Complete Report - University of New South Wales

To measure contact resistance to the boron-diffused layer, a stencil with a specially formed
contact pattern consisting of vertical lines spaced progressively further apart was fabricated,
and contacts printed on similar test structures to that shown in Figure 4.3.2.6. Contact
resistances were measured using the transmission-line method, and are shown in Figure
4.3.2.8.
(a)
(b)
Figure 4.3.2.8. Results of contact resistance measurements with a) Ferro 5540 aluminium
and b) Ferro 33-466 silver thick-fi lm conductive paste fi ngers. Peak fi ring time was 10 s.
For aluminium paste fi red at peak temperature for 10 s, area-specifi c contact resistances as
low as 3.85 mΩ.cm2 were obtained, as shown in Figure 4.3.2.8. For silver-aluminium paste
fi red for 10 s, area-specifi c contact resistances were as low as 13.8 mΩ.cm2. Aluminium
was found to be superior in forming ohmic contact to a boron-diffused layer, however, it was
observed that the benefi t of low contact resistance when using aluminium was sometimes
outweighed by the higher bulk resistivity of these pastes.
Paste rheology in stencil printing is critical, and it was found that the lower viscosity of
commercially available aluminium pastes designed for the formation of the rear contact in
p-type cells led to inferior print quality compared to silver pastes, which generally have a
higher viscosity. This leads to fi ll factor degradation in fi nished cells that were stencil printed
with low-viscosity aluminium. Since the inferior contact resistance of silver paste also leads
to fi ll factor degradation, high viscosity aluminium is an ideal choice of paste for use in stencil
printed n-type solar cells.
Fabrication of n-Type Solar Cells with Stencil Printed Contacts
Optimised printing and fi ring parameters for aluminium stencil-printed front contacts on
boron emitters were applied to full solar cell structures. Figure 4.3.2.9 shows results for
a typical cell. It was found that while high fi ll factors were achievable, overall cell effi ciencies
were relatively low due to poor blue response and high front surface recombination velocities.
Passivation of the boron-diffused surface was found to be very diffi cult. Collaboration with
the Universitat Politecnica de Cataluña (UPC) is underway to devise an effective passivation
scheme for the front surface of our cells using either a silicon nitride or silicon carbide layer.
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