In-situ Doping and Local Overcompensation of High Performance LPCVD Polysilicon Passivated Contacts as Approach to Industrial IBC Cells

Abstract

In this work, we investigate polysilicon passivation contacts, to be used for high performance IBC cells. We demonstrate an LPCVD process for in-situ p-type doped polysilicon. The p-type polysilicon can be locally overcompensated to n-type polysilicon using phosphorous implantation or POCl3 diffusion. Both contacts show excellent surface passivation, with recombination current (Jo) of 12 fA/cm2 for p-poly on a polished Si surface, and less than 10 fA/cm2 for compensated n-poly both on textured and polished surfaces. These polysilicon layers are implemented in the fabrication of n-PERT cells with front and rear polysilicon contacts. The resulting textured half-fabricated cells have an implied Voc of 701 mV after hydrogenation via PECVD deposition of SiNx:H. Different metallization processes, relevant for IBC cells, are applied. A Voc of 685 mV is obtained for the best cell metallized with low-temperature metallization (ITO/low-temperature Ag paste), and 678 mV is obtained for the best cell with industrial screen-printed firing-through contacts. In these n-PERT cells the same thin oxide layer is used for p-type and n-type polysilicon contacts.