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Electromagnetic stirring and retention to improve segregation in silicon for photovoltaics

Authors
Journal
Journal of Crystal Growth
0022-0248
Publisher
Elsevier
Publication Date
Volume
340
Issue
1
Identifiers
DOI: 10.1016/j.jcrysgro.2011.11.074
Keywords
  • A1. Crystallization
  • A1. Electromagnetic Stirring
  • A1. Purification
  • A1. Segregation
  • B1. Electromagnetic Retention
  • B1. Silicon
Disciplines
  • Chemistry
  • Physics

Abstract

Abstract The segregation of impurities was investigated during the crystallization of upgraded metallurgical grade silicon (UMG Si). An experimental solidification system has been developed, including a strong electromagnetic stirring in order to mix the melt, to increase the effective segregation coefficient. Our crucible has a slit-like opening at the bottom of a face, to be adapted to ribbon pulling for future developments of a crystallization system for the photovoltaic industry. The electromagnetic system allowed retaining the liquid silicon inside the crucible during the crystallization of an ingot (thickness ∼5 cm), or make it flow out after the partial crystallization of a bottom layer (thickness ∼2 mm). Starting from UMG Si of known composition, the chemical analysis of the impurity concentrations in that multicrystalline material shows an efficient purification, despite the rather high crystallization speed (estimated between 9 and 20 cm/h). Phosphorous concentration was reduced from 6 to 1.7 ppm during the segregation, which is difficult to achieve for this impurity, which has a segregation coefficient close to unity ( k 0=0.3).

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