JP2008098344A - Film-like solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a film-like solar cell which is superior in production efficiency and which does not have a defect and variation in performance. <P>SOLUTION: When polyamic acid is flow-expanded to a support body and is dried, an imidization rate of one face (A surface) of a polyimide precursor film is set to be IMa and that of the other face (B surface) to be IMb, the polyimide precursor film in which a difference of the imidization rates of IMa and IMb is 5 or below is obtained. A polyimide film is obtained by imidizing the polyimide precursor film by heat. A photoelectric conversion thin film is laminated on the colorless and transparent polyimide film whose light transmission rate in a wavelength 500 nm is 50% or above in the film-like solar cell. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a film-like solar cell that uses a transparent polyimide film that is a heat-resistant polyimide film that is a flexible substrate and has a small difference in front and back.

A solar cell using a silicon thin film provided on a non-flexible substrate such as a glass plate and using a resin thin film such as polyimide as a flexible substrate is known. When manufacturing thin-film silicon solar cells, the feature of using a flexible film substrate is that the necessary thin-film silicon layer can be provided on the substrate by a continuous winding method or a step-by-step method. It is in the point that it is extremely superior to the non-flexible substrate. Furthermore, since amorphous silicon solar cells formed on flexible substrates are film-like, unlike conventional solar cells formed on flexible substrates, the shape of the product can be given to some degree. And its application is expected to expand.

Many amorphous silicon solar cells using a flexible resin thin film as a substrate have already been proposed. The flexible resin thin film needs to have a certain degree of heat resistance in that a silicon thin film layer is laminated. Polyimide film is well known for its heat resistant film, but when used as a solar cell, it can be used as light irradiation from the substrate side. There is a problem that the utilization rate of.
For example, a flexible solar cell having a high conversion efficiency using a polymer film such as a polyimide film as a substrate (see Patent Document 1), manufactured by a reaction of biphenyltetracarboxylic acid and an aromatic diamine component containing phenylenediamine. A solar cell using a silicon thin film as a substrate on a polyimide film (see Patent Document 2), a solar cell having a polyimide resin layer in which insulating fine particles having a particle diameter of 0.1 to 1 μm are blended as a surface layer (Patent Document 3) ), A solar cell having a polyimide film made of pyromellitic acid and oxydianiline as a substrate (see Patent Document 4), a transparent conductive film on a translucent plastic substrate such as polyethersulfone or polyimide, A thin film solar cell in which a photoelectric conversion layer and a back electrode layer are sequentially laminated (see Patent Document 5, for example) It is below.

JP 05-259494 A Japanese Patent Laid-Open No. 11-029645 JP 2000-091606 A JP 2003-073473 A JP, 2005-129713, A As an example which attached importance to translucency, on the cycloolefin system polymer as a flexible substrate, the photovoltaic generation layer which consists of thin film silicon which has a silicon atom and a hydrogen atom as a main component is formed. A thin-film silicon solar cell having excellent translucency in the short wavelength region provided has been proposed (see Patent Document 6). Substrates in conventional solar cells have the disadvantage that polyimide film is inferior in light transmittance and has excellent heat resistance and flexibility, and films other than polyimide film have the advantage in light transmittance and inferior heat resistance The glass substrate had the advantages of excellent heat resistance and light transmittance and the disadvantage of poor flexibility. When the polyimide film was tested, the conversion efficiency of light energy into electrical energy varied depending on the film, and thus a film that could obtain good conversion efficiency had to be studied. JP 2003-031823 A

The present invention is a film-like solar cell having a photoelectric conversion thin film such as a silicon thin film on a flexible substrate. The substrate has high heat resistance, excellent translucency, suitable for a roll-to-roll manufacturing method, and has a high production efficiency. It is an object of the present invention to provide a thin-film solar cell with high conversion efficiency of light energy to electric energy as a good element.

  As a result of intensive studies, the present inventors have used a specific polyimide film having transparency and heat resistance and no difference between the front and back as a substrate, which is suitable for a roll-to-roll manufacturing method and has a high production efficiency. It has been found that a good and practical film-like solar cell can be obtained, and the present invention has been reached.

That is, this invention consists of the following structures.
1. Polyamic acid obtained by reacting diamine and aromatic tetracarboxylic acid is cast and dried on a support, and the imidization ratio of one side (A side) of the polyimide precursor film is set to IMa, When the imidization ratio of the surface (B surface) is IMb, a polyimide precursor film having a difference between IMa and IMb of 5 or less is obtained, and the polyimide precursor film is imidized by heat to obtain a polyimide film A film-like solar cell obtained by laminating a photoelectric conversion thin film on a colorless transparent polyimide film having a light transmittance of 50% or more at a wavelength of 500 nm.
2. The film-like solar cell according to 1 above, wherein the diamine is trans 1,4-diaminocyclohexane.
3. 2. The film-like solar cell as described in 1 above, wherein the diamine is 4,4′-methylenebis (cyclohexylamine).

  The polyamic acid obtained by reacting the diamine and aromatic tetracarboxylic acid of the present invention is cast and dried on a support, and the imidization ratio of one side (A side) of the polyimide precursor film is defined as IMa. When the imidization ratio of the other surface (B surface) is IMb, a polyimide precursor film having a difference between IMa and IMb of 5 or less is obtained, and the polyimide precursor film is imidized by heat. A film-like solar cell industrially obtained by laminating a photoelectric conversion thin film on a colorless transparent polyimide film having a light transmittance of 50% or more at a wavelength of 500 nm, which is a polyimide film to be obtained, uses a specific substrate film As a result, the deformation of the substrate film due to the heat received in the process such as the formation of the photoelectric conversion thin film on the film and the process of the electrode layer formation, Resistant to warping and twisting on the film surface, the production efficiency of the film-like solar cell is excellent, and there are no defects or variations in the performance of the film-like solar cell, which is extremely useful industrially. .

The polyimide film of the present invention is obtained by casting and drying a polyamic acid obtained by reacting a diamine and an aromatic tetracarboxylic acid on a support, and imidation ratio of one side (A side) of the polyimide precursor film Is IMa and the imidization ratio of the other surface (B surface) is IMb, a polyimide precursor film in which the difference between IMa and IMb is 5 or less is obtained, and the polyimide precursor film is imide by heat. If it is a colorless and transparent polyimide film having a light transmittance of 50% or more at a wavelength of 500 nm, the diamine and aromatic tetracarboxylic acid are not particularly limited, Examples of diamines include trans 1,4-diaminocyclohexane (abbreviated as t-CHDA) and / or 4,4′-methylenebis (cyclohexene). Silamine) (abbreviated as MBCA) is preferred.
In the present invention, the diamine is preferably used in an amount of 70 mol% or more, more preferably 85 mol% or more. If it is less than 30 mol% of diamine, it may be a polyimide film using one or more diamines exemplified below in combination.

  4,4′-bis (3-aminophenoxy) biphenyl, bis [4- (3-aminophenoxy) phenyl] ketone, bis [4- (3-aminophenoxy) phenyl] sulfide, bis [4- (3-amino Phenoxy) phenyl] sulfone, 2,2-bis [4- (3-aminophenoxy) phenyl] propane, 2,2-bis [4- (3-aminophenoxy) phenyl] -1,1,1,3,3 , 3-hexafluoropropane, m-phenylenediamine, o-phenylenediamine, p-phenylenediamine, m-aminobenzylamine, p-aminobenzylamine, 3,3′-diaminodiphenyl ether, 3,4′-diaminodiphenyl ether, 4,4′-diaminodiphenyl ether, 3,3′-diaminodiphenyl sulfide, 3,3′-di Mino diphenyl sulfoxide, 3,4'-diaminodiphenyl sulfoxide, 4,4'-diaminodiphenyl sulfoxide, 3,3'-diaminodiphenyl sulfone, 3,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone,

3,3′-diaminobenzophenone, 3,4′-diaminobenzophenone, 4,4′-diaminobenzophenone, 3,3′-diaminodiphenylmethane, 3,4′-diaminodiphenylmethane, 4,4′-diaminodiphenylmethane, bis [ 4- (4-aminophenoxy) phenyl] methane, 1,1-bis [4- (4-aminophenoxy) phenyl] ethane, 1,2-bis [4- (4-aminophenoxy) phenyl] ethane, 1, 1-bis [4- (4-aminophenoxy) phenyl] propane, 1,2-bis [4- (4-aminophenoxy) phenyl] propane, 1,3-bis [4- (4-aminophenoxy) phenyl] Propane, 2,2-bis [4- (4-aminophenoxy) phenyl] propane, 1,1-bis [4- (4-aminopheno) Cis) phenyl] butane, 1,3-bis [4- (4-aminophenoxy) phenyl] butane, 1,4-bis [4- (4-aminophenoxy) phenyl] butane, 2,2-bis [4- (4-aminophenoxy) phenyl] butane, 2,3-bis [4- (4-aminophenoxy) phenyl] butane,

2- [4- (4-Aminophenoxy) phenyl] -2- [4- (4-aminophenoxy) -3-methylphenyl] propane, 2,2-bis [4- (4-aminophenoxy) -3- Methylphenyl] propane, 2- [4- (4-aminophenoxy) phenyl] -2- [4- (4-aminophenoxy) -3,5-dimethylphenyl] propane, 2,2-bis [4- (4 -Aminophenoxy) -3,5-dimethylphenyl] propane, 2,2-bis [4- (4-aminophenoxy) phenyl] -1,1,1,3,3,3-hexafluoropropane, 1,4 -Bis (3-aminophenoxy) benzene, 1,3-bis (3-aminophenoxy) benzene, 1,4-bis (4-aminophenoxy) benzene, 4,4'-bis (4-aminophenoxy) bif Nyl, bis [4- (4-aminophenoxy) phenyl] ketone, bis [4- (4-aminophenoxy) phenyl] sulfide, bis [4- (4-aminophenoxy) phenyl] sulfoxide, bis [4- (4 -Aminophenoxy) phenyl] sulfone, bis [4- (3-aminophenoxy) phenyl] ether, bis [4- (4-aminophenoxy) phenyl] ether,

  1,3-bis [4- (4-aminophenoxy) benzoyl] benzene, 1,3-bis [4- (3-aminophenoxy) benzoyl] benzene, 1,4-bis [4- (3-aminophenoxy) Benzoyl] benzene, 4,4′-bis [(3-aminophenoxy) benzoyl] benzene, 1,1-bis [4- (3-aminophenoxy) phenyl] propane, 1,3-bis [4- (3- Aminophenoxy) phenyl] propane, 3,4'-diaminodiphenyl sulfide, 2,2-bis [3- (3-aminophenoxy) phenyl] -1,1,1,3,3,3-hexafluoropropane, bis [4- (3-Aminophenoxy) phenyl] methane, 1,1-bis [4- (3-aminophenoxy) phenyl] ethane, 1,2-bis [4- (3-aminophene) Xyl) phenyl] ethane, bis [4- (3-aminophenoxy) phenyl] sulfoxide, 4,4′-bis [3- (4-aminophenoxy) benzoyl] diphenyl ether, 4,4′-bis [3- (3 -Aminophenoxy) benzoyl] diphenyl ether, 4,4′-bis [4- (4-amino-α, α-dimethylbenzyl) phenoxy] benzophenone,

  4,4′-bis [4- (4-amino-α, α-dimethylbenzyl) phenoxy] diphenylsulfone, bis [4- {4- (4-aminophenoxy) phenoxy} phenyl] sulfone, 1,4-bis [4- (4-Aminophenoxy) phenoxy-α, α-dimethylbenzyl] benzene, 1,3-bis [4- (4-aminophenoxy) phenoxy-α, α-dimethylbenzyl] benzene, 1,3-bis [4- (4-Amino-6-trifluoromethylphenoxy) -α, α-dimethylbenzyl] benzene, 1,3-bis [4- (4-amino-6-fluorophenoxy) -α, α-dimethylbenzyl ] Benzene, 1,3-bis [4- (4-amino-6-methylphenoxy) -α, α-dimethylbenzyl] benzene, 1,3-bis [4- (4-amino-6-sia) Phenoxy) -α, α-dimethylbenzyl] benzene, 3,3′-diamino-4,4′-diphenoxybenzophenone, 4,4′-diamino-5,5′-diphenoxybenzophenone, 3,4′-diamino -4,5'-diphenoxybenzophenone, 3,3'-diamino-4-phenoxybenzophenone,

  4,4′-diamino-5-phenoxybenzophenone, 3,4′-diamino-4-phenoxybenzophenone, 3,4′-diamino-5′-phenoxybenzophenone, 3,3′-diamino-4,4′-dibi Phenoxybenzophenone, 4,4′-diamino-5,5′-dibiphenoxybenzophenone, 3,4′-diamino-4,5′-dibiphenoxybenzophenone, 3,3′-diamino-4-biphenoxybenzophenone, 4, 4'-diamino-5-biphenoxybenzophenone, 3,4'-diamino-4-biphenoxybenzophenone, 3,4'-diamino-5'-biphenoxybenzophenone, 1,3-bis (3-amino-4- Phenoxybenzoyl) benzene, 1,4-bis (3-amino-4-phenoxybenzoyl) benzene, , 3-bis (4-amino-5-phenoxybenzoyl) benzene, 1,4-bis (4-amino-5-phenoxybenzoyl) benzene, 1,3-bis (3-amino-4-biphenoxybenzoyl) benzene 1,4-bis (3-amino-4-biphenoxybenzoyl) benzene, 1,3-bis (4-amino-5-biphenoxybenzoyl) benzene, 1,4-bis (4-amino-5-bi) Phenoxybenzoyl) benzene, 2,6-bis [4- (4-amino-α, α-dimethylbenzyl) phenoxy] benzonitrile and some or all of the hydrogen atoms on the aromatic ring in the aromatic diamine are halogen atoms, particularly A part of hydrogen atoms of a fluorine atom, an alkyl group or alkoxyl group having 1 to 3 carbon atoms, a cyano group, or an alkyl group or alkoxyl group; All are mentioned halogenated alkyl group or an aromatic diamine substituted with an alkoxyl group having 1 to 3 carbons substituted with a halogen atom and country fluorine atom and the like.

  The tetracarboxylic acid used in the present invention is preferably an aromatic tetracarboxylic anhydride. Specific examples of the aromatic tetracarboxylic acid anhydride include the following, but pyromellitic acid anhydride of Chemical formula 5 and 3,3 ′, 4,4 ′ biphenyltetracarboxylic acid anhydride of Chemical formula 6 are preferable. It is a thing. Pyromellitic anhydride or 3,3 ′, 4,4′biphenyltetracarboxylic anhydride is preferably used in an amount of 70 mol% or more, more preferably 85 mol%. In addition to this pyromellitic acid anhydride and 3,3 ′, 4,4′biphenyltetracarboxylic acid anhydride, if the following aromatic tetracarboxylic acid (anhydride) or non-aromatic tetracarboxylic acid is less than 30 mol% You may use together.

  Butane-1,2,3,4-tetracarboxylic dianhydride, pentane-1,2,4,5-tetracarboxylic dianhydride, cyclobutanetetracarboxylic dianhydride, cyclopentane-1,2,3 , 4-tetracarboxylic dianhydride, cyclohexane-1,2,4,5-tetracarboxylic dianhydride, cyclohex-1-ene-2,3,5,6-tetracarboxylic dianhydride, 3- Ethylcyclohex-1-ene-3- (1,2), 5,6-tetracarboxylic dianhydride, 1-methyl-3-ethylcyclohexane-3- (1,2), 5,6-tetracarboxylic Acid dianhydride, 1-methyl-3-ethylcyclohex-1-ene-3- (1,2), 5,6-tetracarboxylic dianhydride, 1-ethylcyclohexane-1- (1,2) , 3,4-tetracarboxylic dianhydride, -Propylcyclohexane-1- (2,3), 3,4-tetracarboxylic dianhydride, 1,3-dipropylcyclohexane-1- (2,3), 3- (2,3) -tetracarboxylic acid Dianhydride, dicyclohexyl-3,4,3 ′, 4′-tetracarboxylic dianhydride, bicyclo [2.2.1] heptane-2,3,5,6-tetracarboxylic dianhydride, 1- Propylcyclohexane-1- (2,3), 3,4-tetracarboxylic dianhydride, 1,3-dipropylcyclohexane-1- (2,3), 3- (2,3) -tetracarboxylic acid bis Anhydride, dicyclohexyl-3,4,3 ′, 4′-tetracarboxylic dianhydride, bicyclo [2.2.1] heptane-2,3,5,6-tetracarboxylic dianhydride, bicyclo [2 2.2] Octane-2,3,5,6-te Rakarubon dianhydride, bicyclo [2.2.2] oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, and the like. These tetracarboxylic acids (dianhydrides) may be used alone or in combination of two or more.

  The solvent used when polymerizing the diamine (monomer) and the tetracarboxylic acid (monomer) to obtain the polyamic acid is not particularly limited as long as it dissolves both the raw material monomer and the polyamic acid to be produced. Polar organic solvents are preferred, for example, N-methyl-2-pyrrolidone, N-acetyl-2-pyrrolidone, N, N-dimethylformamide, N, N-diethylformamide, N, N-dimethylacetamide, dimethyl sulfoxide, hexa Examples include methylphosphoric amide, ethyl cellosolve acetate, diethylene glycol dimethyl ether, sulfolane, and halogenated phenols. These solvents can be used alone or in combination. The amount of the solvent used may be an amount sufficient to dissolve the monomer as a raw material. As a specific amount used, the mass of the monomer in the solution in which the monomer is dissolved is usually 5 to 40% by mass, The amount is preferably 10 to 30% by mass.

Conventionally known conditions may be applied for the polymerization reaction for obtaining the polyamic acid (hereinafter also simply referred to as “polymerization reaction”). As a specific example, in a temperature range of 0 to 80 ° C., 10 Stirring and / or mixing continuously for 30 minutes. If necessary, the polymerization reaction may be divided or the temperature may be increased or decreased. In this case, the order of adding both monomers is not particularly limited, but it is preferable to add an aromatic tetracarboxylic acid (anhydride) to the aromatic diamine solution. The mass of the polyamic acid in the polyamic acid solution obtained by the polymerization reaction is preferably 5 to 40% by mass, more preferably 10 to 30% by mass, and the viscosity of the solution is measured with a Brookfield viscometer (25 ° C.). From the viewpoint of the stability of liquid feeding, it is preferably 10 to 2000 Pa · s, and more preferably 100 to 1000 Pa · s.
The reduced viscosity (ηsp / C) of the polyamic acid in the present invention is not particularly limited, but is preferably 2.0 dl / g or more, more preferably 3.0 dl / g or more, still more preferably 5.0 dl / g or more. preferable.

Vacuum defoaming during the polymerization reaction is effective for producing a high-quality polyamic acid organic solvent solution. Moreover, you may perform superposition | polymerization by adding a small amount of terminal blockers to aromatic diamine before a polymerization reaction. Examples of the end capping agent include compounds having a carbon-carbon double bond such as maleic anhydride. The amount of maleic anhydride used is preferably 0.001 to 1.0 mol per mol of aromatic diamine.
In order to form a polyimide film from the polyamic acid solution obtained by the polymerization reaction, a green film is obtained by coating the polyamic acid solution on a support and drying, and then subjecting the green film to a heat treatment. The method of imidating reaction is mentioned.

The support to which the polyamic acid solution is applied only needs to have sufficient rigidity to form the polyamic acid solution into a film, and the surface is metal (more preferably, stainless steel that does not rust and has excellent corrosion resistance), plastic Or a drum-like rotating body.
A method using a polymer film having appropriate rigidity is also a preferred embodiment.
The surface of the metal support may be plated with metal such as Cr, Ni, or Sn.
The surface of the support can be provided with any surface uneven structure depending on the required surface uneven structure of the film, and the surface uneven structure on the support may be applied to the entire surface of the support or may be partially applied. Alternatively, it may be directly applied to the drum or belt-like rotating body support, or a film or metal may be bonded and used. The depth of the concavo-convex structure is preferably 0.5 μm to 80 μm, more preferably 0.5 μm to 50 μm, and still more preferably 0.5 μm to 40 μm.
Application of the polyamic acid solution to the support includes, but is not limited to, casting from a slit base, extrusion through an extruder, squeegee coating, reverse coating, die coating, applicator coating, wire bar coating, etc. Conventionally known solution coating means can be appropriately used.
In addition, after applying the polyamic acid solution on the support, a part of the solvent is released by overheating, and the surface uneven structure can be imparted by pressing the uneven roll at a stage where the degree of accident indication is reached.
By these prescriptions, a polyimide film having a concavo-convex structure with Rt of 0.3 μm to 50 μm and RSm of about 0.5 μm to 40 μm can be obtained relatively easily.

The above polyimide film may be provided with a lubricant in the polyimide to give fine irregularities on the film surface to improve the slipping property of the film.
Further, by applying a polyamic acid solution filled with a lubricant at a high density on the surface layer, the surface uneven structure may be applied to the entire surface of the support or may be partially applied, and the depth of the uneven structure is 0. The thickness is preferably 0.5 μm to 80 μm, more preferably 0.5 μm to 50 μm, and still more preferably 0.5 μm to 40 μm.
As the lubricant, inorganic or organic fine particles having an average particle diameter of about 0.03 μm to 1 μm can be used. Specific examples include titanium oxide, alumina, silica, calcium carbonate, calcium phosphate, calcium hydrogen phosphate, calcium hydrogen pyrophosphate. , Magnesium oxide, calcium oxide, clay mineral and the like.
Although the thickness of a polyimide film is not specifically limited, When considering using it for the board | substrate film of a film-form solar cell, it is 3-200 micrometers normally, Preferably it is 10-150 micrometers.
This thickness can be easily controlled by the amount of the polyamic acid solution applied to the support and the concentration of the polyamic acid solution.
The polyimide film of the present invention is usually an unstretched film, but may be stretched uniaxially or biaxially. Here, the unstretched film refers to a film obtained without intentionally applying a mechanical external force in the surface expansion direction of the film by tenter stretching, roll stretching, inflation stretching, or the like.

  One of the polyimide precursor films, which is a polyimide film used for the substrate of the film-like solar cell of the present invention, is cast and dried on a support, polyamic acid obtained by reacting diamine and aromatic tetracarboxylic acid. When the imidation rate of the surface (A surface) is IMa and the imidization rate of the other surface (B surface) is IMb, a polyimide precursor film having a difference between IMa and IMb of 5 or less is obtained. The polyimide film obtained by imidizing the polyimide precursor film with heat is the main point that the drying method obtains the predetermined polyimide precursor film, and the polyimide precursor film is self-supporting in the drying step. The surface of the polyimide precursor film that is in contact with air because the direction in which the solvent evaporates is limited to the surface that is in contact with air. The imidization rate tends to be lower than the imidization rate of the surface in contact with the support, but in order to obtain a polyimide precursor film in which the difference in imidization rate between the front and back of the film is 0.001 or more and 5 or less, drying is performed. It is indispensable to introduce a double-sided drying step before performing the polyimide treatment through the process, and by passing through this double-sided drying step, a polyimide precursor film with a small difference in imidation ratio between the front and back can be obtained. It is essential to imidize this polyimide precursor film.

As a specific method of the double-sided drying step, for example, the precursor film that has finished the first drying step on the support is peeled off from the support, and the precursor is in a state in which the film surface that is in close contact with the support is free. The method of drying a body film is mentioned.
In the precursor film obtained by the method for producing a polyimide film in the present invention, when the imidation rate of one surface (A surface) is IMa and the imidation rate of the other surface (B surface) is IMb The difference between IMa and IMb is 0.001 or more and 5 or less, more preferably 0.001 or more and 1 or less.
When the difference between IMa and IMb of the polyimide precursor film exceeds 5, there are obstacles such as frequent curling when the resulting polyimide film is used as a substrate film for solar cells, and the tensile elastic modulus is less than 2 GPa. It tends to be a polyimide film with poor quality.
As drying conditions in the drying step, for example, when N-methylpyrrolidone is used as a solvent, the drying temperature is preferably 70 to 130 ° C, more preferably 75 to 125 ° C, and further preferably 80 to 120 ° C. is there.
When the drying temperature is higher than 130 ° C., the molecular weight is lowered, and the polyimide precursor film tends to be brittle. Moreover, imidation progresses partially at the time of polyimide precursor film manufacture, and it becomes difficult to obtain a desired physical property at the time of an imidation process. On the other hand, when the temperature is lower than 70 ° C., the drying time becomes long, the molecular weight tends to decrease, and the handling property tends to be poor due to insufficient drying. The drying time is preferably 10 to 90 minutes, more preferably 15 to 80 minutes, although it depends on the drying temperature. When the drying time is longer than 90 minutes, the molecular weight is lowered and the film tends to be brittle. When the drying time is shorter than 10 minutes, the drying property is insufficient and the handling property tends to be poor. Further, in order to improve the drying efficiency or suppress the generation of bubbles at the time of drying, the temperature may be raised stepwise in the range of 70 to 130 ° C. for drying.

As drying conditions in the double-sided drying step, the drying temperature and drying time are preferably in the range of 100 ° C. to 150 ° C. and 1 minute to 15 minutes. When the drying temperature is higher than 150 ° C., ordering of the structure occurs, resulting in insufficient drying of the solvent on the support side, and it is difficult to obtain a double-sided drying effect. On the other hand, when the temperature is lower than 100 ° C., a lot of time is required for removing the solvent, and the productivity is inferior. If the drying time is longer than 15 minutes, the productivity is poor and unsuitable. When the drying time is shorter than 1 minute, drying on the support side is insufficient, and the effect of drying on both sides tends to be difficult to obtain.
A conventionally known drying apparatus can be applied, and examples thereof include hot air, hot nitrogen, far infrared rays, and high frequency induction heating.

In the present invention, the imidization rate on the film (precursor film) surface means an imidization rate evaluated from the film surface to a depth of about 3 μm.
Evaluation of the imidization ratio of the film front and back was performed according to the following ATR measurement procedure.
The film to be measured is sampled to a size of 2 cm × 2 cm, the measurement object surface is brought into close contact with the ATR crystal, set in an IR measurement apparatus, and the following specific wavelength absorbance is measured. Was obtained.
IMx = λ ₁₇₇₈ / λ ₁₄₇₈ (1)
λ ₁₇₇₈ is the absorbance of the measurement surface at the imide specific wavelength ₁₇₇₈ cm ⁻¹ (near), and λ ₁₄₇₈ is the absorbance of the measurement surface near the aromatic ring specific wavelength ₁₄₇₈ cm ⁻¹ .
The ATR measurement conditions used this time are shown below.
The imidation rate IM of the A surface is IMa and the imidation rate IM of the B surface is IMb, and the difference between them is shown with an absolute value.
The measured value is 2 points in the width direction at any point on the film (1/3 and 2/3 of the width), and the measured value is the average value of the two points.
[Measurement condition]
Device name: FT-IR (measuring device: manufactured by Digilab, FTS-60A / 896, etc.) Attachment; golden gate MkII (manufactured by SPECAC)
IRE; Germanium angle of incidence; 45 °
Resolution: 4cm ^-1
Accumulation count: 128 times

The film-like solar cell using the film of the present invention as a substrate as a preferred embodiment of the present invention is formed by forming a laminate including a photoelectric conversion layer made of a semiconductor on the above-described film base. The said laminated body has a photoelectric converting layer which converts the energy of sunlight into an electrical energy as an essential structure, and usually further has an electrode layer etc. for taking out the obtained electrical energy.
Hereinafter, a laminated structure in which a photoelectric conversion layer is sandwiched between a pair of electrode layers will be described as a typical example of the laminated body formed so as to constitute a film-like solar cell. However, the laminated structure formed in the present invention is not limited to the embodiment described below, and the structure of the laminated body of the solar cell of the prior art may be referred to as appropriate, and a protective layer and known auxiliary means are added. Is also good.
One electrode layer (hereinafter also referred to as a back electrode layer) of the pair of electrode layers is preferably formed on one main surface of the film substrate. The back electrode layer is obtained by laminating a conductive inorganic material by a method known per se, for example, a CVD (Chemical Vapor Deposition) method or a sputtering method. Examples of conductive inorganic materials include metal thin films such as Al, Au, Ag, Cu, Ni, stainless steel, In ₂ O ₃ , SnO ₂ , ZnO, Cd ₂ SnO ₄ , ITO (adding Sn to In ₂ O ₃ Oxide semiconductor-based conductive materials and the like. The thickness of the back electrode layer is not particularly limited, and is usually about 30 to 1000 nm. Preferably, the back electrode layer is a metal foil film.

The photoelectric conversion layer for converting the energy of sunlight into electric energy is a layer made of a semiconductor, and a compound semiconductor thin film (chalcopyrite structure semiconductor thin film) made of a group I element, a group III element, and a group VI element, CuInSe2 ( A CIS) film, or a Cu (In, Ga) Se ₂ (CIGS) film (hereinafter, collectively referred to as a CIS film) in which Ga is dissolved, and a silicon semiconductor layer. Examples of the silicon-based semiconductor include a thin film silicon layer, an amorphous silicon layer, and a polycrystalline silicon layer. The photoelectric conversion layer may be a laminate having a plurality of layers made of different semiconductors.
The thin film silicon layer is a silicon layer obtained by a plasma CVD method, a thermal CVD method, a sputtering method, a cluster ion beam method, a vapor deposition method, or the like.
The amorphous silicon layer is a layer made of silicon having substantially no crystallinity. The lack of crystallinity can be confirmed by not giving a diffraction peak even when irradiated with X-rays. Means for obtaining an amorphous silicon layer are known, and examples of such means include a plasma CVD method and a thermal CVD method.
The polycrystalline silicon layer is a layer made of an aggregate of microcrystals made of silicon. The above amorphous silicon layer is distinguished by giving a diffraction peak by irradiation with X-rays. Means for obtaining a polycrystalline silicon layer are known, and such means include means for heat-treating amorphous silicon.
The photoelectric conversion layer used in the present invention is not limited to a silicon-based semiconductor layer, and may be, for example, a thick film semiconductor layer. The thick film semiconductor layer is a semiconductor layer formed from a paste of titanium oxide, zinc oxide, copper iodide or the like.
The means for constituting the semiconductor material as the photoelectric conversion layer may refer to a known method as appropriate. For example, an about 20 nm a-Si (n layer) is formed by performing high-frequency plasma discharge in a gas in which phosphine (PH ₃ ) is added to SiH ₄ at a temperature of 200 to 500 ° C., and then SiH _4. An a-Si (i layer) of about 500 nm can be formed only with gas, and then diborane (B ₂ H ₆ ) can be added to SiH ₄ to form a p-Si (p layer) of about 10 nm. .

Of the pair of electrode layers sandwiching the photoelectric conversion layer, an electrode layer (hereinafter also referred to as a current collecting electrode layer) provided on the side opposite to the film substrate is formed by solidifying a conductive paste containing a conductive filler and a binder resin. It may be an electrode layer or a transparent electrode layer. As the transparent electrode layer, an oxide semiconductor material such as In ₂ O ₃ , SnO ₂ , ZnO, Cd ₂ SnO ₄ , ITO (In ₂ O ₃ added with Sn) can be preferably used.
Thus, a film-like solar cell in which transparent electrode / p-type a-Si / i-type a-Si / n-type a-Si / metal electrode / polyimide film are laminated in this order, which is a preferred embodiment of the present invention. can get. Alternatively, the p layer may be a-Si, the n layer may be polycrystalline silicon, and a thin and doped a-Si layer may be inserted between them. In particular, when an a-Si / polycrystalline silicon hybrid type is used, the sensitivity to the sunlight spectrum is improved.
In manufacturing a solar cell, an antireflection layer, a surface protective layer, or the like may be added in addition to the above structure.

EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated more concretely, this invention is not limited by a following example. In addition, the evaluation method of the physical property in the following examples is as follows except for the above.
1. Reduced viscosity of polyamic acid (ηsp / C)
A solution dissolved in N-methyl-2-pyrrolidone so that the polymer concentration was 0.2 g / dl was measured at 30 ° C. using an Ubbelohde type viscosity tube.
2. Film thickness of polyimide film The thickness of the film was measured using a micrometer (manufactured by Fine Reef, Millitron (registered trademark) 1254D).

3. Light transmittance Measured with a spectrophotometer (manufactured by Shimadzu Corporation, “UV-3150”).
4. Tensile modulus, tensile breaking strength and tensile breaking elongation of polyimide film Tested by cutting the dried film into strips of length 100mm and width 10mm in the longitudinal direction (MD direction) and width direction (TD direction), respectively. Using a tensile tester (manufactured by Shimadzu Autograph (trade name) model name AG-5000A) at a tensile speed of 50 mm / min and a distance between chucks of 40 mm, tensile modulus, tensile breaking strength and tensile breaking elongation I asked for a degree.
5. Linear expansion coefficient (CTE) of polyimide film
The expansion / contraction rate was measured under the following conditions, and the range from 30 to 300 ° C. was divided at 15 ° C. intervals and obtained from the average value of the expansion / contraction rate / temperature of each divided range.
Device name: TMA4000S manufactured by MAC Science
Sample length; 20mm
Sample width: 2 mm
Temperature rise start temperature: 25 ° C
Temperature rising end temperature: 400 ° C
Temperature increase rate: 5 ° C / min
Atmosphere: Argon

6). The imidation ratio of the front and back surfaces of the polyimide precursor film was measured as described in [Best Mode for Carrying Out the Invention].
The difference in the imidization rate between the front and back surfaces of the polyimide precursor film in the present invention is the difference (| IMa−IMb |) between the imidization rate IMa on one surface and the imidation rate IMb on the other surface.

7). Coefficient of linear expansion (CTE) of film substrate
About the film base material to be measured, the stretch rate in the MD direction and the TD direction is measured under the following conditions, and the stretch rate / temperature at intervals of 30 ° C. to 45 ° C., 45 ° C. to 60 ° C.,. Then, this measurement was performed up to 300 ° C., and the average value of all measured values was calculated as CTE. The meanings of the MD direction and the TD direction are as described above.
Device name: TMA4000S manufactured by MAC Science
Sample length; 20mm
Sample width: 2 mm
Temperature rise start temperature: 25 ° C
Temperature rising end temperature: 400 ° C
Temperature increase rate: 5 ° C / min
Atmosphere: Argon

8). Photoelectric conversion characteristics Measured with an Oriel solar simulator whose conversion efficiency was adjusted to AM = 1.

(Examples 1-2, Comparative Examples 1-2)
(Polyamide acid polymerization-1)
In a reaction vessel, 1140 parts by mass of trans 1,4-diaminocyclohexane was dissolved in 34,000 parts by mass of N, N-dimethylacetamide, and then stirred with 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride. 2646 parts by mass of the product powder and 218 parts by mass of pyromellitic dianhydride were gradually added. When the formed salt solution was heated in an oil bath at 150 ° C. with vigorous stirring for 5 minutes, a part of the salt began to dissolve, so the reaction vessel was removed from the oil bath and stirred at room temperature for several hours. A clear and viscous polyamic acid solution (A) was obtained.

(Polyamide acid polymerization-2)
After putting 2100 parts by mass of 4,4′-methylenebis (cyclohexylamine) into a reaction vessel and dissolving in 28600 parts by mass of N-methyl-2-pyrrolidone, pyromellitic dianhydride powder 2180 is stirred with stirring in a nitrogen stream. Mass parts were gradually added and reacted at 35 ° C. for 8 hours to obtain a transparent and viscous polyamic acid solution (B). The logarithmic viscosity of the obtained polyamic acid was 1.3 dl / g.

(Film preparation conditions: Production and imidization of green film of polyamic acid)
Condition 1; As a drying step, drying was performed at three atmospheric hot air drying zones at 90 ° C. × 7 minutes, 90 ° C. × 7 minutes, and 90 ° C. × 7 minutes.
The precursor film which became self-supporting after drying was peeled from the support to obtain a polyimide precursor film having a thickness of 40 μm.
This peeled precursor film was double-sided dried at an atmospheric temperature of 150 ° C. for 10 minutes in a hot air drying zone.
The obtained polyimide precursor film was passed through a continuous drying furnace and heat-treated at 200 ° C. for 3 minutes, then heated to 350 ° C. for about 20 seconds, heat-treated at 350 ° C. for 7 minutes, and 5 Cooled to room temperature over a period of time to obtain a polyimide film.
Condition 2; As a drying step, drying was performed at three atmospheric hot air drying zones at 100 ° C. × 10 minutes, 120 ° C. × 10 minutes, and 130 ° C. × 10 minutes.
The polyimide precursor film which became self-supporting after drying was peeled from the support to obtain a polyimide precursor film having a thickness of 40 μm.
The obtained polyimide precursor film was passed through a continuous drying furnace and heat-treated at 200 ° C. for 3 minutes, then heated to 350 ° C. for about 20 seconds, heat-treated at 350 ° C. for 7 minutes, and 5 Cooled to room temperature over a period of time to obtain a polyimide film.

  Each of the obtained polyamic acid solutions was coated on a surface not containing a lubricant of a polyester film (Cosmo Shine A4100 (manufactured by Toyobo Co., Ltd.)) having a thickness of 188 μm and a width of 800 mm as a support so as to have a width of 740 mm. Each polyimide film was obtained by adapting each film preparation condition. Table 1 shows the performance of each film obtained.

(Manufacture of film-like solar cells)
Using a stainless steel target with a sputtering apparatus, a stainless steel layer having a thickness of 1000 nm was formed on each of the films. Next, a film in which a stainless steel layer is formed is placed between the counter electrode and the support electrode in the vacuum reactor, and the inside of the reactor is once evacuated to 1 × 10 ⁻⁵ Torr, and the temperature of the support electrode is increased to 300 ° C. It was. Then, while applying a 15MHz RF voltage of 30W to the counter electrode and the supporting electrode, SiH ₄ and argon gas was introduced into the reactor was pre-sputtered in an argon atmosphere of 3 mTorr, then diluted to 10% with hydrogen gas Similarly, PH ₃ gas diluted to 1% with hydrogen gas was simultaneously introduced to form a 25 nm n-type amorphous silicon layer on the stainless steel layer in an atmosphere of 1 Torr. Next, only SiH ₄ is introduced, an i-type amorphous silicon layer having a thickness of 500 nm is laminated on the n-type amorphous silicon layer, and further, a mixture containing 1% B ₂ H ₆ in SiH ₄ gas. A p-type amorphous silicon layer having a thickness of 25 nm was formed on the i-type amorphous silicon layer by introducing a gas.
Next, the film on which the pin type amorphous silicon layer was formed was mounted in a vacuum deposition apparatus, and an indium tin oxide layer having a thickness of 100 nm was deposited by an electron beam method to form a hetero electrode layer. Finally, a 100 nm palladium layer was vacuum-deposited in a comb shape on it. Each film-like solar cell was obtained as described above. Table 1 shows the photoelectric conversion characteristics (conversion efficiency) of the obtained film-like solar cells.

  The polyamic acid obtained by reacting the diamine and aromatic tetracarboxylic acid of the present invention is cast and dried on a support, and the imidization ratio of one side (A side) of the polyimide precursor film is defined as IMa. When the imidization ratio of the other surface (B surface) is IMb, a polyimide precursor film having a difference between IMa and IMb of 5 or less is obtained, and the polyimide precursor film is imidized by heat. A film-like solar cell obtained by laminating a photoelectric conversion thin film on a colorless transparent polyimide film having a light transmittance of 50% or more at a wavelength of 500 nm is a polyimide film to be obtained, using a specific substrate film The deformation of the substrate film due to heat in the process such as the formation of the photoelectric conversion thin film on the film and the process of the electrode layer formation, particularly the film surface It has retention warpage and resistant to torsion, excellent production efficiency of the film-like solar cell, and it is assumed faultless and variations in the performance of the film-like solar cell, industrially very useful.

Claims (3)

  Polyamic acid obtained by reacting diamine and aromatic tetracarboxylic acid is cast and dried on a support, and the imidization ratio of one side (A side) of the polyimide precursor film is set to IMa, When the imidization ratio of the surface (B surface) is IMb, a polyimide precursor film having a difference between IMa and IMb of 5 or less is obtained, and the polyimide precursor film is imidized by heat to obtain a polyimide film A film-like solar cell obtained by laminating a photoelectric conversion thin film on a colorless and transparent polyimide long film having a light transmittance of 50% or more at a wavelength of 500 nm.   The film-like solar cell according to claim 1, wherein the diamine is trans 1,4-diaminocyclohexane.   The film-like solar cell according to claim 1, wherein the diamine is 4,4'-methylenebis (cyclohexylamine).