Local rear contacts to silicon solar cells by in-line high-rate evaporation of aluminum

We contact p-type wafers and boron-diffused layers by laser ablation of a passivating aluminum oxide and silicon nitride stack and subsequent in-line high-rate evaporation of aluminum. We measure saturation current densities at the base contacts of 2.5×106–1.9×107 fA/cm2 for base resistivities of 0.5–3.8 Ω cm and 491–905 fA/cm2 for the contacts to boron-diffused layers of sheet resistances of 23–86 Ω/sq. The contact resistivity of Al layers to p-type silicon with surface doping densities of 4×1015–3×1019 cm−3 is in the range of 4–0.1 mΩ cm2, respectively. The measured contact properties allow for the fabrication of highly efficient ‘passivated emitter and rear cells’ (PERC) and ‘passivated emitter and rear totally diffused cells’ (PERT). Numerical simulations show that evaporated rear contacts in combination with screen printed contacts at the front allow for energy conversion efficiencies of 20.6% and of 21.1%, for PERC and PERT cells, respectively. The simulated free energy losses show that such cells are not limited by the in-line evaporated point contacts on the rear side.