Numerical forecast of redzone extension in cast silicon ingots in dependence on the purity level of crucible, coating and feedstock
The extension of the areas with extremely low minority carrier lifetime in cast silicon ingots for photovoltaic application is a crucial parameter for the industrial manufacturers determining the yield of the grown Si ingot material usable for solar cell fabrication. In order to make a forecast of this redzone extension in dependence on the purity levels of the consumables like crucible, Si3N4-coating and Si-feedstock, a numerical 2D model was used to investigate the diffusive iron (Fe) incorporation into the silicon during the directional solidification process. In order to set up this model, the diffusion parameters of Fe in crucible and Si3N4-coating were determined by 1D numerical analysis of annealing experiments. These values were used in a 2D model to simulate several G1 experiments with varying Fe impurity levels of all crucible consumables (SiO2-crucible and internal Si3N4-coating). The 2D model was validated through comparison of the calculated and experimentally observed redzone. The 2D model was scaled up to the industrial G6 dimensions and a forecast of redzone extension considering the use of the same consumables was set up. The results show that as long as the influence of conventional silica ceramic crucibles is present, the use of Si3N4-coatings or Si feedstock with higher purity will show no positive effect on the redzone extension. For setups without impact of the crucible, similar to current setups including a diffusion barrier, and high silicon feedstock purity, the Si3N4-coating becomes the dominating Fe contamination source. At the same time, the effect of high purity Si3N4-coatings becomes negligible, when the assumed Fe concentration level of the silicon feedstock exceeds values expected for aggressive blending scenarios or low grade feedstock scenarios, which could be motivated by an aggressive cost down attitude.