ECN publication
Impact of Metal Contamination in Silicon Solar Cells
Published by: Publication date:
ECN Solar Energy 22-9-2010
ECN report number: Document type:
ECN-M--10-078 Conference Paper
Number of pages: Full text:
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Presented at: 2010 International Conference on Solid State Devices and Materials (SSDM 2010), Tokyo, Japan, 22-24 september 2010.

The impact on solar cell performance of transition metals like iron, chromium, nickel, titanium and copper is the topic of this extended abstract. Each impurity 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 cut from the bottom to the top of these ingots. Adding 50 ppmwt of iron or 40 ppmwt of nickel or chromium to silicon feedstock, results in comparable solar cell performances to reference uncontaminated material in the range 40% to 70% of the ingot height. Addition of 10 ppmwt of titanium dramatically reduces the efficiency along the entire ingot. Impurities like iron, chromium and titanium cause a reduction in the diffusion length. Nickel does not reduce the diffusion length. On the other hand affects strongly the emitter recombination reducing the solar cell performance significantly. Copper has the peculiarity to impact both bulk recombination as well as emitter recombination.
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 base bulk recombination and emitter recombination. The model fits very well the experimental data and has been also successfully validated. 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.

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