Investigation of Thermomechanical Stress in Solar Cells with Multi Busbar Interconnection by Finite Element Modeling
The interconnection of silicon solar cells by soldering causes thermomechanical stress due to different coefficients of thermal expansion of the materials involved, especially copper and silicon. In contrast to the current standard interconnection technology, using three to five flat ribbons to be soldered on continuous or segmented busbars on the solar cells front side and on large contact pads on the solar cells rear side, the Multi Busbar concept uses twelve to fifteen round solder coated copper wires to be soldered on a large number of small contact pads for each polarity. In this paper we present two finite element models, one for a solar cell with a common busbar-based interconnection and a second one for a solar cell using the Multi Busbar interconnection approach to analyze and compare the distribution of the thermomechanical stress induced by the cell interconnection. The results show only a slight vertical deformation of a contacted solar cell of 0.4 mm for the three busbar design and 2.2 mm for the Multi Busbar interconnection technology due to the non-symmetric pad layout and the aluminum metallization on the rear side. However, most parts of the copper interconnector (ribbons and wires) are plastically deformed because the tensile stress exceeds the yield strength of 100 MPa. In addition, the results indicate that there is only slight tensile stress in most parts of the silicon solar cell after the soldering process. For both interconnection technologies we detect local stress peaks and, on the rear side of the solar cells near the outermost contacts, we determine areas where the tensile stress reaches levels above 200 MPa that potentially cause defects.