Abstract:
A method of forming metallic connector patterns for solar cells, whereby an embosser having raised features shaped in the form of a metallic connector pattern is used to attach a portion of a metallic foil to a transparent conductive layer formed on a top transparent surface of a solar cell structure. The raised surfaces of the embosser press the metallic foil portion against the transparent conductive layer. Heat and pressure directed to the metallic foil portion attach the metallic foil portion to the underlying transparent conductive layer, and then the rest of the metallic foil, which is not attached to the transparent conductive layer, is removed.

Description:
BACKGROUND 
       [0001]    The present application claims priority from U.S. Provisional Appln. No. 60/886,078 filed Jan. 22, 2007, the contents of which are incorporated herein by reference in their entirety. 
     
    
     FIELD 
       [0002]    The present invention relates to a method and system for manufacturing thin film solar cells. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970&#39;s there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods. Group IIB-VIA compounds such as CdTe, Group IBIIIAVIA compounds and amorphous Group IVA materials such as amorphous Si and amorphous Si alloys are important thin film materials that are being developed. 
         [0004]    Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Among the family of compounds, best efficiencies have been obtained for those containing both Ga and In, with a Ga amount in the 15-25%. Recently absorbers comprising Al have also been developed and high efficiency solar cells have been demonstrated using such absorbers. 
         [0005]    The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in  FIG. 1 . The device  10  is fabricated on a substrate  11 , such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film  12 , which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown over a conductive layer  13  or a contact layer, which is previously deposited on the substrate  11  and which acts as the electrical ohmic contact to the device. The substrate  11  and the conductive layer  13  form a base  16  on which the active layers of the device are deposited. The most commonly used contact layer or conductive layer in the solar cell structure of  FIG. 1  is Molybdenum (Mo). If the substrate itself is a properly selected conductive material such as a Mo foil, it is possible not to use a conductive layer  13 , since the substrate  11  may then be used as the ohmic contact to the device. The conductive layer  13  may also act as a diffusion barrier in case the metallic foil is reactive. For example, foils comprising materials such as Al, Ni, Cu may be used as substrates provided a barrier such as a Mo layer is deposited on them protecting them from Se or S vapors. The barrier is often deposited on both sides of the foil to protect it well. After the absorber film  12  is grown, a transparent layer  14  such as a CdS, ZnO or CdS/ZnO stack is formed on the absorber film. Radiation  15  enters the device through the transparent layer  14 . Metallic grids (not shown) may also be deposited over the transparent layer  14  to reduce the effective series resistance of the device. The preferred electrical type of the absorber film  12  is p-type, and the preferred electrical type of the transparent layer  14  is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The preferred device structure of  FIG. 1  is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te)2 absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the transparent superstrate side. A variety of materials, deposited by a variety of methods, can be used to provide the various layers of the device shown in  FIG. 1 . 
         [0006]    Thin film photovoltaic devices may be manufactured in the form of monolithically integrated modules where electrical interconnection of individual solar cells with each other is achieved on a single substrate, such as a glass sheet, during the film deposition steps and a module with high voltage is obtained. Alternatively thin film solar cells may be manufactured individually as separate cells and then connected in series, through use of metallic ribbons, soldering or conductive epoxies, like crystalline Si solar cells, to obtain high voltage modules. In this case, solar cells often need to be large area, one dimension being greater than 1″, typically greater than 3″. Such large area requires deposition of finger patterns over the top conducting layer of the solar cell, such as the transparent layer  14  in  FIG. 1 . 
         [0007]    The finger patterns comprising at least one busbar and multiple fingers connecting to the busbar are generally formed by screen-printing a conductive ink typically a silver-based ink followed by a curing step to get rid of the solvent and to conjoin the silver particles together to the optimal packing density. The screen-printing technique has limitations in terms of its ability to produce narrow fingers. Typically the line width of a screen printed finger needs to be greater than 100 micrometers (μm) to obtain a continuous line. Such large finger widths dramatically increase the shadowing loss in solar cells. The curing temperature of the inks used for Si solar cell manufacturing is typically greater than 200° C. This temperature may need to be reduced to below 200° C. for thin film solar cells such as CIGS solar cells due to possible degradation problems with annealing at elevated temperatures. Although, at the present time many of the available inks require high temperatures to cure (such as greater than 200° C.) there are some low temperature curable inks available that typically cure at less than 150° C. Once cured, the inks themselves have conductivities that are 10-20 times the bulk material. For example a typical Ag-based ink when cured at it&#39;s appropriate curing temperature gives a bulk resistivity of about 20-40 micro-ohm-cm (μΩcm). Such high bulk resistivity causes high resistive loses along the fingers and the busbar during operation of the solar cell. A total power loss in a typical screen printed finger pattern comprising fingers and one or two busbars amounts to about 15-20% of the gross power generated by the solar cell. Out of this total, the finger resistive losses are typically 5-6%, the finger shadowing losses add up to about 6-7%, the busbar shadow losses amount to around 3% and the busbar resistive losses are typically less than 1% with an addition of a ribbon material, typically Sn-plated Cu, which is soldered on top of the busbar to enhance its effective conductivity. 
         [0008]    As can be seen from the brief discussion above there is a need to develop new approaches for the formation of low resistance finger patterns on solar cells, at the same time keeping the shadow losses to a minimum. 
       SUMMARY 
       [0009]    A method of forming metallic connector patterns for solar cells, whereby an embosser having raised features shaped in the form of the metallic connector pattern is used to attach a portion of a metallic foil to a transparent conductive layer formed on a top transparent surface of a solar cell structure. The raised surfaces of the embosser press the metallic foil portion against the transparent conductive layer. Heat and pressure directed to the metallic foil portion attaches this portion to the underlying transparent conductive layer, and then the rest of the metallic foil is removed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is the structure of a conventional Group IBIIIAVIA compound photovoltaic cell. 
           [0011]      FIG. 2A  shows a step of depositing a transparent conductive material in a patterned manner on the top surface of a device. 
           [0012]    FIGS.  2 B and  2 BB show a step in the process of forming a finger pattern in accordance with the present invention. 
           [0013]      FIG. 2C  shows the transfer of a finger pattern on a device structure. 
           [0014]      FIG. 2D  shows a device structure with a finger pattern formed in accordance with the present invention. 
           [0015]      FIG. 2E  shows a top view of the device structure of  FIG. 2D . 
           [0016]      FIG. 3A  shows a CIGS(S) solar cell structure in accordance with the present invention. 
           [0017]      FIG. 3B  shows another exemplary solar cell structure fabricated in accordance with the present invention. 
           [0018]      FIG. 3C  is a solar cell structure where the active region of the cell is protected by a dense, transparent and high resistance inorganic layer and the conductivity is provided by a transparent conductive material. 
           [0019]      FIG. 4  shows a roll to roll method for transferring finger patterns on solar cell structures. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    In one embodiment, the present invention forms a highly conductive metallic foil finger pattern on a solar cell structure without causing excessive shadow loss. This is achieved by transferring a highly conductive metal foil on the surface of the solar cell in the form of a finger pattern and employing a transparent conductive layer which has adhesive characteristics to attach the finger pattern on the solar cell surface. The transparent conductive layer is substantially transparent in a wavelength range of 0.45-1.2 micrometers, having an optical transmission of more than about 70%. The method will now be discussed by describing a method of forming a finger pattern or grid pattern on the device  10  which is shown in detail in  FIG. 1 . 
         [0021]    Referring to  FIG. 2A , the device  10  (details of the device are shown in  FIG. 1 ) on which the finger pattern will be formed may comprise a transparent material at its top surface  20 A. As described before in reference to  FIG. 1 , the top surface  20 A may comprise any of the commonly known materials that are deposited on p-type CIGS(S) layers or p-CIGS(S)/buffer layer structures to form solar cells. These transparent materials include, but are not limited to CdS, CdZnS, indium-tin-oxide (ITO), tin-oxide (TO), zinc-oxide (ZnO), indium-zinc-oxide (IZO), or the like. The buffer layers include, but are not limited to CdS, ZnS, CdZnS, ZnSe, (Ga,In)—(S,Se), In—S—O, or the like. 
         [0022]    In forming the finger pattern over the top surface  20 A of the device  10 , a transparent conductive material  21  is first deposited in a patterned manner onto the top surface  20 A. The pattern of the transparent conductive material  21  may substantially match the pattern of, at least, the fingers to be formed. Further, the pattern of the transparent conductive material  21  may preferably match the pattern of the busbar to be formed on the top surface  20 A. 
         [0023]    As shown in  FIG. 2B , an embosser  23  is brought in close proximity of the free surface  24  of the already deposited transparent conductive material  21  and a metallic foil  22  is placed between the free surface  24  and the embosser  23 . The embosser  23  has a pattern that is equivalent to the finger pattern to be formed. This pattern may be formed on the embosser using techniques such as masking and etching, dry etching etc. The embosser material is typically a thermally conductive material, which may be made of materials comprising nickel, magnesium, aluminum, copper, silicon, or the like. The embosser  23  may be pre-heated to a temperature which is typically less than 300° C., preferably in the range of 70-150° C. Referring to  FIGS. 2B and 2C , the embosser  23  may be moved towards the device  10 , pressing a portion of the metallic foil  22  against the free surface  24  of the transparent conductive material  21 . It should be noted that the portion of the metallic foil  22  pushed against the free surface  24  of the transparent conductive material  21  is in the form of the finger pattern to be formed. Combination of the heat and the pressing action applied by the embosser  23  cuts the portion of the metallic foil  22  and adheres it to the free surface  24 , thus forming a finger pattern  22 A on the free surface  24  as the embosser is raised away from the device  20  ( FIG. 2C ). The unused portion  22 B of the metallic foil may then be removed. It should be noted that the transparent conductive material may be deposited on the top surface  20 A using a wet deposition technique such as ink writing, screen printing, roll printing, gravure printing etc. and it may be partially cured or un-cured at the time the portion of the metallic foil  22  is pushed against its free surface  24  by the embosser  23 , which may be heated. This way adhesion of the finger pattern  22 A to the free surface  24  may be improved. 
         [0024]    The invention may also be practiced by providing an adhesive and conductive layer (not shown) on the bottom surface  100  of the metallic foil  22 . This adhesive and conductive layer may cover substantially the whole of the bottom surface  100  or may be patterned so that it is present under the portion of the metallic foil that will later be transferred onto the top surface  20 A. In this case there may not be a need for the formation of transparent conductive material  21  on the top surface  20 A, and the metallic foil  22  with the adhesive and conductive layer on its bottom surface  100  may be pushed directly on the top surface  20 A to form the finger pattern  22 A as depicted in FIG.  2 BB. The adhesive and conductive layer is preferably transparent to visible light but may also be opaque since its excess may remain attached to the unused portion  22 B of the metallic foil  22  when the embosser pulls away from the device. Since the width of the adhesive and conductive layer would be nearly equivalent to the width of the fingers thus formed, the adhesive and conductive layer would not cause any shadow losses in this case. 
         [0025]      FIG. 2D  shows a cross-sectional view of an exemplary solar cell structure  25  fabricated using one of the embodiments of the present invention. The solar cell structure  25  comprises a device  10 , such as a base/CIGS(S)/buffer layer/ZnO or base/p-CIGS(S)/n-CIGS(S)/ZnO stack with a top surface  20 A, the top surface being the surface through which the light  26  will enter the solar cell. The top surface  20 A comprises a conductive material such as doped ZnO, ITO, IZO or the like to carry the electricity generated by the solar cell to the locations where the finger pattern  22 A is formed. Typical CIGS(S) type solar cells use a intrinsic-ZnO(50-200 nm thick)/doped-ZnO (200-1000 nm thick) structure over a buffer layer (such as CdS) to carry the current to the finger pattern. In the solar cell structure  25  of  FIG. 2D , the width “W” of the transparent conductive material  21  may be greater than the width “m” of the fingers of the finger pattern  22 A. As described above, screen printed fingers have a typical width of 100-200 μm. In the solar cell structure  25  of  FIG. 2D , the width “m” of the fingers of the finger pattern  22 A may be less than 100 μm, preferably in the range of 10-50 μm. Although the width “W” of the transparent conductive material  21  may be large this does not cause shadowing loss in the device because the transparent conductive material  21  transmits light to the device  10  efficiently. Furthermore since the conductivity of the metallic foil is much higher (typically 10-20 times higher) than that of screen printed silver material, narrow fingers may carry much higher currents without electrical loss. 
         [0026]      FIG. 2E  is a top view of the exemplary solar cell structure  25  of  FIG. 2D . As can be seen from this figure the finger pattern  22 A comprises fingers  22 B and a busbar  22 C and the transparent conductive material  21  is deposited in a way that it lies under the whole finger pattern  22 A. Alternately, the transparent conductive material  21  may be formed under only the fingers  22 B, but not under the busbar  22 C. 
         [0027]    The metallic foil  22  needs to be soft enough to be cut by the embosser  23  and should have conductivity of about less than one tenth of the transparent conductive material  21  preferably less than 5 μΩcm. The material make up of the foil may include but is not limited to Al, Cu, Ag, Au, W, Ni, Mo and their combinations thereof The metal foil thickness may be less than 100 μm, preferably in the range of 0.1-40 μm and more preferably in the range of 1-20 μm. The lower thickness values and finger width values may give rise to current levels inside the fingers that are close to the electro-migration limits and hence may need to be avoided. The electro-migration stability of the material can also be improved by the addition of dopants including Cu, Si, Ge to increase grain size of the Al and offer grain boundary adhesion protection. 
         [0028]    The transparent conductive material  21  may consist of an organic base with conductive metal or metal oxide particles dispersed in it. The organic base may be epoxy, silicone, EVA, or other transparent materials that can stand the temperature requirements with minimal outgassing during the device encapsulation processes. Lower cure temperatures are preferred for example from room temperature to 150° C. range. Conductive particle materials include but not limited to, ITO, ZnO, SnO, ZnSnO, AlZnO, InZnO, CdSnO, GaZnO, carbon, carbon nanotubes, metallic nano rods etc. Preferably the opaque particles such as metallic particles are nano-structured to improve the conductivity maintaining a high open space between them and thus high transparency. This is accomplished since the particles crosslink to form closed loop structures with high conductivity while leaving open spaces that are transparent. Specifically in such cases a binder material is added on in a subsequent step to planarize the cross linked particles forming a level free surface  24  for subsequent processing and good adhesion to the metal foil  22 . The width “W” of the transparent conductive material  21  may range from 1 mm down to 50 μm, preferably in the range of 75-400 μm. The thickness of the transparent conductive material  21  may be in the range of 5-10000 nm, preferably 50-1000 nm. The transparent conductive material  21  may be printed in one step or several steps using slot die printing, screen printing, gravure printing, flexographic printing, spin coating or other liquid coating processes. 
         [0029]      FIG. 3A  shows a CIGS(S) solar cell  36  constructed in accordance with one embodiment of the present invention. The CIGS(S) solar cell comprises a substrate  30 , a contact layer  31 , a CIGS(S) absorber layer  32 , and an optional buffer layer  33 . A transparent layer  34 A is formed on the buffer layer  33 , the transparent layer comprising a high resistance layer  34  and a low resistance layer  35 . The transparent conductive material  21  and the finger pattern  22 A may be formed over the top surface  20 A as described before. The high resistance layer  34  may be an un-doped ZnO layer with a resistivity value in the range of 1-1000 ohm-com, and the low resistance layer may be a doped ZnO layer such as an Al or In doped ZnO layer. This high/low resistivity layer structure is widely used in CIGS type solar cells since it reduces shunting effects in the device. In the CIGS(S) solar cell  36  of  FIG. 3A , light generated current “I 1 ” flows horizontally through the low resistivity layer  35  to the finger pattern  22 A. The resistivity of the low resistance layer  35  may be in the range of about 4×10 −4 −4×10 −3  ohm-cm in a typical solar cell. The function of the transparent layer  34 A is to protect the active region  32 A of the CIGS(S) solar cell  36  from any impurities, moisture etc. that may originate from the environment or the transparent conductive material  21 , which may be a porous material. The transparent layer  34 A being an inorganic layer with near 100% density is a good barrier to provide such protection. 
         [0030]      FIG. 3B  shows yet another device structure  36 B. Various layers in the device structure  36 B of  FIG. 3B  are similar to those in  FIG. 3A  and are numbered accordingly, same number representing the same layer. The difference in  FIG. 3B  is the fact that the transparent conductive material  21  covers substantially the whole of the top surface  20 A. It should be noted that in the device structure  36 B shown in  FIG. 3B  the thickness of the low resistance layer  35  may be reduced compared to  FIG. 3A  since the generated current “I 2 ” may flow across the low resistance layer  35  as well as the transparent conductive material  21 . For example, the thickness of the low resistance layer in  FIG. 3A  may be in the range of 200-1000 nm, whereas this thickness may be reduced to the range of 50-200 nm in the device structure  36 B of  FIG. 3B . This may lower manufacturing cost of solar cells since inorganic low resistance layers are typically deposited using expensive, low throughput sputtering techniques. Reduction of thickness requirement increases throughput of the sputtering technique and lowers cost. 
         [0031]    In the cell  36 C of  FIG. 3C  the high resistance layer  34  is used as a protective layer for the active region  32 A of the cell and the transparent conductive material  21  is deposited over the high resistance layer  34 . The current “I 3 ” in this case flows mainly across the transparent conductive material  21  to reach the finger pattern  22 A. 
         [0032]      FIG. 4  shows a schematic of a roll-to-roll manufacturing technique using the embossing process described above using an embosser  23 . The embosser  23  may be heated and the device  10  may be moved under the embosser  23  in a first direction while the embosser  23  also rotates. This way transfer of a finger pattern  22 A over the device  10  is achieved. 
         [0033]    Although the present invention is described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.