Rear-Side Contact Opening by Laser Ablation for Industrial Screen-Printed Aluminium Local Back Surface Field Silicon Wafer Solar Cells

Aluminium local back surface field (Al-LBSF) cells are currently being investigated for the next generation of high efficiency industrial silicon wafer solar cells. These Al-LBSF solar cells feature passivating dielectric layers with local contact openings at the rear side of the cell. These local contact openings can be made by inkjet printing, laser ablation or mechanical scribing. Cell efficiencies of up to 19.0% have been achieved for screen-printed p-type Al-LBSF solar cells at SERIS, whereby the dielectric layer was locally opened by laser ablation. This paper reports on the optimisation of the laser process for this kind of solar cell by various characterisation techniques. Optical microscopy and scanning electron microscopy (SEM) are used to determine the degree of ablation of the dielectric layers and to examine the heat affected zone. Photoluminescence measurements are used to study if the electronic quality of the underlying silicon is affected by laser dielectric ablation. We demonstrate that similar solar cell efficiencies can be obtained using either a nanosecond (ns) or a picosecond (ps) laser for the laser ablation process, with the ns laser being an industrially more mature technology. Post-laser etching for laser-induced damage removal is found to be a crucial step to improve the efficiency of those cells which have their contact openings formed with the ns laser.