Cross-Reference to Related Application

This application claims priority from U.S. Provisional Patent Applications Serial No. 61/284,958, filed December 28, 2009, Serial No. 61/284,956 filed December 28, 2009 and Serial No. 61/284,924 filed December 28, 2009 all of which are incorporated herein by reference. Also incorporated by reference in their entireties are the following patents and patent applications: Patent No, 7,194,197, Patent No. 6,690,041 , Serial No. 12/364,440 filed February 2, 2009 and Serial No. 12/587,1 1 1 filed September 30, 2009.

Background

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-Cesar 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.

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.

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.

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. 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.

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 multilayer functional thin-film materials such as photovoltaics.

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. Patent Nos. 6,310,281 , 6,372,538, and 7,194,197, all to Wendt et al. These patents are hereby incorporated into the present disclosure by reference for all purposes.

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.

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.

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

A photovoltaic module includes multiple photovoltaic cells connected in series between protective outer layers. A first conductive grid is attached to a first substantially transparent substrate. A second conductive grid is attached to a second substrate. A plurality of thin film photovoltaic cells are disposed between the first and second substrates with a photoactive side of each cell facing the first substrate. The first and second conductive grids are configured to interconnect the negative side of one cell to the positive side of an adjacent cell in a flexible laminated package formed by the first and second substrates. Methods of producing photovoltaic modules are also provided including preparation of first and second flexible substrates, by applying conductive grid patterns over an adhesive layer on each substrate. Photovoltaic cells are then placed on the grid on the first substrate. Next the second substrate is laminated with the first substrate, sandwiching the cells between, and establishing electrical interconnection between the cells by connecting the grid of the first substrate with the grid on the second substrate. Numerous related intermediate articles of manufacture and sub-methods and processes are also disclosed.

Brief Description of the Figures

Fig. 1 is a plan view of an upper portion of a photovoltaic module.

Fig. 2 is a partial cross-sectional view of the upper portion of Fig. 1 .

Fig. 3 is a plan view of a lower portion of a photovoltaic module.

Fig. 4 is partial cross-sectional view of the lower portion of Fig. 3. Fig. 5 is a plan view of the upper portion of Fig. 1 with a plurality of photovoltaic cells placed over a conductive grid.

Fig. 6 is a perspective exploded view of a photovoltaic module including the upper portion of Fig. 1 , the lower portion of Fig. 3, sandwiching a plurality of photovoltaic cells in a laminated interconnection configuration.

Fig. 7 is a partial cross-sectional view of the module components shown in Fig. 6, after assembly.

Fig. 8 is a flow chart illustrating steps in a process of making a thin-film flexible photovoltaic module.

Fig. 9 is a perspective view of a flexible substrate with a conductive grid pattern for use as a component of a photovoltaic module.

Fig. 10 is a partial top plan view of the substrate and grid pattern shown in Fig. 9, with the addition of dielectric strips covering edge portions of the grid pattern.

Fig. 1 1 is the same plan view of Fig. 10, with the addition of photovoltaic cells positioned on the grid pattern.

Fig. 12 is a perspective exploded view of a photovoltaic module assembly including top and bottom substrates sandwiching a series of photovoltaic cells.

Fig. 13 is a top plan view of the component shown in Fig. 12, after assembly. Selected portions of the module have been cut away for illustration purposes.

Fig. 14 is a cross sectional view of the photovoltaic module shown in Fig. 13.

Fig. 15 is a flow chart illustrating steps in a method for producing a thin-film flexible photovoltaic module.

Detailed Description

Figs. 1 -7 show portions of an interconnected module of photovoltaic cells at various stages of assembly, according to aspects of the present teachings. Fig. 1 is a plan view of an upper portion of a partially assembled photovoltaic module, generally indicated at 100. Upper portion or substrate 100 includes a first or top substrate 102, and an adhesive layer 104 disposed on the surface of substrate 102. A conductive grid 106 and a dielectric strip 108 are disposed on portions of adhesive layer 104 on substrate 102. Fig. 2 is a magnified cross sectional view showing details of the various layers shown in Fig. 1 . Substrate 102 supports adhesive layer 104. Conductive grid 106 is patterned on top of adhesive layer 104. Dielectric strip 108 runs over adhesive layer 104 and portions of conductive grid 106.

Substrate 102 may be a transparent polymer that allows solar radiation to pass through the substrate to a significant degree, and that is conducive to a roll-to- roll manufacturing process. For example, suitable substrates may be constructed from polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyetheretherketone (PEEK), polyimide, or ethylene tetrafluoroethylene (ETFE), among others. Alternative transparent materials, such as glass substrates, also may be used.

Adhesive layer 104 may be applied to substrate 102 as a thin, substantially transparent coating configured to have relatively strong adhesion to the substrate and also to the other components of the module. Suitable adhesives may include, for example, ethylene vinyl acetate (EVA), polyolefins, ionomers, polyvinyl butyral (PVB), and various urethanes, among others.

Conductive grid 106 is applied to the adhesive-coated substrate by any suitable method, such as printing, and typically will be constructed primarily from a conductive metal such as silver or copper. Grid 106 need not be continuous throughout the surface of substrate 102. A lack of continuity may result in improved bonding and electrical contact during subsequent assembly steps, as described below.

Dielectric strip 108 is preferably substantially transparent to increase the amount of the surface area of the cell which is exposed to the sun through upper module portion 100, and is applied in a configuration, for example as depicted in Fig. 1 , to provide electrical separation between photovoltaic cells in a subsequently assembled module. Suitable dielectric materials may include, for example, transparent UV curable dielectric materials, PET, PEN, PEEK, and tapes containing PET, PEN, or PEEK along with a pressure sensitive adhesive.

Fig. 3 is a plan view of a lower portion or substrate, generally indicated at 150.

Lower module portion 150 includes a second or bottom interconnection substrate 152, an adhesive layer 154 disposed on the top surface of substrate 152, and a conductive grid 156 disposed on portions of adhesive layer 154. Grid 156 includes multiple sub-grid patterns, each having bus bar 158 near an edge of the pattern and perpendicular to the long axis of the substrate. Fig. 4 is a magnified cross sectional view through lower portion 150, showing details of how conductive grid 156 and bus bar 158 may be disposed on substrate 152.

Substrate 152 of lower module portion 150 may be similar or identical in its characteristics to substrate 102 of upper module portion 100, and this similarity may help to minimize stresses (such as thermal stress) on the assembled module. Alternatively, or in addition to including a polymer layer, substrate 152 may incorporate a vapor barrier. For reasons that will become apparent below, the bottom interconnection substrate need not be transparent. Accordingly, substrate 152 may be constructed partially or entirely from a non-transparent material such as a polyvinyl fluoride (PVF), polycarbonate, printed circuit board, aluminum foil, or some other metallic foil. In some cases, substrate 152 may include both a metallic foil and a polymer, combined to form a multi-layer laminated structure.

Adhesive layer 154 may be formed from the same adhesive as adhesive layer 104, or layer 154 may be formed from an alternative non-transparent (or less transparent) adhesive with other beneficial properties such as increased vapor resistance relative to the adhesive used in layer 104. Conductive grid 156 will typically be similar to grid 106, except grid 156 also includes conductive bus bars 158. As will be described below, bus bars 158 form electrical connections between adjacent photovoltaic cells in an assembled module.

Fig. 5 is a plan view of upper module portion 100, also showing a plurality of photovoltaic cells 120 attached to upper module portion 100. Cells 120 may be thin film cells manufactured and/or cut in a roll-to-roll process. For example, cells 120 may be of the type described in U.S. Patent No. 6,310,281 , No. 6,372,538, and No. 7,194,197 to Wendt et al., which are hereby incorporated by reference in their entirety.

Each of cells 120 is attached to the upper module portion 100 with its photoactive side (anode or "sunny side") facing conductive grid 106, and is held in place by adhesive layer 104 to minimize movement between cell 120 and substrate 102 during subsequent processing steps and in the final product configuration. In some cases, heat may be applied to further bond each cell 120 to adhesive layer 104. In addition, as Fig. 5 depicts, one edge of each cell 120 is positioned, partially overlapping a strip of dielectric 108, to provide an electrically insulating barrier between the back side (cathode, visible in Fig. 5) of cell 120 and conductive grid 106.

Fig. 6 shows an exploded view of a photovoltaic module 200 including multiple photovoltaic cells 120 sandwiched between upper and lower portions 100 and 150. Upper module portion 100 and lower portion 150 sandwich photovoltaic cells 120. Upper and lower substrates 100 and 150 are aligned so that bus bar 158a on lower portion 150 connects with an edge of a corresponding grid pattern 106a on upper module portion 100. Accordingly, to establish an electrical connection between adjacent cells, the lower module portion is oriented with grid 156 facing the back side of cells 120, and is aligned with the upper module portion so that the parts of grid 106 disposed at the edges of each cell overlap bus bars 158 of grid 156. Additionally, conductive ribbons 130 may be attached at each end of module 200 to make electrical contact with cells 120 disposed at each end, thereby creating positive and negative contacts at opposite ends of the photovoltaic module.

Fig. 7 shows a cross sectional view of a central portion of interconnected photovoltaic module 200 of Fig. 6, including additional details of how electrical contacts are formed between adjacent photovoltaic cells 120 within module 200. More specifically, Fig. 7 shows how one side of each cell 120 (anode side) is in electrical contact with a portion of conductive grid 106, and the other side of each cell (cathode side) is in electrical contact with a portion of conductive grid 156. Furthermore, Fig. 7 shows how edge portions of grid 106 overlap bus bars 158, forming an electrical series connection between each pair of adjacent cells, namely, an electrical connection from the anode, or sunny side of one cell, to the cathode side of an adjacent cell. In this manner, any desired number of photovoltaic cells may be connected to attain a desired module length and voltage.

Fig. 8 is a flowchart depicting a method of assembling an interconnected module of photovoltaic cells, generally indicated at 300. Method 300 is suitable for assembling an interconnected photovoltaic module such as module 200 of Figs. 6-7, among others. Accordingly, materials suitable to be used at each step of method 300 include those described above with respect to Figs. 1 -7.

At step 302, a conductive collection grid is applied to a first substrate coated with an adhesive on one side. The collection grid is applied on the same side as the adhesive. The conductive grid need not be continuous across the entire substrate. A lack of continuity of the grid may result in improved bonding and electrical contact during a subsequent lamination step. At step 304, a transparent dielectric strip is applied to the first substrate at selective locations as shown, for example, at 108 in Fig. 1 . In general, the applied dielectric is configured to overlap the edge portions of subsequently applied photovoltaic cells, to help prevent an electrical short circuit between opposite polarity sides of the same cell.

At step 306, a second conductive collection grid is applied to a second substrate coated with an adhesive on one side. As in the case of step 302, the collection grid of step 306 is applied on the same side as the adhesive, and need not be continuous across the entire substrate. The grid applied in this step includes relatively thicker bus bars as depicted, for example, at 158 in Fig. 3. The bus bars help to establish electrical contact between adjacent cells when the module is fully assembled and laminated together. As described previously, the second substrate may be similar to the first substrate, but need not be transparent because solar radiation need not penetrate the second substrate.

At step 308, photovoltaic cells are placed along the adhesive-coated side of the first substrate, with the photoactive or "sunny" side of each cell facing the conductive collection grid. The cells are positioned so that the long edge of each cell is overlapped by the dielectric that was applied in step 304, to help prevent the cells from short circuiting. Pressure and/or heat may be applied to help form a secure bond between the cells and the substrate.

At step 310, conductive ribbons are positioned to make electrical contact with particular cells, to form strings of a desired length (i.e., a desired number of interconnected cells). One of the ribbons is placed to make electrical contact with a cell through the conductive grid of the first substrate, and the other ribbon is placed so that it will make electrical contact with another cell through the conductive grid of the second substrate after the second substrate is attached to the first substrate in a subsequent step. Thus, the potential difference between the two ribbons will be equal to the voltage generated by the string of photovoltaic cells bounded by the cells to which the ribbons are attached.

At step 312, the second substrate is aligned with and then attached to the first substrate, with the conductive grid of the second substrate facing the back side (the side opposite from the "sunny" side) of the photovoltaic cells. More specifically, the second substrate is aligned with the first substrate so that the free ends of the conductive grid on the first substrate overlap the thick bus bars of the conductive grid on the second substrate. At step 314, the entire interconnected structure may be laminated to fix the components in place and protect them from the elements.

The method described above and shown in Fig. 8 may be modified in many different ways for various purposes. For example, sub-combinations of the steps may be practiced independently or in conjunction with other steps or methods. Additional steps may be added. Steps may be deleted, and the order of steps may also be modified in accordance with principals described in this disclosure.

Figs 9-14 illustrate aspects of another laminated photovoltaic module embodiment. The module design described below uses substantially identical conductive grid patterns on top and bottom substrates, with one pattern inverted relative to the other for interconnecting adjacent photovoltaic cells in one laminated module.

Fig. 9 shows a continuous roll 400 of a flexible substrate material, which will form a component of a laminated photovoltaic module, as explained in more detail below. Substrate 400 is preferably made of a clear polymer. For example, suitable substrates may be constructed from PET, PEN, PEEK, polyimide, ETFE, among others. Adhesive 404 is applied across the surface of substrate 400. Adhesive 404 may be substantially transparent. Suitable adhesives may include, for example, EVA, polyolefins, ionomers, PVB, and various urethanes, among others. Grid patterns 408 are applied on top of adhesive layer 404. Grid patterns 408 are conductive and may, for example, be made of copper or silver. Grid pattern 408 may be applied to the adhesive-coated substrate by any suitable method, such as printing. As shown in Fig. 9, grid pattern 408 includes a plurality of separated and discreet sub-patterns, each sub-pattern having a central array 412 of parallel lines or strips, and a pair of projecting side arms 416 on opposite sides of a given sub-pattern. Each side arm 416 has a plurality of holes 420 which allow adhesive from layer 404 to pass through and adhere to another sheet (not shown in Fig. 9), as shown in subsequent figures, and described in further detail below.

Fig. 10 shows the same substrate component of Fig. 9, including grid 408 on adhesive layer 404 on top of flexible substrate 400. Additionally, dielectric paint or tape 424 is applied to cover edge portions of grid lines 412 while leaving exposed side arms 416. Dielectric strips 424 help to avoid shunts or shorts between positive and negative sides of photovoltaic cells which will be placed on the substrate in subsequent processing steps.

Fig. 1 1 shows the same intermediate article of manufacture shown in Fig. 10, with the addition of photovoltaic cells 430. As shown in Fig. 1 1 , edges 434 of photovoltaic cells 430 overlap dielectric strips 424, however, are separated from side arms 416.

Fig. 12 shows an exploded view of a series of photovoltaic cells 430 sandwiched between substrates 400 and 400a. Bottom substrate 400 includes grid patterns and adhesive, as described above. Top substrate 400a may be prepared in a substantially identical configuration as substrate 400. In an assembly step, photovoltaic cells 430 are sandwiched between the substrates such that the grid pattern for substrate 400a is inverted relative to the grid pattern for substrate 400. In other words, side arms 416 project in an opposite direction from side arms 416a on substrate 400a. Therefore, when the components shown in Fig. 12 are fully assembled, conductive arms 416 of substrate 400 overlap with corresponding arms 416a of substrate 400a, thereby establishing electrical interconnection between the "sunny" side (anode) of one cell and the back side (cathode) of an adjacent cell.

Fig. 13 shows a portion of a photovoltaic module with portions cut away for illustration purposes. Elements of the module shown in Fig. 13 are the same as shown in Fig. 12, except after assembly and from a top plan view. On the left hand side of Fig. 13, substrate 400a includes conductive grid lines 412a and arm 416a which contacts and overlaps arm 416 from lower substrate 400. In the center of Fig. 13, upper substrate 400a has been cut away exposing photovoltaic cells 430. On the right side of Fig. 13, cells have been cut away exposing conductive grid elements 412, dielectric strips 424, and arms 416 of substrate 400.

Fig. 14 shows a cross section through the photovoltaic module of Fig. 13. Bottom substrate 400 supports conductive grid arms 416 which contact and overlap grid arms 416a on substrate 400a. Dielectric strips 424 and 424a overlap the side edges of photovoltaic cell 430.

Fig. 15 illustrates steps in a method of manufacturing photovoltaic modules, for example, as described above. For example, manufacturing method 500 includes a first step 510 of applying a first conductive grid to a first substrate coated with adhesive on one side. In a preferred embodiment, the grid pattern has one or more projecting side arms. In a subsequent processing step 530, dielectric strips are applied on edges of the grid pattern. In a following step 540, photovoltaic cells are placed on the grid pattern. Subsequently in step 550, a second substrate is prepared using the same steps mentioned above with respect to the first substrate. Later step 560 includes positioning conductive ribbons at ends of a desired module length to establish positive and negative contacts. In a final step 570, photovoltaic cells are sandwiched between the first and second substrate by aligning and laminating the second substrate to the first substrate.

The method described above and shown in Fig. 15 may be modified in many different ways for various purposes. For example, sub-combinations of the steps may be practiced independently or in conjunction with other steps or methods. Additional steps may be added. Steps may be deleted, and the order of steps may also be modified in accordance with principals described in this disclosure.

In the embodiment described above, the grid patterns are substantially identical on the front and back sheets, which provides symmetry and simplicity in the manufacturing process. However, it should be appreciated that distinguishable grid patterns on the front and back sheets may be advantageous. For example, the front grid pattern should be optimal for permitting sun to reach the photovoltaic cell and for collecting and transmitting current from the top side of the cell. On the other side there is no need to permit light to reach the backside of the cell. Therefore, the back grid pattern may use thicker more expansive conductive lines to cut down on resistance. The conductive arms may still be identical on the two substrates for overlapping and interconnecting adjacent cells.

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 appropriate conductivity characteristics, and sufficient structural strength to withstand the loads incurred during typical use.

Materials may be selected based on their conductivity, durability, flexibility, weight, and/or aesthetic qualities.

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.