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 bifacial 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.

Bifacial, fully screen-printed n-PERT solar cells with BF2 and B implanted emitters

“…For both solar cell groups E1 and E4, J 0,surface decreases to 5172 fA/cm 2 on the separately-annealed samples. As also reported by Lanterne et al [6], we additionally observe an increase in intercept lifetime of the solar cell precursors upon separate annealing. Both effects and the slightly smaller metallization fraction reduce the J 01 of the solar cells and thus results in a higher V oc .…”

“…Annealing the P implant separately allows for a tailored process and more freedom in designing the phosphorous doping profile. Furthermore, Lanterne et al [6] reported a higher (pseudo) fill factor for separatelyannealed solar cells compared to co-annealed solar cells. In the framework of the two-diodes model, this corresponds to a much lower pre-factor J 02 of the recombination current with an ideality factor of two in the case of separate anneals.…”

“…In the framework of the two-diodes model, this corresponds to a much lower pre-factor J 02 of the recombination current with an ideality factor of two in the case of separate anneals. According to Lanterne et al [6], the resulting J 02 of less than 3 nA/cm 2 are nearly one order of magnitude lower for the separately annealed samples than for the co-annealed solar cells.…”

“…for interdigitated back contact solar cells [1][2][3] or on full-area, e.g. for passivated emitter, rear totally doped (PERT) cells [4][5][6][7][8]. Ion implantation requires a high temperature annealing step to cure implant-induced crystal defects.…”

Screen‐Printed Front Junction n‐Type Silicon Solar Cells

“…Hence, a relatively lower annealing temperature (around 850°C) can recover implantation-induced defects on the silicon wafer surface. A proper combination of implant energy, phosphorus ion dose and post-implantation annealing conditions is needed to obtain high-efficiency screen-printed n-type silicon solar cells [27,28].…”