17. Metal Impurities and Gettering in Crystalline Silicon

Solar-grade Si usually contains a considerable amount of metal impurities and extended defects. The detrimental effect of transition metals on solar cell performance is well documented. This necessitates a study of transition metal properties in Si and their interaction with point and extended defects. Such studies will form the basis for the development of procedures on suppressing the detrimental effects of transition metals. This chapter briefly describes the solubilities, diffusivities, and electrical properties of metal impurities that are most frequently detected in multicrystalline silicon. The diffusion length values necessary for highly efficient crystalline silicon solar cells, which set a limit on the electrically active metal concentration, are estimated. The effects of dislocations and grain boundaries on the effective diffusion length are evaluated. Finally, the efficiency of gettering and hydrogen passivation procedures used in silicon solar cell technology is considered.