Fine line printed and plated contacts on high ohmic emitters enabling 20% cell efficiency

The contact performance of seed-layer printed, fired and plated (SFP) contacts were studied on solar cells with different emitter sheet resistivities. For the seed layer of the SFP-contacts a special metal ink called SISC (seed layer ink for the metallization of solar cells), developed and fabricated at Fraunhofer ISE to contact lowly doped emitters was used. The doping profile and the surface concentration of active phosphorus are determined by secondary ion mass spectroscopy (SIMS) and by electrochemical capacitance voltage (ECV) measurements. Large-area Cz-silicon solar cells 12.5 × 12.5 cm² were fabricated with sheet resistivities between 40 ¿/sq. and 130 ¿/sq. The cells are metalized with SFP-contacts and conventional screen print contacts for comparison. High fill factors above 79% on solar cells with lowly doped emitters (sheet resistance of 130 ¿/sq.) could be achieved with SFP-contacts. In contrast, for screen-printed contacts using standard Ag paste the FFs for such lowly doped emitters are reduced to values around 55%. The difference in contact formation is explained by contact resistance measurements together with images from the metal semiconductor junction. Using the SISC ink solar cells with 45 ¿m wide contact structures were processed on a high efficiency cell structure with a passivated rear surface and a 110 ¿/sq. emitter a FF of 81% was achieved resulting in a cell efficiency of ¿=20.6%.