About Crystalline Silicon PV Cells

Crystalline silicon solar cells were the first type of PV cells to be widely commercialized. They are very stable and do not deteriorate significantly with time.  Silicon cells represented more than 90% of the 927 MW of solar cells sold in 2004.  Silicon is in short supply and increasingly costly.  Approximately 45% of the cost of a silicon cell solar module is determined by the cost of the silicon.  Thus efforts are being made to use less silicon in the manufacture of solar cells.  This is being done by making the cells thinner, making them more efficient and using cell structures other than silicon.

Crystalline silicon cells come in four forms:

Single-Crystalline or Monocrystaline Silicone - Made using cells saw-cut from a single cylindrical crystal of silicon. The principle advantage of monocrystalline cells are their higher efficiencies, typically around 15%, although the manufacturing process required to produce monocrystalline silicon is complicated, resulting in slightly higher costs than other forms of silicone cells. The two production process, Czochralski process or float-zone process produce single crystal cylinders or rods. Once the rods are produced, by either the Cz or FZ method, they must be sliced or sawn to form thin wafers.

Poly-crystalline or Multicrystalline Silicon Made from cells cut from an ingot of melted and recrystallized silicon.   In the manufacturing process, molten silicon is cast into ingots of polycrystalline silicon, these ingots are then saw-cut into very thin wafers and assembled into complete cells.  Multicrystalline cells are less expensive to produce than monocrystalline ones, due to the simpler manufacturing process.  However, they tend to be slightly less efficient, with average efficiencies of around 12%.

Ribbon Silicon - A type of single crystal silicon that differs in the way the single crystals are grown. These single crystals may cost less than other processes, because they form the silicon directly into thin, usable wafers of single-crystal silicon. These methods involve forming thin crystalline sheets directly, thus avoiding the slicing step required of cylindrical rods.

Thin-film Silicon:  Another multicrystalline technology where the silicon is deposited in a continuous process onto a base material giving a fine grained, sparkling appearance. The term "thin-film silicon" typically refers to silicon-based PV devices other than amorphous silicon cells and single-crystalline silicon cells (where the silicon layer is thicker than 200 micrometers). These films have high absorptivity of light and may have cell thicknesses of only a few micrometers or less. Nanocrystalline silicon and small-grained polycrystalline silicon—considered thin-film silicon—may replace amorphous silicon alloys as the bottom cell in multijunction devices.

In single-crystal silicon, the molecular structure—which is the arrangement of atoms in the material—is uniform, because the entire structure is grown from the same crystal. This uniformity is ideal for transferring electrons efficiently through the material.

To create silicon in a single-crystal state, high-purity silicon is melted.  Then it is solidified very slowly in contact with a single crystal "seed."  The silicon adapts to the crystal structure of the single-crystal seed as it cools and gradually solidifies.  Because the process is started from a seed, this process is called "growing" a rod of silicon.

Multicrystalline silicon can be produced in a variety of ways. The most popular commercial method uses a casting process in which molten silicon is directly cast into a mold and allowed to solidify into an ingot. The starting material can be a lower-grade silicon, rather that the higher-grade semiconductor grade required for single-crystal material. The cooling rate is one factor that determines the final size of crystals in the ingot and the distribution of impurities. The mold is usually square, producing an ingot that can be cut and sliced into square cells that fit more compactly into a PV module.

One "ribbon growth" technique—edge-defined film-fed growth (EFG)—starts with two crystal seeds that grow and capture a sheet of material between them as they are pulled from a source of molten silicon. A frame entrains a thin sheet of material when drawn from a melt. This technique does not waste much material, but the quality of the material is not as high as Cz and FZ silicon

One way of fabricating thin-film silicon is from SiH₄ +H₂ source gas using plasma-enhanced chemical vapor deposition (PECVD).  This method creates crystal sizes of microcrystalline silicon that are several orders of magnitude smaller than conventional multicrystalline silicon solar cells, but of comparable performance with a conversion efficiency over 10%.

Frequent increases in production capacity of many producers result in changes in the ranking of production capacity of solar cell suppliers.  The top ten manufacturers of solar cells as of the writing of this post, to the best of my ability to research the data, are:

1. Sharp Electronics Corporation, Photovoltaics Division, Nara Prefecture, Japan - monocrystalline and multicrystalline, 400 MW/yr production capacity as of 01/05.  Leading manufacturer in 2004 with 324 MW of production. High efficiency commercial module with 15.8% conversion efficiency as of 6/1/05.
2. Kyocera Corporation, Solar Energy Division, Kyoto - Japan, Multicrystalline, Production capacity 240 MW/yr as of 8/2005. World record for conversion efficiency 17.7% set in 2004 and remains as of 3/2005, commercial cells 14.1% efficiency,
3. Mitsubishi Electric Corporation, Solar Power, Tokyo, Japan - multicrystalline, Expanding production capacity to 230 MW/yr by 2006
4. BP Solar, Frederick, Maryland - monocrystalline and multicrystalline, expanding to 200 MW/yr capacity by 2006. Manufacturing facilities in Australia, India, North America and Spain,
5. Sanyo Electric Co., Osaka,Japan - amorphous silicon/crystalline silicon hybrid cells, 150 MW/yr, 16% module efficiency.
6. Q cells AG, Thalheim, Germany/Evergreen Solar Inc, Marlboro, MA, USA - monocrystalline and multicrystalline, 105 MW/yr, planning on transition to thin film (~100 micrometer) manufacturing process in Marlboro by end of 2005
7. RWE Shott Solar, Alzeneau, Germany - multicrystalline, 100 MW/yr capacity, Germany  Edge-defined Film-fed Growth (EFG) process.
8. Shell Solar, Camarillo, CA, 80 MW/yr  - monocrystalline and multicrystalline, Manufacturing facilities in United States, Germany, and Portugal.
9. Isofoton SA, Madrid, Spain - monocrystalline
10. GE Energy, Solar Power, Newark, DE, USA - monocrystalline

Resource:  EERE Solar Technologies Program/Technologies/Silicon

Technocrati tags: solar, solar cells, Energy, Renewable, Alternative energy

Comments

[Alan]

Very nice (and accurate) overview -- I would have to split hairs to argue with anything you said. I have to say that you certainly seem to do your homework for all of your posts! Wish more bloggers would do the same....

[Robert McLeod]

Anyone know why Evergreen's ribbon panels seem to be getting fairly low efficiency? They only seem to be running about 10 % or so.

[Jim for The Energy Blog]

Not really an answer, but EERE says that the quality of silicon produced by this technique is lower. Evergreen makes this statement about new technology: "Ongoing technology development shows great promise for even lower cost. Early experiments have produced 15% solar cells on thin 100 micron wafers."

[Ryan McGeorge]

Who are the manufactures of the polycrystalline silicon from which the single crystal is made to make the solar cells?

What is their capacity?

[husky air compressor]

I'm working on a green house project. I'm really interested by this Crystalline Silicon PV Cells and I really would like to know if it's reliable for single housing application.
thanks a lot!
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