Abstract:
A method of fabricating a differential doped solar cell is described. The method includes the following steps. First, a substrate is provided. A doping process is conducted thereon to form a doped layer. A heavy doping portion of the doped layer is partially or fully removed. Subsequently, an anti-reflection coating layer is formed thereon. A metal conducting paste is printed on the anti-reflection coating layer and is fired to form the metal electrodes for the solar cell.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority to Taiwan Application Serial Number 97135526, filed Sep. 16, 2008, which is herein incorporated by reference. 
       FIELD OF THE INVENTION  
       [0002]    The present invention generally relates to a method of fabricating a solar cell. More particularly, this invention relates to a method of fabricating a differential doped solar cell. 
       BACKGROUND OF THE INVENTION  
       [0003]    The solar cell design in widespread use today has a p/n junction formed near the front surface (a surface which receives the light) which creates an electron flow as light energy is absorbed in the cell. A conventional cell design has one set of electrical contacts on the front side of the cell, and a second set of electrical contacts on the rear side of the solar cell. In a typical photovoltaic module these individual solar cells are interconnected electrically in series to increase the voltage. 
         [0004]    This interconnection is typically accomplished by soldering a conductive ribbon from the front side of one solar cell to the rear side of an adjacent solar cell. 
         [0005]    Therefore, the conventional solar cell adopts a p-type substrate and forms a thin n-type semiconductor layer on the p-type substrate. Before the diffusion process, a surface texturing is formed thereon and then an anti-reflection coating layer is coated to reduce the reflection of the light. Subsequently, a screen printing process is conducted to apply the silver (Ag) paste and the alumina (Al) paste on the surfaces of the wafer by a screen printing technology. After that a fire process is conducted in a high temperature oven to sinter the Al and Ag on the wafer surfaces to form the Al—Si alloy and the Ag—Si alloy on the respective surfaces of the wafer with the ohmic contact. Therefore, the conductive electrodes are formed on the surfaces of the wafer and the conventional solar cell is achieved. 
         [0006]    However, while forming the conductive electrodes of the solar cell, a high doping concentration is required to reduce the contact resist on the conductive area. On the contrary, on the emitter area, the doping concentration has to be limited to improve the short wave frequency response for the solar cell. There is a need to balance or choose the doping concentration on the two areas of the solar cell to improve the energy conversion efficiency for the solar cell. 
       SUMMARY OF THE INVENTION  
       [0007]    One objective of the present invention is to provide a method of fabricating a differential doped solar cell to form a high doping concentration in the electrode area and a low doping concentration in the emitter area to improve the energy conversion efficiency of the solar cell. 
         [0008]    To achieve these and other advantages and in accordance with the objective of the present invention, as the embodiment broadly describes herein, the present invention provides a method of fabricating a differential doped solar cell. First, a substrate is provided. A doped layer having a heavily doped portion and a lightly doped portion under the heavily doped portion is formed on the substrate. The heavily doped portion of the doped layer is partially or fully removed. An anti-reflection coating layer is subsequently formed on the substrate, and a patterned metal conducting paste is formed on the anti-reflection coating layer. After that, the patterned metal conducting paste is fired to form metal electrodes for a solar cell. 
         [0009]    An aspect of the present invention is to provide a method of fabricating a differential doped solar cell including the following steps: providing a substrate; forming a doped layer having a heavily doped portion and a lightly doped portion under the heavily doped portion; forming a patterned etching resist layer; acting the etching resist layer as a mask to remove an exposed portion of the heavily doped portion of the doped layer; removing the etching resist layer; forming an anti-reflection coating layer; forming a patterned metal conducting paste on the anti-reflection coating layer; and firing the metal conducting paste to form metal electrodes of a solar cell. 
         [0010]    In addition, the step of forming a patterned etching resist layer further includes applying an etching paste on an etching resist layer and firing the etching paste to form the patterned etching resist layer. It is worth noting that the metal conducting paste is formed above residual portions of the heavily doped portion of the doped layer. The etching resist layer is made of silicon nitride or silicon oxide. 
         [0011]    Another aspect of the present invention is to provide a method of fabricating a differential doped solar cell including the following steps: providing a substrate; forming a doped layer having a heavily doped portion and a lightly doped portion under the heavily doped portion; removing the heavily doped portion of the doped layer; forming a patterned diffusion barrier layer; acting the patterned diffusion barrier layer as a mask to dope an exposed portion of the lightly doped portion of the doped layer to form a heavily doped area; removing the patterned diffusion barrier layer; forming an anti-reflection coating layer; forming a metal conducting paste on the anti-reflection coating layer; and firing the metal conducting paste to form metal electrodes of a solar cell. 
         [0012]    The patterned diffusion barrier layer preferably includes the steps of applying an etching paste on a diffusion barrier layer and firing the etching paste to form the patterned diffusion barrier layer. In addition, the patterned metal conducting paste is formed above the heavily doped area. 
         [0013]    Furthermore, a sheet resistance of the heavily doped portion is about 10-50 Ohm/sq. and a sheet resistance of the lightly doped portion is higher than 50 Ohm/sq. The substrate is a p-type substrate and the doped layer is an n-type doped layer, for example, a phosphorus ion doped layer. 
         [0014]    Therefore, the method of fabricating a differential doped solar cell according to the present invention only needs a single time of applying the etching paste to pattern the resist layer to form desired heavily doped area and lightly doped area on the substrate of the solar cell. Accordingly, the present invention can form a heavily doped area at the desired position for the metal electrodes to effectively reduce the contact resistance thereof and a lightly doped area on the emitter region of the solar cell to effectively improve the short wave frequency response for the solar cell. The heavily doped area can be formed by one or two times doping processes without departing from the spirit and scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0015]    The foregoing aspects and many of the attendant advantages of this invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0016]      FIGS. 1A to 1H  illustrate a preferred embodiment of a method of fabricating a differential doped solar cell according to the present invention; and 
           [0017]      FIGS. 2A to 2H  illustrate another preferred embodiment of a method of fabricating a differential doped solar cell according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    The following description is of the best presently contemplated mode of carrying out the present invention. This description is not to be taken in a limiting sense but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined by referencing the appended claims. 
         [0019]      FIGS. 1A to 1H  illustrate a preferred embodiment of a method of fabricating a differential doped solar cell according to the present invention. First, refer to  FIG. 1A . To manufacture a solar cell differential doped solar cell, a substrate  110  is first doped with ions, for example, phosphorus ions. Since the dopants enter the substrate  110  from the surface of the substrate  110 , a heavily doped portion  112  is formed on the surface of the substrate  110  and a lightly doped portion  111  is therefore formed underneath the heavily doped portion  112 . In a preferred embodiment, the substrate  110  is a p-type substrate and an n-type semiconductor layer is formed thereon through an ion doping process or a high temperature diffusion process. In addition, the doping concentration in the lightly doped portion  111  is lower than the doping concentration in the heavily doped portion  112 . In a preferred embodiment, the sheet resistance of the heavily doped portion  112  is about 10-50 Ohm/sq., and the sheet resistance of the lightly doped portion  111  is higher than 50 Ohm/sq. When the doping concentration of the lightly doped portion  111  is lower than the doping concentration of the heavily doped portion  112 , a higher energy conversion efficiency of the solar cell and a lower contact resistance for the conductive electrodes of the solar cell can be achieved. Subsequently, an etching resist layer  120  is formed on the heavily doped portion  112 . In a preferred embodiment, the etching resist layer  120  can be made of silicon nitride, silicon oxide and so on. 
         [0020]    Further refer to  FIG. 1B . An etching paste  130  is printed on the etching resist layer  120  in a screen printing process. Afterward the etching paste  130  is heated to form an opening  140  in the etching resist layer  120  so as to form a patterned etching resist layer  121 , referring to  FIG. 1C . Alternatively, the etching resist layer  120  can be patterned through a photolithograph process. 
         [0021]    In addition, the patterned etching resist layer  121  acts as a mask to further etch the heavily doped portion  112  of the substrate  110 , for example, using a wet etching process or a dry etching process to remove a part of the heavily doped portion  112  and form the opening  141  in the heavily doped portion  112  so as to expose the lightly doped portion  111 . 
         [0022]    From  FIGS. 1E to 1F , the patterned etching resist layer  121  is removed and an anti-reflection coating layer  150  is then formed on the substrate  110 , that is, on the residual part of the heavily doped portion  112  and the exposed lightly doped portion  111 . 
         [0023]    From the  FIGS. 1G to 1H , a metal paste  160 , for example, a silver paste or an aluminum paste is formed on the anti-reflection coating layer  150  through the screen printing process. The metal paste  160  is then fired at high temperature to etch through the anti-reflection coating layer  150  and form a eutectic structure with the heavily doped portion  112  so as to form metal electrodes  161  for the solar cell. 
         [0024]    Accordingly, this preferred embodiment of the present invention can form a heavily doped portion at the desired position for the metal electrodes to effectively reduce the contact resistance thereof and a lightly doped layer on the emitter region of the solar cell to effectively improve the short wave frequency response for the solar cell. 
         [0025]    Referring  FIGS. 2A to 2H , another preferred embodiment of a method of fabricating a differential doped solar cell according to the present invention is illustrated. First, a substrate  210  for a solar cell is doped with ions, for example, phosphorus ions. Since the dopants enter the substrate  210  from the surface of the substrate  210 , a heavily doped portion  212  is formed on the surface of the substrate  210  and a lightly doped portion  211  is therefore formed underneath the heavily doped portion  212 . Similarly, the substrate  210  is preferably a p-type substrate and an n-type semiconductor layer is formed thereon through the ion doping process or the high temperature diffusion process. In addition, the doping concentration in the lightly doped portion  211  is lower than the doping concentration in the heavily doped portion  212 . In a preferred embodiment, the sheet resistance of the heavily doped portion  212  is about 10-50 Ohm/sq., and the sheet resistance of the lightly doped portion  211  is higher than 50 Ohm/sq. However, when the doping concentration of the lightly doped portion  211  is lower than the doping concentration of the heavily doped portion  212 , a higher energy conversion efficiency of the solar cell and a lower contact resistance for the metal electrodes of the solar cell can be achieved. Subsequently, the heavily doped portion  212  is removed, for example, by etching back process or any other removing process, and the lightly doped portion  211  is still kept. 
         [0026]    Referring to  FIG. 2C , a patterned diffusion barrier layer  221  is formed. The patterned diffusion barrier layer  221  can be formed by the etching paste as shown in  FIG. 1B . Alternatively, the patterned diffusion barrier layer  221  can be formed through a photolithograph process. 
         [0027]    In  FIG. 2D , a second doping process is conducted to further dope the substrate  210  and the lightly doped portion  211  under the opening  222  so as to form a heavily doped area  230  therein. In  FIG. 2E , the patterned diffusion barrier layer  221  is removed. Subsequently, an anti-reflection coating layer  250  is formed thereon, in  FIG. 2F . 
         [0028]    Refer to  FIG. 2G . A metal paste  240 , for example, a silver paste or an aluminum paste, is formed on the anti-reflection coating layer  250  and positioned above the heavily doped area  230  through the screen printing process. Refer to  FIG. 2H . The metal paste  240  is then fired at high temperature to etch through the anti-reflection coating layer  250  and form a eutectic structure with the heavily doped area  230  so as to form metal electrodes  260  for the solar cell. 
         [0029]    This preferred embodiment of the present invention uses two times doping processes to form a heavily doped area at the desired position for the metal electrodes to effectively reduce the contact resistance thereof and a lightly doped area on the emitter region of the solar cell to effectively improve the short wave frequency response for the solar cell. In addition, the two times doping process use only one time etching paste process or photolithograph process to form the metal electrodes for the solar cell so that the etching paste process or the photolithograph process can be saved in the two times doping process and the energy conversion efficiency of the solar cell is also improved. 
         [0030]    As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrative of the present invention rather than limiting of the present invention. It is intended that various modifications and similar arrangements be included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures.