Qualification of laser-weld interconnection of aluminum foil to back-contact silicon solar cells

Laser welding can be used to interconnect high-efficiency back-contact silicon solar cells with low-cost Al foil. This interconnection approach is relatively new and, thus, requires detailed vetting of its reliability before being adopted commercially. In this study, we weld 50-μm-thick Al foil to Sunpower back-contact cells and observe that the laser-weld adhesion, module fill factor, and reliability through thermocycling are all highly correlated to each other. A JMP statistical model built from adhesion data reveals that the statistically significant parameters to improve laser-weld adhesion are the laser pulse energy, pulse density, and pattern. Increasing the laser pulse energy and density improves the foil adhesion to the cell metallization, which is likely because of the improved melting of the Sn capping layer on the Cu electrode of the cells, as identified by cross-sectional microscopy. 94.4% of the modules fabricated using laser welds with a mean adhesion above 0.8 mJ lost less than 5% of their initial maximum power after 200 thermocycles, which is the IEC 61215 criterion for any single accelerated stress test. Additionally, 90.0% of modules fabricated with an as-fabricated module series resistance below 1.9 Ω cm passed thermocycling. Thus, laser-weld adhesion and the as-fabricated module series resistance can be used in the further development of new laser-weld settings and as quality control parameters in the manufacturing of these modules.