Patent Publication Number: US-2007095384-A1

Title: Photovoltaic modules and interconnect methodology for fabricating the same

Description:
BACKGROUND  
      The invention relates generally to solar cells, and more particularly but not exclusively to structures for interconnecting solar cells.  
      Solar cells, also referred to as photovoltaic cells, are well known devices for converting solar radiation to electrical energy. They may be fabricated on a semiconductor wafer using semiconductor processing technology. Solar cells generally include one or more photoactive materials sandwiched between two electrodes. A typical solar cell includes n-doped and p-doped regions fabricated on a silicon substrate. Solar radiation impinging on the solar cell creates electrons and holes that migrate to the p-doped and n-doped regions respectively, creating voltage differentials between the doped regions.  
      Individual solar cells generate only a small amount of power, usually much less power than is required by most applications. Desired voltage and current for practical applications is realized by interconnecting a plurality of solar cells in a series and parallel matrix. This matrix is generally referred to as a solar cell array, and can be used to generate electrical energy from solar radiation for a variety of applications. Currently, conventional crystalline silicon photovoltaic (PV) cells are interconnected using a process known as “stringing” to form a module. Stringing cells together in a series network generally involves connecting the front electrode of one cell to the back electrode of an adjacent cell via a conductive path, such as copper wire, extending from the front side of one cell to the back side of the adjacent cell. The strings of cells are placed in a laminate structure, and then the strings are soldered together, creating a single ended connection of one device to another in a series network. That is to say that an electrical path is provided from only one end of the cell to one end of the adjacent cell.  
      Current industry standards for fabricating solar cell arrays include stringing cells end to end and then forming a laminate consisting of front layer of a glass and backsheets containing materials such as TEDLAR® PVF (polyvinyl fluoride) films (available from E.I. du Pont de Nemours and Company). Such practices are entrenched in the industry because a large database of reliability data exists for such PV laminates. But problems with PV laminates include limited packing density, electrical resistive losses, and lack of automation in assembly that can result in damage and contamination. The strings of cells that are constructed during assembly are fragile, difficult to handle, and require manual operations to repair or rework. Accordingly, standard fabrication techniques lack the automation of the microelectronics electronic card assembly and test infrastructure and exhibit relatively poor yield, inexact placement of the cells, and difficult manual rework operations.  
      Accordingly, a technique is needed to address one or more of the foregoing problems in fabricating solar cell assemblies.  
     BRIEF DESCRIPTION  
      In accordance with one aspect of the invention, a solar cell array including multiple interconnected photovoltaic cells is provided. The photovoltaic cells are configured such that both the front and back contacts of each cell are accessible from the back side of the cell. The array includes multiple front side tabs that are electrically coupled to the front contacts to provide electrical paths from the front contacts to the back sides of the cell. The array also includes interconnect leads coupled from a respective tab on the back side of one photovoltaic cell to a back contact on the back side of another photovoltaic cell.  
      In accordance with another aspect of the present invention, a solar cell array of interconnected photovoltaic cells is provided including at least one double-ended current path on the front side and back side of each photovoltaic cell.  
      In accordance with another aspect of the present invention, a method of manufacturing a photovoltaic cell array is provided. The method includes providing multiple parallel current paths on a front side and a back side of a photovoltaic cell. It also comprises disposing the photovoltaic cells on a laminate. It further includes soldering multiple tabs on the back side of the photovoltaic cells and interconnecting the photovoltaic cells in series via interconnect leads. It further includes heating the photovoltaic cells such that the cells are adhered to the laminate. 
    
    
     DRAWINGS  
      These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:  
       FIG. 1  is an exploded view of a photovoltaic module that may be fabricated in accordance with embodiments of the present invention;  
       FIG. 2  is a cross sectional view of a conventional photovoltaic cell illustrating the front contacts on a front side and back contacts on a back side of the photovoltaic cell;  
       FIG. 3  is a cross-sectional view of an exemplary photovoltaic cell illustrating the adaptation of a front contact point to a back side of a photovoltaic cell in accordance with embodiments of the present invention;  
       FIG. 4  illustrates a portion of a photovoltaic cell array and a mechanism for providing multiple current paths to a back side and front side of a photovoltaic cell in accordance with embodiments of the present invention;  
       FIG. 5  is a plan view of the back side of a portion of a photovoltaic cell array illustrating a double-ended connection scheme achieved by point to point wiring in accordance with embodiments of the present invention;  
       FIG. 6  is a plan view of the back side of a portion of a photovoltaic cell array illustrating the interconnection scheme between photovoltaic cells comprising two front and back contact points in accordance with embodiments of the present invention;  
       FIG. 7  is a plan view of the back side of a portion of a photovoltaic cell array illustrating the interconnection scheme between photovoltaic cells comprising individual back contact cells that are connected to each other by preformed interconnect leads prior to assembly of the array; and  
       FIG. 8  is a flow chart representation of the steps involved in manufacturing a solar cell array in accordance with the double-ended interconnection scheme of  FIG. 4 , in accordance with embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION  
      As discussed in detail below, embodiments of the present invention provide a solar cell array comprising multiple photovoltaic cells, wherein each photovoltaic cell includes a front side and a back side. The solar cell array also includes multiple front contacts and back contacts on the front and back side respectively. One or more conductive tabs are electrically coupled to the front contacts and are configured to provide electrical paths from the front contacts to the back side of the photovoltaic cell. The solar cell array further includes multiple interconnect leads that connect tabs on the back side of each photovoltaic cell to at least one back contact on the back side of another photovoltaic cell. Other embodiments wherein at least two parallel current paths are provided at the back side and the front side of each photovoltaic cell and a method for manufacturing a photovoltaic cell array are also discussed.  
       FIG. 1  is an illustration of a photovoltaic (or solar) module  10  in accordance with an exemplary embodiment of the present invention. As will be appreciated, the terms “photovoltaic” and “solar” are used interchangeably throughout the application. Photovoltaic module  10  includes a solar cell array  12  that includes photovoltaic cells  14  which are laminated between lamination layers  18  and  20 . As a non-limiting example, lamination layers  18  and  20  may comprise sheets of ethylene vinyl acetate (EVA) material, the selection and use of which is known to those skilled in the art. The photovoltaic module  10  may also include outer protective layers  16  and  22 , as well. The outer protective layers  16  and  22  may be configured to protect the solar cell array  12  from environmental conditions such as moisture and humidity, as well as to provide structural durability and protection from certain mechanical forces. The protective layer  16  may comprise a transparent material, such as glass, and the protective layer  22  may comprise a backsheet material comprising one or more layers of polymeric materials, generally including a outermost layer of polyvinyl fluoride, such as TEDLAR® PVF, for example. The selection and use of these materials are known to those skilled in the art. During fabrication, the photovoltaic module  10  may be placed in a laminator such that the lamination layers  18  and  20  may be heated to hermetically seal the photovoltaic module  10 . During operation, the photovoltaic module  10  is oriented such that layer  16  faces the sun. Accordingly, in the present exemplary embodiment, the front side of each of the solar cells  14  of the solar cell array  12  is configured to receive sunlight and is oriented to face the protective layer  16 . The back side of each of the solar cells  14  of the solar cell array  12  is oriented to face the protective layer  22 .  
      An individual solar cell  14  with an interconnect system that is known in the art is shown in  FIG. 2 . The front side  26  of the cell  14  includes a pattern of conductive grid lines  27  that transmit electrical current from the photovoltaic conversion of photons to electrons. The grid lines  27  are electrically connected to a conductive tab  30  on the front side. The back side  28  of the cell  14  includes a uniform coating of conductive material  29  that distributes current from a conductive tab  34  across the back of the photovoltaic cell. Incident solar radiation is converted to electrical energy and results in the generation of current across the cell, as depicted by the current flow indicated by the arrows  31 . This wiring configuration is commonly used for interconnecting silicon solar cells, and is referred to as a single-ended series network. In practice, the cells may vary in size from less then 1 inch to as much as 8 inches or more. Depending on the size the cell, one (as shown in  FIG. 2 ) or more parallel pairs of conductive tabs  30  and  34  may be configured to provide adequate conductivity to collect the current efficiently.  
      As the current  31  is distributed across the front side  26  and back side  28  of the solar cell  14  by means of elements  27  and  29 , the current along the conductive tabs  30  and  34  linearly increases or decreases along their length. Whereas the current is varying, the cross-section and resistance per unit length of the conductive tabs  30  and  34  is typically constant, and greatest power loss per unit length occurs where the current is at a maximum. Thus, adopting a wiring configuration that can limit the maximum current would reduce power loss, and adopting a configuration that provides excess current capacity would increase shadowing as explained below.  
      In the configuration of  FIG. 2 , the conductive tab  30  and grid lines  27  have the effect of obscuring part of the cell from incident radiation and thus reducing its overall efficiency. It is therefore desirable to reduce the total area covered by the tabs  30  and  34 . Reducing the width to height ratio of the conductive tab  30 , also reduces the surface area covered by the conductive tab, but may have deleterious effects on the reliability of the assembly due to the difference in mechanical and thermal properties of conductive tab  30  and the solar cell  14 . Under thermal excursions typical of solar service conditions, these differences create thermal stresses that may initiate cracks or other damages to the cell. Accordingly, the width to height ratio cannot be altered without the consideration of the impact on thermal stresses.  
       FIG. 3  is a cross-sectional view of a photovoltaic cell  14  that may be: coupled to a single-sided contact device, in accordance with embodiments of the present invention. The photovoltaic cell  14  includes a front side  26  and a back side  28 . As will be appreciated, the front side  26  of the photovoltaic cell  14  includes a front side electrode on its surface (not shown). Similarly, the back side  28  of the photovoltaic cell  14  includes a back side electrode on its surface (not shown). To provide electrical contact from the front side electrode to the back side  28  of the photovoltaic cell  14 , a conductive front side tab  30  is disposed on the front side  26  of the photovoltaic cell  14 . The conductive front side tab  30  is electrically coupled to the front side electrode. The conductive front side tab  30  is sized such that the edges of the conductive tab  30  may be wrapped around the edges of the photovoltaic cell  14 . The edges of the conductive front side tab  30  are wrapped around such that they provide remote access to front contacts  32  on the back side  28  of the photovoltaic cell  14 . An insulator, such as tape  33 , may be employed to provide isolation and to act as a spacer between adjacent cells in the series network. Electrically conductive contacts  32  are connected to the front side conductive tab  30  of the photovoltaic cell, but are located physically on the back side of the photovoltaic cell, as illustrated in  FIG. 3 . The term “back contact configuration” thus refers to a photovoltaic cell configuration wherein its electrically conductive contacts are accessible from the back side as exemplified by contacts  32  and  35  in  FIG. 3 . In one embodiment of the present invention, a conventional silicon photovoltaic cell with front and rear contacts as depicted in  FIG. 2  can be restructured into back contact configuration by providing electrically conductive tabs as shown in  FIG. 3 . In a non-limiting example, conductive tabs  30  and  34  may be copper ribbon that is soldered to the front and back side electrodes on the cell. This back contact configuration provides a conductive tab design that minimizes the resistance losses without increasing shadowing or thermal stresses.  
      The presently described back contact configuration provides opportunity for improved manufacturability and durability of cell arrays by enabling the individual cells, such as the photovoltaic cell  14 , to be placed face down individually in the layup process prior to interconnecting them. In conventional fabrication, the cells are connected into a string prior to placing them down, and the string is fragile and difficult to repair. By placing the individual cells face down and connecting the cells via the front contacts  32  and back contacts  35 , the interconnection of the cells in the array is now amenable to a fully automated assembly process.  
      Providing front contacts  32  and back contacts  35  on the back side  28  of the photovoltaic cell  14  enables a double-ended connection scheme. The term “double ended” refers to connection of adjacent photovoltaic cells at both ends of a back side of the photovoltaic cell, rather than connecting at one end, as in conventional single ended series network type arrays. That is, the front electrode of one photovoltaic cell in an array is coupled to the back electrode of an adjacent array through a first connection between a front contact  32  at one end of a photovoltaic cell  14  and a back contact  35  (conductive tab  34 ) of an adjacent cell  14 , as well as through a second parallel connection between another front contact  32  of the photovoltaic cell  14  and another back contact  35  (conductive tab  34 ) of the adjacent cell  14 . The double-ended connection aspect of the present embodiments will be further illustrated and described with reference to  FIGS. 4, 5 ,  6  and  7 .  
      Referring now to  FIG. 4 , a schematic representation of the current paths of adjacent photovoltaic cells is illustrated. As discussed above, the front contacts  32  ( FIG. 3 ) provide a mechanism for providing multiple current paths to an adjacent photovoltaic cell as opposed to a single current path. As illustrated in  FIG. 4 , a solar cell array  12  comprises photovoltaic cells  48  and  50 . The current coming from the front side of the photovoltaic cell  48  (via the front contacts  32  of  FIG. 3 ) and going to the back side of the photovoltaic cell  50  (via the conductive tab  34 ) is essentially split into two current paths  42  and  44 . Thus, current will flow from the front (top) side of the cell  48  around both ends and to the back side of the cell  48 . The current flows from the front contacts  32  ( FIG. 3 ) of the cell  48  to the back contact  35  ( FIG. 3 ) (conductive tab  34 ) on the back side of the cell  50  via two current paths, rather than just one, as in conventional arrays. Advantageously, by providing parallel current paths between adjacent cells, resistance losses may be reduced.  
      Consider an example where 4 Ampere (A) of current (I) flows through the photovoltaic cell. The power loss due to electrical resistance in each photovoltaic cell in the array is equal to I 2 R where R is the resistance in the current path and I is the current. Hence, in this case, the power loss for 4 A current is 4 2 R. In the embodiment referred to in  FIG. 3 , where the current is split in half along two current paths, the power loss is 2 2  R/2 along each path. The calculation is based on the assumption that the additional wiring paths on the back side are of relatively low resistance. Consequently, the total resistance power loss is reduced to 2 2 /4 2  or 25%. Since the current is constant in a series circuit, the total resistance power percentage remains the same for any number of cells. The net effect on total output of this relative improvement is affected by the design and the operating conditions. It is be noted that under maximum current and peak operating temperatures, where losses are highest, the benefit is maximized. Naturally, solar modules are disposed in locations that tend to maximize their solar exposure, thus the peak current and temperatures conditions are often nominal operating conditions.  
       FIG. 5  is a back side plan view of solar cells adapted to have front contacts and back contacts on the back side of each cell and having a double ended interconnection achieved by point-to-point wiring in one of the embodiments. The solar cell array  60  comprises photovoltaic cells  62 ,  64 , and  66 , wherein each cell includes front contacts and back contacts on the back side of the cell, as illustrated in  FIG. 3 . Thus, the photovoltaic cell  62  includes front contacts  68  as well as back contacts  79  on a back side  70  of the cell  62 . The cells  64  and  66  are similarly configured. As illustrated in  FIG. 5 , the front contact  68  of the cell  62  is electrically coupled to the back contact  79  of the cell  64  via an interconnect lead  74 . To provide a second parallel current path from the cell  62  to the cell  64 , the front contact  69  of the cell  62  is electrically coupled to the back contact  79  of the cell  64  via another interconnect lead  76 . Since the front contacts  68  are accessible from the back side  70  of the photovoltaic cells, the interconnection scheme can be double ended, as described above. The wiring pattern can be repeated for an arbitrary number of cells in a series connection, with  77  and  78  depicting the input and output terminals of the network. This is akin to the automated point to point wiring that is widely used in the microelectronics industry for packaging semi-conductors, and the interconnect lead should be selected to minimize resistive losses. The connections can be made after the cells have been physically positioned in their locations for module assembly. Some non-limiting examples of interconnect leads include bus bars as well as insulated wires and ribbons, wherein the wires and ribbons are configured not to overlap with each other.  
      As can be appreciated, because the solar cells are fabricated to include front contacts on the back side of each cell, access to the front side of the solar cells is no longer necessary in interconnecting the cells to form the cell array. Advantageously, the fabrication of the photovoltaic cell array can be automated, and rework, if required, is simplified because only the affected cell has to be removed, without disturbing the adjacent cells.  
       FIG. 6  is a back side plan view of solar cells adapted to have front contacts and back contacts on the back side of each cell and having a double ended interconnection achieved by connecting two points to two points, in accordance with embodiments of the present invention. The solar cell array  80  comprises photovoltaic cells  82 ,  84  and  86 , wherein each cell includes front contacts and back contacts on the back side  90  of the cell, as depicted previously in  FIG. 3 . Thus, the photovoltaic cell is a back contact cell with the contacts  88  and  89  forming the terminals of the cell.  
      As shown in  FIG. 6 , interconnect leads  95  and  96  connect one photovoltaic cell to another. The interconnect leads are constructed of a low resistance material such as copper that may be die cut, machined or etched in any known art. Among other advantages over a single, long interconnect lead, separate interconnect leads require less interconnect material, provide more room for contact points and lower the weight of the solar cell array. Further, the aforementioned embodiment of the present invention requires minimal changes to the existing commercial infrastructure of a photovoltaic cell array. In an exemplary embodiment, the solar cell array  80  can also include passive components (not shown) embedded into the circuitry. A non-limiting example of a passive component includes a bypass diode.  
       FIG. 7  shows an alternate embodiment in which preformed interconnect leads  106  and  108  are attached to individual back contact cells  100  and  103  prior to assembly of the array. The interconnect leads  106  and  108  may be fabricated on a low electrical resistance material such as copper, and formed by etching, punching or other means to match the dimensions of the back contact cells  100  and  103 . Connections are formed through a process such as soldering to contacts  32  and  35 . Soldering can be accomplished by contacting the interconnect leads  106  and  108  to their respective locations, and by fluxing and soldering by supplying sufficient heat to connection following means known in the art. The resulting sub-assembly  100  (or  103 ) can now be tested, and repaired as necessary prior to forming a multi-cell array  101 . Connection of the cell  100  to another cell  103  can be accomplished through two points of contact, namely  102  and  104 . The back side  28 , insulator  33 , and tab  34  are also shown in  FIG. 7  and have the same respective functions as described previously in reference to  FIG. 3 .  
      The process of forming a multi-cell array of cell  101  is accomplished by placing cells in position during the lamination layup step as depicted in  FIG. 1 . Cell to cell connections are made by soldering the contact  102  of one cell, to the contact  104  of the adjacent cell, and so forth, to form a linear series network. In the case where one linear series network is to be connected to a co-linear adjacent network, connection at the ends can be accomplished by adding a shunting constructed of a solderable and conductive material, for example a copper ribbon. The cells can be placed manually or with an automated method, and the soldering can be accomplished by a manual or automated method by applying a heating device such as a soldering iron to the desired location. This process of assembly obviates the need to pre-assemble strings of cells, transport them to the layup station, and then repair or remove them if they have defects. By contrast, the sequential placement of individual cells  100 ,  103  by the means described falls into the methodologies that are employed in electronic card assembly and test manufacturing infrastructure which is characterized by automation for pick and place operations and high yielding soldering processes.  
       FIG. 8  shows a flow chart illustrating the steps involved in a method of manufacturing  120  of a solar cell array. The individual back contact cells are assembled using interconnect leads as in step  122 . Referring to step  124 , back contact photovoltaic cells are disposed individually on a laminate stack. A non-limiting example of the laminate stack includes a backsheet of ethylene vinyl acetate or polyvinyl fluoride on glass. The disposition of the photovoltaic cells on the laminate stack is an automated process. Each photovoltaic cell is individually picked and placed on the laminate stack. Further, the method includes tabbing  126  wherein tabs are soldered on a back side of each photovoltaic cell by an automated soldering apparatus. The tabs are designed to help maintain the spacing between the cells throughout the lamination process. The tabs may be insulated. After the cells have been placed, in a non-limiting example, they are tacked in place by using a hot iron. A slight warming of the cell will tack it to the underlying ethylene vinyl acetate and help retain the cell positions. Further, the cells are interconnected to each other in series via interconnect leads. As an example from the embodiment of  FIG. 4 , the interconnect leads can be insulated wires which may be round or flat. In other embodiments, as described with respect to  FIGS. 6 and 7 , the interconnect leads comprise copper (or alternately other conductive materials) ribbons that are punched or etched or otherwise manufactured into the desired shape. They may or may not be insulated depending on the process requirements. In general, the EVA flows between the ribbon and the cell providing an insulating layer that forms during the lamination process. As a next step  128 , an inspection of the cells is performed to test for a defect. Finally, the lamination process is completed in step  130 , wherein the photovoltaic cells are encapsulated using an encapsulant. Some non-limiting examples of encapsulants include ethylene vinyl acetate.  
      The aforementioned embodiments result in the potential for interconnection between adjacent cells with minimized spacing and thus can be used to improve the packing density and enhance the photovoltaic module output. In a non-limiting example, the permissible spacing between cells is typically at least 1 mm.  
      From the foregoing description, it is believed evident that the present invention has provided improved solar cell arrays with several advantages including reduced power loss in the circuit and a more convenient interconnect methodology that allows easier replacement of defective cells.  
      While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.