Conductive Adhesives for Metallization of Interdigitated Back Contact Solar Cells

The metallization and interconnection of Si solar cells to form a module is usually a multi-step process: cells are metallized individually, soldered into strings, and subsequently assembled into a module. Furthermore, the metallization process for individual cells must be compatible with the doped surfaces being contacted, which creates additional challenges when moving from diffused junctions to heterojunctions and/or passivated contact structures. To mitigate these problems and streamline module manufacturing for the case of interdigitated back contact (IBC) cell modules, we developed an approach wherein unmetallized IBC cells are bonded directly to a circuitized backsheet using a conductive adhesive (CA) that only conducts out-of-plane. The metal/CA/silicon stack is bonded by hot-pressing at 100-200°C.For a CA consisting of ethylene vinyl acetate (EVA) filled with Ag-coated spheres or indium powder we find that contact resistivities in the range of 1-10 Wcm² are obtained, regardless of which conductive filler and which hot pressing parameters are used. XPS revealed that about 1-2 nm EVA residue bonds to the Si in these samples, suggesting the formation of an insulating barrier that is responsible for the high contact resistance. Using indium powder alone, improved contact resistivities <0.6 Wcm², high shunt resistances, and an implied open-circuit voltage loss below 10 mV were obtained. Furthermore, a cost estimate is presented, indicating that substantial cost savings of up to 10m², or about 0.05W_DC, are possible through CA-based IBC module manufacturing.