Recombination characteristics of p+ regions alloyed from screen-printed aluminum pastes containing boron additives

We present a detailed analysis of the recombination characteristics of p+ regions alloyed into silicon from screen-printed aluminum pastes containing boron additives (Al-B pastes). By numerically simulating the saturation current densities corresponding to (i) recombination at the surfaces of the Al-B-codoped p+ Si (Al-B-p+) regions as well as (ii) Auger and (iii) defect recombination within their bulks, we show that the recombination characteristics are versatilely affected by the B additive within the paste: An increase in the effective B percentage, which is the percentage of B additive actually utilized, leads to a significant reduction of surface recombination and of defect recombination, but also to the simultaneous intensification of Auger recombination. The optimal effective B percentage, which has been calculated to 0.03 wt%, constitutes a compromise between these recombination mechanisms. By measuring the saturation current densities j0,p+ of Al-B-p+ regions, the optimal effective B percentage of 0.03 wt% has been verified experimentally. Excellently low j0,p+ values of less than 260 fA/cm2 have been achieved, corresponding to implied open-circuit voltages of more than 664 mV. Our simulations show that Al-B pastes of optimal composition provide a high robustness against variations of the alloying conditions, since their Al-B-p+ saturation current densities are largely independent of the printed paste amount and the firing conditions. This allows for a more flexible fabrication of the front and rear contacts of Si solar cells during a single co-firing process.