Abstract:
A solar panel utilizes at least one and, in one embodiment, three protective layers to eliminate the need for a metal frame. The protective layers may include one inorganic layer and two polymer layers, which are cured onto an underside of the panel. In one embodiment, the protective layers are cured over lateral edges of certain of the layers of the solar panel, including for example the conductor layers, semiconductor junction, and reflector layer. The protective layers may extend to cover an exposed edge along an underside of panel&#39;s superstrate. In one embodiment, the lateral edge of the superstrate is contoured to resist damage from rough handling and/or exposure to the elements. A support platform may be provided, and the solar panel secured thereon by way of interposing an adhesive between an underside of the panel and the support platform.

Description:
RELATED APPLICATION  
       [0001]    The present application relates to U.S. patent application Ser. No. 11/947,543, entitled “Conformal Protective Coating for Solar Panel.” 
     
    
     FIELD OF THE INVENTION  
       [0002]    The present invention relates to thin-film solar photovoltaic panels, modules, fabrication and assembly methods and, more particularly, to a thin-film photovoltaic solar panel without a frame around the edges that achieves the requirements normally provided by a frame. 
       BACKGROUND OF THE INVENTION  
       [0003]    Thin-film photovoltaic solar panels have been constructed with a number of substrates and/or backing materials. Current designs have used glass substrates, glass superstrates, stainless steel substrates, and plastic substrates. In one common prior art design, thin-film glass solar panels are manufactured by starting with a glass superstrate onto which are deposited a set of thin-films that create the solar cell. Sunlight enters through the front surface of the glass superstrate and is absorbed by the thin-films on the back surface and converted by them to electricity. From the sunlight side, or front of the solar panel, the thin-films are protected from the environment by the glass of the glass superstrate. From the back side of the solar panel, the thin-films are protected from the environment by a series of protective layers that are separately fabricated as sheets, and then attached to the panel over the thin-films. The sheet materials are often heavy, expensive, or both. The attachment of these sheets is an extra manufacturing step that is typically a batch process. Finally, a metal frame is added around the entire perimeter of the solar panel, to compress laminated protective layers, to protect electrical connections, to add additional mechanical strength if needed, to protect the superstrate and to provide for an attachment point for mounting the panel on a support structure. 
         [0004]    The metal frames used in the prior art solar panels have several significant deficiencies associated therewith. First, they are expensive and add significant cost to the solar panel. Second, on the back side of the solar panel, they tend to hold water at the edges of the solar panel. This held water tends to encourage water ingress between the protective sheets and the thin-film materials of the solar panel. Third, on the sunlight-facing or front side of the solar panel, they collect water and dirt. This dirt absorbs the sunlight and reduces the amount of sunlight that is converted into electricity. Even if the dirt only collects near frame edges, the resulting power loss can be quite significant because, in many cases, the solar panels are divided into a large number of thin segments that are then connected in series. Since they are connected in series, the current of the entire solar panel is limited by the current of the segment with the smallest current. Therefore, even a thin line of dirt at the edge of the solar panel will significantly reduce the current of the segment closest to that edge—and because of the series connection between segments, the current of the entire solar panel will drop to a much greater extent than the fractional area of the panel that is actually covered by the dirt.. 
         [0005]    Fourth, the frame can wear against the superstrate when wind or thermal expansion and contraction exert repeated stress on the panel, causing micro-cracks to develop in the glass superstrate. These micro-cracks can cause a panel to fail in multiple ways; they can trap dust that blocks light from the panel, admit moisture and contaminants that can attack the functional films, propagate into the functional films and destroy their electrical integrity, or promote panel breakage in extreme conditions such as storms. Fifth, by its very design to hold the glass module tightly, the frame will collect and trap moisture as the ambient temperature and humidity change over daily and seasonal cycles. Collected water can accelerate the deterioration of the solar panels in outdoor environments. It can also accelerate the adhesive failure of the protective back layers to the solar panel. This adhesive failure of he back coats has been a major failure mode of framed solar panels. 
         [0006]    Sixth, the frame can act as a short circuit to ground for the exposed glass surfaces that result in delamination of the thin-films due to sodium migration or an equivalent. Finally, the assembly of the frames onto the solar panel is a process that is relatively expensive to automate. Hand assembly adds additional cost, yield loss and makes the solar panel less reliable. Even when the assembly is automated, steps such as lamination are generally batch processes. Compared to continuous processes, batch processes add expense (1) through requiring a large space where a batch of large panels can be processed simultaneously, and (2) the loss of entire batches whenever the process goes wrong. 
       SUMMARY OF THE INVENTION  
       [0007]    In accordance with an embodiment of the present invention, a solar panel is disclosed. The solar panel comprises, in combination: a transparent superstrate; a first conductor disposed onto the superstrate; wherein the first conductor forms a contact for the solar panel of a first polarity; at least one device layer adapted to convert sunlight to electricity; a second conductor; wherein the second conductor forms a contact for the solar panel of a second, opposite polarity; wherein the at least one device layer is interposed between the first and second conductors; a reflector; wherein the at least one protective layer has been cured on the solar panel. 
         [0008]    In accordance with another embodiment of the present invention, a method for fabricating a solar panel is disclosed. The method comprises: providing a transparent superstrate; disposing a first conductor onto the superstrate; wherein the first conductor forms a contact for the solar panel of a first polarity; providing at least one device layer adapted to convert sunlight to electricity; providing a second conductor; wherein the second conductor forms a contact for the solar panel of a second, opposite polarity; wherein the at least one device layer is interposed between the first and second conductors; providing a reflector located below the second conductor; curing at least one protective layer onto the solar panel below the reflector. 
         [0009]    In accordance with yet another embodiment of the present invention, a solar panel is disclosed. The solar panel comprises, in combination: a transparent superstrate; a first conductor disposed onto the superstrate; wherein the first conductor forms a contact for the solar panel of a first polarity; at least one device layer adapted to convert sunlight to electricity; a second conductor; wherein the second conductor forms a contact for the solar panel of a second, opposite polarity; wherein the at least one device layer is interposed between the first and second conductors; a reflector; at least three protective layers located below the reflector, comprising at least one inorganic layer and at least two polymer layers; wherein the at least three protective layers have been cured on the solar panel; wherein the at least one inorganic layer comprises one of Si 3 N 4  and SiO 2 ; wherein each of the at least two polymer layers comprises one of EVA, polyvinyl fluoride and an acrylate; wherein the at least three protective layers extend over a lateral edge of the first conductor, the second conductor, the semiconductor junction, and the reflector; and wherein adhesion of the at least three protective layers is sufficiently strong that a frame is not required around edges of the solar panel to prevent the at least three protective layers from peeling off the solar panel, beginning at the edge, during use. 
         [0010]    In accordance with a further embodiment of the present invention, a method for converting sunlight into electricity is disclosed. The method comprises: providing a photovoltaic cell comprising, in combination: a transparent superstrate; a first conductor disposed onto the superstrate; wherein the first conductor forms a contact for the solar panel of a first polarity; at least one device layer adapted to convert sunlight to electricity; a second conductor; wherein the second conductor forms a contact for the solar panel of a second, opposite polarity; wherein the at least one device layer is interposed between the first and second conductors; a reflector; at least one protective layer located below the reflector; wherein the at least one protective layer has been r cured on the solar panel; positioning the photovoltaic cell so that sunlight may enter the glass superstrate and thereafter pass through the device layer, where a portion of the sunlight is converted into electricity; and outputting the electricity from the photovoltaic cell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  a side, cross-sectional view of a prior art single junction amorphous silicon thin-film solar panel on a superstrate with protective back coating 
           [0012]      FIG. 2  is a side, cross-sectional view of a single junction amorphous silicon thin-film solar panel on a superstrate with thin-film protective back coatings, consistent with an embodiment of the present invention. 
           [0013]      FIG. 3  is a side, cross-sectional view of a single-junction thin-film solar panel on a superstrate with thin-film protective back coatings, consistent with another embodiment of the present invention. 
           [0014]      FIG. 4(   a ) is a side view illustrating a contoured edge of a glass superstrate portion of a thin-film solar panel consistent with an embodiment of the present invention. 
           [0015]      FIG. 4(   b ) is a side view illustrating a contoured edge of a glass superstrate portion of a thin-film solar panel consistent with another embodiment of the present invention. 
           [0016]      FIG. 4(   c ) is a side view illustrating a contoured edge of a glass superstrate portion of a thin-film solar panel consistent with a further embodiment of the present invention. 
           [0017]      FIG. 5(   a ) is a top view illustrating a plurality of solar panels mounted on an underlying support structure and attached to the support structure away from the edges. 
           [0018]      FIG. 5(   b ) is an end view illustrating attachment of a solar panel to an underlying support structure away from the edges. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    Referring first to  FIG. 1 , a prior art amorphous silicon, single junction, solar panel  10  is illustrated. The solar panel  10  consists of a glass superstrate  12 , a first layer of transparent conductive oxide (e.g., SnO 2 , ZnO, InSnO, etc.)  14 , a p-layer  16 , an I-layer  18  and an n-layer  20  of amorphous silicon, a second layer of transparent conductive oxide  22  and then a layer or layers of metals (e.g., aluminum, silver or silver and titanium, etc.)  24 . The amorphous silicon layers  16 ,  18  and  20  convert the sunlight into electricity. The first conductive oxide layer  12  may be an electrical connection that becomes the positive contact. The second conductive oxide  22  plus the metal layer  24  form an electrical connection that may become the negative contact to the solar cell. 
         [0020]    As shown in  FIG. 1 , a prior art solar panel  10  may further include a first protective layer  26 , which may be of EVA, and a second protective layer  28 , which may be a polyvinyl fluoride such as Tedlar®. In the prior art, the first and second protective layers  26  and  28  are purchased in sheet form and then laminated onto the back of the solar panel. The process of lamination of thick sheets of polymer tends to create a weak adhesion between the polymer sheet and the back of the solar panel  10 . It is especially weak at the edges of the solar panel, where chips, scratches, or repeated temperature or freeze-thaw cycling can loosen the laminations at the edge of the solar panel and encourage it to peel off. In the prior art, a frame (not shown) is required around the edges of the solar panel to apply pressure to the edges of the laminations and to prevent them from de-laminating at the edges. The frame can wear the superstrate edges, resulting in micro cracks that, in turn, will initiate cracks in the panel and ultimate failure of the glass superstrate and the entire solar panel. 
         [0021]    Referring now to  FIG. 2 , a solar panel  40  consistent with an embodiment of the present invention is illustrated. The panel  40  comprises a glass superstrate  42 , a first layer of transparent conductive oxide (e.g., SnO 2 , ZnO, InSnO, etc.)  44 , a p-layer  46 , an I-layer  48  and an n-layer  50  of amorphous silicon, a second layer of transparent conductive oxide  52  and then a back reflector layer  54  (comprising, for example, aluminum, silver or silver and titanium). It should be noted that the solar panel  40  is shown in position for use, with the glass superstrate  42  positioned to received sunlight there through. However, during fabrication, the glass superstrate  42  will generally be laid down first, with remaining layers being deposited thereon, and the completed panel will be inverted for use. It should further be noted that it would be possible to provide a solar panel having a glass substrate rather than a superstrate, with a transparent protective top coating, perhaps comprised of UV resistant plastic, positioned as the top most layer of the solar panel when in use. 
         [0022]    As shown in  FIG. 2 , the solar panel  40  has a single junction of amorphous silicon, though it should be noted that instead of a single junction, it would be possible to use additional layers of amorphous or micro-crystalline silicon and/or additional layers of transparent conductive oxides to form additional junctions and create a multiple junction solar panel. In other embodiments of the present invention, other materials that convert sunlight to electricity could be used instead of amorphous or micro-crystalline silicon, including for example CdTe, CuInGaSe 2 , nano-particles, or carbon nano-tubes, among others. 
         [0023]    In this embodiment, an inorganic protective layer  56  (e.g., Si 3 N 4  or SiO 2 , silicon oxynitride or silicon carbide) is provided below the back reflector layer  54 , and then a first thin-film polymer protective layer  58  and a second thin film polymer protective layer  60  are deposited thereon. The purpose of the inorganic protective layer  56  and the first and second polymer layers  58  and  60  is to protect the set of layers that form the solar cell. The inorganic protective layer  56  can be sputter deposited onto the back reflector layer  54  to create a very strong bond between it and the underlying layers of the solar panel  40 . This layer protects the solar cells from water and pollutant degradation. The inorganic protective layer  56  can consist of one layer as described above or multiple inorganic layers (e.g., Si 3 N 4 , SiO 2 , silicon oxynitride or silicon carbide) to improve or optimize polymer adhesion, abrasion resistance, corrosion resistance and microhardness. 
         [0024]    Unlike the prior art, in which polymers are provided in sheet form and then laminated onto the back of the solar panel as described above, the polymers that will form the first and second polymer layers  58  and  60  are provided as monomers in liquid form, applied as a liquid onto the back of the solar panel  40  by, for example, spin-coating, roll-coating, or slot-coating, and then cured on the solar panel  40  by a suitable means, such as ultra-violet irradiation, heat, or chemical reaction between various components of the liquid. This process of curing the polymer on the solar panel creates a strong, tough conformal bond/coating. The conformal coating process displaces water and air from the panel surface, which the prior-art lamination process does not. This creates an inherently superior bond and interface. While the above example defines two conformal polymer layers  58  and  60  that are cured on the solar panel, multiple layers of polymers can be applied to optimize adhesion, wear, water and pollutant permeability, electrical conductivity and thermal expansion mismatches. 
         [0025]    The process of lamination of thick sheets of polymer tends to create a weak adhesion between the polymer sheet and the back of the solar panel. In contrast, by careful choice of materials for the inorganic protective layer or layers and then the polymer layer or layers, the adhesion of a polymer layer that is cured in place to the inorganic layer can be significantly stronger. The polymer layers  58  and  60  can be formed from, by way of example, monomers of EVA, polyvinylfluoride, acrylates, or other polymer-forming monomers. 
         [0026]    In the embodiment shown in  FIG. 2 , and in contrast to prior art solar panels utilizing laminated sheets of polymers as protective layers (as illustrated in  FIG. 1 ), the adhesion of the inorganic and polymer protective layers  56 ,  58  and  60  is sufficiently strong that a frame is not required around the edges of the solar panel  40  to prevent the protective layers from peeling off the solar panel, beginning at the edges. 
         [0027]    Referring now to  FIG. 3 , a solar panel  70  consistent with another embodiment of the present invention is shown. Like the solar panel  40  shown in  FIG. 2 , the solar panel  70  comprises a glass superstrate  72 , a first layer of transparent conductive oxide  74 , a p-layer  76 , an I-layer  78  and an n-layer  80  of amorphous silicon, a second layer of transparent conductive oxide  82  and then a back reflector layer  84 . In addition, and as shown in  FIG. 2 , an inorganic protective layer or layers  86  is/are provided below the back reflector layer  84 , and then a first thin-film polymer protective layer  88  and a second thin film polymer protective layer  90  are deposited thereon. As in the previous example, multiple layers of polymer can be added to optimize the desired product life. 
         [0028]    In the embodiment of  FIG. 3 , layers  74 ,  76 ,  78 ,  80 ,  82 , and  84  are removed at the perimeter of the solar panel, exposing bare glass from glass superstrate  72  at the edge of solar panel on all four edges (only one edge is shown). The width of the exposure may be in the range of approximately one to 15 mm, with a range of from about three to about 10 mm being preferred in terms of achieving the goals of the present invention. The inorganic and polymer protective layers  56 ,  58  and  60  are deposited on top of the layers that form the solar panel and its conductors as described above with respect to the embodiment of  FIG. 2 . As shown in  FIG. 3 , the inorganic layer/layers  56  is/are deposited over the edges of layers  74 ,  76 ,  78 ,  80 ,  82 , and  84 , and onto the exposed edge of the glass superstrate  72 , preferably extending out to the lateral edge of the glass superstrate  72  on all four edges. The polymer protective layers  58  and  60  are conformally coated, cured on the solar panel and bonded over the inorganic layer and, in one embodiment, extend over lateral edges of layers  78 ,  80 ,  82  and  84 . 
         [0029]    The inorganic layer or layers  56  is/are chosen so that it forms a strong bond to the glass superstrate  72 . Since the glass superstrate  72  and the inorganic and polymer layers  86 ,  88  and  90  are chosen to be very resistant to degradation in outdoor environments, they effectively seal the solar panel  70  and its conductors  74  and  82  from the back and at the edges. 
         [0030]    This seal is located away from the surface of the lateral edge of the glass superstrate  72 , to prevent it from potential damage or other compromise of integrity by nicks or chips in the glass edge. As a result, a frame is not required to protect the edges of the protective layers  86 ,  88  and  90  from minor damage, such as chips or scratches. 
         [0031]    The bus-bar (not shown) that collects electricity at the edges of a solar panel is a critical part of the solar panel that needs to be protected, both from corrosion and from detachment from the films that generate the electricity. In the prior art, the frame protects and promotes adhesion of the bus-bar. In one embodiment of this invention, the bus-bars are ribbons of metal that are welded to the back conductor of the solar cell with an ultrasonic welding system. Ultrasonic welding provides strong adhesion of the bus-bar to the solar panel without the need for a frame. In one embodiment of this invention, the bus-bar is welded to the back of the solar panel before the protective layers  86 ,  88  and  90  are applied. Thus, the bus-bar is protected by the same protective back coatings as the solar panel  70 . Furthermore, the bus-bar metal can be chosen to be aluminum, titanium or another metal that itself is highly resistant to corrosion. Then, with the protective layers  86 ,  88  and  90  over the bus-bar metal, and with the bus-bar metal welded to the solar panel  70 , the probability of corrosion is, relative to prior art designs, significantly lower even without a frame. 
         [0032]    It is noted that solar panel glass is typically fabricated in very large sheets and then cut to size by first scribing the surface where a cut is desired and then encouraging the scratch created by the scribe to propagate into a cleave all the way through the thickness of the glass sheet, usually by stressing the sheet. If the glass has no internal defects such as bubbles, striae, or inclusions in the path of the cleave, the propagated part of the cleave is very smooth, and therefore very strong—in fact, smoother and stronger than most polishing operations produce. However, the region that was scratched by the scribe often has defects because scribing involves abrasion and highly localized pressure on the glass. In addition, glass is sensitive to chips and scratches at its edges, whether from rough handling or airborne debris. In the prior art, the frame protects the edges of the glass from airborne debris or rough handling that might chip an edge of the glass. 
         [0033]    In a solar panel without a frame, as for example shown in  FIGS. 2 and 3 , the edges of the glass are exposed to the environment. To reduce the need for a frame to protect the glass, as shown in  FIGS. 4(   a )-( c ), the edges of a glass substrate  100  may be ground, polished, or otherwise shaped (e.g., with laser shaping or diamond turning) with a contours. The contour may be, for example, elliptical, parabolic, oval, or catenary ( FIG. 4(   a )), semicircular ( FIG. 4(   b )), or with rounded corners ( FIG. 4(   c )). Since the defects and micro-cracks from the glass cutting process tend to be only near the surface that is cut, one can improve the strength and lifetime of the glass merely by grinding or polishing the part of the edge that is near the surface that was cut, as shown by way of example in  FIG. 4(   c ), in which the middle section of the lateral edge of the glass superstrate  100  is left unpolished, thus saving manufacturing cost. However, for strength against airborne debris that might chip the lateral edge, it is preferable to have a contour along the entire lateral edge that is symmetrical from the top surface of the glass to the bottom surface of the glass, as shown by way of example in  FIGS. 4(   a )-( b ). The contours illustrated in  FIGS. 4(   a )-( c ) eliminate micro-cracks caused by the glass cutting process and give the lateral edge of the glass superstrate  100  great strength against airborne debris that might cause minor chips, scratches or other defects at the edges, eliminating the need for a frame to protect the edge of the glass solar panel. 
         [0034]    In the field installation process for a prior art solar panel array, a supporting structure may be built consisting of horizontal beams supported by vertical posts. Then, the metal frames of the framed solar panels are attached to the horizontal beams with mounting brackets and mounting hardware, e.g., bolts. Referring now to  FIGS. 5(   a )-( b ), for frameless solar panels as herein described, a different installation system and method are described. In this embodiment, a pair of supporting beams  120  is provided. An adhesive material  132  is then interposed between a top surface of the supporting beams  120  and an underside of the solar panels  130  that are to be mounted thereon. The choice of adhesive should provide strong adhesion, good compliance to withstand the stress created by differential thermal expansion between the glass solar panels and the material comprising the supporting beams  120  (e.g., metal), and a lifetime in outdoor environments for about 30 years or more. Silicone adhesives are one choice that meets these requirements, at least for typical panel-supporting structures of steel or aluminum. 
         [0035]    An attachment of the solar panels to their supporting structure via an adhesive, as illustrated in  FIGS. 5(   a )-( b ), offers several advantages over the prior art approach of bolting the frames of the solar panels to the supporting structure. A silicone adhesive, for example, is compliant, providing damping to the overall structure that minimizes vibration of the solar panels during times of high wind or percussive precipitation such as sleet, hail, or hard rain. Silicone adhesives are robust in outdoor environments for 30 or more years. By contrast, the metal bolts and nuts used in the prior art tend to corrode over time. The attachment of solar panels via adhesives may be readily automated, while the attaching of bolts and nuts is not readily automated. Automation of the attachment process can lower cost and improve quality. In addition, the preferred areas of adhesive attachment as shown in  FIGS. 5(   a ) and ( b ) are on the back of the solar panel  130 , rather than at the edges. The attachment points are preferably located relatively far from the edges of the solar panel to avoid collecting water or dirt that could encourage corrosion at the edges of the solar panel. Moreover, since the points of adhesive attachments are on the back of the solar panel they cannot shade the front of the solar panel in any way even if they do collect some dirt or debris. 
         [0036]    In summary, in various embodiments of this invention, requirements normally met by a frame around the edges of the solar panel may be met without a frame. These include preventing de-lamination of the protective back layers, preventing damage to the protective back layers at the edges of the solar panel, promoting strong adhesion of the bus-bar, minimizing the risk of cracking the glass due to a scratch or chip at the edge of the solar panel, and providing a robust means for attaching the solar panels to an underlying metal supporting structure. 
         [0037]    A frameless solar panel according to this invention is less expensive in materials cost, and requires less labor to assemble the complete structure, than prior-art framed solar panels. The assembly process can be automated as a continuous process that takes little factory space and minimizes the number of units scrapped in case of a process problem. Without the frame, the panel is lighter in weight; this weight reduction relaxes the load-bearing requirements on supporting structures and tracking mechanisms, reducing their cost as well. A frameless solar panel has a flat front surface all the way to its edges, rather than a frame that may shade part of the panel and reduce its output power and that may collect water or dirt at its edges. 
         [0038]    Although particular embodiments of the invention have been described in detail for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited, except as by the appended claims.