Iron contamination in cast quasi-single crystalline silicon ingots

Abstract

Iron is an important metallic impurity in crystalline silicon for solar cells and it can significantly influence the minority carrier lifetime. We numerically investigated the transport and distribution characteristics of iron impurity in the directional solidification process for quasi-single crystalline (QSC) silicon ingots, and attempted to reveal the relationship between the distributions of iron concentration and the minority carrier lifetime map. Additionally, the seed preservation time was varied to investigate its influence on the diffusion and distribution of iron impurity. The results show that iron diffusion from the silicon melt to the seed crystal during the preservation process can lead to a peak of iron concentration near the seed-crystal interface at the early stage of solidification. This peak gradually disappears during crystal growth, and iron diffusion results in a large region enriched with iron at the ingot bottom after complete solidification. Therefore, we conclude that the iron impurity is related to the large low lifetime region at the ingot bottom. We also discussed the effect of iron contamination on the high-low-high lifetime distribution pattern in the axial direction near the seed-crystal interface, which is a phenomena sometimes reported in QSC silicon ingots. The effect of seed preservation time on iron contamination in the QSC silicon ingot was also investigated. We found that the single-crystalline seed is highly contaminated even after a short time of seed preservation.