Impact of Ag Pads on the Series Resistance of PERC Solar Cells

Screen-printed passivated emitter and rear cells (PERC) require Ag pads on the rear side to enable solderable connections for module integration. These Ag pads are separated from the silicon by a dielectric layer to avoid recombination of minority charge carriers. The drawback of this configuration is an elongated transport path for the majority charge carriers generated above the pads. This results in an increase in series resistance. The strength of this effect depends on charge carrier generation above the Ag pads that critically depends on shading of the cell's front side. Ag pads are usually wider than the busbars or the interconnector ribbons and thus are only partially shaded. We build PERC test structures with various rear side configurations of Ag and Al screen printing as well as with and without laser contact openings (LCO). Using experiments and finite element simulations we investigate the impact of shading the Ag pads by the busbars and other means. While fully shaded regions do not increase the lumped solar cell's series resistance, unshaded Ag pads lead to an increase of about 37%.