the 13C-NMR spectrum correspond to the peak intensities of “CH2” in the structural unit derived from an α-olefin having 3 or more carbon atoms. More specifically, Tαα and Tαβ represent the peak intensities of two types of “CH2” having different locations with respect to tertiary carbon atoms as illustrated in the following general equation (3).




Tαβ/Tαα can be obtained in the following manner. The 13C-NMR spectrum of the ethylene/α-olefin copolymer is measured using an NMR measurement device (for example, trade name “JEOL-GX270” manufactured by JEOL, Ltd.). The measurement is carried out using a solution mixture of hexachlorobutadiene and d6-benzene (at a volume ratio of 2/1) which has been adjusted to provide a specimen concentration of 5 weight % and a standard of d6-benzene (128 ppm) at 67.8 MHz and 25° C. The measured 13C-NMR spectrum is analyzed according to Lindemann-Adams' proposal (Analysis Chemistry, 43, p 1245 (1971)), J. C. Randall (Review Macromolecular Chemistry Physics, C29, 201 (1989)), and Tαβ/Tαα is obtained.
The intensity ratio (Tαβ/Tαα) of Tαβ to Tαα of the ethylene/α-olefin copolymer in the 13C-NMR indicates the coordination state of the α-olefin with respect to the polymerization catalyst during the polymerization reaction. In a case in which the α-olefin is coordinated with respect to the polymerization catalyst in a Tαβ form, the substituent in the α-olefin hinders the polymerization growth reaction of a polymer chain, and tends to promote the generation of a low molecular weight component. As a result, there is a tendency that the sheet becomes sticky and blocked, and the feeding property of the sheet deteriorates. Furthermore, since the low molecular weight component spreads on the sheet surface, adhesion is hindered, and the adhesiveness is degraded.
(Molecular Weight Distribution Mw/Mn)
The molecular weight distribution Mw/Mn of the ethylene/α-olefin copolymer, which is represented by the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) measured through gel permeation chromatography (GPC), is preferably in a range of 1.2 to 3.5, more preferably in a range of 1.7 to 3.0, still more preferably in a range of 1.7 to 2.7, and particularly preferably in a range of 1.9 to 2.4. The molecular weight distribution Mw/Mn of the ethylene/α-olefin copolymer can be adjusted using the metallocene compound described below during the polymerization.
When the Mw/Mn is set to equal to or more than 1.2, it is possible to obtain the catalyst activity for polymerizing the ethylene/α-olefin copolymer in a living polymerization manner. Alternatively, since it becomes unnecessary to separate the low molecular weight component and high molecular weight component of the ethylene/α-olefin copolymer obtained using a well-known polymerization method of the related art, it is possible to decrease the manufacturing cost. In addition, since the temperature width in which molding is possible is narrow, and furthermore, the ejection amount in an extruder becomes uniform, it is possible to obtain a sheet having a uniform thickness, and sheet molding becomes easy.
Generally, it is known that, when the molecular weight distribution Mw/Mn becomes wide, the composition distribution also becomes wide. However, when the Mw/Mn is equal to or less than 3.5, the amount of the low molecular weight component becomes small, and therefore the sheet does not become sticky and is not easily blocked so that the feeding property of the sheet can be improved. In addition, since the low molecular weight component spreads on the sheet surface, it is possible to suppress the degradation of the adhesiveness.
In the embodiment, the ratio (Mw/Mn) of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) was measured in the following manner using a gel permeation chromatography manufactured by Waters (trade name: “ALLIANCE GPC-2000”). Two TSKgel GMH6-HT (trade name) columns and two TSKgel GMH6-HTL (trade name) columns were used as separation columns. Regarding the column size, an inner diameter of 7.5 mm and a length of 300 mm were provided to all columns, the column temperature was set to 140° C., o-dichlorobenzene (manufactured by Wako Pure Chemical Industries, Ltd.) was used as a mobile phase, and 0.025 weight % of BHT (manufactured by Takeda Pharmaceutical Company Limited) was used as an antioxidant. The mobile phase was moved at a rate of 1.0 ml/minute so as to set the specimen concentration to 15 mg/10 ml, the specimen injection amount was set to 500 μl, and a differential refractometer was used as a detector. Polystyrene manufactured by Tosoh Corporation was used as the standard polystyrene for the ethylene/α-olefin copolymer having Mw≦1000 and Mw≧4×106. In addition, polystyrene manufactured by Pressure Chemical Corporation was used as the standard polystyrene for the ethylene/α-olefin copolymer having 1000≦Mw≦4×106. The molecular weight was the value of the ethylene/α-olefin copolymer converted in accordance with each of the used α-olefins through universal correction.
(The Content Ratio of Chlorine Ions)
The content ratio of chlorine ions in the ethylene/α-olefin copolymer, which is detected from an extraction liquid after a solid-phase extraction treatment through ion chromatography, is preferably equal to or less than 2 ppm, more preferably equal to or less than 1.5 ppm, and particularly preferably equal to or less than 1.2 ppm. The content ratio of chlorine ions can be adjusted by adjusting the structure and polymerization conditions of the metallocene compound described below. That is, when the polymerization activity of the catalyst is increased, the amount of a catalyst residue in the ethylene/α-olefin copolymer is decreased, and the ethylene/α-olefin copolymer having a content ratio of chlorine ions within the above-described range can be obtained.
When the content ratio of chlorine ions in the ethylene/α-olefin copolymer is set to equal to or less than 2 ppm, it is possible to prevent a problem of the long-term reliability of the solar cell module being degraded by the corrosion of an electrode made of silver and the like. Meanwhile, when the metallocene compound including no chlorine atom is used, it is possible to obtain the ethylene/α-olefin copolymer substantially containing no chlorine ion.
The content ratio of chlorine ions in the ethylene/α-olefin copolymer can be measured using an extraction liquid, which is obtained by, for example, precisely weighing approximately 10 g of the ethylene/α-olefin copolymer in a glass container that has been sterilized and washed using an autoclave or the like, adding 100 ml of ultrapure water, closely encapsulating the glass container, and then carrying out ultrasonic wave (38 kHz) extraction at room temperature for 30 minutes, and an ion chromatography device manufactured by Dionex Ltd. (trade name “ICS-2000”).
(The Extraction Amount into Methyl Acetate)
The extraction amount of the ethylene/α-olefin copolymer into methyl acetate is preferably equal to or less than 5.0 weight %, more preferably equal to or less than 4.0 weight %, still more preferably equal to or less than 3.5 weight %, and particularly preferably equal to or less than 2.0 weight %. A large extraction amount into methyl acetate indicates that a large amount of a low molecular weight component is contained in the ethylene/α-olefin copolymer and the molecular weight distribution or the composition distribution is wide. Therefore, it is possible to obtain the ethylene/α-olefin copolymer having a small extraction amount into methyl acetate using the metallocene compound described below by adjusting the polymerization conditions.
For example, when the metallocene compound having a decreased polymerization activity is emitted outside a polymerization system by shortening the polymerization retention time in a polymerization vessel, it is possible to suppress the generation of the low molecular weight component. When the extraction amount into methyl acetate is equal to or less than 5.0 weight % in a Soxhlet extraction method, the sheet does not become sticky, and therefore it is possible to suppress blocking and to improve the feeding property of the sheet.
The extraction amount into methyl acetate is computed by, for example, precisely weighing approximately 10 g of the ethylene/α-olefin copolymer, carrying out Soxhlet extraction using an organic solvent which has a low boiling point and serves as a poor solvent for the ethylene/α-olefin copolymer, such as methyl acetate or methyl ethyl ketone, at a temperature that is equal to or higher than the boiling point of methyl acetate, and using the weight difference of the ethylene/α-olefin copolymer before and after the extrusion or the residue amount obtained after the extracted solvent is volatilized.
(Melting Peak)
The differential scanning calorimetry (DSC)-based melting peak of the ethylene/α-olefin copolymer is preferably present in a range of 30° C. to 90° C., more preferably present in a range of 33° C. to 90° C., and particularly preferably present in a range of 33° C. to 88° C. When the melting peak is 90° C. or lower, the degree of crystallinity becomes low, and the flexibility of the obtained encapsulating material for solar cell becomes favorable, and therefore it is possible to prevent the occurrence of cracking in a cell or the chipping of the thin film electrode during the lamination molding of the solar cell module. On the other hand, when the melting peak is 30° C. or higher, it is possible to provide appropriately favorable flexibility to the resin composition, and therefore it is possible to easily obtain a sheet of the encapsulating material for solar cell through extrusion-molding. In addition, it is possible to prevent the sheet from becoming sticky and blocked, and to suppress the deterioration of the feeding property of the sheet.
(Method for Manufacturing the Ethylene/α-Olefin Copolymer)
The ethylene/α-olefin copolymer can be manufactured using, for example, a variety of metallocene compounds described below as a catalyst. Examples of the metallocene compounds that can be used include metallocene compounds described in Japanese Unexamined Patent Publication No. 2006-077261, Japanese Unexamined Patent Publication No. 2008-231265, Japanese Unexamined Patent Publication No. 2005-314680 and the like. However, a metallocene compound having a different structure from those of the metallocene compounds described in the above-described patent documents may also be used, and a combination of two or more metallocene compounds may also be used.
Preferable examples of a polymerization reaction in which the metallocene compound is used include the following aspects.
Ethylene and one or more monomers selected from α-olefins and the like are supplied in the presence of a catalyst for olefin polymerization composed of a well-known metallocene compound of the related art and at least one compound (also referred to as a promoter) selected from a group consisting of (II) (II-1) the organic aluminumoxy compound, (II-2) compounds that react with the metallocene (I) so as to form an ion pair, and (II-3) the organic aluminum compound.
As (II-1) the organic aluminumoxy compound, (II-2) the compounds that react with the metallocene (I) so as to form an ion pair, and (II-3) the organic aluminum compound, for example, the metallocene compounds described in Japanese Unexamined Patent Publication No. 2006-077261, Japanese Unexamined Patent Publication No. 2008-231265, Japanese Unexamined Patent Publication No. 2005-314680 and the like can be used. However, a metallocene compound having a different structure from those of the metallocene compounds described in the above-described patent documents may also be used. The above-described compounds may be individually injected into a polymerization atmosphere or be brought into contact with each other in advance and then injected into a polymerization atmosphere. Furthermore, for example, the compounds may be carried by the fine particle-shaped inorganic oxide carrier described in Japanese Unexamined Patent Publication No. 2005-314680 or the like. Meanwhile, preferably, when the ethylene/α-olefin copolymer is manufactured without substantially using (II-2) the compound that reacts with the metallocene (I) so as to form an ion pair, it is possible to obtain the ethylene/α-olefin copolymer having excellent electrical characteristics.
The polymerization of the ethylene/α-olefin copolymer can be carried out using any one of a well-known gas-phase polymerization method of the related art and a liquid-phase polymerization method such as a slurry polymerization method or a solution polymerization method. The polymerization is preferably carried out using the liquid-phase polymerization method such as the solution polymerization method. In a case in which the ethylene/α-olefin copolymer is manufactured by carrying out the copolymerization of ethylene and an α-olefin having 3 to 20 carbon atoms using the metallocene compound, (I) the metallocene compound is used in an amount in a range of, generally, 10−9 mole to 10−1 mole, and preferably 10−8 mole to 10−2 mole per a reaction volume of one liter.
The compound (II-1) is used in an amount in which the molar ratio [(II-1)/M] of the compound (II-1) to all transition metal atoms (M) in the compound (I) is generally in a range of 1 to 10000, and preferably in a range of 10 to 5000. The compound (II-2) is used in an amount in which the molar ratio [(II-2)/M] of the compound (II-2) to all the transition metal atoms (M) in the compound (I) is generally in a range of 0.5 to 50, and preferably in a range of 1 to 20. The compound (II-3) is used in an amount in a range of, generally, 0 millimoles to 5 millimoles, and preferably approximately 0 millimoles to 2 millimoles per a reaction volume of one liter.
In the solution polymerization method, when ethylene and an α-olefin having 3 to 20 carbon atoms are copolymerized in the presence of the above-described metallocene compound, it is possible to efficiently manufacture an ethylene/α-olefin copolymer having a large content of a copolymer, a narrow composition distribution and a narrow molecular weight distribution. Here, the preliminary molar ratio of ethylene to the α-olefin having 3 to 20 carbon atoms is generally in a range of 10:90 to 99.9:0.1, preferably in a range of 30:70 to 99.9:0.1, and more preferably in a range of 50:50 to 99.9:0.1 (ethylene:α-olefin).
As the α-olefin having 3 to 20 carbon atoms, a straight or branched α-olefin, for example, propylene, 1-butene, 2-butene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 3-methyl-1-pentene, 1-octene, 1-decene or 1-dodecene can be used.
Examples of the α-olefin that can be used in the solution polymerization method also include polar group-containing olefins. Examples of the polar group-containing olefins include α,β-unsaturated carboxylic acids such as acrylic acid, methacrylic acid, fumaric acid and maleic anhydride, and metallic salts thereof such as sodium salts; α,β-unsaturated carboxylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, methyl methacrylate and ethyl methacrylate; vinyl esters such as vinyl acetate and vinyl propionate; unsaturated glycidyl esters such as glycidyl acrylate and glycidyl methacrylate; and the like. In addition, it is also possible to proceed with high-temperature solution polymerization in the co-presence of an aromatic vinyl compound in the reaction system. Examples of the aromatic vinyl compound include styrenes such as styrene, o-methyl styrene, m-methyl styrene, p-methyl styrene, o,p-dimethyl styrene, methoxy styrene, vinyl benzoate, vinyl methyl benzoate, vinyl benzyl acetate, hydroxyl styrene, p-chlorostyrene and divinyl benzene; 3-phenyl propylene, 4-phenyl propylene, α-methyl styrene, and the like.
Among the above-described α-olefins, propylene, 1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene are preferably used. In addition, in the solution polymerization method, a cyclic olefin having 3 to 20 carbon atoms, for example, cyclopentene, cycloheptene, norbornene or 5-methyl-2-norbornene may be jointly used.
The “solution polymerization method” is a collective term for all methods in which polymerization is carried out in a state in which a polymer is dissolved in an inert hydrocarbon solvent described below. In the solution polymerization method, the polymerization temperature is generally in a range of 0° C. to 200° C., preferably in a range of 20° C. to 190° C., and more preferably in a range of 40° C. to 180° C. In a case in which the polymerization temperature fails to satisfy 0° C., the solution polymerization method is not practical in terms of productivity since the polymerization activity is extremely degraded, and the removal of polymerization heat becomes difficult. In a case in which the solution polymerization temperature exceeds 200° C., the solution polymerization method is not practical in terms of productivity since the polymerization activity is extremely degraded.
The polymerization pressure is generally in a range of normal pressure to 10 MPa (gauge pressure), and preferably in a range of normal pressure to 8 MPa (gauge pressure). Copolymerization can be carried out in all of a batch method, a semi-continuous method, and a continuous method. The reaction time (the average retention time in a case in which a copolymer reaction is carried out using a continuous method) varies depending on the conditions such as the catalyst concentration and the polymerization temperature, and can be appropriately selected, but is generally in a range of one minute to three hours, and preferably in a range of ten minutes to 2.5 hours. Furthermore, it is also possible to carry out the polymerization in two or more steps with different reaction conditions. The molecular weight of the obtained ethylene/α-olefin copolymer can be adjusted by changing the concentration of hydrogen or the polymerization temperature in the polymerization system. Furthermore, the molecular weight of the ethylene/α-olefin copolymer can also be adjusted using the amount of the compound (II) being used. In a case in which hydrogen is added, the amount of hydrogen is appropriately in a range of approximately 0.001 NL to 5000 NL per kilogram of the ethylene/α-olefin copolymer being generated. In addition, a vinyl group and a vinylidene group present at the ends of a molecule in the obtained ethylene/α-olefin copolymer can be adjusted by increasing the polymerization temperature and extremely decreasing the amount of hydrogen being added.
A solvent used in the solution polymerization method is generally an inert hydrocarbon solvent, and is preferably a saturated hydrocarbon having a boiling point in a range of 50° C. to 200° C. at normal pressure. Specific examples thereof include aliphatic hydrocarbon such as pentane, hexane, heptane, octane, decane, dodecane and kerosene; and alicyclic hydrocarbon such as cyclopentane, cyclohexane and methylcyclopentane. Meanwhile, aromatic hydrocarbons such as benzene, toluene and xylene and halogenated hydrocarbon such as ethylene chloride, chlorobenzene and dichlorobenzene also belong to the scope of the “inert hydrocarbon solvent”, and the use thereof is not limited.
As described above, in the solution polymerization method, not only the organic aluminumoxy compound dissolved in the aromatic hydrocarbon, which was frequently used in the related art, but also modified methyl aluminoxane dissolved in the aliphatic hydrocarbon or the alicyclic hydrocarbon such as MMAO can be used. As a result, when the aliphatic hydrocarbon or the alicyclic hydrocarbon is employed as the solvent for the solution polymerization, it becomes possible to almost completely eliminate the possibility of the aromatic hydrocarbon being incorporated into the polymerization system or the ethylene/α-olefin copolymer being generated. That is, the solution polymerization method also has characteristics in that the environmental load can be reduced and the influence on human health can be minimized. Meanwhile, to suppress the variation in properties, it is preferable to melt the ethylene/α-olefin copolymer obtained through the polymerization reaction and other components added as desired using an arbitrary method, and to knead, granulate and the like the ethylene/α-olefin copolymer and other components.
(Organic Peroxide)
The encapsulating material for solar cell of the embodiment contains an organic peroxide. The organic peroxide is used as a radical initiator during the graft modification of an ethylenic unsaturated silane compound and the ethylene/α-olefin copolymer, and furthermore, is used as a radial initiator during a crosslinking reaction when the ethylene/α-olefin copolymer is lamination-molded to the solar cell module. When the ethylenic unsaturated silane compound is graft-modified in the ethylene/based copolymer, a solar cell module having a favorable adhesiveness to the transparent surface protective member, the back surface protective member, a cell, and an electrode is obtained. Furthermore, when the ethylene/α-olefin copolymer is crosslinked, a solar cell module having excellent heat resistance and adhesiveness can be obtained.
The content of the organic peroxide in the encapsulating material for solar cell of the embodiment is preferably in a range of 0.1 parts by weight to 3 parts by weight, more preferably in a range of 0.2 parts by weight to 3 parts by weight, particularly preferably in a range of 0.2 parts by weight to 2.5 parts by weight, with respect to 100 parts by weight of the above-described the ethylene/α-olefin copolymer.
When the content of the organic peroxide is equal to or more than 0.1 parts by weight, the degradation of crosslinking characteristics such as the crosslinking degree or crosslinking rate of the encapsulating material for solar cell is suppressed, and the graft reaction of the ethylenic unsaturated silane compound described below into the main chain of the ethylene/α-olefin copolymer is bettered, whereby it is possible to suppress the degradation of the heat resistance and the adhesiveness.
When the content of the organic peroxide is equal to or less than 3.0 parts by weight, a gel is not easily generated in an encapsulating sheet for solar cell obtained from the resin composition during extrusion sheet molding, and therefore it is possible to suppress an increase in the torque of an extruder and to facilitate sheet molding. In addition, since it is possible to suppress the sheet surface becoming uneven due to a gel-form substance generated in the extruder, it is possible to prevent the degradation of the appearance. In addition, since it is possible to prevent the occurrence of cracking in the sheet when a voltage is applied, it is possible to prevent a decrease in the dielectric breakdown voltage. Furthermore, it is also possible to prevent the degradation of moisture permeability. In addition, since it is possible to suppress the sheet surface becoming uneven, the adhesion among the transparent surface protective member, the cell, the electrode, and the back surface protective member becomes favorable during the lamination process of the solar cell module, and the adhesiveness also improves.
There is no particular limitation with the organic peroxide that can be preferably used as long as the organic peroxide is capable of graft-modifying the ethylenic unsaturated silane compound in the ethylene/α-olefin copolymer or crosslinking the ethylene/α-olefin copolymer, and the one-minute half-life temperature of the organic peroxide is preferably in a range of 100° C. to 170° C. in consideration of the balance between the productivity during extrusion sheet molding and the crosslinking rate during the lamination molding of the solar cell module. When the one-minute half-life temperature of the organic peroxide is 100° C. or higher, it becomes difficult for a gel to be generated in a solar cell encapsulating sheet obtained from the resin composition during the extrusion sheet molding, and therefore it is possible to suppress an increase in the torque of the extruder and to facilitate sheet molding. In addition, since it is possible to suppress the sheet surface becoming uneven due to a gel-form substance generated in the extruder, the degradation of the appearance can be prevented. In addition, since it is possible to prevent the occurrence of cracking in the sheet when a voltage is applied, a decrease in the dielectric breakdown voltage can be prevented. Furthermore, the degradation of the moisture permeability can also be prevented. In addition, since it is possible to suppress the sheet surface becoming uneven, the adhesion among the transparent front protective member, a cell, an electrode, and the back surface protective member becomes favorable during the lamination process of the solar cell module, and the adhesiveness also improves. When the extrusion temperature of the extrusion sheet molding is decreased to 90° C. or lower, while the molding is possible, the productivity significantly degrades. When the one-minute half-life temperature of the organic peroxide is 170° C. or lower, it is possible to suppress a decrease in the crosslinking rate during the lamination molding of the solar cell module, and therefore it is possible to prevent the degradation of the productivity of the solar cell module. In addition, it is also possible to prevent the degradation of the heat resistance and adhesiveness of the encapsulating material for solar cell.
A well-known organic peroxide can be used as the organic peroxide. Specific examples of the preferable organic peroxide having a one-minute half-life temperature in a range of 100° C. to 170° C. include dilauroyl peroxide, 1,1,3,3-tetramethylbutyl peroxy-2-ethylhexanoate, dibenzoyl peroxide, t-amylperoxy-2-ethylhexanoate, t-butylperoxy-2-ethylhexanotae, t-butylperoxy isobutyrate, t-butylperoxy maleate, 1,1-di(t-amylperoxy)-3,3,5-trimethylcyclohexane, 1,1-di(t-amylpeoxy)cyclohexane, t-amylperoxy isononanoate, t-amylperoxy normaloctoate, 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-di(t-butylperoxy)cyclohexane, t-butylperoxy isopropyl carbonate, t-butylperoxy-2-ethylhexylcarbonate, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, t-amyl-peroxybenzoate, t-butylproxy acetate, t-butylperoxy isononanoate, 2,2-di(t-butylperoxy)butane, t-butylperoxy benzoate, and the like. Preferable examples thereof include dilauroyl peroxide, t-butylperoxy isopropyl carbonate, t-butyl proxy acetate, t-butylperoxy isononanoate, t-butylperoxy-2-ethylhexyl carbonate, t-butylperoxy benzoate, and the like.
The encapsulating material for solar cell of the embodiment contains at least one crosslinking aid selected from a group consisting of divinyl aromatic compounds, cyanurates, diallyl compounds, triallyl compounds, oximes and maleimides. When encapsulating material for solar cell contains the crosslinking aid, the encapsulating material for solar cell can have an appropriate crosslinking structure, and it is possible to improve crosslinking characteristics, heat resistance, mechanical properties and adhesiveness. The content of the crosslinking aid in the encapsulating material for solar cell of the embodiment is in a range of 0.1 parts by weight to 3 parts by weight, preferably in a range of 0.1 parts by weight to 2.5 parts by weight, and more preferably in a range of 0.2 parts by weight to 2.0 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer.
When the content of the crosslinking aid is equal to or more than 0.1 parts by weight, it is possible to further improve the crosslinking characteristics of the encapsulating material for solar cell. When the content of the crosslinking aid is 3 parts by weight of less, the scorch property is suppressed, no gel is generated during extrusion molding, and the appearance and adhesiveness of the sheet can be improved.
A well-known crosslinking aid that is ordinarily used for an olefin resin can be used as the crosslinking aid. The above-described crosslinking aid is a compound having two or more double bonds in the molecule. Specific examples thereof include divinyl aromatic compounds such as divinyl benzene and di-i-propenyl benzene; cyanurates such as triallyl cyanurate and triallyl isocyanurate; diallyl compounds such as diallyl phthalate; triallyl compounds; oximes such p-quinone dioxime and p,p′-dibenzoyl quinone dioxime, and maleimides such as phenyl maleimide.
Among the above-described crosslinking aids, at least one selected from a group consisting of the triallyl compounds and cyanurates is preferred, and triallyl isocyanurate is particularly preferred due to the generation of air bubbles or the most favorable balance among crosslinking characteristics in the laminated encapsulating material for solar cell.
((Meth)Acrylate-Based Monomer)
The encapsulating material for solar cell of the embodiment contains the (meth)acrylate-based monomer, and the content of the (meth)acrylate-based monomer is in a range of 0.1 parts by weight to 5.0 parts by weight, preferably in a range of 0.1 parts by weight to 4.5 parts by weight, and particularly preferably in a range of 0.2 parts by weight to 3.0 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer. When the content of the (meth)acrylate-based monomer is equal to or more than 0.1 parts by weight, the cured encapsulating material for solar cell has a favorable crosslinking structure, and the volume resistivity improves. Particularly, in a temperature range that is equal to or higher than the melting point of the ethylene/α-olefin copolymer, such as 100° C., the movement of molecular chains is suppressed, and the volume resistivity tends to improve. When the content of the (meth)acrylate-based monomer is equal to or less than 5.0 parts by weight, there is a tendency that the scorch property can be decreased, a gel is suppressed during extrusion molding, and the degradation of the appearance and adhesiveness of the sheet can be suppressed. In addition, it is also possible to suppress a reaction between (meth)acrylate-based monomers, and to improve the adhesiveness or heat resistance of the encapsulating material for solar cell.
Meanwhile, the above-described content in the encapsulating material for solar cell can be analyzed by extracting the (meth)acrylate-based monomer from the encapsulating material for solar cell through solvent extraction and using chromatography such as gas chromatography or liquid chromatography.
A well-known (meth)acrylate-based monomer of the related art can be used as the (meth)acrylate-based monomer. Specific examples thereof include monoacrylates such as t-butyl acrylate, lauryl acrylate, cetyl acrylate, stearyl acrylate, 2-methoxy ethyl acrylate, ethyl carbitol acrylate, methoxy tripropylene glycol acrylate, methoxy polyethylene glycol acrylate, isostearyl acrylate, and 2-acryloyloxy ethyl succinate; monomethacrylates such as t-butyl methacrylate, lauryl methacrylate, cetyl methacrylate, stearyl methacrylate, methoxy ethylene glycol methacrylate, methoxy polyethylene glycol methacrylate, isostearyl methacrylate and 2-methacryloyloxy ethyl succinate; diacrylates such as 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, 9-nonanediol diacrylate, 1,10-decanediol diacrylate, neopentyl glycol diacrylate, diethylene glycol diacrylate, dipropylene glycol diacryalte, triethylene glycol diacrylate, tripropylene glycol diacrylate, tetraethylene glycol diacrylate, tetrapropylene glycol diacrylate, tricyclodecane dimethanol diacrylate, 2-hydroxy-3-acryloyloxy propyl methacrylate, polyethylene glycol diacrylate, polypropylene glycol diacrylate, polytetramethylene glycol diacrylate and glycerine diacrylate; dimethacrylate such as 1,3-butanediol dimethacrylate, 1,6-hexanediol dimethacrylate, 9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, neopentyl glycol dimethacrylate, ethylene glycol dimethacrylate, diehtylene glycol dimethacrylate, triethylene glycol dimethacrylate, dipropylene glycol dimethacrylate, tripropylene glycol dimethacrylate, tricyclodecane dimethanol dimethacrylate, glycerine dimethacrylate, ethoxylated propylene glycol dimathacrylate, polyethylene glycol dimethacrylate, polypropylene glycol dimathacrylate and glycerine dimethacrylate; triacrylates such as ethoxylated glycerine triacrylate, trimethylol propane triacrylate, tetramethylol methane triacrylate and pentaerythritol triacrylate; trimethacrylates such as ethoxylated pentaerythritol tetracrylate, dimethylol propane tetracylate, trimethylol propane trimethacrylate and trimethylol ethane trimethacrylate; tetraacrylates such as pentaerythritol tetraacrylate, tetramethylol methane tetraacrylate and ditrimethylol propane tetracrylate; tetramethacrylates such as pentaerythritol tetramethacrylate, tetrmethylol methane tetramethacrylate and ditrimethylol propane tetramethacylate; polyfunctional acrylates such as dipentaerythritol polyacrylate and dipentaerythritol hexacrylate; polyfunctional methacrylates such as dipentaerythritol polymethacrylate and dipentaerythritol hexamethacrylate; and the like. The above-described (meth)acrylate-based monomer may be solely used, or two or more thereof may be jointly used.
Among the above-described (meth)acrylate-based monomers, at least one selected from a group consisting of mono(meth)acrylate, di(meth)acrylate, tri(meth)acrylate and tetra(meth)acrylate is preferred, and at least one selected from a group consisting of methoxy polyethylene glycol (meth)acrylate, stearyl(meth)acrylate, glycerine di(meth)acrylate, neopentyl glycol di(meth)acrylate trimethylol propane tri(meth)acrylate, and ditrimethylol propane tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate is more preferred.
The molecular weight of the (meth)acrylate-based monomer is preferably equal to or more than 150, and more preferably equal to or more than 180. In addition the molecular weight of the (meth)acrylate-based monomer is not particularly limited, but is preferably equal to or less than 2000, and more preferably equal to or less than 1500.
In addition, the total content of the crosslinking aid and the (meth)acrylate-based monomer is preferably in a range of 0.2 parts by weight to 5.0 parts by weight, more preferably in a range of 0.2 parts by weight to 4.5 parts by weight, and still more preferably in a range of 0.3 parts by weight to 3.0 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer.
The graft amount of the (meth)acrylate-based monomer in the crosslinking-treated sheet of the encapsulating material for solar cell can be obtained through the IR measurement of a sheet piece. When the absorption by other additives such as the ethylenic unsaturated silane compound overlaps the absorption band of a (meth)acrylate group, the graft amount of the (meth)acrylate-based monomer in the crosslinking-treated sheet of the encapsulating material for solar cell may be obtained in the following manner. The content of silicon atoms is obtained through ICP measurement, and the content of the ethylenic unsaturated silane compound is obtained. Separately, the content of the ethylenic unsaturated silane compound and the calibration curve of the IR absorption intensity have been obtained in advance, and the IR absorption band intensity is subtracted by the content of the ethylenic unsaturated silane compound obtained through ICP measurement, whereby it is possible to compute the resulting amount as the graft amount of the (meth)acrylate-based monomer.
(Ethylenic Unsaturated Silane Compound)
The encapsulating material for solar cell of the embodiment preferably further contains an ethylenic unsaturated silane compound. The content of the ethylenic unsaturated silane compound in the encapsulating material for solar cell of the embodiment is preferably in a range of 0.1 parts by weight to 5 parts by weight, more preferably in a range of 0.1 parts by weight to 4 parts by weight, and particularly preferably in a range of 0.1 parts by weight to 3 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer.
When the content of the ethylenic unsaturated silane compound is equal to or more than 0.1 parts by weight, the adhesiveness improves. On the other hand, when the content of the ethylenic unsaturated silane compound is equal to or less than 5 parts by weight, the balance between the costs and performance of the encapsulating material for solar cell is favorable, it is possible to suppress the addition amount of the organic peroxide for causing a graft reaction of the ethylenic unsaturated silane compound in the ethylene/α-olefin copolymer during the lamination of the solar cell module, and a gel is not easily generated in the encapsulating sheet for solar cell obtained from the resin composition during extrusion sheet molding, and therefore it is possible to suppress an increase in the torque of the extruder and to facilitate sheet molding. In addition, since it is possible to suppress the sheet surface becoming uneven due to a gel-form substance generated in the extruder, it is possible to prevent the degradation of the appearance. In addition, since it is possible to prevent the occurrence of cracking in the sheet when a voltage is applied, a decrease in the dielectric breakdown voltage can be prevented. Furthermore, it is also possible to prevent the degradation of the moisture permeability. In addition, since it is possible to suppress the sheet surface becoming uneven, the adhesiveness to the transparent surface protective member, the cell, the electrode, and the back surface protective member becomes favorable during the lamination process of the solar cell module, and the adhesiveness also improves.
A well-known ethylenic unsaturated silane compound of the related art can be used as the ethylenic unsaturated silane compound, and there is no particular limitation. Specific examples thereof that can be used include vinyltriethoxysilane, vinyltrimethoxysilane, vinyltris(β-methoxyethoxy)silane, γ-glycidoxypropyl trimethoxysilane, γ-aminopropyl triethoxysilane and γ-methacryloxypropyl trimethoxysilane. Preferable examples thereof include γ-glycidoxypropyl methoxysilane, γ-aminopropyl triethoxysilane, γ-methacryloxypropyl trimethoxysilane and vinyltriethoxysilane all of which have favorable adhesiveness.
(Ultraviolet Absorber, Light Stabilizer, and Heat-Resistant Stabilizer)
The encapsulating material for solar cell of the embodiment may contain at least one additive selected from a group consisting of an ultraviolet absorber, a light stabilizer, and a heat-resistant stabilizer. The content of the above-described additives is preferably in a range of 0.005 parts by weight to 5 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer. Furthermore, it is preferable to contain at least two additives selected from the above-described three additives, and it is particularly preferable to contain all three additives. When the blending amount of the additives is within the above-described range, an effect that improves the resistance against a high temperature and a high humidity, the resistance against the heat cycle, weather resistance stability and heat resistance stability is sufficiently ensured, and it is possible to prevent the degradation of the transparency of the encapsulating material for solar cell or the adhesiveness among the transparent surface protective member, the back surface protective member, the cell, the electrode and aluminum, which is preferable.
Specific examples of the ultraviolet absorber that can be used include benzophenone-based ultraviolet absorbers such as 2-hydroxy-4-normal-octyloxylbenzophenone, 2-hydroxy-4-methoxybenzophenone, 2,2-dihydroxy-4-methoxybenzophenone, 2-hydroxy-4-methoxy-4-carboxybenzophenone, and 2-hydroxy-4-n-octoxybenzophenone; benzotriazole-based ultraviolet absorbers such as 2-(2-hydroxy-3,5-di-t-butylphenyl)benzotriazole and 2-(2-hydroxy-5-methylpheyl)benzotriazole; salicyclic acid ester-based ultraviolet absorbers such as phenyl salicylate and p-octyl phenyl salicylate.
Examples of the light stabilizer that can be preferably used include hindered amine-based light stabilizers such as bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate, poly[{6-(1,1,3,3-tetramehtylbutyl)amino-1,3,5-triazine-2,4-diyl} {(2,2,6,6-tetramethyl-4-piperidyl)imino}hexamethylene{(2,2,6,6-t etramethyl-4-piperidyl)imino}]; hindered pyperidine-based compounds, and the like.
Specific examples of the heat-resistant stabilizer include phosphite-based heat-resistant stabilizer such as tris(2,4-di-tert-butylphenyl)phosphite, bis[2,4-bis(1,1-dimethylethyl)-6-methylphenyl]ethylester photsphite, tetraquis(2,4-di-tert-butylphenyl)[1,1-viphenyl]-4,4′-diylbisphosphite and bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite; lactone-based heat-resistant stabilizers such as a reaction product of 3-hydroxy-5,7-di-tert-butyl-furan-2-on and o-xylene; hindered phenol-based heat-resistant stabilizers such as 3,3′,3′′,5,5′,5′′-hexa-tert-butyl-a,a′,a′′-(methylene/2,4,6-toryl)tri-p-cresole, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyphenyl)benzylbenzene, pentaerythritol tetraquis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, and thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]; sulfur-based heat-resistant stabilizers; amine-based heat-resistant stabilizers; and the like. In addition, it is possible to use only one of the above-described heat-resistant stabilizers or a combination of two or more heat-resistant stabilizers. Among the above-described heat-resistant stabilizers, the phosphite-based heat-resistant stabilizers and the hindered phenyl-based heat-resistant stabilizers are preferred.
(Other Additives)
The ethylenic resin composition configuring the encapsulating material for solar cell can appropriately contain a variety of components other than the components described above in detail within the scope of the purpose of the invention. For example, other than the ethylene/α-olefin copolymer, a variety of polyolefins, styrene-based or ethylene-based block copolymers, propylene-based polymers, and the like can be contained. The content of a variety of components in the encapsulating material for solar cell is preferably in a range of 0.0001 parts by weight to 50 parts by weight, and more preferably in a range of 0.001 parts by weight to 40 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer. In addition, it is possible to appropriately contain one or more additives selected from a variety of resins other than polyolefins and/or a variety of rubbers, a plasticizer, a filler, a pigment, a dye, an antistatic agent, an antimicrobial agent, an antifungal agent, a flame retardant, a crosslinking aid, a dispersant and the like.
(Encapsulating Material for Solar Cell)
The encapsulating material for solar cell of the embodiment is excellent in terms of the electrical characteristics, is also excellent in terms of the balance among the adhesiveness to a variety of solar cell members such as the transparent surface protective member, the back surface protective member, a thin film electrode, aluminum and the solar cell element, heat resistance, extrusion moldability and crosslinking characteristics, and furthermore, is excellent in terms of transparency, flexibility, appearance, weather resistance, electrical insulating properties, moisture permeability, electrode corrosion properties and process stability. Therefore, the encapsulating material for solar cell of the embodiment is preferably used as an encapsulating material for solar cell for a well-known solar cell module of the related art.
A preferable embodiment of the encapsulating material for solar cell has a sheet shape as the entire shape. In addition, an encapsulating material for solar cell complexed with other layers including at least one sheet made of the above-described resin composition can also be preferably used. The thickness of the encapsulating material for solar cell layer is generally in a range of 0.01 mm to 2 mm, preferably in a range of 0.05 mm to 1.5 mm, more preferably in a range of 0.1 mm to 1.2 mm, still more preferably in a range of 0.2 mm to 1 mm, particularly preferably in a range of 0.3 mm to 0.9 mm, and most preferably in a range of 0.3 mm to 0.8 mm. When the thickness is within the above-described range, it is possible to suppress the breakage of the transparent surface protective member, a solar cell element, a thin film electrode and the like during the lamination step and to sufficiently ensure light transmittance, thereby obtaining a great light power generation amount. Furthermore, the lamination molding of the solar cell module at a low temperature is possible, which is preferable.
(Volume Resistivity)
The volume resistivity of the encapsulating layer formed by crosslinking the encapsulating material for solar cell of the embodiment, which is based on JIS K6911 and measured at a temperature of 100° C. with an applied voltage of 500 V, is preferably in a range of 1.0×1015 Ω·cm to 1.0×1018 Ω·cm. The encapsulating material for solar cell having a small volume resistivity tends to have a characteristic that easily allows the occurrence of PID. Furthermore, while sunlight is being radiated, since there is a case in which the module temperature reaches, for example, 70° C. or higher in a solar cell module of the related art, the volume resistivity is required under a high-temperature condition rather than at room temperature (23° C.) which has been reported in the related art from the viewpoint of long-term reliability, and the volume resistivity at a temperature of 100° C. becomes important.
The volume resistivity is preferably in a range of 1.0×1015 Ω·cm to 1.0×1018 Ω·cm, and more preferably in a range of 5.0×1015 Ω·cm to 1.0×1018 Ω·cm.
When the volume resistivity is less than 1.0×1015 Ω·cm, the PID phenomenon tends to occur within a short period of time (approximately one day) in a constant temperature and humidity test at 85° C. and 85% rh. When the volume resistivity is more than 1.0×1018 Ω·cm, there is a tendency that static electricity is generated in the sheet, trash is easily adsorbed to the sheet, the trash is interfused into the solar cell module, and the degradation of the power generation efficiency or the long-term reliability is caused.
The volume resistivity is measured after the encapsulating material is molded into a sheet, and the sheet is crosslinked and processed into a flat sheet in a vacuum laminator, a hot press, a crosslinking furnace or the like. In addition, for a sheet in the module laminate, the volume resistivity is measured after other layers are removed.
The encapsulating material for solar cell of the embodiment is capable of suppressing a gel-form foreign substance being generated in a crosslinked sheet and satisfying the above-described volume resistivity.
An ordinarily-used method can be used as a method for manufacturing the encapsulating material for solar cell of the embodiment, but the encapsulating material for solar cell is preferably manufactured through melt blend using a kneader, a Banbury mixer, an extruder or the like. Particularly, the encapsulating material for solar cell is preferably manufactured using an extruder capable of continuous production.
A method for molding a sheet of the encapsulating material for solar cell is not particularly limited, and a variety of well-known molding methods (cast molding, extrusion sheet molding, inflation molding, injection molding, compression molding and the like) can be employed. Particularly, when an ethylene-based resin composition obtained by manually blending the ethylene/α-olefin copolymer, the organic peroxide, at least one crosslinking aid selected from a group consisting of divinyl aromatic compounds, cyanurates, diallyl compounds, triallyl compounds, oximes and maleimides, the (meth)acrylate-based monomer, if necessary, the enthylenic unsaturated silane compound, the ultraviolet absorber, the light stabilizer, the heat-resistant stabilizer and other additives in a bag such as a plastic bag or blending them using a stirring and mixing machine such as a Henschel mixer, a tumbler or a super mixer, is injected into an extrusion sheet molding hopper, and extrusion sheet molding is carried out while melting and stirring the mixture, thereby obtaining a sheet-shaped encapsulating material for solar cell, it is possible to improve the adhesiveness and to prevent the deterioration of the light stabilizer so as to prevent degrade of the long-term reliability such as weather resistance or heat resistance, which is more preferable.
The extrusion temperature is preferably in a range of 100° C. to 130° C. When the extrusion temperature is 100° C. or higher, the productivity of the encapsulating material for solar cell can be improved. When the extrusion temperature is 130° C. or lower, gelatinization does not easily occur when the ethylenic resin composition is made into a sheet using an extruder so as to obtain an encapsulating material for solar cell. Therefore, it is possible to prevent an increase in the torque of the extruder and to facilitate sheet molding. In addition, since the sheet surface does not easily become uneven, it is possible to prevent the degradation of the appearance. In addition, since it is possible to suppress the occurrence of cracking in the sheet when a voltage is applied, a decrease in the dielectric breakdown voltage can be prevented. Furthermore, it is also possible to suppress the degradation of the moisture permeability. In addition, since the sheet surface does not easily become uneven, the adhesion to the transparent surface protective member, the cell, the electrode, and the back surface protective member becomes favorable during the lamination process of the solar cell module, and the adhesiveness is excellent.
In addition, in a case in which MFR of the ethylene/α-olefin copolymer is, for example, equal to or less than 10 g/10 minutes, it is also possible to obtain a sheet-shaped encapsulating material for solar cell by carrying out calendar molding using a calendar molder in which a molten resin is rolled using a metal roll (calendar roll) so as to produce a sheet or film having a desired thickness while melting and kneading the ethylene/α-olefin copolymer, the organic peroxide, at least one crosslinking aid selected from a group consisting of divinyl aromatic compounds, cyanurates, diallyl compounds, triallyl compounds, oximes and maleimides, the (meth)acrylate-based monomer, if necessary, the ethylenic unsaturated silane compound, the ultraviolet absorber, the light stabilizer, the heat-resistant stabilizer, and other additives. A variety of well-known calendar molders can be used as the calendar molder, and it is possible to use a mixing roll, a three roll calendar or a four roll calendar. Particularly, I-type, S-type, inverse L-type, Z-type, inclined Z-type calendar rolls can be used as the four roll calendar. In addition, the ethylene-based resin composition is preferably heated to an appropriate temperature before being applied to the calendar roll, and it is also one of preferable embodiments to install, for example, a Banbury mixer, a kneader, an extruder or the like. Regarding the temperature range for the calendar molding, the roll temperature is preferably set in a range of, ordinarily, 40° C. to 100° C.
In addition, the surface of the sheet (or the layer) of the encapsulating material for solar cell may be embossed. When the sheet surface of the encapsulating material for solar cell is decorated through an embossing process, it is possible to prevent the blocking between the sheets of the encapsulating material for solar cell or between the sheet of the encapsulating material for solar cell and other sheets. Furthermore, since embosses decreases the storage elastic modulus of the encapsulating material for solar cell (the encapsulating sheet for solar cell), the embosses serve as cushions for the solar cell element and the like during the lamination of the encapsulating sheet for solar cell and the solar cell element, and the breakage of the solar cell element can be prevented.
The porosity P (%), which is expressed by the percentage ratio VH/VA×100 of the total volume VH of the concave portions per the unit area of a sheet of the encapsulating material for solar cell to the apparent volume VA of the sheet of the encapsulating material for solar cell is preferably in a range of 10% to 50%, more preferably in a range of 10% to 40%, and still more preferably in a range of 15% to 40%. Meanwhile, the apparent volume VA of the sheet of the encapsulating material for solar cell can be obtained by multiplying the unit area by the maximum thickness of the encapsulating sheet for solar cell.
When the porosity P is equal to or more than 10%, it is possible to sufficiently decrease the elastic modulus of the encapsulating material for solar cell, and therefore it is possible to obtain sufficient cushion properties. Therefore, when the lamination process (pressurization step) is carried out in two phases in a step for manufacturing the module, it is possible to prevent the cracking in a silicon cell or a solder fixing a silicon cell and an electrode in a crystalline solar cell and to prevent the cracking of a silver electrode in a thin film-based solar cell. That is, when the porosity of the encapsulating material for solar cell including the sheet made of the ethylene-based resin composition is equal to or more than 10%, convex portions to which the pressure is added are deformed so as to break even when there is a case in which a pressure is locally added to the encapsulating material for solar cell. Therefore, during the lamination process, for example, even when a large pressure is locally added to the silicon cell or the like, it is possible to prevent the cracking of the silicon cell. In addition, when the porosity of the encapsulating material for solar cell is equal to or more than 10%, an air ventilation path can be ensured, and therefore it is possible to favorably exhaust air during the lamination process. Therefore, it is possible to prevent air from remaining in the solar cell module so as to deteriorate the appearance and to prevent the electrode from being corroded due to moisture in the remaining air when the solar cell module is used for a long period of time. Furthermore, since the number of pores generated in the flowed crosslinkable resin composition is decreased during the lamination, the crosslinkable resin composition does not squeeze outside a variety of adherends that are attached to the solar cell module so as to prevent the contamination of the laminator.
On the other hand, when the porosity P is equal to or less than 50%, air can be favorably exhausted during the pressurization in the lamination process, and therefore it is possible to prevent air from remaining in the solar cell module. Therefore, the deterioration of the appearance of the solar cell module is prevented, and the electrode is not corroded due to moisture in the remaining air when the solar cell module is used for a long period of time. In addition, since it is possible to favorably exhaust air during the pressurization in the lamination process, the attaching area between the encapsulating material for solar cell and the adherends to be attached increases, and a sufficient adhering strength can be obtained.
The porosity P can be obtained through the following calculation. The apparent volume VA (mm3) of the embossed encapsulating material for solar cell is computed through the product of the maximum thickness tmax (mm) and unit area (for example, 1 m2=1000=mm×1000 mm=106=mm2) of the encapsulating material for solar cell as described in the following equation (3).

VA(mm3)=tmax(mm)×106(mm2)  (3)

Meanwhile, the actual volume V0 (mm3) of the unit area of the encapsulating material for solar cell is computed by applying the specific weight ρ (g/mm3) of a resin configuring the encapsulating material for solar cell and the actual weight W (g) of the encapsulating material for solar cell per unit area (1 m2) to the following equation (4).

V0(mm3)=W/ρ  (4)

The total volume VH (mm3) of the concave portions per the unit area of the encapsulating material for solar cell is computed by subtracting the “actual volume V0” from the “apparent volume VA of the encapsulating material for solar cell” as described in the following equation (5).

VH(mm3)=VA−V0=VA−(W/ρ)  (5)

Therefore, the porosity (%) can be obtained in the following manner.




  
    
      
        
          Porosity
          ⁢
          
            
          
          ⁢
          
            P
            ⁡
            
              (
              %
              )
            
          
        
        =
        
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              V
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              A
            
          
          ×
          100
        
      
    
  
  
    
      
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              (
              
                
                  V
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        =
        
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          1
          -
          
            
              W
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                    A
                  
                
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        =
        
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                    max
                  
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The porosity (%) can be obtained using the above-described equation, and can also be obtained by photographing a cross-section or embossed surface of an actual encapsulating material for solar cell using a microscope and then processing the image or the like.
The depth of the concave portion formed through the embossing process is preferably in a range of 20% to 95%, more preferably in a range of 50% to 95%, and particularly preferably in a range of 65% to 95% of the maximum thickness of the encapsulating material for solar cell. There is a case in which the percentage ratio of the depth D of the concave portion to the maximum thickness tmax of the sheet is called the “depth ratio” of the concave portion.
The depth of the concave portion by the embossing process indicates the depth difference D between the top portion of the convex portion and the bottom portion of the concave portion in the uneven surface of the encapsulating material for solar cell formed through the embossing process. In addition, the maximum thickness tmax of the encapsulating material for solar cell indicates the distance from the top portion of the convex portion on one surface to the other surface (in the thickness direction of the encapsulating material for solar cell) in a case in which only one surface of the encapsulating material for solar cell is embossed, and indicates the distance from the top portion of the convex portion on one surface to the bottom portion of the proportion portion on the other surface (in the thickness direction of the encapsulating material for solar cell) in a case in which both surfaces of the encapsulating material for solar cell are embossed.
The embossing process may be carried out on a single surface or on both surfaces of the encapsulating material for solar cell. In a case in which the depth of the concave portion through the embossing process is set to be large, the concave portions are preferably formed only on a single surface of the encapsulating material for solar cell. In a case in which the embossing process is carried out only on a single surface of the encapsulating material for solar cell, the maximum thickness tmax of the encapsulating material for solar cell is in a range of 0.01 mm to 2 mm, preferably in a range of 0.05 mm to 1 mm, more preferably in a range of 0.1 mm to 1 mm, still more preferably in a range of 0.15 mm to 1 mm, still more preferably in a range of 0.2 mm to 1 mm, still more preferably in a range of 0.2 mm to 0.9 mm, particularly preferably in a range of 0.3 mm to 0.9 mm, and most preferably in a range of 0.3 mm to 0.8 mm. When the maximum thickness tmax of the encapsulating material for solar cell is within the above-described range, it is possible to suppress the breakage of the transparent surface protective member, the solar cell element, the thin film electrode and the like, and to laminate-mold the solar cell module at a relative low temperature, which is preferable. In addition, the encapsulating material for solar cell is capable of ensuring a sufficient light transmittance, and the solar cell module for which the above-described encapsulating material for solar cell is used has a high light transmittance.
Furthermore, a sheet thereof can be used as an encapsulating material for solar cell in a leaflet form that has been cut in accordance with the size of the solar cell module or in a roll form that can be cut in accordance with the size immediately before the solar cell module is produced. The sheet-shaped encapsulating material for solar cell (the encapsulating sheet for solar cell) which is a preferable embodiment of the embodiment needs to have at least one layer made of the encapsulating material for solar cell. Therefore, the number of the layers made of the encapsulating material for solar cell of the embodiment may be one or multiple. The number of the layers made of the encapsulating material for solar cell of the invention is preferably one since the structure can be simplified so as to decrease the cost, the interface reflection between layers is extremely decreased, and light is effectively used.
The sheet of the encapsulating material for solar cell may be configured only of layers made of the encapsulating material for solar cell of the embodiment, or may have layers other than a layer containing the encapsulating material for solar cell (hereinafter, also referred to as “other layers”). The other layers can be classified based on the purposes into, for example, a hard coat layer for protecting the front surface or the back surface, an adhering layer, an antireflection layer, a gas barrier layer, an anti-contamination layer, and the like. The other layers can be classified based on the material into, for example, an ultraviolet-curable resin layer, a thermosetting resin layer, a polyolefin resin layer, a carboxylic acid-modified polyolefin resin layer, a fluorine-containing resin layer, a cyclic olefin (co) polymer layer, an inorganic compound layer, and the like.
There is no particular limitation with the positional relationship between the layer made of the encapsulating material for solar cell of the embodiment and the other layers, and a preferable layer configuration is appropriately selected in consideration of the relationship with the object of the invention. That is, the other layers may be provided between two or more layers made of the encapsulating material for solar cell, may be provided on the outermost layer of the encapsulating sheet for solar cell, or may be provided at locations other than what has been described above. In addition, the other layers may be provided only on a single surface or on both surfaces of the layer made of the encapsulating material for solar cell. The number of the other layers is not particularly limited, and an arbitrary number of other layers may be provided or no other layer may be provided.
To simplify the structure so as to decrease the cost, extremely decrease the interface reflection between layers, and effectively use light, it is preferable to produce the encapsulating sheet for solar cell only with a layer made of the encapsulating material for solar cell of the embodiment without providing the other layers. However, when there are layers necessary or useful in terms of purpose, the other layers may be provided as appropriate. In a case in which the other layers are provided, there is no particular limitation with a method for laminating a layer made of the encapsulating material for solar cell of the embodiment and the other layers, but a method in which layers are co-extruded using a well-known melt extruder such as a casting molder, an extrusion sheet molder, an inflation molder or an injection molder, thereby obtaining a laminate or a method in which one layer is melted or laminated by heating on the other layer that has been formed in advance, thereby obtaining a laminate is preferred. In addition, the layers may be laminated using a dry laminate method, a heat laminate method or the like in which an appropriate adhesive (for example, a maleic acid anhydride-modified polyolefin resin (product name “ADOMER (registered trademark)” manufactured by Mitsui Chemicals, Inc.), “MODIC (registered trademark)” manufactured by Mitsubishi Chemical Corporation or the like), a low (non) crystalline soft polymer such as an unsaturated polyolefin, an acrylic adhesive including an ethylene/acrylic acid ester/maleic acid anhydride-ternary copolymer (trade name “BONDINE (registered trademark)” manufactured by Sumika CdF Chemical Company Limited.), an ethylene/vinyl acetate-based copolymer, an adhesive resin composition containing what has been described above, or the like) is used. An adhesive having heat resistance in a range of approximately 120° C. to 150° C. is preferably used as the adhesive, and preferable examples thereof include polyester-based adhesives, polyurethane-based adhesive, and the like. In addition, to improve the adhesiveness between both surfaces, for example, a silane-based coupling treatment, a titanium-based coupling treatment, a corona treatment, a plasma treatment or the like may be used.
2. Regarding the Solar Cell Module
An example of the solar cell module is a crystalline solar cell module obtained by, generally, sandwiching a solar cell element formed of polycrystalline silicon or the like using the encapsulating sheet for solar cell, laminating the solar cell element, and furthermore, covering the front and back surfaces with protective sheets. That is, a typical solar cell module has a configuration of a protective sheet for the solar cell module (a transparent surface protective member)/the encapsulating material for solar cell/the solar cell element/the encapsulating material for solar cell/a protective sheet for the solar cell module (a back surface protective member). However, the solar cell module which is one of the preferable embodiments of the invention is not limited to the above-described configuration, and some of the above-described layers can be appropriately removed or layers other than the above-described layers can be appropriately provided within the scope of the object of the invention. Examples of layers other than the above-described layers include an adhering layer, a shock absorbing layer, a coating layer, an anti-reflection layer, a back surface re-reflection layer and a light diffusion layer. The above-described layers are not particularly limited, and can be provided at appropriate locations in consideration of the purpose or characteristics of providing the respective layers.
(Crystalline Silicon-Based Solar Cell Module)
FIG. 1 is a cross-sectional view schematically illustrating an embodiment of the solar cell module of the embodiment. Meanwhile, in FIG. 1, an example of the configuration of a crystalline silicon-based solar cell module 20 is illustrated. As illustrated in FIG. 1, the solar cell module 20 includes a plurality of crystalline silicon-based solar cell elements 22 electrically connected through an interconnector 29 and a pair of a transparent surface protective member 24 and a back surface protective member 26 which sandwich the solar cell elements, and an encapsulating layer 28 is loaded between the protective members and a plurality of the solar cell elements 22. The encapsulating layer 28 is obtained by attaching, heating and pressurizing the encapsulating material for solar cell of the embodiment, and is in contact with electrodes formed on the light-incident surfaces and back surfaces of the solar cell elements 22. The electrodes refer to collector members respectively formed on the light-incident surfaces and the back surfaces of the solar cell elements 22, and the electrode includes collector lines, tab-type busbars, a back surface electrode layer and the like.
FIG. 2 is a plan view schematically illustrating a configuration example of the light-incident surface and the back surface of the solar cell element. In FIG. 2, examples of the configurations of a light-incident surface 22A and a back surface 22B of the solar cell element 22 are illustrated. As illustrated in FIG. 2(A), a number of linearly-formed collector lines 32 and tab-type busbars (busbars) 34A which collect charges from the collector lines 32 and are connected to the interconnector 29 (FIG. 1) are formed on the light-incident surface 22A of the solar cell element 22. In addition, as illustrated in FIG. 2(B), a conductive layer (back surface electrode) 36 is formed on the entire back surface 22B of the solar cell element 22, and tab-type busbars (busbars) 34B which collect charges from the conductive layer 36 and are connected to the interconnector 29 (FIG. 1) are formed on the conductive layer. The line width of the collector line 32 is, for example, approximately 0.1 mm; the line width of the tab-type busbar 34A is, for example, in a range of approximately 2 mm to 3 mm; and the line width of the tab-type busbar 34B is, for example, in a range of approximately 5 mm to 7 mm. The thicknesses of the collector line 32, the tab-type busbar 34A and the tab-type busbar 34B are, for example, in a range of approximately 20 μm to 50 μm respectively.
The collector line 32, the tab-type busbar 34A and the tab-type busbar 34B preferably contain highly conductive metal. Examples of the highly conductive metal include gold, silver, copper and the like, and silver, a silver compound, a silver-containing alloy and the like are preferred due to the high conduction property or high corrosion resistance. The conductive layer 36 preferably contains not only highly conductive metal but also a highly light-reflecting component, for example, aluminum since light incident on the light-incident surface is reflected so as to improve the photoelectric conversion efficiency of the solar cell element. The collector line 32, the tab-type busbar 34A, the tab-type busbar 34B, and the conductive layer 36 are formed by applying a coating material of a conductive material containing the above-described highly conductive metal to the light-incident surface 22A or the back surface 22B of the solar cell element 22 to a thickness of a coated film of 50 μm through, for example, screen printing, then, drying the coated film, and, if necessary, baking the coated film at a temperature in a range of, for example, 600° C. to 700° C.
The transparent surface protective member 24 is disposed on the light-incident surface side, and is thus required to be transparent. Examples of the transparent surface protective member 24 include a transparent glass plate, a transparent resin film and the like. On the other hand, the back surface protective member 26 is not required to be transparent, and a material for the back surface protective member is not particularly limited. Examples of the back surface protective member 26 include a glass substrate, a plastic film, and the like, and a glass substrate is preferably used from the viewpoint of durability or transparency.
The solar cell module 20 can be obtained using an arbitrary manufacturing method. The solar cell module 20 can be obtained using a step in which a laminate of the back surface protective member 26, the encapsulating material for solar cell, a plurality of the solar cell elements 22, the encapsulating material for solar cell and the transparent surface protective member 24 laminated in this order is obtained; a step in which the laminate is pressurized and attached using a laminator or the like, and is heated at the same time as necessary; and a step in which, after the above-described steps, a heating treatment is further carried out on the laminate as necessary so as to cure the encapsulating material.
Generally, a collection electrode for extracting generated electricity is disposed in the solar cell element. Examples of the collection electrode include a busbar electrode, a finger electrode, and the like. Generally, the collection electrode is disposed on the front and back surfaces of the solar cell element; however, when the collection electrode is disposed on the light-incident surface, the collection electrode shields light, and therefore there is a problem in that the power generation efficiency decreases.
Recently, to improve the power generation efficiency, it is considered to use a back contact-type solar cell element which is not required to have the collection electrode disposed on the light-incident surface. In an aspect of the back contact-type solar cell element, p-doped regions and n-doped regions are alternately provided on the back surface side which is provided on the opposite side to the light-incident surface of the solar cell element. In another aspect of the back contact-type solar cell element, a p/n junction is formed in a substrate provided with through holes, a doped layer on the front surface (light-incident surface) side is formed on up to the inner walls of the through holes and the periphery portions of the through holes on the back surface side, and a current on the light-incident surface is extracted on the back surface side.
Generally, in a solar cell system, several or several tens of the above-described solar cell modules are connected in series, and are operated at a voltage in a range of 50 V to 500 V for small-scale residential use, and at a voltage in a range of 600 V to 1000 V for large-scale use called a mega solar power generation system. An aluminum frame is used as the outer frame of the solar cell module for the purpose of holding the strength and the like, and an aluminum frame is frequently earthed (grounded) from the viewpoint of safety. As a result, when the solar cell generates power, a voltage difference is generated by power generation between the surface of the transparent surface protective member having a lower electric resistance compared with the encapsulating material and the solar cell element. As a result, for the encapsulating material for solar cell that encapsulates the space between a power generation cell and the transparent surface protective member or the aluminum frame, there is a demand for favorable electrical characteristics such as high electric insulating properties and high resistance.
(Thin Film Silicon-Based (Amorphous Silicon-Based) Solar Cell Module)
The thin film silicon-based solar cell module can be (1) a module in which the transparent surface protective member (a glass substrate)/a thin film solar cell element/an encapsulating layer/the back surface protective member are laminated in this order; (2) a module in which the transparent surface protective member/the encapsulating layer/the thin film solar cell element/the encapsulating layer/the back surface protective member are laminated in this order; or the like. The transparent surface protective member, the back surface protective member, and the encapsulating layer are the same as in the case of the above-described “crystalline silicon-based solar cell module”.
The thin film solar cell element in the (1) aspect includes, for example, a transparent electrode layer/a pin-type silicon layer/a back surface electrode layer in this order. Examples of the transparent electrode layer include semiconductor-based oxides such as In2O3, SnO2, ZnO, Cd2SnO4, ITO (an oxide obtained by adding Sn to In2O3). The back surface electrode layer includes, for example, a silver thin film layer. The respective layers are formed using a plasma chemical vapor deposition (CVD) method or a sputtering method. The encapsulating layer is disposed so as to be in contact with the back surface electrode layer (for example, a silver thin film layer). Since the transparent electrode layer is formed on the transparent surface protective member, the encapsulating layer is not disposed between the transparent surface protective member and the transparent electrode layer in many cases.
The thin film solar cell element in the (2) aspect includes, for example, the transparent electrode layer/the pin-type silicon layer/a metal foil or a metal thin film layer (for example, a silver thin film layer) disposed on a heat-resistant macromolecular film in this order. Examples of the metal foil include a stainless steel foil and the like. Examples of the heat-resistant macromolecular film include a polyimide film and the like. The transparent electrode layer and the pin-type silicon layer are, similar to those in the (1) aspect, formed using the CVD method or the sputtering method. That is, the pin-type silicon layer is formed on the metal foil or the metal thin film layer disposed on the heat-resistant macromolecular film; and furthermore, the transparent electrode layer is formed on the pin-type silicon layer. In addition, the metal thin film layer disposed on the heat-resistant macromolecular film is also formed using the CVD method or the sputtering method.
In this case, the encapsulating layer is disposed between the transparent electrode layer and the transparent surface protective member; and between the metal foil or the heat-resistant macromolecular film and the back surface protective member respectively. As described above, the encapsulating layer obtained from the encapsulating material for solar cell is in contact with the collector line, the busbar for tab attachment, and the electrode such as the conductive layer of the solar cell element. In addition, compared with the crystalline silicon-based solar cell element, since the silicon layer is thinner in the thin film solar cell element in the (2) aspect, the silicon layer is not easily broken due to pressurization during the manufacturing of the solar cell module or the external impact during the operation of the module. Therefore, the flexibility of the encapsulating material for solar cell used for the thin film solar cell module may be lower than that of the encapsulating material for solar cell used for the crystalline silicon-based solar cell module. Meanwhile, since the electrode of the thin film solar cell element is a metal thin film layer as described above, in a case in which the electrode is deteriorated due to corrosion, there is a concern that the power generation efficiency may significantly decrease.
Therefore, the encapsulating material for solar cell of the embodiment which does not necessarily requires a crosslinking agent which has poorer flexibility than the ethylene/vinyl acetate copolymer (EVA) but serves as a generation source of decomposed gas and contains a sheet composed of the ethylene-based resin composition is more preferably used as a sheet of the encapsulating material for solar cell for a thin film solar cell module.
In addition, there is a solar cell module in which silicon is used for the solar cell element as another aspect of the solar cell module. Examples of the solar cell module in which silicon is used for the solar cell element include a hybrid-type (HIT-type) solar cell module in which crystalline silicon and amorphous silicon are laminated, a multi-junction-type (tandem-type) solar cell module in which silicon layers having different absorption wavelength ranges are laminated, a back contact-type solar cell module in which p-doped regions and n-doped regions are alternately provided on the back surface side which is provided on the opposite side to the light-incident surface of the solar cell element, a spherical silicon-type solar cell module in which a number of spherical silicon particles (having a diameter of approximately 1 mm) and a concave mirror (also serving as the electrode) which has a diameter in a range of 2 mm to 3 mm and improves the light-gathering capability are combined together, and the like.
In addition, examples of the solar cell module in which silicon is used for the solar cell element include a field-effect-type solar cell module having a structure in which the role of an amorphous silicon-type p-type window layer having the pin junction structure of the related art is provided to the “inversion layer being induced by the field effect” instead of the “insulated transparent electrode”, and the like.
In addition, examples thereof include a GaAs-based solar cell module in which monocrystalline GaAs is used for the solar cell element; a CIS or CIGS-based (chalcopyrite-based) solar cell module in which a I-III-VI group compound called a chalcopyrite-based compound made of Cu, In, Ga, Al, Se, S or the like is used as the solar cell element instead of silicon; a CdTe—CdS-based solar cell in which a Cd compound thin film is used as the solar cell element, a Cu2ZnSnS4 (CZTS) solar cell module, and the like. The encapsulating material for solar cell of the embodiment can be used as an encapsulating material for solar cell for all of the above-described solar cell modules.
Particularly, a layer of the encapsulating material laminated below a photovoltaic element configuring the solar cell module is required to have adhesiveness to the layer of the encapsulating material, the electrode, and the back surface protective layer laminated on the top portion of the photovoltaic element. In addition, the encapsulating layer is required to have thermal plasticity to hold the flatness of the back surface of the solar cell element as the photovoltaic element. Furthermore, the encapsulating layer is required to have excellent scratch resistance, shock absorbance and the like to protect the solar cell element as the photovoltaic element.
The layer of the encapsulating material desirably has heat resistance. Particularly, when the solar cell module is manufactured, it is desirable that the ethylene-based resin composition configuring the layer of the encapsulating material not be modified and deteriorated or decomposed due to the heating action in a lamination method or the like in which the encapsulating layer is suctioned in a vacuum, heated and pressurized or the action of heat such as sunlight during the long-term use of the solar cell module and the like. Then, it is possible to prevent the additives and the like contained in the resin from being eluted or prevent decomposed substances from being generated so as to act on the electromotive force surface (the element surface) of the solar cell element, and to prevent the function and performance from being deteriorated. Therefore, the heat resistance is an indispensable characteristic for the layer of the encapsulating material in the solar cell module.
Furthermore, the layer of the encapsulating material preferably has an excellent moisture-proof property. In this case, it is possible to prevent moisture from transmitting from the back surface side of the solar cell module, and to prevent the corrosion and deterioration of the photovoltaic element in the solar cell module.
Unlike the filler layer laminated on the photovoltaic element, the above-described layer of the encapsulating material is not essentially required to have transparency. The encapsulating material for solar cell of the embodiment has the above-described characteristics, and can be preferably used as the encapsulating material for solar cell on the back surface side of the crystalline solar cell module and the encapsulating material for solar cell for the thin film-type solar cell module that is weak against moisture intrusion.
The solar cell module of the embodiment may appropriately include arbitrary members within the scope of the object of the invention. Typically, it is possible to provide an adhering layer, a shock absorbing layer, a coating layer, an anti-reflection layer, a back surface re-reflection layer, a light diffusion layer and the like. The locations at which the above-described layers are provided are not particularly limited, and the layers can be provided at appropriate locations in consideration of the purpose of the provision of the layers and the characteristics of the layers.
(Transparent Surface Protective Member for Solar Cell Module)
There is no particular limitation with the transparent surface protective member for solar cell module used in the solar cell module, but the transparent surface protective member is located on the outermost surface layer of the solar cell module, and thus preferably has performances for ensuring long-term reliability for the outdoor exposure of the solar cell module including weather resistance, water repellency, contamination resistance, and mechanical strength. In addition, the transparent surface protective member is preferably a sheet having a small optical loss and high transparency for the effective use of sunlight.
Examples of a material for the transparent surface protective member for solar cell module include a resin film made of a polyester resin, a fluorine resin, an acryl resin, a cyclic olefin (co)polymer, an ethylene/vinyl acetate copolymer or the like, a glass substrate, and the like. The resin film is preferably a polyester resin having excellent transparency, strength, cost and the like, and particularly preferably a polyethylene terephthalate resin, a fluorine resin having favorable weather resistance, or the like. Examples of the fluorine resin include an ethylene/tetrafluoroethylene copolymer (ETFE), a polyvinyl fluoride resin (PVF), a polyvinylidene fluoride resin (PVDF), a polytetrafluoroethylene resin (PTFE), a fluorinated ethylene/propylene copolymer (FEP) and a poly trifluorochloroethylene resin (PCTFE). The polyvinylidene fluoride resin is excellent from the viewpoint of weather resistance, and the ethylene-tetrafluoroethylene copolymer is excellent in terms of satisfying both weather resistance and mechanical strength. In addition, to improve the adhesiveness to materials configuring other layers such as an encapsulating material layer, it is desirable to carryout a corona treatment and a plasma treatment on the transparent surface protective member. In addition, it is also possible to use a sheet that has been subjected to a stretching treatment, for example, a biaxially stretched polypropylene sheet to improve the mechanical strength.
In a case in which a glass substrate is used as the transparent surface protective member for solar cell module, the total light transmittance of the glass substrate with respect to light having a wavelength in a range of 350 nm to 1400 nm is preferably equal to or more than 80%, and more preferably equal to or more than 90%. It is usual to use as the glass substrate a super white glass plate that only slightly absorbs the infrared region, but a blue glass plate has a small influence on the output characteristics of the solar cell module when the blue glass plate has a thickness of equal to or less than 3 mm. In addition, it is possible to obtain reinforced glass through a thermal treatment to increase the mechanical strength of the glass substrate, but a float glass plate that has not been subjected to a thermal treatment may be used. In addition, the light-incident surface side of the glass substrate may be coated for antireflection to suppress reflection.
(Back Surface Protective Member for Solar Cell Module)
There is no particular limitation with the back surface protective member for solar cell module used in the solar cell module, but the back surface protective member is located on the outermost surface layer of the solar cell module, and thus, similar to the above-described transparent surface protective member, is required to have a variety of characteristics such as weather resistance and mechanical strength. Therefore, the back surface protective member for solar cell module may be configured of the same material as for the transparent surface protective member. That is, a variety of the above-described materials used as the material for the transparent surface protective member can also be used as a material for the back surface protective member. Particularly, it is possible to preferably use a polyester resin and glass. Since the back surface protective member is not required to allow the penetration of sunlight, transparency required by the transparent surface protective member is not essentially required. Therefore, a reinforcement plate may be attached to increase the mechanical strength of the solar cell module or to prevent strain and warpage caused by the temperature change. Examples of the reinforcement plate that can be preferably used include a steel plate, a plastic plate, a fiber reinforced plastic (FRP) plate, and the like.
Furthermore, the encapsulating material for solar cell of the embodiment may be integrated with the back surface protective member for solar cell module. When the encapsulating material for solar cell and the back surface protective member for solar cell module are integrated together, it is possible to shorten a step for cutting the encapsulating material for solar cell and the back surface protective member for solar cell module to a module size when the module is assembled. In addition, when a step in which the encapsulating material for solar cell and the back surface protective member for solar cell module are separately laid up is replaced by a step in which the encapsulating material for solar cell and the back surface protective member are laid up as an integrated sheet, it is also possible to shorten and remove the lay-up step. In a case in which the encapsulating material for solar cell and the back surface protective member for solar cell module are integrated together, there is no particular limitation with the method for laminating the encapsulating material for solar cell and the back surface protective member for solar cell module. Preferable examples of the lamination method include a method in which the encapsulating material for solar cell and the back surface protective member are co-extruded using a well-known melt extruder such as a casting molder, an extrusion sheet molder, an inflation molder or an injection molder, thereby obtaining a laminate; and a method in which one layer is melted or laminated by heating on the other layer that has been previously molded, thereby obtaining a laminate.
In addition, the encapsulating material for solar cell and the back surface protective member may be laminated using a dry laminate method, a heat laminate method or the like in which an appropriate adhesive (for example, a maleic acid anhydride-modified polyolefin resin (product name “ADOMER (registered trademark)” manufactured by Mitsui Chemicals, Inc.), “MODIC (registered trademark)” manufactured by Mitsubishi Chemical Corporation or the like), a low (non) crystalline soft polymer such as an unsaturated polyolefin, an acrylic adhesive including an ethylene/acrylic acid ester/maleic acid anhydride-ternary copolymer (trade name “BONDINE (registered trademark)” manufactured by Sumika CdF Chemical Company Limited.), an ethylene/vinyl acetate-based copolymer, an adhesive resin composition containing what has been described above, or the like) is used.
An adhesive having heat resistance in a range of approximately 120° C. to 150° C. is preferably used as the adhesive, and preferable examples thereof include polyester-based adhesives, and polyurethane-based adhesives. In addition, to improve the adhesiveness between both surfaces, for example, a silane-based coupling treatment, a titanium-based coupling treatment, a corona treatment, a plasma treatment or the like may be used.
(Solar Cell Element)
There is no particular limitation with the solar cell element used in the solar cell module as long as the solar cell element is capable of generating power using a photovoltaic effect of a semiconductor. As the solar cell element, for example, a silicon (monocrystalline silicon, polycrystalline silicon or non-crystalline (amorphous) silicon) solar cell, a compound semiconductor group, II-VI group, or the like) solar cell, a wet-type solar cell, an organic semiconductor solar cell or the like can be used. Among the above-described solar cells, the polycrystalline silicon solar cell is preferred from the viewpoint of the balance between power generation performance and cost.
Both the silicon solar cell element and the compound semiconductor solar cell element have excellent characteristics as the solar cell element, but it is known that both solar cell elements are easily broken due to external stress, impact and the like. Since the encapsulating material for solar cell of the embodiment has excellent flexibility, the encapsulating material for solar cell has a great effect of absorbing the stress, impact and the like against the solar cell element so as to prevent the breakage of the solar cell element. Therefore, in the solar cell module of the embodiment, layers made of the encapsulating material for solar cell of the embodiment are desirably indirect contact with the solar cell element. In addition, when the encapsulating material for solar cell has thermal plasticity, it is possible to relatively easily remove the solar cell element even after the solar cell module has been produced, and the solar cell element obtains excellent recycling properties. Since the ethylene-based resin composition configuring the encapsulating material for solar cell of the embodiment has thermal plasticity, the encapsulating material for solar cell also has thermal plasticity as a whole, which is also preferable from the viewpoint of recycling properties.
(Electrode)
There is no particular limitation with the configuration and material of an electrode used in the solar cell module; however, in a specific example, the electrode has a laminate structure of a transparent conductive film and a metal film. The transparent conductive film is made of SnO2, ITO, ZnO or the like. The metal film is made of metal such as silver, gold, copper, tin, aluminum, cadmium, zinc, mercury, chromium, molybdenum, tungsten, nickel or vanadium. The metal film may be solely used or may be used in a form of a complexed alloy. The transparent conductive film and the metal film are formed using a method such as CVD, sputtering or deposition.
(Method for Manufacturing the Solar Cell Module)
A method for manufacturing the solar cell module of the embodiment includes (i) a step in which the transparent surface protective member, the encapsulating material for solar cell of the embodiment, the solar cell element (cell), the encapsulating material for solar cell, and the back surface protective member are laminated in this order, thereby forming a laminate, and (ii) a step in which the obtained laminate is pressurized and heated so as to be integrated.
In the step (i), a surface of the encapsulating material for solar cell on which an uneven shape (emboss shape) is formed is preferably disposed to be on the solar cell element side.
In the step (ii), the laminate obtained in the step (i) is heated and pressurized using a vacuum laminator or a hot press according to an ordinary method so as to be integrated (encapsulated). During the encapsulating, since the encapsulating material for solar cell of the embodiment has a high cushion property, it is possible to prevent the damage of the solar cell element. In addition, since the encapsulating material for solar cell of the embodiment has favorable deaeration properties, air is not trapped, and it is possible to manufacture high-quality products with a favorable yield.
When the solar cell module is manufactured, the ethylene/α-olefin-based resin composition configuring the encapsulating material for solar cell is cured through crosslinking. The crosslinking step may be carried out at the same time as the step (ii) or after the step (ii).
In a case in which the crosslinking step is carried out after the step (ii), the laminate is heated and pressurized in a vacuum for three to six minutes under conditions of a temperature in a range of 125° C. to 160° C. and a vacuum pressure of equal to or less than 10 Torr; and then, pressurization by the atmospheric pressure is carried out for approximately 1 minute to 15 minutes, thereby integrating the laminate. The crosslinking step carried out after the step (ii) can be carried out using an ordinary method, and, for example, a tunnel-type continuous crosslinking furnace may be used, or a tray-type batch crosslinking furnace may be used. In addition, the crosslinking conditions are generally a temperature in a range of 130° C. to 155° C. for approximately 20 minutes to 60 minutes.
Meanwhile, in a case in which the crosslinking step is carried out at the same time as the step (ii), it is possible to carry out the crosslinking step in the same manner as the case in which the crosslinking step is carried out after the step (ii) except for the fact that the heating temperature in the step (ii) is set in a range of 145° C. to 170° C. and the pressurization time by the atmospheric pressure is set in a range of 6 minutes to 30 minutes. Since the encapsulating material for solar cell of the embodiment contains the specific organic peroxide so as to have excellent crosslinking characteristics, the solar cell module does not need to pass through two phases of an adhering step in the step (ii), is capable of being completed at a high temperature for a short period of time, the crosslinking step carried out after the step (ii) may be removed, and it is possible to significantly improve the productivity of the module.
In any case, the solar cell module of the embodiment may be manufactured by temporarily adhering the encapsulating material for solar cell to the solar cell element or a protection material at a temperature at which a crosslinking agent is not decomposed and the encapsulating material for solar cell of the embodiment is melted, and then increasing the temperature, thereby carrying out sufficient adhering and the crosslinking of the encapsulating material. An additive prescription with which a variety of conditions can be satisfied may be selected, and, for example, the type and impregnation amount of the above-described crosslinking agent and the above-described crosslinking aid may be selected.
In addition, the above-described crosslinking is preferably carried out until the gel fraction of the crosslinked ethylene/α-olefin copolymer becomes 50% to 95%. The gel fraction is more preferably in a range of 55% to 90%, still more preferably in a range of 60% to 90%, and most preferably in a range of 65% to 90%. The gel fraction can be computed using the following method. For example, 1 g of a sample of an encapsulating material sheet is sampled from the solar cell module, and Soxhlet extraction is carried out for ten hours in boiling toluene. An extraction liquid is filtered using a stainless steel mesh having 30 meshes, and the mesh is depressurized and dried at 110° C. for eight hours. The weight of a residue remaining on the mesh is measured, and the ratio (%) of the weight of the residue remaining on the mash to the sample amount (1 g) before the treatment is considered as the gel fraction.
When the gel fraction is equal to or more than the above-described lower limit value, the heat resistance of the encapsulating material for solar cell becomes favorable, and it is possible to suppress the degradation of the adhesiveness in, for example, a constant temperature and humidity test at 85° C.×85% RH, a high-strength xenon radiation test at a black panel temperature of 83° C., a heat cycle test at a temperature in a range of −40° C. to 90° C., and a heat resistance test. On the other hand, when the gel fraction is equal to or less than the above-described upper limit value, the encapsulating material for solar cell obtains high flexibility, and the temperature followability in the heat cycle test at a temperature in a range of −40° C. to 90° C. improves, and therefore it is possible to prevent the occurrence of peeling.
(Power Generation Facility)
The solar cell module of the embodiment is excellent in terms of productivity, power generation efficiency, service life, and the like. Therefore, a power generation facility using the above-described solar cell module is excellent in terms of cost, power generation efficiency, service life, and the like, and has a high practical value. The above-described power generation facility is preferable for long-term indoor and outdoor use so as to be used as an outdoor mobile power supply for camping and the like, which is installed outside houses, or to be used as an auxiliary power supply for automobile batteries.
EXAMPLES
Hereinafter, the invention will be specifically described based on examples, but the invention is not limited to the examples.
(1) Measurement Method
[The Content Ratio of the Ethylene Unit and the α-Olefin Unit]
After a solution obtained by heating and melting 0.35 g of a specimen in 2.0 ml of hexachlorobutadiene was filtered using a glass filter (G2), 0.5 ml of deuterated benzene was added, and the mixture was injected into an NMR tube having an inner diameter of 10 mm. The 13C-NMR was measured at 120° C. using a JNM GX-400-type NMR measurement device manufactured by JEOL, Ltd. The cumulated number was set to equal to or more than 8000 times. The content ratio of the ethylene unit and the content ratios of the α-olefin unit were determined from the obtained 13C-NMR spectra.
[MFR]
MFR of the ethylene/α-olefin copolymer was measured on the basis of ASTM D1238 under conditions of 190° C. and a load of 2.16 kg.
[Density]
The density of the ethylene/α-olefin copolymer was measured on the basis of ASTM D1505.
[Shore A Hardness]
After the ethylene/α-olefin copolymer was heated at 190° C. for four minutes and pressurized at 10 MPa, the ethylene/α-olefin copolymer was pressurized and cooled at 10 MPa to room temperature for five minutes, thereby obtaining a 3 mm-thick sheet. The shore A hardness of the ethylene/α-olefin copolymer was measured on the basis of ASTM D2240 using the obtained sheet. Meanwhile, the shore A hardness was measured after 15 seconds had elapsed from the application of a load to the test specimen sheet.
[The Content of an Aluminum Element]
After the ethylene/α-olefin copolymer was wet-decomposed, the volume was made to be constant using pure water, the amount of aluminum was determined using an ICP emission spectrometer (ICPS-8100 manufactured by Shimadzu Corporation), and the content of an aluminum element was obtained.
[Volume Resistivity]
After the obtained sheet was cut into a size of 10 cm×10 cm, the sheet was laminated using a lamination apparatus (LM-110×160S manufactured by NPC Corporation) at 150° C. for three minutes in a vacuum and under pressurization for 15 minutes, thereby producing a crosslinked sheet for measurement. The volume resistivity (Ω·cm) of the produced crosslinked sheet was measured at an applied voltage of 500 Von the basis of JIS K6911. Meanwhile, during the measurement, the temperature was set to 100±2° C. using a high-temperature measurement chamber “12708” (manufactured by Advantest Corporation), and a microammeter “R8340A” (manufactured by Advantest Corporation) was used.
[Gel Fraction]
After the obtained sheet was cut into a size of 10 cm×10 cm, the sheet was laminated using a lamination apparatus (LM-110×160S manufactured by NPC Corporation) at 150° C. for three minutes in a vacuum and under pressurization for 15 minutes, thereby producing a crosslinked sheet for measurement. Approximately 1 g of the obtained crosslinking sheet sample was weighed (the weighed value is represented by A (g)), Soxhlet extraction was carried out for ten hours in boiling toluene, an extraction liquid was filtered using a stainless steel mesh having 30 meshes, and the mesh is depressurized and dried at 110° C. for eight hours, the residue amount B (g) on the mesh was computed, and the gel fraction was computed using the following formula.

Gel fraction(weight %)=100×B/A 

(2) The Synthesis of the Ethylene/α-Olefin Copolymer
Synthesis Example 1
A toluene solution of methyl aluminoxane was supplied as a co-catalyst at a rate of 8.0 mmol/hr, a hexane slurry of bis(1,3-dimethylcyclopentadienyl)zirconium dichloride and a hexane solution of triisobutylaluminum were supplied at rates of 0.025 mmol/hr and at 0.5 mmol/hr respectively as main catalysts to one supply opening of a continuous polymerization vessel having stirring blades and an inner volume of 50 L, and normal hexane which was used as a catalyst solution and was dehydrated and purified so that the total of the dehydrated and purified normal hexane became 20 L/hr was continuously supplied. At the same time, ethylene, 1-butene and hydrogen were continuously supplied at rates of 3 kg/hr, 15 kg/hr and 5 NL/hr respectively to another supply opening of the polymerization vessel, and continuous solution polymerization was carried out under conditions of a polymerization temperature of 90° C., a total pressure of 3 MPaG, and a retention time of 1.0 hour. The normal hexane/toluene solution mixture of the ethylene/α-olefin copolymer generated in the polymerization vessel was continuously exhausted through an exhaust opening provided in the bottom portion of the polymerization vessel, and was guided to a coupling pipe in which a jacket portion was heated using 3 kg/cm2 to 25 kg/cm2 steam so that the normal hexane/toluene solution mixture of the ethylene/α-olefin copolymer reached a temperature in a range of 150° C. to 190° C. Meanwhile, a supply opening through which methanol that was a catalyst-devitalizing agent was injected was provided immediately before the coupling pipe, and methanol was injected at a rate of approximately 0.75 L/hr so as to combine with the normal hexane/toluene solution mixture of the ethylene/α-olefin copolymer. The normal hexane/toluene solution mixture of the ethylene/α-olefin copolymer maintained at approximately 190° C. in the steam jacket-equipped coupling pipe was continuously sent to a flash chamber by adjusting the degree of the opening of a pressure control valve provided at the terminal portion of the coupling pipe so as to maintain approximately 4.3 MPaG. Meanwhile, when the normal hexane/toluene solution mixture was sent to the flash chamber, the solution temperature and the degree of the opening of the pressure-adjusting valve were set so that the pressure in the flash chamber was maintained at approximately 0.1 MPaG and the temperature of a vapor portion in the flash chamber was maintained at 180° C. After that, a strand was cooled in a water vessel after passing a single screw extruder in which the die temperature was set to 180° C., and the strand was cut using a pellet cutter, thereby obtaining an ethylene/α-olefin copolymer in a pellet form. The yield was 2.2 kg/hr. The properties are described in Table 1.
Synthesis Example 2
An ethylene/α-olefin copolymer was obtained in the same manner as in Synthesis Example 1 except for the fact that a hexane solution of [dimethyl(t-butylamide)(tetramethyl-η5-cyclopentadienyl)silane]titandichloride was supplied as a main catalyst at a rate of 0.012 mmol/hr, a toluene solution of triphenylcarbenium(tetrakis-pentafluorophenyl)borate and a hexane solution of triisobutylaluminum were supplied at rates of 0.05 mmol/hr and 0.4 mmol/hr respectively as co-catalysts, and 1-butene and hydrogen were supplied at rates of 5 kg/hr and 100 NL/hr respectively. The yield was 1.3 kg/hr. The properties are described in Table 1.









TABLE 1






Synthesis Example 1
Synthesis Example 2







α-olefin type
1-butene
1-butene


Content ratio of
14
17


α-olefin unit [mol %]


Content ratio of
86
83


ethylene unit [mol %]


Density [g/cm3]
0.870
0.866


MFR [g/10 minutes]
20
11


Shore A hardness [—]
70
62


Al residue amount [ppm]
102
8









3) Manufacturing of the Encapsulating Material for Solar Cell (Sheet)
Example 1
0.5 parts by weight of γ-methacryloxypropyltrimethoxy silane as the ethylenic unsaturated silane compound, 1.0 part by weight of t-butylperoxy-2-ethylhexyl carbonate having a one-minute half-life temperature of 166° C. as the organic peroxide, 0.2 parts by weight of methoxypolyethylene glycol acrylate as the (meth)acrylate monomer, 1.3 parts by weight of triallyl isocyanurate as the crosslinking aid, 0.4 parts by weight of 2-hydroxy-4-normal-octyloxybenzophenone as the ultraviolet absorber, 0.2 parts by weight of bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate as the hindered amine-based light stabilizer, 0.1 parts by weight of tris(2,4-di-tert-butylphenyl)phosphite as the heat-resistant stabilizer 1, and 0.1 parts by weight of octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate as the heat-resistant stabilizer 2 were blended with 100 parts by weight of the ethylene/α-olefin copolymer of Synthetic Example 1.
A coat hanger T die (with a lip shape: 270 mm×0.8 mm) was mounted in a single screw extruder (with a screw diameter of 20 mmφ, L/D=28) manufactured by Thermoplastic Company, and molding was carried out at a roll temperature of 30° C. and a winding rate of 1.0 m/min under a condition of a die temperature of 100° C. using an embossing roll as a first cooling roll, thereby obtaining an embossed sheet (a sheet of the encapsulating material for solar cell) having a thickness of 500 μm. The porosity of the obtained sheet was 28%. A variety of evaluation results of the obtained sheet are described in Table 2.
Examples 2 to 4 and Comparative Examples 1 and 3
Embossed sheets (sheets of the encapsulating material for solar cell) were obtained in the same manner as in Example 1 except for the fact that the components were blended as described in Table 2. The porosities of the obtained sheets were all 28%. A variety of evaluation results of the obtained sheets are described in Table 2.
Comparative Example 2
An embossed sheet (a sheet of the encapsulating material for solar cell) was obtained in the same manner as in Example 1 except for the fact that the components were blended as described in Table 2. The porosity of the obtained sheet was 28%. However, the torque of the extruder was higher compared with Example 2, a gel was observed on the surface of the obtained sheet, and the appearance of the sheet deteriorated. A variety of evaluation results of the obtained sheets are described in Table 2.














TABLE 2











Comparative
Comparative



Example 1
Example 2
Example 3
Example 4
Comparative Example 1
Example 2
Example 3






















Blending (parts by weight)









Ethylene/α-


olefin copolymer


Synthetic Example 1
100

100
100


Synthetic Example 2

100


100
100
100


Ethylenic unsaturated
0.5
0.5
0.5
0.5
0.5
0.5
0.5


silane compound


Organic peroxide
1.0
1.0
1.0
1.0
1.0
1.0
1.0


(Meth)acrylate monomer 1
0.2


(Meth)acrylate monomer 2

1.0


(Meth)acrylate monomer 3


0.5


5.3


(Meth)acrylate monomer 4



0.5


Ultraviolet absorber
0.4
0.4
0.4
0.4
0.4
0.4
0.4


Hindered amine-
0.2
0.2
0.2
0.2
0.2
0.2
0.2


based light stabilizer


Crosslinking aid
1.3
0.5
0.5
0.5


1.5


Heat-resistant stabilizer 1
0.1
0.05
0.1
0.1
0.1
0.1
0.05


Heat-resistant stabilizer 2
0.1
0.1
0.1
0.1
0.1
0.1
0.1


Evaluation


Sheet appearance
Favorable
Favorable
Favorable
Favorable
Favorable
Gel included
Favorable


Volume resistivity @ 100° C.
4.5 × 1015
1.2 × 1015
1.2 × 1016
1.5 × 1016
4.5 × 1014
9.1 × 1015
8.5 × 1014


[Ω · cm]


Gel fraction [weight %]
68
60
57
60
25
55
75





(Meth)acrylate monomer 1: methoxypolyethylene glycol acrylate


(Meth)acrylate monomer 2: neopentyl glycol diacrylate


(Meth)acrylate monomer 3: trimethylol propane trimethacrylate


(Meth)acrylate monomer 4: ditrimethylol propane tetramethacrylate






The present application claims priority based on Japanese Patent Application No. 2012-026927 filed on Feb. 10, 2012, the contents of which are incorporated herein by reference.

## Claims

1. An encapsulating material for solar cell containing an ethylene/α-olefin copolymer, an organic peroxide, at least one crosslinking aid selected from a group consisting of divinyl aromatic compounds, cyanurates, diallyl compounds, triallyl compounds, oximes and maleimides, a (meth)acrylate-based monomer, and an ethylenic unsaturated silane compound,
wherein a content of the (meth)acrylate-based monomer in the encapsulating material for solar cell is in a range of 0.1 parts by weight to 5.0 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer,
a content of the crosslinking aid in the encapsulating material for solar cell is in a range of 0.1 parts by weight to 3 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer, and
a content of the ethylenic unsaturated silane compound in the encapsulating material for solar cell is in a range of 0.1 parts by weight to 5 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer.

2. The encapsulating material for solar cell according to claim 1,
wherein a content of an aluminum element in the ethylene/α-olefin copolymer is in a range of 10 ppm to 500 ppm.

3. The encapsulating material for solar cell according to claim 1,
wherein a volume resistivity of an encapsulating layer formed by crosslinking the encapsulating material for solar cell, which is based on JIS K6911 and measured at a temperature of 100° C. with an applied voltage of 500 V, is in a range of 1.0×1015 Ω·cm to 1.0×1018 Ω·cm.

4. The encapsulating material for solar cell according to claim 1,
wherein a molecular weight of the (meth)acrylate-based monomer is equal to or more than 150.

5. The encapsulating material for solar cell according to claim 1,
wherein the ethylene/α-olefin copolymer satisfies the following requirements a1) to a4):
a1) a content ratio of a structural unit derived from ethylene is in a range of 80 mol % to 90 mol %, and a content ratio of a structural unit derived from an α-olefin having 3 to 20 carbon atoms is in a range of 10 mol % to 20 mol %;
a2) MFR, which is based on ASTM D1238 and measured under conditions of a temperature of 190° C. and a load of 2.16 kg, is in a range of 0.1 g/10 minutes to 50 g/10 minutes;
a3) a density, which is measured on the basis of ASTM D1505, is in a range of 0.865 g/cm3 to 0.884 g/cm3; and
a4) a shore A hardness, which is measured on the basis of ASTM D2240, is in a range of 60 to 85.

6. The encapsulating material for solar cell according to claim 5,
wherein MFR of the ethylene/α-olefin copolymer, which is based on ASTM D1238 and measured under conditions of a temperature of 190° C. and a load of 2.16 kg, is in a range of 10 g/10 minutes to 50 g/10 minutes.

7. The encapsulating material for solar cell according to claim 1,
wherein a one-minute half-life temperature of the organic peroxide is in a range of 100° C. to 170° C., and
a content of the organic peroxide in the encapsulating material for solar cell is in a range of 0.1 parts by weight to 3.0 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer.

8. The encapsulating material for solar cell according to claim 1,
wherein the ethylene/α-olefin copolymer is prepared by using a catalyst for olefin polymerization containing at least one compound selected from a group consisting of metallocene compounds, organic aluminumoxy compounds and organic aluminum compounds.

9. The encapsulating material for solar cell according to claim 1, further comprising at least one of additives selected from a group consisting of an ultraviolet absorber, a hindered amine-based light stabilizer and a heat-resistant stabilizer
in which a content of the above-described additives in the encapsulating material for solar cell is in a range of 0.005 parts by weight to 5 parts by weight with respect to 100 parts by weight of the ethylene/α-olefin copolymer.

10. The encapsulating material for solar cell according to claim 1 obtained by melting and kneading the ethylene/α-olefin copolymer, the organic peroxide, the (meth)acrylate-based monomer, and the crosslinking aid, and then molding, by extrusion, the mixture into a sheet shape.

11. The encapsulating material for solar cell according to claim 1 having a sheet shape.

12. A solar cell module comprising:
a transparent surface protective member,
a back surface protective member,
a solar cell element, and
an encapsulating layer which is formed by crosslinking the encapsulating material for solar cell according to claim 1 and encapsulates the solar cell element between the transparent surface protective member and the back surface protective member.