Patent Publication Number: US-2012024340-A1

Title: Solar Cells With Localized Silicon/Metal Contact For Hot Spot Mitigation and Methods of Manufacture

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 61/367,912, filed Jul. 27, 2010. 
    
    
     BACKGROUND 
     Embodiments of the present invention generally relate to photovoltaic modules and methods of making photovoltaic modules. Specific embodiments pertain to photovoltaic modules and photovoltaic cells incorporating technology to mitigate hot spot damage and methods of making the same. 
     Thin film solar modules, also called photovoltaic modules, are made up of a plurality of individual thin film solar cells, or photovoltaic cells, connected in series. Photovoltaic modules installed in the field can experience localized shadowing due to, for example, shading by nearby objects (e.g., light poles), by adjacent module strings (e.g., late in the day when the sun angle is low such that one row of modules shades another) as well as by other means. 
     To assure module durability in the event of such shading, modules must pass a “hot spot” test in which a multi-cell module is electrically short-circuited in full sunlight while a subset of cells is shadowed. In this hot spot test, the power photogenerated in the illuminated cells is dissipated in the shadowed cells, heating the shadowed cells. Should the temperature of the shadowed cells be sufficient for the module to fail (e.g., glass breakage, encapsulation melt, junction box detachment, fire, etc.) then the module is said to fail the hot spot test. 
       FIG. 1  shows cross-sections of common process steps in the manufacture of thin-film photovoltaic modules. A substrate  10  is coated with a layer of a transparent conductive oxide (TCO)  20  material. One or more silicon layers  30  are deposited over the TCO layer  20 . These silicon layer(s)  30  is/are the primary light absorbing structure in the photovoltaic module. A metal layer  40  is deposited over the silicon layer(s)  30 . Photovoltaic modules of this type demonstrate good reverse current flow due to the silicon layer  30  to metal layer  40  contact, but poor back contact optical reflection. 
       FIG. 2  shows cross-sections of photovoltaic module processing steps according to other manufacturing processes. In these processes, the substrate  10  is coated with a TCO layer  20  and silicon layers  30 . A second TCO layer  50  is deposited over the silicon layers  30  and a metal layer  40  is deposited over the second TCO layer  50 . Photovoltaic modules of this variety demonstrate good back contact optical reflection due to the silicon layer  30 —second TCO layer  50 —metal layer  40  contact. These devices show poor reverse current flow, and therefore, hot spot problems can occur. 
     Therefore, there is a need in the art for photovoltaic modules and methods of making photovoltaic modules that have good optical reflection and reverse current flow and can more likely pass a hot spot test and remain stable under conditions of use. 
     SUMMARY OF THE INVENTION 
     One or more embodiments of the invention are directed to photovoltaic cells comprising a superstrate with a front contact layer on the superstrate and a photoabsorber layer on the front contact layer. The photoabsorber layer comprises one or more of an n-type layer, a p-type layer and an intrinsic layer. A patterned discontinuous conductive layer is on the photoabsorber layer. A back contact layer is in contact with the photoabsorber layer and the patterned discontinuous conductive layer. A reflective layer on the back contact layer is on the back contact layer. The reflective back contact layer is adapted to reflect incident light not absorbed by the photoabsorber layer. 
     Additional embodiments of the invention are directed to photovoltaic modules comprising a plurality of the photovoltaic cells as described connected in series. 
     Further embodiments of the invention are directed to methods of manufacturing a photovoltaic cell. A front contact layer is deposited onto a superstrate. A photoabsorber layer is deposited onto the front contact. The photoabsorber layer is adapted to convert light energy into electrical current. The photoabsorber layer includes one or more sublayers selected from the group consisting of p-type, n-type and instrinsic sublayers. A patterned discontinuous conductive layer is deposited on the photoabsorber layer. A back contact layer is deposited onto the patterned discontinuous conductive layer. A reflective layer is deposited on the back contact layer. 
     In some embodiments, the patterned discontinuous conductive layer comprises a metal. In specific embodiments, the metal is selected from the group consisting of silver, copper, aluminum, indium, tin, nickel, molybdenum, chromium, tantalum, titanium and combinations thereof. 
     In various embodiments, the patterned discontinuous conductive layer has a pattern selected from the group consisting of random distributions, grid-like distribution, one-dimensional grid, equally spaced dots and combinations thereof. 
     In specific embodiments, the patterned discontinuous conductive layer covers up to about 10% of the photoabsorber layer. The patterned discontinuous conductive layer of some embodiments extends through the back contact layer and contacts the reflective layer. 
     According to various embodiments, one or more of the front contact layer and the back contact layer comprises a transparent conductive oxide. In detailed embodiments, the transparent conductive oxide is aluminum doped zinc oxide. In specific embodiments, the photoabsorber layer comprises silicon. 
     Some detailed embodiments have the reflective layer comprising one or more of a paint layer, a polymer layer impregnated with a white pigment, and a metal selected from the group consisting of silver, copper and combinations thereof. 
     Some embodiments comprise laser scribing the front contact layer, the photoabsorber layer and the back contact layer to create a plurality of individual photovoltaic cells connected in series. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  shows a cross-sectional view of a process for making photovoltaic modules; 
         FIG. 2  shows a cross-sectional view of a process for making photovoltaic modules; 
         FIG. 3A  shows a process for making photovoltaic modules according to one or more embodiments of the invention; 
         FIG. 3B  show a cross-sectional view of a process for making photovoltaic modules according to one or more embodiments of the invention; 
         FIG. 4A  is a side cross-sectional view of a thin film photovoltaic modules according to one or more embodiment of the invention; 
         FIG. 4B  is a side cross-sectional view of a thin film photovoltaic modules according to one or more embodiment of the invention; 
         FIG. 5  is a plan view of a composite photovoltaic module according to one or more embodiment of the invention; 
         FIG. 6  is a side cross-sectional view along Section  6 - 6  of  FIG. 5 ; 
         FIG. 7  is a side cross-sectional view along Section  7 - 7  of  FIG. 5 ; 
         FIG. 8  shows a cross-sectional view of a process for making thin film photovoltaic modules according to one or more embodiment of the invention; 
         FIG. 9  shows a cross-sectional view of a thin film photovoltaic module according to one or more embodiment of the invention; and 
         FIG. 10  shows various patterns for localized metal according to one or more embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Before describing several exemplary embodiments of the invention, it is to be understood that the invention is not limited to the details of construction or process steps set forth in the following description. The invention is capable of other embodiments and of being practiced or being carried out in various ways. 
     As used in this specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly indicates otherwise. For example, reference to a “cell” may also refer to more than one cell, and the like. 
     The term “photovoltaic cell” is used to describe an individual stack of layers suitable for converting light into electricity. The term “solar cell” and the like are used interchangeably with “photovoltaic cell.” The term “photovoltaic module” is used to describe a plurality of photovoltaic cells connected in series. The term “solar module” and the like are used interchangeably with “photovoltaic module.” 
     According to embodiments of the invention, the likelihood of a thin-film photovoltaic module passing the hot spot test is improved by providing a means for a direct silicon to metal back contact in localized areas such that the reverse current conductance is increased so that the maximum temperature of a shaded cell during the hot spot test is lower. 
       FIGS. 3A and 3B  illustrate a typical process sequence  100  used in the manufacture of solar cells. It is to be understood that the invention is not limited to the process sequence illustrated and described below. Other manufacturing processes can be employed without deviating from the spirit and scope of the invention. 
     The process sequence  100  generally starts at step  101  in which a superstrate  102  is loaded into a loading module. The superstrate  102  may be received in a “raw” state where the edges, overall size, and/or cleanliness of the substrates are not well controlled. Receiving “raw” substrates reduces the cost to prepare and store substrates prior to forming a solar module and thus reduces the solar cell module cost, facilities costs, and production costs of the finally formed solar cell module. However, typically, it is advantageous to receive “raw” substrates that have a transparent conducting oxide (TCO) layer already deposited on a surface of the superstrate  102  before it is received into the system in step  101 . If a front contact layer  110 , such as TCO layer, is not deposited on the surface of the “raw” superstrate  102  then a front contact deposition step (step  107 ), which is discussed below, needs to be performed on a surface of the superstrate  102 . 
     The superstrate  102  is often made of glass, but other materials including, but not limited to, polymeric materials can be employed. Additionally, the superstrate  102  can be made of a rigid or flexible material. An exemplary thickness for a glass sheet is about 3 mm. In the art, this superstrate  102  may be referred to as a substrate because a plurality of material layers are deposited onto the superstrate  102 . 
     In step  103 , the surfaces of the superstrate  102  are prepared to prevent yield issues later in the process. The superstrate  102  may be inserted into a front end substrate seaming module that is used to prepare the edges of the superstrate  102  to reduce the likelihood of damage, such as chipping or particle generation from occurring during the subsequent processes. Damage to the superstrate  102  can affect module yield and the cost to produce a usable photovoltaic module. 
     Next, the superstrate  102  is cleaned (step  105 ) to remove any contaminants found on the surface. Common contaminants may include materials deposited on the superstrate  102  during the substrate forming process (e.g., glass manufacturing process) and/or during shipping or storing of the superstrate  102 . Typically, cleaning uses wet chemical scrubbing and rinsing steps to remove any undesirable contaminants, but other cleaning processes can be employed. 
     If the superstrate  102  loaded in step  101  does not have a front contact layer  110  on the surface, a front contact layer  110  is deposited in step  107 . The front contact layer  110  is often a transparent conductive oxide (TCO) layer, and may be referred to as a “first TCO layer” throughout this specification. The superstrate  102  may be transported to a front end processing module in which a front contact formation process, step  107 , is performed on the superstrate  102 . In step  107 , the one or more substrate front contact formation steps may include one or more of preparation, etching, and/or material deposition steps to form the front contact regions on a bare superstrate  102 . Step  107  may comprise one or more physical vapor deposition (PVD) steps or chemical vapor deposition (CVD) steps that are used to form the front contact region on a surface of the superstrate  102 . 
     Suitable materials for the front contact layer  110  include, but are not limited to, aluminum-doped zinc oxide (AZO), indium tin oxide (ITO), indium molybdenum oxide (IMO), indium zinc oxide (IZO) and tantalum oxide. In some embodiments, the front contact region may contain a transparent conducting oxide (TCO) layer  110  that contains a metal element selected from a group consisting of zinc (Zn), aluminum (Al), indium (In), tantalum (Ta) molybdenum (Mo) and tin (Sn). In a specific embodiment, zinc oxide (ZnO) is used to form at least a portion of the front contact layer  110 . 
     In step  109 , separate cells are electrically isolated from one another via scribing processes. Contamination particles on the front contact layer  110  surface and/or on the bare glass superstrate  102  surface can interfere with the scribing procedure. In laser scribing, for example, if the laser beam runs across a particle, it may be unable to scribe a continuous line, resulting in a short circuit between cells. In addition, any particulate debris present in the scribed pattern and/or on the front contact layer  110  of the cells after scribing can cause shunting and non-uniformities between layers. 
     The device superstrate  102  is transported to the scribe module in which step  109 , or a front contact isolation step, is performed on the device superstrate  102  to electrically isolate different regions of the device superstrate  102  surface from each other. In step  109 , material is removed from the device superstrate  102  surface by use of a material removal step, such as a laser ablation process. The success criteria for step  109  are to achieve good cell-to-cell and cell-to-edge isolation while minimizing the scribe area. The front contact isolation step  109  uses a laser scribing process, often referred to as P 1 , which scribes strips  104  through the entire thickness of the front contact layer  110 . The scribed strips are usually 5-10 mm apart, but larger and smaller distances can be used. 
     Next, the device superstrate  102  is transported to a cleaning module in which step  111 , a pre-deposition substrate cleaning step, is performed on the device superstrate  102  to remove any contaminants found on the surface of the device superstrate  102  after performing the cell isolation step  109 . Typically, cleaning uses wet chemical scrubbing and rinsing steps to remove any undesirable contaminants found on the device superstrate  102  surface after performing the cell isolation step. 
     Next, the device superstrate  102  is transported to a processing module in which step  113 , which comprises one or more photoabsorber layer  120  deposition steps, is performed on the device superstrate  102 . In step  113 , the one or more photoabsorber layer  120  deposition steps may include one or more of preparation, etching, and/or material deposition steps that are used to form the various regions of the solar cell device. Step  113  generally comprises a series of sub-processing steps that are used to form one or more p-i-n junctions. In some embodiments, the one or more p-i-n junctions comprise amorphous silicon and/or microcrystalline silicon materials. Step  113  can include multiple p-layers, i-layers and n-layers, as in the case of multi-junction photovoltaic modules. In specific embodiments, the photoabsorber layer  120  comprises one or more of an n-type layer, a p-type layer and an intrinsic layer. In specific embodiments, the photoabsrober layer  120  comprises silicon. 
     A cool down step, or step  115 , may be performed after step  113 . The cool down step is generally used to stabilize the temperature of the device superstrate  102  to assure that the processing conditions seen by each device superstrate  102  in the subsequent processing steps are repeatable. Generally, the temperature of the device superstrate  102  exiting a processing module can vary by many degrees and exceed a temperature of 50° C., which can cause variability in the subsequent processing steps and solar cell performance. 
     Next, the device superstrate  102  is transported to a scribe module in which step  117 , or the interconnect formation step, is performed on the device superstrate  102  to electrically isolate various regions of the device superstrate  102  surface from each other. In step  117 , material is removed from the device superstrate  102  surface by use of a material removal step, such as a laser ablation process. This second laser scribing step, often referred to as P 2 , completely cuts scribes  108  through the photoabsorber layer  120 . 
     Next, the device superstrate  102  may be subjected to one or more substrate back contact formation steps, or step  119 . In step  119 , a back contact layer  130 , also referred to as a second TCO layer, is formed on the photoabsorber layer  120 . The one or more back contact layer  130  formation steps may include one or more of preparation, etching, and/or material deposition steps that are used to form the back contact regions of the solar module. Step  119  generally comprises one or more PVD steps or CVD steps that are used to form the back contact layer  130  on the surface of the photoabsorber layer  120 . In detailed embodiments, the one or more PVD steps are used to form a back contact region that contains a metal layer selected from a group consisting of zinc (Zn), tin (Sn), aluminum (Al), copper (Cu), silver (Ag), nickel (Ni), vanadium (V), molybdenum (Mo), and conductive carbon. 
     According to one or more embodiments, the average temperature of shadowed reverse-biased photovoltaic cells is reduced in a so-called hot spot test by forming localized areas where the metal of a typical transparent conductor/metal back contact directly contacts the silicon, forming a silicon/metal interface that has been shown to have higher reverse-bias conductance while maintaining high forward-bias rectification. A silicon/back contact structure is formed where the major area of the silicon/back contact is a high-reflectance silicon/oxide/metal structure conductive to maximizing photocurrent density and the minor area is a lower-reflectance higher-reverse-conductance silicon/metal structure conducive to conducting reverse current with minimal average cell heating. 
     Without being bound by any particular theory of operation, it is believed that forming a distributed (e.g. a high area density of small areas) high-reverse-bias conductance the reverse current dissipated in a shadowed cell is dissipated in said distributed areas of high-reverse-bias conductance so that the dissipated power is laterally distributed across the shadowed cell thereby better distributing the heat generated by the dissipation, thereby lowering the average cell temperature relative to the case where a low-reverse-bias conductance silicon/oxide/metal contact causes reverse-bias current to preferentially flow only in a few small areas (e.g. where local defects cause higher-than-average reverse-bias conductance) so that local maximum temperatures at or near those local defects is much higher than the average maximum temperatures in cells with higher, distributed reverse-bias conductance. 
     In detailed embodiments, the majority of the silicon may be in contact with a high-reflectance silicon/oxide/metal stack that maximizes photocurrent density while a minority of the silicon may be contacted with a high-conductance silicon/metal stack that minimizes average heating during reverse-bias current flow (e.g., while shadowed in a hot spot test). For example, it may be possible to cover 90+% of the silicon with oxide/metal while covering &lt;10% of the silicon with metal to retain near-maximum photocurrent while assuring hot spot protection. 
     Specific embodiments are directed to structures and methods in which a distributed silicon/metal contact is formed by depositing metal in localized areas prior to depositing a standard oxide/metal back contact over the total area. A method for forming localized silicon/metal contacts is to spray a metal-containing ink or dilute paste—such as silver-containing pastes as are often used for forming c-silicon solar cell grid contacts. In specific embodiments, a mid-frequency (e.g. 10-20 kHz) ultrasonic spray nozzle with a solvent-diluted silver ink is distributed over the surface so that localized silicon/silver splotches occur that would serve as high reverse-bias conductance areas. 
     Some embodiments use other methods to form local silicon/metal contacts, such as shadowing of certain areas during oxide deposition, e.g. close-to-substrate wire shadow masks under an oxide sputtering target so that stripes of silicon are uncovered by the oxide—e.g. ZnO:Al—such that subsequent all-area coverage by Al or Ag creates a majority area of high-photocurrent silicon/oxide/metal and a minority area (e.g. stripes as defined by the above mentioned wire masks) of high-conductance silicon/metal. 
     The localized areas of silicon/metal can be in many patterns. Suitable patterns include, but are not limited to, stripes, dots, webs, geometric patterns, and random patterns. Additionally, the coverage ratios can be varied. Suitable coverage ratios include, but are not limited to, 10:1 silicon/oxide/metal:silicon/metal coverage. 
     In some detailed embodiments, the localized areas of silicon/metal can be formed by patterning the oxide. The oxide can be patterned by, for example, depositing metal first in local areas (e.g. by depositing metal in localized areas prior to depositing any oxide) by masking the silicon in certain areas during oxide deposition. Then removing oxide from the silicon in certain areas (e.g. by mechanical, chemical or ablative means), and/or by diffusing metal through the oxide in certain areas. 
     A contact according to one or more embodiments of the present invention might comprise a single metal used for both the localized and distributed contacts. Additionally, the contact might comprise multiple metals for different purposes. Suitable examples include, but are not limited to, depositing Ag in localized areas to form a high-conductance contact and depositing ZnO:Al in most areas to form a high-reflectance, low-cost contact. 
     An example of one or more embodiments of the inventive method is to dilute Cabot Ag ink in a suitable diluent solvent. The dilute ink can then be sprayed (spritzed) onto a bare silicon plate after P 2  laser patterning and prior to ZnO/Ag. The dilute ink can be dried and cured in any in-line drying oven to form local Si/Ag high-conductance contact areas. Then, ZnO/Ag can be deposited by, for example, physical vapor deposition to form a distributed high-reflectance contact. 
     Next, the device superstrate  102  is transported to a scribe module in which step  121 , or a back contact isolation step, is performed on the device superstrate  102  to electrically isolate the plurality of solar cells contained on the substrate surface from each other. In step  121 , material is removed from the substrate surface by use of a material removal step, such as a laser ablation process. This third scribing process, called P 3 , is used to scribe strips  112  through the back contact layer  130  and the photoabsorber layer  120 . The area between, and including, the P 1  and P 3  scribes results in a dead zone  114  which decreases the overall efficiency of the cell. The dead zone is typically in the range of about 100 μm to about 500 μm, depending on the accuracy of the lasers and optics employed in the scribing processes. 
       FIG. 4A  shows a single junction amorphous silicon photovoltaic cell  104 . The photovoltaic cell  104  shown comprises a superstrate  102  such as a glass substrate, polymer substrate, metal substrate, or other suitable substrate, with thin films formed thereon. In a specific embodiment, the superstrate  102  is a glass substrate that is about 2200 mm×2600 mm×3 mm in size. The solar cell  104  further comprises a first transparent conducting oxide (TCO) layer  110  (e.g., zinc oxide (ZnO), tin oxide (SnO)) formed over the superstrate  102 , a first photoabsorber layer  120 , comprising a p-i-n junction, formed over the front contact layer  110 . A back contact layer  130  is formed over the first photoabsorber layer  120 , and a reflective layer  150 , or stack of layers, is formed over the back contact layer  130 . To improve light absorption by enhancing light trapping, the superstrate  102  and/or one or more of the thin films formed thereover may be optionally textured by wet, plasma, ion, and/or mechanical processes. For example, in the embodiment shown in  FIG. 4A , the front contact layer  110  is textured, and the subsequent thin films deposited thereover generally follow the topography of the surface below it. 
     In the detailed embodiment shown in  FIG. 4A , the first photoabsorber layer  120  comprises a p-type amorphous silicon layer  122 , an intrinsic type amorphous silicon layer  124  formed over the p-type amorphous silicon layer  122 , and an n-type microcrystalline silicon layer  126  formed over the intrinsic type amorphous silicon layer  124 . The p-type amorphous silicon layer  122  may be formed to a thickness between about 60 Å and about 300 Å, the intrinsic type amorphous silicon layer  124  may be formed to a thickness between about 1,500 Å and about 3,500 Å, and the n-type microcrystalline silicon layer  126  may be formed to a thickness between about 100 Å and about 400 Å. The back contact layer  130  is deposited over the photoabsorber layer  120  and is often a second transparent conductive oxide layer. A reflective layer  150  is deposited over the back contact layer  130 . The reflective layer  150  is often considered to be a sublayer in a back contact stack, which can also include the back contact layer  130 . The reflective layer  150  may include, but is not limited to, a material selected from the group consisting of Al, Ag, Ti, Cr, Au, Cu, Pt, Ni, Mo, conductive carbon, alloys thereof, and combinations thereof. In detailed embodiments, the reflective layer  150  comprises one or more of a paint layer, a polymer layer impregnated with a white pigment, and a metal selected from the group consisting of silver, copper and combinations thereof. 
       FIG. 4B  is a schematic diagram of an embodiment of a solar cell  104 , which is a multi-junction solar cell. The solar cell  104  of  FIG. 4B  comprises a superstrate  102 , such as a glass substrate, polymer substrate, metal substrate, or other suitable substrate, with thin films formed thereover. The solar cell  104  may further comprise a first transparent conducting oxide (TCO) layer  110  formed over the superstrate  102 , a first photoabsorber layer  120  formed over the front contact layer  110 , a second photoabsorber layer  160  formed over the first photoabsorber layer  120 , a back contact layer  130  formed over the second photoabsorber layer  160 , and a reflective layer  150  formed over the back contact layer  130 . 
     In the embodiment shown in  FIG. 4B , the front contact layer  110  is textured, and the subsequent thin films deposited thereover generally follow the topography of the surface below it. The first photoabsorber layer  120  may comprise a p-type amorphous silicon layer  122 , an intrinsic type amorphous silicon layer  124  formed over the p-type amorphous silicon layer  122 , and an n-type microcrystalline silicon layer  126  formed over the intrinsic type amorphous silicon layer  124 . In one example, the p-type amorphous silicon layer  122  may be formed to a thickness between about 60 Å and about 300 Å, the intrinsic type amorphous silicon layer  124  may be formed to a thickness between about 1,500 Å and about 3,500 Å, and the n-type microcrystalline silicon layer  126  may be formed to a thickness between about 100 Å and about 400 Å. 
     The second photoabsorber layer  160  may comprise a p-type microcrystalline silicon layer  162 , an intrinsic type microcrystalline silicon layer  164  formed over the p-type microcrystalline silicon layer  162 , and an n-type amorphous silicon layer  166  formed over the intrinsic type microcrystalline silicon layer  164 . In one example, the p-type microcrystalline silicon layer  162  may be formed to a thickness between about 100 Å and about 400 Å, the intrinsic type microcrystalline silicon layer  164  may be formed to a thickness between about 10,000 Å and about 30,000 Å, and the n-type amorphous silicon layer  166  may be formed to a thickness between about 100 Å and about 500 Å. The reflective layer  150  may include, but is not limited to a material selected from the group consisting of Al, Ag, Ti, Cr, Au, Cu, Pt, Ni, Mo, conductive carbon, alloys thereof, and combinations thereof. 
     Next, the device superstrate  102  is transported to a quality assurance module in which step  123 , or quality assurance and/or shunt removal steps, are performed on the device superstrate  102  to assure that the devices formed on the substrate surface meet a desired quality standard and in some cases correct defects in the formed device. In step  123 , a probing device is used to measure the quality and material properties of the formed photovoltaic module by use of one or more substrate contacting probes. 
     Next, the device superstrate  102  is optionally transported to a substrate sectioning module in which a substrate sectioning step  125  is used to cut the device superstrate  102  into a plurality of smaller devices to form a plurality of smaller photovoltaic modules. Instead of directly cutting the device superstrate  102  into smaller sections, the substrate sectioning step  125  may form a series of scored lines. The device superstrate  102  may then be broken along the scored lines to produce the desired size and number of sections needed for the completion of the solar cell devices. 
     The superstrate  102  is next transported to a seamer/edge deletion module in which a substrate surface and edge preparation step  127  is used to prepare various surfaces of the device superstrate  102  to prevent yield issues later on in the process. Damage to the device superstrate  102  edge can affect the device yield and the cost to produce a usable solar cell device. The seamer/edge deletion module may be used to remove deposited material from the edge of the device superstrate  102  (e.g., 10 mm) to provide a region that can be used to form a reliable seal between the device superstrate  102  and the backside glass (i.e., steps  137  and  139  discussed below). Material removal from the edge of the device superstrate  102  may also be useful to prevent electrical shorts in the final formed solar cell. 
     The device superstrate  102  is then transported to a pre-screen module in which optional pre-screen steps  129  are performed on the device superstrate  102  to assure that the devices formed on the substrate surface meet a desired quality standard. In step  129 , a light emitting source and probing device may be used to measure the output of the formed solar cell device by use of one or more substrate contacting probes. If the module detects a defect in the formed device it can take corrective actions or the solar cell can be scrapped. 
     Next the device superstrate  102  is transported to a cleaning module in which step  131 , or a pre-lamination substrate cleaning step, is performed on the device superstrate  102  to remove any contaminants found on the surface of the superstrate  102  after performing the preceding steps. Typically, the cleaning uses wet chemical scrubbing and rinsing steps to remove any undesirable contaminants found on the substrate surface after performing the cell isolation step. 
     The superstrate  102  may then be transported to a bonding wire attach module in which a bonding (or ribbon) wire attach step  133  is performed on the superstrate  102 . Step  133  is used to attach the various wires/leads required to connect various external electrical components to the formed solar cell module. The bonding wire attach module may be an automated wire bonding tool that reliably and quickly forms the numerous interconnects required to produce large solar cells. 
       FIG. 5  shows a plan view that schematically illustrates an example of the rear surface of a formed solar cell module  106  produced by the previously described procedure.  FIG. 6  is a side cross-sectional view of the solar cell module  106  illustrated in  FIG. 5  (see section  6 - 6 ).  FIG. 7  is a side cross-sectional view of a portion of the solar cell module  106  illustrated in  FIG. 5  (see section  7 - 7 ). While  FIG. 7  illustrates the cross-section of a single junction cell similar to the configuration described in  FIG. 4A , this is not intended to be limiting as to the scope of the invention described herein. 
     The solar cell module  106  shown in  FIG. 5-7  contains a superstrate  102 , the solar cell device elements (e.g., reference numerals  110 - 150 ), one or more internal electrical connections (e.g., side-buss  155 , cross-buss  156 ), a layer of bonding material  190 , a back glass substrate  191 , and a junction box  170 . The junction box  170  generally contains two junction box terminals  171 ,  172  that are electrically connected to the leads  173  of the solar cell module  106  through the side-buss  155  and the cross-buss  156 , which are in electrical communication with the reflective layer  150  and active regions of the solar cell module  106 . An edge delete region  161  is shown around the perimeter of the photovoltaic module  106   
       FIG. 6  is a schematic cross-section of a solar cell module  106  illustrating various scribed regions used to form the individual cells within the solar cell module  106 . As illustrated in  FIG. 6 , the solar cell module  106  includes a transparent superstrate  102 , a front contact layer  110 , a first photoabsorber layer  120 , a back contact layer  130  and a reflective layer  150 . Three laser scribes  104 ,  108 ,  112  produce trenches to form a high efficiency solar cell device. Although formed together on the superstrate  102 , the individual cells are isolated from each other by the insulating trench (e.g., scribe  112 ) formed in the back contact layer  130  and reflective layer  150 . In addition, a scribe  108  trench is formed in the first photoabsorber layer  120  so that the reflective layer  150  is in electrical contact with the front contact layer  110  of the adjacent cell. In one embodiment, the P 1  scribe line  104  is formed by the removal of a portion of the front contact layer  110  prior to the deposition of the first photoabsorber layer  120 , back contact layer  130  and reflective layer  150 . Similarly, in one embodiment, the P 2  scribe  108  forms a trench in the first photoabsorber layer  120  by the removal of a portion of the first photoabsorber layer  120  prior to the deposition of the back contact layer  130  and the reflective layer  150 . While a single junction type solar cell is illustrated in  FIG. 6  this configuration is not intended to be limiting to the scope of the invention described herein. 
     In some embodiments, step  133  includes a bonding wire attach module which is used to form the side-buss  155  and cross-buss  156  on the formed back contact. In this configuration, the side-buss  155  may comprise a conductive material that can be affixed, bonded, and/or fused to the reflective layer  150  to form a robust electrical contact. In one embodiment, the side-buss  155  and cross-buss  156  each comprise a metal strip, such as copper tape, a nickel coated silver ribbon, a silver coated nickel ribbon, a tin coated copper ribbon, a nickel coated copper ribbon, or other conductive material that can carry current delivered by the solar cell module  106  and that can be reliably bonded to the reflective layer  150 . In a specific embodiment, the metal strip is between about 2 mm and about 10 mm wide and between about 1 mm and about 3 mm thick. 
     The cross-buss  156 , which is shown electrically connected to the side-buss  155 , can be electrically isolated from the reflective layer  150  of the solar cell module  106  by use of an insulating material  157 , such as an insulating tape. The ends of each of the cross-busses  156  generally have one or more leads  162  that are used to connect the side-buss  155  and the cross-buss  156  to the electrical connections found in a junction box  170 , which is used to connect the formed solar cell module  106  to other external electrical components. 
     As best shown in the partial cross-section view of  FIG. 7 , in the next steps, step  132  and  134 , a bonding material  360  and “back glass” substrate  361  is provided and applied. The back glass substrate  361  is bonded onto the device superstrate  102  by use of a laminating process (step  134  discussed below). In a detailed embodiment of step  132 , a polymeric material is placed between the back glass substrate  361  and the deposited layers on the device superstrate  102  to form a hermetic seal to prevent the environment from attacking the solar cell during its life. 
     The device superstrate  102 , the back glass substrate  191 , and the bonding material  190  are transported to a bonding module in which step  135  and step  139  are performed. Portions of these steps include lamination to bond the backside glass substrate  191  to the device substrate. In step  137 , a bonding material  190 , such as Polyvinyl Butyral (PVB) or Ethylene Vinyl Acetate (EVA), may be sandwiched between the backside glass substrate  191  and the device superstrate  102 . Heat and pressure are applied to the structure to form a bonded and sealed device using various heating elements and other devices found in the bonding module. The device superstrate  102 , the back glass substrate  191 , and the bonding material  190  thus form a composite solar cell structure, as shown in  FIG. 7  that at least partially encapsulates the active regions of the solar cell device. In some embodiments, at least one hole formed in the back glass substrate  191  remains at least partially uncovered by the bonding material  190  to allow portions of the cross-buss  156  or the side-buss  155  to remain exposed so that electrical connections can be made to these regions of the solar cell structure in future steps. 
     Next the composite solar cell structure is transported to an autoclave module in which step  139 , or autoclave steps are performed on the composite solar cell structure to remove trapped gasses in the bonded structure and assure that a good bond is formed. In step  137 , a bonded solar cell structure is inserted in the processing region of the autoclave module where heat and high pressure gases are delivered to reduce the amount of trapped gas and improve the properties of the bond between the device superstrate  102 , back glass substrate  191 , and bonding material  190 . The processes performed in the autoclave are also useful to assure that the stress in the glass and bonding layer (e.g., PVB layer) are more controlled to prevent future failures of the hermetic seal or failure of the glass due to the stress induced during the bonding/lamination process. It may be desirable to heat the device superstrate  102 , back glass substrate  191 , and bonding material  190  to a temperature that causes stress relaxation in one or more of the components in the formed solar cell structure. 
     Additional processing steps  141  may be performed, including but not limited to device testing, additional cleaning, attaching the device to a support structure, unloading modules from processing chambers and shipping. 
     Accordingly, with reference to  FIG. 8 , one or more embodiments of the invention are directed to methods of manufacturing a photovoltaic cell and/or photovoltaic module. For clarity of illustration,  FIG. 8  is shown without any scribe lines  104 ,  108 ,  112 . A front contact layer  110  is deposited onto a superstrate  102 . A photoabsorber layer  120  is deposited onto the front contact layer  110 . The photoabsorber layer  120  is adapted to convert light energy into electrical current and includes one or more sublayers. In specific embodiments, the one or more sublayers are selected from the group consisting of p-type, n-type and instrinsic sublayers. A patterned discontinuous conductive layer  140  is deposited onto the photoabsorber layer  120 . A back contact layer  130  is deposted onto the patterned discontinuous conductive layer  140 , and a reflective layer  150  is deposited onto the back contact layer  130 . 
       FIG. 8  shows an embodiment of the patterned discontinuous conductive layer  140  which extends from the photoabsorber layer  120  into the back contact layer  130 , but does not touch the reflective layer  150 .  FIG. 9  shows an alternate embodiment in which the patterned discontinuous conductive layer  140  extends from the photoabsorber layer  120  through the back contact layer  130  and contacts the reflective layer  150 . Some embodiments of the invention represent a combination of the embodiments shown in  FIGS. 8 and 9 , where a portion of the patterned discontinuous conductive layer  140  extends through the back contact layer  130  to contact the reflective layer  150 , and a portion of the patterned discontinuous conductive layer  140  does not extend through the back contact layer  130 . 
     In detailed embodiments, the patterned discontinuous conductive layer  140  comprises a metal. In specific embodiments, the metal is selected from the group consisting of silver, copper, aluminum, indium, tin, nickel, molybdenum, chromium, tantalum, titanium and combinations thereof. In various embodiments, the patterned discontinuous conductive layer  140  comprises the same material used in the reflective layer  150 . In some embodiments, patterned discontinuous conductive layer  140  comprises a different material than that used in the reflective layer  150   
     The patterned discontinuous conductive layer  140  can have a variety of patterns.  FIG. 10  shows four non-limiting examples of suitable patterns. These patterns are merely illustrative and should not be taken as limiting the scope of the invention.  FIG. 10A  shows a pattern of separated dots or hemispheres in a predictable pattern.  FIG. 10B  shows a two-dimensional grid which may be useful in aiding lateral conduction across a photovoltaic module.  FIG. 100  shows a one-dimensional grid which also may aid in the conductance in a particular direction with minimal coverage.  FIG. 10D  shows a random pattern of dots. Any of these patterns, or others, can be used and may extend through the back contact layer  130 , or not. In specific embodiments, the pattern is selected from the group consisting of random distributions, grid-like distribution, one-dimensional grid, equally spaced dots and combinations thereof. 
     The patterned discontinuous conductive layer  140  can cover a wide range of percentages of the photoabsorber layer  120 . In detailed embodiments, the patterned discontinuous conductive layer  140  covers up to about 10% of the photoabsorber layer  120 . In various embodiments, the patterned discontinuous conductive layer  140  covers up to about 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15% of the photoabsorber layer  120 . In some embodiments, the patterned discontinuous conductive layer  140  covers in the range of about 1% to about 15% of the photoabsorber layer  120 , or in the range of about 2% to about 14% of the photoabsorber layer  120 , or in the range of about 3% to about 13% of the photoabsorber layer  120 , or in the range of about 5% to about 12% of the photoabsorber layer  120 , or in the range of about 7% to about 11% of the photoabsorber layer  120 . 
     Reference throughout this specification to “one embodiment,” “certain embodiments,” “one or more embodiments,” “an embodiment,” “one aspect,” “certain aspects,” “one or more embodiments” and “an aspect” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases such as “in one or more embodiments,” “in certain embodiments,” “in one embodiment,” “in an embodiment,” “according to one or more aspects,” “in an aspect,” etc., in various places throughout this specification are not necessarily referring to the same embodiment or aspect of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or aspects. The order of description of the above method should not be considered limiting, and methods may use the described operations out of order or with omissions or additions. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of ordinary skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.