Fabrication and analysis of high-efficiency String Ribbon Si solar cells

This paper reports on the solar cell efficiencies of 18.3% and 16.8% (both 4 cm2) achieved on String Ribbon Si material using photolithography-defined and screen-printed front grid contacts, respectively, reinforcing the potential of String Ribbon Si for high-performance industrial cells. Rapid co-firing of screen-printed Al on the rear and low-frequency plasma-enhanced chemical vapor deposited single-layer SiNx antireflection coating on the front in a rapid thermal processing unit was combined to enhance the as-grown minority carrier lifetime of 2–3 μs to 91 μs in processed String Ribbon Si cells. The performance of the best screen- printed String Ribbon Si cell (16.8%) was comparable to the 17.0% efficient planar float-zone (FZ) Si cell, which was processed simultaneously in the same run. However, the average performance of String Ribbon Si cells was 0.9%–1.4% lower in absolute efficiency relative to the FZ Si cells. Detailed characterization and analysis revealed that this is due to the presence of distributed electrically active defects, which could not be fully passivated, in spite of very effective defect hydrogenation during the cell processing.