Electroplated and Light-Induced Plated Sn-Bi Alloys for Silicon Photovoltaic Applications

Abstract

Electroplated and light-induced plated Sn-Bi alloys for interconnection of silicon solar cells are reported. The eutectic 42Sn-58Bi alloys, formed on the interconnection wire by electroplating and on the front metal grid of solar cells by bias-assisted light-induced plating, can melt during a typical lamination process and therefore potentially enable electrical connection between adjacent cells in a solar module without the need to use soldering. Plated alloy composition was analyzed by inductively-coupled plasma optical emission spectrometry and energy dispersive spectrometry, and the morphology of the plated alloy was examined using scanning electron microscopy. Although a eutectic alloy could be electroplated readily using a previously-reported bath composition, lightinduced plating of the alloy required a bath with reduced bismuth ion concentration to ensure a eutectic alloy of uniform morphology across the cell surface could be formed at a plating current density at which the solar cell remained forward biased.Alloys of lead and tin have been widely-used in the past for electrical and electronic assembly, especially for the connection of components on the printed circuit boards, due to their advantages of low-cost and low melting point. 1 The prohibition of the use of lead (Pb) in commercial electronics in the European Union in 2006 by the Restriction of Hazardous Substances Directive (RoHS) has resulted in increased interest in the use of lead-free solders, which contain various compositions of tin, zinc, copper, silver, bismuth, indium or other metals. A variety of alloy combinations (Sn-Bi, Sn-In, Sn-Zn, Sn-Ag, Sn-AgBi, Sn-Bi-Cu, etc) have been investigated, 2-4 however most of them have a slightly higher melting point than the eutectic Sn-Pb alloy solder. The increased melting temperatures of Pb-free solder represents a significant challenge for the silicon photovoltaic industry because the use of higher soldering temperatures can compromise adhesion between the metal grids of the solar cells and the interconnection wire. 5 Furthermore, as silicon wafers reduce in thickness, 6 increased localized heating from soldered interconnection points can increase the formation of micro cracks in cells which reduce cell performance and compromise module reliability. 7 For silicon photovoltaic applications, Sn-Bi and Sn-In alloys are attractive alternatives to Pb-based solders because their eutectic temperatures are lower than 150 • C, which is the typical temperature applied in the module lamination process used by most silicon module manufacturers. The eutectic alloy of 42Sn-58Bi alloy has a melting temperature of 138 • C, 8 therefore, if formed on either or both of the metal grid on the cell or/and the wire that interconnects adjacent cells in a module, it can melt during the lamination process and form electrical connection between adjacent cells in a module. This new interconnection method can be advantageous in that: (i) a separate soldering step with its associated heat stresses is eliminated; and (ii) micro...

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“…The silver content in the PERC solar cell is expected to decrease as a result of new developments in metallisation pastes, screen printing processes such as dual printing as well as alternative metallisation designs such as thinner fingers and busbarless configuration [57]. Another major change that can be expected in PV modules is the use of lead-free soldering in 2050, via the substitution of lead with bismuth [64]. Besides that, PV module mounting frames currently consist mainly of aluminium.…”