Study of metallurgical refinement of silicon using directional solidification for photovoltaic system

In this study, the effects of thermal gradient and solidification rate on the metallurgical refinement of low-purity Si for use in inexpensive solar cells, using an improved directional solidification process and equipment, were evaluated. The thermal gradient and solidification rate are the main parameters that determine the shape of the microstructure of solidified Si. The high thermal gradient ahead of the liquid-solid interface prevents the formation of secondary dendrites, giving superior segregation of metallic impurities from the Si matrix. Si was vertically solidified using the directional solidification system with the Stöber process. The concentration of impurities was measured using inductively coupled plasma mass spectrometry. The high thermal gradient increased the solidification rate for an identical level of refinement. Application of these methods in Si refining will produce inexpensive pure Si.