Solidification control of multicrystal silicon ingot for solar battery

本文研究了多晶硅柱状结构的生长机理，以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。在小型坩埚中凝固太阳能电池级高纯硅晶体（内径为20 mm，高度为100 mm），速度为0.075 ~ 9.6 mm/s，温度梯度为20 K/cm。研究了凝固条件对硅多晶体形核和晶粒生长过冷度、晶粒尺寸和晶体取向的影响。在1 mm/min左右的凝固速度下，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。随着生长速度的增加，晶粒尺寸逐渐变小，并开始形成等轴晶组织。 ...更多信息 Ve的速度约为1.8 mm/min。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，晶体生长的过冷度和驱动力的基础上的二维晶核生长制度的模型估计。结果表明，<211>孪晶界定向生长的过冷度降低到定向生长的70%左右<111>，孪晶界的凹角有利于小面生长。基于分子动力学理论的计算机模拟得到了类似的现象。因此，我们认为孪晶界上的弯折位置有利于凝固速度的提高和晶粒尺寸的发展，<211><101>在初始凝固阶段，无论坩埚的形状和结构如何，硅晶体都沿沿着坩埚底部的方向生长。即使在中等尺寸的试样（内径80 mm和高度20 mm）中也出现类似的现象，并且在大尺寸的太阳能电池单元中也出现类似的现象。因此，通过在试样底部覆盖硅晶体，使其在<211><101>凝固初期由于孪晶晶界的存在而向α晶方向凝固，然后以相对较慢的凝固速度向上生长，直至最终凝固，可以控制晶粒尺寸和晶粒取向。少

The mechanisms of columnar structure growth of the multicrystal silicon are investigated to the development of the photoelectric transformation efficiency and the productivity of solar battery cell fabricated by the unidirectional solidification technique.First of all, solar battery- grade high purity silicon crystals were solidified in small size crucibles (the inside diameter of 20 mm and the height of 100 mm) at the velocity of 0.075〜9.6 mm/s and a temperature gradient of 20K/cm by using the Bridgman type furnace. The undercoolings of nucleation and grain growth, the grain sizes and the crystal orientations of silicon multicrystal were investigated in related to the solidification conditions. The columnar structures are observed parallel to the heat flow direction in a velocity about 1 mm/min, and the larger size grains are obtained at the lower solidification speed. The crystal size becomes small with the increase of growth velocity and an equiaxed grain structure start forming abo … More ve a velocity around 1.8 mm/min.Twin boundaries, which have the crystal orientation relationship of sigma 3, grow parallel to the heat flow direction. Thus, the undercooling and driving force for crystal growth are estimated by the model based on two-dimensional nucleus growth regime. It was revealed that the undercooling of the <211> direction growth with twin boundary decreases to about 70 % of the case of <111> direction growth and reentrant corner of twin boundary gives an advantage to the faceted growth. Similar phenomena is obtained by the computer simulation based on the molecular dynamics theory. Therefore we conclude the kink site of the reentrant corner on twin boundary encourages the solidification velocity increasing and the development of grain size.The silicon crystal grows to the orientation between <211> to <101> along to the crucible bottom regardless on the form of the crucible shape and configuration at initial solidification stage. Similar phenomena are appeared even in the middle size specimen (the inside diameter of 80mm and the height of 20 mm) and also in the large size solar battery cell. Therefore, the grain size and grain orientation can be controlled by the covering the specimen bottom with the silicon crystal, which solidified toward the <211>〜<101> orientation due to the twin boundary at initial solidification stage, and then the growing upward at the relatively slow solidification velocity until final solidification stage. Less