Numerical Analysis Of Thermally-induced Stress Caused By Inclusions Embedded In Polycrystalline Silicon Ingot

Polycrystalline silicon is widely used in photovoltaic solar cell industry, because of its high energy conversion efficiency, and lower cost of production compared with monocrystalline silicon. A numerical analysis was performed to investigate the thermally-induced stress caused by SiC and Si3N4, the main hard inclusions in polycrystalline silicon ingot used in fabricating Photovoltaic solar cells. Firstly The initial temperature field and impurities (SiC, Si3N4) distribution after solidification were obtained through the simulation of directional solidification of polycrystalline silicon ingot by using the crystal growth software CGsim. It shows that, the two inclusions are located in small areas beneath the upper surface of silicon ingot. Then based on it, the thermally-induced stress was studied caused by these two kinds of hard inclusions embedded in silicon matrix by using the finite element analysis software ANSYS. Model shape was designed in terms of true shape of the inclusion particles. Cause SiC and silicon are all the cubic structure, the action of SiC inclusion can be treated as isotropy; For Si3N4 inclusion with hexagonal structure, anisotropy of mechanical properties were considered through the coordinate transformation of the elastic stiffness matrix. It indicated that, in the process of cooling of polycrystalline silicon ingot from 1685K to room temperature, the maximum thermally-induced stress caused by inclusions were:16MPa by SiC particle,13-21MPa by Si3N4 particle,15 MPa by SiC cluster and 18-21MPa by multi-particles, respectively. Based on it, the calculation result showed that the minimum size of instability critical crack in polycrystalline silicon ingot was in the range from 286 to 676μm, less than the size of inclusions, so the crack caused by inclusions was easy to appear in the process of cooling of ingot. Possible sources that the C,N impurities come from was also analyzed in this paper, and the suggestion of manufacturing process improvement was given for reducing the impurity content.