JP2016023343A - Production method of ferritic stainless foil for solar cell substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress occurrence of wrinkles, etc. due to buckling of a substrate even after a light absorption layer formation process, in producing a solar cell by using a roll-to-roll method.SOLUTION: Ferritic stainless foil for solar cell substrates can keep sufficient hardness to suppress buckling, etc. in plate passing during production of a solar cell by a roll-to-roll method and is excellent in plate passability. The ferritic stainless foil comprises, by mass%, 14-24% of Cr, 0.1-0.6% of Nb and 2.0% or less of Mo and has a Vickers hardness of Hv 250-450, and its Vickers hardness after a light absorption layer formation process of holding the substrate in the temperature range of 450-650°C for 1 minute or longer is Hv 250-450.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing a ferritic stainless steel foil for a solar cell substrate. The present invention can maintain a sufficient hardness to suppress buckling and the like during sheet feeding, particularly when manufacturing a solar cell by a roll-to-roll method. The present invention relates to a method for producing a ferritic stainless steel foil for a solar cell substrate having excellent threading performance.

  In recent years, a power generation system using sunlight as a new energy source has attracted attention. Crystalline Si solar cells composed of single-crystal Si and polycrystal Si have been put into practical use, and such solar cells play an important role as a solar photovoltaic system for power supply. ing. However, manufacturing a crystalline Si solar cell requires a process for manufacturing a bulk crystal. For this reason, a large amount of raw material is used, crystal growth takes a long time, the manufacturing process is complicated, and a great amount of energy is required, so that the manufacturing cost is extremely high.

  Against this backdrop, thin-film Si solar cells with significantly reduced Si usage, compound thin-film solar cells that do not use any Si (Compound Film Solar Cell), organic thin-film solar cells (Organic Film Solar Cell) In addition, new solar cells such as dye-sensitized solar cells and quantum dot solar cells have been actively researched and put into practical use. Each of these solar cells is a thin film solar cell, and an amorphous Si or a compound semiconductor is formed on a substrate to form a thin-film absorber layer. It is manufactured by forming. Therefore, the manufacturing process is simpler than that of the crystalline Si solar cell, and the manufacturing time can be shortened. Moreover, since the thickness of the light absorption layer is several tens of nm to several μm, it is possible to significantly reduce the raw materials used as compared with the crystalline Si solar cell.

For the above reasons, thin film solar cells are highly expected as next-generation solar cells because of low production costs and high mass productivity. In particular, CIGS solar cells, which are compound thin-film solar cells that use Cu (In _1-X Ga _X ) Se ₂ (hereinafter sometimes abbreviated as CIGS (Copper Indium Gallium DiSelenide)) as the light absorption layer, Among solar cells, it has a high degree of attention because of its high photoelectric conversion efficiency and low manufacturing costs.

  For the substrate of the thin film solar cell, a glass plate such as soda-lime glass, a stainless steel foil, or a plastics film such as polyimide is mainly used. Among these, since the glass plate is not flexible, the roll-to-roll method of continuously processing in a coil state cannot be applied, which is disadvantageous for mass production and cost reduction. Since the synthetic resin film is inferior in heat-resisting property, it has a drawback that the upper limit of the treatment temperature must be limited in the manufacturing process of the solar battery cell.

  On the other hand, the stainless steel foil is excellent in flexibility and heat resistance. Therefore, a roll-to-roll method that is advantageous for mass production and cost reduction can be applied. Since the stainless steel foil has heat resistance superior to that of the synthetic resin film, it is possible to improve the solar cell production efficiency and manufacture a light-weight and flexible thin-film solar battery.

  Since stainless steel foil has excellent flexibility, thin-film solar cells using this as a substrate can be applied to curved surfaces, and further development of solar cells can be expected as so-called flexible solar cells. . In particular, ferritic stainless steel among stainless steels has a coefficient of linear thermal expansion that is similar to CIGS, so it is actively applied as a substrate material for thin-film solar cells. Has been considered.

  A thin film solar cell is formed by sequentially forming a back electrode layer (back contact layer) made of, for example, a Mo layer, a light absorption layer, a buffer layer (transparent layer), and a transparent conductive layer (transparent contact layer) on a substrate. Manufactured. In some cases, an insulating layer is provided between the substrate and the back electrode layer.

  When a stainless steel foil is used as the substrate, a roll-to-roll method that is advantageous for mass production can be applied when forming a light absorption layer or the like on the substrate.

  In the roll-to-roll method, a roll for rewinding a coiled substrate (stainless foil) and a roll for winding the substrate are provided. Further, a thin film manufacturing apparatus for the back electrode layer, a thin film manufacturing apparatus for the light absorption layer, and the like are disposed between the two rolls. A back electrode layer, a light absorption layer, a buffer layer, and a transparent conductive layer are sequentially formed on the substrate conveyed from the rewind roll, and then wound with a take-up roll. Therefore, according to the roll-to-roll method, a plurality of solar cells can be manufactured continuously and in large quantities, and mass production and cost reduction of solar cells can be achieved.

  Here, the stainless steel foil as the solar cell substrate is very thin with a thickness of 20 to 300 μm. For this reason, if the strength (hardness) is insufficient, buckling occurs in the stainless steel foil when passing through the roll-to-roll method, causing wrinkles, breaks, drawing, etc. It becomes easy to do. As described above, in a continuous process such as the roll-to-roll method, if wrinkles or the like are generated on the substrate during sheet feeding, the solar cell cannot be manufactured or the solar cell with reduced photoelectric conversion efficiency is manufactured. Therefore, the stainless steel foil used as the material for the solar cell substrate has sufficient strength (hardness) to suppress the buckling as described above, and has a plateability in a continuous process such as a roll-to-roll method. It is also important to be good.

  Regarding the sheet-passability of a stainless steel foil (substrate) when manufacturing solar cells by a continuous process using a roll-to-roll method, Patent Document 1 discloses cold rolling with a reduction ratio of 50% or more on a stainless steel material. If necessary, heat treatment is performed at 400 to 700 ° C. in an inert gas atmosphere to form a stainless steel foil, so that the tensile strength in the direction perpendicular to the rolling direction of the stainless steel foil is obtained. A technology of over 930 MPa has been proposed. In the technique proposed in Patent Document 1, it is said that a stainless steel foil for a solar cell substrate that is unlikely to buckle can be obtained even when applied to a continuous process using a roll-to-roll method.

Japanese Unexamined Patent Publication No. 2012-138571 (International Publication WO2012 / 0777827)

  According to the technique proposed in Patent Document 1, in the continuous process of the roll-to-roll method, buckling of the stainless steel foil (substrate) can be suppressed to some extent to improve the sheet passing property.

  However, in the technique proposed in Patent Document 1, consideration is not given to the threadability of the stainless steel foil (substrate) after undergoing the absorber layer growth process.

  The temperature of the substrate when the light absorption layer is formed on the substrate on which the back electrode layer is formed depends on the type of material constituting the light absorption layer. For example, when forming a light absorption layer (CIGS layer) of a CIGS-based compound thin film solar cell, a high temperature process of 450 to 650 ° C. is generally used. Therefore, even when a stainless steel foil having a predetermined strength (hardness) is used as the substrate, the substrate (stainless steel foil) is softened by the light absorption layer deposition process, and buckled by the subsequent manufacturing process. This causes a problem that wrinkles, folds, or apertures are likely to occur.

  For the above reasons, the stainless steel foil used as the material for the solar cell substrate has sufficient heat resistance that can suppress softening due to the process of forming the light absorption layer, and in the continuous process of the roll-to-roll method, the light absorption layer It is important that the plate-passability is excellent even after the film forming process. In the technique proposed in Patent Document 1, such a problem is not studied at all.

  Therefore, in the technique proposed in Patent Document 1, in the continuous process of the roll-to-roll method, the stainless foil (substrate) before the light absorption layer film forming process is free from wrinkles and the like, and the plate passing property is good. Even so, the stainless steel foil (substrate) is softened by being heated to a high temperature of 450 to 650 ° C. during the light absorption layer forming process. As a result, generation of wrinkles and the like due to buckling of the stainless steel foil (substrate) is unavoidable in the continuous process after the light absorption layer forming process, and the productivity and photoelectric conversion efficiency of the solar cell are reduced.

  The present invention advantageously solves the above-described problems, such as wrinkles due to buckling of the substrate even in a continuous process after the light absorption layer film forming process at the time of solar cell manufacturing using the roll-to-roll method. It is an object of the present invention to provide a ferritic stainless steel foil for a solar cell substrate that can suppress the occurrence of the above and has good sheet-passability.

  In order to solve the above-mentioned problems, the present inventors have made a continuous process after a high-temperature process such as film formation (film formation process) of a light absorption layer at the time of manufacturing a solar cell using a roll-to-roll method. At present, the inventors have intensively studied a means for suppressing the buckling and undulation of the stainless steel substrate and maintaining the plate-passability. As a result, the presence or absence of buckling or undulation of the stainless steel substrate during the production of solar cells using the roll-to-roll method, that is, whether the plate passing property is good or bad is the Vickers hardness of the stainless steel foil used as the substrate It became clear that it depends heavily on. And it was confirmed that if the stainless steel foil has a Vickers hardness of Hv 250 or more and 450 or less, it shows excellent plate-passability.

  Furthermore, as a result of investigations, the present inventors have conducted a high-temperature process such as a light absorption layer deposition process in the case of manufacturing a solar battery cell using a roll-to-roll method. Is a stainless steel foil having a Vickers hardness of Hv250 or more and 450 or less, after a light absorption layer forming process in which a holding time at an arbitrary temperature selected from a temperature range of 450 ° C or more and 650 ° C or less is 1 minute or more. It has been found that if the hardness is maintained, the substrate (stainless foil) does not easily buckle or swell even after the light absorption layer deposition process, and excellent plate-through properties can be maintained.

  Further, the present inventors have provided means for imparting the above-mentioned hardness characteristics to the stainless steel foil, that is, holding time in a temperature range of Vvs hardness of Hv250 to 450 and 450 ° C to 650 ° C. The inventors sought a means for imparting a hardness characteristic (heat resistance) that can ensure a Vickers hardness of Hv 250 or higher and 450 or lower even after a light absorption layer forming process of 1 minute or longer. As a result, the present inventors have found that it is an effective means to subject a ferritic stainless steel sheet containing predetermined amounts of Cr and Nb to annealing and cold rolling and then to heat treatment under predetermined conditions. And the present inventors specify the component of stainless steel, the rolling reduction of cold rolling, determine the heat treatment temperature of the heat treatment according to the temperature of the substrate during the light absorption layer film formation process during solar cell production, Furthermore, by specifying the heat treatment conditions (temperature increase rate to the heat treatment temperature, holding time at the heat treatment temperature, cooling rate after holding at the heat treatment temperature), the desired hardness characteristics described above can be imparted to the stainless steel foil. I found out.

The present invention is based on the above findings, and the gist of the present invention is as follows.
[1] Ferritic stainless steel sheet containing Cr: 14% or more and 24% or less and Nb: 0.1% or more and 0.6% or less by mass%, and then cold rolling at a reduction rate of 60% or more after annealing. Then, the temperature is raised to a heat treatment temperature T (° C.) at a rate of temperature rise of 10 ° C./s or more and 100 ° C./s or less in an inert gas atmosphere, and the heat treatment temperature T (° C.) is maintained for 1 s or more and 60 s or less. Then, heat treatment is performed to cool at a cooling rate of 5 ° C / s or more and 50 ° C / s or less,
The ferrite system for solar cell substrate, wherein the heat treatment temperature T (° C.) is a temperature satisfying the following formulas (1) and (2) with respect to an arbitrary temperature X selected from a temperature range of 450 ° C. to 650 ° C. Stainless steel foil manufacturing method.

Record
When 450 ℃ ≦ X <600 ℃ 300 ℃ ≦ T ≦ 800 ℃… (1)
When 600 ℃ ≦ X ≦ 650 ℃ X−300 ℃ ≦ T ≦ 800 ℃… (2)
[2] The method for producing a ferritic stainless steel foil for a solar cell substrate according to [1], wherein the ferritic stainless steel sheet further contains Mo: 2.0% or less by mass%.

  According to the present invention, it is possible to suppress the generation of wrinkles due to buckling of the substrate even after the light absorption layer film forming process at the time of manufacturing the solar cell using the roll-to-roll method, and an excellent sheet passing It is possible to obtain a ferritic stainless steel foil for a solar cell substrate having the properties.

  The ferritic stainless steel foil for solar cell substrate of the present invention contains, by mass%, Cr: 14% to 24% and Nb: 0.1% to 0.6%, Vickers hardness of Hv250 to 450, Is characterized by having a Vickers hardness of Hv 250 or more and 450 or less after the light absorption layer forming process in which is kept in a temperature range of 450 ° C. or more and 650 ° C. or less for 1 minute or more.

Cr: 14% or more and 24% or less The ferritic stainless steel foil for solar cell substrate of the present invention is a ferritic stainless steel foil containing 14% or more and 24% or less of Cr by mass%.

  Cr is an essential element for imparting corrosion resistance to the stainless steel foil. If the Cr content is less than 14% by mass, the corrosion resistance that can withstand long-term use of solar cells cannot be ensured. Therefore, when such a stainless steel foil is applied to a substrate, corrosion of the substrate becomes a problem during long-term use of solar cells.

  Basically, the higher the Cr content, the better the corrosion resistance. However, when Nb is contained together with Cr, if the Cr content exceeds 24%, the hot-rolled sheet as a material for the stainless steel foil becomes brittle. The stainless steel foil is usually manufactured by subjecting a slab to hot rolling to form a hot rolled sheet, and subjecting the hot rolled sheet to pickling and annealing as necessary, followed by cold rolling. If the hot-rolled sheet used as the material for the stainless steel foil becomes brittle, it is difficult to perform cold rolling. Therefore, the Cr content of the ferritic stainless steel foil is 14% to 24% by mass. Preferably, it is 14% or more and 20% or less by mass%.

Nb: 0.1% or more and 0.6% or less
Nb has the effect of increasing the recrystallization temperature and preventing softening of the stainless steel foil by the light absorption film forming process. The effect can be obtained by setting the Nb content to 0.1% or more by mass%. However, if the Nb content exceeds 0.6% by mass, the hot-rolled sheet used as the material for the stainless steel foil becomes brittle, making it difficult to perform cold rolling. Therefore, the Nb content of the ferritic stainless steel foil is 0.1% to 0.6% by mass. Preferably they are 0.2% or more and 0.5% or less.

  The ferritic stainless steel foil may further contain Mo: 2.0% or less by mass in addition to Cr and Nb.

Mo: 2.0% or less
Mo is an element effective for improving the corrosion resistance of stainless steel, particularly local corrosion, and for preventing the softening of the stainless steel foil by the light absorption film forming process. In order to obtain this effect, it is preferable to contain 0.1% or more of Mo by mass%. On the other hand, when the Mo content exceeds 2.0% by mass, the surface defects of the stainless steel foil increase and the production yield of solar cells tends to deteriorate. Therefore, the Mo content of the ferritic stainless steel foil is preferably 2.0% or less by mass%. More preferably, it is 0.1% or more and 1.8% or less.

  The ferritic stainless steel foil for solar cell substrate of the present invention may have any component composition as long as the content of Cr and Nb, or Mo further satisfies the predetermined conditions described above.

  The particularly preferable component composition of the ferritic stainless steel foil for solar cell substrates of the present invention is as follows. “%” Representing the following components means mass% unless otherwise specified.

C: 0.12% or less
Since C is combined with Cr in the steel to cause a decrease in the corrosion resistance of the stainless steel foil, the lower the content thereof, the better. However, if the C content is 0.12% or less, the corrosion resistance will not be remarkably lowered. Therefore, the C content is preferably 0.12% or less. More preferably, it is 0.04% or less. In the present invention containing Nb, the C content is more preferably 0.02% or less.

Si: 2.5% or less
Si is an element used for deoxidation, and its effect can be obtained by containing 0.01% or more. However, if it is contained excessively, the ductility of the ferritic stainless steel sheet as a foil material may be lowered, and the productivity may be lowered. Therefore, the Si content is preferably 2.5% or less. More preferably, it is 1.0% or less.

Mn: 1.0% or less
Mn may combine with S in steel to form MnS, which may reduce the corrosion resistance of the stainless steel foil. Therefore, the Mn content is preferably 1.0% or less. More preferably, it is 0.8% or less.

S: 0.030% or less As described above, S combines with Mn to form MnS, thereby reducing the corrosion resistance of the stainless steel foil. Therefore, the S content is preferably 0.030% or less. More preferably, it is 0.008% or less.

P: 0.050% or less
P reduces the ductility of the ferritic stainless steel sheet, which is a foil material, and decreases the productivity. Therefore, the lower the P content, the better. However, if it is 0.050% or less, the ductility is not significantly reduced. Therefore, the P content is preferably 0.050% or less. More preferably, it is 0.040% or less.

Cr: 14% or more and 24% or less As described above, Cr is an essential element for securing the corrosion resistance of the stainless steel foil. In the present invention, the Cr content is set to 14% or more and 24% or less. Preferably they are 14% or more and 20% or less.

Nb: 0.1% or more and 0.6% or less As described above, Nb has an effect of increasing the recrystallization temperature and preventing the softening of the stainless steel foil by the light absorption film forming process. The effect is acquired by making it contain 0.1% or more. However, if the content exceeds 0.6%, the hot-rolled sheet as a material for the stainless steel foil becomes brittle, and it becomes difficult to perform cold rolling. Therefore, the Nb content is 0.1% or more and 0.6% or less. Preferably they are 0.2% or more and 0.5% or less.

N: 0.06% or less
N combines with Cr in the steel to cause a reduction in the corrosion resistance of the stainless steel foil. Therefore, the lower the N content, the better. However, if it is 0.06% or less, the corrosion resistance will not be significantly reduced. Therefore, the N content is preferably 0.06% or less. More preferably, it is 0.02% or less.

  The above is a particularly preferable basic component of the ferritic stainless steel foil for solar cell substrate of the present invention. In the present invention, the following elements can be appropriately contained as required in addition to the above basic components.

Mo: 2.0% or less As described above, Mo is an element effective in improving the corrosion resistance of stainless steel foil, in particular, local corrosion, and preventing softening of the stainless steel foil by the light absorption film forming process. In order to obtain these effects, the Mo content is preferably 0.1% or more. On the other hand, when the Mo content exceeds 2.0%, the surface defects of the stainless steel foil increase, and the production yield of solar cells tends to deteriorate. Therefore, the Mo content is preferably 2.0% or less. More preferably, it is 0.1% or more and 1.8% or less.

Al: 0.20% or less
Al is an element used for deoxidation, and its effect can be obtained by adding 0.001% or more. However, if the content exceeds 0.20%, surface defects are likely to occur in the stainless steel foil, which may reduce the photoelectric conversion efficiency of the solar cell. Therefore, when Al is contained, the content is preferably 0.20% or less. More preferably, it is 0.10% or less.

At least one selected from Ti and Zr: 0.40% or less in total
Ti and Zr are elements useful for fixing the corrosion resistance of stainless steel foil by fixing C and N in the steel as carbides, nitrides, or carbonitrides. In order to sufficiently exhibit such an effect, it is preferable that these elements are contained alone or in combination to contain 0.10% or more in total. However, if the total content exceeds 0.40%, surface defects may occur in the stainless steel foil, which may reduce the photoelectric conversion efficiency of the solar cell. Therefore, it is preferable to limit these elements to 0.40% or less in total in the case of containing alone or in combination.

  In addition, for the purpose of improving the corrosion resistance, one or more of Ni, Cu, V, and W can be contained at 1.0% or less, respectively. Furthermore, for the purpose of improving hot workability, cold workability and surface properties, one or more of Ca, Mg, rare earth elements (also referred to as REM), and B may be contained at 0.1% or less, respectively. it can.

  The balance is Fe and inevitable impurities. Among inevitable impurities, the content of O (oxygen) is preferably 0.02% or less.

Vickers hardness: Hv250 or more and 450 or less If the Vickers hardness of the stainless steel foil is less than Hv250, when manufacturing a solar cell by the roll-to-roll method, the stainless steel foil is buckled, wrinkled, bent, drawn, etc. Is likely to occur. Therefore, the ferritic stainless steel foil for solar cell substrates of the present invention has a Vickers hardness of Hv250 or higher. Preferably it is Hv270 or more. If the hardness is excessively high, waviness is likely to occur and the problem of deterioration of the sheet passing property is caused. Therefore, the Vickers hardness is set to Hv450 or less. Preferably it is Hv410 or less.

Vickers hardness after a predetermined light absorption layer film formation process: Hv 250 or more and 450 or less When a solar cell is manufactured using a roll-to-roll method, the substrate is usually 450 to 650 in the light absorption layer film formation process. Heated to ° C. When the substrate is softened by this heating and the Vickers hardness is reduced to less than Hv250, the substrate is likely to buckle in the subsequent process. And with buckling of a board | substrate, the productivity and photoelectric conversion efficiency of a photovoltaic cell fall. On the other hand, when the hardness is excessively high, undulation is likely to occur, causing a problem that the sheet passing property is deteriorated.

  Therefore, the ferritic stainless steel foil for solar cell substrate of the present invention has a hardness characteristic (heat resistance) such that the Vickers hardness after a predetermined light absorption layer forming process is Hv 250 or more and 450 or less. Preferably it is Hv270 or more and 410 or less.

  As described above, the ferritic stainless steel foil for solar cell substrate of the present invention has a Vickers hardness of Hv250 or more and 450 or less and 450 or less even after the light absorption layer film forming process that is held for 1 minute or more in the temperature range of 450 ° C or more and 650 ° C or less Maintain the hardness of Therefore, in the case of manufacturing solar cells using the roll-to-roll method, if the ferrite-based stainless steel foil for solar cell substrate of the present invention is applied as a substrate, the substrate even after the light absorption layer forming process Therefore, it becomes possible to provide a solar cell with good productivity and photoelectric conversion efficiency.

  Next, the preferable manufacturing method of the ferritic stainless steel foil for solar cell substrates of this invention is demonstrated.

  The ferritic stainless steel foil for solar cell substrate of the present invention can be produced by annealing a steel sheet made of a foil material, then performing cold rolling, and then performing heat treatment in an inert gas atmosphere.

  And the ferritic stainless steel sheet containing Cr: 14% or more and 24% or less and Nb: 0.1% or more and 0.6% or less by mass%, and the rolling reduction of the cold rolling is 60% or more, Heat treatment is heated to a heat treatment temperature T (° C.) at a temperature increase rate of 10 ° C./s or more and 100 ° C./s or less, held at the heat treatment temperature T (° C.) for 1 to 60 seconds, and then 5 ° C. / Heat treatment is performed at a cooling rate of s to 50 ° C./s. Further, the steel plate of the foil material may be a ferritic stainless steel plate containing Mo: 2.0% or less in addition to Cr: 14% or more and 24% or less and Nb: 0.1% or more and 0.6% or less.

  The ferritic stainless steel sheet of foil material, as long as it is a ferritic stainless steel sheet containing Cr: 14% or more and 24% or less and Nb: 0.1% or more and 0.6% or less, or Mo: 2.0% or less in mass% Does not matter. That is, any ferritic stainless steel sheet can be used as the foil material as long as the Cr and Nb content, or the Mo content satisfies the above-described predetermined conditions. The ferritic stainless steel sheet of the foil material is not limited to the component composition as long as the content of Cr and Nb, or further the content of Mo satisfies the above-mentioned predetermined conditions. What is necessary is just to determine according to a component composition.

  The ferritic stainless steel sheet as a foil material is not particularly limited in its manufacturing conditions and can be manufactured according to a conventionally known method. For example, a slab cast by a known casting method such as a continuous casting method, an ingot-bundling rolling method, or a thin slab continuous casting method is hot-rolled to form a hot-rolled sheet. The hot-rolled sheet can be pickled and annealed as necessary, and then cold-rolled to obtain a ferritic stainless steel sheet as a foil material.

  The ferritic stainless steel sheet made of the foil material as described above is annealed and then cold rolled to obtain a stainless steel foil. The annealing conditions are not particularly limited. For example, bright annealing under conditions normally applied to ferritic stainless steel sheets may be performed, or pickling may be performed after annealing.

Cold rolling reduction: 60% or more Work hardening amount when the rolling reduction of the cold rolling following the above annealing (the rolling reduction when cold rolling the ferritic stainless steel sheet of foil material) is less than 60% Is insufficient, and the strength (hardness) of the finally obtained ferrite-based stainless steel foil for solar cell substrate may be insufficient. In such a foil with insufficient strength (hardness), it becomes difficult to suppress the occurrence of buckling during the production of solar cells using the roll-to-roll method, and stable plate-passability cannot be obtained. .

  Therefore, the rolling reduction of cold rolling is preferably 60% or more. More preferably, it is 80% or more. However, if the rolling reduction of the cold rolling becomes excessively high, the working strain (residual strain by machining) becomes very large, and even if heat treatment is applied, the reduction of the working strain becomes insufficient, and the Vickers hardness of the foil exceeds Hv450. Since there is a concern about exceeding, it is preferably 95% or less.

  The thickness of the stainless foil after cold rolling is preferably 20 μm or more and 300 μm or less. More preferably, it is 20 μm or more and 120 μm or less. Even more preferably, it is 30 μm or more and 80 μm or less.

  The stainless steel foil after the cold rolling obtained as described above is subjected to a predetermined heat treatment to obtain a ferritic stainless steel foil for a solar cell substrate. This heat treatment imparts sufficient heat resistance to the stainless steel foil after cold rolling that can suppress softening due to the light absorption layer film forming process, and is excellent in subsequent plateability. It is extremely important in doing so.

  When cold rolling is performed on a ferritic stainless steel sheet of foil material at a reduction rate of 60% or more, a large processing strain accumulates in the stainless steel foil. When the stainless steel foil after cold rolling with such a large processing strain accumulated is used as a solar cell substrate, the processing strain is released as the substrate is heated at a high temperature in the light absorption layer deposition process. The substrate softens.

  Therefore, heat treatment is performed on the stainless steel foil after cold rolling to reduce the amount of processing strain to an appropriate amount to obtain a ferritic stainless steel foil for a solar cell substrate. Thus, by using the stainless steel foil with the processing strain reduced to an appropriate amount in advance as the solar cell substrate, the softening of the substrate due to the release of the processing strain can be effectively suppressed in the light absorption layer film forming process. By leaving an appropriate amount of processing strain, a stainless steel foil having sufficient strength (hardness) to suppress buckling is obtained, and the plate-passability in a continuous process such as a roll-to-roll method is improved.

  The heat treatment is preferably performed by a continuous annealing furnace or the like.

  The heat treatment is preferably performed in an inert gas atmosphere in order to suppress oxidation of the stainless steel foil surface layer. The type of the inert gas is not particularly limited, and nitrogen gas, hydrogen gas, argon gas, ammonia decomposition gas (mixed gas of 75 volume% hydrogen and 25 volume% nitrogen), HN gas (5 volume% hydrogen and 95 volume% nitrogen) And reducing or inert gases such as mixed gas). The dew point of these gases is desirably −30 ° C. or lower.

  Here, the ferritic system for solar cell substrates that gives sufficient heat resistance to the stainless steel foil after cold rolling (that is, a characteristic that the amount of softening by the light absorption layer film forming process is small), and has excellent subsequent plateability. In order to obtain a stainless steel foil, it is necessary to optimize heat treatment conditions. The heat treatment conditions (heat treatment temperature T, temperature increase rate up to heat treatment temperature T, holding time at heat treatment temperature T, cooling rate after holding at heat treatment temperature T) are preferably defined as follows.

Heat treatment temperature T
As described above, this heat treatment is performed for the purpose of removing the processing strain accumulated in the stainless steel foil in advance and suppressing the phenomenon that the processing strain of the substrate is excessively released in the light absorption layer forming process.

  The substrate temperature during the light absorption layer deposition process depends on the type of material constituting the light absorption layer. For example, the substrate when the light absorption layer (CIGS layer) of a CIGS compound thin film solar cell is formed The temperature is generally a temperature selected from a temperature range of 450 to 650 ° C. Therefore, the temperature of the substrate during the light absorption layer forming process is set as X, and the heat treatment temperature T is determined based on this. Specifically, the heat treatment temperature T is set to a heat treatment temperature T (° C.) that satisfies the following expressions (1) and (2).

When 450 ℃ ≦ X <600 ℃ 300 ℃ ≦ T ≦ 800 ℃… (1)
When 600 ℃ ≦ X ≦ 650 ℃ X−300 ℃ ≦ T ≦ 800 ℃… (2)
If the heat treatment temperature T is less than X-300 ° C or less than 300 ° C, the effect of removing the working strain accumulated by cold rolling may not be sufficiently obtained. Therefore, when the stainless steel foil after heat treatment is applied as a solar cell substrate, there is a concern that the substrate is softened in the light absorption layer film forming process. On the other hand, when the heat treatment temperature T exceeds 800 ° C., the processing strain is reduced more than necessary, and the substrate is likely to buckle in the solar cell manufacturing process. And with buckling of a board | substrate, the productivity and photoelectric conversion efficiency of a photovoltaic cell fall.

Holding time at heat treatment temperature T: 1 s or more and 60 s or less, more preferably 1 s or more and 30 s or less If holding time at the above heat treatment temperature T is less than 1 s, the effect of reducing work strain accumulated by cold rolling to an appropriate amount is sufficient May not be obtained. On the other hand, when the holding time at the heat treatment temperature T exceeds 60 s, the effect of removing the processing strain is saturated. For this reason, even if the heat treatment temperature T is maintained for a time exceeding 60 s, the effect of removing further processing strain is small, and the productivity only decreases. For the above reasons, the holding time at the heat treatment temperature T is preferably set to 1 s or more and 60 s or less. Since the heat treatment temperature T varies in the actual heat treatment furnace operation, the holding time may be the time during which the stainless steel foil stays in the temperature range of the heat treatment temperature T ± 20 ° C. More preferably, it is 1 s or more and 30 s or less.

Heating rate up to heat treatment temperature T: 10 ° C / s or more and 100 ° C / s or less The temperature rising rate when heating the cold-rolled stainless steel foil (ie, stainless steel foil at room temperature) to the heat treatment temperature T If it is less than 10 ° C./s, a temper color (thin oxide film) is likely to be generated on the surface of the stainless steel foil, and it may not be used as a solar cell substrate. In addition, when the temperature exceeds 100 ° C / s, the temperature distribution becomes inhomogeneous and deformation such as irregularities on the foil (irregularity, ceter buckle) and end surfaces extending on the waves (edge waves) Therefore, the rate of temperature rise is preferably 10 ° C./s or more and 100 ° C./s or less, and more preferably 20 ° C./s or more and 70 ° C./s or less.

Cooling rate after holding at heat treatment temperature T: 5 ° C / s or more and 50 ° C / s or less Cooling rate when cooling stainless steel foil after holding at heat treatment temperature T to a temperature range of 300 ° C or less is 5 ° C / s If it is less than 1, a temper color is likely to be generated on the surface of the stainless steel foil, and it may not be used as a solar cell substrate. On the other hand, if the cooling rate exceeds 50 ° C./s, the stainless steel foil may be deformed and the shape may be deteriorated, making it difficult to satisfy the dimensional accuracy required for the solar cell substrate. Therefore, the cooling rate is preferably 5 ° C./s or more and 50 ° C./s or less. More preferably, it is 15 ° C./s or more and 35 ° C./s or less.

  By performing the above heat treatment, the processing strain of the stainless steel foil caused by cold rolling is moderately reduced. As a result, the Vickers hardness is Hv250 or more and 450 or less, and the Vickers hardness after the light absorption layer deposition process is maintained in the temperature range of 450 ° C or more and 650 ° C or less for 1 minute or more. A ferritic stainless steel foil for battery substrates is obtained.

  When manufacturing a photovoltaic cell using the ferritic stainless steel foil for solar cell substrates of this invention, manufacturing according to the following method is preferable.

  A thin-film solar battery cell is usually formed by sequentially forming a back electrode layer made of, for example, a Mo layer, a light absorption layer, a buffer layer, and a transparent conductive layer on a substrate, and a grid electrode on the surface of the transparent conductive layer. Manufactured by forming. Further, an insulating layer may be provided between the substrate and the back electrode layer. By providing the insulating layer, an integrated solar cell structure can be obtained. When the (insulating layer), back electrode layer, light absorption layer, buffer layer, and transparent conductive layer are sequentially formed on the substrate, it is preferable to apply a roll-to-roll method that is advantageous for mass production.

  The method for forming the back electrode layer is not particularly limited, and any method such as a PVD method (Physical Vapor Deposition method), a CVD method (chemical Vapor deposition method), or a sputtering method may be employed. An example of the material constituting the back electrode layer is Mo. After the back electrode layer is formed, a light absorption layer is formed on the back electrode layer.

  In forming the light absorption layer, it is extremely important to control the substrate temperature.

  For example, in the case of CIGS solar cells, it is possible to obtain better photoelectric conversion efficiency when the light absorption layer (CIGS layer) is formed at a high temperature. Therefore, the substrate temperature when forming the light absorption layer (CIGS layer) is usually , Selected from a temperature range of 450-650 ° C. On the other hand, in the roll-to-roll process, if the substrate is heated to a high temperature range of 450 to 650 ° C. during film formation of the light absorption layer, the hardness of the substrate decreases, and after the film formation of the light absorption layer There is a concern that the substrate will buckle in this continuous process. When the substrate is buckled in this way, a decrease in productivity and photoelectric conversion efficiency of the solar cell is inevitable.

  However, the ferritic stainless steel foil for solar cell substrates of the present invention is not affected by buckling or undulation of the substrate even after the light absorption layer forming process in which the holding time in the temperature range of 450 ° C. to 650 ° C. The hardness required for suppressing the above-mentioned, that is, the hardness of Vickers hardness Hv 250 or more and 450 or less can be maintained.

  As long as the substrate temperature at the time of film formation is a temperature selected from a temperature range of 450 ° C. or more and 650 ° C. or less, the light absorption layer film formation method is not particularly limited, evaporation method, PVD such as sputtering method, etc. The film can be formed by CVD, electrodeposition method, spin coating method or the like.

  As described above, since the substrate temperature varies in actual operation of the film forming apparatus, X may be a temperature range of the substrate temperature ± 20 ° C. during film formation.

  After the light absorption layer is formed, a buffer layer and a transparent conductive layer are sequentially formed on the light absorption layer. Examples of the material constituting the buffer layer include CdS, InS, and Zn (S, O, OH). Moreover, as a material which comprises a transparent conductive layer, ZnO etc. are mentioned, for example. The film forming means for the buffer layer and the transparent conductive layer is not particularly limited, and can be formed by a CBD method (Chemical bath deposition method), a vapor deposition method, a sputtering method, a CVD method, or the like.

  The substrate of the solar battery has excellent heat resistance that can suppress softening due to the light absorption layer film formation process during the production of the solar battery cell using the roll-to-roll method. It is required to have excellent boardability that can suppress generation of wrinkles and undulations. This is because, in a continuous process such as the roll-to-roll method, if wrinkles or the like are generated on the substrate during feeding, the productivity of the solar cell and the photoelectric conversion efficiency are lowered. Moreover, when waviness arises, the plate-through property deteriorates and the productivity is lowered.

In view of the required characteristics, a sample of a stainless steel foil for a solar cell substrate was prepared, and various tests for evaluating the above characteristics were performed. The sample preparation method and various test / evaluation methods are as follows.
(1) Sample preparation method After bright annealing was applied to a stainless steel plate having the chemical components shown in Table 1, using a 20-stage Zenjimia cold rolling mill (roll diameter: 55 mm) Cold rolling was performed at the rolling reduction shown in Table 2 to obtain a ferritic stainless steel foil having a thickness of 50 μm.

The stainless steel foil having a thickness of 50 μm obtained as described above was degreased and then heat treated in a nitrogen gas at a dew point of −65 ° C. to obtain a sample of a stainless steel foil for a solar cell substrate. Table 2 shows the heat treatment conditions (heat treatment temperature, holding time at the heat treatment temperature, heating rate up to the heat treatment temperature, cooling rate after holding at the heat treatment temperature). Some stainless steel foils were used as solar battery substrate stainless steel foil samples without heat treatment (Sample Nos. 16 and 17 in Table 2).
(2) Solar cell manufacturing method using roll-to-roll method Using various samples prepared in (1) above as a substrate, in a continuous process using the roll-to-roll method, from the Mo layer on the substrate A back electrode (thickness: 1 μm) was formed, and then a light absorption layer (thickness: 2 μm) made of Cu (In _1-X Ga _x ) Se ₂ was formed on the back electrode made of Mo layer. The back electrode made of the Mo layer was formed using a sputtering method. The light absorption layer was formed using a multi-source evaporation method. Table 2 shows the substrate temperature and the film formation time (holding time at the substrate temperature) in the film formation process conditions when forming the light absorption layer.
(3) Hardness test (Vickers hardness evaluation before and after film formation of light absorption layer)
Vickers hardness test (test surface: thickness of sample) in accordance with the provisions of JIS Z 2244 (1998) for the various samples manufactured in (1) above and the various samples after the light absorption layer deposition in (2) above. Direction cross-section).
(4) Evaluation of boardability Evaluation of boardability is made by visually observing the surface of the substrate during continuous process boarding before and after film formation of the light absorption layer, and confirming the presence or absence of wrinkles, creases or squeezing due to buckling. Was done. Passability is evaluated as good (○) when no occurrence of wrinkles, folds or squeezing due to buckling, and poor passability (×) when occurrence of wrinkles, folds or squeezing due to buckling is observed. evaluated. Furthermore, in addition to wrinkles, bends or squeezing, if the surface of the foil has irregularities or ear stretches, or if the flatness of the base material is lost due to the above, and the film forming apparatus makes an abnormal contact, the plate-through property is poor ( X).

  The obtained results are shown in Table 2.

  From Table 2, the following matters are clear.

  The samples (Nos. 1 to 15) of the invention examples have a Vickers hardness of Hv 250 or more and 450 or less before and after the formation of the light absorption layer, and no wrinkles are observed. The plate property is maintained. In contrast, the comparative sample (No. 16, 17) that was not heat-treated had a Vickers hardness of more than Hv450 before film formation of the light absorption layer, and the occurrence of waviness was observed, and the plate-through property Was bad. Samples of comparative examples (Nos. 18 to 21) were not heat-treated or the heat-treatment temperature was outside the range of the present invention, and the Vickers hardness before or after the formation of the light absorption layer was less than Hv250, and generation of wrinkles, etc. Was observed, and the plate passing property was poor.

  In the above examples, the back electrode was formed by sputtering using various samples as a substrate, and the light absorption layer was formed by multi-source deposition. However, in the present invention, even when the back electrode or the light absorption layer is formed by a method other than these, the same effects as in the above-described embodiment (invention example) are exhibited.

  According to the present invention, even when a stainless steel foil that is inexpensive and can be mass-produced is applied to a solar cell substrate, the light absorption layer formation is performed when the solar cell is manufactured using the roll-to-roll method. Even after the film process, the generation of wrinkles and the like due to the buckling of the substrate can be suppressed, and the excellent plate passing property can be maintained. Therefore, it not only contributes to the reduction of the manufacturing cost of the solar battery cell, but is also expected to improve the photoelectric conversion efficiency, and has a remarkable industrial effect.

Claims (2)

Ferritic stainless steel sheets containing Cr: 14% or more and 24% or less and Nb: 0.1% or more and 0.6% or less in mass% are annealed and then cold rolled at a reduction rate of 60% or more, In an inert gas atmosphere, the temperature is raised to a heat treatment temperature T (° C.) at a rate of temperature rise of 10 ° C./s to 100 ° C./s, and held at the heat treatment temperature T (° C.) for 1 s to 60 s. Apply heat treatment to cool at a cooling rate of 5 ℃ / s to 50 ℃ / s,
The ferrite system for solar cell substrate, wherein the heat treatment temperature T (° C.) is a temperature satisfying the following formulas (1) and (2) with respect to an arbitrary temperature X selected from a temperature range of 450 ° C. to 650 ° C. Stainless steel foil manufacturing method.
Record
When 450 ℃ ≦ X <600 ℃ 300 ℃ ≦ T ≦ 800 ℃… (1)
When 600 ℃ ≦ X ≦ 650 ℃ X−300 ℃ ≦ T ≦ 800 ℃… (2)   The method for producing a ferritic stainless steel foil for a solar cell substrate according to claim 1, wherein the ferritic stainless steel sheet further contains Mo: 2.0% or less by mass%.