Analyses of the evolution of iron-silicide precipitates in multicrystalline silicon during solar cell processing
We simulate the precipitation of iron during the multicrystalline ingot crystallization process and the redistribution of iron during subsequent phosphorus diffusion gettering with a 2-D model. We compare the simulated size distribution of the precipitates with the X-ray fluorescence microscopy measurements of iron precipitates along a grain boundary. We find that the simulated and measured densities of precipitates larger than the experimental detection limit are in good agreement after the crystallization process. Additionally, we demonstrate that the measured decrease of the line density and the increase of the mean size of the iron precipitates after phosphorus diffusion gettering can be reproduced with the simulations. The size and spatial distribution of iron precipitates affect the kinetics of iron redistribution during the solar cell process and, ultimately, the recombination activity of the precipitated iron. Variations of the cooling rate after solidification and short temperature peaks before phosphorus diffusion strongly influence the precipitate size distribution. The lowest overall density of iron precipitates after phosphorus diffusion is obtained in the simulations with a temperature peak before phosphorus diffusion, followed by moderate cooling rates.