Nanosecond laser-induced selective removal of the active layer of CuInGaSe_2 solar cells by stress-assisted ablation

摘要

We demonstrate that laser pulses of nanosecond duration (λ = 1064 nm, τ = 25 ns, PRR = 5 kHz) are capable of the clean removal of the CuInGaSe_2 (CIGS) and ZnO:Al layers in the layer structure of chalcogenide-based solar cells, leaving the underlying Mo layer undamaged and producing excellent crater morphology. Our results prove that the material removal process is governed by the thermomechanical stress developing in the CIGS layer due to rapid laser heating. In the mechanical ablation of the active layer, three phenomena play a crucial role, namely, delamination, buckling,and fracture. Morphological and compositional analysis of the laser-processed areas is used to identify the experimental parameters where clean mechanical ablation can be achieved. Numerical calculations, performed in the COMSOL software environment, are also presented to complement the experimental tendencies and verify the proposed model. Our calculation proves the development of a stress distribution that drives the delamination of the CIGS and Mo layers. As the delamination front proceeds radially outward, the separation of the layers ceases in the colder outer regions according to the Griffith's criterion and defines the size of the craters produced afterwards. The free-standing chalcogenide layer continues to deform, and buckling results in a growing tensile stress at the perimeter of the delaminated area, where ultimately fracture will finalize the removal process and facilitate the clean ablation of the laser-irradiated area.