Dose-dependency of contact resistance and sheet resistance of B-implanted emitters for N-type crystalline Si solar cells fabricated using screen-printed fire-through Ag/Al paste metallization process

The contact resistance and sheet resistance of a screen-printed fire-through Ag/Al paste metallization to the implanted B emitters of n-type crystalline Si solar cells were investigated as a function of B doses in the range of 2 × 10¹⁴ to 5 × 10¹⁵ cm⁻² at a fixed implantation energy of 10 keV. For B emitters implanted with a B dose of > 2 × 10¹⁵ cm⁻², the Ag/Al paste contacts showed very low contact resistance (< 4.3 mΩ⋅cm²) and sheet resistance (< 90 Ω/□), which resulted in a negligible contribution to the series resistance (< 0.1 Ω⋅cm²) in a finished cell with contacts made by floating and non-floating screen-printing techniques. A comparison of the sheet resistance measured between the transfer length method and four-point probe analyses indicated an increase in carrier concentration of B emitter surface associated with additional doping through the Al diffusion from the Ag/Al paste into B emitter. Scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy analysis revealed that after the fire-through process, the Al-containing Pb-based glass layer and numerous small Ag/Al crystallites were formed along the emitter surface without the formation of an Al-Si alloy layer.