Challenges for lowly-doped phosphorus emitters in silicon solar cells with screen-printed silver contacts

This work points out that the application of phosphorus emitters with low surface concentration N-surf of a few 10(19) cm(-3) in combination with state-of-the-art screen-printed and fired silver contacts is no more limited by the specific contact resistance rho(c) but by the dark saturation current densities underneath the metal contacts j(0,met). Eight emitter doping profiles have been designed with different N-surf ranging between 3.3.10(19) cm(-3) and 1.2.10(20) cm(-3) which feature very similar junction depths of about 350 nm. The measured rho(c) values for these emitters are determined to be between 2 m Omega cm(2) and 5 m Omega cm(2) for about 50 mu m-wide contact fingers. Their emitter dark saturation current density (alkaline textured and passivated surface) range from 40 fA/cm(2) to 90 fA/cm(2). From these parameters and the current-voltage data of large-area Czochralski-grown silicon solar cells featuring an aluminum back surface field, we estimate j(0,met) to be between 1200 fA/cm(2) and 3900 fA/cm(2). Our results show that for decreasing N-surf, the recombination underneath the metal contacts increases significantly which mainly limits the cell's open-circuit voltage. We thus conclude that the major challenge for emitters with low N-surf consists in achieving reasonable low j(0,met) values in order to benefit on the cell level from the lower recombination activity in the passivated, non-metallized surface area. (C) 2017 The Authors. Published by Elsevier Ltd.