Impact of metal contamination in multicrystalline silicon solar cells: Case study for iron

The impact on solar cell performance of iron has been investigated. Iron has been intentionally added to silicon feedstock used to grow p-type directionally solidified multicrystalline silicon ingots. A state of the art screen print solar cell process has been applied to wafers from the bottom to top of the ingot. Adding 50 ppmwt of iron to silicon feedstock, results in comparable solar cell performances to reference uncontaminated material, in the range 40 to 70% of the ingot height. Iron causes a reduction in the diffusion length, which decreases with the ingot height. A model based on Scheil distribution of impurity has been derived to fit the degradation along the ingot. Solar cell performance has been modeled as function of the diffusion length. The model fits very well the experimental data. Unexpectedly, the Scheil distribution of impurity along the ingot leaves its finger-print also at the end of the solar cell process. A measure of impurity impact has been defined as the level of impurity which causes a degradation of less than 2% up to 90% of the ingot height. The advantage of this parameter is that comprises the different impurities physical characters in one single parameter, easy to compare. In case of iron we estimate C_L as 11 ppm wt.