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.
KeywordsA2: High-performance B2: Multicrystalline Si A1: Directional solidification A1: Casting A1: Dislocation cluster A1: Grain boundary A1: Lifetime A1: Gettering
The Emergence of HP mc-Si
Although high-quality ingots have been demonstrated for both mono-like and dendritic casting techniques, the multiplication and propagation of dislocation clusters due to thermal stress are still very difficult to control during crystal growth, especially for industrial-scale production. As a result, the solar cells fabricated from the wafers grown by both techniques have a very wide distribution in the conversion efficiency, and the low-efficiency tail in the distribution causes significant yield loss in the cell production. During the development of the dendritic casting technique, Lan’s group found that the control of undercooling was also not trivial. The thick quartz crucible wall (>30 mm) in production made the heat extraction from the crucible bottom much less effective. Moreover, the undercooling was sensitive to the silicon nitride coating as well. On the other hand, even large dendrites could be induced, massive dislocation clusters still appeared afterward due to thermal stress (Lan et al. 2012a, b; Yang et al. 2015). As the defect clusters appeared, they multiplied and propagated, so that the upper part of the ingot still had poor quality, i.e., low minority lifetime. Surprisingly, they occasionally induced small grains by controlling the undercooling, and the ingot grown from the small grains turned out to have a much better uniformity (Yang et al. 2015). The defect multiplication and propagation were significantly mitigated. For the wafers without massive dislocation clusters, they also noticed that the correlation between efficiency and grain size was small. This indicated that the GBs were not crucial to the wafer performance.
The Growth of HP mc-Si
The experimental results of HP mc-Si were first presented by Prof. Lan in the 5th International Workshop on Crystal Growth Technology in June of 2011 (Lan 2011a; Lan et al. 2013), and later in the 5th International Workshop on Crystalline Silicon Solar Cells held in Boston in October (Lan 2011b). The sample wafers were also sent to Solarworld for testing right after the Boston conference, and the feedback was excellent. The patent for such grain structures in the ingot and the wafer was filed in 2011 and first granted in 2014 (Lan et al. 2014); both silicon and nonsilicon particle seeds were used for ingot growth in the illustrated examples. After this finding, different approaches for getting such a grain structure have been explored and reported (Zhu et al. 2014; Wong et al. 2014a; Lan et al. 2016a, b; Zhang et al. 2016).
The Performance of HP mc-Si
Growth of HP mc-Si Without Seeding
The Properties of HP mc-Si
Wong et al. (2014a) did a detailed analysis of the grain structures developed from small silicon beads (0.9 mm in diameter), and they also found that the lowest-energy orientation, i.e., (111) tended to dominate during grain competition. However, the twining from the tri-junctions generated new grains with different orientations (Wong et al. 2014a); this might be the reason for more (112) orientation at the top ingot. Indeed, the (111)- or (112)-dominated orientations and the high fraction of random GBs are the typical characteristics nowadays in commercial HP mc-Si wafers. The high-percentage of the random GBs shown in Fig. 11c upsets the previous understanding for high quality mc-Si wafers; however, they played a crucial role in the reduction of dislocation clusters, especially for industrial production. Before Lan’s group reported this finding (Lan et al. 2012a, b, 2014; Yang et al. 2015), most people believed that twins or Σ3 GBs were needed for better lifetime (Fujiwara et al. 2006; Li et al. 2011, 2012; Nakajima et al. 2010a, b; Wang et al. 2009; Yeh et al. 2010), which was the core concept of the dendritic casting approach. In fact, if the wafers from the dendritic casting are carefully examined, one could find that the twin areas often have very few defects (Li et al. 2012; Ryningen et al. 2011); however, this is based on the condition without massive dislocation clusters. During the growth of large ingot, the relaxation of thermal stress for reducing the multiplication of dislocation clusters is crucial, and the large amount of random GBs in HP mc-Si plays a critical role.
Outlook for HP mc-Si
In addition to the dislocation clusters, the control of impurities is also important to ingot quality. The back diffusion of the metals from the silicon seeds increases the red zone and deteriorates the quality of bottom ingot, even though the EPD is the lowest there. Therefore, to further improve HP mc-Si, the reduction of seed layer thickness and the improvement of crucible/coating purity would be important. Recently, the crucibles coated with high purity silica are available in the market; using a diffusion barrier could also be feasible (Hsieh et al. 2014). Eventually, if the seeding layer is replaced by simple coating, while keeping small grains and high fraction of random GBs after nucleation, both quality and yield of HP mc-Si could be improved.
Nowadays, nearly 70% of solar cells are made from mc-Si wafers, and most of them are produced from the HP casting method. The method is very robust, so that it has been widely adopted by industry. In addition to the excellent ingot quality, the yield of the HP casting is also very high due to the much less massive propagation of dislocation clusters. The excellent quality of the HP mc-Si has been reflected on the significant progress of the solar cell efficiency in the recent years. Besides the p-type champion cell with an efficiency higher than 21.23% reported by Trina Solar Inc., the very recent world record (21.9%) on the n-type HP mc-Si solar cell has been made by Fraunhofer ISE in early 2007. Nevertheless, to compete with the monocrystalline silicon solar cells, further improvement of the ingot quality by HP casting can be expected.
CWL is grateful for the generous support by the Ministry of Science and Technology of Taiwan, National Taiwan University, and SAS.
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