John W. Graff

Counterdoping with patterned ion implantation

In this work, we investigate the applicability of counterdoping by ion implantation for the formation of pn-junctions for high efficiency interdigitated back contacted silicon solar cells. Counterdoping offers the possibility of creating the emitter with a blanket implantation and the back surface field with a masked implantation, leading to an elegant process without the need of precise alignment between the two implantation steps. We analyze I-V curves of diodes after implantation and high temperature annealing and compare the results with numerical simulations. Despite the presence of highly doped boron and phosphorous regions in contact to each other, neither trap assisted tunneling nor dominant recombination in the space-charge region is observed in forward direction. This result reflects the excellent removal of implant damage during the co-annealing step. In reverse direction, a sharp breakdown due to band-to-band-tunneling is observed at -8 V. Since it occurs very homogenously across the whole wafer and no local hot spots are observed, no implications for module reliability are implied.