Paste-based silver reduction for iTOPCon rear side metallization

This work presents two approaches to reduce the silver (Ag) amount on the rear side of M2-sized industrial tunnel oxide passivated contact (iTOPCon) solar cells by (partially) substituting conventional Ag pastes with copper (Cu) and aluminum (Al) pastes. The first approach is Cu-based and works by screen printing an Ag layer with ca. 75% Ag paste reduction compared to a conventional Ag layer (functioning as a contact and Cu barrier layer) and screen printing a full-area Cu layer on top of the Ag layer (functioning as a conduction layer compensating the reduced Ag amount). This Cu-based approach yields similar power conversion efficiency (eta) compared to the conventional approach, showing the Cu-based approach is promising for iTOPCon solar cells. The second approach is Al-based and works by laser contact opening of the dielectric layers and screen printing a full-area layer with an Al paste - adapted to properly contact an n+-poly-Si layer -, substituting Ag on the rear side entirely. This Al-based approach exhibits a 0.9%abs eta gap to the conventional approach, mainly stemming from the Al paste damaging the poly-Si layer, while - on a positive note - neither rear p+ layer formation nor tunnel oxide layer penetration by the Al paste is detected. Even though the abovementioned eta gap must be eliminated in the future, it shows the principal functionality and potential of such type of contact for iTOPCon solar cells.