This paper focuses on the latest developments from research on MWT (metal wrap through) solar cells at Fraunhofer ISE. An overview of the current cell results for mc-Si and Cz-Si material with both Al-BSF and passivated rear side is presented. Recent progress in cell technology and the challenges in order to reach efficiencies for pilot-line processed large area MWT solar cells towards 20% are discussed. Up to now MWT cell efficiencies over 18.5% for Cz-Si and over 17% for mc-Si are reached with pilot-line processing. Improvements of the MWT cell design in order to increase cell efficiency further and to allow an easy module assembly are shown. Furthermore first calibrated IV measurements of MWT solar cells are presented.

To achieve higher efficiencies without extra-cost is the major goal of research and development in crystalline silicon solar cell production technology. This goal can be addressed by fine line printing. Therefore, the focus of this work will be the combination of fine line printed front contacts and high emitter sheet resistances. A detailed study of fine line front contacts is presented. Therefore, an electrical analysis based on solar cell results as well as an optical analysis based on microscope pictures are carried out. Furthermore, the contact characteristic of different silver pastes as a function of the emitter sheet resistance is analyzed. As additional approach fine line printed front contacts as seed layer for a two-step metallization are discussed within this work. Both concepts, the one- and two-step metallization, will be compared with each other. Moreover, the use of MWT (metal wrap through) solar cells (1) allows a further efficiency increase (2). Hence, MWT solar cells with fine line screen printed front contacts are developed and characterized in this paper. Efficiencies up to 16.8% are realized with mc-Si MWT solar cells.

On the way to higher efficiencies, back contact solar cells seem to be a promising alternative to conventional screen-printed solar cells. Especially, the MWT (metal wrap through) solar cell concept with only two additional process steps compared with the conventional cell process is appropriate for a fast transfer to industry. Hence, a pilot-line process based on a new contact design was developed and tested. Several batches of about 1000 mc-Si MWT solar cells were successfully produced at ISE's PV-TEC pilot-line. Cell efficiencies up to 16.2% are achieved with an emitter sheet resistance of 65 Ω/sq. Compared with conventionally processed cells made of the same mc Si-block, an efficiency gain up to 0.5% is observed. The main reason for the high efficiency is a high short circuit current due to the absence of the bus bars on the front. Furthermore, a cell interconnection only on the back is possible and thus less serial resistance losses in the tabs. Therefore, an additional efficiency gain of about 0.2%abs. compared with standard module technology is achieved using the new MWT tabbing technology.