Flexible solar cell

Abstract

The purpose of the present invention is to provide a solar cell excellent in flame retardancy in a plane direction. This flexible solar cell having a power generation unit (4) and a flame-retardant layer (3) is characterized in that the thickness of the flame-retardant layer (3) is 75 μm or larger, and the substance volume per unit area of a layer that has lower flame retardancy than VTM-2 in an UL94 flammability test, among layers disposed above the flame-retardant layer (3), is 0.075 cm3 / cm2 or less.

Description

flexible solar cells

[0001] The present invention relates to flexible solar cells.

[0002] Conventionally, solar cells have been actively developed based on stacked bodies in which an N-type semiconductor layer and a P-type semiconductor layer are disposed between opposing electrodes, with inorganic semiconductors such as silicon being primarily used as the N-type and P-type semiconductors. However, such inorganic solar cells have been problematic in that they are costly to manufacture and difficult to enlarge, limiting their range of application. Therefore, in recent years, attention has been focused on perovskite solar cells, which use organic-inorganic perovskite compounds having a perovskite structure with lead, tin, or the like as the central metal in their photoelectric conversion layers (see, for example, Patent Document 1 and Non-Patent Document 1). Perovskite solar cells are expected to have high photoelectric conversion efficiency, and can be manufactured by a printing method, thereby significantly reducing manufacturing costs.

[0003] Meanwhile, in recent years, flexible solar cells using heat-resistant polymer materials such as polyimide and polyester, or metal foil as a substrate have been attracting attention. Flexible solar cells have advantages such as ease of transportation and installation due to their thinness and light weight, and resistance to impact. For example, they are manufactured by laminating, in a thin film form, multiple layers, such as a photoelectric conversion layer that generates current when irradiated with light, on a flexible substrate. Furthermore, if necessary, the upper and lower surfaces of the flexible solar cell are sealed by laminating solar cell encapsulant sheets. For example, Patent Document 2 describes a substrate for a semiconductor device including a sheet-like aluminum substrate, and an organic thin-film solar cell including this substrate for a semiconductor device.

[0004] JP 2014-72327 A JP 2013-253317 A

[0005] M. M. Lee, et al., Science, 2012, 338, 643

[0006] Such flexible solar cells often use organic materials to provide transparency and flexibility. Because organic materials are more flammable than inorganic materials, flexible solar cells require more fire prevention measures than conventional solar cells. However, while conventional solar cells have been proposed that are flame-retardant in the thickness direction, flame retardancy in the planar direction (the surface direction perpendicular to the thickness direction) has not been adequately studied. In particular, in recent years, advances in manufacturing technology have resulted in larger areas per flexible solar cell unit. Therefore, flame retardancy in the planar direction has become even more important, since poor flame retardancy in the planar direction could result in the burning of a large number of solar cells and the risk of the fire spreading to other areas.

[0007] An object of the present invention is to provide a solar cell that is excellent in flame retardancy in the planar direction.

[0008] The present invention includes the following Disclosures 1 to 12. The present invention will be described in detail below. [Disclosure 1] A flexible solar cell having a power generation section and a flame-retardant layer, wherein the flame-retardant layer has a thickness of 75 μm or more, and a layer disposed on the flame-retardant layer that has a flame-retardant property lower than VTM-2 in a UL94 flame test has a material volume per unit area of ​​0.075 cm ３ / cm ２[Disclosure 2] A flexible solar cell according to Disclosure 1, wherein the flame-retardant layer has a flame retardancy of VTM-0 or higher in a UL94 flame test. [Disclosure 3] A flexible solar cell according to Disclosure 1 or 2, wherein, among layers disposed on the flame-retardant layer, a layer having a flame retardancy lower than VTM-2 in the UL94 flame test has a thickness of 750 μm or less. [Disclosure 4] A flexible solar cell according to any one of Disclosures 1 to 3, wherein a survival rate of the flame-retardant layer in a flame spread test performed in accordance with the IEC 61730-2 MST23 standard is 30 wt % or more. [Disclosure 5] A flexible solar cell according to any one of Disclosures 1 to 4, wherein the flame-retardant layer is made of inorganic fibers, organic resin, or a composite reinforcing material having inorganic fibers and an organic resin. [Disclosure 6] A flexible solar cell according to any one of Disclosures 1 to 5, wherein the flame-retardant layer is made of a composite reinforcing material having glass cloth and an organic resin. [Disclosure 7] The flexible solar cell according to any one of Disclosures 1 to 5, wherein the flame-retardant layer is made of a composite reinforcing material having glass cloth and an organic resin, the flame-retardant layer is disposed above the power generation section, and the refractive index ratio between the glass cloth and the organic resin (refractive index of glass cloth / refractive index of organic resin) is 0.95 or more and 1.05 or less. [Disclosure 8] The flexible solar cell according to any one of Disclosures 1 to 7, wherein the flame-retardant layer contains a coupling material. [Disclosure 9] A flexible solar cell having a power generation section and a flame-retardant layer, wherein the flame-retardant layer has a thickness of 75 μm or more and has a flame retardancy of VTM-0 or higher in a UL94 flame test, and wherein a layer disposed above the flame-retardant layer that has a flame retardancy lower than VTM-2 in the UL94 flame test has a thickness of 750 μm or less. [Disclosure 10] A flexible solar cell having a power generation section and a flame-retardant layer, wherein the flame-retardant layer is made of flame-retardant PET, and the thickness of the flame-retardant layer is 75 μm or more.[Disclosure 11] A flexible solar cell having a power generation section and a flame-retardant layer, wherein the flame-retardant layer is a glass film, and the thickness of the flame-retardant layer is 75 μm or more. [Disclosure 12] A flexible solar cell having a power generation section and a flame-retardant layer, wherein the flame-retardant layer is a composite reinforcing material having glass cloth and an organic resin, and the thickness of the flame-retardant layer is 75 μm or more.

[0009] The flexible solar cell of the present invention has a power generation section. The power generation section is a section that converts sunlight into electricity and is composed of a substrate, an electrode, a counter electrode, a photoelectric conversion layer, an electron transport layer, a hole transport layer, etc., and has at least an electrode, a counter electrode, and a photoelectric conversion layer. In this specification, "layer" refers not only to a layer with a clear boundary, but also to a layer with a concentration gradient in which the contained elements gradually change. Elemental analysis of a layer can be performed, for example, by performing FE-TEM / EDS line analysis measurement of a cross section of a solar cell to confirm the element distribution of a specific element. In addition, in this specification, "layer" refers not only to a flat thin-film layer, but also to a layer that can form a complex, intricate structure together with other layers.

[0010] The substrate is not particularly limited as long as it has flexibility, and examples thereof include resin films made of heat-resistant polymers such as polyimide and polyester, metal foils, thin glass sheets, etc. Among these, PET resin films are preferred.

[0011] The materials for the electrodes and counter electrodes are not particularly limited, and examples thereof include 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, and Al / Al ２ O ３Examples of the material include a mixture of Al and LiF, an Al / LiF mixture, etc. Examples also include gold, silver, titanium, molybdenum, tantalum, tungsten, carbon, nickel, chromium, etc. These materials may be used alone or in combination of two or more.

[0012] The thickness of the electrode and the 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 resistance can be reduced while the electrode functions. If the thickness is 1000 nm or less, the light transmittance can be further improved. A more preferred lower limit of the thickness of the electrode and the counter electrode is 50 nm and a more preferred upper limit is 500 nm.

[0013] The photoelectric conversion layer preferably contains an organic-inorganic perovskite compound represented by the general formula AMX (wherein A is an organic base compound and / or an alkali metal, M is a lead or tin atom, and X is a halogen atom). A solar cell in which the photoelectric conversion layer contains the organic-inorganic perovskite compound is also called an organic-inorganic hybrid solar cell. By using the organic-inorganic perovskite compound in the photoelectric conversion layer, the photoelectric conversion efficiency of the flexible solar cell can be further improved.

[0014] A is an organic base compound and / or an alkali metal. Specific examples of the organic base compound include 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 ions thereof (e.g., methylammonium (CH ３ NH ３) and phenethylammonium. Among these, methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine, formamidine, acetamidine, ions thereof, and phenethylammonium are preferred, and methylamine, ethylamine, propylamine, formamidine, and ions thereof are more preferred. Examples of the alkali metal include lithium, sodium, potassium, rubidium, and cesium.

[0015] The metal atom M is a lead or tin atom. These metal atoms may be used alone or in combination of two or more.

[0016] The X is a halogen atom, and examples of the halogen atom include chlorine, bromine, iodine, sulfur, and selenium. These halogen atoms may be used alone or in combination of two or more. By including a halogen in the structure, the organic-inorganic perovskite compound becomes soluble in organic solvents, enabling application to inexpensive printing methods and the like. In particular, X is preferably iodine, since this narrows the energy band gap of the organic-inorganic perovskite compound.

[0017] The organic-inorganic perovskite compound preferably has a cubic 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 the presence of this structure makes it easy to change the orientation of the octahedrons in the crystal lattice, thereby increasing the mobility of electrons in the organic-inorganic perovskite compound and improving the photoelectric conversion efficiency of solar cells.

[0018] The organic-inorganic perovskite compound is preferably a crystalline semiconductor. By crystalline semiconductor, we mean a semiconductor from which a scattering peak can be detected by measuring an X-ray scattering intensity distribution. When the organic-inorganic perovskite compound is a crystalline semiconductor, the mobility of electrons in the organic-inorganic perovskite compound increases, improving the photoelectric conversion efficiency of the flexible solar cell.

[0019] The degree of crystallinity can also be evaluated as an index of crystallization. The degree of crystallinity can be determined by separating the scattering peaks derived from crystalline materials and the halo derived from amorphous portions detected by X-ray scattering intensity distribution measurement by fitting, determining the respective intensity integrals, and calculating the ratio of the crystalline portion to the total. The preferred lower limit of the degree of crystallinity of the organic-inorganic perovskite compound is 30%. A crystallinity of 30% or more increases the mobility of electrons in the organic-inorganic perovskite compound, improving the photoelectric conversion efficiency of the solar cell. The more preferred lower limit of the degree of crystallinity is 50%, and even more preferred is 70%. Methods for increasing the degree of crystallinity of the organic-inorganic perovskite compound include, for example, thermal annealing, irradiation with high-intensity light such as a laser, and plasma irradiation.

[0020] The photoelectric conversion layer may further contain an organic or inorganic semiconductor in addition to the organic-inorganic perovskite compound, as long as the effects of the present invention are not impaired. The organic or inorganic semiconductor may function as a hole transport layer or an electron transport layer. Examples of the organic semiconductor include compounds having a thiophene skeleton, such as poly(3-alkylthiophene). Other examples include conductive polymers having a polyparaphenylene vinylene skeleton, polyvinyl carbazole skeleton, polyaniline skeleton, polyacetylene skeleton, etc. Other examples include compounds having a porphyrin skeleton, such as a phthalocyanine skeleton, a naphthalocyanine skeleton, a pentacene skeleton, or a benzoporphyrin skeleton, or a spirobifluorene skeleton, as well as carbon-containing materials such as carbon nanotubes, graphene, and fullerenes, which may be surface-modified.

[0021] Examples of the inorganic semiconductor include titanium oxide, zinc oxide, indium oxide, tin oxide, gallium oxide, tin sulfide, indium sulfide, zinc sulfide, CuSCN, and Cu. ２ O, CuI, MoO ３ , V ２ O ５ , W.O. ３ , MoS ２ , MoSe ２ , Cu ２ Examples include S.

[0022] When the photoelectric conversion layer contains the organic-inorganic perovskite compound and the organic semiconductor or inorganic semiconductor, it may be a laminate in which a thin-film organic semiconductor or inorganic semiconductor portion is laminated with a thin-film organic-inorganic perovskite compound portion, or a composite film in which an organic semiconductor or inorganic semiconductor portion is composited with an organic-inorganic perovskite compound portion. A laminate is preferred in that it can be produced easily, and a composite film is preferred in that it can improve the charge separation efficiency in the organic semiconductor or inorganic semiconductor.

[0023] The thickness of the photoelectric conversion layer preferably has a lower limit of 5 nm and an upper limit of 5000 nm. If the thickness is 5 nm or more, sufficient light absorption is possible, resulting in high photoelectric conversion efficiency. If the thickness is 5000 nm or less, the occurrence of regions incapable of charge separation can be suppressed, leading to improved photoelectric conversion efficiency. A more preferred lower limit of the thickness is 10 nm, a more preferred upper limit is 1000 nm, an even more preferred lower limit is 20 nm, and an even more preferred upper limit is 500 nm.

[0024] When the photoelectric conversion layer is a composite film in which an organic semiconductor or inorganic semiconductor portion and an organic-inorganic perovskite compound portion are combined, the preferred lower limit of the thickness of the composite film is 30 nm, and the preferred upper limit is 3000 nm. If the thickness is 30 nm or more, sufficient light absorption is possible, resulting in high photoelectric conversion efficiency. If the thickness is 3000 nm or less, charges can easily reach the electrode, resulting in high photoelectric conversion efficiency. A more preferred lower limit of the thickness is 40 nm, a more preferred upper limit is 2000 nm, an even more preferred lower limit is 50 nm, and an even more preferred upper limit is 1000 nm.

[0025] The method for forming the photoelectric conversion layer is not particularly limited, and examples thereof include vacuum deposition, sputtering, chemical vapor deposition (CVD), electrochemical deposition, and printing. Among these, the use of a printing method makes it possible to easily form a large-area solar cell that can exhibit high photoelectric conversion efficiency. Examples of printing methods include spin coating and casting, and examples of methods using printing include roll-to-roll methods.

[0026] The power generation unit may have an electron transport layer between the cathode or counter electrode and the photoelectric conversion layer. The material of the electron transport layer is not particularly limited, and examples thereof include 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. Specific examples thereof include cyano group-containing polyphenylene vinylene, boron-containing polymers, bathocuproine, bathophenanthrene, hydroxyquinolinatoaluminum, oxadiazole compounds, benzimidazole compounds, naphthalene tetracarboxylic acid compounds, perylene derivatives, phosphine oxide compounds, phosphine sulfide compounds, fluoro group-containing phthalocyanines, titanium oxide, zinc oxide, indium oxide, tin oxide, gallium oxide, tin sulfide, indium sulfide, and zinc sulfide.

[0027] The electron transport layer may consist of only a thin-film electron transport layer, but preferably includes a porous electron transport layer. In particular, when the photoelectric conversion layer is a composite film obtained by combining an organic semiconductor or inorganic semiconductor portion with an organic-inorganic perovskite compound portion, it is preferable that the composite film be formed on a porous electron transport layer, since a more complex composite film (a more intricately intricate structure) can be obtained and the photoelectric conversion efficiency can be increased.

[0028] The thickness of the electron transport layer is preferably 1 nm at the lower limit and 2000 nm at the upper limit. If the thickness is 1 nm or more, holes can be blocked sufficiently. If the thickness is 2000 nm or less, resistance during electron transport is unlikely to occur, resulting in high photoelectric conversion efficiency. The thickness of the electron transport layer is more preferably 3 nm at the lower limit and 1000 nm at the upper limit, and even more preferably 5 nm at the lower limit and 500 nm at the upper limit.

[0029] The power generation section may have a hole transport layer between the anode electrode or counter electrode and the photoelectric conversion layer. 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. Specific examples include compounds having a thiophene skeleton, such as poly(3-alkylthiophene). Other examples include conductive polymers having a triphenylamine skeleton, polyparaphenylenevinylene skeleton, polyvinylcarbazole skeleton, polyaniline skeleton, polyacetylene skeleton, etc. Further examples include compounds having a porphyrin skeleton such as a phthalocyanine skeleton, a naphthalocyanine skeleton, a pentacene skeleton, or a benzoporphyrin skeleton, a spirobifluorene skeleton, or the like; molybdenum sulfide, tungsten sulfide, copper sulfide, tin sulfide, or the like; fluoro group-containing phosphonic acid; carbonyl group-containing phosphonic acid; and copper compounds such as CuSCN and CuI.

[0030] The flexible solar cell of the present invention has a flame-retardant layer. By providing a flame-retardant layer on a flexible solar cell, combustion beyond the flame-retardant layer can be suppressed, thereby improving flame retardancy in the thickness direction. The flame-retardant layer may be a single layer, or multiple flame-retardant layers may be provided. The flame-retardant layer may be provided below, above, or both the power generation section. When the flame-retardant layer is located above the power generation section, high transparency is required for the flame-retardant layer from the perspective of photoelectric conversion efficiency, but the material volume per unit area (described below) can be easily satisfied, thereby increasing the design flexibility of the flexible solar cell as a whole. When the flame-retardant layer is located below the power generation section, design constraints are imposed in order to satisfy the material volume per unit area (described below) compared to when the flame-retardant layer is located above the power generation section. However, transparency is not required for the flame-retardant layer, thereby expanding the range of material options. Note that, in this specification, "upper" refers to the direction of the surface onto which light is incident, and "lower" refers to the opposite direction from "upper," i.e., toward the bottom surface of the flexible solar cell. Furthermore, "flame retardant" in this specification means that the flame retardancy in the UL94 combustion test described below is VTM-0, VTM-1 or flame retardancy exceeding VTM-0.

[0031] The material of the flame-retardant layer is not particularly limited as long as it is flexible and flame-retardant, but examples thereof include organic resins, inorganic materials, inorganic fibers, and composite reinforced materials composed of inorganic fibers and organic resins. Examples of organic resins include polycarbonate, polyphenylene sulfide, polyamide, polyvinyl chloride, polyimide, polypropylene, tetrafluoroethylene resin, perfluoroalkoxyalkane, polyvinylidene fluoride, polyacetal, polyetherimide, polyphenylsulfone, polyamideimide, polyetheretherketone, phenolic resin, urea resin, melamine resin, and polychlorotrifluoroethylene. Examples of inorganic materials include metal foil, punched metal, metal steel plate, and inorganic porous materials. Examples of inorganic fibers include glass cloth, silica cloth, aluminum-glass cloth, heat-resistant rock wool, alkaline earth silicate fiber, alumina cloth, carbon fiber, and organic-inorganic hybrid fiber. Among the above flame-retardant materials, the flame-retardant layer is preferably a composite reinforced material having inorganic fibers and organic materials, and more preferably a composite reinforced material having glass cloth and organic resin, because it has high strength, flame retardancy, and transparency.

[0032] When the flame-retardant layer is made of a composite reinforcing material containing glass cloth and an organic resin and is disposed above the power generation section, the refractive index ratio between the glass cloth and the organic resin (refractive index of glass cloth / refractive index of organic resin) is preferably 0.95 or more and 1.05 or less. Having the refractive index ratio between the glass cloth and the organic resin within this range further increases the transparency of the flame-retardant layer, thereby further suppressing a decrease in photoelectric conversion efficiency even when the flame-retardant layer is disposed above the power generation section. The refractive index ratio between the glass cloth and the organic resin is more preferably 0.97 or more and more preferably 1.03 or less.

[0033] The flame-retardant layer preferably contains a coupling material. The inclusion of a coupling material in the flame-retardant layer facilitates impregnation of the resin constituting the sealing layer into the flame-retardant layer, reducing the likelihood of air bubbles forming in the flame-retardant layer. Therefore, when a transparent material is used for the flame-retardant layer, the transparency of the flame-retardant layer can be increased. Even when a transparent material is not used, the adhesive strength between the flame-retardant layer and the sealing layer is increased, further suppressing interlayer delamination. Examples of the coupling material include acrylic silane, amino silane, epoxy silane, mercapto silane, vinyl silane, methacryl silane, ureido silane, alkyl silane, styryl silane, isocyanurate silane, isocyanate silane, acid anhydride silane, and aqueous acrylic emulsion.

[0034] When the photoelectric conversion layer of the flexible solar cell of the present invention contains the organic-inorganic perovskite compound, the coupling material is preferably hydrophobic. The inventors have discovered that in flexible solar cells using an organic-inorganic perovskite compound in the photoelectric conversion layer, using a hydrophilic material as the coupling material causes the photoelectric conversion layer to deteriorate during sealing. Therefore, in the case of flexible solar cells using an organic-inorganic perovskite compound, deterioration of the photoelectric conversion layer can be suppressed by using a hydrophobic coupling material. Examples of hydrophobic coupling materials include acrylic silane, amino silane, epoxy silane, mercapto silane, vinyl silane, methacryl silane, ureido silane, alkyl silane, styryl silane, isocyanurate silane, isocyanate silane, and acid anhydride silane.

[0035] The content of the coupling material is not particularly limited, but from the viewpoint of further increasing the transparency of the flame-retardant layer and suppressing delamination, it is preferably 0.1% by weight or more, more preferably 1% by weight or more, and is preferably 10% by weight or less, and more preferably 3% by weight or less, relative to 100% by weight of the flame-retardant layer.

[0036] The flame-retardant layer may be disposed as the outermost layer on the bottom side (lower side). Depending on the type of flame-retardant layer, disposing the flame-retardant layer as the outermost layer on the bottom side (lower side) can provide additional functionality. For example, if the flame-retardant layer is a composite reinforced material containing inorganic fibers and organic materials with excellent mechanical strength, it can also be used to secure flexible solar cells. Furthermore, the installation surface of the flexible solar cell is prone to puddles and, since it is not exposed to sunlight, takes a long time to dry, so moisture is likely to penetrate from the bottom side of the flexible solar cell. Therefore, disposing a glass film with excellent water vapor barrier performance as a flame-retardant layer as the outermost layer on the bottom side can effectively suppress the penetration of water vapor from the outside.

[0037] The flame-retardant layer preferably has a flame retardancy of VTM-0 or higher in the UL94 flammability test. The UL94 flammability test is a standard for determining the flammability of materials, and evaluates the flame retardancy of materials using a grade of VTM-0 to VTM-2. VTM-0 is the highest flame retardancy grade, and if the flame-retardant layer undergoes the UL94 flammability test and achieves a VTM-0 grade, the flame retardancy can be further enhanced. Examples of materials that satisfy the flame retardancy of VTM-0 or higher include flame-retardant PET, glass cloth, glass film, silica cloth, aluminum-glass cloth, aluminum foil, metal mesh, and composite reinforced materials made of inorganic fibers and organic resins. Specifically, the UL94 flammability test is measured using the following method.

[0038] First, a test specimen (200±5×50±1×0.5 mm) is prepared. The obtained test specimen is rolled into a cylinder with the long side facing the side (200 mm high) and attached vertically to a clamp. Next, cotton is placed under the test specimen to determine ignition by dropped material. A 20 mm flame is then applied to the lower end of the test specimen for 3 seconds. The above test is performed five times, and the flammability (flame retardancy) is determined based on the evaluation criteria in Table 1 below. Note that the glowing time in the table refers to the time the specimen burns like charcoal after ignition. The burning time does not include the time it is in contact with the flame.

[0039]

[0040] When the flame-retardant layer has a flame retardancy of VTM-0 or higher, particularly when the flame-retardant layer is a composite reinforcing material containing flame-retardant PET, a glass film, a glass cloth, and an organic resin, the flame retardancy of the resulting flexible solar cell is enhanced, and therefore, as long as the thickness of the flame-retardant layer is 75 μm or more, excellent flame retardancy in the planar direction can be exhibited regardless of the amount of flammable components laminated on the flame-retardant layer, as described below.

[0014] The present invention also provides a flexible solar cell having a power generation section and a flame-retardant layer, wherein the flame-retardant layer is flame-retardant PET and has a thickness of 75 μm or more; a flexible solar cell having a power generation section and a flame-retardant layer, wherein the flame-retardant layer is a glass film and has a thickness of 75 μm or more; and a flexible solar cell having a power generation section and a flame-retardant layer, wherein the flame-retardant layer is a composite reinforcing material containing glass cloth and an organic resin, and has a thickness of 75 μm or more. Other configurations of the power generation section, flame-retardant layer, sealing layer, front sheet, back sheet, etc. in the present invention are the same as those of the power generation section, flame-retardant layer, sealing layer, front sheet, and back sheet described above or below.

[0041] The thickness of the flame-retardant layer is 75 μm or more. By setting the thickness of the flame-retardant layer within this range, the flame retardancy of the flexible solar cell can be further improved, and flexibility can be ensured. From the viewpoint of a balance between flame retardancy and flexibility, the thickness of the flame-retardant layer is preferably 100 μm or more, and even more preferably 150 μm or more, and is preferably 500 μm or less, more preferably 300 μm or less, and even more preferably 200 μm or less.

[0042] The flexible solar cell of the present invention may have a front sheet at the top. The front sheet can suppress light reflection and improve the drainage performance of the flexible solar cell surface by forming a pattern such as unevenness or arcs on its surface. For example, providing a front sheet with a convex arc with an apex at the center of the flexible solar cell can provide a drainage gradient from the center to the edge of the solar cell, while providing a front sheet with a concave arc with a bottom at the center of the flexible solar cell can provide a water collection area from the edge to the center of the solar cell. By providing such a drainage gradient or water collection area, the accumulation of dirt and the like can be concentrated in a specific location. Furthermore, randomly forming unevenness on the front sheet can improve design.

[0043] The material of the front sheet is not particularly limited as long as it is transparent, and examples thereof include fluorine-containing resins, vinyl chloride resins, polyethylene resins, polycarbonate resins, etc. Specific examples include polycarbonate, polyvinyl chloride, tetrafluoroethylene resin, polyvinylidene fluoride, polychlorotrifluoroethylene, etc. Among these, fluorine-containing resins are preferred because of their excellent weather resistance.

[0044] The thickness of the front sheet is not particularly limited, but is preferably 25 μm or more, more preferably 50 μm or more, and is preferably 1000 μm or less, more preferably 300 μm or less, from the viewpoint of the balance between light transmittance and the functionality of the front sheet. Note that when the material of the front sheet has flame retardancy and the thickness is 50 μm or more, the front sheet is also treated as a flame-retardant layer in this specification.

[0045] The flexible solar cell of the present invention preferably has an encapsulating layer. By encasing the power generation unit in the encapsulating layer, deterioration of the power generation unit due to components in the atmosphere can be suppressed. Examples of materials for the encapsulating layer include thermosetting resins, thermoplastic resins, and inorganic materials. Examples of the 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, and polyisobutylene.

[0046] From the viewpoint of a balance between the protection performance of the power generation section and flexibility, the thickness of the sealing layer is preferably 10 μm or more, more preferably 50 μm or more, and is preferably 1000 μm or less, and more preferably 700 μm or less. Note that when the sealing does not have flame retardancy and the flame-retardant layer is located below the sealing layer, the thickness of the sealing layer is adjusted to satisfy the material volume per unit area described below.

[0047] The flexible solar cell of the present invention may have a backsheet at the bottom. The backsheet serves to prevent the penetration of substances that cannot be prevented by the sealing layer alone, such as moisture, thereby improving the weather resistance of the flexible solar cell. Examples of materials for the backsheet include polyethylene terephthalate.

[0048] The thickness of the back sheet is not particularly limited, but is preferably 50 μm or more, more preferably 100 μm or more, from the viewpoint of the balance between flexibility and functionality of the back sheet, and is preferably 1000 μm or less, more preferably 500 μm or less.

[0049] In the flexible solar cell of the present invention, among the layers disposed on the flame-retardant layer, the layer having a flame retardancy lower than VTM-2 in the UL94 flame test has a material volume per unit area of ​​0.075 cm ３ / cm ２Among the layers disposed on the flame-retardant layer, the layer having a flame retardancy lower than VTM-2 in the UL94 flame test, i.e., the layer containing a flammable component, has a material volume per unit area within the above range, thereby making it possible to suppress large-scale fire spread in the planar direction that occurs when the flexible solar cell is made large, thereby exhibiting excellent flame retardancy. The material volume per unit area is 0.075 cm ３ / cm ２ Preferably, it is 0.05 cm or less. ３ / cm ２ There is no particular lower limit to the material deposition amount per unit area, and the smaller the better. However, taking into consideration the flexibility, weather resistance, and power generation performance of a flexible solar cell, it is preferable to use a material deposition amount of 0.001 cm or less. ３ / cm ２ It is preferable that the thickness is 0.002 cm or more. ３ / cm ２ The material volume per unit area can be calculated by cutting the flexible solar cell up to the measurement position with a microtome (HistoCore AUTOCUT R, manufactured by Leica Microsystems or an equivalent product), enlarging the cross section with an SEM (SU3800, manufactured by Hitachi High-Technologies Corporation or an equivalent product), and measuring the thickness of the layer that is less flame-retardant than VTM-2.

[0050] Here, "flame retardancy lower than VTM-2" refers to flame retardancy at or below VTM-2, i.e., flame retardancy not meeting VTM-0 or VTM-1. Therefore, layers with flame retardancy above VTM-0, VTM-0, and VTM-1 are excluded from the calculation of the volume of material per unit area, even if they are disposed on the flame-retardant layer. The volume of material per unit area is measured at the portion where the power-generating section is formed. When the flexible solar cell has irregularities, the volume of material per unit area is measured to include the thickest portion of the portion where the power-generating section is formed. Furthermore, when multiple flame-retardant layers are formed, the volume of material per unit area on the uppermost flame-retardant layer (the side where light is incident) is measured. Therefore, the higher the flame-retardant layer is located, the easier it is to satisfy the volume of material per unit area, and there is room for using more flammable layers, thereby increasing the design flexibility of the flexible solar cell. In calculating the material volume per unit area, the power generation section other than the substrate is excluded. This is because the parts of the power generation section other than the substrate are significantly thinner than the other layers, and even if a layer with lower flame retardancy than VTM-2 is included, the effect on combustion is small enough to be within the margin of error.

[0051] In the flexible solar cell of the present invention, among the layers disposed on the flame-retardant layer, the layer having a flame-retardant rating lower than VTM-2 in the UL94 flammability test preferably has a thickness of 750 μm or less. Having the thickness of the layer having a flame-retardant rating lower than VTM-2 within the above range can further enhance flame retardancy in the planar direction. The thickness of the layer having a flame-retardant rating lower than VTM-2 is preferably 700 μm or less, and more preferably 200 μm or less. There is no particular lower limit for the material deposition amount per unit area, and the smaller the better. However, in consideration of the flexibility and power generation performance of the flexible solar cell, it is preferably 0 μm or more, and more preferably 10 μm or more.

[0052] In the flexible solar cell of the present invention, the residual rate of the flame-retardant layer in the flame spread test performed according to the IEC 61730-2 MST23 standard is preferably 30% by weight or more. By having the residual rate of the flame-retardant layer in the flame spread test performed according to the IEC 61730-2 MST23 standard within the above range, flame retardancy in the planar direction can be further improved. The residual rate of the flame-retardant layer in the flame spread test is more preferably 50% by weight or more, and even more preferably 80% by weight or more. There is no particular upper limit for the residual rate of the flame-retardant layer in the flame spread test; the higher the better; however, it is typically 100% by weight or less. The residual rate of the flame-retardant layer in the flame spread test is calculated by the following formula, calculated from the weights before and after the flame spread test, after performing the flame spread test according to the IEC 61730-2 MST23 standard: residual rate (%) = weight after flame spread test / weight before flame spread test × 100

[0053] 1(a) to 1(c) are schematic diagrams showing an example of the structure of a flexible solar cell of the present invention. In the flexible solar cell of FIG. 1(a), a power generation unit 4 including an electrode 41, a photoelectric conversion layer 42, a counter electrode 43, and a substrate 44 is laminated on a back sheet 1. The power generation unit 4 is sealed with a sealing layer 5 to prevent the intrusion of atmospheric components, etc., and a flame-retardant layer 3 is laminated on the sealing layer 5, and a front sheet 6 is laminated on the flame-retardant layer 3 via an adhesive layer 2. In the flexible solar cell of FIG. 1(b), the flame-retardant layer 3 is also formed below the power generation unit 4, while in the flexible solar cell of FIG. 1(c), the flame-retardant layer 3 is formed only below the power generation unit 4. Note that the flexible solar cells of FIGS. 1(a) to 1(c) have frame portions at both ends where the power generation unit 4 is not formed, resulting in a larger material volume per unit area than the other portions. However, the frame portion where the power generating unit 4 is not formed is only a small part of the entire flexible solar cell unit, and the frame portion of the flexible solar cell unit is often covered with a fixing member, so even if the material volume per unit area in the frame portion where the power generating unit is not formed is outside the above range, it does not affect the flame retardancy in the planar direction.

[0054] As described above, the present invention provides a flexible solar cell with excellent flame retardancy in the horizontal direction by providing a flame-retardant layer with a certain thickness or greater and by providing layers formed on the flame-retardant layer with a flame retardancy lower than VTM-2 within the above-described range. On the other hand, even when the flame-retardant layer has high flame retardancy and the thickness of the layers with a flame retardancy lower than VTM-2 falls within a specific range, the flexible solar cell can still provide excellent flame retardancy in the horizontal direction. This flexible solar cell includes a power generation unit and a flame-retardant layer, wherein the flame-retardant layer has a thickness of 75 μm or greater and a flame retardancy of VTM-0 or higher in the UL94 flame test, and the layer disposed on the flame-retardant layer with a flame retardancy lower than VTM-2 in the UL94 flame test has a thickness of 750 μm or less. Other components of the power generation unit, flame-retardant layer, sealing layer, front sheet, back sheet, etc. of the present invention are the same as those of the power generation unit, flame-retardant layer, sealing layer, front sheet, and back sheet described above.

[0055] The method for producing the solar cell of the present invention is not particularly limited, and for example, the power generation unit is produced by first laminating the electrode, the photoelectric conversion layer, the counter electrode, etc. on the substrate by a printing method. Subsequently, a sheet in which an adhesive layer, a flame-retardant layer, a sealing layer, etc. are laminated on a back sheet according to the design of the flexible solar cell, and a sheet in which an adhesive layer, a flame-retardant layer, a sealing layer, etc. are laminated on a front sheet according to the design, and the obtained power generation unit is sandwiched between these sheets and laminated to produce the solar cell.

[0056] The flexible solar cell of the present invention has excellent flame retardancy in the planar direction, and therefore the larger the area of ​​the flexible solar cell unit, the greater the effect. Specifically, the effect of the present invention is more pronounced when the size of the flexible solar cell is 1 m or more in width and 1 m or more in length. To better demonstrate the effect of the present invention, the size of the flexible solar cell of the present invention is preferably 1 m or more in width and / or length, more preferably 2 m or more, and even more preferably 4 m or more. There is no particular upper limit to the size of the flexible solar cell of the present invention, but due to manufacturing technology, the width is limited to approximately 10 m. Note that when produced by a printing method, there is no particular upper limit to the length because continuous production is possible.

[0057] It is also preferable to arrange multiple flexible solar cells of the present invention side by side. Specifically, when arranged on an inclined surface such as a roof, it is preferable to arrange the strip-shaped flexible solar cell units of the present invention side by side with their short sides in the inclined direction. This arrangement creates gaps between the flexible solar cell units, which can prevent fire from spreading between the units. Furthermore, the gaps reduce the temperature rate between the solar cell units, thereby further improving fire resistance.

[0058] It is also preferable to arrange the belt-shaped flexible solar cell units of the present invention side by side with their long sides in the oblique direction, since this arrangement forms gaps between the units in the oblique direction, making it easier for dirt to be washed away by rain, etc., thereby further reducing dirt buildup.

[0059] From the viewpoint of further suppressing the spread of fire while increasing power generation efficiency per area, when multiple flexible solar cells of the present invention are arranged side by side, the gap between each unit is preferably 1 mm or more, preferably 2000 mm or less, more preferably 500 mm or less, and even more preferably 10 mm or less.

[0060] According to the present invention, it is possible to provide a solar cell that has excellent flame retardancy in the planar direction.

[0061] 1 is a schematic diagram showing an example of the structure of a flexible solar cell of the present invention.

[0062] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0063] (Measurement of Flammability (Flame Retardancy)) The materials of each layer listed in Tables 2 to 5 were subjected to a UL94 flammability test using the following method to measure flame retardancy. The results are shown in Tables 2 to 5. First, a test specimen (200±5×50±1×0.5 mm) was prepared using the material of each layer. The obtained test specimen was rolled into a cylinder with the long side facing the side and attached vertically to a clamp. Next, cotton was placed under the test specimen to determine ignition by dropped material. After that, a 20 mm flame was applied to the lower end of the test specimen for 3 seconds. The above test was performed five times, and the flammability (flame retardancy) was determined based on the criteria in Table 1 above.

[0064] Example 1 (1) Manufacturing of Flexible Solar Cell A 100 μm-thick PET film was prepared as a substrate. A 200 nm-thick ITO film was formed on the substrate by sputtering as a counter electrode. A 20 nm-thick thin-film electron transport layer was formed on the formed counter electrode by sputtering. Furthermore, a titanium oxide paste containing titanium oxide was applied to the thin-film electron transport layer by spin coating and then dried to form a 100 nm-thick porous electron transport layer. Next, lead iodide, as a metal halide compound, was dissolved in a mixed solvent of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) to prepare a 1 M solution, and this was then formed on the porous electron transport layer by spin coating. Furthermore, methylammonium iodide, as an amine compound, was dissolved in 2-propanol to prepare an 8 wt % solution. This solution was applied onto the lead iodide by spin coating, and annealed at 150° C. for 10 minutes to form a 700 nm thick film of an organic-inorganic perovskite compound, CH ３ NH ３ PbI ３A photoelectric conversion layer containing the above was formed. Next, a chlorobenzene solution containing 2 wt % Spiro-OMETAD (manufactured by Merck) was applied to the photoelectric conversion layer by spin coating and then dried to form a hole transport layer with a thickness of 80 nm. After that, a 100 nm thick Al film was formed as an electrode on the photoelectric conversion layer by sputtering, thereby obtaining a power generation unit consisting of the substrate, counter electrode, electron transport layer, photoelectric conversion layer, hole transport layer, and electrode.

[0065] Next, a 50 μm thick PET sheet was prepared as a front sheet. Sheet A was obtained by applying polyisobutylene as a sealing layer to a thickness of 12.5 μm on the front sheet. Next, a 360 μm thick aluminum-containing back sheet (manufactured by Toyo Aluminum Co., Ltd., FAPL) was prepared as a back sheet, and polyisobutylene (VTM-2) was applied to a thickness of 50 μm on the aluminum-containing back sheet as an adhesive layer. Next, a 100 μm thick glass cloth (manufactured by Nitto Boseki Co., Ltd., WLA120 105 BS N) was laminated on the adhesive layer as a flame-retardant layer B, and polyisobutylene was applied to a thickness of 12.5 μm on the glass cloth as a sealing layer to obtain Sheet B. The power generation unit was then sandwiched between sheets A and B and laminated so that the sealing layer of sheet A faced the base material of the power generation unit and the sealing layer of sheet B faced the electrode of the power generation unit, thereby obtaining a flexible solar cell having the structure shown in Figure 1(c) (referred to as structure (c) in the table).

[0066] (2) Measurement of material volume per unit area The obtained flexible solar cell was cut using a microtome (HistoCore AUTOCUT R, manufactured by Leica Microsystems) up to the portion where the power generation section was formed. The cross section was then observed using an SEM (SU3800, manufactured by Hitachi High-Technologies Corporation), and the material volume per unit area was calculated by measuring the thickness of the flame-retardant lower layer using VTM-2.

[0067] (Examples 2 to 17, 31 to 36, Comparative Examples 1 to 4) Flexible solar cells were obtained in the same manner as in Example 1, except that the configuration of each layer and the width of the flexible solar cell were as shown in Tables 2 to 5, and the material volume per unit area was calculated. Note that in Examples 33 to 36, the flame retardancy of the front sheet was VTM-0, so the front sheet was treated as the flame retardant layer. Details of the materials of the flame retardant layer B and the front sheet in the tables are as follows: Flame-retardant PET: FR321, manufactured by Celanese Corporation Silica cloth: NSC, manufactured by Japan Glass Fiber Industry Co., Ltd. Aluminum glass cloth: Aluminum glass cloth, manufactured by Sanei Kogyo Co., Ltd. Aluminum foil: 1N30, manufactured by UACJ Foil Co., Ltd. Metal mesh: ES-22042, manufactured by Yamawaki Sangyo Co., Ltd. Thin film glass film: G-Leaf, manufactured by Nippon Electric Glass Co., Ltd. Glass cloth-organic resin composite reinforcement: Funen Clear, manufactured by Achilles Corporation, refractive index ratio (refractive index of glass cloth / refractive index of organic resin): 1.02 ETFE: Fluon, manufactured by AGC

[0068] (Examples 18 to 30) Flexible solar cells were obtained in the same manner as in Example 1, except that the material of flame-retardant layer B was surface-treated and 20 wt % of the coupling materials shown in Tables 3 and 4 were attached to 100 wt % of flame-retardant layer B, and the material volume per unit area was calculated. Details of the materials used for the surface treatment are as follows: Acrylic silane: KBM-5103, manufactured by Shin-Etsu Chemical Co., Ltd. Amino silane: KBM-602, manufactured by Shin-Etsu Chemical Co., Ltd. Epoxy silane: KBM-303, manufactured by Shin-Etsu Chemical Co., Ltd. Mercapto silane: KBM-802, manufactured by Shin-Etsu Chemical Co., Ltd. Vinyl silane: KBM-1003, manufactured by Shin-Etsu Chemical Co., Ltd. Methacrylic silane: KBM-502, manufactured by Shin-Etsu Chemical Co., Ltd. Ureido silane: KBE-585A, manufactured by Shin-Etsu Chemical Co., Ltd. Alkyl silane: KBM-5803, manufactured by Shin-Etsu Chemical Co., Ltd. Styryl silane: KBM-1403, manufactured by Shin-Etsu Chemical Co., Ltd. Isocyanurate silane: KBM-9659, manufactured by Shin-Etsu Chemical Co., Ltd. Isocyanate silane: KBE-9007N, manufactured by Shin-Etsu Chemical Co., Ltd. Acid anhydride silane: X-12-967C, manufactured by Shin-Etsu Chemical Co., Ltd. Water-based acrylic emulsion: X-12-1135, manufactured by Shin-Etsu Chemical Co., Ltd.

[0069] Example 37 A power generation unit was obtained in the same manner as in Example 1. Next, a 50 μm-thick PET sheet was prepared as a front sheet, and polyisobutylene was applied to the front sheet to a thickness of 50 μm as an adhesive layer. Next, a 100 μm-thick glass cloth-impregnated resin film was laminated on the adhesive layer as a flame-retardant layer A, and polyisobutylene was applied to the glass cloth to a thickness of 50 μm as a sealing layer, thereby obtaining Sheet A. Next, Sheet B was produced in the same manner as in Example 1, except that the polyisobutylene thickness was 50 μm. The power generation unit was then sandwiched between Sheet A and Sheet B and laminated so that the sealing layer of Sheet A faced the base material of the power generation unit, and the sealing layer of Sheet B faced the electrode of the power generation unit. This resulted in a flexible solar cell having a width of 1000 mm and a length of 1000 mm, and having the structure shown in FIG. 1(b) (referred to as Structure (b) in the table). The material volume per unit area of ​​the obtained flexible solar cell was calculated in the same manner as in Example 1.

[0070] (Example 38) A power generation unit was obtained in the same manner as in Example 1. Next, Sheet A was produced in the same manner as in Example 37. Subsequently, Sheet B was obtained by applying polyisobutylene as a sealing layer to a thickness of 50 μm on the same aluminum-containing back sheet as in Example 1. Thereafter, Sheet A, the power generation unit, and Sheet B were laminated in the same manner as in Example 1 to obtain a flexible solar cell having a width of 1000 mm and a length of 1000 mm and having the structure shown in FIG. 1(a) (referred to as Structure (a) in the table). For the obtained flexible solar cell, the material volume per unit area was calculated in the same manner as in Example 1.

[0071] <Evaluation> The solar cells obtained in the examples and comparative examples were evaluated as follows. The results are shown in Tables 2 to 5.

[0072] (1) Evaluation of flame spread resistance Four of the obtained flexible solar cells were lined up lengthwise to form a length of 4 m and fixed to prepare an evaluation sample. The obtained evaluation sample was subjected to a flame spread test in accordance with the IEC 61730-2 MST23 standard under fire safety class C conditions. After the flame spread test, the evaluation sample was visually observed, and the flame spread resistance was evaluated by assigning "Good" if no part had burned through and fallen, and "Poor" if any part had fallen.

[0073] (2) Evaluation of the Residual Rate of the Flame-Retardant Layer The weights were measured before and after the flame spread test, and the value obtained by the following calculation formula was taken as the residual rate: Weight after flame spread test / Weight before flame spread test×100

[0074] (3) Evaluation of performance after sealing Immediately after the production of the solar cell, a power supply (KEITHLEY, Model 236) was connected between the electrodes of the solar cell, and an intensity of 100 mW / cm ２ The photoelectric conversion efficiency was measured using a solar simulator (manufactured by Yamashita Denso Co., Ltd.) The performance after sealing was evaluated by assigning "good" when the value of photoelectric conversion efficiency after sealing / photoelectric conversion efficiency before sealing was 0.9 or more, and "poor" when it was less than 0.9.

[0075]

[0076]

[0077]

[0078]

[0079] According to the present invention, it is possible to provide a solar cell that has excellent flame retardancy in the planar direction.

[0080] REFERENCE SIGNS LIST 1 Back sheet 2 Adhesive layer 3 Flame retardant layer 4 Power generation section 41 Electrode 42 Photoelectric conversion layer 43 Counter electrode 44 Substrate 5 Sealing layer 6 Front sheet

Claims

1. A flexible solar cell having a power generation section and a flame-retardant layer, The flame-retardant layer exhibits flame retardancy of VTM-0 or higher than VTM-0 in the UL94 combustion test. The thickness of the flame-retardant layer is 75 μm or more. Among the layers placed on the aforementioned flame-retardant layer, the layer whose flame retardancy is lower than VTM-2 in the UL94 combustion test has a unit area material volume of 0.075 cm³. ３ / cm ２ The following is A flexible solar cell characterized by the following features.

2. The flexible solar cell according to claim 1, characterized in that, among the layers disposed on the flame-retardant layer, the thickness of the layer that is less flame-retardant than VTM-2 in the UL94 combustion test is 750 μm or less.

3. The flexible solar cell according to claim 1 or 2, characterized in that the remaining percentage of the flame-retardant layer in the flame propagation test conducted in accordance with the IEC 61730-2 MST23 standard is 30% by weight or more.

4. The flexible solar cell according to claim 1 or 2, characterized in that the flame-retardant layer is made of inorganic fibers, organic resin, or a composite reinforced material having inorganic fibers and organic resin.

5. The flexible solar cell according to claim 1 or 2, characterized in that the flame-retardant layer is made of a composite reinforced material having glass cloth and an organic resin.

6. The flexible solar cell according to claim 1 or 2, characterized in that the flame-retardant layer is made of a composite reinforced material having glass cloth and an organic resin, the flame-retardant layer is positioned above the power generation section, and the refractive index ratio of the glass cloth and the organic resin (refractive index of glass cloth / refractive index of organic resin) is 0.95 or more and 1.05 or less.

7. The flexible solar cell according to claim 1 or 2, characterized in that the flame-retardant layer contains a coupling material.

8. A flexible solar cell having a power generation section and a flame-retardant layer, The flame-retardant layer has a thickness of 75 μm or more. In the UL94 combustion test, it exhibits flame retardancy of VTM-0 or higher than VTM-0. Of the layers placed on the aforementioned flame-retardant layer, the thickness of the layer that is less flame-retardant than VTM-2 in the UL94 combustion test is 750 μm or less. A flexible solar cell characterized by the following features.

9. A flexible solar cell having a power generation section and a flame-retardant layer, The flame-retardant layer is made of flame-retardant PET. A flexible solar cell characterized in that the thickness of the flame-retardant layer is 75 μm or more.

10. A flexible solar cell having a power generation section and a flame-retardant layer, The flame-retardant layer is a glass film. A flexible solar cell characterized in that the thickness of the flame-retardant layer is 75 μm or more.

11. A flexible solar cell having a power generation section and a flame-retardant layer, The flame-retardant layer is a composite reinforced material having glass cloth and organic resin. A flexible solar cell characterized in that the thickness of the flame-retardant layer is 75 μm or more.