Abstract:
A simplified method and system for interconnecting solar cell arrays which does not utilize cause damage to the solar cells while at the same time minimizing process steps. In particular, in accordance with the present invention, interconnection between solar cell are made by way of a conductive epoxy, patterned on a substrate. The use of the epoxy eliminates the need for wire bonding as well as eliminates additional processing steps to interconnect the solar cell arrays.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to solar cells and more particularly and process for interconnecting solar cell arrays in which the interconnections are made by way of a conductive epoxy patterned on the substrate which eliminates the need for wire bonding directly to the solar cells in order to minimize damage to the solar cells and reduce the cost for forming solar cell arrays. 
     2. Description of the Prior Art 
     Solar cells are known to be used in various applications for converting light energy to electrical energy. In fact, solar cell arrays are known to be used as the primary power source in space craft applications due to their self-generating the power by charging from sunlight. However, due to the relatively low energy output of such solar cells, a plurality of solar cells are formed in an array which, in turn, are interconnected with a relatively large number of additional solar cell arrays to form a solar panel that is capable of generating in an electrical power, for example to power a spacecraft. 
     A typical solar cell array module is about 6″×12″×3 mils. Typical solar cell panels can be as large as 14 feet×42 feet. Thus, a significant number of solar array modules must be interconnected to form a solar cell panel. In particular, individual solar cells are connected in series to provide a desired output voltage. The series blocks of solar cells in turn are connected in parallel to provide sufficient current output for the particular application. 
     Solar cells (also known as photo-voltaic cells) are known to be formed from semiconductor processing techniques. Such solar cells are typically formed as thin films on a substrate, typically 3 to 250 mils in thickness. Electrical contact pads are formed on the solar cells in order to enable the cell to be interconnected with other cells to form a solar cell array. Examples of solar cells are disclosed in U.S. Pat. Nos. 5,246,506; 4,892,592; 4,348,254; 4,240,842; 4,392,010; 5,963,790; 5,922,142 and 5,928,437, all hereby incorporated by reference. 
     Various techniques are known for interconnecting the solar cells to form solar cell arrays. Examples of such techniques are disclosed in U.S. Pat. Nos. 5,006,179; 5,011,544; 5,961,737; 5,620,528 and 5,021,099. However, there are problems with such techniques for interconnecting solar cells. In particular, U.S. Pat. Nos. 5,006,179; 5,961,737 and 5,620,528 disclosed interconnecting solar cells to form solar cell arrays by wire bonding electrical conductors to the wire bond pads on the solar cells. Such wire bonding is known to raise the temperature of the wire bond pads to 200° C. or better which can cause damage to the solar cells. In addition, wire bonding techniques are known to result in short p-n junction of the solar cells due to the pressure from the wire bonder. Accordingly, alternate techniques for interconnecting solar cells have been developed. Examples of such alternate techniques are disclosed in U.S. Pat. Nos. 5,021,099 and 5,466,302, hereby incorporated by reference. 
     The &#39;099 patent discloses an interconnection system in which the solar cells are interconnected by way of an interconnect circuit formed on a flexible dielectric substrate, disposed on the back side of the solar cell. The interconnection circuit is adapted to be used with solar cells having front mounted wire bonding pads. The interconnection circuit is formed with a plurality of vertically extending figures which extend in the direction generally perpendicular to the plane of the solar cell. The extending figures are disposed adjacent the edges of the solar cell and are bent to make electrical contact with the wire bond pads on the top side of the solar cell. Such a system, however, involves rather complicated processing techniques and can result in non-uniform contact pressure between the contact fingers and the wire bond pads on the solar cell, thus degrading the power output of the solar cell. 
     The &#39;302 patent solves these problems by utilizing a electrically conductive paste in order to connect copper strips to the wire bond pads on the solar cell. Although the process disclosed in the &#39;302 patent eliminates problems associated with soldering and solar cell degradation, the process disclosed by the &#39;302 patent is relatively expensive and requires the formation of the copper strips for the interconnection. 
     As mentioned above, groups of solar cell arrays modules are interconnected to form solar panels. In order to avoid the interconnection problems discussed above, thermal compression bonding techniques have been developed for interconnecting solar cell arrays to form a solar cell panel. An example of such a technique is illustrated in FIGS. 1-5. Referring to FIGS. 1A and 1B, a conventional solar cell array is shown. In particular, a front or glass side  22  is illustrated in FIG. 1A, while a back or metal side  24  is illustrated in FIG.  1 B. As shown in FIGS. 2A and 2B, various electrical ribbon conductors, identified with the reference numerals  26 ,  28 ,  30  and  32 , are connected to the metal side  24  of the solar cell by conventional thermal compression bonding to form a solar cell assembly  34 . The solar cell assembly  34  is assembled to a substrate  36 , such as an aluminum hollow foil panel carrier, as illustrated in FIG. 3. A thermally conductive (but not electrically conductive) epoxy is patterned on the carrier  36  to form a number of interconnection pads  38 ,  40 ,  42 , and  44  on the substrate  36 . Subsequently, as illustrated in FIG. 4, the ribbon conductors  26 ,  28  and  30  on the solar cell assemblies  34  are aligned with the interconnection pads  38 ,  40 ,  42  and  44  on the carrier  36 . As shown in FIG. 5, the solar cell assembly  34  is disposed on the carrier  36  with the electrical ribbon conductors  26 - 32  aligned with the interconnection pads  38 - 44  and secured thereto by thermal compression by way of a wire bonder. Unfortunately the wire bonder is in contact with the solar cells  20  during this process. The pressure from the wire bonder is known to short out the solar cells. Thus there is a need for a simplified method for interconnecting solar cell arrays which does not damage the solar cells. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a simplified method and system for interconnecting solar cell arrays which does not utilize cause damage to the solar cells while at the same time minimizing process steps. In particular, in accordance with the present invention, interconnection between solar cell are made by way of a conductive and thermal epoxy (non-electrical epoxy) epoxy, patterned on a substrate. The use of the epoxy eliminates the need for wire bonding and isolates the individual cell from electrical shortage, as well as eliminates additional processing steps to interconnect the solar cell arrays. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     These and other advantages of the present invention will be readily understood with reference to the following specification and attached drawings wherein: 
     FIG. 1A is a plan view of the glass side of a conventional solar array. 
     FIG. 1B is a plan view of the opposing or metal side of the solar cell array illustrated in FIG.  1 A. 
     FIG. 2A illustrates a number of conventional ribbon conductors. 
     FIG. 2B illustrates the ribbon conductors illustrated in FIG. 2A attached to the metal side of the solar cell array illustrated in FIG. 1B forming a solar cell assembly. 
     FIG. 3 illustrates a conventional aluminum hollow foil panel carrier, with a thermal (non electrically conductive epoxy) applied thereto to form a plurality of interconnection pads. 
     FIG. 4 illustrates the solar cell assemblies illustrated on FIG. 2B aligned with the carrier illustrated in FIG.  3 . 
     FIG. 5 illustrates the assembly of the solar cell assemblies to the aluminum hollow foil panel carrier. 
     FIG. 6 the schematic diagram illustrating the interconnection of solar cell arrays. 
     FIGS. 7A-7H illustrate the process for interconnecting the solar cell arrays in accordance with the present invention. 
     FIGS. 8A-8H illustrate an alternate embodiment of the invention utilizing thick film screening on a printed circuit board for interconnection. 
     FIGS. 9A-9C illustrate on another alternate embodiment of the invention utilizing an etching printed circuit board for interconnection. 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to a relatively simple and effective method for interconnecting solar cell arrays which does not utilize wire bonding. By eliminating the need for wire bonding for interconnecting solar cell arrays, damage to solar cells as a result of a wire bonding process is virtually non-existent. In addition, interconnection of the solar cells is rather simpler and easier and involves less processing steps, for example, than known interconnection techniques, for example as disclosed in U.S. Pat. No. 5,466,302. By reducing the number of processing steps and materials required, the system and method for interconnecting the solar cells in accordance with the present invention is relatively easier to manufacture and thus less expensive. 
     Three embodiments of the invention are described and illustrated. One embodiment is illustrated in FIGS. 7A-7H which involves forming a thick film interconnection pattern on a substrate, such as an aluminum hollow foil panel carrier, and interconnecting the solar cell bond pads to the electrical interconnection pattern by way of an electrically conductive epoxy. An alternate embodiment of the invention is illustrated in FIGS. 8A-8H which involves forming an interconnection pattern on a printed circuit board by film screening techniques. Another alternate embodiment is illustrated in FIGS. 9A-9C involves etching the copper on a printed circuit board to form an interconnection pattern and interconnecting the solar cells to the interconnection pattern by way of a conductive epoxy. All three embodiments in accordance with the present invention enable interconnections between solar cell arrays to form a solar cell panel. 
     Referring to FIG. 6, an exemplary solar panel is illustrated and generally identified with the reference numeral  50 . The solar panel  50  is shown with an exemplary number of solar cell arrays  52 ,  54 ,  56 ,  58 ,  60  and  62 . Each solar cell array  52 ,  54 ,  56 ,  58 ,  60  and  62  is formed from an exemplary number of solar cells  64 ,  66 ,  68 ,  70 ,  72 ,  74 ,  76 ,  78  and  80 . Various techniques are known for interconnecting individual solar cells, for example, as disclosed in U.S. Pat. Nos. 5,011,544; 5,620,528; 5,961,737; 5,006,179; 5,021,099; 5,466,302 to form a solar cell array, for example, as disclosed in U.S. Pat. Nos. 4,892,592 and 5,928,437, all hereby incorporated by reference. In general, the individual solar cells  64 - 80  are connected in series to form the solar cell arrays  52 - 62 . External connections to the solar cell arrays  52 - 62  are by way of electrical contacts. In particular, each solar cell array  52 - 62 , is provided with a positive polarity contact, generally identified with the reference numeral  82 , and a negative polarity contact, generally identified with the reference numeral  84 . As shown in FIG. 6, multiple solar cell arrays  52 - 62  often need to be connected in series for high voltage applications. Thus, as shown, the solar cell pairs  52 / 58 ;  54 / 60  and  56 / 62  are shown schematically connected in the series, represented by the arrows  86 ,  88  and  90  to form banks of solar cell arrays.  92 ,  94  and  96 . In relatively high current applications, multiple solar cell array banks  92 ,  94  and  96  are connected in parallel. These parallel connections are represented by the arrows  98 ,  100 ,  102 ,  104 ,  106 ,  108  and  110 . 
     Referring to FIGS. 7A-7H, a process for interconnecting solar cell arrays in accordance with the present invention is illustrated. Initially, as illustrated in FIG. 7A, a carrier  112 , such as, an aluminum hollow foil panel carrier, is provided. The carrier  112  is coated with a thermally conductive non-electrically conductive epoxy  114 , such as Ablestik  84 - 3 , as shown in FIG.  7 B. Other thermally conductive epoxies are also suitable; however, the selected epoxy should have a thermal expansion characteristic similar to the thermal expansion characteristic of the glass on the solar cell. As shown in FIG. 7C, a thick film electrical interconnection pattern  116  is printed on top of the epoxy  114  by industrial standard thick film processing techniques. The interconnection pattern  116  is formed as generally shown in FIG.  7 C and includes a plurality of spaced apart interconnective rectangular conductors  118 . The areas outside of the rectangular conductive areas  118  define isolation areas while the area within the conductive pattern conductive rectangles  118  form conductive areas. After the interconnection pattern  116  is formed, a mask (not shown) is used to pattern the non-conductive areas. A non-electrically but thermally conductive epoxy  120 , for example, Loctite® 3536 or equivalent is injected in the isolation areas as generally shown in FIG.  7 D. Subsequently, a mask (not shown) is used to pattern a conductive areas within the conductive rectangles  118 . An electrically conductive epoxy  122 , for example Ablestik® 965-1L or equivalent is injected into the conductive areas as generally shown in FIG.  7 E. After the thermally conductive epoxy  120  and electrically conductive epoxy  122  are disposed in the isolation and conductive areas, a solar cell panel, generally identified with the reference numeral  24 , which consists of a plurality of solar cell arrays, is loaded into a vacuum disk, generally identified with the reference number  126 . An exemplary vacuum disk for receiving multiple solar cell panels is disclosed in U.S. patent application Ser. No. 09/316,991. The vacuum disk  126  is preferably adapted for multiple solar cell panels with mesa patterns for alignment and epoxy high control. As shown in FIG. 7F, the vacuum disk loaded with the solar cell panels is aligned with the carrier  112  and pressed against the carrier  112 . The epoxy is allowed to cure. As shown in step  7 G, the vacuum chuck is removed leaving a solar cell panel with a plurality of interconnected solar cell arrays. Lastly, as shown in FIG. 7H, a non-electrical epoxy, such as Loctite® 3565 or equivalent may be applied along the joint therebetween to secure the solar cell panels to the carrier  112 . 
     An alternative embodiment of the invention is illustrated in FIGS. 8A-8H. In this embodiment, a printed circuit board is used instead of a non-electrically conductive coating, for example, Rogers Duroid 6002 or equivalent on a hollow aluminum foil carrier as discussed above. In this embodiment, a solar cell  124 , for example, as illustrated in FIG. 8A, is adapted to be electrically interconnected with other solar cell arrays by way of interconnection PC board  126  illustrated in FIG.  8 B. The process for forming the solar cell interconnection PC board  126  is illustrated in FIGS. 8C-8F. FIGS. 8G-8H illustrate the principle of an interconnection PC board for interconnecting multiple solar cell arrays. 
     Referring to FIG. 8A, the solar cell array  124  is a conventional solar cell array, for example, as supplied by Solarex Corporation of Rockfield, Md. The interconnection PC board  126  is formed by known thick film screen printing techniques, as discussed above. In particular, referring to FIG. 8C, initially an electrically conductive epoxy, for example, Ablebond® 965-1L or 84-1LMI, is screen printed on the PC board to form a conductive pattern  128  as shown. Subsequently, an isolation area  130  is screen printed on top of the conductive pattern as generally shown in FIG.  8 D. As shown in FIG. 8E, the interconnection patterned formed with the conductive epoxy is formed with a number of rectangles, generally identified with the reference numeral  132  forming conductive areas. These conductive areas  132  are filled with the conductive epoxy as generally shown in FIG.  8 E. Subsequently, the solar cell  124  is placed on the PC board as shown in FIG.  8 F. As shown in FIG. 8F, the conductive pattern  128  facilitates connections between solar cell arrays without the need for wire bonding. 
     The process illustrated in FIGS. 8C-8F may be used to form a solar panel using an interconnection PC board  134 . In particular the interconnection PC board  134  is used to interconnect a plurality of solar cell arrays and facilitate connections of those arrays to other solar cell arrays. FIG. 8H illustrates an assembly of multiple solar cell arrays disposed on a PC board  134 . 
     Alternatively, as shown in FIGS. 9A-9C the copper in a PC board may be etched in lieu of thick film screening. With reference to FIG. 9A, a PC board  140  may be masked, by a mask (not shown) to form an interconnection pattern  142 , generally identified with the reference numeral  142 . The printed circuit board  140  is etched by known techniques to expose the copper and to form the conductive pattern  142  and in particular generally rectangular electrically conductive areas  144  and non-conductive areas  146 . The electrically conductive areas  144  may be filed with an electrically conductive epoxy, for example, Ablestik® 965-1L or equivalent as shown in FIG. 8B, while the non-conductive areas  146  are injected with a thermally conductive non electrically conductive epoxy, for example, Loctite® 3565 or equivalent as generally shown in FIG.  9 B. As shown in FIG. 9C, a solar cell array  148  aligned with the PC board  140  and placed thereon utilizing a vacuum disk as discussed above. 
     Obviously, many modifications and variations of the present intention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.