Eutectic Sn-Bi Alloy for Interconnection of Silicon Solar Cells

Abstract

The shear stress induced by soldering for the interconnection of silicon solar cells can lead to increased micro cracks and decrease manufacturing yields for thinner cells. Concerns remain in transitioning to copper (Cu) metallisation to reduce silver consumption regarding the reliability of using soldered interconnection to Cu-plated cells. This thesis investigates the electrochemical deposition of eutectic tin (Sn) - bismuth (Bi) alloy and describes three applications of the plated alloy for the interconnection of silicon solar cells. In the first application, it is shown that by controlling the plating current density, eutectic Sn-Bi alloy can be deposited from an electrolyte containing both Sn and Bi ions on Cu interconnection tabs as a substitute for Sn-lead solder. Reflowing the alloy after plating can improve the bonding strength with an average force of 1.26 N/mm being measured in a pull test of two bonded Cu tabs where the plated Sn-Bi was reflowed on a hotplate at 200 C and then bonded at 180 C with quenching in ethylene glycol for fast cooling. The second application involves the deposition of eutectic Sn-Bi alloy on n-type and p-type Cu contacts of laser-doped selective-emitter cells using light-induced and field-induced plating, respectively. The importance of uniformly reflowing the alloy after plating is demonstrated, with cells having plated Sn-Bi alloy and interconnected using Smart Wire Connection Technology (with Sn-silver coated wires) experiencing a power loss of 10% after 140 thermal cycles if the alloy was reflowed before lamination and 51% if the alloy was not reflowed. In the third application, the use of plated Sn-Bi is proposed to provide bonding for a backsheet metallisation scheme in which an inkjet-patterned seed layer was plated with Cu and eutectic Sn-Bi alloy to enable bonding to cells. The tapered height of plated Cu is advantageous in reducing the Cu usage for back contact cells. This thesis demonstrates the future potential of plated Sn-Bi alloy to enable cost-effective interconnection of silicon solar cells in a range of different ways. It also highlights the importance of developing methods to accurately characterise the plated alloy composition and uniformly reflowing the alloy before bonding.