Patent Publication Number: US-2012037228-A1

Title: Thin-Film Photovoltaic Cell Having Distributed Bragg Reflector

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application Ser. No. 61/372,101, filed Aug. 10, 2010, which is herein incorporated by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to a thin-film photovoltaic cell. More particularly, the disclosure relates to a thin-film photovoltaic cell having a back reflector. 
     2. Description of Related Art 
     Photovoltaic cells, commonly known as solar cells, are well known devices that convert light energy into electricity. Therefore, how to increase the photoelectric conversion efficiency is always an important issue in photovoltaic system. One way is to place a back reflector beneath photoactive semiconductor layers to reflect light unabsorbed by the semiconductor layers to back through the semiconductor layers for further absorption. Accordingly, the use of a back reflector can increase the cell efficiency of the photovoltaic cells. 
     SUMMARY 
     Accordingly, a novel back reflector is provided for photovoltaic cells. This novel back reflector is a distributed Bragg reflector (DBR) containing conductive nanostructures therein. Therefore, the resistivity of the novel DBR can be largely reduced, and the novel DBR can also serve as a metal back contact at the same time. The conductive nanostructures above can be metal nanoparticles or metal thin films distributed in the multi-layered DBR structure. 
     It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional diagram of a conventional photovoltaic cell. 
         FIG. 2  is a cross-sectional diagram of a photovoltaic cell according to one embodiment of this invention. 
         FIGS. 3A-4B  are cross-sectional diagrams of distributed Bragg reflectors for photovoltaic cells according to embodiments of this invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
       FIG. 1  is a cross-sectional diagram of a conventional photovoltaic cell. In  FIG. 1 , a conventional photovoltaic cell includes the following layers of a glass substrate  110 , a first transparent conductive oxide (TCO) layer  120 , a semiconductor layer  130 , a second transparent conductive oxide layer  140 , and a metal back contact  150 . The semiconductor layer  130  usually includes an p-doped semiconductor layer  132 , an intrinsic semiconductor layer  134 , and a n-doped semiconductor layer  136 . The semiconductor layer  130  can be amorphous silicon layer. The first transparent conductive oxide layer  120  can be a tin dioxide layer. The second transparent conductive oxide layer  140  can be an aluminum doped zinc oxide (AZO) layer or a gallium doped zinc oxide (GZO) layer. 
       FIG. 2  is a cross-sectional diagram of a photovoltaic cell according to an embodiment of this invention. In  FIG. 2 , the second transparent conductive oxide layer  140  and a metal back contact  150  are replaced by a novel distributed Bragg reflector  160 , which serves as a back reflector and a metal back contact for photovoltaic cells. 
       FIGS. 3A-4B  are cross-sectional diagrams of distributed Bragg reflectors for photovoltaic cells according to embodiments of this invention. In  FIGS. 3A-4B , the distributed Bragg reflector  160  comprises multiple first refractive layers  162  and multiple second refractive layers  164 , which are alternatively stacked. The refractive index of the first refractive layers  162  is higher than that of the second refractive layers  164 . Please refer to both  FIGS. 2 and 3A , the semiconductor layer  130  contacts the upmost first refractive layer  162 . Generally, the reflectance (R) of a distributed Bragg reflector can be determined by the following simplified formula. 
     
       
         
           
             R 
             = 
             
               1 
               = 
               
                 4 
                  
                 
                   
                     n 
                     air 
                   
                   
                     n 
                     s 
                   
                 
                  
                 
                   
                     ( 
                     
                       
                         n 
                         L 
                       
                       
                         n 
                         H 
                       
                     
                     ) 
                   
                   
                     2 
                      
                     N 
                   
                 
               
             
           
         
       
         
         
           
             R: reflectance 
             n L : lower refractive index 
             n H : higher refractive index 
             n air : air&#39;s refractive index 
             n s : substrate&#39;s refractive index 
             N: pair number of the first and the second refractive layers 
           
         
       
    
     Accordingly, the reflectance of the distributed Bragg reflector can be increased by increasing the refractive index difference between the first refractive layers  162  and the second refractive layers  164  and the pair number of the first refractive layers  162  and the second refractive layers  164 . Therefore, the reflectance of the distributed Bragg reflector can be adjusted by choosing proper materials for the first refractive layers  162  and the second refractive layers  164 . In addition, the required wavelength range to be reflected is dependent on the material and the thickness of the semiconductor layer  130 . Generally, for single junction photovoltaic cells, the required wavelength to be reflected is about 500-800 nm. For multiple junction photovoltaic cells, the required wavelength to be reflected is about 800-1200 nm. 
     Possible choices for the first refractive layers  162  and the second refractive layers  164  can be silicon-based materials, III-V semiconductor materials, or II-VI semiconductor materials, for example. The silicon-based materials can be amorphous silicon, SiGe, amorphous SiGe, amorphous silicon carbide, or silicon oxide, for example. The III-V semiconductor materials can be GaN, GaP, InN, InP, GaAs, InAs, AIN, AlP, or AlAs, for example. The II-VI semiconductor materials can be ZnO, ZnTe, ZnSe, CdTe, CdSe, HgTe, ZnS, CdS, or HgS, for example. 
     Moreover, the distributed Bragg reflector comprises conductive nanostructures distributed in the interfaces between the first refractive layers  162  and the second refractive layers  164 . The material of the conductive nanostructures can be Ag, Al, Pt, In, or Au, for example. 
     In  FIGS. 3A and 3B , the conductive nanostructures can be metal nanoparticles  166 , for example. The average diameter of the metal nanoparticels  166  is about 1-2000 Å, preferably 1-1000 Å, and more to preferably 1-200 Å. The average height of the metal nanoparticels  166  is about 1-1000 Å, preferably 1-100 Å, and more preferably 1-50 Å. The difference between  FIGS. 3A and 3B  is the distribution density of the metal nanoparticels  166 . The distribution density of the metal nanoparticels  166  can be used to adjust the resistance of the distributed Bragg reflector according to various needs. For example, in  FIG. 3A , the metal nanoparticels  166  are disposed in each interface between the first refractive layers  162  and the second refractive layers  164 . In  FIG. 3B , the metal nanoparticels  166  are disposed in only each two interfaces between the first refractive layers  162  and the second refractive layers  164 . 
     In  FIGS. 4A and 4B , the conductive nanostructures can be metal thin film  168 , for example. The thickness of the metal thin film  168  is about 1-200 Å, preferably 1-100 Å, and more preferably 1-50 Å. The difference between  FIGS. 4A and 4B  is the distribution density of the metal thin film  168 . Similarly, the distribution density of the metal thin film  168  can be used to adjust the resistance of the distributed Bragg reflector according to various needs. For example, in  FIG. 4A , the metal thin film  168  disposed in each interface between the first refractive layers  162  and the second refractive layers  164 . In  FIG. 4B , the metal thin film  168  is disposed in only each two interfaces between the first refractive layers  162  and the second refractive layers  164 . 
     Next, the resistances of the distributed Bragg reflectors in  FIG. 4A  with various thicknesses were measured. When the metal thin film  168  was a silver thin film, the obtained thickness and the resistance are listed in the table below. It can be seen from the table that the resistance can be largely decreased by several orders. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Thickness (nm) 
                 Resistance (Ohm-cm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 0 
                 1 
               
               
                   
                 4 
                 0.0025 
               
               
                   
                 6 
                 0.0008 
               
               
                   
                 8 
                 0.0006 
               
               
                   
                 10 
                 0.00055 
               
               
                   
                   
               
            
           
         
       
     
     Accordingly, since the distributed Bragg reflector above contains conductive nanostructures distributed therein, the novel distributed Bragg reflector provided above can be used as a back reflector and a back metal contact at the same time. 
     The reader&#39;s attention is directed to all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. 
     All the features disclosed in this specification (including any accompanying claims, abstract, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, each feature disclosed is one example only of a generic series of equivalent or similar features.