JP7315047B2 - Encapsulant sheet for solar cell module, solar cell module using same, and method for manufacturing solar cell module - Google Patents

Description

TECHNICAL FIELD The present invention relates to an encapsulant sheet for a solar cell module, a solar cell module using the same, and a method for manufacturing a solar cell module.

Conventionally, ethylene-vinyl acetate copolymer resin (EVA) has been mainly used as a sealing material sheet for use in solar cell modules, including double-sided glass protective substrate type solar cell modules, from the viewpoints of workability, workability, manufacturing cost, and the like. However, EVA resin tends to gradually decompose with long-term use, and may generate acetic acid gas that affects solar cell elements. For this reason, in recent years, there has been an increasing demand for encapsulant sheets for solar cell modules that use polyethylene-based resin instead of EVA resin (see Patent Document 1).

Generally, in a sealing material sheet for a solar cell module using a polyethylene-based resin as a base resin, the transparency and flexibility can be improved by reducing the density. However, the low density causes a problem of insufficient heat resistance. Therefore, in the encapsulant sheet of Patent Document 2, heat resistance is imparted with a cross-linking agent. In this case, the heat resistance is certainly improved, but if the necessary and sufficient degree of cross-linking treatment is performed in order to provide sufficient heat resistance to withstand long-term use under high temperatures, there is a problem that the ability to follow the unevenness of the surface of the facing member (hereinafter referred to as "molding property") cannot be maintained during modularization. In addition, in the manufacturing process that requires a cross-linking treatment, if the cross-linking progresses during molding, the film-forming property will deteriorate, so it is necessary to consider such as performing the molding at a low temperature and performing the cross-linking reaction again after molding.

As an attempt to solve such a problem, research has been conducted on a method of achieving both heat resistance and molding properties by making the encapsulant sheet into a multi-layer structure, selecting the material resins for the core layer and the skin layer, adding additives, and optimizing the melting point of each layer individually (see Patent Document 2).

However, in the production of encapsulant sheets for solar cell modules, it has been recognized as a new problem that when this is made into a multi-layer sheet, if the difference in the melting point of the base resin between the core layer and the skin layer becomes greater than a certain level, thickness unevenness tends to occur near the center and near the periphery of the encapsulant sheet during heat treatment during integration as a solar cell module.

JP 2009-10277 A WO2012/073971

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a sealing material sheet for a solar cell module, which is a sealing material sheet using a polyethylene resin, does not require a cross-linking step, has high productivity, has heat resistance and molding properties, and can suppress the occurrence of thickness unevenness when integrated as a solar cell module.

As a result of intensive studies, the present inventors found that the above problems can be solved by making the encapsulating material sheet into a multi-layered sheet having a configuration of skin layer-core layer-skin layer, and optimizing the composition of the encapsulating material sheet so that the inflection point temperatures at which the rate of change of the coefficient of linear expansion with respect to the resin temperature locally increases exist at two locations within each specific temperature range, thereby completing the present invention. More specifically, the present invention provides the following.

(1) A sealing material sheet for a solar cell module, which uses a polyethylene resin as a base resin and has a density of 0.905 g/cm³0.925 g/cm or more³and a core layer formed on the outermost surface of the encapsulant sheet, which uses a polyethylene resin as a base resin and has a density of 0.875 g / cm³0.905 g/cm or more³In a multi-layer sheet comprising a skin layer containing a silane-modified polyethylene-based resin and having a coefficient of linear expansion measured in accordance with JIS K 7197 and expressed as a function of the resin temperature, there are two inflection point temperatures at which the rate of change in the coefficient of linear expansion locally increases only before and after that temperature, and the first inflection point temperature on the lower temperature side of the two inflection point temperatures is in the range of 55° C. or more and 70° C. or less, and the high temperature side of the inflection point temperatures. A sealing material sheet having a second inflection point in the range of 80° C. or higher and 95° C. or lower.

(2) A sealing material sheet for a solar cell module, which uses a polyethylene resin as a base resin and has a density of 0.905 g/cm³More than 0.925g/cm³The following core layer and a polyethylene resin as a base resin, with a density of 0.875 g / cm³0.905 g/cm or more³a skin layer containing a silane-modified polyethylene-based resin, and a multilayer sheet having a low melting point resin component having a melting point of 60° C. or more and less than 70° C. and a high melting point resin component having a melting point of 95° C. or more and 110° C. or less, both of which are contained in the total resin components of the encapsulant sheet in an amount of 25% or more and 70% or less by mass; A sealing material sheet containing 5% by mass or more and 15% by mass or less of the total resin components of the sealing material sheet.

(3) A vacuum heat lamination method in which the module constituent members containing the sealing material sheet according to (1) or (2) are laminated by vacuum suction and then heated and pressed,
A method for manufacturing a solar cell module, wherein the resin temperature of the encapsulant sheet when starting thermocompression bonding after the vacuum suction is equal to or higher than the first inflection point temperature, and the thermocompression bonding is performed at the second inflection point temperature or higher.

According to the present invention, it is possible to provide an encapsulant sheet for a solar cell module that does not require a cross-linking step, is highly productive, does not require a cross-linking step, has excellent heat resistance and molding properties, and is capable of suppressing unevenness in thickness when integrated into a solar cell module.

BRIEF DESCRIPTION OF THE DRAWINGS It is sectional drawing which shows typically the layer structure of the sealing material sheet|seat of this invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view schematically showing an example of the layer structure of a solar cell module using the encapsulant sheet of the present invention and a thin-film solar cell element. FIG. 3 is a partial enlarged view of FIG. 2 and is a drawing for explaining the molding characteristics of the encapsulant sheet of the present invention when used in a thin-film solar cell module. FIG. 2 is a partially enlarged cross-sectional view of a conventional solar cell module in which a conventional encapsulant sheet having poor molding properties is used for a thin-film solar cell module. It is a graph which shows the correlation of the resin temperature of the sealing material sheet|seat of this invention, and a linear expansion coefficient.

Hereinafter, the encapsulant composition that can be used for producing the encapsulant sheet for the solar cell module of the present invention, the encapsulant sheet for the solar cell module of the present invention, and the solar cell module using the encapsulant sheet of the present invention will be sequentially described.

<Sealant sheet>
The encapsulant sheet of the present invention is an encapsulant sheet for a solar cell module, the base resin of which is a thermoplastic polyethylene resin that can be produced without undergoing a cross-linking treatment, and is a multi-layer sheet comprising a core layer, which is a layer having a relatively high density and a high melting point, and a skin layer, which is a layer having a relatively low density and a low melting point, which is formed on the outermost surface of the encapsulant sheet.

Here, the melting point in the present specification means the average value of the melting points obtained by calculating from the unique melting points of the respective components contained in the object to be measured and their compounding ratios.

For example, the melting point according to this definition of the encapsulant sheet or each resin layer constituting it can be obtained by measuring by differential scanning calorimetry (DSC). When the DSC curve has a plurality of valley peaks, the melting point of the peak with the largest peak area can be taken as the melting point of the encapsulant sheet or each resin layer.

In addition, as another method for specifying the melting point defined above from the sealing material sheet, when the measured linear expansion coefficient measured in accordance with JISK7179 is expressed as a function of the resin temperature, the linear expansion coefficient can be approximated by measuring the linear expansion peak temperature, which is the temperature at the maximum value when it changes from increase to decrease. According to this method, the melting point defined above can be specified from a finished product such as a sealing material sheet within a range of variation of about 2° C. or less.

As shown in the graph of FIG. 5, this encapsulant sheet has two inflection point temperatures at which the rate of change of the linear expansion coefficient locally increases only before and after the two inflection point temperatures when the coefficient of linear expansion measured in accordance with JIS K 7179 is expressed as a function of the resin temperature. is in the range of 80°C or higher and 95°C or lower.

The inflection point temperature in this specification refers to a point at which the rate of change of the linear expansion coefficient clearly changes before and after the resin temperature, such as points T1 and T2 in the graph of Example 1 in FIG. More specifically, it refers to a temperature at which the rate of change in the coefficient of linear expansion between the resin temperature and the resin temperature +3° C. is greater than the rate of change in the linear expansion coefficient between the resin temperature of about −3° C. and the resin temperature, and the rate of increase is at least 1.2 times or more, preferably 1.35 times or more.

In order for the sealing material sheet to have two inflection point temperatures within the specific temperature range as described above, it can be realized by including a low melting point resin component and a high melting point resin component each having a specific melting point range peculiar to the present invention, and an intermediate melting point resin component having an intermediate melting point between them in a predetermined ratio in all the resin components of the sealing material sheet.

The low melting point resin component is a resin component having a melting point of 60° C. or more and less than 70° C., and the high melting point resin component is a resin component having a melting point of 95° C. or more and 110° C. or less. The encapsulant sheet of the present invention may contain 25% by mass or more and 60% by mass or more of the low melting point resin component and the high melting point resin component in the total resin components of the encapsulating material sheet.

The intermediate melting point resin component mentioned above refers to a resin component having a melting point higher than the melting point of the low melting point resin component by 20° C. or more and lower than the melting point of the high melting point resin component by 10° C. or more. The encapsulant sheet of the present invention may contain the intermediate melting point resin component in an amount of 8% by mass or more and 15% by mass or more based on the total resin components of the encapsulant sheet.

The details of the encapsulant composition that can be formed with the content ratio of each resin component having a different melting point within the above range will be described later. In this specification, the melting point of the resin forming each layer of the encapsulant sheet refers to the melting point of the resin that can be measured by differential scanning calorimetry (DSC).

As shown in FIG. 1 , the encapsulant sheet 1 has a core layer 11 and skin layers 12 are formed on both sides of the core layer 11 . However, even a sealing material sheet in which the core layer has a multi-layer structure and other functional layers are arranged in the core layer is within the scope of the present invention as long as it is provided with a core layer and a skin layer having the constituent elements of the present invention and other constituent elements of the present invention. In this specification, the skin layers refer to layers arranged on both outermost surface sides of the multilayer encapsulant sheet, and the core layers refer to inner layers other than the skin layers in the multilayer encapsulant sheet. Although the core layer itself may have a multi-layer internal structure, a typical embodiment of the present invention is a sealing material sheet 1 having a three-layer structure in which skin layers are laminated on both sides of a core layer having a single-layer structure.

The average density of all layers of the encapsulant sheet 1 having a three-layer structure including the core layer 11 and the skin layer 12 is preferably 0.895 g/cm ³ or more and 0.915 g/cm ³ or less. The density of the core layer 11 should be 0.905 g/cm ³ or more and 0.925 g/cm ³ or less, preferably 0.910 g/cm ³ or more and 0.920 g/cm ³ or less. The density of the skin layer 12 should be 0.875 g/cm ³ or more and 0.905 g/cm ³ or less, preferably 0.880 g/cm ³ or more and 0.890 g/cm ³ or less.

The MFR of the sealing material sheet 1 having a three-layer structure including the core layer 11 and the skin layer 12 is preferably 3.0 g/10 min or more and less than 5.0 g/10 min, and preferably 3.3 g/10 min or more and less than 3.8 g/10 min. When the MFR of the sealing material sheet 1 is less than 5.0 g/10 min, the required heat resistance of the sealing material sheet 1 can be provided, and when the MFR is 3.0 g/10 min or more, the required molding properties of the sealing material sheet 1 can be provided.

The MFR in this specification is a value obtained by the following method, unless otherwise specified.
MFR (g/10min): Measured according to JIS K7210. Specifically, a synthetic resin was heated and pressurized at 190°C in a cylindrical container heated by a heater, and the amount of resin extruded per 10 minutes from an opening (nozzle) provided at the bottom of the container was measured. An extrusion type plastometer was used as the test machine, and the extrusion load was 2.16 kg.
In addition, the MFR of the multilayer encapsulant sheet was measured by the above-described treatment while all the layers were integrally laminated, and the obtained measured value was used as the MFR value of the multilayer encapsulant sheet.

The total thickness of the sealing material sheet 1 having a three-layer structure including the core layer 11 and the skin layer 12 is preferably 250 μm or more and 600 μm or less, more preferably 300 μm or more and 550 μm or less. If the total thickness is less than 250 μm, impact cannot be sufficiently mitigated, but if the total thickness is 250 μm or more, for example, even when the sealing material sheet 1 is thinned to a total thickness of about 250 μm, it is possible to combine molding properties and heat resistance at a sufficiently preferable level. If the total thickness exceeds 600 μm, it is difficult to obtain a further improvement in the shock absorbing effect, it is not possible to meet the demand for thinner solar cell modules, and it is uneconomical.

The thickness of the core layer 11 in the sealing material sheet 1 is 200 μm or more and 400 μm or less, preferably 250 μm or more and 350 μm or less. Each layer of the skin layer 12 has a thickness of 30 μm or more and 100 μm or less, preferably 35 μm or more and 80 μm or less. The total thickness of the two skin layers 12 laminated on both sides of the core layer is 1/20 or more and 1/3 or less, preferably 1/15 or more and 1/4 or less of the total thickness of the sealing material sheet 1. By setting the thickness of each layer of the encapsulant sheet 1 within such a range, the heat resistance and molding properties of the encapsulant sheet 1 can be maintained within favorable ranges.

Formation of the encapsulant sheet 1 is performed by various molding methods such as injection molding, extrusion molding, blow molding, compression molding, and rotational molding, which are commonly used for thermoplastic resins. One example of a method for forming a sheet when the encapsulant sheet is a multilayer film is a method of forming by co-extrusion using three types of melt-kneading extruders.

However, in any of the molding methods described above, the melt molding temperature in the production of the encapsulant sheet 1 is preferably the melting point of the base resin of the encapsulant composition for the core layer contained in the encapsulant composition +30° C. or higher. Specifically, a high temperature in the range of 175°C to 230°C is preferable, and a high temperature in the range of 190°C to 210°C is more preferable. Since the encapsulant composition used for the encapsulant sheet 1 is a thermoplastic composition that does not contain a cross-linking agent, it is not necessary to consider the control of undesirable progress of cross-linking during melt molding. As a result, in the production of a sealing material sheet using a polyethylene resin as a base resin, the temperature limit when using a thermosetting sealing material composition that requires a cross-linking treatment, which has been common in the past, is solved. In order to improve productivity, the melt molding temperature can be set in a higher high temperature range. Thereby, the encapsulant sheet 1 can be manufactured with higher productivity than a conventional thermosetting encapsulant sheet.

<Sealant composition>
The encapsulant sheet 1 can be produced by melt-molding a encapsulant composition described in detail below. As for the sealing material composition, a sealing material composition for the core layer and a sealing material composition for the skin layer are used separately for each layer. Then, the encapsulating material sheet 1 can be produced by forming a multi-layer sheet having a three-layer structure in which the core layer is the inner layer and the skin layer is the outermost layer, using the encapsulating material compositions for the core layer and the skin layer.

[Sealant composition for core layer]
The encapsulant composition for the core layer is a thermoplastic encapsulant composition that contains a polyethylene resin as a base resin, does not contain a cross-linking agent, and does not require a cross-linking step during molding of the encapsulant sheet. In addition to the low-density polyethylene resin (LDPE) used as the base resin, other resins such as silane-modified polyethylene resins and other components may be contained in an appropriate amount within a range that does not impair the effects of the present invention.

As the base resin of the sealing material composition for the core layer, a low density polyethylene resin (LDPE), a linear low density polyethylene resin (LLDPE), or a metallocene linear low density polyethylene resin (M-LLDPE) can be preferably used. Among them, from the viewpoint of long-term reliability of the solar cell module, a low-density polyethylene-based resin (LDPE) can be particularly preferably used as the sealing material composition for the core layer. In the present specification, the term "base resin" refers to a resin having the largest content ratio among the resin components of the resin composition containing the base resin.

It is preferable that the sealant composition for the core layer further contains a predetermined amount of a silane-modified polyethylene resin in addition to the base resin. In the sealing material composition for the core layer, the silane-modified polyethylene-based resin is not necessarily an essential component, but when the silane-modified polyethylene-based resin is added to the sealing material composition for the core layer, it is preferable to use the silane-modified polyethylene-based resin having a weight average molecular weight of 70000 or more and a melting point of about 90° C. in terms of polystyrene, and to act as an intermediate melting point resin component in the sealing material sheet.

The density of the sealing material composition for the core layer is 0.905 g/cm ³ or more and 0.925 g/cm ³ or less, preferably 0.910 g/cm ³ or more and 0.920 g/cm ³ or less. By setting the density of the encapsulant composition for the core layer within the above range, the encapsulant sheet 1 can be provided with a good balance of heat resistance and molding properties without undergoing a cross-linking treatment.

The encapsulant composition for the core layer preferably contains the above-mentioned high melting point resin component and low melting point resin component as essential components, and further contains an intermediate melting point resin component. The proportions of these resin components are not limited to specific proportions, but may be appropriately adjusted so that all layers of the sealing material sheet 1, including the skin layer, contain the above three resin components having different melting points in the above specific proportions. As described above, the specific proportion is a proportion of 25% by mass or more and 70% by mass or less for both the low melting point resin component and the melting point resin component, and 5% by mass or more and 15% by mass or less for the intermediate melting point resin component in the total resin components of the encapsulant sheet.

On the premise that the content ratio of each resin component in all layers of the encapsulant sheet is adjusted to the above ratio, the melting point of the encapsulant composition for the core layer is preferably 70° C. or higher and 110° C. or lower, and more preferably 80° C. or higher and 100° C. or lower. As long as the melting point of the core layer 11 of the encapsulant sheet 1 can be maintained within the above range, it is possible to appropriately mix polyethylene-based resins having different melting points to prepare the encapsulant composition for the core layer. For example, according to a resin composition obtained by mixing 15 parts by mass, 8 parts by mass, and 82 parts by mass of three types of polyethylene resins having melting points of 60° C., 90° C., and 105° C., respectively, the melting point of the core layer as a whole can be set to 97° C., and such a compounding example of the material resin can be exemplified as a preferable resin compounding example of the sealing material composition for the core layer.

By maintaining the melting point of the core layer encapsulant composition at 70° C. or higher as described above, the encapsulant sheet 1 can be provided with necessary heat resistance. In addition, the melting point of the sealing material composition for the core layer generally needs to be about 110° C. or lower in relation to the heating conditions during melt molding for sheet formation as a sealing material sheet and during thermal lamination processing for integration as a solar cell module.

The melt mass flow rate (MFR) of the encapsulant composition for the core layer may be 3.0 g/10 min or more and less than 5.0 g/10 min, and as long as the MFR is 0.8 g/10 min or more and less than 5.0 g/10 min, a polyethylene resin may be appropriately mixed and used. By setting the MFR of the sealing material composition for the core layer within the above range, the sealing material sheet 1 can be provided with well-balanced heat resistance and molding properties.

[Sealant composition for skin layer]
Like the core layer sealing material composition, the skin layer sealing material composition is also a thermoplastic sealing material composition that uses a polyethylene resin as a base resin and does not contain a cross-linking agent. Further, it is the same as the sealing material composition for the core layer in that it may contain other components in an appropriate amount within a range that does not impair the effects of the present invention.

As the base resin of the sealing material composition for the skin layer, similarly to the sealing material composition for the core layer, a low density polyethylene resin (LDPE), a linear low density polyethylene resin (LLDPE), or a metallocene linear low density polyethylene resin (M-LLDPE) can be preferably used. Among them, metallocene-based linear low-density polyethylene resin (M-LLDPE) can be particularly preferably used as the composition for the skin layer from the viewpoint of molding properties.

The skin layer encapsulant composition used in the encapsulant sheet 1 preferably further contains a silane-modified polyethylene resin in addition to the base resin. In the sealing material composition for the skin layer, the silane-modified polyethylene-based resin is not necessarily an essential component, but when the silane-modified polyethylene-based resin is added to the sealing material composition for the skin layer, it is preferable to use the silane-modified polyethylene-based resin having a polystyrene-equivalent weight average molecular weight of 70000 or more and a melting point of about 90° C., and to act as an intermediate melting point resin component in the sealing material sheet.

The density of the sealing material composition for the skin layer is 0.880 g/cm ³ or more and 0.910 g/cm ³ or less, more preferably 0.899 g/cm ³ or less. By setting the density of the sealing material composition for the skin layer within the above range, the adhesion of the sealing material sheet 1 can be maintained within a preferable range.

The melting point of the sealing material composition for the skin layer should be 70° C. or higher and 90° C. or lower, preferably 70° C. or higher and 80° C. or lower. As long as the melting point of the skin layer can be maintained within the above range similarly to the core layer, the sealing material composition for the skin layer can be prepared by appropriately mixing polyethylene resins having different melting points. For example, according to a resin composition obtained by mixing 65 parts by mass, 8 parts by mass, and 20 parts by mass of three types of polyethylene-based resins having melting points of 60° C., 90° C., and 97° C., respectively, the melting point of the core layer as a whole can be set to 73° C., but such a compounding example of the material resin can be exemplified as a preferable resin compounding example of the sealing material composition for the core layer. By setting the melting point of the sealing material composition for the skin layer to 70° C. or higher, the necessary heat resistance can be imparted to the sealing material sheet 1 . Further, by setting the melting point of the sealing material composition for the skin layer to 90° C. or less, the molding properties of the sealing material sheet can be maintained within a preferable range when integrated as a solar cell module.

The melt mass flow rate (MFR) of the sealant composition for the skin layer may be 3.0 g/10 min or more and less than 5.0 g/10 min, and as long as the MFR is 0.8 g/10 min or more and less than 5.0 g/10 min, a polyethylene resin may be appropriately mixed and used. By setting the MFR of the sealing material composition for the skin layer within the above range, the sealing material sheet 1 can be provided with well-balanced heat resistance and molding properties.

[Silane-modified polyethylene resin]
The encapsulant sheet 1 preferably contains a silane-modified polyethylene resin in each layer, and the silane-modified polyethylene resin contained in each layer is preferably a "high molecular weight type silane-modified polyethylene resin".

The silane-modified polyethylene-based resin is obtained by graft-polymerizing an ethylenically unsaturated silane compound as a side chain to a linear low-density polyethylene-based resin (LLDPE) or the like as a main chain. The term "silane-modified polyethylene-based resin" used herein refers to a copolymer obtained by copolymerizing at least an α-olefin and an ethylenically unsaturated silane compound as comonomers, and optionally other unsaturated monomers as comonomers, and includes modified products or condensates of the copolymer.

A copolymer of an α-olefin and an ethylenically unsaturated silane compound, or a modified or condensed product thereof, may be obtained by, for example, polymerizing one or more α-olefins and, if necessary, one or more other unsaturated monomers, simultaneously or stepwise, in the presence of a radical polymerization initiator and, if necessary, a chain transfer agent, in a desired reaction vessel, and then adding one or more ethylenically unsaturated silane compounds to the polyolefin polymer produced by the polymerization. is graft-copolymerized, and further, if necessary, the silane compound portion constituting the graft copolymer produced by the copolymer is modified or condensed to produce a copolymer of α-olefin and an ethylenically unsaturated silane compound or a modified or condensed product thereof.

As the silane-modified polyethylene-based resin, any of random copolymers, alternating copolymers, block copolymers, and graft copolymers can be preferably used. Graft copolymers are more preferable, and graft copolymers obtained by polymerizing polyethylene for polymerization as a main chain and ethylenically unsaturated silane compounds as side chains are even more preferable. Such a graft copolymer increases the degree of freedom of silanol groups that contribute to adhesive strength, and thus can improve the adhesion of the encapsulant sheet to other members in the solar cell module, particularly glass substrates and the like.

The content of the ethylenically unsaturated silane compound when composing the silane-modified polyethylene resin is, for example, about 0.001 to 15% by mass, preferably about 0.01 to 5% by mass, particularly preferably about 0.05 to 2% by mass, relative to the total mass of the copolymer. When the content of the ethylenically unsaturated silane compound that constitutes the copolymer of the α-olefin and the ethylenically unsaturated silane compound is within the above range, the adhesion of the encapsulant sheet to glass is remarkably improved. If the content of the silane compound exceeds the above range, the tensile elongation and heat-sealability of the encapsulant sheet tend to deteriorate, which is not preferable.

In the encapsulant sheet 1, among the silane-modified polyethylene resins described above, a "high-molecular-weight type silane-modified polyethylene resin" having a specific molecular weight range is preferably used as an essential addition resin to the encapsulant composition for the skin layer. The molecular weight of this "high-molecular-weight type silane-modified polyethylene-based resin" is 70,000 or more and 120,000 or less, preferably 90,000 or more and 120,000 or less, in polystyrene equivalent weight average molecular weight. If the molecular weight of the silane-modified polyethylene resin exceeds 120,000, the compatibility with the base resin, which is assumed to be preferable when the MFR is about 3.0 g/10 min or more and 5.0 g/10 min or less, is not preferable.

The measurement of the molecular weight of each resin component constituting the sealing material sheet 1 can be performed using a conventionally known GPC method. Since polyolefin is difficult to dissolve in a solvent at room temperature, it is preferable to measure by high-temperature GPC at 140 to 150° C. using trichlorobenzene, o-dichlorobenzene, or the like as a solvent. Particularly in the case of the encapsulant sheet 1, in order to measure the molecular weight of the silane-modified polyethylene resin contained in the skin layer 12, the skin layer of the encapsulant sheet 1, which is a multi-layer sheet, is separated, and the molecular weight of the silane-modified polyethylene resin can be specified by reading the molecular weight corresponding to the component identified by IR by combining molecular weight measurement and component analysis by GPC-FTIR or the like. The number average molecular weight Mn, the weight average molecular weight Mw, and the degree of dispersion d when there are Ni (pieces) of polymers having a molecular weight Mi (g/mol) in the skin layer of the encapsulant sheet 1 are defined by the following equations.
Number average molecular weight Mn = Σ (MiNi) / ΣNi
Weight average molecular weight Mw=Σ(Mi ² Ni)/ΣMiNi
Dispersion d=Mw/Mn

[Other additive ingredients]
Adhesion improvers can be appropriately added to each of the core layer and skin layer sealing material compositions constituting the sealing material sheet 1, the constituting sealing material composition, particularly the sealing material composition for the skin layer. As the adhesion improver, a known silane coupling agent can be used, and a silane coupling agent having an epoxy group (hereinafter also referred to as "epoxy-silane coupling agent") or a silane coupling agent having a mercapto group (hereinafter also referred to as "mercapto-based silane coupling agent") can be particularly preferably used.

Each sealing material composition for the core layer and the skin layer can further contain other components. For example, components such as a weather-resistant masterbatch for imparting weather resistance to the encapsulant sheet, various fillers, light stabilizers, ultraviolet absorbers, and heat stabilizers are exemplified. Although the content of these substances varies depending on the particle shape, density, etc., it is preferable that they are in the range of 0.001% by mass to 5% by mass in the encapsulant composition. By including these additives, it is possible to provide the encapsulant sheet with stable mechanical strength over a long period of time, an effect of preventing yellowing and cracking, and the like.

<Solar cell module>
The encapsulant sheet 1 can be used universally for various conventionally known solar cell modules. In general, in a solar cell module, encapsulant sheets are placed on both sides of a solar cell element in a manner of sealing by sandwiching the solar cell elements, but the encapsulant sheet 1 can be placed on both sides of the solar cell element as encapsulant sheets, or only the encapsulant sheet on either side can be the encapsulant sheet 1. In addition, the encapsulant sheet 1 can be particularly preferably used for a solar cell module such as a thin-film solar cell module in which a relatively large protrusion such as a lead wire is formed on a solar cell element.

FIG. 2 is a cross-sectional view showing an example of the layer structure of a thin-film solar cell module 10 that can be constructed using the encapsulant sheet 1 of the present invention. The solar cell module 10 has a configuration in which a transparent front substrate 2, thin-film solar cell elements 3 arranged on the front surface of the transparent front substrate 2, a sealing material sheet (sealing material sheet 1), and a back protection substrate 4 are laminated in this order from the incident light receiving surface side. In the thin-film solar cell module 10 , the encapsulant sheet (encapsulant sheet 1 ) is laminated on the non-light-receiving surface side of the solar cell elements 3 .

Here, in the solar cell module 10, as shown in FIG. 3, the surface of the solar cell element 3 on the non-light-receiving surface side has unevenness due to the metal electrodes 31 and the lead wires 32 for current collection. In the case of using a conventional encapsulant sheet using a polyethylene-based resin as a base resin, if heat resistance is ensured by cross-linking or simply increasing the density, as shown in FIG.

However, when the encapsulating material sheet 1 that achieves both high levels of heat resistance and molding properties is arranged on this uneven surface, as shown in FIG. In other words, the encapsulant sheet 1 can be particularly preferably used when unevenness formed by protrusions such as the lead wires 32 is present on the surface of the solar cell element as in the solar cell module 10 . When the thickness of the projections of the unevenness is 50% or more and 90% or less of the thickness of the encapsulant sheet 1, the molding properties of the encapsulant sheet are particularly well exhibited, and as described above, it is possible to sufficiently prevent the formation of voids V due to the presence of surface unevenness of the solar cell element.

More specifically, when the lead wire 32 has a thickness (d ₁ ) of about 250 μm or more, the encapsulant sheet 1 exhibits a special effect that is significantly different from that of the conventional product. For example, as shown in FIG. 5, when a thick lead wire 32 is arranged, when a sealing material sheet 1 made of a conventional common polyethylene resin is arranged on the uneven surface, generally, when the thickness (d ₁ ) of the lead wire 32 with respect to the thickness (d ₂ ) of the sealing material sheet 1 exceeds 50% as a rough guideline, the formation of the void V often becomes a problem. However, as shown in FIG. 4, when the sealing material sheet 1 is arranged on such an uneven surface, the formation of the void V can be sufficiently prevented if the thickness (d ₁ ) of the lead wire 32 with respect to the thickness (d ₂ ) of the sealing material sheet 1 is 90% or less. In the present invention, when there is a state in which a plurality of lead wires are stacked, such as when the lead wires are arranged to cross each other, the total thickness of the plurality of lead wires in the stacked portion is considered to be the "thickness of the lead wire", that is, the "thickness of the convex portion".

[Method for manufacturing solar cell module]
The solar cell module 10 can be manufactured by sequentially laminating the constituent members including the encapsulant sheet 1 and then integrating them by vacuum suction or the like, and then thermocompression molding the above members into an integrally molded body by a molding method such as a lamination method.

Here, in a general laminator used in the above manufacturing method, the heating temperature during vacuum suction differs greatly between the central portion and the edge portions of the sealing material sheet, and there is a tendency for the central portion to be overheated and the edge portions to be insufficiently heated. When a conventional thermoplastic multi-layer sheet is used as a sealing material sheet, the degree of melting and softening of the surface of the sealing material sheet at the point immediately before heat-pressing varies greatly due to this, and as a result, the thickness of the sealing material sheet after heat-pressing may vary.

The sealing material sheet 1 achieves both heat resistance and moldability by mixing a high melting point resin component and a low melting point resin component, and further adds an appropriate amount of an intermediate melting point resin component.

In the method for manufacturing a solar cell module of the present invention, the above effects can be obtained by adjusting the heating conditions so that the resin temperature of the encapsulant sheet at the time of starting thermocompression bonding after vacuum suction is equal to or higher than the first inflection point temperature and equal to or lower than the second inflection point temperature.

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples.

<Manufacturing encapsulant sheet for solar cell module>
The raw materials for the encapsulant composition described below were mixed at the ratios (parts by mass) shown in Table 1 below to obtain the encapsulant composition for the core layer and the encapsulant composition for the skin layer of the encapsulant sheets of Examples and Comparative Examples, respectively. Using a film molding machine having an extruder of φ30 mm and a T-die of 200 mm width, each resin sheet for a core layer and a skin layer was produced from each encapsulating material composition at an extrusion temperature of 210° C. and a take-up speed of 1.1 m/min. These resin sheets were laminated to produce encapsulating material sheets with a three-layer structure of Examples and Comparative Examples each having a core layer and skin layers arranged on both outermost surfaces. The total thickness of each encapsulant sheet in Examples and Comparative Examples was 450 μm. Regarding the thickness ratio of each layer of the encapsulant sheets having a three-layer structure in Examples and Comparative Examples, the thickness ratio of skin layer:core layer:skin layer was set to 1:5.5:1 for all encapsulant sheets.

The following raw materials were used as the material resin of the encapsulant composition for molding each resin sheet for the encapsulant sheet.
Polyethylene resins 1 to 4 (represented as "PE1 to 4" in the table)
: Both are metallocene-based linear low-density polyethylene resins (M-LLDPE). Density, melting point, and MFR at 190°C are shown in Table 1, respectively.
Silane-modified polyethylene resin 1 (denoted as "PS1" in the table)
: A silane-modified polyethylene resin obtained by mixing 2 parts by mass of vinyltrimethoxysilane and 0.15 parts by mass of dicumyl peroxide as a radical generator (reaction catalyst) with 100 parts by mass of a metallocene-based linear low-density polyethylene resin having a density of 0.900 g/cm ³ and an MFR of 2.0 g/10 min, followed by melting and kneading at 200°C. Density 0.900 g/cm ³ , MFR 1.0 g/10 minutes. Melting point 90°C.
Silane-modified polyethylene resin 2 (denoted as "PS2" in the table)
: A silane-modified polyethylene resin obtained by mixing 2 parts by mass of vinyltrimethoxysilane and 0.15 parts by mass of dicumyl peroxide as a radical generator (reaction catalyst) with 100 parts by mass of a metallocene linear low-density polyethylene resin having a density of 0.880 g/cm ³ and an MFR of 3.5 g/10 min, followed by melting and kneading at 200°C. Density 0.880 g/cm ³ , MFR 2.0 g/10 minutes. Melting point 60°C.

<Measurement of coefficient of linear expansion>
The coefficient of linear expansion was measured according to JIS K7197 for each sealing material sheet of Examples and Comparative Examples cut into 5 mm×20 mm. The measurement was performed using the following measurement apparatus and measurement conditions.
Measuring device: Thermomechanical device manufactured by Seiko Instruments (TMA/SS-6000)
Constant load tension mode: 20mN, measurement temperature range: -50°C to 110°C
Table 1 shows the linear expansion peak temperature and the linear expansion convergence point temperature of the sealing material sheets of each example and comparative example obtained by the above measurements. FIG. 5 is a graph showing the coefficient of linear expansion measured as described above as a function of the resin temperature, particularly for Example 1 and Comparative Example 2. As shown in FIG. From the figure, it can be seen that only Example 1 has two inflection point temperatures, the first inflection point temperature being in the range of 55° C. or higher and 70° C. or lower (60° C.), and the second inflection point being in the range of 80° C. or higher and 95° C. or lower (83° C.).

<Evaluation Example 1: Molding Properties>
A lead wire (diameter of 250 μm) was placed on the surface of a white plate tempered glass with a flat surface, and the lead wire was further covered, and each sealing material sheet of Examples and Comparative Examples cut to 150 mm × 150 mm was laminated. The resin temperature (ultimate temperature) of the encapsulant sheet during lamination during this heat treatment was 147°C. These samples for solar cell module evaluation were visually observed, and molding characteristics were evaluated according to the following evaluation criteria.
(Evaluation Criteria) A: Completely follow the unevenness of the substrate surface facing the encapsulant sheet. No void formation was observed.
B: Up to 5 bubbles within 2 mm ² were observed.
C: A portion of the encapsulant sheet did not completely conform to the unevenness of the facing substrate surface, and a partial lamination failure portion (void) was formed in the vicinity of the lead wire.
The evaluation results are shown in Table 2 as "molding properties".

<Evaluation Example 2: Heat resistance test>
A thermal creep test was conducted as a heat resistance test. On the same glass plate as in Evaluation Example 1, one sheet of the sealing material sheet of Example and Comparative Example cut into 5 cm × 7.5 cm was superimposed, and the same glass plate of 5 cm × 7.5 cm as in Evaluation Example 1 was superimposed thereon. After that, the large-sized glass was placed vertically and left at 105° C. for 12 hours, and the moving distance (mm) of the 5 cm×7.5 cm glass plate was measured and evaluated. Evaluation was performed according to the following criteria.
(Evaluation criteria) A: 0.0 mm or more and less than 0.5 mm
B: 0.5 mm or more and less than 1.0 mm
C: 1.0 mm or more

<Evaluation Example 3: Uniformity of encapsulant thickness>
On the same glass plate of 40 cm × 40 cm as in Evaluation Example 1 above, one sealing material sheet of Examples and Comparative Examples cut into 40 cm × 40 cm was placed on top of each other. After that, the thickness uniformity of the encapsulant sheet was measured by comparing the film thicknesses at the central portion and the edge portion of the substrate.
The measurement was performed with a "micrometer" manufactured by Mitutoyo Corporation.
Evaluation was performed according to the following criteria.
The resin temperature of the sealing material sheet at the time of starting the thermocompression bonding after vacuum suction was 90° C., the first inflection point temperature of the sealing material sheet of Example 1 was (60° C.) or higher, and the thermocompression bonding temperature was a temperature (157° C.) exceeding the second inflection point.
(Evaluation Criteria) A: The difference (range) between the maximum film thickness and the minimum film thickness is less than 60 μm
B: The difference (range) between the maximum film thickness and the minimum film thickness is 60 μm or more. The evaluation results are shown in Table 2 as “uniformity”.

From Tables 1 and 2 and FIG. 5, it can be seen that the encapsulant sheet of the present invention is an encapsulant sheet for a solar cell module that does not require a cross-linking step, has high productivity, has heat resistance and molding properties, and can also suppress the occurrence of unevenness in thickness when integrated as a solar cell module, although it is an encapsulant sheet that uses a polyethylene-based resin.

REFERENCE SIGNS LIST 1 sealing material sheet 11 core layer 12 skin layer 2 transparent front substrate 3 solar cell element 31 metal electrode 32 lead wire 4 backside protective substrate 10 solar cell module

Claims (3)

A sealing material sheet for a solar cell module,
Contains polyethylene resin,
When the linear expansion coefficient measured in accordance with JIS K7197 is expressed as a function of the resin temperature, there are two inflection point temperatures that are temperatures at which the rate of change in the linear expansion coefficient locally increases only before and after that temperature. There is
encapsulant sheet. A solar cell module using the encapsulant sheet according to claim 1,
A transparent front substrate, a solar cell element, a sealing material sheet, and a back protective substrate are laminated in this order.
solar module. A lead wire is arranged between the solar cell element and the encapsulant sheet,
The lead wire has a thickness of 90% or less with respect to the thickness of the encapsulant sheet.
The solar cell module according to claim 2 .

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