Internal Gettering of Iron at Extended Defects

The role of iron in crystalline silicon solar cells has been extensively investigated, yet the interaction mechanisms with structural defects have not been fully understood. In this work we have investigated a multicrystalline silicon ingot made in a small scale (1.5 kg) vertical gradient freeze (VGF) furnace with the addition of 50 ppma Fe in the polysilicon feedstock. The minority carrier lifetime was qualitatively measured by photoluminescence (PL). Grain morphology and -orientation were determined by electron backscatter diffraction (EBSD). The comparison between the PL and EBSD maps shows high lifetime areas near the grain boundaries, which is explained by an internal gettering mechanism. Furthermore, it is evident that Fe segregates slightly at coincidence site lattice (CSL) boundaries – especially at Σ3, while it segregates heavily at random grain boundaries. These results indicate the dependence of Fe segregation towards defect on grain boundary character. A method to qualitatively and quantitatively identify the segregation profiles and depleted region thickness is developed by image analysis.