Plated metal adhesion to picosecond laser-ablated silicon solar cells: Influence of surface chemistry and wettability

This study investigated the influence of UV picosecond laser fluence, used to ablate the SiNx antireflection coating for Ni/Cu/Ag plated p-type Si solar cells, on busbar and finger adhesion and cell electrical performance. Surface chemistry was characterised post-ablation and post-pre-treatment in 7:1 buffered oxide etch (BOE) using a combination of X-ray photoelectron spectroscopy and contact angle measurements. Although growth of laser-induced Si oxides increases with increasing laser fluence, these oxides are effectively removed in the BOE pretreatment and therefore do not impact plated metal adhesion with busbar pull forces of 1.9 +/- 0.7 N/mm being achieved when a laser fluence of 0.63 J/cm(2) was used to ablate the busbar openings. It is also revealed that the use of high laser fluence leads to a more hydrophobic surface due to reduced residual SiNx, however the complete wetting of the ablated Si surface can be ensured by the use of surfactants in Ni plating electrolytes. Residual SiNx impacts Ni silicidation and reduces the busbar pull force to values of only 0.8 +/- 0.5 N/mm when average laser fluences <= 0.45 J/cm(2) are used. Finger dislodgement forces are interestingly shown not to be affected by laser fluence and presence of residual SiNx, providing an opportunity to optimise the laser ablation process separately for finger and busbar openings. Finally, it is demonstrated that the use of higher laser fluences does not impact the electrical performance of the Al back surface field cells, with open-circuit voltages of >= 637 mV and fill factors >= 80.4% being demonstrated for cells where the average laser fluence used was varied between 0.35 and 0.63 J/cm(2).