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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. What may be surprising, however, is that more multicrystalline versus single-crystal material is currently used in PV modules. Some (a small fraction) 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.

 

Siliconsultant, P.O. Box 1453, Evergreen, CO 80437 USA                                   e-mailted_ciszek@siliconsultant.com

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This page was last updated on June 19, 2016