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

研究了多晶硅柱状结构的生长机理和缠绕对微观结构形成的影响，从而提高了单向光固化技术制备的太阳能电池的光电转换效率和生产效率。利用Bridgman型炉在氩气氛下，以0.075 ~ 9.6 mm/s的速度和20K/cm的温度梯度，在内径为20 mm、高为100 mm的石英坩埚中固化了太阳能电池级高纯硅晶体。在1.2 mm/min左右的凝固速度下，柱状组织平行于热流方向，在较低的凝固速度下获得较大尺寸的晶粒。孪晶界沿热流方向平行生长，晶体取向关系为σ 3。因此，通过<111>和<101>的种子和孪生引入<211>晶体来测量生长的过冷度。<111>的过冷度较大，为2.5 ~ 5.5K, <101>的过冷度较小。而孪晶<211>的过冷度最低，为0.5 ~ 3.5K。由于分子动力学模拟还揭示了引入<211>晶粒的孪晶的凝固速度更快，孪晶界的重入角成为结点，有利于晶粒的多面生长和尺寸的发展。硅晶体沿坩埚底向< 21> ~ <101>方向生长。随着过冷度的增大，初始凝固区域的凝固速度加快（0.6 ~ 2.4mm/min），晶粒尺寸增大。然而，为了保持如此大的增益尺寸，有必要将速度降低到0.12 ~ 0.3mm/min，直至凝固中期和最终阶段。

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.