Patent Publication Number: US-2012031480-A1

Title: Current collection system for a photovoltaic cell

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional patent application No. 61/296,532 filed Jan. 20, 2010. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to thin-film photovoltaic (PV) devices, and more specifically to a PV cell with an improved current collection system. 
     BACKGROUND OF THE INVENTION 
     Thin-film PV cells can be produced by forming thin-film PV semiconductor materials, such as amorphous silicon (a-Si) based thin-film material, on low-cost substrates such as glass, stainless steel, etc. 
       FIG. 1  illustrates an a-Si based thin-film PV cell  10  known in the art made on a metal substrate. The PV cell  10  includes the metal substrate and a back reflection layer (back reflector, BR) which covers the metal substrate. Also included in the PV cell are an a-Si based semiconductor material  12  and a transparent conductive oxide (TCO) front contact layer  14  which are disposed atop the back reflector. Finally, the PV cell includes a current collection system  16  attached to the front contact layer  14 . 
     The current collection system  16  comprises a plurality of grid wires  18  which extend across the front contact layer  14 . Each grid wire  18  has a first end  20  and a second end  22  which terminate on busbars  24 . However, the connection area  23  between the grid wires  18  and the busbars  24  is small which may lead to a decrease in cell efficiency or cell failure. 
     Therefore, a need exists for a PV cell that has increases the connection area between the grid wire and the busbars. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of an embodiment of a PV cell known in the art; 
         FIG. 2  is a top view of an embodiment of a PV cell the present invention; 
         FIG. 3  is a top view of an embodiment of a PV cell the present invention; and 
         FIG. 4  is a top view of an embodiment of a PV cell the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     It is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly stated to the contrary. It should also be appreciated that the specific embodiments and processes illustrated in and described in the following specification are simply exemplary embodiments of the inventive concepts defined in the appended claims. For example, although the present invention will be described in connection with a-Si the present invention is not so limited. As such, the present invention may also be applied to PV cell having at least one single junction (SJ) of cadmium telluride (CdTe), amorphous silicon germanium (a-SiGe), crystalline silicon (c-Si), microcrystalline silicon (μc-Si), nanocrystalline silicon (nc-Si), CIS 2 , or CIGS. Additionally, although the present invention will be described with a substrate it should be appreciated that it may also be utilized in connection with a superstrate. 
       FIG. 2  illustrates a thin-film PV cell  26 , preferably an a-Si based, of the present invention. In an embodiment, the PV cell  26  comprises a metallic substrate (not depicted) such as a stainless steel foil, for an electric back contact substrate, a back reflector (not depicted), a layer of photovoltaic material  28  such as an a-Si based PV semiconductor material deposited over the back contact substrate, and a transparent and conductive front contact layer  30  such as a TCO front contact layer, and a current collection system  32 . 
     The current collection system  32  comprises a wire grid  34  and at least one busbar  36 , preferably a pair of busbars  36 . In an embodiment, the wire grid  34  comprises a conductive wire  38 . In this embodiment, the conductive wire  38  is attached to the front contact layer  30  and the at least one busbar  36 . The at least one busbar  36  is attached to a side portion  40  of the PV cell  26 . 
     The conductive wire  38  may have a lower resistance than the front contact layer  30 . In this embodiment, the conductive wire  38  may be metallic, for example silver, copper, or a combination thereof. In another embodiment, the conductive wire  38  comprises a metal core wire and a carbon coating covering the metal core wire. Examples of the conductive wire  38  of this embodiment can be found in U.S. Pat. Nos. 5,861,324 and 5,681,402, the disclosures of which are fully incorporated by reference. It should also be appreciated that other conductive wire materials and configurations are compatible with the present invention. 
     As shown in  FIG. 2 , the conductive wire  38  comprises a first end  42 , a second end  44 , and portions  46  which extend across the PV cell  26 . In an embodiment, the conductive wire  38  is unbroken over the cell. Thus, in contrast to the plurality of grid wires  18  depicted in  FIG. 1 , the conductive wire  38  of the present invention makes more than one pass across the front contact layer  30 . As such, a single conductive wire may provide all of the current collection for the PV cell front contact layer  30 . In an embodiment, each pass of the conductive wire  38  is equally spaced apart on the front contact layer  30 . 
     The present invention also provides a current collection system  32  with an improved connection area  47  between the wire grid  34  and the at least one busbar  36 . As shown in  FIGS. 2-4 , in an embodiment a portion of the conductive wire  38  comprises a loop portion  48 . In another embodiment, the conductive wire comprises a plurality of loop portions  48 . Each loop portion  48  is connected to two conductive wire portions  50  which extend across the PV cell  26 . Additionally, each loop portion  48  has an end portion  52  which allows the conductive wire  38  to double over on itself. As depicted in  FIG. 2 , in an embodiment the loop portions  48  may have a semicircular end portion  54 . As shown in  FIG. 3 , in another embodiment the loop portions  48  may have an end portion  52  with perpendicular portions  56 . As shown in  FIG. 4 , in yet another embodiment the loop portions  48  may have an end portion  52  having an angle  58  which is less than 90 degrees. In an alternative embodiment (not depicted), the loop portions  48  may have an end portion  52  having an angle which is greater than 90 degrees. It should also be noted that the embodiments of the end portions  52  described above may be combined and incorporated within a PV cell  26 . 
     Each loop portion  48  is attached to a busbar  36 . Thus, in an embodiment a first portion  60  of the conductive wire  38  forms a loop portion  62  adjacent the PV cell side portion  40 , a second portion  64  of the conductive wire  38  extends across the PV cell  26 , a third portion  66  of the conductive wire  38  forms a loop portion  68  adjacent the PV cell side portion  40 , and a fourth portion  70  of the conductive wire  38  extends across the PV cell  26 . In this embodiment, the conductive wire fourth portion  70  is in a parallel spaced apart relationship with the conductive wire second portion  64 . Therefore, in this embodiment, the conductive wire first portion  60  and third portion  66  are each attached to a busbar  36 . 
     The conductive wire loop portions  48  and the conductive wire&#39;s parallel spaced apart relationship may be formed by using a wiring frame. 
     The above detailed description of the present invention is given for explanatory purposes. Thus, it will be apparent to those skilled in the art that numerous changes and modifications can be made without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be construed in an illustrative and not a limitative sense. Therefore, specific dimensions, directions or other physical characteristics relating to the embodiments disclosed are not to be considered as limiting, unless the claims expressly state otherwise.