Abstract:
A method is disclosed for simultaneously forming the reflector of a photovoltaic concentrator and the electrical connections between a plurality of photovoltaic cells. In some embodiments a method for producing a photovoltaic device is disclosed using triangular prisms to concentrate light onto silicon cells, thereby reducing the amount of photovoltaic silicon required for generation of electrical power from sunlight without reducing the amount of light accepted by the device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/660,381, filed Mar. 10 th , 2005. This application is a divisional application of U.S. patent application Ser. No. 11/372,769 filed on Mar. 10 th , 2006 asserting claims drawn to the previously nonelected invention from the reply filed on Jan. 19, 2009. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to the assembly of photovoltaic cells into a photovoltaic device, more specifically, forming desired electrical connections between photovoltaic cells in a photovoltaic device. 
       BACKGROUND 
       [0003]    Photovoltaic (PV) devices convert sunlight into electricity. In their most common use they are mounted on the roofs of buildings to generate electrical power for use within that building. Though these devices are simple to use, and highly reliable, their widespread use has been hindered by their cost. 
         [0004]    Photovoltaic (PV) devices are made up of PV cells that are electrically connected so that the device produces a convenient amount of power at a desired voltage. For instance, a typical Silicon PV cell generates power optimally at approximately 0.5V, but it may be more convenient to operate a system at 18V; in this case a PV device with 36 silicon cells electrically connected in series can be used to obtain the desired voltage. The device also serves to support and protect the silicon cells which are fragile and can degrade when exposed to moisture. 
         [0005]    Traditionally, the electrical connection between PV cells in a device is formed by electrically conductive wires that are soldered to each cell. The process of soldering the cells to the wires is known as cell stringing, and is a costly manufacturing step. 
         [0006]    Typically, each PV cell has two electrical connections. One on the front surface, and one on the back surface. Recently, however, several manufacturers have begun producing silicon PV cells with both electrical contacts on the back of the cell. The back contact cells generally convert light to electricity more efficiently than more traditional cells. This is because no conductors need be present on the front surface that would block light from reaching that surface, and because the internal cell structure can be modified to improve the efficiency of the photo generating process. 
         [0007]    Referring to the publication,  Simplified Module Assembly Using Back - Contact Crystalline - Silicon Solar Cells , by James M. Gee, Stephen E. Garrett and William P. Morgan, and presented at the 26th IEEE Photovoltaic Specialists Conference on Sep. 29-Oct. 3, 1997, in Anaheim, Calif., the publication proposed the technique of using a printed circuit as an interconnect to eliminate the need for stringing back contacted cells to create a module or device. However this method carries with it additional cost. 
         [0008]    Another approach to cost reduction of PV devices is the use of optical components to concentrate light onto the cell. Using this technique less cell area is required to generate a specific amount of energy in a device. PV Cells are the largest single component of cost in a PV device, so reducing the need for these cells contributes to cost savings. PV devices have been developed and previously disclosed that use a rear reflector to concentrate light onto PV cells. 
         [0009]    It would be desirable to at least partially address some or all of the concerns referred to herein to produce a more cost-effective PV device. 
       SUMMARY 
       [0010]    Many of the limitations described above are overcome in accordance with preferred embodiments of the present invention. Some preferred embodiments of the present invention combine the creation of a rear reflector in a concentrating PV device with the electrical connections between the various cells in the PV device to reduce the number of manufacturing steps and amount of materials required for producing the PV device. A variety of ways to form the electrical connections are described and claimed herein. In some preferred embodiments a masking layer is used to form a pattern in a conductive layer deposited onto the photovoltaic module, including gaps in the conductive layer, as is described in detail herein. Some preferred embodiments of the present invention include a parallel array of triangle prism concentrators (“triangular prism concentrator array”) optically coupled to photovoltaic cells, with the photovoltaic cells electrically connected to produce a useful voltage at the device&#39;s electrical terminals. 
     
    
     
       DRAWINGS 
       Drawing Figures 
         [0011]      FIG. 1  is a perspective view of a triangular prism concentrator photovoltaic device; 
           [0012]      FIG. 2  is a perspective view of a triangular prism concentrator array; 
           [0013]      FIG. 3  is a detailed perspective view of the triangular prism concentrator array; 
           [0014]      FIG. 4  is a detailed side-view of a triangular prism device; 
           [0015]      FIG. 5  is a detailed side-view of a triangular prism device showing masking and metal deposition; 
           [0016]      FIG. 6  is an isometric perspective view of a mask layer associated with a triangular prism concentrator array; 
           [0017]      FIG. 7  is an isometric perspective view of a mask layer overlaying a triangular prism concentrator array prior to reflector deposition; and 
           [0018]      FIG. 8  is an isometric perspective view of a finished triangular prism concentrator array with a mask layer removed. 
       
    
    
     REFERENCE NUMERALS IN DRAWINGS 
       [0000]    
       
           100  Triangular prism concentrator array photovoltaic device 
           110  Front glass 
           120  PV cells 
           130  Reflectors 
           140  Module frame 
           210  Flat concentrator front surface 
           220  Multiple triangular prisms on concentrator back surface 
           310  First side of each triangular prism 
           320  Second side of each triangular prism 
           330  Third side of each triangular prism 
           350  Optical coupling gel 
           360  Electrical interconnect means 
           370  Encapsulant film 
           410  Back positive terminal 
           420  Back negative terminal 
           510  Masking material layer 
           520  Reflective and conductive material source 
       
     
       DETAILED DESCRIPTION 
       [0036]      FIG. 1  shows a triangular prism concentrator (TPC) array photovoltaic device  100 . A description of the physical relationships between various components of the device  100  is included here in  FIGS. 1-8  to aid in the understanding of the device  100  before describing in further detail an apparatus and method for electrically connecting cells in a photovoltaic (PV) device.  FIGS. 2 ,  3  and  4  break out and enlarge components of device  100 . A variety of methods for forming a useful, patterned electrically conductive layer to electrically connect photovoltaic cells to for the photovoltaic device are described. 
         [0037]    One embodiment of the photovoltaic device of the present invention is illustrated in  FIG. 1 .  FIG. 1  shows a triangular prism concentrator (TPC) array photovoltaic device  100 . A brief description of the physical relationships between various components of the device  100  is included here to aid in the understanding of the device  100  before being described in greater detail. The description also references  FIGS. 2 ,  3  and  4  which break out and enlarge components of device  100  illustrated in  FIG. 1 . The device  100  is made up of a front glass  110  with a flat front surface  210  and a back surface formed to create multiple triangular prisms  220 . The flat front surface  210  acts as a second side of each triangular prism  320 , as is described in detail below. Photovoltaic cells  120  are arrayed along a first side  310  of each of the prisms of the front glass  110 . A second side  320  of each of the triangular prisms  220  is formed by the flat front surface  210  of the front glass  110 . A reflective surface (Reflectors)  130  is added to a third side  330  of each triangular prism  220 . The reflectors  130  may be formed by coating the third side  330  of each triangular prism  220  with a reflective material. A rigid frame  140  surrounds the device providing mechanical stiffness and offering a surface for bolting to rails mounted on a roof. 
         [0038]    In some preferred embodiments, the front glass  110  is a molded or extruded clear material having an index of refraction greater than one and preferably between 1.48 and 1.52. In some preferred embodiments the front glass  110  is made of UV-enhanced polymethylmethacrylate Acrylic (PMMA). In some embodiments, the PMMA used in the front glass  110  is Atoglas VH Plexiglas produced by Atofina Chemicals, Inc., Philadelphia, Pa. However, in other embodiments the front glass  110  can be fabricated from materials such as glass or polycarbonate plastic, which are substituted for PMMA. 
         [0039]    In some preferred embodiments the third side  330  of each prism  220  is coated with aluminum deposited by vacuum metallization to achieve a reflectance on the order of 95% to form the reflectors  130 . However, the reflectors  130  may be made of any materials that can be formed into this shape and made to be highly reflective and conductive such as other metals, etc. 
         [0040]      FIG. 3  is a detailed perspective view of the triangular prism concentrator array showing additional details of the prism assembly  100 . An optical coupling gel  350  is used. The optical coupling gel  350  is a thixotropic gel with an index of refraction approximately equal to that of the material comprising front glass  110 . The optical coupling gel  350  is sandwiched between photovoltaic cell  120  and the first side of each triangular prism  220  of the front glass  110 . The optical coupling gel  350  is used in part as an adhesive to hold PV cell  120  in place, as well as an optical coupler, thereby eliminating any air gaps between PV cell  120  and the first side of each triangular prism  310  of the front glass  110 . In some preferred embodiments the optical coupling gel is Lightspan SL-1246 optical coupling gel (thixotropic) from Lightspan, LLC, 14 Kendrick Road, Unit #2, Wareham, Mass. In other embodiments, Sylgard 184 Silicone rubber from The Dow Chemical Company, 901 Loveridge Road, Pittsburg, California or the Nye Optical OCK451 curable adhesive from Nye Optical Company, 10309 Centinella Drive, La Mesa, Calif., can be used as the optical coupling gel  350 . In other preferred embodiments the optical coupling gel can be replaced by ethelyne vinyl acetate (EVA) which is available from multiple vendors. 
         [0041]    The PV cells  120  are electrically connected to each other by electrical interconnection means  360 . In preferred embodiments the PV cells  120  have two electrical connections on their back surface (facing away from front glass  110 ). 
         [0042]    The entire back of device  100  is sealed with an encapsulant film  370 . In some embodiments this encapsulant film is a polymer sheet like EVA, ETFE, or Tedlar™, in other embodiments encapsulant film  370  may be applied in vapor or liquid form and may be either a polymer, epoxy, glass, or silicon nitride, or any other material capable of sealing out moisture, withstanding temperatures of approximately 50 degrees Celsius and protecting the back of device  100  from abrasions. 
         [0043]    In  FIG. 4 , as described above, the device  100  includes the front glass  110  with the flat front surface  210 ,  320  and back surfaces  310  and  330  formed to create multiple triangular prisms  220 . Photovoltaic cells  120  are arrayed along the first side  310  of each of the prisms  220 . The thixotropic clear gel  350  fills the space between the cell  120  and the prism  220 . 
         [0044]    The third side  330  of each of the triangular prisms  220  is coated with a reflective and conductive film to form reflector  130 , as described herein. This film is both reflective and electrically conductive and extends to contact a back positive terminal  410  of each PV cell  120  to a back negative terminal  420  of the adjacent PV cell  120  forming an in-series electrical connection between the PV cells  120  to create the desired output voltage for the device  100 , e.g., 18 volts. Back positive terminal  410  of each cell is separated from back negative terminal  420  of the same cell by a gap in the electrically conductive layer  430 . 
         [0045]    Turning to  FIG. 5 , in some embodiments, the PV cells  120 , gel  350 , and front glass  110  are first assembled together prior to the creation of reflector  130 . In some preferred embodiments a masking material layer  510  is then placed so as to cover the space between the electrical contacts on the back of each of the PV cells  120  to prevent undesirable electrical connections being created in the next step, creating the gap in the electrically conductive layer  430 . In the next step, the reflector  130 , which is both reflective and electrically conductive, is then deposited on the entire back side of the assembly from a reflective material source  520 . In some preferred embodiments the reflector is made primarily of aluminum. In some other preferred embodiments the reflector is made primarily of silver. Then the mask and overlying portions of the reflector  130  are removed, leaving both a reflective layer  130  and the desired electrical connections between each of the PV cells  120 . In some alternative embodiments, the reflective and conductive material forming the reflective layer  130  is deposited first, then a protective positive masking layer  510  is deposited over the reflective layer  130 . Finally those portions of the reflective layer  130  that are unprotected by the masking layer are etched away with chemicals, plasma or other known removing means to break undesirable electrical connections such as those between the back positive terminal  410  and back negative terminal  420  initially formed when the reflective layer was deposited. In some alternative embodiments the masking layer is also removed before the PV device  100  is complete. In all cases, forming the final reflective and conductive surfaces  130  are achieved by processes well known in the relevant arts. 
         [0046]    In other alternative embodiments, the reflective and conductive material forming the reflective layer  130  is deposited in the desired pattern by directly writing or applying the reflective layer  130  in the desired pattern. In some embodiments this is accomplished by ink jet-like, electrostaticly-controlled, technology for depositing materials onto a surface, in this case, the PV cells  120 . 
         [0047]    Turning to  FIGS. 6-8 , in some preferred embodiments a process for forming the finished reflective and conductive layer  130  is shown in greater detail. In  FIG. 6 , the masking layer  510  is shown for use with the triangular prism concentrator array photovoltaic device  100 . The masking layer  510  is formed by means, and made of materials, well known in the relevant arts. For example, the mask layer may be made from any suitable plastic. The masking layer is formed and placed onto the back surface of the PV array  100 , closest to the first ( 310 ) and third ( 330 ) sides and furthest from the second ( 320 ) sides. In some preferred embodiments employing a liftoff method, the masking layer is what ultimately prevents the reflective and conductive coating in the next step from becoming attached to certain portions of the PV array  100  where improper electrical connections would otherwise form. In this sense the mask can be considered a negative mask because the reflective and conductive layer  130  is not deposited between the PV array  130  and the masking layer  510 . In other embodiments a positive mask may be used as is known in the relevant arts. 
         [0048]    In  FIG. 7 , the masking layer  510  is shown placed onto the PV cells  120 , the PV cells  120  are attached to the multiple triangular prisms on concentrator back surface  220 . With the masking layer  510  in place, in some preferred embodiments the reflective and conductive layer  130  is then deposited onto the PV array  100  and mask  510 . The reflective and conductive layer  130  is preferably made of aluminum or silver. While aluminum or silver are preferred materials, it is envisioned that the reflective layer can also be made of many other metals, combinations of metals, or any materials that are or can be made reflective, conductive and can withstand the operating temperatures of the PV device  100 , such as −20 to 100 degrees Centigrade. 
         [0049]    The reflective and conductive layer  130  can be deposited onto the PV array  100  by a variety of methods known in the relevant arts. In some preferred embodiments the deposition is performed by the process of vapor deposition in which the assembly is placed in a vacuum chamber and an aluminum or silver filament  520  is heated to vaporize the aluminum or silver which then coats all exposed surfaces that are not masked. In some alternative embodiments the reflective and conductive layer  130  is deposited by sputtering, electroplating, electroless chemical plating or spray coating. The present invention is not limited to any particular method creating the reflective and conductive layer  130  and other known methods for depositing a thin layer of reflective and conductive material may be used as well. 
         [0050]    Turning to  FIG. 8 , following the metal deposition step, the masking material  510  is removed leaving the electrically conductive and optically reflective layer  130  deposited on the desired portions of multiple triangular prisms on concentrator back surface  220 , more specifically, the first side  310  and third side  330 , in a pattern to form a series electrical connection between the PV cells  120  of the device  100 . Other patterns corresponding to particular electrical connections, and hence particular voltages, can also be used. An electrically insulating backcoating is then applied to the surfaces previously covered by the mask  510  and reflective and conductive layer  130 . In some preferred embodiments the backcoating may be EVA in a liquid two-part catalytic solution which is allowed to cure or in sheet form which is laminated to the PV array using a standard vacuum laminating process. Other known insulating coatings and other known techniques for applying all these coatings are envisioned by the present invention which is not limited to any particular coating or method of application. 
         [0051]    It is understood that the forms of the invention shown and described in the detailed description and the drawings are to be taken merely as examples. It is intended that the following claims be interpreted broadly to embrace all the variations of the example embodiments disclosed herein. Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.