Clean Synthesis and Formation Mechanisms of High‐Purity Silicon for Solar Cells by the Carbothermic Reduction of SiC with SiO₂

Abstract

Metallurgical synthesis of solar grade silicon (SoGÀ Si) is an existing challenge due to the strict chemical composition of silicon for solar cells. Here, we report a green and facile approach to synthesize high-purity silicon (99.98 wt%, 0.12 ppmw B and 0.18 ppmw P) for solar cells by the carbothermic reduction of SiC with SiO 2 in a vacuum graphite resistance furnace. Catalyst of Fe 2 O 3 plays the role of reducing the concentration of CO (g) , lowering the energy consumption and improving the purity and yield of silicon. The present work is one-step to obtain silicon with less B and P, but it is composed of silicon synthesis, oxidation refining, vacuum refining and blowing refining, which discloses an innovative chemical concepts of in-situ synthesis combined with refining. According to the characterizations of the reacted SiC particles performed by XRD, Raman, PL, XPS, SEM and TEM, the morphologies and formation mechanisms of the in-situ silicon from the horizons of atomic scale were revealed for the first time. It was evidenced conclusively that the SiÀ C bonding is broken from the (006) planes of 6H-SiC at 1850°C by the reaction of SiC (s) + 2SiO 2(l) = 3SiO (g) + CO (g) , and then the silicon nucleates on the (101) planes of 6H-SiC and grows along the [111] direction at 1950°C via the vapor-solid reaction of SiO (g) + SiC (s) = 2Si (l) + CO (g) .High-purity SiO 2 (low content of B and P) can be achieved relatively easily, whereas it is difficult for carbon materials. Herein, it is urgent to use a high-purity reducing agent. [23,24] In fact, the actual reduction process of reaction (1) is much more complicated, which includes the formation of intermediate species, such as SiO (g) and SiC. Meanwhile, SiC is a significant reactant for the production of silicon. [25,26] The concentration of B and P in SiC can be controlled under 0.1 ppmw, [27] which meets the required level for solar cells. Herein, high-purity SiC is proposed as the reducing agent.

EFFECTS OF BORIC ACID DOSAGE AND REACTION TEMPERATURE ON THE PHASE COMPOSITION AND MICROSTRUCTURE OF Si C-B₄C COMPOSITE POWDERS SYNTHESISED BY THE CARBOTHERMAL REDUCTION METHOD

“…The SiC nucleation reaction between the SiO gas generated in reaction formula (1) and the C particles led to the pressure reduction of the system. According to the principle of chemical equilibrium shift, the equilibrium in reaction (2) occurs at a relatively low temperature, and this analysis result is consistent with the XRD analysis result [24][25]. In the present study, the SiO gas generated by reaction (1) and the C particles were generated by re-action (2) to generate the SiC particles.…”