The corrosive effect of alkaline nickel-phosphorus plating on crystalline silicon

Abstract

In the process of finding alternatives to conventional silver thick-film metallization for silicon solar cells the interest in chemical deposition of nickel layers is rising. Electroless plating of nickel/phosphorus layers is a common surface coating method to protect metallic surfaces from corrosion and is currently examined as an option for solar cell metallization. However, some chemical compositions for this process contain alkaline substances (e.g. sodium hydroxide or ammonium), which are known to chemically react with silicon. This work examines the effect of Ni/P plating on crystalline silicon substrates with varying n-type emitters (20 Ω/sq, 50 Ω/sq, 80 Ω/sq and 120 Ω/sq). Two different chemical bath compositions, both with an alkaline electrolyte, are tested at different temperatures and varying plating durations. By comparing the mass of Cz-Si wafers in different stages of the Ni/P plating process (before plating, after plating with the Ni/P layer intact, after plating with the Ni/P layer removed) one can detect an average silicon etching depth between 40 nm and 80 nm ± 13 nm. ECV measurements suggest an even higher damage to the emitter of >250 nm up to a complete consumption of a shallow emitter. SEM imaging reveals a morphological change of the wafer surface.