Laser lift-off processes have been observed during structuring CIS (copper-indium-diselenide) thin film solar cells with ultra-short laser pulses, if a thin molybdenum film is irradiated from the glass substrate side.
To investigate the underlying physical effects, ultrafast pump-probe microscopy was used to record the transient behavior of the single pulse ablation process. The ablation itself is initiated by a 660 fs pump pulse at a wavelength of 1053 nm. The ultrafast dynamic of the ablation process in the femtosecond and picosecond range is captured by illuminating the laser-material interaction region with an optically delayed 510 fs probe pulse up to 4 ns. Delay times can be extended by a second electronically delayed 600 ps probe laser pulse to record events in the nanosecond and microsecond range, which reveal mechanical deformation.
Time-resolved investigations show that glass side ablation generates a confined gas-liquid mixture after 10 ps. Expansion of the gas and a generated shock wave delaminate the film at about 400 ps. The film then bulges to a maximum at 20 ns, if the fluence is below 0.6 J/cm². At higher fluences an intact Mo disk shears and lifts-off, while at fluences above 0.75 J/cm² a steep reflectivity decrease occurs due to nonlinear absorption in the glass substrate after 1 ps.