Phosphorus-diffused polysilicon contacts for solar cells

This paper describes the optimization of a technique to make polysilicon/SiO_x contacts for silicon solar cells based on doping PECVD intrinsic polysilicon by means of a thermal POCl₃ diffusion process. Test structures are used to measure the recombination current density J_oc and contact resistivity ρ_c of the metal/n⁺ polysilicon/SiO_x/silicon structures. The phosphorus diffusion temperature and time are optimized for a range of thicknesses of the SiO_x and polysilicon layers. The oxide thickness is found to be critical to obtain a low contact resistivity ρ_c, with an optimum of about 1.2 nm for a thermal oxide and ∼1.4 nm for a chemical oxide. A low J_oc≤5 fA/cm has been obtained for polysilicon thicknesses in the range of 32 nm-60 nm, while ρ increases from 0.016 Ω cm to 0.070 Ω-cm due to the bulk resistivity of polysilicon. These polysilicon/SiO contacts have been applied to the rear of n-type silicon solar cells having a front boron diffusion, achieving V=674.6 mV, FF=80.4% and efficiency=20.8%, which demonstrate the effectiveness of the techniques developed here to produce high performance solar cells.