Microstructural analysis and process chain simulation of copper-ribbons for solar cell interconnections

An important process step in manufacturing silicon based photovoltaic solar panels is the electrical contacting of the solar cells by soldering of thin copper-ribbons. During the soldering process, considerable mechanical stresses are induced into the solar cells since the coefficients of thermal expansion of the connected materials are significantly different. This may cause micro cracks in the silicon cell, which is a major reason for cell breakage within the production line. Furthermore, the soldering induced stresses can also lead to breakage of the copper interconnectors during lifetime due to thermo-mechanical fatigue. For these reasons, producers of copper ribbons aim to optimize the mechanical properties to minimize the possibility of damage. Here, low yield stress is favourable to provide a soft ribbon material that tends to deform plastically [1]. The production chain of copper ribbons consists of several steps which are accompanied by large plastic deformations of the material, as illustrated in Fig. 1. For a detailed understanding of the macroscopic material behaviour and its subsequent optimization, it is essential to consider the microstructure evolution along the process chain in the experimental analysis and also within the numerical model.