Abstract:
A photovoltaic structure is provided with an added layer inserted between an emitter layer and a window layer. The added layer includes all elements which are same or different both in the emitter layer and the window layer. The addition of the added layer enhances converted current and voltage that improves the conversion efficiency when the structure is applied to a solar cell.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a photovoltaic structure, and more especially, to a thin-film photovoltaic structure. 
         [0003]    2. Background of the Related Art 
         [0004]    Solar cell, also called photovoltaic cell, is a device for converting sun light into electrical power and has many advantages, such as no noise, no waste, no pollution, high reliability, high duration, and low maintenance cost, suitable for various applications. Most of the solar cells for sale are based on silicon material to have high reliability but in company with the drawback of a high series resistance. Nowadays, some resolutions include reducing the cost of silicon material or assembly, enhancing the efficiency of solar cell or concentration system, or developing a manufacturing technology for new material or the device. 
         [0005]    The materials of thin film solar cell may be single element, binary elements and ternary elements as follows: (1) polycrystalline silicon or amorphous silicon-hydrogen alloy (Si—H) belonging to single element; (2) GaAs or InP solar cell of group III-V compound semiconductor material belonging to binary elements; (3) CuS/CdS solar cell of group II-VI compound semiconductor material; and (4) CuInSe 2 /CdS and CuInS solar cell belonging to ternary elements. 
         [0006]      FIG. 1  is a schematically cross-sectional diagram illustrating traditional thin-film solar cell made by group III-V compound semiconductor material. A back surface field layer  112  (BSF), a base layer  114 , an emitter layer  116 , a window layer  118 , and an ohmic contact layer  120  are sequentially formed on a substrate  110 . 
         [0007]    Gallium arsenide based solar cell has features as follows: (1) good energy band gap (1.42 eV) as well as high efficiency and thermal stability; (2) the material of direct energy band gap as well as strong light absorption suitable for a thin-film solar cell (in thickness of about 2-3 micrometers enough to absorb whole sun light); and (3) light suitable for a space power battery. However, such a battery also has some drawbacks, such as: (1) high cost; and (2) unknown factors in a manufacturing process to affect the battery performance so as to make no effort on efficiency enhancement. Some improvement methods include optimizing each thin film in a battery, integration of thickness and doping concentration in each thin film, or process enhancement. However, it is still limited to improve efficiencies by those methods aforementioned. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is to provide a thin-film photovoltaic structure which includes an added layer sandwiched between an emitting layer and a window layer for photovoltaic enhancement. 
         [0009]    Further, the present invention is to provide a thin-film semiconductor photovoltaic structure which has a quaternary semiconductor alloy composition formed on an emitting layer of a ternary semiconductor alloy composition for power support of a photovoltaic cell. 
         [0010]    Accordingly, one of embodiments of the present invention provides a photovoltaic structure which includes a substrate, a back surface field layer on the substrate, a base layer on the back surface field layer, an emitter layer on the base layer, a window layer on the emitter layer, an added layer between the emitter layer and the window layer, and an ohmic contact layer on the window layer. The emitter layer is an alloy composition including a common component and a first component. The window layer is an alloy composition including the common component and a second component. An added layer is between the emitter layer and the window layer. The added layer is an alloy composition including the common component, the first and the second components. 
         [0011]    These and other aspects, features and advantages of the present invention can be further understood from the accompanying drawings and description of a preferred embodiment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematically cross-sectional diagram illustrating a conventional film cell made by group III-V compound semiconductor material. 
           [0013]      FIG. 2  is a schematically cross-sectional diagram illustrating a photovoltaic structure in accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]      FIG. 2  is a schematically cross-sectional diagram illustrating a photovoltaic structure in accordance with one embodiment of the present invention. It is noted that a group III-V compound semiconductor alloy composition is exemplarily illustrated, such as one mainly based on binary, ternary, or quaternary semiconductor alloy composition. It is understood that a small amount of other component may be added in the semiconductor alloy composition for physical enhancement. However, such a semiconductor alloy composition herein is a binary, ternary, or quaternary. 
         [0015]    Shown in  FIG. 2 , a substrate  10 , which is not involved in a photovoltaic reaction, is provided herein. For example, the group III-V based compound substrate  10 , such as GaAs or InP, may be with various characteristics by different forming method for enhancing the efficiency of a photovoltaic cell. In the embodiment, the substrate  10  is made of n-GaAs. A back surface field layer  12  (BSF layer), such as an n-InAlP ternary semiconductor alloy composition, is positioned on the substrate  10  and configured for blocking carriers (holes) from diffusing into the substrate  10 . A base layer  14 , an emitter layer  16  and a window layer  18  are sequentially formed on the back surface field layer  12 . In the embodiment, the base layer  14  is an n-InGaP layer; the emitter layer  16  is a p-InGaP layer; and the window layer  18  is a p-InAlP layer. Moreover, an ohmic contact layer  20 , such as a p-GaAs, is formed on the window layer  18 . 
         [0016]    Specially, in the present invention, an added layer  17  is sandwiched between the emitter layer  16  and the window layer  18 . In addition to a common component, the emitter layer  16  and the window layer  18 , which are ternary semiconductor alloy compositions, further include a first component and a second component different from each other. It is noted that the common component herein may include one or more same elements. However, the percentages of elements in the common component may be identical or different in the emitter layer  16  and the window layer  18 , respectively. 
         [0017]    Furthermore, the added layer  17  is a quaternary semiconductor alloy composition which includes the common, first and second components in both the emitter layer  16  and the window layer  18 . The added layer  17  may be a p-InGaAlP layer or one layer represented as p-In 0.5 (Ga x Al 1-x ) 0.5 P where x is between zero and 1. In the embodiment, in the added layer  17 , the percentage of one component different from one in the emitter layer  16  is relatively little where is close to the interface of the emitter layer  16  and the added layer  17 . That is, in the added layer  17 , the second component is less than the first component where the added layer  17  is close to the emitter layer  16 . The second component is gradually increased in the added layer  17  where the added layer  17  is gradually away from the emitter layer  16 . Accordingly, the composition of the added layer  17  near the window layer  18  may be identical or similar to the window layer  18  by increasing the second component in the added layer  17 . For example, for the added layer  17  represented as p-In 0.5 (Ga x Al 1-x ) 0.5 P, gallium (Ga) is far more than aluminum (Al) where the added layer  17  is close to the emitter layer  16  of the first ternary semiconductor alloy composition p-InGaP. Where the position of the added layer  17  is away from the emitter layer  16 , gallium (Ga) is gradually decreased while aluminum (Al) is gradually increased. The aluminum (Al) is far more than gallium (Ga) where the added layer  17  is closed to the window layer  18 . That is, the composition of the added layer  17  is similar to the window layer  18  of the second ternary semiconductor alloy composition p-InAlP. 
         [0018]    Next, the thickness of the added layer  17  is not limited, which may be adapted to other layers to have a thickness without influence on a whole thickness. However, the existence of the added layer  17  of the quaternary semiconductor alloy composition makes a device in an effort to convert current and enhance voltage. Compared with some photovoltaic parameters, such as short-circuit current J sc  (mA/cm 2 ), open-circuit voltage V oc  (V), maximum power point J m  (mA/cm 2 ), maximum voltage V m  (V), maximum power P m  (mW), fill factor FF, and conversion efficiency (%), a conventional structure ( FIG. 1 ) is with J sc =12.37 mA/cm 2 , V oc =1.37 V, J m =11.26 mA/cm 2 , V m =1.26 V, FF=0.837, P m =14.19 mW, and conversion efficiency=14.19%. On the condition of sandwiching the added layer  17  between the emitting layer  16  and the window layer  18 , J sc  is between 12.63 and 12.71 mA/cm 2 , V oc  between 1.40 and 1.41 V, J m  between 11.42 and 1.70 mA/cm 2 , V m  between 1.26 and 1.29 V, FF between 0.826 and 0.830, P m  between 14.69 and 14.78 mW, and conversion efficiency between 14.69 and 14.78%. According to the parameters aforementioned, the added layer  17  is profitable to enhance the solar cell output characteristics. 
         [0019]    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.