Mechanical behaviour and fatigue of copper ribbons used as solar cell interconnectors

The soldering of solar cell strings is a critical step in the production of photovoltaic modules. Temperature induced stresses can cause cracking in the cells. During the soldering operation, the cell and the wires are heat up and expand. During the subsequent cooling phase they contract. The differential contraction between the Cu and the Si combined with thermal gradients, causes mechanical stress in the assembly. Moreover the lifetime of solar modules can be limited by the fatigue of the copper-ribbons. Since the front glass and the silicon cells have a significant difference in their coefficients of thermal expansion, temperature fluctuations are able to induce thermo-mechanical stresses in the photovoltaic module. The paper will present the results of the mechanical tests on copper materials. In order to give an explanation for the fatigue behaviour of the material, a correlation to the relevant microstructures will be given. Based on the investigated mechanical behaviour practical implications for proper handling of copper ribbons during solar module production processes will be concluded.