Optimization of BBr3 diffusion processes for n-type silicon solar cells

In this work, we investigate boron diffusion processes for emitter formation on the front side of n-type Cz-Si solar cells with an edge length of 156 mm. The processes are performed in an industrial tube furnace from Tempress Systems using boron tribromide (BBr3) as liquid dopant source. An initial optimization of BBr3 diffusion processes yields a significant improvement in the homogeneity in sheet resistance Rsh across the wafers and from wafer to wafer for full load runs. A standard deviation of the Rsh across the wafer in the range of 3% is achieved for a mean Rsh ≈ 70 Ω/sq. Dark saturation current density j0e = 60 fA/cm² is extracted from lifetime samples with alkaline textured surface and PECVD Al2O3/SiNx passivation after firing. A second optimization aimed at reducing emitter recombination by decreasing the maximum boron doping concentration Nmax near the surface. By adapting the post-oxidation incorporated within the BBr3 diffusion process, Nmax is decreased to 1.8·1019 cm-3 while the junction depth increases to slightly above 800 nm (Rsh ≈ 115 Ω/sq, below 4%). This results in a reduction in j0e to a value of j0e = 30 fA/cm², which corresponds to an open-circuit voltage limit of 717 mV. Despite the fairly low Nmax, low spe-cific contact resistance below 4 mΩcm² is found for screen-printed and fired contacts using commercially available silver-aluminum paste. Hence, post-oxidation is found to be a promising method for manipulating boron doping profiles while maintaining high homogeneity in Rsh