Modeling distribution and impact of efficiency limiting metallic impurities in silicon solar cells

Although metal impurities are known to limit the efficiency of solar cells especially for the case of multicrystalline silicon the quantitative prediction of carrier lifetime and cell parameters as a function of impurity concentration is complex. Metal point defects as well as precipitates of different sizes and spatial distribution contribute to the overall carrier recombination and thus lifetime and cell efficiency. Diffusion and firing steps significantly modify the concentration but also the distribution and configuration of metal impurities. This article presents our modeling results on iron and chromium impurities which take into account these effects and which consistently explain our experimental findings on metal impurity concentration, distribution, and configuration throughout solar cell processing. Together with a cell simulation which is based on injection dependent carrier lifetime results we have now a tool to quantitatively determine the impact of impurities on cell parameters.