Solar cells from upgraded metallurgical grade (UMG) and plasma-purified UMG multi-crystalline silicon substrates

High impurity concentrations do not allow the direct use of upgraded metallurgical grade (UMG) Si for PV production. A newly developed prototype inductive plasma-purification system and process allowed the significant reduction of the elements B, C, O, P, Al, Ca, Fe and Ti, depending on the duration of the treatment. Based on this type of purification, it is shown that subsequent appropriate low-cost cell-processing yields homogeneously distributed energy-conversion efficiencies throughout the cast ingots. Stabilised cell efficiencies of up to 14.7% were already experimentally shown to be attainable on highly B-doped (ρ<0.1 Ωcm) 102 cm² multi-crystalline Si substrates of high purity. On plasma-purified UMG p-type 0.1-0.2 Ωcm ingots, efficiencies of up to 12.38% are reached, to be compared with about 10.12% on the same material without prior plasma treatment. Some light-induced degradation is present on processed samples, which is most likely linked to the presence of metastable boron-oxygen complexes in the material, and results in stabilised efficiencies of, respectively, 12.19% and 10.00%.