Excimer Laser Annealing for Boron Implanted Local BSF for PERC Crystalline Silicon Solar Cells

In this work, the activation of an implanted boron (B) junction by an Excimer Laser Annealing (ELA) process is investigated. The objective is to substitute the standard Aluminium Back Surface Field (Al-BSF) normally used at the rear side of PERC cells by a Boron-BSF, taking advantage of the higher solubility of B in Si compared to Al. The long annealing step used to re-crystallize the amorphized region after implantation is usually performed in an oven at high temperature. In this study, we replaced it by the ELA based on a system from Excico. The laser has a 150 ns pulse duration, a wavelength of 308 nm, a top-hat beam profile with excellent energy uniformity (±2%) and an annealing area up to 400 mm. The samples are analysed by sheet resistance, SIMS profiles and lifetime/J0e for different laser energy densities and implantation conditions (dose and energy), suggesting process conditions for a BBSF implementation. The B-BSF formed by implantation and subsequent ELA shows sheet resistance values as low as 20 Ohm/sq, J0e values down to 200 fA/cm 2 and dopant profiles with surface concentration higher than 10 at/cm.