Low-Cost, Manufacturable, 6-Inch Wafer Bonding Process for Next-Generation 5-Junction IMM+Ge Photovoltaic Devices

We propose the development of a 6-inch wafer bonding process to allow bonding of a multi-junction inverted metamorphic (IMM) tandem solar cell structure to an upright single-junction Ge solar cell. The process will use SU8 epoxy to provide the mechanical connection between the substrates with an embedded metallic grid to provide electrical conductivity across the bonded interface. This process will enable the manufacture of next-generation, five-junction cells with the potential to achieve efficiency >37% under AM0 illumination in high-volume production. The proposed process will be designed for 6-inch substrates and will be low-cost, scalable, and high-yield. Compared to conventional wafer fusion approaches, this technique will be much more tolerant of the rough surfaces typical of metamorphic materials and eliminate the need for any pre-bonding surface polishing or preparation.