- The similarities between monocrystalline silicon and polycrystalline silicon are as follows.
(1) Both are elemental elements silicon
Monocrystalline silicon is elemental silicon, and polycrystalline silicon is also elemental silicon. They are all composed of a single element of silicon and are both elemental semiconductors.
(2) Both consist of unit cells
Both monocrystalline and polycrystalline silicon are crystals composed of numerous “unit cells”.
(3) have a fixed melting point
Both monocrystalline silicon and polycrystalline silicon are crystalline, so they have a fixed melting point. Melting occurs only after the temperature reaches the melting point (1420°C), and the temperature remains unchanged during the melting process. As shown in Figure 1, monocrystalline silicon and polycrystalline silicon (see line A) are solid in section AB, and the temperature is constantly rising due to heating; when entering section BC, the temperature of monocrystalline silicon or polycrystalline silicon reaches the melting point (1420°C). , begins to melt, and is in a state of solid-liquid coexistence, and the temperature remains unchanged at this time; after entering the CD segment, monocrystalline silicon or polycrystalline silicon becomes liquid. On the other hand, amorphous silicon (see line B) gradually softens as the temperature increases and gradually melts into a liquid, and there is no fixed melting point.
(4) are the raw materials for the manufacture of photovoltaic cells
Monocrystalline silicon is the earliest raw material for the production of photovoltaic cells, and it is also the raw material for photovoltaic cells with the highest photoelectric conversion efficiency; polycrystalline silicon is a rising star and is the most used raw material for photovoltaic cell production. It can be said that both monocrystalline silicon and polycrystalline silicon are the most important raw materials for the manufacture of photovoltaic cells.
- The difference between monocrystalline silicon and polycrystalline silicon.
(1) Monocrystalline silicon has a neat and regular geometric shape, while polycrystalline silicon does not have a regular and regular geometric shape
Monocrystalline silicon is composed of basic unit cells with exactly the same size, shape, chemical and physical properties. The unit cells that make up monocrystalline silicon are periodically repeated in three-dimensional space according to certain rules, so they have neat and tidy properties. Regular geometric shape, is a perfect crystal. Its shape has a bright metallic luster (see Figure 2-3). Silicon wafers cut from single-crystal silicon rods are also flat, and after grinding, their appearance also has a bright metallic luster.
Polysilicon is also composed of basic unit cells with exactly the same size, shape, chemical and physical properties, but the unit cells that make up polysilicon are not completely unified, and are periodically repeated according to certain rules, but are composed of unit cells one by one. Grains of different sizes and shapes are composed of many grains with different orientations. Therefore, it can be considered that polycrystalline silicon is formed by random arrangement of many small pieces of single crystal silicon. Therefore polysilicon does not have a neat and regular geometry. Therefore, silicon wafers cut from polycrystalline silicon ingots will have obvious patterns after grinding.
(2) Monocrystalline silicon has the characteristics of anisotropy, and polycrystalline silicon does not have the characteristics of anisotropy
Single crystal silicon has different physical and chemical properties in different directions, that is, the performance values measured in different directions are different. This phenomenon is called anisotropy or heterogeneity. Anisotropy is a common property of complete crystals, especially single crystals, so single crystal silicon is no exception. The anisotropy of the crystal is embodied in the elastic modulus, hardness, fracture resistance, yield strength, thermal expansion coefficient, thermal conductivity, resistivity, electrical displacement vector, electrical polarization, magnetic susceptibility and refractive index in different directions of the crystal. different.
Although polycrystalline silicon is also composed of single crystals (grains), this composition is irregular, and the orientation of each single crystal is different. Although each single crystal in polysilicon still maintains its original characteristics, because the polycrystal is randomly arranged by a large number of single crystals, the advantages of some single crystals are offset by the disadvantages, so that the characteristics of the single crystals in all directions are averaged. After the properties of a single crystal in all directions are averaged, no one direction is more dominant than the other, so it is called isotropic. Therefore, polysilicon has no anisotropic characteristics.
Read more: What aspects affect the performance of photovoltaic cells?
