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Molybdenum thin films on glass substrates play an important role as contact layer for thin film solar cells. They can be ablated by picosecond laser pulses irradiated from the substrate side at low laser fluences of less than 1 J cm−2, while structured trenches remain free from thermal damage and residues. The fluence for that so-called direct induced ablation from the substrate side is in contrast to metal side ablation reduced by approximately one order of magnitude and is far below the thermodynamic limit for heating, melting and evaporating the complete layer. For an extended investigation of the direct induced laser ablation and the underlying mechanism, further thin film materials, chromium, titanium and platinum, with thicknesses between 200 nm and 1 µm were examined. Finally, a simple thermo-dynamical model is able to connect the observed ablation energetics with the mechanical ductility and stress limit of the metal thin films.

We report on the selective structuring of CIS (Cu(In,Ga)(S,Se) 2 ) thin film solar cells applying picosecond lasers at 1064 nm. For a monolithic serial interconnection the thin layers are selectively separated by so called laser patterns 1, 2 and 3 (P1, P2 and P3). We demonstrate that the half micron thick molybdenum back electrode can be structured with a P1 process speed of more than 4 m/s without detectable residues and damages by direct induced laser ablation from the back side. A CIS layer (~2 µm thickness) is structured by standard direct laser ablation at higher energy densities and a process speed up to 200 mm/s. A 1.5 µm thick ZnO front electrode layer can be line separated with P3 speed up to several 1000 mm/s by indirect induced laser ablation. We demonstrate that direct induced (P1) and indirect induced (P3) picosecond laser ablation are not purely thermal processes working at energy densities far below the evaporation enthalpy. To increase the scribing speed elliptical and rectangular beam profiles were investigated. Validation of the processes for functionality within a CIS solar cell will be presented.

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