Impact of transition metals in feedstock on multicrystalline silicon solar cell properties
The effect of metal contamination in multicrystalline silicon ingots is investigated. The impurities have been added on purpose to the silicon feedstock and the solar cell performance of each contaminated ingots has been compared to a reference uncontaminated ingot. A larger crystal defect density is observed in the top and in the bottom of the contaminated ingots with respect to the reference. Adding 50 ppmw of iron or 40 ppmw of nickel or chromium to the silicon feedstock in p-type ingots, the solar cell performances are comparable to the reference in the range 40 to 70 % of the ingot height. This means that about 10 ppmw of Fe, Cr and possibly Ni will be allowed in the silicon feedstock. This is a value which is at least 2 orders of magnitude larger than previously thought.