Patent Publication Number: US-2010122727-A1

Title: Method for fabricating III-V compound semiconductor solar cell and structure thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a solar cell, and more particularly, to a III-V compound semiconductor solar cell and its fabrication method. 
     2. Background of the Related Art 
     The intensity of the incident light onto an absorption layer, indicative of the absorption efficiency, is an important factor affecting the conversion efficiency of the solar cell. In general, the higher absorption efficiency, is, the higher conversion efficiency is. 
     A surface texture process is commonly applied on the surface of the solar cell to increase the absorption efficiency (reducing reflection). For example, a silicon solar cell is soaked into KOH solution of an anisotropic etching process to form pyramid like textures on the surface of the silicon solar cell. Since the III-V compound semiconductor materials, such as GaAs, InP, or InGaP, are being developed to application of the solar cell due to its higher conversion efficiency of photovoltaic feature. To increase the benefit of the application and to enhance the conversion efficiency of the III-V compound solar cell, forming a texture surface on the III-V compound semiconductor solar cell can be applied. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to provide a method for fabricating a III-V compound semiconductor solar cell and a structure thereof. The method comprises applying the lithography and etching process to form a periodic array of hole textures on the surface of III-V compound semiconductor solar cell. The texture on the surface of the solar cell increases the transmission of the incident light and the absorption of the incident light. The sunlight passes through the holes, reaches the absorption layer and is absorbed thereby to increase the generated short-circuit current and open-circuit voltage. Therefore, the conversion efficiency of photovoltaic of the solar cell can be enhanced. 
     An example method for fabricating a III-V compound semiconductor solar cell is described to illustrate an embodiment of this the present invention as follows, which includes providing a solar cell structure comprising a window layer made of III-V compound material formed on the top surface of the solar cell structure, a periodic array of hole textures is formed of the window layer by using the lithography and etching process to form a patterned window layer. Next, an anti-reflection coating film is formed to cover the patterned window layer. 
     An example structure of a III-V compound semiconductor solar cell is described to illustrate an embodiment of the present invention as follows. The solar cell structure comprises the patterned window layer made of III-V compound material formed over the top surface of the solar cell structure. The patterned window layer comprises a periodic array of holes and an anti-reflection coating film covering the patterned window layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  to  FIG. 1   c  illustrates the flow diagrams that illustrate a method for fabricating III-V compound semiconductor solar cell in accordance with an embodiment of the present invention; 
         FIG. 2  shows the SEM picture from the top view of a III-V compound semiconductor solar cell structure in accordance with an embodiment of the present invention; 
         FIG. 3  shows the reflectance of the different process treatments on the top surface of the solar cells as a function of the wavelength. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1   a  to  FIG. 1   c  are the flow diagrams that illustrate a method for fabricating a III-V compound semiconductor solar cell in accordance with an embodiment of the present invention. It should be mentioned that the structure of the solar cell illustrated in the drawings are not illustrated in its actual dimensions. First, referring to  FIG. 1   a , a solar cell structure  10  is provided, which comprises a substrate  12 , an absorption layer  14  formed on the substrate  12 , a window layer  16  formed on the absorption layer  14 , an upper metal electrode  18  formed on the window layer  16  and a rear contact  20  formed below the substrate  12 . The window layer  16  is made of III-V compound material, such as an InAlP layer. Next, referring to  FIG. 1   b , a period of holes  22  is formed of the window layer  16  to form a patterned window layer  16 ′. In an embodiment of the present invention, the holes may be arranged in an array and the process of forming the holes in the window layer  16  includes, for example but not limited to, a photolithography process, which includes using a mask (not shown) and performing an exposure process to transfer a pattern over the top surface of the solar cell structure  10 , a surface of window layer  16  accordingly, and then performing an etching process to form the holes  22  of the window layer  16  to form the patterned window layer  16 ′. Next, referring to  FIG. 1   c , an anti-reflection coating film  24  is formed over the patterned window layer  16 ′ covering the holes  22  by using a suitable method, for example, sputtering or evaporation method. 
       FIG. 1   c  is a schematic plot that shows a III-V compound semiconductor solar cell structure. As shown in  FIG. 1   c , the III-V compound semiconductor solar cell  10  comprises the substrate  12 , the absorption layer  14 , the patterned window layer  16 ′, a front contact  18 , a rear contact  20  and the anti-reflection coating film  24 . The absorption layer  14  is formed on the substrate  12 , and the absorption layer  14  comprises a single-junction structure or a multi-junction structure. The patterned window layer  16 ′ is formed on the absorption layer  14 , wherein the patterned window layer  16 ′ comprises a pattern of a periodic array of holes  22  formed therein. The anti-reflection coating film  24  covers the patterned window layer  16 ′. The front contact  18  is formed over portion of the patterned window layer  16 ′, and the rear contact  20  is formed below the substrate  12 . 
     In the present embodiment, a thickness of the window layer  16  is between 200 nm and 300 nm. Various masks may be applied in the lithography process according to the size of the holes  22  and the density of the periodic array of holes for satisfying the requirement of the solar cell. In the present embodiment, the size of hole is between 5 μm and 20 μm. 
       FIG. 2  shows the SEM picture from the top view of a III-V compound semiconductor solar cell structure. As illustrated in  FIG. 2 , the distribution of the holes is periodic and forms a texture on top of the III-V compound semiconductor solar cell surface  10 . 
     In the present embodiment, the patterned window layer  16 ′ not only increases the surface area of the light incident surface of the III-V compound semiconductor solar cell  10  but also serve to trap the incident light. When the light is incident on the anti-reflection coating film  24 , some of the incident light transmits through the holes  22  and are absorbed by the absorption layer  14 , while some of the incident light may strike on the sidewalls of the holes  22  at different incident angles and are repeatedly reflected between the sidewalls of the holes  22  and are directed toward the absorption layer  14 , which are ultimately absorbed by the absorption layer  14 . Thus, the patterned window layer, which forms the texture surface, serves to trap the incident light and enhance transmission of the incident light. Owing to the different sizes of the hole, the conversion efficiency of the solar cell varies correspondingly. Table 1 lists the parameters and conversion efficiency of different solar cell structures, for example, a traditional solar cell structure without the patterned window layer with array of holes (hereinafter, referred to as traditional structure), a solar cell structure with the patterned window layer including 10 μm holes (hereinafter, referred to as 10 μm structure), and the solar cell structure with the patterned window layer including 5 μm holes (hereinafter, referred to as 5 μm structure). As can be inferred from Table 1, the efficiencies of the traditional structure, 10 μm structure and 5 μm structure are 13.86%, 15.18% and 15.93% under AM 1.5 g (100 mW/cm 2 ) illumination at 25° C., respectively. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 the parameters and conversion efficiency of different solar cell 
               
               
                 structures 
               
            
           
           
               
               
               
               
            
               
                 AM 1.5 g 
                 traditional structure 
                 10 μm structure 
                 5 μm structure 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 J sc  (mA/cm 2 ) 
                 13.36 
                 14.19 
                 14.8 
               
               
                 V oc  (V) 
                 1.33 
                 1.41 
                 1.41 
               
               
                 FF 
                 0.78 
                 0.759 
                 0.763 
               
               
                 J m  (mA/cm 2 ) 
                 12.62 
                 13.02 
                 13.13 
               
               
                 V m  (V) 
                 1.1 
                 1.17 
                 1.16 
               
               
                 P m  (mW) 
                 13.86 
                 15.18 
                 15.93 
               
               
                 Efficiency (%) 
                 13.86 
                 15.18 
                 15.93 
               
               
                   
               
            
           
         
       
     
       FIG. 3  shows the reflectance of the different process treatments on the top surface of the solar cells as a function of the wavelength. Also, the solar spectrum is illustrated by the curve illustrated in  FIG. 3 . As can be seen from  FIG. 3 , the 10 μm structure and the 5 μm structure both have relatively lower reflectance when compared to the traditional structure. 
     In a summary, the present invention proposes forming a texture surface of the solar cell to increase the surface area of the light absorption region and the incident light transmission to enhance the light absorption of a III-% V compound solar cell. Thus, the generated short-circuit current and open-circuit voltage are increased and therefore the conversion efficiency of the III-V compound solar cell are enhanced, respectively. 
     Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that other modifications and variation can be made without departing the spirit and scope of the invention as hereafter claimed.