JP5971076B2 - Solar cell module - Google Patents

Description

  The present invention relates to a solar cell module.

  In recent years, solar cells as a clean energy source have attracted attention due to the growing awareness of environmental issues. Currently, various types of solar cell modules have been developed and proposed. Generally, a solar cell module has a configuration in which a transparent front substrate, a solar cell element, and a back surface protection sheet are laminated via a sealing material sheet.

  As illustrated in FIG. 1, the solar cell module 1 includes a transparent front substrate 2, a front sealing material sheet 3, a solar cell element 4, a back sealing material sheet 5, and a back surface protection sheet from the light receiving surface side of incident light. 6 are laminated in order, and the solar cell element 4 is arranged in such a manner that it is sandwiched between the front sealing material sheet 3 arranged on the light receiving surface side and the back sealing material sheet 5 arranged on the back surface side. ing.

  Here, in a state where the solar cell module 1 does not receive sunlight, as shown in FIG. 2A, the front sealing material sheet 3 and the back sealing material sheet 5 (hereinafter, both are collectively referred to as “sealing”). The stress a on the light-receiving surface side of the solar cell element 4 sandwiched between the “stopping sheet” is substantially equal to the stress b on the back surface side of the solar cell element 4.

  However, in a state where the solar cell module 1 receives sunlight, that is, in a use state of a general solar cell module, the temperature on the light receiving surface side of the solar cell element 4 is the temperature on the back surface side of the solar cell element 4. Higher than. Accordingly, the front sealing material sheet 3 is thermally expanded more than the back sealing material sheet 5. Since the front sealing material sheet 3 has a larger thermal expansion coefficient than the solar cell element 4, the stress a on the light receiving surface side of the solar cell element 4 is the stress on the back surface side of the solar cell element 4 as shown in FIG. 2B. smaller than b.

  As described above, in the state where the solar cell element 4 receives sunlight, an imbalance as shown in FIG. 2B occurs between the stress a and the stress b, which is caused by such an imbalance of stress. As a result, the solar cell element 4 is warped.

  As a solar cell module for preventing such warpage of the solar cell element, a technique of adding a crosslinking agent to the encapsulant sheet has been proposed in order to increase the hardness of the encapsulant sheet (see Patent Document 1).

  Further, in order to enhance the effect of preventing warpage, the stress is increased by making a difference in the degree of crosslinking of the respective sealing materials between the front sealing material sheet 3 and the back sealing material sheet 5. There has also been proposed a solar cell module that absorbs this imbalance and prevents the solar cell element from warping (Patent Document 2).

  On the other hand, as a sealing material sheet for a solar cell module, conventionally, a sealing material having a configuration mainly containing EVA (ethylene-vinyl acetate copolymer) and containing a cross-linking agent is used in terms of transparency and fluidity. The thing has been widely used (refer patent document 3). And it was assumed that those sealing material sheets were formed into an uncrosslinked film and crosslinked by heating or the like in any process after the film formation, such as a modularization process.

  However, EVA has a basic drawback that water vapor barrier properties are inferior, and with the recent increase in the output of solar cells, EVA with polar groups also has a problem of insufficient electrical resistance. It has come to manifest.

  Therefore, it is conceivable to use polyethylene resin as a material to replace EVA. However, cross-linked polyethylene resin that requires a cross-linking treatment after film formation has the disadvantage that the cross-linking speed is slow and the time required for vacuum heating and pressurization (thermal lamination) in the modularization process is very long, resulting in poor productivity. was there.

  Therefore, in order to solve the problem, development of a thermoplastic polyethylene encapsulant sheet that does not require a crosslinking treatment at the time of modularization after film formation has been performed.

Japanese Patent Laid-Open No. 11-61055 JP 2007-294868 A JP 2009-135200 A

  Here, the solar cell module described in Patent Documents 1 and 2 is a cross-linking system that requires a cross-linking process in a process after film formation, such as an EVA sealing material sheet described in Patent Document 3, as a sealing material sheet. It is assumed that a sealing material sheet is used.

  A solar cell module using a thermoplastic polyethylene encapsulant sheet that is excellent in terms of water vapor barrier properties and electrical resistance and has high productivity is substantially not subjected to crosslinking treatment, and the above solar cell element I have not yet found an effective solution to solve the problem of warping.

  The present invention has been made to solve the above-described problems, and the object thereof is to use a thermoplastic polyethylene sealing material sheet having preferable physical properties and high productivity as a sealing material sheet for a solar cell module. An object of the present invention is to provide a solar cell module that can sufficiently prevent warpage of the solar cell element.

  The inventors of the present invention have a molecular weight of the front sealing material sheet 3 disposed on the light receiving surface side of the solar cell element 4 and a back surface sealing material sheet 5 disposed on the back surface side of the solar cell element 4 and a ratio of the molecular weight thereof. The inventors have found that the above problems can be solved by optimizing them, and have completed the present invention. More specifically, the present invention provides the following.

(1) A solar cell element, a front sealing material sheet disposed on the light receiving surface side of the solar cell element, and a back sealing material sheet disposed on the back surface side of the solar cell element, and the front surface The sealing material sheet and the back surface sealing material sheet contain low density polyethylene having a density of 0.940 g / cm ³ or less, and the gel fraction of the front surface sealing material sheet and the back surface sealing material sheet is 0%. The solar cell module is characterized in that the front sealing material sheet has a weight average molecular weight in terms of polystyrene larger than that of the back sealing material sheet.

  (2) The ratio of the weight average molecular weight by polystyrene conversion of the back surface sealing material sheet and the weight average molecular weight of the front surface sealing material sheet is 0.5 or more and less than 1.0. Solar cell module.

(3) The solar cell module according to (1) or (2), wherein the low-density polyethylene is a low-density polyethylene having a density of 0.890 g / cm ³ or less.

(4) A method for manufacturing a solar cell module according to any one of (1) to (3), wherein a sealing material composition containing low density polyethylene having a density of 0.940 g / cm ³ or less is formed into a sheet shape Forming a pre-sealing material sheet into a sheet forming step, and irradiating the pre-sealing material sheet with ionizing radiation to increase the molecular weight, the front sealing material sheet or the back sealing material sheet An ionizing radiation irradiation step to be obtained, and a modularization step of integrating a laminated body including a constituent part in which a solar cell element is sandwiched between the front sealing material sheet and the back sealing material sheet by a thermal laminating process. The front sealing material sheet and the back sealing material sheet are both made of the same sealing material composition, and the irradiation conditions of the ionizing radiation are different conditions. Serial weight average molecular weight terms of polystyrene of the front encapsulant sheet, a method for manufacturing a solar cell module, wherein the is made larger than the molecular weight of the back surface sealing material sheet.

  ADVANTAGE OF THE INVENTION According to this invention, it is a solar cell module using the thermoplastic polyethylene sealing material sheet | seat, Comprising: The solar cell module which can fully prevent the curvature of a solar cell element can be provided.

It is sectional drawing which shows an example of the laminated constitution about the solar cell module using the sealing material sheet of this invention. It is a schematic diagram with which it uses for description about the aspect of the stress which acts on the solar cell element in the solar cell module in the state which sunlight does not irradiate. It is a schematic diagram with which it uses for description about the aspect of the stress and curvature which act on the solar cell element in the solar cell module in case sunlight irradiates.

  The solar cell module of the present invention is mainly characterized in that the front sealing material sheet and the back sealing material sheet, each having a weight average molecular weight optimized to different relative ranges, are arranged at specific positions in the solar cell module. There are features. First, the overall configuration of the solar cell module and the method for manufacturing the solar cell module will be described, the details of each sealing material sheet constituting the sealing material layer will be described later, and other components will be described.

<Solar cell module>
FIG. 1 is a cross-sectional view showing an example of the layer structure of a solar cell module 1 according to an embodiment of the present invention. In the solar cell module 1, a transparent front substrate 2, a front sealing material sheet 3, a solar cell element 4, a back surface sealing material sheet 5, and a back surface protection sheet 6 are sequentially laminated from the light receiving surface side of incident light. In particular, the solar cell element 4 is structurally different in that the solar cell element 4 is laminated in such a manner as to be sandwiched between the front sealing material sheet 3 disposed on the light receiving surface side and the back sealing material sheet 5 disposed on the back surface side. There are features.

  In the present specification, the solar cell element means a minimum structural unit that generates light by receiving light by a photoconductive effect and / or a photovoltaic effect, and has a size of 10 cm × 10 cm square to 20 cm × 20 cm square, for example. Things. In the present specification, one surface that is mainly a light receiving surface in the solar cell element is referred to as a light receiving surface, and the other surface opposite to the light receiving surface is referred to as a back surface. Moreover, a solar cell module means what is comprised by connecting this solar cell element in multiple numbers, for example, about 0.5-50 square x-2. The thing about 0 m x 2.0 m square can be mentioned. In the present specification, the solar cell module includes a solar cell panel that is an assembly of modules.

  The solar cell module 1 has a relatively large weight average molecular weight (hereinafter simply referred to as “weight average molecular weight”) in terms of polystyrene as the front sealing material sheet 3 as compared to the back sealing material sheet 5. The main feature is that the sheet is placed.

  Here, about the fluidity | liquidity at the time of a heating, a sealing material sheet has the characteristic that it becomes difficult to flow, so that a weight average molecular weight is large. Therefore, the front sealing material sheet 3 having a relatively large weight average molecular weight is relatively less likely to thermally expand at the same high temperature than the back sealing material sheet 5 because of the larger weight average molecular weight. Become. Therefore, even if the temperature of the front sealing material sheet 3 becomes higher than the temperature of the back sealing material sheet 5 due to the irradiation of the sunlight S from the light receiving surface side as shown in FIG. The difference in weight average molecular weight between the stop sheet will offset the stress imbalance described above. As a result, according to the solar cell module 1 of the present invention, even when the temperature of the front sealing material sheet 3 is larger than that of the back sealing material sheet 5, as shown in FIG. The stress acting on the upper and lower sides of the element 4 can be kept in a balanced state, whereby the warpage of the solar cell element 4 can be prevented. The details of these sealing material sheets will be described later.

  About the transparent front substrate 2 and the back surface protection sheet 6 which are members other than the front surface sealing material sheet 3 and the back surface sealing material sheet 5, a conventionally well-known material can be especially used without a restriction | limiting. As the transparent front substrate 2, glass or the like, and as the back protective sheet 6, a resin sheet such as ETFE or water-resistant PET can be used. In addition, the solar cell module 1 of this invention may also contain members other than the said member.

  The solar cell element 4 is not particularly limited, but is not limited to a crystalline silicon solar cell manufactured using a single crystal silicon substrate or a polycrystalline silicon substrate, and amorphous silicon, microcrystalline silicon, a chalcopyrite compound, or the like is used. The thin-film solar cell (CIGS) is preferably used, and the configuration of the solar cell module is not only the conventional front contact type but also the back contact type in which a plurality of electrodes having different polarities are provided on the non-light-receiving surface side. This solar cell element can also be preferably used.

  Among these, as the solar cell element 4, a back contact type solar cell element such as a metal wrap through (MWT) method or an emitter wrap through (EWT) method can be used particularly preferably. In the solar cell module using these back contact type solar cell elements, it is necessary to insulate the electrodes provided on the non-light-receiving surface side to prevent a short circuit. Since the solar cell module 1 uses the back surface sealing material sheet 5 made of a highly insulating polyethylene-based resin as the back surface sealing material sheet 5 in contact with the non-light-receiving surface side of the solar cell element 4, the solar cell element A back contact type solar cell element can be particularly preferably used as 4.

<Sealing material sheet>
The front sealing material sheet 3 and the back sealing material sheet 5 (hereinafter, both are also referred to as “sealing material sheet”) are the same as those used in ordinary thermoplastic resins. Are formed into a sheet or film by various molding methods such as injection molding, extrusion molding, hollow molding, compression molding, and rotational molding. In addition, the sheet form in this invention means the film form, and there is no difference in both. Hereinafter, details are sequentially described for each of the encapsulant composition, the front encapsulant sheet, and the back encapsulant sheet.

[Encapsulant composition]
The encapsulant composition used as the material of the encapsulant sheet of the present invention uses low-density polyethylene, which will be described in detail below, as a base resin, and additionally contains a crosslinking agent, a crosslinking aid, and the like as optional components. is there. The sealing material sheet of the present invention is one in which the weight average molecular weight of the front sealing material sheet 3 is larger than the weight average molecular weight of the back sealing material sheet 5. Moreover, it is preferable that the ratio of the weight average molecular weights is also adjusted within a predetermined range.

  The front sealing material sheet 3 and the back sealing material sheet 5 having different weight average molecular weights can be manufactured from different sealing material compositions, respectively, but according to the manufacturing method of the present invention, any one of them is sealed. A stopping material sheet can also be manufactured from the sealing material composition of the same composition. In that case, what is necessary is just to form the same sealing material composition into a film, and to adjust the weight average molecular weight of each sealing material sheet to the optimal range suitably by irradiation of ionizing radiation after film forming.

(Low density polyethylene)
As the main resin of the encapsulant composition, low density polyethylene (LDPE) having a density of 0.940 g / cm ³ or less, preferably linear low density polyethylene (LLDPE) is used. Linear low density polyethylene is a copolymer of ethylene and α- olefin, in the present invention, within the scope thereof density of 0.940 g / cm ³ or less, preferably 0.900 g / cm ³ within the following ranges Moreover, it is more preferably in the range of 0.870 to 0.890 g / cm ³ from the viewpoint of particularly improving transparency.

  In addition, even if the density of the low density polyethylene used as a base is in such a low density range by performing irradiation with ionizing radiation after the molding of the encapsulant composition, the encapsulant sheet has sufficient heat resistance. Can be granted. For this reason, even if it is a low density like the said range, it can be used conveniently.

  In the present invention, it is preferable to use a metallocene linear low density polyethylene. Metallocene linear low density polyethylene is synthesized using a metallocene catalyst which is a single site catalyst. Such polyethylene has few side chain branches and a uniform comonomer distribution. For this reason, molecular weight distribution is narrow, it is possible to make it the above ultra-low density, and a softness | flexibility can be provided with respect to a sealing material sheet. As a result of the flexibility, the adhesion between the sealing material sheet and other members such as the back surface protection sheet 6 is enhanced, and the intrusion of moisture into the solar cell module 1 can be suppressed.

  As the α-olefin of the linear low density polyethylene, an α-olefin having no branch is preferably used. Among these, 1-hexene and 1-heptene which are α-olefins having 6 to 8 carbon atoms are preferable. Or 1-octene is particularly preferably used. When the number of carbon atoms of the α-olefin is 6 or more and 8 or less, the sealing material sheet can be given good flexibility and good strength.

  In addition, the molecular weight of the low density polyethylene used as a raw material is not particularly limited. However, as an example, in the case of the above-described metallocene linear low density polyethylene, the number average molecular weight in terms of polystyrene (hereinafter, also simply referred to as “number average molecular weight (Mn)”) is about 30,000 to 100,000 g / mol, The weight average molecular weight is preferably about 70,000 to 300,000 g / mol, and the dispersity is preferably about 1.4 to 2.6. Here, the degree of dispersion means the ratio (Mw / Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn). When the molecular weight and molecular weight distribution of the low-density polyethylene as a raw material are in this range, various physical properties preferable as a sealing material sheet for a solar cell module by appropriately changing the molecular weight by ionizing radiation irradiation described later It can be easily provided with The molecular weight can be measured by a conventionally known GPC method after dissolving in a solvent such as THF.

  The Shore D hardness of the low density polyethylene is preferably 80 degrees or less, preferably 15 degrees or more and 40 degrees or less, and more preferably 15 degrees or more and 35 degrees or less. When the Shore D hardness of the linear low density polyethylene is in the above range, the flexibility of the encapsulant sheet can be maintained. The MFR of the linear low density polyethylene is preferably from 0.5 g / 10 min to 40 g / 10 min at 190 ° C., more preferably from 2 g / 10 min to 40 g / 10 min. When the MFR is in the above range, the processability during film formation is excellent.

  The sealing material composition may further contain a silane-modified polyethylene resin. The silane-modified polyethylene resin is obtained by graft-polymerizing an ethylenically unsaturated silane compound as a side chain to linear low-density polyethylene (LLDPE) or the like as a main chain. Since such a graft copolymer has a high degree of freedom of silanol groups that contribute to the adhesive force, the adhesion of the sealing material to other members in the solar cell module can be improved.

  The silane-modified polyethylene resin can be produced, for example, by the method described in JP-A-2003-46105. By using the resin as a component of the sealing material composition of the solar cell module, strength and durability can be obtained. In addition, it has excellent weather resistance, heat resistance, water resistance, light resistance, wind pressure resistance, yield resistance, and other characteristics, and is also affected by manufacturing conditions such as thermocompression bonding for manufacturing solar cell modules. Therefore, it is possible to manufacture solar cell modules having extremely excellent heat-fusibility, stably and at low cost, and suitable for various applications.

  Examples of ethylenically unsaturated silane compounds to be graft polymerized with linear low density polyethylene include, for example, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltripropoxysilane, vinyltriisopropoxysilane, vinyltributoxysilane, vinyltripentyloxysilane , One or more selected from vinyltriphenoxysilane, vinyltribenzyloxysilane, vinyltrimethylenedioxysilane, vinyltriethylenedioxysilane, vinylpropionyloxysilane, vinyltriacetoxysilane, and vinyltricarboxysilane be able to.

  The graft amount, which is the content of the ethylenically unsaturated silane compound, is, for example, 0.001 to 15 masses with respect to a total of 100 mass parts of all the resin components in the sealing material composition containing other polyethylene-based resins described later. What is necessary is just to adjust suitably so that it may become a site | part, Preferably 0.01 mass part or more and 5 mass parts, Especially preferably, it is 0.05 mass part or more and 2 mass parts. In the present invention, when the content of the ethylenically unsaturated silane compound is large, the mechanical strength and heat resistance are excellent. However, when the content is excessive, the tensile elongation and the heat fusion property tend to be inferior.

  The sealing material composition may further contain an acid-modified polyethylene resin typified by maleic anhydride modification. The acid-modified polyethylene resin is obtained, for example, by graft polymerization using, for example, maleic anhydride or the like as a side chain on linear low density polyethylene (LLDPE) or the like as a main chain. Such a graft polymer has a high polarity of the acid part which contributes to adhesive force, and can improve the adhesiveness of the sealing material sheet to the metal member in the solar cell module.

  The content of the low density polyethylene contained in the encapsulant composition is preferably 10 parts by mass or more and 99 parts by mass or less, more preferably 50 parts by mass with respect to 100 parts by mass in total of all resin components in the encapsulant composition. It is not less than 99 parts by mass and more preferably not less than 90 parts by mass and not more than 99 parts by mass. Other resin may be included as long as the melting point of the sealing material composition is within a range of less than 80 ° C. These may be used, for example, as an additive resin, or may be used for masterbatching other components described later.

(Crosslinking agent)
The encapsulant sheet according to the present invention does not require a crosslinking treatment, but the encapsulant composition may contain an appropriate amount of a crosslinking agent as necessary. As a result, the heat resistance is improved by advancing a very limited cross-linking in a range where gel is not generated during film formation on the encapsulant sheet, or the transparency after irradiation with ionizing radiation is improved. Effect can be obtained. In this case, the crosslinking agent is not particularly limited. Specific examples include various organic peroxides, azo compounds, silanol condensation catalysts, and the like. These may be used alone or in combination of two or more.

  A well-known thing can be used for a crosslinking agent, It does not specifically limit, For example, a well-known radical polymerization initiator can be used. Examples of radical polymerization initiators include hydroperoxides such as diisopropylbenzene hydroperoxide and 2,5-dimethyl-2,5-di (hydroperoxy) hexane; di-t-butyl peroxide, t-butyl Cumyl peroxide, dicumyl peroxide, 2,5-dimethyl-2,5-di (t-butylperoxy) hexane, 2,5-dimethyl-2,5-di (t-peroxy) hexyne-3, etc. Dialkyl peroxides; diacyl peroxides such as bis-3,5,5-trimethylhexanoyl peroxide, octanoyl peroxide, benzoyl peroxide, o-methylbenzoyl peroxide, 2,4-dichlorobenzoyl peroxide; t-butyl peroxyacetate, t-butyl pero Ci-2-ethylhexanoate, t-butyl peroxypivalate, t-butyl peroxyoctoate, t-butyl peroxyisopropyl carbonate, t-butyl peroxybenzoate, di-t-butyl peroxyphthalate, 2 , 5-dimethyl-2,5-di (benzoylperoxy) hexane, 2,5-dimethyl-2,5-di (benzoylperoxy) hexyne-3, t-butylperoxy-2-ethylhexyl carbonate Oxyesters; organic peroxides such as ketone peroxides such as methyl ethyl ketone peroxide and cyclohexanone peroxide, or azo compounds such as azobisisobutyronitrile and azobis (2,4-dimethylvaleronitrile), dibutyltin Diacetate, dibutyltin dilaurate It can be mentioned dibutyltin dioctoate, dioctyltin dilaurate, dicumyl peroxide, such a silanol condensation catalyst. These may be used alone or in combination of two or more. Specifically, the fast curing method in the vacuum heating lamination process of solar cell module fabrication (the sealing material is highly crosslinked only by vacuum heating lamination, and is allowed to stand at about 150 to 160 ° C. for 30 minutes to 1 hour to post-crosslink the sealing material. T-butylperoxy-2-ethylhexyl carbonate (TBEC) is preferably used from the viewpoint of adapting to the method of omitting the curing step.

  Conventionally, in the case of irradiation with ionizing radiation, it has been considered that a crosslinking agent such as an organic peroxide is unnecessary unlike the case of thermal crosslinking treatment. However, it is preferable that the sealing material composition used in the present invention contains a small amount of a crosslinking agent while being irradiated with ionizing radiation. This is because the addition of this crosslinking agent makes it possible to improve the heat resistance as well as the transparency by irradiation with ionizing radiation. The action of the cross-linking agent in the cross-linking treatment by irradiation with ionizing radiation is not clear, but since ionizing radiation has strong energy, the cross-linking proceeds, but the crystals that cause HAZE do not loosen and do not participate in cross-linking unless the temperature exceeds a certain temperature. It is estimated that this is because it remains.

  The amount of the crosslinking agent added to the encapsulant composition of the present invention is preferably 0.01% by mass or more and 1.5% by mass or less in the encapsulant composition, and 0.3% by mass or more and 0.0. It is more preferably 7% by mass or less, and further preferably 0.3% by mass or more and 0.5% by mass or less. By making the addition amount of the crosslinking agent to the sealing material composition within this range, in addition to heat resistance and adhesion, a sealing material sheet that is particularly excellent in transparency can be obtained. In this case, if the content of the crosslinking agent is less than 0.01% by mass, the effect of improving the transparency is insufficient, and if it exceeds 0.7% by mass, the load during extrusion increases, As a result of gel generation, the film-forming property is lowered and the transparency is also lowered. In addition, although the sealing material sheet of the present invention can be obtained by performing a crosslinking treatment by irradiation with ionizing radiation, in this case, the addition amount of the crosslinking material is different from the conventional thermal crosslinking, Even if the addition amount of the crosslinking material is less than 0.5% by mass, sufficient heat resistance can be obtained. Thereby, the risk of the productivity fall by the gelatinization of the sealing material composition in the sheeting process of a sealing material composition can be reduced, and manufacturing cost can also be reduced by reducing the usage-amount of a crosslinking agent.

(Crosslinking aid)
In the present invention, a polyfunctional monomer having a carbon-carbon double bond and / or an epoxy group may be used as a crosslinking aid. Preferably, the functional group of the polyfunctional monomer is an allyl group, a (meth) acrylate group, or a vinyl group. In the present invention, the crosslinking aid reduces the crystallinity of the low density polyethylene and maintains transparency.

  Specifically, polyallyl compounds such as triallyl isocyanurate (TAIC), triallyl cyanurate, diallyl phthalate, diallyl fumarate, diallyl maleate, trimethylolpropane trimethacrylate (TMPT), trimethylolpropane triacrylate (TMPTA) Poly (meth) acryloxy compounds such as ethylene glycol diacrylate, ethylene glycol dimethacrylate, 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, 1,9-nonanediol diacrylate, Glycidyl methacrylate containing epoxy groups, 4-hydroxybutyl acrylate glycidyl ether and 1,6-hexanediol diglycidyl ether containing two or more epoxy groups, 1 4-butanediol diglycidyl ether, cyclohexanedimethanol diglycidyl ether, and epoxy compounds such as trimethylolpropane polyglycidyl ether. These may be used alone or in combination of two or more.

  Among these, TAIC is preferably used from the viewpoint of good compatibility with low density polyethylene, lowering crystallinity by crosslinking, maintaining transparency, and imparting flexibility at low temperatures. Further, 1,6-hexanediol diacrylate can also be preferably used from the viewpoint of reactivity with the silane coupling agent.

  When the crosslinking aid is added, the content thereof is preferably 0.01% by mass or more and 3% by mass or less, more preferably 0.05% by mass or more and 2.0% by mass in the encapsulant composition. % Or less. Within this range, a substantial crosslinking reaction does not occur even when irradiated with ionizing radiation, and the gel fraction of the encapsulant sheet can be maintained at 0%.

(Radical absorbent)
When adding a crosslinking agent or a crosslinking aid to the encapsulant sheet, the degree of crosslinking is adjusted by using a crosslinking agent as a radical polymerization initiator together with a radical absorbent that quenches it. The gel fraction can be adjusted. Specific examples of the radical absorbent include hindered phenol-based antioxidants, hindered amine-based weather resistance stabilization, and the like. The content of the radical absorbent is preferably included in the sealing material composition in the range of 0.01% by mass to 3% by mass, and more preferably in the range of 0.05% by mass to 2.0% by mass. . If it exists in this range, a crosslinking reaction can be suppressed moderately.

(Other ingredients)
The sealing material composition may further contain other components. For example, components such as a weather resistance masterbatch for imparting weather resistance to the encapsulant sheet, various fillers, a light stabilizer, an ultraviolet absorber, and a heat stabilizer are exemplified. These contents vary depending on the particle shape, density, and the like, but are preferably in the range of 0.001% by mass or more and 5% by mass or less in the sealing material composition. By including these additives, it is possible to impart a long-term stable mechanical strength, an effect of preventing yellowing, cracking, and the like to the encapsulant composition for solar cell modules.

  In addition to the above, other components used in the encapsulant composition include adhesion improvers such as silane coupling agents, nucleating agents, dispersants, leveling agents, plasticizers, antifoaming agents, flame retardants, and the like. Can be mentioned.

[Encapsulant sheet]
The front sealing material sheet 3 and the back sealing material sheet 5 are both formed by melt-molding the above-mentioned sealing material composition at a temperature exceeding its melting point, and the sealing material sheet after film formation Are irradiated with ionizing radiation under different conditions to increase the weight average molecular weight to an optimum value, respectively, and the front sealing material sheet 3 having a relatively large weight average molecular weight and the relatively weight average molecular weight. It can obtain by setting it as the back surface sealing material sheet 5 with small. In addition, the molecular weight of the encapsulant sheet in the present specification means an average value of the molecular weight of the entire encapsulant sheet when there is a variation in the molecular weight value in each part in the encapsulant sheet. Shall.

  About the details of the irradiation conditions of ionizing radiation, individual conditions are not specifically limited. What is necessary is just to set suitably so that the weight average molecular weight of each sealing material sheet may become the predetermined range prescribed | regulated by this invention as follows as a total process result.

  Irradiation with ionizing radiation is not always essential. As long as the encapsulant sheet has a weight average molecular weight adjusted to the predetermined range of the present invention regardless of the treatment, the encapsulant sheet is not necessarily a encapsulant sheet whose weight average molecular weight is increased by irradiation with ionizing radiation. It can be a constituent material of the solar cell module of the invention.

  The encapsulant sheet constituting the solar cell module 1 of the present invention is characterized in that the weight average molecular weight of the front encapsulant sheet 3 is larger than the weight average molecular weight of the back encapsulant sheet 5. Moreover, it is preferable that ratio of the weight average molecular weight of the back surface sealing material sheet 5 and the weight average molecular weight of a front surface sealing material sheet is 0.5 or more and less than 1.0. Further, this ratio is more preferably 0.6 or more and less than 0.85.

  If the ratio of the weight average molecular weight of the front sealing material sheet 3 and the back sealing material sheet 5 is in the range of 0.5 or more and less than 1.0, the solar cell in the solar cell module 1 will be shown later in Examples. The occurrence of warping of the element 4 can be sufficiently suppressed to prevent the element from being damaged.

  The sealing material sheet of this invention should just have the relative ratio of the weight average molecular weight of the front surface sealing material sheet 3 and the back surface sealing material sheet 5 in the said range, and about the absolute value of the molecular weight of each sealing material sheet Is not particularly limited.

  Moreover, the encapsulant sheet of the present invention has increased dispersity by irradiation with ionizing radiation. Such a low-density polyethylene-based sealing material having a specific degree of dispersion cannot be obtained by conventional thermal crosslinking treatment, but can be obtained by crosslinking treatment by irradiation with ionizing radiation. According to the cross-linking treatment by irradiation with ionizing radiation, unlike conventional thermal cross-linking, the cross-linking reaction by irradiation with ionizing radiation in which the cross-linking progresses in the encapsulant while the decomposition reaction proceeds irregularly simultaneously. This is because a unique reaction proceeds. The sealing material sheet of the present invention having such a specific degree of dispersion is a sealing material sheet having heat resistance and unevenness followability superior to conventional products, and transparency.

  Here, the molecular weight can be measured by a conventionally known GPC method after dissolving in a solvent such as THF. The number average molecular weight (Mn), the weight average molecular weight (Mw), and the degree of dispersion d when the polymer having a molecular weight Mi (g / mol) is Ni (pieces) in the encapsulant sheet are defined by the following equations, respectively. It is what is done.

  Number average molecular weight Mn = Σ (MiNi) / ΣNi

Weight average molecular weight Mw = Σ (Mi ² Ni) / ΣMiNi

  Dispersity d = Mw / Mn

  Regarding the gel fraction of the sealing material sheet of the present invention, the gel fraction is maintained at 0% at all stages including the state after post-molding crosslinking treatment, after crosslinking treatment, and even after modularization. It is characterized by having. By maintaining the gel fraction at 0%, it is possible to improve the unevenness followability of the unevenness of other members laminated up and down during modularization, and to reduce the load at the time of melt extrusion with high productivity stably. An encapsulant sheet can be produced.

  In addition, according to the said process which irradiates an ionizing radiation to a sealing material sheet, molecular weight can be made into the above molecular weight ranges, maintaining the gel fraction 0% of a sealing material sheet.

  On the other hand, even when the gel fraction is 0%, by appropriately optimizing the resin density, MFR, cross-linking agent, cross-linking aid content, cross-linking method, etc., shear stress is increased and flow prevention is achieved. be able to.

  Here, the gel fraction (%) means that 0.1 g of a sealing material sheet is put into a resin mesh, extracted with xylene by heating under reflux for 12 hours, taken out together with the resin mesh, weighed after drying, and before and after extraction. The mass ratio of the residual insoluble matter is measured and the gel fraction is obtained. “Gel fraction 0%” means that the residual insoluble matter is substantially 0 and the crosslinking reaction of the encapsulant composition has not substantially started. More specifically, “gel fraction 0%” in the present invention means that the residual insoluble matter is not present at all, and the residual insoluble matter mass% measured by a precision balance is less than 0.05 mass%. Let's say the case.

  The thickness of the encapsulant sheet is preferably 200 μm or more and 600 μm or less, and if it is less than 200 μm, the impact cannot be sufficiently mitigated, and the insulating property becomes insufficient, which is not preferable. Moreover, even if it exceeds 600 micrometers, since the effect beyond it is not acquired and it is uneconomical, it is unpreferable.

[Method for producing sealing material sheet]
The sealing material sheet which comprises the solar cell module of this invention can be manufactured with the following manufacturing methods.

(Sheet making process)
Film formation by melt molding of the sealing material composition is performed by various molding methods such as injection molding, extrusion molding, hollow molding, compression molding, and rotational molding, which are usually used in ordinary thermoplastic resins. At that time, the lower limit of the molding temperature may be a temperature exceeding the melting point of the encapsulant composition, and the upper limit may be a temperature at which crosslinking does not start during film formation depending on the 1-minute half-life temperature of the crosslinking agent used. There is no particular limitation as long as it is within these ranges. In the method for producing a solar cell module sealing material sheet of the present invention, as described above, since a crosslinking agent having a half-life temperature higher than that of the conventional one can be used, the molding temperature is higher than that of the conventional one. By setting to, it is possible to reduce the load applied to the extruder or the like and increase the productivity of the sealing material sheet.

  The molecular weight of the pre-sealing material sheet that has undergone the sheet forming step is not particularly limited, but as an example, the number average molecular weight is 80000 g / mol or more and 120,000 g / mol or less, and the weight average molecular weight is 150,000 g / mol or more and 250,000 g / mol or less. It is preferable that By irradiating the pre-encapsulant sheet in such a molecular weight range with ionizing radiation, the solar cell module of the present invention having a low gel fraction and a molecular weight increased to a predetermined molecular weight range, respectively, is configured. The front surface sealing material sheet or back surface sealing material sheet which can be manufactured in a preferable aspect can be manufactured.

  In addition, about the sealing material composition, this manufacturing method which optimizes each molecular weight by adjusting suitably the irradiation conditions of ionizing radiation using the preliminary sealing material sheet formed from the composition of the same composition According to this, two kinds of encapsulant sheets having different weight average molecular weights can be efficiently produced as compared with the case where a film is separately formed from compositions having different compositions.

(Ionizing radiation irradiation process)
A treatment for increasing the molecular weight is performed by irradiating the pre-sealing material sheet after the sheeting step with ionizing radiation under appropriate conditions. By this step, the weight average molecular weight of the front sealing material sheet and the back sealing material sheet and the ratio thereof can be optimized within a predetermined range specific to the present application.

  Moreover, it is preferable to adjust appropriately the irradiation amount of ionizing radiation, and the addition amount of a crosslinking agent so that it may become 0% also about the gel fraction of a sealing material sheet. Conventionally, when the gel fraction is 0%, the effect of suppressing flow by the crosslinking step before the modularization step is not exhibited, and it is difficult to keep the film thickness of the encapsulant sheet constant due to excessive flow. However, the sealing material sheet of the present invention avoids such a problem by increasing the molecular weight to a predetermined range by irradiation with electron irradiation rays, and thus the sealing material for solar cell modules. It can be preferably used as a sheet.

  The cross-linking treatment can be performed by ionizing radiation such as electron beam (EB), α-ray, β-ray, γ-ray, neutron beam, etc. Among them, it is preferable to use an electron beam. The processing conditions are not particularly limited as long as the molecular weight of each encapsulant sheet and the ratio thereof are within the predetermined range described above as a total processing result.

  In addition, this ionizing radiation irradiation treatment process may be performed continuously in-line following the sheet forming process, or may be performed off-line.

<Method for manufacturing solar cell module>
The front sealing material sheet 3 manufactured by the above manufacturing method of the sealing material sheet is disposed on the light receiving surface side of the solar cell element 4, and the back surface sealing material sheet 5 is disposed on the back surface side of the solar cell element 4. The solar cell element 4 is sandwiched between both sealing material sheets, and other members such as the transparent front substrate 2 and the back surface protective sheet 6 are sequentially laminated and then integrated by vacuum suction or the like, and then lamination is performed. According to a molding method such as the above method, the above-described member can be manufactured by thermocompression molding as an integral molded body.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention is not limited to a following example.

<Manufacture of sealing material sheet for solar cell module>
The encapsulant composition composed of the following materials is mixed and melted, and is formed by extrusion at 220 ° C. so as to have a thickness of 400 μm by a conventional T-die method. A pre-sealing material sheet for 3 was formed.

(Base resin)
Each of the following two types of LLDPE (resin M1 or M2) and 75 parts by mass of silane-modified polyethylene resin (resin S) mixed and melted in accordance with the composition shown in Table 1 are sealed. The base resin of the stopping material composition was used.

LLDPE (Resin M1)
: Polyethylene resin (LLDPE): A metallocene linear low density polyethylene which is a copolymer of ethylene and 1-hexene and has a density of 0.880 g / cm ³ and MFR of ³ g / 10 min. Number average molecular weight 100,000 in terms of polystyrene.
LLDPE (Resin M2)
: Polyethylene resin (LLDPE): A metallocene linear low density polyethylene which is a copolymer of ethylene and 1-hexene and has a density of 0.900 g / cm ³ and MFR of 2 g / 10 min. Number average molecular weight of 120,000 in terms of polystyrene.
Silane-modified polyethylene resin (resin S)
A copolymer of ethylene and 1-hexene, a metallocene linear low density polyethylene resin having a density of 0.880 g / cm ³ and a melt mass flow rate (MFR) at 190 ° C. of 3 g / 10 min, 98 Density obtained by mixing 2 parts by mass of vinyltrimethoxysilane and 0.1 part by mass of dicumyl peroxide as a radical generator (reaction catalyst) with respect to parts by mass, and melting and kneading at 200 ° C. Silane-modified polyethylene resin having 0.884 g / cm ³ and MFR of 2 g / 10 min. Number average molecular weight 110000 in terms of polystyrene.
(Crosslinking agent)
2,5 dimethyl 2,5 di (t-butylperoxy) hexane (trade name Luperox 101, manufactured by Arkema Yoshitomi Co., Ltd.) is used as a crosslinking agent, and only the sealing material compositions of the sealing material sheets 1 and 2 are used. 0.03 part by mass was added.
(Other additives)
0.25 mass part of UV absorbers (Kemipro Kasei Co., Ltd. make, brand name KEMISORB12) were added to the sealing material composition of the sealing material sheets 1-3.
0.6 parts by mass of a weathering stabilizer (manufactured by Ciba Japan Co., Ltd., trade name Tinuvin 770) was added to the sealing material compositions of the sealing material sheets 1 to 3.
An antioxidant (trade name: Irganox 1076, manufactured by Ciba Japan Co., Ltd.) was added to 0.05 parts by mass of the encapsulant composition of the encapsulant sheets 1 to 3.

  Next, the preliminary sealing material sheet was subjected to a crosslinking treatment by irradiation with ionizing radiation to obtain a sealing material sheet 1. Irradiation conditions were as follows: double-sided irradiation with an acceleration voltage of 200 kV and an irradiation dose of 20 kGy for a total of 40 kGy.

  For the sealing material sheets 2 to 3, the irradiation treatment with the ionizing radiation was not performed, and each pre-sealing material sheet after molding was directly used as the sealing material sheet 2 or 3.

  The weight average molecular weights and gel fractions of the sealing material sheets 1 to 3 were measured by the following method. The results are shown in Table 1.

Molecular weight: The encapsulant sheets 1 to 3 were dissolved in o-dichlorobenzene, and the polystyrene-equivalent number average molecular weight, weight average molecular weight, and molecular weight distribution of each encapsulant sheet were measured under the following conditions.
Measurement model: GPC / V2000 manufactured by Waterers
Column: Shodex AT-G + AT-806MS × 2 Eluent: o-dichlorobenzene (with 0.3% BHT)
Temperature: Column and injector 145 ° C
Concentration: about 1.0 g / L
Flow rate 1.0ml / min
Solubility: Complete dissolution Detector: Differential refractometer (RI)

  Gel fraction (%): 0.1 g of the encapsulant sheets 1 to 3 was put into a resin mesh, extracted with 60 ° C. toluene for 4 hours, taken out together with the resin mesh, weighed after drying treatment, and weight comparison before and after extraction And the mass% of the remaining insoluble matter was measured and this was used as the gel fraction. The definition of the gel fraction of 0% is as described above.

<Manufacture of solar cell module evaluation samples>
Moreover, solar cell module evaluation of an Example and a comparative example is used using the said sealing material sheets 1-3 as a front surface sealing material sheet arrange | positioned at the light-receiving surface side, respectively, or a back surface sealing material sheet arrange | positioned at a back surface side. The sample for manufacture was manufactured by the structure shown in Table 2 below.

Samples for solar cell module evaluation are from a transparent front glass substrate (blue plate glass), a front sealing material sheet, a solar cell element, a back surface sealing material sheet, and a back surface protection sheet (Dai Nippon Printing Co., Ltd., model number VPEW 280 μm). These members were sequentially laminated and then manufactured by thermocompression-bonding the above members as an integrally formed body by vacuum heating lamination, and this was used as a solar cell module evaluation sample of Examples and Comparative Examples. Regarding the solar cell elements, solar cell elements described below were prepared, and 42 solar cell elements of the same type were electrically connected in series with each other using one connecting member. The front sealing material sheet and the back sealing material sheet in each evaluation sample were combined as shown in Table 2, and the thermal lamination conditions were as follows.
<Heat lamination conditions> (a) Vacuum drawing: 5.0 minutes
(B) Pressurization (0 kPa to 100 kPa): 1.5 minutes
(C) Pressure holding (100 kPa): 15.0 minutes
(D) Temperature 150 ° C
(Solar cell element)
A crystalline silicon solar cell element manufactured using a polycrystalline silicon substrate. An electrode for collecting current is arranged on the daylighting side. (Q-CELLS, cell Q6LTT-200 / 1520 156mm)

<Evaluation example>
Using the solar cell module evaluation samples of Examples and Comparative Examples, the microcracks generated from the warpage of the solar cell elements were measured. The results are shown in Table 2. The test conditions are as follows.

Microcrack measurement test of solar cell element: Cell microcracks were observed by EL images for the solar cell module evaluation samples of the above-described Examples and Comparative Examples. The test was performed before and after the cycle test described below. The cycle test used the method based on the temperature cycle test of JIS C8917. Increase from 25 ° C. to 90 ° C. over 45 minutes, hold at this temperature for 90 minutes, then decrease to −40 ° C. over 90 minutes, hold at this temperature for 90 minutes, then increase to 25 ° C. over 45 minutes Let This is one cycle (6 hours). This cycle was repeated 400 cycles to perform a cycle test. Observation of the cell microcrack by the EL image was performed as follows. When current is made to conduct in the forward direction with respect to the solar cell element, electrons of minority carriers are introduced into the p layer, and light emission is generated by recombination of the electrons and holes in the p layer. In the detection step, conventionally known light detection means that can detect the light emission characteristics of light from the solar cell element can be used. In the test, a current of 3 A was passed from the output terminal of the module, EL emission was photographed, and microcracks at a level that could not be confirmed visually or with a microscope were confirmed from the emission image. And the number of the cell in which the microcrack has generate | occur | produced in one solar cell module evaluation sample comprised by 42 cells was measured. The number of microcracks generated before the cycle test was 0 for all microcracks. The measured values were evaluated according to the following evaluation criteria. The results are shown in Table 2.
(Evaluation criteria)
A = Number of microcracks generated 0-2
B = number of microcracks generated 3-7
C = Number of microcracks generated is 8 or more

  From Tables 1-2, it turns out that the solar cell module of this invention is the outstanding sealing material sheet provided with the cell protection performance by the original sealing material layer structure.

DESCRIPTION OF SYMBOLS 1 Solar cell module 2 Transparent front substrate 3 Front sealing material sheet 4 Solar cell element 5 Back surface sealing material sheet 6 Back surface protection sheet

Claims (4)

A solar cell element;
A front sealing material sheet disposed on the light-receiving surface side of the solar cell element;
A back surface sealing material sheet disposed on the back surface side of the solar cell element,
The front sealing material sheet and the back sealing material sheet contain low density polyethylene having a density of 0.940 g / cm ³ or less,
The gel fraction of the front sealing material sheet and the back sealing material sheet is 0%,
The said front sealing material sheet has a weight average molecular weight by polystyrene conversion larger than the said back sealing material sheet, The solar cell module characterized by the above-mentioned.   2. The solar cell module according to claim 1, wherein a ratio of a weight average molecular weight in terms of polystyrene of the back surface sealing material sheet and the weight average molecular weight of the front surface sealing material sheet is 0.5 or more and less than 1.0. . The solar cell module according to claim 1 or 2, wherein the low-density polyethylene is low-density polyethylene having a density of 0.890 g / cm ³ or less. It is a manufacturing method of the solar cell module according to any one of claims 1 to 3,
Forming a sealing material composition containing low-density polyethylene having a density of 0.940 g / cm ³ or less into a sheet shape to obtain a preliminary sealing material sheet; and
Ionizing radiation irradiation step for increasing the molecular weight by irradiation of ionizing radiation to the preliminary sealing material sheet to obtain a front sealing material sheet or a back sealing material sheet,
Comprising a modularization step in which a laminate including a constituent part in which a solar cell element is sandwiched between the front sealing material sheet and the back sealing material sheet is integrated by a thermal laminating process;
The front sealing material sheet and the back sealing material sheet are both composed of a sealing material composition having the same composition, and the front surface sealing material sheet and the front sealing material sheet have different ionizing radiation irradiation conditions. The manufacturing method of the solar cell module characterized by making the polystyrene conversion weight average molecular weight of the sealing material sheet larger than this molecular weight of the said back surface sealing material sheet.

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