Patent Publication Number: US-2013247976-A1

Title: Solar cell

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of Taiwanese application no. 101109895, filed on Mar. 22, 2012, which is hereby incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     This invention relates to a solar cell, more particularly to a crystalline silicon solar cell having a local back surface field structure. 
     BACKGROUND OF THE INVENTION 
     Referring to  FIG. 1 , a conventional solar cell includes a silicon substrate  11 , an emitter layer  12 , a dielectric layer  13 , a plurality of local back surface field structures  14 , and a back electrode  15 . 
     The silicon substrate  11  and the emitter layer  12  are formed with a p-n junction therebetween. The dielectric layer  13  is formed on a rear surface  111  of the silicon substrate  11  and is formed with a plurality of spaced-apart circular through holes  131 . The local back surface field structures  14  are formed under the rear surface  111  of the silicon substrate  11  corresponding in position to the through holes  131 . The local back surface field structures  14  have a doping concentration larger than that of the silicon substrate  11 . The back electrode  15  is formed by screen printing aluminum paste on the dielectric layer  13 , followed by firing the aluminum paste at high temperature (about 700° C. to 800° C.). A portion of the aluminum paste flows into the through holes  131  and thus, the rear electrode  15  has a surface layer portion  151  that is laminated on the dielectric layer  13 , and a plurality of contact portions  152  respectively extending into the through holes  131  to contact the silicon substrate  11 . During firing process, the aluminum in the aluminum paste would be mixed with the silicon of the silicon substrate  11  so as to form the local back surface field structures  14  made of Al—Si alloy in the silicon substrate  11 . The local back surface field structures  14  improve carrier collection efficiency and photoelectric conversion efficiency. 
     In practice, at the high firing temperature, silicon of the silicon substrate  11  has high diffusibility in aluminum of the aluminum paste. Since, in the conventional solar cell, the back electrode  15  is formed into a continuous large area, the same cannot provide confinement to silicon diffusibility. It is thus not favorable to the formation of the local back surface field structures  14  since silicon flows outwardly from the silicon substrate  11  into the back electrode  15  and the local back surface field structures  14  has less amount of silicon. If outflow of silicon becomes more severe, a cavity  10  might be formed between the rear surface  111  of the silicon substrate  11  and the back electrode  15  (see  FIG. 2 ). The cavity  10  would adversely influence conduction performance of the back electrode  15  and the quality of the local back surface field structures  14 . Referring to  FIGS. 3 and 4 , another conventional solar cell includes a silicon substrate  11 , a dielectric layer  13 , a plurality of local back surface field structures  14 , and a back electrode  15  formed on the dielectric layer  13 . The back electrode  15  has two linear busbars  153  and three conductive portions  154  separated by the linear busbars  153 . The dielectric layer  13  is formed with a plurality of through holes  131  that are in the form of a slot and that are perpendicular to the linear busbars  153 . Similarly, each of the conductive portions  154  of the back electrode  15  has a surface layer portion  155  and a plurality of contact portions  156  that respectively extend into the through holes  131  to contact the local back surface field structures  14 . As such, diffusion of silicon from the silicon substrate  11  would occur, thereby resulting in formation of a plurality of cavities and drawbacks attributed thereto. 
     Referring to  FIGS. 5 and 6 , to overcome the drawbacks occurred of the conventional solar cell shown in  FIGS. 3 and 4 , the back electrode  15  is modified to be not formed into a continuous area. To be specific, the back electrode  15  includes a plurality of spaced-apart conductive sections  157  each of which has a first portion  158  formed on the dielectric layer  13  and a contact portion  159  that extends into a respective one of the through holes  131 . In this configuration, since the back electrode  15  is divided into a plurality of spaced-apart conductive sections  157 , diffusion of the silicon from the silicon substrate  11  is confined. Besides, each of the conductive sections  157  has a limited size so that saturation of silicon would be quickly achieved and diffusion of silicon would be limited. Accordingly, formation of the cavities would may be alleviated. 
     However, in the configuration shown in  FIGS. 5 and 6 , the back electrode  15  has a relatively small area, thereby resulting in inferior conductivity and high series resistance. Also, as shown in  FIGS. 5 and 6 , current in one of the conductive sections  157  should be transmitted to another one of the conductive sections  157  through the busbars  153 . Accordingly, the current transmission and electrical conductivity are adversely affected. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a solar cell that can overcome cavity problem and inferior conductivity associated with the prior art. 
     Accordingly, a solar cell of this invention includes: 
     a substrate having a front surface and a back surface opposite to the front surface; 
     an emitter layer formed in the substrate under the front surface; 
     a dielectric layer disposed on the back surface and having at least two through holes to expose the back surface; 
     at least two first electrode layers formed on the dielectric layer and respectively filling in the through holes to contact the substrate; 
     at least one second electrode layer entirely formed on the dielectric layer and disposed between the first electrode layers; and 
     at least one third electrode layer filled in a space which is substantially defined by the second electrode layer and one of the first electrode layers so that the at least one third electrode layer interconnects the second electrode layer and the one of the first electrode layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments of the invention, with reference to the accompanying drawings, in which: 
         FIG. 1  is a sectional view of a conventional solar cell; 
         FIG. 2  is a scanning electron microscope (SEM) picture showing a cavity formed in the conventional solar cell; 
         FIG. 3  is a bottom view of another conventional solar cell; 
         FIG. 4  is a sectional view taken long line IV-IV in  FIG. 3 ; 
         FIG. 5  is a bottom view of yet another conventional solar cell; 
         FIG. 6  is a sectional view taken along line VI-VI in  FIG. 5 ; 
         FIG. 7  is a bottom view of a first preferred embodiment of a solar cell according to this invention; 
         FIG. 8  is a sectional view taken along line VIII-VIII in  FIG. 7 ; 
         FIG. 9  is a bottom view showing a modification of the first preferred embodiment, which includes a plurality of first electrode layers, third electrode layers, through holes, spaces, and local back surface field structures; 
         FIG. 10  is a scanning electron microscope (SEM) picture of the first preferred embodiment; 
         FIG. 11  is a fragmentary bottom view of a second preferred embodiment of a solar cell according to this invention; 
         FIG. 12  is a fragmentary bottom view of a third preferred embodiment of a solar cell according to this invention; 
         FIG. 13  is a fragmentary bottom view of a fourth preferred embodiment of a solar cell according to this invention; 
         FIG. 14  is a fragmentary bottom view of a fifth preferred embodiment of a solar cell according to this invention; 
         FIG. 15  is a fragmentary bottom view of a sixth preferred embodiment of a solar cell according to this invention; 
         FIG. 16  is a fragmentary bottom view of a seventh preferred embodiment of a solar cell according to this invention; 
         FIG. 17  is a sectional view taken along line XVII-XVII in  FIG. 16 ; 
         FIG. 18  is a fragmentary bottom view of an eighth preferred embodiment of a solar cell according to this invention; 
         FIG. 19  is a fragmentary bottom view of a ninth preferred embodiment of a solar cell according to this invention; and 
         FIG. 20  is a sectional view taken along line XX-XX in  FIG. 19 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before the present invention is described in greater detail, it should be noted herein that like elements are denoted by the same reference numerals throughout the disclosure. 
     Referring to  FIGS. 7 and 8 , a solar cell of the first preferred embodiment according to this invention includes a substrate  2  made of silicon, an emitter layer  23 , a dielectric layer  3 , two first electrode layers  52 , a second electrode layer  511 , four third electrode layers  512 , and two spaces  6  which are formed on the dielectric layer  3  and each of which is defined between the second electrode layer  511  and a respective one of the first electrode layers  52  to isolate the second electrode layer  511  and a respective one of the first electrode layers  52 . 
     The substrate  2  has a front surface  22  and a back surface  21  opposite to the front surface  22 . The front surface  22  is a light incident surface and can be roughened to improve light incidence efficiency. The emitter layer  23  is formed in the substrate  2  under the front surface  22 . A p-n junction is formed between the emitter layer  23  and the substrate  2 . In this embodiment, the substrate  2  is a p-type silicon substrate and the emitter layer  23  is made of a n-type semiconductor material. However, the conductivity types of the substrate  2  and the emitter layer  23  can be interchanged as long as the p-n junction is formed therebetween. 
     Besides, the emitter layer  23  may be formed with an anti-reflection layer (not shown) made of a material such as silicon nitride (SiN x ) for reducing reflection of incident light and surface recombination velocity (SRV) of carriers, and raising light transmittance. In this embodiment, the solar cell is further formed with a front electrode (not shown) for outputting electric power. Since the front electrode and the anti-reflection layer are well known to a skilled artisan, detailed descriptions thereof are omitted herein for the sake of brevity. 
     The dielectric layer  3  is a passivation layer and is disposed on the back surface  21  of the substrate  2  for compensating surface defects of the substrate  2  so as to reduce carrier combination velocity on the back surface  21  and raise photoelectric conversion efficiency of the solar cell. The dielectric layer  3  is made of a material selected from the group consisting of oxides, nitrides, and the combinations thereof. 
     In this embodiment, the dielectric layer  3  is formed with at least two spaced-apart circular through holes  31  to expose the substrate  2 . It should be noted that the shape of the through holes  31  can vary and is not limited by the disclosure in this embodiment. 
     The substrate  2  further includes two local back surface field structures  4  that are formed in the substrate  2  underneath the back surface  21  and respectively exposed from the through holes  31 . In this embodiment, the back surface field structures  4  are made of p-type semiconductor material of aluminum-silicon alloy, and have a doping concentration higher than that of the substrate  2 . By virtue of the electric field of the back surface field structures  4 , electrons movement in the substrate  2  toward the back surface  21  can be prevented and electrons are collected in the emitter layer  23 , thereby improving carrier collection and photoelectric conversion efficiencies of the solar cell. 
     The first electrode layers  52  are formed on the dielectric layer  3  and respectively extend into the through holes  31  to contact the back surface field structures  4 . The second electrode layer  511  is entirely formed on the dielectric layer  3 . The third electrode layers  512  are disposed in the spaces  6  to interconnect the first electrode layers  52  and the second electrode layer  511 . 
     In this embodiment, each of the spaces  6  and a respective one of the first electrode layers  52  are disposed in concentric relation substantially. That is, each of the first electrode layers  52  is surrounded by a respective one of the spaces  6  and is isolated from the second electrode layer  511  by the respective one of the spaces  6 . Two of the third electrode layers  512  are formed in each of the spaces  6  to interconnect the second electrode layer  511  and a respective one of the first electrode layers  52 . The materials for the first electrode layers  52  and the second electrode layer  511  can be the same or different. The third electrode layers  512  are made of a material different from or the same with that of the first electrode layers  52 . Examples of the material for the third electrode layers  512  include aluminum, silver, zinc oxide, and nickel. Preferably, when the third electrode layers  512  are made of a material different from that of the first electrode layers  52 , the material for the third electrode layers  512  has lower diffusibility for silicon than that of the first electrode layers  52 . In this embodiment, the first electrode layers  52  and the second electrode layer  511  are made of aluminum and are formed by screen-printing aluminum paste, and the third electrode layers  512  are made of silver by another screen-printing and are filled in a part of the spaces  6 . 
     The surface area of the dielectric layer  3  which is occupied by the first, second and third electrode layers  52 ,  511 ,  512  is greater than the area of the spaces  6 . 
     The back surface field structures  4  are formed by mixing aluminum in the aluminum paste and silicon of the substrate  2  during forming the first, second, and third electrode layers  52 ,  511 ,  512  by firing process. 
     In this embodiment, since each of the third electrode layers  512  has a small area, diffusion of the silicon from the substrate  2  to the second electrode layer  511  through the first electrode layers  52  and the third electrode layers  512  can be limited. Moreover, silicon concentration in the third electrode layers  512  is easily saturated and thus silicon diffusion would be limited. Accordingly, enough silicon would be confined near the back surface  21  and mixed with aluminum so as to form superior back surface field structures  4 . Without forming the cavities in the solar cell, photoelectric conversion efficiency and electrical conductivity can be improved. 
     Referring to  FIG. 10 , the scanning electron microscope (SEM) image shows that no cavity is formed between the back surface  21  and the first electrode layers  52  and the back surface field structures  4  have sufficient thickness. 
     It should be noted that, the number of each of the elements, e.g., the first electrode layers  52 , the third electrode layers  512 , the through holes  31 , the spaces  6 , and the local back surface field structures  4 , can vary based on actual requirements and should not be limited to the disclosure in this embodiment. For example, a solar cell of this invention can include a plurality of the first electrode layers  52 , the third electrode layers  512 , the through holes  31 , the spaces  6 , and the local back surface field structures  4  (see  FIG. 7 ). 
     Referring to  FIG. 11 , the second preferred embodiment of the solar cell according to this invention is similar to that of the first preferred embodiment except that the solar cell contains one third electrode layer  512  formed in each of the spaces  6 . 
     Referring to  FIG. 12 , the third preferred embodiment of the solar cell according to this invention is similar to that of the first preferred embodiment except that each of the third electrode layers  512  has curved lateral surfaces. 
     Referring to  FIG. 13 , the fourth preferred embodiment of the solar cell according to this invention is similar to that of the first preferred embodiment except that four spaced-apart third electrode layers  512  are formed in each of the spaces  6 , two of which have a rectangular shape, and the other two of which have a trapezoidal shape. 
     Referring to  FIG. 14 , the fifth preferred embodiment of the solar cell according to this invention is similar to that of the first preferred embodiment except that one third electrode layer  512  is formed in each of the spaces  6  and the third electrode layer  512  has a spiral shape and surrounds a respective one of the first electrode layers  52 . 
     Referring to  FIG. 15 , the sixth preferred embodiment of the solar cell according to this invention is similar to that of the first preferred embodiment except that each of the spaces  6  is completely filled with the respective third electrode layer  512 . It should be noted that, in such configuration, the third electrode layer  512  should be made of a material different from that of the first electrode layers  52 , and the material for the third electrode layer  512  should have lower diffusibility for silicon than that of the first electrode layers  52 . In this embodiment, the first and second electrode layers  52 ,  511  are made of aluminum. The third electrode layers  512  in the spaces  6  are made of silver that exhibits low silicon diffusibility. In this embodiment, since each of the first electrode layers  52 , the second electrode layer  511 , and a respective one of the third electrode layers  512  are connected, current transmission and photoelectric conversion efficiency can be improved. 
     In view of the above, when each of the spaces  6  is partly filled with the third electrode layer(s)  512  (as shown in the first to fifth preferred embodiments), the first electrode layers  52  can be made of a material the same with or different from that of the third electrode layer(s)  512 . On the other hand, when each of the spaces  6  is completely filled with the third electrode layer  512  as shown in the sixth preferred embodiment, the material of the third electrode layers  512  is required to be different from that of the first electrode layers  52  and should have relatively low diffusibility for silicon so as to block diffusion of the silicon from the substrate  2  to the second electrode layer  511 . 
       FIGS. 16 and 17  show the seventh preferred embodiment of the solar cell according to this invention, in which the second electrode layer  511 , the first electrode layers  52 , the through holes  31 , and the spaces  6  respectively extend in a first direction, and the first electrode layers  52 , the third electrode layers  512 , the second electrode layer  511 , and the spaces  6  are juxtaposed in a second direction perpendicular to the first direction. The second electrode layer  511  is disposed between the first electrode layers  52 . A plurality of spaced-apart third electrode layers  512  are formed in each of the two spaces  6 . 
     In this embodiment, the through holes  31  and the spaces  6  are in the form of a slot. The second electrode layer  511  and the first electrode layers  52  are made of aluminum. The third electrode layers  512  are made of silver. 
     Referring to  FIG. 18 , the eighth preferred embodiment of the solar cell according to this invention is similar to that of the seventh preferred embodiment except that each of the spaces  6  is completely filled with a respective one of the third electrode layers  512 . 
     Referring to  FIGS. 19 and 20 , the ninth preferred embodiment of the solar cell according to this invention is similar to that of the eighth preferred embodiment except that the second electrode layer  511  and the third electrode layers  512  are made of the same material, i.e., silver. Each of broken lines in  FIG. 19  is used to indicate a boundary between the second electrode layer  511  and a respective one of the third electrode layers  512 . The first electrode layers  52  are made of aluminum. Each of the first electrode layers  52  has an end extending over a surface of a respective one of the third electrode layers  512  opposite to the dielectric layer  3 . 
     It should be noted that, in each of the preferred embodiments of this invention, the number of each of the elements included in the solar cell can vary based on actual requirements. The rules of material selection for the first, second, and third electrode layers  52 ,  511 ,  512  in the seventh to ninth preferred embodiments are the same with those in the first to sixth preferred embodiments. Moreover, the solar cell in the seventh, eighth, or ninth embodiment may further include a busbar (not shown). 
     Example 
     The conventional solar cells shown in  FIGS. 3 and 5  were used as Comparative examples 1 and 2 to compare with the solar cell of the ninth preferred embodiment with respect to series resistance and cavity percentage. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Series Resistance 
                 Cavity Percentage 
               
               
                   
                 (mΩ) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Comparative 
                 4.38 
                 57.66 
               
               
                   
                 Example 1 
               
               
                   
                 Comparative 
                 6.66 
                 12.38 
               
               
                   
                 Example 2 
               
               
                   
                 Ninth preferred 
                 4.63 
                 28.97 
               
               
                   
                 embodiment 
               
               
                   
                   
               
            
           
         
       
     
     Referring to Table 1, the solar cell of the ninth preferred embodiment has a series resistance much smaller than that of Comparative Examples 2, and has a cavity percentage much lower than that of Comparative Examples 1. In this embodiment, a trade-off between the series resistance and the cavity percentage is reached, thereby simultaneously improving electrical conductivity and photoelectric conversion efficiency of the solar cell. 
     While the present invention has been described in connection with what are considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretations and equivalent arrangements.