Quantitative Understanding and Implementation of Screen Printed p+ Poly-Si/Oxide Passivated Contact to Enhance the Efficiency of p-PERC Cells

Abstract

This paper reports on the modeling, optimization, and implementation of p-TOPCon (tunnel oxide passivating contacts) on the rear side of a PERC to enhance its cell efficiency. Local Al-BSF of a traditional PERC was replaced by p+ poly-Si/oxide passivated contact composed of ~15Å thick chemically grown tunnel oxide, capped with 120-250nm thick p+ poly-Si layer grown by LPCVD. Process optimization resulted in full-area un-metallized recombination current density (J0b, pass) of < 5fA/cm2 for planar surface, nearly independent of poly-Si thickness in this range. Metallized Jo showed an increase with decreased poly-Si thickness and was found to be 9.6 and 25fA/cm2 for 250nm and 120nm poly-Si respectively, with 4.6% direct metal-Si contact fraction, suitable for bifacial cells. A 21.4% baseline PERC cell with local BSF was fabricated and characterized to extract the rear side recombination current density (J0b,) of 65fA/cm2. Detailed analysis and device simulation showed that by replacing this LBSF with 250nm TOPCon developed in the study should produce a Voc enhancement of 9.2mV, consistent with the observed cell Voc increase of 10mV.