Multicrystalline silicon wafers prepared from upgraded metallurgical feedstock

A solution to the problem of the shortage of silicon feedstock used to grow multicrystalline ingots can be the production of a feedstock obtained by the direct purification of upgraded metallurgical silicon by means of a plasma torch. It is found that the dopant concentrations in the material manufactured following this metallurgical route are in the 10¹⁷ cm⁻³ range. Minority carrier diffusion lengths L_n are close to 35 μm in the raw wafers and increases up to 120 μm after the wafers go through the standard processing steps needed to make solar cells: phosphorus diffusion, aluminium–silicon alloying and hydrogenation by deposition of a hydrogen-rich silicon nitride layer followed by an annealing. L_n values are limited by the presence of residual metallic impurities, mainly slow diffusers like aluminium, and also by the high doping level.