Principles of semiconductor solar photovoltaic cell

Crystal structure and electron-hole pairs of semiconductor solar photovoltaic cell

admin on August 3, 2021 0 Comments • Tags: #diamondstructure #intrinsicsemiconductor #latticeconstant #thermalequilibrium

Crystal structure
Solid substances can be divided into two categories, crystalline and non-crystalline, according to the arrangement rules of their particles (atoms, ions, and molecules). Solid substances with a definite melting point are called crystals, such as silicon, gallium arsenide, ice, and general metals; solid substances that have no definite melting point and gradually soften within a certain temperature range when heated are called amorphous materials, such as glass and rosin Wait. All crystals are made up of atoms, molecules, ions or groups of these particles arranged in space according to certain rules. This symmetrical and regular arrangement is called a crystal lattice or crystal lattice, or lattice for short. The smallest crystal lattice is called a crystal packet. The length of the crystal packet in all directions is called the lattice constant; the crystal lattice is arranged periodically and repeatedly to form a crystal. Crystals are divided into single crystals and polycrystals. The whole material is periodically arranged according to the same rule from beginning to end, which is called single crystal. The entire crystal is composed of multiple small crystals (ie, crystal grains) of the same composition and crystal structure, called polycrystals. In polycrystals, the orientation of the order of atoms in each small crystal is different. Amorphous crystals do not have the above characteristics, and the arrangement of the particles that compose them is irregular, but an arrangement of “short-range order and long-range disorder”, so it is also called amorphous. General silicon rods are single crystals, and crude metallurgical silicon and silicon thin films made by evaporation or vapor deposition are polycrystalline silicon, which can also be considered as amorphous silicon.

Figure 1 Atomic structure of silicon
Figure 2 The unit cell structure of crystalline silicon

Figure 1 shows the atomic structure of silicon. Figure 2 shows the unit cell structure of crystalline silicon. It can be regarded as two face-centered cubic unit cells intertwined with each other with a quarter displacement along the diagonal direction. This structure is called a diamond structure. Important semiconductors such as silicon (Si) and germanium (Ge) are all diamond structures. One silicon atom and four adjacent silicon atoms are connected by covalent bonds. These four silicon atoms are exactly on the four vertex corners of a regular tetrahedron, and the center of the tetrahedron is another silicon atom. Silicon atoms can make many planes with the same spacing and parallel to each other, called crystal planes. The direction perpendicular to the normal of the crystal plane is called the crystal direction. All crystal faces with the same crystal orientation are similar, which is called a crystal face family. A crystal can be divided into many crystal face groups. In order to distinguish the different crystal planes and crystal orientations of silicon, it is conceivable to use three mutually perpendicular coordinate axes, and the position of each crystal plane in space is represented by the intersecting relationship of the remaining three coordinate axes. It is usually expressed by the reciprocal of the intercept on each axis, that is, the crystal face index. As shown in Figure 3, where the (111) plane refers to the crystal plane and the coordinate system of the “X, Y, Z” 3 axis of the reciprocal of the intercept is 1 cycle: (110) plane refers to the crystal plane and coordinates The reciprocal of the intercept of the X and Y axes is 1 cycle, and the Z axis of 0 means that it is parallel to the Z axis; (100) plane is any plane that intercepts only the X axis and is parallel to the Y axis and the Z axis. The main crystal planes of common crystalline silicon are these three crystal planes.

Figure 3 Schematic diagram of the main crystal planes of crystalline silicon

Crystals have the characteristics of anisotropy, that is, some physical and chemical properties are very different on different crystal planes.

Electron hole pair
Pure semiconductors are called intrinsic semiconductors. We use the simplified atomic model of the silicon atom to illustrate. When the temperature is T=0K and there is no external excitation, every electron is bound by a covalent bond. At room temperature, or obtain certain energy from the outside (such as light, heating, electromagnetic field excitation, etc.), some valence electrons will gain enough energy to break free from the bondage of covalent bonds and become free electrons, which is called intrinsic excitation . Theories and experiments show that at room temperature (300K), the valence electrons in the silicon covalent bond can be excited into free electrons as long as they obtain an energy greater than the ionization energy (1.1eV), and the free electrons move under the action of an external electric field. The vacancy left in the original covalent bond after the free electron moves is called a hole.

When a hole appears, the valence electron of the neighboring atom is easier to leave the covalent bond where it is located and fill the hole to make a new hole appear in the covalent bond where the valence electron was originally located. This hole may again Filled by the valence electrons of neighboring atoms, new holes appear. The movement of valence electrons filling holes is equivalent to the movement of positively charged holes in both form and effect, and the direction of movement is opposite to the direction of movement of valence electrons. In order to distinguish it from the movement of free electrons, this movement is called hole movement, and holes are regarded as positively charged carriers.

While holes and free electrons are constantly being generated, the original holes and free electrons will continue to recombine to form a balance. Therefore, the conductive substances in semiconductors are free electrons and holes. In the crystal structure of intrinsic semiconductors, each atom is combined with four adjacent atoms. The valence electron of each atom forms an electron pair with a valence electron of another atom. This pair of valence electrons are shared by every two adjacent atoms, and they combine adjacent atoms to form a so-called covalent bond structure, as shown in Figure 4.

Figure 4 Intrinsic silicon covalent bond structure diagram

Free electrons and holes always appear in pairs in intrinsic semiconductors, so they are called “electron-hole pairs”. When free electrons encounter holes in the process of movement, they may be filled in to restore a covalent bond, and at the same time an “electron-hole pair” disappears. This reverse process is called recombination. Under certain temperature conditions, when the number of generated “electron-hole pairs” and the compound “electron-hole pairs” are equal, a relative balance is formed. This kind of relative balance belongs to the dynamic balance when the dynamic balance is reached. Yes” to maintain a fixed number. Unlike metal conductors where there are only free electrons, there are two types of carriers, free electrons and holes, in semiconductors. This is also the difference between semiconductors and conductors in the way they conduct electricity.

If an external effect (such as light) is applied to the semiconductor, the thermal equilibrium condition is destroyed, and the semiconductor is in a state that deviates from the thermal equilibrium state, it is called an unbalanced state. In a non-equilibrium state semiconductor, the part of the carriers that have more carriers than in the equilibrium state is called non-equilibrium carriers.

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