A simple copper metallisation process for high cell efficiencies and reliable modules

This work reports on using copper (Cu) as the main conductor as an alternative to screen printed silver (Ag) front contacts for homogeneous emitter silicon solar cells. The work is not only motivated by the limitations that Ag screen printed contacts have regarding solar cell efficiency (high contact shading, limited line conductivity, and poor contact resistance to moderately doped emitters) but also the PV industries desire to reduce Ag usage for cost reasons. A relatively simple process sequence is investigated consisting of i) defining the front contact pattern by ps-UV laser ablation ii) self-aligned plating of the contacts using Ni/Cu/Ag and finally iii) sintering in N2 for nickel silicidation. The process sequence is applied to 12.5x12.5 cm p-type CZ-Si PERC type solar cells with 1μm deep 120Ω/□ homogeneous emitters. Solar cell efficiencies of up to η=20.3% and average pull tab adhesion results >2N/mm are achieved. Thermal cycling and damp heat reliability data for single cell laminates and small modules are reported meeting extended (1.5x) IEC61215 criteria.