JP7852388B2 - Resin film for current collector sheets, current collector sheets, solar cell elements with current collector sheets, and solar cells - Google Patents

Description

This disclosure relates to a film for current collector sheets, a current collector sheet, a solar cell element with a current collector sheet, and a solar cell.

In recent years, global warming, caused by carbon dioxide, has become a worldwide problem. In response to this issue, solar cells, which utilize solar energy as an environmentally friendly and clean energy source, are attracting attention, and active research and development are underway.

Solar cells are used, for example, in the form of modules that connect multiple solar cell elements. A conventional method of connecting solar cell elements has been to use busbars, which are strip-shaped wires approximately 2 mm to 5 mm wide. However, this busbar method presents a problem: in the solar cell module, sunlight is physically blocked in the areas where the busbars are placed, reducing the amount of sunlight entering the solar cell elements.

In contrast, in recent years, a method called multi-wire connection has begun to be adopted, which uses wires (thin wire-like conductors) with a diameter of approximately 150 μm to 300 μm to connect solar cell elements. In multi-wire connection, one method for fixing the wires to the solar cell elements is to embed the wires in a heat-sealable resin film to form a current collector sheet, and then heat-press the current collector sheet onto the solar cell elements to fix them (Patent Documents 1 and 2). The resin film used for the current collector sheet is also called a connecting film.

International Publication No. 2004/021455 International Publication No. 2017/076735 Japanese Patent Publication No. 2020-174060

In multi-wire connections, when embedding wires in a heat-sealable resin film to create a current collector sheet, it is necessary that the wires are stably held in the resin film to prevent them from detaching before the current collector sheet is heat-pressed onto the solar cell element. Furthermore, when heat-pressing the current collector sheet onto the solar cell element, the resin film must wrap around the wires to secure them to the solar cell element. Therefore, the resin film used in current collector sheets for multi-wire connections requires both adhesion to the wires and the ability to embed the wires effectively.

For example, Patent Document 3 discloses a technology for fixing current collector wires for a solar cell module, having a base layer and a wire holding layer. This technology optimizes the complex viscosity of the wire holding layer within a specific range, thereby avoiding poor electrical conductivity caused by the entire wire becoming embedded in the wire holding layer, while simultaneously providing adequate wire holding force.

However, depending on the wire material, the adhesion of the resin film to the wire may not be sufficient, and there is still room for improvement.

Furthermore, in multi-wire connections, when the solar cell elements with current collector sheets and other components are heat-sealed together to form a solar cell module, the resin film shrinks due to heat, causing the wires to shift relative to the solar cell elements. This wire shifting results in insufficient conductivity between the solar cell elements and the wires, leading to reduced reliability.

This disclosure is made in view of the above circumstances, and its main purpose is to provide a resin film for current collector sheets that exhibits excellent adhesion to wires, excellent wire embedding properties, and excellent thermal dimensional stability, as well as a current collector sheet using the same, a solar cell element with a current collector sheet, and a solar cell.

One embodiment of this disclosure provides a resin film for current collector sheets used in solar cells, comprising a base layer, an adhesive layer, and a polyethylene resin layer in that order, wherein the base layer contains polyethylene terephthalate resin, the melt mass flow rate (MFR) of the polyethylene resin layer at 190°C is 4 g/10 min or more and 8 g/10 min or less, and the thermal shrinkage rate when held at 150°C for 10 minutes is 2.0% or less.

Another embodiment of this disclosure provides a current collector sheet for use in a solar cell, comprising the above-described resin film for current collector sheets and a wire disposed on the side of the polyethylene resin layer of the resin film for current collector sheets.

Another embodiment of this disclosure provides a solar cell element with a current collector sheet, comprising the current collector sheet described above and a solar cell element disposed on the side of the polyethylene resin layer of the current collector sheet and electrically connected to the wire.

Another embodiment of this disclosure provides a solar cell comprising, in this order, a transparent substrate, a first encapsulant, the solar cell element with the current collector sheet described above, a second encapsulant, and a counter substrate.

The resin film for current collector sheets in this disclosure exhibits excellent adhesion to wires, superior wire embedding capabilities, and excellent thermal dimensional stability.

This is a schematic cross-sectional view illustrating an example of a resin film for a current collector sheet in this disclosure. These are schematic plan and cross-sectional views illustrating the current collector sheet in this disclosure. These figures show a schematic perspective view and a cross-sectional view illustrating a solar cell element with a current collector sheet in this disclosure. This is a schematic cross-sectional view illustrating an example of a current collector sheet in this disclosure. This is a schematic cross-sectional view illustrating a solar cell element with a current collector sheet as described in this disclosure. This is a schematic cross-sectional view illustrating a solar cell in this disclosure.

The embodiments of this disclosure will be described below with reference to the drawings and other illustrations. However, this disclosure can be implemented in many different ways and should not be interpreted as being limited to the embodiments described below. Furthermore, while the drawings may schematically represent the width, thickness, shape, etc., of parts in order to clarify the explanation, these are merely examples and should not limit the interpretation of this disclosure. In addition, in this specification and the drawings, elements similar to those described above in previously shown drawings will be denoted by the same reference numerals, and detailed explanations may be omitted as appropriate.

In this specification, when describing a configuration in which one component is placed on top of another component, unless otherwise specified, the terms "above" or "below" include both cases: the other component being placed directly above or below the component so as to be in contact with it, and the other component being placed above or below the component via yet another component. Furthermore, when describing a configuration in this specification in which one component is placed on the surface of another component, unless otherwise specified, the terms "on the surface" or "on the side of the surface" include both cases: the other component being placed directly above or below the component so as to be in contact with it, and the other component being placed above or below the component via yet another component.

Furthermore, in this specification, terms such as "film," "sheet," and "substrate" are not distinguished from each other based on differences in terminology.

The following provides a detailed description of the resin film for current collector sheets, current collector sheets, solar cell elements with current collector sheets, and solar cells as described in this disclosure.

A. Resin film for current collector sheets The resin film for current collector sheets in this disclosure is a resin film for current collector sheets used in solar cells, and comprises a base layer, an adhesive layer, and a polyethylene resin layer in this order, wherein the base layer contains polyethylene terephthalate resin, the melt mass flow rate (MFR) of the polyethylene resin layer at 190°C is 4 g/10 min or more and 8 g/10 min or less, and the thermal shrinkage rate when held at 150°C for 10 minutes is 2.0% or less.

The resin film for current collector sheets in this disclosure will be described with reference to the figures. Figure 1 is a schematic cross-sectional view illustrating the resin film for current collector sheets in this disclosure. As shown in Figure 1, the resin film 10 for current collector sheets has, in this order, a base layer 1 containing polyethylene terephthalate resin, an adhesive layer 2, and a polyethylene resin layer 3. The melt mass flow rate (MFR) of the polyethylene resin layer 3 is within a predetermined range. Furthermore, the thermal shrinkage rate when the resin film 10 for current collector sheets is held at 150°C for 10 minutes is less than or equal to a predetermined value.

Figures 2(a) and 2(b) are schematic plan views and cross-sectional views illustrating a current collector sheet having a resin film for current collectors according to this disclosure. Figure 2(b) is a cross-sectional view taken along line A-A in Figure 2(a). As shown in Figures 2(a) and 2(b), the current collector sheet 20 comprises a resin film for current collectors 10 and wires 11 arranged on the side of the polyethylene resin layer 3 of the resin film for current collectors 10. Thus, the resin film for current collectors 10 is used to support the wires 11. Figure 2(a) shows a schematic plan view of the current collector sheet as seen from the polyethylene resin layer side of the resin film for current collectors.

Figures 3(a) to 3(c) are schematic perspective views and cross-sectional views illustrating a solar cell element with a current collector sheet, which includes a current collector sheet having a resin film for the current collector sheet, as described in this disclosure. Figure 3(b) is a cross-sectional view taken along line A-A in Figure 3(a), and Figure 3(c) is a cross-sectional view taken along line B-B in Figure 3(a). As shown in Figures 3(a) to 3(c), the solar cell element with a current collector sheet 30 includes a current collector sheet 10 and a solar cell element 31 arranged on the side of the polyethylene resin layer 3 of the current collector sheet 10 and electrically connected to a wire 11. Thus, the resin film 10 for the current collector sheet is used to fix the wire 11 electrically connected to the solar cell element 31. Figures 3(a) to 3(c) show an example where the current collector sheet 20 has two resin films 10 for the current collector sheet, and a solar cell element 31 is arranged on each resin film 10.

In this disclosure, the base layer contains polyethylene terephthalate resin. The polyethylene terephthalate resin contained in the base layer has a melting point of approximately 260°C and high heat resistance. Furthermore, when molded into a film, polyethylene terephthalate resin provides good rigidity. Therefore, the base layer containing polyethylene terephthalate resin can impart rigidity to the resin film for the current collector sheet. Thus, when manufacturing a solar cell element with a current collector sheet using a current collector sheet having the resin film for the current collector sheet, the resin film for the current collector sheet can sufficiently press the wire against the solar cell element during thermocompression bonding. Furthermore, in this disclosure, by ensuring that the MFR of the polyethylene resin layer is within a predetermined range, the adhesion of the polyethylene resin layer to the wire and the embedding ability of the wire can be improved. Therefore, in a current collector sheet having the resin film for the current collector sheet, the wire can be securely fixed to the solar cell element by the resin film for the current collector sheet.

Furthermore, when solar cells with current-collecting sheets are used in solar cells, the resin film for the current-collecting sheet allows for good fixation of the wires to the solar cell elements, thereby suppressing the decrease in power generation efficiency due to poor wire adhesion.

Furthermore, in solar cells, by ensuring that the MFR (Metal Fiber Retention) of the polyethylene resin layer is within a predetermined range, good adhesion of the polyethylene resin layer to the solar cell element can be achieved.

Furthermore, as described above, in this disclosure, the resin film for the current collector sheet can have rigidity because the base layer contains polyethylene terephthalate resin. Therefore, thermal shrinkage of the resin film for the current collector sheet can be suppressed. Also, as described above, by having the MFR of the polyethylene resin layer within a predetermined range, the adhesion of the polyethylene resin layer to the wire, the embedding of the wire, and the adhesion to the solar cell element can be improved. Therefore, even when the thickness of the polyethylene resin layer is relatively thin, adhesion to the wire, the embedding of the wire, and the adhesion to the solar cell element can be ensured. When the thickness of the polyethylene resin layer is thin, the thermal shrinkage of the resin film for the current collector sheet is small. Therefore, having the MFR of the polyethylene resin layer within a predetermined range can also contribute to suppressing the thermal shrinkage of the resin film for the current collector sheet. Furthermore, since the resin film for the current collector sheet in this disclosure has a predetermined thermal shrinkage rate, thermal dimensional stability can be improved. Therefore, when manufacturing a solar cell element with a current collector sheet using a current collector sheet having a resin film for the current collector sheet, misalignment of the wire relative to the solar cell element during the heating process can be suppressed. Furthermore, when manufacturing solar cells using solar cell elements with current collector sheets, misalignment of the wires relative to the solar cell elements during the heating process can be suppressed. Moreover, even when the solar cell reaches high temperatures in its operating environment, misalignment of the wires relative to the solar cell elements can be suppressed. Therefore, the reliability of the solar cell can be improved.

Furthermore, since the resin film for current collector sheets in this disclosure exhibits excellent thermal dimensional stability, when manufacturing solar cell elements with current collector sheets using a current collector sheet having the resin film for current collector sheets, and when manufacturing solar cells using solar cell elements with current collector sheets, thermal shrinkage of the resin film for current collector sheets during the heating process can be suppressed, thereby preventing curl deformation.

The following describes the various components of the resin film for current collector sheets in this disclosure.

I. Structure of the resin film for current collector sheets The resin film for current collector sheets in this disclosure has a base layer, an adhesive layer, and a polyethylene resin layer in this order.

1. Polyethylene resin layer The polyethylene resin layer in this disclosure is a component that supports the wire when a resin film for current collection sheets is used as a current collection sheet.

(1) Characteristics of the polyethylene resin layer The melt mass flow rate (MFR) of the polyethylene resin layer at 190°C is preferably 4 g/10 min or more and 8 g/10 min or less, and more preferably 6 g/10 min or more and 8 g/10 min or less. By having the MFR of the polyethylene resin layer within the above range, the adhesion to the wire and the wire embedding ability can be improved. In particular, by having the MFR of the polyethylene resin layer of 6 g/10 min or more, a polyethylene resin layer with excellent adhesion to the wire and wire embedding ability can be obtained regardless of the wire material.

The MFR of the polyethylene resin layer refers to the MFR of the polyethylene resin composition constituting the polyethylene resin layer. Here, the melt mass flow rate (MFR) of the polyethylene resin layer can be measured in accordance with JIS K7210-1. The measurement conditions are a temperature of 190°C and a load of 2.16 kg.

The MFR of the polyethylene resin layer can be adjusted, for example, by the molecular weight of the polyethylene resin contained in the polyethylene resin layer.

Furthermore, the melting point of the polyethylene resin layer is not particularly limited as long as it exhibits the desired heat-welding properties. The melting point of the polyethylene resin layer is preferably, for example, 100°C or higher and 120°C or lower, and more preferably 105°C or higher and 110°C or lower. If the melting point of the polyethylene resin layer is too high, the heating temperature required when pressing the current collector sheet to the solar cell element will need to be increased, potentially increasing manufacturing costs or degrading the solar cell element. On the other hand, if the melting point of the polyethylene resin layer is too low, the polyethylene resin layer may melt in the solar cell's operating environment, making it difficult to fix the wires.

Here, the melting point of the polyethylene resin layer can be determined by differential scanning calorimetry (DSC) in accordance with JIS K7121 (Method for Measuring Transition Temperature of Plastics). If two or more melting point peaks are present, the higher temperature peak can be used as the melting point.

(2) Materials for the polyethylene resin layer (a) Polyethylene resin The polyethylene resin layer contains polyethylene resin. The polyethylene resin is not particularly limited as long as it is possible to obtain a polyethylene resin layer that satisfies the above-mentioned MFR. Examples of polyethylene resins include high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), metallocene-based linear low-density polyethylene (M-LLDPE), and very low-density polyethylene (VLDPE). The polyethylene resin may be used alone or in combination of two or more types. Among these, low-density polyethylene (LDPE) is preferred because of its good flexibility and processability.

The density of the polyethylene resin is not particularly limited, but is preferably, for example, 0.890 g/ ^cm³ or more and 0.930 g/ ^cm³ or less, and more preferably 0.900 g/ ^cm³ or more and 0.925 g/ ^cm³ or less. By having the density of the polyethylene resin within the above range, the flexibility and processability of the polyethylene resin layer can be improved, and the adhesion to the wire and the embedding ability of the wire can be improved.

Here, the density of polyethylene resin can be measured, for example, by the pycnometer method in accordance with JIS K7112.

The polyethylene resin layer may contain only polyethylene resin as its resin component, or it may contain polyethylene resin in addition to other resins. In the latter case, it is preferable that the polyethylene resin layer contains polyethylene resin as its main component. Note that "containing polyethylene resin as its main component" means that polyethylene resin accounts for the largest proportion of all resin components.

The proportion of polyethylene resin to the total resin components in the polyethylene resin layer is, for example, 50% by mass or more, may be 60% by mass or more, or 70% by mass or more. Furthermore, the proportion of polyethylene resin is, for example, 99% by mass or less, may be 95% by mass or less, or 90% by mass or less. The proportion of polyethylene resin may also be 100% by mass.

(b) Adhesion enhancer The polyethylene resin layer in this disclosure may contain an adhesion enhancer. The adhesion enhancer is a component that improves adhesion to wires and to solar cell elements.

Examples of adhesion improvers include silane-modified resins and silane coupling agents.

(i) Silane-modified resins Examples of silane-modified resins include silane-modified polyolefin resins. Silane-modified polyolefin resins are copolymers of α-olefins and ethylenically unsaturated silane compounds. The copolymer may be any of the following: random copolymers, alternating copolymers, block copolymers, and graft copolymers. Among these, the copolymer is preferably a graft copolymer, and more preferably a graft copolymer in which polyolefin is the main chain and ethylenically unsaturated silane compounds are polymerized as side chains. Such graft copolymers have a high degree of freedom for the silanol groups that contribute to adhesion, and thus can further improve adhesion to wires and to solar cell elements.

Examples of α-olefins constituting silane-modified polyolefin resins include ethylene, propylene, 1-butene, isobutylene, 1-pentene, 2-methyl-1-butene, 3-methyl-1-butene, 1-hexene, 1-heptene, 1-octene, 1-nonene, and 1-decene. The α-olefin may be used alone or in combination of two or more. Among these, polyethylene is preferred. That is, the silane-modified polyolefin resin is preferably a silane-modified polyethylene resin. This is because silane-modified polyethylene resin has good compatibility with the polyethylene resin contained in the polyethylene resin layer.

Furthermore, the silane-modified polyethylene resin is preferably a resin in which linear low-density polyethylene (LLDPE) is used as the main chain and ethylenically unsaturated silane compounds are graft-polymerized as side chains.

Examples of the above-mentioned ethylenically unsaturated silane compounds include vinyltrimethoxysilane, vinyltriethoxysilane, vinyltripropoxysilane, vinyltriisopropoxysilane, vinyltributoxysilane, vinyltripentyloxysilane, vinyltriphenoxysilane, vinyltribenzyloxysilane, vinyltrimethylenedioxysilane, vinyltriethylenedioxysilane, vinylpropionyloxysilane, vinyltriacetoxysilane, and vinyltricarboxysilane. The ethylenically unsaturated silane compounds may be used individually or in combination of two or more.

Silane-modified polyolefin resin can be obtained, for example, by the manufacturing method described in Japanese Patent Publication No. 2003-46105.

Silane-modified resins may be used individually or in combination of two or more types.

The content of the silane-modified resin in the polyethylene resin layer is not particularly limited; for example, it may be 25% by mass or less, or 15% by mass or less.

(ii) Silane coupling agent As the silane coupling agent, a silane coupling agent commonly used as a encapsulant for solar cells can be used. The silane coupling agent may be used alone, or two or more types may be used in combination.

The content of the silane coupling agent in the polyethylene resin layer is not particularly limited and may be, for example, 5% by mass or less.

(c) Other components The polyethylene resin layer may contain additives such as antioxidants, antiblocking agents, and lubricants, as needed.

Here, the proportion of each resin component contained in each layer of the current collector sheet resin film can be analyzed, for example, from the peak ratio detected by scanning calorimetry (DSC), infrared spectroscopy (IR), nuclear magnetic resonance (NMR), etc.

(3) Other The thickness of the polyethylene resin layer is not particularly limited as long as it can support the wire when the resin film for current collection sheets is used as a current collection sheet, and can be appropriately selected according to the thickness of the wire. The thickness of the polyethylene resin layer is preferably greater than the thickness of the base material layer described later. This improves the adhesion to the wire and the embedding of the wire. Specifically, the thickness of the polyethylene resin layer is preferably 40 μm or more and 100 μm or less, and more preferably 45 μm or more and 80 μm or less. By having the thickness of the polyethylene resin layer within the above range, the adhesion to the wire and the embedding of the wire can be improved.

The side of the polyethylene resin layer opposite the adhesive layer may be surface-treated. That is, the polyethylene resin layer may have a surface-treated surface on the side opposite the adhesive layer. This can improve adhesion to the wire and to the solar cell element.

The surface treatment is not particularly limited as long as it can improve adhesion to the wire and to the solar cell elements. Examples include corona treatment, plasma treatment, ultraviolet treatment, electron beam treatment, and flame treatment. Among these, corona treatment is preferred in terms of processing cost and reducing damage to the polyethylene resin layer.

2. Substrate Layer The substrate layer in this disclosure contains polyethylene terephthalate resin. The substrate layer is a component that imparts rigidity to the resin film for the current collector sheet.

The base layer may contain various additives as needed.

The thickness of the base layer is not particularly limited and can be appropriately selected depending on the size and application of the solar cell in which the current collector sheet resin film is used. As mentioned above, it is preferable that the thickness of the base layer be thinner than the thickness of the polyethylene resin layer. Specifically, the thickness of the base layer is preferably 12 μm to 38 μm, and more preferably 12 μm to 25 μm. If the thickness of the base layer is too thin, sufficient rigidity may not be obtained. Conversely, if the thickness of the base layer is too thick, the rigidity may become too high, potentially reducing the wire-following ability of the current collector sheet resin film.

The base layer is preferably annealed. This improves thermal dimensional stability. This makes it easier to adjust the thermal shrinkage rate of the resin film for the current collector sheet (described later) to a predetermined range. Therefore, when manufacturing a solar cell with a current collector sheet using a current collector sheet having a resin film for the current collector sheet, misalignment of the wires relative to the solar cell elements during the heating process for fixing the wires to the solar cell elements can be suppressed. Furthermore, when manufacturing a solar cell using a solar cell with a current collector sheet, misalignment of the wires relative to the solar cell elements during the heating process for integrating each component can be suppressed. Moreover, even when the solar cell reaches high temperatures in the operating environment, misalignment of the wires relative to the solar cell elements can be suppressed. As a result, the reliability of the solar cell can be improved.

The annealing temperature of the substrate layer is preferably, for example, 120°C to 250°C.

3. Adhesive Layer The adhesive layer in this disclosure is a component that is placed between the base material layer and the polyethylene resin layer and is used to bond the base material layer and the polyethylene resin layer.

The adhesive used in the bonding layer is not particularly limited as long as it is transparent and capable of bonding the above-mentioned base material layer and polyethylene resin layer. Examples of adhesives commonly used for laminating films include urethane-based adhesives, acrylic-based adhesives, polycarbonate-based adhesives, and phenol-based adhesives. Furthermore, the adhesive may be, for example, a dry laminating adhesive or an anchor coating agent for extrusion lamination.

Furthermore, it is preferable that the adhesive has heat and humidity resistance. This can suppress the decrease in adhesive strength due to hydrolysis. For example, it is preferable that the resin component constituting the adhesive does not contain ester bonds. Specifically, in the case of a urethane-based adhesive, it is preferable that the main component is polycarbonate polyurethane resin.

The thickness of the adhesive layer is not particularly limited as long as it is transparent and capable of bonding the substrate layer and the polyethylene resin layer. For example, a thickness of 0.1 μm to 10 μm is preferred.

4. Other The thickness of the resin film for the current collector sheet in this disclosure is not particularly limited and can be appropriately selected according to the thickness of the wire used in the current collector sheet. For example, the thickness of the resin film for the current collector sheet may be 50 μm or more and 300 μm or less. If the thickness of the resin film for the current collector sheet is too thin, it may become difficult to fix the wire to the solar cell element. On the other hand, if the thickness of the resin film for the current collector sheet is too thick, the transparency may decrease.

II. Physical Properties of Resin Film for Current Collector Sheets 1. Thermal Shrinkage Rate In the resin film for current collector sheets according to this disclosure, the thermal shrinkage rate when held at 150°C for 10 minutes is 2.0% or less, preferably 1.5% or less, and more preferably 1.0% or less. By having the thermal shrinkage rate of the resin film for current collector sheets within the above range, thermal dimensional stability can be improved. As a result, when manufacturing a solar cell with a current collector sheet using a current collector sheet having the resin film for current collector sheets, misalignment of the wires relative to the solar cell elements during the heating process for fixing the wires to the solar cell elements can be suppressed. Also, when manufacturing a solar cell using a solar cell with a current collector sheet, misalignment of the wires relative to the solar cell elements during the heating process for integrating each component can be suppressed. Furthermore, even when the solar cell reaches a high temperature in the operating environment of the solar cell, misalignment of the wires relative to the solar cell elements can be suppressed. As a result, the reliability of the solar cell can be improved.

In particular, as will be described later, if the wire is coated with, for example, a Bi-Sn type solder or a Sn-In-Ag-Bi type solder, the melting point of these solders is relatively high, which tends to increase the heating temperature in the heating process described above. Therefore, in the above case, it is more preferable that the thermal shrinkage rate of the resin film for the current collector sheet be small, specifically, 1.0% or less.

On the other hand, the lower limit of the thermal shrinkage rate mentioned above should be 0% or greater.

Here, the thermal shrinkage rate of the resin film for current collection sheets refers to the larger of the thermal shrinkage rates in the MD direction and the TD direction. In other words, for the resin film for current collection sheets, the larger of the thermal shrinkage rates in the MD direction and the TD direction falls within the above range.

Note that the MD direction of the current collection sheet resin film is usually the longitudinal direction of the current collection sheet resin film. Also, the TD direction of the current collection sheet resin film is usually the short direction of the current collection sheet resin film.

Furthermore, the thermal shrinkage rate of the resin film for the current collector sheet can be measured, for example, by a method conforming to ASTM D1204.

One method for controlling the thermal shrinkage rate of resin films for current collector sheets is to adjust the thermal shrinkage rate of the base layer. Methods for adjusting the thermal shrinkage rate of the base layer include, as mentioned above, annealing the base layer, adjusting the crystallinity of the polyethylene terephthalate resin contained in the base layer, and adjusting the molding method and conditions when forming the polyethylene terephthalate resin into a film. Methods for adjusting the molding method include, for example, applying a relaxation treatment to alleviate stress and strain during molding. Molding conditions include, for example, the stretch ratio.

2. Light transmittance at wavelengths of 400 nm to 1200 nm The light transmittance of the resin film for current collector sheets in this disclosure at wavelengths of 400 nm to 1200 nm is not particularly limited as long as it transmits enough sunlight to enable the solar cell element to generate electricity. The light transmittance of the resin film for current collector sheets at wavelengths of 400 nm to 1200 nm is preferably 75% or more, more preferably 80% or more, and even more preferably 85% or more. By having the above light transmittance within the above range, the light utilization efficiency of the solar cell element can be increased.

Here, the light transmittance in the wavelength range of 400 nm to 1200 nm is the average value of the light transmittance in this range. The light transmittance in the wavelength range of 400 nm to 1200 nm can be measured in accordance with JIS K7361 1. Specifically, the light transmittance in the wavelength range of 400 nm to 1200 nm can be measured using a haze meter HM150 manufactured by Murakami Color Technology Laboratory Co., Ltd.

3. Haze The transparency of the resin film for current collector sheets in this disclosure is not particularly limited as long as it allows sunlight to pass through to a degree that enables the solar cell element to generate electricity. The transparency of the resin film for current collector sheets can be evaluated, for example, by haze. The haze of the resin film for current collector sheets is, for example, 1% or more and 40% or less, may be 5% or more and 30% or less, or 10% or more and 20% or less.

Here, haze is measured in accordance with JIS K7136. Haze can be measured using, for example, a haze meter HM150 manufactured by Murakami Color Technology Laboratory Co., Ltd.

Furthermore, when measuring the haze of the resin film for current collector sheets, a measurement sample will be prepared and used for measurement. The measurement sample will be prepared using the following method. First, the resin film for current collector sheets will be cut to 50 mm x 50 mm to prepare a test piece. Next, the ETFE (tetrafluoroethylene-ethylene copolymer) film, the test piece, and the ETFE film will be laminated in this order, and vacuum lamination will be performed under the conditions of a set temperature of 165°C, vacuum evacuation for 2 minutes, pressing for 2.5 minutes, and a pressure of 100 kPa. This is to eliminate fine irregularities on the surface of the resin film for current collector sheets during the film formation stage. Subsequently, the ETFE film will be removed from both sides of the test piece to prepare the measurement sample.

III. Method for Manufacturing Resin Film for Current Collector Sheets The method for manufacturing a resin film for current collector sheets in this disclosure is not particularly limited as long as a resin film for current collector sheets having a base layer, an adhesive layer, and a polyethylene resin layer in that order can be obtained. For example, a method in which a film-like base layer and a polyethylene resin layer are used and the base layer and polyethylene resin layer are laminated by a dry lamination method via a dry lamination adhesive, or a method in which a film-like base layer is used and the base layer and polyethylene resin layer are laminated by an extrusion lamination method via an extrusion lamination anchor coating agent.

As a method for forming the film-like substrate layer and the polyethylene resin layer, for example, one method involves preparing a resin composition for each layer and then melt-molding the resin composition. Known molding methods can be used as the melt-molding method, such as injection molding, extrusion molding, hollow molding, compression molding, and rotational molding. The molding temperature is, for example, above the melting point of the resin composition. The upper limit of the molding temperature is appropriately adjusted depending on the type of resin composition.

B. Current Collector Sheet The current collector sheet in this disclosure is a current collector sheet used in a solar cell, and comprises the above-mentioned resin film for current collector sheets and a wire arranged on the side of the polyethylene resin layer of the resin film for current collector sheets.

Figures 2(a) and 2(b) are schematic plan and cross-sectional views illustrating the current collector sheet in this disclosure, with Figure 2(b) being a cross-sectional view taken along line A-A in Figure 2(a). As shown in Figures 2(a) and 2(b), the current collector sheet 20 comprises a current collector sheet resin film 10 and wires 11 arranged on the side of the polyethylene resin layer 3 of the current collector sheet resin film 10. Figure 2(a) shows a schematic plan view of the current collector sheet as seen from the polyethylene resin layer side of the current collector sheet resin film.

In the current collector sheet described in this disclosure, the resin film for the current collector sheet allows for secure fixing of the wires to the solar cell elements. Furthermore, the resin film suppresses wire displacement due to heat. Therefore, when the current collector sheet is used in a solar cell, power generation efficiency can be improved and reliability can be enhanced.

I. Structure of the current collector sheet The current collector sheet in this disclosure comprises a resin film for current collector sheets and a wire.

1. Resin film for current collection sheet: The resin film for current collection sheet is a component that supports the wire. It is also a component that fixes the wire to the solar cell element.

The resin film for the current collection sheet can be the same as described in "A. Resin Film for Current Collection Sheet" above, so the explanation here will be omitted.

As described later, if the current collection sheet has multiple resin films for current collection sheets, at least one of the resin films for current collection sheets should be the resin film described above. In this case, the current collection sheet may have other resin films for current collection sheets besides the resin film described above. Among these, it is preferable that all of the multiple resin films for current collection sheets in the current collection sheet are the resin films described above.

2. Wires The wires are positioned on the side of the polyethylene resin layer of the resin film for the current collection sheet. The wires are used, for example, to connect solar cell elements in a solar cell module. They are also used, for example, to collect electricity generated by solar cell elements in a single-cell solar cell. The wires are usually positioned to connect to the electrodes of the solar cell elements.

The cross-sectional shape of a wire is typically circular, such as a perfect circle or an ellipse, but is not limited to these shapes.

The wire thickness, or the size of its cross-section, is not particularly limited as long as it does not obstruct the incidence of sunlight onto the solar cell element. For example, it is between 100 μm and 300 μm. The size of the wire's cross-section refers to, for example, the diameter if the cross-section is circular, the major axis if it is elliptical, and the length of the longest diagonal if it is polygonal.

The wire material is not particularly limited as long as it exhibits the desired conductivity, and can be the same as the wire material used for current collector sheets in general solar cell elements. For example, metallic materials such as copper (Cu) and silver (Ag) can be used as the wire material. Furthermore, the wire may have, for example, a core portion and a skin portion located on the outside of the core portion. In this case, for example, the above-mentioned metallic material can be used as the core portion material. Solder can be used as the skin portion material.

The melting point of the solder is preferably between 70°C and 140°C, and more preferably between 80°C and 135°C. If the solder's melting point is too high, it may degrade the substrate layer and polyethylene resin layer when connecting wires to the solar cell element.

Examples of such solders include Sn-In and Bi-Sn systems.

For example, Bi-Sn solder has a higher melting point compared to Sn-In and In-Bi solders. Therefore, when using Bi-Sn solder, the heating temperature in the heating process when manufacturing solar cell elements with current collectors using current collector sheets, and the heating temperature in the heating process when manufacturing solar cells using solar cell elements with current collector sheets, tend to be higher. Consequently, in these cases, poor wire adhesion and wire misalignment are likely to occur. Therefore, this disclosure is particularly effective in the above cases.

II. Structure of the Current Collector Sheet In the current collector sheet according to this disclosure, it is sufficient that at least one wire is arranged on a single resin film for the current collector sheet. From the viewpoint of improving the conductivity of the current collector sheet, it is preferable that multiple wires are arranged on a single resin film for the current collector sheet.

When a current collector sheet has multiple wires, the arrangement of the wires in a plan view is not particularly limited and can be the same as the arrangement of wires in known current collector sheets. For example, as shown in Figure 2(a), the wires 11 may be arranged in a line, or, although not shown, the wires may be arranged in a grid.

Furthermore, in the current collection sheet, the wire is positioned on the side of the polyethylene resin layer of the current collection sheet resin film. For example, as shown in Figure 2(b), it is preferable that the wire 11 is positioned such that a portion of it is embedded in the polyethylene resin layer 3 of the current collection sheet resin film 10, while a portion of the wire 11 is exposed. This allows for good wire fixation.

Furthermore, for example, as shown in Figure 2(b), the wire 11 may be embedded in the polyethylene resin layer 3 so as not to contact the base layer 1, or as shown in Figure 4, the wire 11 may be embedded in the polyethylene resin layer 3 so as to contact the base layer 1. When the wire is embedded in the polyethylene resin layer so as to contact the base layer, the thickness of the current collector sheet can be reduced, thus enabling the thinning of solar cells using current collector sheets.

The degree of wire embedding, i.e., the degree of wire exposure, is not particularly limited and can be appropriately selected depending on the material and thickness of the polyethylene resin layer, the wire diameter, and the form of the solar cell element on which the current collector sheet is placed.

In a current collector sheet, for example, the same wire may be arranged on multiple current collector resin films. For example, Figure 5 shows an example where the same wire 11 is arranged on two current collector resin films 10A and 10B. Also, as shown in Figure 5, adjacent current collector resin films 10A and 10B may be arranged so that their polyethylene resin layer 3 sides face opposite directions. That is, the polyethylene resin layer 3 side of one current collector resin film 10A and the base material layer 1 side of the other current collector resin film 10B may be arranged in the same direction. By having the above structure, a current collector sheet 20 can be made that allows two solar cell elements 31 to be arranged in series, for example, as shown in Figure 3(a). Although not shown, adjacent current collector resin films may be arranged so that their polyethylene resin layer sides face the same direction.

III. Method for Manufacturing a Current Collector Sheet The method for manufacturing a current collector sheet in this disclosure is not particularly limited as long as it is a method that allows the wire to be embedded to the extent that it can be fixed to the surface side of the polyethylene resin layer of the resin film for the current collector sheet, and known methods can be used. For example, a method can be given in which a wire is placed on the surface side of the polyethylene resin layer of the resin film for the current collector sheet, and the wire is heated to melt a part of the polyethylene resin in the polyethylene resin layer and embed the wire.

C. Solar cell element with current collector sheet The solar cell element with a current collector sheet in this disclosure comprises the current collector sheet described above and a solar cell element disposed on the side of the polyethylene resin layer of the current collector sheet and electrically connected to a wire.

Figures 3(a) to 3(c) are schematic perspective views and cross-sectional views illustrating a solar cell element with a current collector sheet in this disclosure. Figure 3(b) is a cross-sectional view taken along line A-A in Figure 3(a), and Figure 3(c) is a cross-sectional view taken along line B-B in Figure 3(a). As shown in Figures 3(a) to 3(c), the solar cell element 30 with a current collector sheet comprises a current collector sheet 10 and a solar cell element 31 arranged on the side of the polyethylene resin layer 3 of the current collector sheet 10 and electrically connected to a wire 11. Figures 3(a) to 3(c) show an example where the current collector sheet 20 has two resin films 10 for current collector sheets, and a solar cell element 31 is arranged on each resin film 10 for current collector sheets.

In the solar cell element with a current collector sheet described in this disclosure, the wire can be securely fixed to the solar cell element by the resin film for the current collector sheet. Furthermore, the resin film for the current collector sheet can suppress wire displacement due to heat. Therefore, when a solar cell element with a current collector sheet is used in a solar cell, power generation efficiency can be improved and reliability can be enhanced.

I. Configuration of a Solar Cell Element with Current Collector Sheet The solar cell element with a current collector sheet in this disclosure comprises a current collector sheet and a solar cell element.

1. Current Collection Sheet The current collection sheet can be the same as described in "B. Current Collection Sheet" above, so the explanation here will be omitted.

If a solar cell element with current collector sheets has multiple current collector sheets, at least one of them should be the current collector sheet described above. In this case, the solar cell element with current collector sheets may also have other current collector sheets besides the one described above. However, it is preferable that all of the current collector sheets are the current collector sheet described above.

2. Solar Cell Elements Solar cell elements can be the same as those used in general solar cells. Examples of solar cell elements include monocrystalline silicon solar cell elements, polycrystalline silicon solar cell elements, amorphous silicon solar cell elements, compound semiconductor solar cell elements, dye-sensitized solar cell elements, quantum dot solar cell elements, and organic thin-film solar cell elements. The size and shape of the solar cell elements can be appropriately selected according to the application of the solar cell.

II. Structure of a solar cell element with a current collector sheet A solar cell element with a current collector sheet typically has a laminated structure in which the current collector sheet and the solar cell element are stacked.

In a solar cell element with a current collector sheet, when viewed with reference to the surface on which the current collector sheet of the solar cell element is placed, for example as shown in Figure 3(c), the thickness of the resin film 10 for the current collector sheet in the direction _DL perpendicular to the placement surface, that is, the distance from the surface on which the current collector sheet 20 of the solar cell element 31 is placed to the surface of the base layer 1 opposite to the solar cell element 31, is preferably such that the region where the wire 11 is placed is thicker (longer distance) than other regions. By using such a laminated structure, the base layer 1 can effectively press the wire 11 towards the solar cell element 31, and problems such as poor contact between the wire 11 and the solar cell element 31 can be prevented.

In this case, the ratio x/y of the maximum distance y of the wire 11 in the direction _DL perpendicular to the placement surface, that is, the maximum distance y of the wire 11 from the surface on which the current collector sheet 20 of the solar cell element 31 is placed, and the minimum distance x of the resin film 10 for the current collector sheet in the direction _DL perpendicular to the placement surface, that is, the minimum distance x of the distance from the surface on which the current collector sheet 20 of the solar cell element 31 is placed to the surface of the base layer 1 opposite to the solar cell element 31, is preferably 2/3 or less, and more preferably 1/2 or less.

The lower limit of the above ratio x/y is a value that is appropriately adjusted according to the thickness of the polyethylene resin layer and the wire thickness, but for example, it is 1/20 or more. As long as the above ratio x/y is within the above range, the wire can be securely fixed to the solar cell element using the current collection sheet resin film.

The above ratio x/y can be controlled by adjusting the wire thickness (diameter, etc.) and the thickness of the polyethylene resin layer. Furthermore, as described later, the above ratio x/y can also be controlled by adjusting the pressure applied when heat-pressing the current collector sheet to the solar cell element.

Furthermore, as shown in Figure 3(c), for example, in a cross-sectional view of the solar cell element 30 with a current collector sheet, it is preferable that the distance from the surface of the solar cell element 31 on the current collector sheet 20 side to the surface of the substrate layer 1 opposite to the solar cell element 31 gradually decreases from the position of the maximum distance y to the position of the minimum distance x.

Furthermore, when multiple wires are arranged, for example, as shown in Figure 3(c), in a cross-sectional view of the solar cell element 30 with a current collector sheet, it is preferable that the minimum distance between adjacent wires 11 is determined by the distance from the surface of the solar cell element 31 where the current collector sheet 20 is located to the surface of the base layer 1 opposite to the solar cell element 31. This creates a recess between the wires, resulting in a structure where the current collector sheet has an uneven shape, thus increasing the area of the surface of the base layer opposite to the solar cell element. As a result, in a solar cell having a solar cell element with a current collector sheet, the contact area between the base layer and the encapsulant (described later) increases, improving adhesion to the encapsulant.

Furthermore, in solar cell elements with current collector sheets, as described in section "B. Current Collector Sheet" above, the wire 11 may be embedded in the polyethylene resin layer 3 so as to contact the base layer 1, for example, as shown in Figure 3(c). This allows for a reduction in the thickness of the solar cell element with a current collector sheet, thus enabling a thinner solar cell.

A solar cell element with a current collector sheet may consist of at least one solar cell element and a current collector sheet connected to at least one of the positive and negative electrodes of the solar cell element. For example, it may be a solar cell element with a current collector sheet that constitutes a single-cell solar cell, where a current collector sheet is placed on each of the positive and negative electrodes of a single solar cell element. Alternatively, for example, a solar cell element with a current collector sheet may constitute a solar cell module (solar cell module) in which multiple solar cells are connected in parallel or series using current collector sheets.

III. Other The manufacturing method of a solar cell element with a current collector sheet in this disclosure is not particularly limited as long as it is a method that can be obtained in which the wires of the current collector sheet are electrically connected to and fixed to the solar cell element. For example, a manufacturing method can be given which includes a temporary bonding step of temporarily bonding the current collector sheet to the solar cell element, and a fixing step of electrically connecting and fixing the wires of the current collector sheet to the solar cell element by thermocompression bonding the temporarily bonded current collector sheet to the solar cell element. Known methods can be used for the temporary bonding method and the thermocompression bonding method, for example, the vacuum thermal lamination method. Furthermore, the fixing step may be performed simultaneously with the integration step of laminating and integrating the various components of the solar cell, as will be explained later in section D. Solar Cells.

Solar cell elements with current-collecting sheets are typically used as components of solar cells. Furthermore, if a solar cell element with a current-collecting sheet comprises, for example, one solar cell element and a current-collecting sheet connected to only one of the electrodes (either the positive or negative electrode) of the solar cell element, then this solar cell element with a current-collecting sheet can be used, for example, as part of the single-cell type solar cell described above, or as part of a solar cell module.

D. Solar Cell The solar cell in this disclosure comprises a transparent substrate, a first encapsulant, the solar cell element with the current collector sheet described above, a second encapsulant, and a counter substrate, in this order.

Figure 6 is a schematic cross-sectional view illustrating a solar cell in this disclosure. As shown in Figure 6, the solar cell 40 comprises a transparent substrate 41, a first encapsulant 42, a solar cell element 30 with a current collector sheet, a second encapsulant 43, and a counter substrate 44.

The solar cell in this disclosure may be a solar cell module having multiple solar cell elements with current-collecting sheets.

In the solar cell described in this disclosure, by having a solar cell element with a current collector sheet as described above, the wire can be securely fixed to the solar cell element by the resin film for the current collector sheet. Furthermore, the resin film for the current collector sheet can suppress wire displacement due to heat. Therefore, power generation efficiency can be improved and reliability can be enhanced.

I. Solar Cell Configuration The solar cell in this disclosure comprises, in this order, a transparent substrate, a first encapsulant, a solar cell element with a current collector sheet, a second encapsulant, and a counter substrate.

1. Solar cell elements with current collector sheets The solar cell elements with current collector sheets can be described in the same way as in the section "C. Solar cell elements with current collector sheets" above, so the explanation here will be omitted.

2. Transparent Substrate and Opposing Substrate The transparent substrate, together with the opposing substrate, is a component that protects the solar cell element. The transparent substrate is usually placed on the light-receiving side of the solar cell and functions as a front protective plate on the light-receiving side. The transparency of the transparent substrate is not particularly limited as long as it does not hinder the power generation of the solar cell element. The transparent substrate can be the same as the transparent substrate used in general solar cells, so a detailed explanation is omitted here.

The opposing substrate, along with the transparent substrate mentioned above, is a component that protects the solar cell element. The opposing substrate may or may not be transparent. If the opposing substrate is transparent, both sides of the solar cell can be used as light-receiving surfaces. The transparent substrate mentioned above can be used as the opposing substrate. Alternatively, a back-side protective sheet for solar cells can be used as the opposing substrate.

3. First and Second Encapsulating Materials The first and second encapsulating materials are components that enclose the solar cell element. The first encapsulating material is usually placed on the light-receiving side of the solar cell.

The first encapsulant and the second encapsulant contain a thermoplastic resin. The thermoplastic resin used in the first and second encapsulants can be the same as that used in general solar cell encapsulants. For example, encapsulants primarily composed of various olefin resins such as polyethylene resin and ethylene-vinyl acetate copolymer (EVA) can be used. Note that "primarily composed of these resins" means that these resins constitute the largest proportion of the total resin components.

The first encapsulant typically contains an ultraviolet absorber. This can suppress the deterioration of the substrate layer containing polyethylene terephthalate resin due to ultraviolet light, such as yellowing, cracking, and fracture.

If the opposing substrate is transparent, the second encapsulant typically contains an ultraviolet absorber, similar to the first encapsulant.

The UV absorber can be the same as the UV absorbers used in typical solar cell encapsulants.

The thickness of the first and second encapsulants is appropriately selected according to the type and size of the solar cell.

II. Method for Manufacturing Solar Cells The method for manufacturing solar cells in this disclosure can be the same as that for general solar cell manufacturing methods. As an example, a manufacturing method can be cited which includes a laminate formation step of forming a laminate by stacking a transparent substrate, a first encapsulant, a solar cell element with a current collector sheet, a second encapsulant, and a counter substrate in this order, and an integration step of integrating the laminate to form a solar cell by heating and pressurizing the laminate.

The heating and pressurizing processes are not particularly limited and can be the same as those performed during the general manufacturing of solar cells. For example, vacuum thermal lamination is preferred. The conditions for the vacuum thermal lamination are not particularly limited and can be appropriately selected depending on the size of the solar cell, the type of each component, etc. The lamination temperature is preferably, for example, 130°C to 170°C. The lamination time is preferably, for example, 5 minutes to 30 minutes, and more preferably 8 minutes to 15 minutes.

III. Applications The applications of the solar cells described herein include, for example, solar cells for electronic devices and large solar cells for outdoor installation.

This disclosure is not limited to the embodiments described above. The embodiments described above are illustrative, and any configuration that is substantially identical to the technical idea described in the claims of this disclosure and achieves similar effects is included within the technical scope of this disclosure.

[Example 1]
A 12 μm thick polyethylene terephthalate (PET) film (DuPont "LBD") was used as the base layer. A two-component adhesive (anchor coating agent) consisting of a polycarbonate-based main component (Rock Paint "KT-0035") and an isocyanate-based curing agent (Rock Paint "H-039Z2") was used. High-pressure low-density polyethylene (LDPE) with a density of 0.92 g/ ^cm³ , a melting point of 106°C, and an MFR (190°C) of 7 g/10 min was used as the polyethylene resin.

The adhesive was applied at a rate of 0.1 g/ ^m² as an anchoring agent to one side of the base material layer, and the polyethylene resin was extruded to a thickness of 60 μm to form a polyethylene resin layer. Furthermore, the side of the polyethylene resin layer opposite to the base material layer was subjected to corona treatment. This resulted in a resin film for current collector sheets having a base material layer, an adhesive layer, and a polyethylene resin layer in that order.

[Example 2]
A resin film for current collector sheets was prepared in the same manner as in Example 1, except that the base layer was subjected to an annealing treatment at 200°C for 10 seconds.

[Comparative Example 1]
A PET film with a thickness of 12 μm was used as the base layer. A urethane-based adhesive was used as the adhesive (anchor coating agent). Linear low-density polyethylene (LLDPE) with a melting point of 109°C and an MFR (190°C) of 5.7 g/10 min was used as the polyethylene resin.

A resin film for the current collector sheet was prepared in the same manner as in Example 1.

[Comparative Example 2]
A 12 μm thick polyethylene terephthalate (PET) film (Toyobo Co., Ltd. "E5104") was used as the base layer. A two-component adhesive (anchor coating agent) consisting of a polycarbonate-based main component (Rock Paint Co., Ltd. "KT-0035") and an isocyanate-based curing agent (Rock Paint Co., Ltd. "H-039Z2") was used as the adhesive (anchor coating agent). A polyethylene film containing metallocene-based linear low-density polyethylene (M-LLDPE) with a density of 0.915 g/ ^cm³ , a melting point of 105°C, and an MFR (190°C) of 2 g/10 min was used as the polyethylene resin layer.

A resin film for current collector sheets was obtained by bonding a substrate layer and a polyethylene resin layer using a dry lamination method with an adhesive.

[Comparative Example 3]
A resin film for current collector sheets was prepared in the same manner as in Comparative Example ² , except that a polyethylene film containing metallocene-based linear low-density polyethylene (M-LLDPE) with a density of 0.915 g/cm³, a melting point of 110°C, and an MFR (190°C) of 2 g/10 min was used as the polyethylene resin layer.

[Comparative Example 4]
A resin film for current collector sheets was prepared in the same manner as in Example 1, except that the thickness of the polyethylene resin layer was set to 70 μm.

[evaluation]
(1) Thermal shrinkage rate The thermal shrinkage rate of the resin film for current collector sheets in the MD direction and TD direction was measured in accordance with ASTM D1204. The measurement conditions were 150°C for 10 minutes.

(2) Light transmittance at wavelengths of 400 nm to 1200 nm The light transmittance of the resin film for current collector sheets at wavelengths of 400 nm to 1200 nm was measured in accordance with JIS K7361 1 using a haze meter HM150 manufactured by Murakami Color Technology Laboratory Co., Ltd.

(3) Haze A resin film for current collector sheets was cut to 50 mm x 50 mm to prepare a test piece. Next, the ETFE (tetrafluoroethylene-ethylene copolymer) film, the test piece, and the ETFE film were laminated in this order, and vacuum lamination was performed under the conditions of a set temperature of 165°C, vacuuming for 2 minutes, pressing for 2.5 minutes, and a pressure of 100 kPa. Subsequently, the ETFE film was removed from both sides of the test piece to obtain a sample for measurement. The haze of the sample for measurement was then measured in accordance with JIS K7136 using a haze meter HM150 manufactured by Murakami Color Technology Laboratory Co., Ltd.

(4) Wire Adhesion Wire A, coated with SnIn-based solder, and wire B, coated with SnBi-based solder, were used as wires. The diameters of wires A and B were 250 μm each. A resin film for current collector sheets was cut to 100 mm x 100 mm to prepare a test specimen. Next, an ETFE (tetrafluoroethylene-ethylene copolymer) film was placed on the substrate layer side of the test specimen. On the polyethylene resin layer side of the test specimen, five wires A and five wires B were placed at 10 mm intervals, and the ETFE film was placed in order. Then, lamination was performed using a hot roll laminator under the conditions of a set temperature of 120°C and a press pressure of 0.1 MPa. Subsequently, the ETFE film was removed from both sides of the test specimen to obtain evaluation samples. Then, the wires of the evaluation samples were bent 180 degrees and peeled off the resin film for the current collector sheet at a tensile speed of 300 mm/min, and the peel strength of the current collector wires from the resin film for the current collector sheet was measured. Wire adhesion was evaluated according to the following criteria. Note that "N/wire" indicates the peel strength when peeling off a single wire.
A: The peel strength is greater than 0.1 N/wire.
B: The wire adheres tightly to the resin film for the current collection sheet, but the peel strength is 0.1 N/wire or less.
C: The wire does not adhere to the resin film for the current collection sheet and peels off naturally.

(5) Module Reliability First, a current collector sheet was obtained in which wires were fixed to a resin film for current collector sheets, using the same method as for preparing the evaluation samples in the wire adhesion evaluation described above. In this case, a total of 18 wires were placed at an 8 mm pitch on the polyethylene resin layer side of the test piece. For samples in which the wire adhesion evaluation was "C", wire adhesion was ensured by raising the set temperature of the thermal roll laminator to 130°C.

A 3.2 mm thick white tempered glass plate was used as the transparent substrate, a 470 μm thick ethylene-vinyl acetate copolymer (EVA) sheet (Fast Cure EVA, manufactured by Takiron CI Co., Ltd.) was used as the first and second encapsulants, an N-type silicon cell was used as the solar cell element, and an aluminum layer-containing backsheet (VAPE-CW, manufactured by Dai Nippon Printing Co., Ltd.) was used as the counter substrate. Next, the transparent substrate, the first encapsulant, the current collector sheet, the solar cell element, the current collector sheet, the second encapsulant, and the counter substrate were laminated together, and vacuum lamination was performed under the conditions of a set temperature of 150°C, vacuuming for 5 minutes, pressing for 7.5 minutes, and a pressure of 100 kPa. When laminating each component, the current collector sheet was positioned so that the wire side of the current collector sheet faced the solar cell element side. Furthermore, as illustrated in Figure 3(b), an evaluation module was fabricated by arranging the current collector sheets 20 above and below the solar cell elements 31, thereby creating a solar cell module with four solar cell elements connected in series.

The evaluation modules underwent high-temperature and high-humidity testing (85°C, 85% RH, 2000 hours) and temperature cycling testing (-40°C ⇔ 90°C, 200 cycles, 1 cycle = 6 hours). The photovoltaic output was measured before and after each test, and the output degradation rate was determined. Module reliability was evaluated according to the following criteria.
A: The rate of power reduction after both tests is less than 5%.
B: The power output reduction rate after at least one of the tests is 5% or more but less than 10%.
C: The output reduction rate after at least one of the tests is 10% or more.

Table 1 confirms that when the MFR of the polyethylene resin layer in the current collector sheet resin film is within a predetermined range, and the heat shrinkage rate of the current collector sheet resin film is within a predetermined range, wire adhesion and module reliability are good.

Furthermore, in Comparative Examples 1 and 4, the output decreased after the temperature cycling test. This is because a high thermal shrinkage rate of the resin film for the current collector sheet leads to high residual stress after module lamination.

Furthermore, when the PET film used as the base layer in Example 1 and Comparative Example 4 was measured for thermal shrinkage in the same way as the resin film for current collector sheets, the thermal shrinkage in the MD direction was 1.4%, and the thermal shrinkage in the TD direction was 0.3%. This suggests that even if the thermal shrinkage of the base layer itself is 2.0% or less, the overall thermal shrinkage of the resin film for current collector sheets is not necessarily 2.0% or less.

This disclosure provides the following [1] to [11].
[1] A resin film for current collector sheets used in current collector sheets for solar cells,
It has a base layer, an adhesive layer, and a polyethylene resin layer in this order.
The above substrate layer contains polyethylene terephthalate resin,
The melt mass flow rate of the polyethylene resin layer at 190°C is 4 g/10 min or more and 8 g/10 min or less.
A resin film for current collector sheets, having a heat shrinkage rate of 2.0% or less when held at 150°C for 10 minutes.
[2] The resin film for current collector sheets according to [1], wherein the melting point of the polyethylene resin layer is 100°C or more and 120°C or less.
[3] The current collector sheet resin film according to [1] or [2], wherein the thickness of the polyethylene resin layer is greater than the thickness of the base material layer.
[4] A resin film for current collector sheets according to any one of [1] to [3], wherein the thickness of the polyethylene resin layer is 40 μm or more and 100 μm or less.
[5] A resin film for current collector sheets according to any one of [1] to [4], wherein the thickness of the base material layer is 12 μm or more and 38 μm or less.
[6] A resin film for current collector sheets according to any one of [1] to [5], wherein the thickness of the adhesive layer is 0.1 μm or more and 10 μm or less.
[7] A resin film for current collector sheets according to any one of [1] to [6], wherein the base material layer has a surface treatment on the side opposite to the adhesive layer.
[8] A current collector sheet used in solar cells,
A resin film for current collection sheets as described in any of [1] to [7],
A wire arranged on the surface side of the polyethylene resin layer of the resin film for current collection sheet,
A current collector sheet.
The current collection sheet described in [9] and [8],
A solar cell element is arranged on the side of the polyethylene resin layer of the current collector sheet and electrically connected to the wire,
A solar cell element with a current collector sheet.
[10] A solar cell comprising, in this order, a transparent substrate, a first encapsulant, a solar cell element with a current collector sheet as described in [9], a second encapsulant, and a counter substrate.
[11] The solar cell according to [10], wherein the solar cell is a solar cell module having a plurality of solar cell elements with current collecting sheets.

1… Substrate layer 2… Adhesive layer 3… Polyethylene resin layer 10, 10A, 10B… Resin film for current collector sheet 11… Wire 20… Current collector sheet 30… Solar cell element with current collector sheet 31… Solar cell element 40… Solar cell 41… Transparent substrate 42… First encapsulant 43… Second encapsulant 44… Opposing substrate

Claims (11)

A resin film for current collector sheets used in current collector sheets for solar cells,
It has a base layer, an adhesive layer, and a polyethylene resin layer in this order.
The aforementioned substrate layer contains polyethylene terephthalate resin,
The melt mass flow rate of the polyethylene resin layer at 190°C is 4 g/10 min or more and 8 g/10 min or less.
A resin film for current collector sheets, having a heat shrinkage rate of 2.0% or less when held at 150°C for 10 minutes. The resin film for current collector sheets according to claim 1, wherein the melting point of the polyethylene resin layer is 100°C or higher and 120°C or lower. The resin film for a current collector sheet according to claim 1 or 2, wherein the thickness of the polyethylene resin layer is greater than the thickness of the base material layer. The resin film for current collector sheets according to claim 1 or claim 2, wherein the thickness of the polyethylene resin layer is 40 μm or more and 100 μm or less. The resin film for current collector sheets according to claim 1 or claim 2, wherein the thickness of the base material layer is 12 μm or more and 38 μm or less. The resin film for current collector sheets according to claim 1 or claim 2, wherein the thickness of the adhesive layer is 0.1 μm or more and 10 μm or less. The resin film for current collector sheets according to claim 1 or 2, wherein the base material layer has a surface treatment on the side opposite to the adhesive layer. A current collector sheet used in solar cells,
A resin film for current collection sheets according to claim 1 or claim 2,
A wire arranged on the surface side of the polyethylene resin layer of the current collection sheet resin film,
A current collector sheet. The current collection sheet according to claim 8,
A solar cell element is arranged on the side of the polyethylene resin layer of the current collector sheet and electrically connected to the wire,
A solar cell element with a current collector sheet. A solar cell comprising, in this order, a transparent substrate, a first encapsulant, a solar cell element with a current collector sheet as described in claim 9, a second encapsulant, and a counter substrate. The solar cell according to claim 10, wherein the solar cell is a solar cell module having a plurality of solar cell elements with current-collecting sheets.