We study ion implantation for patterned doping of back-junction back-contacted solar cells with polycrystalline- monocrystalline Si junctions. In particular, we investigate the concept of counterdoping, that is, a process of first implanting a blanket emitter and afterward locally overcompensating the emitter by applying masked ion implantation for the back surface field (BSF) species. On planar test structures with blanket implants, we measure saturation current densities J _0,poly of down to 1.0 ± 1.1 fA/cm ² for wafers passivated with phosphorusimplanted poly-Si layers and 4.4 ± 1.1 fA/cm ² for wafers passivated with boron-implanted poly-Si layers. The corresponding implied pseudofill factors pFF _{im pl.} are 87.3% and 84.6%, respectively. Test structures fabricated with the counterdoping process applied on a full area also exhibit excellent recombination behavior (J _0,poly = 0.9 ± 1.1 fA/cm ² , pF F _{im pl.} = 84.7%). By contrast, the samples with patterned counterdoped regions exhibit a far worse recombination behavior dominated by a recombination mechanism with an ideality factor n > 1. A comparison with the blanket-implanted test structures points to recombination in the space charge region inside the highly defective poly-Si layer. Consequently, we suggest introducing an undoped region between emitter and BSF in order to avoid the formation of p ⁺ /n ⁺ junctions in poly-Si.

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