Micro characterization and imaging of spikes in nickel plated solar cells

Annealing induced silicidation of plated nickel contacts can severly lower the solar cell performance due to deep nickel silicide spikes penetrating the space charge region. This work summarizes several attempts to characterize performance limiting deep silicide structures and determines the influence of different passivation layer structuring technologies on the silicide growth. Reverse biased electroluminescence measurements revealed that the deep nickel silicides occur after an anneal and are located along the structured and plated passivation layer openings. Cross-section studies of the plated contacts demonstrated that deep silicide growth is present independently of the applied passivation layer structuring technology. While for laser-ablation the critical silicide structures could be mostly attributed to accelerated silicide growth at laser induced defects, there are also deep silicide structures that appear without any obvious correlation even in the case of defect free wet chemical passivation structuring of the contact openings. By varying the emitter doping profile and the annealing temperature after plating the distance of the space charge region and the surface was found to influence the annealing-induced solar cell performance decrease. Considering the presented results there are three ways to overcome annealing induced degradation of Ni plated solar cells. Either optimized process designs with sufficient contact adhesion and contact resistivity without the need of thermal silicide formation, improved silicide depth control (e.g. Ni source limited growth) or the implementation of a selective emitter design with pn-junction depths well above 1 mm within the contacted area.