Effect of twins on crystal growth of polycrystalline silicon for solar cells

The microstructure of polycrystalline silicon has been investigated through a unidirectional solidification technique, and the mechanism of the twin growth on a reentrant corner has been evaluated. A columnar structure is observed in the rod-like silicone specimens, which are solidified at a velocity from 1.25 to 40 μm/s in an electric resistance furnace with a positive temperature gradient of 20 K/cm. At slow velocity from 1.25 to 2.5 μm/s, the grain sizes increase as solidification proceeds. However, mean grain sizes decrease with increasing solidification velocity, and at the central part of specimens the equiaxed grains appear above a critical velocity of around 40 μm/s. Most of the columnar grains have twin boundaries in them, and grains with several twins grow larger than twin-free grains. To increase grain size, it is necessary to keep the solidification velocity low and to introduce the twins. The supercooling of directional growth is less than 4 K at a velocity from 2.5 to 30 μm/s. A model of two-dimensional nucleation on the reentrant corner has been established and their critical nucleation radii are calculated from 70% to 80% of the radii of those of general two-dimensional nucleation. The decrease of the critical radius on the reentrant corner would reduce the supercooling of grain growth. Therefore, the grains with twins could preferentially grow and enlarge in a competitive growth.