Method of in-line diffusion for solar cells

Abstract

A method is provided for the simultaneous diffusion of dopants of different types on respective sides of a solar cell wafer in a single stage process. The dopants are applied to respective sides of the wafer in wet chemical form preferably by pad printing. The doping materials can be applied to the entire wafer surface or effective area thereof, or can be applied in a pattern to suit the intended solar cell configuration. In a typical embodiment, the dopants are boron and phosphorus.

1 . A method of simultaneously diffusing dopants of different types on respective sides of a silicon wafer comprising the steps of:
 providing the silicon wafer having a first side and a second side directly opposite the first side;   applying a first liquid doping material being of a first type to the first side of the wafer and not applying the first liquid doping material to the second side of the wafer;   applying a second liquid doping material being of a second type different from the first type to a second side of the wafer and not applying the second liquid doping material to the first side of the wafer; and   heating the wafer to diffuse simultaneously the first liquid doping material into the first side of the wafer and the second liquid doping material into the second side of the wafer.   
     
     
         2 . The method of  claim 1  wherein the first liquid doping material includes a Group III dopant. 
     
     
         3 . The method of  claim 1  wherein the second liquid doping material includes a Group V dopant. 
     
     
         4 . The method of  claim 2  wherein the dopant is a boron composition. 
     
     
         5 . The method of  claim 3  wherein the dopant is a phosphorus composition. 
     
     
         6 . The method of  claim 1  wherein the steps of applying the first liquid doping material and the second liquid doping material comprise one of pad printing, spray coating, dip coating, roll coating, gravure coating, rod coating, ink-jet printing, and screen printing. 
     
     
         7 . The method of  claim 1  wherein the steps of applying the first and second liquid doping materials comprise pad printing the first and second doping materials to the respective first and second sides of the wafer. 
     
     
         8 . The method of  claim 1  including the step of applying a capping coating over the first liquid doping material. 
     
     
         9 . The method of  claim 1  including the step of applying a capping coating over the second liquid doping material. 
     
     
         10 . The method of  claim 1  including the steps of applying a capping coating over each of the first and second liquid doping materials. 
     
     
         11 . The method of  claim 1  wherein the heating step comprises in-line heating the wafer during transport through a furnace chamber. 
     
     
         12 . The method of  claim 1  further including the step of removing diffusion glass layers formed on the first and second sides of the wafer during heating and diffusion. 
     
     
         13 . The method of  claim 12  wherein the step of removing the diffusion glass layers includes simultaneously removing the diffusion glass layers from the first and second sides of the wafer. 
     
     
         14 . The method of  claim 13  wherein simultaneously removing the diffusion glass layers from the first and second sides of the wafer is performed by etching both sides of the wafer with a solution of hydrofluoric acid. 
     
     
         15 . The method of  claim 2  wherein the first liquid doping material is a water-based boron solution. 
     
     
         16 . The method of  claim 2  wherein the second liquid doping material is a water-based phosphorus solution. 
     
     
         17 . The method of  claim 1  further including the step of drying either one or both of the first and second liquid doping materials. 
     
     
         18 . The method of  claim 1 , wherein the heating step comprises directing a gas flow from a furnace in a direction away from the first side of the wafer and away from the second side of the wafer, thereby minimizing the cross-contamination of the first liquid doping material on the second side of the wafer and the cross-contamination of the second liquid doping material on the first side of the wafer. 
     
     
         19 . The method of  claim 1 , wherein the heating step comprises directing a gas flow from a furnace at a velocity sufficient to entrain at least one of gaseous dopants and particulate dopants away from the first side of the wafer and away from the second side of the wafer, thereby minimizing the cross-contamination of the first liquid doping material on the second side of the wafer and the cross-contamination of the second liquid doping material on the first side of the wafer.