Impact of boron doping profiles on the specific contact resistance of screen printed Ag-Al contacts on silicon

For the contacting of boron-doped emitters with screen-printed metallization in n-type silicon solar cells, usually a small amount of aluminum is added to the silver pastes. To date, low specific contact resistances rC in the range of a few mO cm2 have only been achieved with these so-called silver-aluminum (Ag-Al) pastes. Within this work, rC is experimentally determined for two Ag-Al pastes for three different boron-doped emitters on alkaline textured and passivated silicon surfaces. The investigated boron doping profiles feature almost identical rising curve progressions up to a depth of ≈60 nm (which corresponds to the depletion zone), with a maximum dopant concentration Nmax≈8·1019 cm−3. However, they have different junction depths between 570 nm and 980 nm. This work shows that the dopant concentrations with values well below Nmax following the depletion zone significantly affect the measured rC: the deeper the junction, the lower rC. This behavior is observed for both investigated Ag-Al pastes. For example, the mean rC for paste Ag-Al1 decreases from rC≈4 mO cm2, determined on a 570 nm deep doping profile, to rC≈2 mO cm2, determined on a 980 nm deep one. By using an analytical model to calculate rC with dependence on e.g. dopant concentration, metal crystallite coverage fraction, and crystallite penetration depth, the lower rC for deeper profiles can be explained by crystallites with penetration depths of several 100 nm. The calculations also reveal that the impact of the depletion zone is negligible with respect to rC for crystallites deeper than ≈100 nm.