TEST & MEASUREMENT
glass sheet, the transparent conductive oxide
electrodes and the active layer are deposited on
the whole substrate surface. Between each
deposition step, the 10 – 50 µm width patterning is
required to separate the layer into cells connected
in serial. The patterns need to prevent possible
contact between the top and bottom electrodes
that would result in a shunt and decrease the
efficiency of the solar module. Specifically, the first
step in creating thin-film solar cells is to coat the
front electrode made of a transparent oxide
(usually the indium tin oxide) onto the glass
substrate. Once the oxide is patterned with scribes,
the panel is coated with a semiconductor (typically
a layer of amorphous silicon) in the chemical vapor
deposition machine. This is then patterned with
second scribes. When the second scribes are
completed, the panel is coated with the back
contact, which is usually aluminum, and then the
final pattern is made.
Solar cell patterning
During edge isolation in c-Si technology, lasers
33
scribe an isolation groove, typically 10 to 20 µm
deep, to eliminate shunt pathways between the
www
front and rear surfaces. The groove must be as
.solar
close as possible to the edges. Otherwise, the cell
efficiency will decrease.
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Roughness created by wet chemical etching
should be as well monitored as to attain the best
efficiency. The rough surface of the wafer traps the
incoming light and reduces backscattering, so that
more elementary p- and n- charges particles are
created in the active layer and efficiency increases. of higher (in 4 to 5times) absorption of shorter- Figure 2. Measurement
Issue I 2009
The roughness needs to be checked in order not wavelength lasers. Shorter wavelength lasers allow of the scribes on the
to compromise the mechanical stability of the solar narrower grooves to be scribed, minimising the surface of the thin-film
panel. “shadow” area around the edges thereby solar cell: 3D View
increasing the efficiency of the cell. (left) and cross-cut
Between each deposition step in thin–film across the scribe
technology, laser patterning is required to separate Solar cell non contact quality control (right)
the layer into cells connected in serial. The The move to still thinner and larger substrates of
patterns 10-30 µm wide should be accurate and solar cells and to the usage of still lesser quantity
prevent possible contact between the top and of semiconductor materials per cell places
bottom electrodes that would result in a shunt and significant requirements on both the tools for
decrease efficiency of the module. The first (front manufacturing solar panels from the grown solar
electrode) pattern is usually scribed with a laser cells and quality control tools as they apply to
operating at 1064 nm wavelength and using different solar cell technologies. Both in
around 12 to 15 W of energy. The second (active manufacturing and quality control, non-contact
layer) and third (back electrode) scribes are made optical devices find widespread usage offering
mostly by green 532 nm lasers. Current systems profound benefits versus any contact-based
rely more heavily on the shorter wavelength (532 alternative. Thinner, larger wafers are increasingly
nm or 355 nm) lasers than on 1064 nm lasers. This fragile and mechanically vulnerable, so any form of
allows highly localised front surface scribing and contact processing can result in lower yields thus
fewer micro-cracks created by shorter wavelength cancelling out technologic advantages related to
lasers due to shallower penetration depth because solar cell production technology. Lasers in
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