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seed crystal is being withdrawn, it is rotated slowly and temperature is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.
Resource Box:
Peak Solar is a company that is trying to
turn the world a little bit green. We offer Solar Kits for home that will help
you to get some income and get you to do something on your part for the earth.
Source: http://www.peaksolar.com/
