Complete Report - University of New South Wales

During 2005, we have devoted signifi cant effort to optimising and upscaling of the AIT glass
texturing process. We found that considerable freedom is available for varying conditions
used in the AIT process, and that these conditions affect the morphology of the resulting
texture. Prime determinants are the Al layer thickness, anneal temperature profi le, as well
as HF-nitric acid ratio and immersion time. For example, Fig. 4.4.3 shows FIB microscope
images of the surface (plus cross section) of 2.3 µm thick SPC poly-Si fi lms deposited by
PECVD onto three different AIT-textured glass sheets. All were AIT-annealed at a maximum
temperature of 630°C, with differing temperature profi les. The Al thickness and HF-nitric
mixture are varied as follows: 200 nm and 1:20 for sample (a), 400 nm and 1:1 for sample
(b), 600 nm and 1:20 for sample (c). The HF-nitric immersion time was kept to the minimum
needed to remove the silicon residue.
All samples had a similar Voc of about 400 mV after the same SPC crystallisation, rapid
thermal anneal (RTA) and hydrogenation steps, despite the large variation in surface
morphology. In another experiment, AIC seed layers were prepared on the above three AIT
textures. The Al was evaporated, whereas the a-Si was deposited by sputtering. Only sample
(b) was free of pinholes, indicating its promising potential for general use with evaporated
silicon solar cells.
Sample (a) Sample (b) Sample (c)
Figure 4.4.3: Microscope images (with milled trenches to reveal the samples’ cross sections)
of 2.3 µm thick PECVD-based SPC poly-Si fi lms on three AIT-textured glass sheets. The
samples’ structure is: 3.3 mm glass /70 nm SiN /100 nm n++ /2100 nm p- /100 nm p+.
Figure 4.4.4 shows the total measured optical absorption 1-R-T (where R is the sample’s
refl ection and T is its transmission) for the three poly-Si samples, along with that for a planar
PECVD-based poly-Si fi lm with the same layer structure and treatment, in the superstrate
confi guration. These curves are compared with a randomising texture with zero refl ection
at the glass-silicon interface and no back refl ector (top curve; calculated with the software
WVASE). In these measurements, light was collected by an integrating sphere and the
Edwards mount was used. In this arrangement, the sample is placed in the centre of the
integrating sphere and the beam is collimated to illuminate only a small, central area of the
sample. All light emerging from the sample is collected, including R, T, and light from the
edges of the glass (the latter can be signifi cant for textured samples at weakly absorbed
wavelengths, particularly with thick glass). Because edge leakage reduces coupling of light
with the sample, the absorption measured at these wavelengths for the textured samples
underestimates what would be absorbed in a larger solar cell module. Thus, we can say that
the centremount-derived curves in Fig. 4.4.4 represent a conservative estimate of the true
absorption for the textured samples.
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