Mechanical stress analysis in photovoltaic cells during the string-ribbon interconnection process

The paper analyzes the mechanical problems of interconnecting individual solar cells in order to create a photovoltaic module. Modern modules increase their produced electrical power steadily at a constant low voltage, which causes high currents through the interconnecting copper wires, also called copper ribbons. The resistance of the interconnections is crucial, because it has a significant influence on the total module efficiency. However, an increased cross-section of the copper ribbons leads to severe mechanical problems, because the thin silicon solar cells would tend to break more easily. In this study the stresses created during the cell interconnection process are analyzed by 3-D FEM-simulations. These simulations are done by applying the commercial code ANSYS. The geometrical model consists of two adjacent cell quarters which are interconnected by one and a half copper ribbons. The geometrical model has a very fine mesh in critical cell sections in order to enable a result evaluation by path plots. The mechanical load is created by a temperature reduction from the solidification temperature of the lead-containing solder to room temperature. The result evaluation by path plots highlights those cell sections, which are the most critical in the productive tabber-stringer process. These results cannot be found by contour plots since the path plots are able to identity positions with high stress gradients. Due to the dominating compressive load of the silicon after the cooling step it is difficult to find possible crack positions. Applying the path plots big stress difference can be found in very small sections which correlate well with experimentally observed failure positions. Now it is possible to understand the complex nature of failure formation in the silicon solar cells applying this result evaluation method.