Polycrystalline silicon is a material made of misaligned (polycrystalline) silicon crystal. It occupies an intermediate position between amorphous silicon, in which there is no long-range order, and monocrystalline silicon.
Polycrystalline silicon has an impurity level of 1 part per billion or less.
For what is polycrystalline silicon?
Polycrystalline silicon is used mainly in the electronics industry and in photovoltaic solar energy.
1. Photovoltaic energy
This type of material is essential for the manufacture of photovoltaic cells and solar energy in general.
Polycrystalline silicon is also used in particular applications, such as solar PV. There are mainly two types of photovoltaic panels that can be monocrystalline or polycrystalline silicon.
Polycrystalline solar panels use polycrystalline silicon cells. On the other hand, monocrystalline solar panels use monocrystalline silicon cells. The choice of one type of panel or another will depend on the performance we want to obtain and the budget.
2. Electronics
This material has discreet metallic characteristics. It often replaces aluminum to produce metal parts within semiconductor electronic devices due to the integrated circuit production process’s improved mechanical strength.
It is also used to make capacitors in an integrated environment: metal plates are made with polysilicon, while silicon oxide makes the dielectric interposed between the plates.
It can be obtained with less sophisticated and less expensive techniques than those required for silicone depositions in electronics. Polycrystalline silicon can also be obtained during silicon manufacturing processes.
Polycrystalline photovoltaic panels
Polycrystalline cells have an efficiency that varies from 12 to 21%.
These solar cells are manufactured by recycling discarded electronic components: the so-called "silicon scraps,” which are remelted to obtain a compact crystalline composition.
These silicon residues are melted inside a crucible to create a homogeneous compound that is then cooled in such a way that it generates a crystallization that develops vertically.
Then a loaf of approximately 150-200 kg is obtained which is then cut vertically into parallelepiped ingots. This time horizontal, with another cut, cuts of a thickness similar to single crystal wafers are obtained. In this case, the wafers are cleaned with a soda connection, and then doped with phosphorus for the realization of the PN junctions.
How is polycrystalline silicon produced?
Most of the world's polycrystalline silicon is produced in the form of gray cylindrical rods with a rough dendritic surface. In general, the rods are divided into fragments, which are packed in clean bags of thick polyethylene (5-10 kg).
Splintered rods are shaped like a shell, similar to the kinks of amorphous materials. The cutting of a polysilicon rod is usually studied by controlling the silicon’s quality and analyzing the technological process.
In the center of the bar is a mono or polysilicon "seed". Previously, seeds were obtained by pulling from the atmosphere of electronic grade polysilicon.
The seed’s purity and electrical resistivity have a decisive influence on the final polycrystalline rod’s purity.
From the seed perpendicular to the generatrix, compact crystallites in the form of short needles sprout, with a cross-section of less than 1 mm. With a high sedimentation rate, polysilicon grains often begin to grow dendritically. During the process, the dendrites can even form scaly scabs.
Polycrystalline silicon and conversion efficiency
The quality and purity of this polysilicon is usually lower.
Production in a fluidized bed is more advantageous due to orders of magnitude of more significant surface deposition and, consequently, more complete consumption of the reaction mixture.
On the other hand, said silicon contains a certain amount of amorphous material and fine particles from the reactor lining.
In general, granular silicon has a significantly lower degree of purity than silicon obtained by deposition on fixed bars. It is used most often for the less demanding production of solar quality glass, that is, to build polycrystalline solar panels.