Electrical and structural properties of multicrystalline silicon grown from reusable silicon nitride crucibles
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The solar cell industry has grown considerably and experiences renewed public interest due to technological advances and awareness of human impact on the environment.However, the industry faces several challenges that must be overcome, such as reduction in ingot production costs without affecting solar cell performance. One promising prospect is by replacing the standardized silica crucibles used in directional solidification, as they can only be used once due to breaking after each ingot solidification and are also known to contaminate silicon ingots with oxygen through diffusion.In this work, oxidized nitride bonded silicon nitride (NBSN) crucibles were investigated as a potential replacement for silica crucibles. The work includes solidification in both NBSN and standardized silica crucibles. NBSN crucible was reused for comparative study against the standard. Two ingots from each crucible type were investigated with respect to the following: Carrier lifetime mapping by µPCD. Resistivity mapping by eddy currents. Resistivity as a function of ingot height was also investigated with FPP. Oi and Cs concentrations by FTIR. Furnace gas composition by in situ gas chromatography.In addition, the contact angle on both uncoated and coated NBSN substrates was measured by the sessile drop method. Coated NBSN substrate displayed satisfactory non-wetting contact with liquid silicon, but the results from µPCD lifetime mapping imply that NBSN crucibles impose considerable thermal strain in the ingot, as verified by unfavorable grain orientations and high dislocation concentration in the ingot center. These trends and consequent loss in carrier lifetime increased after NBSN crucible reuse, to a maximum of 6 µs. Both ingots solidified in NBSN crucible displayed considerably increased Oi and Cs contamination relative to the references. Oi concentration decreased to below 10 ppma levels after reuse, but Cs remained relatively unaffected and above 7 ppma. Overall resistivity converged to 1,5±±0.5 Ωcm, which was deemed acceptable. Increased Oi and Cs contamination was attributed to deoxidization of NBSN crucible and increased CO gas diffusion into the silicon melt, respectively. Keywords: Multi-crystalline, silicon, p-type. solar cells, NBSN crucible, silica crucible, thermal strain, lifetime, resistivity, impurity, interstitial oxygen, substitutional carbon.