Solar‐grade boron emitters by BF₃ plasma doping and role of the co‐implanted fluorine

Abstract

We investigate the electrical properties and dopant profiles of boron emitters performed by plasma immersion ion implantation from boron trifluoride (BF 3 ) gas precursor, thermally annealed and passivated by silicon oxide/silicon nitride stacks. High thermal budgets are required for doses compatible with screen-printed metal pastes, to reach very good activation rates. However, if good sheet resistances and saturation current densities may be obtained, we met strong limitations of the implied open-circuit voltage of the n-type Czochralski silicon substrates, which is incompatible with high-efficiency solar cells. Such limitations are not encountered with beamline where pure B + ions are implanted. Efforts on the passivation quality may improve the implied open-circuit voltage but are not sufficient. We provide experimental comparison between beamline and plasma immersion allowing us to discriminate the causes explaining this observation (implantation technique or ion specie used) and to infer our interpretation: The co-implantation of fluorine seems to indirectly impact the lifetime of the core substrate after thermal annealing.

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

“…Although one might conclude that there is a correlation between the inactive boron dose and the increase in J 01 , we so far cannot exclude other recombination mechanisms, e.g. a different defect structure for the increasing implant doses compared to the lowest, which might also involve the bulk silicon [20]. A hint for this can be seen in the intercept lifetime in Table 3.…”

Low-cost plasma immersion ion implantation doping for Interdigitated back passivated contact (IBPC) solar cells

“…These low J o values show the distinct advantage of implanting into the pc-Si/SiO 2 /c-Si contact structure rather than into the wafer itself, where J o values one order of magnitude higher are typically achieved. [4,21] For the B implanted samples the J o values are between 15 -35 fA/cm 2 . The B 2 H 6 implanted species show a decreasing J o value for increasing dose, while the other samples do not show much of a trend with implant dose.…”

“…[3] However, in some IBC cell designs, an even simpler plasma immersion ion implanter (PIII) can be utilized for significant capEx and production cost savings. [4] Recently an interdigitated back contact cell was introduced using c-Si/SiO 2 /poly-Si passivated contacts formed by ion implantation [5][6][7][8][9] with a record 23.9% efficiency achieved by Rienäcker et al [10] In this cell configuration, the ions are implanted into a-Si or polycrystalline-Si (pc-Si) layers separated from the c-Si wafer by a thin oxide layer, instead of directly into the c-Si wafer. This eliminates the end-of-range damage such as dislocation loops [11] to the wafer, which in turn, lowers the post implantation thermal budget needed to activate and diffuse dopants in the pc-Si layer, rather than anneal out induced defects in the c-Si.…”

New processes for homojunction silicon solar cells doping: From beam line to plasma immersion ion implantation

“…The reason for such degradation is not clearly established and can be due to deep defects after annealing and/or activation of recombination centers linked to the implantation ofF atoms [18].…”