Lattice defects within grain volumes that affect the electrical quality of cast polycrystalline silicon solar-cell materials

Lattice defect structures within large grains of cast polycrystalline silicon have been investigated using optical microscopy, x-ray topography, and electron-beam-induced conductivity (EBIC) measurements. Optical observations through a series of wafers of an ingot indicate that kinks in high-angle grain boundaries develop by forming straight grain boundaries rather than curved boundaries. It is also observed that a junction of three grain forms a kink in the grain boundary. X-ray topographic observations of lattice defects show that cumulative dislocations are generated at these kinks when the material is subjected to thermal stresses at sufficiently elevated temperatures during solidification. Subsequently the dislocations propagate into the grain volume and form subgrain boundaries. In some cases, they pile up in the coherent {111} twin boundary interfaces. Observed EBIC constrast has shown that both dislocations and subgrain boundaries are electrically active and serve to decrease the effective minority carrier diffusion length.