Structural Defects in Polycrystalline Silicon and Their Effect on Solar Cell Efficiency

Abstract

Although extensive electron microscope studies of defect structures in silicon single crystals have been made, it appears that little or no attention has been given to the defect structures in polycrystalline silicon (Polysil) because it has been used almost exclusively as a source material subjected to further purification and growth into device grade single crystals. There is current interest in Polysil for fabrication of less expensive solar cells; however, improvement in the electrical properties will be necessary in order to achieve reasonable solar cell efficiency. Since most structural defects degrade the electrical properties of both p- and n-type silicon, the characterization of their structures will provide a basis for selection of better material and/or processes for reducing defect densities. The present work is part of a program which includes the evaluation and development of Polysil as a solar cell material. The observations reported here were made on Polysil obtained from Texas Instruments and Monsanto and are believed to be more or less representative of the structures observed in other Polysil material.