Patent Publication Number: US-2012024366-A1

Title: Thin film solar cell structure and fabricating method thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to a solar cell structure and the fabricating method thereof. In particular, the invention relates to a thin-film solar cell structure that uses a chemical thin film as its absorbing layer and the fabricating method thereof. 
     2. Related Art 
     In recent years, the solar energy industry gradually turns its research emphasis from conventional wafer manufacturing to thin films. Compound thin films, in particular, receive particular attention. Compound thin film solar cells compared with wafer solar cells have many advantages, such as higher conversion efficiency, lower cost, wider absorbing range, more flexible, and possible for large area applications. Among various chemical compounds, copper indium gallium selenium (CIGS) materials have a wide absorbing spectrum. They can absorb more solar power to increase the conversion efficiency. 
     Please refer to  FIG. 1  for a cross-sectional view of the structure of a conventional compound thin film solar cell. This compound thin film solar cell  10  includes: a substrate  11 , a metal layer  12 , an absorbing layer  13 , a buffer layer  14 , and a window layer  15 . Generally speaking, the most bottom substrate  11  is glass or some flexible material, such as aluminum alloy foil and copper foil. The substrate  11  is then sputtered with Mo to form the metal layer  12  as a back electrode layer. After the metal layer  12  forms, a compound such as CIGS is sputtered onto the metal layer  12  to form the absorbing layer  13 . Afterwards, CdS is deposited on the absorbing layer  13  by chemical bath deposition to form the buffer layer  14 . ZnO is grown on the buffer layer  14  by sputtering to form the window layer  15 . However, after the absorbing layer  13  is cut by a machine or laser, many interface trap densities form on the absorbing layer  13 . They even result in interface binding and greatly lower the power conversion efficiency. 
     In summary, the prior art always has the problem that the power conversion efficiency is affected by the interface trap density. Therefore, it is desirable to provide a better solution. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the specification discloses a thin film solar cell structure and the fabricating method thereof. 
     One embodiment of the disclosed thin film solar cell structure includes: a substrate, a metal layer, an absorbing layer, and a passivation layer. The metal layer is formed on the substrate. The absorbing layer is formed on the metal layer. The passivation layer is formed on the absorbing layer. The surface electric field of the passivation layer passivates the absorbing layer. 
     Another embodiment of the disclosed thin film solar cell structure also includes: a substrate, a metal layer, an absorbing layer, and a passivation layer. The metal layer is formed on the substrate. The absorbing layer is formed on the metal layer. The passivation layer is formed on the metal layer and contacts at least one side of the absorbing layer. The surface electric field of the passivation layer passivates the absorbing layer. 
     The disclosed fabricating method of thin film solar cells includes the steps of: providing a substrate; forming a metal layer on the substrate; forming an absorbing layer on the metal layer; and forming a passivation layer on the absorbing layer, with the surface electric field of the passivation layer passivating the absorbing layer. 
     The disclosed structure and fabricating method differ from the prior art in the following. By embedding the passivation layer in the thin film solar cell, the passivation layer is in contact with the absorbing layer. The surface electric field of the passivation layer thus reduces the interface trap density of the absorbing layer. 
     The invention achieves the goal of increasing power conversion efficiency and protecting the absorbing layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will become more fully understood from the detailed description given herein below illustration only, and thus is not limitative of the present invention, and wherein: 
         FIG. 1  is a cross-sectional view of the structure of a conventional thin film solar cell; 
         FIG. 2  is a cross-sectional view of a first structure of the disclosed thin film solar cell; 
         FIG. 3  is a flowchart of the disclosed fabricating method of a thin film solar cell; 
         FIG. 4  is a cross-sectional view of a second structure of the disclosed thin film solar cell; 
         FIG. 5  is a cross-sectional view of a third structure of the disclosed thin film solar cell; and 
         FIG. 6  is a cross-sectional view of a fourth structure of the disclosed thin film solar cell. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings, wherein the same references relate to the same elements. 
     We first describe the structure of the disclosed thin film solar cell.  FIG. 2  is a cross-sectional view of the first structure of thin film solar cell according to the invention. The thin film solar cell  20  includes: a substrate  21 , a metal layer  22 , an absorbing layer  23 , and a passivation layer  24 . The substrate  21  is made of a flexible material (also called soft material), glass, or polyimide (PI). In practice, the flexible material can be aluminum alloy foil, copper foil, and so on. Besides, the substrate  21  has to be first washed before subsequent sputtering and deposition. 
     The metal layer  22  forms on the substrate  21 . In practice, the metal layer  22  is grown on the substrate  21  by sputtering Mo onto the substrate  21 , and is used as a back electrode layer for conducting electricity. In addition, the metal layer  22  can also be formed by depositing a layer of Mo using electron-beam evaporation (EBE) and connected to the positive electrode. 
     The absorbing layer  23  forms on the metal layer  22 . The material of the absorbing layer  23  is such compound as copper indium gallium selenium (CIGS), copper indium selenium (CIS), or copper gallium selenium (CGS). The absorbing layer  23  can be formed on the metal layer  22  by co-evaporation, sputtering, or printing. The absorbing layer  23  is P-type. In practice, the CIGS thin film can be formed using the vacuum process of four-element co-evaporation or the combination of sputtering and selenium. In particular, co-evaporation can freely control the composition and energy gap of the thin film in order to make high-efficiency thin film solar cells. However, it is harder to control and more difficult in producing large-area products. For the combination of sputtering and selenium, one has to be careful in processing special gas (e.g., HSe). 
     Since CIS can form a thin film between 350° C. to 550° C. Therefore, when using CIS as the absorbing layer  23 , one can use the cheaper soda-lime glass as the substrate  21 . 
     The passivation layer  24  forms on the absorbing layer  23 . The passivation layer  24  carries sufficient positive or negative fixed charges to form a surface electric field in order to passivate the absorbing layer  23 . The passivation refers to the action of filling defects in the absorbing layer  23 . For example, the absorbing layer  23  after laser cutting produces an interface trap density that affects power conversion efficiency. In practice, the passivation layer  24  can be grown from Al 2 O 3  by atomic layer deposition (ALD), low pressure chemical vapor deposition (LPCVD), sputtering, or sol-gel. The growth thickness is pervious to light (e.g., the growth thickness can between 30 nm and 100 nm). As a result, the negative fixed charges on Al 2 O 3  produces a surface electric field so that there is less surface binding on the absorbing layer  23 , rendering a better passivation effect. It should be noted that the invention is not restricted to the above-mentioned thickness of the passivation layer  24 . Moreover, Al 2 O 3  can enclose the absorbing layer  23  or even grow on the metal layer  22 , in contact with at least one side of the absorbing layer  23 . The details will be described later. Besides, the passivation layer  24  prevents moisture and oxygen from directly contacting the absorbing layer. The absorbing layer  23  is thus free from deterioration in power conversion efficiency due to moisture and oxygen. 
       FIG. 3  is a flowchart of the disclosed fabricating method of a thin film solar cell. The method includes the steps of: providing a substrate  21  (step  210 ); forming a metal layer  22  on the substrate  21  (step  220 ); forming an absorbing layer  23  on the metal layer  22  (step  230 ); and forming a passivation layer  24  on the absorbing layer  23 , with the surface electric field of the passivation layer  24  passivating the absorbing layer  23  (step  240 ). The above-mentioned steps embed the passivation layer  24  in the thin film solar cell  20  so that the passivation layer  24  is in contact with the absorbing layer  23 . The surface electric field of the passivation layer  24  reduces the interface trap density of the absorbing layer  23 . 
     Besides, step  240  can be further followed by the step of growing a coating layer of CdS, ZnS, or ZnO on the passivation layer  24  (step  250 ). The coating layer and the passivation layer  24  are both N-type in order to form a P—N junction with the P-type absorbing layer  23 . In practice, since CdS contains poisonous cadmium, one can use ZnS instead. 
     Please refer to  FIG. 4  for a cross-sectional view of the second structure of a thin film solar cell according to the invention. In addition to the structure of the thin film solar cell  20 , Al 2 O 3  can grow on the metal layer  22  to form the passivation layer  24  in practice. The passivation layer  24  touches at least one side of the absorbing layer  23 . Practically, the finished passivation layer  24  is as shown in  FIG. 4 . The metal layer  22  of the thin film solar cell  20   a  is simultaneously grown with the absorbing layer  23  and the passivation layer  24 . After laser cutting the cutting surface of the absorbing layer  23  has an interface trap density. As shown in  FIG. 4 , the passivation layer  24  grown on the cutting surface of the absorbing layer  23  of the thin film solar cell  20   a  produces a surface electric field due to the negative fixed charges of Al 2 O 3 . The interface trap density of the cutting surface of the absorbing layer  23  is thus reduced. The surface binding is reduced to achieve good passivation. 
       FIG. 5  is a cross-sectional view of the third structure of a thin film solar cell according to the invention. In practice, Al 2 O 3  is grown on the absorbing layer  23  by ALD, LPCVD, sputtering, or sol-gel. Its structure can be the passivation layer  24  that encloses the absorbing layer  23 , as shown in the drawing. Therefore, the passivation layer  24  of the thin film solar cell  20   b  can effectively prevent the absorbing layer  23  cut by a machine or laser from directly contacting moisture and oxygen. In other words, in addition to using the negative fixed charges of the aluminum oxide to form a surface electric field to passivate the absorbing layer  23 , the passivation layer  24  further prevents moisture and oxygen from contacting the absorbing layer  23 . Thus, the material of the absorbing layer  23  would not deteriorate to affect the power conversion rate. 
     Please refer to  FIG. 6  for a cross-sectional view of the fourth structure of a thin film solar cell according to the invention. As mentioned before, aluminum oxide can be the passivation layer enclosing the absorbing layer  23  as shown in  FIG. 5 . In practice, it is possible to grow a coating layer of CdS, ZnS, or ZnO on the passivation layer  24 , as shown in  FIG. 6 . For example, suppose the coating layer  25  is CdS or ZnS. The coating layer  25  can then serve as the buffer layer of the thin film solar cell  20   c . If the coating layer  25  is ZnO, then it can be the window layer of the thin film solar cell  20   c.    
     In summary, the invention differs from the prior art in that a passivation layer  24  is embedded in the thin film solar cell  20  to be in contact with the absorbing layer  23 . The surface electric field of the passivation layer  24  reduces the interface trap density of the absorbing layer  23 . This disclosed technique solves problems existing in the prior art and increase the power conversion efficiency as well as protect the absorbing layer. 
     Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments, will be apparent to persons skilled in the art. It is, therefore, contemplated that the appended claims will cover all modifications that fall within the true scope of the invention.