Hans-Joachim Krokoszinski

Pilot-line processing of screen-printed Cz-Si MWT solar cells exceeding 17% efficiency

On the way to higher efficiencies, back contact solar cells are a promising alternative to conventional screen-printed solar cells. Especially, the MWT (metal wrap through) solar cell concept with only a few additional process steps compared to the conventional cell process is appropriate for a fast transfer to industry. The focus of this work is to change the conventional cell process as little as possible and to reduce the time-to-market. Hence, a MWT process with only three screen-printing steps and without efficiency losses is realized by combining the via and the solder pad metallization steps. In comparison with the conventional cell process the number of screen-printing steps is not increased and thus the costs for metallization rise only marginally due to somewhat increased paste consumption. In comparison to a previous approach the additional costs of the MWT process can be significantly reduced. A successful MWT process transfer from Fraunhofer ISEs Photovoltaic Technology Evaluation Center (PV-TEC) pilot-line to the pilot-line of ersol Solar Energy AG is shown. Mean cell efficiencies above 17% and maximum efficiencies of 17.3% are achieved for Cz-Si MWT solar cells within pilot-line production. Moreover, an efficiency gain compared to simultaneously processed, conventionally screen-printed solar cells of 0.3% absolute has already been reached. Furthermore, the influence of p-contact solder pads on the cell efficiency is analyzed in detail by current voltage (IV-) and photoluminescence (PL-) measurements within this work.

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.

Aluminum-Based Rear-Side PVD Metallization for nPERT Silicon Solar Cells

This paper presents the results of a detailed study on Al-based physical vapor deposition metallization for the rear side of nPERT silicon solar cells. Pure Al is compared with a barrier metallization (Al-Si/Al or Ti/Al) in terms of spiking, contact formation and back-side reflection. A degradation of cell performance with pure Al rear-side metallization due to Al spiking after thermal annealing is observed. This can be avoided either by using a spiking barrier or by using a sufficiently deep doping profile. In addition, all metallization schemes have a sufficiently low specific contact resistance <;0.2 mΩ·cm2 on n+-Si with a sheet resistance of ~75 Ω/sq. Furthermore, the widely used front-side contact metal Ti leads to a significant short-circuit current density loss of more than 0.3 mA/cm2 when applied to the rear side of a silicon solar cell due to its low reflectivity of infrared wavelengths.