Device for fabricating a photovoltaic element with stabilised efficiency

Abstract

A method and device for fabricating a photovoltaic element with stabilized efficiency is proposed. The method comprises the following steps: preparing a boron-doped, oxygen-containing silicon substrate; forming an emitter layer on a surface of the silicon substrate; and a stabilization treatment step. The stabilization treatment step comprises keeping the temperature of the substrate during a treatment time within a selectable temperature range having a lower temperature limit of 50° C., preferably 90° C., more preferably 130° C. and even more preferably 160° C. and an upper temperature limit of 230° C., preferably 210° C., more preferably 190° C. and even more preferably 180° C., and generating excess minority carriers in the silicon substrate during the treatment time, for example, by illuminating the substrate or by applying an external voltage. This method can be used to fabricate a photovoltaic element, e.g. a solar cell or a solar module having an efficiency which is stable at a value higher than that of photovoltaic elements fabricated without the stabilization treatment step.

1 . A device for fabricating a photovoltaic element with stabilized efficiency or for stabilizing the efficiency of a photovoltaic element, wherein the device is adapted to perform the following steps:
 receiving a boron-doped, oxygen-containing silicon substrate with an emitter layer formed on a surface of the silicon substrate;   characterized in that the device is further adapted to perform a stabilization treatment step, comprising:   keeping the temperature of the substrate during a treatment time within a selectable temperature range having a lower temperature limit of 50° C. and an upper temperature limit of 230° C. and   generating excess minority charge carriers in the silicon substrate during the treatment time.   
     
     
         2 . The device according to  claim 1 , wherein the silicon substrate comprises electrical contacts formed thereon, wherein the device is further adapted to, in the step of generating excess minority carriers, apply an external electrical voltage to the contacts. 
     
     
         3 . The device according to  claim 2 , wherein the device is adapted to apply the voltage in the conducting direction of the pn junction formed with the silicon substrate and the emitter layer. 
     
     
         4 . The device according to  claim 2 , wherein the applied voltage is higher than 0.4 V, preferably higher than 0.6 V and more preferably higher than 0.7 V. 
     
     
         5 . The device according to  claim 2 , wherein the device is adapted such that the silicon substrate is substantially not illuminated during the application of the external voltage. 
     
     
         6 . The device according to  claim 2 , wherein the device is adapted such that the treatment time t in minutes during which the substrate is held within the selectable temperature range is given by: 
       
         
           
             
               t 
               ≥ 
               
                 
                   a 
                   
                     
                       ( 
                       
                         y 
                         + 
                         b 
                       
                       ) 
                     
                     c 
                   
                 
                 * 
                 
                   exp 
                    
                   
                     ( 
                     
                       x 
                       
                         ( 
                         
                           T 
                           + 
                           273 
                         
                         ) 
                       
                     
                     ) 
                   
                 
               
             
           
         
         where T is the average temperature of the selectable temperature range in ° C. during the treatment time, y is the current density through the photovoltaic element brought about by the applied voltage in A/cm 2  and a=4.247*10 −14 , b=0.00286, c=0.887 and x=12550, preferably a=3.272*10 −14 , b=0.00352, c=0.934 and x=12800. 
       
     
     
         7 . The device according to  claim 1 , wherein the device is adapted to simultaneously receive a plurality of silicon substrates which have been previously stacked one above the other in a space-saving manner. 
     
     
         8 . The device according to  claim 1 , wherein the device is adapted to, in the step of generating the excess minority carriers, illuminate the silicon substrate. 
     
     
         9 . The device according to  claim 8 , wherein the illumination takes place using light having a wavelength shorter than 1180 nm. 
     
     
         10 . The device according to  claim 8 , wherein the illumination takes place using light having a radiation intensity higher than 10 W/m 2 , preferably higher than 100 W/m 2  and more preferably higher than 1000 W/m 2 . 
     
     
         11 . The device according to  claim 8 , wherein the treatment time t in minutes during which the substrate is held within the selectable temperature range is given by: 
       
         
           
             
               t 
               ≥ 
               
                 
                   a 
                   
                     
                       ( 
                       
                         y 
                         + 
                         b 
                       
                       ) 
                     
                     c 
                   
                 
                 * 
                 
                   exp 
                    
                   
                     ( 
                     
                       x 
                       
                         ( 
                         
                           T 
                           + 
                           273 
                         
                         ) 
                       
                     
                     ) 
                   
                 
               
             
           
         
         where T is the average temperature of the selectable temperature range in ° C. during the treatment time, y is the radiation intensity in kW/m 2  and a=2.298*10 −11 , b=0.399, c=1.722 and x=11100, and preferably a=5.355*10 −11 , b=0.355, c=1.349 and x=11000. 
       
     
     
         12 . The device according to  claim 1 , wherein the device is adapted to simultaneously receive a plurality of silicon substrates which have been previously encapsulated in a module. 
     
     
         13 . The device according to  claim 1 , wherein the device comprises a voltage source for applying a voltage corresponding to a desired voltage to be applied to a single solar cell multiplied by the number of solar cells connected in series and received within the device. 
     
     
         14 . The device according to  claim 1 , wherein the device comprises a hot plate and/or a suitably heatable room for storing the silicon substrate and keeping it on an elevated temperature during the stabilization treatment step. 
     
     
         15 . A photovoltaic element comprising a boron-doped, oxygen-containing silicon substrate having an efficiency-stabilized state, wherein the photovoltaic element has a high efficiency such as can be achieved by annealing, characterized in that the efficiency of the solar cell drops by less than 5% relatively, preferably less than 2% relatively, under illumination. 
     
     
         16 . A photovoltaic element with stabilized efficiency, fabricated by a process comprising the steps of:
 providing a boron-doped, oxygen-containing silicon substrate;   forming an emitter layer on a surface of the silicon substrate;   performing stabilization treatment, comprising:   keeping the temperature of the substrate during a treatment time within a selectable temperature range having a lower temperature limit of 50° C. and an upper temperature limit of 230° C.; and   generating excess minority charge carriers in the silicon substrate during the treatment time.   
     
     
         17 . The photovoltaic element according to  claim 16 , fabricated by a process further comprising the step of forming electrical contacts on the silicon substrate, wherein the step of generating excess minority carriers involves applying an external electrical voltage to the contacts, and wherein the treatment time t in minutes during which the substrate is held within the selectable temperature range is given by 
       
         
           
             
               t 
               ≥ 
               
                 
                   a 
                   
                     
                       ( 
                       
                         y 
                         + 
                         b 
                       
                       ) 
                     
                     c 
                   
                 
                 * 
                 
                   exp 
                    
                   
                     ( 
                     
                       x 
                       
                         ( 
                         
                           T 
                           + 
                           273 
                         
                         ) 
                       
                     
                     ) 
                   
                 
               
             
           
         
         where T is the average temperature of the selectable temperature range in ° C. during the treatment time, y is the current density through the photovoltaic element brought about by the applied voltage in A/cm 2  and a=4.247*10 −14 , b=0.00286, c=0.887 and x=12550, preferably a=3.272*10 −14 , b=0.00352, c=0.934 and x=12800. 
       
     
     
         18 . The photovoltaic element according to  claim 16 , wherein the step of generating the excess minority carriers comprises illuminating the silicon substrate, and wherein the treatment time t in minutes during which the substrate is held within the selectable temperature range is given by: 
       
         
           
             
               t 
               ≥ 
               
                 
                   a 
                   
                     
                       ( 
                       
                         y 
                         + 
                         b 
                       
                       ) 
                     
                     c 
                   
                 
                 * 
                 
                   exp 
                    
                   
                     ( 
                     
                       x 
                       
                         ( 
                         
                           T 
                           + 
                           273 
                         
                         ) 
                       
                     
                     ) 
                   
                 
               
             
           
         
         where T is the average temperature of the selectable temperature range in ° C. during the treatment time, y is the radiation intensity in kW/m 2  and a=2.298*10 −11 , b=0.399, c=1.722 and x=11100, and preferably a=5.355*10 −11 , b=0.355, c=1.349 and x=11000.