Patent Publication Number: US-2020292236-A1

Title: Light annealing in a cooling chamber of a firing furnace

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/158,358, filed May 18, 2016, which claims the benefit of U.S. Provisional Patent Application Ser. No. 62/164,235, filed on May 20, 2015. The entireties of U.S. patent application Ser. No. 15/158,358 and U.S. Provisional Patent Application Ser. No. 62/164,235 are incorporated herein by reference. 
    
    
     BACKGROUND 
     Solar cells are commonly fabricated from p-type crystalline silicon (c-Si) doped with boron. Such solar cells suffer from light-induced degradation (LID) that lowers the efficiency of the solar cells. This is generally ascribed to boron-oxygen (B—O) defects in the silicon. 
     One approach to reversing such LID involves a post-process step in which finished solar cells are subjected to a low-temperature thermal annealing step in which the solar cells are heated to a temperature between 50° C. to 230° C. while simultaneously generating excess carriers in the silicon. With this approach to reversing LID, excess carriers are generated in the silicon by either applying an external voltage to the solar cells or by subjecting the solar cells to illumination while the solar cells are heated. This LID-reversal approach is described in the patent application WO 2007/107351. 
     However, this approach for reversing LID is a post-process step, which is performed after fabrication of the solar cell would otherwise be complete. Using such a post-process step adds additional equipment and processing to the fabrication of solar cells. Also, this LID-reversal approach is performed at a low temperature so as to avoid damaging the solar cell that would result from higher-temperature heating. 
     It has been suggested that a similar approach can be used in the middle of solar cell fabrication during the step of passivation of defects in the silicon by hydrogenation. Using such an approach during hydrogenation passivation is described in the patent application WO 2013/173867. With this approach, the silicon wafer is illuminated during both the heating of the wafer and the subsequent cool down of the wafer during hydrogenation passivation. Such illumination during both heating and subsequent cool down can be performed during any subsequent thermal processes that may degrade the quality of hydrogenation passivation. 
     However, this approach for protecting the quality of hydrogenation passivation still typically involves illuminating the silicon while the silicon is being heated. Such an approach may not be suitable for use in high-performance, highly efficient solar cell fabrication lines that require precise and stable thermal profiles during metallization firing. Also, such an approach typically involves illuminating the silicon in the presence of a hydrogen source during cooling, which necessitates including a hydrogen source in the cooling chamber. 
     SUMMARY 
     One embodiment is directed to an apparatus comprising a firing furnace comprising a heating chamber configured to fire a metallization layer of photovoltaic devices and a cooling chamber configured to cool the photovoltaic devices that have been heated by the heating chamber. The cooling chamber comprises lights to light anneal the photovoltaic devices to reduce light induced degradation as the photovoltaic devices are cooled in the cooling chamber. The cooling chamber of the firing furnace is configured to use residual heat from heating performed in the heating chamber of the firing furnace as heat for the light annealing of the photovoltaic devices. Light annealing is not performed in the heating chamber of the firing furnace. 
     Another embodiment is directed to a method of co-firing. The method comprises heating photovoltaic devices in a heating chamber of a firing furnace to co-fire metallization layers of the photovoltaic devices. The method further comprises cooling the photovoltaic devices that have been heated by the heating chamber in a cooling chamber of the firing furnace while light annealing the photovoltaic devices to reduce the effects of light-induced degradation using residual heat from heating the photovoltaic devices in the heating chamber as heat for light annealing. Light annealing is not performed in the heating chamber of the firing furnace. 
    
    
     
       DRAWINGS 
         FIG. 1  is a side plan view of one exemplary embodiment of a firing furnace having a heating chamber and a cooling chamber, where light annealing is integrated into the cooling chamber but not the heating chamber. 
         FIG. 2  is a flow diagram of one exemplary embodiment of a method of co-firing the metallization layers of solar cells or other photovoltaic devices. 
         FIG. 3  is a side plan view of one exemplary embodiment of a combined apparatus in which a drying furnace is integrated with a firing furnace having a heating chamber and a cooling chamber, where light annealing is integrated into the cooling chamber but not into the heating chamber. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a side plan view of one exemplary embodiment of a firing furnace  100  having a heating chamber  102  and a cooling chamber  104 , where light annealing is integrated into the cooling chamber  104  but not the heating chamber  102 . 
     The furnace  100  shown in  FIG. 1  is suitable for use in the firing of metal contacts on photovoltaics device (such as solar cells)  106 . Wafers of photovoltaic devices (also referred to here as “wafers” or “solar cells”)  106  are transported by a conveyor  108  into an entry  110  formed in the firing furnace  100 . After processing, the wafers  106  are transported by the conveyor  108  out of an exit  112  formed in the firing furnace  100 . More specifically, in the exemplary embodiment shown in  FIG. 1 , wafers  106  pass through the entry  110  into the heating chamber  102 , then pass through the heating chamber  102  and the cooling chamber  104 , and then are conveyed out of the furnace  100  through the exit  112 . 
     Although the following description refers to a single conveyor  108  for ease of explanation, it is to be understood that one, two, or more parallel conveyors  108  can be used at the same time in the same furnace  100 . Each separate conveyor  108  is also referred to as a “lane.” In one implementation, the furnace  100  and the conveyor  108  are configured so that each conveyor  108  (and the solar cells  106  thereon) are thermally isolated from one another in order to reduce lane-to-lane influence. 
     As noted above, the furnace  100  is used for the firing of metal contacts on photovoltaic devices  106 . Front and back side metal contacts of photovoltaic devices  106  are initially formed by an electrically conductive metallized paste or ink that is applied, for example, by a screen printing, inkjet spray or aerosol spray process to silicon wafers. Commonly, the front side contact extends in a grid pattern, and the backside contact extends continuously. 
     After the metallized paste has been applied to the silicon wafers  106 , the wafers  106  are dried. The wafers  106  are dried in order to remove any remaining volatile organic compounds (VOCs) (for example, solvent) used in the screen-printing or other paste-application processes. 
     In the exemplary embodiment shown in  FIG. 1 , the solvent removal is decoupled from binder burnout to improve binder retention. This is done by drying the silicon wafers  106  in a heating chamber that is separate from the heating chamber in which the binder burnout is performed. In one example, this is done by using a separate drying furnace (such as a continuous infrared drying furnace) (not shown in  FIG. 1 ) that feeds into the firing furnace  100  shown in  FIG. 1 . In an alternative embodiment described below in connection with  FIG. 3 , the drying furnace is integrated with the firing furnace  100 . 
     In the exemplary embodiment shown in  FIG. 1 , the heating chamber  102  of the firing furnace  100  includes two heating sections  114  and  116 . It is to be understood, however, that a different number of heating sections can be used. 
     In the exemplary embodiment shown in  FIG. 1 , the first heating section  114  is configured for binder burn out (and is also referred to here as the “binder-burn-out heating section”  114 ). In this exemplary embodiment, the second heating section  116  is configured for firing the metallization layers of the solar cells  106  (and is also referred to here as the “metallization heating section”  116 ). In the particular embodiment shown in  FIG. 1 , the furnace  100  is configured to thermally decouple the binder-burn-out heating section  114  from the metallization heating section  116  so that each section  114  and  116  can be independently controlled and optimized for each of the respective process objectives. 
     Exhaust ducts are used to thermally decouple each of the heating sections  114  and  116  from each other and from the exterior environment in the case of the binder-burn-out heating section  114  and from the cooling chamber  104  in the case of the metallization heating section  116 . The exhaust ducts are also used to vent out of the furnace  100  any off-gases produced while the wafers  106  pass through the furnace  100 . 
     In the exemplary embodiment shown in  FIG. 1 , each section  114  and  116  includes multiple pairs of infrared (IR) lamps  120 , where one “upper” IR lamp  120  of each pair is located above the conveyor  108  and the other “lower” IR lamp  120  of each pair is located below the conveyor  108  directly opposite the corresponding upper IR lamp  120 . 
     In one implementation, the upper and lower IR lamps  120  can be separately controlled in order to provide independent control and optimization of conditions in the top and bottom regions of the heating sections  114  and  116  (for example, because different metal pastes are used on the top and bottom surfaces of the solar cells  106 ). 
     In the exemplary embodiment shown in  FIG. 1 , the heating chamber  102  includes two heating sections  114  and  116 , where each of the sections  114  and  116  can be independently controlled (for binder burn out in the case of the section  114  and for firing the metallization layer in the case of section  116 ). It is to be understood, however, that the heating chamber  102  can be configured to have a different number of sections. Also, one or more of the sections of the heating chamber  102  can be further subdivided into smaller zones or microzones, where each such zone or microzone can be independently controlled to provide additional control over the heating in the heating chamber  102 . 
     In the exemplary embodiment shown in  FIG. 1 , the cooling chamber  104  of the firing furnace  100  includes two cooling sections  122  and  124 . It is to be understood, however, that a different number of cooling sections can be used. 
     In the exemplary embodiment shown in  FIG. 1 , the first cooling section  122  uses radiant cooling to cool wafers  106  that pass through the first cooling section  122 , and the second cooling section  124  uses convection cooling to cool wafers  106  that pass through the second cooling section  124 . The first cooling section  122  is also referred to here as the “radiant” cooling section  122 , and the second cooling section  124  is also referred to here as the “convective” cooling section  124 . 
     In the exemplary embodiment shown in  FIG. 1 , the radiant cooling section  122  includes a pair of cooling walls  126 . One of the cooling walls  126  is positioned above the conveyor  108 , and the other one of the cooling walls  126  is positioned below the conveyor  108 . In the exemplary embodiment shown in  FIG. 1 , the cooling walls  126  are water-cooled. Cooled water is circulated through pipes (or other passages) that are in thermal contact with the cooling walls  126 . It is to be understood, however, that the radiant cooling may be implemented in other ways. 
     The silicon wafers  106  that exit the heating chamber  102  and pass through the radiant cooling section  122  are cooled by radiant heat transfer from the wafers  106  to the cooling walls  126  and the water flowing through the pipes. 
     In the exemplary embodiment shown in  FIG. 1 , the convective cooling section  124  includes two sub-sections  128 . Each of the convective cooling sub-sections  128  includes a respective supply fan that draws air from a respective supply duct into the upper part of that cooling sub-section  128  and causes the air to flow down towards the conveyor  108  and to pass the wafers  106 . Some of the air contacts the surface of the passing wafers  106  as it flows downward, thereby heating the flowing air. The air then flows below the conveyor  108  and the passing wafers  106 . Each convective cooling sub-section  128  also includes a return fan that draws the flowing air into a respective return duct and circulates the air back to one of the supply ducts (via ductwork not shown in  FIG. 1 ) to be re-circulated back into the upper part of the corresponding sub-section  128 . 
     In the embodiment shown in  FIG. 1 , a respective heat exchanger is positioned in each sub-section  128  below the conveyor  108 . Air flowing over and around the passing wafers  106  is heated. Heat from the air flowing past the heat exchanger is transferred to the heat exchanger. This cools the air before it is drawn into the return duct and re-circulated into the upper part of the corresponding sub-section  128 . 
     It is to be understood that the particular embodiment of the heating and cooling chambers  102  and  104  shown in  FIG. 1  are merely exemplary. The heating and cooling chambers  102  and  104  can be implemented in other ways. 
     One or more sections  122  and  124  of the cooling chamber  104  include lights  130  for performing light annealing of the solar cell wafers  106  passing through the cooling chamber  104 . 
     The purpose of light annealing is to reduce the effect of light induced degradation (LID) that occurs in the solar cells  106 . Traditionally, this light anneal has involved exposing completed solar cells to intense light at an elevated temperature in a separate, standalone process where the intense illumination occurs, at least in part, in a heating chamber of a furnace. 
     However, with the co-firing furnace  100  described here in connection with  FIG. 1 , light annealing to reduce the effects of LID is integrated into the cooling chamber  104  of the co-firing furnace  100 . Light annealing is not performed in the heating chamber  102  of the co-firing furnace  100 . Instead, residual heat from the heating chamber  102  is used to achieve the required elevated temperature for light annealing in the cooling sections  122  and  124  of the cooling chamber  104 . Also, in this exemplary embodiment, a hydrogen source is not present in the cooling chamber  104 ; instead, light annealing is performed in ambient air. 
     In the exemplary embodiment shown in  FIG. 1 , an array of lights  130  is positioned in both the radiant cooling section  122  and the convective cooling section  124  of the cooling chamber  104  but not in the heating chamber  102 . 
     In the exemplary embodiment shown in  FIG. 1 , for each of the cooling sections  122  and  124 , the lights  130  comprise light emitting diodes (LEDs) that are mounted on a water-cooled plate  132 . Cooled water is circulated through pipes (or other passages) that are in thermal contact with the plate  132 . The plate  132  is water cooled in order to remove heat generated by the LEDs  130  and any heat that is transferred to the LEDs  130  and plates  132  by the passing solar cells  106 . 
     In the exemplary embodiment shown in  FIG. 1 , one plate  132  with LEDs  130  mounted to it is positioned within the radiant cooling section  122 , and another plate  132  with LEDs  130  mounted to it is positioned with the convective cooling section  124 . However, it is to be understood multiple plates  132  with LEDs  130  mounted to them can be positioned within the radiant cooling section  122  or the convective cooling section  124 . Also, a single plate  132  with LEDs  130  mounted to it can be used in both the radiant cooling section  122  and the convective cooling section  124 . That is, the single plate  132  with LEDs  130  mounted to it can span the radiant cooling section  122  and the convective cooling section  124 . 
     In the radiant cooling section  122 , the respective water-cooled plate  132  (with the LEDs  130  mounted to it) is positioned between the upper cooling wall  126  and the conveyor  108  with the light output from the LEDs  130  directed generally downward towards the upper surface of the solar cells  106  passing by on the conveyor  108 . 
     In the convective cooling section  124 , the respective water-cooled plate  132  (with the LEDs  130  mounted to it) is positioned in the upper part of the section  124  above the conveyor  108  with the light output from the LEDs  130  directed generally downward towards the upper surface of the solar cells  106  passing by on the conveyor  108 . The portion of the water-cooled plate  132  that is positioned in the convective cooling section  124  has a shape (and/or openings formed it) to enable air flowing through the convective cooling section  124  to pass through and/or around the water-cooled plate  132  and the mounted LEDs  130 . 
     The water-cooled plate  132  can be mounted within the cooling sections  122  and  124  in any suitable manner (for example by attaching, suspending, or supporting the plate  132  and LEDs  130  to one or more of the side, top, or bottom walls of the furnace  100  or one or more structures within the cooling chamber  104  such as the cooling walls  126 ). 
     A power supply (not shown) is electrically connected to each of the LEDs  130  in order to provide power to the LEDs  130 . In this exemplary embodiment, the power supply is positioned outside of the cooling chamber  104 . 
     The number, size, and arrangement of the LEDs  130  in the array are configured so as to provide sufficiently intense illumination for performing light annealing to reduce LID (for example, by having a radiation intensity in a range between 3,000 Watts/meters 2  and 48,000 Watts/meters 2 ). For example, in one implementation, 10 millimeter by 10 millimeter LEDs are arranged in an array in which there are at least two thousand LEDs in an area that is about 0.3 meters wide by about 3 meters long. It is to be understood, however, that the LEDs can be arranged in other ways. 
     In this exemplary embodiment, the LEDs  130  are commercially available LEDs that output light in the spectrum between 300 nanometers and 900 nanometers (that is, within the visible spectrum). 
     Moreover, one advantage of using LEDs  130  to provide the intense light for light annealing is that the intensity of light output from the LEDs  130  can be adjusted by adjusting the DC voltage supplied to the LEDs  130 . This enables the light intensity to be adjusted as needed to optimize the light annealing process. 
     In the exemplary embodiment shown in  FIG. 1 , the array of LEDs  130  includes multiple zones  134 , where each zone  134  includes a subset of the LEDs  130 . In this exemplary embodiment, the intensity of light output by the LEDs  130  in each of the zones  134  can be independently controlled. The zones  134  can be adjusted so that the intensity of light output by the LEDs  130  in at least one of the zones  134  differs from the intensity of light output by the LEDs  130  in at least one of the other zones  134 . For example, the temperature of the solar cells  106  will be reduced as the solar cells  106  are conveyed through the cooling chamber  104 . As a result, it might be beneficial to adjust the light intensity in the various zones  134  to account for this reduction in temperature as the solar cells are conveyed through the cooling chamber  104 . 
     In general, the process of light annealing for LID reduction can be controlled based on various factors including, without limitation, the speed at which the solar cells  106  are conveyed through the cooling chamber  104 , the length of the cooling chamber  104 , the length of the array of LEDs  130 , the exit temperature of the solar cells  106  as they exit the heating chamber  102  and enter the cooling chamber  104 , the intensity of light output from the LEDs  130  in each of the light zones  134  (or the array of LEDs  130  as a whole where zones  134  are not used), and the number, size, and arrangement of the LEDs  130 . 
     In one implementation, one or more of these factors are controlled so that each solar cell  106  moving through the cooling sections  122  and  124  on the conveyor  108  will be exposed to the intense light from the LEDs  130  for an amount of time between 5 seconds and 45 seconds. In one example, this is done while each solar cell  106  is at a temperature between 700° C. and 240° C. In another example, this is done while each solar cell  106  is at a temperature between 700° C. and 50° C. 
       FIG. 2  is a flow diagram of one exemplary embodiment of a method  200  of co-firing the metallization layers of solar cells or other photovoltaic devices. The exemplary embodiment of method  200  is described here as being implemented using the co-firing furnace  100  of  FIG. 1  (though it is to be understood that it can be implemented in other ways, for example, using the combined apparatus described below in connection with  FIG. 3 ). 
     Method  200  comprises heating solar cells  106  (or other photovoltaic devices) in a heating chamber  102  of a furnace  100  to co-fire the metallization layers of the solar cells  106  (block  202 ). The solar cells  106  are not light annealed as they pass through the heating chamber  102  of the furnace  100 . In the exemplary embodiment described here in connection with  FIG. 2 , the solar cells  106  are first heated in a binder-burn-out section  114  to burn out the binders in the metallization pastes and then are heated in a metallization section  116  in order to co-fire the metallization layers of the solar cells  106 . In this exemplary embodiment, the heating chamber  102  is configured to efficiently provide flexible and precise process control in a high-volume production environment. 
     Method  200  further comprises cooling the heated solar cells  106  (or other photovoltaic devices) in a cooling chamber  104  of the furnace  100  while light annealing the solar cells  106  (or other photovoltaic devices) to reduce the effects of light-induced degradation (block  204 ). In the exemplary embodiment described here in connection with  FIG. 2 , the heated solar cells  106  are first radiantly cooled in the radiant cooling section  122  of the cooling chamber  104  and then are convectively cooled in the convective cooling section  124  of the cooling chamber  104 . In this exemplary embodiment, light annealing is performed by the LEDs  130  positioned in the cooling chamber  104  while the heated solar cells  106  are cooled in both the radiant cooling section  122  and the convective cooling section  124  of the cooling chamber  104 . Also, in this exemplary embodiment, a hydrogen source is not present in the cooling chamber  104 ; instead, light annealing is performed in ambient air. 
     In this exemplary embodiment where the array of LEDs  130  includes multiple zones  134 , the intensity of light output by the LEDs  130  in each of the zones  134  can be independently controlled and adjusted (for example, to adjust the light intensity in the various zones  134  to account for the reduction in temperature as the solar cells  106  are conveyed through the cooling chamber  104 ). 
     By incorporating light annealing to reduce the effects of LID into the cooling chamber  104  of the co-firing furnace  100 , a separate, standalone light annealing process step can be avoided, which avoids adding additional equipment and processing to the fabrication of solar cells. Also, by not performing light annealing in the heating chamber  102  of the co-firing furnace  100 , lights for providing the intense light for light annealing do not need to be placed in the heating sections  114  and  116  of the co-firing furnace  100 . Placing lights for light annealing in the heating sections  114  and  116  could impair the ability to provide flexible and precise process control, especially where one or more of the heating sections  114  and  116  is further sub-divided into high-resolution microzones that are independently controlled since it may be difficult to accommodate both the heating-related equipment for providing the desired thermal properties and the lights for light annealing in such zones and microzones. Also, it is easier to assemble and maintain a co-firing furnace  100  with lights  130  for light annealing integrated in only the cooling chamber  104 . Moreover, the light annealing techniques described here are especially well suited for use in co-firing furnaces  100  that are configured for use in high-volume production environments. Also, the light annealing techniques described here do not require a hydrogen source to be present in the cooling chamber  104 ; instead, light annealing can be performed in ambient air. 
     In the embodiments described above in connection with  FIGS. 1 and 2 , the solar cells  106  are dried in a separate drying furnace that is not shown in  FIG. 1 . However, it is to be understood that the light annealing techniques described above can be used in combined drier/co-firing furnaces. 
       FIG. 3  is a side plan view of one exemplary embodiment of a combined apparatus  350  in which a drying furnace  352  is integrated with a firing furnace  100  having a heating chamber  102  and a cooling chamber  104 , where light annealing is integrated into the cooling chamber  104  but not into the heating chamber  102 . 
     In general, the firing furnace  100  is as described above in connection  FIGS. 1 and 2 , the description of which is not repeated here in connection with  FIG. 3 . 
     In the exemplary embodiment shown in  FIG. 3 , the drying furnace  352  is integrated with the firing furnace  100 . 
     After metallized paste has been applied to the silicon wafers  106 , the wafers  106  are transported by a conveyor  108  into an entry  354  formed in the drying furnace  352 . After processing, the wafers are transported by the conveyor  108  out of an exit  356  formed in the drying furnace  352 . The drying furnace  352  includes a heating chamber  358  in order to dry the solar cells  106  in order to remove any remaining volatile organic compounds (VOCs) (for example, solvent) used in the screen-printing or other paste-application processes. More specifically, in the exemplary embodiment shown in  FIG. 3 , wafers  106  pass through the entry  354  into the heating chamber  358  of the drying furnace  352 , then pass through the heating chamber  358  to dry the solar cells  106  to remove any VOCs, and then are conveyed out of the drying furnace  352  through the exit  356 . 
     In the exemplary embodiment shown in  FIG. 3 , the heating chamber  358  of the drying furnace  352  includes one or more infrared (IR) lamps  362  located in a region close to the entry  354 . In this exemplary embodiment, IR lamps  362  are located above and below the conveyor  108  so as to direct infrared light toward the top and bottom surfaces of the solar cells  106 . 
     Also, in the exemplary embodiment shown in  FIG. 3 , supply ducts  364  are positioned above the conveyor  108  in the regions near the entry  354  and exit  356 . Each supply duct  364  includes a respective inlet at the end of the duct  364  nearest the center of the heating chamber  358 . Heated air is supplied to the supply ducts  364  at the respective inlets and flows towards the other end of the supply ducts  364 . Each of the supply ducts  364  includes a plurality of openings on its bottom wall. A portion of the heated air that flows along each supply duct  364  will flow out of the openings towards the conveyor  108  and any solar cells  106  passing under the supply duct  364 . Heated air is supplied to the supply ducts  364  by a supply fan, one or more heaters, and appropriate ductwork (all of which are not shown in  FIG. 3 ). 
     Heat from the IR lamps  362  and/or the heated air discharged from the openings of the supply ducts  364  heats the solar cells  106  passing by on the conveyor  108  in order to vaporize one or more compounds (such as solvent) used in the metallized pastes. 
     In the exemplary embodiment shown in  FIG. 3 , an exhaust port is positioned generally in the center of the heating chamber  358 . An exhaust blower  366  draws airs from the interior of the heating chamber  358  through the exhaust port and into an exhaust stack  368 . In this exemplary embodiment, a thermal VOC oxidizer  370  is positioned in the exhaust stack  368  in order to thermally oxidize VOCs from the air stream flowing into the exhaust stack  368  before the air exits the drying furnace  352 . 
     It is noted that light annealing is not performed in the drying furnace  352 . 
     In the exemplary embodiment shown in  FIG. 3 , solvent removal is decoupled from binder burnout to improve binder retention. This is done by drying the silicon wafers  106  in the heating chamber  358  of the drying furnace  352 , which is separate and decoupled from the binder-burn-out section  114  of the heating chamber  102  of the co-firing furnace  100 . After the solar cells  106  are dried in the drying furnace, they are conveyed into the entry  110  of the co-firing furnace  100  for firing, where light annealing to reduce the effects of LID is performed in the cooling chamber  104  but not in the heating chamber  102 . 
     After exiting the drying furnace  352 , the dried solar cells  106  are conveyed into the heating chamber  102  of the firing furnace  100  for binder burn out and firing of the metallization layers of the solar cells  106 . After that, the solar cells  106  are conveyed into the cooling chamber  104  to be cooled. While the solar cells  106  are being cooled, the solar cells  106  are light annealed by exposing the solar cells  106  to intense illumination from the LEDs  130 . This is all done as described above in connection with  FIGS. 1 and 2 . 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications to the described embodiments may be made without departing from the spirit and scope of the claimed invention.