Production of high performance multi-crystalline silicon ingots for PV application by using contamination-free SixNy seed particles

In this work a new type of contamination-free SixNy seed particles for the directional solidification of HPM silicon ingots is introduced and evaluated. Several G1 crystallization experiments have been carried out to examine the influence of the seed particle size, the introduced mass and the influence of the composition of the underlying Si3N4 coating at the crucible bottom. Finally, multi-PERC cells over the ingot height from one of the most promising SixNy seeded ingot have been prepared. The results show that an oxygen contamination is not occurring by the new particles. A minimum seed particle density of around 120/cm2 at the crucible bottom is sufficient to generate a HPM like grain structure with random grain boundary fractions R of 65%±5%, which is very close to typical values of classical silicon seeded HPM. A further increase of the particle density doesn't lead to a further increase of the R value due to the competition between the very close packed seeds. While lower oxygen levels of the Si3N4-coating promote significant higher R values in crucible setups without seed particles, the Si3N4-coating composition has almost no influence on the resulting grain structure properties when SixNy seed particles are used. This could be shown even for small seed densities of ~30 cm−2 and a remaining coating area fraction of 80%. The cell analysis shows same efficiencies of the ingots seeded on the SixNy particles and on the silicon feedstock layer, but a higher wafer yield can be achieved in the case of the SixNy seeding method since a full melt process can be used.