Process for the production of a MWT silicon solar cell

1. A process for the production of a metal wrap through silicon solar cell comprising the steps:

(1) providing an n-type silicon wafer for the production of the metal wrap through silicon solar cell, wherein the n-type silicon wafer has (i) holes forming vias between the front-side and the back-side of the wafer and (ii) a p-type emitter extending over the entire front-side and the inside of the holes,

(2) applying a conductive metal paste to the holes of the silicon wafer to provide at least the inside of the holes with a metallization,

(3) drying the applied conductive metal paste, and

(4) firing the dried conductive metal paste, whereby the wafer reaches a peak temperature of 700 to 900° C.,

wherein the conductive metal paste has no or only poor fire-through capability and comprises (a) at least one particulate electrically conductive metal selected from the group consisting of silver, copper and nickel, (b) at least one particulate p-type dopant, and (c) an organic vehicle, wherein the at least one particulate p-type dopant is selected from the group consisting of boron, aluminum and p-type silicon alloys selected from the group consisting of alloys comprising silicon and boron, alloys comprising silicon and aluminum and alloys comprising silicon, boron and aluminum.

2. The process of claim 1 , wherein the n-type silicon wafer has an ARC layer on its front-side that leaves out the inside of the holes.

3. The process of claim 1 , wherein the organic vehicle content is from 10 to 45 wt. %, based on total conductive metal paste composition.

4. The process of claim 1 , wherein the electrically conductive metal is present in the conductive metal paste in a proportion of 50 to 92 wt. %.

5. The process of claim 1 , wherein the electrically conductive metal is silver.

6. The process of claim 1 , wherein the conductive metal paste contains at least one other particulate metal selected from the group consisting of particulate iridium, particulate platinum and particulate palladium in a total proportion of 0.5 to 5 wt. %, based on the total of particulate metals contained in the conductive metal paste.

7. The process of claim 1 , wherein the total content of the at least one p-type dopant in the conductive metal paste is 0.5 to 10 wt.-%.

8. The process of claim 1 , wherein the p-type silicon alloy is eutectic aluminum/silicon alloy (AlSi12).

9. The process of claim 1 , wherein the conductive metal paste comprises as component (d) at least one glass fit selected from the group consisting of (i) lead-free glass frits with a softening point temperature in the range of 550 to 611° C. and containing 11 to 33 wt. % of SiO 2 , >0 to 7 wt. % of Al 2 O 3 and 2 to 10 wt. % of B 2 O 3 and (ii) lead-containing glass frits with a softening point temperature in the range of 571 to 636° C. and containing 53 to 57 wt. % of PbO, 25 to 29 wt. % of SiO 2 , 2 to 6 wt. % of Al 2 O 3 and 6 to 9 wt. % of B 2 O 3 .

10. The process of claim 9 , wherein one or more of the lead-free glass frits contain 40 to 73 wt. % of Bi 2 O 3 .

11. The process of claim 9 , wherein the total content of glass frit selected from the group consisting of types (i) and (ii) in the conductive metal paste is 0.25 to 8 wt. %.

12. The process of claim 1 , wherein the conductive metal paste is applied as a conductive metal layer or as a conductive metal plug.

13. The process of claim 1 , wherein the conductive metal paste is applied by printing.

14. The process of claim 1 , wherein firing is performed as cofiring together with at least one metal paste selected from the group consisting of (1) a conductive metal paste that has been applied to the front-side to form a front-side metallization in the form of thin conductive metal collector lines arranged in a pattern typical for MWT silicon solar cells and (2) a back-side silver paste that has been applied to the back-side to form silver back collector contacts.