Dislocations in Multicrystalline Silicon for Solar Cells: Microstructure and Sources

Abstract

The study of defects especially dislocations and grain boundaries in semiconductors has received a great deal of attention for over 60 years, primarily due to the importance of the defects in microelectronics. The defect sources and their properties are therefore fairly well understood and can be avoided. However, growth of silicon crystals free from dislocations for example, is expensive following the requirements, i.e. the use of very pure silicon materials and the use of high energy consumption crystal growth techniques. Defect level requirements are less stringent on materials for solar cell applications, and most importantly, lowering costs on materials and processes is essential for sustainability and for the contribution of the energy generated from solar cells to be substantial. New materials and new processes have been developed or are being developed with defects and defect properties that are not yet well understood. Continued efforts for understanding these defects are therefore necessary. This thesis is based on microstructure investigations of the defects, particularly grain boundaries, dislocations and stacking faults in standard multicrystalline silicon for solar cells and is primarily aimed at understanding the formation and multiplication mechanism of dislocations.

Investigation of the microstructure of dislocations in the clusters show that the dislocations are in polygonised structures forming walls normal to the glide plane and assuming line directions in near-most possible normal to the surface. These results are in accordance to the recovery process at high temperature where the dislocations acquire lower energy configurations, for minimization of the elastic strain energy of the crystal. Dislocation generation and multiplication processes in directionally solidified industrial multicrystalline silicon for solar cells are therefore most likely occurring at temperatures close to the melting point. Dislocation density reduction by high temperature annealing of as-grown industrial multicrystalline silicon may therefore be difficult and expensive.

In addition to the general high-angle grain boundaries which are generally known to be sources of dislocations and to form barriers to dislocation glide, sub grain boundaries with misorientation <5° as well as stacking faults appear to generate dislocations and to form barriers to dislocation glide as well. These sources appear to be operational during the ingot cooling stage as well, which makes annihilation and re-arrangement difficult following existence of stable configurations of grain boundaries, stacking faults and dislocations generated at higher temperature