Thermochemical and Kinetic Databases for the Solar Cell Silicon Materials

The fabrication of solar cell grade silicon (SOG-Si) feedstock involves processes that require direct contact between solid and a fluid phase at near equilibrium conditions. Knowledge of the phase diagram and thermochemical properties of the Si-based system is, hence, important for providing boundary conditions in the analysis of processes. A self-consistent thermodynamic description of the Si-Ag-Al-As-Au-B-Bi-C-Ca-Co-Cr-Cu-Fe-Ga-Ge-In-Li-Mg-Mn-Mo-N-Na-Ni-O-P-Pb-S-Sb-Sn-Te-Ti-V-W-Zn-Zr system has recently been developed by SINTEF Materials and Chemistry. The assessed database has been designed for use within the composition space associated with the SoG-Si materials. An assessed kinetic database covers the same system as in the thermochemical database. The impurity diffusivities of Ag, Al, As, Au, B, Bi, C, Co, Cu, Fe, Ga, Ge, In, Li, Mg, Mn, N, Na, Ni, O, P, Sb, Te, Ti, Zn and the self diffusivity of Si in both solid and liquid silicon have extensively been investigated. The databases can be regarded as the state-of-art equilibrium relations in the Si-based multicomponent system. The thermochemical database has further been extended to simulate the surface tensions of liquid Si-based melts. Many surface-related properties, e.g., temperature and composition gradients, surface excess quantity, and even the driving force due to the surface segregation are possible to obtain directly from the database. By coupling the Langmuir-McLean segregation model, the grain boundary segregations of the nondoping elements in polycrystalline silicon are also possible to estimate from the assessed thermochemical properties.