[7]
contribute significantly to minority charge carrier recombination. With regard to directional solidification technology, if temperature distribution inside the crystallization furnace is controlled perfectly, higher growth rates for high quality UMG-Si ingot fabricating can be achieved. Thus, 20μm/s can be used for production of high-quality and large Si ingots in the future. Effect of drop-down rate on the growth orientation As can be seen from Fig.3, the columnar crystal grew orderly and the grain size was bigger when the drop-down rate was 20μm/s. Maybe the temperature profiles distribution inside the furnace is suitable for ingot growth at 20μm/s. When the drop-down rate was 10μm/s, the grain size was big since the slower solidification rate and time enough to growth. However, its state of columnar crystal growth was worse than 20μm/s. When the drop-down rate was 30μm/s, the columnar crystals were also obtained. However, the quality of ingot was not better than the ingots prepared at 10 and 20μm/s, respectively. Besides, there were many macro-holes in the middle part of the ingot. The reason is that gases dissolved into the melt and precipitated from the solid and formed bubbles during solidification due to the solubility of gases in the solid phase below the liquid phase. Meanwhile, these bubbles grew large with the crystal growth. They were retained inside the ingot because crystal growth rate was faster than bubbles growth. When the drop-down rate was 40μm/s, since larger temperature gradient and lager residual thermal stress inside the ingot, which result in the crystals splitted. According to the above discussions, the ingot prepared at the drop-down rate of 20μm/s is more favorable for fabricating solar cells with high conversion efficiency. To confirm the above result, XRD analysis was performed for wafers (1,3,5) from silicon ingot prepared at the growth rate is 20μm/s. As shown in Fig.6, the preferential orientation was (111). Since (111) face is a closely packed plane and smooth face in silicon, therefore, it is easy to grow preferentially during the melt crystallization. Meanwhile, inducing grain growth with (111) face or <111> vectors is good to reduce dislocation density during melt growth [12], and is also useful for subsequent cell processing. Conclusions The dislocation density distribution was "V" shape for the ingots prepared at the four drop-down rates, i.e., higher in both ends and lower in the center. When the drop-down rate was 20μm/s, the ingot had low dislocation density, columnar crystal growth sufficiently, lower concentration of transition metal impurities and crystal growth along <111> directions. We anticipate these results will be highly beneficial to fabrication of low cost and high-efficiency UMG-Si solar cells. Acknowledgement This work was supported by Program for New Century Excellent Talents in University (NCET-07-0387) and the Open Fund of State Key Laboratory of Silicon Materials of Zhejiang University (SKL2009-8). The author would like to acknowledge Mr. Hongjun Wu for his helpful discussion.
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