microstructure Development of multicrystal silicon ingot for solar battery by the control of twinning

为了提高定向凝固技术制备太阳能电池的光电转换效率和生产率，研究了多晶硅柱状晶的生长机理以及孪晶对微结构形成的影响，在内径20 mm、高100 mm的石英坩埚中对太阳能电池级高纯硅晶体进行了定向凝固，在Ar气氛下，采用Bridgman炉，在流速为0.075~9.6mm/s，温度梯度为20K/cm的条件下，在流速约为1.2mm/min时，观察到平行于热流方向的柱状晶组织，在较低的凝固速度下获得较大尺寸的晶粒。孪晶晶界具有σ 3的晶体取向关系，平行于热流方向生长。因此，生长的过冷度通过使用和和孪晶引入的晶种来测量。<111><101><211>试样具有较大的过冷度（2.5~5.5K），而具有较小的过冷度（2.5~5.5K）。<111><101>而双晶引入试样的过冷度最低，为0.5~3.5K。<211>由于分子动力学模拟还揭示了引入孪晶的凝固速度更快，孪晶界的凹角成为扭折点，有利于小面生长和晶粒尺寸的发展，硅晶体沿着沿着坩埚底部向~之间的取向生长。<211><211><101>提高过冷度和在初始凝固区加快凝固速度（0.6~2.4mm/min），晶粒尺寸增大。然而，为了继续这样大的增益尺寸，有必要将速度降低到0.12至0.3mm/min，直到中间和最终凝固阶段。

The mechanisms of columnar structure growth of the multicrystal silicon and influence of twining on the microstructure formation are investigated to the development of the photoelectric transformation efficiency and the productivity of solar battery cell fabricated by the unidirectional solidification technique.The solar battery-grade high-purity silicon crystals has been solidified in quartz crucibles, whose inside diameter is 20 mm and the height is 100 mm, at the velocity of 0.075 to 9.6 mm/s and a temperature gradient of 20K/cm by using the Bridgman type furnace at the Ar atmosphere.The columnar structures are observed parallel to the heat flow direction in a velocity about 1.2 mm/min, and the larger size grains are obtained at the lower solidification speed. Twin boundaries, which have the crystal orientation relationship of sigma 3, grow parallel to the heat flow direction. Therefore, the undercoolings for the growth were measured by using seeds of <111> and <101> and twin introduced <211> crystals. The <111> specimens has larger undercooling of 2.5〜5.5K and <101> has smaller. However, twin introduced <211> specimens has lowest undercooling of 0.5〜3.5K. Since it is also revealed that the solidification velocity of twin introduced <211> crystal is higher by the molecular dynamics simulation, the reentrant corner of twin boundary becomes kink site and can give an advantage to the faceted growth and the development of grain size.The silicon crystal grows to the orientation between <211> to <101> along to the crucible bottom. And the grain size was enlarged by increasing the undercooling as well as the faster solidification velocity of 0.6 to 2.4mm/min in the initial solidification region. However, in order to continue such large gain size, it is necessary to reduce the velocity to 0.12 to 0.3mm/min until middle and final solidification stage.