Understanding of the annealing temperature impact on ion implanted bifacial n-type solar cells to reach 20.3% efficiency

Abstract : Ion implantation has the advantage of being a unidirectional doping technique. Unlike gaseous diffusion, this characteristic highlights strong possibilities to simplify solar cell process flows. The use of ion implantation doping for n-type PERT bi-facial solar cells is a promising process, but mainly if it goes with a unique co-annealing step to activate both dopants and to grow a SiO 2 passivation layer. To develop this process and our SONIA cells, we studied the impact of the annealing temperature and that of the passivation layers on the electrical quality of the implanted B-emitter and P-BSF. A high annealing temperature (above 1000°C) was necessary to fully activate the boron atoms and to anneal the implantation damages. Low J 0BSF (BSF contribution to the saturation current density) of 180 fA/cm 2 was reached at this high temperature with the best SiO 2 passivation layer. An average efficiency of 19.7% was reached using this simplified process flow ("co-anneal process") on large area (239 cm 2) Cz solar cells. The efficiency was limited by a low FF, probably due to contaminations by metallization pastes. Improved performances were achieved in the case of a "separated anneals" process where the P-BSF is activated at a lower temperature range. An average efficiency of 20.2% was obtained in this case, with a 20.3% certified cell.