Yacine Boulfrad

Preventing light-induced degradation in multicrystalline silicon

Multicrystalline silicon (mc-Si) is currently dominating the silicon solar cell market due to low ingot costs, but its efficiency is limited by transition metals, extended defects, and light-induced degradation (LID). LID is traditionally associated with a boron-oxygen complex, but the origin of the degradation in the top of the commercial mc-Si brick is revealed to be interstitial copper. We demonstrate that both a large negative corona charge and an aluminum oxide thin film with a built-in negative charge decrease the interstitial copper concentration in the bulk, preventing LID in mc-Si.

Experimental evidence on removing copper and light-induced degradation from silicon by negative charge

In addition to boron and oxygen, copper is also known to cause light-induced degradation (LID) in silicon. We have demonstrated previously that LID can be prevented by depositing negative corona charge onto the wafer surfaces. Positively charged interstitial copper ions are proposed to diffuse to the negatively charged surface and consequently empty the bulk of copper. In this study, copper out-diffusion was confirmed by chemical analysis of the near surface region of negatively/positively charged silicon wafer. Furthermore, LID was permanently removed by etching the copper-rich surface layer after negative charge deposition. These results demonstrate that (i) copper can be effectively removed from the bulk by negative charge, (ii) under illumination copper forms a recombination active defect in the bulk of the wafer causing severe light induced degradation.

Modeling of Lifetime Distribution in a Multicrystalline Silicon Ingot

Lifetime distribution of a multicrystalline silicon ingot of 250 mm diameter and 100 mm height, grown by unidirectional solidification has been modeled. The model computes the combined effect of interstitial iron and dislocation distribution on minority carrier lifetime of the ingot based on Shockley Read Hall (SRH) recombination model for iron point defects and Donolato’s model for recombination on dislocations. The iron distribution model was based on the solid state diffusion of iron from the crucible and coating to the ingot during its solidification and cooling, taking into account segregation of iron to the melt and back diffusion after the end of solidification. Dislocation density distribution is determined from experimental data obtained by PVScan analysis from a vertical cross section slice. Calculated lifetime is fitted to the measured one by fitting parameters relating the recombination strength and the local concentration of iron