Abstract:
A method for efficiently producing closely-spaced parallel gridlines and perpendicular bus bar structures on a substrate during a single pass of a multi-nozzle printhead assembly over the substrate. A first section of the parallel gridlines is printed adjacent to one edge of the substrate while moving the printhead assembly in a first direction. The printhead assembly is then reciprocated in a second direction (X-axis) orthogonal to the first direction, whereby the extruded material forms a bus bar structure extending perpendicular to the gridlines. Movement of the printhead assembly in the first direction is then resumed to form a second section of the gridlines. The second direction reciprocation process is repeated for each desired bus bar structure. The entire gridline/bus bar printing process is performed without halting the extrusion of material (i.e., using a continuous bead).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is related to extrusion systems, and more particularly to micro-extrusion systems for extruding closely spaced lines of material on a substrate. 
       BACKGROUND 
       [0002]      FIG. 9  is a simplified diagram showing an exemplary conventional solar cell  40  formed on a semiconductor substrate  41  that converts sunlight into electricity by the inner photoelectric effect. Solar cell  40  is formed on a semiconductor substrate  41  that is processed using known techniques to include an n-type (or p-type) doped upper region  41 A and an oppositely p-type (or n-type) doped lower region  41 B such that a pn-junction is formed near the top of substrate  41 . Disposed on an upper surface  42  of semiconductor substrate  41  are a series of parallel metal gridlines (fingers)  44  (shown in end view) that are electrically and mechanically connected to n-type region  41 A. A substantially solid conductive layer  46  is formed on a lower surface  43  of substrate  41 , and is electrically and mechanically connected to p-type region  41 B. An antireflection coating  47  is typically formed over upper surface  42  of substrate  41 . Solar cell  40  generates electricity when a solar photon from sunlight beams L 1  (with an energy greater than the semiconductor band gap) passes through upper surface  42  into substrate  41  and interacts with a semiconductor material atom. This interaction excites an electron (“−”) in the valence band to the conduction band, allowing the electron and an associated hole (“+”) to flow within substrate  41 . The pn-junction separating n-type region  41 A and p-type region  41 B serves to prevent recombination of the excited electrons with the holes, thereby generating a potential difference that can be applied to a load by way of gridlines  44  and conductive layer  46 , as indicated in  FIG. 9 . 
         [0003]      FIG. 10  is a perspective view showing the front contact pattern of solar cell  40  in additional detail. The front contact pattern solar cell  40  consists of a rectilinear array of parallel gridlines  44  and one or more wider collection lines (bus bars)  45  that extend perpendicular to gridlines  44 , both disposed on upper surface  42 . Gridlines  44  collect electrons (current) from substrate  41  as described above, and bus bars  45  which gather current from gridlines  44 . In a photovoltaic module, bus bars  45  become the points to which metal ribbon (not shown) is attached, typically by soldering, with the ribbon being used to electrically connect one cell to another. 
         [0004]    Conventional methods for producing the front contact pattern of solar cell  40  typically involve screen-printing both gridlines  44  and bus bars  45  in a single pass using a metal-bearing ink. Conventional screen printing techniques typically produce gridlines having a roughly rectangular cross-section with a width W of approximately 130 μm and a height H of approximately 15 μm, producing an aspect ratio of approximately 0.12. A problem associated with screen printing in the context of solar cells is this relatively low aspect ratio causes gridlines  44  to generate an undesirably large shadowed surface area (i.e., gridlines  44  prevent a significant amount of sunlight from passing through a large area of upper surface  22  into substrate  21 , as depicted in  FIG. 9  by light beam L 2 ), which reduces the ability of solar cell  20  to generate electricity. However, simply reducing the width of gridlines  44  (i.e., without increasing the gridlines&#39; cross-sectional area by increasing their height dimension) could undesirably limit the current transmitted to the applied load, and forming high aspect ratio gridlines using screen printing techniques would significantly increase production costs. 
         [0005]    More recently, a method was introduced for producing front contact patterns for solar cells in which a metal-bearing material is extrusion printed directly onto a semiconductor substrate. Although the extrusion printing method addressed the shadowing problem of screen printed front contact patterns by providing gridlines having relatively high aspect ratios, this alternative production method requires two separate steps: one to apply the gridlines, and a second step, (either previous to or subsequent to the gridline application), to apply the bus bars. For example, as illustrated in  FIGS. 11(A) to 11(C) , a solar cell  40 A similar to that described with reference to  FIG. 10  is formed by moving an extrusion printhead (not shown) in a Y-axis direction relative to a substrate  41 A while printing bus bars  45 A on upper surface  42 A (see  FIG. 11(A) ). Substrate  41 A (or the printhead) is then turned 90° as shown in FIG.  11 (B)), and then, as shown in  FIG. 11(C) , gridlines  44 A are printed on substrate surface  42 A and on bus bars  45 A using the printhead. Although providing higher aspect ratio gridlines, advantages of extrusion printing over screen printing are partially offset by the increased process complexity and product handling involved in writing or printing gridlines  44 A and bus bars  45 A as separate steps, as illustrated in  FIGS. 11(A) to 11(C) . 
         [0006]    Referring again to  FIG. 11(C) , another problem with extrusion printing the front metallization of conventional H-pattern solar cell  40  is the uneven topography on the bus bars  45  (i.e., where bus bars  45  are crossed by the gridlines  44 ). This topography does not impact the cell performance, but it can create a weak solder joint between the subsequently applied metal ribbon (not shown) and the top of bus bar  45  because there is insufficient solder to fill in the gaps between gridlines  44 . 
         [0007]    What is needed is a micro extrusion printing method and associated apparatus for producing solar cells that facilitates the formation of extruded gridlines and bus bars for solar cells at a low cost that is acceptable to the solar cell industry and addresses the problems described above. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is directed to a micro-extrusion system and method for producing solar cells (and other electric electronic and devices) in which a printhead is used to produce continuous lines (beads) that include both straight (gridline) sections and switchback (wavy) sections that are alternately formed on a substrate during a single pass of the printhead assembly over the substrate surface. The straight sections of each continuous line are aligned in a first direction to form a set of parallel gridlines, with each adjacent pair of gridline sections being connected by an associated switchback section. The switchback sections include several connected switchback segments that extend generally in a second direction, and collectively form relatively wide switchback structures that extend generally perpendicular to the gridlines. The invention thus facilitates the formation of the front solar cell metallization pattern (gridlines and buses) using a single pass of an extrusion head, thereby eliminating the added time and cost associated with separate printing steps for gridline and bus bar formation, as required in the prior art. In addition, because the gridline material is deposited during a single pass, the gridlines do not cross the bus bar structures, thereby avoiding the weak solder joint problem associated with conventional extrusion processes. 
         [0009]    In accordance with an embodiment of the present invention, a method for forming front beads method involves positioning the printhead assembly over a predetermined region of the substrate (e.g., adjacent to a side edge of the substrate), and starting the extrusion process while moving the printhead assembly at an initial speed in a straight-line first (Y-axis) direction (i.e., while keeping the substrate stationary) for a predetermined distance such that the extruded line forms first gridline sections on the substrate surface. Next, while maintaining relative movement of the printhead assembly and substrate in the first (Y-axis) direction, but at a slower speed, the method involves reciprocating the printhead assembly relative to the substrate in a second (X-axis) direction, whereby the extruded material associated with each gridline forms an associated bus bar section extending in the second (X-axis) direction such that the bus bar sections collectively form a bus bar structure. Upon completing the bus bar structure, the printhead assembly is again moved at the first speed in the in the straight-line first (Y-axis) direction such that the extruded line foams second gridline sections on the substrate surface. The process of alternately forming gridline sections and bus bar structures is repeated to produce as many bus bar structures as desired. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, where: 
           [0011]      FIG. 1  is a perspective view showing a simplified extrusion printhead assembly and printed structure formed on a substrate in accordance with an embodiment of the present invention; 
           [0012]      FIG. 2  is a side view showing a portion of a micro-extrusion system including a micro-extrusion printhead assembly utilized in accordance with an embodiment of the present invention; 
           [0013]      FIG. 3  is a side view showing the micro-extrusion system of  FIG. 2  in additional detail; 
           [0014]      FIG. 4  is an exploded cross-sectional side view showing generalized micro-extrusion printhead assembly utilized in the system of  FIG. 2 ; 
           [0015]      FIG. 5  is a partial side view showing the micro-extrusion printhead assembly of  FIG. 4  during operation; 
           [0016]      FIG. 6  is a cross-sectional assembled side view showing a portion of the micro-extrusion printhead assembly of  FIG. 4  during operation; 
           [0017]      FIGS. 7(A) ,  7 (B),  7 (C) and  7 (D) are partial perspective views showing the system of  FIG. 2  during the production of a solar cell in accordance with an embodiment of the present invention; 
           [0018]      FIGS. 8(A) and 8(B)  are plan views showing printed patterns formed on a substrate in accordance with alternative embodiments of the present invention; 
           [0019]      FIG. 9  is a simplified cross-sectional view showing a solar cell during operation; 
           [0020]      FIG. 10  is a perspective view showing a conventional solar cell; and 
           [0021]      FIGS. 11(A) ,  11 (B) and  11 (C) are partial perspective views showing a conventional method for extrusion printing bus lines and grid lines for conventional solar cells. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    The present invention relates to an improvement in micro-extrusion systems. The following description is presented to enable one of ordinary skill in the art to make and use the invention as provided in the context of a particular application and its requirements. As used herein, directional terms such as “upper”, “top”, “lower”, “bottom”, “front”, “rear”, and “lateral” are intended to provide relative positions for purposes of description, and are not intended to designate an absolute frame of reference. As used herein, the term “generally perpendicular” is intended to mean that the respective elongated structures are aligned at an angle in the range of 45 to 90 degrees. As used herein, the term “integrally connected” is intended to mean that the related structures are formed during a single fabrication process (e.g., extrusion or molding) step, whereas the term “connected” without the modifier “integrally” is intended to mean the two related structures are either integrally connected, or are separately formed and then connected by means of a fastener, weld or other connective mechanism. Various modifications to the preferred embodiment will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed. 
         [0023]      FIG. 1  is a perspective view showing the front contact pattern of simplified solar cell  40 A formed on an upper surface  42 A of a semiconductor substrate  41  in accordance with an embodiment of the present invention. Similar to conventional solar cell  40  (described above with reference to  FIGS. 9 and 10 ), the front contact pattern of solar cell  40 A consists of narrower parallel gridlines  44 A- 1 ,  44 A- 2  and  44 A- 3  extending in a Y-axis (first) direction, and relatively wide bus bar structures  45 A- 1  and  45 A- 2  that extend in a X-axis (second) direction (i.e., generally perpendicular to gridline  44 A- 1  to  44 A- 3 ). Also similar to conventional solar cell  40 , gridlines  44 A- 1  to  44 A- 3  collect electrons (current) from substrate  41 A as described above, and bus bar structures  45 A- 1  and  45 A- 2  gather current from gridlines  44 A- 1  to  44 A- 3 . In a photovoltaic module, bus bar structures  45 A- 1  and  45 A- 2  serve as points to which metal ribbons (not shown) are attached, typically by soldering, with the ribbon being used to electrically connect one cell to another. 
         [0024]    In accordance with an aspect of the present invention, solar cell  40 A differs from conventional solar cell  40  (described above) in that both gridlines  44 A- 1 ,  44 A- 2  and  44 A- 3  and bus bar structures  45 A- 1  and  45 A- 2  are produced by integral extruded structures (beads)  55  during a single pass of a micro-extrusion printhead assembly  100  over substrate  41 A in the Y-axis direction. Referring to the upper portion of  FIG. 1 , printhead assembly  100  defines nozzle outlets  169 - 1  to  169 - 3  from which beads  55 - 1  to  55 - 3  are respectively extruded. Beads  55 - 1  to  55 - 3  comprise an electrically conductive material that is forced through nozzle outlets  169 - 1  to  169 - 3  in the manner described below. As indicated by continuous extruded structures  55 - 1  to  55 - 3  disposed on upper surface  42 A and as described in additional detail below, beads  55  are extruded continuously during the formation of both gridlines  44 A- 1 ,  44 A- 2  and  44 A- 3  and bus bar structures  45 A- 1  and  45 A- 2 . 
         [0025]    As shown in  FIG. 1  and described in additional detail below, printhead assembly  100  is moved relative to substrate  41 A by a positioning mechanism  70  during the extrusion process to produce substantially collinear gridline sections that form gridlines  44 A- 1 ,  44 A- 2  and  44 A- 3 , and intervening switchback sections that form bus bar structures  45 A- 1  and  45 A- 2 . For example, continuous extruded structure  55 - 1  includes a first section  55 - 11  that forms a first elongated, substantially straight gridline section  44 A- 11 , a second section  55 - 12  that forms a first serpentine-shaped switchback section  45 A- 11 , a third section  55 - 13  that forms a second gridline section  44 A- 12 , a fourth section  55 - 14  that forms a second switchback section  45 A- 12 , a fifth section  55 - 15  that forms third gridline section  44 A- 13 . Similarly, continuous extruded structures  55 - 2  and  55 - 3  respectively include first sections  55 - 21  and  55 - 31  forming first gridline sections  44 A- 21  and  44 A- 31 , second sections  55 - 22  and  55 - 32  forming first switchback sections  45 A- 21  and  45 A- 31 , third sections  55 - 23  and  55 - 23  forming second gridline sections  44 A- 22  and  44 A- 32 , fourth sections  55 - 24  and  55 - 34  forming second switchback sections  45 A- 22  and  45 A- 32 , and fifth sections  55 - 25  and  55 - 35  forming third gridline sections  44 A- 23  and  44 A- 33 . Each collinear set of gridline sections collectively forms an associated gridline extending across substrate  41 A in the Y-axis direction (e.g., gridlines sections  44 A- 11 ,  44 A- 12  and  44 A- 13  collectively form gridline  44 A- 1 , gridlines sections  44 A- 21 ,  44 A- 22  and  44 A- 23  collectively form gridline  44 A- 2 , and gridlines sections  44 A- 31 ,  44 A- 32  and  44 A- 33  collectively form gridline  44 A- 3 ). Similarly, each set of switchback sections aligned in the X-axis direction collectively forms an associated bus bar structure that extends across substrate  41 A in the X-axis direction (e.g., switchback sections  45 A- 11 ,  45 A- 12  and  45 A- 13  collectively form bus bar structure  45 A- 1 , and bus bar sections  45 A- 21 ,  45 A- 22  and  45 A- 23  collectively form bus bar structure  45 A- 2 ). 
         [0026]    According to an aspect of the present invention, because integral extruded structures  55 - 1  to  55 - 3  are continuously formed during a single pass of printhead assembly  100  over substrate  41 A, each switchback section comprises a serpentine-like continuous line of material that is integrally connected between an associated pair of gridline sections. For example, referring to the lower left portion of  FIG. 1 , switchback section  45 A- 11  is integrally connected between gridline sections  44 A- 11  and  44 A- 12 . In particular, a first end of switchback section  45 A- 11  is integrally connected to (i.e., continuously formed with) gridline section  44 A- 11 , a second end of switchback section  45 A- 11  is integrally connected to gridline section  44 A- 12 , and a central portion of switchback section  45 A- 11  includes several switchback segments  45 A- 11 A that extend generally in the X-axis direction, and are integrally connected by way of 180° bend structures  145 A- 11 B. 
         [0027]    According to an embodiment of the present invention, a method for producing solar cell  40 A includes positioning multi-nozzle extrusion printhead assembly  100  over the surface  42 A such that nozzle outlets  169 - 1  to  169 - 3  are located adjacent to and parallel with side edge  41 A- 1 , and then, while causing printhead assembly  100  to continuously extrude material (i.e., such that beads  55 - 1  to  55 - 3  are directed toward substrate  41 A), sequentially moving printhead assembly  100  relative to the target substrate in a manner that alternately forms the gridline segments and switchback segments that are described above. In particular, printhead assembly  100  is first moved in a straight line along the (first) Y-axis direction such that first extrusion line portions  55 - 11 ,  55 - 21  and  55 - 31  are deposited to respectively form a set of parallel first gridline sections  44 A- 11 ,  44 A- 21  and  44 A- 31 . Next, printhead assembly  100  is reciprocated back and forth in the X-axis (second) direction such that second extrusion line portions  55 - 12 ,  55 - 22  and  55 - 32  collectively form a first set of bus bar segments  45 A- 11 ,  45 A- 21  and  45 A- 31  that are aligned in the X-axis direction (i.e., extend generally parallel to edge  41 A- 1 ). Note that the extrusion of material forming integral extruded structures  55 - 1 ,  55 - 2  and  55 - 3  remains continuous during the transition between printing first extrusion line portions  55 - 11 ,  55 - 21  and  55 - 31  and second extrusion line portions  55 - 12 ,  55 - 22  and  55 - 32 , whereby bus bar segments  45 A- 11 ,  45 A- 21  and  45 A- 31  are integrally connected to ends of first gridline sections  44 A- 11 ,  44 A- 21  and  44 A- 31 , respectively. Note also that, according to the disclosed embodiment, the movement of printhead assembly  100  in the X-axis direction during the formation of bus bar segments  45 A- 11 ,  45 A- 21  and  45 A- 31  is selected such that adjacent bus bar segments (e.g., segments  45 A- 11  and  45 A- 21 ) contact each other to form continuous bus bar structure  45 A- 1  extending in the X-axis direction. Next, printhead assembly  100  is returned to a straight line movement along the Y-axis direction such that third extrusion line portions  55 - 13 ,  55 - 23  and  55 - 33  are deposited to respectively form a set of parallel second gridline sections  44 A- 12 ,  44 A- 22  and  44 A- 32 . In one embodiment, printhead assembly  100  is positioned relative to substrate  41 A during deposition of third extrusion line portions  55 - 13 ,  55 - 23  and  55 - 33  such that second gridline sections  44 A- 12 ,  44 A- 22  and  44 A- 32  are respectively aligned with first gridline sections  44 A- 11 ,  44 A- 21  and  44 A- 31 . Printhead assembly is then again reciprocated back and forth in the X-axis (second) direction such that fourth extrusion line portions  55 - 14 ,  55 - 24  and  55 - 34  collectively form a second set of bus bar segments  45 A- 12 ,  45 A- 22  and  45 A- 32 . Finally, printhead assembly  100  is returned once more to a straight line movement along the Y-axis direction such that fifth extrusion line portions  55 - 15 ,  55 - 25  and  55 - 35  are deposited to respectively form a set of parallel third gridline sections  44 A- 13 ,  44 A- 23  and  44 A- 33 . The flow of extrusion material through printhead assembly  100  is then terminated. 
         [0028]    In accordance with an embodiment of the present invention, positioning mechanism  70  controls the relative movement of printhead assembly  100  and substrate  41 A such that printhead assembly  100  moves in the Y-axis direction at a first speed during formation of the gridline sections, and moves in the Y-axis at a second (slower) speed during formation of the bus bar segments. For example, during the first phase of the printing process, printhead assembly  100  is moved in a straight-line along the Y-axis direction at a relatively fast first speed such that first bead portions  55 - 11 ,  55 - 21  and  55 - 31  are deposited on surface  42 A to form first parallel gridline sections  44 - 11 ,  44 - 21  and  44 - 31 . Next, during the second phase of the printing process, movement of printhead assembly  100  in the Y-axis direction is slowed down while printhead assembly  100  is reciprocated back and forth in the X-axis direction, thereby causing second extrusion line portions  55 - 12 ,  55 - 22  and  55 - 32  to collectively form a first set of bus bar segments  45 A- 11 ,  45 A- 12  and  45 A- 13  that are aligned in the X-axis direction (i.e., extend generally parallel to edge  41 A- 1 ). Then, at the end of the second phase and the beginning of the third printing phase, movement of printhead assembly  100  in the Y-axis direction is again sped up to the first speed to facilitate rapid printing of third bead portions  55 - 13 ,  55 - 23  and  55 - 33 , thereby forming second gridline sections  44 - 12 ,  44 - 22  and  44 - 32  that extend parallel to (and respectively collinear with) first gridline sections  44 - 11 ,  44 - 21  and  44 - 31 . 
         [0029]    As set forth above, a preferred embodiment of the present invention involves the formation of gridlines and bus bar structures using a micro-extrusion system. An exemplary micro-extrusion system is set forth below. 
         [0030]      FIG. 2  is a simplified side view showing a portion of a generalized micro-extrusion system  50  for performing the extrusion printing process in accordance with a specific embodiment of the present invention. Micro-extrusion system  50  includes a material feed system  60  that is operably coupled to extrusion printhead assembly  100  (mentioned above with reference to  FIG. 1 ) by way of at least one feedpipe  68  and an associated fastener  69 . The materials are applied through pushing and/or drawing techniques (e.g., hot and cold) in which the materials are pushed (e.g., squeezed, etc.) and/or drawn (e.g., via a vacuum, etc.) through extrusion printhead assembly  100 , and nozzle outlets  169  that are respectively defined in a lower portion of printhead assembly  100 . Micro-extrusion system  50  also includes a X-Y-Z-axis positioning mechanism  70  including a mounting plate  76  for rigidly supporting and positioning printhead assembly  100  relative to substrate  41 A, and a base  80  including a platform  82  for supporting substrate  41 A in a stationary position as printhead assembly  100  is moved in a predetermined (e.g., Y-axis) direction over substrate  41 A. In alternative embodiment, printhead assembly  100  is stationary and base  80  includes an X-Y axis positioning mechanism (shown in dashed lines) for moving substrate  41 A under printhead assembly  100 . In either case, an electronic controller (e.g., a PC or other computer) supplies control signals to the positioning mechanism using known techniques such that the positioning mechanism is caused to perform the novel printing process described herein. 
         [0031]      FIG. 3  shows material feed system  60 , X-Y-Z-axis positioning mechanism  70  and base  80  of micro-extrusion system  50  in additional detail. The assembly shown in  FIG. 3  represents an experimental arrangement utilized to produce solar cells on a small scale, and those skilled in the art will recognize that other arrangements would typically be used to produce solar cells on a larger scale. Referring to the upper right portion of  FIG. 3 , material feed system  60  includes a housing  62  that supports a pneumatic cylinder  64 , which is operably coupled to a cartridge  66  such that material is forced from cartridge  66  through feedpipe  68  into printhead assembly  100 . Referring to the left side of  FIG. 3 , X-Y-Z-axis positioning mechanism  70  includes a Z-axis stage  72  that is movable in the Z-axis (vertical) direction relative to target substrate  41 A by way of a housing/actuator  74  in response to control signals received from an electronic controller  90 . Mounting plate  76  is rigidly connected to a lower end of Z-axis stage  72  and supports printhead assembly  100 , and a mounting frame  78  is rigidly connected to and extends upward from Z-axis stage  72  and supports pneumatic cylinder  64  and cartridge  66 . Referring to the lower portion of  FIG. 3 , base  80  includes supporting platform  82 , which supports target substrate  41 A as an X-Y mechanism moves printhead assembly  100  in the X-axis and Y-axis directions (as well as a couple of rotational axes) over the upper surface of substrate  41 A in accordance with the techniques described herein. 
         [0032]    As shown in  FIG. 2  and in exploded form in  FIG. 4 , layered micro-extrusion printhead assembly  100  includes a first (back) plate structure  110 , a second (front) plate structure  130 , and a layered nozzle structure  150  connected therebetween. Back plate structure  110  and front plate structure  130  serve to guide the extrusion material from an inlet port  116  to layered nozzle structure  150 , and to rigidly support layered nozzle structure  150  such that extrusion nozzles  163  defined in layered nozzle structure  150  are pointed toward substrate  41 A at a predetermined tilted angle θ 1  (e.g., 45°), whereby extruded material traveling down each extrusion nozzle  163  toward its corresponding nozzle orifice  169  is directed toward target substrate  41 A. 
         [0033]    Each of back plate structure  110  and front plate structure  130  includes one or more integrally molded or machined metal parts. In the disclosed embodiment, back plate structure  110  includes an angled back plate  111  and a back plenum  120 , and front plate structure  130  includes a single-piece metal plate. Angled back plate  111  includes a front surface  112 , a side surface  113 , and a back surface  114 , with front surface  112  and back surface  114  forming a predetermined angle  82  (e.g., 45°; shown in  FIG. 1 ). Angled back plate  111  also defines a bore  115  that extends from a threaded countersunk bore inlet  116  defined in side wall  113  to a bore outlet  117  defined in back surface  114 . Back plenum  120  includes parallel front surface  122  and back surface  124 , and defines a conduit  125  having an inlet  126  defined through front surface  122 , and an outlet  127  defined in back surface  124 . As described below, bore  115  and plenum  125  cooperate to feed extrusion material to layered nozzle structure  150 . Front plate structure  130  includes a front surface  132  and a beveled lower surface  134  that form predetermined angle θ 2  (shown in  FIG. 1 ). 
         [0034]    Layered nozzle structure  150  includes two or more stacked plates (e.g., a metal such as aluminum, steel or plastic) that combine to form one or more extrusion nozzles  163 . In the embodiment shown in  FIG. 4 , layered nozzle structure  150  includes a top nozzle plate  153 , a bottom nozzle plate  156 , and a nozzle outlet plate  160  sandwiched between top nozzle plate  153  and bottom nozzle plate  156 . Top nozzle plate  153  defines an inlet port (through hole)  155 , and has a (first) front edge  158 - 1 . Bottom nozzle plate  156  is a substantially solid (i.e., continuous) plate having a (third) front edge  158 - 2 . Nozzle outlet plate  160  includes a (second) front edge  168  and defines an elongated nozzle channel  162  extending in a predetermined first flow direction F 1  from a closed end  165  to an nozzle orifice  169  defined through front edge  168 . When operably assembled (e.g., as shown in  FIG. 6 ), nozzle outlet plate  160  is sandwiched between top nozzle plate  153  and bottom nozzle plate  156  such that elongated nozzle channel  162 , a front portion  154  of top nozzle plate  153 , and a front portion  157  of bottom nozzle plate  156  combine to define elongated extrusion nozzle  163  that extends from closed end  165  to nozzle orifice  169 . In addition, top nozzle plate  153  is mounted on nozzle outlet plate  160  such that inlet port  155  is aligned with closed end  165  of elongated channel  162 , whereby extrusion material forced through inlet port  155  flows in direction F 1  along extrusion nozzle  163 , and exits from layered nozzle structure  150  by way of nozzle orifice  169  to form bead  55  that is deposited on substrate  41 A. 
         [0035]    Referring again to  FIG. 2 , when operably assembled and mounted onto micro-extrusion system  50 , angled back plate  111  of printhead assembly  100  is rigidly connected to mounting plate  76  by way of one or more fasteners (e.g., machine screws)  142  such that beveled surface  134  of front plate structure  130  is positioned close to parallel to upper surface  42 A of target substrate  41 A. One or more second fasteners  144  are utilized to connect front plate structure  130  to back plate structure  110  with layered nozzle structure  150  pressed between the back surface of front plate structure  130  and the back surface of back plenum  120 . In addition, material feed system  60  is operably coupled to bore  115  by way of feedpipe  68  and fastener  69  using known techniques, and extrusion material forced into bore  115  is channeled to layered nozzle structure  150  by way of conduit  125 . 
         [0036]    In a preferred embodiment, as shown in  FIG. 2 , a hardenable material is injected into bore  115  and conduit  125  of printhead assembly  100  in the manner described in co-owned and co-pending U.S. patent application Ser. No. 12/267,194 entitled “DEAD VOLUME REMOVAL FROM AN EXTRUSION PRINTHEAD”, which is incorporated herein by reference in its entirety. This hardenable material forms portions  170  that fill any dead zones of conduit  125  that could otherwise trap the extrusion material and lead to clogs. 
         [0037]      FIG. 5  is a partial side view showing a portion of system  50  including printhead assembly  100 , and  FIG. 6  is a simplified cross-sectional side view showing a portion of printhead assembly  100  during operation. As indicated in these figures, during operation printhead assembly  100  is maintained above substrate  41 A and moved in the Y-axis direction as extruded material is injected through inlet port  116  into bottom plate assembly  110 , and through back plenum  120  to layered nozzle assembly  150 , from which beads  55  are extruded onto surface  42 A. As shown in additional detail in  FIG. 6 , the extrusion material exiting conduit  125  of back plenum  120  enters the closed end of nozzle  163  by way of inlet  155  and closed end  165  (both shown in  FIG. 3 ) of nozzle  163 , and flows in direction F 1  down nozzle  163  toward outlet  169 . The extrusion material flowing in the nozzle  163  is directed through the nozzle opening  169 . Referring back to  FIG. 2 , the extruded material is guided at the tilted angle θ 2  as it exits nozzle orifice  169 , thus being directed toward substrate  41 A in a manner that facilitates high volume solar cell production. 
         [0038]      FIGS. 7(A) to 7(D)  illustrate the production of the front contact pattern for a solar cell  40 B according to another specific embodiment of the present invention. The production process illustrated in these figures utilizes a co-extrusion printhead assembly  100 B, which is similar to printhead assembly  100 B (described above), but simultaneously extrudes a metal-bearing (gridline) material  51 B- 1  and a non-conductive sacrificial material  51 B- 2  using co-extrusion techniques such as those described in co-owned and co-pending U.S. patent application Ser. No. 12/267,069, entitled “DIRECTIONAL EXTRUDED BEAD CONTROL”, which is incorporated herein by reference in its entirety. As with the previously described embodiments, the printing process illustrated in  FIGS. 7(A) to 7(D)  involves a single pass of printhead  100 B over the surface of substrate  41 B. As indicated in  FIG. 7(A) , after printing first gridline sections  44 B- 1 , printhead  100 B is reciprocated (oscillated) in the X-axis direction in order to print switchback sections that form first bus bar structure  45 B- 1  (shown in  FIG. 7(B) ). Similarly, as indicated in  FIGS. 7(C) and 7(D) , after printing second gridline sections  443 - 2 , printhead  100 B is again reciprocated in the X-axis direction to print second bus bar structure  45 B- 2 , then translated in the Y-axis direction to print third gridline sections  44 B- 3 , then reciprocated to print third bus bar structures  44 B- 3 , then translated in the Y-axis direction to print fourth gridline sections  44 B- 4 . The resulting solar cell  40 B is shown in  FIG. 7(D) . 
         [0039]      FIGS. 8(A) and 8(B)  illustrate exemplary switchback patterns that are generated by extruded lines  55 C and  55 D in accordance with alternative embodiments of the present invention utilizing techniques similar to those described above. These figures illustrate that by reducing the speed of translation in the Y-axis direction between printing straight sections  44 C- 1 / 44 C- 2  and  44 D- 1 / 44 D- 2 , while at the same time oscillating either the device or the printhead in the X-axis direction, a bus bar structure pattern can be defined that is continuous or nearly continuous (e.g., bus bar structure  45 D- 1 ; see FIG.  8 (B)), or open to various degrees (e.g., bus bar structure  45 C- 1 ; see  FIG. 8(A) ). Such a pattern allows the fingers and the buses to be written in a single pass while allowing additional features to be designed into the bus, for example reducing the use of ink (extruded material) or optimizing the surface area available for subsequent lead wire attachment. Alternatively, the pattern may be pre-defined using laser ablation, the principle of oscillation of the write head or the substrate around the direction of travel being the same as for the direct application of ink. 
         [0040]    Although the present invention has been described with respect to certain specific embodiments, it will be clear to those skilled in the art that the inventive features of the present invention are applicable to other embodiments as well, all of which are intended to fall within the scope of the present invention. For example, instead of, or in addition to, oscillating the device or the print head to form the bus areas, the width of the central, metal feature of the extruded line may be varied by altering the relative pressure between the metal-bearing ink and the non-metal bearing ink in the invention described in co-owned and co-pending U.S. patent application Ser. No. 11/282,882, filed Nov. 17, 2005, entitled “Extrusion/Dispensing Systems and Methods”, and in co-owned and co-pending U.S. patent application Ser. No. 11/282,882, filed Nov. 17, 2005, entitled “Extrusion/Dispensing Systems and Methods”, which are incorporated herein by reference in their entirety. Maximizing the width of the metal bearing ink in the bus region, with or without oscillation can be used to provide the solderable bus area required. Some process sequences use a pattern that has been pre-written using a laser to define the contact area. This can also be accomplished using the present invention. Clearly, any number of different patterns can be obtained by appropriate manipulation of the printhead and the device to obtain a pattern that is continuous and may be applied by a single pass of the printhead.