Mechanical stress in solar cells with multi busbar interconnection — Parameter study by FEM simulation

The interconnection of silicon solar cells is commonly realized by soldering copper ribbons or wires with a solder coating onto screen-printed silver contacts. Due to the difference of the coefficient of thermal expansion (CTE) of copper and silicon, thermomechanical stress is induced after the soldering process during cooling down to room temperature. In the first part of this work, a model is introduced based on the Finite Element Method (FEM) to determine the influence of crucial interconnector properties on the deformation of a section of a silicon solar cell with Multi Busbar (MBB) layout, connected on one side. The deformation is easy to measure and serves as a visible indicator for the mechanical stress in the solar cell. We determine the diameter of the wire to have the largest influence on the cell deformation. However, it has to be taken into account that, if the wire diameter is changed also the number of wires for the interconnection of an entire MBB solar cell has to be adapted. Moreover, the considerable influences of the Young's modulus and the yield strength of the copper wire on the cell bow are shown. Additionally, the results of a FEM model to determine the distribution of the thermomechanical stress in a MBB solar cell, connected on both sides, are shown. The stress distribution in the interconnecting wires reveals that they undergo plastic deformation during the cooling down to room temperature after the solder process. We determine maximum tensile stress values in the wires of about 162 MPa, located adjacent to the outermost pads of a pad row on the front side or a pad group on the back side of the MBB solar cell. In addition, we find areas with large tensile stresses within the silicon solar cell. Due to the stress level near the outermost pads on both sides of the MBB solar cell we expect these areas to be most sensitive to defects like cracks in the silicon or adhesive failures of the pad metallization.