Solidification control of multicrystal silicon ingot for solar battery

研究了多晶硅柱状结构的生长机理，对提高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具有重要意义。首先，利用Bridgman型电炉，以0.075 ~ 9.6 mm/s的速度和20K/cm的温度梯度，在小尺寸坩埚（内径20 mm，高度100 mm）中固化太阳能电池级高纯硅晶体。研究了硅多晶的过冷形核和晶粒生长、晶粒尺寸和晶体取向与凝固条件的关系。在1 mm/min左右的凝固速度下，晶粒平行于热流方向形成柱状组织，在较低的凝固速度下晶粒尺寸较大。随着生长速度的增加，晶粒尺寸逐渐变小，当生长速度在1.8 mm/min左右时，晶粒开始形成等轴结构。孪晶界沿热流方向平行生长，晶体取向关系为σ 3。因此，晶体生长的过冷度和驱动力可以通过基于二维核生长机制的模型来估计。结果表明，<211>方向晶界生长的过冷性比<111>方向晶界生长的过冷性降低约70%，孪晶界的可入角有利于面形晶界生长。基于分子动力学理论的计算机模拟也得到了类似的现象。因此，孪晶界上可入角处的扭结部位促进了凝固速度的提高和晶粒尺寸的发展。无论坩埚初始凝固阶段的形状和形态如何，硅晶体沿坩埚底部向<211> ~ <101>取向方向生长。即使在中等尺寸的样品（内径为80mm，高度为20mm）和大尺寸的太阳能电池电芯中也出现了类似的现象。因此，可以通过在试样底部覆盖硅晶体来控制晶粒尺寸和晶粒取向，硅晶体在初始凝固阶段由于孪晶界向<211> ~ <101>取向凝固，然后以相对缓慢的凝固速度向上生长，直至最终凝固阶段。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