20.5% Efficiency on Large Area N-type PERT Cells by Ion Implantation

We developed a high efficiency N-type PERT (Passivated Rear Totally Diffused) bifacial structure based on B and P ion implantation doping, SiO2 passivation and conventional screen-printing metallization. Two process flows were compared: a “co-anneal” process and a process using separated anneals for B and P activation. We highlight the impact of the variations of the B- emitter and P- BSF profiles on the solar cells performance. The impact of the boron implantation dose was studied allowing to optimize this parameter. Concerning the BSF, two temperature ranges were studied for the P activation leading to very different BSF profiles. A shallower profile enables to reach high implied Voc while keeping low contact resistivity. The overall optimization was integrated into a simplified and industrial process flow on large area Cz-Si solar cells (239cm2). An average efficiency of 19.7% was reached using the “co-annealing” process. The efficiency in this case was limited by a low PFF. This limitation was solved using the “separated anneal” process where an average efficiency of 20.2% was obtained on a 15 cells batch with a 20.5% champion cell.