Analyses of warpage effects induced by passivation and electrode coatings in silicon solar cells

The price of silicon on the world market has quintupled within the past five years. As a consequence, the wafer producers developed strategies to save costs, e.g. by manufacturing thinner wafers. While in the year 2004 wafer thicknesses were about 300 μm, currently thicknesses are around 200 μm. In the future, wafers with thicknesses of 150 μm and below will be processed. Especially the firing step during processing of such thin wafers may lead to a serious warpage of the cells which is due to differences in the thermo-mechanical behavior of silicon and electrode layer materials. The present handling systems are not adapted for such bows, so that collisions occur and the breakage rates increase. Therefore, optimized electrode geometries are to be realized in such a way that warpage of the cells is minimized. A numeric model based on experimentally evaluated data was developed to enable the calculation and prediction of the warpage of solar cells having different kinds of metallization geometries. It could be shown that warpage can be significantly reduced by modifying the geometrical electrode configuration