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Nickel silicide contacts formed by excimer laser annealing for high efficiency solar cells


Correspondence: Loic Tous, imec, Kapeldreef 75, 3001 Heverlee, Belgium.



Driven by the relatively high cost of silver (Ag), interest has grown in the photovoltaic (PV) industry to substitute conventional screen printed (SP) Ag front contacts with copper (Cu) plated contacts. The approach chosen here applies selective laser ablation of the front anti-reflection coating (ARC), then forming self-aligned nickel silicides (NiSix) contacts, and thickening the lines by Cu plating to achieve the desired line conductivity. A successful implementation of this scheme requires annealing to form NiSix with low contact resistance. However, it has been shown that industrial shallow emitters can be damaged severely upon conventional annealing of nickel. In this paper, we show that by using large area excimer laser annealing (ELA), NiSix contacts can be formed on industrial shallow emitters without the associated junction degradation. On the basis of sheet resistance, transmission electron microscopy, and lifetime measurements, we demonstrate that NiSix formation by ELA can be achieved in narrow contact openings without damaging the passivation and reflectance properties of the neighboring ARC. In addition, the thresholds for NiSix formation for different Ni thicknesses are quantified by rigorous finite element simulations and compared with experimental data. Finally, high efficiency passivated emitter and rear cell type solar cells featuring a shallow 85 Ω/sq emitter have been processed on large area CZ–Si using laser ablation of the ARC and subsequent NiSix formation by ELA. These cells show an average efficiency gain of 0.4%abs compared with cells processed with reference SP contacts. In this work, the best performing cell with the ELA process reached 20.0% energy conversion efficiency. Copyright © 2013 John Wiley & Sons, Ltd.