Highly n-doped surfaces on n-type silicon wafers by laser-chemical processes

For a novel cell concept based on the combination of silicon heterojunction (SHJ) with metal wrap through (MWT) metallization [1] a highly doped area underneath the metallization grid is required. In the past years, alternative processes for realizing defined doping profiles in n- and p-type silicon solar cells were published [2,3]. Laser doping offers the advantage of a locally confined heat impact causing diffusion of phosphor atoms into the molten silicon.Aim of this work is the development of a low cost laser-chemical doping process resulting in a shallow phosphor profile with high concentrations of electrically active phosphor on the surface. Application of the phosphor source and the diffusion of phosphor into the silicon is done in a one-step process with a single laser type. Sheet resistances of the laser doped areas are below 15 Ohm/square when using a pulsed wave (picosecond) laser source in the infrared region.