ForTeS – Passivated Contacts for High-Efficiency Silicon Solar Cells

Due to ongoing improvements in material quality and surface passivation, recombination at the metal contacts has become one of the dominant loss mechanisms in silicon solar cells. This loss can be reduced by usingso-called passivated contacts, which suppress the recombination of minority charge carriers and simultaneously allow lossless majority charge carrier transport. On the basis of an extremely thin dielectric tunnel oxide, we have succeeded in producing a very effective, passivated contact (TOPCon), which simultaneously passivates the surface excellently and constitutes low resistance to charge carrier transport.

To increase the efficiency of silicon solar cells, the PERC concept (Passivated Emitter and Rear Cell) is being transferredto industrial production. Higher voltages can be achieved due to the dielectric surface passivation and the reduction of the metallized area down to contact points. However, the gain in voltage is accompanied by higher series resistance, as the charge carriers must travel longer distances within the silicon. Optimising this type of structure demands a trade-off between high voltage (long distance between contact points) and high fill factor (short distance between contact points).

One possibility to avoid this trade-off is to use carrierselective contacts which cover the whole surface, also called “passivated contacts”. The challenge associated with these contacts is that they must both suppress the recombination of minority charge carriers and simultaneously allow lossless majority charge carrier transport. At Fraunhofer ISE, this type of contact was developed on the basis of an extremely thin tunnel oxide and a thin silicon layer (Tunnel Oxide Passivated Contact, TOPCon), and implemented in high-efficiency solar cell structures.

The long effective lifetimes measured for the TOPCon structure confirm the very good passivation properties of the layer stack used. As the silicon oxide which is used as the tunnel oxide also forms a barrier to the majority charge carriers in principle, it must be kept so thin that the charge carriers can overcome it by quantum mechanical tunnelling processes. In order to investigate this charge carrier transport, highly efficient n-type silicon solar cells were produced with a diffused front-surface boron emitter and the TOPCon contact covering the entire back surface. Very high fill factors of more than 82% and voltages exceeding 700 mV confirm that the charge carrier transport through the tunnel oxide is almost completely free of losses. This demonstrates that a selective and excellently passivated contact was implemented with the TOPCon structure. Altogether, the TOPCon structure has been determined to be a simple und non-structured back contact, with which we were able to achieve efficiency values of up to 24% for high-efficiency n-type silicon solar cells.