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 (η) 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 η 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 η gap must be eliminated in the future, it shows the principal functionality and potential of such type of contact for iTOPCon solar cells.