Investigation of Aluminum-boron Doping Profiles Formed by Coalloying from Screen-printed Pastes

摘要

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 T_(peak, 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 T_(peak,eff) approx < 770 °C, the increased defect density prevails for T_(peaic,eff) approx > 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 T_(peak,eff)= 765 °C, we have achieved a saturation current density of 253 fA/cm~2, 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.