Solar cell

Abstract

The purpose of the present invention is to provide a solar cell from which heavy metal is unlikely to flow to the outside even if large-scale damage occurs.　The present invention is a solar cell having a power generation unit (1) and a sealing material (2). The sealing material (2) covers the entirety of the power generation unit (1) in a plan view of the power generation unit (1) from the light receiving surface side and the installation surface side, and has a water expansion coefficient of 7.0% or more.

Description

solar cells

[0001] This invention relates to a solar cell.

[0002] Conventionally, laminated solar cells, in which an N-type semiconductor layer and a P-type semiconductor layer are arranged between opposing electrodes, have been actively developed, and inorganic semiconductors such as silicon have been mainly used as the N-type and P-type semiconductors. However, such inorganic solar cells have problems such as high manufacturing costs and difficulty in scaling up, which limits their range of application. Therefore, in recent years, perovskite solar cells have attracted attention, which use organic-inorganic perovskite compounds having a perovskite structure with lead, tin, etc. as the central metal as the photoelectric conversion layer (for example, Patent Document 1, Non-Patent Document 1). Perovskite solar cells can be expected to have high photoelectric conversion efficiency, and since they can be manufactured by printing, manufacturing costs can be significantly reduced.

[0003] Japanese Patent Publication No. 2014-72327

[0004] M. M. Lee, et al., Science, 2012, 338, 643

[0005] On the other hand, conventional solar cells using inorganic semiconductors utilize heavy metals such as lead, selenium, and cadmium in their power generation section. Since solar cells are mostly installed outdoors, there are concerns that if a solar cell is damaged, heavy metals may dissolve into rainwater and be released into the environment. Furthermore, while perovskite solar cells, which mainly use organic materials, utilize lead and tin as the central metal in the organic-inorganic perovskite compound, the amount is small. In addition, organic-inorganic perovskite compounds are very susceptible to moisture, so they are structured to be as isolated from the outside as possible to prevent degradation. Therefore, large-scale metal leakage is unlikely to occur due to damage from normal use or degradation over time. However, if a perovskite solar cell is severely damaged, such as being cut, there are concerns about metal leakage. From an environmental perspective, there is a need for solar cells that are less likely to release heavy metals into the environment even in the event of large-scale damage.

[0006] The present invention aims to provide a solar cell that is less likely to release heavy metals into the environment even in the event of large-scale damage.

[0007] The present invention includes the following disclosures 1 to 6. The present invention will be described in detail below. [Disclosure 1] A solar cell having a power generation unit and a sealing material, wherein the sealing material covers the entire power generation unit when viewed from the light-receiving surface side and the installation surface side, and has a water expansion rate of 7.0% or more. [Disclosure 2] The solar cell according to disclosure 1, wherein the sealing material has a water expansion rate of 15.0% or less. [Disclosure 3] The solar cell according to disclosure 1 or 2, wherein the power generation unit contains an organic inorganic perovskite compound. [Disclosure 4] The solar cell according to any one of disclosures 1 to 3, wherein the sealing material has a thickness of 0.5 μm or more on the light-receiving surface side of the power generation unit. [Disclosure 5] The solar cell according to any one of disclosures 1 to 4, wherein the sealing material has a thickness of 0.5 μm or more on the installation surface side of the power generation unit. [Disclosure 6] The solar cell according to any one of disclosures 1 to 5, wherein a second sealing material is provided between the power generation unit and the sealing material.

[0008] The solar cell of the present invention has a power generation unit. The power generation unit is the part that converts sunlight into electricity and is composed of electrodes, counter electrodes, a photoelectric conversion unit, an electron transport layer, a hole transport layer, etc., and has at least electrodes, a photoelectric conversion unit, and a counter electrode. In this specification, "layer" means not only layers with clear boundaries, but also layers with a concentration gradient in which the contained elements change gradually. Elemental analysis of a layer can be performed, for example, by performing FE-TEM / EDS line analysis of the cross-section of the solar cell to confirm the elemental distribution of specific elements. Furthermore, in this specification, "layer" means not only flat, thin-film layers, but also layers that can form a complex, interwoven structure together with other layers. In this specification, "upper" refers to the direction of the light incident surface in the thickness direction of the solar cell, and "lower" refers to the opposite direction, i.e., the direction of the installation surface.

[0009] The materials for the electrodes and counter electrodes are not particularly limited and include, for example, FTO (fluorine-doped tin oxide), ITO (tin-doped indium oxide), AZO (aluminum zinc oxide), IZO (indium zinc oxide), GZO (gallium zinc oxide), sodium, sodium-potassium alloy, lithium, magnesium, aluminum, magnesium-silver mixture, magnesium-indium mixture, aluminum-lithium alloy, Al / Al ２ O ３ Examples include mixtures, Al / LiF mixtures, etc. Also, gold, silver, titanium, molybdenum, tantalum, tungsten, carbon, nickel, chromium, etc. These materials may be used individually or in combination of two or more.

[0010] The thickness of the electrode and counter electrode is not particularly limited, but a preferred lower limit is 10 nm and a preferred upper limit is 1000 nm. If the thickness is 10 nm or more, the electrode can function while suppressing resistance. If the thickness is 1000 nm or less, the light transmittance can be further improved. A more preferred lower limit for the thickness of the electrode and counter electrode is 50 nm and a more preferred upper limit is 500 nm.

[0011] The photoelectric conversion material constituting the above-mentioned photoelectric conversion unit only needs to be able to convert sunlight into electricity, and may be an inorganic semiconductor such as conventional silicon, or an organic-inorganic hybrid semiconductor such as an organic-inorganic perovskite compound. In particular, it is preferable that the above-mentioned photoelectric conversion unit, i.e., the power generation unit, contains an organic-inorganic perovskite compound because it allows for a thin and lightweight solar cell.

[0012] The above organic-inorganic perovskite compound is represented by the general formula AMX (where A is an organic base compound and / or alkali metal, M is a lead or tin atom, and X is a halogen atom). By using the above organic-inorganic perovskite compound in the photoelectric conversion section, the photoelectric conversion efficiency of the solar cell can be improved.

[0013] The above A is an organic base compound and / or alkali metal. Specifically, the above organic base compounds include, for example, methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine, dimethylamine, diethylamine, dipropylamine, dibutylamine, dipentylamine, dihexylamine, trimethylamine, triethylamine, tripropylamine, tributylamine, tripentylamine, trihexylamine, ethylmethylamine, methylpropylamine, butylmethylamine, methylpentylamine, hexylmethylamine, ethylpropylamine, ethylbutylamine, formamidine, acetamidine, guanidine, imidazole, azole, pyrrole, aziridine, azirine, azetidine, azeto, azole, imidazoline, carbazole and their ions (for example, methylammonium (CH4) ３ NH ３ Examples include ) and phenethylammonium. Among these, methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine, formamidine, acetamidine and their ions and phenethylammonium are preferred, and methylamine, ethylamine, propylamine, formamidine and their ions are more preferred. Examples of the alkali metals mentioned above include lithium, sodium, potassium, rubidium, cesium, etc.

[0014] The above M is a metal atom, which is either lead or tin. These metal atoms may be used individually or in combination of two or more. In particular, it is preferable that M contains lead because it can further increase the photoelectric conversion efficiency.

[0015] The above X is a halogen atom, and examples of halogen atoms include chlorine, bromine, iodine, sulfur, and selenium. These halogen atoms may be used individually or in combination of two or more. By including a halogen in the structure, the above organic-inorganic perovskite compound becomes soluble in organic solvents, making it possible to apply it to inexpensive printing methods and the like. In particular, X is preferably iodine because it narrows the energy band gap of the above organic-inorganic perovskite compound.

[0016] The above organic-inorganic perovskite compound preferably has a cubic crystal structure in which a metal atom M is located at the body center, an organic base compound or alkali metal A is located at each vertex, and a halogen atom X is located at the face center. Although the details are not clear, it is presumed that having the above structure allows the orientation of the octahedra in the crystal lattice to change easily, thereby increasing the electron mobility in the organic-inorganic perovskite compound and improving the photoelectric conversion efficiency of the solar cell.

[0017] The above organic-inorganic perovskite compound is preferably a crystalline semiconductor. A crystalline semiconductor is a semiconductor in which the X-ray scattering intensity distribution can be measured and a scattering peak can be detected. The crystalline nature of the organic-inorganic perovskite compound increases the electron mobility within it, thereby improving the photoelectric conversion efficiency of the solar cell.

[0018] Furthermore, the degree of crystallinity can also be evaluated as an indicator of crystallization. The degree of crystallinity can be determined by separating the scattering peaks originating from the crystalline portion and the halos originating from the amorphous portion, detected by X-ray scattering intensity distribution measurement, by fitting them together, calculating the integral of each intensity, and then calculating the ratio of the crystalline portion to the whole. The preferred lower limit for the degree of crystallinity of the above organic-inorganic perovskite compound is 30%. When the degree of crystallinity is 30% or higher, the electron mobility in the above organic-inorganic perovskite compound increases, improving the photoelectric conversion efficiency of the solar cell. A more preferred lower limit for the degree of crystallinity is 50%, and an even more preferred lower limit is 70%. In addition, methods for increasing the degree of crystallinity of the above organic-inorganic perovskite compound include, for example, thermal annealing, irradiation with high-intensity light such as lasers, and plasma irradiation.

[0019] The above photoelectric conversion unit may further contain an organic semiconductor or an inorganic semiconductor in addition to the above organic-inorganic perovskite compound, as long as the effects of the present invention are not impaired. Here, the organic semiconductor or inorganic semiconductor mentioned may serve as a hole transport layer or an electron transport layer. Examples of the above organic semiconductor include compounds having a thiophene skeleton such as poly(3-alkylthiophene). Further, examples also include conductive polymers having a polyphenylene vinylene skeleton, a polyvinyl carbazole skeleton, a polyaniline skeleton, a polyacetylene skeleton, etc. Furthermore, examples include compounds having a porphyrin skeleton such as a phthalocyanine skeleton, a naphthalocyanine skeleton, a pentacene skeleton, a benzoporphyrin skeleton, a spirobifluorene skeleton, etc., and carbon-containing materials such as carbon nanotubes, graphene, fullerenes, etc. which may be surface-modified.

[0020] Examples of the above inorganic semiconductor include titanium oxide, zinc oxide, indium oxide, tin oxide, gallium oxide, tin sulfide, indium sulfide, zinc sulfide, CuSCN, Cu ２ O, CuI, MoO ３ , V ２ O ５ , WO ３ , MoS ２ , MoSe ２ , Cu ２ S, etc.

[0021] When the above photoelectric conversion unit contains the above organic-inorganic perovskite compound and the above organic semiconductor or the above inorganic semiconductor, it may be a laminate in which a thin film-like organic semiconductor or inorganic semiconductor part and a thin film-like organic-inorganic perovskite compound part are laminated, or it may be a composite film in which an organic semiconductor or inorganic semiconductor part and an organic-inorganic perovskite compound part are compounded. A laminate is preferable in terms of the simplicity of the manufacturing method, and a composite film is preferable in terms of being able to improve the charge separation efficiency in the above organic semiconductor or the above inorganic semiconductor.

[0022] The thickness of the above photoelectric conversion part preferably has a lower limit of 5 nm and an upper limit of 5000 nm. If the above thickness is 5 nm or more, light can be sufficiently absorbed, and the photoelectric conversion efficiency is increased. If the above thickness is 5000 nm or less, the generation of regions where charge separation cannot occur can be suppressed, leading to an improvement in the photoelectric conversion efficiency. A more preferable lower limit of the above thickness is 10 nm, and a more preferable upper limit is 1000 nm. An even more preferable lower limit is 20 nm, and an even more preferable upper limit is 500 nm.

[0023] When the above photoelectric conversion part is a composite film in which an organic semiconductor or inorganic semiconductor part and an organic-inorganic perovskite compound part are combined, the preferable lower limit of the thickness of the above composite film is 30 nm, and the preferable upper limit is 3000 nm. If the above thickness is 30 nm or more, light can be sufficiently absorbed, and the photoelectric conversion efficiency is increased. If the above thickness is 3000 nm or less, charges can easily reach the electrode, leading to a high photoelectric conversion efficiency. A more preferable lower limit of the above thickness is 40 nm, and a more preferable upper limit is 2000 nm. An even more preferable lower limit is 50 nm, and an even more preferable upper limit is 1000 nm.

[0024] The method for forming the above photoelectric conversion part is not particularly limited, and examples include a vacuum evaporation method, a sputtering method, a gas phase reaction method (CVD), an electrochemical deposition method, a printing method, etc. Among these, by adopting the printing method, a solar cell capable of exhibiting high photoelectric conversion efficiency can be easily formed over a large area. Examples of the printing method include a spin coating method, a casting method, etc., and examples of the method using the printing method include a roll-to-roll method, etc.

[0025] The power generation unit may have an electron transport layer between the electrode acting as the cathode or the counter electrode and the photoelectric conversion unit. The material of the electron transport layer is not particularly limited and includes, for example, N-type conductive polymers, N-type low molecular weight organic semiconductors, N-type metal oxides, N-type metal sulfides, alkali metal halides, alkali metals, surfactants, etc. Specifically, examples include cyano group-containing polyphenylene vinylene, boron-containing polymers, vasocuproin, vasophenanthrene, hydroxyquinolinatoaluminum, oxadiazole compounds, benzimidazole compounds, naphthalenetetracarboxylic acid compounds, perylene derivatives, phosphine oxide compounds, phosphine sulfide compounds, fluoro group-containing phthalocyanines, titanium dioxide, zinc oxide, indium oxide, tin oxide, gallium oxide, tin sulfide, indium sulfide, zinc sulfide, etc.

[0026] The electron transport layer described above may consist only of a thin-film electron transport layer, but it is preferable to include a porous electron transport layer. In particular, when the photoelectric conversion portion is a composite film formed by combining an organic semiconductor or inorganic semiconductor portion with an organic-inorganic perovskite compound portion, a more complex composite film (a more intricately interwoven structure) can be obtained, and the photoelectric conversion efficiency is higher, so it is preferable that the composite film is fabricated on a porous electron transport layer.

[0027] The preferred lower limit for the thickness of the electron transport layer is 1 nm, and the preferred upper limit is 2000 nm. If the thickness is 1 nm or more, holes can be sufficiently blocked. If the thickness is 2000 nm or less, it will not be a resistance during electron transport, and the photoelectric conversion efficiency will be high. A more preferred lower limit for the thickness of the electron transport layer is 3 nm, a more preferred upper limit is 1000 nm, an even more preferred lower limit is 5 nm, and an even more preferred upper limit is 500 nm.

[0028] The power generation unit may have a hole transport layer between the electrode that acts as the anode or the counter electrode and the photoelectric conversion unit. The material of the hole transport layer is not particularly limited, and the hole transport layer may be made of an organic material. Examples of materials for the hole transport layer include P-type conductive polymers, P-type low molecular weight organic semiconductors, P-type metal oxides, P-type metal sulfides, surfactants, etc. Specifically, examples include compounds having a thiophene skeleton such as poly(3-alkylthiophene). Also, examples include conductive polymers having a triphenylamine skeleton, poly(p-phenylenevinylene) skeleton, polyvinylcarbazole skeleton, polyaniline skeleton, polyacetylene skeleton, etc. Furthermore, examples include compounds having phthalocyanine skeletons, naphthalocyanine skeletons, pentacene skeletons, porphyrin skeletons such as benzoporphyrin skeletons, spirobifluorene skeletons, molybdenum sulfide, tungsten sulfide, copper sulfide, tin sulfide, fluoro group-containing phosphonic acids, carbonyl group-containing phosphonic acids, copper compounds such as CuSCN and CuI, etc.

[0029] The above-mentioned power generation unit may be formed on a substrate. Examples of the substrate include resin films made of polyimide or polyester-based heat-resistant polymers, metal foils, thin glass sheets, etc. In particular, from the viewpoint of flexibility and transparency, the substrate is preferably made of polyethylene terephthalate, polyethylene, polypropylene, polyethylene naphthalate, polymethyl methacrylate, polystyrene, or polycarbonate.

[0030] The above substrate is preferably 30 μm or more and 200 μm or less in thickness. Having the substrate thickness within this range allows for greater flexibility. The substrate is more preferably 50 μm or more in thickness, and even more preferably 70 μm or more. From the viewpoint of further increasing flexibility, the substrate is more preferably 150 μm or less in thickness, and even more preferably 100 μm or less.

[0031] The solar cell of the present invention has a sealing material, which covers the entire power generation section when viewed from the light-receiving surface side and the installation surface side. Here, the light-receiving surface side refers to the surface on which light enters the solar cell, and the installation surface side refers to the side opposite to the light-receiving surface, that is, the surface facing the installation surface when the solar cell is installed. By covering both sides of the power generation section with the sealing material, deterioration of the power generation section due to components in the atmosphere can be suppressed. Preferably, the sealing material covers 105% or more of the upper and lower surfaces of the power generation section, and more preferably 110% or more. Note that the sealing material does not necessarily need to be in direct contact with the power generation section.

[0032] The above-mentioned sealant has a water expansion coefficient of 7.0% or higher. Regardless of the type of solar cell, such as silicon-based inorganic solar cells or organic-inorganic hybrid solar cells, heavy metals harmful to the human body, such as arsenic, chromium, and lead, are used to increase photoelectric conversion efficiency. Although the amount of these heavy metals is small, if a solar cell is severely damaged and the power generation part is exposed to the atmosphere, there is a risk that they will dissolve in rainwater and other substances and be released into the environment. This invention suppresses the leakage of heavy metals even if a solar cell is severely damaged by setting the water expansion coefficient of the sealant within the above range. The reason why setting the water expansion coefficient of the sealant within the above range suppresses the leakage of heavy metals is not entirely clear, but it is thought that the sealant near the photoelectric conversion part that is exposed when the solar cell is damaged absorbs moisture from the atmosphere or rainwater and expands, sealing the exposed part. The water expansion coefficient of the above-mentioned sealant can be adjusted by the type of sealant, the molecular weight of the sealant, the addition of a crosslinking agent, etc. The above-mentioned water expansion coefficient can be measured by the following method.

[0033] First, the sealant is cut into 5cm x 5cm pieces, and multiple pieces are stacked as needed to obtain a test specimen with a thickness of 500μm or more. The obtained test specimen is laminated to a low-water-absorbent film such as ETFE (ethylene tetrafluoroethylene). Next, the thickness of the sealant is measured at five points using a digital indicator (Teclock Co., Ltd., PC-485S3-f or equivalent). Then, the entire sealant is immersed in pure water at 23°C for three days. After immersion, the thickness of the sealant is measured at the same points as before immersion using the digital indicator, and the rate of change in thickness before and after immersion is defined as the water expansion coefficient.

[0034] The water expansion rate of the above-mentioned sealant is preferably 7.2% or higher, more preferably 7.5% or higher, and even more preferably 8.0% or higher, in order to suppress the leakage of heavy metals even if the solar cell is severely damaged. There is no particular upper limit to the water expansion rate of the above-mentioned sealant, but if a sealant with excessively high water swelling properties is used, moisture can easily penetrate into the sealant, so from the viewpoint of suppressing deterioration of power generation performance, it is preferably 15.0% or lower, more preferably 13.0% or lower, and even more preferably 11.0% or lower.

[0035] The above sealing material has a water vapor transmission rate of 1.0 × 10 －３ g / m ２ It is preferable that the water vapor permeability of the sealing material is less than or equal to 1.0 × 10⁻⁶. A high level of condensation prevention can be achieved by having the water vapor permeability of the sealing material within the above range. The water vapor permeability of the sealing material is 1.0 × 10⁻⁶. －４ g / m ２ It is preferable that it be less than or equal to 1.0 × 10 －５ g / m ２ It is more preferable that it be less than or equal to / day. The lower limit of the water vapor permeability of the above sealing material is not particularly limited, and the lower the better, but from a manufacturing technology standpoint, 0.5 × 10 －６ g / m ２ It is approximately per day. The above water vapor transmission rate can be measured by the differential pressure method in accordance with the gas chromatography method of JIS K 7129, under conditions of a temperature of 85°C, a humidity of 85%, and a sample thickness of 100 μm to 200 μm, with the surface located on the outside of the solar cell being used as the water vapor introduction surface.

[0036] The sealing material that forms the main component of the above sealing material should be able to seal the power generation section and, when additives are added as needed, should have a water expansion coefficient within the above range. Specific examples of the above sealing material include thermosetting resins, thermoplastic resins, or inorganic materials. Examples of thermosetting resins or thermoplastic resins include epoxy resins, acrylic resins, silicone resins, phenolic resins, melamine resins, and urea resins. Other examples include butyl rubber, polyester, polyurethane, polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyvinyl alcohol, polyvinyl acetate, ABS resin, polybutadiene, polyamide, polycarbonate, polyimide, polyisobutylene, and polyisoprene. In particular, when the power generation section contains an organic-inorganic perovskite compound in the photoelectric conversion section, the durability of the power generation layer over time can be further enhanced, so it is preferable that the above sealing material is at least one selected from the group consisting of polybutadiene, polyisobutylene, and polyisoprene.

[0037] The above-mentioned encapsulant may contain a crosslinking agent. By including a crosslinking agent in the encapsulant, the water expansion rate can be easily adjusted to the above range. Examples of the above-mentioned crosslinking agent include quinone oximes, bisphenols, and alkylphenol resins.

[0038] The above-mentioned encapsulant preferably has a thickness of 0.5 μm or more on the light-receiving surface side of the power-generating section. Having the above-mentioned thickness on the light-receiving surface side of the encapsulant makes it less likely for the power-generating section to be exposed when the solar cell suffers major damage, thus further suppressing the leakage of heavy metals. Furthermore, it further suppresses the intrusion of external oxygen and moisture, thereby increasing the durability of the solar cell. The thickness of the above-mentioned encapsulant on the light-receiving surface side is more preferably 10 μm or more, and even more preferably 25 μm or more. There is no particular upper limit to the thickness of the above-mentioned encapsulant on the light-receiving surface side, but from the viewpoint of thinning and weight reduction, it is preferably 500 μm or less.

[0039] The above-mentioned sealing material preferably has a thickness of 0.5 μm or more on the side facing the power generation section. Having the above-mentioned thickness on the side facing the installation section of the sealing material makes it less likely for the power generation section to be exposed when the solar cell suffers major damage, and further suppresses the leakage of heavy metals. Furthermore, it further suppresses the intrusion of external oxygen and moisture, thereby increasing the durability of the solar cell. The thickness of the above-mentioned sealing material on the side facing the installation section is more preferably 10 μm or more, and even more preferably 25 μm or more. There is no particular upper limit to the thickness of the above-mentioned sealing material on the side facing the installation section, but from the viewpoint of thinning and weight reduction, it is preferably 500 μm or less.

[0040] The above sealing material has a volume of 1.0 × 10 per unit area. －４ cm ３ / cm ２ It is preferable that the above conditions are met. When the volume of the sealing material per unit area is within the above range, that is, when a large amount of sealing material is used, the power generation part can be made less likely to be exposed when a large-scale damage occurs to the solar cell, and the leakage of heavy metals can be further suppressed. From the viewpoint of balancing the suppression of heavy metal leakage and thinning, the volume of the sealing material per unit area is 2.0 × 10⁻⁶. －３ cm ３ / cm ２ It is more preferable that the above be the case, 5.0 × 10 －３ cm ３ / cm ２ It is even more preferable that the above be the case, 5.0 × 10 －２ cm ３ / cm ２ The following is preferable:

[0041] The above sealing material has a density of 0.85 g / cm³. ３ 1.20g / cm or more ３ The following is preferable. A density of the encapsulant within the above range makes it less likely for the power generation section to be exposed when a large-scale damage occurs to the solar cell, and further suppresses the leakage of heavy metals. From the viewpoint of balancing the suppression of heavy metal leakage with flexibility, the density of the encapsulant is 0.87 g / cm³. ３ It is more preferable that the amount be greater than or equal to 0.90 g / cm³. ３ It is even more preferable that the concentration be greater than or equal to 1.10 g / cm³.３ It is more preferable that the following is the case: 1.00 g / cm³ ３ The following is even more preferable:

[0042] The solar cell of the present invention may have a second sealing material between the power generation section and the sealing material. By sealing the power generation section with the second sealing material and further sealing the outside of the second sealing material with the sealing material, new performance can be imparted while suppressing the outflow of heavy metals from the power generation section. For example, by adding a flame retardant to the second sealing material, a solar cell that is less flammable while suppressing the outflow of heavy metals can be made, and by using an inorganic material for the sealing material constituting the second sealing material, a solar cell with excellent water vapor barrier performance while suppressing the outflow of heavy metals can be made. Note that, since the second sealing material plays a role in suppressing the outflow of heavy metals, it is not necessary for its water expansion coefficient to be within the above range.

[0043] The sealing material constituting the second sealing material described above may be the same as that of the sealing material described above, or it may be different. Examples of sealing materials other than the sealing material described above include resins having at least one skeleton selected from the group consisting of polybutadiene, polyisobutylene, and polyisoprene.

[0044] If the photoelectric conversion section of the power generation unit contains an organic-inorganic perovskite compound, the total thickness of the sealing material, or, if the second sealing material is present, the total thickness of the sealing material and the second sealing material, is preferably 10 μm or more and 1000 μm or less (hereinafter simply referred to as the total thickness of the sealing material). By setting the total thickness of the sealing material within the above range, the balance between the protective performance of the power generation unit and the thinning of the unit can be further improved. The total thickness of the sealing material is more preferably 50 μm or more, even more preferably 100 μm or more, even more preferably 200 μm or more, even more preferably 1000 μm or less, and even more preferably 700 μm or less.

[0045] The solar cell of the present invention may have a front sheet at the uppermost part of the light-receiving surface. The front sheet plays a role in suppressing light reflection and improving the drainage performance of the solar cell surface by forming patterns such as irregularities and arcs on its surface. For example, if a front sheet has a convex arc with its apex at the center of the solar cell, a drainage slope can be provided from the center to the edges of the solar cell. If a front sheet has a concave arc with its lowest point at the center of the solar cell, a water collection area can be provided from the edges to the center of the solar cell. By providing such drainage slopes and water collection areas, the accumulation of dirt and other debris can be concentrated in specific locations. Furthermore, the aesthetic appeal can be improved by randomly forming irregularities on the front sheet.

[0046] The material of the front sheet mentioned above is not particularly limited as long as it is transparent, and examples include fluorine-containing resins, vinyl chloride resins, polyethylene resins, and polycarbonate resins. Specifically, examples include polycarbonate, polyvinyl chloride, tetrafluoroethylene resin, polyvinylidene fluoride, and polychlorotrifluoroethylene. Among these, fluorine-containing resins are preferred because they have excellent weather resistance.

[0047] The thickness of the front sheet is not particularly limited, but from the viewpoint of balancing light transmittance and the functionality of the front sheet, it is preferably 25 μm or more, more preferably 50 μm or more, preferably 1000 μm or less, and more preferably 300 μm or less.

[0048] The solar cell of the present invention may have a backsheet at the bottom of the mounting surface. The backsheet plays a role in improving the weather resistance of the solar cell by preventing the intrusion of substances that cannot be prevented by the sealing layer alone, such as moisture. Examples of materials for the backsheet include polyethylene terephthalate.

[0049] The thickness of the backsheet is not particularly limited, but from the viewpoint of balancing flexibility and the functionality of the backsheet, it is preferably 50 μm or more, more preferably 100 μm or more, preferably 1000 μm or less, and most preferably 500 μm or less.

[0050] Here, schematic diagrams illustrating an example of the structure of the solar cell of the present invention are shown in Figures 1 to 3. As shown in Figure 1, the solar cell of the present invention has a power generation unit 1 consisting of electrodes, a photoelectric conversion unit, a counter electrode, etc., and the entire power generation unit 1 is sealed with a sealing material 2, with a front sheet 3 and a back sheet 4 arranged above and below the sealing material 2. The sealing material 2 has a water expansion coefficient within a specific range, and by using such a sealing material, even if the solar cell is severely damaged, the leakage of heavy metals contained in the power generation unit into the environment can be suppressed. Furthermore, as shown in Figure 2, the solar cell of the present invention may have a second sealing material 5 between the power generation unit 1 and the sealing material 2. By using a desired sealing material or additive in the second sealing material 5, desired performance such as flame retardancy and water vapor barrier properties can be achieved, and in the event of damage to the solar cell, the leakage of heavy metals can be suppressed by the sealing material 2 arranged outside the second sealing material 5. Furthermore, a solar cell using such a second encapsulant may have a structure in which the encapsulant 2 encloses the second encapsulant 5, as shown in Figure 2, or it may have a structure in which two encapsulants 2 are laminated on both sides of the layer of the second encapsulant 5 that encloses the power generation unit 1, as shown in Figure 3.

[0051] The method for manufacturing the solar cell of the present invention is not particularly limited, but a preferred method is to laminate a power generation unit between a sheet (sheet A) having a sealing material and optionally a front sheet, and a sheet (sheet B) having a sealing material and optionally a back sheet, as this method facilitates manufacturing over a large area.

[0052] One example of a method for laminating sheet A, the power generation unit, and sheet B is the roll-to-roll method. By using the roll-to-roll method, large-area solar cells can be manufactured continuously.

[0053] According to the present invention, it is possible to provide a solar cell in which heavy metals are less likely to leak out even if large-scale damage occurs.

[0054] This is a schematic diagram showing an example of the structure of the solar cell of the present invention. This is a schematic diagram showing an example of the structure of the solar cell of the present invention. This is a schematic diagram showing an example of the manufacturing process of the solar cell of the present invention.

[0055] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0056] (Example 1) (1) A polyethylene terephthalate (PET) film with a thickness of 100 μm was prepared as the manufacturing substrate layer for the solar cell. An ITO film with a thickness of 200 nm was formed on the block layer by sputtering to serve as a counter electrode. A thin-film electron transport layer with a thickness of 20 nm was formed on the formed counter electrode by sputtering. Furthermore, a titanium dioxide paste containing titanium dioxide was applied to the thin-film electron transport layer by spin coating and then dried to form a porous electron transport layer with a thickness of 100 nm. Next, lead iodide was dissolved as a metal halide compound in a mixed solvent of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) to prepare a 1 M solution, which was then formed on the porous electron transport layer by spin coating. Furthermore, methylammonium iodide was dissolved in 2-propanol as an amine compound to prepare an 8% by weight solution. This solution is applied to the lead iodide mentioned above by spin coating, and annealed at 150°C for 10 minutes to form a 700 nm thick organic-inorganic perovskite compound CH ３ NH ３ PbI ３ A photoelectric conversion section containing the above was formed. Next, a chlorobenzene solution containing 2% by weight of Spiro-OMETAD (manufactured by Merck) was applied to the photoelectric conversion section by spin coating and then dried to form a hole transport layer with a thickness of 80 nm. Subsequently, an Al film with a thickness of 100 nm was formed on the photoelectric conversion section as an electrode by sputtering, and a power generation section consisting of a counter electrode, electron transport layer, photoelectric conversion section, hole transport layer, and electrode was obtained on a block layer.

[0057] Next, a PET sheet with a thickness of 200 μm was prepared as the front sheet. Then, polyisobutylene A (number average molecular weight Mw: 100,000, density: 0.92 g / cm³) was added.３ Sheet A was obtained by applying a encapsulant made of ) to a front sheet to a thickness of 100 μm. On the other hand, a 360 μm thick aluminum-reinforced backsheet (FAPL, manufactured by Toyo Aluminum Co., Ltd.) was prepared as a backsheet, and sheet B was obtained by applying a encapsulant made of polyisobutylene A to a thickness of 100 μm. Then, the encapsulant of sheet A and the encapsulant of sheet B were placed opposite each other, and the power generation section was sandwiched between sheet A and sheet B and laminated to obtain a solar cell having the structure shown in Figure 1.

[0058] (2) Measurement of the water expansion rate of the sealant The sealant was removed from the obtained solar cell, cut into 5 cm x 5 cm pieces, and multiple pieces were stacked to obtain a test piece with a thickness of 500 μm. The obtained test piece was laminated to a film with low water absorption, such as ETFE (ethylene tetrafluoroethylene). Next, the thickness of the sealant was measured at five points using a digital indicator (Teclock Co., Ltd., PC-485S3-f). Then, the entire sealant was immersed in pure water at 23°C for three days. After immersion, the thickness of the sealant was measured at the same points as before immersion using a digital indicator, and the rate of change in thickness before and after immersion was defined as the water expansion rate. The results are shown in Table 1.

[0059] (Examples 2-9, Comparative Examples 1-3) Solar cells were obtained in the same manner as in Example 1, except that the type and thickness of the encapsulant used were as shown in Table 1, and the water expansion coefficient of the encapsulant was measured. Details of polyisobutylene B-E are as follows. Also, the thickness of the encapsulant on the side above the power generation section and the thickness on the side below the power generation section are the same, that is, half the thickness shown in the table. Polyisobutylene B: Mw = 500,000, density = 0.92 g / cm³ ３ Polyisobutylene C: Mw = 1,000,000, Density = 0.92 g / cm³ ３ Polyisobutylene D: Mw = 1,500,000, Density = 0.92 g / cm³ ３ Polyisobutylene E: Mw = 3,000,000, Density = 0.92 g / cm³ ３

[0060] <Evaluation> The solar cells obtained in the examples were evaluated as follows. The results are shown in Table 1.

[0061] (1) Evaluation of heavy metal leaching: In accordance with JWWA Z108:2016 Water supply equipment - Leaching test method, leaching time 3 days, leaching temperature 23°C, contact area ratio 50 cm². ２ A leaching test was conducted under the condition of 0.001 mg / L, and the amount of heavy metal (Pb) leached out was measured. As a test sample, the obtained solar cell was cut diagonally with scissors so that the power generation part was exposed on the cut surface. Furthermore, all values ​​below the detection limit were considered to be less than 1 ppb. In addition, the detection standard for lead according to JWWA Z108:2016 is 0.001 mg / L or less (1 ppb or less).

[0062]

[0063] According to the present invention, it is possible to provide a solar cell in which heavy metals are less likely to leak out even if large-scale damage occurs.

[0064] 1. Power generation unit 2. Sealing material 3. Front sheet 4. Back sheet 5. Second sealing material

Claims

1. A solar cell comprising a power generation unit and a sealing material, wherein the sealing material covers the entire power generation unit when viewed from the light-receiving surface side and the installation surface side, and the water expansion rate is 7.0% or more.

2. The solar cell according to claim 1, characterized in that the sealing material has a water expansion rate of 15.0% or less.

3. The solar cell according to claim 1 or 2, characterized in that the power generation section contains an organic-inorganic perovskite compound.

4. The solar cell according to any one of claims 1 to 3, characterized in that the sealing material has a thickness of 0.5 μm or more on the light-receiving surface side of the power generation section.

5. The solar cell according to any one of claims 1 to 4, characterized in that the sealing material has a thickness of 0.5 μm or more on the side where the power generation unit is installed.

6. A solar cell according to any one of claims 1 to 5, characterized in that it has a second sealing material between the power generation section and the sealing material.

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