Laser applications on mono- and polycrystalline solar cells

Drilling of wafers

The efficiency of solar cells can be increased by eliminating the front side
contact grids and bus bars, which would otherwise block quite a substantial
amount of light. By means of EWT and MWT, electrical contacts of the front
side are transferred to the reverse of the wafer. The realization of this process
requires to drill holes of different sizes and numbers. Hole diameters from
30-100 μm are produced by percussion drilling. Larger hole diameters
require a relative movement between laser beam and wafer (trepanning,
cutting). Extremely high removal rates can be achieved with qs-disc lasers
with very high TEM00-average power. These lasers realize processing
speeds of up to 5000 holes/second for percussion drilling and up to 25
holes/second for trepanning.

Cutting of wafers

Fast cutting of mono and polycrystalline silicon wafers can be conducted
with very high precision and low heat input by using the same ablation process
as for edge isolation and drilling. In the past, flash-lamp pumped Nd:YAG
lasers were used to melt cut silicon in a single pass with a coaxial gas jet.
Due to rapid cooling of the melt layer at the cut edge, micro cracks were formed.
New approaches indicate that a multi-pass cutting process without assist gas
gives a better surface quality at the edge. With a qs-disc laser, users can
expect typical cutting speeds of up to 150 mm/s for a wafer thickness of 0.2 mm.
With wafer thicknesses above 400 μm and in production areas with a low
degree of automation, silicon wafers are not cut completely, but scribed to a
depth of 30-50% of the cross section. To separate the wafer, a subsequent
snapping, either manual or fully-automated, is required. Typical scribing
speeds are in the range of 50-300 mm/s.