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 Nsurf of a few 1019 cm-3 in combination with state-of-the-art screen-printed and fired silver contacts is no more limited by the specific contact resistance rC but by the dark saturation current densities underneath the metal contacts j0,met. Eight emitter doping profiles have been designed with different Nsurf ranging between 3.3·1019 cm-3 and 1.2·1020 cm-3 which feature very similar junction depths of about 350 nm. The measured rC values for these emitters are determined to be between 2 mOcm2 and 5 mOcm2 for about 50 µm-wide contact fingers. Their emitter dark saturation current density (alkaline textured and passivated surface) range from 40 fA/cm² to 90 fA/cm². From these parameters and the current-voltage data of large-area Czochralski-grown silicon solar cells featuring an aluminum back surface field, we estimate j0,met to be between 1200 fA/cm² and 3900 fA/cm². Our results show that for decreasing Nsurf, 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 Nsurf consists in achieving reasonable low j0,met values in order to benefit on the cell level from the lower recombination activity in the passivated, non-metallized surface area.