Silicon
Photovoltaic Material - General Information
Unlike
silicon crystals used in the electronics industry, crystal perfection,
purity, and uniformity are not necessarily highest on the list of
desirable attributes for crystalline Si incorporated into commercial
PV modules. Tradeoffs
are routinely made, weighing these attributes against cost, throughput,
energy consumption, and other economic factors.
In fact, such tradeoffs for PV use have spawned far more
alternative growth methods for silicon than the many decades of
semiconductor technology development has. Semiconductor applications use the well-known Czochralski
(CZ) technique almost exclusively, with a small contribution (on the order
of 10%-15%) from float-zone (FZ) growth.
Of
course, some FZ material is used in the PV industry, and the highest
recorded silicon solar cell efficiency (the ratio of cell output electrical
power to solar power incident on the cell), 24%, has been achieved with
devices fabricated on FZ wafers (Zhao et
al., 1995). But the device-processing procedures needed to achieve the
high efficiencies are expensive and time consuming. So, as in the semiconductor industry, more CZ wafers
than FZ wafers are used for PV modules. What
may be surprising, however, is that more multicrystalline versus
single-crystal material is currently used in PV modules. Some of this multicrystalline Si is not in the form of wafers from ingots but, rather, ribbons or sheets of silicon solidified in a planar geometry.
Of the 152 peak megawatts (MWp) of PV modules sold throughout the
world in 1998, 132 MWp, or 87%, employed crystalline Si. Comprising this 87% was ~39% fabricated from single-crystal Si
ingots, ~44% made from multicrystalline Si ingots, and ~4% based on
multicrystalline Si ribbons or sheets (Maycock, 1999). The remaining 13% of modules sold are largely amorphous silicon or
non-silicon thin films, which are not discussed here. There is an increasing PV research effort focused on thin-layer
polycrystalline Si deposited on foreign substrates. These approaches have not yet reached a commercialization stage.
An
issue common to all the Si PV growth approaches is the availability of
low-cost polycrystalline Si feedstock. The PV industry has in the past relied on reject silicon from the
electronics industry for use as feedstock. But the PV industry has been growing at an average rate of 20% over
the last five years, which is faster than the growth rate of the
electronics industry. So, the point has been reached where the supply of
reject silicon is insufficient.
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Zhao, J., Wang, A., Altermatt, P., and Green, M.A. (1995) Appl.
Phys. Lett. 66, 3636.
Maycock,
P.D., Ed. (1999) PV News,
February. |
