Low-cost solar cell metallization over tco and methods of their fabrication

Abstract

Methods for fabricating busbar and finger metallization over TCO are disclosed. Rather than using expensive and relatively resistive silver paste, a high conductivity and relatively low cost copper is used. Methods for enabling the use of copper as busbar and fingers over a TCO are disclosed, providing good adhesion while preventing migration of the copper into the TCO. Also, provisions are made for easy soldering contacts to the copper busbars.

1 . A solar cell, comprising:
 a substrate having a back surface and a front surface, the front surface designed for facing the sun;   a photovoltaic structure formed on the front surface;   a transparent conductive oxide (TCO) layer formed over the photovoltaic structure; and, front metallization layer formed over the TCO, the front metallization layer comprising a copper layer.   
     
     
         2 . The solar cell of  claim 1 , wherein the TCO comprises an indium tin oxide (ITO). 
     
     
         3 . The solar cell of  claim 1 , wherein the front metallization layer further comprises an adhesion/barrier layer formed between the TCO and the copper layer. 
     
     
         4 . The solar cell of  claim 3 , wherein the front metallization layer further comprises a cap layer formed over the copper layer. 
     
     
         5 . The solar cell of  claim 3 , wherein the front metallization layer further comprises a seed copper layer formed between the adhesion/barrier layer and the copper layer. 
     
     
         6 . The solar cell of  claim 3 , wherein the adhesion/barrier layer comprises one of nickel, chromium, or titanium or alloy thereof. 
     
     
         7 . The solar cell of  claim 6 , wherein the adhesion/barrier layer comprises one of electroplated nickel or electroplated chromium. 
     
     
         8 . The solar cell of  claim 6 , wherein the adhesion/barrier layer comprises one of sputtered nickel or sputtered titanium. 
     
     
         9 . The solar cell of  claim 1 , wherein the copper layer comprises electroplated copper or an alloy comprising copper. 
     
     
         10 . The solar cell of  claim 4 , wherein the cap layer comprises tin. 
     
     
         11 . The solar cell of  claim 1 , wherein the photovoltaic structure comprises a doped amorphous silicon layer. 
     
     
         12 . The solar cell of  claim 11 , wherein the photovoltaic structure further comprises an intrinsic amorphous silicon layer between the substrate and the doped amorphous silicon. 
     
     
         13 . The solar cell of  claim 11 , wherein the substrate comprises a doped silicon, and wherein the doped amorphous silicon layer is doped opposite the doping of the substrate. 
     
     
         14 . A method for fabricating a solar cell, comprising:
 fabricating a photovoltaic structure over a substrate;   fabricating a transparent conductive oxide (TCO) layer over the photovoltaic structure; and,   fabricating front metallization layer in electrical contact with the TCO, the front metallization layer comprising a copper layer or an alloy with 50% or more of copper.   
     
     
         15 . The method of  claim 14 , wherein fabricating front metallization layer comprises electroplating the copper layer. 
     
     
         16 . The method of  claim 15 , further comprising treating the TCO to become hydrophilic prior to fabricating the metallization layer. 
     
     
         17 . The method of  claim 16 , wherein fabricating front metallization layer further comprises fabricating a barrier/adhesion layer prior to electroplating the copper layer. 
     
     
         18 . The method of  claim 17 , wherein fabricating front metallization layer further comprises fabricating a cap layer over the copper layer. 
     
     
         19 . The method of  claim 17 , wherein fabricating a barrier/adhesion layer comprises sputtering a metal layer. 
     
     
         20 . The method of  claim 19 , wherein fabricating front metallization layer further comprises forming a mask to delineate the metallization design. 
     
     
         21 . The method of  claim 14 , wherein fabricating front metallization layer comprises electroplating the metallization layer directly over the photovoltaic structure.