Collector grid, electrode structures and interrconnect structures for photovoltaic arrays and methods of manufacture
The invention teaches novel structure and methods for producing electrical current collectors and electrical interconnection structure. Such articles find particular use in facile production of modular arrays of photovoltaic cells. The current collector and interconnecting structures may be initially produced separately from the photovoltaic cells thereby allowing the use of unique materials and manufacture. Subsequent combination of the structures with photovoltaic cells allows facile and efficient completion of modular arrays. Methods for combining the collector and interconnection structures with cells and final interconnecting into modular arrays are taught
This application is a Continuation-in-Part of U.S. patent application Ser. No. 11/824,047 filed Jun. 30, 2007, entitled Collector Grid, Electrode Structures and Interconnect Structures for Photovoltaic Arrays and other Optoelectric Devices, which is a Continuation-in-Part of U.S. application Ser. No. 11/404,168 filed Apr. 13, 2006, entitled Substrate and Collector Grid Structures for Integrated Photovoltaic Arrays and Process of Manufacture of Such Arrays, which is a Continuation-in-Part of U.S. application Ser. No. 10/776,480 filed Feb. 11, 2004, entitled Methods and Structures for the Continuous Production of Metallized or Electrically Treated Articles, now abandoned, which is a Continuation-in-Part of U.S. patent application Ser. No. 10/682,093 filed Oct. 8, 2003 entitled Substrate and Collector Grid Structures for Integrated Series Connected Photovoltaic Arrays and Process of Manufacture of Such Arrays, which is a Continuation-in-Part of U.S. patent application Ser. No. 10/186,546 filed Jul. 1, 2002, entitled Substrate and Collector Grid Structures for Integrated Series Connected Photovoltaic Arrays and Process of Manufacture of Such Arrays, now abandoned, which is a Continuation-in-Part of U.S. patent application Ser. No. 09/528,086, filed Mar. 17, 2000, entitled Substrate and Collector Grid Structures for Integrated Series Connected Photovoltaic Arrays and Process of Manufacture of Such Arrays, and now U.S. Pat. No. 6,414,235, which is a Continuation-in-Part of U.S. patent application Ser. No. 09/281,656, filed Mar. 30, 1999, entitled Substrate and Collector Grid Structures for Electrically Interconnecting Photovoltaic Arrays and Process of Manufacture of Such Arrays, and now U.S. Pat. No. 6,239,352. The entire contents of the above identified applications are incorporated herein by this reference.BACKGROUND OF THE INVENTION
This invention teaches novel structure and methods for achieving efficient collection and conveyance of power from photovoltaic power generating devices.
Photovoltaic cells have developed according to two distinct methods. The initial operational cells employed a matrix of single crystal silicon appropriately doped to produce a planar p-n junction. An intrinsic electric field established at the p-n junction produces a voltage by directing solar photon produced holes and free electrons in opposite directions. Despite good conversion efficiencies and long-term reliability, widespread energy collection using single-crystal silicon cells is thwarted by the high cost of single crystal silicon material and interconnection processing.
A second approach to produce photovoltaic cells is by depositing thin photovoltaic semiconductor films on a supporting substrate. Material requirements are minimized and technologies can be proposed for mass production. Thin film photovoltaic cells employing amorphous silicon, cadmium telluride, copper indium gallium diselenide, dye sensitized solar cells (DSSC), printed silicon inks and the like have received increasing attention in recent years.
Despite significant improvements in individual cell conversion efficiencies for both single crystal and thin film approaches, photovoltaic energy collection has been generally restricted to applications having low power requirements. One factor characteristic of many optoelectric devices and photovoltaic cells in particular is that electrical energy is produced over a relatively expansive surface area. Thus a challenge to implementing bulk power systems is the problem of economically collecting the photogenerated power from an expansive surface. In particular, photovoltaic cells can be described as high current, low voltage devices. Typically individual cell voltage is less than about two volts, and often less than 0.6 volt. The current component is a substantial characteristic of the power generated. Efficient power collection from expansive photovoltaic cell surfaces must minimize resistive losses associated with the high current characteristic. In the specific case of most photovoltaic cells, the upper surface is normally formed by a transparent conductive oxide (TCO). However, these TCO layers are relatively resistive compared to pure metals and have a surface resistivity on the order of 10 to 100 ohms per square. Thus the conductive surface itself is limited in its ability to collect and transport current and efforts must be made to minimize resistive losses in transport of current through the TCO layer. This problem increases in severity as individual cell sizes increase. One solution is to simply reduce individual cell size (and thus accumulated current from an individual cell) to a point where the transparent conductive oxide alone can handle the current. Where larger individual cell sizes are the norm, it is common practice to augment the transparent conductive oxide with a current collector structure comprising a pattern of highly conductive traces extending over substantially the entire surface from which current is to be collected. Often the structure is in the form of a grid or lattice pattern. The current collector structure reduces the distance that current must be transported by the transparent conductive oxide before it reaches a highly conductive conveyance path off the surface. Thus the current collector structure collects current from a surface having relatively low surface conductivity. Many current collector structures or grids are conventionally prepared by first applying metal wires, fused silver filled pastes or silver filled ink traces to the cell surface and then covering the surface with a sealing material in a subsequent operation. These highly conductive traces may lead to a collection buss such as a copper foil strip which also functions as a tab extending to the back electrode of an adjacent cell. The wire approach requires positioning and fixing of multiple fine fragile wires which makes mass production difficult and expensive. Silver pastes are expensive and require high fusion temperatures which not all photovoltaic semiconductors can tolerate. A silver filled ink, as compared to a fuseable paste, is simply dried or cured at mild temperatures which do not adversely affect the cell. However, this ink approach requires the use of relatively expensive inks because of the high loading of finely divided silver particles. In addition, batch printing on the individual cells is laborious and expensive. Finally, the silver filled ink is relatively resistive compared to a fuseable silver paste or metal wire. Typical silver filled inks have intrinsic resistivities in the range 0.00002 to 0.01 ohm-cm.
Thus there remains a need for improved materials and structure for collecting the current from the top light incident surface of photovoltaic cells.
Normally one envisions a photovoltaic power collection device much larger than the size of an individual cell. Therefore, an arrangement must be supplied to collect power from multiple cells. This is normally accomplished by interconnecting multiple cells in series. In this way, voltage is stepped through each cell while current and associated resistive losses are minimized. Such interconnected multi-cell arrangements are commonly referred to as “modules” or “arrays”. However, it is readily recognized that making effective, durable series connections among multiple small cells can be laborious, difficult and expensive. Regarding traditional crystalline silicon cells, the individual cells are normally discrete and comprise rigid wafers approximately 200 micrometers thick and approximately 230 square centimeters in area. A common way to convert multiple such cells into modules is to use a conventional “string and tab” arrangement. In this process multiple discrete cells are arranged in “strings” and the topside current collector electrodes of cells are connected to backside electrodes of adjacent cells using “tabs” or ribbons of conductive material. The cell connections often involve tedious manual operations such as soldering and handling of multiple interconnected cells. Next, unwieldy flexible leads from the terminal cells must be directed and secured in position for outside connections, again a tedious operation. Finally, weight and assembly concerns limit the ultimate size of the module. These limitations impede adoption of the modules for large scale power generation.
In order to approach economical mass production of modules of series connected individual cells, a number of factors must be considered in addition to the type of photovoltaic materials chosen. These include the substrate employed and the process envisioned. Since thin films can be deposited over expansive areas, thin film technologies offer additional opportunities for mass production of interconnected modules compared to inherently small, discrete single crystal silicon cells. Thus a number of U.S. patents have issued proposing designs and processes to achieve series interconnections among thin film photovoltaic cells. Many of these technologies comprise deposition of photovoltaic thin films on glass substrates followed by scribing to form smaller area individual cells. Multiple steps then follow to electrically connect the individual cells in series while maintaining the original common glass substrate. These “common” substrate approaches have come to be known as “monolithic integration”. Examples of these proposed processes are presented in U.S. Pat. Nos. 4,443,651, 4,724,011, and 4,769,086 to Swartz, Turner et al. and Tanner et al. respectively. While expanding the opportunities for mass production of interconnected cell modules compared with inherently discrete approaches for crystal silicon cells, monolithic integration employing common glass substrates must inherently be performed on an individual batch basis. In addition, many monolithic approaches are not compatible with the use of a current collector grid and therefore cell sizes (in the direction of current flow) are constrained. Typically, cell widths for monolithic integration between 0.5 cm. and 1.0 cm. are taught in the art. However, as cell widths decrease, the width of the area between individual cells (interconnect area) should also decrease so that the relative portion of inactive surface of the interconnect area does not become excessive. These small cell widths demand very fine interconnect area widths, which dictate delicate and sensitive techniques to be used to electrically connect the top TCO surface of one cell to the bottom electrode of an adjacent series connected cell. Furthermore, achieving good stable ohmic contact to the TCO cell surface has proven difficult, especially when one employs those sensitive techniques available when using the TCO only as the top collector electrode.
More recently, developers have explored depositing wide area films using continuous roll-to-roll processing. This technology generally involves depositing thin films of photovoltaic material onto a continuously moving web. However, a challenge still remains regarding subdividing the expansive films into individual cells followed by interconnecting into a series connected array. For example, U.S. Pat. No. 4,965,655 to Grimmer et. al. and U.S. Pat. No. 4,697,041 to Okamiwa teach processes requiring expensive laser scribing and interconnections achieved with laser heat staking. In addition, these two references teach a substrate of thin vacuum deposited metal on films of relatively expensive polymers. The electrical resistance of thin vacuum metallized layers may significantly limit the active area of the individual interconnected cells.
It has become well known in the art that the efficiencies of certain promising thin film photovoltaic junctions can be substantially increased by high temperature treatments. These treatments involve temperatures at which even the most heat resistant plastics suffer rapid deterioration. Use of a metal foil as a substrate allows high temperature treatments and continuous roll-to-roll processing. However, the subsequent conversion to an interconnected module of multiple cells has proven difficult, in part because the metal foil substrate is electrically conducting.
U.S. Pat. No. 4,746,618 to Nath et al. teaches a design and process to achieve interconnected arrays using roll-to-roll processing of a metal web substrate such as stainless steel. The process includes multiple operations of cutting, selective deposition, and riveting. These operations add considerably to the final interconnected array cost.
U.S. Pat. No. 5,385,848 to Grimmer teaches roll-to-roll methods to achieve integrated series connections of adjacent thin film photovoltaic cells supported on an electrically conductive metal substrate. The process includes mechanical or chemical etch removal of a portion of the photovoltaic semiconductor and transparent top electrode to expose a portion of the electrically conductive metal substrate. The exposed metal serves as a contact area for interconnecting adjacent cells. These material removal techniques are troublesome for a number of reasons. First, many of the chemical elements involved in the best photovoltaic semiconductors are expensive and environmentally unfriendly. This removal subsequent to controlled deposition involves containment, dust and dirt collection and disposal, and possible cell contamination. This is not only wasteful but considerably adds to expense. Secondly, the removal processes are difficult to control dimensionally. Thus a significant amount of the valuable photovoltaic semiconductor is lost to the removal process. Ultimate module efficiencies are further compromised in that the spacing between adjacent cells grows, thereby reducing the effective active collector area for a given module area.
Thus there remains a need for acceptable mass manufacturing processes and articles to achieve effective integrated interconnections among photovoltaic cells.
A further issue that has impeded adoption of photovoltaic technology, especially for bulk power collection in the form of solar farms, involves installation of multiple modules over expansive regions of surface. Traditionally, modules have been mounted individually on supporting mounts, normally at an incline to horizontal appropriate to the latitude of the site. Conducting leads from each module are then physically coupled with leads from an adjacent module in order to interconnect multiple modules. This arrangement results in a string of modules each of which is coupled to an adjacent module. At one end of the string, the power is transferred from the end module to be conveyed to a separate site for further treatment such as voltage adjustment. This arrangement avoids having to run conductive cabling from each individual module to the separate treatment site.
The traditional solar farm installation described in the above paragraph has some drawbacks. First, traditional modules are limited in size due to weight and manufacturing constraints. This fact increases the number of individual modules required to cover a desired surface area. Next, the module itself comprises a string of individual cells. In the conventional module lead conductors in the form of flexible wires or ribbons are attached to an electrode on the two cells positioned at each end of the string in order to convey the power from the module. After mounting the individual modules on their support at the installation site, the respective leads from adjacent modules must be connected in order to couple adjacent modules, and the connection must be protected to avoid environmental deterioration or separation. These are intrinsically tedious manual operations. Finally, since the module leads and cell interconnections are not of high current carrying capacity, the adjacent cells are normally connected in series arrangement. Thus voltage builds up to high levels even at relatively short strings of modules. While not an overriding problem security and insulation must be appropriate to eliminate a shock hazard.
Thus there remains a need for improved module form factors and complimentary installation structure to reduce the cost and complexity of achieving large area “utility” scale photovoltaic installations.
In a somewhat removed segment of technology, a number of electrically conductive fillers have been used to produce electrically conductive polymeric materials. This technology generally involves mixing of a conductive filler such as silver particles with the polymer resin prior to fabrication of the material into its final shape. Conductive fillers may have high aspect ratio structure such as metal fibers, metal flakes or powder, or highly structured carbon blacks, with the choice based on a number of cost/performance considerations. More recently, fine particles of intrinsically conductive polymers have been employed as conductive fillers within a resin binder. Electrically conductive polymers have been used as bulk thermoplastic compositions, or formulated into paints and inks. Their development has been spurred in large part by electromagnetic radiation shielding and static discharge requirements for plastic components used in the electronics industry. Other known applications include resistive heating fibers and battery components and production of conductive patterns and traces. The characterization “electrically conductive polymer” covers a very wide range of intrinsic resistivities depending on the filler, the filler loading and the methods of manufacture of the filler/polymer blend. Resistivities for filled electrically conductive polymers may be as low as 0.00001 ohm-cm. for very heavily filled silver inks, yet may be as high as 10,000 ohm-cm or even more for lightly filled carbon black materials or other “anti-static” materials. “Electrically conductive polymer” has become a broad industry term to characterize all such materials. In addition, it has been reported that recently developed intrinsically conducting polymers (absent conductive filler) may exhibit resistivities comparable to pure metals.
In yet another separate technological segment, coating plastic substrates with metal electrodeposits has been employed to achieve decorative effects on items such as knobs, cosmetic closures, faucets, and automotive trim. The normal conventional process actually combines two primary deposition technologies. The first is to deposit an adherent metal coating using chemical (electroless) deposition to first coat the nonconductive plastic and thereby render its surface highly conductive. This electroless step is then followed by conventional electroplating. ABS (acrylonitrile-butadiene-styrene) plastic dominates as the substrate of choice for most applications because of a blend of mechanical and process properties and ability to be uniformly etched. The overall plating process comprises many steps. First, the plastic substrate is chemically etched to microscopically roughen the surface. This is followed by depositing an initial metal layer by chemical reduction (typically referred to as “electroless plating”). This initial metal layer is normally copper or nickel of thickness typically one-half micrometer. The object is then electroplated with metals such as bright nickel and chromium to achieve the desired thickness and decorative effects. The process is very sensitive to processing variables used to fabricate the plastic substrate, limiting applications to carefully prepared parts and designs. In addition, the many steps employing harsh chemicals make the process intrinsically costly and environmentally difficult. Finally, the sensitivity of ABS plastic to liquid hydrocarbons has prevented certain applications. ABS and other such polymers have been referred to as “electroplateable” polymers or resins. This is a misnomer in the strict sense, since ABS (and other nonconductive polymers) are incapable of accepting an electrodeposit directly and must be first metallized by other means before being finally coated with an electrodeposit. The conventional technology for electroplating on plastic (etching, chemical reduction, electroplating) has been extensively documented and discussed in the public and commercial literature. See, for example, Saubestre, Transactions of the Institute of Metal Finishing, 1969, Vol. 47, or Arcilesi et al., Products Finishing, March 1984.
Many attempts have been made to simplify the process of electroplating on plastic substrates. Some involve special chemical techniques to produce an electrically conductive film on the surface. Typical examples of this approach are taught by U.S. Pat. No. 3,523,875 to Minklei, U.S. Pat. No. 3,682,786 to Brown et. al., and U.S. Pat. No. 3,619,382 to Lupinski. The electrically conductive film produced was then electroplated. None of these attempts at simplification have achieved any recognizable commercial application.
A number of proposals have been made to make the plastic itself conductive enough to allow it to be electroplated directly thereby avoiding the “electroless plating” process. As noted above, it is common to produce electrically conductive polymers by incorporating conductive or semiconductive fillers into a polymeric binder. Investigators have attempted to produce electrically conductive polymers capable of accepting an electrodeposited metal coating by loading polymers with relatively small conductive particulate fillers such as graphite, carbon black, and silver or nickel powder or flake. Heavy such loadings are sufficient to reduce volume resistivity to a level where electroplating may be considered. However, attempts to make an acceptable electroplateable polymer using the relatively small metal containing fillers alone encounter a number of barriers. First, the most conductive fine metal containing fillers such as silver are relatively expensive. The loadings required to achieve the particle-to-particle proximity to achieve acceptable conductivity increases the cost of the polymer/filler blend dramatically. The metal containing fillers are accompanied by further problems. They tend to cause deterioration of the mechanical properties and processing characteristics of many resins. This significantly limits options in resin selection. All polymer processing is best achieved by formulating resins with processing characteristics specifically tailored to the specific process (injection molding, extrusion, blow molding, printing etc.). A required heavy loading of metal filler severely restricts ability to manipulate processing properties in this way. A further problem is that metal fillers can be abrasive to processing machinery and may require specialized screws, barrels, and the like.
Another major obstacle involved in the electroplating of electrically conductive polymers is a consideration of adhesion between the electrodeposited metal and polymeric substrate (metal/polymer adhesion). In most cases sufficient adhesion is required to prevent metal/polymer separation during extended environmental and use cycles. Despite being electrically conductive, a simple metal-filled polymer offers no assured bonding mechanism to produce adhesion of an electrodeposit since the metal particles may be encapsulated by the resin binder, often resulting in a resin-rich “skin”.
A number of methods to enhance electrodeposit adhesion to electrically conductive polymers have been proposed. For example, etching of the surface prior to plating can be considered. Etching can be achieved by immersion in vigorous solutions such as chromic/sulfuric acid. Alternatively, or in addition, an etchable species can be incorporated into the conductive polymeric compound. The etchable species at exposed surfaces is removed by immersion in an etchant prior to electroplating. Oxidizing surface treatments can also be considered to improve metal/plastic adhesion. These include processes such as flame or plasma treatments or immersion in oxidizing acids. In the case of conductive polymers containing finely divided metal, one can propose achieving direct metal-to-metal adhesion between electrodeposit and filler. However, here the metal particles are generally encapsulated by the resin binder, often resulting in a resin rich “skin”. To overcome this effect, one could propose methods to remove the “skin”, exposing active metal filler to bond to subsequently electrodeposited metal.
Another approach to impart adhesion between conductive resin substrates and electrodeposits is incorporation of an “adhesion promoter” at the surface of the electrically conductive resin substrate. This approach was taught by Chien et al. in U.S. Pat. No. 4,278,510 where maleic anhydride modified propylene polymers were taught as an adhesion promoter. Luch, in U.S. Pat. No. 3,865,699 taught that certain sulfur bearing chemicals could function to improve adhesion of initially electrodeposited Group VIII metals.
For the above reasons, electrically conductive polymers employing metal fillers have not been widely used as bulk substrates for electroplateable articles. Such metal containing polymers have found use as inks or pastes in production of printed circuitry. Revived efforts and advances have been made in the past few years to accomplish electroplating onto printed conductive patterns formed by silver filled inks and pastes.
An additional physical obstacle confronting practical electroplating onto electrically conductive polymers is the initial “bridge” of electrodeposit onto the surface of the electrically conductive polymer. In electrodeposition, the substrate to be plated is often made cathodic through a pressure contact to a metal rack tip, itself under cathodic potential. However, if the contact resistance is excessive or the substrate is insufficiently conductive, the electrodeposit current favors the rack tip to the point where the electrodeposit will not bridge to the substrate.
Moreover, a further problem is encountered even if specialized racking or cathodic contact successfully achieves electrodeposit bridging to the substrate. Many of the electrically conductive polymers have resistivities far higher than those of typical metal substrates. Also, many applications involve electroplating onto a thin (less than 25 micrometer) printed substrate. The conductive polymeric substrate may be relatively limited in the amount of electrodeposition current which it alone can convey. Thus, the conductive polymeric substrate does not cover almost instantly with electrodeposit as is typical with metallic substrates. Except for the most heavily loaded and highly conductive polymer substrates, a large portion of the electrodeposition current must pass back through the previously electrodeposited metal growing laterally over the surface of the conductive plastic substrate. In a fashion similar to the bridging problem discussed above, the electrodeposition current favors the electrodeposited metal and the lateral growth can be extremely slow and erratic. This restricts the size and “growth length” of the substrate conductive pattern, increases plating costs, and can also result in large non-uniformities in electrodeposit integrity and thickness over the pattern.
This lateral growth is dependent on the ability of the substrate to convey current. Thus, the thickness and resistivity of the conductive polymeric substrate can be defining factors in the ability to achieve satisfactory electrodeposit coverage rates. When dealing with selectively electroplated patterns long thin metal traces are often desired. Electrodeposited metal thicknesses of from 1 to 25 micrometer are often typical. The metal traces must normally be of relatively uniform thickness and have a minimum of internal stress. Further, an electrically conductive polymer “seed” pattern defining the traces is often relatively thin, less than about 25 micrometers, and therefore may have relatively low current carrying capacity. These factors of course often work against achieving the desired result.
This coverage rate problem likely can be characterized by a continuum, being dependent on many factors such as the nature of the initially electrodeposited metal, electroplating bath chemistry, the nature of the polymeric binder and the resistivity of the electrically conductive polymeric substrate. As a “rule of thumb”, the instant inventor estimates that coverage rate issue would demand attention if the resistivity of a bulk conductive polymeric substrate rose above about 0.001 ohm-cm. Alternatively, a “rule of thumb” appropriate for thin film substrates would be that attention is appropriate if the substrate film to be plated had a surface “sheet” resistance of greater than about 0.075 ohm per square.
The least expensive (and least conductive) of the readily available conductive fillers for plastics are carbon blacks. Attempts have been made to electroplate electrically conductive polymers using carbon black loadings. Examples of this approach are the teachings of U.S. Pat. Nos. 4,038,042, 3,865,699, and 4,278,510 to Adelman, Luch, and Chien et al. respectively. These earlier efforts were directed primarily at achieving decorative electroplated articles with the substrate fully encapsulated with electrodeposit.
Adelman taught incorporation of conductive carbon black into a polymeric matrix to achieve electrical conductivity required for electroplating. The substrate was pre-etched in chromic/sulfuric acid to achieve adhesion of the subsequently electroplated metal. A fundamental problem remaining unresolved by the Adelman teaching is the relatively high resistivity of carbon loaded polymers. The lowest “microscopic resistivity” generally achievable with carbon black loaded polymers is about 1 ohm-cm. This is about five to six orders of magnitude higher than typical electrodeposited metals such as copper or nickel. Thus, the electrodeposit bridging and coverage rate problems described above remained unresolved by the Adelman teachings.
Luch in U.S. Pat. No. 3,865,699 and Chien et al. in U.S. Pat. No. 4,278,510 also chose carbon black as a filler to provide an electrically conductive surface for the polymeric compounds to be electroplated. The Luch U.S. Pat. No. 3,865,699 and the Chien U.S. Pat. No. 4,278,510 are hereby incorporated in their entirety by this reference. However, these inventors further taught inclusion of materials to increase the rate of metal coverage or the rate of metal deposition on the polymer. These materials can be described herein as “electrodeposit growth rate accelerators” or “electrodeposit coverage rate accelerators”. An electrodeposit coverage rate accelerator is a material functioning to increase the electrodeposition coverage rate over the surface of an electrically conductive polymer independent of any incidental affect it may have on the conductivity of an electrically conductive polymer. In the embodiments, examples and teachings of U.S. Pat. Nos. 3,865,699 and 4,278,510, it was shown that certain sulfur bearing materials, including elemental sulfur, can function as electrodeposit coverage or growth rate accelerators to overcome problems in achieving electrodeposit coverage of electrically conductive polymeric surfaces having relatively high resistivity or thin electrically conductive polymeric substrates having limited current carrying capacity.
In addition to elemental sulfur, sulfur in the form of sulfur donors such as sulfur chloride, 2-mercapto-benzothiazole, N-cyclohexyle-2-benzothiaozole sulfonomide, dibutyl xanthogen disulfide, and tetramethyl thiuram disulfide or combinations of these and sulfur were identified. Those skilled in the art will recognize that these sulfur donors are the materials which have been used or have been proposed for use as vulcanizing agents or accelerators. Since the polymer-based compositions taught by Luch and Chien et al. could be electroplated directly they could be accurately defined as directly electroplateable resins (DER). These directly electroplateable resins (DER) can be generally described as electrically conductive polymers with the inclusion of a growth rate accelerator.
Specifically for the present invention, specification, and claims, directly electroplateable resins, (DER), are characterized by the following features:
- (a) presence of an electrically conductive polymer;
- (b) presence of an electrodeposit coverage rate accelerator;
- (c) presence of the electrically conductive polymer and the electrodeposit coverage rate accelerator in the directly electroplateable composition in cooperative amounts required to achieve direct coverage of the composition with an electrodeposited metal or metal-based alloy.
In his Patents, Luch specifically identified unsaturated elastomers such as natural rubber, polychloroprene, butyl rubber, chlorinated butyl rubber, polybutadiene rubber, acrylonitrile-butadiene rubber, styrene-butadiene rubber etc. as suitable for the matrix polymer of a directly electroplateable resin. Other polymers identified by Luch as useful included polyvinyls, polyolefins, polystyrenes, polyamides, polyesters and polyurethanes.
Using the materials and loadings reported, the carbon black/polymer formulations of Adelman, Luch and Chien referenced above would be expected to have intrinsic “microscopic” resistivities of less than about 1000 ohm-cm. (i.e. 1 ohm-cm., 10 ohm-cm., 100 ohm-cm., 1000 ohm-cm.). When used alone, the minimum workable level of carbon black required to achieve “microscopic” electrical resistivities of less than 1000 ohm-cm. for a polymer/carbon black mix appears to be about 8 weight percent based on the combined weight of polymer plus carbon black. The “microscopic” material resistivity generally is not reduced below about 1 ohm-cm. by using conductive carbon black alone. This is several orders of magnitude larger than typical metal resistivities or resistivities associated with common silver filled inks.
It is understood that in addition to carbon blacks, other well known, highly conductive fillers can be considered to decrease the “microscopic” resistivity of DER compositions. Examples include but are not limited to metallic fillers or flake such as silver. In these cases the more highly conductive fillers can be used to augment or even replace the conductive carbon black. Furthermore, one may consider using intrinsically conductive polymers to supply the required conductivity. For example, an intrinsically conductive polymer in particulate form may be considered as a conductive filler.
The “bulk, macroscopic” resistivity of conductive carbon black filled polymers can be further reduced by augmenting the carbon black filler with additional highly conductive, high aspect ratio fillers such as metal containing fibers. This can be an important consideration in the success of certain applications. Furthermore, one should realize that incorporation of non-conductive fillers may increase the “bulk, macroscopic” resistivity of conductive polymers loaded with finely divided conductive fillers without significantly altering the “microscopic resistivity” of the conductive polymer “matrix” encapsulating the non-conductive filler particles. It has been found that DER formulations can include substantial quantities of non-conductive fillers. In particular, loading of DER formulations with glass fibers has been shown to dramatically reduce mold shrinkage and increase stiffness of these formulations.
Regarding electrodeposit coverage rate accelerators, both Luch and Chien et al. in the above discussed U.S. patents demonstrated that sulfur and other sulfur bearing materials such as sulfur donors and vulcanization accelerators function as electrodeposit coverage rate accelerators when using an initial Group VIII metal electrodeposit “strike” layer. Thus, an electrodeposit coverage rate accelerator need not be electrically conductive, but may be a material that is normally characterized as a non-conductor. The coverage rate accelerator need not appreciably affect the conductivity of the polymeric substrate. As an aid in understanding the function of an electrodeposit coverage rate accelerator the following is offered:
- a. A specific conductive polymeric structure is identified as having insufficient current carrying capacity to be directly electroplated in a practical manner.
- b. A material is added to the conductive polymeric material forming said structure. Said material addition may have insignificant affect on the current carrying capacity of the structure (i.e. it does not appreciably reduce resistivity or increase thickness). The material need only be present at the substrate surface.
- c. Nevertheless, inclusion of said material greatly increases the speed at which an electrodeposited metal laterally covers the electrically conductive surface.
It is contemplated that a coverage rate accelerator may be present as an additive, as a species absorbed on a filler surface, or even as a functional group attached to the polymer chain. One or more growth rate accelerators may be present in a directly electroplateable resin (DER) to achieve combined, often synergistic results.
A hypothetical example might be an extended trace of conductive ink having a dry thickness of 1 micrometer. Such inks typically include a conductive filler such as silver, nickel, copper, conductive carbon etc. The limited thickness of the ink reduces the current carrying capacity of this trace thus preventing direct electroplating in a practical manner. However, inclusion of an appropriate quantity of a coverage rate accelerator may allow the conductive trace to be directly electroplated in a practical manner.
One might expect that other Group 6A elements, such as oxygen, selenium and tellurium, could function in a way similar to sulfur. In addition, other combinations of electrodeposited metals, such as copper and appropriate coverage rate accelerators may be identified. It is important to recognize that such an electrodeposit coverage rate accelerator is important in order to achieve direct electrodeposition in a practical way onto polymeric substrates having low conductivity or very thin electrically conductive polymeric substrates having restricted current carrying ability.
It has also been found that the inclusion of an electrodeposit coverage rate accelerator promotes electrodeposit bridging from a discrete cathodic metal contact to a DER surface. This greatly reduces the bridging problems described above.
Due to multiple performance problems associated with their intended end use, the instant inventor is unaware of any recognizable commercial success for attempts to directly electroplate electrically conductive polymers in applications intended to produce decorative “bright” electroplated objects. Nevertheless, electroplating in a selective manner onto insulating substrates for functional applications remains an intriguing possibility for many applications. This is because electroplating is selective between conductive and insulating surfaces and is inexpensive. Further, a wide variety of metals and alloys can be deposited by electroplating and the deposition rates are relatively rapid. There are a number of techniques available to achieve selective electrodeposited patterns on insulating substrates. Most involve initial formation of a “seed” pattern. The “seed” pattern is formed from a material that has the ability to assist in subsequent metal electrodeposition. Typical “seed” patterns comprise metals, polymers containing electroless plating catalysts, and electrically conductive polymers. Examples of such processes follow in subparagraphs 1 through 3.
(1) An electrically conductive polymer is formed into a “seed” pattern by printing from an ink formulation onto the surface of an insulating substrate. This electrically conductive polymer “seed layer” pattern, when dried, is then subjected to a metal electroplating process to cover the pattern with a conductive metal.
(2) A polymeric composition containing a catalyst suitable for initiating chemical metal deposition is printed into a “seed layer” pattern. After appropriate activation, the article is subjected to a chemical metal deposition “electroless” plating bath. Following coverage with electroless metal, the “seed pattern”, now comprising a layered structure of polymer and chemically deposited metal, is subjected to an electroplating process to cover the pattern with electrodeposited metal.
(3) An insulating substrate is coated in its entirety with a thin film of metal. This uniform coating may be achieved, for example, using vacuum metallizing, sputtering, chemical metal deposition processing. As a next step, a mask is applied having a pattern the reverse of the eventual desired selective metal pattern. The remaining exposed pattern (reverse of the mask pattern) retains its conductive surface and thereby forms a “seed” pattern for subsequent further metal electrodeposition. This subsequent electrodeposition increases metal thickness and also may apply a final coat resistant to an eventual etch. The mask is then removed and the article etched to completely remove the metal that had been covered by the mask.
As previously noted, the current inventor is unaware of any recognizable success in attempts to use DER technology to produce decorative “bright” electroplated objects. Nevertheless, the current inventor has persisted in personal efforts to overcome certain performance deficiencies associated with the initial DER technology. Along with these efforts has come a recognition of unique and eminently suitable applications employing the DER technology for functional applications. These functional applications often have requirements, such as selectivity, fine patterning and relatively thin electrodeposits, which differ substantially from the requirements of purely decorative electroplating. Some examples of these unique applications for electroplated articles include solar cell electrical current collection grids and interconnect structures, electrodes, electrical circuits, electrical traces, circuit boards, antennas, capacitors, induction heaters, connectors, switches, and resistors. One readily recognizes that the demand for such functional applications for electroplated articles is relatively recent and has been particularly explosive during the past decade.
It is important to recognize a number of important characteristics of directly electroplateable resins (DER's) which may facilitate certain embodiments of the current invention. One such characteristic of the DER technology is its ability to employ polymer resins and formulations generally chosen in recognition of the fabrication process envisioned and the intended end use requirements. In order to provide clarity, examples of some such fabrication processes are presented immediately below in subparagraphs 1 through 9.
- (1) Should it be desired to electroplate an ink, paint, coating, or paste which may be printed or formed on a substrate, a good film forming polymer, for example a soluble resin such as an elastomer, can be chosen to fabricate a DER ink (paint, coating, paste etc.). For example, in some embodiments thermoplastic elastomers having an olefin base, a urethane base, a block copolymer base or a random copolymer base may be appropriate. In some embodiments the coating may comprise a water based latex. Other embodiments may employ more rigid film forming polymers. The DER ink composition can be tailored for a specific process such flexographic printing, rotary silk screening, gravure printing, ink jet printing, flow coating, spraying etc. Furthermore, additives such as tackifiers or curatives can be employed to improve the adhesion of the DER ink to various substrates.
- (2) Very thin DER traces often associated with collector grid structures can be printed and then electroplated due to the inclusion of the electrodeposit growth rate accelerator. Traces as thin as 1.5 micrometer have been demonstrated as practical. Silk screening has produced trace widths as little as 150 micrometers.
- (3) Should it be desired to cure the DER substrate to a 3 dimensional matrix, an unsaturated elastomer or other “curable” resin may be chosen.
- (4) DER inks can be formulated to form electrical traces on a variety of flexible substrates. For example, should it be desired to form electrical structure on a laminating film, a DER ink adherent to the sealing surface of the laminating film can be effectively electroplated with metal and subsequently laminated to a separate surface.
- (5) Should it be desired to electroplate a fabric, a DER ink can be used to coat all or selected portion of the fabric intended to be electroplated. Furthermore, since DER's can be fabricated out of the thermoplastic materials commonly used to create fabrics, the fabric itself could completely or partially comprise a DER. This would eliminate the need to coat the fabric.
- (6) Should one desire to electroplate a thermoformed article or structure, DER's would represent an eminently suitable material choice. DER's can be easily formulated using olefinic materials which are often a preferred material for the thermoforming process. Furthermore, DER's can be easily and inexpensively extruded into the sheet like structure necessary for the thermoforming process.
- (7) Should one desire to electroplate an extruded article or structure, for example a sheet or film, DER's can be formulated to possess the necessary melt strength advantageous for the extrusion process.
- (8) Should one desire to injection mold an article or structure having thin walls, broad surface areas etc. a DER composition comprising a high flow polymer can be chosen.
- (9) Should one desire to vary adhesion between an electrodeposited DER structure supported by a substrate the DER material can be formulated to supply the required adhesive characteristics to the substrate. For example, the polymer chosen to fabricate a DER ink can be chosen to cooperate with an “ink adhesion promoting” surface treatment such as a material primer or corona treatment. In this regard, it has been observed that it may be advantageous to limit such adhesion promoting treatments to a single side of the substrate. Treatment of both sides of the substrate in a roll to roll process may adversely affect the surface of the DER material and may lead to deterioration in plateability. For example, it has been observed that primers on both sides of a roll of PET film have adversely affected plateability of DER inks printed on the PET. It is believed that this is due to primer being transferred to the surface of the DER ink when the PET is rolled up.
All polymer fabrication processes require specific resin processing characteristics for success. The ability to “custom formulate” DER's to comply with these changing processing and end use requirements while still allowing facile, quality electroplating is unique among methods to electroplate onto polymeric forms.
Another important recognition regarding the suitability of DER's for the teachings of the current invention is the simplicity of the electroplating process. Unlike many conventional electroplated plastics, DER's do not require a significant number of process steps prior to actual electroplating. This allows for simplified manufacturing and improved process control. It also reduces the risk of cross contamination such as solution dragout from one process bath being transported to another process bath. The simplified manufacturing process will also result in reduced manufacturing costs.
Yet another recognition of the benefit of DER's for the teachings of the current invention is the ability they offer to selectively electroplate metal onto an article or structure. The desired metal structures of the invention often involve long yet fine metal traces. Further, the articles of the invention often consist of such metal patterns selectively positioned in conjunction with flexible insulating materials. Such selective positioning of metals can often be expensive and difficult. As discussed previously, the coverage rate accelerators included in DER formulations allow for such extended surfaces to be covered with electrodeposit in a relatively rapid and simple manner.
Yet another recognition of the benefit of DER's is their ability they to be continuously electroplated. As will be shown in later embodiments, it is often desired to continuously electroplate metal onto “seed” patterns defining specific structure. DER's are eminently suitable as “seed” patterns for such continuous electroplating.
Yet another recognition of the benefit of DER's for the teachings of the current invention is their ability to withstand the pre-treatments often required to prepare other materials for plating. For example, were a DER to be combined with a metal, the DER material would be resistant to many of the pre-treatments such as cleaning which may be necessary to electroplate the metal.
Another important recognition is the suitability of metal electrodeposition for producing articles of the current invention. Electroplating is a rapid and inexpensive metal deposition process. Electroplating allows selective deposition of a wide variety of metals and alloys. Single or multi-layered deposits may be chosen for specific attributes. Examples may include copper for conductivity and nickel, silver or gold for corrosion resistance. Electrodeposition further allows a wide range of appropriate deposit thickness to be achieved relatively quickly. The articles of the invention may require metal traces varying from about 0.1 micrometer to greater than about 100 micrometer (i.e. 0.1 micrometer, 1 micrometer, 10 micrometer 25 micrometer, 100 micrometer etc.). Such thicknesses may be readily achieved in reasonable time using metal electrodeposition.
These and other attributes of DER's and electroplating may contribute to successful articles and processing of the instant invention. However, it is emphasized that the DER technology, and more broadly electroplating onto conductive polymeric “seed” layers, is but one of a number of alternative metal deposition or positioning processes suitable to produce many of the embodiments of the instant invention. Other approaches, such as electroless metal deposition, selective metal etching, placement of metal forms such as wires or strips, stamping metal patterns and selective vacuum or chemical deposition may be suitable alternatives for producing the selectively positioned metal structures of the invention. These choices will become clear to the skilled artisan in light of the teachings to follow in the remaining specification, accompanying figures and claims.
Another important aspect of the embodiments of the current invention is the inclusion of web processing to achieve structure and combinations in a facile and economic fashion. A web is a generally planar or sheet-like structure, normally flexible, having thickness much smaller than its length or width. This sheet-like structure may also have a length far greater than its width. A web of extended length may be conveyed through one or more processing steps in a way that can be described as “continuous”, thereby achieving the advantages of continuous processing. “Continuous” web processing is well known in the paper and packaging industries. It is normally accomplished by supplying web material from a feed roll of extended length to the process steps. The product resulting from the process is often continuously retrieved onto a takeup roll following processing, in which case the process may be termed roll-to-roll or reel-to-reel processing.
An advantage of web processing is that the web can comprise many different materials, surface characteristics and forms. The web may comprise layers for packaging material options such as pressure sensitive or hot melt adhesive layers, environmental barriers, and as support for printing and other features. The web can constitute a nonporous film or may be a fabric and may be transparent or opaque. Combinations of such differences over the expansive surface of the web can be achieved. Indeed, as will be shown, the web itself can comprise materials such as conductive polymers or even metal fibers which will allow the web itself to perform electrical function. The web material may remain as part of the final article of manufacture or may be removed after processing, in which case it would serve as a surrogate or temporary support during processing.
In order to eliminate ambiguity in terminology, for the present invention the following definitions are supplied:
While not precisely definable, for the purposes of this specification, electrically insulating materials may generally be characterized as having electrical resistivities greater than 10,000 ohm-cm. Also, electrically conductive materials may generally be characterized as having electrical resistivities less than 0.001 ohm-cm. Also electrically resistive or semi-conductive materials may generally be characterized as having electrical resistivities in the range of about 0.01 ohm-cm to about 10,000 ohm-cm. The characterization “electrically conductive polymer” covers a very wide range of intrinsic resistivities depending on the filler, the filler loading and the methods of manufacture of the filler/polymer blend. Resistivities for electrically conductive polymers may be as low as 0.00001 ohm-cm. for very heavily filled silver inks, yet may be as high as 10,000 ohm-cm or even more for lightly filled carbon black materials or other “anti-static” materials. “Electrically conductive polymer” has become a broad industry term to characterize all such materials. Thus, the term “electrically conductive polymer” as used in the art and in this specification and claims extends to materials of a very wide range of resistivities from about 0.00001 ohm-cm. to about 10,000 ohm-cm and higher.
An “electroplateable material” is a material having suitable attributes that allow it to be coated with a layer of electrodeposited material.
A “metallizable material” is a material suitable to be coated with a metal deposited by any one or more of the available metallizing process, including chemical deposition, vacuum metallizing, sputtering, metal spraying, sintering and electrodeposition.
“Metal-based” refers to a material or structure having at least one metallic property and comprising one or more components at least one of which is a metal or metal-containing alloy.
“Alloy” refers to a substance composed of two or more intimately mixed materials.
“Group VIII metal-based” refers to a substance containing by weight 50% to 100% metal from Group VIII of the Periodic Table of Elements.
A “bulk metal foil” refers to a thin structure of metal or metal-based material that may maintain its integrity absent a supporting structure. Generally, metal films of thickness greater than about 2 micrometers may have this characteristic. Thus, in most cases a “bulk metal foil” will have a thickness between about 2 micrometers and 250 micrometers (i.e. 2 micrometer, 5 micrometer, 10 micrometer, 25 micrometer, 100 micrometer, 250 micrometer) and may comprise a laminate of multiple layers.
The term “monolithic” or “monolithic structure” is used in this specification and claims as is common in industry to describe an object that is made or formed into or from a single item or material.
The term “continuous form” refers to a material structure having one dimension of sufficient size such that it can be fed to or retrieved from a process over an extended time period without interruption of the material structure.
The term “continuous process” refers to a process or method in which at least one of the material feed components or a product of the process has a continuous form.
A web is a generally planar or sheet-like structure, normally flexible, having thickness much smaller than its length or width.
“Web processing” is a process wherein a web itself is altered by the process or wherein structure supported by the web is added, altered or otherwise modified by the process.
A “reel to reel” or “roll to roll” process is one wherein at least one of the feed components to a process is supplied in a continuous roll form and the product of the process is retrieved on a takeup roll.OBJECTS OF THE INVENTION
An object of the invention is to eliminate the deficiencies in the prior art methods of producing expansive area, series or parallel interconnected photovoltaic modules and arrays.
A further object of the present invention is to provide improved substrates to achieve series or parallel interconnections among photovoltaic cells.
A further object of the invention is to provide structures useful for collecting current from an electrically conductive surface.
A further object of the invention is to provide structures useful in collecting current from a surface of limited current carrying capacity such as those of many optoelectric devices including photovoltaic cells. The current collector structure comprising a pattern of highly conductive traces normally extends over a preponderance of the surface from which current is to be collected.
A further object of the present invention is to provide improved processes whereby interconnected photovoltaic modules can be economically mass produced.
A further object of the invention is to provide a process and means to accomplish interconnection of photovoltaic cells into an integrated array through continuous processing.
Other objects and advantages will become apparent in light of the following description taken in conjunction with the drawings and embodiments.SUMMARY OF THE INVENTION
The current invention provides a solution to the stated needs by producing the active photovoltaic cells and interconnecting structures separately and subsequently combining them to produce the desired interconnected array or module. One embodiment of the invention contemplates deposition of thin film photovoltaic junctions on metal foil substrates which may be heat treated following deposition if required in a continuous fashion without deterioration of the metal support structure. In a separate operation, interconnection structures are produced. In an embodiment, interconnection structures are produced in a continuous roll-to-roll fashion. In an embodiment, the interconnecting structure is laminated to the foil supported photovoltaic cell and conductive connections are applied to complete the array. Application of a separate interconnection structure subsequent to cell manufacture allows the interconnection structures to be uniquely formulated using polymer-based materials. Interconnections are achieved without the need to use the expensive and intricate material removal operations currently taught in the art to achieve interconnections.
In another embodiment, a separately prepared current collector grid structure is taught. In an embodiment the current collector structure is produced in a continuous roll-to-roll fashion. The current collector structure comprises conductive material pattern positioned on a first surface of a laminating sheet or positioning carrier sheet. This combination is prepared such that a first surface of the laminating or positioning sheet and the conductive material can be positioned in abutting contact with a conductive surface. In one embodiment the conductive surface is the light incident surface of a photovoltaic cell. In another embodiment the conductive surface is the rear conductive surface of a photovoltaic cell.
The various factors and details of the structures and manufacturing methods of the present invention are hereinafter more fully set forth with reference to the accompanying drawings wherein:
Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. In the drawings, like reference numerals designate identical, equivalent or corresponding parts throughout several views and an additional letter designation is characteristic of a particular embodiment.
In its simplest form, a photovoltaic structure combines an n-type semiconductor with a p-type semiconductor to from a p-n junction. Often an optically transparent “window electrode”, such as a thin film of zinc oxide, tin oxide indium tin oxide or the like is employed to minimize resistive losses involved in current collection.
“Window electrode” 18 normally comprises a conductive metal oxide. These materials are applied as a thin layer, normally having a thickness less than about 1 micrometer. In addition, these materials have much higher intrinsic resistivity (in the range 0.001 ohm-cm.) than metals such as copper. Thus the light incident surface of the cell, despite being characterized as conductive, has limited current carrying capacity.
In the following, photovoltaic cells having a “bulk” metal based support foil will be used to illustrate many of the embodiments and teachings of the invention. The metal based foil may often serve as the back electrode of the cell. However, those skilled in the art will recognize that many of the applications of the instant invention may not use a “bulk” foil as represented in
Those skilled in the art will readily realize that the deposition process 19 of
Referring now to
Lamination of such sheetlike films employing such sealing materials is a common practice in the packaging industry. In the packaging industry lamination is known and understood as applying a film, normally polymer based and normally having a surface comprising a sealing material, to a second surface and sealing them together with heat and/or pressure. However, while a combination of heat and pressure is often used in the lamination process, the instant invention is applicable to laminating materials, such as pressure sensitive adhesives, which may be applied using pressure alone. Suitable sealing materials may be made tacky and flowable, often under heated conditions, and retain their adhesive bond to many surfaces upon cooling and/or release of pressure. A wide variety of laminating films with associated sealing materials is possible, depending on the surface to which the adhesive seal or bond is to be made. Sealing materials such as olefin copolymers or atactic polyolefins may be advantageous, since these materials allow for the minimizing of materials which may be detrimental to the longevity of a solar cell with which it is in contact. Additional layers 74, 76 etc. may comprise materials which assist in support or processing such as polypropylene, polyethylene terepthalate and polycarbonate. Additional layers 74, 76 may comprise barrier materials such as fluorinated polymers, biaxially oriented polypropylene (BOPP), Saran, and thin compound layers such as Siox. Saran is a tradename for poly (vinylidene chloride) manufactured by Dow Chemical Corporation. Siox refers to a thin film of silicon oxide often vapor deposited on a polymer support. Additional layers 74, 76 etc. may also comprise materials intended to afford protection against ultraviolet radiation and may also comprise materials to promote curing. The instant invention does not depend on the presence of any specific material for layers 72, 74, or 76. In many embodiments substrate 70 may be generally be characterized as a laminating material. For example, the invention has been successfully demonstrated using standard laminating films sold by GBC Corp., Northbrook, Ill., 60062.
In the embodiment of
While a pattern of “fingers” and “busses” is shown in the
As indicated, structure 71 may be produced and processed extending continuously in the length “Y-71” direction. Repetitive multiple “finger/buss” arrangements are shown in the
“Fingers” 84 and “busses” 86 may comprise electrically conductive material. Examples of such materials are metal wires and foils, stamped or die cut metal patterns, conductive metal containing inks and pastes such as those having a conductive filler comprising silver or stainless steel, patterned deposited metals such as etched metal patterns or masked vacuum deposited metals, intrinsically conductive polymers and DER formulations. In a preferred embodiment, the pattern of “fingers and “busses” comprise electroplateable material such as DER or an electrically conductive ink which will enhance or allow subsequent metal electrodeposition. “Fingers” 84 and “busses” 86 may also comprise non-conductive material which would assist accomplishing a subsequent deposition of conductive material in the pattern defined by the “fingers” and “busses”. For example, “fingers” 84 or “busses” 86 could comprise a polymer which may be seeded to catalyze chemical deposition of a metal in a subsequent step. An example of such a material is seeded ABS. Patterns comprising electroplateable materials or materials facilitating subsequent electrodeposition are often referred to as “seed” patterns or layers. “Fingers” 84 and “busses” 86 may also comprise materials selected to promote adhesion of a subsequently applied conductive material. “Fingers” 84 and “busses” 86 may differ in actual composition and be applied separately. For example, “fingers” 84 may comprise a conductive ink while “buss/tab” 86 may comprise a conductive metal foil strip. Alternatively, fingers and busses may comprise a continuous unvarying monolithic material structure forming portions of both fingers and busses. Fingers and busses need not both be present in certain embodiments of the invention.
One will recognize that while shown in the embodiments as a continuous void free surface, “buss” 86 could be selectively structured. Such selective structuring may be appropriate to enhance functionality, such as flexibility, of article 71 or any article produced there from. Furthermore, regions of substrate 70 supporting the “buss” regions 86 may be different than those regions supporting “fingers” 84. For example, substrate 70 associated with “buss region” 86 may comprise a fabric while substrate 70 may comprise a solid transparent film in the region associated with “fingers” 84. A “holey” structure in the “buss region” would provide increased flexibility, increased surface area and increased structural characteristic for an adhesive to grip.
The embodiment of
While shown as two layers 88, 90, it is understood that this conductive material could comprise more than two layers or be a single layer. In addition, while each additional conductive layer is shown in the embodiment as having the same continuous monolithic material extending over both the buss and finger patterns, one will realize that selective deposition techniques would allow the additional “finger” layers to differ from additional “buss” layers. For example, as best shown in
It has been found very advantageous to form surface 98 of “fingers” 84 or top surface 100 of “busses” 86 with a material compatible with the conductive surface with which eventual contact is made. In preferred embodiments, electroless deposition or electrodeposition is used to form a suitable metallic surface. Specifically electrodeposition offers a wide choice of potentially suitable materials to form the top surface. Corrosion resistant materials such as nickel, chromium, tin, indium, silver, gold and platinum are readily electrodeposited. These functional top coatings, sometimes referred to as “flash” coatings, are often thin, less than about two micrometer (i.e. 0.1 micrometer, 1 micrometer, 2 micrometer). The “flash” coatings normally need not exhibit exceptional current carrying capacity since the bulk of the current may be carried by the underlying material such as the above described electroplated metals such as copper. When compatible, of course, surfaces comprising metals such as copper or zinc or alloys of copper or zinc may be considered. Alternatively, surfaces 98 and 100 may comprise a conversion coating, such as a chromate coating, of a material such as copper or zinc. Further, as will be discussed below, it may be highly advantageous to choose a material to form surfaces 98 or 100 which exhibits adhesive or bonding ability to a subsequently positioned abutting conductive surface. For example, it may be advantageous to form surfaces 98 and 100 using an electrically conductive adhesive. Such an adhesive could be applied intermittently, for example as a series of “dots” over the underlying conductive surface. Alternatively, it may be advantageous to form surfaces 98 of “fingers” 84 or 100 of “busses” 86 with a conductive material such as a low melting point metal such as tin, tin containing alloys, indium, lead etc. in order to facilitate electrical joining to a complimentary conductive surface. Such low melting point materials can be caused to melt at temperatures below that of many polymer processing operations such as lamination. These processes are normally carried out at temperatures below about 325 degree C. (i.e. 100 degree C., 150 degree C., 250 degree C.). One will note that materials forming “fingers” surface 98 and “buss” surface 100 need not be the same. It is emphasized that many of the principles taught in detail with reference to
“Fingers” 84a and “buss/tab” 86a of
The invention contemplates a particularly attractive conductive joining that may be achieved through a technique described herein as a laminated contact. In light of the teachings to follow one will recognize that the structures shown in
In the process of producing a laminated contact, the exposed surface of a conductive material pattern positioned on or embedded in the surface of an adhesive is brought into facing relationship with a second conductive surface to which electrical joining is intended. Heat and/or pressure are applied to soften the adhesive which then may flow around edges or through openings in the conductive pattern to also contact and adhesively “grab” the exposed second surface portions adjacent the conductive pattern. When heat and pressure are removed, the adhesive adjacent edges of the conductive pattern firmly fixes features of conductive pattern in secure mechanical contact with the second surface.
The laminated contact is particularly suitable for the electrical joining requirements of many embodiments of the instant invention. A simplified depiction of structure to assist understanding the concept of a laminated contact is embodied in
In order to achieve the laminated contact, articles 350 and 362 are brought together in the facing relationship depicted and heat and pressure are applied. The adhesive layer 351 softens and flows to contact surface 360. In the case of the
Figures illustrates a process 92 by which the current collector grids of
A “tabbed cell stock” combination 112 has a number of fundamental advantageous attributes. First, the laminated current collector electrodes protect the surfaces of the cell from defects possibly introduced by the further handing associated with final interconnections. Moreover, “tabbed cell stock” can be produced as a continuous form using a continuous cell “strip” originally produced in the “machine” direction (“Y” direction of
a. It would be anticipated that since the raw cell “strip” is continuous in the “machine” direction, performance variation would be minimized and the “tabbed cell stock” could be accumulated in takeup roll form.
b. Should there be excessive performance variation along the length of the raw cell “strip”, the “tabbed cell stock” combination may be continuously characterized, cut to standard lengths and automatically sorted prior to assembly into a module. Excessively defective cell material could be readily identified and discarded prior to final interconnection into a module. Acceptable lengths could be automatically sorted according to defined parameters and placed in appropriate cassette feeders prior to assembly into final modules.
The lamination process 92 of
It will be understood that while the process 92 of
The sectional drawings of
Structure 124 may be produced, processed and extend continuously in the length “Y-124” direction.
The embodiment of
“Fingers” 84b and “busses” 86b may comprise electrically conductive material. Examples of such materials are metal wires and foils, stamped metal patterns, conductive metal containing inks and pastes such as those having a conductive filler comprising silver or stainless steel, patterned deposited metals such as etched metal patterns or masked vacuum deposited metals, intrinsically conductive polymers and DER formulations. In a preferred embodiment, the “fingers and “busses” comprise electroplateable material such as DER or an electrically conductive ink which will enhance or allow subsequent metal electrodeposition. “Fingers” 84b and “busses” 86b may also comprise non-conductive material which would assist accomplishing a subsequent deposition of conductive material in the pattern defined by the “fingers” and “busses”. For example, “fingers” 84b or “busses” 86b could comprise a polymer which may be seeded to catalyze chemical deposition of a metal in a subsequent step. An example of such a material is seeded ABS. Patterns comprising electroplateable materials or materials facilitating subsequent electrodeposition are often referred to as “seed” patterns or layers. “Fingers” 84b and “busses” 86b may also comprise materials selected to promote adhesion of a subsequently applied conductive material. “Fingers” 84b and “busses” 86b may differ in actual composition and be applied separately. For example, “fingers” 84b may comprise a conductive ink while “buss/tab” 86b may comprise a conductive metal foil strip. Alternatively, fingers and busses may comprise a continuous unvarying monolithic material structure forming portions of both fingers and busses. Fingers and busses need not both be present in certain embodiments of the invention.
The embodiments of
It is important to note however that the laminating current collector structures of the instant invention may be manufactured utilizing continuous, bulk roll to roll processing. While the collector grid embodiments of the current invention may advantageously be produced using continuous processing, one will recognize that combining of grids or electrodes so produced with mating conductive surfaces may be accomplished using either continuous or batch processing. In one case it may be desired to produce photovoltaic cells having discrete defined dimensions. For example, single crystal silicon cells are often produced having X-Y dimensions of 6 inches by 6 inches. In this case the collector grids of the instant invention, which may be produced continuously, may then be subdivided to dimensions appropriate for combining with such cells. In other cases, such as production of many thin film photovoltaic structures, a continuous roll-to-roll production of an expansive surface article can be accomplished in the “Y” direction as identified in
While each additional conductive material is shown the
It has been found very advantageous to form surface 98b of “fingers” 84b or top surface 100b of “busses” 86b with a material compatible with the conductive surface with which eventual contact is made. In preferred embodiments, electroless deposition or electrodeposition is used to form a suitable surface 98b or 100b. Specifically electrodeposition offers a wide choice of potentially suitable materials to form these top surfaces. Corrosion resistant materials such as nickel, chromium, tin, indium, silver, gold and platinum are readily electrodeposited. When compatible, of course, surfaces comprising metals such as copper or zinc or alloys of copper or zinc may be considered. Alternatively, the surface 98b or 100b may comprise a conductive conversion coating, such as a chromate coating, of a material such as copper or zinc. Further, as will be discussed below, it may be highly advantageous to choose a material to form surfaces 98b or 100b which exhibits adhesive or bonding ability to a subsequently positioned abutting conductive surface. For example, it may be advantageous to form surfaces 98b and 100b using an electrically conductive adhesive or low melting point metal or solder. For example, forming surfaces 98b or 100b with a conductive hot melt adhesive or with materials such as tin or tin alloyed with element such as lead, bismuth or indium could result in a surface material having a melting point less than the temperature of a subsequent lamination process. This would facilitate electrical joining during the subsequent lamination steps. One will note that materials forming “fingers” surface 98b and “buss” surface 100b need not be the same.
Referring now to
Referring now to
As embodied in
Continuing reference to
It is seen in
One method of combining the current collector stock 226 embodied in
As embodied in
Yet another form of the instant invention is embodied in
As in prior embodiments, “fingers” 252 and 254 and “buss” 256 may comprise electrically conductive material. Examples of such materials are metal wires and metal foils, conductive metal containing inks and pastes, patterned metals such as etched metal patterns or masked vacuum deposited metals, intrinsically conductive polymers, conductive inks and DER formulations. In a preferred embodiment, the “fingers and “busses” comprise material such as DER or an electrically conductive ink such as silver containing ink which will enhance or allow subsequent metal electrodeposition. “Fingers” 252 and 254 and “buss” 256 may also comprise non-conductive material which would assist accomplishing a subsequent deposition of conductive material in the pattern defined by the “fingers” and “busses”. For example, “fingers” 252 and 254 or “buss” 256 could comprise a polymer which may be seeded to catalyze chemical deposition of a metal in a subsequent step. An example of such a material is “seeded” ABS. “Fingers” 252 and 254 and “buss” 256 may also comprise materials selected to promote adhesion of a subsequently applied conductive material.
The sectional views of
The sectional view of
Thus, it is seen in the
The embodiments of
In the present specification lamination has been shown as a means of combining the collector grid or electrode structures with a conductive surface. However, one will recognize that other application methods to combine the grid or electrode with a conductive surface may be appropriate such as transfer application processing. For example, in the embodiments such as those of
Using a laminating approach to secure the conductive grid materials to a conductive surface involves some design and performance “tradeoffs”. For example, if the electrical trace or path “finger” 84 comprises a wire form, it has the advantage of potentially reducing light shading of the surface (at equivalent current carrying capacity) in comparison to a substantially flat electrodeposited, printed, etched or die cut foil member. However, the relatively higher profile for the wire form must be addressed. It has been taught in the art that wire diameters as small as 50 micrometers (0.002 inch) can be assembled into grid like arrangements onto photovoltaic cell surfaces prior to applying sealing materials. Thus when laid on a flat surface such a wire would project above the surface 50 micrometers (0.002 inches). For purposes of this instant specification and claims, a structure projecting above a surface less than 50 micrometers (0.002 inches), i.e. 1 micrometer, 5 micrometer, 10 micrometer 25 micrometer, 50 micrometer, will be defined as a low profile structure. Often a low profile structure may be further characterized as having a substantially flat surface.
A potential cross sectional view of a wire form 84d after being laminated to a surface is depicted in
A low profile structure such as depicted in
Electrical contact between conductive grid “fingers” or “traces” 84 and a conductive surface (such as cell surface 59) may be further enhanced by coating a conductive adhesive formulation onto “fingers” 84 and possibly “busses” 86 prior to or during the lamination process such as taught in the embodiment of
In the case of a low profile form such as depicted in
For example, a suitable conductive “hot melt” adhesive may be deposited from solution onto the surface of the “fingers” and “busses” by conventional paint electrodeposition techniques. Alternatively, should a condition be present wherein the exposed surface of fingers and busses be pristine (no oxide or tarnished surface), the well known characteristic of such a surface to “wet” with water based formulations may be employed to advantage. A freshly activated or freshly electroplated metal surface will be readily “wetted” by dipping in a water-based polymer containing fluid such as a latex emulsion containing a conductive filler such as carbon black. Application selectivity would be achieved because the exposed polymeric sealing surface 80 would not wet with the water based latex emulsion. The water based material would simply run off or could be blown off the sealing material using a conventional air knife. However, the water based film forming emulsion would cling to the freshly activated or electroplated metal surface. This approach is similar to applying an anti-tarnish or conversion dip coating to freshly electroplated metals such as copper and zinc.
Alternatively, one may employ a low melting point metal-based material as a constituent of the material forming either or both surfaces 98 and 100 of “fingers” and “busses”. In this case the low melting point metal-based material, or alloy, melts during the temperature exposure of the process 92 of
In yet another embodiment, one or more of the layers 84, 86, 88, 90 etc. may comprise a material having magnetic characteristics. Magnetic materials include nickel and iron. In this embodiment, either a magnetic material in the cell substrate or the material present in the finger/grid collector structure is caused to be permanently magnetized. The magnetic attraction between the “grid pattern” and magnetic component of the foil substrate of the photovoltaic cell (or visa versa) creates a permanent “pressure” contact.
In yet another embodiment, the “fingers” 84 and/or “busses” 86 comprise a magnetic component such as iron or nickel and a external magnetic field is used to maintain positioning of the fingers or busses during the lamination process depicted in
A number of methods are available to employ the current collecting and interconnection structures taught hereinabove with photovoltaic cell stock to achieve effective interconnection of multiple cells into arrays. A brief description of some possible methods follows. A first method envisions combining photovoltaic cell structure with current collecting electrodes while both components are in their originally prepared “bulk” form prior to subdivision to dimensions appropriate for individual cells. An expansive surface area of photovoltaic structure such as embodied in
Another method of combining the collector electrodes and interconnect structures taught herein with photovoltaic cells involves a first step of manufacture of multiple individual current collecting structures or electrodes. A suitable method of manufacture is to produce a bulk continuous roll of electrodes using roll to roll processing. Examples of such manufacture are the processes and structures embodied in the discussion of
Alternate methods to achieve interconnected arrays according to the instant invention comprise first manufacturing multiple current collector structures in bulk roll to roll fashion. In this case the “current collector stock” would comprise electrically conductive current collecting structure on a supporting sheetlike web essentially continuous in the “Y” or “machine” direction. Furthermore, the conductive structure is possibly repetitive in the “X” direction, such as the arrangement depicted in
Having separately prepared rolls of “current collector stock” and unit “cell stock”, multiple assembly processes may be considered to assemble modules as follows. In one form of array assembly process, a roll of unit “current collector stock” is produced, possibly by subdividing a bulk roll of “current collector stock” to appropriate width for the unit roll. The rolls of unit “current collector stock” and unit “cell stock” are then combined in a continuous process to produce a roll of unit “tabbed stock”. The “tabbed” stock therefore comprises cells, which may be extensive in the “Y” dimension, equipped with readily accessible contacting surfaces for either or both the top and bottom surfaces of the cell. The “tabbed” stock may be assembled into an interconnected array using a multiple of different processes. As examples, two such process paths are discussed according to (A) and (B) following.Process Example (A)
Multiple strips of “tabbed” stock are fed to a process such that an interconnected array of multiple cells is achieved continuously in the machine (original “Y”) direction. This process would produce an interconnected array having series connections of cells whose number would correspond to the number of rolls of “tabbed” stock being fed. In this case the individual strips of “tabbed” stock would be arranged in appropriate overlapping fashion as dictated by the particular embodiment of “tabbed” stock. The multiple overlapping tabbed cells would be electrically joined appropriately using electrical joining means, surface mating through laminating or combinations thereof as has been taught above. Both the feed and exit of such an assembly process would be substantially in the original “Y” direction and the output of such a process would be essentially continuous in the original “Y” direction. The multiple interconnected cells could be rewound onto a roll for further processing.Process Example (B)
An alternative process is taught in conjunction with
While the feed material in the process embodiment of
It will be appreciated that using the processing as embodied in
It is further pointed out that additional optional operations may be achieved after or during the formation of modular arrays such as those of
Finally, the modularization processes described by way of example above in paragraphs 245 through 249 are very scaleable since they adopt laminating processing firmly established in the packaging industry.Example 1
A standard plastic laminating sheet from GBC Corp. 75 micrometer (0.003 inch) thick was coated with DER in a pattern of repetitive fingers joined along one end with a busslike structure resulting in an article as embodied in
The finger/buss pattern thus produced on the lamination sheet was then electroplated with nickel in a standard Watts nickel bath at a current density of 50 amps. per sq. ft. Approximately 4 micrometers of nickel thickness was deposited to the overall pattern.
A photovoltaic cell having surface dimensions of 1.75 inch wide by 2.0625 inch long was used. This cell was a CIGS semiconductor type deposited on a 0.001 inch stainless steel substrate. A section of the laminating sheet containing the electroplated buss/finger pattern was then applied to the top, light incident side of the cell, with the electroplated grid finger extending in the width direction (1.75 inch dimension) of the cell. Care was taken to ensure that the buss region of the conductive electroplated metal did not overlap the cell surface. This resulted in a total cell surface of 3.61 sq. inch. (2.0625″×1.75″) with about 12% shading from the grid, (i.e. about 88% open area for the cell).
The electroplated “finger/buss” on the lamination film was applied to the photovoltaic cell using a standard Xerox office laminator. The resulting completed cell showed good appearance and connection.
The cell prepared as above was tested in direct sunlight for photovoltaic response. Testing was done at noon, Morgan Hill, Calif. on Apr. 8, 2006 in full sunlight. The cell recorded an open circuit voltage of 0.52 Volts. Also recorded was a “short circuit” current of 0.65 Amps. This indicates excellent power collection from the cell at high efficiency of collection.Example 2
Individual thin film CIGS semiconductor cells comprising a stainless steel supporting substrate 0.001 inch thick were cut to dimensions of 7.5 inch length and 1.75 inch width.
In a separate operation, multiple laminating collector grids were prepared as follows. A 0.002 inch thick film of Surlyn material was applied to both sides of a 0.003 inch thick PET film to produce a starting laminating substrate as embodied in
While many of the embodiments of the invention refer to “current collector” structure, one will appreciate that similar articles could be employed to collect and convey other electrical characteristics such as voltage.
One application of the modules made practical by the teachings above is expansive area photovoltaic energy farms or expansive area rooftop applications. The instant invention envisions facile manufacture and installation of large sheetlike modules having area dimensions suitable for covering expansive surface areas. Practical module widths may be 2 ft., 4 ft., 8 ft etc. Practical module lengths may be 2 ft., 4 ft., 10 ft., 50 ft, 100 ft., 500 ft., or larger. The “sheetlike” modules may be produced in a wide range of forms. The longer lengths can be characterized as “continuous” and be shipped and installed in a roll format. In another application, the sheetlike modules may be adhered to a rigid supporting member such as plywood, polymeric sheet or a honeycomb structure. The sheetlike modules may be produced having terminal bars at two opposite terminal ends of the module. These terminal bars are easily incorporated into the modules using the same continuous processes used in assembly of the bulk module. It is noted that in the hereinbefore teachings, the terminal bars may have oppositely facing conductive surface regions with electrical communication between them. This is an advantage for certain embodiments of the instant invention, in that an upward facing conductive surface for the terminal bars may facilitate electrical connections.
Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications, alternatives and equivalents may be included without departing from the spirit and scope of the inventions, as those skilled in the art will readily understand. Such modifications, alternatives and equivalents are considered to be within the purview and scope of the invention and appended claims.
1. A method of manufacture of an article combining a photovoltaic cell structure and a current collector structure suitable for collecting current from light incident surfaces of photovoltaic cells, said method comprising the steps of,
- providing a photovoltaic cell structure having semiconductor material positioned between a top light incident cell surface and bottom cell surface,
- providing current collector structure, said current collector structure comprising a flexible, polymer based sheetlike substrate having a first surface and a second surface and comprising a pattern of electrically conductive material supported by said substrate and positioned over said first surface, said current collector structure being manufactured separately and distinctly from said cell structure,
- combining a portion of said current collector structure and said cell structure such that said first substrate surface of said collector structure portion and said top light incident surface of said cell structure face each other and said pattern of conductive material associated with said collector structure portion extends over and is in electrical contact with a preponderance of said top light incident cell surface.
2. The method of claim 1 wherein said current collector structure is supplied to said combining step in a form wherein said substrate is characterized as continuous.
3. The method of claim 2 wherein said photovoltaic cell structure is supplied to said combining step as a continuous form.
4. The method of claim 1 wherein said combining step comprises lamination of said current collector structure portion to said light incident cell surface.
5. The method of claim 2 wherein said combining step is characterized as roll-to-roll processing.
6. A method of manufacture of an article combining a photovoltaic cell structure and a current collector structure suitable for collecting current from light incident surfaces of photovoltaic cells, said method comprising the steps of,
- providing a photovoltaic cell structure having a light incident surface,
- providing a separately prepared current collector structure, said current collector structure comprising a sheetlike substrate having a first surface and a pattern of electrically conductive material extending over said first surface, said current collector structure being initially separate and distinct from said photovoltaic cell structure,
- combining said photovoltaic cell structure and a portion of said current collector structure such that said light incident surface of said cell structure and said first substrate surface of said collector structure portion not covered by said conductive material face each other and are adhesively bonded to each other, and said pattern of electrically conductive material associated with said collector structure portion is in electrical contact with said light incident surface.
7. The method of claim 6 wherein said light incident surface of said cell portion has substantially uniform composition.
9. The method of claim 6 wherein said first surface is formed by a polymer based adhesive.
10. The method of claim 6 wherein said pattern of electrically conductive material associated with said collector structure portion extends over a preponderance of said light incident surface of said cell structure.
11. The method of claim 6 wherein said sheetlike substrate comprises multiple layers.
12. The method of claim 6 wherein said sheetlike substrate comprises a layer of glass.
13. The method of claim 11 wherein said sheetlike substrate comprises multiple polymer based layers.
14. The method of claim 6 wherein said sheetlike substrate is flexible.
15. The method of claim 6 wherein said photovoltaic cell structure comprises thin film semiconductor material.
16. The method of claim 6 wherein said pattern of electrically conductive material comprises an electrically conductive adhesive to adhesively bond portions of said material to said light incident surface.
17. The method of claim 6 wherein said pattern of electrically conductive material has surface portions formed by a low melting point metal-based material.
18. The method of claim 17 wherein said low melting point metal-based material comprises Indium.
19. A method of manufacture of an article combining a photovoltaic cell structure and a current collector structure suitable for collecting current from light incident surfaces of photovoltaic cells, said method comprising the steps of,
- providing a separately prepared current collector structure, said current collector structure comprising a flexible polymer based sheetlike substrate having a first surface and comprising a pattern of electrically conductive material extending over said first surface, said current collector structure being initially separate and distinct from said photovoltaic cell structure,
- applying photovoltaic cell structure over at least a portion of said current collector structure, said cell structure comprising a light incident surface, a back surface and semiconductor material positioned between said light incident and back surfaces, said application being such that a preponderance of said light incident surface faces and is in electrical communication with said pattern of conductive material associated with said portion of current collector structure.
20. The method according to claim 19 wherein said current collector structure provided has a form wherein said substrate is characterized as continuous.
21. A method of manufacture of an article combining a photovoltaic cell structure and a current collector structure suitable for collecting current from light incident surfaces of photovoltaic cells, said method comprising the steps of,
- providing a separately prepared current collector structure, said current collector structure comprising a transparent or translucent substrate having a length dimension and having a first surface, said current collector structure further comprising multiple distinctly separated patterns of electrically conductive material supported on said first surface, each of said patterns being elongate in said length dimension and having a width dimension perpendicular to said length dimension, said multiple patterns being repetitive in said width dimension, said patterns being distinctly separated from each other by a region of substrate absent conductive material extending in said length dimension,
- positioning photovoltaic cell structures over at least a portions of each of said patterns of conductive material, each of said cell structures comprising a light incident surface, a back surface and semiconductor material positioned between said light incident and back surfaces, said positioning being such that a preponderance of said light incident surface of each of said cell structures faces and is in electrical contact with a said pattern of conductive material.
Filed: Nov 5, 2008
Publication Date: Apr 30, 2009
Inventor: Daniel Luch (Morgan Hill, CA)
Application Number: 12/290,896
International Classification: H01L 21/04 (20060101);