Impact of metal contamination in silicon solar cells
The impact of the transition metals iron, chromium, nickel, titanium and copper on solar-cell performance is investigated. Each impurity is intentionally added to the silicon feedstock used to grow p-type, directionally solidified, multicrystalline silicon ingots. A state-of-the-art screen-print solar-cell process is applied to this material. Impurities like iron, chromium and titanium cause a reduction in the diffusion length. Nickel does not reduce the diffusion length significantly, but strongly affects the emitter recombination, reducing the solar-cell performance significantly. Copper has the peculiarity of impacting both base-bulk recombination as well as emitter recombination. Two models based on the Scheil distribution of impurities are derived to fit the degradation along the ingot. Solar-cell performances are modelled as a function of base-bulk recombination and emitter-bulk recombination. The model fits the experimental data very well and is also successfully validated. Unexpectedly, the distribution of impurities along the ingot, due to segregation phenomena (Scheil distribution), leaves its finger-print even at the end of the solar-cell process. A measure of impurity impact is defined as the level of impurity that causes a degradation in cell performance of less than 2% up to 90% of the ingot height. The advantage of this impurity-impact metric is that it comprises the different impurities physical characters in one single parameter, which is easy to compare.