Ultrasonic Tinning of Al Busbars for a Silver-Free Rear Side on Bifacial Silicon Solar Cells

Reducing the silver consumption of photovoltaics (PV) is a major aspect in recent solar cell research. For bifacial PERC+ solar cells silver is used for the front contact. On the rear side aluminum metallization provides the contact to the silicon. The native oxide of aluminum prohibits a standard soldering process. Therefore, rear side silver pads are typically used for the cell-to-cell interconnections with copper wires. Silver can be avoided when using ultrasonic soldering for wetting the aluminum metallization to form tin solder pads. We demonstrate mechanically stable soldering of interconnects to the silver-free solder pads with a median adhesion up to 3 N/mm. We observe a penetration of the native aluminum oxide layer by the ultrasonic tinning process and the formation of metal-to-metal contacts from the aluminum to the solder. Resistance measurements demonstrate a reduced series resistance of the ultrasonically prepared contact when compared with using silver pads. For PERC+ cells, we can thus fully avoid rear side silver pads for a standard stringing process to reduce the silver consumption by 20%–40%. We fabricate mini modules that reach the same efficiency as reference modules with standard silver pads on the rear. The efficiency degradation of the modules with the ultrasonic interconnection is less than 3.6% after 200 humidity-freeze cycles and less than 2.2% after 600 temperature cycles.