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

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.