Residual and bending stress measurements by X-ray diffraction and synchrotron diffraction analysis in silicon solar cells

Abstract

The presence of residual stresses in multicrystalline silicon solar cells has become a problem of growing importance, especially in view of silicon wafer thickness reduction. Without increasing the wafer strength, this leads to a high fracture rate during subsequent handling and processing steps. The most critical processing step during the manufacture of screen-printed solar cells is the firing of metallic contacts. In this work we evaluate the development of mechanical stresses in metallic contacts (Al, Ag and Al/Ag bus bars) with respect to different processing steps. For this purpose we combine X-ray diffraction (XRD) stress measurements, Synchrotron measurements, cell bowing measurements with a laser scanning device and in-situ bending tests. Synchrotron diffraction analysis showed that there is a stress gradient in both Ag and Al layers. It was found that the Al back contact layer represents a very porous/loose microstructure, which does not affect the mechanical stability of the solar cell. It was also found that the thickness and composition of the eutectic layer are the most important factors influencing the bowing of a complete solar cell. Furthermore, residual stresses and stresses developing during cell bending in Ag, Al/Ag bus bars are measured and discussed in detail in this work.