Patent Publication Number: US-2011053307-A1

Title: Repatterning of polyvinyl butyral sheets for use in solar panels

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/239,376 (APPM/014180L), filed Sep. 2, 2009, which is herein incorporated by reference. This application is related to U.S. application Ser. No. 12/202,199, filed Aug. 29, 2008 (Attorney Docket No. APPM/11141), U.S. application Ser. No. 12/201,840, filed Aug. 29, 2008 (Attorney Docket No. APPM/11141.02), and U.S. Provisional Application Ser. No. 61/149,942, filed Feb. 4, 2009 (Attorney Docket No. APPM/13847L). 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention generally relate to a method of forming solar cell devices. In particular, embodiments of the invention relate to methods of processing encapsulant layers of a solar cell device and forming solar cell devices using processed encapsulant layers. 
     2. Description of the Related Art 
     Photovoltaic (PV) devices or solar cells are devices which convert sunlight into direct current (DC) electrical power. Typical thin-film-type PV devices, or thin-film solar cells, have one or more p-i-n junctions. Each p-i-n junction comprises a p-type layer, an intrinsic-type layer, and an n-type layer. When the p-i-n junction of the solar cell is exposed to sunlight (consisting of energy from photons), the sunlight is converted to electricity through the PV effect. 
     Typically, a thin-film solar cell includes active regions, or photoelectric conversion units, and a transparent conductive oxide (TCO) film disposed as a front electrode and/or as a backside electrode. The photoelectric conversion unit includes a p-type silicon layer, an n-type silicon layer, and an intrinsic-type (i-type) silicon layer sandwiched between the p-type and n-type silicon layers. Several types of silicon films, including microcrystalline silicon film (μc-Si), amorphous silicon film (a-Si), polycrystalline silicon film (poly-Si), and the like, may be utilized to form the p-type, n-type, and/or i-type layers of the photoelectric conversion unit. The backside electrode may contain one or more conductive layers. There is a need for an improved process of forming a solar cell that has good interfacial contact, low contact resistance, and high overall performance. 
     With traditional energy source prices on the rise, there is a need for a low-cost way of producing electricity using a low-cost solar cell device. Conventional solar cell manufacturing processes are highly labor-intensive and have numerous interruptions that can affect the production line throughput, solar cell cost, and device yield. For instance, a major challenge in encapsulation of large-sized solar cells is achieving bubble-free lamination results across locally stepped topography, such as from internal electrical connections, e.g., cross- and side-buss ribbons that collect power from individual solar cells on the front glass. The large size (2.2 m×2.6 m) of substrates and the cross- and side-buss wires on the glass make the lamination process particularly sensitive to bubble formation. Bubble formation during lamination may create paths from edge to center, causing delamination or environmental encroachment, such as rain, to seep into the area and damage the solar cell. 
     One method to prevent bubble formation is patterning the back glass to decrease air entrapment between the encapsulant material and the back glass. Chemical etchants may be used to pattern the back glass. However, patterning the glass tends to weaken the glass substrate. Another method to prevent bubble formation is to machine encapsulant material to form a pattern. The machining process typically uses a mill to cut the pattern in the encapsulant. However, machining encapsulant tends to be expensive, very difficult, and creates a gummy final product. Therefore, there is a need for a method of decreasing bubble formation during lamination of large-size substrates used in the manufacture of solar cells along a production line having a suite of modules and improve solar cell quality. 
     SUMMARY OF THE INVENTION 
     In one embodiment, a method of forming a composite solar cell structure includes preparing a device substrate, wherein the device substrate includes a glass substrate, a transparent conductive layer deposited over the glass substrate, one or more silicon layers deposited over the transparent conductive layer, a back contact layer deposited over the one or more silicon layers, and one or more internal electrical connections disposed on the back contact layer. The method also includes forming a mating pattern on a bonding material to match a topography of an exposed surface of the device substrate, the exposed surface comprising the back contact layer and the one or more internal electrical connections. The method also includes positioning the bonding material over the exposed surface, disposing a back glass substrate over the bonding material to form a composite structure, and compressing the composite structure. 
     In another embodiment, a method of preparing a pre-patterned bonding material for a solar cell assembly includes placing a bonding material over a work surface having an embossment, wherein at least a portion of the embossment corresponds to a topography of an exposed surface of a device substrate, heating the bonding material, and pressing the bonding material onto the embossment to form a mating pattern. 
     In another embodiment, a method of preparing a pre-patterned bonding material for a solar cell assembly, includes passing a bonding material between at least two rollers, wherein at least one roller has an embossment, at least a portion of the embossment corresponding to a topography of an exposed surface of a device substrate, heating the bonding material, and pressing the bonding material onto the embossment to form a mating pattern as the bonding material passes through the two rollers. 
    
    
     
       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  illustrates a process sequence for forming a solar cell device according to one embodiment described herein. 
         FIG. 2  illustrates a plan view of a solar cell production line according to one embodiment described herein. 
         FIG. 3A  is a side cross-sectional view of a thin-film solar cell device according to one embodiment described herein. 
         FIG. 3B  is a side cross-sectional view of a thin-film solar cell device according to one embodiment described herein. 
         FIG. 3C  is a plan view of a composite solar cell structure according to one embodiment described herein. 
         FIG. 3D  is a side cross-sectional view along Section A-A of  FIG. 3C . 
         FIG. 3E  is a side cross-sectional view of a thin-film solar cell device according to one embodiment described herein. 
         FIG. 4  is a schematic cross-sectional view of a bonding module according to one embodiment. 
         FIG. 5  illustrates a side cross-sectional view of one embodiment of an autoclave module and supporting equipment. 
         FIG. 6A  is a plane view of a composite solar cell structure without the junction box attached according to one embodiment described herein. 
         FIG. 6B  is a close-up isometric view of the cross-buss, side-buss, and isolation tape shown on the composite solar cell structure in  FIG. 6A . 
         FIG. 7  is a cross-sectional view of a composite solar cell structure in  FIG. 6A  along lines B-B. 
         FIG. 8  is a work surface having an embossment according to one embodiment described herein. 
         FIGS. 9A-9B  illustrate a set of rollers having an embossment according to one embodiment described herein. 
         FIGS. 10A-10C  illustrate a matting pattern formed on a bonding material according to embodiments described herein. 
         FIG. 11  is a cross-sectional view of a bonding material having a material pattern matched with the cross-buss. 
         FIG. 12  is a partial cross-sectional isometric view of the pre-patterned bonding material over the internal electrical connections prior to lamination. 
         FIG. 13  schematically illustrates a method of forming a composite solar cell structure according to one embodiment described herein. 
         FIG. 14  schematically illustrates a method of preparing a pre-patterned bonding material for a solar cell assembly according to one embodiment described herein. 
         FIG. 15  schematically illustrates a method of preparing a pre-patterned bonding material for a solar cell assembly according to one embodiment described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention provide a method of pre-patterning a bonding material to match a topography of an exposed surface of the device substrate. In one embodiment, the bonding material is pre-patterned before use in the formation of a solar cell by embossing a pattern that matches the topography of an exposed surface comprising a back contact layer and one or more internal electrical connections. In one embodiment, the bonding material is pressed on a work surface having an embossment that corresponds to the topography of the exposed surface. In one embodiment, the bonding material is passed between at least two rollers where at least one roller has an embossment that corresponds to the topography of the exposed surface. In one embodiment of the present invention, a method for forming a composite solar cell structure is provided. 
     Embodiments of the present invention generally relate to a system used to form solar cell devices using processing modules adapted to perform one or more processes in the formation of the solar cell devices. In one embodiment, the system is adapted to form thin-film solar cell devices by accepting a large unprocessed substrate, such as about 5.7 m 2 , and performing multiple deposition, material removal, cleaning, sectioning, bonding, and various inspection and testing processes to form multiple complete, functional, and tested solar cell devices that can then be shipped to an end user for installation in a desired location to generate electricity. 
     While the discussion below primarily describes the formation of thin-film solar cell devices, this configuration is not intended to be limiting as to the scope of the invention since the apparatus and methods disclosed herein can also be used to form, test, and analyze other types of solar cell devices, such as III-V-type solar cells, thin-film chalcogenide solar cells (e.g., CIGS, CdTe cells), amorphous or nanocrystalline silicon solar cells, photochemical-type solar cells (e.g., dye sensitized), crystalline silicon solar cells, organic-type solar cells, or other similar solar cell devices. 
       FIG. 1  illustrates one embodiment of a process sequence  100  that contains a plurality of steps (i.e., steps  102 - 142 ) that are each used to form a solar cell device using a novel solar cell production line  200  described herein. The configuration, number of processing steps, and order of the processing steps in the process sequence  100  is not intended to be limiting to the scope of the invention described herein. 
       FIG. 2  is a plan view of the production line  200 , which is intended to illustrate the flow of substrates through the system and other aspects of the system design. Examples and information regarding various process sequence and hardware configurations may also be found in U.S. patent application Ser. No. 12/202,199, filed Aug. 29, 2008 (Attorney Docket No. APPM/11141), U.S. patent application Ser. No. 12/201,840, filed Aug. 29, 2008 (Attorney Docket No. APPM/11141.02), and U.S. Provisional Patent Application Ser. No. 60/967,077. 
     In general, a system controller  290  may be used to control one or more components found in the solar cell production line  200 . The system controller  290  is generally designed to facilitate the control and automation of the overall solar cell production line  200  and typically includes a central processing unit (CPU) (not shown), memory (not shown), and support circuits (or I/O) (not shown). The CPU may be one of any form of computer processors that are used in industrial settings for controlling various system functions, substrate movement, chamber processes, and support hardware (e.g., sensors, robots, motors, lamps, etc.), and monitor the processes (e.g., substrate support temperature, power supply variables, chamber process time, I/O signals, etc.). The memory is connected to the CPU, and may be one or more of a readily available memory, such as random access memory (RAM), read only memory (ROM), floppy disk, hard disk, or any other form of digital storage, local or remote. Software instructions and data can be coded and stored within the memory for instructing the CPU. The support circuits are also connected to the CPU for supporting the processor in a conventional manner. The support circuits may include cache, power supplies, clock circuits, input/output circuitry, subsystems, and the like. 
     A program (or computer instructions) readable by the system controller  290  determines which tasks are performable on a substrate. In one embodiment, the system controller  290  also contains a plurality of programmable logic controllers (PLC&#39;s) that are used to locally control one or more modules in the solar cell production, and a material handling system controller (e.g., PLC or standard computer) that deals with the higher-level strategic movement, scheduling, and running of the complete solar cell production line 
     Examples of a solar cell  300  that can be formed using the process sequence(s) illustrated in  FIG. 1  and the components illustrated in the solar cell production line  200  are illustrated in  FIGS. 3A-3E .  FIG. 3A  is a simplified schematic diagram of a single-junction amorphous or micro-crystalline silicon solar cell  300  that can be formed and analyzed in the system described below. 
     As shown in  FIG. 3A , the single-junction amorphous or micro-crystalline silicon solar cell  300  is oriented toward a light source or solar radiation  301 . The solar cell  300  generally comprises a substrate  302 , such as a glass substrate, polymer substrate, metal substrate, or other suitable substrate, with thin films formed thereover. In one embodiment, the substrate  302  is a glass substrate that is about 2200 mm×2600 mm×3 mm in size. 
     The solar cell  300  further comprises a first transparent conducting oxide (TCO) layer  310  (e.g., zinc oxide (ZnO), tin oxide (SnO)) formed over the substrate  302 , a first p-i-n junction  320  formed over the first TCO layer  310 , a second TCO layer  340  formed over the first p-i-n junction  320 , and a back contact layer  350  formed over the second TCO layer  340 . To improve light absorption by enhancing light trapping, the substrate 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. 3A , the first TCO layer  310  is textured, and the subsequent thin films deposited thereover generally follow the topography of the surface below it. 
     In one configuration, the first p-i-n junction  320  may comprise a p-type amorphous silicon layer  322 , an intrinsic-type amorphous silicon layer  324  formed over the p-type amorphous silicon layer  322 , and an n-type amorphous silicon layer  326  formed over the intrinsic-type amorphous silicon layer  324 . In one example, the p-type amorphous silicon layer  322  may be formed to a thickness between about 60 Å and about 300 Å, the intrinsic-type amorphous silicon layer  324  may be formed to a thickness between about 1,500 Å and about 3,500 Å, and the n-type amorphous silicon layer  326  may be formed to a thickness between about 100 Å and about 500 Å. The back contact layer  350  may include, but is not limited to, a material selected from the group consisting of Al, Ag, Ti, Cr, Au, Cu, Pt, alloys thereof, and combinations thereof. 
       FIG. 3B  is a schematic diagram of an embodiment of a solar cell  300 , which is a multi-junction solar cell that is oriented toward the light or solar radiation  301 . The solar cell  300  comprises a substrate  302 , such as a glass substrate, polymer substrate, metal substrate, or other suitable substrate, with thin films formed thereover. The solar cell  300  may further comprise a first transparent conducting oxide (TCO) layer  310  formed over the substrate  302 , a first p-i-n junction  320  formed over the first TCO layer  310 , a second p-i-n junction  330  formed over the first p-i-n junction  320 , a second TCO layer  340  formed over the second p-i-n junction  330 , and a back contact layer  350  formed over the second TCO layer  340 . 
     In the embodiment shown in  FIG. 3B , the first TCO layer  310  is textured, and the subsequent thin films deposited thereover generally follow the topography of the surface below it. The first p-i-n junction  320  may comprise a p-type amorphous silicon layer  322 , an intrinsic-type amorphous silicon layer  324  formed over the p-type amorphous silicon layer  322 , and an n-type microcrystalline silicon layer  326  formed over the intrinsic-type amorphous silicon layer  324 . In one example, the p-type amorphous silicon layer  322  may be formed to a thickness between about 60 Å and about 300 Å, the intrinsic-type amorphous silicon layer  324  may be formed to a thickness between about 1,500 Å and about 3,500 Å, and the n-type microcrystalline semiconductor layer  326  may be formed to a thickness between about 100 Å and about 400 Å. 
     The second p-i-n junction  330  may comprise a p-type microcrystalline silicon layer  332 , an intrinsic-type microcrystalline silicon layer  334  formed over the p-type microcrystalline silicon layer  332 , and an n-type amorphous silicon layer  336  formed over the intrinsic-type microcrystalline silicon layer  334 . In one example, the p-type microcrystalline silicon layer  332  may be formed to a thickness between about 100 Å and about 400 Å, the intrinsic-type microcrystalline silicon layer  334  may be formed to a thickness between about 10,000 Å and about 30,000 Å, and the n-type amorphous silicon layer  336  may be formed to a thickness between about 100 Å and about 500 Å. The back contact layer  350  may include, but is not limited to, a material selected from the group consisting of Al, Ag, Ti, Cr, Au, Cu, Pt, alloys thereof, and combinations thereof. 
       FIG. 3C  is a plan view that schematically illustrates an example of the rear surface of a formed solar cell  300  that has been produced in the production line  200 .  FIG. 3D  is a side cross-sectional view of a portion of the solar cell  300  illustrated in  FIG. 3C  (see section A-A). While  FIG. 3D  illustrates the cross-section of a single-junction cell similar to the configuration described in  FIG. 3A , this is not intended to be limiting as to the scope of the invention described herein. 
     As shown in  FIGS. 3C and 3D , the solar cell  300  may contain a substrate  302 , the solar cell device elements (e.g., reference numerals  310 - 350 ), one or more internal electrical connections (e.g., side-buss  355 , cross-buss  356 ), a layer of bonding material  360 , a back glass substrate  361 , and a junction box  370 . The junction box  370  may generally contain two connection points  371 ,  372  that are electrically connected to portions of the solar cell  300  through the side-buss  355  and the cross-buss  356 , which are in electrical communication with the back contact layer  350  and active regions of the solar cell  300 . To avoid confusion relating to the actions specifically performed on the substrates  302  in the discussion below, a substrate  302  having one or more of the deposited layers (e.g., reference numerals  310 - 350 ) and/or one or more internal electrical connections (e.g., side-buss  355 , cross-buss  356 ) disposed thereon is generally referred to as a device substrate  303 . Similarly, a device substrate  303  that has been bonded to a back glass substrate  361  using a layer of bonding material  360  is referred to as a composite solar cell structure  304 . 
       FIG. 3E  is a schematic cross-section of a solar cell  300  illustrating various scribed regions used to form the individual cells  382 A- 382 B within the solar cell  300 . As illustrated in  FIG. 3E , the solar cell  300  includes a transparent substrate  302 , a first TCO layer  310 , a first p-i-n junction  320 , and a back contact layer  350 . 
     Three laser scribing steps may be performed to produce trenches  381 A,  381 B, and  381 C, which are generally required to form a high-efficiency solar cell device. Although formed together on the substrate  302 , the individual cells  382 A and  382 B are isolated from each other by the insulating trench  381 C formed in the back contact layer  350  and the first p-i-n junction  320 . In addition, the trench  381 B is formed in the first p-i-n junction  320  so that the back contact layer  350  is in electrical contact with the first TCO layer  310 . 
     In one embodiment, the insulating trench  381 A is formed by the laser scribe removal of a portion of the first TCO layer  310  prior to the deposition of the first p-i-n junction  320  and the back contact layer  350 . Similarly, in one embodiment, the trench  381 B is formed in the first p-i-n junction  320  by the laser scribe removal of a portion of the first p-i-n junction  320  prior to the deposition of the back contact layer  350 . While a single-junction-type solar cell is illustrated in  FIG. 3E , this configuration is not intended to be limiting to the scope of the invention described herein. 
     General Solar Cell Formation Process Sequence 
     Referring to  FIGS. 1 and 2 , the process sequence  100  generally starts at step  102  in which a substrate  302  is loaded into the loading module  202  found in the solar cell production line  200 . In one embodiment, the substrates  302  are received in a “raw” state where the edges, overall size, and/or cleanliness of the substrates  302  are not well controlled. Receiving “raw” substrates  302  reduces the cost to prepare and store substrates  302  prior to forming a solar device and thus reduces the solar cell device cost, facilities costs, and production costs of the finally formed solar cell device. However, typically, it is advantageous to receive “raw” substrates  302  that have a transparent conducting oxide (TCO) layer (e.g., first TCO layer  310 ) already deposited on a surface of the substrate  302  before it is received into the system in step  102 . If a conductive oxide layer, such as TCO layer, is not deposited on the surface of the “raw” substrates, then a front contact formation step (step  107 ), which is discussed below, needs to be performed on a surface of the substrate  302 . 
     In one embodiment, the substrates  302  or  303  are loaded into the solar cell production line  200  in a sequential fashion, and thus do not use a cassette or batch-style substrate loading system. In the next step, step  104 , the surfaces of the substrate  302  are prepared to prevent yield issues later on in the process. In one embodiment of step  104 , the substrate is inserted into a front end substrate seaming module  204  that is used to prepare the edges of the substrate  302  or  303  to reduce the likelihood of damage, such as chipping or particle generation, from occurring during the subsequent processes. 
     Next, the substrate  302  or  303  is transported to the cleaning module  206 , in which step  106 , or a substrate cleaning step, is performed on the substrate  302  or  303  to remove any contaminants found on the surface thereof. Common contaminants may include materials deposited on the substrate  302  or  303  during the substrate forming process (e.g., glass manufacturing process) and/or during shipping or storing of the substrates  302  or  303 . Typically, the cleaning module  205  uses wet chemical scrubbing and rinsing steps to remove any undesirable contaminants. 
     In the next step, or step  108 , separate cells are electrically isolated from one another via scribing processes. Contamination particles on the TCO surface and/or on the bare glass surface can interfere with the scribing procedure. In one embodiment, the cleaning module  205  is available from the Energy and Environment Solutions division of Applied Materials, Inc. of Santa Clara, Calif. 
     Referring to  FIGS. 1 and 2 , in one embodiment, prior to performing step  108  the substrates  302  are transported to a front end processing module (not illustrated in  FIG. 2 ) in which a front contact formation process, or step  107 , is performed on the substrate  302 . In one embodiment, the front end processing module is similar to the processing module  218  discussed below. In step  107 , the one or more substrate front contact formation steps may include one or more preparation, etching, and/or material deposition steps that are used to form the front contact regions on a bare solar cell substrate  302 . In one embodiment, step  107  generally comprises one or more PVD steps that are used to form the front contact region on a surface of the substrate  302 . In one embodiment, the front contact region contains a transparent conducting oxide (TCO) layer that may contain metal element selected from a group consisting of zinc (Zn), aluminum (Al), indium (In), and tin (Sn). In one example, a zinc oxide (ZnO) is used to form at least a portion of the front contact layer. In one embodiment, the front end processing module is an ATONTM PVD 5.7 tool available from Applied Materials, Inc. of Santa Clara, Calif. in which one or more processing steps are performed to deposit the front contact formation steps. In another embodiment, one or more CVD steps are used to form the front contact region on a surface of the substrate  302 . 
     Next, the device substrate  303  is transported to the scribe module  208  in which step  108 , or a front contact isolation step, is performed on the device substrate  303  to electrically isolate different regions of the device substrate  303  surface from each other. In step  108 , material is removed from the device substrate  303  surface by use of a material removal step, such as a laser ablation process. 
     Next, the device substrate  303  is transported to the cleaning module  210  in which step  110 , or a pre-deposition substrate cleaning step, is performed on the device substrate  303  to remove any contaminants found on the surface of the device substrate  303  after performing the cell isolation step  108 . Typically, the cleaning module  210  uses wet chemical scrubbing and rinsing steps to remove any undesirable contaminants found on the device substrate  303  surface after performing the cell isolation step. In one embodiment, a cleaning process similar to the processes described in step  105  above is performed on the device substrate  303  to remove any contaminants on the surface(s) of the device substrate  303 . 
     Next, the device substrate  303  is transported to the processing module  212  in which step  112 , which comprises one or more photoabsorber deposition steps, is performed on the device substrate  303 . In step  112 , the one or more photoabsorber deposition steps may include one or more preparation, etching, and/or material deposition steps that are used to form the various regions of the solar cell device. Step  112  generally comprises a series of sub-processing steps that are used to form one or more p-i-n junctions. In one embodiment, the one or more p-i-n junctions comprise amorphous silicon and/or microcrystalline silicon materials. In general, the one or more processing steps are performed in one or more cluster tools (e.g., cluster tools  212 A- 212 D) found in the processing module  212  to form one or more layers in the solar cell device formed on the device substrate  303 . 
     In one embodiment, the device substrate  303  is transferred to an accumulator  211 A prior to being transferred to one or more of the cluster tools  212 A- 212 D. In one embodiment, in cases where the solar cell device is formed to include multiple junctions, such as the tandem junction solar cell  300  illustrated in  FIG. 3B , the cluster tool  212 A in the processing module  212  is adapted to form the first p-i-n junction  320  and cluster tools  212 B- 212 D are configured to form the second p-i-n junction  330 . In such an embodiment, the device substrate  303  may optionally be transferred into a spectrographic inspection module  215  for a corresponding film characterization step  115  following processing in the first cluster tool  212 A. In one embodiment, the optional inspection module  215  is configured within the overall processing module  212 . 
     In one embodiment of the process sequence  100 , a cool-down step, or step  113 , is performed after step  112  has been performed. The cool-down step is generally used to stabilize the temperature of the device substrate  303  to assure that the processing conditions seen by each device substrate  303  in the subsequent processing steps are repeatable. 
     In one embodiment, the cool-down step  113  is performed in one or more of the substrate supporting positions found in one or more accumulators  211 . In one configuration of the production line, as shown in  FIG. 2 , the processed device substrates  303  may be positioned in one of the accumulators  211 B for a desired period of time to control the temperature of the device substrate  303 . In one embodiment, the system controller  290  is used to control the positioning, timing, and movement of the device substrates  303  through the accumulator(s)  211  to control the temperature of the device substrates  303  before proceeding down stream through the production line. 
     Next, the device substrate  303  is transported to the scribe module  216  in which step  114 , or the interconnect formation step, is performed on the device substrate  303  to electrically isolate various regions of the device substrate  303  surface from each other. In step  114 , material is removed from the device substrate  303  surface by use of a material removal step, such as a laser ablation process. As shown in  FIG. 3E , in one embodiment, the trench  381 B is formed in the first p-i-n junction  320  layers by use of a laser scribing process. 
     Next, the device substrate  303  is transported to the processing module  218  in which one or more substrate back contact formation steps, or step  118 , are performed on the device substrate  303 . In step  118 , the one or more substrate back contact formation steps may include one or more preparation, etching, and/or material deposition steps that are used to form the back contact regions of the solar cell device. In one embodiment, step  118  generally comprises one or more PVD steps that are used to form the back contact layer  350  on the surface of the device substrate  303 . In one embodiment, 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), and vanadium (V). In one example, a zinc oxide (ZnO) or nickel vanadium alloy (NiV) is used to form at least a portion of the back contact layer  350 . In one embodiment, the one or more processing steps are performed using an ATONTM PVD 5.7 tool available from Applied Materials, Inc. of Santa Clara, Calif. In another embodiment, one or more CVD steps are used to form the back contact layer  350  on the surface of the device substrate  303 . 
     In one embodiment, the solar cell production line  200  has at least one accumulator  211  positioned after the processing module  218 . During production, the accumulators  211 D may be used to provide a ready supply of substrates to the scribe modules  220 , and/or provide a collection area where substrates coming from the processing module  218  can be stored if the scribe modules  220  go down or cannot keep up with the throughput of the processing module  218 . 
     Next, the device substrate  303  is transported to the scribe module  220  in which step  120 , or a back contact isolation step, is performed on the device substrate  303  to electrically isolate the plurality of solar cells contained on the substrate surface from each other. In step  120 , material is removed from the substrate surface by use of a material removal step, such as a laser ablation process. As shown in  FIG. 3E , in one embodiment, the trench  381 C is formed in the first p-i-n junction  320  and back contact layer  350  by use of a laser scribing process. 
     Next, the device substrate  303  is optionally transported to the substrate sectioning module  224  in which a substrate sectioning step  124  is used to cut the device substrate  303  into a plurality of smaller device substrates  303  to form a plurality of smaller solar cell devices. 
     Referring back to  FIGS. 1 and 2 , the device substrate  303  is next transported to the seamer/edge deletion module  226  in which a substrate surface and edge preparation step  126  is used to prepare various surfaces of the device substrate  303  to prevent yield issues later on in the process. 
     Next, the device substrate  303  is transported to the pre-screen module  228  in which optional pre-screen steps  128  are performed on the device substrate  303  to assure that the devices formed on the substrate surface meet a desired quality standard. Next, the device substrate  303  is transported to the cleaning module  230  in which step  130 , or a pre-lamination substrate cleaning step, is performed on the device substrate  303  to remove any contaminants found on the surface of the substrates  303  after performing steps  122 - 127 . 
     Next, the substrate  303  is transported to a bonding wire attach module  231  in which step  131 , or a bonding wire attach step, is performed on the substrate  303 . Step  131  is used to attach the various wires/leads required to connect the various external electrical components to the formed solar cell device. Typically, the bonding wire attach module  231  is an automated wire bonding tool that is advantageously used to reliably and quickly form the numerous interconnects that are often required to form the large solar cells formed in the production line  200 . 
     In one embodiment, the bonding wire attach module  231  is used to form the side-buss  355  ( FIG. 3C ) and cross-buss  356  on the formed back contact region (step  118 ). In this configuration the side-buss  355  may be a conductive material that can be affixed, bonded, and/or fused to the back contact layer  350  found in the back contact region to form a good electrical contact. 
     The cross-buss  356 , which is electrically connected to the side-buss  355  at the junctions, can be electrically isolated from the back contact layer(s) of the solar cell by use of an insulating material  357 , such as an insulating tape. The ends of each of the cross-busses  356  generally have one or more leads that are used to connect the side-buss  355  and the cross-buss  356  to the electrical connections found in a junction box  370 , which is used to connect the formed solar cell to the other external electrical components. 
     In the next step, step  132 , a bonding material  360  ( FIG. 3D ) and “back glass” substrate  361  are prepared for delivery into the solar cell formation process (i.e., process sequence  100 ). The preparation process is generally performed in the glass lay-up module  232 , which generally comprises a material preparation module  232 A, a glass loading module  232 B, a glass cleaning module  232 C, and a glass inspection module  232 D. The back glass substrate  361  is bonded onto the device substrate  303  formed in steps  102 - 131  above by use of a laminating process (step  134  discussed below). In general, step  132  requires the preparation of a polymeric material that is to be placed between the back glass substrate  361  and the deposited layers on the device substrate  303  to form a hermetic seal to prevent the environment from attacking the solar cell during its life. 
     Referring to  FIG. 2 , step  132  generally comprises a series of sub-steps in which a bonding material  360  is prepared in the material preparation module  232 A, the bonding material  360  is then placed over the device substrate  303 , and the back glass substrate  361  is loaded into the loading module  232 B. The back glass substrate  361  is washed by the cleaning module  232 C. The back glass substrate  361  is placed over the bonding material  360  and the device substrate  303 . 
     In one embodiment, the material preparation module  232 A is adapted to receive the bonding material  360  in a sheet form and perform one or more cutting operations to provide a bonding material, such as Polyvinyl Butyral (PVB) or Ethylene Vinyl Acetate (EVA) that is sized to form a reliable seal between the back glass substrate  361  and the solar cells formed on the device substrate  303 . In general, when using bonding materials  360  that are polymeric, it is desirable to control the temperature (e.g., 16-18° C.) and relative humidity (e.g., RH 20-22%) of the solar cell production line  200  where the bonding material  360  is stored and integrated into the solar cell device to assure that the attributes of the bond formed in the bonding module  234  (discussed below) are repeatable and the dimensions of the polymeric material is stable. It is generally desirable to store the bonding material prior to use in a temperature- and humidity-controlled area (e.g., T=6-8° C.; RH=20-22%). 
     The tolerance stack-up of the various components in the bonded device (Step  134 ) can be an issue when forming large solar cells. Therefore accurate control of the bonding material properties and tolerances of the cutting process are required to assure that a reliable hermetic seal is formed. In one embodiment, PVB may be used to advantage due to its UV stability, moisture resistance, thermal cycling, good US fire rating, compliance with International Building Code, low cost, and reworkable thermo-plastic properties. 
     In one example, a 30-gauge or a 45-gauge PVB material sheet is used to bond the back glass substrate  361  to the device substrate  303 . In one part of step  132 , the bonding material is transported and positioned over the back contact layer  350  and side-buss  355  ( FIG. 3C ) and cross-buss  356  ( FIG. 3C ) elements of device substrate  303  using an automated robotic device. In one embodiment, a robot  232 D, which can be a conventional robotic device (e.g., 6-axis robot), is used to pick up and place the bonding material  360  on the device substrate  303 . The device substrate  303  and bonding material  360  are then positioned to receive a back glass substrate  361 , which can be placed thereon by use of the same automated robotic device used to position the bonding material  360 , or a second automated robotic device. In one embodiment, a glass loading robot  232 E, which can be a conventional robotic device (e.g., 6-axis robot), is used to place the back glass substrate  361  on the device substrate  303  and bonding material  360 . 
     In the next sub-step of step  132 , the back glass substrate  361  is transported to the cleaning module  232 C in which a substrate cleaning step is performed on the substrate  361  to remove any contaminants found on the surface of the substrate  361 . The prepared back glass substrate  361  is then positioned over the bonding material and device substrate  303  by use of an automated robotic device. 
     Next, the device substrate  303 , the back glass substrate  361 , and the bonding material  360  are transported to the bonding module  234  in which step  134 , or lamination steps, are performed to bond the backside glass substrate to the solar cell devices formed in steps  102 - 130  discussed above. In step  134 , a bonding material  360 , such as Polyvinyl Butyral (PVB) or Ethylene Vinyl Acetate (EVA), is sandwiched between the backside glass substrate  361  and the solar cells, and heat and pressure is applied to the structure to form a bonded and sealed device using various heating elements and other devices found in the bonding module  234 , which are discussed below. The device substrate  303 , the back glass substrate  361 , and bonding material  360  thus form a composite solar cell structure  304  ( FIG. 3D ) that at least partially encapsulates the active regions of the solar cell  300 . In one embodiment, at least one hole formed in the back glass substrate  361  remains at least partially uncovered by the bonding material  360  to allow portions of the cross-buss  356  or the side-buss  355  to remain exposed so that electrical connections can be made to these regions of the solar cell device in future steps (i.e., step  138 ). The processes and apparatus used to perform step  134  and some of its sub-steps are further described below in conjunction with FIGS.  4  and  6 - 13 . 
     Next, the composite solar cell structure  304  is transported to the autoclave module  236  in which step  136 , or autoclave steps are performed on the composite structure to remove trapped gases in the bonded structure and assure that a good bond is formed during step  134 . In step  134 , a bonded 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 substrate  302 , back glass substrate, and bonding material  360 . 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. In one embodiment, it may be desirable to heat the substrate, back glass substrate  361 , and bonding material  360  to a temperature that causes stress relaxation in the one or more of the components formed in the composite solar cell structure  304 . 
     Next, the solar cell structure  304  is transported to the junction box attachment module  238  in which junction box attachment steps  138  are performed on the formed solar cell structure  304 . The junction box attachment module  238 , used during step  138 , is used to install a junction box  370  ( FIG. 3C ) on a partially formed solar cell. The installed junction box  370  acts as an interface between the external electrical components that will connect to the formed solar cell, such as other solar cells or a power grid, and the internal electrical connection points, such as the leads, formed during step  131 . In one embodiment, the junction box  370  contains one or more junction box terminals, such as connection points  371 ,  372 , so that the formed solar cell can be easily and systematically connected to other external devices to deliver the generated electrical power. 
     Next, the solar cell structure  304  is transported to the device testing module  240 , in which device screening and analysis steps  140  are performed on the solar cell structure  304  to assure that the devices formed on the solar cell structure  304  surface meet desired quality standards. In one embodiment, the device testing module  240  is a solar simulator module that is used to qualify and test the output of the one or more formed solar cells. 
     Next, the solar cell structure  304  is transported to the support structure module  241 , in which support structure mounting steps  141  are performed on the solar cell structure  304  to provide a complete solar cell device that has one or more mounting elements attached to the solar cell structure  304  formed using steps  102 - 140  to a complete solar cell device that can easily be mounted and rapidly installed at a customer&#39;s site. 
     Next, the solar cell structure  304  is transported to the unload module  242  in which step  142 , or device unload steps, are performed on the substrate to remove the formed solar cells from the solar cell production line  200 . 
     In one embodiment of the solar cell production line  200 , one or more regions in the production line are positioned in a clean room environment to reduce or prevent contamination from affecting the solar cell device yield and useable lifetime. In one embodiment, as shown in  FIG. 2 , a class 10,000 clean room space  250  is placed around the modules used to perform steps  108 - 118  and steps  130 - 134 . 
     Referring to  FIGS. 1 and 2 , in one embodiment of the solar cell production line  200 , one or more accumulators  211 A- 211 D are inserted to provide buffering capability at various points within the solar cell production line  200  to achieve a desired throughput during steady state and fault state conditions (e.g., one or more modules  202 - 241  is in a fault state). As shown in  FIG. 2 , in one embodiment, the solar cell production line  200  has at least one accumulator  211  (e.g., accumulator  211 A) positioned before the one or more cluster tools  212 A- 212 D found in the processing module  212  and at least one accumulator  211  (e.g., accumulator  211 B) positioned after the one or more cluster tools  212 A- 212 D. During the production of solar cells, it is generally desirable to load the accumulators  211 A with two or more substrates to assure that the one or more cluster tools  212 A- 212 D have a ready supply of substrates, and provide a collection area where substrates coming from the upstream processes can be stored if one or more of the cluster tools  212 A- 212 D goes down. 
     Bonding Module Design and Processes 
     As noted above, during step  134 , or the lamination step, one or more process steps are performed to bond the backside glass substrate to the solar cell devices formed in steps  102 - 132  to form a bonded composite solar cell structure  304  ( FIG. 3D ). Step  134  is thus used to seal the active elements of the solar cell from the external environment to prevent the premature degradation of a formed solar cell during its useable life. An exemplary bonding module  234  and method of using the same are further described in U.S. patent application Ser. No. 12/359,250, filed Jan. 23, 2009, which is herein incorporated by reference. 
       FIG. 4  illustrates one or more embodiments of a bonding module  234  which may be useful to perform the lamination process, discussed below.  FIG. 4  is a schematic cross-sectional view of the bonding module  234  that illustrates some of the common components found within this module. Generally, the bonding module  234  contains a preheat module  411 , a lamination module  410 , a system controller  290 , and a conveyor system  422 . The conveyor system  422  generally contains a plurality of support rollers  421  that are designed to support, move, and/or position a device substrate  303 , the back glass substrate  361 , and the bonding material  360 , or hereafter composite solar cell structure  304 . As discussed in more detail below, a pre-patterned bonding material  360  may be provided to help prevent bubble formation during the lamination process. As shown in  FIG. 4 , a solar cell can be transferred into and through the bonding module  234  following the paths Ai and Ao. 
     The preheat module  411  generally contains a plurality of support rollers  421 , a plurality of heating elements  401 A,  401 B, two or more temperature sensors (e.g., temperature sensors  402 A,  402 B), and one or more compression rollers  431 A. The plurality of support rollers  421  are adapted to support the composite solar cell structure  304  while it is positioned within the processing region  415  of the preheat module  411  and are configured to withstand the temperatures created by the heating elements  401 A,  401 B during normal processing. In one embodiment, the preheat module  411  also contains a fluid delivery system  440 A that is use to deliver a desired flow of a fluid, such as air or nitrogen (N2), through the processing region  415  during processing. 
     The plurality of heating elements  401 A,  401 B are typically lamps (e.g., IR lamps), resistive heating elements, or other heat generating devices that are controlled by the system controller  290  to deliver a desired amount of heat to desired regions of the composite solar cell structure  304  during processing. In one embodiment, a plurality of heating elements  401 A are positioned above the composite solar cell structure  304 , and a plurality of heating elements  401 B are positioned below the composite solar cell structure  304 . In one embodiment, the heating elements  401 A,  401 B are oriented substantially perpendicular to the direction of travel of the substrate, and the energy delivered by the lamps creates a uniform temperature profile across the substrate as it is continually moved through the processing region  415 . 
     The compression rollers  431 A are adapted to provide a desired amount of force F to the composite solar cell structure  304  to help remove the air bubbles found within the composite solar cell structure  304  and evenly distribute the bonding material within the composite solar cell structure  304  after performing the preheat process step. The compression rollers  431 A are generally configured to receive the composite solar cell structure  304  after it has been sufficiently heated in the preheat module  411 . 
     Referring to  FIG. 4 , the preheat module  411  also contains two temperatures sensors  402 A,  402 B that are adapted to measure the temperature of regions of the composite solar cell structure  304  during the preheat process. The temperature sensors may be a non-contact-type temperature sensor, such as a conventional pyrometer, or a conventional contacting-type of temperature sensor. In one embodiment, the preheat module  411  contains a top temperature sensor  402 A that is adapted to measure the temperature of the top of the composite solar cell structure  304  and a bottom temperature sensor  402 B that is adapted to measure the temperature of the bottom of the composite solar cell structure  304  during or after processing. In one embodiment, the top temperature sensor  402 A and a bottom temperature sensor  402 B are positioned over one another so that the difference in temperature between the top side and bottom side of the composite solar cell structure  304  at the same position on the substrate can be simultaneously measured. 
     In general, during the preheat process the composite solar cell structure  304  is controllably heated as it passes through the processing region  415  by use of the one more of the heating elements  401 A,  401 B disposed therein. In one embodiment, at least one of the top heating elements  401 A and at least one of the bottom heating elements  401 B are close loop controlled using the system controller  290  and at least one temperature sensor  402 B positioned on the top of the substrate and at least one temperature sensor  402 B positioned on the bottom of the substrate. After the substrate is preheated, a desired force is applied to one or more sides of the preheated substrate by use of the one or more compression rollers  431 A using one or more controlled force generating elements. The applied force supplied by the one or more compression rollers  431 A may be between about 200 [N/cm] and about 600 [N/cm]. 
     The lamination module  410  generally contains a plurality of support rollers  421 , a plurality of heating elements  401 C,  401 D, two or more temperature sensors (e.g., temperature sensors  402 C,  402 D), and one or more compression rollers  431 B. The plurality of support rollers  421  are adapted to support the composite solar cell structure  304  while it is positioned within the processing region  416  of the lamination module  410  and are configured to withstand the temperatures achieved during normal thermal processing. In one embodiment, the lamination module  410  also contains a fluid delivery system  440 B that is used to deliver a desired flow of a fluid through the processing region  416  during processing. In one embodiment, the fluid delivery system  440 B is a fan assembly that is adapted to deliver a desired flow of air across one or more surfaces of the substrate disposed within the processing region  416  by use of commands sent from the system controller  290 . 
     The plurality of heating elements  401 C,  401 D are typically lamps (e.g., IR lamps), resistive heating elements, or other heat-generating devices that are controlled by the system controller  290  to deliver a desired amount of heat to desired regions of the composite solar cell structure  304  during processing. In one embodiment, a plurality of heating elements  401 C are positioned above the composite solar cell structure  304 , and a plurality of heating elements  401 D are positioned below the composite solar cell structure  304 . In one embodiment, the heating elements  401 C,  401 D are oriented substantially perpendicular to the direction of travel of the substrate, and the energy delivered by the lamps creates a uniform temperature profile across the substrate as it is moved through the processing region. 
     The one or more compression rollers  431 B are adapted to provide a desired amount of force F to the composite solar cell structure  304  (i.e., composite structure) to help remove the air bubbles found within the composite solar cell structure  304  and evenly distribute the bonding material within the composite solar cell structure  304 . The compression rollers  431 B are generally configured to receive the composite solar cell structure  304  after it has been sufficiently heated in the lamination module  410 . In one embodiment, as shown in  FIG. 4 , a pair of compression rollers  431 B are used to remove any trapped air from the substrate by applying a force F to both sides of the composite solar cell structure  304  by the compression rollers  431 B by use of a conventional electric or pneumatic force generating element. 
     Referring to  FIG. 4 , the lamination module  410  also contains two temperatures sensors  402 C,  402 D that are adapted to measure the temperature of regions of the composite solar cell structure  304  during the lamination process. The temperature sensors may be non-contact-type temperature sensor, such as a conventional pyrometer, or a conventional contact-type temperature sensor. In one embodiment, the lamination module  410  contains a top temperature sensor  402 C that is adapted to measure the temperature of the top of the composite solar cell structure  304  and a bottom temperature sensor  402 D that is adapted to measure the temperature of the bottom of the composite solar cell structure  304  during or after processing. In one embodiment, the top temperature sensor  402 C and a bottom temperature sensor  402 D are positioned one over another, so that the difference in temperature between the top side and bottom side of the composite solar cell structure  304  can be simultaneously measured. In one embodiment, an array of pairs of temperature sensors  402 C,  402 D are positioned over desired areas of the composite solar cell structure  304  so that top and bottom temperature readings at different areas of the composite solar cell structure  304  can be measured. 
     Therefore, after performing the preheat process, a lamination process is performed in the lamination module  410 . During the lamination process, the composite solar cell structure  304  is controllably heated as it passes through the processing region  416  by use of the one more of the heating elements  401 C,  401 D disposed therein. In one embodiment, at least one of the top heating elements  401 C and at least one of the bottom heating elements  401 D are close loop controlled, using the system controller  290  and at least one temperature sensor  402 C positioned on the top of the substrate and at least one temperature sensor  402 D positioned on the bottom of the substrate. After the substrate is heated in the lamination module, a desired force is applied to one or more sides of the composite substrate by use of the one or more compression rollers  431 B, using one or more controlled force generating elements. The applied force supplied by the one or more compression rollers  431 B may be between about 200 [N/cm] and about 600 [N/cm]. 
     Autoclave Module Design and Processes 
     As discussed above, in step  134 , the composite solar cell structure is inserted in the processing region of the autoclave module, where heat and pressure is applied to the partially formed solar cells to reduce the amount of trapped gas disposed between bonding material  360  and the back glass substrate  361 , substrate  302  or the back contact layer  350  to prevent environmental attack of portions of the solar cell device through the regions of trapped gas. Use of the autoclave process is also used to improve the properties of the bond between the substrate  302 , back glass substrate and bonding material  360 . 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. 
       FIG. 5  illustrates a side cross-sectional view of an autoclave module  236  and supporting equipment. The autoclave module  236  will generally contain a vessel assembly  510 , one or more substrate racks  520 , and a loading system  530 . The vessel assembly  510  generally contains a fluid movement device  511 , compressor  512 , heating unit  513 , cooling unit  514 , and a vessel  515 . The vessel  515  has a door  516  that is configured to enclose the substrate racks  520  and composite solar cell structures  304  disposed thereon in a processing region  517  during processing. As shown in  FIG. 5 , the door  516  is closed and sealed against the vessel  515 . The compressor  512 , system controller  290 , and pressure sensor “P” are used in combination to deliver and actively control the pressure within the processing region  517  during the autoclave process by controlling the delivery and release of a high-pressure fluid from a fluid pump  512 A, valve  512 B and relief valves  512 C. In one embodiment, the compressor  512  is adapted to provide compressed air at pressure greater than about 13 Bar to the processing region  517  of the autoclave module  236  during processing. In another embodiment, the compressor  512  is adapted to provide compressed air at pressure between about 13 Bar and about 15 Bar to the processing region  517  during processing. 
     To control the temperature of the composite solar cell structures  304  during the autoclave process, the system controller  290  and temperature sensor “T” are used in combination to control the amount of heat that is transferred to the composite solar cell structures  304  positioned in the processing region  517  by use of the components contained in the heating unit  513  and the cooling unit  514 . The heating unit  513  generally contains a heater controller  513 A and a plurality of heating elements  513 B (e.g., thermally-controlled resistance heating elements) that are in thermal communication with the composite solar cell structures  304  disposed within the processing region  517 . Similarly, the cooling unit  514  contains a cooling unit controller  514 A and a plurality of cooling elements  514 B that are in thermal communication with the composite solar cell structures  304  disposed within the processing region  517 . The cooling elements  514 B may comprise a series of fluid-containing channels, in which a fluid exchanging medium is provided from the cooling unit controller  514 A, to cool the components contained in the processing region  517 . In one example, the heating elements  513 B and/or cooling elements  514 B are disposed within the processing region  517  and are adapted to add and/or remove heat from the composite solar cell structures  304  by convective heat transfer supplied by movement of the high-pressure gas contained in the processing region  517  during processing by use of the fluid movement device  511  (e.g., mechanical fan). The fluid movement device  511  is configured to provide motion to the fluid contained in the processing region  517  during processing to also reduce the variation in temperature throughout the processing region  517 . In one embodiment, the temperature in the processing region is maintained between about 140° C. and about 160° C. for a time between about 1 and about 4 hours. The autoclave processing temperatures, pressures, and times will vary by the type of bonding material that is used, and as one or more of the process variables are altered. 
     The loading system  530  is generally configured to deliver and remove one or more of the racks  520  to the processing region  517  of the vessel  515  prior to and after processing. The loading system  530  generally contains an automated material handling device  531 , for example, a conveyor or a robotic device, which is used to transfer the racks  520  to and from the processing region  517  of the vessel  515  in an automated fashion. 
     The one or more substrate racks  520  generally include one or more regions of shelves  521  that are adapted to support the composite solar cell structures  304  during processing. In one embodiment, each substrate rack  520  contains wheels  521 A that allows the racks to be easily moved and positioned within the production line  200 . Each of the composite solar cell structures  304  are spaced a desired distance apart to assure that temperature uniformity and pressures applied to the composite solar cell structures  304  are uniform. In one embodiment, to assure that the substrates see the same processing conditions, one or more spacers  522  are disposed between and in contact with both adjacent composite solar cell structures  304  to assure that the spacing between the adjacent composite solar cell structures  304  is uniform. In one embodiment, three or more spacers are positioned between adjacent composite solar cell structures  304 . In one example, the spacers  522  are adapted to space adjacent composite solar cell structures  304  between about 5 mm and about 15 mm apart. 
     In general, the autoclave module  236  may be transferrably connected to the automation device  281 , positioned after the bonding module  234 , to receive and perform an autoclave process on one or more of the formed composite solar cell structures  304 . The autoclave module  236  may also be transferrably connected to the automation device  281  positioned before the junction box attachment module  238  so that the processed substrates can be transferred to the down stream processing modules. 
     In one embodiment, as shown in  FIG. 2A , the composite solar cell structures  304  leaving the bonding module  234  are transferred to a substrate rack  520  that is then transferred to the autoclave module  236  for processing, and then transferred to a position near the junction box attachment module  238  after processing. As shown in  FIG. 2A , a plurality of substrate racks  520  are positioned to receive substrates from the automation device  281  positioned after the bonding module  234 . In one embodiment, one or more robots  235 A (e.g., 6-axis robot) are positioned to transfer the composite solar cell structures  304  from the automation device  281 , which is positioned after the bonding module  234 , and on to a moveable substrate rack  520  by use of a robotic device (e.g., automated material handling device  531 ). Similarly, in one embodiment, the substrate racks  520  are moved from the autoclave module to a position where a robot  235 B (e.g., 6-axis robot) is able to transfer the composite solar cell structures  304  from a substrate rack  520  and on to the automation device  281  positioned before the junction box attachment module  238 . In one embodiment, the substrate rack  520  may be moved to and from the autoclave module  236  in an automated fashion. In some cases it is desirable to minimize the need for and/or the amount of human intervention. 
     Method of Forming Pre-Patterned Bonding Material Used for Forming a Composite Solar Cell Structure 
     As previously set forth, steps  131 - 136  of the processing sequence  100  may be used to form a composite solar cell structure  304  from the device substrate  303  using the bonding wire attach module  231 , the bonding module  234 , and the autoclave module  236 .  FIG. 6A  is a plane view of a composite solar cell structure  304  without the junction box attached but after lamination and autoclaving the solar cell structure  304 . The back contact layer  350  may be seen through the back glass substrate  361 . As previously discussed, the internal electrical connections disposed on the back of contact layer  350  may comprise the cross-buss  356  and side-buss  355  as shown in  FIG. 6A . A layer of isolation type may be placed beneath the cross-buss  356 . During lamination, bubbles  610  may inadvertently form between the back glass substrate  361  and the previously exposed surface of back contact layer  350  and side- and cross-busses  355 ,  356 , creating delamination paths or even environmental exposure paths leading to device failure. Formation of bubbles during lamination may be particularly problematic around the increased step topography created by the side- and cross-busses, especially at their intersection. 
       FIG. 6B  is a close-up isometric view of the cross-buss  356 , side-buss  355 , and isolation tape  357  shown on the composite solar cell structure in  FIG. 6A . The close-up  6 A though does not show the back contact layer  350 . As can be seen, the area where the cross- and side-busses overlap may be particularly troublesome for bubble formation because of the thickness of the busses. 
       FIG. 7  is a cross-sectional view of a composite solar cell structure  304  in  FIG. 6A  along lines B-B. The composite solar cell  304  shown in  FIG. 7  is a single-junction-type solar cell. Although a single-junction-type solar cell is shown, it should be understood that other types of solar cells may also use embodiments of the invention during their formation. A transparent conductive oxide layer  310  is deposited over a glass substrate  302 . A single-junction layer  320 , including one or more silicon layers, is deposited over the transparent conductive layer  310 . A back contact layer  350  is deposited over the single-junction layer  320 . One or more internal electrical connections, such as side- and cross-busses  355 ,  356 , are disposed on the back contact layer  350 . In some embodiments, an isolation tape  357  may be disposed between the cross-buss  356  and the back contact layer  350 . At this point in the solar cell formation process, a device substrate has been formed. 
     Next, a bonding material  360  and backing glass substrate  361  are laminated together, forming a composite solar cell structure  304 . As can be seen from the cross-sectional side view of  FIG. 6A , the large thickness differences between the cross-buss  356 , the side-buss  355 , the isolation tape  357 , and the back contact layer  350  creates an exposed surface topography  710  that increases the chance for air to be trapped between the bonding material  360  and the adjacent layers. To decrease the likelihood of bubble formation, especially at these corners near the busses and back contact layer, one embodiment of the invention provides a bonding material  360  that has been pre-patterned to match the topography  710  of an exposed surface of the device substrates. One method of embossing a mating pattern in the bonding material is shown in  FIG. 8 . 
       FIG. 8  is a work surface having an embossment according to one embodiment described herein. In this embodiment of preparing a pre-patterned bonding material  360  for a solar cell assembly, a bonding material  360  is placed over a work surface  800  having an embossment  808 , wherein at least a portion of the embossment  808  corresponds to a topography  710  of an exposed surface of a device substrate  303 . The embossment  808  may have the same pattern as the busses  355 ,  356 . For example, the embossment  808  includes a side-buss surface projection  810  corresponding in length and thickness to the side-buss  355 . A cross-buss surface projection  812  roughly mirrors the length and thickness of the cross-buss  356 . The bonding material thickness may be 45 gauge, or 1.14 ml. In other embodiments, the bonding material thickness may be 30 gauge or 0.7 ml. 
     A central hole  815  corresponds to a hole in the back substrate glass  361  and bonding material  360  necessary to electrically connect the junction box with the cross-buss  356 , as previously described in step  138 . The bonding material may be pre-cut into sheets that match the size of the device substrate  303 . Pre-cutting the bonding material may take place at material perpetration module  232 A discussed above in connection with process steps  131 - 136 . 
     After the bonding material  360  is placed on the work surface, the bonding material  360  is heated and then pressed onto the embossment  808  to form a mating pattern. The heating of the bonding material may include locally heating a portion of the bonding material corresponding to the topography of the exposed surface. In one embodiment, only the embossment  808  is heated to create the localized heating of the bonding material  360  along the areas of the mating pattern. Localized heating may be useful because heating up the entire bonding material  360  may cause shrinkage and negatively impact the ability of the bonding material  360  to sufficiently cover the exposed surface of the device substrate  303 . The localized heating may heat portions of the bonding material to a temperature between 30 and 95° C. for imprinting the pattern on the bonding material. 
     In one embodiment a platen may be pressed down on the bonding material, creating a die to form the mating pattern. In other embodiments a roller may roll across the work surface to imprint the mating pattern on the bonding material  360 . Additionally, various pressures may be used to press the bonding material  360  on the embossment  808  to form the mating pattern. The pressure and time of embossing are interrelated and can be adjusted to provide the desired mating pattern without negatively affecting the physical and chemical properties of the bonding material  360 . Any of these disclosed methods of pre-patterning the bonding material  360  may be performed at material perpetration module  232 A discussed above in connection with process steps  131 - 136 . 
     A plurality of holes  805  may be used to inject air from the bottom of the work surface  800  to prevent sticking of the bonding layer  360  after embossing the mating pattern. The bonding material may easily stick to various surfaces, and injecting air through holes  805  can be used to remove the bonding material  360  from the work surface  800 . In another embodiment, the embossment  808  and work surface  800  may have a coating that prevents sticking. These stick-prevention features may be particularly desirable for PVB, which tends to stick very well, especially when heated. Furthermore, local heating around the area so embossed will help minimize the overall heating instability of the bonding material  360 , such as PVB. 
     In other embodiments of the invention, pre- and post-processing steps may be performed. One type of pre-processing step would be relaxing the bonding material  360  before placing the bonding material  360  over the work surface  800 . Relaxing the bonding material, such as PVB, may help the PVB layer to flow when it passes though the rollers in the lamination step previously discussed, further helping to prevent bubble formation. Relaxing the bonding material may also prevent the edges from pulling away from the glass substrate during lamination and further increasing the chance of atmospheric contamination. Additionally, relaxed bonding material may not move as much, preventing inadvertent movement of the cross- and side-busses during lamination. Bonding material that moves too much may push and fight against the busses and increase the chance for bubble formation. 
     Relaxing the bonding material may be performed in two steps, a heating step, and a quick cooling step. One type of post-processing step may be chilling the bonding material  360  after pressing the bonding material  360  onto the embossment  808 . Chilling also helps to prevent the edge problems. The bonding material may be chilled after embossment to between 18 and 25° C. Combined pre- and post-processing improves the physical characteristics of the bonding material and increases the likelihood of bubble prevention during lamination. 
     Once the bonding material has been embossed, the will need to know the orientation of the glass and bonding material so that the mating pattern formed in the bonding material will properly align, angularly and axially, with the device substrate  303 . An automated vision system may match the bonding material  360  with the back glass substrate  361  prior to lamination. 
       FIGS. 9A-9B  illustrate a set of rollers  900  having an embossment  908  according to one embodiment described herein. In this method of preparing a pre-patterned bonding material for a solar cell assembly, the bonding material  360  passes between at least two rollers, such as upper roller  902  and lower roller  904 . At least one roller, such as lower roller  904 , has an embossment  908 , where at least a portion of the embossment  908  corresponds to a topography  710  of an exposed surface of a device substrate  303 . 
     The embossment  908  may have the same pattern as the busses  355 ,  356 . For example, the embossment  908  includes a side-buss surface projection  910  corresponding in length and thickness to the side-buss  355 . A cross-buss surface projection  912  roughly mirrors the length and thickness of the cross-buss  356 . A central hole  915  corresponds to a hole in the back substrate glass  361  and bonding material  360  necessary to electrically connect the junction box with the cross-buss  356 , as previously described in step  138 . The bonding material may be pre-cut into sheets that match the size of the device substrate  303 . Pre-cutting the bonding material may take place at material perpetration module  232 A discussed above in connection with process steps  131 - 136 . 
     The bonding material  360  is heated and passed through the rollers  900  to form a mating pattern. The heating of the bonding material may include locally heating the rollers or a portion of the bonding material corresponding to the topography of the exposed surface. In one embodiment, only the embossment  908  is heated to create the localized heating of the bonding material  360  along the areas of the mating pattern. The localized heating may heat portions of the bonding material to a temperature between 30 and 95° C. for imprinting the pattern on the bonding material. 
     Similar pre- and post-pattern processing steps may be performed to prepare the bonding material for lamination. In one embodiment, the bonding material  360  may continuously roll off a reel of the bonding material and pass between the rollers  900 . In this method, the bonding material would not need to be pre-cut, but could come of another reel and cut to size just before placement over the exposed surface of the cross- and side-busses and the back contact layer prior to lamination. A separate embossment table or work surface would be unnecessary. 
       FIGS. 10A-10B  illustrate a mating pattern  1010  formed on a bonding material  360  according to embodiments described herein. Various cross-sections of the mating pattern may be formed from the embossment.  FIG. 10A  shows a shallow groove  1011  may be formed in the bonding material  360 . Embossing the bonding material may also form a bump or protrusion  362  that may be necessary to flatten some prior to lamination.  FIG. 10B  shows a groove  912  having sidewalls angling toward the center to help prevent bubbles and better match the profile of the busses.  FIG. 10C  shows a groove  1013  that is deeper and has filleted corners for a smoother transition between the bonding material  360  and the busses. However, an oversized pattern compared to the topography of the surface may otherwise create bubbles. 
       FIG. 11  is a cross-sectional view of a bonding material  360  placed over the cross-buss  356  and isolation tape  357 . By matching the topography of the exposed surface, including the busses, the prevention of bubbles during lamination is increased.  FIG. 12  is a partial cross-sectional isometric view of the pre-patterned bonding material  360  over the internal electrical connections prior to lamination. The large variation in step topography of the exposed surface at the overlap area of the side-buss  355  and cross-buss  356  may create the greatest concern for bubble formation. As illustrated, a sheet of bonding material  360  that has been pre-patterned with a mating pattern to match the topography of the surface, overlays and covers the transition corners between the busses and the isolation tape, thereby reducing the bubble formation during lamination. 
       FIG. 13  schematically illustrates a method  1300  of forming a composite solar cell structure according to one embodiment described herein. The method  1300  includes preparing a device substrate, box  1310 , wherein the device substrate includes a glass substrate, a transparent conductive layer deposited over the glass substrate, one or more silicon layers deposited over the transparent conductive layer, a back contact layer deposited over the one or more silicon layers, and one or more internal electrical connections disposed on the back contact layer, box  1315 . The method  1300  also includes forming a mating pattern on a bonding material to match a topography of an exposed surface of the device substrate, the exposed surface comprising the back contact layer and the one or more internal electrical connections, box  1320 . The method also includes positioning the bonding material over the exposed surface, box  1325 , disposing a back glass substrate over the bonding material to form a composite structure, box  1330 , and compressing the composite structure, box  1335 . 
       FIG. 14  schematically illustrates a method  1400  of preparing a pre-patterned bonding material for a solar cell assembly according to one embodiment described herein. The method includes placing a bonding material over a work surface having an embossment, wherein at least a portion of the embossment corresponds to a topography of an exposed surface of a device substrate, box  1410 , heating the bonding material, box  1415 , and pressing the bonding material onto the embossment to form a mating pattern, box  1420 . 
       FIG. 15  schematically illustrates a method  1500  of preparing a pre-patterned bonding material for a solar cell assembly according to one embodiment described herein. The method includes passing a bonding material between at least two rollers, wherein at least one roller has an embossment, at least a portion of the embossment corresponding to a topography of an exposed surface of a device substrate, box  1510 , heating the bonding material, box  1515 , and pressing the bonding material onto the embossment to form a mating pattern as the bonding material passes through the two rollers, box  1520 . 
     The above described embodiments can be readily implemented along an automated solar cell production line and help prevent bubble formation during lamination of large area solar cell substrates. Prevention of bubble formation improves efficiency due to decreased exposure to environmental conditions, such as heat and humidity. Large-sized solar cells present a challenge due to the locally stepped topography from cross- and side-busses, which embodiments of the invention help overcome. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.