Crystal Defects - What are 3D Defects?
  1. What are crystal defects? What are the types of crystal defects?

An ideal crystal is composed of a basic unit cell with exactly the same size, shape, chemical and physical properties. The unit cell is in a three-dimensional space and is completely arranged according to certain rules and is strictly periodically repeated, so it has neat rules. The geometric shape is an extremely perfect crystal. In nature, such perfectly perfect crystals do not exist.

However, in actual crystals, due to the influence of crystal formation conditions, thermal motion of atoms and other conditions, the arrangement of atoms cannot be so complete and regular, and there are often regions that deviate from the ideal crystal structure more or less. These deviations from the full periodic lattice structure are defects in the crystal, which destroy the symmetry of the crystal.

There are many reasons for the formation of defects due to the nature of crystals, so there are also many defects in crystals. According to the types of crystal defects, it can be divided into four types: point defects, line defects, surface defects and bulk defects. According to the reasons for the formation of crystal defects, it can be divided into three types: thermal defects, impurity defects, and non-stoichiometric defects. According to the three-dimensional space occupied by crystal defects, it can be divided into zero-dimensional defects, one-dimensional defects, two-dimensional defects and three-dimensional defects.

  1. What is the relationship between crystal defects and photovoltaic cells?

Due to the existence of crystal defects, crystals exhibit various properties. The reason why silicon photovoltaic cells can generate electricity is completely realized by using the special properties brought by the impurity defects of silicon crystals. When the silicon crystal has impurity defects, P-type silicon and N-type silicon are formed, and the PN junction is formed. With the PN junction, the pure silicon material becomes a silicon photovoltaic cell, which can truly convert light into electricity.

In addition to the positive effects of crystal defects on silicon photovoltaic cells, there are many negative effects. When crystal defects such as dislocations, inclined grain boundaries, and torsional grain boundaries appear in the crystals of silicon photovoltaic cells, the conversion efficiency of photovoltaic cells will be affected, and in severe cases, the entire cell may even be scrapped.

It can be said that crystal defects are very important for silicon photovoltaic cells, which have both decisive positive and negative effects. Therefore, it is necessary to study crystal defects, which is of great significance for both photovoltaic cell fabrication and improvement of photovoltaic cell quality.

  1. What is a 3D defect?

A three-dimensional defect is a crystal defect. In addition to three-dimensional defects, there are zero-dimensional defects, one-dimensional defects and two-dimensional defects.
(1) Three-dimensional defects The so-called three-dimensional generally refers to the space formed by the three axes of length, width and height (X, Y, Z). Three-dimensional defects refer to defects in the crystal in three-dimensional space. It can be said that the crystal defects themselves have length, width and height, that is, a bulk defect. For example, the voids and pits appearing on the crystal belong to the three-dimensional defects of the crystal. These three-dimensional defects are the fatal damage to the crystal, which has a great impact on the crystal itself. In severe cases, the crystal will be scrapped.

(2) Zero-dimensional defect The so-called zero-dimensional defect means that the defect of the crystal is very small, and it is close to zero on the three axes of X, Y, and Z. In fact, it is also a point defect. In fact, points also have length, width, and height, but they are smaller. Generally, the length, width, and height of point defects are on the order of atoms, only a few angstroms. Therefore, zero-dimensional defects are also three-dimensional defects, but they are smaller and can also be said to be miniature three-dimensional defects.

Zero-dimensional defects mainly include self-interstitial atoms, vacancies (Schottky defects), interstitial site impurities, substitution site impurities and Frenkel defects, as shown in Figure 1.

Figure 1 - Zero-dimensional defect map of a crystal
Figure 1 – Zero-dimensional defect map of a crystal

Self-interstitial atoms are the simplest point defects in crystals and refer to silicon atoms that exist in the interstitial spaces of the silicon lattice. A vacancy is the absence of an atom in a lattice position. When the silicon atoms on the lattice enter the gap and form self-interstitial atoms, no atoms occupy the original lattice position, which becomes a space point, that is, a vacancy. Such as a lattice vacancy, if an atom in a normal position runs to the surface and generates a lattice vacancy in the body, this defect is called a Schottky defect; if a lattice atom enters the gap and generates a vacancy, the gap and The vacancies are created at the same time, and this defect is called a Frenkel defect.

Interstitial site impurities refer to foreign atoms present in the interstitial spaces of the silicon lattice.
A substitution site impurity is when a foreign atom occupies the site of a silicon atom on the crystal lattice.
In the process of crystal growth and processing, it is inevitable to contaminate some impurities. These impurities can occupy normal positions in the crystal lattice, or they can exist in interstitial spaces. The existence of these impurities destroys the integrity of the lattice, causes the distortion of the lattice, has an important influence on the electrical properties of the crystal, and changes the properties of the original crystal. For crystals, it may be a disadvantage, but for photovoltaic cells, it is necessary, because silicon photovoltaic cells are made by impurities that cause lattice distortion in the silicon crystal.

(3) One-dimensional defects The so-called one-dimensional defects mean that the defects of the crystal only extend in one direction, that is, the size on the X axis is large, while the size on the Y and Z axes is small, close to zero, it can be said that Just a line. Therefore, it is also called a line defect. Dislocations are line defects commonly found in crystals. In the crystal, some atomic arrangement deviates from strict periodicity, and the relative positions are disordered, resulting in dislocations. Dislocations are generally classified into two types: edge dislocations and screw dislocations.

Dislocations in the crystal can be assumed to be formed by slippage, after which the two parts of the crystal realign. In the slipped crystal plane, dislocations are formed at the boundary between the slipped portion and the non-slipped portion. When the dislocation line is perpendicular to the slip vector, such a dislocation is called an edge dislocation [see Fig. 2(a)]; if the dislocation line is parallel to the slip vector, it is a screw dislocation [see Fig. 2(b)] ].

Figure 2 - Schematic diagram of one-dimensional defects
Figure 2 – Schematic diagram of one-dimensional defects

(4) Two-dimensional defects The so-called two-dimensional defects refer to the defects generated by the crystal deviating from the periodic and regular arrangement in the ideal crystal in the two-dimensional direction, that is, the defects extend in two directions, and in the third direction. The direction is very small, like a plane, so it is also called a surface defect.
There are many kinds of two-dimensional defects, mainly including inclined grain boundaries [see Fig. 3(a)], twisted grain boundaries [see Fig. 3(b)], stacking faults and so on.

Figure 3 - Schematic diagram of two-dimensional defects, Figure 4 - schematic diagram of stacking faults
Figure 3 – Schematic diagram of two-dimensional defects, Figure 4 – schematic diagram of stacking faults

The most obvious two-dimensional defect is the grain boundary of polycrystalline, and the grain boundary is a transition region of atomic dislocation. The ideal crystal is composed of basic unit cells with exactly the same size, shape, chemical and physical properties. The unit cells are in three-dimensional space and are strictly periodic A, B, C layer by layer according to certain rules. Repeated arrangement, but sometimes the arrangement of crystals breaks the periodicity of ideal crystals, and there are fewer rows [see Figure 4(a)], such as layer A after layer C, and layer B is missing; there are also multiple rows [see Figure 4(b)], for example, layer A should be layer B after layer A, but a layer C is added between layers A and B. This kind of disorder due to the level is called stacking fault, or stacking fault for short. Stacking faults with fewer rows are called extraction-type stacking faults, and stacking faults with multiple rows are called insertion-type stacking faults.

Read more: What are the characteristics of light?

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