Phosphorus-diffused polysilicon contacts for solar cells

This paper describes the optimization of a technique to make polysilicon/SiOx 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 Joc and contact resistivity ρc of the metal/n+ polysilicon/SiOx/silicon structures. The phosphorus diffusion temperature and time are optimized for a range of thicknesses of the SiOx and polysilicon layers. The oxide thickness is found to be critical to obtain a low contact resistivity ρc, with an optimum of about 1.2 nm for a thermal oxide and ∼1.4 nm for a chemical oxide. A low Joc≤5 fA/cm2 has been obtained for polysilicon thicknesses in the range of 32 nm–60 nm, while ρc increases from 0.016 Ω cm2 to 0.070 Ω-cm2 due to the bulk resistivity of polysilicon. These polysilicon/SiOx contacts have been applied to the rear of n-type silicon solar cells having a front boron diffusion, achieving Voc=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.