Silicon of the Earth
Silicon (Si) is the second most abundant element in the Earth's crust. It makes up about 28% of the crust, always combined with other elements, especially
oxygen (the most abundant element at 46%). The sand on a beach,
silicon dioxide, is one such combination. The ready
availability of this environmentally friendly material is a key factor in why silicon is the
most-used semiconductor for solar or PV cells. In comparison, some
thin-film materials have abundances in the Earth's crust of 0.000015% (Cd);
0.00001% (In); 0.000009% (Se); .0.0000002% (Te). More than 90% of PV
modules sold are based on crystalline silicon. |
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. While less-expensive multicrystalline
wafers from directionally-solidified ingots were dominant for a time,
improvements in low-cost CZ growth have been made. With the additional
advantage of producing higher solar cell efficiencies, CZ material now plays a
dominant role in PV manufacturing. A small
fraction of Si wafers for PV come not from ingots but, rather, ribbons or sheets of silicon solidified in a planar geometry.
The low throughput of most sheet technologies limits their
viability. And those with high throughput generally have poor minority
carrier lifetime and yield low solar cell efficiencies.
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
faster than the growth rate of the electronics industry. Tremendous advances
have been made recently in producing high-purity polycrystalline feedstock at
low costs on the order of $12/kg. Crystalline Si solar cells continue to
dominate over ones formed from thin film, non-silicon materials.
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Zhao, J., Wang, A., Altermatt, P., and Green, M.A. (1995) Appl.
Phys. Lett. 66, 3636.
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