JP5251904B2 - Stainless foil coated with silica-based inorganic polymer film and silicon thin film solar cell using the same - Google Patents

Description

  The present invention relates to a stainless steel foil coated with a silica-based inorganic polymer film and a silicon thin film solar cell using the same.

  Solar power generation is attracting attention as one of new energy sources. As solar cells, single-crystal Si or polycrystalline Si bulk solar cells are the mainstream, but there are many points where thin-film solar cells are considered to be more advantageous, such as weight reduction, large area, and low price due to mass production. .

  Glass is used as the substrate of the thin film solar cell, but a resin substrate is also being studied from the viewpoint of weight reduction and flexibility. However, since the resin has low heat resistance, the manufacturing process temperature of the thin film solar cell is limited.

For this reason, a stainless steel foil substrate with an insulating film on the surface has been proposed (see Patent Document 1). As a solar cell substrate, insulation of at least MΩ · cm ² order is required so that a certain performance can be obtained by arranging a plurality of cells. Therefore, this level of insulation is also required for the film on the stainless steel foil. It is done.

  Although there is an example using what applied organic resin paints, such as a highly insulating polyimide paint, on metal foil (refer to patent documents 2), organic resin is 200-350 ° C at the time of photovoltaic cell formation. Many of them deteriorate with heat treatment.

  Among thin film solar cells, crystalline Si thin film solar cells that are expected to have high conversion efficiency have recently attracted attention. Conventionally, since this material has a small absorption coefficient in the visible region, it has been thought that it is not suitable for a thin film material. However, even in crystalline Si, in order to obtain a long optical path length, It has been found that high conversion efficiency is achieved by reflecting light on the surface and confining it inside.

For example, SnO ₂ is provided on a glass substrate as a back surface reflection layer having an uneven texture structure, and an n-type Si layer, an Si layer serving as an active layer, a p-type Si layer, and an ITO film are sequentially laminated thereon, A cell is produced. The back reflective layer has a structure that also serves as an electrode (see Non-Patent Document 1).

  In addition, in order to increase the conversion efficiency of amorphous Si, as a result of simulating the case where the surface of the amorphous Si layer has a concavo-convex texture structure on each of the back and earning the optical path length, the effectiveness of the concavo-convex texture structure is demonstrated. Is shown (see Non-Patent Document 2).

At present, the mainstream of the uneven texture structure is obtained by wet etching with acid after producing SnO ₂ or ZnO as a transparent electrode on a glass substrate by CVD or sputtering.

However, when considering the use of a metal foil substrate from the viewpoint of weight reduction, flexibility, heat resistance, etc., the wet etching process with SnO ₂ or ZnO acid causes a problem of corrosion of the metal itself. I couldn't make it.

  Although it is conceivable to form a metal such as Al in a shape having an uneven texture structure as the back electrode, in reality, the technique of depositing Al or the like at a submicron level that exhibits a light confinement effect is Not established.

Japanese Patent Application No. 2002-43362 JP 2001-127323 A

K. Yamamoto, et al. , IEEE Trans. Elect. Devices, Vol. 46, p. 2041 (1999) Supervised by the Photovoltaic Technology Research Association, Basics and Applications of Thin Film Solar Cells, p. 82-91, Ohmsha (2001)

The present invention provides a stainless steel foil coated with a silica-based inorganic polymer film capable of producing a back reflective layer having an uneven texture structure as a thin film solar cell, with an insulation property of 1 MΩ · cm ² order or more and thermal stability. To do.

  In order to achieve the above object, the present invention uses the following means.

(1) A stainless steel foil coated with a silica-based inorganic polymer film mainly composed of siloxane bonds, which is an insulating substrate for a solar cell, wherein at least a part of Si constituting the silica-based inorganic polymer film is organic are chemically bonded to one or both of the group or hydrogen, it has an uneven structure due to phase separation of the silica-based inorganic polymer formation process on the surface of the silica-based inorganic polymer film, the uneven structure in the surface roughness Ra 0 A stainless steel foil coated with a silica-based inorganic polymer film, characterized by being 1 μm or more and 0.8 μm or less.

  (2) The stainless steel foil coat | covered with the silica type inorganic polymer film as described in (1) whose surface roughness Rmax of the said silica type inorganic polymer film is 3 micrometers or less.

  (3) The silica-based inorganic film according to (1) or (2), wherein the thickness of the silica-based inorganic polymer film is 0.6 μm or more and 40 μm or less, and the thickness of the stainless steel foil is 40 μm or more and 200 μm or less. Stainless steel foil coated with polymer film.

  (4) About Si which comprises the said silica type inorganic polymer film, the molar ratio of Si which is D nucleus with respect to all Si is 0.3 or more, and it is chosen from Al, Ti, Zr, Nb, Ta, and W Any one of (1) to (3) characterized in that one or both of one or more kinds of metal elements or Si of Q nuclei includes 0.025 or more and 0.25 or less as a molar ratio with respect to total Si. A stainless steel foil coated with the silica-based inorganic polymer film described in 1.

(5) The silica-based inorganic polymer film is coated with the silica-based inorganic polymer film according to any one of (1) to (4), wherein the organic group chemically bonded to Si of the silica-based inorganic polymer film is a methyl group. Stainless steel foil.
(6) The stainless steel foil coated with the silica-based inorganic polymer film according to any one of claims 1 to 5, wherein the uneven structure is a lamellar uneven structure.

  (7) A silicon thin film solar cell, wherein the substrate is a stainless steel foil coated with the silica-based inorganic polymer film according to any one of (1) to (6).

  According to the present invention, a stainless steel foil coated with a silica-based inorganic polymer film having a concavo-convex structure on its surface is obtained, and a lightweight and flexible insulating substrate is provided for various electric and electronic parts including solar cells. can do.

  Especially when the stainless steel foil of the present invention is used as a thin film solar cell substrate, a back surface reflection layer having a concavo-convex structure can be obtained, so that the optical path length can be increased and the conversion efficiency of the solar cell can be improved.

It is a figure which shows the surface SEM photograph (10000 time) of the stainless steel foil coat | covered with the silica type inorganic polymer film | membrane which has the uneven structure of this invention.

An inorganic polymer is a polymer in which the main skeleton is composed of inorganic bonds of M (metal or metalloid) -O (oxygen) -M. In particular, when M is Si, it is called a siloxane bond. When all of the four bond hands of Si are connected by a Si—O—Si bond, it becomes SiO ₂ glass. SiO ₂ glass is hard and easily cracked.

In general, the surface roughness of the stainless steel foil surface is Ra = 0.03 to 0.6 μm and Rmax = 0.35 to 5.0 μm. Therefore, insulation cannot be maintained without a certain film thickness. In any film formation method such as sputtering, CVD, or sol-gel, it is difficult to form a SiO ₂ film having no cracks and a thickness of nearly 1 μm.

In order to reduce the hardness of the SiO ₂ glass, when an inorganic polymer film mainly composed of siloxane bonds into which Si—R (R is an organic group or hydrogen) bond is used, flexibility is imparted to the film. Even if it is a film, cracks are less likely to occur, and an insulation resistance value of about 1 MΩ · cm ² can be obtained.

Si, like C, can be directly chemically bonded to an organic group or H like Si—CH ₃ , Si—C ₆ H ₅ , and Si—H. Of the four bonds of Si, two of them form Si—R (R is an organic group or H) bond, and the remaining two are Si—O—Si bonds, that is, R ₂ Si (— O—Si) ₂ is a D nucleus, one Si—R (R is an organic group or H) bond, and the remaining three are Si—O—Si bonds, that is, RSi (− O—Si) _{3 is referred} to as a T nucleus, and the case of Si (—O—Si) ₄ in which all of the four bonds are connected by Si—O—Si bonds is referred to as a Q nucleus.

  These nuclides can be examined by NMR. When the types of R are the same, it is most hydrophobic when the siloxane skeleton consists of D nuclei. When R consists of unbonded Q nuclei, it is the most hydrophilic, and the T nuclei are located between these.

  The inventors have found that in the sol-gel method, when a sol in which these nuclides are mixed is applied and heat-treated under specific conditions, phase separation occurs, and as a result, a submicron uneven structure appears on the surface. It was.

  When a precursor of a hydrophilic siloxane skeleton and a precursor of a hydrophobic siloxane skeleton coexist, if a common good solvent such as alcohol is present in a certain amount or more, both of them are dissolved and a transparent sol is obtained. It is thought that after application, the solvent becomes immiscible as the solvent evaporates, resulting in phase separation in the emulsion state, which dries and the surface becomes uneven.

  For example, in the case of a sol synthesized from polydimethylsiloxane and an alkoxide of Ti, a substance having a network structure in which polydimethylsiloxane is three-dimensionally cross-linked by Ti is obtained. When heat treatment at 120 ° C. is performed for 12 hours, FIG. As shown in (1), a state in which a hydrophilic convex portion rich in Ti as a crosslinking component and a hydrophobic concave portion rich in polydimethylsiloxane are separated.

  Depending on the amount of Ti and the molecular weight of polydimethylsiloxane, a lamellar uneven structure may be obtained or a structure in which convex portions are distributed in an island shape may be obtained.

  By using a common good solvent, a transparent sol can be prepared, and it can be estimated that there are no particles or droplets of several tens of nm or more at the level where Rayleigh scattering does not occur at the sol stage. Therefore, when heat treatment is performed to accelerate the evaporation of the solvent, the same level of homogeneity as the sol is maintained, so that a smooth surface is obtained.

  The concavo-convex structure on the surface may be subjected to an etching process in order to emphasize the concavo-convex structure. In the case of a film containing a metal element other than Si as an inorganic component, the siloxane skeleton is selectively etched by etching with an alkali.

  In the case of a film obtained from polydimethylsiloxane and Ti alkoxide, by etching with an aqueous sodium hydroxide solution, the siloxane polymer in the recesses is etched, and the protrusions with much Ti can be emphasized more.

  The surface roughness of the silica-based inorganic polymer film is preferably such that the center line average roughness Ra is 0.1 μm or more and 0.8 μm or less, and the maximum height Rmax is 3 μm or less. When Ra is smaller than 0.1 μm, it is difficult to obtain a sufficient uneven texture structure for the back reflective layer provided on the inorganic polymer film, so it is difficult to increase the optical path length and increase the conversion efficiency of the solar cell.

  When Ra exceeds 0.8 μm and Rmax exceeds 3 μm, delamination tends to occur when a thin-film solar cell is stacked on the inorganic polymer film. More preferably, Ra is 0.3 μm or more and 0.6 μm or less, and Rmax is 1.5 μm or less.

  The film thickness of the silica-based inorganic polymer film is desirably 0.6 μm or more and 40 μm or less, and the thickness of the stainless steel foil is desirably 40 μm or more and 200 μm or less. When a film thinner than 0.6 μm is formed on the stainless steel foil, it is difficult to obtain sufficient insulation.

  When a film thicker than 40 μm is formed, the inorganic polymer film is easily cracked even if it contains an organic component. A more preferable range of the film thickness is 1.5 μm or more and 5 μm or less.

  When the thickness of the stainless steel foil is less than 40 μm, there is no strength and handling as a substrate is difficult. When the thickness of the stainless steel foil exceeds 200 μm, the advantage over the glass substrate that it is lightweight and flexible is reduced.

  The molar ratio of Si that is D nuclei to all Si is 0.3 or more, and one or both of one or more metal elements selected from Al, Ti, Zr, Nb, and W or Si of Q nuclei, It is desirable that the sum of these be included in the range of 0.025 or more and 0.25 or less as a molar ratio to the total Si.

  When the molar ratio of Si as D nuclei to all Si is smaller than 0.3, it is difficult to increase the film thickness because of the hard film composition. Since Si as the D nucleus is highly hydrophobic, not only is the film thickness easy when the molar ratio of Si as the D nucleus to the total Si is 0.8 or more, but layer separation is also promoted. preferable.

  By including one or both of one or more metal elements selected from Al, Ti, Zr, Nb, Ta, and W or Si of Q nucleus, it becomes easy to form a hydrophilic precursor, and phase separation It becomes easy to cause.

  When the molar ratio of one or both of one or both of metal elements selected from Al, Ti, Zr, Nb, Ta, and W or Q nucleus to all Si is smaller than 0.025, the effect of causing phase separation is obtained. small. When this molar ratio exceeds 0.25, the film becomes too hard and cracks are likely to occur.

  The organic group directly bonded to Si is particularly preferably a methyl group from the viewpoint of high heat resistance and low degassing.

  The stainless steel foil coated with the silica-based inorganic polymer film of the present invention is hydrolyzed using polysiloxane and one or more metal alkoxides selected from Si, Al, Ti, Zr, Nb, Ta, and W as starting materials. The prepared sol can be applied to a stainless steel foil, dried and gelled.

  Examples of the polysiloxane include polydimethylsiloxane, polymethylphenylsiloxane, and polydiphenylsiloxane. Both ends of the polysiloxane may be modified with carbinol or amino, in addition to a silanol group. In particular, when polydimethylsiloxane is used, it is excellent in heat resistance and crack resistance.

  The mass average molecular weight of the polysiloxane is desirably 500 or more and 15000 or less. When the mass average molecular weight is less than 500, it is difficult to obtain a surface uneven structure, and when it exceeds 15000, it takes time to cure the film, and the resulting film is also soft and easily damaged.

  In an organic solvent, the molar ratio of one or more metal alkoxides selected from Si, Al, Ti, Zr, Nb, Ta, and W to Si in polydimethylsiloxane is 1/40 or more and 1/4 or less. It is particularly desirable to disperse and then hydrolyze.

  When the molar ratio of the metal alkoxide to Si in the polydimethylsiloxane is less than 1/40, the three-dimensional network structure is difficult to develop and the film is likely to be insufficiently cured. If this molar ratio exceeds 1/4, the film becomes too hard and cracks are likely to occur.

  As the Si alkoxide, organoalkoxysilane can be used in addition to tetraethoxysilane and tetramethoxysilane. Examples of the organoalkoxysilane include methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, propyltrimethoxysilane, propyltriethoxysilane, isobutyltrimethoxysilane, isobutyltriethoxysilane, dimethoxydimethylsilane, Diethoxydimethylsilane, phenyltrimethoxysilane, phenyltriethoxysilane, methacryloxypropyltrimethoxysilane, methacryloxypropyltriethoxysilane, glycidoxypropyltrimethoxysilane, glycidoxypropyltriethoxysilane, aminopropyltrimethoxysilane And aminopropyltriethoxysilane.

  Since all metal alkoxides of Al, Ti, Zr, Nb, Ta, and W are more reactive than alkoxysilane, some alkoxy groups are β-diketone, β-ketoester, alkanolamine, alkylalkanolamine. Alternatively, an alkoxide derivative substituted with an organic acid or the like may be used.

  A sol can be prepared by performing hydrolysis in an organic solvent capable of uniformly dispersing and dissolving the starting material for producing the above-described film solids such as alkoxide and polysiloxane. As the organic solvent, for example, various alcohols such as methanol, ethanol, propanol and butanol, acetone, toluene, xylene and the like can be used.

  The mass of the organic solvent that dissolves the raw materials having different hydrophilicity and hydrophobicity is preferably 20% or more of the solid content mass including the amount of alcohol generated as a by-product during hydrolysis. When the amount of the solvent is small, it becomes an emulsion-like sol at the end of hydrolysis, and defects such as repellency and pinholes are likely to occur during film formation.

  Hydrolysis is performed by adding 0.5 to 2 times as much water as the number of moles of all alkoxy groups in the starting material. If necessary, an acid may be added as a hydrolysis catalyst. As the acid, both inorganic acids and organic acids can be used. The prepared sol can be made uneven by utilizing the particle shape by adding inorganic particles such as fumed silica having a particle size of submicron or less.

  It is particularly desirable that the drying and gelation be performed under conditions such that the sol forms an emulsion state by evaporation of the common good solvent on the coated substrate, generally 70 ° C. or more and 200 ° C. or less. When the drying / gelation temperature is lower than 70 ° C., it takes time to cure the film. When the temperature exceeds 200 ° C., the solvent evaporates instantaneously while maintaining the homogeneity at the sol stage, so that phase separation does not occur and a smooth surface tends to be obtained.

  Coating on the stainless steel foil can be performed by a bar coating method, a roll coating method, a spray coating method, a dip coating method, a spin coating method, or the like. In particular, when the stainless steel foil has a coil shape, it is preferable to apply with a roll coater by an offset method or a gravure method because continuous processing is easy.

  The stainless steel foil exhibits good adhesion without any pretreatment, but it can be pretreated before application, if necessary. Typical pretreatments include pickling, alkali degreasing, chemical conversion treatment such as chromate, grinding, polishing, blasting, and the like, and these can be performed alone or in combination as necessary.

  Examples of the stainless steel foil material include ferritic stainless steel foil, martensitic stainless steel foil, and austenitic stainless steel foil. The surface of the stainless steel foil may be subjected to a surface treatment such as bright annealing or buffing.

  The stainless steel foil coated with the silica-based film of the present invention will be specifically described by the following examples.

Example 1
2 mol of ethyl acetoacetate and 1 mol of tetraethoxytitanium were dispersed in 2 mol of ethanol, 0.5 mol of polydimethylsiloxane having an average molecular weight of 3000 was modified by both-end carbinol modification, and the mixture was stirred. A mixed solution of 2 mol of ethanol and 2 mol of water was added dropwise to prepare a sol.

  A SUS430 substrate having a thickness of 100 μm was coated with a number 16 bar coater, dried and gelled at 150 ° C. for 24 hours, and then heat-treated for film curing at 200 ° C. for 4 hours in vacuum. . The surface of the film showed a submicron level lamellar uneven structure. Ra was 0.4 μm, Rmax was 1.5 μm, and the film thickness was 2.5 μm.

When an insulation resistance was measured at an applied voltage of 100 V with a 1 cm ² upper electrode attached, a stainless steel foil of 80 MΩ · cm ² and coated with an insulating film having a submicron level uneven structure was obtained.

On top of this, an Al electrode, an n-type silicon layer, an Si layer as an active layer, a p-type silicon layer, an ITO film are sequentially laminated, and finally, the conversion efficiency of the thin-film microcrystalline silicon cell in which the comb-type electrode is formed is It was higher than the conversion efficiency of a silicon cell produced by a similar process on a stainless steel foil coated with an uneven SiO ₂ insulating film produced by plasma CVD.

(Example 2)
2 moles of ethyl acetoacetate and 1 mole of tetrabutoxyzirconium were dispersed in 2 moles of 2-ethoxyethanol, and 2 moles of polymethylphenylsiloxane having an average molecular weight of 1500 was added to both ends by carbinol modification and stirred. A mixed solution of 2 mol of 2-ethoxyethanol and 2 mol of water was dropped to prepare a sol.

  A SUS304 substrate having a thickness of 70 μm was used, and coating was performed with a dip coater having a substrate pulling rate of 0.6 mm / sec. After drying and gelling at 120 ° C. for 24 hours, heat treatment for film curing at 200 ° C. for 3 hours was performed in the air. The film surface showed a submicron level uneven structure. Ra was 0.2 μm, Rmax was 1 μm, and the film thickness was 1.7 μm.

It was measured insulation resistance at an applied voltage of 100V with a top electrode of 1 cm ^2, a 10MΩ · cm ^2, stainless steel foil coated with an insulating film having a submicron level in the rugged structure is obtained.

  The conversion efficiency of the solar battery cell produced using this stainless steel foil in the same manner as in Example 1 was higher than that produced on the stainless steel foil coated with the insulating film having no uneven structure shown in Example 1.

(Example 3)
1 mol of tetramethoxysilane was dispersed in 3 mol of 2-ethoxyethanol, and 1 mol of polydiphenylsiloxane having an average molecular weight of 6000 was added to both ends silanol and stirred. A mixed solution of 3 mol of 2-ethoxyethanol and 2 mol of water was dropped to prepare a sol.

  Using a 70 μm thick SUS304 substrate, it was applied with a No. 7 bar coater, dried and gelled at 100 ° C. for 24 hours, and then subjected to heat treatment for film hardening at 250 ° C. for 1 hour in a vacuum. The film surface showed a submicron level uneven structure. Ra was 0.3 μm, Rmax was 1 μm, and the film thickness was 1.5 μm.

It was measured insulation resistance at an applied voltage of 100V with a top electrode of 1 cm ^2, a 10MΩ · cm ^2, stainless steel foil coated with an insulating film having a submicron level in the rugged structure is obtained.

  The conversion efficiency of the solar battery cell produced using this stainless steel foil in the same manner as in Example 1 was higher than that produced on the stainless steel foil coated with the insulating film having no uneven structure shown in Example 1.

(Comparative Example 1)
2 mol of ethyl acetoacetate and 1 mol of tetraethoxytitanium were dispersed in 4 mol of ethanol, and 10 mol of tetramethoxysilane was added and stirred. A liquid for mixing 4 mol of ethanol and 2 mol of water was added dropwise to obtain a transparent solution. A SUS304 substrate having a thickness of 70 μm was used, and coating was performed with a dip coater having a substrate pulling rate of 0.6 mm / sec. After drying and gelling at 120 ° C. for 24 hours, heat treatment for film curing at 200 ° C. for 3 hours was performed in the air. The film thickness was 0.6 μm.

When the insulation resistance was measured at an applied voltage of 100 V with the 1 cm ² upper electrode attached, the insulation resistance was 1 MΩ · cm ² . As a result of SEM observation, the film surface was smooth and Ra was 0.02 μm or less.

  The conversion efficiency of the solar battery cell produced in the same manner as in Example 1 using this stainless steel foil was equivalent to that produced on the stainless steel foil coated with the insulating film having no uneven structure shown in Example 1. .

Claims (7)

A stainless steel foil coated with a silica-based inorganic polymer film mainly composed of siloxane bonds as an insulating substrate for a solar cell, wherein at least part of Si constituting the silica-based inorganic polymer film is an organic group or hydrogen one or both and are chemically bonded, has an uneven structure due to phase separation of the silica-based inorganic polymer formation process on the surface of the silica-based inorganic polymer film, the relief structure 0.1μm or more surface roughness Ra of A stainless steel foil coated with a silica-based inorganic polymer film, characterized by being 0.8 μm or less.   The stainless steel foil coated with the silica-based inorganic polymer film according to claim 1, wherein the silica-based inorganic polymer film has a surface roughness Rmax of 3 μm or less.   3. The silica-based inorganic polymer film according to claim 1, wherein the silica-based inorganic polymer film has a thickness of 0.6 μm or more and 40 μm or less, and the stainless steel foil has a thickness of 40 μm or more and 200 μm or less. Stainless steel foil. About Si which comprises the said silica type inorganic polymer film, the molar ratio of Si which is D nucleus with respect to all Si is 0.3 or more, and 1 or more types chosen from Al, Ti, Zr, Nb, Ta, and W of one or both of Si metal elements or Q nucleus, according to any one of claims 1 to 3, the sum of these is characterized in that it comprises 0.025 to 0.25 as the molar ratio of total Si Stainless steel foil coated with silica-based inorganic polymer film. Stainless steel foil coated with a silica-based inorganic polymer film according to any one of claims 1 to 4, Si chemically bonded to have an organic group of the silica-based inorganic polymer film characterized in that it is a methyl group . The relief structure, lamellar stainless steel foil coated with a silica-based inorganic polymer film according to claim 1, characterized in that an uneven structure. Substrate, a silicon thin film solar cell which is a stainless steel foil coated with a silica-based inorganic polymer film according to any one of claims 1-6.