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Silicon Info


Silicon Info - General: 

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 Valley

Another reason why silicon is a popular choice for PV energy generation is the large technology base that has built up over the past 35 years for silicon used in the semiconductor industry.  It is the backbone of nearly all of modern electronics.

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.
Zhao, J., Wang, A., Altermatt, P., and Green, M.A. (1995) Appl. Phys. Lett. 66, 3636.


♦♦♦♦♦          ted_ciszek @ siliconsultant.com (remove spaces)             ♦♦♦♦♦