Copper and Silicon: Implications for the Long-term Stability of Plated Contacts for Silicon Solar Cells

Employing copper (Cu) plating as an alternative to screen-printed silver (Ag) for silicon (Si) solar cells can offer significant cost benefits. However, concerns that Cu may penetrate through nickel (Ni) barrier layers into Si where it impacts cell performance and module durability have contributed to the limited adoption of plated metallisation in manufacturing. This thesis aimed to investigate the thermal stability of Cu-plated Si solar cells with a particular focus on cells with Ni/Cu grids plated on openings formed by selective-ablation of the antireflection coating using a UV ps laser. The formation of Cu precipitates in float-zone-Si was investigated using advanced lifetime spectroscopy of temperature and injection-dependent lifetime curves, with two defect levels identified at Et-Ei = −(0.14-0.16) eV and Et-Ei = (0.08-0.11) eV. Different capture cross-section ratios were obtained for Cu precipitates formed under illumination (k = 1.4 ± 0.2) and ingot-grown precipitates (k = 5.6 ± 1.4). A power-law dependence of the capture cross section was identified (with α=1.35-2.54) and α was observed to increase with increasing precipitate size/density, the latter being theorised to be related to the faster precipitation rate under illumination generating smaller precipitates. A physics of failure methodology was used to characterise Cu diffusion from plated contacts and degradation in Cu-plated p-type PERC and n-type PERT solar cells during thermal testing. Significant Cu diffusion through Ag capping layers was observed during treatments at 200 °C, the extent of the diffusion depending on the capping layer microstructure. Voids which formed as a result of Cu diffusion distorted Suns-VOC measurements, leading to 5-10% errors in the measured pseudo fill factor of the cells. Cross-sectional imaging (FIB/TEM/EDS) and LA-ICP-MS were used to identify that Ni and Cu can penetrate into Si on heating, with increased Ni/Cu detection correlated with increased electrical degradation. Finally, thermal treatments both in the dark and under illumination revealed increased degradation of p-type PERC cells under illumination due to Cu-LID, whilst non-uniformities in the laser-ablation and Ni barrier formation were proposed as the most probable reasons for the observed variable thermal stability of Cu-plated contacts to the p-type PERC and n-type PERT cells