N-type silicon RST ribbon solar cells

Abstract

In this work, we report for the first time on the electronic properties and the solar cell fabrication on N-type silicon ribbons grown by the RST method. We have investigated the majority charge carriers' parameters (resistivity and mobility) and the minority carrier lifetime (MCL) of N-type silicon RST ribbons. The MCL values are deduced using the photoconductivity decay method on RST wafers, the surface of which was passivated by an iodine/ethanol solution. The effect of hydrogen plasma treatment on the electronic properties of the as-grown RST ribbons was also studied. We show that the bulk passivation following hydrogen plasma treatment, carried out at temperatures above 350°C for more than 1h, strongly improves the MCL. Conventional solar cells were prepared on RST ribbons grown in optimized conditions and passivated by plasma hydrogenation. Two types of boron emitters were tested, namely those obtained by boron-spin-coating and by ion implantation, while the N+ region serving as a back surface field was made by phosphorus spin coating. The highest conversion efficiency values obtained for boron-implantation (B-II) and boron-spin-coating (B-SOD) emitters on a ∼120μm thick RST were 13.8% and 12.7% respectively, with no surface texturing, or optimized surface passivation. Suns-Voc method applied to these RST cells gave, for B-II and B-SOD emitters conversion efficiencies of about 15.1% and 13.6%, respectively. The former value represents to our knowledge the highest efficiency value obtained for ultrathin N-type multi-crystalline silicon ribbon based cells.