Damage-free Ablation Process for Back-contacted Silicon Heterojunction Solar Cells

This work explores the feasibility of ultra-short femto-second laser pulses in the green and infrared wavelength range (515nm,1030nm), and nano-second pulses (355nm) in the ultra-violet wavelength range to process back-contacted silicon heterojunction solar cells. Two hardmasks -a single silicon dioxide (SiO2) layer and a combination of silicon dioxide and a hydrogenated amorphous silicon (a-Si:H) layer -were compared for a damage-free structuring process. We characterized the quality of ablation and its impact on the device properties by utilizing light and atomic force microscopy, Raman spectroscopy, and photoluminescence imaging. Light and atomic force microscopies were used to identify surface modification, ablation and damage regions. Threshold fluence for these zones was evaluated for both the hardmasks. From Raman spectroscopy no modification in the crystalline structure of the silicon substrate was observed with SiO2+a-Si:H mask after laser irradiation with single pulses. Square regions of 10x10mm 2 were structured in the hard mask by separate and partially overlapping pulses. Using photoluminescence lifetime imaging, we successfully obtained damage-free processing windows of laser fluences and optimal spot overlap distances between adjacent pulses for the back-side emitter and base contact patterning of back-contacted silicon heterojunction solar cells.