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

Abstract

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 ρC in the range of a few mΩcm2 have only been achieved with these so-called silver–aluminum (Ag–Al) pastes. Within this work, ρC 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 ≈60nm (which corresponds to the depletion zone), with a maximum dopant concentration Nmax≈8·1019cm−3. However, they have different junction depths between 570nm and 980nm. This work shows that the dopant concentrations with values well below Nmax following the depletion zone significantly affect the measured ρC: the deeper the junction, the lower ρC. This behavior is observed for both investigated Ag–Al pastes. For example, the mean ρC for paste Ag–Al1 decreases from ρC≈4mΩcm2, determined on a 570nm deep doping profile, to ρC≈2mΩcm2, determined on a 980nm deep one. By using an analytical model to calculate ρC with dependence on e.g. dopant concentration, metal crystallite coverage fraction, and crystallite penetration depth, the lower ρC for deeper profiles can be explained by crystallites with penetration depths of several 100nm. The calculations also reveal that the impact of the depletion zone is negligible with respect to ρC for crystallites deeper than ≈100nm.