Improvement of solder interconnections applied on back contact solar cells with low-temperature copper paste busbars

This work addresses the solderability and the reliability of n-type IBC ZEBRA cells with screen printed copper paste busbars. Improvement of the solderability by Sn60Pb40 solder alloy coated PV ribbon using an industrial automated IR stringer is reported. For qualification of the module reliability, climate chamber thermal cycling (TC) and damp heat tests (DH) were performed on mini modules with variations in material combinations: low temperature silver and copper pastes, each of them demanding dedicated soldering process parameters and two different encapsulation materials: polyolefin elastomers (POE) and ethylene vinyl acetate (EVA). All tested modules passed 3 x IEC TC and 2 x IEC DH, while the modules with EVA encapsulant passed DH 3000 as well. The copper paste metallized modules with EVA encapsulant showed no degradation in pseudo fill factor (pFF) and open circuit voltage (VOC), proving the statement that no copper diffuses into the silicon bulk.