Abstract:
Flexible interconnects for attaching overlapping strings that can be part of a photovoltaic module. The interconnects can absorb strain caused by non-uniform heating and other loads encountered by the photovoltaic module.

Description:
BACKGROUND 
       [0001]    Solar panels typically include one or more strings of solar cells. Adjacent solar cells in a string may overlap one another in a cascading arrangement. For example, continuous strings of solar cells that form a solar panel are described in U.S. patent application Ser. No. 14/510,008, filed Oct. 8, 2014, and entitled “Module Fabrication of Solar Cells with Low Resistivity Electrodes,” the disclosure of which is incorporated herein by reference in its entirety. Producing solar panels with a cascaded cell arrangement can reduce the resistance due to inter-connections between the strips, and can increase the number of solar cells that can fit into a solar panel. 
         [0002]    One method of making such a panel includes sequentially connecting the busbars of adjacent cells and combining them. One type of panel (as described in the above-noted patent application) includes a series of cascaded strips created by dividing complete solar cells into strips, and then cascading the strips to form one or more strings. 
         [0003]    In some environments, photovoltaic (PV) modules exhibit great strain due to effects of heating, and in particular effects of heating components, such as copper busbars, that have a disparate coefficient of expansion in comparison to other components. This strain can be exacerbated by non-uniform heating of the PV module, often caused by partial sun shading of the PV panel. These heating effects can result in non-uniform cyclical loading onto joints of a strip, resulting in premature failure of joints. 
       SUMMARY 
       [0004]    To address the issues noted above, strips can be assembled using flexible interconnects that can absorb movement of strips. These interconnects can be arranged between strips and can be connected at edges of the strips. The strips can be arranged to overlap only at their edges. The interconnects can be flexible and for example, take the form of a ribbon, wire, several wires, and specialized shapes formed from foil. The interconnect can have one more folds or bends to facilitate strain absorption and flexibility. 
         [0005]    A flexible interconnect can be connected at one portion to only a first strip and at a second portion only to a second strip. Movement of one strip can cause the ribbon to move between the connected portions, but not transfer significant force to the other strip. Accordingly, the interconnect can compensate for strain within a PV module cause by, for example, non-uniform heating of the module. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIGS. 1A and 1B  respectively show cross-sectional and top views of photovoltaic structures, according to some embodiments of the invention. 
           [0007]      FIG. 1C  shows a side view of a string constructed from strips, according to some embodiments of the invention. 
           [0008]      FIGS. 2-5  show perspective views of various types of flexible interconnects between strips, according to some embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    The following description is intended to convey a thorough understanding of the embodiments described by providing a number of specific embodiments and details involving flexible interconnects for strings. It should be appreciated, however, that the present invention is not limited to these specific embodiments and details, which are exemplary only. It is further understood that one possessing ordinary skill in the art, in light of known systems and methods, would appreciate the use of the invention for its intended purposes and benefits in any number of alternative embodiments, depending upon specific design and other needs. 
         [0010]      FIG. 1A  shows one example of a photovoltaic structure. Photovoltaic structure  100  in this example can include N type lightly doped crystalline silicon (c-Si) base layer  130 , intrinsic tunneling layer  132 , N type heavily doped amorphous silicon (a-Si) surface field layer  134 , transparent conductive oxide (TCO) layer  136 , and front-side busbar  138 . On the backside, the structure can include intrinsic tunneling layer  140 , P type a-Si emitter layer  142 , TCO layer  144 , and backside busbar  146 . The backside tunneling junction, formed by P type a-Si emitter layer  140 , intrinsic tunneling layer  140 , and N type c-Si base layer  130 , can transport away the majority carriers generated by base layer  130 . The front side tunneling junction, formed by N type heavily doped a-Si surface field layer  134 , intrinsic tunneling layer  132 , and base layer  130 , can transport away the minority carriers generated by base layer  130 , thereby reducing the amount of carrier recombination in base layer  130 . Tunneling layers  132  and  140  can passivate the interface between base layer  130  and the two heavily doped a-Si layers while still allowing carriers generated by base layer  130  to enter these a-Si layers due to tunneling effect. 
         [0011]    Some conventional solar panels include a single string of serially connected un-cleaved photovoltaic structures. As described in U.S. patent application Ser. No. 14/563,867, which is incorporated by reference herein, it can be more desirable to have multiple (such as 3) strings, each string including cascaded strips, and connect these strings in parallel. Such a multiple-parallel-string panel configuration provides the same output voltage with a reduced internal resistance. 
         [0012]      FIG. 1B  shows photovoltaic structure  100  including three photovoltaic strips  102 . 1 ,  102 . 2 , and  102 . 3 , which can be the result of photovoltaic structure  100  having an electroplated copper electrode that exhibits low contact resistance. Each strip can include a number of substantially parallel finger lines, such as finger lines  106 , arranged in the X direction. These finger lines can collect the carriers generated by the photovoltaic structure and allow them to move toward a busbar. The busbar can be any electrically conductive element such as a metallic strip, often wider than a finger line, arranged in the Y direction. The busbar then can aggregate the current collected by the finger lines. Each strip can include two busbars, one on each surface, positioned on opposite edges. For example, strip  102 . 1  can have busbar  204 . 1  on the top surface, and busbar  15 . 1  on the bottom surface. Similarly, strip  202 . 2  can have busbars  104 . 2  and  105 . 2  on the top and bottom surfaces, respectively, and strip  202 . 3  can have busbars  104 . 3  and  105 . 3  on the top and bottom surfaces, respectively. 
         [0013]      FIG. 1C  shows a cascaded arrangement of three strips assembled as a string. In this example, three strips  102 . 1 ,  102 . 2 , and  102 . 3  can be arranged in a cascaded manner, such that the positive-side busbar of one strip overlaps and is electrically coupled to the negative-side busbar of the neighboring strip. 
         [0014]    Often, assembly of a string is performed by adhering each busbar using a conductive adhesive, which is an effective method to assemble an efficient PV modules. 
         [0015]    However, in some environments, PV modules exhibit great strain due to effects of heating, and in particular effects of heating components, such as copper busbars, that have a disparate coefficient of expansion. This can be exacerbated by non-uniform heating of the PV modules, often caused by partial sun shading of the PV modules. These heating effects can result in cyclical loading onto joints J between busbars, resulting in cracking and eventual failure of the joint. Interconnects as disclosed herein can alleviate these effects. 
         [0016]      FIG. 2  shows a perspective view of a portion of string  200 . String  200  includes first strip  202  overlaying second strip  204 . Each strip can be a portion or segment of a photovoltaic structure, such as a solar cell. A solar cell may be divided into a number of strips. A strip may have any shape and any size. The width and length of a strip may be the same or different from each other. 
         [0017]    First strip  202  overlays second strip  204  within overlay portion  206 . Overlay portion  206  can serve as an area for both physical and electrical connection between first strip  202  and second strip  204 . Overlay portion  206  can be bounded between first strip edge  208  and second strip edge  210 , and can include portions of busbars of each first strip  202  and second strip  204 . 
         [0018]    Interconnect  212  provides a mechanical and electrical interconnection between first strip  202  and second strip  204 . Lower interconnection portion  214  electrically and mechanically connects to a busbar portion of second strip  204 , but in some cases is not mechanically connected to first strip  204 . Upper interconnection portion  216  electrically and mechanically connects to a busbar portion of first strip  202 , but in some cases is not mechanically connected to second strip  204 . 
         [0019]    The lower and upper connection portions can be electrically and mechanically connected to the strips by soldered joints, weld joints, or by use of conductive adhesive. Portions of interconnect  212  can be masked with a material, e.g., polyimide film, to prevent such portions from adhering to busbars during soldering in a reflow oven. Soldering can be accomplished by many different methods, including inductive heating, air/convective heating, conductive heating. 
         [0020]    Low temperature solders can be used to form the joints. Such solders can include alloys like Bismuth (e.g. Bismuth-Tin (BiSn)) to reduce melting temperature as compared to conventional solders. Low temperature solders can have reflow or relatively low melt temperatures around 140° C., whereas typical silver solders melt at 180° C. and greater. By lowering the melt temperature to within the range of lamination temperatures it can be possible to produce a PV module with fewer steps. 
         [0021]    The use of low temperature solders can allow the assembly of a PV module in essentially one step, including formation of strings. In practice, this can be achieved by layering strips as strings between laminates (such as a back sheet or glass sheet). The strips can have interconnects as disclosed herein with low temperature solder paste on connection surfaces of the interconnects and busbars of the strips. Hence, increasing temperature of this arrangement will cause the low temperature solder to reflow and form permanent electrical and mechanical connections. 
         [0022]    In addition, the laminate surfaces can include layers of heat cured adhesive, which cure at approximately the same temperature as the low temperature solder. Thus, all of the major parts of the PV module can be stacked and heated to simultaneously melt the solder and cure the encapsulate using a single heating step. Traditionally, an ethylene-vinyl acetate (EVA) adhesive is used to form laminates. However, in some cases, such adhesives may not be compatible with solder fluxes of low-temperature solders. In some embodiments, ionomer encapsulates, such as PV5400 sheets by Dupont, can be used in lieu of or in conjunction with traditional adhesives because ionomer encapsulates can be resistant to fluxes used in low temperature solders. 
         [0023]    Interconnect  212  includes an elongated lower portion  218  that extends from lower interconnection portion  214  to fold  220 . Elongated upper portion  222  extends between fold  220  and upper interconnection portion  216 .  FIG. 2  shows interconnect  212  configured as a flat ribbon of an electrically conductive material. However, other configurations of material are possible, such as a strand or wire, in a braided or solid configuration. 
         [0024]    Interconnect  212  can be folded back upon itself to form fold  220 . Hence, lower interconnection portion  214 /elongated lower portion  218  can respectively contact upper interconnection portion  216 /elongated upper portion  222 , albeit in a floating manner such that these upper and lower portions are not mechanically connected to one another. This can provide flexibility in the X, Y, and Z directions between first strip  202  and second strip  204 , such that forces applied to second strip  204  by first strip  202 , as well as forces applied to first strip  202  by second strip  204 , are mitigated by interconnect  212 . Often, such forces are caused by thermal expansion of one or more portions of an associated PV module. 
         [0025]    While  FIG. 2  shows only one of interconnect  212 , more than one interconnect  212  can be used to connect first strip  202  and second strip  204 . Further, one or more of interconnect  212  can be located at locations of first strip  202  and second strip  204  that experience a particular amount of strain, and used in conjunction with portions of directly connected joints, such as conductive adhesive joints between first strip  202  and second strip  204 , that experience lesser strain. In addition, interconnect  212  can be used in conjunction with other types of interconnects, including, but not limited to, any of the other types of interconnects disclosed herein. 
         [0026]      FIG. 2  also shows interconnect  224  adjacent to interconnect  212 . Although these interconnects can be used side-by-side, placement is shown only for economy of this disclosure. Interconnect  224  provides a mechanical and electrical interconnection between first strip  202  and second strip  204 . First interconnection portion  226  electrically and mechanically connects to a busbar portion of first strip  202 , but in some cases is not mechanically connected to second strip  204 . Second interconnection portion  228  electrically and mechanically connects to a busbar portion of second strip  204 , but in some cases is not mechanically connected to first strip  202 . 
         [0027]    The connection portions can be electrically and mechanically connected to the strips by soldered joints, weld joints, or by use of conductive adhesive. Portions of interconnect  224  can be masked with a material, e.g., polyimide film, to prevent such portions from adhering to busbars during soldering in a reflow oven. Soldering can be accomplished by many different methods, including inductive heating, air/convective heating, conductive heating. 
         [0028]    Interconnect  224  includes elongated portions  230  that each can extend from first interconnection portion  226  and second interconnection portion  228  to folds  232 . Intermediary portion  234  can extend between folds  232 .  FIG. 2  shows interconnect  224  configured as a flat ribbon of an electrically conductive material. However, other configurations of material are possible, such as a strand or wire, in a braided or solid configuration. Further, while folds  232  are shown to be symmetrical, folds  232  can also be configured in an asymmetrical pattern. And while two folds are shown, more than two folds can be used, as well as series of closely arranged folds, e.g., folds arranged in accordion patterns. Interconnect  224  can also be constructed from a single sheet of material formed with no folds, e.g., die or laser cut from foil, and include reliefs or other features to promote flexibility. 
         [0029]    While  FIG. 2  shows only one of interconnect  224 , more than one interconnect  224  can be used to connect first strip  202  and second strip  204 . Further, one or more of interconnect  224  can be located at locations of first strip  202  and second strip  204  that experience a particular amount of strain, and used in conjunction with portions of directly connected joints, such as conductive adhesive joints between first strip  202  and second strip  204 , that experience lesser strain. In addition, interconnect  224  can be used in conjunction with other types of interconnects, including, but not limited to, any of the other types of interconnects disclosed herein. 
         [0030]      FIG. 3  shows a perspective view of a portion of string  300 . String  300  can include first strip  302  overlaying second strip  304 . Each strip can be a portion or segment of a photovoltaic structure, such as a solar cell. A solar cell may be divided into a number of strips. A strip may have any shape and any size. The width and length of a strip may be the same or different from each other. 
         [0031]    First strip  302  overlays second strip  304  within overlay portion  306 . Overlay portion  306  can serve as an area for both physical and electrical connection between first strip  302  and second strip  304 . Overlay portion  306  can be bounded between first strip edge  308  and second strip edge  310 , and can include portions of busbars of each first strip  302  and second strip  304 . 
         [0032]    Interconnect  312  provides a mechanical and electrical interconnection between first strip  302  and second strip  304 . First interconnection portion  314  electrically and mechanically connects to a busbar portion of first strip  302 , but in some cases is not directly mechanically connected to second strip  304 . Second interconnection portion  316  electrically and mechanically connects to a busbar portion of second strip  304 , but in some cases is not directly mechanically connected to first strip  302 . The first and second connection portions can be electrically and mechanically connected to the strips by soldered joints, weld joints, or by use of conductive adhesive. 
         [0033]    Interconnect  312  can include an elongated middle portion  318  that extends from first interconnection portion  314  to second interconnection portion  316 . Elongated middle portion  318  may in some cases be not directly connected to either strip, and therefore can provide flexibility between the first interconnection portion  314  to second interconnection portion  316 , to alleviate strain between the strips. Thus, interconnect  312  can provide flexibility in the X, Y, and Z directions between first strip  302  and second strip  304 , such that forces applied to second strip  304  by first strip  302 , as well as forces applied to first strip  302  by second strip  304 , are mitigated by flexure of elongated middle portion  318 . 
         [0034]      FIG. 3  shows interconnect  312  configured as a plurality of round wires of an electrically conductive material arranged in a side-by-side formation, in a braided or solid configuration. However, other configurations of material are possible, such as ribbon wire. Further, while interconnect  312  is shown constructed from a plurality of directly adjacent wires, in some embodiments, only one wire is used or a plurality of wires separated by gaps. 
         [0035]    While  FIG. 3  shows only one of interconnect  312 , more than one interconnect  312  can be used to connect first strip  302  and second strip  304 . Further, one or more of interconnect  312  can be located at locations of first strip  302  and second strip  304  that experience a particular amount of strain, and used in conjunction with portions of directly connected joints, such as conductive adhesive joints between first strip  302  and second strip  304 , that experience lesser strain. In addition, interconnect  312  can be used in conjunction with other types of interconnects, including, but not limited to, any of the other types of interconnects disclosed herein. 
         [0036]      FIG. 4A  shows a perspective view of a portion of string  400 . String  400  can include first strip  402  overlaying second strip  404 . Each strip can be a portion or segment of a photovoltaic structure, such as a solar cell. A solar cell may be divided into a number of strips. A strip may have any shape and any size. The width and length of a strip may be the same or different from each other. 
         [0037]    First strip  402  overlays second strip  404  within overlay portion  406 . Overlay portion  406  can serve as an area for both physical and electrical connection between first strip  402  and second strip  404 . Overlay portion  406  can be bounded between first strip edge  408  and second strip edge  410 , and includes portions of busbars of each first strip  302  and second strip  404 . 
         [0038]    Interconnect  412  provides an elongated mechanical and electrical interconnection between first strip  402  and second strip  404 . First interconnection portions  414  electrically and mechanically connects to a busbar portion of first strip  402 , but in some cases is not directly mechanically connected to second strip  404 . Second interconnection portion  416  electrically and mechanically connects to a busbar portion of second strip  404 , but in some cases is not directly mechanically connected to first strip  402 . The first and second connection portions can be electrically and mechanically connected to the strips by soldered joints, weld joints, or by use of conductive adhesive. Portions of interconnect  412  can be masked with a material, e.g., polyimide film, to prevent such portions from adhering to busbars during soldering in a reflow oven. Soldering can be accomplished by many different methods, including inductive heating, air/convective heating, conductive heating. 
         [0039]    Interconnect  412  includes flexible portions  418  that each extend between first interconnection portions  414  and second interconnection portions  416 . Flexible portions  418  in some cases may not be not directly mechanically connected to either strip and can extend laterally from overlay portion  416  at varying distances by elongated wire portions  420 , and therefore can provide flexibility between first interconnection portions  414  and second interconnection portions  416 , to alleviate strain between the strips. Thus, interconnect  412  provides flexibility in the X, Y, and Z directions between first strip  402  and second strip  404 , such that forces applied to second strip  404  by first strip  402 , as well as forces applied to first strip  402  by second strip  404 , are mitigated by flexure of flexible portions  418 . 
         [0040]    Like flexible portions  418 , elongated wire portions  420  in some cases may not be directly connected to either strip to provide flexibility between the strips. As mentioned above, elongated wire portions  420  can vary in length. For example, at the embodiment shown at  FIG. 4B , which is substantially the same as the embodiment shown at  FIG. 4A , elongated wire portions  422  are significantly longer than elongated wire portions  420 , thus laterally extending flexible portions  418  father away from second strip edge  410 . In some cases, this arrangement can provide more comparative flexibility than the arrangement shown at  FIG. 4A . 
         [0041]      FIGS. 4A and 4B  show interconnect  412  configured as an elongated wire having alternating U-shaped curves formed from an electrically conductive material, in a braided or solid configuration. However, other configurations of material are possible, such as square or triangular shaped portions. The upper curves of interconnect  412  form the flexible portions  418 , and the lower curves form the first interconnection portions  414  and second interconnection portions  416 , which alternate. Thus, every other lower curve is one of the first interconnection portions  414  or second interconnection portions  416 . 
         [0042]    Interconnect  412  can extend fully between strip  402  and  404  along overlay portion  406 , or only partially extend. While  FIGS. 4A and 4B  show only one of interconnect  412 , more than one interconnect  412  can be used to connect first strip  402  and second strip  404 . Further, one or more of interconnect  412  can be located at locations of first strip  402  and second strip  404  that experience a particular amount of strain, and used in conjunction with portions of directly connected joints, such as conductive adhesive joints between first strip  402  and second strip  404 , that experience lesser strain. In addition, interconnect  412  can be used in conjunction with other types of interconnects, including, but not limited to, any of the other types of interconnects disclosed herein. 
         [0043]      FIG. 5  shows a perspective view of a portion of string  500 . String  500  includes first strip  502  overlaying second strip  504 . Each strip can be a portion or segment of a photovoltaic structure, such as a solar cell. A solar cell may be divided into a number of strips. A strip may have any shape and any size. The width and length of a strip may be the same or different from each other. 
         [0044]    First strip  502  can overlay second strip  504  within overlay portion  506 . Overlay portion  506  can serve as an area for both physical and electrical connection between first strip  502  and second strip  504 . Overlay portion  506  can be bounded between first strip edge  508  and second strip edge  510 , and can include portions of busbars of each first strip  302  and second strip  504 . 
         [0045]    Interconnect  512  provides an elongated mechanical and electrical interconnection between first strip  502  and second strip  504 . First interconnection portions  514  electrically and mechanically connects to a busbar portion of first strip  502 , but in some cases is not directly mechanically connected to second strip  504 . Second interconnection portion  516  electrically and mechanically connects to a busbar portion of second strip  404 , but in some cases is not directly mechanically connected to first strip  502 . The first and second connection portions can be electrically and mechanically connected to the strips by soldered joints, weld joints, or by use of conductive adhesive. Portions of interconnect  512  can be masked with a material, e.g., polyimide film, to prevent such portions from adhering to busbars during soldering in a reflow oven. Soldering can be accomplished by many different methods, including inductive heating, air/convective heating, conductive heating. 
         [0046]    Interconnect  512  can include flexible portions  518  that each extend between first interconnection portions  514  and second interconnection portion  516 . Flexible portions  518  may not always be directly connected to either strip and can extend laterally from overlay portion  506  at varying distances by elongated portions  520 , and therefore can provide flexibility between first interconnection portions  514  and second interconnection portions  516 , to alleviate strain between the strips. Thus, interconnect  512  can provide flexibility in the X, Y, and Z directions between first strip  502  and second strip  504 , such that forces applied to second strip  504  by first strip  502 , as well as forces applied to first strip  502  by second strip  504 , are mitigated by flexure of flexible portions  518 . 
         [0047]    Like flexible portions  518 , elongated portions  520  also are not directly connected to either strip. Elongated portions  520  can vary in length. For example, elongated portions  520  can be significantly longer shown, or alternate in length along the length of interconnect  512 . 
         [0048]      FIG. 5  shows interconnect  512  configured as an flat, planar sheet of material from an electrically conductive material, for example, die or laser cut from metal foil. In particular, first interconnection portions  514 , second interconnection portions  516 , and elongated portions  520  are depicted as contiguous lower rectangular tabs. Every other tab can form part of one of the first interconnection portions  414  or second interconnection portions  416 . While only one of second interconnection portion  516  is depicted, a plurality of second interconnection portion  516  can be used, including arrangements with equal numbers of first interconnection portions  514  and second interconnection portions  516 . 
         [0049]    In addition, other configurations of the tabs are possible to tune flexibility and/or promote manufacturability, such as tabs that taper narrowly or increasingly in width towards flexible portions  518 , and/or tabs including various cuts or openings along its length. Further, flexible portions  518  are depicted uniform elongated tabs, however, other variations are possible to tune flexibility and/or promote manufacturability. For example, flexible portions  518  can taper narrowly or increasingly in width between elongated portions  520 , and/or flexible portions  518  can include various cuts or openings along its length. 
         [0050]    Interconnect  512  can extend fully between strip  502  and  504  along overlay portion  506 , or only partially. While  FIG. 5  shows only one of interconnect  512 , more than one interconnect  512  can be used to connect first strip  502  and second strip  504 . Further, one or more of interconnect  512  can be located at locations of first strip  502  and second strip  504  that experience a particular amount of strain, and used in conjunction with portions of directly connected joints, such as conductive adhesive joints between first strip  502  and second strip  504 , that experience lesser strain. In addition, interconnect  512  can be used in conjunction with other types of interconnects, including, but not limited to, any of the other types of interconnects disclosed herein. 
         [0051]    The foregoing descriptions of embodiments of the invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the invention to the forms disclosed. Accordingly, many modifications and variations may be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the invention. The scope of the invention is defined by the appended claims.