Optimized Grain Size of Seed Plates for High Performance Multicrystalline Silicon
Two silicon ingots with different seed plate configurations were crystallized by directional solidification. Seed plates with high performance multicrystalline structure varying in their mean grain area were used as well as seed plates with comparable mean grain area varying in their grain structure (standard and high performance multicrystalline). All grown materials show an increase of grain size and saturation at different levels with ingot height. Additionally, a dependence of the growth behavior on the mean grain area was observed implying a faster coarsening for smaller grains. The material grown on the standard multicrystalline seed plate shows a larger area fraction limited by dislocations than the material with high performance multicrystalline seed plates, showing less dislocation structures on samples with smaller grains. The minority carrier lifetimes measured on passivated samples after a boron diffusion, as used in high efficiency solar cell processes for n-type silicon, showed a benefit for the material grown on a high performance multicrystalline seed plate with smaller grains compared to the other materials. The harmonic mean lifetime values are even higher than corresponding ones for high performance multicrystalline material nucleated on granular beads. The same trend was seen in ELBA, a combination of injection-dependent lifetime measurements on processed samples with PC1D simulations, which estimates material-related efficiency losses of only 0.4 % compared with the solar cell limit of an n-type solar cell structure on material nucleated on the fine grained seed plate with high performance multicrystalline structure.