Complete Report - University of New South Wales

Solar Cell Design
PL was used to characterize a new method of edge isolation for our n-type cells that involves
a laser formed isolation trench through the emitter and around the solar cell’s edge. Several
test wafers were prepared, half including the new edge isolation technique and the other
half employing the traditional technique of laser-cleaving the edge. Figure 4.3.2.16 shows
the results of Suns-PL and PL imaging used to analyze the effectiveness of the edge isolation
technique. The image in Figure 4.3.2.16a shows the sample in high injection, revealing where,
in order to create a recombination active area, we have deliberately scratched the surface
outside the isolation trench with a diamond pen. Figure 4.3.2.16b shows the same sample in
lower injection, demonstrating the effectiveness of the new process in providing isolation of
the main solar cell area from the edges and scratches.
Figure 4.3.2.16: Photoluminescence images of
1 Ωcm, 240µm thick, n-type wafers with phosphorus
diffusion on the rear side and boron diffusion on
the front side with isolation trench after various
processing steps: a) at ~ 1 Sun ; b) ~ 0.1 Suns
Figure 4.3.2.17 shows the pseudo dark IV and dark local ideality factor of
the samples shown in Figure 4.3.2.16. The curves were generated using the
Suns-PL technique. All of the curves labeled a) have the new trench isolation
technique applied. These curves compare to the non-isolated cases b), c)
and d) in which the effect of edge enhanced junction recombination and edge
shunting are apparent. In this case, PL techniques provided almost immediate
feedback in assessing the effectiveness of the new edge isolation process.
Figure 4.3.2.17: (top) Pseudo dark I-V curves obtained from QSS-PL
measurements on a 1 Ωcm, 240µm thick, n-type wafer with phosphorus
diffusion on the rear side and boron diffusion on the front side after various
processing steps (see text for details); (bottom) corresponding local ideality
factor.
Silicon Nitride – Wafer Handling and Preparation
PL imaging is particularly powerful for developing silicon nitride as a passivation layer because,
unlike thermal oxide fi lms, silicon nitride fi lms show a rich variety of spatially resolved effects,
even in samples with exceptional quality passivation layers.
In 2005, we investigated the application of PL imaging to the development of improved quality
silicon nitride fi lms. We found that PL imaging can be used to understand the deposition
process, to identify and address key manual handling issues, to characterize thermal annealing
processes, and to study the effect of post-deposition chemical processing.
PL imaging can be used to help identify how sample handling can affect the passivation
quality of the SiN fi lms. Figure 4.3.2.18 shows a few PL images that highlight handling related
issues.
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