Patent Publication Number: US-9419236-B2

Title: Photovoltaic textiles

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made using funds obtained from the US Government (US Army, Contract Nos. W911QY-10-C-0015 and W911QY-14-C-0075), and the US Government therefore has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to textiles designed for converting electrical charges, such as charges generated through solar energy, into usable electricity. More specifically the invention is directed to electrical charge transfer textiles, photovoltaic systems, solar textiles, and sub-components which reduce electrical resistance for improved performance. 
     2. Description of Related Art 
     Photovoltaic systems convert sunlight into electricity through the action of photovoltaic cells. Large solar arrays currently in use typically have numerous panels or modules, each with many photovoltaic cells. Such arrays have been made from rigid components. More recently, flexible photovoltaic components have been developed that may be incorporated into textiles as alternatives to rigid cells and modules. 
     Flexible solar energy technology such as polymer photovoltaics (PPV&#39;s) holds great promise for many applications. The freedom of movement provided by textiles has the potential for making solar energy conversion structures that are more easily transported and erected than comparable rigid solar structures. Such systems could be used to bring much needed electricity to remote or disaster ridden areas that would otherwise be without power. In other applications, efficient solar textiles integrated into common articles such as hats, garments, tents, and coverings could potentially provide electric power on a smaller scale. 
     Small cross-section photovoltaic fibers used for solar textile applications provide uniformity and fabric-like flexibility. Inexpensive and flexible polymer photovoltaics (PPVs) are well suited for use as fibers. However, attempts at producing efficient solar textiles from existing PPV components have been constrained by fundamental technical barriers relating to their inherent electrical resistance. 
     Present PPV fibers of coaxial construction rely on a centered inner conductor and a transparent external conductor, such as ITO (Indium Tin Oxide) or a conducting polymer such as PEDOT (Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate)), to move charge along the fiber. However, when such fibers are incorporated into a textile, the low electrical conductivity of the external, optically transparent electrode causes significant voltage drop in the available electricity. The voltage drop results because the transparent electrode provides two critical but contradictory functions. The first function is to pass solar flux unimpeded through the transparent electrode into the optically active photoelectric layers beneath the surface. The second function is to move or transport electric charge axially along the sheet dimension of the transparent electrode with minimum voltage drop or loss. Efficient optical transmission requires maximum optical clarity, implying a relatively thin electrode. However, efficient charge transport requires sufficient thickness to provide a low electrical resistance path. While one function optimizes with increasing thickness the other optimizes with decreasing thickness. Currently available optically transparent compounds, such as ITO and many of the new polymer-based substances such as PEDOT, do not simultaneously satisfy the optical clarity and electrical conductivity requirements. For PPV components made from these substances, acceptable optical transmission results in excessive electrical sheet resistance for use in solar textiles. 
     Other photovoltaic fiber designs rely on dual internal conductors throughout their length. However, the movement of power through textiles made from such fibers is generally more complicated and less reliable because of the need to make and maintain additional electrical connections with external circuitry. Furthermore, the small cross sectional dimension typical of internal conductors restricts charge flow. Similar to co-axial fibers, charge transport along the axis of dual internal conductor fibers yields large voltage drop, thereby diminishing the performance of textiles in which such fibers are used. 
     Attempts at producing photovoltaic (PV) fibers for textiles have reported power conversion efficiencies of only 0.01% with electrical fill factors of 24%. ( A Photovoltaic Fiber Design for Smart Textiles, Textile Research Journal  Vol. 80 (11); 1065-1074 DOI. It has also been reported that: “An n-type carrier counter electrode that is both highly conductive and optically transparent has not been reported. Even indium-tin oxide coatings with a resistivity as low as 10 ohm/cm2 cannot transport the photocurrent generated with 1 sun irradiance over more than 10 to 15 mm without incurring electrical losses.” ( Solar Power Wires Based on Organic Photovoltaic Materials , Science Magazine, 10 Apr. 2009.) 
     In addition to poor efficiency, solar textile modules made from photovoltaic fibers known in the art are subject to malfunction from shorting of conductors, particularly at connections where charges from multiple fibers are merged. Unless substantially fortified, the delicate nature of the small connections allows them to be damaged from minor impacts or abrasions. Depending on the design, a single short circuit could impair the function of multiple cells, or even adjacent solar modules. Similarly, solar textile modules made from other existing photovoltaic components such as thin films are either too fragile or rigid and are still largely unproven for exploiting the advantages of solar textiles. 
     A prior design developed and disclosed in PCT application serial number PCT/US2012/054866 provides a combination of components arranged to form a textile capable of generating electricity from solar energy. That prior design used a combination of highly conductive bus bars serving as conduits in contact with photovoltaic tapes or fibers to move charge in and out of textile unit cells. The bus bar conduits are arranged to minimize charge transport resistance throughout the textile by providing multiple, durable electrical contacts with charged surfaces along the length of the photovoltaic components. Unfortunately, the configurations of combinations of PV tapes and bus bar conduits woven together in an interlacing manner is inefficient in light of the present invention. Specifically, the PV tapes am overshadowed as a result of the over-and-under routing of the bus bar conduits as they pass over the top electrodes of the PV tapes. This limitation can result in shadowing of about 50% of the functional PV tape area. Textiles utilizing the prior design disclosed in the cited PCT application are also difficult to manufacture because they rely on multiple components, which increases the cost and complexity of fabrication and reduces the likelihood of widespread commercialization. 
     These and other technical problems relating to existing photovoltaic components and systems continue to inhibit the rapid commercialization of new applications for solar textiles. Therefore, there is a need in the art for more efficient photovoltaic textile components and materials in general particularly those that are durable but still flexible enough to exhibit the properties of fabrics. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to novel photovoltaic systems, components, and methods of manufacturing related to solar textiles. In general, the invention improves solar textile performance by taking a systems approach to the overall design. Textiles in accordance with the present invention employ integrated tape structures containing both the charge converting PV layer and conductive charge transport layers. The prior art system disclosed in regard to the PCT application referenced above comprises two distinct components, the PV tapes and the conductive bus bar conduits. The two were interwoven and so incapable of utilizing the entire surface area of the solar textile to collect solar energy. The present invention improves manufacturing options and substantially eliminates the shadowing limitation by integrating solar energy conversion and charge transport into a single tape structure. Specifically, the single tape structure comprises first and second electrically conductive layers sandwiched around an insulating substrate. The insulating substrate may be a unitary layer or it may be formed of a plurality of layers. A PV layer is applied on a top surface of the first electrically conducting layer. The PV layer may be a unitary layer or it may be formed of a plurality of layers. The PV layer provides the photovoltaic functionality while the second conductive layer performs as a low-resistance transport channel to permit low-loss charge movement along the axis of the tape. One or more conductive contacts may be positioned on an upper surface of the PV layer spaced from the first electrically conducting layer by the PV layer. This design overcomes the limitations previously described by permitting collection of solar energy across the entire surface area of the textile. In addition, by simultaneously providing the needed low-resistance path to move charge in and out of the textile weave, the single tape structure enables the formation of sufficiently efficient charge carrying textiles suitable for powering devices. 
     The single tape structure of the present invention can be used as both the warp and weft components and enables the formation of a solar textile without requiring a separate bus bar conduit. The tape structure of the present invention enables the fabrication of a textile from a weave of a plurality of such tape structures so that various circuit combinations are available and the output of those circuits is better than what is available when separate solar energy converting and charge transport components are used. 
     While the description of the present invention is directed to a tape structure that is suitable for use in a textile system that converts solar energy to electricity and the effective transport of that electricity, it is not limited strictly to “solar” textiles. Instead, it is directed to the inclusion of the tape structure in any textile that has one or more flexible and elongated charge carrying components, which may be photovoltaic components but that also may be other forms of charge carrying components, including, but not limited to, other forms of flexible conductive elements. 
     The foregoing and related aspects and embodiments and other advantages and features of the invention will be readily apparent to those skilled in the art after review of the following detailed description of the invention, drawings, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross section of an integrated tape structure including a PV portion and two electrically conducting layers spaced from one another by an insulation layer.  FIG. 1A  is a cross section of the PV conversion layer of the integrated tape structure. 
         FIG. 2  is a cross section of the integrated tape structure of the present invention with the insulation layer comprised of a plurality of sublayers. 
         FIG. 3A  is a top perspective view of a portion of a solar textile of the present invention formed with the integrated tape structure of  FIG. 1  and  FIG. 3B  is a bottom perspective view of the portion of the solar textile of  FIG. 3A . 
         FIG. 4  is a perspective view of the solar textile of the present invention formed with the integrated tape structure of  FIG. 1  with terminations at both tape ends. 
         FIG. 5  is a cross section of an alternative embodiment of the tape structure showing a curved version of the second electrically conducting layer. 
         FIG. 6  is another alternative embodiment of the tape structure showing the second electrically conducting layer spaced from the insulation layer. 
         FIG. 7  is cross section side view of the embodiment of the tape structure of  FIG. 6 . 
         FIG. 8  is a bottom perspective view of a textile including the tape structure configuration of  FIG. 6 . 
         FIG. 9  is a top perspective view of an embodiment of a textile including the tape structure of the present invention including a way to increase contact between adjacent tapes. 
         FIG. 10  is a top perspective view of the textile of  FIG. 9  showing internal expansion within the tape structure to enhance contact between adjacent tapes. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The term “deposit” covers all technologies used in coating a surface with a material including but not limited to spraying, dipping, spin coating, vacuum and chemical deposition, printing including, but not limited to, inkjet printing. 
     The term “light” includes a range of electromagnetic radiation known as visible light and portions of infrared and ultraviolet spectrums applicable to generating electricity from photovoltaic components. 
     The term “photovoltaic” means having a capacity to contribute to the production of electricity across opposite electrodes as a result of being exposed to electromagnetic radiation. 
     The abbreviation “PPV” is short for polymer photovoltaic and describes a component or system having photovoltaic function which includes a structural polymer and other materials. 
     The abbreviation “PV” is short for photovoltaic and describes a component or system having photovoltaic function, wherein the photovoltaic may not be a polymeric component. 
     The term “PV layer” refers to one or more photoactive layers that when combined with another will facilitate the generation and movement of electric charge. 
     The term “solar textile” includes textiles, cloths, fabrics, and other woven assemblies incorporating photovoltaic components and having varying degrees of flexibility. 
     The term “tape” refers to a relatively thin and narrow component with two opposing planar surfaces that is particularly longer than it is wide. 
     The term “transparent” means allowing light to pass through and includes translucent as well as transparent. 
     Photovoltaic systems according to the present invention comprise textiles made by weaving together integrated textile tape structures of the form described herein. 
     In preferred embodiments, systems of the present invention incorporate conductive tape structures of the configuration described herein. Shown in  FIG. 1  is a cross section of an integrated single tape structure  10  of the present invention. The tape structure  10  includes a PV conversion layer  12  that may be a polymeric photovoltaic tape but is not limited to the inclusion of polymeric material in its fabrication. The PV conversion layer  12  contacts a first electrically conducting layer  14  that may be a structural support of the PV conversion layer  12 . The first electrically conducting layer  14  is flexible and conductive and defines the active width of the PV conversion layer  12 . The first electrically conductive layer  14  can be a metal, such as aluminum, a conductive polymer or a stack of layers of metal and conductive polymers chosen to achieve simultaneously, good electrical conductivity, mechanical strength and flexibility. As shown in  FIG. 1A , the PV conversion layer  12  is fabricated of an electron absorbing sublayer  22  in contact with the first electrically conducting layer  14 , a photon absorbing photoactive sublayer  24 , and an optically transparent hole transport sublayer  24 . The optically transparent hole transport sublayer  26  serves as an upper electrode  26  of the tape structure  10 . The first electrically conductive layer  14  and the PV conversion layer  12  are sequentially in physical and electrical contact with one another, forming the junction structure needed for photovoltaic conversion. In general other structures may provide the photovoltaic conversion provided by the PV conversion layer  12  without deviating from the invention. 
     The optically transparent upper electrode  26  may be formed of ITO or PEDOT or any suitable optically transparent material having some conductivity. That is, the upper electrode  26  is fabricated of a material that is selected to allow light  28  representing solar energy to pass therethrough. The light  28  passes through the upper electrode  26  and reaches the photon absorbing photoactive layer  24 , which enables movement of electrons to the electron absorbing layer  22 . The photoactive layer  24  may be fabricated of P3HT:PCBM (poly-3-hexylthiophene and [6,6]-phenyl C61 butyric acid methylester blend). The electron absorbing layer  22  may be fabricated of Zinc Oxide. Those of skill in the art will recognize that other options for forming the indicated sublayers of the PV conversion layer  12  are possible. The PV conversion layer  12  may simultaneous overlay both the first electrically conducting layer  14  and a portion of the insulating substrate  32  in such a manner as to establish an insulating margin around the edges of the first electrically conducting layer  14  to ensure electrical separation between otherwise highly conductive layers of the tape structure  10  itself or highly conductive layers of adjacent tape structure layers used to form a solar textile. That optional configuration of the PV conversion layer  12  and the first electrically conducting layer  14  is shown in  FIG. 5 . 
     The photoactive and accompanying layers of materials that form the PV conversion layer  12  may be deposited on the first electrically conductive layer  14  and assembled together using present deposition methods known in the art. One or more contacts  30  may be affixed to an upper surface of the upper electrode  20  and may be used to establish an electrical connection to the upper electrode  26 . Contacts  30  are made from a highly conductive material, such as the same material as the first electrically conducting layer  14 . 
     The remainder of the tape structure  10  includes an insulation layer  32  and a second electrically conducting layer  34 . The insulation layer  32  is fabricated of one or more materials selected to electrically insulate the second electrically conducting layer  34  from the first electrically conducting layer  14  of the tape structure  10 . The insulation layer  32  may be fabricated as a unitary structure or it may be fabricated of two or more sublayers. For example, as shown in  FIG. 2 , the insulation layer  32  may be formed of a first insulation sublayer  36  and a second insulation sublayer  38 . The first insulation sublayer  36  and the second insulation sublayer  38  may be formed of the same or different materials. They may be physically connected together so that they move in unison when the tape structure  10  moves or they may be allowed to move differently with respect to one another to enhance the flexibility of the tape structure  10  as part of a textile. The insulation layer  32  may be fabricated of a nonmetallic material such as polyester, polyimide, polyethylene terephthalate, for example, or other material having structural characteristics suitable for maintaining the structural integrity of the tape structure  10  when used for its intended purpose and that is also sufficiently electrically insulative so that no short occurs between the first electrically conducting layer  14  and the second electrically conducting layer  34 . 
     The second electrically conducting layer  34  is formed of a material and with dimensions similar to that provided for the first electrically conducting layer  14 . That is, it is flexible and conductive and may extend to the perimeter or be recessed at its perimeter with respect to its width versus the width of the insulation layer  32 . It can be a metal, such as aluminum, for example, a conductive polymer or a stack of layers of metal and conductive polymers chosen to achieve simultaneously good electrical conductivity, mechanical strength and flexibility. The second electrically conducting layer  34  of one of the tape structures  10  is arranged to make electrical contact with the PV conversion layer  12  of an adjacent different one of the tape structures  10  when a plurality of the tape structures are combined such as by weaving into a textile. That is, the second electrically conducting layer  34  of one of the tape structures  10  has a potential substantially corresponding to that of the upper electrodes  26  of the PV conversion layer  12  of another one of the tape structures with which it makes contact. The first electrically conducting layer  14  in that instance has a lower potential than that of the second electrically conducting layer  34 . 
     When finished, the tape structure  10  can be rolled for storage or shipment, for example, and then be unrolled and fed as needed info looms of various types to create numerous textile weaves. 
       FIGS. 3A and 3B  show an embodiment of a textile  100  of the present invention using a plurality of the tape structures  10  of  FIG. 1  or  FIG. 2  as an assembled weave whereby each of the plurality of tape structures  10  has the PV conversion layer  12  with one or more optional contacts  30  on an upper side  40  thereof and the second electrically conducting layer  34  on a bottom side  42  thereof. The first electrically conducting layer  14  lies between the PV conversion layer  12  and the insulation layer  32 , while the second electrically conducting layer  34  is on the opposite side of the insulation layer  32 , forming the bottom side  42  of the tape structure  10 . The second electrically conducting layer  34  makes contact either with the contacts  30  or directly to the PV conversion layer  12 . The contacts  30  are highly conductive and no significant voltage drop is expected along the length of the tape structure  10  when two tape structures  10  are put together. 
     Electrical termination with external circuitry may be accomplished by exposing ends  44  of the first electrically conducting layers  14 . A bulk clamping system can also be used to establish the electrical connection to external circuitry. For example, since the tape structure  10  is of a uniform bipolar, bidirectional nature, a simple bipolar, mechanical compression clamp may be used to make electrical connections to complete an electrical circuit. When the connections are made, electrical current travels in a circuit from the terminal established at the interface between the PV conversion layer  12 , or the contacts  30  if present, and the second electrically conducting layer  34  of separate cross-woven tape structures  10  and returns through the relatively lower potential terminal established at the ends  44  of the first electrically conducting layer  14 . 
     The textile incorporating the tape structure  10  of  FIGS. 1 and 2  can be scaled to larger sizes as shown in  FIG. 4 . Note that the electrical terminations are bulk macroscopic terminations without the need to separate individual polarities since the tops of the tape structures  10  are always one polarity and the intermediate sections of the tape structures  10  at the ends  44  of the first electrically conducting layer  14  are effectively the other polarity. Additionally,  FIG. 4  shows that terminations can be made at both ends  44  of the tape structures  10 , which should minimize voltage drop. It is also evident from these figures that this tape structure configuration eliminates the shadowing limitation evident with the prior design since all exposed surfaces of the textile are photoactive. This configuration is also easier to manufacture than prior designs requiring the use of two separate structures, one for converting solar energy and the other for transporting the convened electrical signals. 
     An alternative embodiment of the tape structure  10  is represented in  FIG. 5 . In this embodiment, the second electrically conducting layer  34  is curved or arched to enable its conformance with an underlying adjacent tape structure  10  that is curved as a result of its flexibility and any movement of the manufactured textile including the tape structure  10 . This serves multiple purposes: 1) the curved surface approximately matches the curvature of the under arching tape, thereby increasing the effective contact area between the tapes, and 2) contact pressure is more uniformly distributed over the area of contact. The result is a more uniform and controlled electrical interface between the over and under arching tapes. 
       FIG. 6  illustrates a design alternative that maintains the desirable electrical properties of the two metal electrodes while minimizing the impact on tape stiffness. This is accomplished by separating the second electrically conducting layer  34  from the insulation layer  32 , albeit with some physical connection maintained, thereby enabling mechanical shear to occur between the layers. This eliminates I-beam behavior and reduces layer stress arising from tape flexure.  FIG. 7  shows the same separated design shown in  FIG. 6  but with a curved cross section of the second electrically conducting layer  34 . Other cross section designs are also possible.  FIG. 8  shows the bottom curved electrode that is the second electrically conducting layer  34  as it would be implemented in a weave. 
     An aspect of achieving desired good tape-to-tape electrical contact in a textile comprising a plurality of the tape structures  10  combined together is to force the tapes to establish more surface area contact with one another. An applied force packs the adjoining tapes closer together, and through tape flexure, increases contact pressure between the over and under arching tapes arising from the lighter radius of curvature of the tapes that can be established. It is also possible to increase tape-to-tape contact pressure by expanding the tape cross section.  FIG. 9  shows a simple rectangular cross section tape arranged to include an expansion element  50  in an unexpended configuration.  FIG. 10  shows the same tape of  FIG. 9  but with the expansion element  50  in an expanded configuration. The expanded form of the expansion element  50  causes the tape to take on a convex shape so that adjacent tapes that are so expanded will have greater contact between their respective second electrically conducting layers  34  and PV conversion layers  12 . One example of a way to provide the expansion element  50  is to form a pouch within or adjacent to the insulation layer  32  and insert a fluid to expand the pouch. Another way is to form a portion, as shown in  FIGS. 9 and 10 , of the insulation layer  32  of an expandable material, such as an expandable temperature-active foam and apply fluid pressure to the foam. The insulation layer  32  may also be fabricated of such a material and thereby become the expansion element  50 . 
     Embodiments of the tape structure  10  for textiles described thus far are only examples of the many that may be constructed according to the present invention. Textiles having other weaves attaining functional objectives of the present invention are possible using the same PV conversion layer  12 , first electrically conducting layer  14 , insulation layer  32  and second electrically conducting layer  34 . Moreover, a multitude of textile products are made possible by varying the geometric structures of the tape structures  10  as well as their orientations with respect to one another. 
     As described herein, persons skilled in the art will understand that novel systems, components, and methods for fabricating improved solar textiles are herein disclosed which resolve significant shortcomings in the prior art. The embodiments provided are intended only as exemplary illustrations and not for the purpose of limiting the scope of claims which might be sought to the present invention. Various changes, modification, and equivalents in addition to those shown or described will become apparent to those skilled in the art and are similarly intended to fall within the spirit and scope of the invention whether or not they presently exist in the following or are later made in amended claims.