Abstract:
Thin film PV cells and strings of such cells that may be electrically joined with electrical conductors or electroconductive patterns are disclosed. The electrical conductors wrap or fold around the PV cells to form an electrical series connection among those cells. The electrical conductors may be formed or deposited on an electrically insulating sheet, which is then wrapped or folded around those cells. By constructing the electrical conductor and positioning the cells appropriately, an electrical connection is formed between one polarity of a given cell and the opposite polarity of the adjacent cell when the sheet is folded over. One or more dielectric materials may be applied or attached to exposed edges of the cells or conductive traces prior to folding the electrical conductors and/or electrically insulating sheet to prevent shorts or failure points.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from U.S. Provisional Patent Application Ser. No. 61/284,924, filed Dec. 28, 2009, Ser. No. 61/284,958 filed Dec. 28, 2009 and Ser. No. 61/284,956 filed Dec. 28, 2009 all of which are incorporated herein by reference. Also incorporated by reference in their entireties are the following patents and patent applications: U.S. Pat. No. 7,194,197, U.S. Pat. No. 6,690,041, Ser. No. 12/364,440 filed Feb. 2, 2009, Ser. No. 12/424,497 filed Apr. 15, 2009 and Ser. No. 12/587,111 filed Sep. 30, 2009. 
     
    
     BACKGROUND 
       [0002]    The field of photovoltaics generally relates to multi-layer materials that convert sunlight directly into DC electrical power. The basic mechanism for this conversion is the photovoltaic effect, first observed by Antoine-César Becquerel in 1839, and first correctly described by Einstein in a seminal  1905  scientific paper for which he was awarded a Nobel Prize for physics. In the United States, photovoltaic (PV) devices are popularly known as solar cells or PV cells. Solar cells are typically configured as a cooperating sandwich of p-type and n-type semiconductors, in which the n-type semiconductor material (on one “side” of the sandwich) exhibits an excess of electrons, and the p-type semiconductor material (on the other “side” of the sandwich) exhibits an excess of holes, each of which signifies the absence of an electron. Near the p-n junction between the two materials, valence electrons from the n-type layer move into neighboring holes in the p-type layer, creating a small electrical imbalance inside the solar cell. This results in an electric field in the vicinity of the metallurgical junction that forms the electronic p-n junction. 
         [0003]    When an incident photon excites an electron in the cell into the conduction band, the excited electron becomes unbound from the atoms of the semiconductor, creating a free electron/hole pair. Because, as described above, the p-n junction creates an electric field in the vicinity of the junction, electron/hole pairs created in this manner near the junction tend to separate and move away from junction, with the electron moving toward the electrode on the n-type side, and the hole moving toward the electrode on the p-type side of the junction. This creates an overall charge imbalance in the cell, so that if an external conductive path is provided between the two sides of the cell, electrons will move from the n-type side back to the p-type side along the external path, creating an electric current. In practice, electrons may be collected from at or near the surface of the n-type side by a conducting grid that covers a portion of the surface, while still allowing sufficient access into the cell by incident photons. 
         [0004]    Such a photovoltaic structure, when appropriately located electrical contacts are included and the cell (or a series of cells) is incorporated into a closed electrical circuit, forms a working PV device. As a standalone device, a single conventional solar cell is not sufficient to power most applications. As a result, solar cells are commonly arranged into PV modules, or “strings,” by connecting the front of one cell to the back of another, thereby adding the voltages of the individual cells together in electrical series. Typically, a significant number of cells are connected in series to achieve a usable voltage. The resulting DC current then may be fed through an inverter, where it is transformed into AC current at an appropriate frequency, which is chosen to match the frequency of AC current supplied by a conventional power grid. In the United States, this frequency is 60 Hertz (Hz), and most other countries provide AC power at either 50 Hz or 60 Hz. 
         [0005]    One particular type of solar cell that has been developed for commercial use is a “thin-film” PV cell. In comparison to other types of PV cells, such as crystalline silicon PV cells, thin-film PV cells require less light-absorbing semiconductor material to create a working cell, and thus can reduce processing costs. Thin-film based PV cells also offer reduced cost by employing previously developed deposition techniques for the electrode layers, where similar materials are widely used in the thin-film industries for protective, decorative, and functional coatings. Common examples of low cost commercial thin-film products include water impermeable coatings on polymer-based food packaging, decorative coatings on architectural glass, low emissivity thermal control coatings on residential and commercial glass, and scratch and anti-reflective coatings on eyewear. Adopting or modifying techniques that have been developed in these other fields has allowed a reduction in development costs for PV cell thin-film deposition techniques. 
         [0006]    Furthermore, thin-film cells have exhibited efficiencies approaching 20%, which rivals or exceeds the efficiencies of the most efficient crystalline cells. In particular, the semiconductor material copper indium gallium diselenide (CIGS) is stable, has low toxicity, and is truly a thin film, requiring a thickness of less than two microns in a working PV cell. As a result, to date CIGS appears to have demonstrated the greatest potential for high performance, low cost thin-film PV products, and thus for penetrating bulk power generation markets. Other semiconductor variants for thin-film PV technology include copper indium diselenide, copper indium disulfide, copper indium aluminum diselenide, and cadmium telluride. 
         [0007]    Some thin-film PV materials may be deposited either on rigid glass substrates, or on flexible substrates. Glass substrates are relatively inexpensive, generally have a coefficient of thermal expansion that is a relatively close match with the CIGS or other absorber layers, and allow for the use of vacuum deposition systems. However, when comparing technology options applicable during the deposition process, rigid substrates suffer from various shortcomings during processing, such as a need for substantial floor space for processing equipment and material storage, expensive and specialized equipment for heating glass uniformly to elevated temperatures at or near the glass annealing temperature, a high potential for substrate fracture with resultant yield loss, and higher heat capacity with resultant higher electricity cost for heating the glass. Furthermore, rigid substrates require increased shipping costs due to the weight and fragile nature of the glass. As a result, the use of glass substrates for the deposition of thin films may not be the best choice for low-cost, large-volume, high-yield, commercial manufacturing of multi-layer functional thin-film materials such as photovoltaics. 
         [0008]    In contrast, roll-to-roll processing of thin flexible substrates allows for the use of compact, less expensive vacuum systems, and of non-specialized equipment that already has been developed for other thin film industries. PV cells based on thin flexible substrate materials also exhibit a relatively high tolerance to rapid heating and cooling and to large thermal gradients (resulting in a low likelihood of fracture or failure during processing), require comparatively low shipping costs, and exhibit a greater ease of installation than cells based on rigid substrates. Additional details relating to the composition and manufacture of thin film PV cells of a type suitable for use with the presently disclosed methods and apparatus may be found, for example, in U.S. Pat. Nos. 6,310,281, 6,372,538, and 7,194,197, all to Wendt et al. The complete disclosures of those patents are hereby incorporated by reference for all purposes. 
         [0009]    As noted previously, a significant number of PV cells often are connected in series to achieve a usable voltage, and thus a desired power output. Such a configuration is often called a “string” of PV cells. Due to the different properties of crystalline substrates and flexible thin film substrates, the electrical series connection between cells may be constructed differently for a thin film cell than for a crystalline cell, and forming reliable series connections between thin film cells poses several challenges. For example, soldering (the traditional technique used to connect crystalline solar cells) directly on thin film cells exposes the PV coatings of the cells to damaging temperatures, and the organic-based silver inks typically used to form a collection grid on thin film cells may not allow strong adherence by ordinary solder materials in any case. Thus, PV cells often are joined with wires or conductive tabs attached to the cells with an electrically conductive adhesive (ECA), rather than by soldering. An example of joining PV cells with conductive tabs is disclosed in U.S. Patent Application Publication No. 2009/0255565 to Britt et al. The complete disclosure of that application publication is hereby incorporated by reference for all purposes. 
         [0010]    However, even when wires or tabs are used to form inter-cell connections, the extremely thin coatings and potential flaking along cut PV cell edges introduces opportunities for shorting (power loss) wherever a wire or tab crosses over a cell edge. Furthermore, the conductive substrate on which the PV coatings are deposited, which typically is a metal foil, may be easily deformed by thermo-mechanical stress from attached wires and tabs. This stress can be transferred to weakly-adhering interfaces, which can result in delamination of the cells. In addition, adhesion between the ECA and the cell back side, or between the ECA and the conductive grid on the front side, can be weak, and mechanical stress may cause separation of the wires or tabs at these locations. Also, corrosion can occur between the molybdenum or other coating on the back side of a cell and the ECA that joins the tab to the solar cell there. This corrosion may result in a high-resistance contact or adhesion failure, leading to power losses. 
         [0011]    Advanced methods of joining thin film PV cells with conductive tabs or ribbons may largely overcome the problems of electrical shorting and delamination, but may require undesirably high production costs to do so. Furthermore, all such methods—no matter how robust—require that at least some portion of the PV string be covered by a conductive tab, which blocks solar radiation from striking that portion of the string and thus reduces the efficiency of the system. As a result, there is a need for improved methods of interconnecting PV cells into strings, and for improved strings of interconnected cells. Specifically, there is a need for strings and methods of their formation that reduce interconnection costs and reduce the fraction of each PV cell that is covered by the interconnection mechanism, while maintaining or improving the ability of the cell to withstand stress. 
       SUMMARY 
       [0012]    The present teachings disclose thin film PV cells and strings of such cells that may be electrically joined with electrical conductors or electroconductive patterns. The electrical conductors wrap or fold around the PV cells to form an electrical series connection among those cells. The electrical conductors may be formed or deposited on an electrically insulating sheet, which is then wrapped or folded around those cells. By constructing the electrical conductor and positioning the cells appropriately, an electrical connection is formed between one polarity of a given cell and the opposite polarity of the adjacent cell when the sheet is folded over. One or more dielectric materials may be applied or attached to exposed edges of the cells or conductive traces prior to folding the electrical conductors and/or electrically insulating sheet to prevent shorts or failure points. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a top view of an electrically insulating sheet with electrical conductors in accordance with aspects of the present disclosure. 
           [0014]      FIG. 2  is a top view of the electrically insulating sheet of  FIG. 1  shown with first and second PV cells supported on the sheet aligned with trailing portions of the electrical conductors. 
           [0015]      FIG. 3  is a top view of the electrically insulating sheet of  FIG. 1  shown with PV cells supported on the sheet and aligned with trailing portions of the electrical conductors. 
           [0016]      FIG. 4  is a partial view of the electrically insulating sheet of  FIG. 3  shown with dielectric material attached to portions of the electrical conductors and cells. 
           [0017]      FIG. 5  is a top view of the electrically insulating sheet of  FIG. 1  with dashed lines to indicate where the electrically insulating sheet will be folded. 
           [0018]      FIG. 6  is a top view of the electrically insulating sheet of  FIG. 1  with portions of the electrically insulating sheet folded along the dashed lines shown in  FIG. 5 . 
           [0019]      FIG. 7  is sectional view of the electrically insulating sheet of  FIG. 1  taken along lines  7 - 7  in  FIG. 6 . 
           [0020]      FIG. 8  is a sectional view of the electrically insulating sheet of  FIG. 1  taken along lines  8 - 8  in  FIG. 6 . 
           [0021]      FIG. 9  is a top view of six strings of PV cells prior to connection and lamination. 
           [0022]      FIG. 10  is a flowchart depicting methods of manufacturing strings or modules of photovoltaic cells according to aspects of the present teachings. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  shows an electrically insulating substrate or sheet  20  for a string of photovoltaic cells. The sheet may be configured to support a plurality of PV cells. Electrically insulating sheet  20  (also may be referred to as “electrically insulating backing” or “filler sheet”) may be made of any suitable materials, such as thermoplastic materials including olefin-based polymers or polyolefins, ethylene vinyl acetate (EVA), ionomers, and fluoropolymers. The electrically insulating sheet may be selected based on its recyclability (separable from other components of a spent PV cell module), lamination time, degradation of manufacturing equipment and PV cells, adhesive properties, vapor permeability, and/or other suitable properties. 
         [0024]    Electrically insulating sheet  20  may include electrical conductors or electroconductive patterns  22 , as shown in  FIG. 1 . Electrical conductors  22  may include a trailing portion  24  and one or more leading portions  26 . The electrical conductors may be deposited on sheet  20  via printing, plating, and/or other suitable methods. Additionally, electrical conductors  22  may be made of any suitable materials. For example, low-temperature solder (such as tin-bismuth-silver) may be plated on sheet  20  to form the electrical conductors. The solder forms a metallurgical bond with front and backside contacts of the module upon lamination. Alternatively, electrical conductors  20  may be printed using a B-stage conductive epoxy. Alternatively, nickel may be plated on sheet  20  to form electrical conductors  22  and conductive epoxy or solder paste may be added in certain areas to ensure electrical contact. The electrical conductors may be deposited on only a single side of sheet  22 . 
         [0025]      FIGS. 2-3  show placement of thin film PV cells  28 , such as a first thin film PV cell  30  and a second thin film PV cell  32  on sheet  20 . First cell  30  includes a top surface  34 , a bottom surface  36 , and a conducting grid  38 . Similarly, second cell  32  includes a top surface  40 , a bottom surface  42 , and a conducting grid  44 . Top surface  34  of first cell  30  is positioned on trailing portion  24  of electrical conductor  22  such that the trailing portion contacts top surface  34  and/or conducting grid  38 . Top surface  40  of second cell  32  is positioned adjacent to but spaced from the first cell, such as on the trailing portion of an adjacent electrical conductor such that that trailing portion contacts top surface  40  and/or conducting grid  44 . The cells may be heat-tacked on or otherwise attached to the sheet. Although the first and second cells are shown to include conducting grids, those grids may alternatively be deposited on sheet  20  as part of electrical conductors  22 . 
         [0026]      FIG. 4  shows dielectric material  46  that may be attached to portions of electrical conductors  22  and cells  28  of sheet  20  to protect against short circuits or failure points. The dielectric material may be attached via taping, printing, coating, or other suitable methods prior to folding sheet  20 . Dielectric material  46  may be any suitable shape(s), such as a linear stripe shown in  FIG. 4 . 
         [0027]      FIG. 5  shows, in dashed lines at  47 , where sheet  20  may be folded to connect leading portions  26  to bottom surfaces of cells  28 . Sheet  20  may be referred to as having a base portion  46  that includes the cells and the trailing portions of the electrical conductors, and one or more folded portions  48  that are folded on to the base portion. Although sheet  20  is shown to include two folded portions, the sheet may alternatively include any suitable number of folded portions. For example, sheet  20  may include a single folded portion that covers any suitable part(s) of the base portion when folded. 
         [0028]      FIG. 6  shows sheet  20  with folded portions in which leading portions  26  contact bottom surfaces of cells  28  to form electrical series connections among cells  28  resulting in a string of connected PV cells  50 . For example, trailing portion  24  contacts top surface  34  of first cell  30  such that the trailing portion is electrically connected to conducting grid  38  of the first cell. Additionally, leading portion  26  contacts bottom surface  42  of second cell  32  such that the leading portion is electrically connected to conducting grid  44  of the second cell. Electrical conductors  22  at end portions of sheet  20  are exposed at  51  to provide positive and negative contacts for the cell. The folded portions of sheet  20  may be heat-tacked or otherwise attached to cells  28 . 
         [0029]      FIGS. 7-8  show sheet  20  and leading portion  26  of electrical conductor  22  wrapping or folding around second cell  32 . Trailing portion  24  is adjacent top surface  34  and spaced from bottom surface  36  of first cell  30 , such as within a plane parallel to the bottom surface. The trailing portion includes a first part  52  that may extend longitudinally across the cell and one or more second parts  54  that may extend transversely across the cell toward an adjacent cell (shown in  FIG. 5 ). From the trailing portion to the leading portion, electrical conductor  22  wraps or folds around the second cell such that at least a substantial portion of leading portion  26  is adjacent bottom surface  42  and spaced from top surface  40  of second cell  32  (such as within a plane parallel to the top surface) relative to the trailing portion. Leading portion  26  may extend longitudinally along the second cell when folded. The leading portion includes a first part  56 , a second part  57 , and a third part  58 . Prior to folding sheet  20 , first part  56  is spaced from third part  58  by second part  57  (such as shown in  FIG. 4 ). When sheet  20  is folded, first part  56  is in contact with third part  58 , as shown in  FIG. 8 . This may provide redundant contact to prevent the folded portion of the electrical conductor from becoming a failure point. 
         [0030]      FIG. 9  shows a plurality of strings  50  prior to lamination as a module  60 . As part of the lamination, the strings are electrically connected, such as via connection ribbons. For example, a first connection ribbon may connect the left side of the upper three strings in  FIG. 9  as the positive module connection. A second connection ribbon may connect the left side of the lower three strings in  FIG. 9  as the negative module connection. Finally, a third ribbon may connect all six strings on the right side to place the upper group of three strings and lower group of three strings in a series connection. Strings  50  may alternatively be connected via other suitable method(s). 
         [0031]    A number of methods of manufacturing strings and modules of PV cells are contemplated by the present teachings, and an illustrative method is depicted in  FIG. 10  and generally indicated at  100 . While  FIG. 10  shows illustrative steps of a method according to one embodiment, other embodiments may omit, add to, and/or modify any of the steps shown in that figure. At step  102 , electroconductive pattern(s) are deposited on an electrically insulating sheet, such as via printing or plating. At step  104 , the top surface of a first cell is positioned on an electroconductive pattern. The top surface of the first cell may be positioned on a trailing portion of the pattern. At step  106 , the top surface of a second cell is positioned adjacent to, but spaced from, the first cell. The top surface of the second cell may be positioned on a trailing portion of an adjacent electroconductive pattern. At step  108 , the sheet is folded such that one or more leading portions of the electroconductive pattern contacts the bottom surface of the second cell to form an electrical series connection between the first and second cells. 
         [0032]    Method  100  also may include one or more other steps. For example, at step  110 , conducting grids are deposited on the electrically insulating sheet where the first and second cells will be positioned. The conducting grids may be deposited via printing, plating, or other suitable methods. At step  112 , heat is applied to bond the first and second cells to the sheet. At step  114 , dielectric material is attached to portion(s) of the first and second cells and the pattern. At step  116 , heat is applied to folded portion(s) of the sheet to bond those portions to the first and second cells. 
         [0033]    The various structural members disclosed herein may be constructed from any suitable material, or combination of materials, such as metal, plastic, nylon, rubber, or any other materials with sufficient structural strength to withstand the loads incurred during use. Materials may be selected based on their durability, flexibility, weight, and/or aesthetic qualities. 
         [0034]    It is believed that the following claims particularly point out certain combinations and subcombinations that are directed to one of the disclosed inventions and are novel and non-obvious. Inventions embodied in other combinations and subcombinations of features, functions, elements and/or properties may be claimed through amendment of the present claims or presentation of new claims in this or a related application. Such amended or new claims, whether they are directed to a different invention or directed to the same invention, whether different, broader, narrower or equal in scope to the original claims, are also regarded as included within the subject matter of the inventions of the present disclosure.