Abstract:
A method and apparatus for improving a thin film scribing procedure is presented. Embodiments of the invention include a method and apparatus for determining a scribe setting for removal of an absorber layer of a photovoltaic device that improves contact resistance between a back contact layer and a front contact layer of the device.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the present invention generally relate to photovoltaic devices and fabrication processes thereof. In particular, embodiments of the present invention relate to apparatus and methods for the fabrication of thin-film photovoltaic devices. 
         [0003]    2. Description of the Related Art 
         [0004]    Photovoltaic (PV) devices are devices that convert sunlight into direct current (DC) electrical power. A PV device may be classified as monocrystalline, polycrystalline, or thin-film depending on how the device is fabricated. 
         [0005]    Monocrystalline PV devices are produced by slicing wafers from a single crystal, high-purity, silicon boule. Polycrystalline PV devices are produced by sawing a cast block of silicon into bars, then wafers. Thin-film PV devices are produced by depositing thin layers of materials, such as amorphous silicon, microcrystalline silicon, or copper indium gallium selenide (CIGS) onto a suitable substrate. 
         [0006]    Although monocrystalline and polycrystalline silicon have traditionally produced PV devices with higher efficiencies, the high cost of crystalline silicon wafers has led the industry into increased use and development of thin-film PV devices. 
         [0007]    Accordingly, as the production of thin-film PV devices matures, the need for increased process control has developed. This need is driven by both yield and uniformity considerations. Additionally, fast and effective ramp-up of new or duplicated production lines is needed as well. 
         [0008]    Therefore, a need exists for apparatus and methods for optimizing processes for the fabrication of thin-film photovoltaic devices. 
       SUMMARY OF THE INVENTION 
       [0009]    In one embodiment of the present invention, a method for improving thin film scribe parameters comprises preparing a control sample having first and second back contact pads formed over a front contact layer, preparing first and second test samples having an absorber layer deposited over a front contact layer, removing the absorber layer of the first test sample at a first scribe setting, removing the absorber layer of the second test sample at a second scribe setting, depositing first and second back contact pads on each of the first and second test samples, determining the contact resistance of each of the control sample, the first test sample, and the second test sample, comparing the contact resistance of each of the first and second test samples to the contact resistance of the third test sample, and determining the preferred scribe setting between the first scribe setting and the second scribe setting. 
         [0010]    In another embodiment of the present invention, a method for improving scribe parameters for the production of thin film photovoltaic devices comprises depositing a front contact layer on a large area substrate comprising a control section and a plurality of test sections, depositing an absorber layer over the front contact layer of each of the test sections, removing the absorber layer from each of the test sections, wherein a different scribe setting is used for removing the absorber layer from each individual test section, depositing a plurality of back contact pads onto the control section, depositing a plurality of back contact pads onto each of the test sections, determining the contact resistance between the back contact pads and the front contact layer of the control section, determining the contact resistance between the back contact pads and the front contact layer of each of the test sections, comparing the determined contact resistance of each of the test sections to the control section, and determining the preferred scribe setting. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    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. 
           [0012]      FIG. 1  is a schematic cross-section of a thin-film PV module showing series connections between individual cells. 
           [0013]      FIG. 2  is an enlarged, schematic view of section A of the module in  FIG. 1 . 
           [0014]      FIG. 3  is a schematic block chart depicting a method for determining an improved device setting for scribing a groove in an absorber layer of a thin-film solar module according to one embodiment of the present invention. 
           [0015]      FIGS. 4A-4F  are schematic, cross-sectional views of a control panel and one of a plurality of test panels illustrating a progression of steps according to the method depicted in  FIG. 3 . 
           [0016]      FIGS. 5A-5F  are schematic, top views of the control panel and one of the plurality of test panels illustrating a progression of steps according to the method depicted in  FIG. 3 . 
           [0017]      FIG. 6  is a top view of a mask for depositing back contact pads onto the control panel and each of the plurality of test panels according to one embodiment of the present invention. 
           [0018]      FIG. 7  is a schematic, cross-sectional view of a panel, which may be either a control panel or one of the test panels in  FIGS. 4A-4F  and  5 A- 5 F. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    In thin-film PV module fabrication, individual PV cells may be formed and interconnected into a module via scribing the layers of the cell. Scribing the layers of a solar cell may be performed by laser ablation, water-jet cutting, wheel grinding, or other similar methods.  FIG. 1  is a schematic cross-section of a thin-film PV module  100  showing series connections between individual cells  101 . PV module  100  comprises a substrate  102 , such as a glass or polymer substrate, with thin films formed thereover. 
         [0020]    In the PV module  100  fabrication process, a front contact layer  105 , which may be a transparent conducting oxide (TCO) layer, may be formed over the substrate  102 . The front contact layer  105  may comprise tin oxide, zinc oxide, indium tin oxide, cadmium stannate, combinations thereof, or other suitable materials. In a first scribing step, a first insulating groove  112  may be created in the front contact layer  105  to interrupt lateral current flow. An absorber layer  115  may then be formed over the front contact layer  105 . The absorber layer  115  may comprise layers of thin films, including but not limited to, amorphous silicon, microcrystalline silicon, copper indium gallium selenide (CIGS), or combinations thereof. Next, a second scribing may create a groove  124  in the absorber layer  115 , which may be filled during the subsequent step of forming a back contact layer  125  over the absorber layer  115 . This results in an interconnection between the front contact layer  105  and the back contact layer  125 . The back contact layer  125  may comprise a conductive layer and/or reflective coating. The conductive layer may be an aluminum doped zinc oxide (AZO) layer. The reflective coating may comprise metallic materials including, but not limited to Al, Ag, Ti, Cr, Au, Cu, Pt, alloys thereof, combinations thereof as well as other conductive and reflective materials. Finally a second insulating groove  136  may be formed in the back contact layer  125  to interrupt lateral current flow in the back contact layer  125 . 
         [0021]    As a result, each individual PV cell  101  of the module  100  is electrically connected in series with each adjacent cell  101 .  FIG. 1  depicts five PV cells  101  connected in series via the aforementioned scribing process. However, any number of PV cells  101  may be linked in this manner to form the PV module  100 . 
         [0022]    In the production of thin-film PV modules, such as the module  100 , one critical parameter that dictates the ultimate efficiency of the module  100  is the contact resistance between the front contact layer  105  and the back contact layer  125 . The contact resistance between the front contact layer  105  and the back contact layer  125  is, in turn, dependent upon optimum scribing of the absorber layer  115 . 
         [0023]      FIG. 2  is an enlarged, schematic view of section A of the module  100  in  FIG. 1 . As previously described, the absorber layer  115  is scribed, and the back contact layer  125  is deposited thereover, filling the groove  124 , and creating an electrical pathway between the front contact layer  105  and the back contact layer  125  at a front contact surface  107 . 
         [0024]    If the groove  124  is laser scribed at a laser power setting that is too high, the surface of the front contact layer  105  overheats resulting in an alteration in the contact surface  107 . Similarly if the groove  124  is scribed at a water-jet or a grinding wheel power setting that is too high, the surface of the front contact layer  105  is altered as well. This alteration produces an undesirable change in morphology, which results in poor conductive contact between the front contact layer  105  and the later deposited back contact layer  125 . The result is higher contact resistance between the front contact layer  105  and the back contact layer  125  than may be achieved with scribing of the absorber layer with an improved power setting. 
         [0025]    If, however, the groove  124  is scribed at a power setting that is too low, the absorber layer  115  is not completely removed, resulting in a plurality of absorber layer particles remaining on the front contact surface  107 . The absorber layer particles on the front contact surface  107  prevent full conductive contact between the front contact layer  105  and the later deposited back contact layer  125 . The result, again, is higher contact resistance between the front contact layer  105  and the back contact layer  125  than may be achieved with laser scribing of the absorber layer  115  with an improved power setting. 
         [0026]    At an improved power setting, the groove  124  is scribed such that the absorber layer  115  is completely removed, yet the contact surface  107  is not altered from its as deposited state. The result of scribing at the improved power setting is full conductive contact between the front contact layer  105  and the back contact layer  125 . When such contact is achieved, the contact resistance between the front contact layer  105  and the back contact layer  125  is minimized, and the efficiency of the PV module  100  is maximized. 
         [0027]      FIG. 3  is a schematic block chart depicting a method  300  for determining an improved setting for scribing a groove in an absorber layer of a thin-film solar module according to one embodiment of the present invention.  FIGS. 4A-4F  are schematic, cross-sectional views of a control panel  400  and one of a plurality of test panels  500 , and  FIGS. 5A-5F  are corresponding schematic, top views of the control panel  400  and one of the plurality of test panels  500  illustrating a progression of steps according to the method  300  depicted in  FIG. 3 . 
         [0028]    In one embodiment, at step  310  of  FIG. 3  and depicted in  FIGS. 4A , and  5 A, a front contact layer  405  is deposited onto a substrate  402  of the control panel  400 . An identical front contact layer  505  is deposited onto a substrate  502  of each of the test panels  500 . Although, according to one embodiment, a plurality of test panels  500  is used in the method  300 , the  FIGS. 4A-4F  and  5 A- 5 F depict a single test panel  500 . In one embodiment, the control panel  400  and the test panels  500  are separately constructed from a plurality of individual, sample-sized substrate panels, such as about 10 cm by about 7 cm rectangular substrates for instance. In another embodiment, the control panel  400  and the plurality of test panels  500  are formed on a single large substrate panel, such as a rectangular substrate having a width of about 2.6 m and a length of about 2.2 m. 
         [0029]    At step  320 , as shown in  FIGS. 3 ,  4 B, and  5 B, an absorber layer  515  is deposited onto the front contact layer  505  of each of the test panels  500 . At step  330  of  FIG. 3  and as depicted in  FIGS. 4C and 5C , the entire surface of the absorber layer  515  is removed in a scribing operation, such as that used to form groove  124  in  FIGS. 1 and 2 . In one embodiment, the entire surface of each individual test panel  500  is removed using a constant scribe setting. However, the scribe settings are varied from one test panel  500  to the next test panel  500 . For instance, for a first test panel  500  the entire surface of the absorber layer  515  is removed using a first scribe power setting, and the surface of the absorber layer  515  of each successive test panel  500  is removed using a succession of increasing and/or decreasing scribe power settings. Therefore, the surface of the absorber layer  515  of each test panel  500  is removed with a different scribe power setting. Although the power setting is varied in the above-described embodiment, a number of other parameters of the scribing device may be varied instead. In one embodiment, laser power, current, or frequency may be altered. In another embodiment, water-jet power, pressure, or flow may be altered. In yet another embodiment, grinding wheel power, pressure, force, or speed may be altered. 
         [0030]    At step  340  of  FIG. 3 , a mask  600  is placed onto the control panel  400  and each of the test panels  500  as shown in  FIGS. 4D and 5D , respectively.  FIG. 6  is a top view of the mask  600  according to one embodiment of the present invention. In one embodiment, the mask body  610  is a rectangular sheet member approximately the size of the individual panels  400 ,  500 . In one embodiment mask  600  is comprised of a metal, such as stainless steel or aluminum. In one embodiment, the mask  600  is comprised of a non-metallic material, such as a ceramic or polymeric material. The mask  600  has a plurality of apertures  620  formed therethrough. In one embodiment, the apertures  620  are formed with varying distances (d 1 -d 5 ) between each successive aperture as depicted in  FIG. 6 . Although the mask  600  depicted in  FIG. 6  has seven apertures  620  formed therethrough, any number of apertures  620  may be utilized as long as the mask  600  has at least two apertures  620  and a distance (d 1 ) therebetween. In one embodiment, the apertures  620  are rectangular in shape. However, the shape of the aperture  620  is not limited and may be of any shape. 
         [0031]    At step  350  of  FIG. 3  and as shown in  FIGS. 4E and 5E , a back contact layer  425 ,  525  is deposited over the mask  600  located on the control panel  400  and each of the test panels  500  such that the back contact layer  425 ,  525  only contacts the front contact layer  405 ,  505  at the locations of the apertures  620  of the mask  600 . 
         [0032]    At step  360  of  FIG. 3 , the mask  600  is removed from the control panel  400  and each of the test panels  500 . As a result, the only portions of the back contact layer  425 ,  525  remaining on the control panel  400  and each of the test panels  500  are pads  450 ,  550 , separated by distances (d 1 -d 5 ), as shown in  FIGS. 4F and 5F . 
         [0033]    At step  370  of  FIG. 3 , the resistance between the various pads  450  of the control panel  400  is measured by one of a variety of methods known in the art, and a baseline contact resistance between the back contact layer  425  and the front contact layer  405  is determined as subsequently described. Since the control panel  400  never had an absorber layer deposited on the front contact layer  405 , the determined baseline contact resistance represents the minimum contact resistance achievable between the back contact layer  425  and the front contact layer  405 . 
         [0034]    At step  380 , the resistance between the various pads  550  of each of the test panels  500  is measured, and a contact resistance between the back contact layer  525  and the front contact layer  505  is determined for each of the plurality of test panels  500  as subsequently described. Since the absorber layer  515  previously deposited onto each of the test panels  500  was removed using different scribe parameters for each test panel  500 , it follows that each test panel  500  has a different amount of either over-scribing of the absorber layer  515 , resulting in various amounts of altered morphology of the surface of the front contact layer  505 , or under-scribing of the absorber layer  515 , resulting in various amounts of absorber layer  515  particles remaining on the surface of the front contact layer  505 . Therefore, each test panel  500  will have a different determined contact resistance, which corresponds to the scribe parameters that were varied during the removal process. 
         [0035]    At step  390 , the contact resistance determined for each of the test panels  500  are compared to the baseline contact resistance determined for the control panel  400 . In one embodiment, the scribe parameters used on the test panel  500  having a contact resistance that most closely matches the contact resistance of the control panel  400  are selected as the scribe settings for production. In one embodiment, steps  310 - 390  may be repeated with refined scribe settings to more closely match that of the baseline contact resistance established for the control panel  400 . 
         [0036]    The contact resistance for each panel  400 ,  500  may be determined as follows.  FIG. 7  is a schematic, cross-sectional view of a panel  700 , which may be either a control panel  400  or one of the test panels  500  in  FIGS. 4A-4F  and  5 A- 5 F. The panel  700  comprises a substrate  702  with a front contact layer  705  deposited thereover. Back contact layer pads  750  are deposited over the front contact layer  705  using the mask  600  described with respect to  FIG. 6 . 
         [0037]    As shown in  FIG. 7 , the pads  750  are deposited onto the front contact layer  705  having varying distances (d 1 -d 3 ) therebetween. The resistance (R 1 ) between two successive pads is measured. The contact resistance (Rc) may then be determined according to the following equation: 
         [0000]        R 1=2( Rc )+ d 1( Rs ) 
         [0038]    where Rs=sheet resistance of the front contact layer. 
         [0039]    As previously stated, once the contact resistance for each of the test panels  500  is determined, it is then compared with the contact resistance of the control panel  400 . The scribe parameters used for the removal of the absorber layer  515  from the test panel  500  having a contact resistance most closely matching the contact resistance of the control panel  400  may then be selected for the improved scribing parameters used in production. 
         [0040]    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.