Patent Publication Number: US-2012027923-A1

Title: Seal for photovoltaic module

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to provisional application 61/368,503, filed Jul. 28, 2010, which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to seals for photovoltaic modules, methods for manufacturing photovoltaic modules, and methods for manufacturing seals. 
     BACKGROUND 
     A photovoltaic module can include a substrate layer and a superstrate layer. To bind the substrate layer to the superstrate layer, a sealant layer can be added between the layers. By improving the quality of the sealant layer, the module&#39;s durability and reliability can be improved by providing greater protection against moisture ingress and delamination. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exploded view of a photovoltaic module. 
         FIG. 2  is a perspective view of a sealant application process. 
         FIG. 3  is a top view showing an overlay of a known sealant layer and a new sealant layer. 
         FIG. 4  is a top view of a known nozzle path and a known sealant layer. 
         FIG. 5  is a top view of a known nozzle path and a known sealant layer. 
         FIG. 6  is a perspective view of a photovoltaic module with a known sealant layer. 
         FIG. 7  is a top view of a new nozzle path and a new sealant layer. 
         FIG. 8  is a top view of a new nozzle path and a new sealant layer. 
         FIG. 9  is a perspective view of a photovoltaic module with a new sealant layer. 
         FIG. 10  is a cross sectional side view of a photovoltaic cell. 
         FIG. 11  is a flow chart showing a method for manufacturing a photovoltaic module. 
         FIG. 12  is a flow chart showing a method for generating electricity using a photovoltaic module. 
     
    
    
     DETAILED DESCRIPTION 
     To protect the photovoltaic module from moisture ingress, the sealant layer may be applied near the perimeter of the module. In particular, the sealant layer may be inserted between the superstrate layer and a substrate layer. The sealant layer may serve as an adhesive between the superstrate and substrate layers. However, over time, the sealant layer may fail in bonding the superstrate layer to the substrate layer. For example, as a result of thermal cycling in the field, delamination of the superstrate and substrate may occur proximate to the sealant layer. Delamination is undesirable, since it can lead to premature failure of the module. To improve bonding between the layers and to avoid delamination, a new photovoltaic module and methods of manufacturing photovoltaic modules and sealant layers have been developed and are set forth herein. 
     In one aspect, a method for manufacturing a photovoltaic module may include providing a first layer including a perimeter and four corner areas. The method may also include forming a sealant layer adjacent to the first layer by dispensing sealant from a nozzle as the nozzle follows a nozzle path proximate to the perimeter of the first layer. The nozzle path may include an acute angle at each of the four corner areas. The method may further include forming a second layer adjacent to the sealant layer. The sealant may include an inner edge and an outer edge. The outer edge may be substantially parallel to the perimeter of the first layer. The outer edge of the sealant layer may be about 0 mm to about 6 mm from the perimeter of the first layer. The first layer may be a superstrate layer, and the second layer may be a substrate layer. Alternately, the first layer may be a substrate layer, and the second layer may be a superstrate layer. The sealant layer may include a flowable rubber. The flowable rubber comprises butyl rubber. The method may include heating the sealant prior to dispensing the sealant. The sealant may be heated to a temperature of about 100° C. to about 200° C. Preferably, the sealant may be heated to a temperature of about 150° C. to about 175° C. The nozzle may travel along the nozzle path at a rate of about 0.1 ft/sec to about 2.0 ft/sec. Preferably, the nozzle may travel along the nozzle path at a rate of about 0.5 ft/sec to about 1.0 ft/sec. The sealant may be dispensed at a flow rate of about 0.1 in 3/sec to about 2.0 in 3/sec. Preferably, the sealant is dispensed at a flow rate of about 0.15 in 3/sec to about 0.3 in 3/sec. 
     In another aspect, a method for forming a sealant layer may include providing a surface including a perimeter and four corner areas. The method may also include forming a sealant layer adjacent to the surface by dispensing sealant from a nozzle as the nozzle follows a nozzle path proximate to the perimeter of the surface. The nozzle path may include an acute angle at each of the four corner areas. The sealant layer may include an inner edge and an outer edge, and the outer edge may be substantially parallel to the perimeter of the surface. The outer edge of the sealant layer may be about 0 mm to about 6 mm from the perimeter of the surface. The sealant may include a flowable rubber. The method may include heating the sealant prior to dispensing the sealant. The sealant may be heated to a temperature of about 100° C. to about 200° C. The nozzle may travel along the nozzle path at a rate of about 0.1 ft/sec to about 2.0 ft/sec. The sealant may be dispensed at a flow rate of about 0.1 in 3 /sec to about 2.0 in 3 /sec. 
     As shown in  FIG. 1 , a photovoltaic module  200  may include an optically transparent superstrate layer  215 . A plurality of solar cells  205  may be formed adjacent to the superstrate layer  215 . A sealant layer  220  may be formed between the superstrate layer  215  and a substrate layer  210 , where the substrate layer  210  functions as a protective back cover for the module  200 . The sealant layer  220  may bind the substrate  210  to the superstrate  215  and serve as a barrier to protect the plurality of solar cells  205  from moisture and debris. 
     The sealant layer  220  may be disposed between the perimeters of the superstrate layer  210  and the substrate layer  215 . During application, the sealant layer  220  may be applied to the superstrate layer  215  as shown in  FIG. 2 . For example, the sealant layer  220  may be applied to the superstrate layer  215  and then the substrate layer  210  may be positioned against the sealant layer  215 . Alternately, the sealant layer  220  may be applied to the substrate layer  210 . For example, the sealant layer  220  may be applied to the substrate layer  210  and then the superstrate layer  215  may be positioned against the sealant layer  220 . 
     The sealant layer  220  may provide suitable adhesion properties while also being resistant to degradation resulting from exposure to ultraviolet light. The sealant layer may be applied at room temperature, or it may be heated prior to application to reduce viscosity and improve flow through a nozzle  305 , as shown in  FIG. 2 . For example, the sealant may be heated to a temperature of about 100° C. to about 200° C. Preferably, the sealant may be heated to a temperature of about 150° C. to about 175° C. The sealant may be heated prior to entering the nozzle, while in the nozzle, or a combination thereof. The sealant layer  220  may be any suitable material such as, for example, polyisoprene, silicone, polyurethane, polysulfide, styrene-butadiene rubber (SBR), acrylic or polyacrylate, isoprene, polyisobutylene, vinyl, or nitrile compounds. 
     As shown in  FIG. 2 , a nozzle  305  may be used to apply the sealant layer  220 . The nozzle  305  may include an orifice having any suitable shape for dispensing sealant. For example, the orifice shape may be designed to dispense a sealant layer  220  having a tubular shape or a tape-like shape as shown in  FIG. 3 . The nozzle  305  may be manually controlled, or it may be attached to an automated applicator  310  that is computer-controlled. The nozzle  305  may dispense a continuous bead of sealant around a perimeter of the substrate or superstrate layers ( 210 ,  215 ) to form the sealant layer  220 . The sealant may be dispensed at a flow rate of about 0.1 in 3 /sec to about 2.0 in 3 /sec. Preferably, the sealant may be dispensed at a flow rate of about 0.15 in 3 /sec to about 0.3 in 3 /sec. During the dispensing process, the nozzle may travel at a rate of about 0.1 ft/sec to about 2.0 ft/sec relative to the target layer. Preferably, the nozzle may travel at a rate of about 0.5 ft/sec to about 1.0 ft/sec. 
     The automated applicator  310  may be programmed to move the nozzle  305  around a perimeter  250  of the superstrate layer  215  and dispense a continuous bead of sealant. When dispensing sealant near the perimeter  250 , the nozzle  305  may be programmed to leave a gap  240  between the outer edge  221  of the sealant layer  220  and the perimeter  250  of the superstrate layer  215 . The gap  240  may range from about 0 mm to about 6 mm. Preferably, the gap may range from about 1 to about 2 mm. Upon assembly of the module  200 , the gap  240  provides an area for the sealant to flow when the sealant layer is laminated between the substrate  210  and superstrate layers  215 . As a result, the sealant does not overflow the perimeter  250 , so a subsequent edge clean-up step can be avoided. 
     To illustrate the differences between a known process and a new process,  FIG. 3  shows an overlay of a known sealant layer and a new sealant layer. The corner of the known sealant layer is shown in dotted lines and was created by following a known nozzle path  405  that is shown in  FIGS. 4 and 5 . Conversely, the new sealant layer, shown in solid lines, was created by following the new nozzle path  705  shown in  FIGS. 7 and 8 . Two shaded regions ( 1005 ,  1010 ) highlight differences between the resulting sealant layers. For instance, the first shaded region  1005  shows how corner coverage is improved by following new nozzle path  705 . The second shaded region  1010  shows how the new nozzle path results in less encroachment of sealant into the interior surface area  1015  of the substrate or superstrate layer. Due to less encroachment, the plurality of cells  205  may be positioned closer to the sealant layer  220 , thereby allowing for more active area within a module having the same outer dimensions. Although  FIG. 3  shows an open area between the outer perimeter of the plurality of cells  205  and the inner edge  222  of the sealant layer  220 , this is not limiting. For example, the sealant layer may abut or overlap the outer edge of the plurality of cells  205 . 
     Known methods of applying sealant follow a known nozzle path  405 , as shown in  FIGS. 4 and 5 . The nozzle path  405  is shown as a dotted line. When following the known nozzle path  405 , the nozzle  305  travels in a straight line and, upon reaching a corner, the nozzle  305  rotates 90 degrees counterclockwise while its direction of travel also rotates 90 degrees counterclockwise. As a result, an arc of sealant is dispensed near the corner. In  FIG. 4 , seven exemplary nozzle positions (e.g.  410 ,  415 ) are shown. The nozzle path  405  intersects the midpoint of each nozzle position along the nozzle path  405 . 
     As shown in  FIG. 5 , upon turning 90 degrees near a first corner, the nozzle path  405  continues in a straight line until it reaches the next corner where it again rotates 90 degrees counterclockwise as described above. Upon traveling around the perimeter of the substrate or superstrate layer, the sealant layer  220  is created as shown in  FIG. 5 . Unfortunately, since the nozzle  305  scribes an arc near each of the four corners, sealant is not distributed out to the corner areas  505  of the superstrate or substrate layers. As a result, surface area that could be used for bonding is left unutilized. To further illustrate this point,  FIG. 6  shows a perspective view of a module  200  where the sealant layer  220  does not extend to the corner areas  505  of the substrate or superstrate layers. In addition to forming a weak bond, the configuration shown in  FIG. 6  is undesirable because water may enter the corner voids and freeze, thereby causing delamination between the module&#39;s layers. Furthermore, the corners of the substrate and superstrate layers may be prone to breakage where there is no support from the sealant layer. Therefore, it is desirable to add sealant to the corner areas  505  without adding any additional steps to the manufacturing process, since additional steps can add cost and complexity to the process. 
       FIGS. 7 and 8  depict a new nozzle path  705 . In particular,  FIG. 7  shows the nozzle path  705  in detail near one corner of the target layer (e.g.  210 ,  215 ). The nozzle path  705  is depicted as a dashed line. When the nozzle  305  follows the nozzle path  705 , sealant is distributed to the corner areas of the layer without adding any additional steps to the manufacturing process. A sealant layer  220  is formed having an inner edge  222  and an outer edge  221 . Although the nozzle path  705  is described with respect to a counterclockwise travel path herein, a clockwise travel path, or combination thereof, may also be used. 
     To illustrate the dispensing process, exemplary nozzle positions (e.g.  710 ,  715 ) are shown along the nozzle path  705 . The nozzle path  705  is defined as a path that intersects the midpoint of each nozzle position (e.g.  710 ,  715 ). The nozzle first travels along a straight path  740  towards the corner area  505 . As the nozzle approaches the corner are  505 , the nozzle  305  begins to rotate counterclockwise. Simultaneously, the nozzle path  705  deviates from its straight path  740  towards the corner area  505  along an arced path  745 . Upon rotating 45 degrees counterclockwise and entering the corner area  505 , the nozzle  305  withdraws from the corner area  505  and travels along a second arced path  750  before continuing along a second straight path  755 . The second straight path is substantially perpendicular to the straight path taken when approaching the corner area  505 . As shown in  FIG. 8 , the nozzle path  705  has a shape that resembles a rectangle. However, the nozzle path differs from a rectangle because the corners of the nozzle path  705  are acute angles instead of right angles. 
     Upon traveling around the entire perimeter of the substrate or superstrate layer, a sealant layer  220  is produced as shown in  FIG. 8 , where the sealant extends toward the corner area of the substrate or superstrate layer. As noted above, the sealant layer  220  may have an outer edge  221  and an inner edge  222 . The outer edge  222  of the sealant layer  220 , as shown in  FIG. 8 , may be approximately rectangular. In other words, the outer corners of the sealant layer  220  have little or no radius, so they are nearly right angles when compared to the rounded corners of the sealant layer shown in  FIG. 5 . 
       FIG. 9  shows a module  200  that includes the new sealant layer  220  using the new nozzle path  705 . Unlike the module shown in  FIG. 5 , the module  200  in  FIG. 9  has no corner voids. As a result, bonding between the substrate layer  215  and the superstrate  210  layers is improved, and the module  200  is less susceptible to delamination and breakage. 
       FIG. 1  shows a photovoltaic module  200  containing a simplified example of a plurality of photovoltaic cells. To provide greater detail about the cells,  FIG. 10  depicts a cross-sectional view of an example photovoltaic cell. In particular, the photovoltaic cell  100  may include an anti-reflective coating  105  formed on a superstrate  110 . The anti-reflective coating  105  may be designed to reduce reflection and increase transmission. For instance, reflections are minimized if the coating is approximately one-quarter-wavelength thick with respect to the wavelengths of incident photons. Since CdTe has a bandgap energy of 1.48 eV, the anti-reflective coating  105  may have a thickness of about 0.15 microns. The anti-reflective coating  105  may contain, for example, aluminum oxide, titanium dioxide, magnesium oxide, silicon monoxide, silicon dioxide, or tantalum pentoxide. Since the anti-reflective coating only optimizes transmission at a single wavelength, it may be desirable to modify the surface of the superstrate  110  to improve overall transmission. For instance, the superstrate  110  may be textured prior to adding the anti-reflective coating  105  to enhance light trapping. 
     The superstrate  110  may be formed from an optically transparent material such as soda-lime glass. Since quality and cleanliness of a glass superstrate can have a significant effect on performance of the device, polishing the glass with cerium oxide powder may be desirable to increase transmission. A barrier layer  112  may be formed adjacent to the superstrate  110  to lessen diffusion of sodium or other contaminants from the superstrate  110 . The barrier layer  112  may include silicon dioxide or any other suitable material. 
     A transparent conductive oxide (TCO) layer  115  may be formed between the barrier layer  112  and a buffer layer  120  and may serve as a front contact for the photovoltaic device. In forming the TCO layer  115 , it is desirable to use a material that is both highly conductive and highly transparent. For example, the TCO layer  115  may include tin oxide, cadmium stannate, or indium tin oxide. To further improve transparency, the TCO layer  115  may be about 1 micron thick. If cadmium stannate is used, application of the cadmium stannate may be accomplished by mixing cadmium oxide with tin dioxide using a 2:1 ratio and depositing the mixture onto the superstrate  110  using radio frequency magnetron sputtering. A buffer layer  118  may be formed between the TCO layer  115  and a n-type window layer  120  to decrease the likelihood of irregularities occurring during formation of the n-type window layer. 
     The n-type window layer  120  may include a very thin layer of cadmium sulfide. For instance, the n-type window layer  120  may be 0.1 microns thick and may be deposited using any suitable thin-film deposition technique. For example, the n-type window layer  120  may be deposited using a metal organic chemical vapor deposition (MOCVD). To reduce surface roughness of the n-type window layer  120 , it may be annealed at approximately 400 degrees Celsius for about 20 minutes. The annealing process may improve the boundary between the n-type window layer  120  and the CdTe layer  125  by reducing defects. By reducing defects and improving the boundary, the efficiency of the photovoltaic device is improved. 
     The p-type absorber layer  125  may be formed adjacent to the n-type window layer  120  and may include cadmium telluride. The p-type absorber layer  125  may be deposited using any suitable deposition method. For instance, the p-type absorber layer  125  may be deposited using atmospheric pressure chemical vapor deposition (APCVD), sputtering, atomic layer epitaxy (ALE), laser ablation, physical vapor deposition (PVD), close-spaced sublimation (CSS), electrodeposition (ED), screen printing (SP), spray, or MOCVD. Following deposition, the p-type absorber layer  125  may be heat treated at a temperature of about 420 degrees Celsius for about 20 minutes in the presence of cadmium chloride, thereby improving grain growth and reducing grain boundary trapping effects on minority carriers. By reducing trapping effects within the p-type absorber layer  125 , open-circuit voltage is increased. 
     A p-n junction  122  is formed where the p-type absorber layer  125  meets the n-type window layer  120 . The p-n junction  122  contains a depletion region characterized by a lack of electrons on the n-type side of the junction and a lack of holes (i.e. electron vacancies) on the p-type side of the junction. The width of the depletion region is equal to the sum of the diffusion depths located on the p-type side and the n-type side. The respective lack of electrons and holes is caused by electrons diffusing from the n-type window layer  120  to the p-type absorber layer  125  and holes diffusing from the p-type absorber layer  125  to the n-type window layer  120 . As a result of the diffusion process, positive donor ions are formed on the n-type side and negative acceptor ions are formed on the p-type side. The positive donor ions may be phosphorous atoms locked in a silicon lattice that have donated an electron, and the negative acceptor ions may be boron atoms locked in a silicon lattice that have gained an electron. The presence of a negative ion region near a positive ion region establishes a built-in electric field across the p-n junction  122 . When the photovoltaic device  100  is exposed to sunlight, photons are absorbed within the junction region. As a result, photo-generated electron-hole pairs are created. Movement of the electron-hole pairs are influenced by the built-in electric field, which produces current flow. The current flow occurs between a first terminal  116  attached to the TCO layer  115  and a second terminal  131  attached to a back contact  130 . 
     The back contact  130  may be formed adjacent to the p-type absorber layer  125 . The back contact  130  may be a low-resistance ohmic contact that maintains good contact with the p-type absorber layer  125  throughout temperature cycling. To ensure stability of the contact, a rear surface of the p-type absorber layer  125  may be etched with nitric-phosphoric (NP) to create a layer of elemental Te on the rear surface, and the back contact  130  may cover the entire back surface of the p-type absorber layer  125 . The back contact  130  may include aluminum applied through evaporation that is subsequently annealed. Alternately, the back contact  130  may include molybdenum or any other suitable low-resistance material. 
     The various layers formed between the superstrate layer  110  and substrate layer  140  may be covered by an interlayer  135 . For example, the interlayer  135  may cover the TCO layer, buffer layer, n-type window layer, p-type absorber layer, and back contact  130  as shown in  FIG. 10 . The interlayer  135  may protect the layers from moisture and water ingress and may provide containment of potentially harmful materials if the photovoltaic device is physically damaged. The interlayer  135  may include a polymer material such as, for example, ethylene-vinyl acetate (EVA), but any other suitable material may be used. To form the interlayer  135 , the previously formed layers may be laminated with a sheet of EVA. 
     A sealant layer  145 , as described above, may be formed around the perimeter of the interlayer  135 . Lastly, the substrate  140  may be formed adjacent to the interlayer  135  and may further protect the rear side of the device. The protective back substrate  1 . 40  may include any suitable material such as, for example, soda-lime glass, plastic, carbon fiber, or resin. 
     As shown in  FIG. 11 , a method for manufacturing a photovoltaic module may include providing a first layer  1105  of a photovoltaic module. The first layer may be a substrate or superstrate layer. In addition, the first layer may be an optically transparent material, such as soda lime glass. The method may further include forming a sealant layer adjacent to the first layer by dispensing sealant from a nozzle along a nozzle path  1110  as shown in  FIGS. 7 and 8 . The method may further include forming a second layer adjacent to the sealant layer  1115 . The second layer may be a substrate or superstrate layer. In addition, the second layer may be an optically transparent material, such as soda lime glass. 
     As shown in  FIG. 12 , a method for generating electricity may include illuminating a photovoltaic module  1205  to generate a photocurrent. The method may further include collecting the photocurrent from the photovoltaic module  1210 . “Collecting” may refer to storage or using the current. For example, “collecting” may refer to storing the current in a storage device, such as a battery. Alternately, “collecting” may refer to using the current to power an electrical load. 
     Details of one or more embodiments are set forth in the accompanying drawings and description. Other features, objects, and advantages will be apparent from the description, drawings, and claims. Although a number of embodiments of the invention have been described, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. In particular, steps depicted in the figures may be executed in orders differing from the orders depicted. For example, steps may be performed concurrently or in alternate orders from those depicted. It should also be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features and basic principles of the invention.