Growth rate and impurity distribution in multicrystalline silicon for solar cells

The solidification rate of multicrystalline silicon made by directional solidification has been determined by in situ measurements of the solid/liquid interface position in a pilot-scale furnace. Two experiments were conducted where silicon was solidified vertically from the bottom with a nearly planar interface and cooled after solidification at two different rates. The average solidification rate was found to be 4 × 10−6 m/s, which fits well compared to values calculated from temperature measurements beneath the crucible. The solidified silicon was examined to determine the carbon and oxygen distribution and the electron lifetime vertically in the ingots. The carbon distribution was quite similar in both ingots with a concentration of about 4 ppma in the middle of the ingots. A higher oxygen concentration was found in the ingot with slow cooling. This was a result of poor coating which increased oxygen diffusion from the crucible. The electron lifetime was found to be about 10 μs in the material with fast cooling, whereas the material with slow cooling had an electron lifetime of 2 μs. More diffusion of iron from the crucible may be the reason for the low lifetime in the material with slow cooling.