Abstract:
A flexible metallic strip product including a ferritic chromium strip steel material having a coating of an electrically insulating layer of zirconia stabilized with yttrium, said coating being intended to receive a second coating of an electrically conducting layers is disclosed. This coating is very efficient as an electrically insulating layer in flexible solar cells and solid state thin film batteries because it has practically the same thermal expansion coefficient as the steel substrate.

Description:
The present invention relates to a method of manufacturing a metal oxide coated metal strip product in a roll-to-roll process and in particular to a coated metallic substrate material suitable for manufacturing of thin film devices. This is achieved by producing a metallic strip with an electrically insulating oxide layer, in accordance with claim  1 . 
     BACKGROUND 
     Several materials are used as substrate material and are under further development for the production of thin film flexible products. There exist several advantages to use flexible substrate materials, one being the possibility of roll-to-roll production processes, which lead to a more cost efficient production compared to batch type processes. Moreover, the thin film flexible products will have several technical advantages, for instance, they may be folded or rolled into compact packages and they may be used for making light weight products, which are required for portable, spatial and military applications. The common materials used for thin film flexible products include, e.g., plastic foils such as polyamide, metallic strips or foils such as stainless steel, titanium, copper, molybdenum, aluminum and nickel foils, bearing in mind that they all have to fulfil certain criteria. Thus, the substrate material should be thermally resistant in order to withstand further process steps in the production of thin film products, and this may include heat treatments at elevated temperatures under corrosive atmosphere. However, all the electrical conducting metallic strip materials used as substrate material for the production of thin film flexible products may need to be electrically insulated, if modules with integrated series connections are to be produced. Conventionally, the conducting metallic strip or foil material is coated with oxides such as aluminum oxide or silicon oxide, which are good electrical insulating oxides. However, these oxide layers usually have a large mismatch in the thermal expansion compared with the underlying metallic substrate. Thus, it is essential that the thermal expansion coefficient (TEC) of the substrate material should be as close as possible to the TEC of the electrically insulating metal oxide layer(s), in order to avoid thermal cracking or spallation of the insulating metal oxide layer. 
     The mismatch in thermal expansion (MTE) can be defined as follows: (TEC ss −TEC ox )/TEC ss  where the TEC ss  is the thermal expansion of the metallic strip substrate and TEC ox  is the thermal expansion of the metal oxide layer. 
     Ferritic stainless steels and yttrium-stabilized zirconia (abbreviated YSZ) are two materials commonly used in the fabrication of Solid Oxide Fuel Cells because these two materials have an extremely low mismatch in their thermal expansion. However, the YSZ in Solid Oxide Fuel Cells are not in the form of a thin coating but in the form of a thin plate, which is used as an electrolyte. 
     Common conventional solutions are:
         Depositing the electrically conducting back contact directly onto the metallic strip;   Depositing insulating layers such as aluminum oxide, silicon oxide and silicon nitride onto metal strips,       

     Both these solutions have their respective disadvantages. If the conducting back contact is directly deposited onto the flexible metal strip substrate, then it will limit the production of modules with integrated series connections. Furthermore, the material often used as insulating layers, such as Al 2 O 3 , Si 4 N 3 , SiO x  or SiO 2 , have a larger TEC mismatch with the underlying metallic strip, which may lead to the formation of cracks and pinholes due to heating during the following process steps. In table 1 the thermal expansion coefficients of some insulating materials and some steels have been summarized. Using the values of the thermal expansions given in Table 1, it is possible to calculate the thermal expansion mismatch (MTE) between the metallic strip substrate and the insulating coating. In Table 2 the MTE values between some of the steels and the listed insulating materials are given. 
     It is therefore a primary object of the present invention to provide a zirconium oxide coated metallic strip product, where the oxide layer and the metallic strip have a very low mismatch in thermal expansion, said product being suitable as a substrate material for the production of thin film flexible products, such as flexible Cu(In; Ga)Se 2  (abbreviated CIGS) solar cells and solid state thin film batteries. 
     Yet another object of the present invention is to provide a flexible substrate for thin film products that is inexpensive and which may be produced in a continuous roll-to-roll process. 
     These and other objects have been attained in a surprising manner by creating a coated metallic strip product with the features according to the characterizing clause of claim  1 . Further preferred embodiments are defined in the dependent claims. 
     BRIEF DESCRIPTION 
     Thus, the above objects and further advantages are achieved by applying a thin continuous, uniform, electrically insulating layer of zirconia, the zirconia being stabilized with yttrium oxide or any other suitable metal oxide normally used for this purpose, on the top of a metal strip serving as substrate. The absolute composition of the zirconia layer can be tailor-made so that the thermal expansion matches the thermal expansion of the chosen metallic strip substrate. The zirconia layer should be smooth and dense in order to avoid any pinholes, which may otherwise function as pathways for electrical conduction when the material is further processed. In order to ensure safe electrical insulation from the metal strip substrate, multi-layers (ML) of zirconia can be deposited. The advantage of a ML structure is that it will terminate any pinholes or conducting pathways through the insulating oxide layers. Furthermore, by depositing a continuous uniform dense zirconia layer on top of the metallic substrate, it is easier to control the insulating properties as well as the thickness of the zirconia layer, compared to for instance anodized oxide layers on metallic strips. Moreover, the zirconia layer will also have an enhanced adhesion to the substrate, in comparison with thermally grown oxide layers. If needed, the zirconia layer may be deposited on both sides of the strip material, in order to totally eliminate any curling of the coated strip during heat treatments due to any small thermal expansion differences. 
     Further, if so required, on top of said zirconia layer may then be deposited a metal layer, this for obtaining a back electrical contact. If the final product is a CIGS flexible thin film solar cell, then this metal should be molybdenum. However, if the final product is a thin film battery, this metal back contact may be aluminum or copper. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic cross-section of a first embodiment of the present invention. 
         FIG. 2  shows a schematic cross-section of a second embodiment of the present invention. 
         FIG. 3  shows a schematic cross-section of a third embodiment of the present invention. 
         FIG. 4  shows a schematic cross-section of fourth embodiment of the present invention. 
         FIG. 5  shows schematically a production line for the manufacturing of a coated metal strip material according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The Metal Strip to be Coated 
     The metal strip to be coated can be any metallic strip material with a low thermal expansion mismatch with the zirconia coating, preferably a stainless steel and most preferably a ferritic chromium steel. Such steel substrate materials are produced by ordinary metallurgical steel making with a chromium content of at least 10% b.w. suitably more than 14% b.w., and most preferably in the range 16-25% b.w. Some examples of possible steel substrates are grade ASTM 430 with a chromium content of 16% b.w., and the Sandvik grade 0C404 with a chromium content of 20% b.w., and an aluminum content of 5.5% b.w. The ferritic strip is hot rolled down to an intermediate size, and thereafter cold-rolled in several steps with a number of recrystallization steps between said rolling steps, until a final thickness and a width of maximally 1000 mm are achieved. Another important parameter is the surface roughness of the metal strip, which should be as smooth as possible; a Ra value of less than 0.2 μm is suitable, preferably less than 0.1 μm. The surface of the substrate material is then cleaned in an appropriate way to remove all oil residuals from the rolling. The key issue for using a ferritic steel is that its thermal expansion coefficient (TEC) matches very well with the deposited zirconia coating, although other steel types fulfilling this criterion are also useful. Thus, it is desirable that the thermal expansion mismatch of the metallic strip substrate and the zirconia coating is lower than ±25% in the temperature range up to 1000° C. Furthermore, ferritic steels have also a very good corrosion resistance to withstand the environment in which the final thin film product will work. The physical shape of the metallic substrate is a strip or foil with a thickness in the range of 5 to 300 μm, preferably 10 to 100 μm. 
     The Insulating Zirconia Layer 
     The electrically insulating zirconia layers should adhere well to the metallic strip, in order to ensure highest possible flexibility of the thin film product without the zirconia layer flaking off or cracking. This is achieved by careful pre-treatment of the metal strip prior to the coating, first by cleaning it in a suitable way to remove all oil residues, etc., which may affect the efficiency of the coating process, and the adhesion and quality of the coating. Thereafter, the metal strip is treated by means of an in-line ion assisted etching process and, if necessary, a thin bond-coat layer may be deposited between the metal strip and the zirconia layer. Preferably, the bond-coat should consist essentially of a metal such as Ti, Zr, Ni or Cr, to enhance the adhesion of the zirconia layer to the substrate. Moreover, the zirconia layer should also be a good electrical insulator in order to avoid any electrical connection between the metallic strip and the molybdenum (or Al or Cu) back contact. This can be achieved by depositing a dense and smooth zirconia layer to bring about better insulating properties, it possibly being repeated so that multi-layered structures are deposited. The number of individual zirconia layers in a multi-layered structure can be 10 or less, preferably less than 5 and at best only two layers. As mentioned above, a multi-layered zirconia structure will terminate any pinholes or electrical pathways through the overall metal oxide layer and ensure good electrical insulation of the metallic strip. The thickness of each individual zirconia layer may be between 10 nm and 2 μm, preferably between 0.1 and 1.5 μm. The total thickness of the overall zirconia layer may be up to 20 μm, preferably 0.5 to 5 μm. 
     The preferred chemical composition of the zirconia layer is ZrO 2  stabilized with Y 2 O 3 , the percentage of Y 2 O 3  in the zirconia can be from 0-25% b.w., preferably 3-20% b.w., or most preferably 5-15% b.w., although other stabilizing metal oxides are feasible (both stoichiometric and non-stoichiometric). The zirconia could also possibly be stabilized by for example Al-oxide. 
     Description of Back Contact Layer 
     When required, a top conducting metallic layer can be applied on top of the insulating zirconia layer. Depending on the final thin film product, different metals may be deposited, including aluminum, molybdenum, nickel, cobalt, copper, silver, gold and platinum. This metallic top layer should be dense and adhere well to the underlying, previously deposited oxide layer. The thickness of this metallic layer should be 0.01-5.0 μm. 
     Description of Coating Method 
     A broad variety of coating techniques may be used as long as they provide a continuous uniform and adherent layer. These may be techniques such as spraying, High Velocity Oxygen Fuel (HVOF), dipping, Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD) or any other known technique for applying a fluid gel or powder of a Zr-containing compound on the surface of the ferritic steel, preferably by PVD in a roll-to-roll Electron Beam Evaporation (EB) process as disclosed in WO 98/08986. If necessary, the PVD process can also be plasma-activated to achieve even better insulating oxide layers. It is also possible to apply the coating in the form of a fine grain powder. The conditions for applying and forming the zirconia layer on the surface may have to be determined experimentally for each individual case. The coating will be affected by factors such as temperature, time drying, time heating, composition and properties, as well as by the ferritic steel and the Zr-containing compound. 
     Advantageously, the coating method is integrated in a conventional roll-to-roll strip production line. 
     Advantageous Embodiment 
     Firstly, the substrate materials are produced by ordinary metallurgical steel-making to a chemical composition as described above. Subsequently, they are hot-rolled down to an intermediate size, and thereafter cold-rolled in several steps with a number of recrystallization steps between said rolling steps, until a final thickness of 0.005-0.3 mm and a width of up to 1000 mm are achieved. The surface of the substrate material is then cleaned in a proper way to remove all oil residuals from the rolling. 
     In  FIG. 1 , a typical cross section of a zirconia coated flexible ferritic steel  1  is shown, one side of which has been coated with a YSZ layer  3  for the production of thin film products. The substrate material  2  is a flexible ferritic steel strip, which has a thermal expansion mismatch lower than ±25% with the zirconia coating in the temperature range 0-1000° C. The surface roughness of the ferritic steel strip should be kept as low as possible. The thickness of the ferritic steel should be in the range of 5-300 μm, preferably 10-100 μm, to ensure good flexibility. 
     On top of the surface of the ferritic steel strip  2  is situated an electrically insulating yttrium stabilized zirconia layer  3  deposited in a roll-to-roll process. The yttrium stabilized zirconia layer should be well adherent to the metal strip, as well as dense and smooth. Well adherent is in this context considered to mean that the coated metal strip is able to be bent 90° over a radius equal to the thickness of the strip without the coating showing any tendency of flaking, spalling or the like. 
     In a variation of the above described flexible metallic substrate being coated on one side only with YSZ, also the other side of the metallic strip may be coated with YSZ, to produce a product  4 , which is illustrated in  FIG. 2 . In the  FIG. 2 , the substrate material  2  has been coated on both sides with YSZ  3  and the two YSZ layers  3  should have the same thickness to achieve a total elimination of any thermal deformation of the coated strip due to the YSZ coatings in further process steps. 
     According to a further variation of the invention, the strip product  5  consists of a flexible metallic strip  2  coated with a multi-layer structure  6  of YSZ, as illustrated in  FIG. 3 . The number of individual zirconia layers in the multi-layered structure  6  can be 10 or less, preferably less than 5 and at best only two layers. As mentioned before, a multi-layered zirconia structure will terminate any pinholes or electrical pathways through the overall metal oxide layer and ensure good electrical insulation of the metallic strip. The thickness of each individual zirconia layer may be between 10 nm and up to 2 μm, preferably between 0.1 and 1.5 μm. The total thickness of the overall zirconium oxide layer may be up to 20 μm, preferably 1 to 5 μm. 
     In order to create a conducting back contact for the production of thin film products, a metal layer  7  can be deposited on top of the electrically insulated, YSZ-coated (8) ferritic steel strip  2 , as illustrated in  FIG. 4 . Depending on the final thin film product, the metal layer  7  can consist of different metals, including aluminum, molybdenum, nickel, cobalt, copper, silver, gold and platinum; the preferred metal being aluminum, molybdenum, silver and/or copper, or alloys consisting mainly of molybdenum. The metal layer  7  should be dense and well adherent to the zirconia coating to avoid cracking or spallation. Furthermore, the metal layer  7  should have a thickness of between 0.1-5 μm, preferably 0.2-2 μm, and most preferably around 0.5 μm. 
     The roll-to-roll electron beam evaporation process is illustrated in  FIG. 5 . The first part of such a production line is the uncoiler  9  within a vacuum chamber  10 , then the in-line ion assisted etching chamber  11 , followed by a series of EB evaporation chambers  12 , the number of EB evaporation chambers needed can vary from 1 up to 10 chambers, this to achieve the wanted multi-layered metal oxide structure. All the metal oxide EB evaporation chambers  12  are equipped with EB guns  13  and crucibles  14  for the evaporation. The following chamber is a separate chamber  15  for the EB evaporation of a metal top layer, this chamber is also equipped with an EB gun  16  and a crucible  17  for the metal melt. The need for a separate EB evaporation chamber for the metal top layer can be excluded if only metal oxide coated strips are to be produced. After this chamber comes the exit vacuum chamber  18  and the recoiler  19  for the coated strip material, the recoiler being located within vacuum chamber  18 . The vacuum chambers  10  and  18  may also be replaced by an entrance vacuum lock system and an exit vacuum lock system, respectively. In the latter case, the uncoiler  9  and the coiler  19  are placed in the open air. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Material 
                 Temperature Range 
                 Thermal Expansion 
               
               
                   
               
             
             
               
                   
                 Si 3 N 4 (α) 
                 25-1000 K 
                  2.9 × 10 −6  K −1   
               
               
                   
                 Si 3 N 4 (β) 
                 25-1000 K 
                 2.25 × 10 −6  K −1   
               
               
                   
                 Al 2 O 3  (polycryst.) 
                  0-1027° C. 
                 8.12 × 10 −6  K −1   
               
               
                   
                 TiO 2  (polycryst.) 
                 25-1000° C. 
                 8.83 × 10 −6  K −1   
               
               
                   
                 SiO 2  (vitreous) 
                 25-1000° C. 
                 0.564 × 10 −6  K −1   
               
               
                   
                 YSZ 
                 25-1000° C. 
                   11 × 10 −6  K −1   
               
               
                   
                 Fe 80 Cr 20   
                 25-1000° C. 
                 13.5 × 10 −6  K −1   
               
               
                   
                 Fe 70 Cr 30   
                 25-1000° C. 
                 13.0 × 10 −6  K −1   
               
               
                   
                 Sandvik grade 0C44 
                 30-900° C. 
                 11.10 × 10 −6  K −1   
               
               
                   
                 ASTM 430 
                 30-900° C. 
                 12.05 × 10 −6  K −1   
               
               
                   
                 St1.4016 
                 25-1000° C. 
                 13.2 × 10 −6  K −1   
               
               
                   
                 St1.4742 
                 25-1000° C. 
                 14.3 × 10 −6  K −1   
               
               
                   
                 St1.4749 
                 25-1000° C. 
                 13.7 × 10 −6  K −1   
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Steel 
                 Insulating layer 
                 Calculated MTE (%) 
               
               
                   
               
             
             
               
                   
                 St1.4016 
                 SiO 2   
                 95.7%   
               
               
                   
                 St1.4016 
                 Si 3 N 4 (α) 
                 78% 
               
               
                   
                 St1.4016 
                 Al 2 O 3   
                 38% 
               
               
                   
                 St1.4016 
                 TiO 2   
                 33% 
               
               
                   
                 St1.4016 
                 YSZ 
                 16.7%   
               
               
                   
                 Fe 80 Cr 20   
                 SiO 2   
                 96% 
               
               
                   
                 Fe 80 Cr 20   
                 Si 3 N 4 (α) 
                 78.5%   
               
               
                   
                 Fe 80 Cr 20   
                 Al 2 O 3   
                 40% 
               
               
                   
                 Fe 80 Cr 20   
                 TiO 2   
                 35% 
               
               
                   
                 Fe 80 Cr 20   
                 YSZ 
                 19% 
               
               
                   
                 0C44 
                 SiO 2   
                 95% 
               
               
                   
                 0C44 
                 Si 3 N 4 (α) 
                 74% 
               
               
                   
                 0C44 
                 Al 2 O 3   
                 27% 
               
               
                   
                 0C44 
                 TiO 2   
                 20% 
               
               
                   
                 0C44 
                 YSZ 
                  1% 
               
               
                   
                 St1.4742 
                 SiO 2   
                 80% 
               
               
                   
                 St1.4742 
                 Si 3 N 4 (α) 
                 96% 
               
               
                   
                 St1.4742 
                 Al 2 O 3   
                 43% 
               
               
                   
                 St1.4742 
                 TiO 2   
                 38% 
               
               
                   
                 St1.4742 
                 YSZ 
                 23%