Raman spectroscopy characterization of residual stress in multicrystalline silicon solar wafers and solar cells: Relation to microstructure, defects and processing conditions

Stress in multicrystalline silicon (mc-Si) is a critical issue for the mechanical stability of the material and it has become a problem of growing importance, especially in view of silicon wafer thickness reduction. Without increasing the wafer strength, the high fracture rate during handling and subsequent processing steps leads to excessive losses. A non-uniform stress distribution could be expected in critical areas such as grain boundaries, wire-saw-damaged layer and areas near the metallization and soldered contacts. Therefore, a non-destructive method to locally determine stress in mc-Si solar cells is of technological importance. In this paper stress characterization based on a combination of Raman spectroscopy, electroluminescence imaging, cell bowing measured with a laser scanning device, confocal microscopy and ex-situ bending tests will be presented. The most critical processing steps during the manufacture of screen-printed solar cells are wafer cutting, firing of metallic contacts and the soldering process. In this work the development of mechanical stress in silicon wafers as a result of different processing steps will be evaluated. Furthermore, residual stress and stress developing during silicon cell bending are measured in relation to microstructure and defect density. It was found that there is an inhomogeneous distribution of stress along grain boundaries and metallic inclusions. It was found that at a certain load grain boundaries in mc-Si wafers experience a higher stress (~50 MPa) than grains themselves (~30 MPa). A significant Raman shift was observed in samples with a wire-saw-damaged layer and in the areas close to Ag fingers and Al/Ag bus bars. Raman scanning was also performed along the solar cell cross section with different metallization patterns.