Monocrystalline silicon is the base material for the silicon chips used in practically all current electronic equipment. In the field of solar energy, monocrystalline silicon is also used to manufacture photovoltaic cells due to its ability to absorb radiation.
Monocrystalline silicon consists of silicon in which the crystalline lattice of the entire solid is continuous, does not break at its edges and is free of any grain limit. Monocrystalline silicon can be prepared as an intrinsic semiconductor that is composed only of very pure silicon, or can be doped by the addition of other elements such as boron or phosphorus to make p-type or n-type silicon.
Monocrystalline silicon in solar cells
Monocrystalline silicon is also used in the field of solar energy, specifically for the manufacture of high-performance photovoltaic panels. Since there are less stringent demands on structural imperfections compared to microelectronics applications, lower quality solar quality silicon (Sog-Si) is often used for photovoltaic cells.
Efficiency of monocrystalline silicon
With a registered single cell laboratory efficiency of 26.7%, monocrystalline silicon has the highest confirmed conversion efficiency of all commercial photovoltaic technologies. The efficiencies of the solar module for monocrystalline silicon, which are always lower than those of their corresponding cells.
The high efficiency is largely attributable to the lack of recombination sites in the single crystal and better photon absorption due to its black color, compared to the blue hue characteristic of poly-silicon. Since they are more expensive than their polycrystalline counterparts, mono-Si cells are useful for applications where the main considerations are weight limitations or available area, such as in spacecraft or satellites powered by solar energy, where efficiency can be further improved by combining other technologies, such as multi-layer solar cells.
Manufacture of monocrystalline silicon
In addition to the low production rate, there are also concerns about the material wasted in the manufacturing process. The creation of space-saving solar panels requires cutting circular wafers (a product of cylindrical ingots formed through the Czochralski process) into octagonal cells that can be packaged together. The surplus material is not used to create photovoltaic cells and is discarded or recycled, returning to the production of ingots for fusion. In addition, although monocrystalline silicon cells can absorb most of the photons within 20 μm of the incident surface, the limitations in the ingot sawing process mean that the commercial thickness of the wafer is generally around 200 μm.
Production of monocrystalline silicon
Monocrystalline silicon is generally created by one of several methods involving the fusion of high purity semiconductor grade silicon (only a few parts per million impurities) and the use of a seed to initiate the formation of a single continuous crystal. This process is normally carried out in an inert atmosphere, such as argon, and in an inert crucible, such as quartz, to avoid impurities that would affect the uniformity of the crystal.
The most common production method of monocrystalline silicon is the Czochralski process, which immerses a rod-mounted seed crystal with precision in the molten silicon. The bar is then slowly pulled up and rotated simultaneously, allowing the stretched material to solidify into a monocrystalline cylindrical ingot up to 2 meters long and weighing several hundred kilograms. Magnetic fields can also be applied to control and suppress turbulent flow, further improving the uniformity of crystallization.
Other methods are the growth of the floating zone, which passes a polycrystalline silicon rod through a radiofrequency heating coil that creates a localized fused zone, from which a seed glass ingot grows, and Bridgman techniques, They move the crucible through a temperature gradient to cool it from the end of the container containing the seed. The solidified ingots are cut into thin sheets for further processing.
Compared with the molding of polycrystalline ingots, the production of monocrystalline silicon is very slow and expensive. However, the demand for monocrystalline silicon continues to increase due to superior electronic properties - the lack of grain boundaries allows for a better flow of the carrier load and prevents the recombination of electrons, which allows better performance of the integrated circuits and the photovoltaic energy.
Last review: September 3, 2018