Abstract:
Photovoltaic cells and methods for the manufacture of photovoltaic cells are described. Operative layers of the photovoltaic cell are deposited onto a superstrate having a plurality of spaced ramps, allowing for the individual cells to be connected in series with minimal loss of the efficiency due to dead space between the cells.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application No. 61/119,426, filed Dec. 3, 2008, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Embodiments of the present invention generally relate to photovoltaic cells and methods for making photovoltaic cells. Specific embodiments pertain to photovoltaic cells and methods of making photovoltaic cells having a substantially minimized dead zone. 
     A typical manufacturing process for solar cells is shown in  FIG. 1 . Starting at  100 , solar cells are manufactured by starting with a glass sheet or substrate  114 . An exemplary thickness for the glass sheet is about 3 mm. In the art, this glass substrate is typically called a glass superstrate because sunlight will enter through this support glass. During the manufacture of a solar cell, shown in step  102 , a continuous, uniform layer of a transparent conductive oxide (TCO)  116  is deposited on the glass substrate  114 . The thickness of the TCO layer  116  is typically a few hundred nanometers. The TCO layer  116  eventually forms the front electrodes of the solar cell. Suitable materials for the TCO layer  116  include, but are not limited to, aluminum-doped zinc oxide (AZO), indium tin oxide (ITO), indium molybdenum oxide (IMO), indium zinc oxide (IZO) and tantalum oxide. The TCO layer  116  can be deposited by any suitable process, such as chemical vapor deposition (CVD). 
     In step  104 , after the deposition of the TCO layer  116 , a laser scribing process, which is often referred to as P 1 , scribes strips  118  through the entire thickness of the TCO layer  116 . The scribed strips are usually 5-10 mm apart. After the scribing process P 1 , a p- and n-type silicon layer  120  is deposited over the TCO layer  116 , as shown in step  106 . The total thickness of the silicon layer  120  is typically on the order of 2-3 μm, and this layer is usually deposited by chemical vapor deposition or other suitable processes. 
     Referring to step  108 , the silicon deposition step is followed by a second laser scribing step, often referred to as P 2 , which completely cuts strips  122  through the silicon layer  120 . As shown in step  110 , a metal layer  124  that forms the rear electrode is deposited over the silicon layer  120 . The metal layer  124  can be deposited by any suitable deposition process, such as physical vapor deposition (PVD). Referring now to step  112 , a third scribing process, called P 3 , is used to scribe strips  126  through the metal layer  124  and the silicon layer  120 . The panel is then typically sealed with a rear surface glass lamination (not shown). The area between, and including, the P 1  and P 3  scribes results in a dead zone  128  which decreases the overall efficiency of the cell. The dead zone is typically in the range of about 100 μm to about 500 μm, depending on the accuracy of the lasers and optics employed in the scribing processes. 
     Therefore, there is a need to provide methods to improve the efficiency of photovoltaic cells. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention relates to a method of making a photovoltaic cell comprising providing a superstrate having a front side and a back side, the back side having a plurality of spaced ramps thereon, the spaced ramps including ramped surfaces and vertical faces which are substantially perpendicular to the back side of the superstrate; depositing a transparent conductive oxide layer onto the back side of the superstrate such that substantially none of the transparent conductive oxide coats the vertical faces of the spaced ramps; depositing a silicon layer on the transparent conductive oxide layer such that the silicon layer deposited on the vertical faces of the spaced ramps does not contact the transparent conductive oxide layer on top on the ramped surfaces of the spaced ramps; depositing a metal layer on the silicon layer, resulting in a plurality of peaks extending from the ramped surfaces through the metal layer; and removing at least a portion of the peaks extending through the metal layer to achieve a substantially flat surface extending across the superstrate, to produce a photovoltaic cell. 
     In one embodiment, a method further comprises applying a polymer lamination layer to the flattened metal layer followed by a layer of glass. 
     In one embodiment, the spaced ramps are separated so that there is a region of flat superstrate between each spaced ramp. In one embodiment, the transparent conductive oxide layer is deposited by physical vapor deposition. In one embodiment, the transparent conductive oxide layer is deposited on an angle such that the vertical faces of the spaced ramps are shielded by the ramped surfaces. 
     According to an embodiment, the silicon layer is deposited by chemical vapor deposition. In an embodiment, the peaks in the metal layer are flattened by one or more process including buffing, grinding and cutting. 
     In one embodiment, a method further comprises cleaning the vertical face of the spaced ramps after deposition of the transparent conductive oxide layer by laser ablation performed at a grazing angle to hit substantially only the vertical surfaces. According to an embodiment, the superstrate is glass or plastic, and the spaced ramps are formed on the superstrate by one or more of intaglio, rotogravure, etching, engraving, relief printing and lithography. 
     Another aspect of the invention pertains to a photovoltaic cell comprising a superstrate having a front side and a back side, the back side having a plurality of spaced ramps including ramped surfaces and vertical faces which are substantially perpendicular to the back side of the superstrate; a layer of a transparent conductive oxide on the back side of the superstrate; a layer of amorphous silicon overlying the transparent conductive oxide layer; and a layer of metal overlying the silicon layer, the metal layer having a front side facing the silicon layer and a back side. In one embodiment, the spaced ramps extend from the superstrate through at least a portion of the layers and the metal layer is smoothed resulting in a substantially flat back surface and exposing portions of the silicon layer. 
     In one embodiment, silicon layer does not extend above the vertical side of the spaced ramps. In one embodiment, the photovoltaic cell further comprises a polymer laminate on the metal layer and glass on the polymer laminate layer. In an embodiment, the separation between the spaced ramps is in the range of about 5 mm to about 10 mm. According to an embodiment, the spaced ramps extend outwardly from the back side of the superstrate up to a height of about 5 microns. 
     In an embodiment, the superstrate is about 3 mm thick. The superstrate may comprise glass or plastic, according to one or more embodiments. In one embodiment, the photovoltaic cell has a dead zone smaller than about 100 microns. 
     The foregoing has outlined rather broadly certain features and technical advantages of the present invention. It should be appreciated by those skilled in the art that the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures or processes within the scope present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  shows steps for making photovoltaic cells using a laser scribing technique according to the prior art; 
         FIG. 2  shows a superstrate being patterned with spaced ramps by a rotogravure-type process in accordance with one embodiment of the invention; 
         FIG. 3  shows steps for making photovoltaic cells using a superstrate having spaced ramps according to an embodiment of the invention; 
         FIG. 4  shows an enlarged view of the top of the spaced ramps, including a transparent conductive oxide layer and a silicon layer; and 
         FIG. 5  shows a photovoltaic cell prepared using a patterned superstrate according to one or more embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Before describing several exemplary embodiments of the invention, it is to be understood that the invention is not limited to the details of construction or process steps set forth in the following description. The invention is capable of other embodiments and of being practiced or being carried out in various ways. 
     One or more embodiments of the invention are directed to photovoltaic cells and methods of making photovoltaic cells. With reference to  FIG. 2 , a superstrate  200  is provided which has a front side  202  and a back side  204 . A series of spaced ramps  206  are provided on the back side  204  of the superstrate  200 . The spaced ramps  206  include ramped surfaces  208  and vertical faces  210  which are substantially perpendicular to the back side  204  of the superstrate  200 . The spaced ramps  206  extend through any material layers applied to the back side  204  of the superstrate  200 . The spaced ramps  206  extend outwardly from the surface of the back side  204  of the substrate  200  to a height of less than about 10 μm. In detailed aspects, the spaced ramps  206  extend from the superstrate  200  backside  204  to a height of about 5 μm. 
     The spaced ramps  206  can be formed by any suitable technique. Non-limiting examples of suitable techniques include intaglio, rotogravure, etching, engraving, relief printing and lithography.  FIG. 2  shows a rotogravure type process where the superstrate  200 , moving from right to left  212 , passes between a flat roller  214  and a patterned roller  216 . The flat roller  214  is shown rotating in a counter-clockwise direction  218  and the patterned roller  216  rotates in a clockwise direction  220  causing the superstrate  200  to move in the desired direction  212 . While the process shown in  FIG. 2  has the superstrate moving from right to left, this should not be interpreted as a limitation on the direction of superstrate movement. In one or more embodiments, the spaced ramps may be formed in the superstrate during formation of the superstrate when the glass or plastic sheet material is in a softened state, for example, while the glass or plastic sheet is being formed during a sheet formation operation such as drawing the sheet from a furnace or lehr. Alternatively, a flat glass or plastic sheet may be heated to soften at least the surface so that the spaced ramps can be formed on the sheet. It will be appreciated that for certain materials and processes such as etching, heating may not be required to form the ramps. 
     The spacing  222  between the spaced ramps  206  can be changed according to the desired size of the resulting solar cells. The spacing  222  is generally less than about 20 mm. Detailed aspects of the invention have the spacing  222  between the spaced ramps  206  of less than about 10 mm. More detailed aspects have the spacing  222  between about 5 and about 10 mm. Other detailed aspects have no spacing  222  between the spaced ramps  206 . Where there is a space between the ramps  206 , the space  222  may be a substantially flat region  224  on the superstrate  200 . 
     The superstrate  200  can be any suitable material, for example, glass or plastic, and can be any thickness as desired by the intended application. Detailed aspects of the invention include a superstrate which is less than about 5 mm thick. According to other detailed aspects, the superstrate is about 3 mm thick. 
       FIG. 3  shows steps for preparing a photovoltaic cell according to one or more embodiments of the invention. A superstrate  300  is prepared having spaced ramps  302  with vertical faces  304 , as previously described. A transparent conductive oxide (TCO) layer  310  is deposited onto the back side of the superstrate  300 . Suitable TCOs are known to those skilled in the art. Non-limiting examples of transparent conductive oxides include aluminum-doped zinc oxide (AZO), indium tin oxide (ITO), indium molybdenum oxide (IMO), indium zinc oxide (IZO) and tantalum oxide. The TCO layer  310  is deposited in a manner such that substantially none of the TCO coats the vertical faces  304  of the spaced ramps  302 . The spaced ramps  302  on the superstrate  300  extend through the TCO layer  310 . 
     The TCO layer  310  can be deposited by any suitable means, as would be known to those skilled in the art. Detailed aspects of the invention have the TCO layer  310  deposited by physical vapor deposition. In other aspects, the TCO layer  310  is deposited on an angle toward the ramped surfaces  306  of the spaced ramps  302 . By depositing the TCO on an angle, the vertical faces  304  of the spaced ramps  302  are partially shielded, resulting in a decreased likelihood that the TCO will coat the vertical faces  304 . 
     According to some detailed aspects, the TCO layer  310  is up to about 500 nm thick. In other detailed aspects, the TCO layer  310  is about 300 nm thick. 
     In some detailed embodiments, laser ablation, or other suitable techniques, are used to clean the vertical faces  304  of the spaced ramps  302  after the TCO layer  310  has been deposited. The laser ablation cleaning may be done by directing the laser at the TCO coated superstrate  300  on a grazing angle so that the laser hits substantially only the vertical faces  304  of the spaced ramps  306 . 
     After depositing the TCO layer  310 , a silicon layer  320  is deposited on the transparent conductive oxide layer  310 . The silicon layer  320  is deposited between the spaced ramps  302  in a manner such that the silicon layer  320  does not contact the transparent conductive oxide layer  310  on top on the ramped surfaces  306 . For instance, the silicon layer  320  does not extend substantially above the peak  308  of the vertical face  304  of the spaced ramps  302 . The thickness of the silicon layer  320  is slightly smaller than the height of the individual spaced ramps  302 , which is typically about 2 to 3 μm.  FIG. 4  shows an expanded view of the top region of one of the spaced ramps  302  with the TCO layer  310  and silicon layer  320  deposited thereon. The spaced ramps  302  on the superstrate  300  extend through the TCO layer  310  and the silicon layer  320 . The silicon layer  320  can be deposited by any suitable methods. Detailed aspects of the invention have the silicon layer  320  deposited by chemical vapor deposition. 
     After deposition of the silicon layer  320 , a metal layer  330  is deposited on the silicon layer  320 . The spaced ramps  302  of the superstrate  300  extend through the TCO layer  310 , the silicon layer  320  and the metal layer  330 , resulting in a plurality of peaks  308  projecting through the metal layer  330 . The metal layer of some aspects is less than about 2 μm thick. In other detailed aspects, the metal layer is less than about 1 μm thick. Suitable metals for use with photovoltaic cells are known to those skilled in the art. Non-limiting examples include aluminum, molybdenum and combinations thereof. 
     After deposition of the metal layer  330 , at least a portion of the peaks  308  extending from the superstrate  300  through the TCO layer  310 , the silicon layer  320  and the metal layer  330  are removed as shown in the last step of  FIG. 3 . Removal of these protruding peaks  308  results in a substantially flat back surface  340  with portions of the silicon layer  320  being exposed. Methods and techniques for removal of the protruding peaks are known to those skilled in the art. Suitable methods include, but are not limited to, buffing, grinding and cutting. 
       FIG. 5  shows a photovoltaic cell  500  made according to one or more embodiments of the described methods. The photovoltaic cell  500  comprises a superstrate  510  having spaced ramps  512  thereon. The photovoltaic cell  500  is not drawn to scale, the height of the spaced ramps  512  being exaggerated for illustrative purposes. A transparent conductive oxide layer  520  is deposited on the superstrate  510 . A silicon layer  530  is deposited on the TCO layer  520 , and a metal layer  540  is deposited on the silicon layer  530 . The back side of the metal layer  540  is shown after it has been smoothed. A polymer lamination layer  550  may be applied followed by a layer of glass  560 , or other suitable material. 
     The resultant photovoltaic cell  500  is a collection of a plurality of individual photovoltaic cells  570  attached in series. The individual photovoltaic cells  570  extend from the vertical face  514  of one spaced ramp  512  to the vertical face  514  of the adjacent spaced ramp  512 . The individual photovoltaic cells  570  are connected to the adjacent cells by a series connection. That is, the TCO layer  520  of one cell  570  connects to the metal layer  540  of the adjacent cell  570 . 
     In the conventional process, what may be referred to as a “dead zone” results between the P 1  and P 3  laser scribed gaps between individual photovoltaic cells. These dead zones are typically on the order of 100 to 500 μm. The dead zone resulting from the methods and photovoltaic cells described herein is smaller than about 100 microns. The dead zone of specific aspects is less than about 75 μm. The dead zone or other specific aspects is less than about 50 μm. This decrease in the size of the dead zone may result in significantly less waste in the resultant photovoltaic cells. 
     Reference throughout this specification to “one embodiment,” “certain embodiments,” “one or more embodiments,” “an embodiment,” “one aspect,” “certain aspects,” “one or more embodiments” and “an aspect” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases such as “in one or more embodiments,” “in certain embodiments,” “in one embodiment,” “in an embodiment,” “according to one or more aspects,” “in an aspect,” etc., in various places throughout this specification are not necessarily referring to the same embodiment or aspect of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or aspects. The order of description of the above method should not be considered limiting, and methods may use the described operations out of order or with omissions or additions. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of ordinary skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.