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 J(oc) and contact resistivity rho(c) 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 rho(c), with an optimum of about 1.2 nm for a thermal oxide and similar to 1.4 nm for a chemical oxide. A low J(oc) <= 5 fA/cm(2) has been obtained for polysilicon thicknesses in the range of 32 nm-60 nm, while pc increases from 0.016 Omega cm(2) to 0.070 Omega-cm(2) 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 V-oc = 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. (C) 2015 Elsevier B.V. All rights reserved.