Abstract:
A copper/indium/gallium/selenium (CIGS) solar cell structure and a method for fabricating the same are provided. The CIGS solar cell structure includes a substrate, a molybdenum thin film layer, an alloy thin film layer, and a CIGS thin film layer. The alloy thin film layer is provided between the molybdenum thin film layer and the CIGS thin film layer, serving as a conductive layer of the CIGS solar cell structure. The alloy thin film layer is composed of a variety of high electrically conductive materials (such as molybdenum, copper, aluminum, and silver) in different atomic proportions.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This is a continuation-in-part application for the application Ser. No. 12/407,780 filed on Mar. 19, 2009, which is incorporated herewith by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to a copper/indium/gallium/selenium (CIGS) solar cell structure and a method for fabricating the same, and more particularly, to a CIGS solar cell structure including an alloy thin film layer disposed between a molybdenum thin film layer and a CIGS thin film layer, and a method for fabricating the same. 
         [0004]    2. The Prior Arts 
         [0005]    CIGS thin film solar cells are being expected as one type of the most potentially low cost solar cells. Comparing with the other current thin film battery technologies, a CIGS thin film solar cell has higher efficiency. Currently, a small size CIGS thin film solar cell unit may achieve an efficiency of up to 19%, and a large size one may achieve an efficiency of up to 13%. Further, the CIGS thin film solar cell can be fabricated by a chemical vapor deposition (CVD) process which is adapted for low cost and large size processing. Furthermore, the CIGS thin film solar cell is radiation resistible and light weighted. 
         [0006]      FIG. 1  is a schematic diagram illustrating a conventional CIGS thin film solar cell  1 . Referring to  FIG. 1 , the CIGS thin film solar cell  1  includes a substrate  10 , a molybdenum thin film layer  20 , and a CIGS thin film layer  80 . The molybdenum thin film layer  20  is deposited by sputtering on the substrate  10  for serving as a back electrode. The CIGS thin film layer  80  is then configured by a synchronizing evaporation deposition and selenylation process on the molybdenum thin film layer  20  for serving as a light absorbing layer. 
         [0007]    However, the CIGS thin film layer  80  directly deposited upon the molybdenum thin film layer  20  often peels off therefrom and is featured with unsatisfactory conductivity and resistance coefficient. 
       SUMMARY OF THE INVENTION 
       [0008]    A primary objective of the present invention is to provide a CIGS solar cell structure. The CIGS solar cell structure includes a substrate, a molybdenum thin film layer, an alloy thin film layer, and a CIGS thin film layer. According to the present invention, the alloy thin film layer is provided between the molybdenum thin film layer and the CIGS thin film layer, serving as a conductive layer of the CIGS solar cell structure. The alloy thin film layer is composed of a variety of high electrically conductive materials (such as molybdenum, copper, aluminum, and silver) in different atomic proportions. 
         [0009]    A further objective of the present invention is to provide a method for fabricating a CIGS solar cell structure. The method includes sputtering a molybdenum thin film layer upon a substrate, and then continuously depositing an alloy thin film layer onto the molybdenum thin film layer by bombarding targets toward the molybdenum thin film layer with a sputtering machine. The sputtering machine is adapted for precisely performing the thin film deposition and improving the uniformity of the alloy mixed by different metals for preparing the alloy thin film layer. The targets include high electrically conductive materials, such as molybdenum, copper, aluminum, and silver. Thereafter, a CIGS thin film layer is then deposited on the alloy thin film layer. 
         [0010]    Accordingly, the present invention is adapted for solving the problems of the conventional technologies as discussed above, so as to improve the electrical conductivity, reduce the resistance coefficient of the molybdenum thin film layer, thus reducing the thickness thereof so as to avoid the peeling off of the CIGS thin film layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will be apparent to those skilled in the art by reading the following detailed description of a preferred embodiment thereof, with reference to the attached drawings, in which: 
           [0012]      FIG. 1  is a schematic diagram illustrating a conventional CIGS thin film solar cell; 
           [0013]      FIG. 2  is a structural diagram illustrating a CIGS solar cell structure according to an embodiment of the present invention; 
           [0014]      FIG. 3  is a schematic diagram illustrating a first embodiment of the present invention; 
           [0015]      FIG. 4  is a schematic diagram illustrating a second embodiment of the present invention; 
           [0016]      FIG. 5  is a schematic diagram illustrating a third embodiment of the present invention; and 
           [0017]      FIG. 6  is a schematic diagram illustrating a fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         [0019]      FIG. 2  is a structural diagram illustrating a CIGS solar cell structure according to an embodiment of the present invention. Referring to  FIG. 2 , the CIGS solar cell structure includes a substrate  10 , a molybdenum thin film layer  20 , an alloy thin film layer  50 , and a CIGS thin film layer  80 . The substrate  10 , the molybdenum thin film layer  20 , the alloy thin film layer  50 , and the CIGS thin film layer  80  are sequentially bottom-up stacked one upon another. The substrate  10  is a glass substrate or a flexible metal substrate. The composition of the CIGS thin layer  80  is Cu y In (1-x) Ga x Se 2 , where y=0.75˜1.0 and x=0.15˜0.35. 
         [0020]    According to an aspect of the current embodiment, the alloy thin film layer  110  for example includes molybdenum and aluminum, in which the atomic proportion of molybdenum to aluminum is about 6˜9:1˜2. When the alloy thin film layer  50  has a thickness ranging from 0.1 to 0.25 μm, the electrical conductivity of the alloy thin film layer  50  ranges from 20×10 6 /mΩ to 25×10 6 /mΩ. 
         [0021]    According to another aspect of the current embodiment, the alloy thin film layer  110  for example includes molybdenum and copper, in which the atomic proportion of molybdenum to copper is about 5˜8:1˜4. When the alloy thin film layer  50  has a thickness ranging from 0.1 to 0.25 μm, the electrical conductivity of the alloy thin film layer  50  ranges from 30×10 6 /mΩ to 35×10 6 /mΩ. 
         [0022]    According to a further aspect of the current embodiment, the alloy thin film layer  50  for example includes molybdenum, copper and aluminum, in which the atomic proportion of molybdenum, copper, and aluminum is about 5˜7:3˜5:1˜2. 
         [0023]    When the alloy thin film layer  50  has a thickness ranging from 0.1 to 0.25 μm, the electrical conductivity of the alloy thin film layer  50  ranges from 30×10 6 /mΩ to 35×10 6 /mΩ. 
         [0024]    According to still another aspect of the current embodiment, the alloy thin film layer  50  for example includes molybdenum, copper, aluminum, and silver, in which the atomic proportion of molybdenum, copper, aluminum, and silver is about 5˜7:3˜4:1˜1.5:2˜2.5. When the alloy thin film layer  50  has a thickness ranging from 0.1 to 0.25 μm, the electrical conductivity of the alloy thin film layer  50  ranges from 35×10 6 /mΩ to 40×10 6 /mΩ. 
         [0025]    It should be noted that the CIGS thin film layer  80  of the CIGS solar cell structure is configured on the alloy thin film layer by a synchronizing evaporation deposition and selenylation process. 
         [0026]      FIG. 3  is a schematic diagram illustrating a first embodiment of the present invention. Referring to  FIG. 3 , at first, a molybdenum thin film layer  20  is deposited upon a substrate  10  by a sputtering process. The substrate  10  together with the molybdenum thin film layer  20  configured thereupon are secured on a roller set  90 , and driven to move along a direction indicated by the arrow. A sputtering machine  200  is provided over the substrate  10  having the molybdenum thin film layer  20  configured thereupon. The sputtering machine  200  includes a plurality of ejector sets. Each of the ejector sets includes a molybdenum target ejector  211 , and an aluminum target ejector  231 . A molybdenum target  212  is provided in a molybdenum target chamber  210 . An aluminum target  232  is provided in an aluminum target chamber  230 . The molybdenum target ejector  211  is positioned beneath the molybdenum target chamber  210 , and the aluminum target ejector  231  is positioned beneath the aluminum target chamber  230 . Each of the target chambers is provided with a sputtering gun (not shown in the drawings). The powers of the sputtering guns can be adjusted. According to the powers of the sputtering guns, the amounts of the targets ejected from the target ejectors can be adjusted for adjusting the alloy mixing proportion. Preferably, the target ejectors are adapted for continuously sputtering on the molybdenum thin film layer  20 . In such a way, an alloy thin film layer  51  can be configured with an improved uniformity. The alloy thin film layer  51  preferably has a thickness ranging from 0.1 to 0.25 μm, in which the alloy proportion between the molybdenum and the aluminum ranges from 9:1 to 3:1. 
         [0027]      FIG. 4  is a schematic diagram illustrating a second embodiment of the present invention. Referring to  FIG. 4 , the current embodiment is similar to the first embodiment as shown in  FIG. 3 , except that copper is employed for substituting the aluminum employed in the first embodiment so that the a molybdenum/copper alloy thin film layer  53  is obtained instead of the molybdenum/aluminum alloy thin film layer  51 . The copper target  232  is provided in the copper target chamber  230 , while the copper target ejector  221  is positioned beneath the copper target chamber  230 . The sputtering ejectors continuously sputter until the obtained molybdenum/copper alloy thin film layer  53  achieves a thickness ranging from 0.1 to 0.25 μm, in which the alloy atomic proportion between the molybdenum and the copper ranges from 8:1 to 1.25:1. 
         [0028]      FIG. 5  is a schematic diagram illustrating a third embodiment of the present invention. Referring to  FIG. 5 , the current embodiment is similar to the first embodiment as shown in  FIG. 3 , except that molybdenum, copper, and aluminum are employed for configuring molybdenum/copper/aluminum alloy thin film layer  55  instead of that the molybdenum and aluminum are used for configuring the molybdenum/aluminum alloy thin film layer  51 . The sputtering ejectors continuously sputter until the obtained molybdenum/copper/aluminum alloy thin film layer  55  achieves a thickness ranging from 0.1 to 0.25 μm, in which the atomic proportion of molybdenum, copper, and aluminum is about 5˜7:3˜5:1˜2. 
         [0029]      FIG. 6  is a schematic diagram illustrating a fourth embodiment of the present invention. Referring to  FIG. 6 , the current embodiment is similar to the first embodiment as shown in  FIG. 3 , except that molybdenum, copper, aluminum, and silver are employed for configuring molybdenum/copper/aluminum/silver alloy thin film layer  57  instead of that the molybdenum and aluminum are used for configuring the molybdenum/aluminum alloy thin film layer  51 . The sputtering ejectors continuously sputter until the obtained molybdenum/copper/aluminum/silver alloy thin film layer  57  achieves a thickness ranging from 0.1 to 0.25 μm, in which the atomic proportion of molybdenum, copper, aluminum, and silver is about 5˜7:3˜4:1˜1.5:2˜2.5. 
         [0030]    Although the present invention has been described with reference to the preferred embodiments thereof, it is apparent to those skilled in the art that a variety of modifications and changes may be made without departing from the scope of the present invention which is intended to be defined by the appended claims.