Incorporation of deep laser doping to form the rear localized back surface field in high efficiency solar cells

Solar cells with rear localized contacts formed with and without the use of deep boron laser doping are compared as an approach to achieve a more repeatable process for high efficiency solar cells. In particular the paper investigates the impact of the belt firing conditions and screen printable Al paste on the quality of localized contact formation. By adjusting the firing profile, cells incorporating deep boron laser doping on the rear surface are shown to better avoid Kirkendall void formation at contact regions, while the cells without need to balance the percentage of Kirkendall void formation against the localized back surface field (LBSF) thickness. The boron laser doping can be integrated into the laser ablation process used to open the rear dielectric, which minimizes the impact on the process steps. Standard Al paste used for commercial screen printed solar cells is shown to be applicable on the boron laser doped solar cells with a peak firing temperature of 700 °C. On the contrary, a special designed Al paste for LBSF applications has to be used on the cells without boron doping. An average cell efficiency of 19.8% is achieved on boron laser doped solar cells using both types of Al pastes.