Pilot Processing of 18.6% Efficient Rear Surface Passivated Silicon Solar Cells with Screen Printed Front Contacts

Abstract

We apply the recently introduced Silicon Nitride Thermal Oxidation (SiNTO) process for the industrial fabrication of silicon solar cells that feature a thermal oxide-passivated rear surface. The SiNTO process utilises a SiNX anti-reflection layer for masking the front side of the solar cell during the thermal oxidation process. This masking layer limits the growth of the thermal oxide to the uncoated rear surface. Laser fired contact (LFC) technology is applied to form the local rear contacts. An efficiency of 18.6% (annealed) and 18.4 % (stable, independently confirmed) is achieved for a PERC device fabricated from boron-doped Czochralski-silicon by means of the SiNTO process. The average efficiency of a batch of 34 SiNTO cells is 18.2%, measured after fabrication (not stabilised). Parallel processed Al-BSF references reach average efficiencies of 17.7%. Thus, the SiNTO approach enables an efficiency increase of 0.5% absolute compared to conventional Al-BSF technology. When introducing soldering pads, the efficiency gain for SiNTO compared to Al-BSF cells even increases to 0.8% absolute. Finally, we use a comprehensive analytical model to estimate the optimum bulk resistivity for locally contacted devices fabricated from conventional Czochralski silicon material. These calculations account for the bulk recombination caused by the formation of boron-oxygen complexes under carrier injection.