Investigation of aluminum-boron doping profiles formed by coalloying of screen-printed pastes

We present a detailed study on aluminum-boron doping profiles formed in silicon by alloying from screen-printed aluminum pastes containing boron additives. We show that an increase in the effective peak temperature Tpeak,eff (determined from phase diagram calculations) of the alloying process leads to higher concentrations of both Al and B atoms within the alloyed p+ region, resulting in (i) higher potential barriers for electrons, but also (ii) increased densities of recombination-active Al defects. While the improved potential barrier predominates for Tpeak,eff ≲ 770 °C, the increased defect density prevails for Tpeak,eff ≲ 800 °C, thus defining an optimal effective peak temperature. Furthermore, we show that, by increasing the amount of elemental B added to the paste, the acceptor concentration can be increased without affecting the defect density. Therefore, the optimal printing and firing conditions comprise high B amounts and low, adapted effective peak temperatures. For a B content of 0.9 wt% and Tpeak,eff = 765 °C, we have achieved a saturation current density of 253 fA/cm2, corresponding to an implied open-circuit voltage of 665 mV, which demonstrates the high potential of B additives within Al pastes to further improve the efficiency of Si solar cells with Al-alloyed p+ rear.