Long-term and Annealing Stable, Solderable PVD Metallization with Optimized Al Diffusion Barrier

Since passivated emitter and rear cells (PERC) and other silicon solar cell concepts with evaporated aluminum (Al) as rear metallization are incompatible with a common solder process, in this work an annealing stable,solderable and long-term stable metallization scheme deposited by physical vapor deposition (PVD) is developed. The solder stack that complements the Al metallization consists of sputter deposited TiN/Ti/Ag or TiN/NiV/Ag, whereby the TiN layer serves as a diffusion barrier against Al. It is therefore optimized by varying sputter parameters and by stuffing the grain boundaries with oxygen. On the optimized stack a cell-interconnector can be conventionally soldered even after a strong annealing step of 15 min at 425 °C, which sets this concept apart from other PVD metallization approaches. Cell efficiency is not influenced by the solder stack compared to a reference rear metallization by plain evaporated Al. Additionally, long-term stability of the solder-joints on the metallization schemeis investigated by thermal aging of solder-joints and thermal cycling of demo moduleswith PERC cells.