Impact of the Presence of Busbars During the Fast Firing Process on Contact Resistances

Uniform and low contact resistances between Ag grids and Si emitters are very important for reducing series resistance losses in screen-printed silicon solar cells. In this paper, we studied systematically the contact resistance of firing-through Ag pastes on samples that were processed in a fast firing furnace with and without the presence of busbars. We fabricated p-type full-area aluminum back surface field and n-type passivated emitter rear totally diffused solar cells with various doping concentrations and surface morphologies. For all tested conditions, lower contact resistances were obtained when firing without busbars. For instance, by omitting the busbars during firing, on a 138 $\Omega$ /sq phosphorus emitter, the median contact resistance dropped from 7.5 to 0.7 m $\Omega$ $\cdot$ cm $^{2}$ , and on an 85 $\Omega$ /sq boron emitter, the median contact resistance decreased from 80 to 1 m $\Omega$ $\cdot$ cm $^{2}$ . Furthermore, we found that the pure Ag paste outperformed the AgAl paste in contacting boron emitters. An explanation of these results might be the short-circuit effect, which is a fundamental mechanism that strongly influences contact formation. In addition, in spite of the significant impact of the presence of busbars during firing on contact resistances, its impact on the $J_{0,\text{met}}$ was found to be only marginal.