Patent Document

BACKGROUND 
     The present invention relates to solar cells, and more specifically, to a method and resultant structure that improves the efficiency of fabricating solar cells. 
     Solar cells are fabricated using a number of processes. For example, an annealing process is often used to create a metallic contact grid that contacts doped regions of a cell substrate. During manufacturing, the annealing process allows the metal of the contact grid, initially formed on top of an anti-reflective coating layer, to diffuse through the anti-reflective coating layer and come in contact with the underneath cell substrate. The annealing process typically includes heating the cell components for a time period that is affected and/or dictated by the thickness of the anti-reflective coating layer. 
     BRIEF SUMMARY 
     According to one embodiment of the present invention, a method for fabricating a cell structure includes doping a substrate to form a N-region and a P-region, disposing a first anti-reflective layer on the substrate, disposing a metallic contact paste on the first anti-reflective layer, drying the metallic contact paste to form contacts, disposing a second anti-reflective layer on the first anti-reflective layer and the metallic contacts, and heating the cell structure, wherein heating the cell structure results in metallic contact material penetrating the first anti-reflective layer and contacting the substrate. 
     According to another embodiment of the present invention, a solar cell includes a substrate having an N-region and a P-region, a first anti-reflective layer disposed on the substrate, a metallic contact disposed on the first anti-reflective layer, a second anti-reflective layer disposed on the first anti-reflective layer and the metallic contact, and a region partially defined by the first anti-reflective layer and the second anti-reflective layer having diffused metallic contact material operative to form a conductive path to the substrate through the first anti-reflective layer, the metallic contact, and the second anti-reflective layer. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  illustrates a perspective view of an exemplary embodiment of a cell structure according to present invention. 
         FIGS. 2A-2F  illustrate a cross-sectional view of an exemplary fabrication method of the cell structure of  FIG. 1  according to one embodiment of present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary embodiment of a cell structure  100  having a substrate with a doped P++ region  110 , a P− region  120 , and an N++ region  130 . An anti-reflective coating layer  140  (AR) is disposed on the substrate on top of N++ region  130 , contacts  150  are arranged in a grid via AR layer  140  and in contact with the region  130 . P++ region  110  has a back surface field  160  and bottom contacts  170  are arranged on the back surface field  160 . 
     In a conventional fabrication process of fabricating solar cells, which may create a structure of solar cell similar but not identical to the one shown in  FIG. 1 , an AR layer, similar to AR layer  140 , is disposed on a cell substrate at a thickness x with the thickness x meeting the designed specifications for the desired anti-reflective properties of the solar cell structure. A silver paste (Ag) is then disposed on the AR layer to define a contact grid similar to the contacts  150 . Once the Ag paste dries, the structure is subjected to a high temperature annealing process of approximately 900 degrees C. for typically over one hour. The annealing process causes the Ag paste to diffuse or penetrate the AR layer underneath and contact the region below, and in the meantime remain at least partially exposed on the top for electrical connections. Generally, the time of annealing may be calculated by the equation t 1 =(AR layer thickness) 2 /D, where D is a coefficient value determined by various other factors. The thickness, which may be denoted by x, is a factor in determining the duration of the annealing process in that a greater x results in a greater annealing process time t 1 , depending on the actual thickness of the AR layer. The exemplary methods described below reduce the annealing process time t. 
       FIGS. 2A-2F  illustrate a cross-sectional view of an exemplary fabrication method of a cell structure  200  according to one embodiment of present invention. Referring to  FIG. 2A , a p-type silicon substrate  202  is doped to form an N region  204 . The N region may be an N+ region in alternate embodiments, while the p-type substrate  202  may include P− and/or P++ regions. 
     Referring to  FIG. 2B , a first anti-reflective coating layer  206  (AR layer) is disposed on the N region  204 . The first anti-reflective coating layer  206  may include for example, a dielectric layer of silicon nitride (SiN y ), a silicon oxide (SiO y )/SiN y  combination, or other suitable materials. The first AR layer  206  may be deposited to have a thickness less than the total desired thickness x of the AR layer for the completed cell structure  200 . The total desired thickness x is, for example, between 50 and 100 nm, and is uniquely dependant on, and determined by the refractive index of the AR layer, for example, some factors such as the wavelength of light applied to the solar cell may interact. In one embodiment, for example, the first AR layer  206  may have a thickness of about half the thickness x, that is, ½x. However, embodiments of the present invention may include other thickness as well, that are larger or smaller than ½x. 
     Referring to  FIG. 2C , Ag paste or other types of paste that are suitable for making contacts for cell structure  200  is disposed on the first AR layer  206  by, for example, a lithographic patterning and deposition process, and dried to define contacts  208 . The paste is disposed on the AR layer  206 , instead of directly on top of N region  204 , to avoid contamination of the N region  204  in the deposition process. 
     Referring to  FIG. 2D , a second AR layer  210  is disposed on the first AR layer  206  and the contacts  208 . The second AR layer  210  may have a thickness which, when being combined together with the thickness of the first AR layer  206 , may provide the desired total thickness x, to function as a single AR layer, for the completed cell structure  200 . For example, in one embodiment when the first AR layer  206  has a thickness of ½x, the second AR layer  210  may have a thickness ½x as well to make a total thickness x of AR layers. 
     Referring to  FIG. 2E , the structure  200  is heated in an annealing process. The annealing process includes heating the structure to approximately 900 degrees C. or any other suitable temperatures for a time duration t 2 . According to one embodiment of the present invention, since contacts  208  need to be in contact with N region  204  and also be exposed on the top for electrical connection, the annealing may be determined by the larger of time needed to diffuse both first AR layer  206  and second AR layer  210 , and may be calculated by the equation t 2 =(layer thickness) 2 /D, where D is a coefficient value determined by various other factors. In the above illustrated embodiment, the time t 2  of annealing maybe reduced, compared to t 1 , by making the thickness of first AR layer  206  approximately the same as that of second AR layer  210 , both being equal to approximately ½x. Other combinations of thicknesses are also possible, although the reduction in time of annealing will be less. Based upon the above equation, the time t 2  for annealing first AR layer  206  (and second AR layer  210 ) with a thickness of ½x is approximately ¼ of the time t 1  that will otherwise be required to anneal an AR layer of thickness x. The annealing process results in contact  208  material diffusing or penetrating through the first AR layer  206  to contact the N region  204 , and penetrating through the second AR layer  210  in the regions above the contacts  208 . Since contact  208  material diffuses into surrounding AR layers, the size of contacts  208  becomes larger, resulting in outer regions  220  of dielectric material, having contact  208  material being diffused therein, that promotes electrical conductivity, as shown in  FIG. 2E . A resultant AR layer  212  comprising the first AR layer  206  and the second AR layer  210  having a combined thickness x is also shown in  FIG. 2E . In the illustrated embodiment, the first and second AR layers  206  and  210  are about the same thickness, however other embodiments may include combinations of AR layers having different thickness (e.g., ¼x and ¾x) above and below contacts  208 . Although such an arrangement would still reduce the time t 2  for annealing as discussed above, AR layers with similar thicknesses offer an even larger reduction in annealing time t 2 . 
     Referring to  FIG. 2F , a back side contact  214  is formed on the underside of the p-type silicon substrate  202 . The graph  201  illustrates the concentration gradient of contact material in the regions  220 . A slight diffusion of the contact material is also present in the N region  204 , which promotes electrical contact with the N region  204 . The arrow  221  illustrates an example of a conductive path including the region  220  through the second anti-reflective layer  210 ; the contact material  208 ; and the region  220  through the first anti-reflective layer  206 , which contacts the N-region  204  of the substrate. 
     The embodiments illustrated above include the deposition of two AR layers; however, other embodiments may include the deposition of a plurality of AR layers that will result in a resultant AR layer  212  having a thickness x. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one ore more other features, integers, steps, operations, element components, and/or groups thereof 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated 
     While the preferred embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.

Technology Category: 4