Dopant-Related Recombination Centers

Compensation can reduce the recombination activity of impurities in silicon by reducing the net doping. However, if the dopant atoms themselves form part of a recombination – active defect, the presence of additional dopant atoms could result in a greater concentra­tion of defects, although their recombination activity will still be determined by the net doping. This could result in an overall reduction in the carrier lifetime. Perhaps the most important dopant-related defect in silicon solar cells is the well-known boron-oxygen defect [61]. These defects are prevalent in boron-doped p-type silicon containing sig­nificant quantities of oxygen, such as Czochralski-grown silicon, and to a lesser degree in multicrystalline silicon [62]. In noncompensated silicon, the defect concentration has been found to be linearly related to the boron concentration. Surprisingly, however, in compensated p-type silicon, it was found to be determined by the net doping, rather than the boron concentration [63], meaning that the additional boron in compensated silicon does not lead to a greater concentration of boron-oxygen defects.

Other dopant-related recombination centers that have been reported in silicon for solar cells are FeB and CrB pairs. However, in these cases, compensation is not expected to affect the concentration of the metal-dopant pairs, since this is determined solely by the concentrations of the metals, which are orders of magnitude lower than the dopant concentrations. Finally, compensated dopant atoms themselves have been proposed as possible recombination centers [59], although this remains experimentally unproven.