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The
PV industry is expected to continue to grow at an annual rate of more than
30%. The well-established technology base and ready availability,
proven performance, and salubrity of silicon, coupled with economies of
scale in larger factories, will likely allow Si to remain the dominant PV
material for the foreseeable future. The
remarkable cost reductions in Siemens-process polycrystalline silicon feedstock
production for PV use is a key contributor to this.
In
ingot growth, the trend for single crystals will be away from the smaller
100- and 125-mm-dia. sizes with more focus on larger diameters.
Despite the potential advantages of FZ material, it is unlikely
that its role in PV will increase significantly because of higher costs
for the crack-free, long cylindrical feedstock it requires and the difficulty
in producing the larger FZ diameters.
In CZ growth, we are likely to see an increased effort to make hot
zones more energy efficient, to grow larger diameters, and to achieve
continuously melt-replenished long growth runs.
There will be a
continuing effort to achieve more wafers per length of ingot, and to take
advantage of potentially higher cell efficiencies afforded by thinner
wafers when back surface fields are used in the cell design.
Multicrystalline casting, directional solidification, and
electromagnetic casting, while playing a dominant role in the Si PV
market during the late 1990's (53% of all ingot-based modules sold in 1998 were multicrystalline)
will likely continue to give way to the higher quality CZ growth which
allows higher cell efficiencies.
In
the ribbon- and sheet-growth technologies, a challenge for dendritic web
growth and edge-supported pulling was to increase areal throughput via
wider ribbons, multiple ribbons, or other approaches.
Even though these methods have the advantage of minimal silicon
consumption and elimination of wafering, it is unlikely that they can
effectively compete due to their current low throughput of 1-2 m2/day.
This is because the effective areal throughputs of CZ growth
are >30 m2/day.
While capillary die growth of octagons produces about 20 m2/day,
the lower material quality and fragility of handling limit their
applicability and the technique has largely been phased out. We may see continued progress in
large-area solid/liquid interface sheets by some variant of the methods
discussed in Silicon Ribbon and Sheet Growth
because the throughput potential is enormous and one growth
furnace could easily generate material for 35 MWp/year or more of solar
cell production. However, enormous improvement in material quality
and minority charge carrier lifetime wil be required.
The
future is expected to bring continued exploration of thin-layer Si growth
approaches, in search of ones that have significant economic advantages
over the best ingot and sheet techniques.
Successful ones will have fast deposition rates, large grain sizes,
high cell efficiencies, compatibility with
low-cost substrates, and amenability to low-cost cell-fabrication schemes.
It is not likely that production of thin-layer Si PV modules will
be a significant fraction of the mainstream PV market in the near term, although they, like the ingot and sheet approaches, would have
substantial advantages over many other thin-film PV approaches.
These include the simple chemistry and relative abundance of the Si
starting material.
The
Earth's crust contains 27.7% Si, in contrast to 0.00002% Cd, 0.00001%
In, 0.000009% Se, and 0.0000002% Te (commonly used thin-film elements).
In addition, crystalline Si benefits from an extremely
well-established technology base, compatibility with SiO2 surface
passivation, relative salubrity with respect to toxicity, and stability
under light exposure. These facts coupled with the amazing cost
reductions achieved in polycrystalline feedstock production and Czochralski
crystal growth are likely to allow it to be the dominant PV technology for the
foreseeable future.
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Roy, A., Chen, Q.S., Zhang, H., Prasad, V., Mackintosh,
B., and Kalejs, J.P. (1999) Presentation at the 11th American
Conference on Crystal Growth & Epitaxy, Tucson August 1-6.
To be published in J. Crystal Growth. |
