Recombination at local aluminum-alloyed silicon solar cell base contacts by dynamic infrared lifetime mapping

The application of local aluminum (Al)-alloyed contacts to the p-type base of silicon solar cells reduces minority charge carrier recombination due to the formation of a local back surface field (LBSF). We study the recombination properties and formation of base contacts, which are realized by local laser ablation of a dielectric stack (laser contact opening - LCO) and subsequent full area screen printing of Al paste. Based on charge carrier lifetime measurements using the camera-based and calibration-free dynamic infrared lifetime mapping (ILM) technique, we determine contact recombination velocities at the contacts as low as S_cont = 65 cm/s on 200 Ωcm float-zone silicon (FZ-Si) and corresponding reverse saturation current densities of J_0,cont = 900 fA/cm² on 1.5 Ωcm FZ-Si. As a result we show that local contact geometries with point contact radii r > 100 μm and line contact widths a > 80 μm are appropriate for lowest contact recombination employing local Al alloyed contacts. Furthermore, complete and high quality laser ablation of the dielectric stack is necessary for the formation of a sufficiently thick LBSF.