Phosphorus replenishment to improve the contact resistivity of light-induced Al contacts on Si solar cells

Abstract

A method to repair laser-induced phosphorus loss from the n-type emitter of Si solar cells during light-induced Al plating is demonstrated. The ability to add a water-reactive phosphorus compound, PCl5, into the Al plating bath gives a unique advantage to Al over Cu for front contacts on Si solar cells. The contact resistivity between light-induced Al and Si emitter is reduced to as low as 0.1 mΩ-cm2. Cyclic voltammetry performed on a plating bath composed of 81 wt% 1-ethyl-3-methylimidazolium tetrachloroaluminate, 19 wt% AlCl3, and 0.2 wt% PCl5 reveals multiple reduction reactions of PCl5, although incomplete decomposition of PCl5 is possible. A decrease in nucleation loop size during cyclic voltammetry is also observed suggesting that PCl5, while being consumed, modifies the growth mechanism during Al plating. Structural analysis confirms that the addition of PCl5 produces an Al film which is levelized and compact. A pulsed voltage profile coupled with chemical analysis verifies that phosphorus incorporation is electrolytically activated and is likely the cause for the low contact resistivity.