Laser microwelding of thin Al layers for interconnection of crystalline Si solar cells: analysis of process limits for ns and μs lasers

Abstract

We investigated a laser welding process for contacting aluminum-metallized crystalline silicon solar cells to a 10-μm-thick aluminum layer on a glass substrate. We analyzed the threshold for laser-induced damage in dependence on the solar cell metallization thickness by applying the process on SiNx passivated lifetime samples. In addition, we measured the mechanical failure stress of the laser welds by perpendicular tear-off. We applied two types of laser processes; one used single or multiple 20-ns-laser pulses at 355 nm with fluences between 12 and 40 J∕cm2, and the other used single 1.2-μs-laser pulses at 1064 nm with 33 to 73 J∕cm2. The ns-laser pulses were able to contact down to 1-μm-thick aluminum layers on silicon without inducing laser damage to the silicon and led to a sufficiently strong mechanical contact. In the case of μs-laser pulses, the limiting thickness was 2 μm. Both laser systems were compared and discussed with respect to the theory of laser-induced forward transfer. The application was shown by contacting solar cells with 2-μm Al metallization on the rear side using μs-laser pulses proving a damage-free contacting.