Abstract:
The present inventions provide structures and methods for manufacturing high electrical conductivity grid patterns having minimum shadowing effect on the illuminated side of the solar cells. In a particular aspect, a width of an effective channel region is greater than a spacing that exists between conductive elements in adjacent grid patterns that exist along a lengthwise direction of a continuous workpiece.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to U.S. Provisional Application No: 61/169673 filed Apr. 15, 2009 entitled “Reel to Reel Plating of Conductive Grids for Flexible Thin Film Solar Cells”, the entirety of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Inventions 
         [0003]    The present inventions generally relate to solar cell fabrication and, more particularly, to fabrication of flexible thin film solar cells. 
         [0004]    2. Description of the Related Art 
         [0005]    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. 
         [0006]    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 CuIn 1−x Ga x  (S y Se 1−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%. It should be noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se) 2  means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1. 
         [0007]    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. 1A . A photovoltaic cell  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. An 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 contact layer, which is previously deposited on the substrate  11  and which acts as the electrical contact to the device. The substrate  11  and the conductive layer  13  form a base  20  on which the absorber film  12  is formed. Various conductive layers comprising Mo, Ta, W, Ti, and their nitrides have been used in the solar cell structure of  FIG. 1A . If the substrate itself is a properly selected conductive material, it is possible not to use the conductive layer  13 , since the substrate  11  may then be used as the ohmic contact to the device. After the absorber film  12  is grown, a transparent conductive layer  14  such as a CdS, ZnO, CdS/ZnO or CdS/ZnO/ITO stack is formed on the absorber film  12 . Radiation  15  enters the device through the transparent conductive layer  14 . As shown in  FIG. 1B  in top view, metallic grids  30  may also be deposited over top surface  16  of the transparent layer  14  to reduce the effective series resistance of the device. The top surface  16  forms the illuminated surface of the solar cell  10 . The preferred electrical type of the absorber film  12  is p-type, and the preferred electrical type of the transparent conductive 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. 1A  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. 
         [0008]    If the substrate  11  of the CIGS(S) type cell shown in  FIG. 1A  is a metallic foil, then under illumination, a positive voltage develops on the substrate  11  with respect to the transparent layer  14 . In other words, an electrical wire (not shown) that may be attached to the substrate  11  would constitute the (+) terminal of the solar cell  10  and a lead (not shown) that may be connected to the metallic grid  30  would constitute the (−) terminal of the solar cell. 
         [0009]    After fabrication, individual solar cells are typically assembled into solar cell strings or circuits by interconnecting them in series electrically, i.e. by connecting the (+) terminal of one cell to the (−) terminal of a neighboring cell. This way the total voltage of the solar cell circuit is increased. The solar cell circuit is then laminated into a protective package to form a photovoltaic module. 
         [0010]    As shown in  FIG. 1B  the metallic grid  30  or the grid pattern is deposited on the illuminated side of the solar cell device and includes one or more busbars  32  and multiple fingers  34  to carry the current from various parts of the device to the busbars  32 . Busbars  32  and fingers  30  generally comprise metals with low electrical resistivity such as silver or silver alloys, which can be ink-deposited or screen printed over the illuminated surfaces using silver-based inks or pastes. 
         [0011]    Although the low electrical resistivity of such materials plays an important role in their choice, in operation, there is a trade off relationship between their size, i.e. height and width, and their electrical resistance, which critically depends on the cross sectional area of the fingers and the busbars. Since the fingers are spread over the illuminated surface, in order to reduce the shadowing effect caused by their presence on the illuminated surface, their width needs to be minimized while their height needs to be maximized to keep the cross sectional area high and therefore the resistance low. However, in ink deposition or screen printing approaches, when the width of the finger is reduced to minimize the shadowing loss, the height of the finger also gets reduced due to the nature of these processes and the nature of the inks and pastes used. Therefore, for narrow fingers the cross sectional area gets reduced and the resistance of the finger increases causing the overall efficiency of the solar cell to go down despite the fact that more light enters the device. It should be noted that resistivity and bulk resistivity mean the same in this application and they have the units of “ohm-cm”. Sheet resistance of a layer is defined as the resistivity of the material making up the layer divided by the thickness of the layer and has the units of “ohms per square”. The resistance of a conductive line, which has the units of “ohms” is equal to the resistivity of the material making up the line times the length divided by the cross sectional area of the line. 
         [0012]    From the foregoing, there is a need in the thin film solar cell industry for improved grid structures and manufacturing methods that allows fabrication of narrow fingers with low resistance so that the conversion efficiency of the solar cells may be improved. 
       SUMMARY 
       [0013]    The present inventions provide structures and methods for manufacturing high electrical conductivity grid patterns having minimum shadowing effect on the illuminated side of the solar cells. 
         [0014]    In a particular aspect, a width of an effective channel region is greater than a spacing that exists between conductive elements in a adjacent grid patterns that exist along a lengthwise direction of a continuous workpiece. 
         [0015]    In a preferred aspect there is described a method of roll to roll manufacturing low electrical resistivity conductive grids having reduced shading effect for solar cells, comprising: providing a flexible continuous workpiece, the flexible continuous workpiece comprising a continuous flexible substrate, a bottom contact layer disposed atop the continuous flexible substrate, an absorber layer disposed atop the bottom contact layer, a transparent conductive layer disposed atop the absorber layer, and a first conductive film having a first resistivity disposed atop predetermined areas of a top surface of the transparent conductive layer and in electrical communication with the transparent conductive layer to form a raised grid pattern along a length of the flexible continuous workpiece, wherein the raised grid pattern includes a plurality of adjacent grids, with each grid having a predetermined grid width, a predetermined grid length, and a predetermined spacing between adjacent grids along the length direction of the flexible continuous workpiece, wherein a sheet resistance of the first conductive film is less than the sheet resistance of the transparent conductive layer, and wherein the top surface of the transparent conductive layer and the raised grid pattern disposed thereon form a front surface of the flexible continuous workpiece; applying an electrodeposition solution onto an effective plating region established on a portion of the front surface, including a part of the first conductive film, and onto an anode placed across from the portion of the front surface, the effective plating region having a length that is substantially the same as a width of the workpiece and a predetermined width that is at least longer than the predetermined spacing between adjacent grids; applying a voltage between the anode and the part of the first conductive film; selectively electrodepositing a conductive material from the electrodeposition solution onto the first conductive film and not the transparent conductive layer to form a second conductive film having a second resistivity atop the first conductive film, thereby forming the low electrical resistivity conductive grids having reduced shading effect, wherein the first resistivity is greater than the second resistivity; and moving the front surface, including the part of the first conductive film, through the effective plating region, during the steps of applying the electrodeposition solution, applying the voltage, and selectively electrodepositing. 
         [0016]    These and other aspects and advantages are described further herein 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1A  is a side schematic view of a solar cell of the prior art; 
           [0018]      FIG. 1B  is a top schematic view of the solar cell with a conductive grid over the top surface; 
           [0019]      FIG. 2  is a perspective depiction of a reel to reel electroplating system processing a workpiece to form a raised conductive grid according to a preferred embodiment; 
           [0020]      FIG. 3  is a schematic side view of a portion of a solar cell structure including a raised conductive grid formed on the transparent conductive layer; 
           [0021]      FIG. 4  is a schematic view of a top portion of the workpiece; and 
           [0022]      FIG. 5A  is schematic side view of an electroplating apparatus of the electroplating system; 
           [0023]      FIG. 5B  is schematic frontal view of the electroplating apparatus; and 
           [0024]      FIG. 5C  is schematic top view of the electroplating apparatus. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    Described herein are methods and apparatus to form low electrical resistance grid patterns over illuminated side of photovoltaic cells or solar cells. In one embodiment, initially a conductive grid pattern is formed, preferably by a screen printing or ink deposition technique, over a transparent conductive layer of a solar cell structure. In the following step, a conductive material is selectively electroplated over the conductive grid pattern using the electroplating apparatus. The electroplated conductive material increases the height of the conductive grid pattern and reduces its electrical resistance. It should be noted that the resistivity of an electroplated conductor such as electroplated Cu or Ag is lower than the resistivity of screen printed or ink deposited conductors such as Ag pastes. 
         [0026]      FIG. 2  shows a depiction of a roll to roll or reel to reel electrodeposition system  100  to selectively deposit a conductor onto a first conductive film  102  shaped as a plurality of grid patterns formed on a front surface  104 A at a front side  101 A of a workpiece  105 , with only those components necessary of this description to be understood illustrated, and it being understood that the actual roll to roll or reel to reel electrodeposition system  100  will have additional components therein. The conductor material may include silver or silver alloy or another low electrical resistance material. Each grid pattern may form a top terminal of a solar cell after the electroplating step and after cutting individual solar cells out of the workpiece. The first conductive film includes a conductive metal such as silver or a silver alloy or compound which may be deposited by techniques such as screen printing and ink jet printing. During the process, the work piece  105  is advanced from a supply spool  106 A in a process direction ‘P’, passed through a deposition unit  108  and wrapped around a receiving spool  106 B. The conductor is electrodeposited on the first conductive film  102  as the workpiece  105  is passed through the deposition unit  108 . Both the electrodeposited conductive material and the first conductive film  102  underneath, form a raised conductive film  110  having the shape of the grid patterns, which will be called final grid patterns hereinafter. During the process a back side  101 B of the workpiece  105  of the workpiece is supported by various support means such as support plates or rollers. 
         [0027]      FIG. 3  shows a detailed cross sectional view of an exemplary portion of the workpiece  105  ( FIG. 2 ) after the electrodeposition process. As shown in  FIG. 3 , the conductor deposited by the electrodeposition process forms a second conductive film  103  on the first conductive film  102 . Therefore, the raised conductive film  110  comprises the first conductive film  102  deposited on the front surface  104 A of the workpiece  105  and the second conductive film  103  selectively deposited on the first conductive film  102 . As mentioned above, each grid pattern on the front side  101 A of the workpiece  105  forms a top terminal for the future solar cells. Accordingly, the layers under each grid pattern form the structural components of the future solar cells as well. In this respect, the front surface  104 A includes the surface of a transparent conductive layer  112 , such as a buffer-layer/TCO stack, formed on an absorber layer  114  which may be a Group IBIIIAVIA absorber layer such as a CIGS absorber layer. TCO stands for transparent conductive oxide such as a ZnO layer, an indium tin oxide (ITO) layer or a stack comprising both ZnO and ITO. An exemplary buffer layer may be a (Cd, Zn)S layer. The absorber layer  114  is formed on a base  115  including a flexible substrate  118  and contact layer  116  formed on the flexible substrate  118 . A preferred flexible substrate material may be a metallic material such as stainless steel, aluminum (Al) or the like. An exemplary contact layer material may comprise molybdenum (Mo). 
         [0028]      FIG. 4  shows a portion of the front side  101 A of the workpiece  105  during an instant of the electroplating process. The portion of the front side  101 A includes the grid patterns of the first conductive film  102  and a final grid pattern of the raised conductive film  110  located at both sides of an effective plating region  120  in which the workpiece  105  is advanced so that the grid patterns of first conductive film  102  are selectively electroplated with the conductor to form the grid patterns of raised conductive film  110  or the final grid patterns. As will be described below the effective plating region  120  is an area that an electrodeposition device (see  FIGS. 5A-5C ) in a preferred embodiment can deposit the conductor on the grid patterns of the first conductive film  102  as they moved through the effective area  120 , thus forming the final grid patterns. 
         [0029]    As shown in  FIG. 4 , each grid pattern of the first conductive layer  102  includes busbars  122  and fingers  124 . After electrodepositing the conductor within the effective plating region  120 , the busbars  122  become raised busbars  123  and the fingers  124  become raised fingers  125 , both the raised busbars and fingers forming the final grid pattern. As will be appreciated, the raised busbars  123  and the raised fingers  125  comprise the first conductive film  102  and the second conductive film  103 . It is critical that, in order to electrodeposit the conductor onto the first conductive film  102 , the sheet resistance of the first conductive film  102  must be less than the sheet resistance of the front surface  104 A which is the surface of the transparent conductive layer  112 . The sheet resistance of the first conductive film  102  deposited in the form of a finger pattern on the transparent conductive layer  112  is less than about one tenth, preferably less than about one hundredth of the sheet resistance of the transparent conductive layer, which is typically in the range of 5-20 ohms per square. 
         [0030]    The width ‘W’ of the effective plating region is greater than the distance ‘d’ between the grid patterns of the first conductive film  102 . This way it is assured that a portion of the first conductive film  102  or a portion of the already plated grid pattern is always in the effective plating region  120 . Since the resistances of the first and second conductive films  102  and  103  are much lower than that of the transparent conductive layer  112 , the plating current preferentially passes through the fingers  124  and/or the raised fingers  125 , depositing material there rather than on the transparent conductive layer. It should be noted that the bulk resistivity of the Ag-based material forming the first conductive film  102  is in the range of 10-30 micro-ohm-cm, whereas the resistivity of materials forming the transparent conductive layer  112  ( FIG. 3 ) is in the range of 200-500 micro-ohm-cm. Furthermore the thickness of the first conductive film  102  is in the range of 1-10 microns, whereas the thickness of the transparent conductive layer  112  is typically in the range of 0.1-0.5 microns. As a result, the sheet resistance of the transparent conductive layer  112  is typically 100-5000 times larger than the sheet resistance of the first conductive layer. This differential facilitates the preferential plating on the first conductive layer  102  if there is, at all times, a section of the grid pattern within the effective plating region  120  and there is at least one electrical contact made to that grid pattern as will be further described. It should also be noted that the electroplated conductor or the second conductive film  103  typically has a very low resistivity in the range of 2-10 micro-ohm-cm, and therefore its thickness can be lower than the thickness of the first conductive film  102 . For example, the thickness of the second conductive film  103  may be in the range of 1-5 microns. 
         [0031]      FIGS. 5A ,  5 B and  5 C show in side, top and front view, respectively, an electrodeposition apparatus  130  through which the workpiece  105  is advanced in the process direction ‘P’, during the electrodeposition process. A support member  131 , such as a plate or a series of rollers, mechanically supports the workpiece portion that is being processed by the apparatus  130 . As shown in  FIG. 5A , the electrodeposition process applied by the apparatus  130  forms the raised fingers  125  from the fingers  124  by electrodepositing the conductive material onto the fingers  124 , and thereby increasing its thickness and conductivity, while the workpiece  105  is advanced. The electrodeposition apparatus  130  is located in the deposition unit  108  of the electrodeposition system  100  shown in  FIG. 2 . As its components shown in  FIGS. 5A and 5C , the electrodeposition apparatus  130  includes an electrodeposition cell  132 , surface contacts  134  and a power supply  138 . The electrodeposition cell  132  includes a substantially rectangular chamber  140  (see  FIG. 5C ) including long side walls  142 A and  142 B and short side walls  142 C and  142 D. The long side walls  142 A and  142 B extend along the width of the workpiece  105  and are separated by the distance ‘W’ which is also the width of effective plating region  120  shown in  FIG. 5C  and also in  FIG. 4  in this embodiment. Adjacent the lower ends of the long side walls  142 A and  142 B, an entrance opening  149 A and an exit opening  149 B are located respectively. The short side walls  142 C and  142 D which are parallel to the edges of the workpiece  105  complete the rectangular chamber  140  which retains an electrodeposition electrolyte  146  and an electrode  148  or anode immersed into the electrolyte  146 .  FIG. 5B  shows in front view the long side wall  142 A, the entrance opening  149 A and the position of the workpiece  105  entering through the entrance opening  149 A of the electrodeposition cell  132  as the workpiece is advanced in the process direction ‘P’. As shown in  FIGS. 5A and 5C , during the process the workpiece  105  enters the electrodeposition cell  132  through the entrance opening  149 A and leaves the electrodeposition cell through the exit opening  149 B while being supported by the support  131 . In a preferred embodiment, a sufficient amount of the electrolyte  146  is maintained in the chamber  140  by being continuously or periodically filled from the top of the chamber  140  at an overall rate that accounts for the removal of the electrolyte  146  through the entrance and exit slits  147 A and  149 A, although it will be understood that other arrangements could be used to maintain the environment necessary for the electrodeposition to occur. 
         [0032]    As the workpiece  105  is advanced through the electrodeposition cell  132 , the electrodeposition electrolyte  146  flows towards the front side  101 A of the workpiece  105 , contacts it and flows out of both the entrance opening  149 A and the exit opening  149 B. The electrolyte  146  is pumped into the chamber  140  from an electrolyte supply tank (not shown) and the used electrolyte leaves the cell through the entrance opening  149 A and the exit opening  149 B. This used electrolyte may be flowed into a recycling tank (not shown) to filter and replenish it. The replenished electrolyte is then redirected into the electrodeposition cell  132  or the electrolyte supply tank (not shown). In this embodiment, the side walls  142 A and  142 B of the rectangular chamber  140  and the edges of workpiece as they pass through the plating chamber define the effective plating region  120 . 
         [0033]    The surface contacts  134  may be made of conductive rollers or brushes which negatively polarize the surface  104 A and the first conductive film  102  which is shown as the finger  124  in  FIG. 5 . As shown in  FIGS. 5A and 5C , there may be at least two surface contacts positioned at both sides of the cell  132  and they may extend along the width of the workpiece  105 . If the surface contacts are made of conductive rollers, they roll on the surface as the workpiece travels. The anode electrode  148  and the surface contacts  134  are electrically connected to a positive and negative terminals of the power supply  130 , respectively. 
         [0034]    As can be seen in  FIGS. 5A and 5C , the effective plating region  120  defined by the distance ‘W’ between the long side walls  142 A,  142 B and the edges of the workpiece within the electrodeposition cell  132  and thus the electrodeposition occurs in this region. As shown, the distance W is kept greater than the distance ‘d’ between the grids of the first conductive layer so as to leave at least a portion of the finger  124  or the raised finger  125  within the effective plating region. Since the sheet resistance of the finger  124  is lower than the sheet resistance of the surface  104 A, the conductive material only deposits onto the fingers. Referring to  FIG. 4  and  FIGS. 5A and 5C , position of the surface contacts  134  is also predetermined depending on the length ‘L’ of the grid pattern so that at least one of the surface contacts  134  stays on the grid patterns. Further, the distance between the surface contacts should be less than or equal to the length of the fingers so that when a portion of a finger is in the effective plating region that particular finger is always contacted at least one surface contact outside the effective plating region. 
         [0035]    Therefore, in one embodiment a finger plating or grid plating method comprises the steps of: i) providing a continuous flexible workpiece with two edges and a width, the workpiece comprising multiple solar cell structures on its front surface, each solar cell structure having a conductive grid pattern with fingers which are parallel to the two edges of the workpiece, ii) applying an electrodeposition solution onto an effective plating region on the front surface of the workpiece and onto an anode placed across from the front surface of the workpiece, the effective plating region having a length that is substantially the same as the width of the workpiece and a predetermined width that is larger than a distance between the grid patterns of adjacent solar cell structures, iii) applying a voltage between the anode and two contacts that touch the front surface of the workpiece while moving the workpiece and the effective plating region with respect to each other and in a direction that is substantially parallel to the fingers of the grid patterns thus causing electrodeposition of a conductive material from the electrodeposition solution onto the conductive grid patterns of solar cell structures, wherein the two contacts are provided on two sides of the effective plating region and a distance between the two contacts is less than or equal to the total length of each of the fingers of the grid patterns. 
         [0036]    Although the present inventions are described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.