7. Growth of Multicrystalline Silicon for Solar Cells: The High-Performance Casting Method

The emergence of high-performance multicrystalline silicon (HP mc-Si) in 2011 has made a significant impact to photovoltaic (PV) industry. In addition to the much better ingot uniformity and production yield, HP mc-Si also has better material quality for solar cells. As a result, the average efficiency of solar cells made from HP mc-Si in production increased from 16.6% in 2011 to 18.5% or beyond in 2016. With an advanced cell structure, an average efficiency of more than 20% has also been reported. More importantly, the efficiency distribution became much narrower; the difference even from various wafer producers became smaller as well. Unlike the conventional way of having large grains and electrically inactive twin boundaries, the crystal growth of HP mc-Si by directional solidification is initiated from uniform small grains having a high fraction of random grain boundaries (GBs). The grains developed from such grain structures significantly relax thermal stress and suppress the massive generation and propagation of dislocation clusters. The gettering efficacy of HP mc-Si is also superior to the conventional one, which also increases solar cell efficiency. Nowadays, most of commercial mc-Si is grown by this approach, which could be implemented by either seeded with silicon particles or controlled nucleation, e.g., through nucleation agent coating. The future improvement of this technology is also discussed in this chapter.