Copper diffusion through plated nickel barrier layers for laser-doped selective-emitter silicon solar cells

Nickel barrier layers are commonly-used in metallisation of silicon-based microelectronic and photovoltaic devices to prevent copper diffusion from metal contacts to the underlying silicon. This thesis compares the effectiveness of nickel barrier layers, formed using: (i) electroless plating; and (ii) light-induced plating (LIP), in preventing copper diffusion into mono-crystalline silicon laser-doped selective emitter (LDSE) solar cells.Energy dispersive X-ray spectroscopy was used in conjunction with transmission electron microscopy for chemical identification of copper-rich regions in silicon under the nickel/copper metal contacts of heat-treated cells. It was observed that copper can penetrate through both electroless-plated and LIP Ni barrier layers in LDSE cells that have been heated for 5 hours at 200 °C or 400 °C, followed by quenching in ethylene glycol (fast cooling).Current-voltage measurements were used to characterise the electrical performance of LDSE cells with different nickel barrier layers. Significant degradations in cell efficiency, fill factor and open-circuit voltage were observed for LDSE cells with electroless-plated nickel barrier layers after heating at 200 °C and subsequent quenching in ethylene glycol. In the worst case, the fill factor was reduced from 66% to 31%, causing severe efficiency degradation. However, the fill factor of cells with LIP nickel barrier layers were only reduced from 66% to 58% under the same conditions. When cells were cooled slowly the electrical performance of the cells was only slightly affected and was independent of the type of nickel barrier layer. Furthermore, LDSE cells with electroless-plated nickel barrier layers that were originally heat-treated followed by fast cooling, were exposed to a second heat treatment at 250 °C for 2 hours, followed by slow cooling. This resulted in a significant improvement in the lifetime.These results suggest that copper may diffuse through both electroless plated and LIP nickel barrier layers at temperatures as low as 200 °C. However, on slow cooling (which is likely in the field), the copper can out-diffuse to regions of the cell where the cell’s electrical performance is not degraded. These observations suggest that some commonly-accepted views on the ability of nickel barrier layers to prevent copper diffusion in silicon solar cells may need to be challenged and further investigated