Mechanical and electrical properties of wave-shaped wires for low-stress interconnection of solar cells
In the manufacturing process of a common solar module, crystalline silicon solar cells are interconnected by soldering. Copper-based interconnectors, coated with a solder alloy are soldered on both sides of the solar cells on the contact metallization. Subsequently, the solar cell strings are embedded in between two sheets of encapsulant, commonly ethylene-vinyl acetat (EVA), which itself attaches to a front glass and a back sheet during the lamination process. Thermomechanical stress, due to the mismatch of the coefficients of thermal expansion (CTE) of copper and silicon, causes bowing of a back-contact solar cell after the soldering process. Additionally, thermomechanical stress can result in the failure of solder joints. By adapting the mechanical properties of an interconnector, thermomechanical stress can be reduced. Shaping a straight wire with a round cross section to a wave-like form reduces its effective tensile mechanical properties and leads to changes of its effective electrical conductivity. This study investigates the mechanical and electrical properties of wave-shaped wires with different amplitudes, periods and diameters. In addition, microscopic imaging reveals the geometry and potential defects due to the shaping process. The influence of the shaping process on the yield limit of commercially available copper-based wires is determined. For this reason, an algorithm is designed that allows the automatic and precise determination of the yield limit of these interconnectors. In this study, a maximum yield limit reduction of 88.5% compared to a straight wire is found. Furthermore, the influence of the shaping process on the electrical resistance is analyzed. For this purpose, the influence of wire damaging due to the shaping process, as well as the wires longitudinal length change is determined. An increase of the electrical resistance between 3.5% and 82.7% is measured. To choose an optimal interconnection concept for solar cells, the mechanical, as well as the electrical properties of the interconnectors have to be considered. This study correlates the advantageous longitudinal softness of wave-shaped wires, which potentially leads to a significant decrease of the thermomechanical stress in solar cells and of the bowing of back-contact solar cells, with the disadvantageous increase of the electrical resistance caused by the shaping process. The findings enable an optimization of a wire-based interconnection concept for back-contact solar cells.