Impurity Gettering by Diffusion-doped Polysilicon Passivating Contacts for Silicon Solar Cells

We report direct experimental evidence for the strong impurity gettering effects associated with the formation of diffusion-doped polysilicon passivating contacts. Iron is used as a marker impurity in silicon to quantify the gettering effectiveness. By monitoring the iron redistribution from the silicon wafer bulk to the polysilicon surface layers, via a combination of carrier lifetime, secondary ion mass spectrometry (SIMS), and transmission electron microscopy (TEM) techniques, the respective gettering sites in the phosphorus and boron diffusion-doped polysilicon contacts are identified. In phosphorus-doped polysilicon, iron moves to the heavily doped polysilicon layer; and in the boron-doped structure, iron is gettered to the boron-rich layer. Both gettering processes occur via an impurity segregation mechanism. Lastly, the gettering of iron to the polysilicon surface layers is found to have no impact on the passivation quality of the polysilicon contacts.