Low-Temperature Boron Gettering for Improving the Carrier Lifetime in Fe-Contaminated Bifacial Silicon Solar Cells with n+pp+ Back-Surface-Field Structure

Gettering kinetics of Fe contaminant by doped boron during low-temperature annealing is discussed to improve the minority carrier lifetime in bifacial silicon solar cells with an n+pp+ back surface field (BSF) structure composed of a boron-doped p-base and a boron diffused p+ layer. A model for Fe-gettering by boron is introduced and computer simulations are carried out for the change in minority carrier lifetime along the thermal process in cell fabrication. Lifetime behavior shows good consistency with experimental results when “Fe-behavior parameters” and proper boundary conditions of the initial Fe concentration being higher than the solubility limit at the gettering temperature are taken into account. As a consequence, low-temperature boron gettering employed after boron diffusion for BSF fabrication is found to markedly improve the carrier lifetime cooperating with the phosphorous gettering associated with the pn junction formation, and can recover the initial high lifetimes before cell fabrication. Additionally, a certain condition of short-time heat treatment at higher temperature is found for firing which does not deteriorate the recovered lifetimes.