How to Obtain Solderable Al/Ni:V/Ag Contacts

We investigate process sequences for obtaining solderable Al/Ni:V/Ag contacts to PERC-type crystalline Si solar cells by in-line Al evaporation. For a high cell efficiency the evaporated aluminum must be annealed at 350 °C for about 5 min. We find that annealing the Al/Ni:V/Ag metallization stack at temperatures above 150 °C destroys the solderability of the wetting layer. A solution for this problem is to first deposit the 2.5 μm Al layer by evaporation, then anneal the cell at 350 °C for 10 min, and finally sputter a double layer of Ni:V/Ag with respective thickness values of 200 nm and 25 nm. This process leads to a contact resistivity lower than 1 mΩcm2. The solderablility is proven by a peel force greater than 3 N/mm. We present a solderable PERC cell with Al/Ni:V/Ag rear side metallization and an efficiency of 18.9%.