Silver-bismuth non-contact metallization pastes for silicon solar cells

Abstract

Metallization pastes for use with semiconductor devices are disclosed. The pastes contain silver particles, low-melting-point base-metal particles, organic vehicle, and optional crystallizing agents. Specific formulations have been developed that produce stratified metal films that contain less silver than conventional pastes and that have high peel strengths. Such pastes can be used to make high contact resistance metallization layers on silicon.

1 . A solar cell comprising:
 a silicon substrate having a front surface and a back surface;   a plurality of fine grid lines on the front surface of the silicon substrate;   at least one front busbar layer on the front surface of the silicon substrate, the front busbar layer in electrical contact with at least a portion of the plurality of fine grid lines;   an aluminum layer on a portion of the back surface of the silicon substrate; and   at least one rear tabbing layer on a portion of the back surface of the silicon substrate;   wherein the rear tabbing layer comprises a stratified film that comprises at least two sublayers:
 a first sublayer over the silicon substrate, the first sublayer comprising a low-melting-point base-metal; and 
 a second sublayer over the first sublayer, the second sublayer comprising silver; 
 wherein the second sublayer has an exterior surface, and the exterior surface is exposed to an outside environment; 
 wherein the stratified film contains a low temperature base-metal fraction greater than 20%. 
   wherein edges of the aluminum layer and edges of the rear tabbing overlap to form overlap regions that comprise:   a solid aluminum-silicon eutectic layer on the silicon layer;   a modified rear tabbing layer over the solid aluminum-silicon eutectic layer; and   a portion of the aluminum layer over the modified rear tabbing layer.   
     
     
         2 . The solar cell of  claim 1 , wherein the stratified film further comprises (MO x ) y (SiO 2 ) z , crystallites wherein 0≤x≤3, 1≤y≤10, and 0≤z≤1 and M is a material selected from the group consisting of bismuth, tin, tellurium, antimony, lead, silicon, and alloys, composites, and combinations thereof. 
     
     
         3 . The solar cell of  claim 1 , wherein the first sublayer comprises a material selected from the group consisting of bismuth, aluminum, carbon, tin, tellurium, antimony, lead, silicon, and alloys, oxides, composites, and combinations thereof. 
     
     
         4 . The solar cell of  claim 1 , wherein the stratified film comprises at least 0.5 wt % aluminum. 
     
     
         5 . The solar cell of  claim 1 , wherein the stratified film comprises at least 1 wt % tellurium. 
     
     
         6 . The solar cell of  claim 1 , wherein the stratified film has a thickness between 1 μm and 15 μm. 
     
     
         7 . The solar cell of  claim 1 , wherein the first sublayer has a thickness between 0.01 μm and 5 μm. 
     
     
         8 . The solar cell of  claim 1 , wherein the second sublayer has a thickness between 0.5 μm and 10 μm. 
     
     
         9 . The solar cell of  claim 1 , wherein the second sublayer exterior surface comprises at least 70 wt % silver. 
     
     
         10 . The solar cell of  claim 1 , wherein the portion of the aluminum layer in the overlap region has an exterior surface, and the exterior surface is exposed to an outside environment. 
     
     
         11 . The solar cell of  claim 1 , wherein the overlap region is between 10 μm and 500 μm wide. 
     
     
         12 . The solar cell of  claim 1 , wherein the rear tabbing layers have a peel strength of more than 1 N/mm when soldered to tin-coated copper tabbing ribbons. 
     
     
         13 . The solar cell of  claim 1 , wherein a contact resistance between the rear tabbing layer and the aluminum layer is between 0 and 5 mΩ. 
     
     
         14 . A solar cell comprising:
 a silicon substrate having a front surface and a back surface;   a plurality of fine grid lines on the front surface of the silicon substrate;   at least one front busbar layer on the front surface of the silicon substrate, the front busbar layer in electrical contact with at least a portion of the plurality of fine grid lines;   an aluminum layer on a portion of the back surface of the silicon substrate; and   at least one rear tabbing layer on a portion of the back surface of the silicon substrate;   wherein at least one of the front busbar layers or the rear tabbing layer comprises a stratified film that comprises at least two sublayers:   a first sublayer over the silicon substrate, the first sublayer comprising a low-melting-point base-metal; and   a second sublayer over the first sublayer, the second sublayer comprising silver;   wherein the second sublayer has an exterior surface, and the exterior surface is exposed to an outside environment;   wherein the stratified film contains a low temperature base-metal fraction greater than 20%