Reduction of soldering induced stresses in solar cells by microstructural optimization of copper-ribbons

Soldering of solar cell strings is a critical step in the production of photovoltaic modules. During the soldering process significant mechanical stresses are induced in the stringed cell assembly. Since silicon has a much smaller coefficient of thermal expansion than copper it is compressed by the copper-ribbon during the cooling phase. The resulting stresses can cause micro-cracks in the silicon cell, which are a major reason for cell breakage within the production line. Furthermore those stresses may lead to a delayed failure of the solder interconnections or cell cracking in the field. Therefore ribbon manufacturers try to create very soft ribbon material, which tends to be rather plastically deformed than generating stresses such that the silicon is prevented from damage. Nevertheless, the general tendency of using thinner wafers in cell production and the projected step towards the usage of lead-free solders increase the mechanical requirements on the cell interconnectors and make systematic scientific investigations inescapable. The purpose of this work is to analyze the micro-structure of ribbon in detail and to correlate it with its mechanical material behavior. An electron backscatter diffraction method was used to evaluate grain sizes and orientations in various annealing steps of the ribbon. These results were compared to their mechanical properties, achieved by conventional mechanical testing. As a result of these investigations the annealing process of the ribbon was optimized on laboratory scale to achieve highly adjusted material properties. Finally the benefit was verified by numerical simulation of the soldering process.