Casted monocrystalline silicon

Seeded cast silicon is also known as cast-mono (or quasi-mono) crystalline silicon. It is grown in a large crucible similar to multicrystalline silicon (mc-Si). But unlike mc-Si production, seeds with 156 × 156 mm2 dimension are placed at the bottom of the crucible. Due to a thermal gradient during cooling, the bricks that are grown along the outer blocks have partially monocrystalline and multicrystalline structure, while the inner blocks are almost entirely monocrystalline. As the oxygen concentration in mc-Si is typically lower than monocrystalline silicon, these wafers are less susceptible to recombination due to boron-oxygen defects. However, a significant disadvantage of cast-mono is the high presence of dislocations. As the cast-mono wafers have no grain boundaries, the thermal stress which occurs during the casting process is relieved by the formation of dislocations. These dislocations, which cannot be seen with optical techniques, are recombination active and can dramatically reduce the performance of the solar cell. Furthermore, a high dislocation density (above 106 cm-3) can also lower the efficacy of gettering processes. The gettering process which occurs during high-temperature diffusion processes removes impurities from the bulk as most impurities (e.g. iron) segregate to the highly doped phosphorus or boron regions of the solar cell, reducing the impurity concentration in the bulk.

Figure 1: Picture of a casted monocrystalline silicon ingot taken at the SNEC exhibition in 2018.

The seeded cast process is shown schematically in the animation below.