JP5219332B2 - Coated stainless steel foil and thin film solar cell - Google Patents

Description

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

  Solar power generation is attracting attention as one of the new energy sources. As solar cells, bulk solar cells of single crystal Si or polycrystal Si 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 cost reduction by 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 (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. There is an example using a metal foil coated with an organic resin paint such as a highly insulating polyimide paint (Patent Document 2), but the organic resin is a heat treatment at 200 to 350 ° C. during the formation of solar cells. There are many things that deteriorate.

Among thin film solar cells, crystalline Si thin film solar cells that are expected to have high conversion efficiency have recently attracted attention. Conventionally, this material has been considered to be unsuitable for thin film materials because of its small absorption coefficient in the visible region. However, in order to obtain a long optical path length even in crystalline Si, the backside of the crystalline Si layer and 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 reflective layer having an uneven texture structure, and an n-type Si layer, an i layer serving as an active layer, a p-type Si layer, and an ITO film are sequentially stacked on the glass substrate. Is making. The back reflective layer has a structure that also serves as an electrode (Non-Patent Document 1). In addition, in order to increase the conversion efficiency in amorphous Si, as a result of simulating the case where the surface of the amorphous Si layer has an uneven texture structure on each of the back surface and earning the optical path length, the effectiveness of the uneven texture structure is confirmed. (Non-Patent Document 2).

At present, the uneven texture structure is mainly obtained by producing SnO ₂ or ZnO as a transparent electrode on a glass substrate by CVD or sputtering and then wet etching with acid. However, when considering the use of metal foil substrates 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, so that an uneven texture structure is used. I couldn't make it. Although it is conceivable to form a metal such as Al in a shape having a concavo-convex texture structure as the back electrode, in reality, an electrode film having concavo-convex of submicron level that exhibits a light confinement effect is formed of Al or the like. The technology for film formation has not been established.

JP2003-247078 JP 2001-127323 A K. Yamamoto, et al., IEEE Trans. Elect. Devices, vol.46, p.2041 (1999) Supervised by Photovoltaic Technology Research Association, Basics and Applications of Thin Film Solar Cells, p82-91, Ohmsha (2001)

The present invention relates to a stainless steel foil and thin film coated with a silica-based inorganic polymer film, capable of producing a back reflection layer having an uneven texture structure as an insulating property, thermal stability, and a thin film solar cell of 1 MΩ · cm ² order or more. A solar cell is provided.

The above object uses the following means in order to achieve the above object.
(1) A stainless steel foil having an insulating film on the surface, the outermost surface of the insulating film, Si-R bond (R is an organic group or hydrogen) composed mainly of siloxane bonds containing, and, 3 [mu] m inclusive diameter 300nm It has a surface uneven structure with convex portions approximated as a part of the sphere, and the convex portions are formed in situ by a sol-gel method,
The convex part area of the convex part approximated as a part of a sphere having a diameter of 300 nm or more and 3 μm or less is 90% or more of the total convex part area, and at least a part of the convex part is in a 5 μm square region arbitrarily set. A coated stainless steel foil characterized by being an existing inorganic polymer film.
(2) The ratio of the convex part area of the convex part approximated as a part of a sphere in the arbitrarily set 5 μm square region is 5% or more and 90% or less. (1) Coated stainless steel foil.
(3) R in the Si-R bond is one or both of a methyl group and a phenyl group, and a molar ratio of R to Si is 0.5 or more and 2.2 or less (1) Or the coated stainless steel foil as described in (2).
(4) The coated stainless steel foil according to any one of (1) to (3), wherein the insulating film has a thickness of 4 μm to 300 μm.
(5) A thin film solar cell comprising the coated stainless steel foil according to any one of (1) to (4) as a substrate.

  According to the present invention, a stainless steel foil coated with an inorganic polymer film mainly composed of a siloxane bond having a concavo-convex structure on its surface is obtained, and it is lightweight and flexible for various electric and electronic parts including solar cells. An insulating substrate can be provided. Especially when the stainless steel foil of the present invention is used as a thin film solar cell, especially a silicon 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. Therefore, with the stainless steel foil of the present invention, the current density of the thin film solar cell is increased, and the conversion efficiency as the solar cell can be improved.

  In the present invention, an insulating film is formed on the surface of the stainless steel foil. Although various materials are assumed as the insulating film, heat-resistant resin materials such as polyimide, ceramic materials such as silica and alumina, siloxane-based inorganic polymers, etc. are used in terms of heat resistance that can withstand the solar cell formation process. desirable. The siloxane-based inorganic polymer is a polymer in which a siloxane skeleton developed in a three-dimensional network is modified with hydrogen or an organic group. The modification of the siloxane skeleton with an organic group is due to a Si—R bond (R is an organic group or hydrogen). Specific examples of R include methyl group, ethyl group, propyl group, phenyl group, glycidoxypropyl group, amino group, and trifluoropropyl group. By including R, the film becomes softer than ceramics, so that the crack resistance is improved. The inorganic main component is silica glass, but may contain Ti, Zr, Al, Ta, Nb, W, etc. in addition to Si.

  The insulating film does not need to be a single layer. In the case of a multilayer film, the outermost surface of the insulating film is a siloxane-based inorganic polymer layer that generates less degassing due to thermal decomposition. As an insulating film, for example, a two-layer structure in which a siloxane-based inorganic polymer is superimposed on a polyimide layer, a two-layer structure in which two siloxane-based inorganic polymers having different compositions are stacked, and a siloxane-based inorganic polymer having a different composition on a polyimide A three-layer structure in which two kinds are stacked, or a three-layer structure in which a siloxane-based inorganic polymer is stacked on a film formed of polyimide and alumina may be used.

  In the present invention, the upper part from the plane where the flat surface without the convex part and the sphere intersect is defined as the convex part. The convex part is approximated as a part of a sphere from the curvature of the convex part. As shown in FIG. 1, the outermost surface of the insulating film has a surface concavo-convex structure 11, and each convex portion has a shape approximated by spheres 12 and 13. The diameter of the sphere approximating the convex part is 20 nm or more and 10 μm or less. When this diameter is smaller than 20 nm, no contribution to the surface relief structure is observed. When the diameter is larger than 10 μm, it becomes difficult to form a solar cell. A preferable range of the diameter of the sphere that approximates the convex portion is 100 nm to 5 μm, and a more preferable range is 300 nm to 3 μm. The convex portions are uniformly distributed so that at least a part of the convex portions is always present in an arbitrarily set 5 μm square region.

  Regarding the size of the convex portion approximated as a part of a sphere, the convex area of the convex portion approximated as a part of a sphere having a diameter of 300 nm to 3 μm is 90% or more of the total convex area. It is desirable that it is 100% or less. The convex area is the area of a circle formed by intersecting the flat surface when the convex is approximated by a sphere. The total convex area is the sum of the convex areas for all the convex portions. When the convex part area of the convex part approximated as a part of a sphere having a diameter of 300 nm or more and 3 μm or less is less than 90% of the total convex part area, light confinement reflecting the surface concave-convex structure tends to be insufficient. .

  In an arbitrarily set 5 μm square region, it is desirable that the ratio of the convex portion area of the convex portion approximated as a part of a sphere is 5% or more and 90% or less. When the ratio of the convex area is less than 5%, the light confinement in the 5 μm square region is at the same level as the flat surface. When this ratio exceeds 90%, it means that there are huge projections or projections of various sizes are aggregated, and it is difficult to obtain the light confinement effect or good Cell formation may be difficult. A preferable range of the ratio of the convex area of the convex portion approximated as a part of a sphere in an arbitrarily set 5 μ square region is 10% or more and 70% or less, more preferably 20% or more and 60% or less.

  R in the Si—R bond is preferably one or both of a methyl group and a phenyl group. Both the methyl group and the phenyl group are easily bonded to Si and are organic groups having a high thermal decomposition temperature, and therefore there is no risk of thermal decomposition during the solar cell formation process. The molar ratio of R to Si is preferably 0.5 or more and 2.2 or less. At this time, R means one or both of a methyl group and a phenyl group. When the molar ratio of R to Si is less than 0.5, the effect of improving crack resistance due to organic components hardly appears. When this ratio exceeds 2.2, the film is too soft and wrinkles easily occur. A preferable range of R with respect to Si is 1.9 or more and 2.1 or less.

  The thickness of the insulating film is desirably 4 μm or more and 300 μm or less. The thickness of the insulating film is the thickness from the SUS foil surface to the flat surface. Here, the flat surface is a surface represented by 14 in FIG. When the thickness of the insulating film is less than 4 μm, it is difficult to obtain a sufficient insulating effect. If it exceeds 300 μm, there will be no difference in characteristics, but the cost of the insulating film material will increase.

Although the insulating film of the present invention may be a single layer, it is necessary that the outermost surface layer has a surface uneven structure with convex portions approximated as a part of a sphere. Moreover, it is desirable that the distribution of the convex portions is uniform. A film having such a surface concavo-convex structure can be formed using a sol-gel method, applying a coating liquid, and performing a process of drying and heat treatment . When a coating solution to which spherical fine particles of an appropriate size are added is applied in order to impart a concavo-convex structure , a great deal of effort is required for designing the component of the coating solution to prevent aggregation of the added particles.

  In the present invention, a siloxane polymer and one or more metal alkoxides selected from Si, Al, Ti, Zr, Nb, Ta, and W are used as starting materials, and a sol prepared by hydrolysis under an acid catalyst is used for coating. It was found that spherical protrusions can be formed in-situ in the drying step, and a film in which spherical protrusions are dispersed without aggregation is found. Examples of the siloxane polymer include polydimethylsiloxane, polymethylphenylsiloxane, polydiphenylsiloxane, and polydimethyldiphenylsiloxane. Both ends of the polysiloxane may be modified with carbinol or amino, in addition to a silanol group. The mass average molecular weight of the polysiloxane is preferably 500 or more and 15000 or less, and particularly preferably 2500 or more and 10,000 or less. When the mass average molecular weight is smaller than 500, it becomes a low-viscosity coating solution, and it is difficult to obtain a sufficient film thickness. When the mass average molecular weight exceeds 15000, it takes time to cure the film, and the resulting film is also soft and easily damaged.

  Dispersed in an organic solvent so that the molar ratio of one or more metal alkoxides selected from Si, Al, Ti, Zr, Nb, Ta, and W to Si in the siloxane polymer is from 1/40 to 1/4. It is particularly desirable to hydrolyze after it has been allowed to. If the molar ratio of metal alkoxide to Si in the siloxane polymer 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 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, 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, methyl ethyl ketone, methyl isobutyl ketone and the like can be used. In order to form the convex portions in-situ, the faster drying tends to have a uniform diameter when the convex portions are approximated to a sphere, so that it is easier to produce using a solvent having a low boiling point.

  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.

  It is essential to add an acid as a catalyst for hydrolysis. When no acid is added, the convex shape is not generated. As the acid, both inorganic acids and organic acids can be used. Hydrochloric acid is particularly desirable, and in the case of hydrochloric acid, the molar ratio is 0.01 or more and 0.5 or less, particularly 0.1 or more and 0.35 or less with respect to one or more metal alkoxides selected from Si, Al, Ti, Zr, Nb, Ta, and W. Then, it becomes easy to form a convex portion approximated by a sphere having a diameter of 20 nm to 10 μm.

  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. In any method, by forming a thin sol, it is easy to form a convex portion approximated by a sphere having a diameter of 20 nm to 10 μm. Forming a thin film of 1 μm or less tends to have a uniform diameter and a small diameter.

  In the case where sufficient insulation cannot be obtained only by a film having a surface uneven structure in which convex portions are formed in situ, a multilayer film structure can be used, and a polyimide film, a siloxane inorganic polymer film, or the like can be used as a base film. The siloxane-based inorganic polymer used as the base film can be prepared by almost the same process as the type of siloxane polymer-based film that forms the protrusions in situ. Decomposition can be performed.

  The stainless steel foil exhibits good adhesion without any pretreatment, but it can be pretreated before application, if necessary. Typical pretreatment includes 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.

When the coated stainless steel foil of the present invention is used for a substrate, for example, an Al electrode, an n-type silicon layer, an i layer serving as an active layer, a p-type silicon layer, and an ITO film are sequentially laminated, and finally a comb-shaped electrode is formed. Thus, a solar battery cell using thin film microcrystalline silicon can be manufactured. According to the present invention, a solar cell having characteristics equivalent to those of a conventional SnO ₂ textured glass substrate can be formed on the coated stainless steel foil. Since the coated stainless steel foil is lighter and harder to break than a glass substrate and can be easily increased in area, it is suitable as a solar cell substrate for building materials such as a roof. Moreover, application as a flexible solar cell is also expected.

(Example 1)
2 moles of ethyl acetoacetate and 1 mole of tetraisopropoxytitanium were dispersed in 2 moles of 2-ethoxyethanol, and 0.2 moles of polydimethyldiphenylsiloxane having an average molecular weight of 6000 by silanol modification at both ends was added and stirred. Thereto, a mixed solution of 2 mol of 2-ethoxyethanol and 2 mol of water was dropped, and hydrolysis was performed while stirring. Thereafter, 5 mol of methyl isobutyl ketone was added to prepare an underlayer sol. It is coated on a YUS190 (SUS444 standard) substrate with a thickness of 120μm and finished with Super Bright, using a dip coater at a lifting speed of 20mm / s, dried at 150 ° C for 2 minutes, and then at 150 ° C for 2 hours. Subsequently, heat treatment was performed at 300 ° C. for 6 hours to produce a base film. The thickness of the obtained base film was about 6 μm. Both the sol for forming the base film and the base film were transparent.

  Next, 5.6 g of ethyl acetoacetate and 17.1 g of tetraisopropoxytitanium were dispersed in 10.8 g of 2-ethoxyethanol, and 27 g of polydimethyldiphenylsiloxane having an average molecular weight of 6000 modified with silanol at both ends was added and stirred. Hydrolysis was performed while dropwise adding a mixed solution of 10.8 g of 2-ethoxyethanol, 0.54 g of water, and 2.8 g of 8N hydrochloric acid. Further, 12.0 g of methyl isobutyl ketone was added to prepare Sol A. The prepared sol was yellow and transparent.

  The obtained sol was applied onto the base film with a No. 3 bar coater, dried at 150 ° C. for 2 minutes, and then baked at 150 ° C. for 2 hours and 300 ° C. for 2 hours. The film thickness obtained was about 0.8 μm. Although sol A was transparent, milky white turbidity was observed visually on the film surface.

  2 and 3 show an SEM photograph of the produced film surface and an SEM photograph of the surface as viewed obliquely from above. The obtained convex part has a substantially hemispherical shape with a diameter of 0.1 to 3.0 μm. The convex portions are uniformly distributed in the plane, and at least a part of the convex portion that is approximated as a part of a sphere having a diameter of 20 nm or more and 10 μm or less always exists in an arbitrarily set region of 5 μm square.

The insulation resistance of the film was obtained from a minute current flowing with a voltage of 1000 V using 10 platinum electrodes with an electrode area of 1 cm ² as the upper electrode and stainless steel foil as the lower electrode. As a result of measuring 10 formed upper electrodes, the sheet resistance of each was 1 MΩ · cm ² or more.

(Comparative Example 1)
The same base film as in Example 1 was produced.

  Next, 0.07 mol of tetraethoxytitanium, 0.21 mol of acetic acid and 0.7 mol of glycidoxypropyltriethoxysilane were mixed for 24 hours, then 0.3 mol of tetraethoxysilane was added, and 1 mol of water and 6 mol of ethanol were mixed. Was added slowly to effect hydrolysis. A sol C was prepared by adding 7.7 g of aminopropyltriethoxysilane, 270.0 g of water, and 184.3 g of ethanol to 58.0 g of the obtained hydrolysis solution. 1.5 g of spherical silica having a diameter of 1 μm was added to 100 g of sol C.

  This sol was applied onto the base film using a dip coater at a lifting speed of 1 mm / s, dried at 100 ° C. for 5 minutes, and baked at 250 ° C. for 5 minutes. The thickness of each of the formed uneven films was 0.5 μm.

  A SEM photograph of the surface is shown in FIG. The obtained convex portion has a shape in which a part of spherical particles having a diameter of 1 μm appears on a flat surface. Aggregation is seen in the convex part.For example, when a 5 μm square is set as shown by a white square in FIG. 5, a part of a sphere having a diameter of 20 nm or more and 10 μm or less does not exist in the region, and a flat surface is obtained. ing.

(Comparative Example 2)
The SUS foil produced up to the base film of Example 1 was used as it was as a substrate for solar cell formation.

About the stainless steel foil having the insulating film produced in the above-mentioned Examples and Comparative Examples, a silicon thin film photovoltaic cell was formed, and the efficiency was compared with the silicon thin film photovoltaic cell produced on the glass substrate with texture by SnO ₂ . . When the efficiency is 1, the performance is the same as that of the textured glass substrate, and when it exceeds 1, the efficiency is higher than that. The results are shown in Table 1.

The convexity distribution represents the number of 5 μm squares that have some convex parts that are approximated as part of a sphere having a diameter of 20 nm or more and 10 μm or less, out of 10 arbitrarily set 5 μm squares. In the case of (5), it is shown that there is a part of a convex portion approximated as a part of a sphere having a diameter of 20 nm or more and 10 μm or less in all 5 μm squares. In the example of the present invention, since the convex portion distribution was good, a solar cell with higher efficiency than the glass substrate with SnO ₂ could be produced. On the other hand, in the comparative examples, sufficient unevenness could not be obtained, and the efficiency was low.

The uneven surface structure of the present invention. 2 is an SEM photograph of the film surface produced in Example 1 as seen from directly above. The SEM photograph which looked at the film | membrane surface produced in Example 1 from diagonally upward. The SEM photograph which looked at the film surface produced in comparative example 1 from right above.

Explanation of symbols

11 Uneven structure on the outermost surface
12 Sphere approximating convex part
13 Sphere approximating convex part
14 Flat surface

Claims (5)

It is a stainless steel foil having an insulating film on the surface, and the outermost surface of the insulating film is a sphere having a Si—R bond (R is an organic group or hydrogen) as a main component and a diameter of 300 nm to 3 μm. It has a surface concavo-convex structure with a convex part approximated as a part, and the convex part is formed in situ by a sol-gel method,
The convex part area of the convex part approximated as a part of a sphere having a diameter of 300 nm or more and 3 μm or less is 90% or more of the total convex part area, and at least a part of the convex part is in a 5 μm square region arbitrarily set. A coated stainless steel foil characterized by being an existing inorganic polymer film.   2. The coated stainless steel according to claim 1, wherein in the arbitrarily set 5 μm square region, the ratio of the convex portion area of the convex portion approximated as a part of a sphere is 5% or more and 90% or less. Foil.   The R of the Si-R bond is one or both of a methyl group and a phenyl group, and the molar ratio of R to Si is 0.5 or more and 2.2 or less. The coated stainless steel foil described.   The thickness of the said insulating film is 4 micrometers or more and 300 micrometers or less, The covering stainless steel foil of any one of Claims 1-3 characterized by the above-mentioned.   A thin film solar cell comprising the coated stainless steel foil according to any one of claims 1 to 4 as a substrate.