Solar Cell Silicon: Silicon Impurity Removal and Refining
Sponsored by: TMS Extraction and Processing Division, TMS: Recycling and Environmental Technologies Committee, TMS: Energy Committee
Program Organizers: Shadia Ikhmayies, Al Isra University; Neale Neelameggham, Ind LLC

Thursday 8:30 AM
March 2, 2017
Room: 19
Location: San Diego Convention Ctr

Session Chair: Neale Neelameggham, IND LLC; Christian Liebscher, Max-Planck-Institut für Eisenforschung GmbH

8:30 AM  
Effect of Magnesium Addition on Removal of Impurities from Silicon by Hydrometallurgical Treatment: Stine Espelien1; Gabriella Tranell1; Jafar Safarian1; 1NTNU
    Hydrometallurgical treatment of silicon is an effective process for the removal of metallic impurities for producing solar grade silicon feedstock. In the present research, the removal of impurities from silicon is studied through the addition of Mg into a commercial silicon, its solidification, and acid leaching. The effect of particle size, concentration of HCl and temperature on the removal of impurities are studied. It is shown that the impurities such as B, P, Fe, Al, Ti, Ca,… are effectively removed through the applied method and Mg improves the segregation and the removal of the impurities. Scanning electron microscopy examination of the samples indicates the disintegration and dissolution of Mg2Si phase in acid leaching which causes the chemical and physical removal of the impurities existing in the phases between the silicon grains. It is shown that the leaching process rate can be optimized through the control of the acid leaching conditions.

8:50 AM  
Evaporation Removal of Boron in Molten Silicon Using Reactive Fluxes: Ye Wang1; Kazuki Morita2; 1SIchuan University; 2The University of Tokyo
     In order to optimize the B removal during slag-refining process for solar-grade silicon production, a new process for the removal of B in molten Si was proposed based on the principle of oxidized chlorination and evaporation. B can be generated and evaporated in the form of B-containing gas (BOCl) from molten Si using CaO-SiO2-CaCl2 slag system. Compared with the binary systems of CaO-CaCl2 and SiO2-CaCl2 slag, the ternary slag system CaO-SiO2-CaCl2 showed a better potential (90 pct) for B removal at 1723K for 2 hours. Moreover, the diffusion coefficient (D) of B in the slag was also determined using tube-molten pool method. Through this, it was found that the rate-limiting step of B-removal in the slag refining process is controlled by B transfer in the slag interface and surface.

9:10 AM  
Study on the Segregation Behavior of Impurities during Solvent Refining Process: Li Jiayan1; Tan Yi1; 1Dalian University of Technology
    Solvent refining is a low-temperature and high refining efficiency Si purification process, in which Si recrystallization takes place from the supersaturated Si-based alloy melt depending on the retrograde behavior of impurity. More attentions have been focused on Si-Al alloy system, which is an economical alloy and can effectively lower the content of B and P. However, the B segregation mechanisms during the alloy solidification process is lack, and the refined silicon can not be separated effectively from the alloy, resulting in a waste of energy and time. In this work, Al and Sn were used as solvent metals to purify metallurgical grade silicon and the segregation behaviours of B were initially clarified. Moreover, the effect of direct electric current on redistribution and microstructure of primary silicon in alloy simple has been studied. This method was attempted by the directional solidification method in order to separate Si from the alloy.

9:30 AM  
Topological Impurity Segregation at Faceted Silicon Grain Boundaries: Christian Liebscher1; Andreas Stoffers2; Oana Cojocaru-Mirédin2; Baptiste Gault1; Christina Scheu1; Gerhard Dehm1; Dierk Raabe1; 1Max-Planck-Institut für Eisenforschung GmbH; 2RWTH Aachen University
    One of the most detrimental lattice defects in multicrystalline Silicon (mc-Si) are grain boundaries. Both, the grain boundary structure but also the local chemistry at the interface strongly degrade the electrical performance by increased charge carrier recombination. In our study we establish a direct connection of the atomic structure and chemistry of faceted, symmetrical tilt grain boundaries extracted from a mc-Si wafer. The one-to-one correlation of atomic resolution aberration-corrected scanning transmission electron microscopy (STEM) and 3D atom probe tomography (APT) unravels a novel segregation phenomenon associated with the faceted nature of the boundaries. STEM observes the arrangement of the nanometer sized facets and the underlying atomic structural units. The segregation of Iron and Carbon impurities as determined by APT is confined to the linear junctions of coherent and incoherent grain boundary segments. An outlook on the connection of local strain state and segregation behavior is presented.