Abstract:
The present invention, in one aspect, is directed to methods for manufacturing solar or photovoltaic modules for better environmental stability. In another aspect, the present invention is directed to environmentally stable solar or photovoltaic modules. These method and apparatus use a moisture barrier film to form a moisture-resistant surface on the circuit, preferably on an illuminating surface of solar cells, or an entire side of a circuit formed of a plurality of solar cells that includes the illuminating surface of solar cells. In certain embodiments, the moisture-resistant film is applied conformally, and in other embodiments the moisture-resistant film is substantially transparent.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims priority to and incorporates by reference herein U.S. Provisional Appln. Ser. No. 60/786,902 filed Mar. 28, 2006 entitled “Technique For Manufacturing Photovoltaic Modules.” 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to method and apparatus for manufacturing solar or photovoltaic modules for better environmental stability.  
       BACKGROUND  
       [0003]     Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970&#39;s there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.  
         [0004]     Amorphous Si [a-Si], cadmium telluride [CdTe] and copper-indium-(sulfo)selenide [CIGS(S), or Cu(In,Ga)(S,Se) 2  or CuIn (1-x) Ga x (S y Se (1-y) ) k , where 0≦x≦1, 0≦y≦1 and k is approximately 2], are the three important thin film solar cell materials. The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a CIGS(S) thin film solar cell is shown in  FIG. 1 . The device  10  is fabricated on a substrate  11 , such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film  12 , which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te) 2 , is grown over a conductive layer  13  or a contact layer, which is previously deposited on the substrate  11  and which acts as the electrical ohmic back contact to the device. The most commonly used contact layer or conductive layer  13  in the solar cell structure of  FIG. 1  is molybdenum (Mo). If the substrate itself is a properly selected conductive material such as a Mo foil, it is possible not to use a conductive layer  13 , since the substrate  11  may then be used as the ohmic contact to the device. The conductive layer  13  may also act as a diffusion barrier in case the metallic foil is reactive. For example, foils comprising materials such as Al, Ni, Cu may be used as substrates provided a barrier such as a Mo layer, a W layer, a Ru layer, a Ta layer etc., is deposited on them protecting them from Se or S vapors. The barrier is often deposited on both sides of the foil to protect it well. After the absorber film  12  is grown, a transparent layer  14  such as a CdS, transparent conductive oxide (TCO) such as ZnO or CdS/TCO stack is formed on the absorber film. Radiation, R, enters the device through the transparent layer  14 . Metallic grids (not shown) may also be deposited over the transparent layer  14  to reduce the effective series resistance of the device. The preferred electrical type of the absorber film  12  is p-type, and the preferred electrical type of the transparent layer  14  is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The preferred device structure of  FIG. 1  is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te) 2  absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the transparent superstrate side. A variety of materials, deposited by a variety of methods, can be used to provide the various layers of the device shown in  FIG. 1 .  
         [0005]     Solar cells have relatively low voltage of typically less than 2 volts. To build high voltage power supplies or generators, solar cells are interconnected to form circuits which are then packaged into modules. There are two ways to interconnect thin film solar cells to form circuits and then fabricate modules with higher voltage and/or current ratings. If the thin film device is formed on an insulating surface, monolithic integration is possible. In monolithic integration, all solar cells are fabricated on the same substrate and then integrated or interconnected on the same substrate by connecting negative terminal of one cell to the positive terminal of the adjacent cell (series connection). A monolithically integrated Cu(In,Ga,Al)(S,Se,Te) 2  compound thin film circuit structure  20  comprising series connected cell sections  18  is shown in  FIG. 2A . In this case the contact layer is in the form of contact layer pads  13   a  separated by contact isolation regions or contact scribes  15 . The compound thin film is also in the form of compound layer strips  12   a  separated by compound layer isolation regions or compound layer scribes  16 . The transparent conductive layer, on the other hand, is divided into transparent layer islands  14   a  by transparent layer isolation regions or transparent layer scribes  17 . As can be seen in  FIG. 2A , the contact layer pad  13   a  of each cell section  18  is electrically connected to the transparent layer island  14   a  of the adjacent cell section. This way voltage generated by each cell section is added to provide a total voltage of V from the circuit structure  20 .  
         [0006]     The second way of integrating thin film solar cells into circuits is to first fabricate individual solar cells and then interconnect them through external wiring. This approach is not monolithic, i.e. all the cells are not on the same substrate.  FIG. 2B  schematically shows integration of three CIGS(S) solar cells  10  into a circuit  21  section, wherein the CIGS(S) cells  10  may be fabricated on conductive foil substrates with a structure similar to the one depicted in  FIG. 1 .  
         [0007]     Irrespective of the integration approach used, after the solar cells are electrically interconnected into a circuit such as the circuit  21  shown in  FIG. 2B , the circuit needs to be packaged to form an environmentally stable and physically well-protected product which is a module.  FIG. 3  shows an exemplary form of a package after the integrated cells of  FIG. 2B  are encapsulated in a protective package. The structure in  FIG. 3  is a flexible module structure that is very attractive in terms of its flexibility and light weight. Some of the commonly used layers in the structure of  FIG. 3  are a top film  30 , a flexible encapsulant  31 , and a backing material  32 . The top film  30  is a transparent durable layer such as TEFZEL® manufactured by DuPont. The most commonly used flexible encapsulant is slow cure or fast cure EVA (ethyl vinyl acetate). The backing material  32  may be a TEFZEL® film, a TEDLAR® film (produced by DuPont) or any other polymeric film with high strength. It should be noted that since the light enters from the top, the backing material  32  does not have to be transparent and therefore it may comprise inorganic materials such as metals.  
         [0008]     Although desirable and attractive, the flexible thin film photovoltaic module of  FIG. 3  may have the drawback of environmental instability. Specifically, the commercially available and widely used top films and flexible encapsulants are semi-permeable to moisture and oxygen therefore corrosion and cell deterioration may be observed after a few years of operation of the flexible module in the field. Therefore, there is a need to develop alternative packaging techniques for modules to provide resistance to moisture absorption and diffusion to the active regions of the circuit.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention, in one aspect, is directed to methods for manufacturing solar or photovoltaic modules for better environmental stability.  
         [0010]     The present invention, in another aspect, is directed to environmentally stable solar or photovoltaic modules.  
         [0011]     In a particular embodiment, there is described a method of manufacturing a photovoltaic module by providing at least two solar cells, each of the at least two solar cells having a top illuminating surface and two terminals. There then follows the steps of electrically interconnecting the at least two solar cells with a conductor between at least one of the terminals of each of the at least two solar cells to form a circuit, and coating at least an entire side of the circuit that corresponds to and includes the top illuminating surface of the at least two solar cells with a moisture barrier film to form a moisture-resistant surface on the circuit.  
         [0012]     In another embodiment, there is described a method of manufacturing a photovoltaic module that includes coating at least an illuminating surface of solar cells with a moisture barrier film to form solar cells with moisture-resistance; electrically interconnecting any two of the solar cells using a conductor between at least one of the terminals of each of the any two solar cells to form a circuit, and encapsulating the circuit in a package.  
         [0013]     In a further embodiment, described is a module that includes at least two solar cells, each of the at least two solar cells having a top illuminating surface and two terminals; an electrical conductor that electrically interconnects the at least two solar cells with a conductor between at least one of the terminals of each of the at least two solar cells, and a moisture barrier film that coats at least an entire side of the circuit that corresponds to and includes the top illuminating surface of the at least two solar cells to form a moisture-resistant surface on the circuit.  
         [0014]     In a further embodiment, described is a module that includes at least two moisture-resistant solar cells each having an illuminating surface that is coated with a moisture barrier film; a conductor that electrically interconnects any two of the moisture-resistant solar cells using a conductor between at least one of the terminals of each of the any two moisture-resistant solar cells to form a circuit, and encapsulating materials that encapsulates the circuit in a package.  
         [0015]     In certain embodiments, the moisture-resistant film is applied conformally, and in other embodiments the moisture-resistant film is substantially transparent. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     These and other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein:  
         [0017]      FIG. 1  is a cross-sectional view of a solar cell employing a Group IBIIIAVIA absorber layer.  
         [0018]      FIG. 2A  is a cross-sectional view of a circuit obtained by monolithic integration of solar cells.  
         [0019]      FIG. 2B  is a cross-sectional view of a circuit obtained by non-monolithic integration of solar cells.  
         [0020]      FIG. 3  shows a module structure obtained by encapsulating the circuit of  FIG. 2B  in a protective package.  
         [0021]      FIGS. 4A and 4B  show solar cells first coated with a transparent moisture barrier layer and then integrated into a circuit according to two different embodiments of the invention.  
         [0022]      FIGS. 5A and 5B  show solar cells first integrated into a circuit and then coated with a transparent moisture barrier layer according to two different embodiments of the invention.  
         [0023]      FIG. 6  shows a module structure obtained by encapsulating the circuit of  FIG. 5A . 
     
    
     DETAILED DESCRIPTION  
       [0024]     In one embodiment of the present invention, each solar cell in the circuit is individually covered by a transparent moisture barrier material layer before the cells are integrated into circuits and then packaged into modules.  FIG. 4A  shows two exemplary CIGS(S) solar cells  40  with all the components and layers indicated in  FIG. 1 . For example, the solar cells  40  may be fabricated on flexible foil substrates i.e. substrate  11  of  FIG. 1  may be a metallic foil. The solar cells  40  are covered by a transparent moisture barrier material layer  41 , which as shown in  FIG. 4A  covers the entire cell  40  including top and bottom surfaces, and in  FIG. 4B  covers the top illuminating surface  42  of the cell where the light enters the device. This top illuminating surface  42  is the most sensitive surface to protect from moisture and in some cases oxygen. The transparent moisture barrier material layer  41  may optionally wrap around to the back surface  43  of the foil substrate as shown in  FIG. 4A . After obtaining the moisture barrier-covered solar cells, integration or interconnection is carried out as shown in  FIG. 2B  using metallic ribbons or wires  44 . For interconnection, the (−) terminal of one cell is electrically connected to the (+) terminal of the other one. This can be achieved through use of soldering wires or ribbons as shown in  FIG. 4A . Alternately the cells maybe directly interconnected by overlapping their respective edges and electrically connecting the front electrode of one cell (which is the negative terminal in the case of the device structure shown in  FIG. 1 ) with the back electrode of the next one. It should be noted that if the barrier material layer  41  is highly insulating and thick it should be at least partially removed from the connection points  45  so that good electrical contact may be obtained between the cell electrode and the ribbon or wire.  
         [0025]     In another approach shown in FIGS.  5 ( a ) and  5 ( b ), the solar cells are first electrically interconnected with a conductor, such as through soldering wires or ribbons, to form a circuit like the one shown in  FIG. 2B , and then the whole circuit is covered with a transparent moisture barrier material layer  41 , the moisture barrier material  41  either covering the entire circuit, top and bottom, as illustrated in  FIG. 5A  or as illustrated in  FIG. 5B , covering only the side of the circuit that contains the top surface where light enters the device. Some of the advantages of this approach are: i) Since the cells are already interconnected, the step of removing the barrier material layer from the connection points is avoided, ii) since the moisture barrier material layer is deposited after interconnection of the solar cells, the barrier material layer covers all portions of the circuit including the connection points and ribbons or wires. The approach as shown in  FIG. 5A  provides total encapsulation or coverage by the moisture barrier layer around the entire circuit, whereas encapsulation and coverage are provided in the  FIG. 5B  approach on that side where such protection is most needed. Either approach reduces the possibility of moisture or oxygen diffusion through any crack or opening.  
         [0026]     After the circuit is covered by at least one transparent moisture barrier material layer, the structure obtained is a moisture resistant circuit ( FIGS. 4A and 4B  and  FIGS. 5A and 5B ). The modules may then be fabricated by various methods such as encapsulating the moisture resistant circuits by a top film  30 , an encapsulant  31  and a backing material  32  as shown in  FIG. 6 . The flexible module obtained by such an approach has a moisture resistant circuit within the module packaging and therefore is environmentally much more stable. It should be noted that use of a backing material  32  is optional in this case. Also the moisture barrier capability of the top film and the backing material is not as important in the module structure of  FIG. 6  compared to the structure of  FIG. 3 , because of the presence of a transparent moisture barrier layer  41  encapsulating the whole circuit. It should also be noted that the transparent moisture barrier layers may also be used to coat the monolithically integrated structures similar to that shown in  FIG. 2A  before such monolithically integrated circuits are packaged to form modules.  
         [0027]     The transparent moisture barrier material layer may comprise at least one of an inorganic material and a polymeric material. Polyethylene, polypropylene, polystyrene, poly(ethylene terephthalate), polyimide, parylene or poly(chloro-p-xylylene), BCB or benzocyclobutene, polychlorotrifluoroethylene are some of the polymeric materials that can be used as moisture and oxygen barriers. Various transparent epoxies may also be used. Inorganic materials include silicon or aluminum oxides, silicon or aluminum nitrides, silicon or aluminum oxy-nitrides, amorphous or polycrystalline silicon carbide, other transparent ceramics, and carbon doped oxides such as SiOC. These materials are transparent so that when deposited over the transparent conductive contact of the solar cell they do not cause appreciable optical loss. It should be noted that polymeric and inorganic moisture barrier layers may be stacked together in the form of multi-layered stacks to improve barrier performance. Layers may be deposited on the solar cells or circuits by a variety of techniques such as by evaporation, sputtering, e-beam evaporation, chemical vapor deposition (CVD), plasma-enhanced CVD (PECVD), organometallic CVD, and wet coating techniques such as dipping, spray coating, doctor blading, spin coating, ink deposition, screen printing, gravure printing, roll coating etc. It is also possible to melt some of the polymeric materials at temperatures below 200 C, preferably below 150 C and coat the melt on the cells and circuits. Thickness of the moisture barrier layers may vary from 50 nm to several hundred microns. One attractive technique is vapor deposition which has the capability of conformal and uniform deposition of materials such as parylene. Parylene has various well known types such as parylene-N, parylene-D and parylene-C. Especially parylene-C is a good moisture barrier that can be vapor deposited on substrates of any shape at around room temperature in a highly conformal manner, filling cracks and even the high aspect ratio (depth-to width ratio) cavities of submicron size effectively. Thickness of parylene layer may be as thin as 50 nm, however for best performance thicknesses higher than 100 nm may be utilized. Another attractive method for depositing moisture barrier layers is spin, spray or dip coating, which, for example may be used to deposit barrier layers of low temperature curable organosiloxane such as P1DX product provided by Silecs corporation. PECVD is another method that may be used to deposit layers such as BCB layers.  
         [0028]     Although the present invention is described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.