Directional solidification of photovoltaic silicon in re-useable graphite crucibles

Graphite crucibles are potentially interesting for the directional solidification processing of photovoltaic silicon, because, contrarily to standard silica crucibles, they can be used many times. In the present work, two types of graphite crucibles are studied: i) graphite directly coated with the Si3N4 powder classically used as a releasing layer for standard silica crucibles, and ii) graphite protected by a dense SiC layer before deposition of the releasing coating. In both cases, spontaneous detachment of the silicon ingot is observed to occur during cooling. Results are given and discussed concerning the impurities introduced in the silicon during the melting-solidification cycle, as well as their consequences on the photo-electrical characteristics of silicon. These results are compared with those obtained using the standard silica crucible. It is shown that re-useable SiC protected graphite crucibles lead to the same performances as standard silica. Reproducible results are found after four runs with the same crucible, and no degradation of the crucible is observed.