Solar cell substrate made of stainless steel foil and method for manufacturing the same

Abstract

Provided are a solar cell substrate made of stainless steel foil which contains 7% to 40% by mass Cr and has a coefficient of linear expansion of 12.010.sup.6/ C. or less at 0 C. to 100 C. and a thickness of 20 m to 200 m is subjected to the preparatory heat treatment for stress relief in an atmosphere consisting of one or more selected from an N.sub.2 gas, an H.sub.2 gas, an Ar gas, an AX gas, and an HN gas within the range of 250 C. to 1,050 C.; a back-contact made of a Mo layer is formed on a surface of the stainless steel foil subjected to the preparatory heat treatment or an insulating coating is formed on a surface of the stainless steel foil followed by forming the back-contact including the Mo layer thereon; and an absorber layer made of Cu(In.sub.1-xGa.sub.x)Se.sub.2 is formed on the back-contact by performing coating formation heat treatment.

Claims

1. A method for manufacturing a solar cell substrate made of stainless steel foil, comprising stainless steel foil which contains 7% to 40% by mass Cr and 0.36% to 1.5% by mass Nb, and which has a coefficient of linear expansion of 12.010.sup.6/ C. or less at 0 C. to 100 C. and a thickness of 20 m to 200 m and which is subjected to preparatory heat treatment for stress relief in an atmosphere containing nitrogen within the range of 250 C. to 1,050 C., the preparatory heat treatment being performed before any layer is formed on a surface of the stainless steel foil; forming a back-contact made of a Mo layer on the surface of the stainless steel foil subjected to the preparatory heat treatment or an insulating coating on the surface of the stainless steel foil followed by forming the back-contact made of the Mo layer thereon; and forming an absorber layer made of Cu(In.sub.1-xGa.sub.x)Se.sub.2 on the back-contact by performing coating formation heat treatment such that a width and a length of the stainless steel foil before and after the coating formation heat treatment satisfy the following inequalities (1) and (2):
100|(W.sub.1W.sub.0)|/W.sub.00.037%(1);
100|(L.sub.1L.sub.0)|/L.sub.00.037%(2), wherein W.sub.0 is a width in mm of the stainless steel foil before being subjected to the forming of the back-contact, W.sub.1 is a width in mm of the stainless steel foil after being subjected to the forming of the back-contact, L.sub.0 is a length in mm of the stainless steel foil before being subjected to the forming of the back-contact, and L.sub.1 is a length in mm of the stainless steel foil after being subjected to the forming of the back-contact.

2. The method for manufacturing a solar cell, substrate made of stainless steel foil according to claim 1, wherein the preparatory heat treatment is performed within the range of 250 C. to 900 C.

3. The method for manufacturing a sour cell substrate made of stainless steel foil according to claim 1, wherein the preparatory heat treatment is performed within the range of 250 C. to 700 C.

4. The method for manufacturing a solar cell substrate made of stainless steel foil according to claim 1, wherein the coating formation heat treatment is performed within the range of 450 C. to 700 C.

5. A solar cell substrate made of stainless steel foil produced in such a way that stainless steel foil which contains 7% to 40% by mass Cr and 0.36% to 1.5% by mass Nb, and which has a coefficient of linear expansion of 12.010.sup.6/ C. or less at 0 C. to 100 C. and a thickness of 20 m to 200 m and which is subjected to the preparatory heat treatment for stress relief in an atmosphere containing nitrogen within the range of 250 C. to 1,050 C., the preparatory heat treatment being performed before any layer is formed on a surface of the stainless steel foil; a back-contact made of a Mo layer is formed on the surface of the stainless steel foil subjected to the preparatory heat treatment or an insulating coating is formed on the surface of the stainless steel foil followed by forming the back-contact made of the Mo layer thereon; and an absorber layer made of Cu(In.sub.1-xGa.sub.x)Se.sub.2 is formed on the back-contact by performing coating formation heat treatment, wherein width and length of the stainless steel foil before and after being subjected to the coating formation heat treatment satisfy the following inequalities (1) and (2):
100|(W.sub.1W.sub.0)|/W.sub.00.037%(1)
100|(L.sub.4L.sub.0)|/L.sub.00.037%(2) where W.sub.0 is the width (mm) of the stainless steel foil before being subjected to the coating formation heat treatment, W.sub.1 is the width (mm) of the stainless steel foil after being subjected to the coating formation heat treatment, L.sub.0 is the length (mm) of the stainless steel foil before being subjected to the coating formation heat treatment, and L.sub.1 is the length (mm) of the stainless steel foil after being subjected to coating formation heat treatment.

6. The solar cell substrate made of stainless steel foil according to claim 5, wherein width and length of the stainless steel foil before and after being subjected to the coating formation heat treatment satisfy the following inequalities (3) and (4):
100|(W.sub.1W.sub.0)|/W.sub.00.018%(3)
100|(L.sub.1L.sub.0)|/L.sub.00.018%(4).

7. The solar cell substrate made of stainless steel foil according to claim 5, wherein the temperature of the preparatory heat treatment ranges from 250 C. to 700 C.

8. The solar cell substrate made of stainless steel foil according to claim 5, wherein the temperature of the coating formation heat treatment ranges from 450 C. to 700 C.

9. The method for manufacturing a solar cell substrate made of stainless steel foil according to claim 2, wherein the preparatory heat treatment is performed within the range of 250 C. to 700 C.

10. The method for manufacturing a so cell substrate made of stainless steel foil according to claim 2, wherein the coating formation heat treatment is performed within the range of 450 C. to 700 C.

11. The method for manufacturing a solar cell substrate made of stainless steel foil according to claim 3, wherein the coating formation heat treatment is performed within the range of 450 C. to 700 C.

12. The solar cell substrate made of stainless steel foil according to claim 6, wherein the temperature of the preparatory heat treatment ranges from 250 C. to 700 C.

13. The solar cell substrate made of stainless steel foil according to claim 6, wherein the temperature of the coating formation heat treatment ranges from 450 C. to 700 C.

14. The solar cell substrate made of stainless steel foil according to claim 7, wherein the temperature of the coating formation heat treatment ranges from 450 C. to 700 C.

15. The method for manufacturing a solar cell substrate made of stainless steel foil according to claim 9, wherein the coating formation heat treatment is performed within the range of 450 C. to 700 C.

16. The solar cell substrate made of stainless steel foil according to claim 12, wherein the temperature of the coating formation heat treatment ranges from 450 C. to 700 C.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic sectional view of an example of a solar cell substrate made of stainless steel foil according to the present invention.

(2) FIG. 2 is a schematic sectional view of another example of a solar cell substrate made of stainless steel foil according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(3) FIGS. 1 and 2 each shows a schematic sectional view of an example of a solar cell substrate made of stainless steel foil according to the present invention. In the present invention, a manufacturing method prior to the foil rolling to produce stainless steel foil 2 which is a material of a solar cell substrate made of stainless steel foil 1 is not particularly limited.

(4) First, components of the stainless steel foil 2 are described.

(5) When the content of Cr is less than 7% by mass, the corrosion resistance during long-term use is insufficient and the durability of the solar cell substrate made of stainless steel foil is deteriorated. However, when the content of Cr is more than 40% by mass, there is a problem in that the toughness of a hot-rolled steel sheet which is a partly-finished product in a step of manufacturing the stainless steel foil is significantly reduced and therefore the hot-rolled steel sheet cannot pass through a production line. Therefore, the content of Cr needs to be 7% to 40% by mass.

(6) The stainless steel foil 2 may contain Nb and/or Mo in the above composition.

(7) In the case where the stainless steel foil 2 contains Nb, when the content of Nb is less than 0.05% by mass, a deformation resistance effect during crystallization heat treatment is reduced. However, when Nb is more than 1.5% by mass, there is a problem in that weldability during the manufacture of the hot-rolled steel sheet, which is the partly-finished product in the step of manufacturing the stainless steel foil, is significantly reduced and therefore the hot-rolled steel sheet cannot pass through the production line. Accordingly, the content of Nb preferably ranges from 0.05% to 1.5% by mass.

(8) In the case where the stainless steel foil 2 contains Mo, when the content of Mo is less than 0.3% by mass, a deformation resistance effect during heat treatment for coating formation is reduced. However, it is more than 3.0% by mass, there is a problem in that hot workability during the manufacture of the hot-rolled steel sheet, which is the partly-finished product in the step of manufacturing the stainless steel foil, is reduced and therefore the hot-rolled steel sheet cannot pass through the production line. Accordingly, the content of Mo preferably ranges from 0.3% to 3.0% by mass, being more preferably 2.0% by mass or less.

(9) Examples of such steel include SUS 430 (17% Cr steel), SUS 447J1 (30% Cr-2% Mo steel), 9% Cr steel, 20% Cr-5% Al steel, and SUS 304 (18% Cr-8% Ni steel).

(10) Added preferable components other than Cr, Nb, and Mo described above are as described below. Incidentally, all percentages defining the composition of steel refer to mass percent.

(11) C: 0.12% or Less

(12) C combines with Cr in steel to cause a reduction in corrosion resistance and therefore is preferably as low as possible. When it is 0.12% or less, the corrosion resistance is not significantly reduced. Therefore, it is preferably 0.12% or less and more preferably 0.04% or less.

(13) Si: 2.5% or Less

(14) Si is an element used for deoxidation. When Si is excessively contained, a reduction in ductility is caused. Therefore, it is preferably 2.5% or less and more preferably 1.0% or less.

(15) Mn: 1.0% or Less

(16) Mn combines with S to form MnS and causes a reduction in corrosion resistance. Therefore, it is preferably 1.0% or less and more preferably 0.8% or less.

(17) S: 0.030% or Less

(18) As described above, S combines with Mn to form MnS and causes a reduction in corrosion resistance. Therefore, it is preferably 0.030% or less and more preferably 0.008% or less.

(19) P: 0.050% or Less

(20) P causes a reduction in ductility and therefore is preferably as low as possible. When it is 0.050% or less, the ductility is not significantly reduced. Therefore, it is preferably 0.050% or less and more preferably 0.040% or less.

(21) Essential components are as described above. In the present invention, elements below may be appropriately contained.

(22) At Least One Selected from Ti and Zr: 1.0% or Less in Total

(23) Both Ti and Zr are elements that fix C and N in steel in the form of a carbide, a nitride, or a carbonitride and are useful in improving corrosion resistance. However, when more than 1.0% of Ti and/or Zr is contained, a reduction in ductility is significant. Therefore, these elements are limited to 1.0% or less in the case of either single addition or combined addition. In order to sufficiently exhibit the effect of adding these elements, 0.02% or more of Ti and/or Zr is preferably contained.

(24) Al: 0.20% or Less

(25) Al is an element used for deoxidation. When Al is excessively contained, a reduction in ductility is caused. Therefore, it is preferably 0.20% or less and more preferably 0.15% or less. However, this does not apply to the case of using 20Cr-5Al steel or the like intentionally containing Al.

(26) N: 0.05% or Less

(27) N combines with Cr in steel to cause a reduction in corrosion resistance and therefore is preferably low. When it is 0.05% or less, the corrosion resistance is not significantly reduced. Therefore, it is preferably 0.05% or less and more preferably 0.015% or less.

(28) In addition, 1.0% or less of each of Ni, Cu, V, and W may be contained for the purpose of improving the corrosion resistance. Furthermore, 0.1% or less of each of Ca, Mg, REMs (rare-earth metals), and B may be contained for the purpose of improving the hot workability.

(29) The remainder are Fe and inevitable impurities. Among the inevitable impurities, O (oxygen) is preferably 0.02% or less.

(30) When the coefficient of linear expansion of the stainless steel foil 2 is more than 12.010.sup.6/ C. at 0 C. to 100 C., an absorber layer peel, off by heat treatment for coating formation. Thus, the coefficient of linear expansion of the stainless steel foil 2 is 12.010.sup.6/ C. or less at 0 C. to 100 C.

(31) Such stainless steel foil can be obtained by foil-rolling stainless steel corresponding to, for example, JIS SUS 430 (so-called 17% Cr steel), SUS 444 (so-called 18Cr-2Mo steel), SUS 447J1 (so-called 30Cr-2Mo steel), or the like.

(32) Next, the thickness of the stainless steel foil 2 is described below.

(33) When the thickness of the stainless steel foil 2 is less than 20 m, the stainless steel foil 2 is extremely easily to be folded or ripped; hence, a crack extends from a fold or rip in the stainless steel foil 2 into an insulating coating or absorber layer described below and therefore the insulating coating or the absorber layer tend to peal off. However, when it is more than 200 m, it is higher in cost than glass substrates. Thus, the thickness of the stainless steel foil 2 ranges from 20 m to 200 m.

(34) Before the insulating coating 3, a back-contact 5, or the like is formed on the stainless steel foil 2, which has such a composition and thickness, the stainless steel foil 2 is subjected to the preparatory heat treatment, whereby the residual stress caused in a step of rolling the stainless steel foil 2 is removed. The preparatory heat treatment is performed in an atmosphere consisting of one or a mixture of two or more selected from an N.sub.2 gas, an H.sub.2 gas, an Ar gas, an AX gas, and an HN gas. Since thermal pretreatment is performed in such an inert or reducing gas atmosphere, the oxidation of the stainless steel foil 2 is prevented. Incidentally, the AX gas refers to a mixed gas of 75% by volume H.sub.2 and 25% by volume N.sub.2 and the HN gas refers to a mixed gas of 3% to 10% by volume H.sub.2 and 97% to 90% by volume N.sub.2 as the balance.

(35) When the temperature of preparatory heat treatment is lower than 250 C., the residual stress in the stainless steel foil 2 cannot be sufficiently removed. However, when it is higher than 1,050 C., grains in the stainless steel foil 2 are coarsened and surfaces of the stainless steel foil 2 are roughened, thereby causing a reduction in conversion efficiency. In addition, when Nb or Mo is not contained, transformation occurs at higher than 900 C. and therefore sufficient corrosion resistance cannot be achieved. Furthermore, in order to maintain the hardness of the stainless steel foil 2 without softening the stainless steel foil 2 and in order to increase the stiffness of the stainless steel foil 2, it is preferably 700 C. or lower. Thus, the temperature of preparatory heat treatment ranges from 250 C. to 1,050 C. It is preferably 250 C. to 900 C. and more preferably 250 C. to 700 C. Performing the preparatory heat treatment within this temperature range allows the deformation of the stainless steel foil 2 to be prevented and also allows residual stress to be removed. In order to exhibit such effects remarkably, holding several seconds at an attained temperature is sufficient regarding the time taken for thermal pretreatment.

(36) In an integration type shown in FIG. 2, after the preparatory heat treatment is performed, the insulating coating 3 is formed on the stainless steel foil 2. A method for forming the insulating coating 3 is not particularly limited. A material for the insulating coating 3 is not particularly limited and is preferably an alumina coating which has been conventionally and widely used. Next, the back-contact 5 is formed on the insulating coating 3 so as to include a Mo layer and a coating of Cu(In.sub.1-xGa.sub.x)Se.sub.2 is formed thereon as the absorber layer 4.

(37) In a grid type shown in FIG. 1, after the preparatory heat treatment is performed, the back-contact 5 is formed on the stainless steel foil 2. Next, the Cu(In.sub.1-xGa.sub.x)Se.sub.2 coating is formed on the back-contact 5 as the absorber layer 4.

(38) In either case, before the insulating coating 3 or the back-contact 5 is formed on the stainless steel foil 2 subjected to the preparatory heat treatment, a Cr layer or the like referred to as a barrier layer may be formed in advance. In this case, the configuration of the insulating coating, the back-contact, and the absorber layer other than the barrier layer is the same as above.

(39) In the present invention, a method for forming each of the insulating coating 3, the back-contact 5, and the absorber layer 4 is not particularly limited. Examples of a heat treatment method for coating formation for the absorber layer 4 include (A) a solid-phase method, (B) a gas-phase method, and (C) an evaporation method. These methods are summarized below.

(40) (A) Solid-phase Method

(41) A method in which after Se/In/(CuGa) is formed in such a way that In/(CuGa) and then Se are deposited on a substrate by sputtering and by vapor deposition, respectively, heat treatment is performed at 450 C. to 500 C., whereby a coating of Cu(In.sub.1-xGa.sub.x)Se.sub.2 is formed.

(42) (B) Gas-phase Method

(43) A method in which after a CIGS film (copper indium gallium diselenide film) is formed by heating In/(CuGa) in an atmosphere containing an H.sub.2Se gas, heat treatment is performed at a temperature of about 500 C., whereby a coating of Cu(In.sub.1-xGa.sub.x)Se.sub.2 is formed.

(44) (C) Evaporation Method

(45) A method in which after In, Ga, and Se are applied to a substrate at a temperature of 350 C. to 500 C., the temperature of the substrate is increased to 500 C. to 550 C., Se and Cu only are applied, and In, Ga, and Cu are then applied at 500 C. to 550 C. or In, Ga, Se, and Cu are vapor-deposited on the substrate at about 500 C. together, whereby a coating of Cu(In.sub.1-xGa.sub.x)Se.sub.2 is formed.

(46) When the temperature of heat treatment for coating formation is lower than 450 C., crystals of Cu(In.sub.1-xGa.sub.x)Se.sub.2 are not sufficiently grown. However, when it is higher than 700 C., the deformation of the stainless steel foil 2 is increased and therefore the insulating coating 3, the back-contact 5, and the absorber layer 4 tend to peel off. Thus, the temperature of heat treatment for coating formation preferably ranges from 450 C. to 700 C. Performing heat treatment for coating formation within this temperature range allows the deformation of the stainless steel foil 2 to be prevented and also allows the Cu(In.sub.1-xGa.sub.x)Se.sub.2 crystals to be grown.

(47) The solar cell substrate made of stainless steel foil 1 (FIG. 2) that is obtained in such a way that the insulating coating 3 is formed on the stainless steel foil 2 and the back-contact 5 and the absorber layer 4 are formed on the insulating coating 3 is reduced in deformation caused by heat treatment for coating formation because residual stress is removed by performing the preparatory heat treatment before the formation of the insulating coating 3. Alternatively, the solar cell substrate made of stainless steel foil 1 (FIG. 1) that is obtained in such a way that the back-contact 5 is formed on the stainless steel foil 2 and the absorber layer 4 is formed on the back-contact 5 is reduced in deformation caused by heat treatment for coating formation because residual stress is removed by performing the preparatory heat treatment before the formation of the back-contact 5.

(48) That is, supposing the width of the stainless steel foil 2 before being subjected to coating formation heat treatment is W.sub.0 (mm), the width of the stainless steel foil 2 after being subjected to coating formation heat treatment is W.sub.1 (mm), the length of the stainless steel foil 2 before being subjected to coating formation heat treatment is L.sub.0 (mm), and the length of the stainless steel foil 2 after being subjected to coating formation heat treatment is L.sub.1 (mm), the changes in width and length satisfy the following inequalities (1) and (2) and the peeling off of the absorber layer 4, the back-contact 5, and the insulating coating 3 can be suppressed:
100|(W.sub.1W.sub.0)|/W.sub.00.037%(1)
100|(L.sub.1L.sub.0)|/L.sub.00.037%(2).

(49) In order to further enhance the effect of suppressing the exfoliation of the absorber layer 4, the back-contact 5, and the insulating coating 3, the deformation of the stainless steel foil 2 due to coating formation heat treatment needs to be further reduced and therefore the changes in width and length of the stainless steel foil 2 are preferably kept within ranges given by the following inequalities (3) and (4):
100|(W.sub.1W.sub.0)|/W.sub.00.018%(3)
100|(L.sub.1L.sub.0)|/L.sub.00.018%(4).

(50) As described above, in the solar cell substrate made of stainless steel foil 1 according to the present invention, the deformation of the stainless steel foil 2 due to coating formation heat treatment is reduced and therefore the peel off of the absorber layer 4, the back-contact 5, and the insulating coating 3 can be suppressed. As a result, a solar cell manufactured from the solar cell substrate made of stainless steel foil 1 according to the present invention can convert sunlight into electricity efficiently.

EXAMPLES

(51) A solar cell substrate made of stainless steel foil 1 shown in FIG. 1 was manufactured and the deformation of stainless steel foil 2 was investigated. Furthermore, a solar cell was manufactured by attaching electrodes to the substrate and the conversion efficiency thereof was investigated. The procedure is described below.

(52) After the stainless steel foil 2 (a thickness of 50 m), which contained components shown in Table 1, was subjected to the preparatory heat treatment (a temperature of 400 C. to 700 C.) and a Mo layer was formed as a back-contact 5, then a coating of Cu(In.sub.1-xGa.sub.x)Se.sub.2 was formed as an absorber layer 4 by performing coating formation heat treatment by a solid-phase method. The temperature of heat treatment in the solid-phase method was 550 C. The solar cell substrate made of stainless steel foil 1 was manufactured as described above. In this procedure, before coating formation heat treatment was performed, ruled lines were drawn on a central portion of the stainless steel foil 2 in a widthwise direction and in a lengthwise direction. After coating formation heat treatment was performed, the ruled lines were measured for length and 100(W.sub.1W.sub.0)/W.sub.0 and 100(L.sub.1L.sub.0)/L.sub.0 were calculated, whereby the deformation of the stainless steel foil 2 was evaluated. The results are shown in Table 2.

(53) A CdS coating was formed on the solar cell substrate made of stainless steel foil 1, which was obtained as described above, as a buffer layer. Next, a ZnO coating was formed as a transparent conducting film. Finally, a NiAl electrode was formed by vacuum deposition, whereby the solar cell was manufactured. The conversion efficiency of the solar cell was measured. This is referred to as an inventive example.

(54) Incidentally, the conversion efficiency of converting sunlight into electricity is calculated by dividing the output obtained from the measurement of the generated current and voltage by the intensity of incident light.

(55) On the other hand, a solar cell was manufactured through the same steps as those of the inventive example except that thermal pretreatment was not performed. The conversion efficiency thereof was measured. This is referred to as a comparative example.

(56) For each of the inventive example and the comparative example, the deformation of the stainless steel foil and the conversion efficiency of the solar cell are shown in Table 2. Herein, in the changes in width and length of the stainless steel foil 2, the symbol + represents expansion and the symbol represents contraction.

(57) TABLE-US-00001 TABLE 1 Stainless steel Components (mass percent) foil C Si Mn P S Cr Cu Nb Mo a 0.037 0.23 0.51 0.028 0.003 16.2 b 0.006 0.20 0.20 0.025 0.005 9.4 0.4 c 0.010 0.23 0.44 0.025 0.003 17.7 0.42 d 0.004 0.34 0.20 0.025 0.003 14.8 0.52 1.58 e 0.005 0.29 0.15 0.025 0.003 18.9 0.36 1.84 f 0.008 0.09 0.17 0.025 0.003 17.9 1.20

(58) TABLE-US-00002 TABLE 2 Deformation of stainless steel foil Solar cell Change in Change in Conversion width length Absorber efficiency (%) (%) layer (%) Inventive 0 to +0.001 to Not 12.7 to 13.0 example 0.001 0.017 Peeling off Comparative +0.005 to 0.019 to Peeling 6.6 to 7.8 example 0.038 0.046 off

(59) As is clear from Table 2, the deformation of the stainless steel foil of the inventive example is small. As a result, the peeling off of the absorber layer is suppressed and therefore the conversion efficiency of the solar cell is high.

(60) According to the present invention, the peeling off of a coating for an absorber layer formed in a solar cell substrate (that is, a solar cell substrate made of stainless steel foil) including stainless steel foil which is inexpensive and which can be mass-manufactured can be prevented. This contributes to the reduction of production costs of solar cells, and enables the conversion efficiency of converting sunlight into electricity to be increased. Therefore, industrially particularly advantageous effects can be produced.

REFERENCE SIGNS LIST

(61) 1 Solar cell substrate made of stainless steel foil

(62) 2 Stainless steel foil

(63) 3 Insulating coating

(64) 4 Absorber layer

(65) 5 Back-contact