Phosphorus-diffused polysilicon contacts for solar cells

Abstract

This paper describes the optimization of a technique to make polysilicon/SiO <inf>x</inf> contacts for silicon solar cells based on doping PECVD intrinsic polysilicon by means of a thermal POCl<inf>3</inf> diffusion process. Test structures are used to measure the recombination current density J<inf>oc</inf> and contact resistivity ρ <inf> c </inf> of the metal/n+ polysilicon/SiO <inf>x</inf> /silicon structures. The phosphorus diffusion temperature and time are optimized for a range of thicknesses of the SiO <inf>x</inf> and polysilicon layers. The oxide thickness is found to be critical to obtain a low contact resistivity ρ <inf> c </inf>, with an optimum of about 1.2nm for a thermal oxide and ~1.4nm for a chemical oxide. A low J <inf> oc </inf>≤5fA/cm2 has been obtained for polysilicon thicknesses in the range of 32nm-60nm, while ρ <inf> c </inf> increases from 0.016Ωcm2 to 0.070Ω-cm2 due to the bulk resistivity of polysilicon. These polysilicon/SiO <inf>x</inf> contacts have been applied to the rear of n-type silicon solar cells having a front boron diffusion, achieving V<inf>oc</inf> =674.6mV, FF=80.4% and efficiency=20.8%, which demonstrate the effectiveness of the techniques developed here to produce high performance solar cells.