Laser applications on thin film solar cells

Selective ablation

Thin film solar cells are produced through a sequence of vapor deposition and scribing processes. The integrated circuits are generated between the different deposition steps by selective ablation of single layers to achieve electrical isolation. Best beam quality lasers (TEM₀₀) with very high repetition rates of up to 200 kHz are used to ablate 10-50 μm wide lines at scribing speeds of up to 2000 mm/s, without damaging the glass substrate or layers underneath. Thus, Nd:Vanadate lasers with short ns-pulse duration (up to 100 ns) are the standard laser type for this kind of application. The optimum wavelength for the various processes depends on the type of layer. Fundamental (1064nm) and second-harmonic (532nm) and third-harmonic (355nm) wavelength are commonly used in the production of a-Si, CIS and CdS/CdTe solar cells.

Edge deletion

For electrical isolation and hermetic sealing of the module, the complete removal of all layers from the edges of fully processed thin film solar cells on glass substrates is required. In order to meet production requirements, removal rates have to be high (up to 50 cm²/s). Here, the laser challenges conventional techniques, like sand blasting and grinding. Since standard TEM₀₀-lasers (like Nd:Vanadate lasers used for scribing) do not provide sufficient ablation rates for this application, especially developed high-power qs-lasers are applied. Those diode-pumped Nd:YAG lasers generate an average power of up to 850 Watt at 30 kHz which is guided through a 600μm step-index fiber, in order to produce a homogenous, flat-top intensity profile. Typical ablation widths are between 0.7 and 1.5 mm at processing speeds of up to 6000 mm/s.

Square Fibers

As the nature of the process is a shot by shot application a square spot geometry has the advantage, that the overlapping of several pulses is constant across the processing direction. Square homogeneous spots are offered by square fibers. Processing with square homogeneous spots from square fibers allows the optimisation of removal applications (size 1.2mm). Overlapping of pulses is realized by a displacement of subsequent pulses less then one spot width. Square pulses have the advantage that the overlap is constant from the centre of the spot to the edge. Efficient and homogeneous removal results are achieved easily.