Insight into Metal Induced Recombination Losses and Contact Resistance in Industrial Silicon Solar Cells

Despite of recent improvements in silver paste technology, the open-circuit voltages $( V_{oc})$ of silicon solar cells with screen printed and firing-through silver contacts continue to be limited by the high recombination currents at the metal contacts $( J_{{0} {,met}})$. To maximize solar cell efficiencies, the contact resistance $( \rho_{c})$ and $J_{{0} {,met}}$ must be simultaneously minimised. In this study we investigated the origin of $J_{{0} {,met}}$ for screen printed silver pastes on $p^{+}$ and $n^{+}$ doped regions and correlate this with contact formation phases during the firing process. We show that, during the contact firing process, the $J_{{0} {,met}}$ significantly increases and even saturates to its final value at temperatures well below 700°C, which is a temperature range below that is needed for contact formation. The same is observed on both $p^{+}$ and $n^{+}$ diffused junctions with planar or random pyramids textured Si surfaces passivated by a SiO2/SiNx layer stack. This show that most of the metal induced recombination loses originates from the etching of the dielectric layers by the glass frit and less during the contact formation process where $\rho_{c}$ is minimised. Furthermore, we demonstrate that increasing the SiNx passivating layer thickness leads to a significant reduction in $J_{{0} {,met}}$, possibly due to an incomplete etching of dielectric layers under the contact, whereas the $\rho_{c}$ remain low and constant under optimum firing conditions. These findings could help design metallization pastes optimised to reduce dielectric etching, and hence $J_{{0} {,met}}$, without affecting $\rho_{c}$.