Electrical properties and recombination activity of copper, nickel and cobalt in silicon

Among the transition metals in silicon, copper, nickel, and cobalt have the highest diffusivity and solubility in silicon. They can be easily introduced into the bulk of silicon wafers during any heat treatment. The electrical properties of Cu, Ni and Co are poorly understood in spite of intensive research. The existing data on energy levels related to these metals, and on their recombination activity are controversial. This article combines an extensive literature review of new experimental data on properties of Cu, Ni and Co and their precipitates in silicon with a discussion of experimental data recently obtained by the authors. Special attention is paid to the recombination activity of these metals. It is shown that the recombination activity of Cu, Ni, Co and their complexes is low in p-type Si, compared to that of other transition metals such as iron. However, it increases markedly upon formation of precipitates or decoration of existing lattice defects. This is explained by the formation of a precipitate-related defect band and of an attractive potential for minority charge carriers by the charged precipitates. The role of Cu, Ni and Co in the degradation of multicrystalline solar cells efficiency is discussed in a separate section. It is suggested that recently reported intragranular microdefects, which decrease the lifetime of solar cells, may be microprecipitates of these metals.