Determination of the effective distribution coefficient (K) for silicon impurities

Abstract

For the production of photovoltaic cells, the silicon purity can be intermediate between metallurgical grade silicon (MG-Si, 98%–99.9% pure) and electronic grade silicon (>99.9999% pure). This silicon, with intermediate purity and that still meets solar cell requirements, is called upgraded metallurgical grade silicon (UMG-Si). One method of producing UMG-Si is applying a controlled solidification process, like unidirectional solidification (heat exchange method), zone melting (or zone refining), or Czochralski growth to MG-Si. These processes use the impurities solubility difference in solid and liquid silicon known as effective distribution coefficient (K). For these reasons, to study the solidification process, it is necessary to determine K for silicon impurities, which is the objective of this study. MG-Si (99.85% purity or 1500 ppm of impurities) was processed by 1 pass of zone melting at 1 mm/min using an electron beam furnace with water cooled copper crucible. The effective distribution coefficient (K) for impurities with Ko ≤ 10−1 was found to follow the relation K = 0.03 Ko−0.063. For boron, K = 0.8. Impurities with Ko between 10−3 and 10−8 presented similar effective distribution coefficients (K = 0.07 ± 0.02), meaning that the effective distribution coefficient of a specific impurity depends on the total amount of impurities. The measured impurities profiles in silicon were compared with those obtained by Pfann’s equations using the effective distribution coefficients and showed comparative results.

Solar cells from upgraded metallurgical-grade silicon purified by metallurgical routes

“…In previous works, Pires et al 18-20 and Mei et al 15,21,22 reported a significant reduction of impurities using VD and ZM techniques. Yuge et al 23 described the purification of MG-Si to SG-Si using the NEDO (New Energy and Industrial Technology Development Organisation) melt-purification process, which adopts a multi-step process involving a sequence of VD, DS, and plasma melting.…”

A mathematical model for distribution of calcium in silicon by vacuum directional solidification

“…The amount of calcium that transfers from solid phase to liquid phase in the time of t can be expressed by: (1) where C l is the content of calcium in the solid-liquid interface, C s is the content of calcium in solid phase. The segregation coefficient of calcium, k 0 , can be express as follows [18] (2)…”

Comparisons of the effects of solute interactions on partition coefficient, kD, in selected binary immiscible solvents: a case of oxalic acid and succinic acid

“…Generally, when the k D is less than 1 (k D <1), the compound distributes into the aqueous phase more than the organic phase. 23 When k D = 1, the solute separates equally in the two phases that are involved, and it could be removed completely from the mixture. This applies when the bulk contaminant where the solute is seized is different from the two liquids that are used to form the binary solvent.…”

“…The Partition coefficient technique has been applied in finding the ‘effective partition coefficient’ (k D ) for silicon impurities. 23 As reported, the process uses the impurities solubility difference in solid and liquid silicon called effective partition coefficient (k D ). The measured impurities profiles in silicon are compared with the theoretically calculated impurities profiles to determine the effective partition coefficient.…”

“…Data for the partition coefficient of oxalic acid in diethyl ether-water at 30°C and atmospheric pressure for the plot of equation(23).…”