Patent Publication Number: US-2012042923-A1

Title: Thin Film Solar Cell and Thin Film Solar Cell System

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims priority to and the benefit of Taiwan Patent Applications No. TW98142753, filed Dec. 14, 2009, No. TW98142754, filed Dec. 14, 2009, and No. TW98143232, filed Dec. 16, 2009, the contents of which are incorporated herein in their entireties by reference. 
     FIELD OF INVENTION 
     The present invention relates to a thin film solar cell and a manufacturing method thereof, and more particularly to a thin film solar cell with higher photoelectric conversion efficiency and a manufacturing and optimization method thereof. 
     BACKGROUND OF THE INVENTION 
     With the raise of the consciousness of environmental protection, the concept of energy saving and carbon dioxide reduction has gradually drawn attention, and the development and utilization of renewable energy have become the focus in the world. A solar cell which converts solar light into electricity is the most promising in energy industry nowadays, so that manufacturers devote themselves to the manufacturing of the solar cell. Currently, the key issue of the solar cell is the improvement of the photoelectric conversion efficiency thereof. Therefore, to improve the photoelectric conversion efficiency of the solar cell means enhancing the product competitiveness. 
     Solar cells using monocrystalline silicon or polycrystalline silicon account for more than 90% in the solar cell market. However, these solar cells are made from silicon wafers of 150 μm to 350 μm thick, and the process cost thereof is higher. In addition, the raw materials of solar cells are silicon ingots with high quality. The silicon ingots face the shortage problem as the usage quantity thereof is increased significantly in recent years. Therefore, the thin film solar cell has been the new focus due to the advantages of low cost, easy for large-area production and simple module process, etc. 
     Generally speaking, in a conventional thin film solar cell, an electrode layer, a photovoltaic layer and another electrode layer are sequentially blanket-stacked on a substrate. During the process of stacking these layers, these layers are patterned by performing laser cutting processes, so as to form a plurality of sub cells connected in series. When a light enters the thin film solar cell from outside, free electron-hole pairs are generated in the photovoltaic layer by the solar energy, and the internal electric field formed by the PN junction makes electrons and holes respectively move toward two layers, so as to generate a storage state of electricity. Meanwhile, if a load circuit or an electronic device is connected, the electricity can be provided to drive the circuit or device. 
     However, the conventional thin film solar cell still has considerable room to improve the photoelectric conversion efficiency. Thus, how to improve the photoelectric conversion efficiency and performance of a thin film solar cell in order to improve the overall competitiveness of the product become the issues of concern. 
     SUMMARY OF THE INVENTION 
     The present invention provides a thin film solar cell having a higher photoelectric conversion efficiency. 
     The present invention further provides a thin film solar cell system, in which photo current respectively generated by the plurality of thin film solar cell modules can be current matching and a better photoelectric conversion efficiency can be obtained. 
     The present invention provides a thin film solar cell including a substrate, a plurality of photovoltaic cells, and at least one control unit. The photovoltaic cells are disposed on the substrate and each generates a photocurrent respectively. Each photovoltaic cell includes a first conductive layer, a photovoltaic layer and a second conductive layer. The first conductive layer is disposed on the substrate. The photovoltaic layer is disposed on the first conductive layer and having an opening exposing the first conductive layer. The second conductive layer is disposed on the photovoltaic layer through the opening and electrically connected to the first conductive layer of the adjacent photovoltaic cell. The control unit is electrically connected to the photovoltaic cells. Wherein, when the control unit examines that at least one photocurrent generated by the photovoltaic cells is different from the photocurrents generated by the other photovoltaic cells, the control unit provides a compensable current to the photovoltaic cells in order to obtain current matching of photocurrents generated by the overall photovoltaic cells. 
     The present invention further provides a thin film solar cell including a substrate, a plurality of first photovoltaic cells and at least one second photovoltaic cell. The first photovoltaic cells are disposed on the substrate and each of them is adapted to generate a photocurrent respectively. Wherein, each of the first photovoltaic cells includes a first conductive layer, a photovoltaic layer and a second conductive layer. The first conductive layer is disposed on the substrate. The photovoltaic layer is disposed on the first conductive layer and having an opening exposing the first conductive layer. The second conductive layer is disposed on the photovoltaic layer through the opening and electrically connected to the first conductive layer of the adjacent first photovoltaic cell. The second photovoltaic cell is disposed on the substrate. When the photocurrents generated by the first photovoltaic cells are different, the second photovoltaic cell is electrically connected to at least a part of the first photovoltaic cells in order to obtain current matching of the photocurrents generated by the overall first photovoltaic cells. 
     The present invention also provides a thin film solar cell system including a plurality of thin film solar cell modules and at least one current matching module. The thin film solar cell modules are connected in electrical series with one another and each providing a photocurrent respectively. Each of the thin film solar cell modules at least includes a substrate, a first conductive layer, a photovoltaic layer and a second conductive layer. The first conductive layer is disposed on the substrate. The photovoltaic layer is disposed on the first conductive layer. The second conductive layer is disposed on the photovoltaic layer. When the photocurrent provided by at least one of the thin film solar cell modules is different from the photocurrents provided by the other thin film solar cell modules, the current matching module is electrically connected to the thin film solar cell module in order to obtain current matching of the photocurrents provided by the thin film solar cell modules. 
     The present invention further provides a thin film solar cell system including a plurality of thin film solar cell modules. The thin film solar cell modules are connected in electrical series with one another and each providing a photocurrent respectively. Each of the thin film solar cell modules at least includes a substrate, a plurality of first photovoltaic cells and at least a second photovoltaic cell. The first photovoltaic cells are disposed on the substrate, and each of the first photovoltaic cells includes a first conductive layer, a photovoltaic layer and a second conductive layer. The first conductive layer is disposed on the substrate. The photovoltaic layer is disposed on the first conductive layer. The second conductive layer is disposed on the photovoltaic layer. At least a second photovoltaic cell is disposed on the substrate. When the photocurrent provided by at least one of the thin film solar cell modules is different from the photocurrents provided by the other thin film solar cell modules, the second photovoltaic cell of the thin film solar cell module is electrically connected in parallel to at least a part of the first photovoltaic cells of the thin film solar cell modules in order to obtain current matching of the photocurrents provided by the thin film solar cell modules. 
     In view of the above, the thin film solar cell of the present invention is designed with the control unit. Thus, when the photocurrents provided by the first photovoltaic cells are different, the current matching can be obtained by electrically connecting the second photovoltaic cell to the part of the first photovoltaic cell. And the overall photoelectric conversion efficiency can be improved. 
     Moreover, the thin film solar cell of the present invention is designed with a second photovoltaic cell. When the photocurrents provided by the first photovoltaic cells are different, the second photovoltaic cell can be electrically connected to a part of the first photovoltaic cells in order to obtain current matching of the photocurrents to improve the overall photoelectric conversion efficiency. 
     Moreover, the thin film solar cell system of the present invention includes at least a current matching module. When the photocurrents provided by the plurality of thin film solar cells are different, the current matching can be electrically connected to the thin film solar cell modules in order to obtain current matching of the photocurrents provided by the thin film solar cell modules. In addition, each of the thin film solar cell modules in an embodiment of the present invention includes a second photovoltaic cell. When the photocurrents provided by the thin film solar cell modules are different, the second photovoltaic cell of the thin film solar cell module can be electrically connected in parallel to a part of the first photovoltaic cells of the thin film solar cell modules in order to obtain the current matching of the photocurrents provided by the thin film solar cell modules. 
     In order to make the aforementioned and other objects, features and advantages of the present invention comprehensible, a preferred embodiment accompanied with figures is described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  schematically illustrates a thin film solar cell according to an embodiment of the present invention. 
         FIG. 2  schematically illustrates a cross-sectional view along the B-B′ line of  FIG. 1  of the thin film solar cell. 
         FIG. 3  schematically illustrates a process flow of manufacturing and optimization of a thin film solar cell according to an embodiment of the present invention. 
         FIG. 4  schematically illustrates a process flow of the photovoltaic cell of  FIG. 3  formed on the substrate. 
         FIG. 5  schematically illustrates a top view of a thin film solar cell according to an embodiment of the present invention. 
         FIG. 6  schematically illustrates a cross-sectional view of a thin film solar cell of  FIG. 5  along the B-B′ line. 
         FIG. 7  schematically illustrates a cross-sectional view of a thin film solar cell of  FIG. 5  along the C-C′ line, wherein the first photovoltaic cell and the second photovoltaic cell is not electrically connected. 
         FIG. 8  schematically illustrates a cross-sectional view of  FIG. 5  along the C-C′ line, an embodiment of wherein the first photovoltaic cell and the second photovoltaic cell are electrically connected. 
         FIG. 9  schematically illustrates a top view of a thin film solar cell according to an embodiment of the present invention. 
         FIG. 10  schematically illustrates a top view of a thin film solar cell according to an embodiment of the present invention. 
         FIG. 11  schematically illustrates a top view of a thin film solar cell according to an embodiment of the present invention. 
         FIGS. 12A to 12G  schematically illustrate a process flow of manufacturing a thin film solar cell according to an embodiment of the present invention. 
         FIGS. 13A and 13B  schematically illustrate a method of electrically connecting between the first photovoltaic cell and the second photovoltaic cell according to an embodiment of the present invention. 
         FIG. 14  schematically illustrates a thin film solar cell system according to an embodiment of the present invention. 
         FIG. 15  schematically illustrates a cross-sectional view of a thin film solar cell of  FIG. 14  along the A-A′ line. 
         FIG. 16  schematically illustrates an embodiment of the electrically connecting of the first photovoltaic cell and the second photovoltaic cell. 
         FIG. 17  schematically illustrates a top view of a thin film solar cell system according to an embodiment of the present invention, wherein the current matching module and the thin film solar cell module are electrically connected in another way. 
         FIG. 18  schematically illustrates a top view of a thin film solar cell system according to an embodiment of the present invention. 
         FIG. 19  schematically illustrates a cross-sectional view of a thin film solar cell module of  FIG. 18  along the B-B′ line. 
         FIG. 20  schematically illustrates a cross-sectional view of  FIG. 18  along the C-C′ line, an embodiment of wherein the first photovoltaic cell and the second photovoltaic cell are electrically connected. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
       FIG. 1  schematically illustrates a thin film solar cell according to an embodiment of the present invention.  FIG. 2  schematically illustrates a cross-sectional view along the B-B′ line of  FIG. 1  of the thin film solar cell. Referring to  FIG. 1  and  FIG. 2 , a thin film solar cell  200  includes a substrate  210 , a plurality of photovoltaic cells  202  and at least a control unit  204 . In this embodiment, the substrate  210  can be a transparent substrate, such as a glass substrate. 
     The photovoltaic cells  202  are disposed on the substrate  210 , and each of them generates photocurrents  202   a  and  202   b  after illuminated respectively. In which each of the photovoltaic cells  202  includes a first conductive layer  220 , a photovoltaic layer  230  and a second conductive layer  240 . In details, the first conductive layer  220  is disposed on the substrate  210 . The photovoltaic layer  230  is disposed on the first conductive layer  220  and having an opening H exposing the first conductive layer  220 . The second conductive layer  240  is disposed on the photovoltaic layer  230  through the opening H and electrically connected to the first conductive layer  220  of the adjacent photovoltaic cell  202 , as shown in  FIG. 2 . Precisely speaking, the above mentioned photovoltaic cells  202  are connected in electrical series to one another, for example. 
     In this embodiment, the first conductive layer  220  is a transparent conductive layer, for example, and the material thereof can be at least one of the zinc oxide, indium tin oxide (ITO), indium zinc oxide (IZO), indium tin zinc oxide (ITZO), aluminium tin oxide (ATO), aluminium zinc oxide (AZO), cadmium indium oxide (CIO), cadmium zinc oxide (CZO), gallium zinc oxide (GZO) and fluorine tin oxide (FTO). In another embodiment (not shown), the first conductive layer  220  can be a stacked layer of a reflective layer (not shown) and the above-mentioned transparent conductive layer, and the reflective layer is disposed between the transparent conductive layer and the substrate. The material of the reflective layer can be a metal with higher reflectivity, such as aluminium (Al), silver (Ag), molybdenum (Mo) or copper (Cu). 
     In this embodiment, the material of the photovoltaic layer  230  can be a semiconductor thin film in Group IV elements of the Periodic Table, Group III-V compound semiconductor thin film, Group II-VI compound semiconductor thin film, organic semiconductor thin film or compound thereof. In details, the semiconductor thin film in Group IV elements of the Periodic Table is at least one of a carbon thin film, a silicon thin film, a germanium thin film, a silicon carbide thin film and a silicon germanium thin film, each of which may be in monocrystalline form, polycrystalline form, amorphous form or microcrystalline form, or a combination thereof. For example, the compound semiconductor thin film in Group III-V of the Periodic Table is at least one of gallium arsenide (GaAs) thin film and indium gallium phosphide (InGaP) thin film, or a combination thereof. The compound semiconductor thin film in Group II-VI, for example, includes at least one of a copper indium diselenide (CIS) thin film, a copper indium gallium diselenide (GIGS) thin film and a cadmium telluride (CdTe) thin film, or a combination thereof. Furthermore, the above mentioned organic compound semiconductor thin film can be a mixture of a conjugated polymer donor and PCBM acceptor. 
     In addition, the film structure of the above mentioned photovoltaic layer  230  can be a PN single layer of photoelectric conversion structure composed of P-type semiconductor and N-type semiconductor or a PIN single layer of photoelectric conversion structure composed of P-type semiconductor, intrinsic layer and N-type semiconductor. However, the present invention is not limited thereto. In another embodiment, the film structure of the photovoltaic layer  230  can be a stacked structure of a tandem junction, a triple junction or more than three-layers of photoelectric conversion film structure. 
     In this embodiment, the material of the above-mentioned transparent conductive layer can be used in the second conductive layer  240 , and the details are not iterated herein. In this embodiment, the second conductive layer  240  can further include a reflective layer disposed on the transparent conductive layer. It is noted that when the second conductive layer  240  includes a reflective layer, the first conductive layer  220  can only be a transparent conductive layer. On the contrary, when the first conductive layer  220  includes a reflective layer, the second conductive layer  240  can only be a transparent conductive layer without a reflective layer thereon. In an embodiment, each of the first conductive layer  220  and the second conductive layer  250  can be a single transparent conductive layer without a reflective layer thereon. In other words, the design of the first conductive layer  220  and the second conductive layer  240  can be adjusted according to the users&#39; requirements (e.g. for manufacturing a thin film solar cell with double-sided illumination or a thin film solar cell with one-sided illumination). The design of the first conductive layer  220  and the second conductive layer  240  described above is provided only for illustration purposes, and is not construed as limiting the present invention. 
     Referring to  FIG. 1 , the control unit  204  is electrically connected to the photovoltaic cell  202 , wherein the photocurrent generated by the photovoltaic cell  202  after illuminated  202   a  can be readily detected by the control unit  204 . Photocurrents  202   a  generated by some of the photovoltaic cells  202  may be different in magnitude. It may due to the process variation or other factors in the manufacturing process and result the photocurrent unmatching condition. In other words, when at least one of the photocurrents  202   b  generated by the photovoltaic cell  202  is different in magnitude from the photocurrents  202   a  generated by the other photovoltaic cell  202 , the control unit  204  can automatically provide a compensable current  204   a  to the photovoltaic cells  202  which generates the photocurrents  202   b  in order to obtain the current matching of the photocurrents generated by the overall photovoltaic cells  202 . Wherein in order to make the current to be superimposed, the control unit  204  is electrically connected in parallel to each of the photovoltaic cells  202 . 
     In other words, since the photovoltaic cells  202  are electrically connected in series with one another in the thin film solar cell  200 , the overall photoelectric conversion efficiency will be restricted due to the current unmatching condition resulted by when the magnitude of the photocurrent  202   b  generated by some of the photovoltaic cells  202  is less than the photocurrent  202   a  generated by other photovoltaic cells  202 . Thus, the control unit  204  of this embodiment not only can readily detect the photocurrents  202   a  and  202   b  generated by the photovoltaic cells  202 , but also can provide a compensable current  204   a  to the photovoltaic cells  202  which generate the smaller photocurrent  202   b . Wherein the control unit  204  and the photovoltaic cell  202  are electrically connected in parallel, so the output of the photocurrent  202   b  generated by the photovoltaic cell  202  can be improved to make all the photocurrents  202   a  and  202   b  generated by the photovoltaic cells  202  which are connected in series to be current matching. In this way, the photoelectric conversion efficiency of the thin film solar cell  200  can be improved. 
     In this embodiment, the control unit  204  is an Application-Specific Integrated Circuit (ASIC), for example. Wherein, the control unit  204  can be connected to each of the photovoltaic cells  202  by means of external electrical connection, such as a conducting wire or a bonding wire method. Additionally, since the control unit  204  is electrically connected to each of the photovoltaic cells  202  in parallel, the anode and cathode of the control unit  204  is electrically connected to the first conductive layer  220  and the second conductive layer  240 , respectively.  FIG. 1  illustrates the number of control units  204  is one as an example. But in another embodiment, the control units  204  can be in a plurality. This means that every single photovoltaic cell  202  can be electrically connected in parallel to a control unit  204  in order to control the photocurrent generated by each of the photovoltaic cells  202 . The number of control units  204  can be decided according to the users&#39; requirement and design, the present invention is not limited thereto. In addition, when the number of control units  204  is more than two, the control units  204  can be placed at the same side or different side of the photovoltaic cells  202 . It means that the control unit  204  can be placed around the photovoltaic cells  202 . In another embodiment, the control unit  204  can also be obtained by the way the semiconductor manufacturing process of integrating the above mentioned ASIC to the layer of photovoltaic cell  202 . 
     The following describes the method of manufacturing and optimization of the above mentioned thin film solar cell  200 .  FIG. 3  schematically illustrates a process flow of manufacturing and optimization of a thin film solar cell according to an embodiment of the present invention.  FIG. 4  schematically illustrates a process flow of the photovoltaic cell of  FIG. 3  formed on the substrate. Referring to  FIG. 3  and  FIG. 4 , first of all, the above mentioned substrate  210  is provided, and the substrate  210  is a glass substrate, for example. 
     And then step  302  is proceeded to and formed a plurality of above mentioned photovoltaic cells  202  on the substrate  210  as shown in  FIG. 2 . In this embodiment, the method to form the photovoltaic cell  202  is as illustrated in the step flow chart of  FIG. 4 . In details, referring to step  302   a  of  FIG. 4 , the first conductive material layer is formed on the substrate  210  (not shown), wherein the above mentioned transparent conductive material, for example, is used in the first conductive material layer and the method to form the first conductive material layer is sputtering, chemical vapour deposition (CVD) or evaporation. 
     Then, step  302   b  of  FIG. 4  is proceeded to. The first conductive material layer is patterned to form the above mentioned first conductive layer  220  of each of the photovoltaic cells  202  as shown in  FIG. 2 . In this embodiment, laser etching method is taken as an example for the method to pattern the first conductive material layer and any other appropriate etching process can be used in another embodiment. Afterward, step  302   c  of  FIG. 4  is proceeded to. Photovoltaic material layer is formed on the substrate  210  (not shown) to cover the first conductive layer  220  of the photovoltaic cells  202 . In this embodiment, the method to form the photovoltaic layer  230 , for example, can be Radio Frequency Plasma Enhanced Chemical Vapour Deposition (RF PECVD), Very High Frequency Plasma Enhanced Chemical Vapour Deposition (VHF PECVD) or Microwave Plasma Enhanced Chemical Vapour Deposition (MW PECVD). 
     After that, step  302   d  of  FIG. 4  is proceeded to. Photovoltaic material layer is patterned to form a plurality of openings H, wherein the openings H are exposing the first conductive layer  220  of the photovoltaic cells  202 , respectively, as shown in  FIG. 2 . In this embodiment, the method to form the number of openings H is, for example, using the laser cutting, etching or mechanical removal process. And proceed to step  302   e  of  FIG. 4 , the second conductive material layer is formed on the substrate  210  to cover the photovoltaic material layer. In which the second conductive layer  240  is generally used as the back contact of the photovoltaic cells  202  as shown in  FIG. 2 . In this embodiment, the method to form the second conductive layer  240  is, for example, sputtering, chemical vapour deposition (CVD) or evaporation and the material can be the above mentioned transparent conductive material. The details are not iterated herein. 
     Hereafter, step  302   f  of  FIG. 4  is proceeded to. The second conductive material layer and the photovoltaic material layer is patterned to form the second conductive layer  240  and the photovoltaic layer  230  of the photovoltaic cell  202  as shown in  FIG. 2 . Wherein, the second conductive layer  240  of each of the photovoltaic cells  202  is electrically connected to the first conductive layer  220  of the adjacent photovoltaic cell  202  through the opening H. At this point, the formation of photovoltaic cell  202  on the substrate  210  which is shown in step  302  of  FIG. 3  can be completed by following the steps  302   a  to  302   f  of  FIG. 4  in sequence. 
     After completing above mentioned step  302 , step  303  of  FIG. 3  is proceeded to. At least one of the control units  204  is electrically connected to the photovoltaic cells  202  as shown in  FIG. 1 . In this embodiment, the method of electrically connecting the control unit  204  to the photovoltaic cell  202  can be the laser welding process or wire bonding process. In another embodiment, the method of electrically connecting the control unit  204  to the photovoltaic cell  202  can be integrating the control unit  204  into the layer of photovoltaic cell  202 . Wherein, the anode and the cathode of the control unit  204  is electrically connected to the first conductive layer  220  and the second conductive layer  240  of each of the photovoltaic cells  202 , respectively. 
     Then, referring to  FIG. 1  and step  304  of  FIG. 3 , the above mentioned control unit  204  is used to detect the magnitude of photocurrents  202   a  and  202   b  generated by each of the photovoltaic cells  202  after illuminated. Finally, in step  305 , when at least one of the photocurrent  202   a  generated by the photovoltaic cell  202  is different from the photocurrent  202   b  generated by other photovoltaic cells  202 , the control unit  204  will provide the above mentioned compensable current  204   a  to the photovoltaic cell  202  in order to enable the current matching of the photocurrents generated by the overall photovoltaic cells  202 . Wherein, the details of current matching mechanism are illustrated in above mentioned embodiment for reference and the details are not iterated herein. After completing the steps  301  to  305 , method of manufacturing and optimization of a thin film solar cell is completed. 
       FIG. 5  schematically illustrates a top view of a thin film solar cell according to an embodiment of the present invention.  FIG. 6  schematically illustrates a cross-sectional view of a thin film solar cell of  FIG. 5  along the B-B′ line.  FIG. 7  schematically illustrates a cross-sectional view of a thin film solar cell of  FIG. 5  along the C-C′ line, wherein the first photovoltaic cell and the second photovoltaic cell is not electrically connected. And  FIG. 8  schematically illustrates a cross-sectional view of  FIG. 5  along the C-C′ line, an embodiment of wherein the first photovoltaic cell and the second photovoltaic cell are electrically connected. Referring to  FIG. 5  to  FIG. 7 , a thin film solar cell  200 ′ includes a substrate  210 ′, a plurality of photovoltaic cells  202 ′ and a second photovoltaic cell  204 ′. In this embodiment, the substrate  210 ′ can be a transparent substrate, such as a glass substrate. The second photovoltaic cell  204 ′ expands in an expansion direction D 2 ′. And the first photovoltaic cell  202 ′ expands in an expansion direction D 1 ′, for example, wherein the expansion direction D 2 ′ is perpendicular to the expansion direction D 1 ′. This means that the first photovoltaic cell  202 ′ can be arranged in the expansion direction D 2 ′. The above description depends on the user&#39;s requirement, it is provided only for illustration purposes, and is not construed as limiting the present invention. 
     The first photovoltaic cells  202 ′ are disposed on the substrate  210 ′, and each of them generates photocurrents after illuminated respectively. In which each of the first photovoltaic cells  202 ′ includes a first conductive layer  220 ′, a photovoltaic layer  230 ′ and a second conductive layer  240 ′. In details, the first conductive layer  220 ′ is disposed on the substrate  210 ′. The photovoltaic layer  230 ′ is disposed on the first conductive layer  220 ′ and having an opening H′ exposing the first conductive layer  220 ′. The second conductive layer  240 ′ is disposed on the photovoltaic layer  230 ′ through the opening H′ and electrically connected to the first conductive layer  220 ′ of the adjacent photovoltaic cell  202 ′, as shown in  FIG. 6 . Precisely speaking, the above mentioned first photovoltaic cells  202 ′ are connected in electrical series to one another, for example. 
     In this embodiment, the first conductive layer  220 ′ is a transparent conductive layer, for example, and the material thereof can be at least one of the zinc oxide, indium tin oxide (ITO), indium zinc oxide (IZO), indium tin zinc oxide (ITZO), aluminium tin oxide (ATO), aluminium zinc oxide (AZO), cadmium indium oxide (CIO), cadmium zinc oxide (CZO), gallium zinc oxide (GZO) and fluorine tin oxide (FTO). In another embodiment (not shown), the first conductive layer  220 ′ can be a stacked layer of a reflective layer (not shown) and the above-mentioned transparent conductive layer, and the reflective layer is disposed between the transparent conductive layer and the substrate. The material of the reflective layer can be a metal with higher reflectivity, such as aluminium (Al), silver (Ag), molybdenum (Mo) or copper (Cu). 
     In this embodiment, the material of the photovoltaic layer  230 ′ can be a semiconductor thin film in Group IV elements of the Periodic Table, Group III-V compound semiconductor thin film, Group II-VI compound semiconductor thin film, organic semiconductor thin film or compound thereof. In details, the semiconductor thin film in Group IV elements of the Periodic Table is at least one of a carbon thin film, a silicon thin film, a germanium thin film, a silicon carbide thin film and a silicon germanium thin film, each of which may be in monocrystalline form, polycrystalline form, amorphous form or microcrystalline form, or a combination thereof. For example, the compound semiconductor thin film in Group III-V of the Periodic Table is at least one of gallium arsenide (GaAs) thin film and indium gallium phosphide (InGaP) thin film, or a combination thereof. The compound semiconductor thin film in Group II-VI, for example, includes at least one of a copper indium diselenide (CIS) thin film, a copper indium gallium diselenide (CIGS) thin film and a cadmium telluride (CdTe) thin film, or a combination thereof. Furthermore, the above mentioned organic compound semiconductor thin film can be a mixture of a conjugated polymer donor and PCBM acceptor. 
     In addition, the film structure of the above mentioned photovoltaic layer  230 ′ can be a PN single layer of photoelectric conversion structure composed of P-type semiconductor and N-type semiconductor or a PIN single layer of photoelectric conversion structure composed of P-type semiconductor, intrinsic layer and N-type semiconductor. However, the present invention is not limited thereto. In another embodiment, the film structure of the photovoltaic layer  230  can be a stacked structure of a tandem junction, a triple junction or more than three-layers of photoelectric conversion film structure. 
     In this embodiment, the material of the above-mentioned transparent conductive layer can be used in the second conductive layer  240 ′, and the details are not iterated herein. In this embodiment, the second conductive layer  240 ′ can further include a reflective layer disposed on the transparent conductive layer. It is noted that when the second conductive layer  240 ′ includes a reflective layer, the first conductive layer  220 ′ can only be a transparent conductive layer. On the contrary, when the first conductive layer  220 ′ includes a reflective layer, the second conductive layer  240 ′ can only be a transparent conductive layer without a reflective layer thereon. In an embodiment, each of the first conductive layer  220 ′ and the second conductive layer  250 ′ can be a single transparent conductive layer without a reflective layer thereon. In other words, the design of the first conductive layer  220 ′ and the second conductive layer  240 ′ can be adjusted according to the users&#39; requirements (e.g. for manufacturing a thin film solar cell with double-sided illumination or a thin film solar cell with one-sided illumination). The design of the first conductive layer  220 ′ and the second conductive layer  240 ′ described above is provided only for illustration purposes, and is not construed as limiting the present invention. 
     Referring to  FIG. 5  and  FIG. 7 , the second photovoltaic cell  204 ′ is disposed on the substrate  210 ′. Wherein, when the photocurrents generated by the first photovoltaic cells  202 ′ is different, the second photovoltaic cell  204 ′ can be electrically connected to at least part of the first photovoltaic cells  202 ′ in order to obtain current matching of the photocurrents generated by the first photovoltaic cells  202 ′. For example, since the first photovoltaic cells  202 ′ are electrically connected in series with one another in the thin film solar cell  200 ′, the overall photoelectric conversion efficiency will be restricted due to the current unmatching condition resulted by when the magnitude of the photocurrent generated by some of the first photovoltaic cells  202 ′ is less than the photocurrent generated by other first photovoltaic cells  202 ′. Thus, the second photovoltaic cell  204 ′ of this embodiment and the first photovoltaic cell  202 ′ can be electrically connected in parallel, so the output of the photocurrent generated by the first photovoltaic cell  202 ′ can be improved to make all the photocurrents generated by the first photovoltaic cells  202 ′ which are connected in series to be current matching. In this way, the photoelectric conversion efficiency of the thin film solar cell  200 ′ can be improved. 
     In this embodiment, the second photovoltaic cell  204 ′ includes a first conductive layer  220   a ′, a photovoltaic layer  230   a ′ and a second conductive layer  240   a ′. Similar to the above mentioned first photovoltaic cell  202 ′, the first conductive layer  220   a ′ of the second photovoltaic cell  204 ′ is disposed on the substrate  210 ′. The photovoltaic layer  230   a ′ is disposed on the first conductive layer  220   a ′ and the second conductive layer  240   a ′ is disposed on the photovoltaic layer  230   a ′. In an embodiment, if the magnitude of photocurrent generated by one of the above mentioned first photovoltaic cells  202 ′ is smaller than photocurrent generated by other first photovoltaic cells  202 ′, the second photovoltaic cell  204 ′ is adapted to electrically connect to the first photovoltaic cell  202 ′ in order to make the photocurrent generated by the first photovoltaic cell  202 ′ and the photocurrent generated by other first photovoltaic cell  202 ′ be current matching. In details, the electrical connection of the first photovoltaic cell  202 ′ and the second photovoltaic cell  204 ′ is illustrated in  FIG. 8 . Wherein, for example, the first conductive layer  220   a ′ of the second photovoltaic cell  204 ′ is electrically connected to the first conductive layer  220 ′ of the first photovoltaic cell  202 ′ through the welding zone W 1 ′, and the second conductive layer  240   a ′ of the second photovoltaic cell  204 ′ is electrically connected to the second conductive layer  240 ′ of the first photovoltaic cell  202 ′ through the welding zone W 2 ′. That is the second photovoltaic cell  204 ′, for example, is electrically connected in parallel to the first photovoltaic cell  202 ′ which generates the smaller photocurrent, wherein each of the welding zone W 1 ′ and W 2 ′ represents a welding point. The present invention is not limited thereto. 
       FIG. 9  schematically illustrates a top view of a thin film solar cell according to an embodiment of the present invention. Referring to  FIG. 9 , the component of thin film solar cell  300 ′ is similar to above mentioned thin film solar cell  200 ′. In which the same component is illustrated in the same symbol and the details are not iterated herein. 
     In this embodiment, the thin film solar cell  300 ′ includes three second photovoltaic cells  304   a ′,  304   b ′ and  304   c ′ and every four of first photovoltaic cells  202 ′ corresponds to each of the second photovoltaic cells  304   a ′,  304   b ′ and  304   c ′. For example, when the photocurrent generated by each of the first photovoltaic cell  202 ′ which is one of the four first photovoltaic cells  202 ′ corresponding to the second photovoltaic cell  304   a ′ is different in magnitude, the second photovoltaic cell  304   a ′ can be electrically connected to the first photovoltaic cell  202 ′, the one which generates the smallest photocurrent, in order to make the photocurrents of that four first photovoltaic cells  202 ′ current matching. Similarly, the same way can be used to obtain current matching by electrically connecting the second photovoltaic cell  304   b ′ and the second photovoltaic cell  304   c ′ to an adjacent first photovoltaic cell  202 ′, respectively. In this way, the overall photoelectric conversion efficiency of the thin film solar cell  300 ′ can be improved. 
     However, neither the number of second photovoltaic cells  304   a ′,  304   b ′ and  304   c ′ nor that of first photovoltaic cells  202 ′ which correspond to second photovoltaic cells  304   a ′,  304   b ′ and  304   c ′ is not limited in present invention. In other embodiment, the number of second photovoltaic cells  304 ′ can be two, three or more. And the number of first photovoltaic cells  202 ′ which correspond to second photovoltaic cells  304   a ′,  304   b ′ and  304   c ′ can also be changed according to users&#39; requirement. 
     On the other hand, in the thin film solar cell  300 ′, each of the second photovoltaic cells  304   a ′,  304   b ′ and  304   c ′ includes a photovoltaic zone P 1 ′, P 2 ′ and P 3 ′, respectively. And areas of each of the photovoltaic zone P 1 ′, P 2 ′ and P 3 ′ of the second photovoltaic cells  304   a ′,  304   b ′ and  304   c ′ are the same. But the present invention is not limited thereto.  FIG. 10  schematically illustrates a top view of a thin film solar cell according to an embodiment of the present invention.  FIG. 11  schematically illustrates a top view of a thin film solar cell according to an embodiment of the present invention. In another embodiment as shown in  FIG. 10 , the areas of photovoltaic zone P 4 ′ of second photovoltaic cells  404   a ′, photovoltaic zone P 5 ′ of second photovoltaic cells  404   b ′ and photovoltaic zone P 6 ′ of second photovoltaic cells  404   c ′ are not the same with each other. 
     In the embodiment illustrated in  FIG. 9 , all of the second photovoltaic cells  304   a ′,  304   b ′ and  304   c ′ are placed at one side  202   a ′ of first photovoltaic cell  202 , i.e., at one end of the first photovoltaic cell  202 ′ of the expansion direction D 1 &#39;. And in the embodiment illustrated in  FIG. 10 , all of the second photovoltaic cells  404   a ′,  404   b ′ and  404   c ′ are placed at one side  202   a ′ of first photovoltaic cell  202 ′. Yet the thin film solar cell  500 ′ of another embodiment as illustrated in  FIG. 11 , wherein, the second photovoltaic cell  504   a ′ is placed at one side  202   a ′ of first photovoltaic cell  202 ′ and the second photovoltaic cell  504   b ′ is placed at the other side  202   b ′ of first photovoltaic cell  202 ′. This means that the second photovoltaic cell  504   a ′ and the second photovoltaic cell  504   b ′ are placed at the opposite ends of first photovoltaic cell  202 ′ of the expansion direction D 1 &#39;. Thus the locations of the second photovoltaic cells are not limited in present invention. In some embodiments the second photovoltaic cells can placed at different sides of first photovoltaic cells. 
     In other embodiment (not shown), when the photocurrents generated by first photovoltaic cells are in good current matching conditions, the second photovoltaic cells can be electrically connected in series to first photovoltaic cells. Otherwise, the second photovoltaic cells can be divided into a plurality of subunits. Each of the subunits can be electrically connected in parallel to the first photovoltaic cells respectively in order to make full use of the second photovoltaic cells to generate photocurrents. In this way, the areas of the second photovoltaic cells being occupied in the thin film solar cell will not be wasted. 
     The following describes the manufacturing method of the above mentioned thin film solar cell  200 ′ with the illustrations of the cross-sectional structure along the B-B′ line and C-C′ line of  FIG. 5  and the steps, accordingly. 
       FIGS. 12A to 12G  schematically illustrate a process flow of manufacturing a thin film solar cell according to an embodiment of the present invention. Referring to  FIG. 12A , at first the substrate  210 ′ is provided. And the substrate  210 ′ can be a glass substrate, for example. 
     After that, as shown in  FIG. 10B , the first conductive material layer C 1 ′ is formed on the substrate. In this embodiment, the first conductive material layer C 1 ′ can be the above mentioned transparent conductive material, for example. And the method to form the first conductive material layer is sputtering, chemical vapour deposition (CVD) or evaporation. 
     Then, as shown in  FIG. 10C , first conductive material layer C 1 ′ is patterned to form first conductive layer  220 ′ of each of the first photovoltaic cells  202 ′. In this embodiment, laser etching method is taken as an example for the method to pattern the first conductive material layer C 1 ′ and any other appropriate etching process can be used in another embodiment. 
     Afterward, as shown in  FIG. 10D , photovoltaic material layer M′ is formed on the substrate to cover the first conductive layer  220 ′ of the first photovoltaic cells  202 ′. In this embodiment, the method to form the photovoltaic layer  230 ′, for example, can be Radio Frequency Plasma Enhanced Chemical Vapour Deposition (RF PECVD), Very High Frequency Plasma Enhanced Chemical Vapour Deposition (VHF PECVD) or Microwave Plasma Enhanced Chemical Vapour Deposition (MW PECVD). 
     After that, as shown in  FIG. 10E , photovoltaic material layer M is patterned to form a plurality of openings H′, wherein the openings H′ are exposing the first conductive layer  220 ′ of the first photovoltaic cells  202 ′, respectively. In this embodiment, the method to form the number of openings H′ is, for example, using the laser cutting, etching or mechanical removal process. 
     And as shown in  FIG. 10F , second conductive material layer C 2 ′ is formed on the substrate  210 ′ to cover the photovoltaic material layer M′. In which the second conductive layer  240 ′ is generally used as the upper electrode of the photovoltaic cells  202 ′. In this embodiment, the method to form the second conductive layer  240 ′ is, for example, sputtering, chemical vapour deposition (CVD) or evaporation and the material can be the above mentioned transparent conductive material. The details are not iterated herein. 
     Hereafter, as shown in  FIG. 10G , the second conductive material layer C 2 ′ and the photovoltaic material layer M′ is patterned to form the second conductive layer  240 ′ and the photovoltaic layer  230 ′ of the first photovoltaic cell  202 ′. Wherein, the second conductive layer  240 ′ of each of the first photovoltaic cells  202 ′ is electrically connected to the first conductive layer  220 ′ of the adjacent first photovoltaic cell  202 ′ through the opening H′. 
     In this embodiment, it has to be specified that in the process of patterning which is mentioned in  FIG. 12G , laser process, etching or mechanical removal process can be used to separate the second photovoltaic cells  204 ′ and the first photovoltaic cells  202 ′. This means that the first photovoltaic cells  202 ′ and the second photovoltaic cells  204 ′ are formed on the substrate  210 ′ simultaneously, in this embodiment. 
     Next, the magnitude of photocurrents generated by the first photovoltaic cells  202 ′ is detected. In this embodiment, the method of detecting the magnitude of photocurrent is illuminating a uniform light to each of the first photovoltaic cells  202 ′ and detecting with the photocurrent detecting device. The above description is provided only for illustration purposes. In other possible embodiment, persons skilled in the art can use any other appropriate detecting methods to detect the photocurrent. The details are not iterated herein. 
     After that, the second photovoltaic cell  204 ′ is electrically connected to one of the first photovoltaic cells  202 ′ in order to obtain current matching of the photocurrents generated by the overall first photovoltaic cells  202 ′. Wherein, the implementation method of electrically connection is illustrated in the following, but the present invention is not limited thereto. 
       FIGS. 13A and 13B  schematically illustrate a method of electrically connecting between the first photovoltaic cell and the second photovoltaic cell according to an embodiment of the present invention. 
     First, referring to  FIG. 13A , the first conductive layer  220   a ′ of the second photovoltaic cell  204 ′ is electrically connected to the first conductive layer  220 ′ of the first photovoltaic cell  202 ′ which generates the smaller photocurrent with laser welding process, and it is illustrated as the welding zone W 1 ′ in the figure. Then, referring to  FIG. 13B , the second conductive layer  240 ′ of the second photovoltaic cell  204 ′ is electrically connected to the second conductive layer  240   a ′ of the first photovoltaic cell  202 ′ which generates the smaller photocurrent with laser welding process, and it is illustrated as the welding zone W 2 ′ in the figure. At this point, the manufacturing process of the above mentioned thin film solar cell  200 ′ illustrated in  FIG. 5  is completed. 
       FIG. 14  schematically illustrates a thin film solar cell system according to an embodiment of the present invention.  FIG. 15  schematically illustrates a cross-sectional view of a thin film solar cell of  FIG. 14  along the A-A′ line.  FIG. 16  schematically illustrates an embodiment of the electrically connecting of the first photovoltaic cell and the second photovoltaic cell. 
     Referring to  FIG. 14  and  FIG. 15 , the thin film solar cell system  200 ″ includes a plurality of thin film solar cell modules  210 ″ and a current matching module  220 ″. In which the number of current matching modules  220 ″ is illustrated one as an example. The number of current matching modules  220 ″ depends on the users&#39; requirement and is not limited in the thin film solar cell system  200 ″ in the present invention. 
     The thin film solar cell modules  210 ″ are connected in electrical series with one another and each providing a photocurrent respectively. Each of the thin film solar cell modules  210 ″ at least includes a substrate  212 ″, a first conductive layer  214 ″, a photovoltaic layer  216 ″ and a second conductive layer  218 ″. In this embodiment, the substrate  212 ″ can be a transparent substrate, for example, a glass substrate. The first conductive layer  214 ″ is disposed on the substrate  212 ″. The photovoltaic layer  216 ″ is disposed on the first conductive layer  214 ″. The second conductive layer  218 ″ is disposed on the photovoltaic layer  216 ″. 
     In this embodiment, each of the thin film solar cell  200 ″ is composed with a plurality of thin film solar cells  210   a ″ electrically connecting in series with each other. This means that the second conductive layer  218 ″ of each of the thin film solar cells  210   a ″ is electrically connected to the first conductive layer  214 ″ of the adjacent thin film solar cell  210   a ″ through the opening H″ as shown in  FIG. 15 . It is worth mentioning that the number of thin film solar cells  210   a ″ of each of thin film solar cell system  210 ″ is not limited in present invention. This means that in other possible embodiment, the thin film solar cell system  210 ″ can include a single thin film solar cell  210   a″.    
     In this embodiment, the first conductive layer  214 ″ is a transparent conductive layer, for example, and the material thereof can be at least one of the zinc oxide, indium tin oxide (ITO), indium zinc oxide (IZO), indium tin zinc oxide (ITZO), aluminium tin oxide (ATO), aluminium zinc oxide (AZO), cadmium indium oxide (CIO), cadmium zinc oxide (CZO), gallium zinc oxide (GZO) and fluorine tin oxide (FTO). In another embodiment (not shown), the first conductive layer  214 ″ can be a stacked layer of a reflective layer (not shown) and the above-mentioned transparent conductive layer, and the reflective layer is disposed between the transparent conductive layer and the substrate. The material of the reflective layer can be a metal with higher reflectivity, such as aluminium (Al), silver (Ag), molybdenum (Mo) or copper (Cu). 
     In this embodiment, the material of the photovoltaic layer  216 ″ can be a semiconductor thin film in Group IV elements of the Periodic Table, Group III-V compound semiconductor thin film, Group II-VI compound semiconductor thin film, organic semiconductor thin film or compound thereof. In details, the semiconductor thin film in Group IV elements of the Periodic Table is at least one of a carbon thin film, a silicon thin film, a germanium thin film, a silicon carbide thin film and a silicon germanium thin film, each of which may be in monocrystalline form, polycrystalline form, amorphous form or microcrystalline form, or a combination thereof. For example, the compound semiconductor thin film in Group III-V of the Periodic Table is at least one of gallium arsenide (GaAs) thin film and indium gallium phosphide (InGaP) thin film, or a combination thereof. The compound semiconductor thin film in Group II-VI, for example, includes at least one of a copper indium diselenide (CIS) thin film, a copper indium gallium diselenide (CIGS) thin film and a cadmium telluride (CdTe) thin film, or a combination thereof. Furthermore, the above mentioned organic compound semiconductor thin film can be a mixture of a conjugated polymer donor and PCBM acceptor. 
     In addition, the film structure of the above mentioned photovoltaic layer  216 ″ can be a PN single layer of photoelectric conversion structure composed of P-type semiconductor and N-type semiconductor or a PIN single layer of photoelectric conversion structure composed of P-type semiconductor, intrinsic layer and N-type semiconductor. However, the present invention is not limited thereto. In another embodiment, the film structure of the photovoltaic layer  216 ″ can be a stacked structure of a tandem junction, a triple junction or more than three-layers of photoelectric conversion film structure. 
     In this embodiment, the material of the above-mentioned transparent conductive layer can be used in the second conductive layer  218 ″, and the details are not iterated herein. In this embodiment, the second conductive layer  218 ″ can further include a reflective layer disposed on the transparent conductive layer. It is noted that when the second conductive layer  218 ″ includes a reflective layer, the first conductive layer  214 ″ can only be a transparent conductive layer. On the contrary, when the first conductive layer  218 ″ includes a reflective layer, the second conductive layer  218 ″ can only be a transparent conductive layer without a reflective layer thereon. In an embodiment, each of the first conductive layer  214 ″ and the second conductive layer  218 ″ can be a single transparent conductive layer without a reflective layer thereon. In other words, the design of the first conductive layer  214 ″ and the second conductive layer  218 ″ can be adjusted according to the users&#39; requirements (e.g. for manufacturing a thin film solar cell with double-sided illumination or a thin film solar cell with one-sided illumination). The design of the first conductive layer  214 ″ and the second conductive layer  218 ″ described above is provided only for illustration purposes, and is not construed as limiting the present invention. 
     Referring to  FIG. 14  and  FIG. 15 , the current matching module  220 ″ is a thin film solar cell, for example, and includes a substrate  222 ″, a first conductive layer  224 ″, a photovoltaic layer  226 ″ and a second conductive layer  228 ″. The first conductive layer  224 ″ is disposed on the substrate  222 ″ of the current matching module  220 ″. The photovoltaic layer  226 ″ is disposed on the first conductive layer  224 ″ of the current matching module  220 ″. The second conductive layer  228 ″ is disposed on the photovoltaic layer  226 ″ of the current matching module  220 ″. The material used in the substrate  222 ″, the first conductive layer  224 ″, the photovoltaic layer  226 ″ and the second conductive layer  228 ″ is generally the same with the above mentioned substrate  212 ″, first conductive layer  214 ″, photovoltaic layer  216 ″ and second conductive layer  218 ″. The details are not iterated herein. 
     In this embodiment, the disposing of current matching module  220 ″ can improve the overall current output efficiency of the thin film solar cell system  200 ″. For example, when the photocurrent C 203 ″ provided by at least one of the thin film solar cell modules  210 ″ is different from the photocurrents C 201 ″ provided by the other thin film solar cell modules  210 ″, the current matching module  220 ″ can be electrically connected to the thin film solar cell module  210 ″ which generates smaller photocurrents in order to obtain current matching of the photocurrents provided by the thin film solar cell modules  210 ″ (i.e., to make the photocurrent C 203 ″ and the photocurrent C 201 ″ equal). In this way, the overall current output efficiency of the thin film solar cell system  200 ″ can be improved. 
     The following illustrates an example of the method of electrically connecting in parallel between the above current matching module  220 ″ and the thin film solar cell module  210 ″. The first conductive layer  224 ″ of the current matching module  220 ″ is electrically connected to the first conductive layer  214 ″ of the thin film solar cell module  210 ″ through a cable C 1 ″. And the second conductive layer  228 ″ of the current matching module  220 ″ is electrically connected to the second conductive layer  218 ″ of the thin film solar cell module  210 ″ through another cable C 1 ″ as shown in  FIG. 16 . In an embodiment, the current matching module  220 ″, for example, is electrically connected to one of the thin film solar cells  210   a ″ of the thin film solar cell module  240 ″. Or in another embodiment, the current matching module  220 ″ can also be electrically connected in parallel with the whole thin film solar cell module  210 ″ as illustrated in  FIG. 17 . 
     Referring to  FIG. 14 , in this embodiment, a photocurrent detecting device  230 ″ can be selectively disposed in the thin film solar cell system  200 ″ to detect the photocurrent generated by each of the thin film solar cell modules  210 ″. In which the photocurrent detecting device  230 ″ can be any other appropriate detecting device which is chosen by person skilled in the art and the details are not iterated herein. However, the disposing of photocurrent detecting device  230 ″ is not essential. In other embodiment, the thin film solar cell system can be without a photocurrent detecting device. 
     In addition, in another embodiment, the above mentioned current matching module  220 ″ can be an external power supply unit. The current matching module  220 ″ is electrically connected in parallel to at least one of the thin film solar cell modules  210 ″ in order to obtain current matching of the photocurrents provided by the thin film solar cell modules  210 ″ (i.e., to make the photocurrent C 203 ″ and the photocurrent C 201 ″ equal). This means that it is not limited in present invention that the current matching module  220 ″ is the above mentioned thin film solar cell. For example, when the current matching module  220 ″ is an external power supply unit, only an external electric current is needed to provide to one of the thin film solar cell modules  210 ″ which generates smaller photocurrent C 203 ″ in order to make the photocurrents of overall thin film solar cell modules  210  current matching (i.e., to make the photocurrent C 203 ″ and the photocurrent C 201 ″ equal). In this way, the whole current output efficiency of the thin film solar cell system  200 ″ can be improved. 
     Since the thin film solar cell system  200 ″ includes the above mentioned current matching module  220 ″, when the photocurrents provided by the thin film solar cell modules  210 ″ are different from each other, the current matching modules  220 ″ can provide the current matching of the photocurrents of the thin film solar cell modules  210 ″ to improve the current output and thus the whole photoelectric conversion efficiency is ameliorated. 
       FIG. 18  schematically illustrates a top view of a thin film solar cell system according to an embodiment of the present invention.  FIG. 19  schematically illustrates a cross-sectional view of a thin film solar cell module of  FIG. 18  along the B-B′ line.  FIG. 20  schematically illustrates a cross-sectional view of  FIG. 18  along the C-C′ line, an embodiment of wherein the first photovoltaic cell and the second photovoltaic cell are electrically connected. 
     Referring to  FIG. 18  and  FIG. 19 , the thin film solar cell system  300 ″ includes a plurality of thin film solar cell modules  310 ″. The thin film solar cell modules  310 ″ are connected in electrical series with one another and each providing a photocurrent respectively. Each of the thin film solar cell modules  310 ″ at least includes a substrate  312 ″, a plurality of first photovoltaic cells  310   a ″ and at least a second photovoltaic cell  320 ″. In this embodiment, the substrate  312 ″ is a transparent substrate, for example, a glass substrate. 
     The first photovoltaic cells  310   a ″ are disposed on the substrate  312 ″. Each of the first photovoltaic cells  310   a ″ includes a first conductive layer  314 ″, a photovoltaic layer  316 ″ and a second conductive layer  318 ″. The first conductive layer  314 ″ is disposed on the substrate  312 ″. The photovoltaic layer  316 ″ is disposed on the first conductive layer  314 ″. The second conductive layer  318 ″ is disposed on the photovoltaic layer  316 ″. In which each of the second conductive layer  318 ″ of the first photovoltaic cells  310   a ″ is electrically connected to the first conductive layer  314 ″ of the adjacent first photovoltaic cell  310   a ″ through the opening H″ in order to let the first photovoltaic cells  310   a ″ be connected in series to each other. The material used in the first conductive layer  314 ″, the photovoltaic layer  316 ″ and the second conductive layer  318 ″ is generally the same with the above mentioned embodiment of first conductive layer  214 ″, photovoltaic layer  216 ″ and second conductive layer  218 ″. The details are not iterated herein. 
     Referring to  FIG. 20 , the second photovoltaic cell  320 ″ is disposed on the substrate  312 ″. In this embodiment, each of the second photovoltaic cells  320 ″ includes a first conductive layer  324 ″, a photovoltaic layer  326 ″ and a second conductive layer  328 ″. The first conductive layer  324 ″ is disposed on the substrate  312 ″. The photovoltaic layer  326 ″ is disposed on the first conductive layer  324 ″. The second conductive layer  328 ″ is disposed on the photovoltaic layer  326 ″. Similarly, the material used in the first conductive layer  324 ″, the photovoltaic layer  326 ″ and the second conductive layer  328 ″ is generally the same with the above mentioned embodiment of first conductive layer  214 ″, photovoltaic layer  216 ″ and second conductive layer  218 ″. The details are not iterated herein. 
     In the thin film solar cell system  300 ″, when the photocurrents C 303 ″ generated by at least one of the thin film solar cell modules  310 ″ are different from the photocurrents C 301 ″ generated by other thin film solar cell modules  310 ″, the second photovoltaic cell  320 ″ of the thin film solar cell module  310 ″ can be electrically connected in parallel to at least a part of the first photovoltaic cell  310   a ″ in order to obtain the current matching of the photocurrents generated by the overall thin film solar cell modules  310 ″ (i.e., to make the photocurrent C 203 ″ and the photocurrent C 201 ″ equal). In which when the second photovoltaic cell  320 ″ is electrically connected to at least a part of the first photovoltaic cell  310   a ″, the first conductive layer  324 ″ of the second photovoltaic cell  320 ″ can be electrically connected to the first conductive layer  314 ″ of the first photovoltaic cells  310   a ″ through the welding zone W 1 ″, for example, and the second conductive layer  328 ″ of the second photovoltaic cell  320 ″ can be electrically connected to the second conductive layer  318 ″ of the first photovoltaic cells  310   a ″ through the welding zone W 2 ″. 
     In another embodiment (not shown), when the photocurrents generated by first photovoltaic cells  310   a ″ are in good current matching conditions, the second photovoltaic cells  320 ″ can be electrically connected in series to first photovoltaic cells  310   a ″. Otherwise, the second photovoltaic cells  320 ″ can be divided into a plurality of subunits. Each of the subunits can be electrically connected in parallel to the first photovoltaic cells  310   a ″ respectively in order to make full use of the second photovoltaic cells  320 ″ to generate photocurrents. In this way, the areas of the second photovoltaic cells  320 ″ being occupied in the thin film solar cell modules  310 ″ will not be wasted. 
     In summary, the thin film solar cell of the present invention is designed with the control unit. Thus, when the photocurrents provided by the photovoltaic cells are different, the control unit can be electrically connected to the part of the first photovoltaic cells in order to improve the current matching of the photocurrents which are in series. In other words, the thin film solar cell of an embodiment of present invention has a better photoelectric conversion efficiency. Besides, the manufacturing and optimization method of the thin film solar cell of an embodiment of present invention can form the above mentioned control unit under the condition of without increasing the manufacturing process. Thus, the performance of the thin film solar cell can be improved in a simple way. 
     Since the thin film solar cell of the present invention is designed with a second photovoltaic cell, when the photocurrents provided by the first photovoltaic cells are different, the second photovoltaic cell can be electrically connected to a part of the first photovoltaic cells in order to improve the current matching of the photocurrents which are in series. In other words, the thin film solar cell of an embodiment of present invention has a better photoelectric conversion efficiency. Besides, the manufacturing method of the thin film solar cell of an embodiment of present invention can form the above mentioned second photovoltaic cell under the condition of without increasing the manufacturing process. Thus, the performance of the thin film solar cell can be improved in a simple way. 
     Since the thin film solar cell system of the present invention is designed with a current matching module, when the photocurrents provided by the current matching modules are different, the current matching module can be electrically connected to a part of the thin film solar cell modules in order to improve the current matching and the current output of the photocurrents which are in series. In other words, the thin film solar cell system of an embodiment of present invention has a better photoelectric conversion efficiency. In an embodiment, since the thin film solar cell system of the present invention is designed with a second photovoltaic cell, when the photocurrents provided by the thin film solar cell modules are different, the second photovoltaic cell can be electrically connected to a part of the first photovoltaic cells in order to improve the current matching of the photocurrents which are in series. 
     The present invention has been disclosed above in the preferred embodiments, but is not limited to those. It is known to persons skilled in the art that some modifications and innovations may be made without departing from the spirit and scope of the present invention. Therefore, the scope of the present invention should be defined by the following claims.