Impurities influence on multicrystalline photovoltaic Silicon

The formation of the grain structure in multicrystalline photovoltaic Silicon relies on basic phenomena which are not well understood and mastered in industrial processes. Nucleation, growth modes (facetted or not), grain competition and kinetics, coarsening are at the origin of the mean grain size and of the morphology of the grain structure which impacts drastically on the photovoltaic properties. During solidification, solute rejection (especially carbon and nitrogen) at the growth interface leads to an increase of the impurity concentration in the liquid phase and then to the precipitation of silicon nitride (Si3N4) and silicon carbide (SiC). As a consequence, the grain structure of the ingot changes from columnar to small grains, also known as grits. A new electromagnetic levitation set up has been developed in order to measure the undercooling versus impurity concentration. The impurity concentration in the levitated Si droplet is controlled by the partial pressure of nitrogen or hydrocarbon gas. The concentration of nitrogen and carbon dissolved will be compared with theoretical predictions (phase diagram). And a new analytical model is proposed to understand the precipitation of SiC during the solidification process of an ingot.