Laser Texturing for Multicrystalline Silicon
A method of laser texturing has been developed as
a possible solution to the problem of texturing multicrystalline
silicon. The texture was created by scanning a pulsed laser beam
across the silicon surface. Each pulse causes an ablation pit to
form and, with appropriate spacing a matrix of inverted cones is
formed. After the laser sculpting, the surface can be etched isotropically
in a solution of HF and HNO3. This etch removes the
laser slag, reduces the laser damage and produces smooth features.
Scanning Electron Microscope (SEM) images of the finished textures
on a multicrystalline substrate are shown below.
Plan view of laser textures shows the matrix
of ablation pits (50 µm scale)
Various experiments have explored the basic parameters
of the texturing, including depth of the ablation pits, the spacing
between pits and the etch times.The spectral reflection of various
textures with varying pit depth (no Anti- Reflection coating) were
measured.The spectral-weighted average reflection of the best textures
are comparable with the chemically etched, random pyramids used
on monocrystalline silicon.
Cross-sectional view of laser textures (200 µm
scale)
Textures on high lifetime wafers were surface passivated
with a phosphorus diffusion and dry thermal oxidation then measured
using the generalised photoconductance decay technique. This provided
information on the effect the texture may have on the electrical
properties of the cell. The carrier lifetime and implied open-circuit
voltage after texturing are over 3 ms and 690 mV respectively,
indicating the efficiency potential of laser textured surfaces
for MC wafers. Completed DSBC solar cells on Float Zoned (ZF) wafers
have short-circuit current densities greater than 38 mA/cm2 and
open-circuit voltages of greater than 660 mV, clearly demonstrating
the performance potential of this technique. The texturisation
is being applied to our n-type MC cell fabrication efforts. |
