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.

Reusability of contaminated seed crystal for cast quasi-single crystalline silicon ingots

“…Therefore, the paper mainly studies if the recycled seed crystal with typical iron concentration can be reused. For the solidification process with standard crucible and standard feedstock, the iron profile in the recycled seed crystal varies around the value of 1.0 Â 10 15 atoms/cm 3 , as shown in our previous study [4]. However, the profile is not unique and it varies with the change of preservation time.…”

“…Some studies have been carried out to study the transport and contamination of iron impurity in the casting process for QSC silicon ingots [4][5][6]. The results show that iron contamination is the main factor for the large low lifetime region at the ingot bottom and the total iron concentration in the seed crystal is more than 10 14 atoms=cm 3 after solidification.…”

“…Recent study shows that the iron transport during casting process can lead to a large region enriched with iron impurity at the bottom of a QSC silicon ingot, which corresponds to the low lifetime region in this location [4]. The seed crystal is also highly contaminated during the casting process.…”

Electronic Quality Improvement of Highly Defective Quasi‐Mono Silicon Material by Phosphorus Diffusion Gettering

“…A significant fraction of QM-Si ingots are typically discarded as unusable due to iron contamination from the seed layer and the crucible wall. [10,11,14,15] A PDG process with phosphorus diffusion at 870 °C for 60 min followed by a low temperature anneal (LTA) significantly increased the harmonic minority carrier lifetime of a QM-Si wafers cut directly from the edge of an ingot from 17 to 178 µs.…”

“…Many research groups have identified experimentally and numerically that iron contamination is the main cause of the red zone for casting silicon. [14,15] While the iron contamination is also a problem in traditional mc-Si, it is particularly aggravated for seed-assisted silicon casting due to the back diffusion from the seed, [16][17][18] which leads to an extended red zone for the QM-Si.…”