Atominstitut der Österreichischen Universitäten
Atomic Institute of the Austrian Universities
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The aim of this work was to produce multicrystalline silicon solar cells
and to investigate new front contact patterns for them. On the one hand
we designed patterns similar to the industrially used standard grid (often
called "H"-grid) (for example the patterns "rope", "ellipse") and on the
other hand some looking completely different (such as "bubbles", "UFO").
Besides we investigated the "hexagon" -pattern which is wholly separated
in the visual appearance.
The first intermediate stage was to develop a standard process
for the production of multicrystalline silicon solar cells of a size of
103 x 103 mm². The parameters investigated were for example: etching time, etching mixture, diffusion time and temperature profile, screen printing parameters,
... The final process gave us the opportunity to produce solar
cells good enough for our investigations. We made two series of wafers each with four different patterns.
For statistical reasons each pattern was printed on three cells together
with one standard pattern for comparison purposes.
The first criteria of our investigation were the maximum current and
the maximum power for each front contact pattern. To be able to compare
the different grids we chose the H pattern as a standard. Due to the fact
that the different contact grids shade a different area of the surface
of the silicon wafer we had to make some mathematical corrections to a
normalised cell area. We found that the corrected values for maximum current
and maximum power do not differ too much between the patterns. Only the
"bubbles" show increased and the "rope" decreased values.
Evaluating the current-voltage-curves of the solar cells gave us the
results of series resistance and shunt resistance. The series resistance
of about 150 mOhm is quite in an acceptable range facing the fact of the
technological limitations during the production process. The shunt resistance
(about 400 to 600 mOhm) is much too small which explains the very low
fill factors of the cells (about 35%). We assume an improvement by an optimisation
of the production process (especially the diffusion process).
For an estimation of the width of the space charge region we investigated
the dark I/V-curve. Calculating the J02, the current coming
from the diffusion of charge carries into the space charge region from
the adjacent volumes, we found that our width was in average 100 nm thick
which is rather small.
Dr. Viktor Schlosser
and the members of his team at the Ludwig Boltzmann Institute Vienna made
some additional measurements on our silicon wafers. They characterised
some small parts of solar cells (approximately 1 x 1 mm²). Their results
show:
| Specific wafer resistivity | 0,7 | Ohm.cm |
| Carrier density of the Boron dopant | 3,5.1022 | m-3 |
| Hall mobility | 235.10-4 | m2V-1s-1 |
| Life time of minority charge carriers | 3 | µs |
| Diffusion length of minority charge carriers | 140 | µm |
| Diffusion constant of minority charge carriers | 6,5.10-3 | m2s-1 |
Concluding we can say that the way to work with multicrystalline silicon
is a very likely one for the future. The different front contact patterns
and the results presented in this work let us lead to the conclusion that
with some steps of optimisation there is a good chance to increase the
efficiency of multicrystalline silicon solar cells with the bus bar - based
as well as with the hexagon pattern.
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