Saw Damage Gettering for industrially relevant mc‐Si feedstock

Abstract

The silicon photovoltaic industry is currently shifting towards lightly doped emitters. These have electrical properties that benefit solar cells, compared to the traditional heavily doped emitters. This move brings new challenges, as gettering efficiencies of impurities are lowered as the doping reduces. This is particularly problematic in multicrystalline silicon (mc-Si) since cell performance is typically boosted by the effective gettering of such impurities. In prior work, we proposed the novel gettering technique, saw damage gettering (SDG), which improved effective carrier lifetime of standard performance mc-Si red zone material. In this work, we expand the study of SDG to various types of industrially relevant mc-Si: upgraded metallurgical grade (UMG), high performance bottom red zone (HPRZ), and diamond sawn high performance (DHP). The optimal condition for SDG is found to be an annealing temperature of 850 8C. With this condition it was demonstrated that the effective carrier lifetime can be increased in all silicon types upon SDG. The largest increase was observed for HPRZ material by a factor of 10, and the largest final effective lifetime post SDG was that of UMG, with t eff ¼ 61.3 ms. SDG is a potentially viable gettering method to work in conjunction with lightly doped emitters in removing the impurities of mc-silicon feedstock and thus, improving the efficiency of the cells made therefrom.

Black silicon significantly enhances phosphorus diffusion gettering

“…They fabricated sacrificial porous silicon (PS) layers on Si wafers and demonstrated that the harmful impurities were gettered from the bulk material to the porous layer during thermal annealing due to the high surface reactivity of PS. Similarly, mechanical 19 or saw damage 20 , 21 has been used as effective gettering sites. Thus, b-Si could promote the removal of impurities via near-surface precipitation, in addition to enhanced segregation.…”

Low‐Temperature Saw Damage Gettering to Improve Minority Carrier Lifetime in Multicrystalline Silicon

“… is probably the most similar to our material, although it is likely to have a lower dislocation density than our top material. For this material, average lifetime improved from ∼10 to 37 μs by SDG at 850 °C . Considering just the samples annealed at ≥500 °C for reasons discussed above, our best lifetime after SDG was 25.2 μs at 600 °C which is an improvement of 63% relative to the average as‐received lifetime in sister samples (15.5 μs).…”

“…The “standard performance bottom red zone” used in Ref. is probably the most similar to our material, although it is likely to have a lower dislocation density than our top material. For this material, average lifetime improved from ∼10 to 37 μs by SDG at 850 °C .…”

“…This approach, called saw damage gettering (SDG), has been recently studied at high temperatures (≥800 °C) by a group at the University of Oxford, who have demonstrated an improvement in lifetime, with the best results achieved at 850 °C followed by a relatively slow cool . However, adding another high‐temperature process step may be commercially unattractive in the context of cell production.…”

“…It is not straightforward to compare our results to high temperature SDG experiments due to the different material types, different surface passivation schemes, and because the high temperature SDG studies used a relatively slow cool from the peak processing temperature during which impurity gettering occurs. The “standard performance bottom red zone” used in Ref.…”