Laser applications on mono- and polycrystalline solar cells

Ablation rates

Processing mono- and polycrystalline silicon wafers with high intensity,
nanosecond laser pulses, absorption always takes place on the surface.The
material is ablated by vapor pressure-induced melt ejection. One of the most
decisive parameters for the ablation depth is pulse duration. Tests have
shown that for power densities well above ablation threshold (typ. 108 W/cm2)
scribing depth is nearly a linear function of pulse duration.

Edge isolation

The decisive factor for solar cell performance is the minimization of recombination
possibilities. In order to obtain high efficiency, front and rear side must
be electrically isolated on the edges. The separation of p-type layers is done
by cutting trenches with qs-Nd:YAG or qs-Nd:Vanadate lasers. Compared
to plasma etching, the productive advantages of laser scribing are better inline
processing and improved material flow. Apart from that, there is no need
for costly etching gases and their disposal. High power density is necessary in
order to effectively eject the melt out of the kerf and to avoid re-deposition
of the molten material. Typical scribing speeds are in the range of 400 to
800 mm/s.

Marking

When it comes to encoding solar cells with the laser, demands on the marking result are high. Microglyph codes are innovative 2 dimensional codes. Other than any conventional matrix or barcode, the basic principle of this technology uses tiny, 45 degree diagonal lines (the micro "glyphs" ) for encoding binary data on the solar cell surface without impairing its electrical conductivity. The encoding is fully readable in spite of the reflection properties of polycrystalline silicon.