The effect of internal stresses on the recombination activity of structural defects in multi-crystalline solar silicon

Abstract

Internal stresses in multi-crystalline silicon (mc-Si) materials for solar cells are the result of the superposition between the thermally induced residual stresses and the defect-related stresses. In the present contribution, we attempt to clarify the influence of internal stresses on the electrical activity of extended structural defects in edge-defined film-feed (EFG) ribbons and block-cast mc-Si wafers. For this purpose, we apply at identical positions a combination of micro-Raman, electron beam induced current (EBIC), defect etching, and electron backscatter diffraction (EBSD) techniques. Since the materials are cut into wafers and then into small pieces, the thermally induced residual stresses relax to a large extent by the creation of free surfaces. Thus, the measured internal stresses are mainly produced by defects. These stresses are too small (several tens of MPa) to influence directly the electrical activity, but they can enhance it via stress-induced accumulation of metallic impurities at defects.