Effect of Boron Codoping and Phosphorus Concentration on Phosphorus Diffusion Gettering

Compared with phosphorus diffusions, conventional boron diffusions for n-type solar cells are not effective at impurity gettering without the presence of a boron-rich layer. In this paper, we investigate the gettering effectiveness of light phosphorus diffusions for removing Fe impurities, applied on an underlying boron diffusion, similar to the buried emitter concept, as an option for achieving effective gettering on boron diffused substrates. Our experimental results on monocrystalline silicon samples demonstrate that the underlying boron diffusion does not affect the gettering effectiveness of the phosphorus diffusion, even though much of the phosphorus diffused region is overdoped by the boron diffusion. Furthermore, we investigate the gettering effectiveness of low surface concentration phosphorus diffusions that can result in reduced recombination in the n+ region. Our results show that the gettering effectiveness decreases when the surface phosphorus concentration is reduced, either through manipulating the deposition gas flows or through subsequent driving in. Driving in the surface phosphorus concentration from 2 × 10²⁰ to 3.5 × 10¹⁹ cm^-3 decreased the gettering effectiveness by about one order of magnitude.