Abstract:
Described herein is a photovoltaic module and method of manufacturing a photovoltaic module to isolate potentially stress-damaged portions of cells from non-stress-damaged portions thereof. The module has a plurality of columnar photovoltaic cells, and at least one isolation scribe at a first edge of an active area of the photovoltaic module and extending across a photovoltaic cell in a direction perpendicular to a length of the columnar cells, where the at least one isolation scribe is deep enough to electrically isolate portions of the photovoltaic cell on opposite sides of the at least one isolation scribe.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/570,043 filed on Dec. 13, 2011, which is hereby incorporated by reference in its entirety herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Disclosed embodiments relate to the field of photovoltaic (PV) power generation systems, and more particularly to a photovoltaic module and manufacturing method thereof. 
       BACKGROUND 
       [0003]    Photovoltaic modules convert the energy of sunlight directly into electricity by the photovoltaic effect. Photovoltaic modules can include a plurality of photovoltaic cells or devices. As one example, a photovoltaic module can include multiple layers created on a transparent substrate (or superstrate), such as a glass. For example, a photovoltaic module can include a transparent conductive oxide (TCO) layer, a buffer layer, and semiconductor layers formed in a stack on a substrate. The semiconductor layers can include a semiconductor window layer, such as a zinc oxide layer or a cadmium sulfide layer, formed on the buffer layer and a semiconductor absorber layer, such as a cadmium telluride layer, formed on the semiconductor window layer. The semiconductor window layer can allow the penetration of solar radiation to the absorber layer, which converts solar energy to electricity. A conductor may be deposited adjacent to the semiconductor absorber layer to serve as a back contact for the module. To complete the module, a back support, typically formed of glass, is provided over the back contact. 
         [0004]    A long field operation lifespan, without failure, of over about 20 years is desirable for PV modules. Generally, in the field, four external clamps  100 , shown in  FIG. 1 , are used to hold a PV module  10  to an underlying supporting structure. During normal operation, a high voltage differential may occur between cells within the PV module, which may have voltages as high as 1000V, and the external clamps  100 , which are at OV. This high voltage differential is believed to cause sodium (Na) diffusion from the glass substrate to other active areas within the module, which may cause various stress defects in the module near the area of the clamps  100 . For example, too much sodium can build up at the interface of layers and can push apart the interfaces, which causes structural damage. Additionally, sodium can diffuse into the other layers and cause current leakage. Although the region with structural damage is typically highly localized within a small area, it may cause much larger areas of the module to be affected electrically. 
         [0005]      FIG. 2  illustrates a conventional photovoltaic module  10  with a peripheral edge area  200 , where no photovoltaic cells are present, and an area of columnar series connected cells  300 . A conventional photovoltaic module  10 , like that shown in  FIG. 2 , can exhibit performance issues related to stress defects. If any portion of a columnar cell  300  is damaged by stress near the location of a clamp  100 , for example, the damage can spread to other parts of the cell more spatially removed from clamps  100 . A method and apparatus are accordingly desired, to mitigate the effect of stress defects in areas of the module held to a supporting structure by external clamps  100 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  illustrates a photovoltaic module set-up. 
           [0007]      FIG. 2  illustrates a conventional photovoltaic module. 
           [0008]      FIG. 3  is a schematic of a conventional photovoltaic module with localized structural damages caused by stressing. 
           [0009]      FIG. 4  illustrates a photovoltaic module according to a first embodiment. 
           [0010]      FIG. 5  is a schematic of a photovoltaic module according to a first embodiment with stress damage. 
           [0011]      FIG. 6  illustrates a photovoltaic module according to a second embodiment. 
           [0012]      FIG. 7  illustrates a photovoltaic module according to a third embodiment. 
           [0013]      FIG. 8  illustrates a photovoltaic module according to a fourth embodiment. 
           [0014]      FIG. 9  illustrates a simulated current-voltage curve for a photovoltaic module before stress. 
           [0015]      FIG. 10  illustrates a simulated current-voltage curve for a conventional photovoltaic module after stress. 
           [0016]      FIG. 11  illustrates a simulated current-voltage curve for a photovoltaic module according to the first embodiment, after stress. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. It should be understood that like reference numbers represent like elements throughout the drawings. Embodiments are described in sufficient detail to enable those skilled in the art to make and use them, and it is to be understood that structural, material, electrical, and procedural changes may be made to the specific embodiments disclosed, only some of which are discussed in detail below. 
         [0018]    Described herein is a photovoltaic module including isolation scribes, which isolate portions of cells that may be subject to stress defects from the rest of the module. The isolation scribes segregate stress-damaged portions of cells from healthy portions of the cells in an active area of a PV module, thus preventing damage from spreading to maximize the total undamaged, usable active area of the PV module. According to the embodiments described herein, the active area sacrificed by adding the isolation scribes is negligible because the isolation scribes only have a width of about 50 μm and the amount of active area that is sacrificed by adding the isolation scribes is only about 0.008% of the total active area (for a 2 ft×4 ft photovoltaic cell, for example). Thus, using isolation scribes according to the embodiments of the invention increases the overall efficiency of the PV module by segregating portions of cells, which may become damaged, from healthy portions of cells to keep the majority of the cells healthy and functional. 
         [0019]    A photovoltaic module includes a set of columnar cells  300 , as shown, for example, in  FIG. 3 , which are connected in series. A typical photovoltaic module is about 2 ft×4 ft and has around 118 columnar cells (each about 1 cm×2 ft). According to the design of a photovoltaic cell  300 , localized damage within a cell affects the performance of the entire cell. In other words, local damage in a cell, e.g., at area  300   b  near the location of an external clamp  100 , may cause electrical degradation of other parts of the cell, e.g., areas  300   a,  and may eventually cause degradation of the entire cell. It has been found that one of the most stressed areas for a cell during operation are near the clamps  100  that hold module  10  to a supporting structure in the field. Although the structural damage caused by high voltage biasing are generally confined near the clamp areas (such as shown by  300   b ), the associated degradation in electrical performance extends to much larger areas (such as shown by  300   a ). Embodiments described herein help mitigate stress damage in PV modules. The isolation scribes of the disclosed embodiments act to prevent the spread of detrimental effects from a damaged portion of a cell to the rest of cell. 
         [0020]    Referring to  FIG. 4 , a first embodiment is now described with reference to the manufacture of a photovoltaic module. Photovoltaic module  20  has edge area  200 , where no photovoltaic cells (active area) are present, and columnar solar cells  300 , connected in series. Isolation scribes  400  are formed at the top and bottom of photovoltaic module  20 . The distance between the top and bottom edge areas  200  and isolation scribes  400  can be determined by the size of the stress-damaged area, which is also affected by clamp size. The distance from the isolation scribe  400  to the edge area  200  is between about 1 mm to about 4 cm. The isolation scribes  400  can be formed by laser, mechanical, and any other scribing methods. The scribe, or patterning, depth should be deep enough to produce an electrical isolation between areas of the cells  300  adjacent the edge area  200  of the module  20  and the remainder of the cells  300 . It can be done by cutting through at least one of the following layers: a contact layer including TCO, and a semiconductor layer. After isolation scribes  400  are formed, the area between the isolation scribes  400  and the edge area  200  remains an active area (because the cells within that area are still electrically connected in series). For example in this embodiment, in any given columnar cell  300 , once the isolation scribes  400  are made, the columnar cell  300  will then be made of two smaller columnar cells  301  (i.e., the cells in the area between the edge  300  and the top isolation scribe  400  and the cells in the area between the edge  300  and the bottom isolation scribe  400 ) and the columnar cell between the top and bottom isolation scribes  400 . All three columnar cells will then be connected in parallel and all will remain a part of the active area. The only part of the columnar cell  300  that is not an active area is the area of the isolation scribes  400 . Thus, the only area sacrificed by adding the isolation scribes is equal to the width of the isolation scribe itself (about 50 μm) multiplied by the length of the isolation scribe (in this case, the length of the module  20 ). In this way, the disclosed embodiments can segregate areas of the columnar cells  300  that are likely to be damaged during use of the PV module while only sacrificing a negligible amount of active area. 
         [0021]      FIG. 5  is a schematic diagram of a stress-damaged photovoltaic module  20  manufactured according to the first embodiment. Stress-damaged areas  300   b  are located around where the clamps  100  will be located. Although the region with structural damage is typically highly localized within a small area, it may cause much larger areas of the module to be affected electrically. Photovoltaic module  20  is manufactured with isolation scribes  400 , to isolate the cells in the stress-damaged areas  300   b  and to prevent them from affecting the cells in the remaining healthy areas  300   a.  For example, the isolation scribes  400  may prevent sodium (Na) diffusion from the glass substrate across the scribes to other active areas within the cell  300  and module  20 . Additionally, the scribe lines isolate the damaged areas, so that current will continue to flow through the cells having the damaged areas, but not through the damaged areas themselves. In this way, the cells are shunted, and only the damaged areas are isolated, while the other regions of the cells are protected. Without the isolation scribes  400 , the structural damage in the localized areas  300   b  could cause the degradation of much larger areas (e.g., such as shown by  300   a ). 
         [0022]    Referring to  FIG. 6 , a second embodiment is now described with reference to the manufacture of a photovoltaic module. Photovoltaic module  30  has edge area  200 , where no photovoltaic cells (active area) are present, and columnar cells  300 . In this embodiment, only the portions of the active area corresponding to locations where cells are likely to be damaged are scribed. Thus, in contrast to the first embodiment, isolation scribes  410  only isolate cells around clamp areas  100 , which are the ones that are most likely to be damaged during operation of the module (e.g., areas  300   b;    FIG. 5 ). In this case, the length of isolation scribes  410  should be long enough to include at least one cell on either side of the areas  300   b  that is unlikely to be damaged. Clamps  100  are typically about 4 to 6 inches long. Preferably, the length of isolation scribes  410  should be longer than the length of clamps  100  by about ½ inch on either side. Thus, the scribes will be long enough to isolate the clamp areas even if the clamp  100  placement is offset during module installation. Again, the width of isolation scribes  410  is about 50 μm and the distance from the isolation scribes  410  to the edge area  200  is between about 1 mm to about 4 cm. This embodiment will minimize current crowding (i.e., a non-homogeneous distribution of current density) through the shunted cells by raising the resistance. 
         [0023]    Referring to  FIG. 7 , a third embodiment is now described with reference to the manufacture of a photovoltaic module. Photovoltaic module  40  has edge area  200 , where no photovoltaic cells (active area) are present, and columnar cells  300 . According to this embodiment, multiple isolation scribes  420  are utilized. Thus, in contrast to the first and second embodiments, which each only have one isolation scribe  400 ,  410  on each side of the module, this embodiment uses multiple isolation scribes  420  on each side. Again, the width of isolation scribes  420  is about 50 μm and the distance from the isolation scribes  420  to the edge area  200  is between about 1 mm to about 4 cm. The distance between adjacent isolation scribes  420  may also be between about 1 mm to about 4 cm. Stress damage can spread and cover larger areas over time. Thus, to mitigate against stress damage spreading past the single scribe, this embodiment utilizes multiple isolation scribes in case any of the isolation scribes  420  are defective and are unable to achieve electrical isolation or if the stress damage occurs further from the module edge area  200  than is protected by a single scribe. 
         [0024]    Referring to  FIG. 8 , a fourth embodiment is now described with reference to the manufacture of a photovoltaic module. Photovoltaic module  50  has edge area  200 , where no photovoltaic cells (active area) are present, and columnar cells  300 . This embodiment uses multiple isolation scribes  430  that only isolate the damaged areas  300   b  around clamp  100 . The length of isolation scribes  430  should be longer than the damaged areas to include at least one healthy cell on either side of the damaged area  300   b.  Clamps  100  are typically about 4 to 6 inches long. Preferably, the length of isolation scribes  430  should be longer than the length of clamps  100  by about ½ inch on either side. Thus, the scribes will be long enough to allow for any offset of clamp  100  placement during module installation. Again, the width of isolation scribes  430  is about 50 μm and the distance from the isolation scribes  430  to the edge area  200  is between about 1 mm to about 4 cm. The distance between adjacent isolation scribes  430  may also be between about 1 mm to about 4 cm. Similar to the embodiment of  FIG. 7 , the embodiment of  FIG. 8  utilizes multiple isolation scribes to ensure that the damaged area does not spread past the single isolation scribes. 
         [0025]    Thus, according to the embodiments described herein, the isolation scribes isolate healthy portions of cells of a PV module from potentially stress-damaged portions of cells of the PV module. The potentially stress-damaged portions remain active areas in the overall circuit, but can lower the overall output of the photovoltaic module. The isolation scribes described herein, confine the potentially stress-damaged areas so that the stress damage does not spread or extend to healthy portions of the cell where they might cause a lower output of the photovoltaic module. 
         [0026]    Referring to  FIGS. 9 to 11 , several current-voltage (I-V) curves are shown, which illustrate the beneficial effects on device performance of using isolation scribes to isolate damaged areas.  FIG. 9  illustrates a simulated I-V curve for a photovoltaic module prior to any stress. According to the simulation, the fill factor (FF) is about  69 . 3 . Fill factor is a parameter which, in conjunction with open-circuit voltage (V OC ) and short-circuit current (I SC ), determines the maximum power or energy yield from a photovoltaic module. The fill factor is defined as the ratio of the maximum power from the photovoltaic module to the product of V OC  and I SC . Graphically, the fill factor is a measure of the “squareness” of the photovoltaic module, and is also the area of the largest rectangle that will fit under the I-V curve.  FIG. 10  illustrates a simulated I-V curve for a conventional photovoltaic module (without isolation scribes) after stress damage. The fill factor is 39.9.  FIG. 11  illustrates a simulated I-V curve for a photovoltaic module with isolation scribes according to the first embodiment described herein, after stress damage. As seen in  FIG. 11 , there is obvious V OC  and fill factor improvement compared to  FIG. 10  with the conventional photovoltaic module. 
         [0027]    While disclosed embodiments have been described in detail, it should be readily understood that the invention is not limited to the disclosed embodiments. Rather, the disclosed embodiments can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described.