The Effect of Al and Fe Doping on Solar Cells Made from Compensated Silicon

Abstract

State of the art Silicon feedstock refined by the metallurgical route readily achieves impurity and dopant concentrations well below 1 ppmw. To avoid costly excessive feedstock refinement, relevant threshold concentrations need to be established for impurities known to have a deleterious effect on the cell performance and reliability. Since compensation may reduce the recombination activity of some impurities, this study evaluates the impact of two important contaminants, Aluminum and Iron, in compensated Silicon. For this, model materials based on electronic grade feedstock are produced and processed to solar cells. The feedstock is co-doped with B (0.25 ppmw) and P (0.43 ppmw) to simulate compensated material, the feedstock was additionally contaminated with Al and Fe and cast to p-type multicrystalline ingots. In cells processed from these materials, Fe has been effectively gettered and the tolerance to Al contamination is found to be much higher than expected from theoretical extrapolations in the literature: for 1 ppmw Al, the efficiency is reduced by about 3% relative. Further, Al doped ingots show weaker light induced degradation than the reference ingots. This is explained by a preferential formation of Al-O complexes in competition with the B-O complex. Breakdown behavior of the pn-junction is influenced by Al doping. No interaction is observed between Al and Fe. Based on the results a threshold concentration close to 0.5 ppmw Al in compensated silicon feedstock is concluded as a safe level in terms of cell performance and reliability.