Effect of interconnect plasticity on soldering induced residual stress in thin crystalline silicon solar cells

Soldering of copper interconnects at ~220 °C induces residual stresses in the crystalline silicon solar cells. These residual stresses may become detrimental in the context of thinner silicon cells (<;180μm) causing cracking, performance loss and failures. In such a scenario a systematic evaluation of these residual stresses considering realistic material properties is required to assess the risk levels. In the present work, soldering induced residual stresses in the back contact c-Si solar cells were evaluated using finite element simulations. Effect of plasticity of the copper interconnects on the cell residual stress was studied. It was observed that the linear elastic material property of interconnect and solder over predicts the cell stress significantly and hence elastic-plastic material properties are needed for realistic predictions. Simulated stresses were also compared with those of synchrotron X-ray micro-diffraction experiments for cell thickness of 180μm to validate the finite element model. Further residual stress was evaluated as a function of silicon cell thickness in order to assess the risk of thinning cells for the current soldering process. These results are expected to enable optimization of soldering and interconnect parameters to mitigate high residual stress and thereby cell fractures.