Friday, 30 August 2013

Poly Crystalline VS Mono Crystalline Modules

The difference starts during the process of creation.  Monocrystalline silicon is created by slowly pulling a monocrystalline silicon seed crystal out of melted monocrystalline silicon using the Czochralski method to form an ingot of silicon.  A seed crystal is a small piece of silicon which is used as a foundation for the molten molecules.  By having a foundation, the molten molecules are able to connect together faster to form an ingot.  While the seed crystal is being withdrawn, it is rotated slowly and temperature is lowered slowly.  This helps form the cylindrical shape until it has the right diameter which is when temperature remains constant. 

The monocrystalline form is used in the semiconductor device fabrication since grain boundaries would bring discontinuities and favor imperfections in the microstructure of silicon, such as impurities and crystallographic defects, which can have significant effects on the local electronic properties of the material. On the scale that devices operate on, these imperfections would have a significant impact on the functionality and reliability of the devices. Without the crystalline perfection, it would be virtually impossible to build Very Large-Scale Integration (VLSI) devices (figure at right), in which millions (up to billions, circa 2005) of transistor-based circuits, all of which must reliably be working, are combined into a single chip to get e.g. a microprocessor. Therefore, electronic industry has invested heavily in facilities to produce large single crystals of silicon.

Monocrystalline silicon is also used in the manufacturing of high performance solar cells. Since, however, solar cells are less demanding than microelectronics for as concerns structural imperfections, monocrystaline solar grade (Sog-Si) is often used, single crystal is also often replaced by the cheaper polycrystalline or multicrystalline silicon. Monocrystalline solar cells can achieve 17% efficiency whereas other types of less expensive cells including thin film and polycrystalline are only capable of achieving around 10% efficiency.

Few solar charger companies use monocrystalline solar panels because of the higher cost to produce the solar cells, although these higher efficiency products are starting to pop up as consumers demand more efficient products. The 2010 Consumer Electronics Show showcased one of these high-efficiency monocrystalline chargers known as the JOOS Orange and awarded it the 2010 Best of Innovations Award.

Polycrystalline silicon is made through a simpler method.  Instead of going through the slow and more expensive process of creating a single crystal, molten silicon is just put into a cast and cooled with a seed crystal.  By using the casting method, the crystal surrounding the seed isn’t uniform and branches into many, smaller crystals, thus "polycrystalline". 

Currently, polysilicon is commonly used for the conducting gate materials in semiconductor devices such as MOSFETs; however, it has potential for large-scale photovoltaic devices. The abundance, stability, and low toxicity of silicon, combined with the low cost of polysilicon relative to single crystals makes this variety of material attractive for photovoltaic production. Grain size has been shown to have an effect on the efficiency of polycrystalline solar cells. Solar cell efficiency increases with grain size. This effect is due to reduced recombination in the solar cell. Recombination, which is a limiting factor for current in a solar cell, occurs more prevalently at grain boundaries.

The resistivity, mobility, and free-carrier concentration in monocrystalline silicon vary with doping concentration of the single crystal silicon. Whereas the doping of polycrystalline silicon does have an effect on the resistivity, mobility, and free-carrier concentration, these properties strongly depend on the polycrystalline grain size, which is a physical parameter that the material scientist can manipulate. Through the methods of crystallization to form polycrystalline silicon, an engineer can control the size of the polycrystalline grains which will vary the physical properties of the material.

Here are the bullet point differences between the two methods:
  • Price
Monocrystalline solar cells cost more than polycrystalline for the same size.
  • Efficiency
Monocrystalline cells have a higher efficiency than polycrystalline cells due to the structure being made from one large crystal as opposed to many small ones.  In addition to having an overall better efficiency, monocrystalline panels can perform up to 10% better than polycrystalline panels in high ambient temperatures.
  • Size
Since monocrystalline panels are more efficient per area, the size of the solar panel kits is less than a polycrystalline solar panel for the same wattage.  If you are limited on size and want to get the most energy possible, monocrystalline panels are the better choice. That is the main reason Monocrystalline solar kits are more popular.
  • Looks
In terms of looks, monocrystalline panels have a nice uniform color and have a more circular cell shape.  Polycrystalline cells are in squares and have inconsistencies in the color sort of like granite. 
  • Longevity
Even though a monocrystalline panel has the potential to last up to 50 years, most warranties only go up to 25 years which polycrystalline panels are able to reach just fine.
Overall, the production process for monocrystalline silicon is mature, and the process for polycrystalline in still maturing. As purity and process tolerances for polycrystalline Si improves, the performance gaps between the two are narrowing.
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