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.

This work focuses on simplifying the fabrication process of our near-industrial n-type silicon solar cells with screen-printed aluminium-alloyed rear emitter. We investigate the structural and electrical properties of Al emitters alloyed on differently prepared Si surfaces. We demonstrate that the formation of a proper emitter neither requires a planar nor a non-diffused rear surface, thus allowing both-sided surface texturing and both-sided phosphorous doping during the n front surface field diffusion without the need of an additional protecting rear masking layer. On textured surfaces a careful choice of the printing and alloying conditions is essential to obtain Al-p emitters without shunts in form of locally non-alloyed regions. By adjusting the alloying conditions or by applying a simple rear surface conditioning step, shunts can be effectively prevented, thereby simultaneously increasing the internal reflectance and reducing the Al-p emitter saturation current density. Al alloying on P-diffused rear surfaces is shown to be uncritical. In summary, we demonstrate that Al alloying for p emitter formation can be easily realised on textured and P-diffused surfaces leading to a much more flexible fabrication of our n-type silicon solar cells.

The influence of the base dopand on the cell performance in a cell type with selective front phosphorus diffusion and an alloyed aluminum rear doping is investigated in this work. First this was done by using two dimensional device simulations to vary the doping species (n- or p-type) and the concentration over a broad range. For n-type base material we found that for a given front side system (diffusion and passivation) the fill factor tends to increase while the short circuit current decreases with increasing base doping concentration. The ideal base doping is therefore a function of the quality of the front side diffusion and passivation. In comparison to the n-type cells the dependence of the p-type cell results on the base doping concentration is much weaker. Based on the simulations a base doping concentration of 8 □cm was chosen to process n-type solar cells, which were then compared with identically processed p-type cells. For our front and rear screen-printed large-area n- as well as for our p-type cells we have achieved efficiencies up to 17.6 %.