Complete Report - University of New South Wales

To illustrate the signifi cance of edge leakage, an absorption curve is also shown in Fig. 4.4.4
for sample (a), based on separate R and T measurements with the beam similarly illuminating
a small central area only. Here, edge leakage is not collected by the sphere and absorption is
overestimated. The true value of absorption therefore lies between these two limits.
Figure 4.4.4: Measured absorption
in the three 2.3 µm thick AITtextured
poly-Si samples shown in
Figure 4.4.3. Absorption attained
in decreasing order in the midband
range (around 800 nm) is for
sample (a) (separate R and T scans),
then (a), (c) and (b) (combined R+T
scans). Also shown is absorption
of a planar control sample
(lowest curve, combined R+T
scan) and calculated absorption
with random scatter (top curve).
Absorption
1
0.8
0.6
0.4
0.2
planar
0
400 500 600 700 800 900 1000 1100 1200
Wavelength (nm)
Random scattering
(calculated)
Comparisons of absorption in the measured samples are only relevant where the Si fi lm
absorbs signifi cantly more than the glass sheet. In the random model, the silicon and glass
absorb equally at around 950 nm. This means that comparisons of absorption for each
texture are best limited to the band 500-900 nm. Here, our AIT-textured samples absorb
midway between the planar and random values, see Fig. 4.4.4. Although absorption can be
seen to increase with sample roughness, the difference is not very large. This is important
because crystallisation tends to favour a low level of roughness with evaporated material. At
long wavelengths, the textured samples appear to absorb more than the random model. We
assume this is due to the WVASE modelling relying upon a T scan which includes only a small
amount of absorption at these wavelengths. This high uncertainty in absorption measurement
is passed on to the k value of the silicon, and this uncertainty is further magnifi ed by pathlength
enhancement in the random model. Long-wavelength absorption measured in the samples
is attributed to either defects and grain boundaries, or modifi cation of the glass properties
during RTA and/or hydrogenation.
As an overall estimate of the benefi t from texturing, we have calculated the 1-Sun Jsc available
from the absorption in the wavelength range 400-900 nm. For the planar sample in Fig. 4.4.4
this is 17.3 mA/cm 2 , whereas sample (b) provides 22.4 mA/cm 2 , a boost of 30%. It should
be noted that these estimations of current densities are obtained without a back refl ector.
In summary, good progress has been obtained in 2005 with fi nding AIT process parameters
that provide a texture topography that is well suited for both PECVD-based and evaporationbased
poly-Si fi lms. Furthermore, good progress has been made with upscaling of the process
to larger glass sheets. As a result of this work, AIT-textured glass sheets with a size of 10x10
cm 2 are now routinely produced by the Group.
57