Abstract:
The present inventions generally relate to thin film solar cell fabrication, and more particularly, to techniques for interconnecting solar cells based on Group IBIIIAVIA thin film semiconductors. In a particular embodiment, a system is described that positions solar cells and conductive leads with respect to each other so that application of a conductive adhesive and formation of an assembled solar cell string, followed by curing and cooling of the conductive adhesive, can occur in a repeatable manner.

Description:
RELATED APPLICATIONS 
     This application is related to and claims priority from U.S. Provisional Application No. 61/149,269 entitled “Method And Apparatus For Stringing Thin Film Solar Cells” filed on Feb. 2, 2009, which application is expressly incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field of the Inventions 
     The present inventions generally relate to thin film solar cell fabrication, more particularly, to techniques for interconnecting solar cells based on Group IBIIIAVIA thin film semiconductors. 
     2. Description of the Related Art 
     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. 
     Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as 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, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. It should be noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se) 2  means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1. 
     The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te) 2  thin film solar cell is shown in  FIG. 1 . A photovoltaic cell  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. An absorber film  12 , which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te) 2 , is grown over a contact layer  13 , which is previously deposited on the substrate  11  and which acts as the electrical contact to the absorber film  12  of the device. The substrate  11  and the contact layer  13  form a base  20  on which the absorber film  12  is formed. Various conductive layers comprising Mo, Ta, W, Ti, and their nitrides have been used in the solar cell structure of  FIG. 1 . If the substrate itself is a properly selected conductive material, it is possible not to use the contact layer  13 , since the substrate  11  may then be used as the ohmic contact to the device. After the absorber film  12  is grown, a transparent layer  14  such as a CdS, ZnO, CdS/ZnO or CdS/ZnO/ITO stack is formed on the absorber film  12 . Radiation  15  enters the device through the transparent layer  14 . A metallic grid pattern or finger pattern (not shown) comprising busbars and fingers 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. 
     If the substrate  11  of the CIGS(S) type cell shown in  FIG. 1  is a metallic foil, a positive voltage develops on the substrate  11  with respect to the transparent layer  14  under illumination. In other words, an electrical wire (not shown) that may be attached to the substrate  11  would constitute the (+) terminal of the solar cell  10  and a lead (not shown) that may be connected to the transparent layer  14  (or to a bulbar of the metallic grid pattern that may be deposited on the transparent layer  14 ) would constitute the (−) terminal of the solar cell. 
     After fabrication, individual solar cells are typically assembled into solar cell strings and circuits by interconnecting them (usually in series) electrically, i.e. by connecting the (+) terminal of one cell to the (−) terminal of a neighboring cell. This way the total voltage of the solar cell circuit is increased. The solar cell circuit is then laminated into a protective package to form a photovoltaic module. 
     For a device structure of  FIG. 1 , if the substrate  11  is a conductive metallic foil, series interconnection of cells may be carried out by connecting the substrate  11  at the back or un-illuminated side of one particular cell to the busbar of the finger pattern (not shown) at the front or illuminated side of the adjacent cell. A common industry practice is to use conductive wires, preferably in the form of strips of flat conductors or ribbons to interconnect a plurality of solar cells to form first a circuit and then a module as described before. Such ribbons are typically made of copper, coated with tin and/or silver. For standard crystalline Si-based technology, ribbons are attached to the front and back sides of the cells in the module structure using a suitable soldering material since both the top grid pattern of the cell and the bottom contact of the cell comprise easily solderable metallic materials such as silver. High temperature solders with processing temperatures in excess of 200° C., typically in excess of 300° C., may be used in the interconnection of Si cells to form “strings” which may then be interconnected by a process called “bussing” to form the module circuit. 
     Unlike Si solar cells, the thin film Group IBIIIAVIA compound solar cell of  FIG. 1  may be fabricated on a metallic foil substrate such as a flexible stainless steel web or aluminum alloy foil. These materials may not be easily soldered, especially since the process temperature for this type of solar cell is limited to less than about 250° C., preferably less than 200° C. Therefore, conductive adhesives are usually employed to attach the Cu ribbons to the busbar of the grid pattern and the back contact or the back surface of the substrate of such solar cells during their interconnection. Although such techniques are in use in products, the contact resistance of the electrical contacts attached by conductive adhesives to metal foil based thin film solar cells still needs to be reduced. Adhesion of the contact to the back surface of the metallic foil substrates also needs improvement. A number of solar cells are connected together typically in series via a number of electrically conductive wires or ribbons to form what is commonly called a cell “string.” Each string has a voltage equal to the sum of the voltages of the individual cells in that string. There may be one or more conducting wires connecting each successive pair of cells depending on the electrical current collection pattern which in turn depends on the size and the shape of the cells. One common way to attach ribbons to cells is to use special conductive inks that when cured, i.e. when heated and maintained at appropriate curing temperature for sufficient time, form mechanically strong bonds that conduct electricity with low resistance. 
     The stringing step is a significant part of the total PV module fabrication. With respect to conventional stringing, there have been limited automated tools developed for stringing thin foil solar cells. These conventional tools are also inefficient, as they rely on complex handling systems for the cells and the strings since the strings are not rigid bodies. These systems also may rely on repeated detection and positioning of the cells or the strings at various process steps, again since the strings are not rigid and also because the bond between the ribbons and the cells are extremely weak before the conductive ink is cured. Additionally, transportation of the string in these systems before the ink is cured requires elaborate measures to prevent reliability problems that could be introduced as a result weak adhesion between the ribbons and the cells. 
     Therefore, there is a need to develop systems and methods that will achieve secure handling and transportation of the strings during string formation while fixing the relative positions of the cells in the string with respect to each other. 
     SUMMARY 
     The present inventions generally relate to thin film solar cell fabrication, and more particularly, to techniques for interconnecting solar cells based on Group IBIIIAVIA thin film semiconductors. 
     In a particular embodiment, a system is described that positions solar cells and conductive leads with respect to each other so that application of a conductive adhesive and formation of an assembled solar cell string, followed by curing and cooling of the conductive adhesive, can occur in a repeatable manner. 
     In a particular embodiment is described a system to interconnect a plurality of solar cells to form a solar cell string for solar cell modules, comprising: a support device including a plurality of support members arranged in a row and extending along a horizontal plane of the support device; an assembly station including an assembly platform to receive the support device and form an assembled solar cell string in the support device by placing the plurality of solar cells into the support device such that each solar cell is placed over one of the support members of the plurality of support members and each solar cell is electrically interconnected to any solar cell located adjacent thereto by conductive leads, the conductive leads being attached to the solar cells with a conductive adhesive; a curing station including a heating platform to receive the support device with the assembled solar cell string from the assembly station, wherein the heating platform heats the support members carrying the solar cells to cure the adhesive attaching the conductive leads to the solar cells of the assembled solar cell string; a cooling station including a cooling platform to receive the support device with the assembled solar cell string from the curing station, wherein the cooling platform cools the support members carrying the solar cells to solidify the adhesive attaching the conductive leads to the solar cells, thereby forming the solar cell string; and a carrier to transport the support device among the assembly station, the curing station and the cooling station. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects and features will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures, wherein: 
         FIG. 1  is a side schematic view of a prior-art solar cell; 
         FIG. 2  is a schematic side view of a solar cell string formed by a process according to one of the preferred embodiments; 
         FIGS. 3A-3C  are schematic side views of an embodiment of a stringing system including an assembly support device to perform a stringing process according to one of the preferred embodiments; 
         FIG. 4  is a schematic view of the assembly support device held by a carrier of the system shown in  FIGS. 3A-3C ; 
         FIG. 5  is schematic plan view of a portion of the assembly support device; and 
         FIGS. 6A-6B  are schematic views describing the assembly process according to one of the preferred embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments described herein are for a method and apparatus for manufacturing solar cell strings, circuit or modules by interconnecting the solar cells with interconnects, conductive leads or ribbons. The embodiments will be described using a stringing process or interconnection process to form a solar cell string or array of preferably thin film CIGS solar cells formed on flexible metallic foil substrates. The stringing process may comprise an assembly step to assemble the solar cell string by utilizing solar cells, interconnects and an adhesive in an assembly support device; a curing or heating step to cure the adhesive while the assembled string is held in the assembly support device; and a cooling step to cool the manufactured string while the assembly string is held in the assembly support device. The assembly support device retains the solar cells and the interconnects in their predetermined positions during manufacturing of the string. The assembly support device may be moved to various process locations by a carrier. 
     The stringing tool comprises: an assembly station where components of a string are assembled, for example, interconnects are aligned and connected to the solar cells with a conductive adhesive to form a string in the assembly support device; a curing station where the conductive adhesive is cured so that the interconnects and the solar cells are mechanically and electrically connected while the string is held in the assembly support device; and a cooling station where the manufactured string is cooled down to room temperature while the string is still held in the assembly support device, before exiting the stringing tool for subsequent processing steps such as string testing and binning. The stringing system described herein further includes a carrier to handle and transport the assembly support device, which retains the solar cell string, among the process stations. In one embodiment, the assembly support device comprises a plurality of assembly support members to contain each solar cell of the string during manufacturing. Each assembly support member is configured to hold at least one solar cell in a predetermined position during the manufacture of the solar cell string. The assembly support device retaining the solar cell string is held by the carrier and transported among the process stations. 
       FIG. 2  shows, in side view, a solar cell string  100  manufactured using a stringing system  200  shown in  FIGS. 3A-3C . The solar cell string  100  includes solar cells  102  with a front surface  104 A and a back surface  104 B. As described in the background section the front surface  104 A is the light receiving side of the solar cells and the back surface is a surface of a metallic substrate of the solar cell. In the exemplary solar cell string  100 , there are 5 interconnected solar cells, namely solar cells  102 A,  102 B,  102 C,  102 D and  102 E. In solar cell string  100 , each front surface  104 A is connected to the back surface  104 B of one of the solar cells  102  next to it by employing at least one or more interconnects  106  or conductive ribbons between them. As shown in  FIG. 2 , for example a first portion  107 A of one of the interconnects  106  is attached to the front surface  104 A of the solar cell  102 A, and a second portion  107 B of the same interconnect is attached to the back surface  104 B of the solar cell  102 B, the next solar cell. The interconnects  106  are attached to the front and back surfaces  104 A and  104 B using a conductive adhesive  108 . Further, the front surface  104 A may include busbars and a finger patterns (not show) attached to the busbars, and the interconnect  106  are typically attached to busbars on the front surface  104 A with the adhesive  108 . The interconnects  106  may be directly attached to the metallic material of the back surface  104 B with the adhesive  108 . 
       FIG. 3A  shows an embodiment of a stringing system  200  of the present invention in side view. In this embodiment, the stringing system  200  comprises an assembly station  202  where the conductive leads  106  and solar cells  102  are aligned and connected with the conductive adhesive  108  to form the string  100  shown in  FIG. 2 , a curing station  204  where the conductive adhesive  108  is cured and a cooling station  205  where the manufactured string is cooled down to room temperature before exiting the stringing system  200 . The assembly station  202  includes a stringing platform  208  or assembly platform including a series of platform vacuum holes (not shown) placed in predetermined positions and connected to a vacuum source. As will be described more fully below an assembly support device  214  is placed over predetermined positions over the stringing platform  208 . The assembly support device  214  may comprise at least one assembly support member  215  including a series of support holes (shown in  FIG. 5 ) connected to the vacuum supply through the platform vacuum holes (not shown) and side holders  216  attached to both sides of the assembly support member  215  through spacers  217  or insulators. Alternatively, there may be a single side holder to hold the assembly support members  215 . Each assembly support member  215  may be configured to hold at least one solar cell to form a string. For example if a string including 5 cells is manufactured, there may be only 5 assembly support members  215 . The assembly support device  214  may be formed of individual pieces of the assembly support members  215  or alternatively integrated as a single piece including 5 separate support sections separated by gaps. 
     The assembly support members  215  are made of thermally conductive materials, such as metals, preferably aluminum or aluminum alloys, to enable heating or cooling of the string during the process. Interconnects  106  and solar cells  102  are initially loaded to the assembly support device  214  to form the string  100  while the assembly support members  215  are held on the stringing platform  208  as in the manner that will be described below. The adhesive  108  (see  FIG. 2 ) applied on the string  100  when the string  100  is in the assembly station  202  is not cured yet (in  FIGS. 3A-3C , the string is shown positioned perpendicular to the plane of page). The curing of the adhesive  108  is performed on a hot plate  210  of the curing station  204  and cooled at a cooling plate  212  of the cooling station  205  while the string  100  is still over the assembly support device  214 . During the curing process, heat is transferred from the hot plate to the interconnected cells through the assembly support members  215 . Curing temperature and time depend on the adhesive and are normally recommended by the supplier. Typical curing temperature is in the range of 130 to 150 degrees C. The advantage of hot plate is that curing temperature is achieved in less than one minute, and another one or two minutes of keeping at the curing temperature may be sufficient for curing the adhesive. Cooling may be even faster. A string is typically assembled in approximately 3 minutes or more depending on the number of cells. Therefore heating is normally not the limiting step. If it were, a second hot plate could be added. 
     During at least a portion of the manufacturing process, the string  100  may be held and transported by a carrier  206 . The carrier  206  holds the assembly support device  214  having the solar cell string and transports the assembly support device  214  to the curing station  204  and cooling station  205  and return the empty assembly support device to assembly station  202  or a queue station after the manufactured string is unloaded. The carrier  206  may move vertically and horizontally by a moving mechanism (not shown), that is preferably computer-controlled, such as a robotic arm. Alternatively, the assembly support device  214  may be transported manually by operators. It is also possible that a second assembly support device may be used to assemble another solar cells string while a first assembly support device still carry a previously assembled solar cell string to the curing or cooling stations. An auxiliary station  203  or supply station may be located between the assembly station  202  and the curing station  204 . The auxiliary station  203  may comprise a solar cell supply station  203 A including a solar cell loader (not shown), an adhesive supply station  203 B including an adhesive applicator (not shown), an interconnect  203 C or ribbon supply station including an interconnect loader (not shown), a weight supply station  203 D including a weight loader/un-loader (not shown), solar cell and interconnect aligners (not shown), and their moving mechanisms (not shown) such as robotic arms or the like. A control system (not shown) connected to a computer controls the flow of predetermined process steps. A system gantry (not shown) may surround the stringing system  200 . 
     As shown in  FIG. 3B , after the string  100  is assembled at the assembly station  202  in the assembly step, the freshly assembled string  100  and the assembly support device  214  in which the string is held are removed from the stringing platform by the carrier  206 . Use of assembly support device  214  eliminates the need for realigning the string with uncured adhesive before, after or during the transportation. As a last step in the assembly station, at least one weight may be placed on the string or at least one weight may be placed on each solar cell to keep the cells and interconnects properly connected throughout the stringing process. The weight is designed such that it has limited surface area in contact with the cell in order to minimize the amount of heat transferred to the weight. In addition, there are pockets in the weight for the ribbons plus the necessary room of about 50 to 500 microns in height for the adhesive. Weights are designed such that they are mechanically restricted to move sideways within the string assembly device. The weight can weigh 100 to 1000 grams. The assembly support members  215  are attached to the side holders  216  through the spacers  217  which are made of a thermally insulating material so as not to transfer heat to the holders  216  when the assembly support device  214  is placed on the hot plate  210 . 
     As shown in  FIG. 3C , the solar cell string  100  held by the assembly support device  214  is placed over the hot plate  210  at the curing station  204  to cure the adhesive  108  in the curing step. Preferably, the width of the assembly support members  215  may be made equal to the width of the hot plate to apply heat only to the assembly support members  215 . The assembly support device  214  containing the assembled string may be placed on the hot plate  210  or they may be kept on the hot plate  210  while still attached to the carrier  206 . At this stage, the heat from the hot plate is transformed through the assembly support members  215  to the string  100 . In the following cooling step, the string  100  with the cured adhesive is cooled down on the cooling plate  22  and the string, and if added previously, the weights are unloaded from the assembly support device  214 . After this step, the assembly support device  214  may be returned back to the assembly station  202  for another string assembly. The system  200  may have more than one assembly support device and carrier to process multiple solar cell strings. 
       FIG. 4  shows in side view the assembly support device  214  with the assembly support members  215 , namely  215 A,  215 B,  215 C,  215 D,  215 E, held by the carrier  206 . The support members  215  are lined up along a horizontal axis of the support device  214  within a horizontal plane. Each assembly support member  215  carries one of the solar cells  102  and is removably attached to the carrier  206  by the side holders  216 . The side holders  216  are also made of a metal, preferably aluminum. As mentioned above, the assembly support members  215  are attached to the side holders  216  through the spacers  217  which are made of thermally insulating materials such as ceramics or heat resistant rigid polymers. The spacers  217  forms a part of the side holders  216 , and the support members  215  may be attached to spacers  217  by conventional means such as screwing, gluing or others. The thermally insulating material of the spacers  217  does not allow heat to transfer from the support members to side holders and to the carrier, and thereby they are protected from unwanted effects of heat. Alternatively, although metallic materials such as aluminum and its alloys, the side holders  216  may also be made of heat resistant materials and may be designed such that they may not contact the surface of the hot plate. In  FIGS. 3A-4 , weights or interconnects are not shown for clarity. Each assembly support member  215  is separated from the next assembly support member by a predetermined gap  218 . The gap  218  compensates the expansion of the assembly support members  215  when they are heated during the curing process. 
     As shown in  FIG. 5 , each assembly support member  215 , for example  215 A and  215 B, includes a body  220  which may be a rectangular plate having side extensions  222  attached to the side holders  216  through the spacers  217  or insulators (see  FIG. 4 ). The assembly support members may be made of aluminum and may have a thickness 0.5-3 millimeters (mm), preferably 1-1.5 mm. The length of the assembly support members may be at least equal to the length of the solar cells. As previously mentioned the width of the assembly support members  215  may be equal to the width of the hot plate  210  ( FIGS. 3A-3C ) so that the heating of them can be done efficiently. The body  220  of the assembly support member  215  may include interconnect grooves  224  to retain the second portion  107 B of the interconnects  106  during the assembly of the string  100  (see  FIG. 2 ). Each groove  224  include groove vacuum holes  226 A to keep the interconnects  106  within the grooves by vacuum suction. The body  222  of each assembly support member  215  may also include support vacuum holes  226 B to keep the solar cell  102  in a predetermined position when the vacuum is applied. The support vacuum holes  226 B are distributed in accordance with the area that the solar cell occupies on the body  220  to effectively hold the solar cell  102  during the stringing process. It is also possible to include a recess (not shown) in the body  222  of the assembly support members  215  to better retain the solar cells of the string on the assembly support members during the assembly process. Alternatively, dividers (not shown), such as protrusions extending from the side holders  217  towards the interconnect grooves  224  (but not intercepting them) to restrict the movement of the solar cells on the assembly support members. During the assembly process, when needed, the vacuum holes  226 A and  226 B are connected to a vacuum supply through the assembly platform  208  shown in  FIGS. 3A-3C , and controlled by the controller (not shown). 
       FIGS. 6A and 6B  exemplify an assembly process performed at the assembly station  202  ( FIG. 3A ) using two solar cells.  FIGS. 6A and 6B  show the solar cell  102 A is loaded on the assembly support members  215 A and the solar cell  102 B is loaded on the assembly support members  215 B in side view and plan view respectively. Referring to  FIG. 6A , in one preferred process sequence, initially a second portion  107 B of a first interconnect set  106 A is placed in the grooves  224  of the assembly support plate  215 A by a interconnect loader (not shown), and vacuum is applied to the groove holes  226 A to hold the interconnects in the grooves (see  FIG. 5 ). Next, an adhesive applicator (not shown) applies the adhesive  108  (see  FIG. 2 ) onto the second portions  107 B of the first interconnect set  106 A, and the back surface  104 B of the solar cell  102 A is placed on the second portions  107 B of the first interconnect set  106 A, and vacuum through the support holes  226 B (see  FIG. 5 ) is applied to hold the solar cell  102  in place. The solar cell  102 A is transported from the solar cell supply station  203 A (shown in  FIGS. 3A-3C ) by a solar cell loader (not shown). Referring to  FIGS. 6A and 6B , in the next process step, first the adhesive is applied along the predetermined position of the interconnects on the front surface  104 A, and a first portion  107 A of a second interconnect set  106 B is placed on the adhesive applied regions of the front surface  104 A while a second portion  107 B of the second interconnect set  106 B is placed into the grooves  224  in the assembly support member  215 B and held by vacuum. In the following step a weight (not shown) may be placed on the first portion  107 A of the second interconnect set  106 B. The same process steps are repeated to interconnect the solar cell  102 B to  102 A. 
     The stringing system  200  described above may be constructed in alternative tool configurations. For example, in an alternative stringing system embodiment, a process station (not shown) may include the curing  204  and cooling stations  205 , and other stations to handle the manufactured string so that the string assembled in the assembly station  202  is taken into the process station for curing and cooling processes. 
     Although the present inventions are described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.